Reduced Spin Polarization Research Articles

Magnetic order and magnetotransport in half-metallic ferrimagnetic MnyRuxGa thin films

The ruthenium content of half-metallic ${\mathrm{Mn}}_{2}{\mathrm{Ru}}_{x}\mathrm{Ga}$ thin films, with a biaxially strained inverse Heusler structure, controls the ferrimagnetism that determines their magnetic and electronic properties. An extensive study of ${\mathrm{Mn}}_{y}{\mathrm{Ru}}_{x}\mathrm{Ga}$ films on MgO (100) substrates with $1.8\ensuremath{\le}y\ensuremath{\le}2.6$ and $x=0.5, 0.7 \mathrm{or} 0.9$, including crystallographic, magnetic order, magneto-transport, and spin polarization, is undertaken to map specific composition-dependent properties in this versatile ternary system. A comparison of experimental densities obtained from x-ray reflectivity with calculated densities indicates full-site occupancy for all compositions, which implies chemical disorder. All moments lie on a Slater-Pauling plot with slope 1 and all except $x=0.5$, $y=2.2$ exhibit magnetic compensation at ${T}_{\text{comp}}$ below 500 K. The coercivity near ${T}_{\text{comp}}$ exceeds 10 T. Increasing the Mn or Ru content raises ${T}_{\text{comp}}$, but increasing Ru also decreases the spin polarization determined by point contact Andreev reflection. Molecular-field theory is used to model the temperature dependence of the net ferrimagnetic moment and three principal exchange coefficients are deduced. Marked differences in the shape of anomalous Hall and net magnetization hysteresis loops are explained by substantial canting of the small net moment by up to ${40}^{\ensuremath{\circ}}$ relative to the $c$ axis in zero field, which is a result of slight noncollinearity of the ${\mathrm{Mn}}^{4c}$ sublattice moments due to competing intrasublattice exchange interactions arising from antisite disorder and excess Mn in the unit cell. Consequences are reduced-spin polarization and an enhanced intrinsic contribution to the anomalous Hall effect. The systematic investigation of the physical properties as a function of $x$ and $y$ will guide the selection of compositions to meet the requirements for magnonic and spintronic MRG-based devices.

Sub-nanosecond switching in a cryogenic spin-torque spin-valve memory element with a dilute permalloy free layer

We present a study of pulsed current switching characteristics of spin-valve nanopillars with in-plane magnetized dilute permalloy and undiluted permalloy free layers in the ballistic regime at low temperatures. The dilute permalloy free layer device switches much faster: the characteristic switching time for a permalloy (Ni0.83Fe0.17) free layer device is 1.18 ns, while that for a dilute permalloy ([Ni0.83Fe0.17]0.6Cu0.4) free layer device is 0.475 ns. A ballistic macrospin model can capture the data trends with a reduced spin-torque asymmetry parameter, reduced spin polarization, and increased Gilbert damping for the dilute permalloy free layer relative to the permalloy devices. Our study demonstrates that reducing the magnetization of the free layer increases the switching speed while greatly reducing the switching energy and shows a promising route toward even lower power magnetic memory devices compatible with superconducting electronics.

Correlation between spin transport signal and Heusler/semiconductor interface quality in lateral spin-valve devices

We show direct evidence for the impact of Heusler/semiconductor interfaces atomic structure on the spin transport signals in semiconductor-based lateral spin-valve (LSV) devices. Based on atomic scale Z-contrast scanning transmission electron microscopy and energy dispersive x-ray spectroscopy we show that atomic order/disorder of ${\mathrm{Co}}_{2}{\mathrm{FeAl}}_{0.5}{\mathrm{Si}}_{0.5}$ (CFAS)/$n$-Ge LSV devices is critical for the spin injection in Ge. By conducting a postannealing of the LSV devices, we find 90% decrease in the spin signal while there is no difference in the electrical properties of the $\mathrm{CFAS}\text{/}n$-Ge contacts and in the spin diffusion length of the $n$-Ge layer. We show that the reduction in the spin signals after annealing is attributed to the presence of intermixing phases at the Heusler/semiconductor interface. First-principles calculations show how that intermixed interface region has drastically reduced spin polarization at the Fermi level, which is the main cause for the significant decrease of the spin signal in the annealed devices above $300{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$.

