Measurement of magnetic parameters of nanometer-thick conducting magnetic films using anisotropic magnetoresistive effect

The angular dependences of anisotropic magnetoresistance (AMR) are measured in conducting ferromagnetic films of nanometer thickness and layered structures containing such films and having the shape of narrow ribbons. These structures are used for preparing spin-dependent magnetic tunnel junctions possessing a giant magnetoresistance. The possibility of determining the main magnetic parameters, which are important for preparing magnetic junctions, by AMR angular measurements is demonstrated experimentally. The magnetic anisotropy axis, the saturating magnetic field, and the coercivity are determined in a 25-nm-thick permalloy (Py) film, in the structures FeMn film (15 nm)-Py film (10 nm) deposited by RF magnetron sputtering on a oxidized silicon substrate, as well as in the structure FeMn (15 nm)-Py (10 nm)-SiC (1.5 nm)-Py (10 nm) deposited on a sitall substrate. It is shown that, under the same conditions of Py films deposition, the magnetic anisotropy axis in the FeMn-Py structure is turned through 90° relative to the anisotropy axis of Py in structures without FeMn layers. The value of the exchange bias fields of the magnetization reversal measured in the structure FeMn (15 nm)-Py (10 nm)-SiC (1.5 nm)-Py (10 nm) by the AMR method is in good agreement with the result of measurement by the inductive method.     

Anisotropic interface magnetoresistances in Pt(111)/Co n /Pt(111)

It is shown in terms of a fully relativistic spin-polarized ab initio-type approach that in Pt/Co/Pt trilayers two types of anisotropic magnetoresistance (AMR) have to be distinguished: an in-plane and an out-of-plane AMR. The obtained results, namely the magnetic field dependence as well as the thickness dependence of both AMR types are in very good agreement with a very recent experimental study, in which the in-plane as well as the out-of-plane AMR was reported for this system. The difference between the two types of AMR is visualized in terms of layer-resolved resistivities. In particular, it is confirmed that the anisotropic interface magnetoresistance (AIMR) introduced in the recent publication mainly originates in the vicinity of the Co/Pt interfaces.     

Temperature dependence of relaxation times in electron focusing and antidot structures made from In0.53Ga0.47As/InP heterojunctions

The magnetoresistance of antidot lattices and the magnetic field dependence of the three-terminal resistance of transverse electron focusing (TEF) devices is studied in a 2DEG in the lattice-matched In_0.53Ga_0.47As/InP heterojunction system, as a function of temperature. Ballistic effects are observed in both types of mesoscopic devices at temperatures exceeding 100 K, and are considerably more robust than Shubnikov–de Haas (SdH) oscillations. Two effects influence the decay in the amplitude of the magnetoresistance peaks in the antidot structures, and of the focusing peaks in the TEF devices: the thermal smearing of the Fermi surface, and the increase in the electron scattering time with increasing temperature due to the increase in electron–phonon scattering. We deduce the temperature dependencies of the scattering times for the different geometries.Copright 1998 Academic Press     

Planar Hall effect in electrodeposited CoCu/Cu multilayer

Electrodeposited CoCu/Cu multilayers were investigated by measuring both anisotropic magnetoresistance (AMR) and planar Hall effect (PHE) simultaneously. Studies have been carried out on a [Co(3 nm)/Cu(4 nm)]₅₀ multilayer sample, where a maximum of ?8.8 % GMR was observed at room temperature. A direct comparison of AMR and PHE output has been made both as a function of field and its relative orientation with respect to the current. Marked changes in PHE loops were observed at different angles (between magnetic field and applied current) whereas no noticeable changes could be found for AMR results. Such PHE outputs are the manifestations of complex spin reorganization due to strong antiferromagnetic-coupling between adjacent magnetic layers. In case of angular dependence output, when the applied field is less than the coercive field, the PHE output shows a deviation from the Sin2θ dependence that can be correlated to the domain wall propagation.     

From antidot to dot superlattices: Magnetoresistance anomalies

For low Fermi energies and large antidot diameters an antidot superlattice changes into a dot superlattice. A negative magnetoresistance (NMR) for low magnetic fields coexisting with a commensurability peak is observed in this transition regime. We reproduce these features for both classical and quantum-mechanical calculations in a model, which traces the NMR back to saddle points with varying heights between the antidots. 1997 Elsevier Science B.V. All rights reserved.     

