Rare Metals 2013,32(03),213-224

Research progress in anisotropic magnetoresistance

Chong-Jun Zhao Lei Ding Jia-Shun HuangFu Jing-Yan Zhang Guang-Hua Yu

Department of Materials Physics and Chemistry, University of Science and Technology Beijing

School of Materials and Chemical Engineering, Hainan University

×÷Õß¼ò½é£ºGuang-Hua Yu e-mail:ghyu@mater.ustb.edu.cn;

ÊÕ¸åÈÕÆÚ£º20 March 2013

»ù½ð£ºfinancially supported by the National Natural Science Foundation of China (Nos. 51071023 and 51101047);the Natural Science Foundation of Hainan Province (No. 512114);the Ph.D. Programs Foundation of Ministry of Education (No. 20120006130002);Program for Changjiang Scholars and Innovative Research Team in University;

Research progress in anisotropic magnetoresistance

Abstract£º

Anisotropic magnetoresistance (AMR) is an important physical phenomenon that has broad application potential in many relevant fields. Thus, AMR is one of the most attractive research directions in material science to date. In this article, we summarize the recent advances in AMR, including traditional permalloy AMR, tunnel AMR, ballistic AMR, Coulomb blockade AMR, anomalous AMR, and antiferromagnetic AMR. The existing problems and possible challenges in developing more advanced AMR were briefly discussed, and future development trends and prospects were also speculated.

Keyword£º

Anisotropic magnetoresistance; Spin¨Corbit coupling; Spintronics;

Received£º 20 March 2013

1 Introduction

Spintronics,which is based on spin-polarized and spin dependent transport processes,have brought revolutionary applications in microelectronic devices[1¨C4].To ou knowledge,an electron is the carrier of both charge and spin Research shows that the spin degrees of freedom in low dimensional nanoscale systems are superior to the charge in many ways.Obtaining new generation microelec tronic devices with powerful functionality,convenien manipulation,and fast processing speed is possible by taking advantage of the spin properties of electrons.

Anisotropic magnetoresistance(AMR),which was discovered by William[5]in ferromagnetic metals,is an important physical phenomenon in spintronics.The application of AMR in magnetic recording[6]shows the considerable advancement of this phenomenon almost a century after its discovery.The application potentials of AMR have been further explored and used in various fields like sensors[7¨C11].The AMR possessing small magnetoresistance(MR)values was used in read heads and now has been replaced by spin valves and other magnetic electronic components which are based on giant magnetoresistance(GMR)and tunnel magnetoresistance(TMR)[12,13].However,the irreplaceable advantages of significant directional and positional sensitivity lead to promising applications,which have been the focus of research in related fields.In recent years,with the emergence of new members in the AMR family[14¨C18],these new AMR shows a rich phenomenology that opens new directions in spintronics research.

AMR refers to resistance changes in ferromagnetic metals,in which the resistance relies on the relationship between the axis of the current flow and magnetization orientation(previously called the¡®¡®orientation¡¯¡¯effect or ferromagnetic resistivity anisotropy).The origin of the effect is based on the spin¨Corbit coupling induced density of states and spin-dependent scattering anisotropy.This origin differentiates AMR from other MR effects that depend on the injection and detection of spin-polarized electrons(such as GMR and TMR).

To understand the internal mechanism of AMR and develop its potential applications,comprehensive investigations and research have been launched.Given the significant advantages of Ni Fe(permalloy)in weak field measurements,recent research has mainly focused on the Ni Fe film system[19¨C25].(Other related materials such as Ni Co and Fe Co will not be discussed in this study.)AMR can also be found in ferromagnetic semiconductor tunneling structures[14].Moreover,considerable progress on AMR has also been made in nanowires[15],single-electron transistor(SET)structures[16],perovskite-type Mn O[17],and antiferromagnetic(AFM)-based tunneling structures[18].Research progress on the new and traditional AMRs is described in the following sections.

2 Research progress in traditional AMR films

Before introducing the research progress in traditional AMR films,defining the relationships between the parameters that need to be optimized in AMR sensors is important.

The new generation of high-quality sensors require materials with high magnetic field sensitivities(S_v)and signal-to-noise ratios(or low noise),where high S_vis the key parameter(Fig.1).To obtain high S_v,the MR value of the material should be increased while minimizing the saturation field.The thickness of the Ni Fe layer should be10 nm or less to reduce the demagnetization effect.

