Minority Spin Band Research Articles

Digital switch and femtotesla magnetic field sensor based on Fano resonance in a spin field effect transistor

We show that a spin field effect transistor, realized with a semiconductor quantum wire channel sandwiched between half-metallic ferromagnetic contacts, can have Fano resonances in the transmission spectrum. These resonances appear because the ferromagnets are half-metallic, so that the Fermi level can be placed above the majority but below the minority spin band. In that case, the majority spins will be propagating, but the minority spins will be evanescent. At low temperatures, the Fano resonances can be exploited to implement a digital binary switch that can be turned on or off with a very small gate voltage swing of few tens of microvolts, leading to extremely small dynamic power dissipation during switching. An array of 500 000×500 000 such transistors can detect ultrasmall changes in a magnetic field with a sensitivity of 1 fT∕Hz, if each transistor is biased near a Fano resonance.

Defects-driven appearance of half-metallic ferrimagnetism in Co–Mn-based Heusler alloys

Half-metallic ferromagnetic full-Heusler alloys containing Co and Mn, having the formula Co 2MnZ where Z is a sp element, are among the most studied Heusler alloys due to their stable ferromagnetism and the high Curie temperatures which they present. Using state-of-the-art electronic structure calculations we show that when Mn atoms migrate to sites occupied in the perfect alloys by Co, these Mn atoms have spin moments antiparallel to the other transition metal atoms. The ferrimagnetic compounds, which result from this procedure, keep the half-metallic character of the parent compounds and the large exchange-splitting of the Mn impurities atoms only marginally affects the width of the gap in the minority-spin band. The case of [Co 1− x Mn x ] 2MnSi is of particular interest since Mn 3Si is known to crystallize in the Heusler L 2 1 lattice structure of Co 2MnZ compounds. Robust half-metallic ferrimagnets are highly desirable for realistic applications since they lead to smaller energy losses due to the lower external magnetic fields created with respect to their ferromagnetic counterparts.

Pulsed laser deposition of Co-based Tailored-Heusler alloys

Thin films of nonstoichiometric Heusler alloys Co 2MnSb x Sn 1− x ( x = 0.2; 0.4; 0.6; 0.8) have been grown by pulsed laser deposition (double-target/double beam configuration) on Si (1 0 0) substrates using a KrF excimer laser ( λ = 248 nm, τ = 20 ns). The substrate temperature was held at 300 K in all experiments to prevent interface interdiffusion of the species. A comparison between the compositions of films and corresponding targets has been done through energy dispersive X-ray spectroscopy (EDS) analysis showing a very satisfactory match. Scanning electron microscopy (SEM) imaging served to investigate the morphology of the films in order to determine the size and density of droplets which may influence the optical data. Optical conductivity derived from reflectivity measurements shows absorption onsets close to 1 eV, which corresponds to the onset of valence-to-conduction transitions in the minority spin bands theoretically predicted. The values of the saturation magnetisation measured at 300 K on the quaternary alloys are very close to those of ternary ones for which either half-metallic properties or high spin polarisation were theoretically predicted.

Van Hove Singularity and Spontaneous Fermi Surface Symmetry Breaking in Sr3Ru2O7

The most salient features observed around a metamagnetic transition in Sr3Ru2O7 are well captured in a simple model for spontaneous Fermi surface symmetry breaking under a magnetic field, without invoking a putative quantum critical point. The Fermi surface symmetry breaking happens in both a majority and a minority spin band but with a different magnitude of the order parameter, when either band is tuned close to van Hove filling by the magnetic field. The transition is second order for high temperature T and changes into first order for low T. The first order transition is accompanied by a metamagnetic transition. The uniform magnetic susceptibility and the specific heat coefficient show strong T dependence, especially a log T divergence at van Hove filling. The Fermi surface instability then cuts off such non-Fermi liquid behavior and gives rise to a cusp in the susceptibility and a specific heat jump at the transition temperature.

Spin-dependent transport properties of fully epitaxial Co 2MnSi/MgO/Co 50Fe 50 tunnel junctions

Spin-dependent transport properties were investigated for fully epitaxial magnetic tunnel junctions (MTJs) having a cobalt-based full-Heusler alloy Co 2MnSi (CMS) thin film as a lower electrode, an MgO tunnel barrier, and a Co 50Fe 50 upper electrode. In the bias voltage dependence of the differential conductance ( G=d I/d V) for the antiparallel and parallel magnetisation configurations ( G AP and G P, respectively), a clear crossover between G P and G AP occurred with increasing negative bias voltage at both room temperature and 4.2 K (the bias voltage was defined with respect to the lower CMS electrode). This results in G AP becoming larger than G P in a bias voltage region beyond the characteristic voltage of about −0.4 V at which the crossover occurs. A possible transport mechanism leading to the observed crossover between G P and G AP for the CMS/MgO/Co 50Fe 50 MTJs is direct tunneling that reflects the electronic density of states in the minority-spin band of the CMS electrode.

