Antiparallel Magnetic Moments Research Articles

Negative Te spin polarization responsible for ferromagnetic order in the doped topological insulator V0.04(Sb1−xBix)1.96Te3

Ferromagnetic topological insulators have emerged as a promising platform for quantum anomalous Hall (QAH) effect with a dissipationless edge transport. However, the observation of QAH effect has so far been restricted to extremely low temperatures. We investigate the microscopic origin of ferromagnetism coupled with topological insulators in vanadium-doped (Sb, Bi)(2)Te-3 employing the x-ray magnetic circular dichroism, angle-resolved two-photon photoemission spectroscopy, combined with first-principles calculations. We found a negative spin polarization, and thus an antiparallel magnetic moment at the Te site with respect to that of the vanadium dopants, which plays the key role in the ferromagnetic order. We ascribe it to the hybridization between Te p and V d majority spin states at the Fermi energy (E-F), being supported by a Zener-type p-d exchange interaction scenario. The substitution of Bi at the Sb site suppresses the bulk ferromagnetism by introducing extra electron carriers in the majority spin channel of Te p states that compensates the antiparallel magnetic moment on the Te site. Our findings reveal important clues to designing magnetic topological insulators with higher Curie temperature that work under ambient conditions.

A Method for Studying the Magnetic Susceptibility of Colloidal Solutions in Ferrofluidic Cells

A technique for studying the magnetic susceptibility of colloidal solutions in a hermetically sealed glass volume (a ferrofluidic cell) is proposed. Experimental investigations have shown that Curie’s law holds in the temperature range of 283–323 K at which ferrofluidic cells are used. A further increase in the temperature of a colloidal solution in a ferrofluidic cell leads to an increase in the Curie constant C, which is due to the fact that the thermal motion of molecules destroys conglomerates with antiparallel magnetic moments of nanoparticles that have a binding energy of the order of kТ.

Ground state and magnetic properties of the Cr-doped Ni-Mn-(Ga, Ge, In, Sn) alloys: Insights from ab initio study

By using first-principles calculations in combination with supercell approach a series of Ni8Mn2Cr2Z4 (Z = Ga, Ge, In, Sn) are studied with partial replacing of Mn by Cr to investigate the effect of such substitution on their structural and magnetic properties. Two 16-atom supercells with different distribution of Cr in the Mn sublattice and magnetic ordering are considered. Crystal structure optimization shows that the magnetic ground state of Ni8Mn2Cr2Z4(Z = Ga, In) alloys is the state with pairwise antiparallel magnetic moments of Cr atoms. While for Ni8Mn2Cr2Z4 (Z = Ge, Sn) alloys is it ferromagnetic state with parallel magnetic moments of Cr and Mn atoms. Based on ground state energies it is shown that martensitic phase can be realized only in Ni8Mn2Cr2Z4 (Z = Ga, Ge) alloys. For the stable crystal structures with associated magnetic order magnetic exchange constants are calculated, to estimate Curie temperatures with the help of mean-field approximation. Obtained results are in qualitative agreement with experimental and theoretical data.

Toroidal and magnetic Fano resonances in planar THz metamaterials

The toroidal dipole moment, a localized electromagnetic excitation of torus magnetic fields, has been observed experimentally in metamaterials. However, the metamaterial based toroidal moment was restricted at higher frequencies by the complex three-dimensional structure. Recently, it has been shown that toroidal moment could also be excited in a planar metamaterial structure. Here, we use asymmetric Fano resonators to illustrate theoretically and experimentally the underlying physics of the toroidal coupling in an array of planar metamaterials. It is observed that the anti-parallel magnetic moment configuration shows toroidal excitation with higher quality (Q) factor Fano resonance, while the parallel magnetic moment shows relatively lower Q factor resonance. Moreover, the electric and toroidal dipole interferes destructively to give rise to an anapole excitation. The magnetic dipole-dipole interaction is employed to understand the differences between the toroidal and magnetic Fano resonances. We further study the impact of intra unit-cell coupling between the Fano resonator pairs in the mirrored and non-mirrored arrangements. The numerical and theoretical approach for modelling the near-field effects and experimental demonstration of toroidal and magnetic Fano resonances in planar systems are particularly promising for tailoring the loss in metamaterials across a broad range of the electromagnetic spectrum.

Quantification of magneto-optically active nanorods and inactive aggregates in nickel nanorod colloids

Nickel nanorods of sufficiently small size exhibit collinear ferromagnetic and optical anisotropy which allows the measurement of their field-dependent orientation in colloidal suspension by optical transmission of linearly polarized light. Coagulation of such extended dipolar particles favors the formation of flux-closing structures such as nanorod dimers with anti-parallel magnetic moments. We present an experimental approach to separate and quantify the fraction of magneto-optically active nanorods and inactive non-magnetic aggregates. The analysis was applied to monitor the coagulation of nanorod colloids which were deliberately destabilized by addition of NaCl.

Ultrathin planar chiral metasurface for controlling gradient phase discontinuities of circularly polarized waves

We develop a gradient phase discontinuities ultrathin planar metasurface based on chiral branched gammadion structure (CBGS) that provides extreme control of electromagnetic wavefronts across single-sheet design. The introduction of the branch can provide antiparallel magnetic moment, which tunes the chirality of the CBGS. In the CBGS metasurface, the transmission phase of a circularly polarized wave can be varied from −180° to +180°. We experimentally demonstrate a beam-refracting CBGS metasurface with refracting a normally incident plane wave to an angle of 17°. The performances of the proposed metasurface at oblique incidence are also presented. The CBGS metasurface can find a wide range of applications over the entire electromagnetic spectrum including single-surface lenses, fully controlling light in direction, and polarization controlling devices.