2.1 Effect of different factors on AMR

The AMR value is generally affected by the microstructure properties of the material,including composition,film thickness,grain size,internal stress,texture,impurity atoms,and seed layer.Microstructure properties are strongly influenced by various process factors,such as applied external magnetic field,deposition temperature and rate,base/working pressure,and heat treatment.

Mitchell et al.[26]reported the compositional and thickness dependence of the AMR values of Ni Fe films in the early stages.They also compared the AMR of thin films and bulk materials as a composition function.

Lee et al.[27]found that thin films with better texture generally corresponds to larger MR values,whereas low MR values can be attributed to the random crystal orientation of grains.Lee et al.[27,28],Lin et al.[29],and Sheng et al.[30]all significantly enhanced the AMR ratio in sputtered permalloy thin films by obtaining strong(111)textures through the seed layer growth of Ni Fe Cr.

Fig.1 Parameters to be optimized as well as their relationship of AMR sensors

Some groups conducted systematic experimental studies on the influence of third elements on AMR.Chen et al.[31]reported that the addition of the element X(i.e.,Al,Nb,Si and Zr)into the Ni Fe film generally decreases the MR value while increasing the resistivity q of the film.However,the addition of notable elements such as Au,Pd,and Pt into permalloy films has less influence on the MR value Other studies[32]indicated that the addition of Sc,Ti,V Cr,Ge,Zr,Nb,Mo,Ru,Ta,and W to Ni Fe rapidly decreases the AMR.By contrast,the decrease in AMR is relatively slow during the addition of Co,Cu,Pd,Ag,Pt and Au.Studies proposed that the influence of elements situated near Ni in the periodic table is smaller than the elements situated far from Ni.

Mao et al.[33]deposited thin Ta/Ni Fe films by using three different deposition techniques:ion beam deposition(IBD),pulsed magnetron sputtering,and static magnetron sputtering.IBD films exhibit relatively high MR values particularly high MR changes at low Ni Fe thicknesses.IBD films also exhibit high MR ratios,whereas minimal difference is observed among different deposition techniques in the sheet resistance of Ni Fe films.

Funaki et al.[34]prepared Ni₈₀Fe₂₀films with 20 nm thicknesses.These films exhibit an improved MR of 3.5%after appropriate post-annealing treatment.This improvement is due to the reduction of the zero-field resistivity caused by the remarkable grain growth in the films.Considering the diffusive electron scattering at the film surface the MR value of the film is thought to be very close to the bulk value.Wu et al.[35]showed that Ni₈₃Fe₁₇films prepared at the substrate temperature above 350¡ãC at a post-annealing temperature of 650¡ãC exhibit good MR properties.They demonstrated that the decrease in resistivity and increase in AMR ratio are caused by the decrease in point defects,increase in grain size,and improvement of grain lattice integrity of the films.

Wang et al.[36]prepared a series of Ni₈₁Fe₁₉films by using the magnetron sputtering method under lower test conditions.They obtained excellent results and showed that the Ni₈₁Fe₁₉films prepared at the substrate temperature of400¡ãC exhibit higher AMR ratios and lower saturation fields.

When the film thickness approaches the electron mean free path,the relative contribution of the surface scattering to the resistivity becomes prominent.Electron scattering at the grain boundary and interface rapidly increases film resistivity,thus resulting in a sharp decrease in MR(Dq/q)Considerable interest is given on improving the electron scattering at the interface to enhance MR properties effectively by increasing the resistivity change Dq and decreasing the resistivity q of the film[23¨C25,37¨C39].Dieny et al.[40]stated that improving the specular electron reflection coefficient p at the interface reduces the sheet resistivity of the material effectively,thereby improving ultrathin film AMR(shown in Fig.2).

2.2 Effect of Co Fe Oxlayers on the AMR

Kamiguchi et al.[41]first reported that the MR can reach as high as 16%by oxidizing partially pinned Co Fe layers or free Co Fe layers in the spin valve.This work raised widespread attention in spintronics.Based on this finding,Wang et al.[39]inserted a nano-oxide layer Co Fe O_xinto the Ta/Ni Fe interface(Fig.3).The results showed that a Ni Fe thickness of 15 nm or less weakens the contributions of grain size and texture on MR value and significantly enhances the specular reflection of conduction electrons thereby increasing the MR value.

