Majority Spin Electrons Research Articles

Spin-Polarized Density of States and Electron Tunnelling from the CO/Al2O3 Interface

ABSTRACTIn order to elucidate the mechanism of spin-polarized electron tunneling in thin-film ferromagnet-insulator junctions, self-consistent band structure calculations of the CO/Al2O3 interface have been performed using a new LMTO technique. Since the results of the calculations are very sensitive to the distance between the Co and Al planes, we have minimised the total energy with respect to this distance. Our calculations show that at the Fermi energy a strong bonding between the 3d-electrons of Co with the sp-electrons of Al at the interface can have an important influence on the spin polarization of the layer-projected density of states (LPDOS) of inner Al and O layers. Since the Fermi energy lies within the minority-spin d-band of Co but above the majority-spin d-band, the sp-d bonding results in a smaller LPDOS of the minority-spin electrons of the interfacial Al layers in comparison to that of the majority-spin electrons. This asymmetry in the LPDOS extends to the inner Al2O3 layers implying a positive spin polarization of the tunneling density of states. The result is consistent with experimental observations on tunnelling from cobalt through alumina where positive values of the spin polarization of the tunnelling current were measured.

Spin polarized photoemission studies of the 3s core level in ferromagnetic systems (abstract)

Spin polarized core level photoemision studies are capable of providing local site specific magnetic information. 3s core level photoemission spectra are characterized by a main peak and satellite whose separation reflects the exchange interaction between the core level and the net spin in the valence bands. However, there is currently considerable disagreement as to whether the splitting between these two peaks may be taken as a measure of the local moment. An alternative approach is to examine the intensities of the different peaks in the spectra. Indeed analysis of the multiplet intensities in the final state suggests that it should be possible to obtain a reasonable measure of the local magnetic moment through measurement of the spin dependent intensities in the multiplet structure. In detailed spin polarized photoemission studies of the 3s core level of Fe and Co films grown on Ag(001) and Cu(001) substrates, respectively, we find that this is indeed the case. This suggests that studies of the spin polarization of the 3s core level photoemission may provide an important new technique for the measurement of magnetic moments in thin films. Lineshape fitting reveals new information about the lifetime of the core holes. Previous studies have suggested that a majority spin core hole will be shorter lived than a minority spin core hole because of the higher density of majority spin electrons in the valence bands. However, our studies indicate that the lifetime is more complicated and that it shows a strong dependence on the LS in the final state.

Spin polarized tunneling in half-metallic ferromagnets (abstract)

We present a study of spin polarized tunneling in tunnel junctions in which one of the conducting layers is a half-metallic ferromagnet (HMF). HMF are unique in that the Fermi level of these materials intersect the majority spin electron band, while the minority band has an energy gap near the Fermi level. Hence, HMF simultaneously have both metallic and semiconducting characteristics, and theory predicts that the conduction electrons are 100% spin polarized. As a result, the magnetoresistance in magnetic multilayers or trilayer tunnel junctions is expected to be significantly higher than with conventional ferromagnetic materials. Two important parameters affecting the performance of these junctions are the smoothness of the HMF surface and its surface composition. Tunnel junctions consisting of a layer of NiMnSb, a barrier layer of Al2O3, and a layer of aluminum were prepared and studied for their tunneling properties. Surface analysis of HMF films was done using Auger depth profiling and AFM.

First principles calculations of electrical conductivity and giant magnetoresistance of periodic multilayers and spin valves (invited)

We used the Layer Korringa Kohn Rostoker technique to calculate the electronic structure of cobalt-copper multilayers and spin valves from first principles within the local spin density approximation. Using this electronic structure together with a phenomenological self-energy which may vary from layer to layer, we calculated the non-local layer-dependent conductivity by means of the Kubo linear response formalism. By calculating the majority and minority conductivities for parallel and anti-parallel alignment of the moments in the cobalt layers we determined the giant magnetoresistance (GMR). Several interesting features emerge from the calculations. When the scattering rates are relatively high, we find that the contributions to the GMR are largely non-local, with the largest contributions arising from changes in the currents carried in a cobalt plane next to copper due to fields sensed in the cobalt layer on the other side of copper. When scattering rates are relatively low (comparable to that of cobalt and copper at room temperature), there are important contributions to the GMR from local conduction in the copper layers. This effect arises from the fact that when the component of the majority spin electron momentum parallel to the layers exceeds a certain value, it gets trapped in the copper layers. If the scattering rate is lower in the copper than in the cobalt there is a significant enhancement in the majority spin conductivity and in the GMR. This effect is analogous to the channeling of light by an optical waveguide.

