Ferromagnetic Polarization Research Articles

Bead-like Co/C@MoS2Composites connected by Multi-Walled carbon nanotubes for high-efficiency electromagnetic wave absorption

This study presents a novel approach to synthesize a composite material composed of bead-like Co/C nanoparticles anchored on MWCNTs, encapsulated by MoS2 nanosheets through a combination of carbonization and hydrothermal treatment using ZIF-67 as the precursor. The resulting composite demonstrates exceptional electromagnetic wave (EMW) absorption performance, with a minimum reflection loss (RL) of −63 dB at a thickness of 2.44 mm and an effective absorption bandwidth (EAB) of 7.2 GHz when RL is below 10 dB together with an expanded EAB of 12.3 GHz (d = 5 mm).The superior absorption capabilities arise from a well-thought-out design of material composition and structure. The synergy of Co/C and MWCNTs enhances ferromagnetic resonance, conductivity, and polarization loss, while sulfur vacancies in MoS2 act as polarization centers, facilitating dipole polarization and efficient conversion of electromagnetic wave energy. The core–shell structure and heterogeneous interfaces introduce various energy loss mechanisms, including multiple polarizations, scattering effects, and dielectric loss. This work introduces a facile method for synthesizing a composite material with outstanding EMW absorption performance. The synergistic combination of unique material components and the carefully engineered structure make this composite a promising candidate for advanced applications in electromagnetic wave absorption technology.

What lies beneath? Investigations of atomic force microscopy-based nano-machining to reveal sub-surface ferroelectric domain configurations in ultrathin films

Multiferroic materials, encompassing simultaneous ferroelectric and ferromagnetic polarization states, are enticing multi-state materials for memory scaling beyond existing technologies. Aurivillius phase B6TFMO (Bi6TixFeyMnzO18) is a unique room temperature multiferroic material that could ideally be suited to future production of revolutionary memory devices. As miniaturization of electronic devices continues, it is crucial to characterize ferroelectric domain configurations at very small (sub-10 nm) thickness. Direct liquid injection chemical vapor deposition allows for frontier development of ultrathin films at fundamental (close to unit cell) dimensions. However, layer-by-layer growth of ultrathin complex oxides is subject to the formation of surface contaminants and 2D islands and pits, which can obscure visualization of domain patterns using piezoresponse force microscopy (PFM). Herein, we apply force from a sufficiently stiff diamond cantilever while scanning over ultrathin films to perform atomic force microscopy (AFM)-based nano-machining of the surface layers. Subsequent lateral PFM imaging of sub-surface layers uncovers 45° orientated striped twin domains, entirely distinct from the randomly configured piezoresponse observed for the pristine film surface. Furthermore, our investigations indicate that these sub-surface domain structures persist along the in-plane directions throughout the film depth down to thicknesses of less than half of an Aurivillius phase unit cell (˂ 2.5 nm). Thus, AFM-based nano-machining in conjunction with PFM allows demonstration of stable in-plane ferroelectric domains at thicknesses lower than previously determined for multiferroic B6TFMO. These findings demonstrate the technological potential of Aurivillius phase B6TFMO for future miniaturized memory storage devices. Next-generation devices based on ultrathin multiferroic tunnel junctions are projected.

Half-metallic p0-d half-Heusler alloys with stable structure in ferromagnetic state

We investigate the structural, magnetic, thermodynamic, and mechanical stability of p0-d half-Heusler compounds composed of p0 = Li, Be, Na, Mg alkali and alkaline earth metals; 3d transition metals; and group V (pnicogens) & VI (chalcogens) sp atoms using the DFT based full-potential Wien2k package. Ferromagnetic, antiferromagnetic, and non-magnetic calculations are performed to ascertain the stable magnetic state. From 280 compounds under study, 16 of them show half-metallic properties in the minimum-energy half-Heusler phase at the optimized lattice constant. Calculations show that compounds are stable in the half-Heusler β-phase and the ferromagnetic state. The band structure and the density of states are obtained for both GGA and GGA + U exchange correlation approximations. Furthermore, in order to study the thermodynamic and the mechanical (dynamic) stability of the compounds, the formation energy, the hull distance, and the elastic constants of half-metallic compounds are calculated. The estimated coupling constants and Curie temperatures from Monte Carlo simulations show that these compounds preserve the ferromagnetic state and magnetic polarization at temperatures much higher than the room temperature and may well be suitable materials for spintronic applications. The phonon dispersion curve for NaVBi as the compound that has the highest Curie temperature among half-metallic compounds shows no imaginary frequencies and testifies to the dynamic stability of the compound in the ferromagnetic half-Heusler β-phase as expected from the elastic constants.