Fig.2 Theoretical calculation of resistivity a and AMR expressed in terms of relative change b or absolute change c of resistivity as a function of thickness of film for various values of coefficient of specular reflection on outer boundaries[40]

Fig.3 Values of DR/R of Ni Fe films without Co Fe Oxlayer(I)and with Co Fe Oxlayer(II)vs.Ni Fe layer thickness[39]

2.3 Effect of Au(or Pt)layers on the AMR

Considering that the AMR in ferromagnetic alloys originates from spin¨Corbit coupling,the Au(or Pt)layer with a strong spin¨Corbit coupling at different interfaces that show distinct magnetic behaviors[42]is inserted at the interface of the tri-layers Ta/Ni Fe/Ta[43].This approach leads to a significant increase in AMR value,which can be attributed to the strong electron spin¨Corbit scattering at Au/Ni Fe and Ni Fe/Au interfaces(Fig.4).Furthermore,the Pt layers can also reduce Ta and Ni Fe interdiffusion and result in negligible momentary loss and AMR degradation after annealing[25].

2.4 Effect of Mg O layers on the AMR

Major breakthroughs in spintronics phenomena are achieved with the development of spintronics.To obtain highperformance spintronic devices,Mg O is often used in various magnetic multilayer structures.Both Parkin et al[44]and Yuasa et al.[45]found considerable enhancement in TMR by introducing the Mg O barrier.Zhang et al.[46]found that the Co/Pt multilayers sandwiched by two Mg O layers considerably increase the anomalous Hall effect Fukuma et al.[47]achieved significant enhancements in spin accumulation and long-distance spin precession in Mg O-based metallic lateral spin valves.

Fig.4 MR value as a function of Au thickness(t nm)for(I)Ta 5 nm/Au(t nm)/Ni Fe 10 nm/Ta 5 nm and(II)Ta 5 nm/Ni Fe 10 nm/Au(t nm)/Ta 5 nm films[43]

Ding et al.[24]recently demonstrated MR enhancement in ultrathin Ni Fe films by introducing Mg O intercalations and annealing at 450¡ãC(Fig.5).The MR value of the10 nm Ni Fe films increases from 1.89%(S_v=6.0%/m T)for the as-grown samples to 3.5%(S_v=21%/m T)for the annealed samples,thus exceeding the theoretical value predicted by Dieny et al.[40].When multilayered films are fabricated into sensor elements that are 30 lm in width,the S_vof the Ta/Mg O/Ni Fe/Mg O/Ta elements achieves33 m V?(V?m T)^-1,which is very close to the value of some TMR elements.Microstructure analysis indicates that considerable Mg O crystallization after annealing plays a significant role on the specular reflection of conduction electrons and significantly improves MR and S_v.This finding is a big step in advancing a new generation of highsensitivity AMR sensor designs.

Although different interpretations have been proposed to explain the functionalities of Mg O layers,the mechanism of the Mg O contribution to the electron spin behavior in artificial thin films remains indefinite.An in-depth understanding of this issue can help elucidate the effects of Mg O in other spintronic materials and devices and provide better quality control to the Mg O layer,thereby optimizing spintronic materials and devices.

Ni Fe films sandwiched by two Mg O layers exhibit subtle and complex MR behavior by systematic annealing[48].Annealing temperature and time significantly affect the sample MR(Fig.6).Lower annealing temperatures(200¨C300¡ãC)lead to a slow increase in sample MR whereas higher annealing temperatures(450¨C550¡ãC)lead to rapid increases in MR within several seconds.When the annealing temperature reaches 450¡ãC,the sample MR ratio reaches 3.25%in 100 s.Contrary to the significant MR change after rapid thermal annealing,the magnetic properties of the thin film samples do not exhibit appreciable variation(Fig.6b).

Considering that positron annihilation spectroscopy(PAS)is one of the most powerful and well-established techniques for defect evaluation,PAS is employed to show the microstructural evolution of Mg O/Ni₈₁Fe₁₉/Mg O during thermal annealing and its effect on the sample MR ratio.The variations of the parameters(Fig.7)show that the defects in the Mg O layer play a determinant role in determining the sample MR.Findings indicate that the ionic interstitials in the Mg O layers recombine with the nearby vacancies and lead to a slow increase in sample MR at 200¨C300¡ãC.By contrast,vacancy defects agglomerate and ordering accelerations in Mg O occur at 450¨C550¡ãC.The improved Mg O and Mg O/Ni Fe interfaces account for the observed significant MR enhancement.