Valence-band photoemission spectroscopy of a single-crystal FePt 3

The valence-band electronic structure of a single-crystal FePt 3 alloy in a paramagnetic phase has been investigated by photoemission spectroscopy using synchrotron radiation. A well-characterized (1 0 0) surface was carefully prepared in an ultrahigh vacuum chamber. A sharp truncated peak was clearly observed just below the Fermi level at photon energy of 190 eV (Pt Cooper minimum). This peak corresponds to the tail of Fe 3d minority-spin band and enhances its intensity around the X point in the Brillouin zone as is predicted by band calculation. Several broad peaks were also observed and their dispersion were in accord with the Brillouin zone.

Spin-dependent tunneling characteristics of fully epitaxial magnetic tunneling junctions with a full-Heusler alloy Co2MnSi thin film and a MgO tunnel barrier

Fully epitaxial, exchange-biased magnetic tunnel junctions (MTJs) were fabricated with a Co-based full-Heusler alloy Co2MnSi (CMS) thin film as a lower electrode, a MgO tunnel barrier, and a Co50Fe50 upper electrode. The microfabricated CMS∕MgO∕Co50Fe50 MTJs exhibited relatively high tunnel magnetoresistance ratios of 90% at room temperature and 192% at 4.2K. The bias voltage dependence of differential conductance (dI∕dV) for the parallel and antiparallel magnetization configurations suggested the existence of a basic energy gap structure for the minority-spin band of the CMS electrode with an energy difference of about 0.4eV between the bottom of the vacant minority-spin conduction band and the Fermi level.

Half metallic state of (Fe,Co)Si with B20-type structure

Electronic structures of ( Fe 1 - x Co x ) Si with a B20-type structure are calculated by the linear muffin-tin orbital method with the atomic sphere approximation. It has been shown that the excess 3d electrons on replaced Co atom for x < 0.5 occupy only the majority spin band and the Fermi level lies still in the band gap of the minority spin band. This is consistent with the observed concentration dependence of the spontaneous magnetization at small x. This fact means that the present system may be a ferromagnetic half metal.

Spin-polarized electron energy loss spectroscopy on Fe(100) thin films grown onAg(100)

We report sharp spin-dependent energy loss features in electron scattering frombcc Fe(100) thin films grown on Ag(100). Majority spin features are observed at∼1.8 and 2.5 eV energy loss, and a minority spin feature is observed at∼2.0 eV energy loss. The majority spin peaks are attributed to spin-flip exchange scatteringfrom the Fe films. The minority spin peak is attributed to non-flip exchange scattering withan energy corresponding to the separation between occupied and unoccupied minority spinbands. The observed peak energies compare favourably with electronic structurecalculations for Fe and with spin-resolved photoemission and inverse photoemission results.

Spin-dependent electronic structure of transition-metal atomic chains adsorbed on single-wall carbon nanotubes

We present a systematic study of the electronic and magnetic properties of transition-metal (TM) atomic chains adsorbed on the zigzag single-wall carbon nanotubes (SWNTs). We considered the adsorption on the external and internal wall of SWNT and examined the effect of the TM coverage and geometry on the binding energy and the spin polarization at the Fermi level. All those adsorbed chains studied have ferromagnetic ground state, but only their specific types and geometries demonstrated high spin polarization near the Fermi level. Their magnetic moment and binding energy in the ground state display interesting variation with the number of $d-$electrons of the TM atom. We also show that specific chains of transition metal atoms adsorbed on a SWNT can lead to semiconducting properties for the minority spin-bands, but semimetallic for the majority spin-bands. Spin-polarization is maintained even when the underlying SWNT is subjected to high radial strain. Spin-dependent electronic structure becomes discretized when TM atoms are adsorbed on finite segments of SWNTs. Once coupled with non-magnetic metal electrodes, these magnetic needles or nanomagnets can perform as spin-dependent resonant tunnelling devices. The electronic and magnetic properties of these nanomagnets can be engineered depending on the type and decoration of adsorbed TM atom as well as the size and symmetry of the tube. Our study is performed by using first-principles pseudopotential plane wave method within spin-polarized Density Functional Method.

Ferromagnetism in transition-metal-doped II–VI compounds

We report systematical density-functional calculations of ternary transition-metal compounds based on zincblende ZnTe and CdTe semiconductor. Some of them are half-metallic at their optimized cell volumes. The effect of atomic position optimization (and cell volume re-optimization) on the electronic structures is to widen the energy bands near the Fermi energy and to reduce the density of states there. As a result, the Fermi level moves upward and the energy gap of the minority-spin bands at the Fermi energy becomes narrower. Therefore, the half-metallic gaps are reduced, two of them even being closed. These compounds are compatible with the II–VI semiconductors, and could be useful in spin-based electronics.