Axion-matter coupling in multiferroics

Multiferroics (MFs) are materials with two or more ferroic orders, like spontaneous ferroelectric and ferromagnetic polarizations. Such materials can exhibit a magnetoelectric effect whereby magnetic and ferroelectric polarizations couple linearly, reminiscent of, but not identical to the electromagnetic $\mathbit{E}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbit{B}$ axion coupling. Here we point out a possible mechanism in which an external dark matter axion field couples linearly to ferroic orders in these materials without external applied fields. We find the magnetic response to be linear in the axion-electron coupling. At temperatures close to the ferromagnetic transition fluctuations can lead to an enhancement of the axion-induced magnetic response. Relevant material candidates such as the Lu-Sc hexaferrite family are discussed.

Half- and quarter-metals in rhombohedral trilayer graphene.

Ferromagnetism is most common in transition metal compounds where electrons occupy highly localized d orbitals. However, ferromagnetic order may also arise in low-density two-dimensional electron systems1-5. Here we show that gate-tuned van Hove singularities in rhombohedral trilayer graphene6 drive spontaneous ferromagnetic polarization of the electron system into one or more spin and valley flavours. Using capacitance and transport measurements, we observe a cascade of transitions tuned to the density and electronic displacement field between phases in which quantum oscillations have fourfold, twofold or onefold degeneracy, associated with a spin- and valley-degenerate normal metal, spin-polarized 'half-metal', and spin- and valley-polarized 'quarter-metal', respectively. For electron doping, the salient features of the data are well captured by a phenomenological Stoner model7 that includes valley-anisotropic interactions. For hole filling, we observe a richer phase diagram featuring a delicate interplay of broken symmetries and transitions in the Fermi surface topology. Finally, we introduce a moiré superlattice using a rotationally aligned hexagonal boron nitride substrate5,8. Remarkably, we find that the isospin order is only weakly perturbed, with the moiré potential catalysing the formation of topologically nontrivial gapped states whenever itinerant half- or quarter-metal states occur at half- or quarter-superlattice band filling. Our results show that rhombohedral graphene is an ideal platform for well-controlled tests of many-body theory, and reveal magnetism in moiré materials4,5,9,10 to be fundamentally itinerant in nature.

Evidence for new enantiospecific interaction force in chiral biomolecules

Evidence for new enantiospecific interaction force in chiral biomolecules

Spin filtering and spin separation in 2D materials by topological spin Hall effect

The needs of high speed performance electronic devices for various applications require novel materials and new physical phenomena. For these purposes we propose to study new physical effects based on electron scattering on magnetic skyrmions and vortices distributed in a 2D ferromagnetic material. We show that the topological spin Hall effect can be efficiently employed for the filtering, switching, and separation of spin currents. For some values of the parameters (conduction electron concentrations, and skyrmion/vortex sizes) it is possible to separate Hall currents for different electron spin projections as it is like for different carrier charges (electrons and holes) in the normal Hall effect. The calculations are performed using the Boltzmann kinetic equation for the nonequilibrium distribution function and the Lippmann–Schwinger equation for the transition matrix in the whole range of the adiabaticity parameter. The spin filtering due to the skyrmion/vortex scattering can be several orders of magnitude more efficient in the narrow range of the electron concentrations than that of the ordinary ferromagnetic spin polarization in spintronics.

A Josephson phase battery.