Anomalous interlayer magnetoresistance in bilayer crystals

The interlayer magnetotransport of a model layered metal is calculated semiclassically. Each layer contains parallel quasi-1D wires but the orientation of wires within each layer is perpendicular to the orientation of wires in adjacent layers. The model has a highly anisotropic amplitude for interlayer electron transfer and is used to illustrate simply the effects that this anisotropy has on the magnetotransport. Strong positive magnetoresistance is calculated for magnetic fields parallel to the current, with the size of magnetoresistance varying inversely with the interlayer hopping amplitude. For fields perpendicular to the current, the magnetoresistance depends qualitatively on the orientation of the field: it scales linearly with the field strength B when the field points toward intersections of 1D Fermi surfaces belonging to individual layers, and scales as √B when the field points between intersections. In a weak field, the resistance is maximum when the field is orientated parallel to the current and minimum when it is perpendicular to the current. Magnetoresistance oscillations are also studied. The implications for more general models of multilayer metals are discussed.     

多层膜巨磁电阻特性及电流最佳测量

测量了不同方向外磁场和温度下多层膜巨磁电阻的磁阻特性,给出了巨磁电阻模拟传感器用于电流测量的最佳磁偏置.结果表明:外磁场强度相同但方向不同,对巨磁电阻的作用效果不同,巨磁电阻饱和时,阻值与外磁场方向无关.温度不同,巨磁电阻的阻值不同,磁电阻变化率也有改变.     

Size effect of the longitudinal magnetoresistance of thin films of n- type Ge

In a study of the longitudinal magnetoresistance as a function of the magnetic field in samples of n-type Ge of various thicknesses at 300°K and 77°K, the surface of the samples was oriented parallel to the (001) plane. The directions of the current and the magnetic field were along either the <100> direction or the <110> direction. As the thickness of the samples was reduced, regardless of the orientation of the current and the magnetic field with respect to the crystallographic directions, a decrease was observed in the magnetoresistance over the entire range of magnetic fields studied, 1–280 kOe. The experimental results are attributed to a screening-length size effect and to “sliding” orbits in the enriched surface layers, which lead to an increase in the electron mobility in a strong magnetic field.     

Extremely large magnetoresistance in boron-doped silicon

Boron-doped Si-SiO2-Al structures are fabricated to study extremely large magnetoresistance (MR) effects. Current-voltage characteristics show a nonlinear behavior, dominated by an autocatalytic process of impact ionization. At low temperatures, the magnetic field postpones the onset of impact ionization to higher electric fields. This results in a symmetric positive MR of over 10,000% at 400 kA/m. Applying a magnetic field leads to an increase of the acceptor level compared to the valence band as deduced by admittance spectroscopy. A macroscopic transport model is introduced to describe how the MR is controlled by voltage, electrode spacing, and oxide thickness.     

Ballistic transport in bilayer nano-graphite ribbons under gate and magnetic fields

This study uses the tight-binding model to examine the ballistic transport of short and infinitely long bilayer nano-graphite ribbons for different stacked structures, AA and AB, under perpendicularly applied gate and magnetic fields. In the small bias region, the conduction of the AB-stacked ribbon is better than for the AA. Under a gate field with small bias, the AB-stacked ribbon exhibits a significant current peak at the zero gate field point, similar to the graphene ribbon. On the contrary, this current peak is not found in the AA-stacked case. Under a perpendicular magnetic field with small bias, the magnetoresistance ratio in both stacked graphene ribbons are proportional to the square of the magnetic field.     

