Low Spin Polarization Research Articles

Spin-polarized resonant surface state in (1 1 1) Sm1−xGdxAl2, a zero-magnetization ferromagnet

The electronic structure of (1 1 1) Sm1−xGdxAl2, a zero-magnetization ferromagnet, is investigated by angle- and spin- resolved photoemission spectroscopy. An intense electron pocket strongly localized around and close to the Fermi level is observed and analyzed in detail. Its various characteristics, combined with electronic structure calculations, reveal a resonant surface state of 5d character and Λ1 symmetry, likely built on bulk states developing around L points. It exhibits moreover a low temperature positive spin polarization at the Fermi level, of strong interest for spin-dependent transport properties in Sm1−xGdxAl2-based spintronic devices.

Mn2CrGa-based Heusler alloys with low net moment and high spin polarization

Mn2CrGa-based Heusler compounds combining small magnetization with high spin polarization, fabricated by Fe and Pt substitution for Cr, are investigated experimentally and discussed in terms of sublattice site occupancies. X-ray diffraction and electron diffraction indicate that the parent alloy and Mn2Cr0.6Fe0.4Ga are normal cubic Heusler alloys with substantial chemical disorder, whereas Mn2Cr0.6Pt0.4Ga crystallizes in the tetragonal D022 structure. Magnetization measurements indicate that the spin structure of the parent alloy is essentially ferrimagnetic, but alloying with Fe or Pt reduces the net magnetization per formula unit from 0.40 µB in the parent alloy to nearly zero in the Fe case. The Curie temperature, the magnetoresistance, and the Andreev-reflection transport-spin polarization are less affected by the substitutions, the last changing from 65% in Mn2CrGa to 60% and 61% in the Fe- and Pt-substituted alloys, respectively. The properties of these alloys are favorable for spintronics applications.

Novel Graphene-like Co2VAl (111): Case Study on Magnetoelectronic and Optical Properties by First-Principles Calculations

The electronic, magnetic, and optical properties of the Co2VAl (111) graphene-like (GL) monolayer as well as the (101) and (011) terminations have been calculated on the basis of the density functional theory (DFT) and FP-LAPW+lo method. The GL (111) Co2VAl has been grown in the (111)-crystallographic direction, leading to various and interesting physical properties compared to those of their considered directions. The films grown in the (101) and (011) directions have shown metallic behavior with low spin-polarization at the Fermi level, while the GL case has shown a perfect half-metallic behavior with an integer amount of magnetic moment (1.00 μB) and 0.5 eV spin-flip gap. Thus, it can be a good candidate for spintronic applications. Although all of these thin films behave similarly in the ultraviolet region, the GL case appears to be different in the infrared and visible regions. The main plasmonic energies occur in the 12–13.8 eV energy range in the parallel (xx) and perpendicular (zz) directions. Our...

Partial spin polarization of a conductance in a bi-layer [formula omitted] heterostructure based nanowire for the rectangular and the smooth lateral confinement potentials

We simulate the electron transport in a vertical bi-layer nanowire in order to study an influence of the lateral confinement's shape on a spin polarization of wire's conductance. The active part of considered quantum wire constitutes a double inverted heterojunction In0.52Al0.48As/In0.53Ga0.47As which nanostructure can be fabricated in molecular beam epitaxy process while the lateral confinement potential can be finally formed by means of cleaved overgrowth or surface oxidization methods giving the desired rectangular and smooth lateral confinement. In calculations we take into account interaction between charge carriers using DFT within local spin density approximation. We show that if the magnetic field is perpendicular to the wire axis, the pseudogaps are opened in energy dispersion relation E(k) what in conjunction with spin Zeeman shift of spin-up and spin-down subbands may enhance the spin polarization of conductance with reference to a single layer wire. For nanowire with rectangular lateral confinement potential we found that the electron density has two maximums localized at wire edges in each layers. This modificates strongly all magnetosubbands giving up to four energy minimums in lowest subband and considerably diminishes widths of pseudogaps what translates into low maximal spin polarization of conductance, not exceeding 40%. This drawback is absent in wire with smooth lateral confinement. However, in order to gain a large spin polarization simultaneous tuning of magnetic field as well as the Fermi energies in both layers of nanowire are required.