Effect of chemical order on the magnetic and electronic properties of epitaxial off-stoichiometryFexSi1−xthin films

Off-stoichiometry, epitaxial $\mathrm{F}{\mathrm{e}}_{x}\mathrm{S}{\mathrm{i}}_{1\ensuremath{-}x}$ thin films $(0.5lxl1.0)$ exhibit $D{0}_{3}$ or $B2$ chemical order, even far from stoichiometry. Theoretical calculations show the magnetic moment is strongly enhanced in the fully chemically disordered $A2$ phase, while both theoretical and experimental results show that the magnetization is nearly the same in the $B2$ and $D{0}_{3}$ phases, meaning partial chemical disorder does not influence the magnetism. The dependencies of the magnetic moments are directly and nonlinearly linked to the number of Si atoms, primarily nearest neighbor but also to a lesser extent (up to 10%) next nearest neighbor, surrounding Fe, explaining the similarities between $B2$ and $D{0}_{3}$ and the strong enhancement for the $A2$ structure. The calculated electronic density of states shows many similarities in both structure and spin polarization between the $D{0}_{3}$ and $B2$ structures, while the $A2$ structure exhibits disorder broadening and a reduced spin polarization.

Photoinduced Spin Precession and Ultrafast Demagnetization in Co2FeAl Films With Crossover From In-Plane to Perpendicular Magnetic Easy Axis

Laser-induced spin precession and ultrafast demagnetization were investigated in MgO-capped Co2 FeAl (MO-CFA) films. Both the Gilbert damping parameter α and the demagnetization time τ $_m$ monotonically increased with the increasing perpendicular magnetic anisotropy as CFA thickness $x$ was reduced to 0.6 nm. The increase in α may be explained by a stronger spin-orbit coupling. The τ $_m$ of an $L2_{1}$ Co2FeAl film is larger than that of thick MO-CFA, but smaller than that of MO-CFA with $x \le 0.9$ nm, indicating that the restored orbital angular momentum may have a stronger and opposite effect on τ $_m$ compared to the reduced spin polarization in thin MO-CFA.

Atomic-scale structure and properties of highly stable antiphase boundary defects in Fe3O4.

The complex and intriguing properties of the ferrimagnetic half metal magnetite (Fe3O4) are of continuing fundamental interest as well as being important for practical applications in spintronics, magnetism, catalysis and medicine. There is considerable speculation concerning the role of the ubiquitous antiphase boundary (APB) defects in magnetite, however, direct information on their structure and properties has remained challenging to obtain. Here we combine predictive first principles modelling with high-resolution transmission electron microscopy to unambiguously determine the three-dimensional structure of APBs in magnetite. We demonstrate that APB defects on the {110} planes are unusually stable and induce antiferromagnetic coupling between adjacent domains providing an explanation for the magnetoresistance and reduced spin polarization often observed. We also demonstrate how the high stability of the {110} APB defects is connected to the existence of a metastable bulk phase of Fe3O4, which could be stabilized by strain in films or nanostructures.

Understanding Cr segregation at the He bubble surface in Fe

Although Cr segregation at the free Fe surface is weak, noticeable segregation of Cr at the He bubble surface in Fe has recently been observed. To understand the driving force for Cr segregation, we have carried out first-principles density functional theory calculations on the energetics of solute Cr atoms at the He bubble surface, which was modeled by a Fe/He interface. We find that both the compressive stress produced by the He bubble and the direct interfacial interaction promote Cr segregation from inside the bulk to the bubble surface, along with reduced spin polarization. Electronic structure analyses show that at the Fe/He interface, Cr is more compressible than Fe due to having more empty eg orbitals and, accordingly, the Fe surface gets energetically more favorable for Cr than in the bulk. On the other hand, the segregation of Cr increases the charge density at the bubble surface, and thus hinders assimilation of further He atoms.

The effects of Schottky barrier profile on spin dependent tunneling in a ferromagnet-insulator-semiconductor system

The insertion of a tunnel barrier between a ferromagnetic (FM) metal source lead and a semiconductor (SC) layer has proved effective in achieving high spin injection efficiency at the FM-SC interface. We investigate the spin transport across a FM-I (insulator)-SC interface, under the influence of a Schottky barrier which arises in the SC layer close to the interface. The spin transport in the presence of an applied voltage is calculated via the nonequilibrium Green’s function (NEGF) tight binding model. The NEGF formalism systematically accounts for: (i) the spatial profile of the Schottky barrier, (ii) the coupling between the FM lead and the SC layer, and (iii) the effect of the entire semi-infinite lead, which can be reduced to a self-energy term. We investigate several parameters (e.g., doping concentration, built-in potential and applied bias) which affect the Schottky barrier profile, and hence the spin current across the FM/I/SC system. It is shown that the spin polarization of current can be significantly improved by having a low Schottky barrier height, but a high built-in potential. A high doping density increases the current density by decreasing the Schottky barrier height and the depletion width, but at the cost of reduced spin polarization.

