Ferromagnetic Polarization Research Articles

Multiferroic Compounds with Double-Perovskite Structures.

New multiferroic compounds with double-perovskite structures were synthesized. Bi2NiMnO6 was synthesized in bulk form by high-pressure synthesis and also in a thin-film form by epitaxial growth. The material showed both ferromagnetic and ferroelectric properties, i.e., the multiferroic property at low temperature. Bi2FeCrO6 was also fabricated in a (1 1 1) oriented BiFeO3/BiCrO3 artificial superlattice, with a 1/1 stacking period. The superlattice film showed ferromagnetic behavior and polarization switching at room temperature. In the compounds, Bi3+ ion, located at the A site in the perovskite structure, caused ferroelectric structural distortion, and the B-site ordering of the Ni2+ and Mn4+ ions (Fe3+ and Cr3+ ions) in a rock-salt configuration led to ferromagnetism according to the Kanamori-Goodenough rule.

Characteristics of a high temperature vertical spin valve

We demonstrate high temperature electrical spin injection and detection in degenerately p-doped GaAs in vertical spin valves using valence band electron tunneling. The maximum measured magnetoresistance at 10 and 300 K is 40% and ∼1%, respectively. Spin relaxation in these devices was found to be relatively insensitive to temperature (T) for T>125 K. The spin injection and detection efficiencies are mostly dominated by the ferromagnetic contact polarization and spin independent transport at the ferromagnet/semiconductor interface.

Oscillatory Spin Polarization and Magneto-Optical Kerr Effect inFe3O4Thin Films on GaAs(001)

The spin dependent properties of epitaxial Fe₃O₄ thin films on GaAs(001) are studied by the ferromagnetic proximity polarization (FPP) effect and magneto-optical Kerr effect (MOKE). Both FPP and MOKE show oscillations with respect to Fe₃O₄ film thickness, and the oscillations are large enough to induce repeated sign reversals. We attribute the oscillatory behavior to spin-polarized quantum well states forming in the Fe₃O₄ film. Quantum confinement of the t(2g) states near the Fermi level provides an explanation for the similar thickness dependences of the FPP and MOKE oscillations.

Spin Control of Drifting Electrons Using Local Nuclear Polarization in Ferromagnet-Semiconductor Heterostructures

We demonstrate methods to locally control the spin rotation of moving electrons in a GaAs channel. The Larmor frequency of optically injected spins is modulated when the spins are dragged through a region of spin-polarized nuclei created at a MnAs/GaAs interface. The effective field created by the nuclei is controlled either optically or electrically using the ferromagnetic proximity polarization effect. Spin rotation is also tuned by controlling the carrier traverse time through the polarized region. We demonstrate coherent spin rotations of 5π rad during transport.

Charge transport and magnetization profile at the interface between the correlated metalCaRuO3and the antiferromagnetic insulatorCaMnO3

A combination of spectroscopic probes was used to develop a detailed experimental description of the transport and magnetic properties of superlattices composed of the paramagnetic metal CaRuO$_3$ and the antiferromagnetic insulator CaMnO$_3$. The charge carrier density and Ru valence state in the superlattices are not significantly different from those of bulk CaRuO$_3$. The small charge transfer across the interface implied by these observations confirms predictions derived from density functional calculations. However, a ferromagnetic polarization due to canted Mn spins penetrates 3-4 unit cells into CaMnO$_3$, far exceeding the corresponding predictions. The discrepancy may indicate the formation of magnetic polarons at the interface.

