Tunnel anisotropic magnetoresistance in graphene with Rashba spin-orbit interaction

Ballistic transport in a graphene-based normal/ferromagnetic barrier/normal junction in the presence of Rashba-type spin-orbit interaction (RSOI) is investigated by the non-equilibrium Green's function approach. It is found that due to the interplay between ferromagnetic exchange coupling and RSOI, the energy dispersion in the ferromagnetic barrier depends on the magnetization direction. The conductance changes by varying the magnetization direction, resulting in a tunnel anisotropic magnetoresistance (TAMR). The predicted TAMR effect oscillates with the RSOI strength or on-site energy, which is efficiently controllable by the gate voltage, making this junction very promising in spintronics applications.     

Magnetization transport and quantized spin conductance

We analyze transport of magnetization in insulating systems described by a spin Hamiltonian. The magnetization current through a quasi-one-dimensional magnetic wire of finite length suspended between two bulk magnets is determined by the spin conductance which remains finite in the ballistic limit due to contact resistance. For ferromagnetic systems, magnetization transport can be viewed as transmission of magnons, and the spin conductance depends on the temperature T. For antiferromagnetic isotropic spin-1/2 chains, the spin conductance is quantized in units of order (gmu(B))(2)/h at T=0. Magnetization currents produce an electric field and, hence, can be measured directly. For magnetization transport in electric fields, phenomena analogous to the Hall effect emerge.     

Universal relationship between giant magnetoresistance and anisotropic magnetoresistance in spin valve multilayers

We measure the giant magnetoresistance (GMR) with the current both parallel and perpendicular to the direction of the magnetization in the ferromagnetic (FM) layers and thus probe the anisotropy of the effective mean free paths for the spin-up and spin-down electrons, seen in the anisotropic magnetoresistance. We find that the difference of the GMR in the two configurations, when expressed in terms of the sheet conductance, displays a nearly universal behavior as a function of GMR. On interpreting the results within the Boltzmann transport formalism we demonstrate the importance of bulk scattering for GMR.     

Electromagnetic soliton in an anisotropic ferromagnetic medium under nonuniform perturbation

We study the propagation of electromagnetic wave (EMW) in a linear as well as in a nonlinear anisotropic ferromagnetic medium which are assumed to be free from electric charges by making a nonuniform perturbation analysis. It is found that as the EMW propagates through the linear anisotropic ferromagnetic medium, the magnetic induction and hence the magnetic field component of the EMW are being modulated in the form of solitons. Also, the magnetization of the ferromagnetic medium is excited in the form of solitons. While the magnetic induction soliton is restricted to the plane normal to the direction of propagation, the magnetization excitations are not restricted to any particular plane. Unsaturated nonlinear ferromagnetic media is also found to give similar results.     

Giant magnetocrystalline anisotropies of 4d transition-metal monowires

The magnetocrystalline anisotropy energy (MAE) for ferromagnetic and antiferromagnetic freestanding monowires of 4d transition metals is investigated on the basis of first-principles calculations. Across the 4d series, the easy axis of the magnetization oscillates between two directions: perpendicular and along the wire axis. The largest values of the MAE occur at the end of the series. Giant values of 30-60 meV/atom can be obtained upon stretching Ru or Rh wires. Ru and Pd chains change the magnetization direction upon wire stretching, opening new perspectives in controlling the spin-dependent ballistic conductance in these structures.     

Andreev reflection in a quantum dot coupled to ferromagnetic and superconductor leads under magnetic fields

By employing the nonequilibrium Green's function, we investigate the spin-dependent linear Andreev reflection (AR) resonant tunneling through a quantum dot connected to a ferromagnetic lead and a superconducting lead, where the magnetization direction in the ferromagnetic lead can be tuned by one. We focus our attention on the effects of the magnetic fields on the AR conductance. One high conductance peak and one low conductance peak are developed in the linear AR conductance when a stronger magnetic field is considered. The interplay between the spin-flip scattering and the magnetic fields on the AR conductance are also studied.     

Exchange bias for ferromagnetic/antiferromagnetic bilayers with the uniaxial anisotropy being misaligned with the exchange anisotropy

Using the principle of minimal energy and S-W model, the exchange bias for ferromagnetic/antiferromagnetic bilayers has been investigated when the uniaxial anisotropy is misaligned with the exchange anisotropy. According to the relation between the energy of the bilayer and the orientation of ferromagnetic magnetization, it is found that the bilayer will be in the monostable state or bistable state when the external field is absent in the initial magnetization state. The monostable state or bistable state of the bilayer, which determines the angular dependence of exchange bias directly, is controlled by the competition between the exchange anisotropy and uniaxial anisotropy. When the applied field is parallel to the intrinsic easy axes and intrinsic hard axes, one of the switching fields of the hysteresis loop shows an abrupt change, while the other keep continuous by analyzing the magnetization reversal processes. Consequently, the exchange bias field and the coercivity will show a jump phenomenon. The numerical calculations indicate that both the magnitude and direction of the exchange anisotropy will significantly affect the angular dependence of exchange bias. The jump phenomenon of exchange bias is an intrinsic property of the bilayer, which is dependent on the interfacial exchange-coupling constant, the orientation of the exchange anisotropy, the thickness and uniaxial anisotropy constant of the ferromagnetic layer.     

