Conduction Electron Polarization Research Articles

Sign Reversal of Spin-Transfer Torques.

Spin-transfer torque (STT) and spin-orbit torque (SOT) form the core of spintronics, allowing for the control of magnetization through electric currents. While the sign of SOT can be manipulated through material and structural engineering, it is conventionally understood that STT lacks a degree of freedom in its sign. However, this study presents the first demonstration of manipulating the STT sign by engineering heavy metals adjacent to magnetic materials in magnetic heterostructures. Spin torques are quantified through magnetic domain-wall speed measurements, and subsequently, both STT and SOT are systematically extracted from these measurements. The results unequivocally show that the sign of STT can be either positive or negative, depending on the materials adjacent to the magnetic layers. Specifically, Pd/Co/Pd films exhibit positive STT, while Pt/Co/Pt films manifest negative STT. First-principle calculations further confirm that the sign reversal of STT originates from the sign reversal of spin polarization of conduction electrons.

Magnetoelastic anisotropy in Heusler-type Mn2−δCoGa1+δ films

Perpendicular magnetization is essential for high-density memory application using magnetic materials. High-spin polarization of conduction electrons is also required for realizing large electric signals from spin-dependent transport phenomena. Heusler alloy is a well-known material class showing the half-metallic electronic structure. However, its cubic lattice nature favors in-plane magnetization and thus minimizes the perpendicular magnetic anisotropy (PMA), in general. This study focuses on an inverse-type Heusler alloy, Mn$_{2-\delta}$CoGa$_{1+\delta}$ (MCG) with a small off-stoichiometry ($\delta$) , which is expected to be a half-metallic material. We observed relatively large uniaxial magnetocrystalline anisotropy constant ($K_\mathrm{u}$) of the order of 10$^5$ J/m$^3$ at room temperature in MCG films with a small tetragonal distortion of a few percent. A positive correlation was confirmed between the $c/a$ ratio of lattice constants and $K_\mathrm{u}$. Imaging of magnetic domains using Kerr microscopy clearly demonstrated a change in the domain patterns along with $K_\mathrm{u}$. X-ray magnetic circular dichroism (XMCD) was employed using synchrotron radiation soft x-ray beam to get insight into the origin for PMA. Negligible angular variation of orbital magnetic moment ($\Delta m_\mathrm{orb}$) evaluated using the XMCD spectra suggested a minor role of the so-called Bruno's term to $K_\mathrm{u}$. Our first principles calculation reasonably explained the small $\Delta m_\mathrm{orb}$ and the positive correlation between the $c/a$ ratio and $K_\mathrm{u}$. The origin of the magnetocrystalline anisotropy was discussed based on the second-order perturbation theory in terms of the spin--orbit coupling, claiming that the mixing of the occupied $\uparrow$- and the unoccupied $\downarrow$-spin states is responsible for the PMA of the MCG films.

Coexisting superconductivity and ferromagnetism in the (V0.60Ti0.40)-Gd alloys

We present here, the effect of microstructure on the magnetic, electrical and thermal properties of (V0.60Ti0.40)-Gd alloys. Gadolinium is found to be immiscible in V0.60Ti0.40 alloys where it precipitates with size <1.2μm. These precipitates enhance the grain boundary density. The (V0.60Ti0.40)-Gd alloys become ferromagnetic below Tmc = 295 K with an increase in the superconducting transition temperature (Tsc). Though the disorder increases with increasing Gd content, the electronic thermal conductivity (κe(H=0)) reduces by at most 15% which is in contrast with the 80% decrease of the phononic thermal conductivity (κl(H=0)). Our analysis suggests that the magnetic moments of the Gd precipitates polarize the conduction electrons along and around the grain boundaries leading to increase in the mean free path of the electrons. The partial suppression of spin fluctuations in the (V0.60Ti0.40)-Gd alloy by the conduction electron polarization enhances the Tsc.

Current-Driven Domain Wall Dynamics in Ferrimagnetic Nickel-Doped Mn4N Films: Very Large Domain Wall Velocities and Reversal of Motion Direction across the Magnetic Compensation Point.