Chemical vapour deposition of graphene on Ni(111) and Co(0001) and intercalation with Au to study Dirac-cone formation and Rashba splitting

We show in detail monitoring by photoelectron spectroscopy how graphene can be grown by chemical vapour deposition on the transition-metal surfaces Ni(111) and Co(0001) and intercalated by a monoatomic layer of Au. For both systems, a linear E( k) dispersion of massless Dirac fermions appears in the graphene π-band in the vicinity of the Fermi energy. In order to study ferromagnetism and spin-orbit effects by spin- and angle- resolved photoelectron spectroscopy, the sample must be magnetized in remanence. To this end, a W(110) substrate is prepared, its cleanliness verified by photoemission from W(110) surface states and surface core levels, and epitaxial Ni(111) and Co(0001) thin films are grown on top. Spin-resolved photoemission from the π-band shows that the ferromagnetic polarization of graphene/Ni(111) and graphene/Co(0001) is negligible and that graphene on Ni(111) is after intercalation of Au spin-orbit split by the Rashba effect.

Magnetic structure of an imbalanced Fermi gas in an optical lattice

We analyze the repulsive fermionic Hubbard model on square and cubic lattices with spin imbalance and in the presence of a parabolic confinement. We analyze the magnetic structure as a function of the repulsive interaction strength and polarization. In the first part of the article, we perform unrestricted Hartree-Fock calculations for the two-dimensional (2D) case and find that above a critical interaction strength ${U}_{c}$ the system turns ferromagnetic at the edge of the trap, which is in agreement with the ferromagnetic Stoner instability of a homogeneous system away from half-filling. For $U<{U}_{c}$, we find a canted antiferromagnetic structure in the Mott region in the center and a partially polarized compressible edge. The antiferromagnetic order in the Mott plateau is perpendicular to the direction of the imbalance. In this regime, the same qualitative behavior is expected for 2D and three-dimensional (3D) systems. In the second part of the article, we give a general discussion of magnetic structures above ${U}_{c}$. We argue that spin conservation leads to nontrivial textures, both in the ferromagnetic polarization at the edge and for the N\'eel order in the Mott plateau. We discuss differences in magnetic structures for 2D and 3D cases.

Localization in an inhomogeneous quantum wire

We study interaction-induced localization of electrons in an inhomogeneous quasi-one-dimensional system--a wire with two regions, one at low density and the other high. Quantum Monte Carlo techniques are used to treat the strong Coulomb interactions in the low density region, where localization of electrons occurs. The nature of the transition from high to low density depends on the density gradient--if it is steep, a barrier develops between the two regions, causing Coulomb blockade effects. Ferromagnetic spin polarization does not appear for any parameters studied. The picture emerging here is in good agreement with measurements of tunneling between two wires.

Numerical study oft2gorbital system with ferromagnetic polarization

Finite-temperature orbital state in a ferromagnetic Mott insulator with triply degenerate ${t}_{2g}$ orbital is investigated numerically. We employ the quantum Monte Carlo simulation with the loop algorithm. Indications for conventional staggered-type orbital order are not remarkable down to the lowest temperature to which the present simulation can get access. Physical parameters monitoring the off-diagonal orbital order, which is characterized by a linear combination of the $({d}_{yz},{d}_{zx},{d}_{xy})$ orbital-wave functions with equal weights, are not conspicuous. It is found that an orbital gaplike behavior appears in the uniform orbital susceptibility. This is supported by a threshold behavior in the staggered correlation function in a calculation with the additional Ising-type interaction. Some rigorous remarks for the long-range orbital order are also presented.

Spin-flip scattering in time-dependent transport through a quantum dot: Enhanced spin-current and inverse tunneling magnetoresistance

We study the effects of spin-flip scatterings on the time-dependent transport properties through a magnetic quantum dot attached to normal and ferromagnetic leads. The transient spin-dynamics as well as the steady-state tunneling magnetoresistance (TMR) of the system are investigated. The absence of a definite spin quantization axis requires the time-propagation of two-component spinors. We present numerical results in which the electrodes are treated both as one-dimensional tightbinding wires and in the wide-band limit approximation. In the latter case we derive a transparent analytic formula for the spin-resolved current, and transient oscillations damped over different timescales are identified. We also find a novel regime for the TMR inversion. For any given strength of the spin-flip coupling the TMR becomes negative provided the ferromagnetic polarization is larger than some critical value. Finally we show how the full knowledge of the transient response allows for enhancing the spin-current by properly tuning the period of a pulsed bias.