Magnetic anisotropy and anisotropic ballistic conductance of thin magnetic wires

The magnetocrystalline anisotropy of thin magnetic wires of iron and cobalt is quite different from the bulk phases. The spin moment of monatomic Fe wire may be as high as 3.4 μ_B, while the orbital moment as high as 0.5 μ_B. The magnetocrystalline anisotropy energy (MAE) was calculated for wires up to 0.6 nm in diameter starting from monatomic wire and adding consecutive shells for thicker wires. I observe that Fe wires exhibit the change sign with the stress applied along the wire. It means that easy axis may change from the direction along the wire to perpendicular to the wire. We find that ballistic conductance of the wire depends on the direction of the applied magnetic field, i.e. shows anisotropic ballistic magnetoresistance. This effect occurs due to the symmetry dependence of the splitting of degenerate bands in the applied field which changes the number of bands crossing the Fermi level. We find that the ballistic conductance changes with applied stress. Even for thicker wires the ballistic conductance changes by factor 2 on moderate tensile stain in our 5×4 model wire. Thus, the ballistic conductance of magnetic wires changes in the applied field due to the magnetostriction. This effect can be observed as large anisotropic BMR in the experiment.     

Conductance properties in spin field-effect transistors

Conductance properties in spin field-effect transistors (SFET) are studied by taking into account the Rashba spin-orbit coupling strength, the presence of external magnetic field, the angle between the direction of magnetization and the conductance band mismatch between the ferromagnetic contacts and the channel. It is shown that the conductance of the SFET has high peaks while the value of external magnetic field varies. These peaks become more and more pronounced with the potential barriers strength increasing. The conductance peaks also appear by increasing the strength of the spin-orbit coupling. It is found that the conductance exhibits quantum oscillating behavior when varying the angle between the direction of magnetization in the two contacts. The influence of conductance band mismatch between the contacts and channel is also discussed.     

Fe84Zr3.5Nb3.5B8Cu1薄带的动态磁化过程

由于铁磁性样品的交流磁化率虚部χ″随磁场强度H的变化是非线性的，在低场(0～12 mT)有一个与铁磁共振信号强度相当的低场非共振信号. 利用ESR谱仪测量交流磁化率虚部χ″对磁场强度H的一次微分随磁场强度的变化dχ″/dH～H，研究Fe₈₄Zr_3.5Nb_3.5B₈Cu₁合金薄带的动态磁化特性. 合金薄带样品是各向异性的，易磁化轴（易轴）在薄带的横向方向，外加磁场H在易轴方向. 样品在可逆磁化区域(0～2.0 mT)和趋近饱和的磁化区域(9.0 mT以上)，dχ″/dH=0；在不可逆畴壁移动过程中，当H为4.2 mT 时, χ″(H)达到最大值χ_max；在磁畴转动过程中，χ″(H)正比于H²（瑞利区）；而实验中却发现，在某些区域交流磁化率虚部χ″(H) 与磁场强度H的n次方即Hⁿ(n≥3)有关；而且发现，在一定区域，有三段不可逆畴壁移动和磁畴转动交替出现的现象. 在这一过程中，dχ″/dH为常数的磁场范围分别为4.8～5.2mT, 5.8～6.4 mT, 8.0～8.5 mT, 其常数相对值分别为1：0.85：0.60. 样品的交流磁化率虚部χ″对磁场H的微分dχ″/dH随磁场H的这一变化规律反映了不可逆畴壁移动和磁畴转动交替发生的微观过程.     

Magnetism-induced ballistic conductance changes in palladium nanocontacts

We present first-principles calculations of the effects of magnetism on the ballistic conductance of a model Pd nanocontact, made of a short Pd monatomic stretched chain placed between two Pd leads, simulated by semi-infinite (100) slabs. The stretching makes the suspended Pd chain generally ferromagnetic. The spin-resolved ballistic conductance, calculated according to the Landauer-Büttiker formula is found to be 0.85G₀ for the spin-up and 1.15G₀ for the spin-down electrons (G₀ = 2e²/h is the conductance quantum). The total conductance ~2G₀ is lower, but still relatively close to that of the nonmagnetic Pd nanocontact with the same geometry, calculated to be 2.3G₀. To illustrate how magnetism and conductance depend on structural details, we change the three atom chain docking from the top to a hollow surface site, where at the same stress the Pd contact is nonmagnetic and the conductance decreases to 1.8G₀. Overall we find these calculated ballistic conductance values of very similar magnitude to the first histogram peak in the experimental data obtained for Pd at low temperature in mechanically controllable break junctions. We conclude that the 15% conductance changes caused by the onset or the demise of local magnetism, similar in magnitude to geometry-related conductance changes, are probably too small to be used as a diagnostic for the presence or absence of nanocontact magnetism.     

