Ferromagnetic Spin Polarization Research Articles

Switchable Quantum Anomalous Hall State in a Strongly Frustrated Lattice Magnet

We establish that the interplay of itinerant fermions with localized magnetic moments on a checkerboard lattice leads to magnetic flux phases. For weak itineracy the flux phase is coplanar and the electronic dispersion takes the shape of graphenelike Dirac fermions. Stronger itineracy drives the formation of a noncoplanar, chiral flux phase, in which the Dirac fermions acquire a topological mass that is proportional to a ferromagnetic spin polarization. Consequently the system self-organizes into a ferromagnetic quantum anomalous Hall state in which the direction of its dissipationless edge currents can be switched by an applied magnetic field.

Bias-induced destruction of ferromagnetism and disorder effects in GaMnAs heterostructures

The magneto-electric properties of resonant tunneling double barrier structures using GaMnAs for the quantum well is investigated within a self-consistent Green’s function approach and a tight-binding electronic structure model. The magnetic state of the well is determined self-consistently by the tunneling current which controls the hole spin density and, hence, the degree of exchange splitting of the subbands inside the well. Prompted by recent experiments we compare model systems of increasing defect concentration (substitutional disorder) regarding their I–V curve, magnetic state, and spin polarization. We predict that, near resonance, the ferromagnetic order which may be present at zero bias in the GaMnAs well tends to be destroyed. Resonance peaks are found to be more sensitive to disorder than ferromagnetic ordering and spin polarization of the steady-state current.

Observation of Nonconventional Spin Waves in Composite-Fermion Ferromagnets

We find unexpected low energy excitations of fully spin-polarized composite-fermion ferromagnets in the fractional quantum Hall liquid, resulting from a complex interplay between a topological order manifesting through new energy levels and a magnetic order due to spin polarization. The lowest energy modes, which involve spin reversal, are remarkable in displaying unconventional negative dispersion at small momenta followed by a deep roton minimum at larger momenta. This behavior results from a nontrivial mixing of spin-wave and spin-flip modes creating a spin-flip excitonic state of composite-fermion particle-hole pairs. The striking properties of spin-flip excitons imply highly tunable mode couplings that enable fine control of topological states of itinerant two-dimensional ferromagnets.

Novel spintronic device: terahertz magnon-photon laser

A novel method of generating THz radiation is proposed. The method is based on pumping of non-equilibrium electrons into the upper (spin-down) subband of spin-polarized half-metallic ferromagnets or ferromagnetic semiconductors. The electrons rapidly emit magnons with THz frequency, pass into highly excited states of the spin-up subband, and fall into the ground state due to interaction with the equilibrium spin-up electrons or emitting optical phonons. The mechanism of magnon generation is similar to a three-level laser, and at a critical pumping intensive magnon lasing begins. Merging of two magnons generates a THz photon. Thus, a magnon laser becomes a THz photon laser, the generated power being of orders of milliwatt vs. several microwatt power generated by contemporary devices. The generated frequency is material dependent, and can be changed by tuning the magnetic field and the bias. The proposed device may have many applications in different fields including medicine (cancer surgery), molecular imaging, security problems etc.

Theory of superconductor-ferromagnet point-contact spectra: The case of strong spin polarization

We study the impact of spin-active scattering on Andreev spectra of point contacts between superconductors (SC) and strongly spin-polarized ferromagnets (FM) using recently derived boundary conditions for the quasiclassical theory of superconductivity. We describe the interface region by a microscopic model for the interface scattering matrix. Our model includes both spin filtering and spin mixing and is nonperturbative in both transmission and spin polarization. We emphasize the importance of spin-mixing caused by interface scattering, which has been shown to be crucial for the creation of exotic pairing correlations in such structures. We provide estimates for the magnitude of this effect in different scenarios and discuss its dependence on various physical parameters. Our main finding is that the shape of the interface potential has a tremendous impact on the magnitude of the spin-mixing effect. Thus, all previous calculations, being based on delta-function or box-shaped interface potentials, underestimate this effect gravely. As a consequence, we find that with realistic interface potentials the spin-mixing effect can easily be large enough to cause spin-polarized subgap Andreev bound states in SC/FM point contacts. In addition, we show that our theory generalizes earlier models based on the Blonder-Tinkham-Klapwijk approach.