Electron transmission through ultra-thin metal layers and its spin dependence for magnetic structures

We present a new set of experiments in which the attenuation of a ‘monoenergetic’, possibly spin-polarized, free-electron beam is measured by direct transmission through an ultra-thin metal layer. The self-supported metal target is either a reference gold sample or a ferromagnetic structure. The overall thickness is of the order of 25 nm. The magnetic structure consists of a 1 nm thick cobalt film sandwiched between 21-2 nm thick gold layers, with perpendicular magnetization. Measurements are performed throughout a wide energy range, with incident electron energies 2–1000 eV above the Fermi level. The transmission of the gold layer is found to be substantially higher than that of the magnetic structure. In the latter case, at low energy, close to the clean surface vacuum level, we find that the majority spin electrons are more easily transmitted than the minority spin electrons. Cesium deposition on the exit side or on both sides of the target increases the overall transmitted current by almost an order of magnitude. In the case of the magnetic structure, this also increases the transmission spin asymmetry from 16 to about 40%. Such structures appear to be well-suited to the construction of convenient and compact spin-detectors.

Spin-dependent electron scattering in ferromagnetic Co layers on Cu(111).

By means of spin-resolved photoemission we have studied the spin and energy dependence of electron scattering in ferromagnetic Co overlayers epitaxially grown on Cu(111). Taking advantage of the tunability of the synchrotron radiation, the kinetic energy of the electrons propagating inside the solid could be varied between 7 and 50 eV. For all the coverages studied we found that the attenuation length is larger for majority-spin electrons than for minority-spin electrons. A comparison with theoretical models indicates that the observed spin-dependent effects originate from inelastic rather than elastic-scattering processes.

Inverse magnetoresistance in the simple spin-valve system Fe1-xVx/Au/Co.

Magnetoresistance (MR) measurements have been performed on Fe/Au/Co and ${\mathrm{Fe}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{V}}_{\mathit{x}}$/Au/Co trilayers with perpendicular orientation of the magnetization of the thin ferromagnetic layers. While the MR is larger for an antiparallel arrangement of the magnetizations than for a parallel one in Fe/Au/Co (the usual spin-valve-type MR), an inverse spin-valve effect is observed in ${\mathrm{Fe}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{V}}_{\mathit{x}}$/Au/Co. This inverse MR is attributed to an increase of the density of states at the Fermi level for majority-spin electrons when Fe is alloyed with V.

Spin-dependent transmission of low-energy electrons through ultrathin magnetic layers.

We present an experiment in which the attenuation of a spin-polarized free electron beam is measured by direct transmission through an ultrathin ferromagnetic layer. The self-supported metal target consists of a 1-nm-thick cobalt film sandwiched between 21- and 2-nm-thick gold layers. Measurements are performed throughout a wide energy range (incident electron energies from 4 eV to 50 eV above the Fermi level). At low energy, close to the clean surface vacuum level, we find that the transmission coefficient of minority spin electrons is about 0.7 times that of majority spin electrons.

Temperature-dependent non-Heisenberg exchange coupling of ferromagnetic layers (invited)

We consider with temperature the exchange coupling of two identical ferromagnetic films separated by a paramagnetic metal of thickness d. The minority-spin band of the ferromagnets is matched to the band of the paramagnet. Majority-spin electrons experience a repulsive potential of maximum height equal to the exchange gap of the ferromagnets. A spin-flip current, which flows as a result of a virtual rotation of the ferromagnetic moments, is calculated at temperature T, assuming single-particle excitations only. This current, equated to the torque exerted by one ferromagnet on the other, defines the exchange energy as an infinite sum of terms with coefficients A12(d,T),B12(d,T),..., extending beyond the Heisenberg-like A12(d,T) term. For certain d, the biquadratic B12(d,T) term is of the proper magnitude and sign to cause intrinsic 90° coupling between the moments of the two ferromagnets. Additionally, we calculate the T dependence of A12(d,T) and B12(d,T) for different Fermi energy to exchange-gap ratios, showing that, in general, a nonmonotonic dependence of both A12(d,T) and B12(d,T) vs T is expected. Monotonic T dependence occurs in the asymptotic regime of large d.

Two oscillatory behaviors as functions of ferromagnetic layer thickness in Fe/Cr(100) multilayers

Oscillatory magnetoresistance as a function of the Fe layer thickness has been found in Fe/Cr(100) multilayers deposited on MgO(100). It is mainly attributed to an oscillatory interlayer exchange coupling between adjacent Fe layers as a function of the Fe layer thickness. Further, oscillatory variation of the saturation resistivity with Fe layer thickness was also observed. Both the oscillation periods are approximately 8 \AA{}. These two oscillations can be understood in terms of the partial confinement of the perpendicular motion to the layer plane of Fe majority spin electrons.