High-resolution angle-resolved photoemission study of Fe (1 1 0)

High-resolution low-temperature angle-resolved photoemission spectroscopy of Fe(1 1 0) single crystal has been done to evaluate many-body interactions on the quasi-particles at the Fermi level. A kink structure in the minority-spin band (B ↓) on the Fermi surface (FS) around the Γ point has been clearly observed at the binding energy of ∼40 meV, but it is absent for the majority-spin band (B ↑) on the FS around the Γ point. Additional spectral feature has been observed for B ↑, indicating a coexistence of a surface resonance state. By the analyzes of the imaginary part of the self-energy, we have confirmed that the kink structure is derived from the electron–phonon interaction.

Temperature-driven band motion prior to the phase transition of an itinerant ferromagnet

Tunneling magnetoresistance spectroscopy measurements have been performed on spin-valve magnetic tunnel junctions where the free magnetic electrode is a Cu38Ni62 alloy with a Curie temperature of around 240K. These measurements allow us to follow the temperature dependence of the energy of the bottom of a minority-spin band close to the Fermi energy, which is partly responsible for the tunneling conductance. The temperature dependence of the tunneling spin polarization of the CuNi electrode has also been extracted using the Julliere formula. We show that the tunneling spin polarization as a function of temperature can be related to the temperature-dependent magnetization using an extension of the commonly applied Stearns model that incorporates the motion of the band close to the Fermi level. This model reproduces the nonlinear relation between tunneling spin polarization and magnetization obtained for Cu38Ni62.

Comparison of hybrid density functional, Hartree–Fock, and GW calculations on NiO

AbstractThe electronic band structure of NiO in the ferromagnetic state is calculated using the B3LYP hybrid density functional, the Hartree–Fock (HF) method, and the GW approximation (GWA) with dielectric functions constructed using either B3LYP or HF wave functions and energy eigenvalues. The band structure from the GWA calculation based on B3LYP wave functions is quite similar to the parent B3LYP band structure; the main differences are in valence bandwidth and in the upward shift of the empty minority spin Ni 3d bands relative to the same bands in the B3LYP calculation. The band structure from the GWA calculation based on HF wave functions differs in that there are large upward shifts in valence band positions in the GWA calculation relative to the HF calculation, which result from screening of the bare exchange term in the Fock operator. Matrix elements of the HF exchange operator are obtained using wave functions from self‐consistent HF or B3LYP calculations. Magnitudes of these matrix elements for several states at the Γ point of the Brillouin zone are compared, and it is found that the only major difference occurs in the empty minority spin bands derived from Ni 3d states of e symmetry. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006

High-resolution Angle-resolved Photoemission Spectroscopy with Synchrotron Radiation: Study of Fine Electronic Structures near the Fermi Level

High-resolution angle-resolved photoemission spectroscopy (ARPES) using tunable synchrotron radiation has recently developed rapidly. New advances in high energy and angular resolution measurements enabled us to examine quantitatively fine spectral features near the Fermi level that are directly related to the low-energy excitation. We present a high-resolution ARPES study of an itinerant ferromagnet Ni. A kink structure was found in the dispersion of the minority-spin bands, while it was not clearly observed for the majority-spin band. On the basis of the self-energy analyses, the kink structure was attributed to the electron-phonon interaction. We found that the electron-phonon interaction and electron correlation contribute to the effective mass enhancement in a different way depending on the energy band, and spin-direction. The analyses is applicable to the surface derived-electronic states. In the case of Cu, the electron-phonon coupling constant was found to be equivalent at the surface and in the bulk.

Doping dependence of transport properties inFe1−xCoxSi

The positive magnetoresistance has been investigated for ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{Si}$ single crystals in a wide range of doping $(0.05\ensuremath{\leqslant}x\ensuremath{\leqslant}0.7)$. Most of the magnetoconductivity data are found to scale well with the magnetization. This is inconsistent with the quantum interference scenario proposed by Manyala et al. [Nature 404, 581 (2000)]. We have shown that the decrease of density of the minority spin band with high mobility in the course of Zeeman splitting is relevant to the positive magnetoresistance. The nearly half-metallic nature in this system seems to enhance the magnetoresistance. The pressure dependence of resistivity has been measured for ${\mathrm{Fe}}_{0.7}{\mathrm{Co}}_{0.3}\mathrm{Si}$. $T$-linear behavior has been found in the resistivity above $7\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, where the helical spin order is completely suppressed. This temperature dependence reproduces that of the hypothetical resistivity of the nonmagnetic state deduced by the analysis of the magnetoresistance. We have investigated the large Hall conductivity in ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{Si}$ ($\ensuremath{\sim}40\phantom{\rule{0.3em}{0ex}}{\ensuremath{\Omega}}^{\ensuremath{-}1}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ at a maximum). The doping dependence of the Hall conductivity is almost parallel with those of the critical field and the wave vector of the helical spin state. This suggests that the Hall conductivity is proportional to the effective spin-orbit interaction. We have also observed the doping dependence of the Seebeck coefficient for ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{Si}$. In the underdoped region $(x\ensuremath{\leqslant}0.1)$, the negative Seebeck coefficient is enhanced at low temperature below $100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, corresponding to the steep doping variation of the resistivity in this temperature region. In the higher doping region $(x\ensuremath{\geqslant}0.2)$, the Seebeck coefficient shows a gradual upturn at low temperatures $(\ensuremath{\lesssim}100\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. This is caused by the electronic structural change occurring with the transition from the paramagnetic to the ferromagnetic state.

