Enhanced Magnetotransport in Nanopatterned Manganite Nanowires

Letter pubs.acs.org/NanoLett

Enhanced Magnetotransport in Nanopatterned Manganite Nanowires Lorena Marín,†,‡,§ Luis Morellón,*,†,‡ Pedro A. Algarabel,‡,§ Luis A. Rodríguez,†,‡ César Magén,†,‡,∥ José M. De Teresa,†,‡,§ and Manuel R. Ibarra†,‡ †

Laboratorio de Microscopı ́as Avanzadas (LMA) - Instituto de Nanociencia de Aragón (INA), Universidad de Zaragoza, 50018 Zaragoza, Spain ‡ Departamento de Física de la Materia Condensada, Universidad de Zaragoza, 50009 Zaragoza, Spain § Instituto de Ciencia de Materiales de Aragón (ICMA), Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain ∥ Fundación ARAID, 50018 Zaragoza, Spain S Supporting Information *

ABSTRACT: We have combined optical and focused ion beam lithographies to produce large aspect-ratio (length-to-width >300) single-crystal nanowires of La2/3Ca1/3MnO3 that preserve their functional properties. Remarkably, an enhanced magnetoresistance value of 34% in an applied magnetic field of 0.1 T in the narrowest 150 nm nanowire is obtained. The strain release at the edges together with a destabilization of the insulating regions is proposed to account for this behavior. This opens new strategies to implement these structures in functional spintronic devices.

KEYWORDS: Magnetoresistance, focused ion beam lithographies, manganite, nanowires, metal−insulator transition

T

patterning29−34 processes which allow the patterning of microwires, nanoconstrictions, and microbridges on magnetic oxides. The goal when using one or a combination of these techniques to fabricate nanodevices is that they preserve or even improve the properties of the system as compared with its thin film or bulk specimens. Here, we have selected La2/3Ca1/3MnO3 (LCMO), which presents a metal−insulator transition at TMI and the CMR effect. This behavior has been extensively studied, introducing external parameters such as the magnetic field, pressure, doping, and epitaxial strain.35,36 We will focus on the nanofabrication of LCMO single-crystal nanowires by combining optical and focused ion beam lithographies. Remarkably, we have obtained bulk-like properties in all of our nanowires, supporting our nanofabrication process and, interestingly, an enhanced magnetoresistance value of 34% in an applied magnetic field of 0.1 T in our narrowest nanowire (thickness = 150 nm, aspect ratio = length-to-width ratio >300). We propose that a combination of both a strain release at the edges of the nanowire with decreasing width together with a destabilization of the insulating regions due to finite size effects around TMI account for the observed behavior. To our knowledge our device is the narrowest and highest

ransition metal oxides exhibit an incredibly rich variety of physical phenomena, including high-temperature superconductivity, colossal magnetoresistance (CMR), spintronic effects and multiferroicity. In recent years, this has been accompanied by the advance in high-quality epitaxial thin film growth of these materials,1−5 leading to new engineered functionalities and to the discovery of emergent phenomena at the interfaces. 6−8 In addition, upon reduction of the dimensionality, new confinement effects are expected, bringing about new properties in nanoscale systems. For instance, recent breakthroughs related to the applications of oxide nanowires in spintronics from the perspectives of both material candidates and device fabrication have been reviewed.9 Among the transition metal oxides, the CMR manganese perovskites of the form R1−xAxMnO3 (R = trivalent rare earth, A = divalent alkaline element) have been intensively investigated.10,11 However, in these highly spin-polarized materials, the CMR effect takes place in applied magnetic fields of several Teslas, too high for technological applications in spintronic devices. Recent advances based on the fabrication of nanostructures12 open new routes in search of new strategies and better understanding of the electronic transport mechanisms in manganites.13 The available techniques for micro- and nanofabrication include optical lithography (OL),14,15 AFM lithography,16 electron beam lithography (EBL),17−27 chemical growth and etching, 28 and focused ion beam (FIB) © 2014 American Chemical Society

