Magnetoelectrical Transport Improvements of Postgrowth Annealed

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Magneto-electrical transport improvements of postgrowth annealed iron-cobalt nanocomposites: a possible route for future room-temperature spintronics Marcos Vinicius Puydinger dos Santos, Sven Barth, Fanny Béron, Kleber R. Pirota, Andre Pinto, João Paulo Sinnecker, Stanislav Moshkalev, José Alexandre Diniz, and Ivo Utke ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00581 • Publication Date (Web): 19 Jun 2018 Downloaded from http://pubs.acs.org on June 25, 2018

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ACS Applied Nano Materials

Magneto-electrical

transport

improvements

of

postgrowth annealed iron-cobalt nanocomposites: a possible

route

for

future

room-temperature

spintronics Marcos V. Puydinger dos Santos,¥,‡,* Sven Barth,ǁ Fanny Béron,¥ Kleber R. Pirota,¥ André L. Pinto,§ João P. Sinnecker,§ Stanislav Moshkalev,‡ José A. Diniz,‡ and Ivo Utke†

¥

Institute of Physics Gleb Wataghin, University of Campinas, Rua Sérgio Buarque de Holanda

777 Cidade Universitária, 13083-859 Campinas-SP, Brazil. ‡

Faculty of Electrical and Computing Engineering and Center for Semiconductor Components and

Nanotechnologies, University of Campinas, Av. Albert Einstein 400, 13083-852 Campinas-SP,

Brazil. ǁ

Institute of Materials Chemistry, TU Wien, Getreidemarkt 9/BC/02, A-1060 Vienna.

§

Centro Brasileiro de Pesquisas Físicas – CBPF, Rua Dr. Xavier Sigaud 150 Urca, 22290-270

Rio de Janeiro-RJ, Brazil.

ACS Paragon Plus Environment

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ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

†

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EMPA, Swiss Federal Laboratories for Materials Science and Technology, Laboratory for

Mechanics of Materials and Nanostructures, Feuerwerkerstrasse 39, 3602 Thun, Switzerland.

KEYWORDS: nanofabrication,

Focused-electron-beam-induced thermally-induced

oxygen

deposition, tuning,

iron-cobalt

thermally-induced

nanocomposites, graphitization,

magnetoresistance, spin-based devices.

ABSTRACT

Focused-electron-beam-induced deposition (FEBID) constitutes a direct-write 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 magneto-transport properties of Fe–Co–C–O deposits, were reproducibly tuned upon ex-situ postgrowth annealing at 100 °C, 200 °C, and 300 oC in a highvacuum system. 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


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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 magneto-transport properties of the nano-sized FEBID material degrades 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. It makes our material especially interesting and advantageous for applications in high density magnetic recording devices, nanoelectronics, MEMS-based sensors and logic devices.

INTRODUCTION Recently, focused-electron-beam-induced deposition (FEBID) has been proven to be a versatile direct-write technique for local deposition of nanomaterials with tunable morphology in predefined shapes and dimensions (0D – 3D), representing an important advantage over conventional lithography.1–13 FEBID-derived nanodevices have been recently demonstrated for many material science applications, such as magnetic14–16 thermal,17 and strain sensors,18 as well as high-purity ferromagnetic nanostructures.19–21 Furthermore, the ability of defining high aspect ratio 3D nanostructures turns FEBID into 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


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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 non-volatile deposit, such as metas, and volatile byproducts.7,14,34–36 A large development has been made towards FEBID deposit growth with high metal content and improved electrical and magnetic properties.1,3,4,37,38 However, nanoscale metal deposits typically contain non-volatile carbon contamination from the incomplete dissociation of the organometallic precursor molecule or hydrocarbons from background gases.3,7,14,39–42 The undesired co-deposited 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 towards removal of residual carbon from Pt and W deposits. Additionally, high-purity W deposits (around 95 at.%) and high-aspect ratio carbon pillars were achieved using impact-enhanced desorption of residual organic ligands by means of 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 fine tune 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 in order to obtain pure metallic deposits. Moreover, beam-induced local heating


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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 Highvacuum annealing is preferred for non-noble metals because of the deposit’s oxidation in contrast to water- and 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 in order to produce a metallic alloy with improved both electrical and magnetic properties. Our aim was to obtain granular material embedded in a carbon matrix and investigate its magneto-transport 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 pre-fabricated semiconductor and 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 high-vacuum 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 magneto-transport properties.