Spin-transfer torque (STT) and spin-orbit torque (SOT) are spintronic phenomena allowing magnetization manipulation using electrical currents. Beyond their fundamental interest, they allow developing new classes of magnetic memories and logic devices, in particular based on domain wall (DW) motion. In this work, we report the study of STT-driven DW motion in ferrimagnetic manganese nickel nitride (Mn4-xNixN) films, in which magnetization and angular momentum compensation can be obtained by the fine adjustment of the Ni content. Large domain wall velocities, approaching 3000 m/s, are measured for Ni compositions close to the angular momentum compensation point. The reversal of the DW motion direction, observed when the compensation composition is crossed, is related to the change of direction of the angular momentum with respect to that of the spin polarization. This is confirmed by the results of ab initio band structure calculations.

Probing interband and intraband transitions in magneto-optical FeT (T = Cr, Co, Ni) alloys from electronic structure studies

Photoelectron spectroscopy as a function of temperature is used to study interband and intraband transitions in magneto-optical FeT (T = Cr, Co, Ni) alloys. Valence band studies on these alloys indicates a strong d-d hybridization between Fe and T atoms which gives rise to localized valence states and contributes to direct interband transition while the s-d hybridized band contribute to the intraband transition and give rise to the spin polarized conduction electrons. The analysis of core levels shows that the spin orbit coupling is influenced by the near neighbour interaction between Fe and T atoms. The exchange interaction has the temperature dependence which drives the itinerant electron magnetism in FeT alloys. We find that the large density of the itinerant states at the Fermi edge and the large separation between the localized and the itinerant electronic state in FeCr are crucial for the large magneto-optical property reported in this system. In FeCo and FeNi, the overlapped localized and itinerant electronic states make these materials optically anisotropic.

Accumulation of spin-polarized states of charge carriers and a spintronic battery

Spin valves based on materials in which the spin-flip is suppressed by the spatial separation of charge carriers, while maintaining electric neutrality in the valve volume, are considered. The possibility of using these valves as electric batteries is discussed. Regulating the potential difference on the valve, one can expect the effects of incommensurability of the type of “devil's staircase” associated with the Coulomb interaction and redistribution of electrons occurring while the battery is charged and discharged. The effects of the emergence and vanishing of spontaneous spin polarization of conduction electrons with a change in the Fermi level in the valve are predicted. Such spin valves can also be used in implementing spintronic memory cells, supercapacitors, and similar devices.

Spin Polarization of Nonequilibrium Conduction Electrons in Magnetic Junctions

An original equation for nonequilibrium spin polarization in magnetic junctions based on dynamic equations for magnetic moment was obtained. Spatial nonuniformity of the distribution of carriers is taken into account in the equations, and the magnetic moment is averaged over the ensemble of nonequilibrium spin-injected electrons. The solution to the dynamic equations for the magnetic moment is used to estimate the probability of spin-flip electron transitions upon spin injection, and the solution to the equation for the nonequilibrium spin polarization in magnetic junctions is used to calculate frequencies of photon emission or absorption at energies corresponding to the energy of effective exchange splitting of spin subbands.

Spin current polarization and electrical conductivity in metal helimagnets

The peculiarities of spin and charge kinetics in helical magnetic metals that are due to forces acting on the magnetic moment of conduction electrons in inhomogeneous magnetic field have been considered. Analyzing the equations of motion for non-equilibrium spin density shows that an electric field directed along the axis of the helix produces conduction-electron spin polarization along this direction. Under the same conditions, the directions of polarization of the spin current and polarization of the locally equilibrium spin density are collinear. The specific structure of the effective exchange field acting on the conduction electrons in helical magnets causes two additional spin relaxation mechanisms to emerge. In addition to the mechanism of spin-lattice relaxation, “diffusion” and “precession” mechanisms of spin relaxation exist in conductive helical magnets. The diffusion mechanism is analogous to the Dyakonov-Perel spin relaxation mechanism. The key point is that the inhomogeneities of a magnetic field initiate a coupling between the spin and charge systems. It has been shown that, in a helical metal, the electrical conductivity decreases due to the action of the non-uniform exchange magnetic field.