Nanostructuring of NiMnSb(110): Influence on surface magnetic properties

The magnetic properties, crystal structure, and topography at the (110) surface of the ferromagnetic half-Heusler alloy NiMnSb have been investigated by means of magneto-optical Kerr effect, spin-resolved appearance potential spectroscopy, low-energy electron diffraction, and atomic force microscopy. A standard sputter-anneal cleaning procedure leads to a nanostructuring of the surface with consequences for the magnetic properties. This finding sheds light on the reduced spin polarization measured by surface-sensitive techniques on NiMnSb surfaces.

Magnetite: a search for the half-metallic state

We present a detailed study of the spin-dependent electronic structure of thinepitaxial magnetite films of different crystallographic orientations. Using spin- andangle-resolved photoelectron spectroscopy at room temperature, we determine for epitaxialFe3O4(111) films a maximum spinpolarization value of −(80 ± 5)% near EF. The spin-resolved photoelectron spectra for binding energies between 1.5 eV andEF show good agreement with the spin-split band structure from density functional theory (DFT)calculations which predict an overall energy gap in the spin-up electron bands in highsymmetry directions, thus providing evidence for the half-metallic ferromagnetic state ofFe3O4 in the [111] direction.In the case of the Fe3O4(100) surface, both the spin-resolved photoelectron spectroscopy experiments and the DFT density ofstates give evidence for a half-metal to metal transition: the measured spin polarization of about−(55 ± 10)% atEF and the theoreticalvalue of −40% are significantlylower than the −100% predicted by local spin density approximation (LSDA) calculations for the bulk magnetite crystal as well as the−(80 ± 5)% obtainedfor the Fe3O4(111) films. The experimental findings were corroborated by DFT calculations as due to a surfacereconstruction leading to the electronic states in the majority-spin band gap and thus tothe reduced spin polarization.

On phonon-wind-driven spin injection and the conductivity mismatch problem

Spin injection from a ferromagnetic metal into a semiconductor driven by a constant phonon flow is considered. It is shown that diffusive (as opposed to electrical) spin injection gives rise to a highly spin-polarized current through the semiconductor. In other words, phonon-wind-driven injection under certain conditions eliminates the conductivity mismatch responsible for reduced spin polarization of the electric current injected from a ferromagnetic metal into a semiconductor.

Microstructure and spin polarization of quaternary Co 2Cr 1− xV xAl, Co 2V 1− xFe xAl and Co 2Cr 1− xFe xAl Heusler alloys

The microstructures, magnetic properties and spin polarization of quaternary Co 2Cr 1− x V x Al, Co 2V 1− x Fe x Al and Co 2Cr 1− x Fe x Al alloys were investigated. Phase separation into A2 and B2/L2 1 structure occurs in Co 2CrAl and Co 2Cr 0.6Fe 0.4Al, whereas Co 2FeAl exhibits a single-phase B2 structure. The ordered L2 1 structure becomes more stable with increasing vanadium concentration ( x ⩾ 0.35). The saturation magnetization measured at 5 K for Co 2Cr 1− x V x Al alloy changes from 1.4 to 2.0 μ B when x increases from 0.0 to 0.5 and then becomes 1.4 μ B for x = 1.0. This behavior can be attributed to the variation in the local magnetic moment of Co atoms. The saturation magnetization of Co 2V 1− x Fe x Al and Co 2Cr 1− x Fe x Al alloys increases with increasing Fe concentration. The spin polarization decreases from 0.62 to 0.56 with increasing x for Co 2Cr 1− x Fe x Al alloy. Also, the spin polarization decreases with increasing x for Co 2Fe 1− x V x Al and Co 2Cr 1− x V x Al alloys. Possible reasons for the reduced spin polarization in these alloys are discussed.

Anisotropic magnetoresistance and spin polarization of La0.7Sr0.3MnO3∕SrTiO3 superlattices

The crystalline structure, anisotropic magnetoresistance (AMR), and magnetization of La0.7Sr0.3MnO3∕SrTiO3 (LSMO/STO) superlattices grown by a rf sputtering system are systematically analyzed to study the spin polarization of manganite at interfaces. The presence of positive low-temperature AMR in LSMO/STO superlattices implies that two bands of majority and minority character contribute to the transport properties, leading to a reduced spin polarization. Furthermore, the magnetization of superlattices follows the T3∕2 law and decays more quickly as the thickness ratio dSTO∕dLSMO increases, corresponding to a reduced exchange coupling. The results clearly show that the spin polarization is strongly correlated with the influence of interface-induced strain on the structure.

