Giant magnetoresistance of iron-chromium superlattices at microwaves

Interaction between an rf electromagnetic field and the Fe/Cr superlattice placed in a rectangular waveguide so that a high-frequency current passes in the plane of superlattice layers is considered. The transmission coefficient versus the magnetic field strength is found at centimeter waves, and a correlation between this dependence and the field dependence of the dc magnetoresistance is established. It is shown that a change in the transmission coefficient may greatly exceed the giant magnetoresistance of the superlattice. The frequency dependence of the microwave measurements has an oscillatory character. The oscillation frequencies are analyzed in terms of wavelet transformation. Two types of oscillation periods are found to exist, one of which corresponds to the resonance of waves traveling in the superlattice along the direction parallel to the narrow wall of the waveguide.     

Investigation of the effect of annealing on the interface structure in Fe/Cr superlattices by conversion electron Mössbauer spectroscopy

The effect of vacuum annealing on the structure of the Fe/Cr superlattice, exhibiting giant magnetoresistance effect, has been investigated by Mössbauer spectroscopy. It is shown that annealing-induced activation of diffusion leads to a decrease in the thickness of “pure” Cr and Fe layers, redistribution of atoms in the interface regions, and a change in the giant magnetoresistance. The existence of the angular spatial dispersion of the magnetic anisotropy field at the interface has also been revealed.     

Giant magnetoresistance caused by magnetic-field-sensitive resonant tunnelling in a random multibarrier semiconductor superlattice

By using a transmission probability formalism, we perform a calculation for the conductance related to the tunnelling of electrons through a semiconductor multibarrier superlattice with special thickness randomness in the presence of a magnetic field. Because of the sharp resonance of the tunnelling in such a structure, the current is highly sensitive to the variation of the in-plane magnetic field. The calculated results show a giant magnetoresistance even for small strength of the field. Possible applications of this effect are discussed.     

InGaN/GaN超晶格厚度对Si衬底GaN基蓝光发光二极管光电性能的影响

采用有机金属化学气相沉积技术在Si(111)衬底上生长蓝光多量子阱发光二极管(LED) 结构, 通过在量子阱下方分别插入两组不同厚度的InGaN/GaN超晶格, 比较了超晶格厚度对LED光电性能的影响. 结果显示: 随超晶格厚度增加, 样品的反向漏电流加剧; 300 K下电致发光仪测得随着电流增加, LED发光光谱峰值的蓝移量随超晶格厚度增加而减少, 但不同超晶格厚度的两个样品在300 K下的电致发光强度几乎无差异. 结合高分辨X射线衍射仪、扫描电子显微镜、透射电子显微镜对样品的位错密度和V形坑特征分析, 明确了两样品反向漏电流产生巨大差异的原因是由于超晶格厚度大的样品具有更大的V形坑和V形坑密度, 而V形坑可作为载流子的优先通道, 使超晶格更厚的样品反向漏电流加剧. 通过对样品非对称(105)面附近的X射线衍射倒易空间图分析, 算得超晶格厚度大的样品其InGaN量子阱在GaN上的弛豫度也大, 即超晶格厚度增加有利于减小InGaN量子阱所受的应力. 综合以上影响LED发光效率的消长因素, 导致两样品最终的发光强度相近.     

Magnetoresistance of an asymmetric quantum-size structure in a parallel magnetic field: Field asymmetry independent of the current direction

A new phenomenon, viz., field-asymmetric transverse magnetoresistance of a doped asymmetric quantum-size structure discovered in a magnetic field parallel to the heteroboundary planes, is studied experimentally and theoretically. The magnetoresistance asymmetry relative to the field direction, which is independent of the direction of transport current, is observed when a lateral electric field is embedded in the structure with the help of alloyed metallic contacts. In the theoretical part of the paper, it is shown that the contribution to current, which is asymmetric in the magnetic field, can be consistently described in the framework of the theory of spontaneous current states and photovoltaic effect in systems without an inversion center; the reason behind the emergence of this current is associated with the asymmetry of the energy spectrum of charge carriers relative to the quasimomentum. It is shown that the change in the size and shape of Fermi contours in a magnetic field determines the magnitude of the strong negative magnetoresistance associated with the intersubband scattering under investigation and is found to be responsible for the emergence of a qualitatively new effect mentioned in the title of this paper.     