Conduction electrons in aperiodic versus periodic structures: An ESR study of quasicrystalline i -Y(Gd)-Cd and its approximant Y(Gd)Cd6

A formal description of collective electronic states in condensed-matter systems lacking long-range periodicity remains a theoretical challenge. To experimentally explore the differences in electronic and magnetic behavior between metallic quasicrystals (QCs) and their conventional crystalline analogs [quasicrystal approximants (QCAs)], we have grown single crystals of ${\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Gd}}_{x}{\mathrm{Cd}}_{6}$ (QCA) together with their QC counterparts $i\text{\ensuremath{-}}{\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Gd}}_{x}\text{\ensuremath{-}}\mathrm{Cd}$ for $x=0.006$, 0.01, 0.1, and 1.00, and we carried out comparative $T$-dependent electron spin resonance (ESR) measurements. On the high Gd concentration side, $x=1.00$, we confirm that ${\mathrm{GdCd}}_{6}$ adopts an antiferromagnetic ground state below ${T}_{N}\ensuremath{\sim}22\phantom{\rule{4pt}{0ex}}\mathrm{K}$, whereas $i$-Gd-Cd presents spin-glass-like behavior showing similar local and dynamical properties from the point of view of ESR. For the diluted samples, our ESR experimental results show similar local conduction electron polarization behavior at the ${\mathrm{Gd}}^{3+}$ site in all QC/QCA pairs investigated, supporting the validity of using QCAs as periodic representations of QCs in terms of short-range electronic interactions. However, there is a measurable difference in the Korringa relaxation rate (spin-flip relaxation process between the localized ${\mathrm{Gd}}^{3+}4f$ electron and the delocalized $s$-type conduction electrons at the Fermi surface) between the QC/QCA pairs probably associated with the lack of periodicity. We expect that our comparative ESR study may provide support and motivation for the development of new theoretical approaches toward a generalized band-structure theory, contemplating condensed-matter systems beyond the scope of traditional periodicity.

Dynamics of Spatially Inhomogeneous Spin Polarization of Nonequilibrium Conduction Electrons in Magnetic Transitions

The equation of the dynamics of the magnetic moment motion that has been averaged over the ensemble of nonequilibrium spin-injected electrons in a ferromagnetic transition in the presence of exchange interaction, interaction with an external electromagnetic field, and a thermostat has been obtained taking into account the spatial inhomogeneity of the charge carrier distribution. It has been shown that taking into account the spatial inhomogeneity of the charge carrier distribution in the equation of the dynamics of the magnetic moment motion allows for describing the various processes of electron transfer in the magnetic transition. The probability of quantum transitions of electrons with opposite spin directions, which determine spin relaxation in interaction with a thermostat, has been estimated. It has been shown that the anisotropy of the radiation medium is determined not only by the anisotropy of the sd-exchange tensor, but also by the additional anisotropy that is caused by the electron impulse derivatives of this tensor. The considered phenomena have a great potential for the detection of new effects and development of new devices based on them, including compact tunable radiation sources in the terahertz frequency range, which is obviously difficult to generate.

Unusual negative permeability of single magnetic nanowire excited by the spin transfer torque effect

Due to the effect of spin transfer torque, negative imaginary parts of permeability (μ″ < 0) are reported in a ferromagnetic nanowire. It is found that negative μ″ values are resulted from the interaction of spin polarized conduction electrons with the spatially non-uniform distributed magnetic moments at both ends of nanowires. The results are well explained from the effect of spin transfer torque on the precession of magnetization under the excitation of both the pulsed magnetic field and static electric field.

Estimate of the Degree of the Spin Polarization of a Ferromagnet from Data on the Superconductor/Ferromagnet Proximity Effect

The superconductor/ferromagnet proximity effect in the Pb/Co2Cr1–xFe x Al bilayer systems has been studied. Thin films of the Heusler alloy Co2Cr1–xFe x Al have been prepared at different substrate temperatures. It has been established using Andreev spectroscopy of point contacts that the degree of spin polarization of conduction electrons in the Heusler alloy is on the order of 30 and 70% for the films prepared at a substrate temperature of 300 and 600 K, respectively. It has been found that the dependence of the superconducting transition temperature on the thickness of the Pb layer at a fixed thickness of the Heusler layer is determined by the degree of spin polarization of the conduction band in the ferromagnetic layer.