Influence of pressure on the stability of the magnetically ordered states in alloys of the system Mn2−xFexAs0.5P0.5

The phase diagram of the system Mn2−xFexAs0.5P0.5 under pressure is investigated experimentally and theoretically. It is found that the spontaneous and magnetic-field-induced low-temperature phase in the region 0.5⩽x<0.8 does not suffer significant changes under hydrostatic pressure up to 2kbar. Based on ab initio calculations of the electronic structure of the alloys Mn1.5Fe0.5As0.5P0.5, it is established that the degree of filling of the 3d electron band changes upon ferromagnetic polarization and compression of the crystal lattice. A model is proposed by which one can take into account the main features of the antiferromagnetic and canted ferromagnetic structures. The parameters of the model are the degree of filling of the d band, the nonmagnetic electronic density of states, and the intra-atomic exchange integral. Their values are estimated directly from the data of the first-principles electronic structure calculations. It is shown in the framework of the model that the stability of the magnetic characteristics of the canted ferromagnetic structure with respect to pressure is due to an increase of the number of electrons in the magnetically active band upon a decrease of the unit cell volume.

Inversion of Ferromagnetic Proximity Polarization by MgO Interlayers

We investigate the spin-dependent reflection properties in Fe/MgO/GaAs heterostructures by optical pump-probe measurement of the ferromagnetic proximity polarization (FPP). As a function of MgO thickness, the FPP is initially enhanced (<2.0 A) and then exhibits an unexpected sign reversal at approximately 5.0 A. The identification of two competing thresholds in the intensity dependence of FPP and the observation of FPP sign reversal in Fe/Mg/GaAs suggest that the inversion of FPP is related to an interfacial bonding effect.

Coexistence of ferromagnetism and superconductivity in Sn nanoparticles

We report on the observations of ferromagnetic spin polarized moment and superconductivity in Sn nanoparticles. Ferromagnetic spin polarization, which is characterized by the appearance of Langevin magnetic-field profiles for magnetization, persists even at room temperature. At temperatures below $15\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, magnetic hysteresis emerges in the weak applied magnetic-field regime. Enhanced superconductivity is found for particles with diameters smaller than $16\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ but larger than $9\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The presence of a magnetic field in the superconducting phase reveals a regime in which the magnetic susceptibility and magnetization increase with decreasing temperature. We attribute these behaviors to the coexistence of ferromagnetic spin polarized moment and superconductivity at low temperatures.

Synthesis, Structure, and Magnetic Properties of a New Three‐Dimensional Iron Phosphite, [C4N2H12][Fe4(H2O)3(HPO3)7]·(H2O)xx = 0.6

AbstractA new open‐framework iron phosphite, [C4N2H12][Fe4(H2O)3(HPO3)7]·(H2O)x x = 0.6 (1), has been prepared by the hydrothermal reaction between FeCl3·6H2O, H3PO3, and piperazine. The three‐dimensional structure is comprised of a network of FeO6, FeO5(H2O), and HPO3 polyhedral units connected through their vertices. The structure has layers formed by the well‐known 4.6.12‐net connectivity and is stacked in an AAAA... sequence, penetrated by the one‐dimensional pillar units, forming a three‐dimensional pillared‐layer structure. This connectivity gives rise to two independent 12‐ring channel systems. Magnetic studies indicate antiferromagnetic behavior at high temperatures, with a small ferromagnetic polarization at low temperatures.(© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2008)

Spin Polarization and Surface Effects on Au- and Some Other Noble Metal Nanoparticles

Ferromagnetic spin polarizations on the samples of Au nanoparticles surrounded by protectives were observed by MCD method on X-ray (XMCD) of Au L3-edgge to investigate the origin of the spin polarization. The consistency between both magnetization data by SQUID and XMCD are quite well and the result means that the observed magnetization is originated from the Au-nanoparticle itself. The protectives make some slight effect on the magnetization, but the protective of DT (Dodecane thiol) makes quite big reduction of the magnetization. The protective dependence directly means that the origin of the spin polarization arises from the surface of the nanoparticles. Diameter dependence of the surface magnetization on Au-nanoparticles is linearly decreasing with increasing diameter and the data sharply close to the value of bulk state Au-metal at the diameter of 4 nm. The results also make us infer close relation between surface and magnetization and the possibility of wide applications of nanosized materials.

