Magnetoelectrical Transport Improvements of Postgrowth Annealed

Article Cite This: ACS Appl. Nano Mater. 2018, 1, 3364−3374

www.acsanm.org

Magnetoelectrical Transport Improvements of Postgrowth Annealed Iron−Cobalt Nanocomposites: A Possible Route for Future Room-Temperature Spintronics Marcos V. Puydinger dos Santos,*,†,‡ Sven Barth,§ Fanny Béron,† Kleber R. Pirota,† André L. Pinto,∥ João P. Sinnecker,∥ Stanislav Moshkalev,‡ José A. Diniz,‡ and Ivo Utke*,⊥

ACS Appl. Nano Mater. 2018.1:3364-3374. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 01/15/19. For personal use only.

†

Institute of Physics Gleb Wataghin, University of Campinas, Rua Sérgio Buarque de Holanda 777 Cidade Universitária, Campinas, 13083-859 São Paulo, Brazil ‡ Faculty of Electrical and Computing Engineering and Center for Semiconductor Components and Nanotechnologies, University of Campinas, Av. Albert Einstein 400, Campinas, 13083-852 São Paulo, Brazil § Institute of Materials Chemistry, TU Wien, Getreidemarkt 9/BC/02, A-1060 Vienna, Austria ∥ Centro Brasileiro de Pesquisas FísicasCBPF, Rua Dr. Xavier Sigaud 150 Urca, Rio de Janeiro, 22290-270 Rio de Janeiro, Brazil ⊥ Laboratory for Mechanics of Materials and Nanostructures, EMPA, Swiss Federal Laboratories for Materials Science and Technology, Feuerwerkerstrasse 39, 3602 Thun, Switzerland S Supporting Information *

ABSTRACT: Focused-electron-beam-induced deposition (FEBID) constitutes a direct-writing maskless technique, which has been employed to prepare nanodots, nanolines, as well as laminar and even three-dimensional nanostructures with potential in many technological fields. Here, we report direct-writing of functional nanostructures with the HFeCo3(CO)12 bimetallic carbonyl precursor. The metal content as well as the magnetotransport properties of Fe−Co−C−O deposits were reproducibly tuned upon ex situ postgrowth annealing at 100, 200, and 300 °C in a high-vacuum system. The atomic composition obtained by energy-dispersive X-ray spectroscopy (EDX) analysis revealed that carbonyl groups release during annealing, although principally sole oxygen is released from the deposits, yielding an atomic ratio of Co:Fe:C:O = 52:17:22:9 with respect to the atomic composition of as-grown Co:Fe:C:O = 41:13:26:20. Interestingly, the amorphous carbon contained in the as-grown material turns into graphite nanocrystals with an average size of around 11 nm with annealing at moderate temperatures, as suggested by Raman analysis. These compositional and microstructural changes permit tuning the deposits’ electrical resistivity over 2 orders of magnitude from 4200 down to 65 μΩ cm. The anisotropic magnetoresistance (AMR) of the annealed deposits is about 1.2%, which represents the highest value so far reported for FEBID-grown materials. In addition, as a key feature for technological applications of the postgrowth treatment presented herein, the magnetotransport properties of the nanosized FEBID material degrade minimally after being stored at ambient conditions for more than one year. It turns out that both the iron and cobalt are protected from being oxidized under ambient atmosphere by the graphitic matrix. Furthermore, the incorporation of carbon atoms in ferromagnetic films allows for consistent improvements in their magnetic coercivity and reversing fields. This makes our material especially interesting and advantageous for applications in highdensity magnetic recording devices, nanoelectronics, nanoelectromechanical system-based (NEMS-based) sensors, and logic devices. KEYWORDS: focused-electron-beam-induced deposition, iron−cobalt nanocomposites, nanofabrication, thermally induced oxygen tuning, thermally induced graphitization, magnetoresistance, spin-based devices

■

INTRODUCTION Recently, focused-electron-beam-induced deposition (FEBID) has been proven to be a versatile direct-writing technique for © 2018 American Chemical Society

Received: April 5, 2018 Accepted: June 19, 2018 Published: June 19, 2018 3364

DOI: 10.1021/acsanm.8b00581 ACS Appl. Nano Mater. 2018, 1, 3364−3374

Article

ACS Applied Nano Materials

properties. Our aim was to obtain granular material embedded in a carbon matrix and investigate its magnetotransport properties outperforming the ones with higher metal content. Therefore, we rather chose experimental conditions where we expected a relatively high carbonaceous matrix content, since we could transform it from amorphous to nanocrystalline graphitic with a slight improvement in metal content. Furthermore, annealing temperatures higher than 300 °C present severe limitations for direct-writing of such material into prefabricated semiconductor and micro-/nanoelectromechanical system (MEMS/NEMS) infrastructures. Therefore, we restricted ourselves to short annealing times and temperatures up to 300 °C in this study. Finally, we consider this exceptionally reproducible highvacuum annealing procedure as adequate for the creation of magnetic complex nanocomposite alloys for a wide variety of technological applications, as the annealing process allows controllable alteration of material magnetotransport properties.

local deposition of nanomaterials with tunable morphology in predefined shapes and dimensions (0D−3D), representing an important advantage over conventional lithography.1−13 FEBIDderived nanodevices have been recently demonstrated for many material science applications, such as magnetic,14−16 thermal,17 and strain sensors,18 as well as high-purity ferromagnetic nanostructures.19−21 Furthermore, the ability of defining high-aspectratio 3D nanostructures turns FEBID into an adequate technique for tridimensional nanoprinting,22−25 providing access to geometries for plasmonics,26,27 advanced gas sensing,28 as well as the fabrication of high-resolution magnetic scanning probe tips.29−32 It is worth mentioning that local Pt, W, and Co local depositions have also been realized onto transparent flexible polycarbonate substrates,33 making possible the fabrication of flexible nano-optic, nanoplasmonic, and electronic devices. In FEBID, the precursor molecules adsorb on the substrate before being dissociated by both the primary electron beam and its generated secondary electron cascade, yielding the desired nonvolatile deposit, such as metals, and volatile byproducts.7,14,34−36 A large development has been made toward FEBID deposit growth with high metal content and improved electrical and magnetic properties.1,3,4,37,38 However, nanoscale metal deposits typically contain nonvolatile carbon contamination from the incomplete dissociation of the organometallic precursor molecule or hydrocarbons from background gases.3,7,14,39−42 The undesired codeposited organic compounds of this composite may either degrade the deposit electrical and magnetic properties, hence seriously limiting their applicability, or, when adequately controlled, generate granular or percolated material systems opening new fields of application.43−46 Therefore, recent developments have focused on purification procedures consisting of postgrowth e-beam irradiation under an oxidizing environment, such as O2 or H2O,3,9,39−41,47,48 growth under O2 flow,42 as well as laser-assisted thermally induced dissociation49−51 toward removal of residual carbon from Pt and W deposits. Additionally, high-purity W deposits (around 95 atom %) and high-aspect-ratio carbon pillars were achieved using impactenhanced desorption of residual organic ligands by means of a thermally energized, supersonic argon gas jet to deliver the precursor molecules.52,53 This procedure is especially interesting to both substantially in situ enhance the growth rate and finetune the deposit carbon content by changing the properties of the jet delivering the precursor. Other surface science studies presented the autocatalysis-induced deposition of carbonyl precursors, such as Fe(CO)554,55 and Co2(CO)8,56,57 to obtain pure metallic deposits. Moreover, beam-induced local heating has already been reported as the main effect responsible for increasing the metallic content in Co deposits.7,58−60 Nevertheless, a postgrowth annealing protocol under high vacuum for electrical conductivity improvement of Co−C deposits has been recently proposed,61,62 suggesting that metal−carbon nanocomposite systems can achieve electrical properties similar to pure metals. In addition, this protocol has been successfully demonstrated for 3-dimensional Co−C−O deposits, yielding to high-purity FEBID material with magnetization close to the bulk value.63 High-vacuum annealing is preferred for non-noble metals because of the deposit’s oxidation in contrast to waterand oxygen-based purification methods for noble metal deposits that remain metallic. In this work, we extend this postgrowth protocol, based on annealing under high vacuum, to deposits obtained from a heteronuclear carbonyl HFeCo3(CO)12 precursor,64,65 to produce a metallic alloy with improved both electrical and magnetic