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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 thermionic tungsten filament was employed for FEBID of FexCoyCwOz rectangular patterns of 20 µm × 2 µm, and 160 nm-thick. They bridge pre-defined 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 Supporting Information, Supp.1) was impinging on the substrate via a capillary with a 760 µm inner diameter by heating the gas injection system (GIS) up to 65 ºC.


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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 in order to increase the deposits metal content on as-deposited material was subsequently performed inside the SEM chamber during 10 minutes at 100 ºC, 200 ºC and 300 ºC upon stabilization, with ramp rate of ca. 15 ºC 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 take-off angle of around 32o. 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 2.7 cm−1. The microstructure of the Fe and Co containing deposits was investigated using a Jeol EM2100F transmission electron microscope (TEM). TEM lamellae were prepared for all samples with different annealing


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temperatures in a TESCAN LYRA 3 FIB, 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 of 2 – 300 K and -3 – 3 T, respectively. An EG&G 5210 lock-in amplifier was employed to measure the root-meansquare (rms) voltage, while a function waveform generator Agilent 33250A was synchronously used in series with a 100 kΩ resistance in order 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 self-heating effects and thus, changes in their atomic composition. It is worth mentioning that the samples were measured with the magnetic field both applied 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 Supporting Information, Supp.2), in order to avoid introducing in the FEBID material an additional shape anisotropy, which is present in the zigzag-shaped cross-section of the deposits as seen in Figure 1.

EXPERIMENTAL RESULTS AND DISCUSSION Deposit shape with annealing temperature


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AFM of the as-grown 162 ± 9 nm-thick material (see Supporting Information, Supp.3) 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 one order of magnitude lower 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 nm, 148 ± 12 nm and 140 ± 5 nm for annealing at 100 oC, 200 oC and 300 oC, 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.


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


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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 varied 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 intensity. Figure 2b presents a tunable atomic composition range of FexCoyCwOz, with 0.13 < x < 0.16 for iron, 0.41 < x < 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 asdeposited samples improves the metal (Fe + Co) content from 54 at.% for as-grown films to 69 at.% at 300 ºC due to 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 at.% to 9 at.%, while the carbon only very slightly decreases from 26 at.% to 22 at.%. This annealing-based purification mechanism is similar to the one reported for 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 at.%, compared to 80 at.% 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).


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Figure 2. (a) Superposed EDX spectra for different annealing temperatures and (b) atomic percentages as function of postgrowth annealing temperature. EDX measurements were taken using 3 keV electrons in order 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, turning 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 Supporting Information, Supp.4). The purification mechanism occurs predominantly via thermally-induced oxygen release, while the C-content stays constant within the 2 at.% error margin. 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


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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 metalcarbonyls 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 electron beam 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 Schottky electron emitter.67 Furthermore, electron-induced 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 Robertson75 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 upwards shift of the G band towards higher wavenumbers corresponds to an increase in graphitization, and nanocrystal


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ordering. In addition, in 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 D/G integrated peak intensity ratio has been used as a quantitative factor for the graphite crystallites size determination in carbon-based structures.77 Therefore, we consider both the G peak shift and the D/G ratio as complimentary 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 later permits the determination of the nanocrystallite size (see Supporting Information, Supp.5).

Figure 3. (a) Raman spectra in the carbon peak range of FEBID material from the HFeCo3(CO)12 precursor as function of the post-growth 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 function of annealing temperature indicate that the amorphous carbon matrix partially converts into graphite nanocrystals.