A classical battery converts chemical energy into a persistent voltage bias that can power electronic circuits. Similarly, a phase battery is a quantum device that provides a persistent phase bias to the wave function of a quantum circuit. It represents a key element for quantum technologies based on phase coherence. Here we demonstrate a phase battery in a hybrid superconducting circuit. It consists of an n-doped InAs nanowire with unpaired-spin surface states, that is proximitized by Al superconducting leads. We find that the ferromagnetic polarization of the unpaired-spin states is efficiently converted into a persistent phase bias φ0 across the wire, leading to the anomalous Josephson effect1,2. We apply an external in-plane magnetic field and, thereby, achieve continuous tuning of φ0. Hence, we can charge and discharge the quantum phase battery. The observed symmetries of the anomalous Josephson effect in the vectorial magnetic field are in agreement with our theoretical model. Our results demonstrate how the combined action of spin-orbit coupling and exchange interaction induces a strong coupling between charge, spin and superconducting phase, able to break the phase rigidity of the system.

Bulk and surface DFT investigations of the electronic and magnetic properties of CsXNO (X [formula omitted] Mg, Ca and Sr) quaternary Heusler alloys

The structural, electronic and magnetic properties of the bulk and (001) surfaces of CsXNO (X = Mg, Ca and Sr) quaternary Heusler alloys are investigated. It is found that all of the compounds are half-metallic ferromagnets with a magnetic moment of 2μB which obeys the Slater–Pauling rule. The half-metallic gaps are calculated to be 0.84, 0.80 and 0.82eV for CsMgNO, CsCaNO and CsSrNO, respectively. It seems that the shift of the bands due to the rather strong p–p exchange interaction leads to half-metallic ferromagnetism in these compounds. They can also maintain their half-metallic properties under great stresses and the calculated Curie temperatures are 705.14, 504.85 and 488.82 K for CsMgNO, CsCaNO and CsSrNO, respectively. The (001) surfaces of the compounds are also simulated using the symmetric slab model. The results of the calculation of surface energies indicate that NO-terminated surface is more stable than CsX-terminated surface. The half-metallicity is also well preserved on the NO-terminated surface, however the bulk half-metallicity is destroyed by the surface states on the CsX-terminated slab. Considering the 100 % ferromagnetic spin polarization in bulk and surface beside high Curie temperature make these materials excellent candidates for spintronic applications.

Experimental observation of coupled valley and spin Hall effect in p‐doped WSe2 devices

AbstractIt is generally accepted that perpendicular magnetic anisotropy (PMA) magnets are preferred over in‐plane magnetic anisotropy (IMA) magnets in data storage applications owing to their large thermal stability even at ultra‐scaled dimensions. To efficiently and deterministically switch a PMA, out‐of‐plan spin current is required. However, conventional giant spin Hall effect (GSHE) materials only produce spin currents with in‐plane polarization. Our work, on the other hand, demonstrated for the first time that out‐of‐plane spin currents can indeed be generated by applying electric filed to monolayer WSe2 at room temperature and the spin information was detected by a graphene channel. Supported by theoretical calculations, we found that the interfacial polarization between graphene and WSe2 to be ~38% assuming the ferromagnetic detector's polarization efficiency to be ~4%. Moreover, the spin polarization is determined by the polarity of the applied electric field.image

Anisotropic current induced in topological surface states due to spin-polarized tunneling from a ferromagnet

In clean tunneling junctions (TJs), the momentum parallel to the barrier is conserved in the tunneling process. We predict that this phenomenon can be observed in a TJ between a ferromagnet (FM) and a topological insulator (TI). The FM possesses spin-polarized electron states, while the TI has spin-momentum locked electron states. The tunneling probability is determined by the spin-matrix element between these electron states and leads to the injection of electrons into the TI with finite momentum parallel to the topological surface state. Due to the spin polarization of the electrons in the FM, the resulting current in the TI is not isotropic. The magnitude of the current is calculated quantitatively in terms of the ferromagnetic polarization and should be experimentally detectable using only an electrical setup.

Magneto-thermal transport behavior in freestanding Ni80Fe20/Au bilayer thin films

Spin polarization, when induced in a non-ferromagnetic material, can change the underlying material behavior especially if the spin diffusion length is of the same order as the sample dimension such as thickness. In this study, we experimentally demonstrate thermal hysteretic behavior induced by spin polarization in Ni80Fe20 (10 nm)/Au (100 nm) bilayer freestanding sample. The thermal hysteresis behavior is uncovered using magneto thermal characterization based on self-heating 3ω method. The third harmonic voltage shows diverging behavior and thermal hysteresis during cooling and heating of the sample under an applied magnetic field, which is attributed to the spin accumulation. The spin accumulation and thermal hysteresis in Au occurs due to ferromagnetic proximity polarization from Ni80Fe20 layer. The observed hysteresis behavior is also attributed to freestanding thin film structure and absence of substrate effects leading to longer spin diffusion length. This study demonstrates experimental evidence of ferromagnetic proximity polarization and resulting changes in thermal transport behavior in Au thin films.