Spin-resolved and high-energy-resolution XPS studies of cobalt metal and a cobalt magnetic glass.

The role of electron spin in photoelectric emission from the 2s, 2${\mathit{p}}_{1/2}$, 2${\mathit{p}}_{3/2}$, and 3p core levels of cobalt metal is investigated through a combination of spin-resolved x-ray-photoelectron spectroscopy (SRXPS) and high-energy-resolution XPS (HRXPS) studies. The sensitivity of the Co core-level spectra to a variation in the valence spin polarization is revealed by comparing the Co-metal data with SRXPS and HRXPS spectra from the amorphous cobalt-based ferromagnetic glass ${\mathrm{Co}}_{66}$${\mathrm{Fe}}_{4}$${\mathrm{Ni}}_{1}$${\mathrm{B}}_{14}$${\mathrm{Si}}_{15}$. The results demonstrate explicitly the role of intra-atomic exchange in deep-core-level photoemission from Co, and reveal core-hole spin dependencies to core-hole-induced electron-hole-pair excitation, to the ${\mathit{L}}_{1}$${\mathit{L}}_{2,3}$V Coster-Kronig transition rate, and to the rates of Auger transitions involving the valence band. The data also suggest that the reduction of the cobalt magnetic moment in the ${\mathrm{Co}}_{66}$${\mathrm{Fe}}_{4}$${\mathrm{Ni}}_{1}$${\mathrm{B}}_{14}$${\mathrm{Si}}_{15}$ glass results from the transfer of majority-spin 3d electron density into the minority-spin 3d band caused by a hybridization-induced weakening of the 3d electron-electron interaction.

Spin exchange between ion probes and localized moments in ferromagnets as the origin of transient fields.

The transient field phenomenon has been ascribed to a polarization transfer between the electrons of the ionic projectiles and the surplus of majority spin electrons of the ferromagnetic host over the minority spin electrons. Earlier attempts to explain this crucial process failed to account for the order of magnitude of the experimentally observed transient field strengths. A recent model which proposes spin exchange scattering between bound projectile electrons and quasifree host electrons as the mechanism of polarization transfer arrives at the correct orders of magnitude but is in conflict with the weak velocity dependence of the experimental polarization, exhibiting a strongly decreasing behavior with increasing velocity. The new model presented here proposes spin exchange between the ionic shell and localized electrons of the ferromagnet as a more adequate approach to the problem. It is shown that calculations involving hydrogenlike ions explain the size of the experimentally observed polarization effects as well as their velocity dependence for various ion probes traversing thin iron foils.

Mechanism for non-Heisenberg-exchange coupling between ferromagnetic layers.

We consider a free-electron model of exchange coupling between transition-metal ferromagnetic films separated by a paramagnetic metal spacer. The minority-spin energy bands in the ferromagnets are matched to those of the paramagnetic spacer. The majority-spin electrons experience a repulsive potential arising from the lack of corresponding states in the spacer. The height of the potential barrier is equal to the exchange-energy gap in the ferromagnets. We show how the model, applicable to a trilayer film such as Fe/Cr/Fe or Co/Ru/Co, generates an infinite sum of terms with coefficients ${\mathit{A}}_{12}$,${\mathit{B}}_{12}$, . . . in the coupling energy, extending beyond the Heisenberg-like ${\mathit{A}}_{12}$ term of bilinear coupling between the moments of the ferromagnets. Both the ${\mathit{A}}_{12}$ and non-Heisenberg (biquadratic) ${\mathit{B}}_{12}$ exchange terms oscillate with spacer-layer thickness. We find that the phase of ${\mathit{B}}_{12}$ relative to ${\mathit{A}}_{12}$ shifts with this thickness, so that at certain regions of spacer-layer thickness, the magnitude of ${\mathit{B}}_{12}$ is greater than that of ${\mathit{A}}_{12}$, and ${\mathit{B}}_{12}$ is of the proper sign to cause 90\ifmmode^\circ\else\textdegree\fi{} coupling between the ferromagnets. These special thicknesses are predicted to appear over a broad range of the ratio of Fermi to exchange-gap energies. Thus, we show from our model that biquadratic, or 90\ifmmode^\circ\else\textdegree\fi{}, coupling between the ferromagnets is intrinsic to itinerant-electron exchange across the paramagnetic spacer.