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.

The influence of localized excitations on spin-selective electron scattering and auger emission in transition metals

Observed spin asymmetry of polarized electrons backscattered from ferromagnetic metallic glasses and spin polarization of Auger electrons in magnetic transition metals can be explained qualitatively in terms of spin-selective scattering involving intermediate excitation of a transient 3p → 3d localized state. Consideration of photoemission data and of prior theoretical models indicates that this state decays selectively by Auger emission, producing the observed main MVV Auger peak. The final state of this transition is identified with the state whose direct excitation is observed in photoemission for the transition metals as a 6–7 eV satellite.

Spin-Dependent Cooper Pair Phase and Pure Spin Supercurrents in Strongly Polarized Ferromagnets

We study heterostructures of singlet superconductors and strongly spin-polarized ferromagnets and show that a relative phase arises between the superconducting proximity amplitudes in the two ferromagnetic spin bands. We find a tunable pure spin supercurrent in a spin-polarized ferromagnet contacted with only one superconductor electrode. We show that Josephson junctions are most effective for a spin polarization P approximately 0.3, and that critical currents for positive and negative bias differ for a high transmission Josephson junction, due to a relative phase between single and double pair transmission.

Temperature dependence of half-metallic hybridization gaps

The temperature dependence of the spin polarization of half-metallic ferromagnets is investigated by model calculations. Emphasis is on changes in the density of states, especially on the closure of the hybridization gap. The Stoner-type calculation yields a simple one-electron expression for the gap as a function of temperature and a corresponding transition temperature T*, above which the spin polarization is drastically reduced. The small parameter determining the finite-temperature breakdown of the spin splitting is the width of the hybridization gap relative to other bandwidth parameters. The model predictions are consistent with the experiment and with first-principles electronic structure calculations, and we argue that such band-structure effects are not limited to specific materials but rather widespread in hybridization-gap half-metals.

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.

Finite-temperature depolarization in half metals

The temperature dependence of the spin polarization of type-I half-metallic ferromagnets isinvestigated and compared with that of other magnetic materials, such as semimetals,strong and weak ferromagnets, and exchange-enhanced Pauli paramagnets. Stable atomicmoments, as realized by strong intra-atomic exchange, exhibit a nonzero spin-down densityof states (DOS) at finite temperatures. This thermal spin mixing means that theconductivity of the ‘insulating’ spin channel is always nonzero and that half-metallicferromagnetism is an idealized limit. At zero temperature, similar effects are caused byintersublattice interactions, spin–orbit coupling and crystal imperfections. With increasinginteratomic hopping, the moment becomes unstable, and Stoner-type thermal excitationsyield an additional reduction of the spin polarization. In the Stoner limit, the hybridizationgap closes far below the Curie temperature, and the corresponding transition temperatureT* increases with increasing hybridization gap and decreasing band width. Correlations areanalysed by a version of the Kondo model and by an unrestricted Hartree–Fockapproximation, and it is argued that correlations are less important than the leadingone-electron contributions.

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

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Integral and fractional quantum Hall Ising ferromagnets

We compare quantum Hall systems at filling factors $\ensuremath{\nu}=2$ to $\ensuremath{\nu}=\frac{2}{3}$ and $\frac{2}{5}$, corresponding to the exact filling of two lowest electron or composite fermion (CF) Landau levels. The two fractional states are examples of CF liquids with spin dynamics. There is a close analogy between the ferromagnetic (spin polarization $P=1$) and paramagnetic $(P=0)$ incompressible ground states that occur in all three systems in the limits of large and small Zeeman spin splitting. However, the excitation spectra are different. At $\ensuremath{\nu}=2$, we find spin domains at half-polarization $(P=\frac{1}{2})$, while antiferromagnetic order seems most favorable in the CF systems. The transition between $P=0$ and 1, as seen when, e.g., the magnetic field is tilted, is also studied by exact diagonalization in toroidal and spherical geometries. The essential role of an effective CF-CF interaction is discussed, and the experimentally observed incompressible half-polarized state is found in some models.

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.

