Ferromagnetic Spin Polarization Research Articles

Direct observation of ferromagnetic spin polarization in gold nanoparticles.

We report the first direct observation of ferromagnetic spin polarization of Au nanoparticles with a mean diameter of 1.9 nm using x-ray magnetic circular dichroism (XMCD). Owing to the element selectivity of XMCD, only the gold magnetization is explored. Magnetization of gold atoms as estimated by XMCD shows a good agreement with results obtained by conventional magnetometry. This evidences intrinsic spin polarization in nanosized gold.

Diameter dependence of ferromagnetic spin moment in Au nanocrystals

Au nanoparticles exhibit ferromagnetic spin polarization and show diameter dependence in magnetization. The magnetic moment per Au atom in the particle attains its maximum value at a diameter of about 3 nm in the magnetization-diameter curve. Because Au metal is a typical diamagnetic material, its ferromagnetic polarization mechanism is thought to be quite different from the ferromagnetism observed in transition metals. The size effect strongly suggests the existence of some spin-correlation effect at the nanoscale. The so-called ``Fermi hole effect'' is the most probable one given in the free-electron gas system. Ferromagnetism in Au nanoparticles is discussed using this model.

Atomic disorder effects on half-metallicity of the full-Heusler alloysCo2(Cr1−xFex)Al:A first-principles study

We investigate the atomic disorder effects on the half-metallicity of the full-Heusler alloy ${\mathrm{Co}}_{2}({\mathrm{Cr}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x})\mathrm{Al}$ from first principles by using the Korringa-Kohn-Rostoker method with the coherent potential approximation. Our results show that disorder between Cr and Al does not significantly reduce the spin polarization of the parent alloy ${\mathrm{Co}}_{2}\mathrm{CrAl},$ while disorder between Co and Cr makes a considerable reduction of the spin polarization. It is observed that the spin polarization of ${\mathrm{Co}}_{2}({\mathrm{Cr}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x})\mathrm{Al}$ decreases with increasing Fe concentration x in both the ordered ${L2}_{1}$ and the disordered $B2$ structures, and that the effects of the disorder on the spin polarization is significant at low Fe concentrations. The results suggest that a highly spin-polarized ferromagnet with high Curie temperature will be obtained if a ${\mathrm{Co}}_{2}({\mathrm{Cr}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x})\mathrm{Al}$ with the ordered ${L2}_{1}$ structure can be fabricated at low Fe concentrations.

Inter-granular Magneto-resistance of Half-Metallic Ferromagnet CrO2 with Cr2 O5 Barriers

Magneto-resistances (MR) were measured at 77 K and 300 K for spin-polarized half-metallic ferromagnet CrO2 nano-granular samples. They were prepared from the precursor of CrO3 in a micro-reactor at temperatures between 598K and 723K under oxygen gas pressures of 0.5 and 10 MPa. Room temperature MR ratios of -0.13 to -0.45% and 77 K MR ratios of -2.3 to -3.4% were observed for CrO2 with Cr2O5 and/or Cr2O3 impurities. The results showed that the Cr205 forms well tunnel barriers between CrO2 nano-grains, probably due to the weak magnetic property at 300K.

Finite-temperature spin polarization in half-metallic ferromagnets

The temperature dependence of the spin polarization of half-metallic ferromagnets is investigated. A unitary spinor transformation shows that the corresponding spin mixing goes beyond finite-temperature smearing of the Fermi level, leading to a nonzero density of states in the gap of the insulating spin channel. As a consequence, the resistance ratio of the two spin channels changes from infinity to some finite value and, in a strict sense, half-metallic ferromagnetism is limited to zero temperature. Bloch-type spin waves and crystal imperfections contribute to the density of states in the gap but only partly explain the pronounced changes at about 0.2 TC observed in various half-metallic magnets. In the case of NiMnSb, the spin structure depends on a nearly dispersionless transverse optical mode that occurs at about 28 meV. In terms of 3 kBT, this corresponds to 103 K—very close to the temperature at which there is a dramatic loss in the Ni and Mn magnetization in NiMnSb. Similar modes exist in other potential half-metallic systems.

Competition between ferromagnetic and antiferromagnetic spin ordering in nuclear matter

In the framework of a Fermi liquid theory it is considered the possibility of ferromagnetic and antiferromagnetic phase transitions in symmetric nuclear matter with Skyrme effective interaction. The zero temperature dependence of ferromagnetic and antiferromagnetic spin polarization parameters as functions of density is found for SkM$^*$, SGII effective forces. It is shown that in the density domain, where both type of solutions of self--consistent equations exist, ferromagnetic spin state is more preferable than antiferromagnetic one.

GW study of half-metallic electronic structure of La 0.7Sr 0.3MnO 3

Half-metallic systems will be quite useful in future spin electronics. The GW approximation is employed to study electronic structure of half-metallic La 0.7Sr 0.3MnO 3, to show that the lowest quasiparticle energy of the unoccupied minority spin states is far above the Fermi energy compared to that in the LSDA. As a bulk, this system is predicted to be a fully spin-polarized half-metallic ferromagnet.