■

EXPERIMENTAL DETAILS

Precursor Characteristics. HFeCo3(CO)12 was synthesized following a modified procedure originally published by Chini et al.66,67 After recrystallization of the compound in toluene, the remaining solvent on the crystal surface is removed under reduced pressure, and no solvent molecules are incorporated in the crystals. The solid HFeCo3(CO)12 precursor is stable under oxygen-free conditions and should be stored in dark vials or protected against sunlight. FEBID Experiments. A Hitachi S-3600 scanning electron microscope (SEM) with a thermionic tungsten filament was employed for FEBID of FexCoyCwOz rectangular patterns of 20 μm × 2 μm, and 160 nm thick. They bridge predefined 100 nm thick Au electrodes obtained by standard UV-lithography on Si wafers with a 200 nm thick SiO2 insulating layer. The beam parameters were 15 kV acceleration voltage, 1.5 nA beam current, and electron flux of 2.6 × 1018 s−1 cm−2. The rectangular scan of 20 μm × 2 μm was performed in a serpentine fashion, with 20 nm pixel-to-pixel distance, 10 μs dwell-time per pixel, 9.6 ms refreshment time, and 800 repetitions. The SEM base pressure was initially 8 × 10−6 mbar, which increased to 3 × 10−5 mbar during deposition. During all the experiments, the substrate was kept at room temperature. A precursor flux of around 1.5 × 1016 molecules s−1 cm−2 (see the Supporting Information, Suppl. 1 section) was impinging on the substrate via a capillary with a 760 μm inner diameter by heating the gas injection system (GIS) up to 65 °C. The in-plane distance between the capillary and the deposition target area was about 50 μm, while a 200 μm gap was fixed between the capillary and the substrate. Postgrowth annealing to increase the deposits’ metal content on asdeposited material was subsequently performed inside the SEM chamber during 10 min at 100, 200, and 300 °C upon stabilization, with a ramp rate of ca. 15 °C min−1, and at pressures around 3−5 × 10−5 mbar. The atomic percentages of the constituent Fe−Co−C−O FEBID materials were monitored by energy-dispersive X-ray spectroscopy (EDX) analysis, using an acceleration voltage and a sample current of 3 kV and 150 pA (extracted using a Faraday cup in the sample holder), respectively. EDX analyses were carried out during 100 s with a takeoff angle of around 32°. The residual carbon signal, originating from the electron-beam-induced contamination by the SEM during the EDX acquisition, was obtained using a standard SiO2/Si sample and subtracted from all carbon atomic content values of the deposits. Furthermore, EDAX TEAM software was used to subtract the detector background signal from the EDX spectra after the acquisition process. The atomic composition of the thin FEBID material was directly calculated from the EDX software, as the substrate signal from SiO2 was not present in the spectra. Atomic force microscopy (AFM, NT-MDT NTEGRA Spectra) was employed to monitor the deposits’ topography. On the other side, Raman spectroscopy was carried out using an upright NT-MDT NTEGRA Raman microscope featuring a laser source with a wavelength of 532 nm and a 50× objective lens with a numerical aperture (NA) of 0.75. The exposure time was fixed at 30 s for all the spectra, which were recorded at a spectral resolution of 3365


Article


Figure 1. Optical microscopy and the corresponding SEM images (gray scale) at higher magnification showing the FEB-induced deposits of 20 × 2 μm2 area and thickness of around 160 nm bridging 4-probe gold electrodes on SiO2/Si substrate for all the annealing temperatures. It is observed that the deposit changes color and sharpens its edges as the annealing temperature increases from parts a to d.

growth rates than those with high electron dose (0.12 μC μm−2) using a Schottky electron emitter.67 AFM of the annealed samples gave thicknesses of 153 ± 4, 148 ± 12, and 140 ± 5 nm for annealing at 100, 200, and 300 °C, respectively, yielding to a related thickness shrinkage of 4%, 8%, and 12% with respect to the as-grown material. The shrinkage can be mainly attributed to oxygen release and metal agglomeration into larger crystals, as will be discussed in the next section. We could observe an average thickness fluctuation of around 8% for the six samples measured at each temperature. The thickness fluctuations may be attributed to process parameters, such as electron-beam flux variation due to astigmatism and focus refinements, as well as molecule flux variations due to thermal gradients in the GIS. Optical microscopy revealed contrast changes between the deposits (Figure 1), which could be attributed to changes in FexCoyCwOz composition. Chemical Characterization. Figure 2a shows EDX spectra containing Fe Lα1,2, Co Lα1,2, O Kα1, and C Kα1 X-ray emission edges obtained at each annealing temperature. The spectra were not normalized, as both SEM and EDX parameters nearly did not vary during each acquisition. The Fe and Co peaks, as well as their ratio, remain almost constant with annealing temperature, while the oxygen and carbon peaks reduce in intensity. Figure 2b presents a tunable atomic composition range of FexCoyCwOz, with 0.13 < x < 0.16 for iron, 0.41 < y < 0.53, for cobalt, 0.26 < w < 0.22, for carbon, and 0.20 > z > 0.09 for oxygen, depending on the postgrowth annealing temperature. Annealing the as-deposited samples improves the metal (Fe + Co) content from 54 atom % for as-grown films to 69 atom % at 300 °C because of the release of oxygen, as well as (CO)x species, thus increasing the relative (Fe + Co):C content. In the same interval, the oxygen percentage decreases from 20 to 9 atom %, while the carbon only very slightly decreases from 26 to 22 atom %. This annealing-based purification mechanism is similar to the one reported for

2.7 cm−1. The microstructure of the Fe- and Co-containing deposits was investigated using a Jeol JEM−2100F transmission electron microscope (TEM). TEM lamellae were prepared for all samples with different annealing temperatures in a TESCAN LYRA 3 FIB instrument, after covering the deposits with a Pt−C protecting layer by FEBID. Techniques including selected area electron diffraction (SAED) and high-resolution TEM (HRTEM) were employed to investigate the crystallographic characteristics of FEBID nanodeposits from HFeCo3(CO)12, as well as its grain size distribution as a function of the postgrowth annealing temperature. Finally, a standard four-probe technique in a physical property measurement system (PPMS) was used to measure the electrical resistivity, ρ, of the Fe−Co−C−O deposits as a function of the temperature, T, as well as the external magnetic field, μoH, over the range 2−300 K and from −3 to 3 T, respectively. An EG&G 5210 lockin amplifier was employed to measure the root-mean-square (rms) voltage, while a function waveform generator Agilent 33250A was synchronously used in series with a 100 kΩ resistance to limit the 500 nA rms drive current with frequency of 17 Hz. The power dissipation in the deposits was limited to 100 nW, hence avoiding selfheating effects and, thus, changes in their atomic composition. It is worth mentioning that the samples were measured with the magnetic field applied both parallel and perpendicular with respect to the current direction, allowing the determination of the anisotropic magnetoresistance (AMR) magnitude for each sample. A distinct design of electrodes was employed for these measurements (see the Supporting Information, Suppl. 2 section), to avoid introducing in the FEBID material an additional shape anisotropy, which is present in the zigzagshaped cross-section of the deposits as seen in Figure 1.

■

EXPERIMENTAL RESULTS AND DISCUSSION Deposit Shape with Annealing Temperature. AFM of the as-grown 162 ± 9 nm thick material (see the Supporting Information, Suppl. 3 section) revealed an average FEBID growth rate of 1.5 ± 0.1 nm min−1 per rectangle area of 40 μm2, which is in good agreement with the low electron dose applied (0.03 μC μm−2),61 representing about 1 order of magnitude lower 3366


Article


Figure 2. (a) Superposed EDX spectra for different annealing temperatures and (b) atomic percentages as a function of postgrowth annealing temperature. EDX measurements were taken using 3 keV electrons to keep the electron range inside the FEBID material, thus eliminating the substrate signal. The L-edges of Co (776 eV) and Fe (705 eV) partially overlap, making the evaluation of the Co:Fe atomic ratio imprecise. Therefore, it is the total metal content (Co + Fe) that is plotted. The Co:Fe ratio of 3 is obtained by considering the Co and Fe K-edges taken at higher primary electron energy (see the Supporting Information, Suppl. 4 section). The purification mechanism occurs predominantly via thermally induced oxygen release, while the C content stays constant within the 2 atom % error margin.