Organometallic Benzene-Vanadium Wire: A One-Dimensional Half-Metallic Ferromagnet

Using density functional theory we perform theoretical investigations of the electronic properties of a freestanding one-dimensional organometallic vanadium-benzene wire. This system represents the limiting case of multidecker Vn(C6H6)(n+1) clusters which can be synthesized with established methods. We predict that the ground state of the wire is a 100% spin-polarized ferromagnet (half-metal). Its density of states is metallic at the Fermi energy for the minority electrons and shows a semiconductor gap for the majority electrons. We find that the half-metallic behavior is conserved up to 12% longitudinal elongation of the wire. Ab initio electron transport calculations reveal that finite size vanadium-benzene clusters coupled to ferromagnetic Ni or Co electrodes will work as nearly perfect spin filters.

Crossed Andreev Reflection-Induced Magnetoresistance

We show that very large negative magnetoresistance can be obtained in magnetic trilayers in a current-in-plane geometry owing to the existence of crossed Andreev reflection. This spin valve consists of a thin superconducting film sandwiched between two ferromagnetic layers whose magnetization is allowed to be either parallelly or antiparallelly aligned. For a suitable choice of structure parameters and nearly fully spin-polarized ferromagnets, the magnetoresistance can exceed -80%. Our results are relevant for the design and implementation of spintronic devices exploiting ferromagnet-superconductor structures.

Spin injection into MgB2

We report spin injection from nearly 100 % spin-polarized ferromagnet La0.67Ca0.33MnO3 into hot isostatically pressed MgB2. The injected spin-polarized current suppresses the supercurrent in MgB2 in the proportion of about 100 units of critical current for each unit of the injected current. Such unusually high current gain could pave the way for using MgB2 in spintronics applications. We overcome incompatibility of MgB2 and La0.67Ca0.33MnO3 by preparing a small-area ab-plane mechanical contact.

High spin polarization in the ferromagnetic filled skutteruditesKFe4Sb12andNaFe4Sb12

The spin polarization of ferromagnetic alkali-metal iron antimonides $\mathrm{K}{\mathrm{Fe}}_{4}{\mathrm{Sb}}_{12}$ and $\mathrm{Na}{\mathrm{Fe}}_{4}{\mathrm{Sb}}_{12}$ is studied by point-contact Andreev reflection using superconducting Nb and Pb tips. From these measurements an intrinsic transport spin polarization ${P}_{t}$ of 67% and 60% for the K and Na compound, respectively, is inferred, which establishes these materials as a class of highly spin-polarized ferromagnets. The results are in accord with band structure calculations within the local spin density approximation that predict nearly 100% spin polarization in the density of states. We discuss the impact of calculated Fermi velocities and spin fluctuations on ${P}_{t}$.

Theory of spin-polarized transport in ferromagnet–semiconductor structures: Unified description of ballistic and diffusive transport

A theory of spin-polarized electron transport in ferromagnet–semiconductor heterostructures, based on a unified semiclassical description of ballistic and diffusive transport in semiconductors, is outlined. The aim is to provide a framework for studying the interplay of spin relaxation and transport mechanism in spintronic devices. Transport inside the (nondegenerate) semiconductor is described in terms of a thermoballistic current, in which electrons move ballistically in the electric field arising from internal and external electrostatic potentials, and are thermalized at randomly distributed equilibration points. Spin relaxation is allowed to take place during the ballistic motion. For arbitrary potential profile and arbitrary values of the momentum and spin relaxation lengths, an integral equation for a spin transport function determining the spin polarization in the semiconductor is derived. For field-driven transport in a homogeneous semiconductor, the integral equation can be converted into a second-order differential equation that generalizes the spin drift-diffusion equation. The spin polarization in ferromagnet–semiconductor structures is obtained by matching the spin-resolved chemical potentials at the interfaces, with allowance for spin-selective interface resistances. Illustrative examples are considered.

Design of new ferromagnetic materials with high spin moments by first-principlescalculation

We have searched for a new highly spin-polarized ferromagnet which has ahigher spin moment than that of known half-metallic transition metal pnictideswith the zinc-blende structure by first-principles calculations. To generate thehigh spin moment we focus on Gd compounds. Our calculation shows that a(GdN)1/(CrAs)1 structure is a ferromagnetic material. The total magnetic moment of this ferromagnet is over9.9 µB per chemical formula.