Tunneling anisotropic magnetoresistance: a spin-valve-like tunnel magnetoresistance using a single magnetic layer

We introduce a new class of spintronic devices in which a spin-valve-like effect results from strong spin-orbit coupling in a single ferromagnetic layer rather than from injection and detection of a spin-polarized current by two coupled ferromagnets. The effect is observed in a normal-metal-insulator-ferromagnetic-semiconductor tunneling device. This behavior is caused by the interplay of the anisotropic density of states in (Ga,Mn)As with respect to the magnetization direction and the two-step magnetization reversal process in this material.     

Electric flow through an ideal ferromagnet–superconductor junction

It is investigated the possibility of controlling the electric flow through a ferromagnet–superconductor junction by spin polarization, within a simple, ideal model of a perfect ferromagnetic–superconductor junction. The ferromagnetic and superconducting properties as well as the Andreev reflection are briefly reviewed and the electrical resistance of the junction is computed both in the diffusive and ballistic regime for the ferromagnetic sample. It is shown that the resistance of the junction increases with increasing magnetization, including both positive or negative jumps on passing from the ballistic to the diffusive regime.     

Gate-Voltage-Induced Magnetization Reversal and Tunneling Anisotropic Magnetoresistance in a Single Molecular Magnet with Temperature Gradient

We study the control of gate voltage over the magnetization of a single-molecule magnet(SMM) weakly coupled to a ferromagnetic and a normal metal electrode in the presence of the temperature gradient between two electrodes.It is demonstrated that the SMM's magnetization can change periodically with periodic gate voltage due to the driving of the temperature gradient.Under an appropriate matching of the electrode polarization,the temperature difference and the pulse width of gate voltage,the SMM's magnetization can be completely reversed in a period of gate voltage.The corresponding flipping time can be controlled by the system parameters.In addition,we also investigate the tunneling anisotropic magnetoresistance(TAMR) of the device in the steady state when the ferromagnetic electrode is noncollinear with the easy axis of the SMM,and show the jump characteristic of the TAMR.     

Magnetization reversal processes in layered high-temperature superconductors with ferromagnetic impurities

The self-consistent interaction of a vortex system of a high-temperature superconductor and ferromagnetic impurities, including single impurities and their clusters, has been considered in the model of a layered high-temperature superconductor. For different temperatures and concentrations of ferromagnetic impurities, the magnetization reversal loops have been calculated by the Monte Carlo method taking into account an ensemble of ferromagnetic particles with different orientations of their easy magnetization axes with respect to the direction of an external magnetic field and for different magnetic anisotropy energies. It has been demonstrated that there is a nonlinear interaction of the high-temperature superconductor with ferromagnetic impurities, in which the initially thermodynamically reversible character of the magnetization reversal of the ferromagnetic ensemble can become irreversible. For a periodic lattice of clusters of ferromagnetic impurities, the magnetization curves of the high-temperature superconductor have been calculated for different sizes and configurations of the clusters. It has been revealed that, when extended defects are oriented parallel to the direction of the entrance of vortices in the sample, the length of the defects does not affect the remanent magnetization. It has been shown that the inclusion of the interaction between the magnetic moments inside the impurity cluster leads to a decrease in the magnetization reversal loop, the coercivity, and, accordingly, the energy loss due to magnetization reversal.     

Giant magnetoresistance effect in hybrid ferromagnetic/semiconductor nanosystems

We theoretically investigate the giant magnetoresistance (GMR) effect in general magnetically modulated semiconductor nanosystems, which can be realized experimentally by depositing two parallel ferromagnetic strips on the top of a heterostructure. Here the exact magnetic profiles and arbitrary magnetization direction of ferromagnetic strips are emphasized. It is shown that a considerable GMR effect can be achieved in such nanosystems due to the significant transmission difference for electrons tunneling through parallel and antiparallel magnetization configurations. It is also shown that the magnetoresistance ratio is strongly influenced by the magnetization direction of ferromagnetic strips in nanosystems, thus possibly leading to tunable GMR devices.     