Received: August 2, 2013 Revised: November 21, 2013 Published: January 1, 2014 423

dx.doi.org/10.1021/nl402911w | Nano Lett. 2014, 14, 423−428

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subsequent fabrication steps of the nanowires, since the resin that protects the wires (positive resist AZ 6624 from MicroChemicals GmbH) is highly insulating. Figure 2b shows a scanning electron microscopy (SEM) image of the LCMO in the V1−V2 area protected by a layer of resin of approximately 2.5 μm thick. The nanowires were fabricated by FIB using an FEI Dual Beam Helios 650 on the area between electrode voltage V1−V2. The Ga+ ion column is placed at an angle of 52° relative to the electron column. The sample is positioned at the coincidence point of both beams (the point where the ion and electron beam axes intersect), and the eucentric height is precisely determined so that the sample remains in focus even after stage tilting. When the sample is tilted to 0° (52°) degrees, the electron (ion) beam is perpendicular to the surface of the sample. Our specimen was tilted to 52° for removing material by FIB, and the nanowires were defined as shown in the SEM image of Figure 2c. The resist above the nanowires minimizes the ion beam damage, and its presence is crucial for the success of the process. We start by removing initially a rectangular area of approximately 1.5 μm in width at both sides of the microwire, thus defining a microwire of 2 μm (0.23 nA and 30 kV working current and acceleration voltage, respectively). Afterward, we etch with a cleaning cross-section procedure (at fixed 30 kV and varying current from 40 pA down to 7.7 pA as the width of the nanowire is decreased). Once the fabrication of the nanowire is finished, the remaining resist is removed using acetone and ultrasound. Next, we anneal the whole structure at 800 °C in a pure oxygen atmosphere for 1 h to recover the insulating state of the STO substrate. Finally, a second step of OL was used to deposit metallic contacts. A 10-nm-thick layer of Cr is first deposited to improve the adhesion of the metallic layer, and a 100-nm-thick layer of Al is subsequently grown, both by e-beam evaporation. Finally, the sample is immersed in acetone to remove the remaining resist and those regions of metal with resist underneath (lift-off process). The pads are finally connected by ultrasonic wire bonding to a chip carrier compatible with the Physical Property Measurement System (PPMS) from Quantum Design. Following these steps we have reproducibly fabricated LCMO single-crystal nanowires with widths between w = 5 μm and 150 nm, 50 μm in length (along the [100] direction), and 30 nm in thickness. The examination of the nanostructure after the patterning process was carried out by High Resolution Transmission Electron Microscopy (HRTEM) on a crosssection lamella extracted from the middle part of the narrowest wire (we checked previously that the width is essentially constant along the full length). The HRTEM image recorded at intermediate magnification reveals that the narrowest wire has a width of approximately 152 nm (see Figure 2d). Furthermore, a high magnification HRTEM image demonstrates that the wire maintains its crystallinity and epitaxial structure after the nanopatterning process (see Figure 2e) with negligible amorphization, structural defects, or Ga+ implantation. We will demonstrate that, even the narrowest nanowire of LCMO, in addition to its high crystallinity, preserves its electronic properties. Other protection routes during FIB patterning were not as effective. Also, although we were able to pattern down to 50 nm in width, the nanowires were no longer functional. We will continue with the characterization of the electrical resistivity in selected nanowires with varying widths. In Figure 3a, we show curves of zero-field resistivity ρ(T) as a function of temperature for selected micro- and nanowires with varying widths (w = 5000, 700, 290, 152 nm), including the