Spin filtering in an electromagnetic structure

We investigate possible spin-dependent resonant tunneling through double-barrier electromagnetic structure. We point out that previous related studies contain serious errors in the calculation and the correct ones show much reduced spin polarization in the tunneling. In this study, we show that the significant amount of spin polarization with nice resonant structures can be achieved by applying a proper electric field subject to the given magnetic barriers. The electric potential U, which is crucial in our model, is chosen to exactly cancel the A2 term originated from the applied magnetic field. In this way, we can remove unwanted obstacles and expedite the tunneling of electrons.

Growth and magnetic properties of single crystal Co/sub 2/MnX (X=Si,Ge) Heusler alloys

Half-metallic (HM) ferromagnetic materials have recently drawn intense interests due to their potential use in magnetoelectronic devices. Co-based Heusler alloys of the type CO/sub 2/MnX (X=Si,Ge), predicted to be HM by first principles band structure calculations, are of particular interest since they alone possess Curie temperatures in excess of 900 K. Since the spin polarization (P) is believed to be sensitive to antisite defects that are likely to occur in vapor-quenched thin film synthesis, single crystals of Co/sub 2/MnX (X=Si, Ge) were prepared using the tri-arc Czochralski method. X-ray diffraction, including Laue backscattering, was employed to determine the high crystalline quality of these crystals. SQUID magnetometry measured a magnetic moment per formular unit that is close to the calculated value indicating that these alloys may in fact be HM. However, point contact Andreev reflection, a technique that has reliably measured high P in CrO/sub 2/, measures P values for these crystals of 50-60%, well below their theoretical values. The reduced spin polarization may be due to the effects of crystal symmetry breaking at the surface or the presence of anti-site defects, or that these materials are not truly half-metallic.

Effect of Fe doping on high field magnetoresistance and low field magnetoresistance at zero field in polycrystalline La0.7Sr0.3Mn1−xFexO3 (x=0–0.12) thin films

Polycrystalline La0.7Sr0.3Mn1−xFexO3 thin films, with x=0–0.12, have been prepared on (001)-Si substrates using pulsed laser deposition. The films consist of fine grains with an average size of 60–80 nm. For those films, the metal–insulator transition temperature, Tp, is much lower than the Curie temperature, TC. The high field magnetoresistance, HFMR, is nearly temperature independent for x<0.08, whereas the extrapolated low field magnetoresistance at zero field, LFMR*, decreases rapidly with increasing temperature. Moreover, Fe doping significantly decreases LFMR* and enhances HFMR at low temperatures. We propose that for the Fe-doped films, both the reduced spin polarization of conduction electrons and the increased spin-flip scattering are responsible for the decrease of LFMR*, while the weakened ferromagnetic spin interaction at the grain boundaries is responsible for the enhanced HFMR.

Electron spin resonance of the two-dimensional electron system inAlxGa1−xAs/GaAsat subunity filling factors

The electron spin resonance (ESR) of a two-dimensional electron system (2DES) in ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ in the regime of fractional filling (\ensuremath{\nu}1) of the lowest Landau level is investigated by a photoconductivity technique at millimeter wave frequencies. By performing the experiments in a standing wave the ESR is shown to be a magnetic-dipole transition. This is in agreement with a calculation based on an 8-band model and k\ensuremath{\cdot}p theory. The (single-electron) theory also yields excellent agreement for the experimental and theoretical magnetic field dependences of the ESR transition energy, i.e., single-electron transition energies are measured and Kohn's theorem is not violated. Nevertheless, due to electron-electron interaction the ground state of the 2DES can be a many-particle state with similar amplitudes of spin-up and spin-down contributions. Evidence for such a reduced spin polarization is found in a striking reduction of the ESR strength when decreasing the temperature from 1.6 to 0.3 K.

Formation of a ferromagnetic silicide at the Fe/Si(100) interface

The interplay between magnetism and chemistry at the Fe/Si(100) interface has been examined by spin- and angle-resolved photoemission. A ferromagnetically ordered metallic silicide of \ensuremath{\sim}20 $\AA{}$ thickness is formed by deposition of Fe on Si at room temperature. This interface layer is ferromagnetic in-plane with a reduced spin polarization in comparison to bulk Fe. Its electronic structure indicates an Fe-rich composition close to ${\mathrm{}\mathrm{Fe}}_{3}\mathrm{Si}$. The Fe/Si and Si/Fe interface are inequivalent with respect to silicide formation and to the resulting magnetic properties.