Equilibrium Chemical Disorder at the Surface of a Single-Crystal ${\rm C1}_{b}$ NiMnSb Half-Heusler Alloy: Implications for Spintronics

The structural and chemical short-range order in a NiMnSb Heusler-type single crystal is measured using extended X-ray absorption fine structure (EXAFS). A parameterized nonlinear least squares fitting of the Ni and Mn K and Sb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</sub> I EXAFS using theoretical EXAFS data generated via a multiple scattering approximation are compared with results from neutron diffraction data. EXAFS results are consistent with a C1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> structure with a quantifiable Ni-Mn chemical short-range disorder in the surface region of the Czochralski-grown single crystal that explains the often-reported low spin polarization.

Spin-charge separation in bipolar spin transport in (111) GaAs quantum wells

We present a microscopic theory for transport of the spin polarized charge density wave with both electrons and holes in the $(111)$ GaAs quantum wells. We analytically show that, contradicting to the commonly accepted belief, the spin and charge motions are bound together only in the fully polarized system but can be separated in the case of low spin polarization or short spin lifetime even when the spatial profiles of spin density wave and charge density wave overlap with each other. We further show that, the Coulomb drag between electrons and holes can markedly enhance the hole spin diffusion if the hole spin motion can be separated from the charge motion. In the high spin polarized system, the Coulomb drag can boost the hole spin diffusion coefficient by more than one order of magnitude.

The role of structure on magneto-transport properties of Heusler Co2MnSi films deposited on MgO(001)

We present an experimental study identifying structural reasons that degrade spin-polarization of Co2MnSi thin films deposited on MgO(001) substrates. Through the fabrication of magnetic tunnel junctions, we measure a range of values for tunneling magneto-resistance (TMR) ratios following post-deposition annealing and epitaxial crystallization of the Heusler film. These TMR ratios reflect qualitatively the change in spin polarization of the Co2MnSi thin films. Low-temperature annealing results in low spin-polarization due to a high fraction of an amorphous phase. As annealing temperatures increase, the fraction of L21 and B2 chemically ordered phases increases, thus improving significantly the spin-polarization. However, for samples annealed at higher temperatures, significant degradation in the cubic magneto-crystalline anisotropy is observed, which we attribute to the detection of manganese diffusion into the MgO substrate. This Mn diffusion is manifested in a reduction of the value of the TMR ratio, namely, the spin polarization. Additionally, the maximum TMR ratio measured here, approximately 65% at room-temperature, is limited because the semi-coherent interface of Co2MnSi with the MgO substrate terminates with a Mn-Si layer.

Grain boundary magnetism and conductivity in polycrystalline Ti1−xMnxN films by reactive sputtering

Polycrystalline Ti1−xMnxN films with different Mn atomic fractions (x) were grown by reactive facing-target sputtering. The preferred lattice orientation changes from (200) to (111) with the increase of x, which makes the surface morphology evolve from spherical to triangular-pyramid-like islands. Weak grain-boundary ferromagnetism with a Curie temperature of higher than 305K has been observed. The saturation magnetization (Ms) and coercivity do not show a monotonous change with x. Asymmetric M–H curves can be observed at low temperatures, and the shift of M–H curves decreases with the increase of temperature because of the relaxation of the low-temperature pinned moments. All of the films show semiconducting-like behavior with a mechanism of thermal fluctuation-induced tunneling. Magnetoresistance (MR) is negative, but is very small. The small Ms and MR can be ascribed to the weakly ferromagnetic interaction of moments at grain boundaries and low spin polarization.