非晶Si/SiO2超晶格结构的交流电致发光

设计并用磁控溅射方法制备了非晶Ｓｉ／ＳｉＯ２超晶格结构,以高纯多晶Ｓｉ为靶材,当以Ａｒ＋Ｏ２为溅射气氛时,得到ＳｉＯ２膜,仅以Ａｒ为气氛时,得到Ｓｉ膜。重复地开和关Ｏ２气,便交替地得到ＳｉＯ２和Ｓｉ膜。衬底在靶前往返平移,改变平移的速度或者改变溅射的功率,可以控制膜的厚度。通过透民镜的照片可以看出ＳｉＯ２和Ｓｉ膜具有均匀的周期结构,用低角Ｘ－射线反射谱表征了超晶格的周期结构和各层的厚度。透射光谱表     

Magnetoresistance and magnetization studies through a Cu interlayer in spin valves of NiFe/Cu/NiFe/FeMn

The influence of the Cu layer thickness on the magnetic and magnetotransport properties has been investigated in Ta/NiFe/Cu/NiFe/FeMn spin valves. The magnetization and magnetoresistance measurements were carried out for magnetic field applied along the easy-axis direction. A phenomenological model, which assumes formation of a planar domain wall at the anti-ferromagnetic side of the interfaces as well as bilinear coupling between the ferromagnetic layers, was used to derive the anisotropy characteristics and orientation of each NiFe layer magnetization. The anisotropy and spin valve magnetoresistance were simulated numerically and compared with the experiment. It was found that the anisotropy magnetoresistance is negligible and that there is a poor agreement for the spin-valve one, which was attributed to the model (valid for ferromagnetic layers in single-domain state only) used for its calculation. It was found that the increase of the Cu layer thickness provokes a decrease of the interdiffusion between the NiFe and FeMn layers, and, as consequence, changes of the uniaxial anisotropy of the pinned NiFe layer, of the exchange interaction between the pinned NiFe layer and the FeMn ones, as well as of the exchange-bias field of the pinned NiFe layer.     

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.     

Quantitative study of magnetization reversal by spin-polarized current in magnetic multilayer nanopillars

We have studied magnetic switching by spin-polarized currents and also the magnetoresistance in sub-100-nm-diam thin-film Co/Cu/Co nanostructures, with the current flowing perpendicular to the plane of the films. By independently varying the thickness of all three layers and measuring the change of the switching currents, we test the theoretical models for spin-transfer switching. In addition, the changes in the switching current and magnetoresistance as a function of the Cu layer thickness give two independent measurements of the room-temperature spin-diffusion length in Cu.     

Linear magnetoresistance effect in lateral magnetic superlattices

Two-dimensional electron gas systems modulated by a lateral magnetic superlattice are proposed and the related magnetoresistance effect is described in this work. It is found that the magnetoresistance (MR) ratio of the given structures depends strongly on the uniform magnetic field, and the peaks of the MR ratio depressed linearly with the increase of the uniform magnetic field. This feature can be utilized in practical linear magnetoresistance (LMR) devices.     

Field-asymmetric transverse magnetoresistance in a nonmagnetic quantum-size structure

A new phenomenon is observed experimentally in a heavily doped asymmetric quantum-size structure in a magnetic field parallel to the quantum-well layers — a transverse magnetoresistance which is asymmetric in the field (there can even be a channge in sign) and is observed in the case that the structure has a built-in lateral electric field. A model of the effect is proposed. The observed asymmetry of the magnetoresistance is attributed to an additional current contribution that arises under nonequilibrium conditions and that is linear in the gradient of the electrochemical potential and proportional to the parameter characterizing the asymmetry of the spectrum with respect to the quasimomentum. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 5, 380–385 (10 September 1998)     

Magnetic transition in Co/(Gd–Co) multilayers

[Co/Gd_0.36Co_0.64]₄/Co multilayers with Co termination layer have been prepared by rf sputtering. They form macroscopic ferrimagnets with a compensation temperature (T_comp) determined by the thickness ratio of the layers. In low fields the magnetization of Co and Gd–Co layers are along the axis of the applied field. Increasing field makes the moments of both the Co and Gd–Co layers deviate from the axis of the field giving rise to a transition into a twisted state. These magnetic transitions were studied by vibrating sample magnetometer (VSM), magneto-optic Kerr effect and magnetoresistance measurements at various temperatures. The nucleation and evolution of surface- and bulk-twisted magnetic states were also observed in these multilayers.     

Magnetic properties of multiferroics-semiconductors Eu(1-x)Ce(x)Mn2O5

Studies of magnetization, magnetoresistance, and magnetic oscillations in semiconductor-multiferroics Eu(1-x)Ce(x)Mn2O5 (x = 0.2-0.25) (ECMO) at temperatures ranging from 5 to 350 K in magnetic fields up to 6 T are presented. It is shown that phase separation and charge carrier self-organization in the crystals give rise to a layered superstructure perpendicular to the c axis. An effect of magnetic field cycling on the superstructure, magnetization, and magnetoresistance is demonstrated. X-ray diffraction studies of ECMO demonstrating the effect of magnetic field on the superstructure are presented. The de Haas-van Alphen magnetization oscillations in high magnetic fields and the temperature-induced magnetic oscillations in a fixed magnetic field are observed at low temperatures. Below 10 K the quantum corrections to magnetization due to the weak charge carrier localization in 2D superlattice layers occur. It is shown that at all the temperatures the Eu(1-x)Ce(x)Mn2O5 magnetic state is dictated by superparamagnetism of isolated ferromagnetic domains.     

