Giant positive magnetoresistance in ultrathin films of mixed phase manganitesSung H. Yun, Tara Dhakal, Devesh Goswami, Guneeta Singh, Arthur Herbard, and Amlan Biswas

Citation: Journal of Applied Physics 103, 07E317 (2008); doi: 10.1063/1.2837277 View online: http://dx.doi.org/10.1063/1.2837277 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/103/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Unusual giant anisotropic magnetoresistance in manganite strips Appl. Phys. Lett. 104, 242405 (2014); 10.1063/1.4883889 Giant intrinsic tunnel magnetoresistance in manganite thin films etched with antidot arrays Appl. Phys. Lett. 104, 082414 (2014); 10.1063/1.4867083 X-ray photoemission study in Re 0.7 Ca 0.3 MnO 3 epitaxial films J. Appl. Phys. 105, 07D505 (2009); 10.1063/1.3062821 Tunnel magnetoresistance in La 0.7 Ca 0.3 Mn O 3 Pr Ba 2 Cu 3 O 7 La 0.7 Ca 0.3 Mn O 3 Appl. Phys. Lett. 88, 022512 (2006); 10.1063/1.2162674 The special magnetoresistive effect in trilayered films made of manganite perovskites Appl. Phys. Lett. 72, 2475 (1998); 10.1063/1.121385

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Giant positive magnetoresistance in ultrathin films of mixed phasemanganites

Sung H. Yun,a� Tara Dhakal, Devesh Goswami, Guneeta Singh, Arthur Herbard, andAmlan BiswasDepartment of Physics, University of Florida, Gainesville, Florida, 32611, USA

�Presented on 9 November 2007; received 20 September 2007; accepted 8 November 2007;published online 20 March 2008�

Magnetic tunnel junctions �MTJs� based on fully spin polarized ferromagnetic manganites havegenerated a lot of interest due to their enhanced field sensitivity at low temperatures. However, thetunneling magnetoresistance �TMR� drops rapidly with increasing temperature due to the reductionof spin polarization at the manganite-insulator interface. We have devised a method for creatingintrinsic tunnel barriers by tuning the phase competition in manganites using substrate inducedstrain. Ultrathin films �7.5 nm� of the mixed phase manganite �La0.5Pr0.5�0.67Ca0.33MnO3 �LPCMO�grown on the substrate �110� NdGaO3 using pulsed laser deposition show positivemagnetoresistance �MR� of about 30% at magnetic fields less than 1 T. Unlike the fabricated MTJdevices, this MR effect has its maximum value close to the insulator to metal transition temperatureand reduces with decreasing temperature. To find out the mechanism leading to this positive MR, theeffect of three orientations of the magnetic field on the LPCMO thin films were studied: �1�perpendicular to the plane of the film, �2� parallel to the plane of the film and applied current, and�3� parallel to the plane of the film but perpendicular to the applied current. The effect of fieldorientation suggests that a possible mechanism for the positive MR is tunneling magnetoresistancedue to the spin conserving tunneling process across the insulating regions separating theferromagnetic metallic regions. The voltage dependence of the MR also supports this mechanism.Our results suggest a novel method for obtaining enhanced TMR in manganite based MTJs bycreating strain induced intrinsic tunnel barriers. © 2008 American Institute of Physics.�DOI: 10.1063/1.2837277�

I. INTRODUCTION

The low field magnetoresistance �LFMR� effect in hole-doped manganese oxides �manganites� has been studied ex-tensively due to the potential for applications.1–3 There is astrong connection between the magnetization and electronictransport properties of manganites, which affects propertiesat the microscopic level such as intrinsic tunneling due to thecomplex interdependence of spin, charge, and lattice degreesof freedom.4 Although colossal magnetoresistance is a char-acteristic of manganites, the required high fields ��1 T� arean impediment for practical applications. Several studieshave attempted to overcome this limitation by studying themicroscopic origin of the transport mechanism �e.g., spindependent tunneling� at low fields. Here, we report an intrin-sic positive, anisotropic LFMR of a simple ultrathin film of�La0.5Pr0.5�0.67Ca0.33MnO3 �throughout this paper LPCMOwill indicate only this composition�.

We cautiously suggest that the LFMR may be due totunneling magnetoresistance �TMR�. We believe that thisstudy of ultrathin films will lead to an understanding of TMRin more sophisticated manganite structures.

II. EXPERIMENTAL DETAILS

LPCMO thin films were grown on the �110� NdGaO3

�NGO� substrates by pulsed laser deposition �KrF 248 nm�with a molecular oxygen pressure of 440 mTorr. The NGOsubstrates were kept at 820 °C throughout the deposition.This procedure results in high quality epitaxial films. Thick-nesses of 75 and 300 Å were used for this experiment. In thecase of the 75 Å thick film, due to its high resistance, thecircuit shown in Fig. 1 was used for magnetoresistance mea-surements. The marked voltage for each result indicates thetotal bias across the circuit �VB�, i.e., the voltage across thesample and load resistance. Therefore, the real applied biasacross the sample will be much smaller for most cases. Forthe 300 Å thickness film, a regular four-probe resistancemeasurement method was used.