Hybrid density functional theory studies on the magnetic interactions and the weak covalent bonding for the phenalenyl radical dimeric pair.

The phenalenyl radical (1) is a prototype of the hydrocarbon radical. Recently, the single crystal of 2,5,8-tri-tert-butylphenalenyl (2) was isolated and showed that the two phenalenyl radicals form a staggered dimeric pair, giving rise to strong antiferromagnetic interactions. The origin of the antiferromagnetic interactions and the nature of the chemical bond for the dimeric pair are challenging issues for chemists. First, spin-polarized hybrid DFT (Becke's half and half LYP (UB2LYP)) and CASSCF calculations were performed for 2 and its simplified model, the staggered-stacking phenalenyl radical dimeric pair (3a), to elucidate the origin of the strong antiferromagnetic coupling and the characteristics of the chemical bond. The calculated results showed that a SOMO-SOMO overlap effect was responsible for the strong antiferromagnetic interactions and weak or intermediate covalent bonding between phenalenyl radicals. The tert-butyl groups introduced at three beta-positions hardly affected the magnetic coupling, mainly causing steric hindrances in the crystalline state. Next, to obtain insight into ferromagnetic stacking, we investigated the stacking effect of staggered (3a)- and eclipsed (3b)-stacking phenalenyl radical dimeric pairs with a change of the SOMO-SOMO overlap on the basis of the extended McConnell model. We found that the stacking mode of the dimeric pair with both a small SOMO-SOMO overlap and a ferromagnetic spin polarization effect provided a ferromagnetic coupling.

EXAFS and neutron diffraction study of the Heusler alloyCo2MnSi

Spintronic devices, devices that rely upon the spin of the conduction electron to control device properties, are of increasing scientific and commercial interest. These devices commonly are heterostructures with ferromagnetic metals used as spin injectors. The performance of these heterostructured devices can be optimized by using a 100% spin-polarized ferromagnet. Recent band-structure calculations predict that the Heusler alloy ${\mathrm{Co}}_{2}\mathrm{MnSi}$ should be a 100% spin-polarized ferromagnet. However, band-structure calculations further suggest that chemical disorder in the ${\mathrm{Co}}_{2}\mathrm{MnSi}$ lattice should lower the degree of spin polarization. In this work, we present neutron diffraction and extended x-ray-absorption fine-structure measurements (EXAFS) on bulk ${\mathrm{Co}}_{2}\mathrm{MnSi},$ which reveal extensive chemical disordering even in pure-phase samples and discuss the limitations of EXAFS in measuring this sort of disorder.

Disorder and interactions in quantum Hall ferromagnets: effects of disorder in Skyrmion physics

We present a Hartree–Fock study of the competition between disorder and interactions in quantum Hall ferromagnets near ν=1. We find that the ground state at ν=1 evolves with increasing interaction strength from a quasi-metallic paramagnet, to a partially spin-polarized ferromagnetic Anderson insulator, and to a fully spin-polarized ferromagnet with a charge gap. Away from ν=1, the ground state evolves from a conventional Anderson insulator, to a conventional quasiparticle glass, and finally to a ferromagnetic Skyrmion quasiparticle glass. These different regimes can be measured in low-temperature transport and NMR experiments. We present calculations for the NMR spectra in different disorder regimes.

Magnetic-field scaling of the conductance of epitaxial cuprate-manganite bilayers

Conductance-voltage characteristics of epitaxial interfaces between oxide ferromagnets and oxide superconductors have been measured as a function of temperature and magnetic field. Their functional form is similar to that predicted by theories of transport across nearly transparent contacts between highly spin-polarized ferromagnets and d-wave superconductors. However, their magnetic field dependencies scale in striking and unusual ways, challenging our current understanding. Existing theories fail to account for apparent nonequilibium effects that are natural for spin injection in such geometries.

Transport properties of normal and ferromagnetic atomic-size constrictions with superconducting electrodes

We present a theoretical approach to the transport properties of normal (N) and ferromagnetic (F) atomic-size contacts between superconducting (S) electrodes. This approach is based on non-equilibrium Green function techniques and a Hamiltonian description of the electron transfer processes in the constriction region. For a normal atomic-size contact the theory allows to relate the conduction channels to the atomic orbital structure. In the ferromagnetic case spin polarization gives rise to a spin dependent transmission distribution. For the case of S–N–S constrictions this theory predicts a d.c. current and shot noise in remarkable agreement with recent experimental results. We also analyze S–F and S–F–S constrictions. In this last case the current–voltage characteristic exhibits a very peculiar subgap structure associated to the competition between the multiple Andreev reflection mechanism and the ferromagnetic spin polarization.