aspect ratio single crystalline manganite nanowire with such an enhanced low-field response. This opens a promising route toward its implementation in spintronic devices operating in low applied magnetic fields. Micro- and nanowires were patterned from epitaxial LCMO thin films deposited on (001)-oriented SrTiO3 (STO) substrates of 1/2 × 1/2 in. by pulsed laser deposition (PLD). The deposition of the LCMO on STO was realized at a substrate temperature of 820 °C and an oxygen pressure of 400 mTorr. After deposition, the sample was cooled down at 700 Torr to obtain an optimal oxygen stoichiometry. The crystal structure of the film was analyzed by X-ray diffraction (XRD). A θ−2θ scan in the vicinity of the (002) reflection of STO (see Figure 1 of the Supporting Information) shows the high crystalline quality evidenced by the finite-size Laue oscillations. The calculated out-of-plane lattice parameter is 3.803(4) Å, yielding a ratio of (a − c)/a = 0.026 [the misfit between STO and LCMO is f = (aSTO − aLCMO)/aLCMO = +0.93%]. The rocking curve (ω-scan) around the (002) LCMO Bragg peak yields a full-width at half-maximum (fwhm) of 0.027(3)° similar to that of the substrate. The film thickness was determined to be 30 nm by X-ray reflectivity (XRR). The quality of our films is illustrated in Figure 1 by high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) carried out in a probe aberration corrected FEI Titan 60-300 operated at 300 kV.

Figure 1. HAADF-STEM image in the vicinity of the STO//LCMO interface (left). On the right a cartoon of the tensile distortion of the LCMO lattice to match that of the STO is shown. The lattice mismatch is +0.93%, yielding a ratio (a − c)/a = 2.6%.

The fabrication of the nanowires was carried out by combining two techniques: OL and FIB. We started the patterning process using OL to define a microwire connected to eight electrodes (Hall-bar configuration, see Figure 2a). The first lithography process ends when the film is ion etched with Ar+ ions accelerated at 250 eV that remove LCMO material in areas unprotected by the resin. This microdevice, due to its well-defined geometry, allows standard four-probe electronic transport measurements avoiding the influence of contact resistances. The pads for injecting the current are at both ends of the microwire and the contacts for measuring the voltage drop are placed longitudinally in the same side of the wire (V1−V2−V3) for (magneto)resistance measurements.37 In this microwire of 5 μm in width we define two working areas: the first between the electrode voltage V1−V2 and the second between V2−V3, as indicated in Figure 2a; the distance between consecutive electrodes is 50 μm. After the ion milling process, the insulating STO substrate develops surface conductivity.38 This is helpful to avoid charging effects in 424


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Figure 2. Images on the left-hand side illustratrating the steps of the fabrication method: (a) Optical lithography, (b) protection of the sample with resin (image taken with ions at an incident angle of 52° relative to the electron column), and (c) FIB etching. The images on the right-hand side show: (d) a TEM image of a cross-section of the narrowest wire and (e) a HRTEM image around the substrate−sample interface. The inset of part d shows a fast Fourier transform (FFT) of a small region inside LCMO revealing that crystallinity and epitaxy are conserved after the fabrication processes.

Figure 3. (a) Zero-field resistivity ρ(T) as a function of temperature for selected samples in zero field. (b) A cartoon showing the relative dimensions of the different nanowires and the proposed mechanism of strain release at the edges are shown. (c) High-temperature ρ(T) at T > TMI. The inset shows the activation energy vs the nanowire width. (d) Fits of the low-temperature T < TM−I resistivity to an expression ρ(T) = ρ0 + ρ2T2 + ρ4.5T4.5. The inset shows the residual resistivity ρ0 vs the nanowire width as extrapolated from experiments (open circles) and from the fits (open squares).

reference measurement for a thin film of 5 × 5 mm2 (Van der Pauw). All systems exhibit the typical metal−insulator (M−I) behavior of the LCMO system,39 where a peak signals the crossover from the low-temperature metallic (ferromagnetic) to the high-temperature insulating (paramagnetic) phase, the temperature of the maximum in ρ(T) corresponds to the M−I

transition temperature (TMI). The main observation is that TMI shifts to higher temperature with decreasing width, from TMI,film = 177 K, for the square 5 × 5 mm2 thin film, up to TMI = 233 K, for the nanowire of 152 nm width. The reduced TMI in the thin film of 30 nm as compared with the bulk LCMO (TMI, bulk = 260 K) is expected due to both elastic and orbital effects.35 The 425


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Figure 4. Resistivity as a function of the temperature at selected magnetic field values of 0, 1, 3, 6, and 9 T for w = 5000 nm (a) and 150 nm (b). Parts c and d show the magnetoresistance MR = [R(0) − R(H)]/R(0) for w = 5000 nm (c) and 150 nm (d). The dashed line marks TMI (H = 0), and the open circles indicate the different TMI (H) in H = 0.1, 1, 3, 6, and 9 T.