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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 oC, 200 o

C and 300 oC, 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 oC Fe-Co-C-O containing FEBID materials. In addition, the thermal conversion of the carbon at 300 o

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 evidenced by the sharp increase in the D/G ratio above 100 oC. 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 Au thin film mainly occurred above 300 oC. 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


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reported, revealing a superior graphitisation 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 magneto-transport 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 and annealed samples at 100 °C, 200 °C, and 300 °C. From the bright field images shown in Figures 4a and 4b the as-grown and annealed samples at 100 oC 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 sub-nanometric grains dispersed in the


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amorphous matrix for the as-grown material (see Supporting Information, Supp.6), similarly to previous works using low electron density FEBID experiments.58,61,81

Figure 4. High-resolution TEM micrographs of FEBID material (a) as-deposited and annealed at (b) 100 ºC, (c) 200 ºC and (d) 300 ºC. 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 oC (inset in d). For indexation details of the SAED patterns, see Supporting Information, Supp.7. 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 oC, evidenced by the diffraction rings (see Supporting Information, Supp.7, for more details). Interestingly, the absence of


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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 FEG SEMs 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 o

C (Figure 4c) and 300 oC (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-Navarro63 and our previous work on Co2(CO)8.61 The Moiré patterns visible in Figures 4c and 4d were caused by the overlap of single crystalline grains in this densely packed, polycrystalline metallic film. Moreover, the diffraction pattern taken at 300 oC annealing temperature (see inset of Figure 4d and Supporting Information, Supp.7) indicates the presence of fcc Co crystals, hence suggesting thermally-induced reduction of CoOx at 300 oC, 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 oC, thus suggesting a postgrowth oxidation effect of small 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


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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 inferred by AFM) as a function of the postgrowth annealing temperature, revealing electrical transport improvements from 4200 (as-grown) down to 65 µΩ cm (300 ºC annealing). This last value is only 50 % larger than the value of the purest Fe-Co material (metal content: 84 at.%) defined by FEBID67 and a factor of 2.5 larger than the resistivity of Co-C FEBID material annealed at 300 o

C under high vacuum with similar dimensions (metal content: 85 at.%).61 Moreover, despite an

approximately constant 69 at.% metal content between 200 ºC and 300 ºC (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 oC during 30 minutes83 is larger than our 300 oC 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. Important to note that one should not expect the Fe crystal oxidation during annealing to consistently contribute to increase the overall resistivity, as both FeO and Fe3O4 phases already co-exist for all the annealing temperatures.


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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 best resistivity values reported for FEBID material obtained from Fe(CO)5, Co2(CO)8 and HFeCo3(CO)12. 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 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 due to lattice vibrations.85 In addition, regions II (80 K < temperature < 200 K) and III (temperature < 80 K) show, respectively,


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intrinsic– and extrinsic–like 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 oC, 200 oC and 300 oC annealing temperature, respectively. This last residual normalized resistivity is about 44% lower compared to deposits with similar dimensions and higher Fe + Co content (84 at.% vs 69 at.%).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 depositon (PVD).88 It suggests that this annealing protocol is a suitable tool for achieving pure metal-like properties for non-noble 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 than CoOx species.


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Figure 6. Temperature-dependent normalized electrical resistivity of (a) as-grown and (b) annealed at 100 oC, 200 oC and 300 oC FEBID deposits from HFeCo3(CO)12 of length about 20 µm, width 2 µm and thickness 160 nm during sample cooling from 300 K down to 2 K. Inset: region I (T > 200 K) presents a lattice-scattering-limited conduction, in which the carriers scatter essentially due to 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. Furthermore, the electrical transport properties preservation with minimum degradation over time represents one of the key features for technological applications of this postgrowth annealing 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. Magneto-transport 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 as-deposited and 100 oC annealed deposits were below our detection limit (~ 0.1 %). On the other hand, the samples annealed at 200 oC and 300 oC presented an AMR signal (measured at the saturation field ~ 3 T) of about 0.6 % (Figure 7a) and 1.2 % (Figure 7b),


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respectively. These measurements are highly reproducible as they were performed on five samples fabricated under same annealing conditions (see Supporting Information, Supp.2). For the 200 oC annealed sample, no detectable MR signal was found for 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 literature for the FEBID material from the same HFeCo3(CO)12 precursor and dimensions, using 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.