Co2(CO)8, regarding the predominance of oxygen release.61 On the other hand, the as-deposited FEBID material, with an atomic composition of Fe0.13Co0.41C0.26O0.20, showed a lower total metal content of 54 atom %, compared to 80 atom % for deposits obtained from a Schottky electron emitter with a composition of Fe0.20Co0.60C0.10O0.10.67 This reduction may be attributed to very local beam-induced heating effects in finely focused beams45,46,58,59 and/or to the higher amount of precursor injected in our system, which causes an increased process pressure (3.0 × 10−5 mbar against 4.2 × 10−6 mbar from Porrati et al.,67 with a SEM base pressure of 8.0 × 10−6 mbar and 4.1 × 10−6 mbar, respectively). It turns out that both deposit shape and composition were found to be strongly affected by small variations in the process pressure for the Co2(CO)8 precursor,46 a relation that can also be observed, to a lesser degree, for HFeCo3(CO)12.67 Variations in the evaporation rate and, thus, a higher precursor pressure may be attributed to the use of different GIS with specific dimensions and heating systems, as well as to the vacuum system characteristics. A higher precursor flux impinging on the surface can increase the growth rate, but also bury incompletely dissociated adsorbates, yielding to higher carbonyl content deposits. In contrast, studies where individual gas molecules were exposed to monoenergetic electrons in the few eV range presented a complete cleavage of carbonyl ligands in HFeCo3(CO)12. This CO dissociation from the metal atoms was attributed to dissociative electron attachment as well as dissociative ionization.68,69 Surface science studies of electron interaction with condensed films of metal carbonyls showed that incomplete dissociation could also be observed.70−72 In these systems a complementary surface science study using the same precursor revealed the interplay between electronbeam-induced deposition and thermal effects in the decomposition process, leading to pure metallic FeCo.73 On the contrary, we found a metallic content ratio Co:Fe of 3:1 in our deposits, which follows the precursor molecule stoichiometry and compares to experiments carried out with high electron flux using a Schottky electron emitter.67 Furthermore, electroninduced decomposition of COx produces amorphous and graphitic carbon, as well as oxygen species, which can produce

metal oxides.72 However, we did not observe such behavior in our previous Co−C−O FEB deposits.61 The Raman spectra show the carbon peaks from carbon fragments generated by the pristine adsorbate precursor molecules’ dissociation (Figure 3a) and present similar trends with respect to the G peak shift, and integrated intensity ratio of the D and G peaks (ID/IG) (Figure 3b). According to Tuinstra and Koenig,74 Robertson,75 and Ferrari,76 the D and G bands correspond, respectively, to the breathing mode of sp2 sites in rings and to the stretching vibration of sp2 sites in chains or aromatic rings. In other words, the G band has a strong dependence on the presence of graphitic crystallites in the carbon structure. It typically ranges from 1520 to 1600 cm−1 depending on the amount and ordering of the graphitic nanocrystals.77 An upward shift of the G band toward higher wavenumbers corresponds to an increase in graphitization, and nanocrystal ordering. In addition, in the case of amorphous carbon with small grain sizes, or correlation lengths (La), the D mode is proportional to the probability of finding 6-fold rings in the graphite cluster, thus being proportional to the cluster area. Therefore, in amorphous carbon, the rising of a D peak indicates ordering. It is worth mentioning that the D/G integrated peak intensity ratio has been used as a quantitative factor for graphite crystallite size determination in carbon-based structures.77 Therefore, we consider both the G peak shift and the D/G ratio as complementary information to characterize the thermally induced transformation of the amorphous carbon into graphite occurring in our work. While the first reveals the long-range ordering of graphite in the sample, the latter permits the determination of the nanocrystallite size (see the Supporting Information, Suppl. 5 section). A Lorentzian peak fitting was used to deconvolve the D and G carbon bands, allowing their individual integrated intensities to be further extracted. The ID/IG ratios were determined as approximately 0.44, 0.56, 1.29, and 1.79 for the as-grown and annealed deposits at 100, 200, and 300 °C, respectively. It was shown that the ID/IG ranges from 0 (amorphous carbon) to around 2.5 (fully nanocrystalline graphite).74−78 Therefore, values of around 0.5 represent principally amorphous carbon, the main characteristic of our as-grown and annealed at 100 °C Fe−Co−C−O-containing FEBID materials. In addition, the 3367


Article


Figure 3. (a) Raman spectra in the carbon peak range of FEBID material from the HFeCo3(CO)12 precursor as a function of the postgrowth annealing temperature. Both the characteristic disordered, D (1350 cm−1), and graphitic, G (1580 cm−1), bands are shown. (b) The G band peak position and the ratio of intensities (ID/IG) as a function of annealing temperature indicate that the amorphous carbon matrix partially converts into graphite nanocrystals.

and annealed samples at 100, 200, and 300 °C. From the brightfield images shown in Figure 4a,b the as-grown and annealed samples at 100 °C present an amorphous matrix containing crystalline grains of ∼4−6 nm size. A precise determination of the average grain size was hampered by the presence of a heterogeneous contrast of densely agglomerated small grains and particles. However, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images revealed the predominance of subnanometric grains dispersed in the amorphous matrix for the as-grown material (see the Supporting Information, Suppl. 6 section), similarly to previous works using low-electron-density FEBID experiments.58,61,81 SAED analysis performed at different sample regions of the samples shows the presence of face-centered cubic (fcc) FeO and CoO, spinel-type Co3O4 and Fe3O4, as well as body-centered cubic (bcc) Fe nanocrystals for annealing temperatures up to 200 °C, made evident by the diffraction rings (see the Supporting Information, Suppl. 7 section, for more details). Interestingly, the absence of the formation of intermetallic FexCoy compounds is in sharp contrast to the results from Porrati et al.,67 which contained only one bimetallic oxide crystal phase Co2FeO4 and one intermetallic FeCo crystal phase. We may hypothesize that the 2−3 orders higher electron flux in field emission gun (FEG) SEM may lead to an in situ curing of the grown structure volume.4,45,82 Dedicated experiments would be needed to shed light on this observation. Upon annealing, a larger grain structure started to form as the temperature increased up to 200 °C (Figure 4c) and 300 °C (Figure 4d). Coalescence and percolation of metallic grains took place, yielding crystal sizes of around 10−20 nm. It is clear that these crystals do not present a regular shape, although with a reasonable size distribution, being similar to other FEBID works, including, for instance, Porrati,67 Pablo-Navarro,63 and our previous work on Co2(CO)8.61 The Moiré patterns visible in Figure 4c,d were caused by the overlap of single crystalline grains in this densely packed, polycrystalline metallic film. Moreover, the diffraction pattern taken at 300 °C annealing temperature (see inset of Figure 4d and the Supporting Information, Suppl. 7 section) indicates the presence of fcc Co crystals, hence suggesting thermally induced reduction of CoOx at 300 °C, this being consistent with our previous findings for Co FEBID.61 On the other hand, the Fe rings vanished for the deposits annealed at 300 °C, thus suggesting a postgrowth oxidation effect of small

thermal conversion of the carbon at 300 °C suggests the formation of nanocrystalline graphite clusters with size of around 11 nm, in a disordered carbon matrix.74,75,78 We already observed a graphitization of amorphous carbon leading to similar cluster sizes in FEBID materials using the Co2(CO)8, Me2Au(acac), and Cu(hfac)2 metal precursors.61,62 Interestingly, a strong graphitic domain ordering, and thus nanocrystalline graphite conversion, is made evident by the sharp increase in the D/G ratio above 100 °C. This graphitization mechanism differs from the previous work from Kulkarni et al.,77 where a larger graphitization process of amorphous carbon FEBID nanostructures grown on top of a Au thin film mainly occurred above 300 °C. It is worth recalling from the same work that the interface between the carbon and the metal from the substrate might act as a nucleation site for the graphitic crystallite formation inside the amorphous carbon during annealing. Thermally induced interfacial stress can act as a trigger for this process in materials with different thermal properties. In our case, the increased number of metal−carbon interfaces of our grainy Fe−Co−C−O FEBID material (as will be presented in the next section) suggests a significant increase of this process and thus a reduction of the specific temperature at which the graphitic carbon starts forming nanocrystallites. Moreover, the catalytic graphitization ability of transition metals has been reported, revealing a superior graphitization ability for metals like Fe and Co when compared to Au.62,79 Finally, it is noteworthy that amorphous carbon can also be converted into nanocrystalline graphite using low-power light irradiation by means of plasmon-assisted heating at the interface between the amorphous carbon and the substrate. Since the substrate material plays an important role in the graphitization ability,80 it represents an alternative method for improving the graphitization efficiency in FEBID materials. It is worth recalling here that our objective in the present work was to obtain granular material embedded in a carbonaceous matrix and investigate its magnetotransport properties outperforming the ones with higher metal content, as will be shown in the next sections. Therefore, we rather chose the aforementioned experimental conditions, where we expected a relatively high carbonaceous matrix content that we could transform from amorphous to nanocrystalline graphitic with a slight improvement in metal content. Microstructural Characterization. Figure 4 presents HRTEM images of the deposits’ microstructure for the as-grown 3368


Article


Figure 4. High-resolution TEM micrographs of FEBID material (a) as-deposited and annealed at (b) 100, (c) 200, and (d) 300 °C. The metallic nanocrystals coalesce, and crystal sizes increase to form a percolated polycrystalline material as the annealing temperature increases. Thermally induced reduction of CoOx into fcc Co occurred at 300 °C (inset in d). For indexation details of the SAED patterns, see the Supporting Information, Suppl. 7 section.