Above TMI, we have fitted the activated behavior of ρ(T) to a model of small polaron hopping in the adiabatic approximation42 ρ(T) = BT exp(Ea/kT) as reported in epitaxial LCMO films,43,44 where k is the Boltzmann constant, Ea is the polaron hopping energy, and B is inversely proportional to the density of charge carriers. The fits to our data are plotted in Figure 3c. We have obtained values of the order Ea = 140−125 meV. Reduction of the lateral dimension decreases Ea, so that the small polaron hopping is favored, consistent with the concomitant increase in TMI. Additionally, the resistivity coefficient, B, is also lower in the narrowest nanowires. Both are in good agreement with previous results in annealed38 and oxygen deficient44 LCMO thin films. At low temperature, the resistivity in zero field was fitted to ρ(T) = ρ0 + ρ2T2 + ρ4.5T4.5, where ρ0 is the residual resistivity, ρ2 is related to electron− electron scattering, and ρ4.5 corresponds to the magnon− magnon scattering. In addition, we have determined the residual resistivity from a current−voltage curve between ±1 μA at 4.2 K (see Supporting Information). In all cases we have found a linear current vs voltage behavior for all of our nanowires. ρ0 has been found to decrease when w decreases, as shown in the inset of the Figure 3d. This value of residual resistivity 2.0 × 10−4 Ω cm for w = 150 nm is typical of highquality epitaxial thin films43 and bulk polycrystalline samples, again indicating the negligible damage in the fabrication process. One of the most interesting features of the manganese perovskites is the existence of colossal magnetoresistance (CMR).10,11 In Figure 4 (top panels), the temperature dependences of the resistivity of the widest and narrowest wires (w = 5000 and 152 nm) under different magnetic fields applied parallel to the wire axis ([100] direction) are shown.

decrease in the nanowire width makes TMI approach that of bulk LCMO. This behavior is in contrast with previous reports on LCMO microbridges where TMI was found to strongly decrease with w, the device of w = 500 nm already displaying an insulating behavior over the whole temperature range.21,22 This supports the high quality of the nanowires fabricated with our method. Furthermore, upon reduction of w, a “shoulder” appears at temperatures lower than TMI (region 1 in Figure 3a). This anomaly coincides with the metal−insulator transition of the unpatterned film, TMI,film and distinct from the higher temperature and sharp TMI (region 2 in Figure 3a). Reemergent metal−insulator transitions have been reported in phase-separated (La,Pr,Ca)MnO315 and LaBaMnO328 wires at lower temperatures than in the unpatterned film and ascribed to spatial confinement effects in the wire geometry. Strain effects are observed to play a key role in this behavior. The two transition temperatures observed in our nanowires are not due to the FIB patterning since irradiation damage or Ga+ implantation is detrimental to the magnetotransport properties of manganite thin films and nanostructures40,41 and would shift TMI to lower temperatures instead of higher. We believe that, upon reduction of the wire dimensions (decreasing width), the epitaxial strain which is responsible for the reduced TMI is released at the edges of the nanowire. Therefore, we propose that our nanowires have an epitaxial strain gradient toward the edges of the wire and therefore a higher metal−insulator transition at the edges that in the highly strained center of the nanowire with a TMI = TMI, film (see schematic representation of the proposed mechanism in Figure 3b). This effect is maximized in the narrowest nanowires. 426