Half-metallic properties ofCo2(Cr1−xFex)GaHeusler alloys

Magnetic and half-metallic properties of ${\mathrm{Co}}_{2}({\mathrm{Cr}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x})\mathrm{Ga}$ Heusler alloys have been investigated. In the entire concentration range, the $L{2}_{1}$-type ordered phase is obtained. The saturation magnetic moment ${M}_{s}$ at $4.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for $x=0.0$ $(\ensuremath{\equiv}{\mathrm{Co}}_{2}\mathrm{Cr}\mathrm{Ga})$ is $3.01{\ensuremath{\mu}}_{B}∕\mathrm{f.u.}$, in agreement with the generalized Slater-Pauling line and consistent with the theoretical value. The experimental value of ${M}_{s}$ increases with increasing $x$, slightly larger than that of the theoretical value above $x=0.60$. The Curie temperature ${T}_{C}$ also increases with $x$, and qualitatively accords with the theoretical values estimated from the effective exchange constant ${J}_{0}$. The present theoretical results disclose that not only the $L{2}_{1}$-type ordered phase but also the $B2$-type phase exhibits high values of the spin polarization ratio $P$ in a low concentration range of $x$. The ratio $P$ of the $L{2}_{1}$- and $B2$-type phases decreases with increasing $x$, because the large peak of the density of states at the Fermi energy for Cr in the majority spin band becomes smaller and a strong hybridization between $\mathrm{Co}\text{\ensuremath{-}}3d$ and $\mathrm{Fe}\text{\ensuremath{-}}3d$ makes a gap narrower in the minority spin band.

Negative Spin Polarization and Large Tunneling Magnetoresistance in EpitaxialCo|SrTiO3|CoMagnetic Tunnel Junctions

We perform an ab initio study of spin-polarized tunneling in epitaxial Co/SrTiO(3)/Co magnetic tunnel junctions with bcc Co(001) electrodes. We predict a large tunneling magnetoresistance in these junctions, originating from a mismatch in the majority- and minority-spin bands both in bulk bcc Co and at the Co/SrTiO(3)/Co interface. The intricate complex band structure of SrTiO(3) enables efficient tunneling of the minority d electrons which causes the spin polarization of the Co/SrTiO(3)/Co interface to be negative in agreement with experimental data. Our results indicate that epitaxial Co/SrTiO(3)/Co magnetic tunnel junctions with bcc Co(001) electrodes are a viable alternative for device applications.

Spin-Polarized Tunneling Spectroscopy in Tunnel Junctions with Half-Metallic Electrodes

We have studied the magnetoresistance (TMR) of tunnel junctions with electrodes of La(2/3)Sr(1/3)MnO3 and we show how the variation of the conductance and TMR with the bias voltage can be exploited to obtain precise information on the spin and energy dependence of the density of states. Our analysis leads to a quantitative description of the band structure of La(2/3)Sr(1/3)MnO3 including the energy gap delta between the Fermi level and the bottom of the t(2g) minority-spin band, in good agreement with data from spin-polarized inverse photoemission experiments. This shows the potential of magnetic tunnel junctions with half-metallic electrodes for spin-resolved spectroscopic studies.

Half-metallic alloys: electronic structure, magnetism and spin polarization

Using the state-of-the-art screened Korringa-Kohn-Rostoker Green function method we study the electronic and magnetic properties of NiMnSb and similar Heusler alloys. We show that all these compounds are half-metals, e.g. the minority-spin band is semiconducting and the Fermi level falls within this gap resulting in 100% spin polarization at the Fermi level. The total spin moment M(t) shows the so-called Slater-Pauling behaviour and scales with the total valence charge Z(t) following the rule M(t) = Z(t) - 18 for half and M(t) = Z(t) - 24 for full Heusler alloys. These rules are connected to the origin of the gap. Finally we show that the inclusion of the spin-orbit interaction in our calculations kills the half-metallic gap but the spin-polarization at the Fermi level can be still very high, approximately 99% for NiMnSb, but much lower for a half-metallic compound like zinc-blende MnBi (77%).