value is only 50% larger than the value of the purest Fe−Co material (metal content: 84 atom %) defined by FEBID67 and a factor of 2.5 larger than the resistivity of Co−C FEBID material annealed at 300 °C under high vacuum with similar dimensions (metal content: 85 atom %).61 Moreover, despite an approximately constant 69 atom % metal content between 200 and 300 °C (see Figure 2b), the electrical resistivity decreased from 171 down to 65 μΩ cm, respectively. This can be primarily attributed to the coalescence and percolation of metallic grains, yielding to an overall increase in the grain size as shown in Figure 4. Additionally, the reduction of CoOx, as well as the thermally induced transformation of the amorphous carbon into graphite nanocrystals, can also contribute to the resistivity reduction, as was observed in our previous work with Co FEBID.61 It is worth mentioning that the typical resistivity found in graphite FEBID films annealed at 350 °C during 30 min83 is larger than that in our 300 °C annealed FEBID material by a factor of 10. Therefore, the resistivity reduction observed during our postgrowth annealing process can be predominantly attributed to the grain growth of metallic nanocrystals, reduction of CoOx species, and oxygen release. It is important to note that one should not expect the Fe crystal oxidation during annealing to consistently contribute to an increase in the overall resistivity, as both FeO and Fe3O4 phases already coexist for all the annealing temperatures. Temperature-dependent resistance measurements indicated a semiconductor-like conduction behavior for the as-grown FEBID material (Figure 6a), while all the annealed deposits present a metallic conduction mechanism (Figure 6b). The normalized conductivity plotted as a function of the inverse of the temperature (inset of Figure 6a) shows three regions with distinct conduction mechanisms.4,6,84 Region I (temperature > 200 K) presents a lattice-scattering-limited conduction, in which the carriers scatter essentially because of lattice vibrations.85 In addition, regions II (80 K < temperature < 200 K) and III (temperature < 80 K) show, respectively, intrinsic- and extrinsiclike conduction, such as in doped semiconductors.85−87 On the other hand, the annealed samples’ residual-normalized resistivity (measured at 2 K) decreased from 0.88 to 0.80 and finally 0.53 at 100, 200, and 300 °C annealing temperature, respectively.

embedded Fe clusters, most likely from the oxygen contained in the material. Finally, the root-mean-square (rms) surface roughness found via AFM on a 2 × 2 μm2 scanned area increased during the annealing step from 1.5 ± 0.3 nm (as-grown) to 1.5 ± 0.4 nm (100 °C), 2.0 ± 0.5 nm (200 °C), and 2.8 ± 0.4 nm (300 °C). We measured the roughness values for six different samples at each postgrowth annealing condition. This overall deposit surface roughness increase was expected as oxygen releases from the deposits, and metal grains percolate/coalesce to form larger grains during annealing. Electrical Transport Characterization. Figure 5 shows the electrical resistivity values (calculated from the deposit profiles

Figure 5. Room-temperature resistivity of FEBID material from HFeCo3(CO)12 as a function of the postgrowth annealing temperature (4 different samples were measured at each annealing condition). Insets present SEM images taken at 5 keV of the FEBID material bridging four-point gold electrodes. The dashed lines present the best resistivity values reported for FEBID material obtained from Fe(CO) 5, Co2(CO)8, and HFeCo3(CO)12.

inferred by AFM) as a function of the postgrowth annealing temperature, revealing electrical transport improvements from 4200 (as-grown) down to 65 (300 °C annealing) μΩ cm. This last 3369


Article


Figure 6. Temperature-dependent normalized electrical resistivity of (a) as-grown and (b) annealed at 100, 200, and 300 °C FEBID deposits from HFeCo3(CO)12 of length of about 20 μm, width 2 μm, and thickness 160 nm during sample cooling from 300 down to 2 K. Inset: region I (T > 200 K) presents a lattice-scattering-limited conduction, in which the carriers scatter essentially because of lattice vibrations, while regions II (80 K < T < 200 K) and III (T < 80 K) show, respectively, intrinsic- and extrinsic-like conduction, such as in doped semiconductors.

Figure 7. Room-temperature magnetoresistance measurements in both longitudinal and transversal directions of the (a) 200 °C and (b) 300 °C annealed FEBID material from HFeCo3(CO)12. Magnetoresistance is defined as MR = 100 × [ρ(H) − ρ(H = 0)]/ρ(H = 0), while the anisotropic magnetoresistance (AMR) signal can be expressed as AMR = 100 × (ρ∥ − ρ⊥)/ρ. The insets show the relative orientations between the electron current density, J, and the external magnetic field, H.

procedure. Storing the samples at ambient conditions during one year, it turns out that the graphitic matrix provides sufficient protection to prevent the oxidation of metals, as verified by the reproducibility of the resistivity values. Magnetotransport Characterization. Magnetoresistance (MR) in ferromagnetic materials is the electrical resistivity modulation as they are magnetized by means of an external magnetic field. In addition, the anisotropic magnetoresistance (AMR) measures the difference in electrical resistivity when the magnetization lies either perpendicular or parallel to the current density direction, arising from the spin−orbit coupling (SOC) phenomenon. It is highly sensitive to the magnetic domain structure, yielding information about the magnetization reversal process.89−92 MR measurements were carried out on the Fe−Co−C−O deposits at room temperature. The MR signals for both asdeposited and 100 °C annealed deposits were below our detection limit (∼0.1%). On the other hand, the samples annealed at 200 and 300 °C presented an AMR signal (measured at the saturation field ∼3 T) of about 0.6% (Figure 7a) and 1.2% (Figure 7b), respectively. These measurements are highly reproducible as they were performed on five samples fabricated under same annealing conditions (see the Supporting Information,

This last residual-normalized resistivity is about 44% lower compared to those of deposits with similar dimensions and higher Fe + Co content (84 versus 69 atom %).67 Furthermore, the residual resistivity ratio of 0.53 found in our 300 °C annealed deposits is close to the ratio of 0.46 measured for pure 140 nm thick polycrystalline Co films defined by physical vapor deposition (PVD).88 This suggests that this annealing protocol is a suitable tool for achieving pure metal-like properties for nonnoble-metal-containing FEBID material.63 We attribute the residual resistivity reduction for the annealed samples to the combination of oxygen releasing from the deposit (partially by means of CoOx reduction), thermally induced graphitization of amorphous carbon, and grain growth of metallic nanocrystals during the annealing process, hence reducing the grain-boundary scattering of electrons. On the other hand, the discrepancy observed between pure Co metal and our FeCox-containing nanocomposites can be mainly attributed to the presence of residual FeOx in the deposits, which is not as easily reduced under low-vacuum conditions and moderate annealing temperatures as compared to CoOx species. Furthermore, the electrical transport properties’ preservation with minimum degradation over time represents one of the key features for technological applications of this postgrowth annealing 3370


Article

ACS Applied Nano Materials Suppl. 2 section). For the 200 °C annealed sample, no detectable MR signal was found for the longitudinal magnetic field, while a perpendicular MR signal of ca. −0.6% was present. In this case, a coherent magnetization reversal process (namely, uniform rotation of the magnetization) was observed in the symmetric perpendicular MR curve. This result is similar in shape, but larger by a factor of 10, when compared to those found in the literature for the FEBID material from the same HFeCo3(CO)12 precursor and dimensions, using a Schottky electron emitter.67 Additionally, our AMR signal was about the same as those reported for high-purity Co59,93 and Fe44,57 FEBID materials produced with a Schottky electron emitter. Moreover, the 300 °C annealed sample showed parallel and perpendicular MR signals of about 0.3% and −1.0%, respectively. Hysteresis was present only for perpendicular magnetic fields, indicating that the magnetization reversal did not occur via a coherent way for the whole cycle, differently from the observed perpendicular MR results of 200 °C annealed deposits. Those curves present a reversible magnetization rotation, as revealed by the overlapped resistivity curves, together with sudden drops near 0.3 T. We argue that the magnetization reversal begins with the nucleation of a domain wall followed by its propagation through the whole deposit. Then, the successive domain wall pinning and depinning lead to jumps in the MR curves. In other words, by reversing the magnetic field direction, domain walls are nucleated, then propagate, and are pinned by defects/nonmagnetic impurities (such as graphite crystals embedded in the nanocomposite).94,95 This result is consistent with the grain growth of the ferromagnetic material in our deposits with annealing temperature, hence increasing their exchange interactions. Remarkably, our 300 °C annealed Fe−Co−C−O deposits showed the largest MR signal so far reported in the literature for magnetic materials produced by FEBID and was about 20−40% larger than the AMR values obtained for pure Co nanowires.94,96,97 It is worth mentioning that the magnetocrystalline anisotropy is negligible in our FEBID material, given its polycrystalline nature. Therefore, the shape anisotropy was the main factor responsible for the AMR signal in the samples. Furthermore, we attribute the improved AMR signals of our annealed FEBID material from HFe3Co(CO)12 to enhanced SOC by the presence of graphite impurities mixed with metals, mainly for annealing temperatures higher than 200 °C. Such enhancement in carbon-based structures, like graphene, has been recently investigated in combination with surface and interface effects with metals.98,99 On the other hand, new physical phenomena in magnetism have been recently described in graphene−ferromagnet interfaces with low SOC.101,102 Moreover, another work using X-ray magnetic circular dichroism (XMCD) in combination with magnetic force microscopy (MFM) suggests that partially graphitized carbon films exhibit magnetic moment improvements, which can enhance the anisotropy, as well as the magnitude of the orbital moment, and hence the magnetization anisotropy.100 Thus, the changes in magnetic properties in a composite of graphitized carbon and metal particles, when compared to composites with amorphous carbon, are evident, but the exact physical description of these effects is not part of this work and requires in-depth studies. It is also noteworthy from Chen et al.103 that the incorporation of 15 vol % of carbon in ferromagnetic FePt films yields an increase in both coercivity and nucleation fields of around 50% and 125%, respectively, being especially desirable for high-density magnetic recording technologies.104,105 Therefore, our FEBIDbased method for the growth of ferromagnetic metal−graphite

nanosized crystals can be considered as a route for future applications that require large coercivity and reversal fields.