Fig.5 MR and Svas a function of annealed temperature for Ta/Mg O/Ni Fe/Mg O/Ta a,and b MR transfer curves for elements without and with Mg O layers[24]

Fig.6 MR¨CH curves a and hysteresis loops b for Ta(5 nm)/Mg O(4 nm)/Ni Fe(10 nm)/Mg O(3 nm)/Ta(3 nm)(Sample 1)and Ta(5 nm)/Ni Fe(10 nm)/Ta(3 nm)(Sample 2)multilayers before and after annealing at 450¡ãC for 100 s(inset of a showing Sample 1 resistivity change as a function of annealing time at 450¡ãC),and c annealing time dependence of MR for Sample 1 at different temperatures[48]

2.5 Effect of Coulomb blockade(CB)on AMR

Mg O defects produce severe diffuse scattering of the conduction electrons and limit MR improvement[48].Therefore,reducing diffuse electron scattering and increasing specular electron scattering lead to larger MR for ultrathin Ni Fe films.

Some special physical characteristics such as CB and spin accumulation[49¨C53]are observed in granular materials,i.e.,nanoscopic particle composites embedded into a matrix of different types(metallic into insulating,etc.),which are mainly attributed to specific interactions at the granule¨Chost interface and(long-range)between granules Yang et al.[54]inserted an ultrathin Co Fe layer in the middle of the Mg O layer in the structure of Co Fe/Mg O/Co Fe.When the Co Fe layer is less than 2 nm,a discontinuous layer of granules is formed,thus affecting the magneto-transport properties of the TMR considerably because of the CB effect.The barrier resistances become larger than the quantum resistance h/e²=25.8 k X(h is Planck¡¯s constant and e is the electron charge),and wave functions are confined to the respective electrodes.

Based on this phenomenon,Huangfu et al.[55]prepared an AMR film with the structure of Mg O/Ni Fe(I)/Mg O/Ni Fe/Mg O/Ni Fe(I)/Mg O by vacuum annealing.The experiment results show that the AMR values of the 5 nm Ni Fe films can be significantly improved by the addition of an ultrathin Ni Fe(I)layer in the middle of the Mg O layer.The improvement is due to the improved specular reflections of the electrons at both the Ni Fe/Mg O interfaces(Fig.8).

Fig.7 Doppler a S-and b W-parameters plotted vs.positron incident energy for Ta/Mg O/Ni Fe/Mg O/Ta in as-deposited state and after annealing100 s at different temperatures(solid lines being guide for the eye),and c S¨CW diagram for Ta/Mg O/Ni Fe/Mg O/Ta in as-deposited state and after annealing 100 s for different temperatures(in order to increase clarity,the points corresponding to the substrate being removed,leaving the dashed circle as a marker)[48]

2.6 Design of the all-metal AMR sensor

Lee et al.[27]and Lin et al.[29]reported that a Ni Fe film grown on a Ni Fe Cr(or Ni Cr)seed layer has high MR value.Ding et al.[56]designed a Ni Fe Cr/Ni Fe/Pt/Ta structure with high-sensitivity for the first time.This design can be attributed to the likely epitaxial relationship at the Ni Fe Cr/Ni Fe interface and the strong electron spin¨Corbit scattering between Ni Fe and Pt interfaces.The magnetic sensitivity reaches 31 m V?(V?m T)^-1,which is very close to the value of some TMR elements,when fabricated into sensor elements.

Although the MR value and S_vin all-metal Ni Fe materials designed by Liu et al.[25]and Ding et al.[56]are lower than the materials with Mg O barriers and Ni Fe layers,high-temperature annealing,which is more conducive to the actual production,is not required in all-metal Ni Fe materials.Furthermore,all-metal Ni Fe materials have good application prospects because of the relatively close thermal expansion coefficient between the metals,which avoid high noise.