Figure 7. Room-temperature magnetoresistance measurements in both longitudinal and transversal directions of the (a) 200 oC and (b) 300 oC annealed FEBID material from HFeCo3(CO)12. Magnetoresistance is defined as 100 0 ⁄ 0 , while the anisotropic magnetoresistance (AMR) signal can be expressed as 100


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‖ ⏊ ⁄ . The insets show the relative orientations between the electron current density, J, and the external magnetic field, H. Moreover, the 300 oC 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 oC 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/non-magnetic 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 oC annealed Fe–Co–C–O deposits showed the largest MR signal so far reported in 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 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,


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mainly for annealing temperatures higher than 200 oC. 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 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 FEBID-based method for the growth of ferromagnetic metal–graphite nano-sized 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 at.% in an amorphous carbonaceous matrix and


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transformed to metallic behavior with a metal content of 69 at.% in a graphitic matrix. The resistivity could be tuned by 2 orders of magnitude upon annealing and reduced to 65 µΩ cm by releasing co-deposited oxygen from 20 at.% down to 9 at.%, 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. It 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 magneto-transport 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 towards the realization of new ferromagnetic metal–carbon nanocomposites for the fabrication of high density magnetic recording devices, MEMS-based sensors and logic devices.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Tel: +55 19 997088551. †

E-mail: [email protected]; Tel: +41 587656257

Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources All authors acknowledge the financial support from Conselho Nacional de Desenvolvimento e Científico 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). Notes The authors declare no competing financial interest. Supporting information The Supporting Information is available free of charge on the ACS Publications website. AFM analysis; Raman analysis; Dark-field images; Calculations for the precursor gas flux; Samples for magnetoresistance characterization; EDX analysis; Crystallographic analysis.

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, CNPEM, Campinas, Brazil for their support with the use of the JEM 2100F TEM-FEG and the Balzer BA510 sputtering. The work was financially supported by the Brazilian funding agencies Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) through the internship program Science without Borders, and in part by the COST Action


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CELINA CM1301, and the Swiss State Secretariat for Education, Research and Innovation SERI (project number C14.0087).

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Table of Contents Graphic


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Optical microscopy and the corresponding SEM images (grey scale) at higher magnification showing the FEB induced deposits of 20 × 2 µm2 area and thickness of around 160 nm bridging 4-probes gold electrodes on SiO2/Si substrate for all the annealing temperatures. Observe that the deposit changes color and sharpens its edges as the annealing temperature increases from a) to d). 190x167mm (300 x 300 DPI)



(a) Superposed EDX spectra for different annealing temperatures and (b) atomic percentages as function of postgrowth annealing temperature. EDX measurements were taken using 3 keV electrons in order 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, turning 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 Supporting Information, Supp.4). The purification mechanism occurs predominantly via thermally-induced oxygen release, while the C-content stays constant within the 2 at.% error margin. 89x37mm (300 x 300 DPI)


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(a) Raman spectra in the carbon peak range of FEBID material from the HFeCo3(CO)12 precursor as function of the post-growth 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 function of annealing temperature indicate that the amorphous carbon matrix partially converts into graphite nanocrystals. 87x35mm (300 x 300 DPI)



High-resolution TEM micrographs of FEBID material (a) as-deposited and annealed at (b) 100 oC, (c) 200 oC and (d) 300 oC. 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 oC (inset in d). For indexation details of the SAED patterns, see Supporting Information, Supp.7. 144x97mm (300 x 300 DPI)


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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 best resistivity values reported for FEBID material obtained from Fe(CO)5, Co2(CO)8 and HFeCo3(CO)12. 163x123mm (300 x 300 DPI)



Temperature-dependent normalized electrical resistivity of (a) as-grown and (b) annealed at 100 oC, 200 oC and 300 oC FEBID deposits from HFeCo3(CO)12 of length about 20 µm, width 2 µm and thickness 160 nm during sample cooling from 300 K down to 2 K. Inset: region I (T > 200 K) presents a lattice-scatteringlimited conduction, in which the carriers scatter essentially due to 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. 86x34mm (300 x 300 DPI)


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Room-temperature magnetoresistance measurements in both longitudinal and transversal directions of the (a) 200 oC and (b) 300 oC 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 × (ρǁ-ρ⏊)⁄ρ. The insets show the relative orientations between the electron current density, J, and the external magnetic field, H. 88x36mm (300 x 300 DPI)


Magnetoelectrical Transport Improvements of Postgrowth Annealed

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