Lattice instability and enhancement of superconductivity in YB6

The superconducting and normal state characteristics of yttrium hexaboride (YB$_6$) have been investigated for the single crystals with a transition temperatures $T_c$ ranging between 6 K and 7.6 K. The extracted set of microscopic parameters [the coherence length $\xi$(0) $\sim$ 320$\div$340 ${\AA}$, the penetration depth $\lambda$(0) $\sim$ 1100$\div$1600 ${\AA}$ and the mean free path of charge carriers $l$ = 31$\div$58 ${\AA}$, the Ginzburg-Landau-Maki parameters $\kappa$$_{1,2}$(0) $\sim$ 3.3$\div$4.8 and the superconducting gap $\Delta$(0) $\sim$ 10.3$\div$14.8 K] confirms the type II superconductivity in "dirty limit" ($\xi$$\gg$ $l$) with a medium to strong electron-phonon interaction (the electron-phonon interaction constant $\lambda_{e-ph}$ = 0.93$\div$0.96) and $s$-type pairing of charge carriers in this compound [2$\Delta$(0)$/k_BT_c$ $\approx$ 4]. The comparative analysis of charge transport (resistivity, Hall and Seebeck coefficients) and thermodynamic (heat capacity, magnetization) properties in the normal state in YB$_6$ allowed to detect a transition into the cage-glass state at $T^*$ $\sim$ 50 K with a static disorder in the arrangement of the Y$^{3+}$ ions. We argue that the significant $T_c$ variations in the YB$_6$ single crystals are determined by two main factors: (i) the superconductivity enhancement is related with the increase of the number of isolated vacancies, both at yttrium and boron sites, which leads to the development of an instability in the hexaboride lattice; (ii) the $T_c$ depression is additionally stimulated by the spin polarization of conduction electrons emerged and enhanced by the magnetic field in the vicinity of defect complexes in the YB$_6$ matrix.

Electronic structure and magnetic moment of dilute transition metal impurities in semi-metallic CaB6

Employing the all electron augmented plane wave+local orbital (APW+lo) method based on the density function theory (DFT), we have performed ab-initio calculations of electronic structure and magnetic properties of transition metal impurities doped in semi-metallic CaB6 host. Here, we present our results on the density of states (DOS), local magnetic moment and hyperfine field of 3d (Sc–Ni), 4d (Y–Pd) and 5d (La–Pt) in CaB6. Our results reveal strong localization of impurity d electrons yielding high local moments for most of the transition metal atoms, including the 5d atoms Ta, Re, Os and Ir. The results presented here would be useful towards a comprehensive understanding of the origin of weak ferromagnetism observed in CaB6. We believe that besides B or B6 vacancies which have been argued to be the main source of magnetism in CaB6, the local moments associated with different transition metal impurities can interact via conduction electron polarization and give rise to the weak ferromagnetism observed in this material.

Microscopic theory of spin-orbit torques in two dimensions

We formulate a general microscopic approach to spin-orbit torques in thin ferromagnet/heavy-metal bilayers in linear response to electric current or electric field. The microscopic theory we develop avoids the notion of spin currents and spin-Hall effect. Instead, the torques are directly related to a local spin polarization of conduction electrons, which is computed from generalized Kubo-St\v{r}eda formulas. A symmetry analysis provides a one-to-one correspondence between polarization susceptibility tensor components and different torque terms in the Landau-Lifshitz-Gilbert equation for magnetization dynamics. The spin-orbit torques arising from Rashba or Dresselhaus type of spin-orbit interaction are shown to have different symmetries. We analyze these spin-orbit torques microscopically for a generic electron model in the presence of an arbitrary smooth magnetic texture. For a model with spin-independent disorder we find a major cancelation of the torques. In this case the only remaining torque corresponds to the magnetization-independent Edelstein effect. Furthermore, our results are applied to analyze the dynamics of a Skyrmion under the action of electric current.