3 Research progress in new AMR materials

3.1 Research progress in tunnel AMR(TAMR)

TAMR is an AMR phenomenon generated in a single ferromagnetic layer and originates from an anisotropic density of states with respect to the magnetic moment induced by strong spin¨Corbit coupling.

Devices relying on spin manipulation are expected to provide low-dissipative alternatives for microelectronics Furthermore,spintronics can potentially lead to the full integration of information processes and storage functionalities,thus opening attractive prospects for the realization of instant on-and-off computers.To achieve this goal,a scheme for such a device must include a tunnel barrier between two ferromagnetic semiconductors such as(Ga Mn)As/(Al,Ga)As/(Ga,Mn)structures[57,58].

To understand completely the physics of tunneling into(Ga,Mn)As and realize the full potential of these systems Gould et al.[14]investigated the TAMR effect.The results showed that a TAMR effect below 20 K exists in a structure containing a single ferromagnetic(Ga,Mn)As layer fitted with a tunnel barrier Al O_xand a non-magnetic metal contact.They introduced a new class of spintronic devices in which a spin-valve-like effect results from strong spin¨Corbit coupling in a single ferromagnetic layer rather than from the injection and detection of a spinpolarized current by two coupled ferromagnets.The removal of the second ferromagnetic layer is more suitable for high-temperature operations(such as annealing,etc.).The rich physics phenomena and value in potential applications of TAMR with a relatively small value(about 3%),gained considerable research interests for the exploration of new directions in spintronics research.

Fig.8 DR/R as a function of t*for S1 and S2 a,DR/R curves for t*=0 nm and t*=1.5 nm in S1 b,and c schematic illustration of Ni Fe(I)nanoparticles and motion of conduction electrons in Ni Fe layer.S1 Mg O(4)/Ni Fe(t*)/Mg O(2)/Ni Fe(5)/Mg O(2)/Ni Fe(t*)/Mg O(2);S2Mg O(4)/Ni Fe(t*)/Mg O(2)/Ni Fe(10)/Mg O(2)/Ni Fe(t*)/Mg O(2)(in nm)[55]

Subsequently,comprehensive theoretical and experimental research on TAMR in different systems is conducted.Ru¡§ster et al.[59]reported the existence of a large TAMR effect exceeding 100,000%in an epitaxially grown(Ga,Mn)As/Ga As/(Ga,Mn)as structure at a low temperature of 1.7 K.(Note the distinguishing TAMR and anisotropic TMR.The explored structure above has two ferromagnetic layers where the spin injection may occur;therefore,the structure should belong to the anisotropic TMR[60].)Saito et al.[61]investigated spin-dependent transport in fully epitaxial Ga_1-xMn_xAs/Zn Se/Ga_1-xMn_xAs heterostructures.They proved that TAMR originates from the anisotropic density of states at the Fermi energy D(EF)of Ga_1-xMn_xAs with respect to magnetization because of the strong spin¨Corbit coupling interactions of Ga_1-xMn_xAs in the valence band.Shick et al.[62]theoretically studied the density of states and magnetocrystalline anisotropies in Co Pt structures.The result showed that the calculated density of state anisotropy of the Co Pt system is greater than that of(Ga,Mn)As system,thus suggesting that a sizable TAMR effect exists in room-temperature metallic ferromagnets.This result promotes the development of new spintronic devices.Moser et al.[63]observed a TAMR in the epitaxial metal¨Csemiconductor system Fe/Ga As/Au with a value of less than 0.5%because of the weak spin¨Corbit coupling in iron.Gao et al.[64]reported that a TAMR effect can be observed in magnetic tunnel junctions with 3d transition metal ferromagnetic electrodes for both crystalline Mg O(001)and amorphous Al₂O₃tunnel barriers Chantis et al.[65]utilized the Rashba shift of interface resonant states and demonstrated a large TAMR effect of up to 20%with the predicted values for the Fe(001)surface at small bias voltages.

Park et al.[66]investigated the TAMR effect in multilayer-(Co/Pt)/Al O_x/Pt structures and provided strategies for future research.Shick et al.[67]investigated the multilayer bimetallic structure of 3d¨C4(5)d Mn/W(001).By using the first principle,they predicted that large magnetic anisotropy energy,as well as the density of states anisotropy,will produce larger TAMR effects.