Disorder and interactions in quantum Hall ferromagnets nearν=1

We report on a finite-size Hartree-Fock study of the competition between disorder and interactions in a two-dimensional electron gas near Landau level filling factor $\nu=1$. The ground state at $\nu=1$ evolves with increasing disorder from a fully spin-polarized ferromagnet with a charge gap, to a partially spin-polarized ferromagnetic Anderson insulator, to a quasi-metallic paramagnet at the critical point between $i=0$ and $i=2$ quantum Hall plateaus. Away from $\nu=1$, the ground state evolves from a ferromagnetic Skyrmion quasiparticle glass, to a conventional quasiparticle glass, and finally to a conventional Anderson insulator. We comment on signatures of these different regimes in low-temperature transport and NMR lineshape and peak position data.

Spin-polarized tunneling of La 0.67Sr 0.33MnO 3/YBa 2Cu 3O 7− δ junctions

The transport properties in contacts between ferromagnets and high- T c superconductors were investigated in La 0.67Sr 0.33MnO 3/YBa 2Cu 3O 7− δ cross-strip junctions. The conductance spectra show zero-bias conductance peaks (ZBCPs), reflecting charge transport along the ab plane. When an external magnetic field is applied to the junctions, the conductance spectra show two notable features, i.e., an increase in background conductance and an asymmetric field response of ZBCP whose width responds nonlinearly to the applied field. These features are shown to be qualitatively consistent with a theoretical prediction of tunneling spectroscopy, in which presence of a ferromagnetic insulator layer between a spin-polarized ferromagnet and a d-wave superconductor is assumed. Possible origins for the ferromagnetic insulator are discussed.

Spin-dependent transport in La 0.67Sr 0.33MnO 3/YBa 2Cu 3O 7− δ junctions

Transport properties across the interface between a ferromagnet and a high- T c superconductor are studied in La 0.67Sr 0.33MnO 3/YBa 2Cu 3O 7− δ (LSMO/YBCO) junctions. We observed magnetic-field response of conductance spectra peculiar to LSMO/YBCO junctions, i.e., increase of background conductance and asymmetric zero-bias conductance peak splitting. It is shown that these features are consistent with a theory which assumes the presence of a ferromagnetic insulating layer between a spin-polarized ferromagnet and a d-wave superconductor.

Preparation and characterization of thin ferromagnetic CrO 2 films for applications in magnetoelectronics

The theoretically predicted high spin polarization of half-metallic ferromagnets like CrO 2 and NiMnSb make them to promising materials for magnetoelectronic applications. Highly textured CrO 2 films have been prepared by chemical vapor deposition and molecular beam epitaxy. The temperature-dependent magnetotransport data has been correlated with electronic properties of CrO 2. Over a wide temperature range (150–330 K) the resistivity follows a T 2 behaviour, consistent with electron-electron scattering. Near the Curie temperature of CrO 2 no metal-insulator transition, a magnetoresistance of −7% ( B=9 T ) and an enhancement of the resistance due to electron-magnon scattering are observed.

Spin Imbalance and Magnetoresistance in Ferromagnet/Superconductor/Ferromagnet Double Tunnel Junctions

We theoretically study the spin-dependent transport in a ferromagnet/super- conductor/ferromagnet double tunnel junction. The tunneling current in the antiferromagnetic alignment of the magnetizations gives rise to a spin imbalance in the superconductor. The resulting nonequilibrium spin density strongly suppresses the superconductivity with increase of bias voltage and destroys it at a critical voltage Vc. The results provide a new method not only for measuring the spin polarization of ferromagnets but also for controlling superconductivity and tunnel magnetoresistance (TMR) by applying the bias voltage.

Magnetoresistance in La/sub 0.8/Sr/sub 0.2/MnO/sub 3-δ/ biepitaxial grain boundary junction

La/sub 0.8/Sr/sub 0.2/MnO/sub 3-/spl delta// (LSMO) films with 45/spl deg/ in-plane grain boundaries were fabricated using two types of biepitaxial multilayers. The low field MR (magnetoresistance) ratio of 32% at 77 K is a significant value due to the artificial grain boundary effect and it maybe useful for developing spin-dependent tunneling junctions using a LSMO biepitaxial grain boundary. The magnetoresistive properties at two different magnetic fields, parallel or perpendicular to the grain boundary junction, can be interpreted by the different ferromagnetic spin-polarization ordering in ab-plane.

Magnetic circular dichroism in photoemission by linear polarized light

For light in a general state of polarization incident in a general direction on a ferromagnetic cubic (001) surface, magnetic dichroism and spin polarization in valence-band photoemission normal to the surface are investigated analytically and numerically, taking into account the optical response of the solid in the classical Fresnel approximation. Evaluation of relativistic dipole matrix elements leads to explicit expressions which reveal details of the underlying physical mechanisms and provide general semiquantitative results. Using a fully relativistic Green function method, numerical results have been obtained for fct Ni(001) with magnetization perpendicular and parallel to the surface. For perpendicular magnetization and linearly polarized light with the electric field vector rotated out of the incidence plane, the optical response leads to substantial magnetic dichroism, the spectral shape of which is the same as in the case of circular polarization of the incident light.

Anomalous variation of optical spectra with spin polarization in double-exchange ferromagnet: La1-xSrxMnO3.

Anomalous variation of optical spectra with spin polarization in double-exchange ferromagnet: La1-xSrxMnO3.