■

CONCLUSION In summary, we have performed ex situ high-vacuum postgrowth annealing of deposits grown by FEBID from the heteronuclear HFeCo3(CO)12 precursor. Our results demonstrate that the electrical transport properties of the as-written Fe−Co−C−O nanocomposite were semiconductor-like for a metal content of 54 atom % in an amorphous carbonaceous matrix and transformed to metallic behavior with a metal content of 69 atom % in a graphitic matrix. The resistivity could be tuned by 2 orders of magnitude upon annealing and reduced to 65 μΩ cm by releasing codeposited oxygen from 20 down to 9 atom %, CoOx reduction, and turning the codeposited amorphous carbon into graphite nanocrystals. No intermetallic FexCoy compound formation was observed during annealing. Magnetoresistance measurements at room temperature of the annealed material showed a signal of around 1.2%, which represents the highest value so far reported for FEBID-grown materials. This is presumably attributed to the SOC enhancement by the graphite nanocrystal formation with interface effects with metals. Furthermore, the nanosized material was proven to maintain its magnetotransport properties after more than one year shelf life without encapsulation and being directly exposed to air. The results shown represent a promising avenue for further FEBID-based works for new room-temperature applications in spintronics, being a forward step for functional nanostructure fabrication of unique spin-based properties that are robust to thermal fluctuations. Finally, we believe that our method can be used toward the realization of new ferromagnetic metal−carbon nanocomposites for the fabrication of high-density magnetic recording devices, MEMS-/NEMS-based sensors, and logic devices.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b00581. AFM analysis; Raman analysis; dark-field images; calculations for the precursor gas flux; samples for magnetoresistance characterization; EDX analysis; and crystallographic analysis (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: puyding@ifi.unicamp.br. Phone: +55 19 997088551. *E-mail: [email protected]. Phone: +41 587656257. ORCID

Marcos V. Puydinger dos Santos: 0000-0001-9971-5994 Sven Barth: 0000-0003-3900-2487 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

All authors acknowledge the financial support from Conselho ́ Nacional de Desenvolvimento e Cientifico e Tecnológico (CNPq, Brazil) through the internship program Science without Borders (Process 200864/2015-7), the COST Action CELINA CM1301, and the Swiss State Secretariat for Education, Research and Innovation SERI (Project C14.0087). 3371


Article

ACS Applied Nano Materials Notes

(15) Gabureac, M. S.; Bernau, L.; Boero, G.; Utke, I. Single Superparamagnetic Bead Detection and Direct Tracing of Bead Position Using Novel Nanocomposite Nano-Hall Sensors. IEEE Trans. Nanotechnol. 2013, 12 (5), 668−673. (16) Boero, G.; Utke, I.; Bret, T.; Quack, N.; Todorova, M.; Mouaziz, S.; Kejik, P.; Brugger, J.; Popovic, R. S.; Hoffmann, P. Submicrometer Hall Devices Fabricated by Focused Electron-Beam-Induced Deposition. Appl. Phys. Lett. 2005, 86 (4), 2003−2006. (17) Edinger, K.; Gotszalk, T.; Rangelow, I. W. Novel High Resolution Scanning Thermal Probe. J. Vac. Sci. Technol., B: Microelectron. Process. Phenom. 2001, 19 (6), 2856−2860. (18) Dukic, M.; Winhold, M.; Schwalb, C. H.; Adams, J. D.; Stavrov, V.; Huth, M.; Fantner, G. E. Direct-Write Nanoscale Printing of Nanogranular Tunnelling Strain Sensors for Sub-Micrometre Cantilevers. Nat. Commun. 2016, 7, 12487-1−12487-7. (19) Gavagnin, M.; Wanzenboeck, H. D.; Belic, D.; Bertagnolli, E. Synthesis of Individually Tuned Nanomagnets for Nanomagnet Logic by Direct Write Focused Electron Beam Induced Deposition. ACS Nano 2013, 7 (1), 777−784. (20) Fernández-Pacheco, a.; De Teresa, J. M.; Szkudlarek, A.; Córdoba, R.; Ibarra, M. R.; Petit, D.; O’Brien, L.; Zeng, H. T.; Lewis, E. R.; Read, D. E.; Cowburn, R. P. Magnetization Reversal in Individual Cobalt Micro- and Nanowires Grown by Focused-Electron-BeamInduced-Deposition. Nanotechnology 2009, 20 (47), 475704. (21) Al Mamoori, M.; Keller, L.; Pieper, J.; Barth, S.; Winkler, R.; Plank, H.; Müller, J.; Huth, M. Magnetic Characterization of DirectWrite Free-Form Building Blocks for Artificial Magnetic 3D Lattices. Materials 2018, 11 (2), 289. (22) Fowlkes, J. D.; Winkler, R.; Brett, L.; Fernández-Pacheco, A.; Skoric, L.; Sanz-Hernández, D.; Stanford, M. G.; Mutunga, E.; Rack, P. D.; Plank, H. High-Fidelity 3D-Nanoprinting via Focused Electron Beams: Growth Fundamentals. ACS Appl. Nano Mater. 2018, 1 (3), 1028−1041. (23) Jesse, S.; Borisevich, A. Y.; Fowlkes, J. D.; Lupini, A. R.; Rack, P. D.; Unocic, R. R.; Sumpter, B. G.; Kalinin, S. V.; Belianinov, A.; Ovchinnikova, O. S. Directing Matter: Toward Atomic-Scale 3D Nanofabrication. ACS Nano 2016, 10 (6), 5600−5618. (24) Fowlkes, J. D.; Winkler, R.; Lewis, B. B.; Stanford, M. G.; Plank, H.; Rack, P. D. Simulation-Guided 3D Nanomanufacturing via Focused Electron Beam Induced Deposition. ACS Nano 2016, 10 (6), 6163− 6172. (25) Keller, L.; Al Mamoori, M. K. I.; Pieper, J.; Gspan, C.; Stockem, I.; Schröder, C.; Barth, S.; Winkler, R.; Plank, H.; Pohlit, M.; Müller, J.; Huth, M. Magnetic Characterization of Direct-Write Free-Form Building Blocks for Artificial Magnetic 3D Lattices. Sci. Rep. 2018, 8, 6160. (26) Winkler, R.; Schmidt, F. P.; Haselmann, U.; Fowlkes, J. D.; Lewis, B. B.; Kothleitner, G.; Rack, P. D.; Plank, H. Direct-Write 3D Nanoprinting of Plasmonic Structures. ACS Appl. Mater. Interfaces 2017, 9 (9), 8233−8240. (27) Höflich, K.; Jurczyk, J.; Zhang, Y.; Puydinger dos Santos, M. V.; Götz, M.; Guerra-Nuñez, C.; Best, J. P.; Kapusta, C.; Utke, I. Direct Electron Beam Writing of Silver-Based Nanostructures. ACS Appl. Mater. Interfaces 2017, 9 (28), 24071−24077. (28) Arnold, G.; winkler, R.; Stermitz, M.; Orthacker, A.; Noh, J.-H.; Fowlkes, J. D.; Kothleitner, G.; Huth, M.; Rack, P. D.; Plank, H. Tunable 3D Nano-Resonators for Gas Sensing Applications. Adv. Funct. Mater. 2018, 28, 1707387. (29) Belova, L. M.; Hellwig, O.; Dobisz, E.; Dan Dahlberg, E. Rapid Preparation of Electron Beam Induced Deposition Co Magnetic Force Microscopy Tips with 10 Nm Spatial Resolution. Rev. Sci. Instrum. 2012, 83 (9), 9−13. (30) Utke, I.; Hoffmann, P.; Berger, R.; Scandella, L. High-Resolution Magnetic Co Supertips Grown by a Focused Electron Beam. Appl. Phys. Lett. 2002, 80 (25), 4792−4794. (31) Fernández-Pacheco, A.; Serrano-Ramón, L.; Michalik, J. M.; Ibarra, M. R.; De Teresa, J. M.; O’Brien, L.; Petit, D.; Lee, J.; Cowburn, R. P. Three Dimensional Magnetic Nanowires Grown by Focused Electron-Beam Induced Deposition. Sci. Rep. 2013, 3, 1492.