In recent years,research on the TAMR was successively reported.The theoretical modeling of Wimmer et al.[68]showed that the interplay of the orbital effect of a magnetic field and the Dresselhaus spin¨Corbit coupling in the Ga As barrier independently contributes to the TAMR effect with uniaxial symmetry,whereas the Bychkov¨CRashba spin¨Corbit coupling does not participate.Matos-Abiague et al.[60]showed that the interplay of the Bychkov¨CRashba and the Dresselhaus spin¨Corbit coupling cause a twofold symmetry of the TAMR effect,as supported by their model calculations.The dependence of MR on the absolute orientation of magnetization in ferromagnets is deduced from general symmetry considerations.Jia et al.[69]found that the TAMR in a Schottky junction is not directly related to magnetocrystalline anisotropy,whereas the uniaxial-type TAMR primarily originates from the combination of Rashba-type and Dresselhaus-type spin¨Corbit interactions.After investigating an epitaxial Co Fe layer deposited on Asor Ga-terminated Ga As,Uemura et al.[70]suggested that the anisotropy of the electronic structure is the likely cause of the TAMR effect rather than the structural anisotropy.Writing the magnetic state is achieved via current-induced switching,and state reading is conducted by the TAMR effect.Mark et al.[71]produced a read¨Cwrite device out of the ferromagnetic semiconductor(Ga,Mn)As.The production of this device is the first step toward a fundamentally new information-processing paradigm.

Numerous studies on TAMR were also reported[72¨C78].The limitation on length prevents the presentation of tautology.

3.2 Research progress in ballistic AMR(BAMR)

The AMR phenomenon known as BAMR is observed in1D nanomaterials when the constriction width becomes comparable to the Fermi wavelength.BAMR originates from strong spin¨Corbit interaction to the quantized conductance,and the resultant number change of bands N crossing the Fermi energy depends on the magnetization direction.

Velev et al.[15]predicted a change in the ballistic conductance with the direction of magnetization,i.e.,the occurrence of BAMR with a predicted MR value of 17%.When the dimensions approach less than the mean free path of the electrons,the electronic transport greatly changes in nanometer-scale constrained geometries.In such a case,electron transport becomes ballistic,and conductance is quantized with the unit 2e²/h for nonmagnetic materials.However,the unit for magnetic materials should be e²/h because of the lift in spin degeneracy by the exchange interaction.A recent study on Ni ballistic nanocontacts indicated that the MR signal depends on whether the current and the magnetic field are parallel or perpendicular.This kind of behavior is interpreted as the traditional signature of AMR[79].The BAMR is different from the AMR obtained in bulk materials because of the absence of electron scattering.The ballistic conductance is expressed by the equation G=Ne²/h,where N is more strongly influenced by the spin¨Corbit interaction in nanoconstrained geometries than in bulk materials.The coupling between spins and the orbital angular momentum cause anisotropic effects because they cause changes in the projection of the spin along with the magnetization direction.Therefore,ballistic conductance can be tuned by changing the number of bands on the Fermi surface by adjusting the magnetization.

Research on ballistic transport has made considerable progress to date[80¨C86].Rocha et al.[86]achieved a BAMR up to 450%in their experiment.This effect has great appeal because of the considerable MR effect.

3.3 Research progress in CB-AMR

The CB-AMR are AMR signals produced in SETs with magnitudes approaching resistance variations because of CB oscillations.The origin of the effect is the large chemical potential shifts induced by magnetization rotations in ferromagnets with strong spin¨Corbit coupling,thus giving unprecedented characteristics to the ferromagnetic SET[16]

Wunderlich et al.[16]observed the CB-AMR effect on a ferromagnetic semiconductor SET.The low-field hysteretic MR emerges in a(Ga,Mn)As SET with a CB-AMR value of up to 1,000%.The effect provides new concepts in functionality such as combining electrical transistor like actions with permanent information storages or utilizing magnetization controlled transistor characteristics in a single nanoscale element.

Many groups have made several subsequent studies on the CB-AMR,namely,theoretical works on anisotropic magneto-Coulomb effects and magnetic SET action in a single nanoparticle system[87¨C90].Bernand-Mantel et al[88]found that magneto-Coulomb effects are caused by the interaction between the charges and spin degrees of freedom.To date,the focus of research has been mainly theoretical with limited experimental results.