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The application of a magnetic field shifts TMI to higher temperatures, TMI (w = 150 nm, H = 90 kOe) = 303 K. This produces the CMR plotted in Figure 4c and d curves for w = 5000 nm and w = 150 nm, respectively. The magnetoresistance is defined as MR = [R(0) − R(H)]/R(0). The dashed line indicates the TMI transition for H = 0, and the circles mark the TMI transition for the different applied magnetic fields. The high-field MR value at TMI is virtually the same in the microwire and in the narrowest nanowire, MR (9 T, w = 5000, 150 nm) ≅ 98%. Remarkably, in sharp contrast the values at low fields are very different. For example, MR (1 T, w = 5000 nm) ≅ 40% and MR (1 T, w = 150 nm) ≅ 76%. This difference is strikingly maximum at the lowest measured magnetic field of 0.1 T with MR (0.1 T, w = 5000 nm) ≅ 5% and MR (0.1 T, w = 150 nm) ≅ 34%, that is, roughly a factor ≈7 of enhancement in the low field resistance. To our knowledge, this value of MR (0.1 T, w = 150 nm) ≅ 34% has never been reported and is of the order of the intergrain tunnelling magnetoresistance in the ferromagnetic phase of LCMO nanowires.45 It is worth mentioning that the concomitant increase in both the TMI value and the MR at TMI makes the incorporation of these materials in roomtemperature devices in low-applied magnetic fields more feasible. In Figure 5 we summarize the values of MR (TMI) for all measured nanostructures fabricated in three different inde-

is applied, the interfacial regions between the conductive and the insulating regions will be displaced to reach a lower resistance state. This process will be favored if the amount of such interfacial areas decreases. Thus, it will be possible to reach such low-resistance state with lower magnetic fields. This is what will occur in the narrowest nanowires studied here, where the edges will be more conductive than the central part and the interfacial area will be less compared to a bulk-like situation (see an sketch in the Supporting Information). In fact, we observe that the resistance state under high magnetic field is similar to that of a bulk-like situation but can be reached with lower magnetic fields. As a summary, we report a reliable method for patterning nanowires of CMR manganites that we have demonstrated that preserve their bulk-like electronic properties down to lateral widths of 150 nm. We have investigated the magnetotransport properties of nanopatterned LCMO devices. We have found the resistivity and the metal−insulator transition temperature TMI to strongly depend on the width of the wires. A decrease of the activation energy, and the residual resistivity has been observed by making these nanowires narrower. Remarkably, an enhancement of the magnetoresistance at TMI and in low applied magnetic field is reported opening new strategies to implement these structures in functional spintronic devices.

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ASSOCIATED CONTENT

S Supporting Information *

Description of the material. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +34 976761212. Fax: +34 976 761229. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Spanish Ministry of Science through project no. MAT2011-27553-C02 including FEDER funding and by the Aragón Regional Government through project E26 and Instituto de Nanociencia de Aragón, Universidad de Zaragoza, PIF-UZ-2009-CIE-02. The authors thank L. Casado and R. Cordoba for their help in the experiements using FIB.

Figure 5. Magnetoresistance at T = TMI (H = 0) as a function of the wire width and in different applied magnetic fields.

pendent runs (run 1: 5000, 2000, 700, 290, and 152 nm; run 2: 250 and 195 nm; run 3: 170 nm). The CMR effect around TMI is thought to be related to intrinsic phase separation of metallic and insulating regions both at the nano- and submicrometer scales.46 In our thinnest nanowires below c.a. 250 nm we are certainly approaching the typical characteristic length scale. In the present manganite nanowires, the resistance will substantially decrease when metallic percolation paths form. This will occur when the ferromagnetic metallic regions grow to become physically connected under the influence of the magnetic field. The enhancement of the low-field magnetoresistance observed for the narrowest nanowires can be qualitatively explained taking into account the strain release at the nanowire edges (see a sketch in the Supporting Information). Without magnetic field, at TMI the sample is expected to be formed by coexisting nanometric conductive and insulating regions.47 The colossal magnetoresistance effect in manganites is generally explained by the expansion of the ferromagnetic conductive regions at the expenses of the insulating regions, as observed by Lorentz microscopy.48 Assuming this, when the magnetic field

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REFERENCES

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