Solids of quantum Hall skyrmions in graphene

Partially filled Landau levels host competing orders, with electron solids prevailing close to integer fillings before giving way to fractional quantum Hall liquids as the Landau level fills. Here, we report the observation of an electron solid with noncolinear spin texture in monolayer graphene, consistent with solidification of skyrmions---topological spin textures characterized by quantized electrical charge. In electrical transport, electron solids reveal themselves through a rapid metal-insulator transition in the bulk electrical conductivity as the temperature is lowered, accompanied by the emergence of strongly nonlinear dependence on the applied bias voltage. We probe the spin texture of the solids using a modified Corbino geometry that allows ferromagnetic magnons to be launched and detected. We find that magnon transport is highly efficient when one Landau level is filled ($\nu=1$), consistent with quantum Hall ferromagnetic spin polarization; however, even minimal doping immediately quenches the the magnon signal while leaving the vanishing low-temperature charge conductivity unchanged. Our results can be understood by the formation of a solid of charged skyrmions near $\nu=1$, whose noncolinear spin texture leads to rapid magnon decay. Data near fractional fillings further shows evidence for several fractional skyrmion crystals, suggesting that graphene hosts a vast and highly tunable landscape of coupled spin and charge orders.

Spin polarization in the phase diagram of a Li\u2013Fe\u2013S system

Divalent and trivalent states of Fe ions are known to be stable in inorganic compounds. We focus a novel LixFeS5 cathode, in which the Li content (x) changes from 2 to 10 by an electrochemical technique. As x increases from 2, a Pauli paramagnetic conductive Li2FeS5 phase changes into a superparamagnetic insulating Li10FeS5 phase. Density functional theory calculations suggest that Fe+ ions in a high-x phase are responsible for ferromagnetic spin polarization. Reaching the monovalent Fe ion is significant for understanding microscopic chemistry behind operation as Li-ion batteries and the original physical properties resulting from the unique local structure.

Local magnetism and spin fluctuation behavior of extremely dilute Fe impurity in Pd-U binary alloys: An experimental and theoretical investigation

We report the magnetic behavior of isolated Fe impurity atoms in Pd1−xUx (0 ≤ x ≤ 0.12) alloys studied through measurements of the local susceptibility (χloc) and spin relaxation time for 54Fe nuclear probe using the time differential perturbed angular distribution (TDPAD) technique. With an increase of the U concentration in Pd-U alloys, the magnetic response of Fe, exhibiting Curie-Weiss type χloc(T), reflects a steady decrease of the giant magnetic moment arising from ferromagnetic polarization of host Pd-4d band electrons. Concomitantly, we find an exponential rise in the spin fluctuation temperature TSF. The experimental observation of U induced suppression of giant magnetic moment of Fe in Pd is supported by ab intio electronic structure calculations based on density functional theory. The results presented here support the idea that sign and strength of spin polarization of host conduction band electrons play a crucial role on the spin fluctuation rate and hence moment stability.

Hydrogen Storage, Magnetism and Electrochromism of Silver Doped FAU Zeolite: First-Principles Calculations and Molecular Simulations.

The complex configuration, H2 adsorption binding energy, magnetic, and optical properties of FAU zeolites with Ag cations loaded by ion exchange in the vacant dielectric cavities were investigated by employing the first-principles calculations with all-electron-relativistic numerical atom-orbitals scheme and the Metropolis Monte Carlo molecular simulations. The visible absorption spectrum peaked at distinct wavelengths arranging from red or green to blue colors when changing the net charge load, due to the produced various redox states of Ag cations exchanging at multiple Li+-substituted sites. The spin population analyses indicate the ferrimagnetic coupling between Al–O–Si framework and Ag cations originates from the major ferromagnetic spin polarization in Ag cation cluster coordinating with sodalite cages, with the net spins appreciably depending on the Ag content and exchange site. The H2 adsorption capacities and binding energies represent significant dependence on the content, location, and electronic property of Ag cations introduced into the FAU zeolites. The evident decrease of H2 adsorption binding energy with increased loading concentration demonstrates repulsive interaction between H2 molecules and heterogeneous adsorption configuration on Ag cations. The adsorption sites of H2 sorted by the binding energy with different adsorption configurations were correlated with exchange sites of Ag cations under different Ag loading to comprehend the H2 adsorption mechanism.