锯齿形石墨烯反点网络加工与输运性质研究

具有特定边界的石墨烯纳米结构在纳电子学、自旋电子学等研究领域表现出良好的应用前景.然而石墨烯加工成纳米结构时,无序的边界不可避免地会降低其载流子迁移率.氢等离子体各向异性刻蚀技术是加工具备完美边界石墨烯微纳结构的一项关键技术,刻蚀后的石墨烯呈现出规则的近原子级平整的锯齿形边界.本文研究了氮化硼上锯齿形边界石墨烯反点网络的磁输运性质,低磁场下可以观测到载流子围绕着一个空位缺陷运动时的公度振荡磁阻峰.随着磁场的增大,朗道能级简并度逐渐增大,载流子的磁输运行为从Shubnikov-de Haas振荡逐渐向量子霍尔效应转变.在零磁场附近可以观测到反点网络周期性空位缺陷的边界散射所导致的弱局域效应.研究结果表明,在氮化硼衬底上利用氢等离子体刻蚀技术加工锯齿形边界石墨烯反点网络,其样品质量会明显提高,这种简单易行的方法为后续高质量石墨烯反点网络的输运研究提供了新思路.     

Observation of commensurability oscillations of thermopower in an antidot lattice

Commensurability oscillations of thermopower in a square antidot lattice are observed. The oscillations are attributed to the geometrical resonances of the classical electron motion in a magnetic field and are much more pronounced than the corresponding magnetoresistance oscillations. The off-diagonal component of the thermopower tensor (the Nernst-Ettingshausen effect) changes sign at resonances. Additional measurements of magnetoresistance verify the correctness of the method used for thermopower measurements and provide information on the temperature distribution in the sample.     

Anomalous giant piezoresistance in AlAs 2D electron systems with antidot lattices

An AlAs two-dimensional electron system patterned with an antidot lattice exhibits a giant piezoresistance effect at low temperatures, with a sign opposite to the piezoresistance observed in the unpatterned region. We suggest that the origin of this anomalous giant piezoresistance is the nonuniform strain in the antidot lattice and the exclusion of electrons occupying the two conduction-band valleys from different regions of the sample. This is analogous to the well-known giant magnetoresistance effect, with valley playing the role of spin and strain the role of magnetic field.     

Reversing the training effect in exchange biased CoO/Co bilayers

We performed a detailed study of the training effect in exchange biased CoO/Co bilayers. High-resolution measurements of the anisotropic magnetoresistance (AMR) display an asymmetry in the first magnetization reversal process and training in the subsequent reversal processes. Surprisingly, the AMR measurements as well as magnetization measurements reveal that it is possible to partially reinduce the untrained state by performing a hysteresis measurement with an in-plane external field perpendicular to the cooling field. Indeed, the next hysteresis loop obtained in a field parallel to the cooling field resembles the initial asymmetric hysteresis loop, but with a reduced amount of spin rotation occurring at the first coercive field. This implies that the antiferromagnetic domains, which are created during the first reversal after cooling, can be partially erased.     

Characterization and Magnetic Properties of Iron-Based Alloy Antidot Arrays

Fe29Cerl and Fe19Ni81 antidot arrays, with different dimensions, are prepared with the rf magnetron sputtering method onto the porous alumina substrate. The size and shape of antidot arrays are characterized by scanning electron microscopy. The glancing angle x-ray diffraction patterns of Fe29C071 and FelgNi81 antidot arrays indicate the bcc and fcc structures, respectively. The coercivities of both the alloys show abnormal thickness dependence, which are discussed qualitatively by considering the pinning and the thickness effect to the films.     

Spin-controlled negative magnetoresistance resulting from exchange interactions

We studied conductivity of AlGaAs–GaAs quantum well structures (where centers of the wells were doped by Be) at temperatures higher than 4 K in magnetic fields up 10 T. Throughout all the temperature region considered the conductivity demonstrated activated behavior. At moderate magnetic fields 0.1 T < H < 1 T, we observed negative isotropic magnetoresistance, which was linear in magnetic field while for magnetic field normal with respect to the plane of the wells the magnetoresistance was positive at H > 2T. To the best of our knowledge, it was the first observation of linear negative magnetoresistance, which would be isotropic with respect to the direction of magnetic field. While the isotropic character of magnetoresistance apparently evidences role of spins, the existing theoretical considerations concerning spin effects in conductance fail to explain our experimental results. We believe that such a behavior can be attributed to spin effects supported by exchange interactions between localized states.     