Effect of magnetocrystalline anisotropy on the temperature characteristics of magnetostatic waves in ferromagnetic films

The temperature dependence of the eigenfrequencies of magnetization oscillation in a single-crystal film is shown to depend strongly on the temperature dependence of the magnetocrystalline anisotropy field. The conditions are considered under which the temperature coefficient of the ferromagnetic resonance frequency in a film with cubic anisotropy changes sign as the orientation of the magnetization vector is changed from the 〈111〉 to the 〈100〉 direction. The spectrum of surface magnetostatic waves is investigated experimentally in 〈110〉-oriented yttrium iron garnet films. It is established that, due to magnetic anisotropy of the ferrite, the temperature dependence of the long-wavelength limit of the frequency spectrum becomes nonmonotonic when a film of this material is magnetized along the 〈100〉 axis.     

Influence of a metallic substrate’s magnetic state on the huge magnetoresistance of a ferromagnet-polymer structure

The association between the threshold value of conductance switching in an external magnetic field and the initial magnetic state of a ferromagnetic plate substrate is shown for a ferromagnet/polymer/nonmagnetic metal system. The threshold magnetic field change is explained by appearance of residual magnetization after being held in an external magnetic field and, correspondingly, by the change in the initial state upon further remagnetization.     

Experimental investigations of the relation between electrical conductivity and natural magnetic anisotropy in ferrites containing cobalt

A deep relation between the crystalline structure, the nature and energy of the interatomic interaction, and the different physicochemical properties is exhibited with special clarity in ferrites. Therefore a comprehensive discussion of the physical properties of ferrites with the peculiarities of magnetic transitions in ferrites taken into account, along with an investigation of the structure, is of great significance. In this connection, we posed the problem of investigating the effect of magnetic anisotropy on the electrical properties of ferrites containing various amounts of cobalt ions, which make, as is well known, an anomalously large contribution to the magnetic anisotropy constant of ferrite-spinels and lead to a change in sign. Investigations of the electrical properties of ferrites have shown that a change in the activation energy of the electrical conductivity [1] and an anomaly in the temperature behavior of the spontaneous Hall field [2] are observed upon the transition of ferritesfrom a ferri- to a paramagnetic state. It is natural to assume that these anomalous changes are related to the disappearance of magnetic order inside a ferromagnetic semiconductor, which indicates a relation of the conductivity electrons to the magnetic structure. However, the magnetic order in crystals of ferro- and ferrimagnets can be altered in other ways besides a transition through the Curie point. The magnetic structure can be altered in a definite way also upon a change in the sign of the constant of natural magnetocrystalline anisotropy ₁ (i.e., when the direction of weak magnetization is changed in a crystal). Investigations of the effects of these kinds of changes in the magnetic structure on the motion of current carriers in ferrites are of great interest. It is important in this connection to carry out investigations in the temperature region in which the investigated object is in the ferrimagnetic state, i.e., the spontaneousmagnetization is not destroyed, and its direction in the crystal was altered only as a result of a change in the sign of ₁. It is completely permissible to assume that there occurs in the ferrite a change in the intracrystalline field, which affects in a definite way the wave functions of the electrons participating in the conductivity. Thus, even these simple qualitative discussions indicate that such investigations can give valuable information on the effect of a magnetic subsystem on kinetic phenomena in magnetic semiconductors — ferrites. Our investigations of the effect of a magnetic subsystem on the transport phenomenon differ from the experiments of a number of authors [3, 4, 5] in that these authors have investigated the temperature dependences of electrical resistance in the temperature range in which the sign of the magnetic anisotropy constant changes, i.e., the direction of the magnetization vector is altered not due to the effect of external magnetic fields, but as a result of a change in the anisotropic properties upon being heated up.Deceased.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 7–13, July, 1979.     

Revealing the origin of the vertical hysteresis loop shifts in an exchange biased Co/YMnO(3) bilayer

We have investigated exchange bias effects in bilayers composed of the antiferromagnetic o-YMnO(3) and ferromagnetic Co thin film by means of SQUID magnetometry, magnetoresistance, anisotropic magnetoresistance and the planar Hall effect. The magnetization and magneto-transport properties show pronounced asymmetries in the field and magnetization axes of the field hysteresis loops. Both exchange bias parameters, the exchange bias field H(E)(T) as well as the magnetization shift M(E)(T), vanish around the Néel temperature T(N)???45?K. We show that the magnetization shift M(E)(T) is also measured by a shift in the anisotropic magnetoresistance and planar Hall resistance having a similar temperature dependence as the one obtained from magnetization measurements. Because the o-YMnO(3) film is highly insulating, our results demonstrate that the M(E)(T) shift originates at the interface within the ferromagnetic Co layer. To show that the main results obtained are general and not because of some special characteristics of the o-YMO(3) layer, similar measurements were done in Co/CoO micro-wires. The transport and magnetization characterization of the micro-wires supports the main conclusion that these effects are related to the response of the ferromagnetic Co layer at the interface.