Magnetically induced dynamical stability of a Fe monolayer on W(110)

The magnetic and structural properties of the pseudomorphic iron monolayer on tungsten substrate were studied within the density functional generalized gradient approximation. The existing strains originating from significant mismatch between iron and tungsten layers were determined for both magnetic and nonmagnetic systems. By studying the spectrum of surface phonons within the direct method, a strong dependence of lattice dynamics on magnetic interactions has been revealed. In the nonmagnetic state, the in-plane surface modes soften along the [001] direction and lead to surface reconstruction. The ferromagnetic interactions were found to be the crucial element in stabilizing the iron monolayer on tungsten. The mechanism enhancing the ferromagnetic polarization of the surface and the soft mode behavior are discussed on the basis of the calculated changes in the electronic density of states.

Synthesis, Structure, and Magnetic Properties of a Novel Pillared Layered Iron(III) Arsenate, [4,4‘-bpyH2]3[Fe9(H2O)6F3(HAsO4)12(AsO4)2]·2H2O

A three-dimensional iron arsenate [4,4'-bpyH2]3[Fe9(H2O)6F3(HAsO4)12(AsO4)2].2H2O, 1, has been synthesized using 4,4'-bipyridine as the templating agent under hydrothermal conditions. The structure is formed by FeO6, FeO3F3 octahedral units connected with HAsO4 and AsO4 units, forming one- and two-dimensional units in which the one-dimensional units act as a pillar. The presence of face-shared Fe-dimer units in the one-dimensional unit is noteworthy. Detailed magnetic studies indicate two possible magnetic interactions: antiferromagnetic interactions within the layer and a weak ferromagnetic polarization between the layers at very low temperatures through the Fe-dimer units.

Direct Observation of the Ferromagnetic Spin Polarization in Gold Nanoparticles: A Review

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From quantum point contacts to quantum wires: Density-functional calculations with exchange and correlation effects

We numerically analyze the conductance and spin polarization of realistic quantum point contacts (QPCs) using density-functional theory, including both exchange and correlation effects. The self-consistent calculations are performed as a function of split gate voltage, for different temperatures and QPC lengths.We show that in short enough QPCs $(100\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ there is no spontaneous spin polarization, and the conductance for up-spin and down-spin electrons is the same. As the length of the QPC increases, so does the spin polarization and the difference in conductance between up-spin and down-spin electrons, resulting in an anomalous structure in the total conductance---the 0.7 anomaly. This structure moves from around 0.9 (in units of $2{e}^{2}∕h$) for a $200\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ QPC to slightly below 0.5 for a $400\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ QPC. Due to the strong ferromagnetic spin polarization in a long QPC, it will effectively work as a spin filter. The temperature dependence of the conductance is discussed in relation to the ``Reilly model,'' whose underlying assumption, regarding the shape of the spin gaps, is investigated using the self-consistent results.

Ab-initio study of giant moment reduction of Fe impurity in dilute [formula omitted

Electronic structure calculations based on density functional theory (DFT) using LSDA + U method are performed in order to investigate the local magnetic behaviour of isolated Fe impurity in dilute Pd 0.95 V 0.05 alloy. The calculations performed for different near neighbour configurations reveal that the giant magnetic moment of Fe in Pd is drastically suppressed in Pd 0.95 V 0.05 mainly due to reduction of the induced ferromagnetic polarization of the host Pd atoms and the development of a negative spin moment on the V atoms. The results are compared with experimental data obtained from γ -ray perturbed angular distribution measurements.