Spin dynamics with inertia in metallic ferromagnets

Non-adiabatic contribution of environmental degrees of freedom yields effective inertia of spin in effective spin dynamics. In this paper, we study several aspects of the inertia of spin in metallic ferromagnets. (i) a concrete expression of the spin inertia $m_s$: $m_s=\hbar S_c/(2g_{\rm sd})$, where $S_c$ is the spin polarization of conduction electrons and $g_{\rm sd}$ is the $sd$ coupling constant. (ii) dynamical behavior of spin with inertia, discussed from viewpoints of a spinning top and of a particle on a sphere. (iii) behavior of spin waves and domain walls in the presence of inertia, and behavior of spin with inertia in the case of a time-dependent magnetic field.

ChemInform Abstract: Spin Transfer Torque Switching and Perpendicular Magnetic Anisotropy in Full Heusler Alloy CO2FeAl‐Based Tunnel Junctions

AbstractReview: 57 refs.

Anomalous Galvanomagnetic Effects due to Spontaneous Spin Polarization of Electrons in Crystal with Low Concentration of 3d-Transition Element Impurities

The results of experimental observations at room temperature of anomalous Hall effect associated with existence of a spontaneous spin polarization of conduction electrons in mercury selenide crystals doped with chromium and vanadium in the concentration range from 1·1018 to 1·1019 sm-3 are reported. Comparison of the experimental data together with the previously reported results related to crystals with iron, cobalt and nickel impurities shows that the relative contribution to the anomalous Hall effect is maximal for vanadium impurity (≈ 13%) and minimal for iron impurity (≈ 5%). Also, transverse magnetoresistance with a characteristic dependence on the magnetic field strength, on which an unusual hysteresis in the field dependence was observed, was found in investigated crystals at room temperature. Theoretical interpretation of the observed effects is developed based on the concepts of thermodynamic nature of anomalous galvanomagnetic phenomena in electron systems with spontaneous spin polarization.

Attempt to measure magnetic hyperfine fields in metallic thin wires under spin Hall conditions using synchrotron-radiation Mössbauer spectroscopy

Measurement of the magnetic hyperfine fields in metallic thin wires under spin Hall conditions was attempted using the emerging technique, synchrotron-radiation Mössbauer spectroscopy. A Mössbauer probe layer of 57Fe (0.2 nm), 57Fe (0.6 nm), or 119Sn (0.6 nm) was embedded as an electron spin detector near the surfaces of V, Au, Pt, and 56Fe wires. The magnitudes of the magnetic hyperfine fields at the 57Fe and 119Sn nuclear sites that could be enhanced by non-equilibrium conduction-electron spin polarization were measured both without and with the application of an electric current along the wire. Changes in the Mössbauer spectra were not clearly observed, indicating that the magnetic hyperfine field induced by non-equilibrium spin polarization is smaller than the detection limit at least for the measured systems and conditions.

Analysis of Switchable Spin Torque Oscillator for Microwave Assisted Magnetic Recording

A switchable spin torque oscillator (STO) with a negative magnetic anisotropy oscillation layer for microwave assisted magnetic recording is analyzed theoretically and numerically. The equations for finding the STO frequency and oscillation angle are derived from Landau-Lifshitz-Gilbert (LLG) equation with the spin torque term in spherical coordinates. The theoretical analysis shows that the STO oscillating frequency remains the same and oscillation direction reverses after the switching of the magnetization of the spin polarization layer under applied alternative magnetic field. Numerical analysis based on the derived equations shows that the oscillation angle increases with the increase of the negative anisotropy energy density (absolute value) but decreases with the increase of spin current, the polarization of conduction electrons, the saturation magnetization, and the total applied magnetic field in thezdirection. The STO frequency increases with the increase of spin current, the polarization of conduction electrons, and the negative anisotropy energy density (absolute value) but decreases with the increase of the saturation magnetization and the total applied magnetic field in thezdirection.