Spin Polarized Photoemission from Fe On CU(100)

AbstractThe spin resolved electronic structure of ultra-thin Fe films on Cu(100) was investigated using spin polarized angle resolved photoemission spectroscopy. All exchange splitting of the Fe ∆s band of 2.5 eV is observed for photon energies between 20 and 30 eV. ∆ peak at 6 eV binding energy which has been previously identified as a many-electron resonance was observed only after contamination of the films with oxygen. In addition, the spin dependent attenuation lengths for electrons in Fe were measured at 11, 19, and 40 eV above Ef. The attenuation length for the minority spin electrons was found to be shorter than that of the majority spin electrons. The difference between the two attenuation lengths was shown to increase at low energy. Short attenuation lengths of ≃3 monolayer were measured at II eV. The large increase of the attenuation length at low energy which is expected from the “universal curve” is not observed in Fe.

Electronic and structural properties of thin films of Fe on Cu(1 1 1) and of the system Cu/Fe/Cu(1 1 1)

We have prepared thin films of Fe (0.5-5 monolayers) on Cu(1 1 1) and the sandwich system Cu(0.5-2 monolayers)/ Fe(1–5 monolayers)/Cu(1 1 1). Low-energy electron diffraction, Auger and angle-resolved photoelectron spectroscopy exhibited epitaxial growth of the films in the fcc structure at room temperature. Annealing leads to a rearrangement of the deposit in the form of three-dimensional Fe and Cu islands on top of the substrate. The Cu electronic states are not affected essentially by the Fe deposit. The Fe 3d states are characterized by a main peak at -∮.6 eV below the Fermi level, which we assign to the maximum density of states of the majority spin electrons.

Electronic structure and magnetic effects at the ideal (110) Fe/Ge interface

Ge and bcc Fe are well lattice matched, and therefore present an excellent prototype system to study magnetic effects at a metal/semiconductor interface. We study the electronic structure of the (110) Fe/Ge interface using a superlattice geometry and a parametrized tight-binding approach. We find that seven layers of Fe and 11 layers of Ge are necessary to identify unambiguously the interface bands in this system. The Fe minority-spin projected band structure has a large gap around the M̄ point (corner of the two-dimensional Brillouin zone) which spans the fundamental gap in Ge. We find four distinct interface bands which lie in this mutual gap and a corresponding large density of states at the Fermi level. For the majority-spin electrons we also find a large density of interface resonances just above the Fermi level which are strongly localized on the Ge atoms at the interface. These results are consistent with the very reactive nature of this interface seen in experimental studies.

Electronic structure of the Fe/Ge(110) interface.

Ge and bcc Fe have lattice spacings which are well matched, and therefore these elements present an excellent prototype system to study magnetic effects at a metal-semiconductor interface. We study the electronic structure of the (110) Fe/Ge interface using a parametrized tight-binding approach. The necessary tight-binding parameters were determined by fitting to first-principles band structures of Ge, spin-polarized bcc, Fe, and a CsCl-structure FeGe compound. We have implemented a generalized Slater-Koster scheme suitable to handle arbitrary atomic geometries, which allows the number of layers of each material to be increased easily to study convergence of interface properties. We find that seven layers of Fe and eleven layers of Ge are necessary to identify unambiguously the interface bands in this system. The Fe minority-spin projected band structure has a large gap around the M\ifmmode\bar\else\textasciimacron\fi{} point which spans the fundamental gap in Ge; there are eight interface bands which lie in this mutual gap. For the majority-spin electrons we also find a large density of interface resonances just above the Fermi level which are strongly localized on the Ge atoms at the interface. These results are consistent with the reactive nature of this interface seen in experimental studies.

CrO2predicted as a half-metallic ferromagnet

The band structure of chromium dioxide, which is an important material because of its magnetic properties, has been calculated with the augmented spherical wave method by performing self-consistent spin-polarised calculations. The majority-spin electrons are metallic, whereas the minority-spin electrons are semiconducting. A new class of half-metallic ferromagnets is found in the rutile structure.

Self-consistent electronic structure of disordered Fe0.65Ni0.35

We present the results of the first ab initio calculation of the electronic structure of the disordered alloy Fe0.65Ni0.35. The calculation is based on the multiple-scattering coherent-potential approach (KKR-CPA) and is fully self-consistent and spin polarized. Magnetic effects are included within local-spin-density functional theory using the exchange-correlation function of Vosko–Wilk–Nusair. The most striking feature of the calculation is that electrons of different spins experience different degrees of disorder. The minority spin electrons see a very large disorder, whereas the majority spin electrons see little disorder. Consequently, the minority spin density of states is smooth compared to the very structured majority spin density of states. This difference is due to a subtle balance between exchange splitting and charge neutrality.

RuFeSi — A new magnetic system

A ternary system consisting of Ru, Fe and Si is identified in which the hyperfine field at Fe is seen to evolve at a characteristic temperature, T ∗, far below the magnetic ordering temperature, T m. It is speculated that the magnetic ordering at T m is itinerant and T ∗ corresponds to the localization of the majority spin electrons at the Fe atoms.