Localized magnetic moments in metallic SrB6 single crystals

The specific heat of metallic SrB6 single crystals shows an anomalous behavior for K which varies strongly with an applied magnetic field. This is consistent with a two-level Schottky system. We ascribe the excess of in this temperature range to localized magnetic moments. In addition, features that are attributable to a partial ferromagnetic polarization of a conduction electron gas are observed. These results are supported by magnetization measurements and are compatible with the transport properties reported previously (Stankiewicz 2016 Phys. Rev. B 94 125141).

Strong ferromagnetic proximity polarization in ferromagnetic metal MnGa/n-type GaAs quantum well junction

By biasing a ferromagnetic metal MnGa/10 nm-thick, n-type GaAs quantum well (QW) junction from negative to positive, it is found that its spin dynamics at zero magnetic field is composed of two components with opposite signs. One is excited by a circularly polarized pump beam. The other is induced by ferromagnetic proximity polarization (FPP), which is continuously enhanced as the bias increases towards the positive direction. The time-resolved Kerr rotations have also been measured at a magnetic field of 0.9 Tesla. A phase reversion of Larmor precession is observed as the bias passes through +0.5 V. Following simple quantum mechanics, we become aware of the fact that the transmission and reflection rates of electrons at the interface of MnGa/n-type GaAs QW are enhanced by a factor of , which is the attempting frequency of electron onto a ferromagnet/semiconductor interface. That gives a reasonable explanation why the FPP effect in our MnGa/n-type GaAs QW junction is greatly enhanced as biasing it into forward direction.

Electrically Controllable Magnetism in Twisted Bilayer Graphene.

Twisted graphene bilayers develop highly localized states around AA-stacked regions for small twist angles. We show that interaction effects may induce either an antiferromagnetic or a ferromagnetic (FM) polarization of said regions, depending on the electrical bias between layers. Remarkably, FM-polarized AA regions under bias develop spiral magnetic ordering, with a relative 120° misalignment between neighboring regions due to a frustrated antiferromagnetic exchange. This remarkable spiral magnetism emerges naturally without the need of spin-orbit coupling, and competes with the more conventional lattice-antiferromagnetic instability, which interestingly develops at smaller bias under weaker interactions than in monolayer graphene, due to Fermi velocity suppression. This rich and electrically controllable magnetism could turn twisted bilayer graphene into an ideal system to study frustrated magnetism in two dimensions.

Metal-ion induced ferromagnetic polarization in a mixed-spin system.

Three new 3d-3d heterometallic complexes, [CuMII(hmb)6(OH)]ClO4·H2O (M = Zn (1), Ni (2) and Co (3)) (Hhmb = 2-hydroxy-3-methoxy-benzaldehyde), have been synthesized. Structural analysis reveals that the three complexes are isostructural. Three CuII ions are in a perfect trigonal geometrical frustration and form a tetrahedral frustrated system with the introduced fourth ion ZnII/NiII/CoII. Magnetic studies indicate the antiferromagnetic coupling between metal ions in all 1, 2 and 3. Due to the geometrical frustration, spin-frustrated magnetism is also a typical feature of 1-3 and the magnetostructural correlation is far more complicated than their structures. The detailed magnetic investigation found that the ground state spin of 2 cannot be simply determined by the antiferromagnetically linear arrangement of spins, because the strong antiferromagnetic coupling between CuII and NiII ions quenches the spin frustration and ferromagnetically polarizes the spins of the [Cu] unit. Contrarily, the antiferromagnetic coupling between CuII and CoII ions is not strong enough in 3, so there is no similar behaviour compared to 2.