Semiclassical and quantum transport in the two-dimensional electron gas in a hard-wall antidot lattice

Commensurate peaks of magnetoresistance and Shubnikov-de Haas and Aharonov-Bohm oscillations in the two-dimensional electron gas (2DEG) in a lattice of antidots with hard potential walls have been experimentally studied. The behavior of both classical magnetoresistance peaks and quantum oscillations has been shown to fundamentally depend on the lattice period and the antidot size, as well as on the smoothness of the potential at the 2DEG-antidot interface. This result indicates the necessity of revising the interpretation of all numerous experiments with antidot lattices, since this effect has been explicitly or implicitly neglected in them.     

Magnetoresistance anisotropy in a hexagonal lattice of Co antidots obtained by thermal evaporation

Patterned soft magnetic materials are eligible for use in magnetic random access memories. A hexagonal-lattice pattern of circular antidots was produced by optical lithography in a Co film. In order to test the effect of geometry on the local magnetisation configuration of such a structure, we performed room-temperature angle-resolved magnetisation measurements aimed to check the pinning of domain walls by the pattern's lattice. Magnetoresistance (MR) room-temperature measurements were performed at various angles between the magnetic field direction and the macroscopic electrical current vector, to clarify whether and how the local current density configuration affects the MR response. We found that the magnetoresistance is of anisotropic type (AMR) and has a local origin. Furthermore, the largely unsaturating behaviour of MR at high fields may be explained only by considering that tiny portions of the pattern constitute highly frustrated regions and align their magnetisation at rather high fields. A simplified model based on a local anisotropy term is shown to account for the experimental results for both M and MR.     

Mini-gaps and novel giant negative magnetoresistance in InAs/GaSb semimetallic superlattices

We have studied the temperature dependence of the magnetoresistance of semimetallic InAs/GaSb structures with magnetic field applied parallel to the layers. We present the first unambiguous evidence for the presence of a mini-gap at the crossing point between the electron and hole dispersion relations. The resistivity is found to change from semiconductor-like behaviour with a strong temperature dependence at low parallel magnetic fields to that of a semimetal with a weak temperature dependence at high field. Furthermore, the magnetoresistance, for intrinsic samples, is found to decrease with field by as much as 70% at low temperatures. As the parallel magnetic field is increased the centres of the electron and hole dispersions are shifted apart in k-space and at sufficiently high field the mini-gap is destroyed and the bands overlap fully. Finally, a theoretical model allows us to estimate that the mini-gap is of order 7 meV.     

垂直各向异性Ho3Fe5O12薄膜的外延生长与其异质结构的自旋输运

垂直磁各向异性稀土-铁-石榴石纳米薄膜在自旋电子学中具有重要应用前景.本文使用溅射方法在(111)取向掺杂钇钪的钆镓石榴石(Gd_0.63Y_2.37Sc₂Ga₃O₁₂,GYSGG)单晶衬底上外延生长了2—100 nm厚的钬铁石榴石(Ho₃Fe₅O₁₂,HoIG)薄膜,并进一步在HoIG上沉积了3 nm Pt薄膜.测量了室温下HoIG的磁各向异性和HoIG/Pt异质结构的自旋相关输运性质.结果显示,厚度薄至2 nm的HoIG薄膜(小于2个单胞层)在室温仍具有铁磁性,且由于外延应变,2—60 nm厚HoIG薄膜都具有很强的垂直磁各向异性,有效垂直各向异性场最大达350 mT;异质结构样品表现出非常可观的反常霍尔效应和“自旋霍尔/各向异性”磁电阻效应,前者在HoIG厚度小于4 nm时开始缓慢下降,而后者当HoIG厚度小于7 nm时急剧减小,说明相较于反常霍尔效应,磁电阻效应对HoIG的体磁性相对更加敏感;此外,自旋相关热电压随HoIG厚度减薄在整个厚度范围以指数方式下降,说明遵从热激化磁振子运动规律的自旋塞贝克效应是其主要贡献者.本文结果表明HoIG纳米薄膜具有可调控的垂直磁各向异性,厚度大于4 nm的HoIG/Pt异质结构具有高效的自旋界面交换作用,是自旋电子学应用发展的一个重要候选材料.