Oscillations of the magnetoresistance of a two-dimensional electron gas in a GaAs quantum well with AlAs/GaAs superlattice barriers in a microwave field

The effect of microwave radiation in the frequency range from 1.2 to 10 GHz on the magnetoresistance of a high-mobility two-dimensional electron gas has been studied in a GaAs quantum well with AlAs/GaAs superlattice barriers. It has been found that the microwave field induces oscillations of this magnetoresistance, which are periodic in the reciprocal magnetic field (1/B). It has been shown that the period of these oscillations in the frequency range under study depends on the microwave radiation power.     

Description of the Colossal Magnetoresistance of La 1.2 Sr 1.8 Mn 2 O 7 Based on the Spin-Polaron Conduction Mechanism in the Ferromagnetic Temperature Range

We have analyzed the resistance of La1.2Sr1.8Mn2(1 – z)O7 single crystal in magnetic fields from 0 to 90 kOe in the ferromagnetic temperature range. The observed magnetoresistance of La1.2Sr1.8Mn2O7 is described based on the spin-polaron conduction mechanism. The magnetoresistance is determined by the change in the sizes and magnetic moment directions of magnetic inhomogeneities (polarons). It is shown that the colossal magnetoresistance is ensured by an increase (along the magnetic field) of the polaron linear size. It is found using the method for separating the contributions of different conduction mechanisms to the magnetoresistance that the contribution to the magnetoresistance from the orientation mechanism at 80 K in low magnetic fields is close to 50%. With increasing magnetic field, this contribution decreases and becomes small in fields exceeding 30 kOe. The comparable contributions to the conductivity from the orientational and spin-polaron mechanisms unambiguously necessitate the inclusion of both conduction mechanisms in the magnetoresistance calculations. We have calculated the temperature variation of the polaron size (in relative units) in zero magnetic field and in a magnetic field of 90 kOe.     

Structural study and magnetoresistance effect of epitaxial La0.67Sr0.33MnO3–δ and Pr0.7Ca0.3MnO3−δ multilayer films

La_0.67Sr_0.33MnO_3−δ (LSMO) and Pr_0.7Ca_0.3MnO_3−δ (PCMO) multilayer epitaxial films, which were fabricated with different LSMO and PCMO layer thickness on LaAlO₃ single crystal substrates of (0 0 1) orientation by a direct current magnetron sputtering technique, were studied further, after the structure, magnetoresistance effect and magnetic properties of LSMO/PCMO/LSMO (LPL) trilayer epitaxial films were systemically studied. The superlattice structures of multilayer films were observed according to the diffraction peaks of X-ray diffraction patterns at small angles. The metal–insulator transition temperature (T_P) and peak resistivity (ρ_max) obviously changed when we altered the thickness of PCMO middle layer and the intra-field related with the thickness of those layers and their interaction. Considering the effect of the distribution of electrical field and current, and the interaction among the layers of LSMO and PCMO, an effective fact n* was introduced to replace n (the number of layer). All the calculated values of ρ (the resistivity of multilayer films) accorded with the experimental values.     

Magnetoresistance in a constricted domain wall

We show that a thin Gd layer inserted between two thicker layers of permalloy contains an in-plane domain wall whose width can be controlled by varying the thickness of the Gd layer. The magnetoresistance of this structure has been measured with the current perpendicular to the plane, thus eliminating spurious contributions which have complicated previous measurements. This is the first measurement to show unambiguously that the domain wall contributes an additional resistance whose magnitude is in good agreement with theory.     

Effect of inter-miniband tunneling on current resonances due to the formation of stochastic conduction networks in superlattices

We study the effects of inter-miniband electron tunneling and electric field domains on the current–voltage and conductance–voltage curves of biased semiconductor superlattices under the action of a magnetic field that is tilted relative to the plane of the layers. For this geometry, electrons in the superlattice minibands exhibit a unique type of stochastic semiclassical motion. At certain critical values of the electric field within the superlattice layers, the stochastic trajectories change abruptly from fully localized to completely unbounded, and map out an intricate web-like mesh of conduction channels in phase space. Delocalization of the electron paths produces a series of strong resonant peaks in the electron drift velocity versus electric field curves. We use these drift velocity characteristics to make self-consistent drift-diffusion calculations of the current–voltage and differential conductance–voltage curves of the superlattices, which reveal strong resonant features originating from the sudden delocalization of the stochastic single-electron paths. We show that this delocalization has a pronounced effect on the distribution of space charge and electric field domains within the superlattices. Inter-miniband tunneling greatly reduces the amount of space-charge buildup, thus enhancing the domain structure and both the strength and number of the current resonances.     

Angular and temperature‐related specific features of averaging of hole effective masses in p‐Ge at low temperatures

Microwave magnetoresistance of lightly doped (nondegenerate) p‐Ge has been studied by the electron spin resonance method, which can record the derivative of the microwave absorption with respect to the magnetic field. The change in the absorption is proportional to that in the conductivity of the semiconductor in the magnetic field (magnetoresistance). It was found that the averaging time of the light and heavy holes effective masses depends on temperature and on the magnetic field direction in a sample. An analysis of the derivative made it possible to determine regions of the fastest effective mass averaging.