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are grateful to IFGW/UNICAMP, CCSNano/ UNICAMP, and EMPA staff for the experimental support, as well as the Electron Microscopy Laboratory at the Brazilian Nanotechnology National Laboratory (LNNano/CNPEM), Campinas, Brazil, for their technical support with the use of the Jeol JEM 2100F TEM-FEG and the Balzer BA510 sputtering. The work was financially supported by the Brazilian funding agencies Fundaçaõ de Amparo à Pesquisa do Estado de São Paulo (FAPESP) and Conselho Nacional de Desenvolví e Tecnológico (CNPq) through the internship mento Cientifico program Science without Borders, and in part by the COST Action CELINA CM1301, and the Swiss State Secretariat for Education, Research and Innovation SERI (Project C14.0087).

■

REFERENCES

(1) Van Dorp, W. F.; Hagen, C. W. A Critical Literature Review of Focused Electron Beam Induced Deposition. J. Appl. Phys. 2008, 104 (8), 81301−81342. (2) Noh, J. H.; Stanford, M. G.; Lewis, B. B.; Fowlkes, J. D.; Plank, H.; Rack, P. D. Nanoscale Electron Beam-Induced Deposition and Purification of Ruthenium for Extreme Ultraviolet Lithography Mask Repair. Appl. Phys. A: Mater. Sci. Process. 2014, 117 (4), 1705−1713. (3) Botman, A.; Mulders, J. J. L.; Hagen, C. W. Creating Pure Nanostructures from Electron-Beam-Induced Deposition Using Purification Techniques: A Technology Perspective. Nanotechnology 2009, 20 (37), 372001. (4) Huth, M.; Porrati, F.; Schwalb, C.; Winhold, M.; Sachser, R.; Dukic, M.; Adams, J.; Fantner, G. Focused Electron Beam Induced Deposition: A Perspective. Beilstein J. Nanotechnol. 2012, 3 (1), 597− 619. (5) Bernau, L.; Gabureac, M.; Utke, I. Variation of the Metallic Content of Focused Electron Beam Induced Deposition of Cobalt. Angew. Chem., Int. Ed. 2010, 49 (47), 8880−8884. (6) Utke, I.; Moshkalev, S.; Russell, P. Nanofabrication Using Focused Ion and Electron Beams: Principles and Applications, 1st ed.; Oxford University Press: New York, 2012. (7) Utke, I.; Hoffmann, P.; Melngailis, J. Gas-Assisted Focused Electron Beam and Ion Beam Processing and Fabrication. J. Vac. Sci. Technol. B Microelectron. Nanom. Struct. 2008, 26 (4), 1197−1276. (8) Toth, M.; Lobo, C.; Friedli, V.; Szkudlarek, A.; Utke, I. Continuum Models of Focused Electron Beam Induced Processing. Beilstein J. Nanotechnol. 2015, 6 (1), 1518−1540. (9) Elbadawi, C.; Toth, M.; Lobo, C. J. Pure Platinum Nanostructures Grown by Electron Beam Induced Deposition. ACS Appl. Mater. Interfaces 2013, 5 (19), 9372−9376. (10) Wnuk, J. D.; Rosenberg, S. G.; Gorham, J. M.; Van Dorp, W. F.; Hagen, C. W.; Fairbrother, D. H. Electron Beam Deposition for Nanofabrication: Insights from Surface Science. Surf. Sci. 2011, 605 (3−4), 257−266. (11) Huth, M.; Porrati, F.; Dobrovolskiy, O. V. Focused Electron Beam Induced Deposition Meets Materials Science. Microelectron. Eng. 2018, 185−186, 9−28. (12) Fowlkes, J. D.; Rack, P. D. Fundamental Electron-PrecursorSolid Deposition Simulations and Experiments. ACS Nano 2010, 4 (3), 1619−1629. (13) Gabureac, M. S.; Bernau, L.; Utke, I. Nanosynthesis of Tunable Composite Materials by Room-Temperature Pulsed Focused Electron Beam Induced Chemical Vapour Deposition. J. Nanosci. Nanotechnol. 2011, 11 (9), 7982−7987. (14) Gabureac, M.; Bernau, L.; Utke, I.; Boero, G. Granular Co−C Nano-Hall Sensors by Focused-Beam-Induced Deposition. Nanotechnology 2010, 21 (11), 115503. 3372


Article

ACS Applied Nano Materials (32) Utke, I.; Cicoira, F.; Jaenchen, G.; Hoffmann, P.; Scandella, L.; Dwir, B.; Kapon, E.; Laub, D.; Buffat, P.; Xanthopoulos, N.; Mathieu, H. J. Focused Electron Beam Induced Deposition of High Resolution Magnetic Scanning Probe Tips. In MRS Proceedings; Cambridge University Press, 2001; Vol. 706. (33) Peinado, P.; Sangiao, S.; De Teresa, J. M. Focused Electron and Ion Beam Induced Deposition on Flexible and Transparent Polycarbonate Substrates. ACS Nano 2015, 9 (6), 6139−6146. (34) Fernández-Pacheco, A.; De Teresa, J. M.; Córdoba, R.; Ibarra, M. R.; Petit, D.; Read, D. E.; O’Brien, L.; Lewis, E. R.; Zeng, H. T.; Cowburn, R. P. Domain Wall Conduit Behavior in Cobalt Nanowires Grown by Focused Electron Beam Induced Deposition. Appl. Phys. Lett. 2009, 94 (19), 192509-1−192509-3. (35) Winkler, R.; Szkudlarek, A.; Fowlkes, J. D.; Rack, P. D.; Utke, I.; Plank, H. Toward Ultraflat Surface Morphologies during Focused Electron Beam Induced Nanosynthesis: Disruption Origins and Compensation. ACS Appl. Mater. Interfaces 2015, 7 (5), 3289−3297. (36) Böhler, E.; Warneke, J.; Swiderek, P. Control of Chemical Reactions and Synthesis by Low-Energy Electrons. Chem. Soc. Rev. 2013, 42 (24), 9219−9231. (37) Serrano-Ramón, L.; Fernández-Pacheco, a.; Córdoba, R.; Magén, C.; Rodríguez, L. a.; Petit, D.; Cowburn, R. P.; Ibarra, M. R.; De Teresa, J. M. Improvement of Domain Wall Conduit Properties in Cobalt Nanowires by Global Gallium Irradiation. Nanotechnology 2013, 24 (34), 345703. (38) Belova, L. M.; Dahlberg, E. D.; Riazanova, a.; Mulders, J. J. L.; Christophersen, C.; Eckert, J. Rapid Electron Beam Assisted Patterning of Pure Cobalt at Elevated Temperatures via Seeded Growth. Nanotechnology 2011, 22 (14), 145305. (39) Geier, B.; Gspan, C.; Winkler, R.; Schmied, R.; Fowlkes, J. D.; Fitzek, H.; Rauch, S.; Rattenberger, J.; Rack, P. D.; Plank, H. Rapid and Highly Compact Purification for Focused Electron Beam Induced Deposits: A Low Temperature Approach Using Electron Stimulated H2O Reactions. J. Phys. Chem. C 2014, 118 (25), 14009−14016. (40) Fowlkes, J. D.; Geier, B.; Lewis, B. B.; Rack, P. D.; Stanford, M. G.; Winkler, R.; Plank, H. Electron Nanoprobe Induced Oxidation: A Simulation of Direct-Write Purification. Phys. Chem. Chem. Phys. 2015, 17 (28), 18294−18304. (41) Mulders, J. J. L. Purity and Resistivity Improvements for Electron-Beam-Induced Deposition of Pt. Appl. Phys. A: Mater. Sci. Process. 2014, 117 (4), 1697−1704. (42) Sachser, R.; Reizh, H.; Huzel, D.; Winhold, M.; Huth, M. Catalytic Purification of Directly Written Nanostructured Pt Microelectrodes. ACS Appl. Mater. Interfaces 2014, 6 (18), 15868−15874. (43) Porrati, F.; Keller, L.; Gspan, C.; Plank, H.; Huth, M. Electrical Transport Properties of Ga Irradiated W-Based Granular Nanostructures. J. Phys. D: Appl. Phys. 2017, 50 (21), 215301. (44) Lavrijsen, R.; Córdoba, R.; Schoenaker, F. J.; Ellis, T. H.; Barcones, B.; Kohlhepp, J. T.; Swagten, H. J. M.; Koopmans, B.; De Teresa, J. M.; Magén, C.; Ibarra, M. R.; Trompenaars, P.; Mulders, J. J. L. Fe: O: C Grown by Focused-Electron-Beam- Induced Deposition: Magnetic and Electric. Nanotechnology 2011, 22 (2), 25302. (45) Córdoba, R.; Sharma, N.; Kölling, S.; Koenraad, P. M.; Koopmans, B. High-Purity 3D Nano-Objects Grown by FocusedElectron-Beam Induced Deposition. Nanotechnology 2016, 27, 355301. (46) Pablo-Navarro, J.; Sanz-Hernández, D.; Magén, C.; FernándezPacheco, A.; Teresa, J. M. De Tuning Shape, Composition and Magnetization of 3D Cobalt Nanowires Grown by Focused Electron Beam Induced Deposition (FEBID). J. Phys. D: Appl. Phys. 2017, 50 (18), 18LT01. (47) Plank, H.; Noh, J. H.; Fowlkes, J. D.; Lester, K.; Lewis, B. B.; Rack, P. D. Electron-Beam-Assisted Oxygen Purification at Low Temperatures for Electron-Beam-Induced Pt Deposits: Towards Pure and High- Fidelity Nanostructures. ACS Appl. Mater. Interfaces 2014, 6 (2), 1018−1024. (48) Lewis, B. B.; Stanford, M. G.; Fowlkes, J. D.; Lester, K.; Plank, H.; Rack, P. D. Electron-Stimulated Purification of Platinum Nanostructures Grown via Focused Electron Beam Induced Deposition. Beilstein J. Nanotechnol. 2015, 6 (1), 907−918.