3.4 Research progress in abnormal AMR

Abnormal AMR refers to the AMR effect found in perovskite-type Mn O,which is several orders of magnitude higher than traditional AMR.The abnormal AMR originates from the broken symmetry caused by a distortion on the lattice structure from cubic to orthorhombic;this distortion creates an AMR response to the external field and the consequent remarkable magneto-transport behavior[17].

Li et al.[17]accounted for the abnormal AMR effect in La_0.69Ca_0.31Mn O₃by using a proposed simple phenomenological model.The effect is a non-monotonic relationship between the temperature and magnetic field with a MR value higher than GMR and TMR,and several orders of magnitude higher than traditional AMR.A small crystalline anisotropy stimulates a¡®¡®colossal¡¯¡¯AMR near the metal¨Cinsulator transition phase boundary of the system,thus revealing the intimate interplay between magneto-and electronic-crystalline couplings.The abnormal AMR provides not only detailed information for fundamental physics such as spin¨Corbit and magneto-elastic coupling in the material,but also guidelines for the study of new AMR materials and applications.

Yu et al.[91]investigated the relationship between the magnetic domain configuration and crystallographic orientation in a colossal magnetoresistive material La_0.69Ca_0.31Mn O₃.However,this study lacks follow-up research.

3.5 Research progress in AFM-TAMR

The TAMR phenomenon was only found in ferromagnets in previous studies,which rely on the relative orientation changes of magnetic moments in a single ferromagnetic electrode.However,Shick et al.[92]and Jungwirth et al.[93]predicted an alternate route toward AFM spintronics based on the relativistic spin¨Corbit coupling phenomenon,particularly on the TAMR.Park et al.[18]predicted another route on the development of AFM spintronics based on spin¨Corbit coupling,namely,the MR in an AFM system,and realized the AFM-TAMR phenomenon in Ni Fe/Ir Mn/Mg O/Pt structure.

In theory,no apparent physical limitation exists on the possibility of realizing the AMR effect induced by spin¨Corbit coupling at high temperatures.The tunneling element designed by Park et al.[18]contains only one Ir Mn AFM-magnetic electrode and a Pt electrode separated by a nonmagnetic Mg O barrier layer.

The TAMR value is very large at low temperature but is reduced to a few percent at 100 K.This finding requires subsequent works to optimize inpidual layers and interfaces in the AFM-TAMR structures to establish the limits of high-temperature operations.

The AFM-TAMR can also be used as an experimental tool to the study magnetic characteristics of AFM films by measuring electronic transport properties.AFM spintronics is likely to be an attractive alternative[94,95]to conventional spintronics based on ferromagnets.Synthesizing a ferromagnetic semiconductor with high-Curietemperature is a challenging.Similarly,finding the AFM with a safe ordering temperature at higher room temperature is an enormous challenge in the magnetic counterpart of the most common semiconductor compounds.

4 Conclusion

The development of AMR is still on the preliminary stage and research on this topic faces a number of challenges.As a new physical phenomenon,AMR research has a long way to go before it can be used for practical applications Nevertheless,AMR especially the new AMR has attracted widespread interest in the field of spintronics because of its great application prospect.

To use conventional permalloy AMR successfully in the new generation of high-sensitive magnetic field sensors decreasing signal noise while continuously increasing sensitivity is necessary.Given that the potential application value of TAMR still needs to be explored,corresponding fundamental theories and application studies should be continued.BAMR,which is anticipated to have a value as high as 10,000%,needs to be investigated and realized Achieving permanent information storage is plausible by using the combination of CB-AMR with electron transistors.Anomalous AMR exhibits very high MR in low magnetic fields and the investigation of its mechanism and application aspects is urgently needed.AFM-AMR has attracted considerable attention since its discovery,and the corresponding research on theory and application should be pursued.

Limited studies on novel MR are incompatible with the increasing demand for advanced functional materials and devices in China.Interdisciplinary and multidisciplinary studies on the MR effect are recommended.On-going indepth research will eventually advance applications in spintronic devices.

Acknowledgments This study was financially supported by the National Natural Science Foundation of China(Nos.51071023 and51101047),the Natural Science Foundation of Hainan Province(No.512114),the Ph.D.Programs Foundation of Ministry of Education(No.20120006130002),and Program for Changjiang Scholars and Innovative Research Team in University.

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