Figure 1 shows the temperature dependence of the resis-tance in different magnetic fields. In this measurement, thefield was applied perpendicular to the plane of the film. Atzero field, the metal insulator transition temperature, TIM isaround 65 K and it increases as the field is increased. Thehysteresis, seen for lower fields in Fig. 1, reduces dramati-cally around 4 T. Colossal magnetoresistive behavior is ob-served around TIM. The discrete steps, marked by arrows inthe figure, can be due to the electronic phase separation,which is related to the fact that this film’s thickness is smallenough to confine the single-phase domain along the c axis.5

This R versus T was taken before the field sweep �Ra�Electronic mail: udcstb@gmail.com. Tel.: 352-392-3667. FAX: 352-392-

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JOURNAL OF APPLIED PHYSICS 103, 07E317 �2008�

0021-8979/2008/103�7�/07E317/3/$23.00 © 2008 American Institute of Physics103, 07E317-1

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versus H� measurements. After the R versus H measurement,R versus T showed a different behavior in which no clear TIM

could be seen, which indicates the occurrence of irreversiblechanges in this system.

Figure 2�a� shows the LFMR with field perpendicular tothe plane of the film. For clarity, the metamagnetic transition�MMT� part has been removed from each LFMR result. Fig-ure 2�d� describes one example of a MMT for the conditionsin Fig. 2�a�. The arrows indicate the field sweep direction.The irreversible MMT is clearly seen with two different re-sistances at zero field. These MMTs were accompanied by asharp drop in resistance at a certain field �in this case at1.8 T�. The first LFMR after the MMT is circled. This set ofLFMR data was obtained for one MMT at 50 K. It is ob-served that these LFMR data have a strong dependence onthe history of the sample. The magnetoresistance �RH

−R0� /R0 for each temperature was shifted by a constantamount for clarity, where RH and R0 are resistances with andwithout magnetic field, respectively.

FIG. 1. �Color online� Resistance vs temperature curves for different mag-netic fields. The magnetic field is applied perpendicular to the plane of thefilm. The load resistance RL was 10 M� and the bias voltages across thewhole circuit used for each field are given in the figure. The inset shows thecircuit diagram used for the measurement and Rsample= �VB−VL� / �VL /RL�.

FIG. 2. �Color online� Low field magnetoresistance when the field is applied �a� perpendicular to the film plane with VB=2 V, RL=10 M�, �b� parallel to thecurrent with VB=5 V, RL=10 M�, and �c� parallel to the plane of the film and perpendicular to the current with VB=5 V, RL=10 M�. �d� Metamagnetictransition for the conditions of �a�.

07E317-2 Yun et al. J. Appl. Phys. 103, 07E317 �2008�

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The origin of the positive magnetoresistance �PMR� canbe interpreted as a spin flipping process between two ferro-magnetic metallic domains with different magnetization di-rections. The temperature dependence of this PMR can beinterpreted as follows: after FMM domains and insulatingregions are formed due to the MMT, the size of each metallicdomain increases as the temperature is decreased, which re-sults in a larger Hf �Hf indicates the magnetization flippingfield for one domain and it is marked in the figures�. TheLFMRs for a magnetic field in the plane of the film and indirections parallel and perpendicular to the current are shownin Figs. 2�b� and 2�c�, respectively. In the case in which thefield is parallel to the current, the Hf ��500 G� is similar inmagnitude to the coercive field when the field is applied inthe plane of the film, which is found in magnetization mea-surements for the thicker film �300 �. If the coercive fielddoes not change significantly within this range of thickness,it will support the possibility that this PMR is due to spindependent tunneling magnetoresistance.

The different Hf for the two in-plane field directions ofFigs. 2�b� and 2�c� is possibly due to the current assisteddomain wall motion resulting from the spin torque in themagnetic domain under simultaneous field and currentapplications.6

For both the in-plane field directions Hf increases astemperature decreases, but the magnitude of PMR has a dif-ferent behavior than the magnetic field applied perpendicularto the plane of the film, as shown in Fig. 2�a�. The differencein behavior between the in-plane and out-of-plane magneticfields still needs to be addressed keeping in mind the micro-scopic domain formation and correlation between the spinalignment and current. We have measured LFMR for the300 Å film and observed a similar PMR effect in that case,although the detailed behavior is different from the LFMR ofthe 75 Å film.

III. CONCLUSIONS

The LFMR effect for LPCMO thin films on NGO wasmeasured for three different field directions: field perpen-dicular to the plane of the film, field parallel to the film planebut perpendicular to the current, and field parallel to the cur-rent.

Each direction has a distinctive PMR. The difference inthe temperature dependence of the PMR magnitude between

the case in which the field is perpendicular to the film �Fig.2�a�� and that in which the field is parallel to the film �Figs.2�b� and 2�c�� may reflect the different domain formationmechanisms due to the intrinsic film properties such as thedifferent strains for these two directions. The difference ofthe PMR for each of the three directions may also indicate ananisotropic magnetoresistive �AMR� response of the materialsimilar to that found in other compositions of manganites.7,8

This AMR is generated not only by the structural anisotropybut also by the dynamical response of the domains to thesimultaneous application of the current and magnetic fieldsand the resultant spin-orbit scattering which is due to theanisotropic electron mean free path’s dependence on the spinand the angle between current and the magnetization.9,10

The origin of LFMR in this film can be considered as anadditional magnetoresistance resulting from spin dependentscattering, which appears due to the fact that the polarizationaxis of the conduction electrons cannot be arranged adiabati-cally along the local magnetic field within the domain wallwhen its thickness approaches the spin diffusion length.9

Whether or not, this spin-dependent scattering at the domainboundary is related to the TMR, which is found in otherperovskite-type transition-metal oxides,11 is still a mystery.The microscopic description of the domain formation andspin and electron dynamics needs to be studied further.

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