(49) Roberts, N. A.; Fowlkes, J. D.; Magel, G. a.; Rack, P. D. Enhanced Material Purity and Resolution via Synchronized Laser Assisted Electron Beam Induced Deposition of Platinum. Nanoscale 2013, 5 (1), 408−415. (50) Roberts, N. A.; Gonzalez, C. M.; Fowlkes, J. D.; Rack, P. D. Enhanced by-Product Desorption via Laser Assisted Electron Beam Induced Deposition of W(CO)6 with Improved Conductivity and Resolution. Nanotechnology 2013, 24 (41), 415301. (51) Stanford, M. G.; Lewis, B. B.; Noh, J. H.; Fowlkes, J. D.; Roberts, N. A.; Plank, H.; Rack, P. D. Purification of Nanoscale Electron-BeamInduced Platinum Deposits via a Pulsed Laser-Induced Oxidation Reaction. ACS Appl. Mater. Interfaces 2014, 6 (23), 21256−21263. (52) Henry, M. R.; Kim, S.; Fedorov, A. G. High Purity Tungsten Nanostructures via Focused Electron Beam Induced Deposition with Carrier Gas Assisted Supersonic Jet Delivery of Organometallic Precursors. J. Phys. Chem. C 2016, 120 (19), 10584−10590. (53) Henry, M. R.; Kim, S.; Rykaczewski, K.; Fedorov, A. G. Inert Gas Jets for Growth Control in Electron Beam Induced Deposition. Appl. Phys. Lett. 2011, 98 (26), 263109-1−263109-3. (54) Walz, M. M.; Schirmer, M.; Vollnhals, F.; Lukasczyk, T.; Steinrück, H. P.; Marbach, H. Electrons as “Invisible Ink”: Fabrication of Nanostructures by Local Electron Beam Induced Activation of SiOx. Angew. Chem., Int. Ed. 2010, 49 (27), 4669−4673. (55) Gavagnin, M.; Wanzenboeck, H. D.; Shawrav, M. M.; Belic, D.; Wachter, S.; Waid, S.; Stoeger-Pollach, M.; Bertagnolli, E. Focused Electron Beam-Induced CVD of Iron: A Practical Guide for Direct Writing. Chem. Vap. Deposition 2014, 20 (7−9), 243−250. (56) Muthukumar, K.; Jeschke, H. O.; Valentí, R.; Begun, E.; Schwenk, J.; Porrati, F.; Huth, M. Spontaneous Dissociation of Co2(CO)8 and Autocatalytic Growth of Co on SiO2: A Combined Experimental and Theoretical Investigation. Beilstein J. Nanotechnol. 2012, 3 (1), 546−555. (57) Porrati, F.; Sachser, R.; Walz, M. M.; Vollnhals, F.; Steinrueck, H. P.; Marbach, H.; Huth, M. Magnetotransport Properties of Iron Microwires Fabricated by Focused Electron Beam Induced Autocatalytic Growth. J. Phys. D: Appl. Phys. 2011, 44 (42), 425001. (58) Utke, I.; Michler, J.; Gasser, P.; Santschi, C.; Laub, D.; Canton, M.; Buffat, P. A.; Jiao, C.; Hoffmann, P. Cross Section Investigations of Compositions and Sub-Structures of Tips Obtained by Focused Electron Beam Induced Deposition. Adv. Eng. Mater. 2005, 7 (5), 323− 331. (59) Fernández-Pacheco, A.; De Teresa, J. M.; Córdoba, R.; Ibarra, M. R. Magnetotransport Properties of High-Quality Cobalt Nanowires Grown by Focused-Electron-Beam-Induced Deposition. J. Phys. D: Appl. Phys. 2009, 42 (5), 55005. (60) Rosenberg, S. G.; Landheer, K.; Hagen, C. W.; Fairbrother, D. H. Substrate Temperature and Electron Fluence Effects on Metallic Films Created by Electron Beam Induced Deposition. J. Vac. Sci. Technol., B: Nanotechnol. Microelectron.: Mater., Process., Meas., Phenom. 2012, 30 (5), 51805. (61) Puydinger dos Santos, M. V.; Velo, M. F.; Domingos, R. D.; Zhang, Y.; Maeder, X.; Guerra-Nuñez, C.; Best, J. P.; Béron, F.; Pirota, K. R.; Moshkalev, S.; Diniz, J. A.; Utke, I. Annealing-Based Electrical Tuning of Cobalt−Carbon Deposits Grown by Focused-ElectronBeam-Induced Deposition. ACS Appl. Mater. Interfaces 2016, 8, 32496−32503. (62) Puydinger dos Santos, M. V.; Szkudlarek, A.; Rydosz, A.; GuerraNuñez, C.; Béron, F.; Pirota, K. R.; Moshkalev, S.; Diniz, J. A.; Utke, I. Comparative Study of Post-Growth Annealing of Cu(hfac)2, Co2(CO)8 and Me2Au(acac) Metal Precursors Deposited by FEBID. Beilstein J. Nanotechnol. 2018, 9, 91−101. (63) Pablo-Navarro, J.; Magen, C.; De Teresa, J. M. M. Purified and Crystalline Three-Dimensional Electron-Beam-Induced Deposits: The Successful Case of Cobalt for High Performance Magnetic Nanowires. ACS Appl. Nano Mater. 2018, 1 (1), 38−46. (64) P, R. K. T.; Weirich, P.; Hrachowina, L.; Hanefeld, M.; Bjornsson, R.; Hrodmarsson, H. R.; Barth, S.; Fairbrother, D. H.; Huth, M.; Ingólfsson, O. Electron Interactions with the Heteronuclear Carbonyl Precursor H2FeRu3(CO)13 and Comparison with HFeCo33373