Spin-resolved bunching and noise characteristics in double quantum dots coupled to ferromagnetic electrodes

We study spin-resolved noise in Coulomb blockaded double quantum dots coupled to ferromagnetic electrodes. The modulation of the interdot coupling and spin polarization in the electrodes gives rise to an intriguing dynamical spin ↑–↑ (↓–↓) blockade mechanism: bunching of up (down) spins due to dynamical blockade of an up (down) spin. In contrast to the conventional dynamical spin ↑–↓ bunching (bunching of up spins associated with a dynamical blockade of a down spin), this new bunching behavior is found to be intimately associated with the spin mutual-correlation, i.e. the noise fluctuation between opposite spin currents. We further demonstrate that the dynamical spin ↑–↑ and ↑–↓ bunching of tunneling events may be coexistent in the regime of weak interdot coupling and low spin polarization.

Generation and Detection of a Pure Spin Current Using Co-Based Heusler-Alloy Spin Injector and Detector: Comparison of Co2MnSi and Co2FeSi

We have demonstrated the electrical generation and detection of giant pure spin currents using lateral spin-valve devices (LSVs) with highly-ordered Co2FeSi (CFS) electrodes, where the CFS films were grown on Si(111) by low-temperature molecular beam epitaxy (LT-MBE). In this paper, we study the performance of LSVs with Co2MnSi (CMS) electrodes. Unlike CFS, we cannot obtain high-quality CMS films on Si(111) even in the same LT-MBE condition. As a result, it is difficult for CMS grown on Si(111) to generate a large pure spin current in LSVs because of a relatively poor crystal quality with low spin polarization compared with CFS grown on Si(111).

Exploiting level anti-crossings for efficient and selective transfer of hyperpolarization in coupled nuclear spin systems

Spin hyperpolarization can be coherently transferred to other nuclei in field-cycling NMR experiments. At low magnetic fields spin polarization is redistributed in a strongly coupled network of spins. Polarization transfer is most efficient at fields where level anti-crossings (LACs) occur for the nuclear spin-states. A further condition is that field switching to the LAC positions is non-adiabatic in order to convert the starting population differences into spin coherences that cause time-dependent mixing of states. The power of this method has been demonstrated by studying transfer of photo-Chemically Induced Dynamic Nuclear Polarization (photo-CIDNP) in N-acetyl-tryptophan. We have investigated the magnetic field dependence and time dependence of coherent CIDNP transfer and directly assessed nuclear spin LACs by studying polarization transfer at specific field positions. The proposed approach based on LACs is not limited to CIDNP but is advantageous for enhancing NMR signals by spin order transfer from any type of hyper-polarized nuclei.

Ultrafast magnetization dynamics in the half‐metallic Heusler alloy Co2Cr0.6Fe0.4Al

AbstractUltrafast demagnetization upon excitation with intense laser pulses has been observed for a variety of ferromagnetic materials. In half‐metals, long demagnetization times are expected due to the band gap for one spin direction. We have investigated the theoretically half‐metallic Heusler alloy Co2Cr0.6Fe0.4Al (CCFA), using the time‐resolved magnetooptical Kerr effect. A demagnetization time in the range of typical transition metal ferromagnets with lower spin polarization has been found, while magnetization recovery proceeds on a slower time scale. The results are discussed in the context of recent models and experimental results. We propose that for Heusler alloys the initial stage of recovery could be a better measure for the spin polarization than the demagnetization time. A simple estimate leads to a bulk spin polarization value of 0.86 for CCFA.

Mn3−xGa (0 ≤ x ≤ 1): Multifunctional thin film materials for spintronics and magnetic recording