Article


Applications in Carbon Nanoelectronics. Appl. Phys. A: Mater. Sci. Process. 2014, 117 (4), 1659−1674. (84) Huth, M. Granular Metals: From Electronic Correlations to Strain-Sensing Applications. J. Appl. Phys. 2010, 107 (11), 113709-1− 113709-7. (85) Kittel, C. Introduction to Solid State Physics, 8th ed.; Johnson, S., McFadden, P., Eds.; John Wiley & Sons: Hoboken, 2005. (86) Gersten, J. I.; Smith, F. W. The Physics and Chemistry of Materials, 1st ed.; John Wiley & Sons: New York, 2001. (87) Ashcroft, N. W.; Mermin, N. D. Solid State Physics; Cengage Learning, Inc: California, 1976. (88) De Vries, J. W. C. Temperature and Thickness Dependence of the Reistivity of Thin Polycrystalline Aluminium, Cobalt, Nickel, Palladium, Silver and Gold Films. Thin Solid Films 1988, 167 (1−2), 25−32. (89) Mcguire, T. R.; Potter, R. I. Anisotropic Magnetoresistance in Ferromagnetic 3d Alloys. IEEE Trans. Magn. 1975, 11 (4), 1018−1038. (90) Malozemoff, A. P. Anisotropic Magnetoresistance of Amorphous and Concentrated Polycrystalline Iron Alloys. Phys. Rev. B: Condens. Matter Mater. Phys. 1985, 32 (9), 6080−6083. (91) Malozemoff, A. P. Anisotropic Magnetoresistance with Cubic Anisotropy and Weak Ferromagnetism: A New Paradigm. Phys. Rev. B: Condens. Matter Mater. Phys. 1986, 34 (3), 1853−1863. (92) Berger, L. Influence of Spin-Orbit Interaction on the Transport Processes in Ferromagnetic Nickel Alloys, in the Presence of a Degeneracy of the 3d Band. Physica 1964, 30 (6), 1141−1159. (93) Begun, E.; Dobrovolskiy, O. V.; Kompaniiets, M.; Sachser, R.; Gspan, C.; Plank, H.; Huth, M. Post-Growth Purification of Co Nanostructures Prepared by Focused Electron Beam Induced Deposition. Nanotechnology 2015, 26 (7), 75301. (94) Dumpich, G.; Krome, T. P.; Hausmanns, B. Magnetoresistance of Single Co Nanowires. J. Magn. Magn. Mater. 2002, 248 (2), 241−247. (95) Pignard, S.; Goglio, G.; Radulescu, a.; Piraux, L.; Dubois, S.; Declémy, a.; Duvail, J. L. Study of the Magnetization Reversal in Individual Nickel Nanowires. J. Appl. Phys. 2000, 87 (2), 824−829. (96) Brands, M.; Dumpich, G. Experimental Determination of Anisotropy and Demagnetizing Factors of Single Co Nanowires by Magnetoresistance Measurements. J. Appl. Phys. 2005, 98 (1), 143091−14309-5. (97) Leven, B.; Dumpich, G. Resistance Behavior and Magnetization Reversal Analysis of Individual Co Nanowires. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 71 (6), 64411. (98) Soumyanarayanan, A.; Reyren, N.; Fert, A.; Panagopoulos, C. Emergent Phenomena Induced by Spin-Orbit Coupling at Surfaces and Interfaces. Nature 2016, 539, 509−517. (99) Han, W.; Kawakami, R. K.; Gmitra, M.; Fabian, J. Graphene Spintronics. Nat. Nanotechnol. 2014, 9, 794−807. (100) Ohldag, H.; Tyliszczak, T.; Höhne, R.; Spemann, D.; Esquinazi, P.; Ungureanu, M.; Butz, T. Π-Electron Ferromagnetism in Metal-Free Carbon Probed By Soft X-Ray Dichroism. Phys. Rev. Lett. 2007, 98 (18), 187204. (101) Hervé, M.; Dupé, B.; Lopes, R.; Böttcher, M.; Martins, M. D.; Balashov, T.; Gerhard, L.; Sinova, J.; Wulfhekel, W. Stabilizing Spin Spirals and Isolated Skyrmions at Low Magnetic Field Exploiting Vanishing Magnetic Anisotropy. Nat. Commun. 2018, 9, 1015. (102) Yang, H.; Chen, G.; Cotta, A. A. C.; N’Diaye, A. T.; Nikolaev, S. A.; Soares, E. A.; Macedo, W. A. A.; Liu, K.; Schmid, A. K.; Fert, A.; Chshiev, M. Significant Dzyaloshinskii−Moriya Interaction at Graphene−ferromagnet Interfaces due to the Rashba Effect. Nat. Mater. 2018, 17, 605−609. (103) Chen, J. S.; Lim, B. C.; Hu, J. F.; Liu, B.; Chow, G. M.; Ju, G. Low Temperature Deposited L10 FePt−C (001) Films with High Coercivity and Small Grain Size. Appl. Phys. Lett. 2007, 91, 132506. (104) Rausch, T.; Gage, E.; Dykes, J. Heat Assisted Magnetic Recording. Springer Proc. Phys. 2015, 159 (11), 200−202. (105) Stamps, R. L.; Breitkreutz, S.; Åkerman, J.; Chumak, A. V.; Otani, Y.; Bauer, G. E. W.; Thiele, J.-U.; Bowen, M.; Majetich, S. A.; Kläui, M.; Prejbeanu, I. L.; Dieny, B.; Dempsey, N. M.; Hillebrands, B. The 2014 Magnetism Roadmap. J. Phys. D: Appl. Phys. 2014, 47 (33), 333001.

(CO)12: From Fundamental Gas Phase and Surface Science Studies to Focused Electron Beam. Beilstein J. Nanotechnol. 2018, 9, 555−579. (65) Ingólfsson, O.; Ragesh Kumar, T. P.; Unlu, I.; Fairbrother, D. H.; Barth, S. Electron Induced Fragmentation and Deposit Formation from Nano-Meter Thin Surface Layers of HFeCo3(CO)12 Adsorbed on Gold Surfaces. J. Phys.: Conf. Ser. 2017, 875, 62041. (66) Chini, P.; Colli, L.; Peraldo, M. Preparazione a Proprietà Dell’idrocarbonile HFeCo3 (CO)12 E Di Alcuni Composti Derivati Dall’anione [FeCo3 (CO)12]. Gazz. Chim. Ital. 1960, 90, 1005−1020. (67) Porrati, F.; Pohlit, M.; Müller, J.; Barth, S.; Biegger, F.; Gspan, C.; Plank, H.; Huth, M. Direct Writing of CoFe Alloy Nanostructures by Focused Electron Beam Induced Deposition from a Heteronuclear Precursor. Nanotechnology 2015, 26 (47), 475701. (68) T P, R. K.; Barth, S.; Bjornsson, R.; Ingólfsson, O. Structure and Energetics in Dissociative Electron Attachment to HFeCo3(CO)12. Eur. Phys. J. D 2016, 70 (8), 163. (69) T P, R. K.; Barth, S.; Bjornsson, R.; Ingólfsson, O. Formation and Decay of Negative Ion States up to 11 eV above the Ionization Energy of the Nanofabrication Precursor HFeCo3 (CO)12. Chem. Sci. 2017, 8 (9), 5949−5952. (70) Rosenberg, S. G.; Barclay, M.; Fairbrother, D. H. Electron Induced Reactions of Surface Adsorbed Tungsten Hexacarbonyl (W(CO)6). Phys. Chem. Chem. Phys. 2013, 15, 4002−4015. (71) Hauchard, C.; Rowntree, P. A. Low-Energy Electron-Induced Decarbonylation of Fe(CO)5 Films Adsorbed on Au(111) Surfaces. Can. J. Chem. 2011, 89 (10), 1163−1173. (72) Spencer, J. A.; Rosenberg, S. G.; Barclay, M.; Wu, Y. C.; McElwee-White, L.; Fairbrother, D. H. Understanding the ElectronStimulated Surface Reactions of Organometallic Complexes to Enable Design of Precursors for Electron Beam-Induced Deposition. Appl. Phys. A: Mater. Sci. Process. 2014, 117 (4), 1631−1644. (73) T P, R. K.; Unlu, I.; Barth, S.; Ingólfsson, O.; Fairbrother, D. H. Electron Induced Surface Reactions of HFeCo3(CO)12, a Bimetallic Precursor for Focused Electron Beam Induced Deposition (FEBID). J. Phys. Chem. C 2018, 122, 2648−2660. (74) Tuinstra, F.; Koenig, L. Raman Spectrum of Graphite. J. Chem. Phys. 1970, 53 (3), 1126−1130. (75) Robertson, J. Diamond-like Amorphous Carbon. Mater. Sci. Eng., R 2002, 37 (4−6), 129−281. (76) Ferrari, A. C.; Robertson, J. Interpretation of Raman Spectra of Disordered and Amorphous Carbon. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 61 (20), 14095−14107. (77) Kulkarni, D. D.; Rykaczewski, K.; Singamaneni, S.; Kim, S.; Fedorov, A. G.; Tsukruk, V. V. Thermally Induced Transformations of Amorphous Carbon Nanostructures Fabricated by Electron Beam Induced Deposition. ACS Appl. Mater. Interfaces 2011, 3 (3), 710−720. (78) Cançado, L. G.; Takai, K.; Enoki, T.; Endo, M.; Kim, Y. A.; Mizusaki, H.; Jorio, A.; Coelho, L. N.; Magalhães-Paniago, R.; Pimenta, M. A. General Equation for the Determination of the Crystallite Size La of Nanographite by Raman Spectroscopy. Appl. Phys. Lett. 2006, 88 (16), 163106-1−163106-3. (79) O̅ ya, A.; O̅ tani, S. Catalytic Graphitization of Carbons By Various Metals. Carbon 1979, 17 (2), 131−137. (80) Kulkarni, D. D.; Kim, S.; Fedorov, A. G.; Tsukruk, V. V. LightInduced Plasmon-Assisted Phase Transformation of Carbon on Metal Nanoparticles. Adv. Funct. Mater. 2012, 22 (10), 2129−2139. (81) Córdoba, R.; Fernández-Pacheco, R.; Fernández-Pacheco, A.; Gloter, A.; Magén, C.; Stéphan, O.; Ibarra, M. R.; De Teresa, J. M. Nanoscale Chemical and Structural Study of Co-Based FEBID Structures by STEM-EELS and HRTEM. Nanoscale Res. Lett. 2011, 6 (1), 592. (82) Botman, A.; Thiel, B. L.; Hagen, C. W.; Mulders, J. J. L.; Randolph, S. J.; Dunn, K. A.; Toth, M.; Li, J. Electron Postgrowth Irradiation of Platinum-Containing Nanostructures Grown by Electron-Beam-Induced Deposition from Pt(PF3)4. J. Vac. Sci. Technol. B Microelectron. Nanom. Struct. 2009, 27 (6), 2759−2763. (83) Fedorov, A. G.; Kim, S.; Henry, M.; Kulkarni, D.; Tsukruk, V. V. Focused-Electron-Beam-Induced Processing (FEBIP) for Emerging 3374


Magnetoelectrical Transport Improvements of Postgrowth Annealed

Recommend Documents