AbstractTetragonal Mn3−xGa (0 ≤ x ≤ 1) epitaxial films possess exceptional magnetic and electronic properties. Stoichiometric Mn3Ga crystallizes in the D022 structure (abstract figure) and is a collinear ferrimagnet with an easy c‐axis. It exhibits a unique combination of low magnetization, high uniaxial anisotropy, high Curie temperature and high spin polarization, which suit the requirements for spin torque memories down to 10 nm in size. Mn2Ga, on the other hand, exhibits much higher magnetization, high perpendicular anisotropy and high Curie temperature but a lower spin polarization, which make it a potential candidate for high density bit‐patterned recording with areal densities up to 10 Tb inch−2 (∼15 kb µm−2) and 10‐year thermal stability. The flexibility of the D022 structure allows a variety of magnetic materials to be synthesized with varying x to suit specific magnetic applications. Hexagonal D019‐Mn3Ga films are antiferromagnetic, which could be useful for exchange bias. magnified image (a) The tetragonal D022 unit cell of Mn3Ga. Ga atoms are ordered in a body‐centred tetragonal structure and Mn atoms occupy 2b (red) and 4d (green) sites. (b) The hexagonal D019 unit cell of Mn3Ga.

Observation of Higher Suppression of Superconducting Transition Temperature in YBa2Cu3O7−δ /La0.5Sr0.5CoO3 as Compared to YBa2Cu3O7−δ /La0.7Sr0.3MnO3 Bilayers

We report the experimental investigations of the suppression of superconductivity due to the pair breaking effect induced by ferromagnetic layer in YBa2Cu3O7−δ /La0.5Sr0.5CoO3 and YBa2Cu3O7−δ /La0.7Sr0.3MnO3 bi-layers fabricated by pulsed laser deposition. The current dependent electrical transport studies in the bilayers exhibit a significant reduction in the superconducting transition temperature in accord to I 2/3 law as compared to single YBa2Cu3O7−δ layer, and moreover the superconducting transition temperature in YBa2Cu3O7−δ /La0.7Sr0.3MnO3 bilayer is surprisingly found to be much larger than the YBa2Cu3O7−δ /La0.5Sr0.5CoO3. It appears that the current driven from a low spin polarization (−11%) material like La0.5Sr0.5CoO3 can also suppress the superconductivity to a larger extent. This indicates that the degree of spin polarization of the ferromagnetic electrode is not the only criteria to determine the suppression of superconductivity by pair breaking effect in superconductor/ferromagnet hybrid structures; rather the transparency of the interface for the spin polarization, the formation of vortex state due to the stray field of ferromagnetic layer and the ferromagnetic domain patterns might play significant roles to determine such effect.

Spin injection and circular polarized electroluminescence from InAs-based spin-light emitting diode structures

We have investigated circularly polarized electroluminescence (EL) from hybrid II-Mn-VI/III–V light emitting diodes (LED’s) at low temperatures in magnetic fields upto 10 T. Both magnetic (the Brillouin paramagnet Cd1−xMnxSe) and nonmagnetic (CdSe) injectors were studied. Electrons, spin unpolarized (n-CdSe) or spin-polarized (n-CdMnSe), were injected into wide InAs quantum wells, where they recombined with unpolarized holes injected from p-type InAs/AlAsSb layers. Detailed measurements and modeling of the circular polarization of the resulting midinfrared EL were carried out to explore and quantify the additional complexities of this materials system compared with the extensively studied GaAs-based spin-LED structures. We show that optical and spin polarization in narrow gap semiconductors such as InAs are not simply related to each other. To analyze the complex relationship, we have developed and used a detailed rate equation model, which incorporates the band-structure of electrons and holes in a magnetic field, a finite ratio of recombination and spin-flip times, and the spin polarization of the CdMnSe spin-aligner as a function of injection current. The latter was determined in situ by circular polarized photoluminescence measurements on the injector material. Experimentally, the circular polarization degrees of magnetic and nonmagnetic structures are observed to be very similar, when the magnetic samples have low effective Mn incorporation. This results from a combination of the consequently low spin polarization of the aligner and comparable spin and recombination life times in InAs.

Spin Filtering by Field-Dependent Resonant Tunneling

We consider theoretically transport in a spinful one-channel interacting quantum wire placed in an external magnetic field. For the case of two pointlike impurities embedded in the wire, under a small voltage bias the spin-polarized current occurs at special points in the parameter space, tunable by a single parameter. At sufficiently low temperatures complete spin polarization may be achieved, provided repulsive interaction between electrons is not too strong.

Off-stoichiometry in Co2FeSi thin films sputtered from stoichiometric targets revealed by nuclear magnetic resonance

Co2FeSi is predicted to be a half-metallic ferromagnet with an extraordinary high magnetic moment and Curie temperature. However, a low tunnel magneto-resistance ratio, a lower spin polarization and a lower magnetic moment were experimentally observed in thin film samples. Consequently, thin Co2FeSi films of different groups were studied using spin-echo nuclear magnetic resonance (NMR). NMR probes the local hyperfine fields of the active atoms, which strongly depend on the local environment. NMR is thus able to reveal the next neighbouring shells of the 59Co nuclei in the Co2FeSi thin films. As expected, our NMR study shows the main resonance line corresponding to 59Co nuclei in the L21 environment but also additional resonance lines at the high frequency site of the main line with a spacing between adjacent resonance lines of 32 MHz. The additional resonance lines correspond to 59Co with more Fe next neighbours than expected for the L21 type ordering, which is interpreted as the formation of an off-stoichiometric film yielded by sputtering from a stoichiometric target. The analysis of the NMR data and a comparison with off-stoichiometric bulk samples reveals the composition of the thin films to be about Co2Fe(Si0.92Fe0.08). This off-stoichiometry might explain the observed deviations from the expected behaviour.

Effect of Singwi–Tosi–Land–Sjölander local field correction on spin relaxation in n-type GaAs quantum wells at low temperature

We study the effect of the Singwi–Tosi–Land–Sjölander local field correction on spin relaxation/dephasing in n-type GaAs quantum wells at low temperature by constructing and numerically solving the kinetic spin Bloch equations. We calculate the local field factor G ( q ) in quantum wells by numerically solving three equations which link the local field factor, the structure factor, and the dielectric function, self-consistently. Such a correction reduces both the electron–electron Coulomb scattering and the Coulomb Hartree–Fock term. We compare the spin relaxation times with and without this correction under different conditions such as temperature, electron density, well width and spin polarization. We find that this correction leads to a decrease/increase of the spin relaxation time in the strong/weak scattering limit. At high spin polarization, it reduces the Hartree–Fock term and consequently tends to decrease the spin relaxation time. The modification of the spin relaxation time by the local field correction is more or less moderate either due to the coexistence of scattering other than the Coulomb scattering at low spin polarization and/or due to the competing effects from the Coulomb scattering and the Coulomb Hartree–Fock term at high polarization.

Microwave generation in MgO magnetic tunnel junctions due to spin transfer effects (invited)

Low resistance-area product and high spin polarization lead to current-driven precession of the magnetization in CoFeB∕MgO∕CoFeB magnetic tunnel junctions due to spin-transfer torque (STT) effects. Current-driven precession of the magnetization leads to pronounced narrow peaks in the frequency range of 4–7GHz with a full width at half maximum as low as 21MHz. The peak amplitudes have a threshold dependence on the dc bias current. Experimental results show that the STT-driven microwave generation can also occur in MgO-based junctions at maximum resistance state but at opposite current polarity, which corresponds to precession of the magnetization of the reference layer (RL) electrode. This conclusion is supported by the peak frequency dependence on magnetic field. The maximum generated power was 35nW at a peak frequency of about 6GHz. The estimated maximum angle change of the RL in-plane magnetization rotation is 19° and corresponds to a large angle precession.

Ab initio Studies on Magnetism of 3d Transition Metal Dimers

Ab initio calculations with the self-consistent full-potential linearized augmented-plane-wave method (FLAPW), under generalized gradient approximation, have been carried out to describe the electronic and magnetic properties of 3d transition metal dimers. It predicted the antiferromagneticity of Cr2 and ferromagneticity of other species. The Mn2 dimer was shown to be ferromagnetic coupling with a local magnetic moment of 5μB, retaining the value of its free atom state. The V2 and Ni2 exhibited low spin-polarization with local magnetic moment of only 1μB, per atom. On the other hand, Fe2 and Co2 were highly spin-polarized with local magnetic moments of 3 and 2μB.