Investigation of electronic structure, magnetic and transport properties of half-metallic Mn2CuSi and Mn2ZnSi Heusler alloys

The electronic and magnetic properties of Mn2CuSi and Mn2ZnSi Heusler alloys have been investigated using full-potential linearized augmented plane wave method. The optimized equilibrium lattice parameters in stable F-43m configuration are found to be 5.75Å for Mn2CuSi and 5.80Å for Mn2ZnSi. Spin-resolved calculations show that the Mn atoms at inequivalent Wyckoff positions have different contributions to the total magnetic moment in the unit cell. The anti-parallel magnetic moments of inequivalent Mn atoms sum to an integer with total magnetic moment per unit cell. The 100% spin-polarization at Fermi energy together with the total magnetic moment of 1.0µB for Mn2CuSi and 2.0µB for Mn2ZnSi per unit cell, predict that the materials follow MT=ZT – 28 Slater–Pauling rule. Both the materials under study exhibit half-metallicity with an energy gap in the spin-down channels. In the study, we predict a rather fine value of Seebeck coefficient. Further, the decreasing electrical conductivity with temperature shows a metallic character in spin-up configurations, while the electrical conductivity of spin-down states follows a semiconductor-like trend.

Measurement of the thermodynamic temperature using a paramagnetic thermometric material

It is suggested that thermodynamic temperature T be measured with magnetic susceptibility χ of a colloid solution of single-domain ferromagnet nanoparticles. The magnetic susceptibility is taken as a thermometric parameter, since χ and temperature are related by Curie’s law. It is shown that for Curie’s law to be satisfied, the magnetic susceptibility should be determined from the formula χ = Mμ0/B, where M is the magnetization and B is the mean magnetic induction in the colloid solution. Experimental verification carried out for a magnetite nanoparticle solution in the temperature range 273–373 K shows that when the temperature grows, the Curie constant either remains almost unchanged or increases by several percent. The increase in the Curie constant upon heating can be explained by the decomposition of conglomerates containing nanoparticles with antiparallel magnetic moments.

Ab initiostudy of electronic, magnetic, and spectroscopic properties inA- andB-site-ordered perovskiteCaCu3Fe2Sb2O12

Electronic structure, magnetism, x-ray absorption spectroscopy (XAS), and x-ray magnetic circular dichroism (XMCD) spectra of $A$- and $B$-site-ordered quadruple perovskite $B$-site-ordered perovskite $\mathrm{Ca}{\mathrm{Cu}}_{3}{\mathrm{Fe}}_{2}{\mathrm{Sb}}_{2}{\mathrm{O}}_{12}$ (CCFSO) were studied using first-principles calculations with inclusion of spin-orbit coupling. The calculated XAS and XMCD spectra for Cu and Fe at the ${L}_{3,2}$ edges were consistent with a recent experiment and indicate ferrimagnetic ordering with antiparallel magnetic moments between Cu and Fe. The magnetic exchange coupling constants were also calculated to clarify the detailed mechanism of the ferrimagnetic ordering in CCFSO. The coupling constants for Cu-Cu, Fe-Fe, and Cu-Fe pairs were found to be moderate ferromagnetic, weak antiferromagnetic, and strong antiferromagnetic, respectively. From the analysis of the calculated density of states and magnetization density, the microscopic origin of the strong antiferromagnetic coupling between Cu and Fe was successfully elucidated by the superexchange mechanism via the $\mathrm{Cu}({d}_{{x}^{2}\ensuremath{-}{y}^{2}})\ensuremath{-}\mathrm{O}({p}_{x})\ensuremath{-}\mathrm{Fe}({t}_{2g})$ exchange path.

La-Co Pair Substituted Strontium Ferrite Films With Perpendicular Magnetization

La-Co pair substitution is proposed to be an effective way to improve the magnetic properties of strontium ferrite films. Based on ab initio simulations, the structure parameters and magnetic performance of La-Co pair substituted Sr-M (SrFe 12 O 19 ) are investigated. La-Co pair substituted Sr-M (Sr 1-x La x Fe 12-y Co y O 19 ) thin films with perpendicular magnetic anisotropy were also deposited on fused quartz and single-crystal Al 2 O 3 (001) substrates by radio frequency magnetron sputtering. The structure and crystallographic changes were analyzed by using the X-ray diffraction patterns and the morphology was evaluated from the atomic force microscope images. The magnetic characteristics were measured using a physical property measurement system. The experimental results and calculations show that: 1) the lowest energy configuration of La (Sr) Co (2a) is that Co atoms substituted at 2a site of Fe atoms, and shows the same magnetic moment as the 4f1, 4f2, 12k sites; 2) the increment of Co substitution amount to a certain extent will lead more Co 2+ (3 μ B ) preferentially occupy Fe 3+ (5 μ B ) in the 4f2 site, which results in significant decrease in the antiparallel magnetic moment than the parallel; and 3) the coercivity of Sr-M films could be enhanced by the Co substitution for the 2a and 4f2 sites of Fe.

Induced spin-accumulation and spin-polarization in a quantum-dot ring by using magnetic quantum dots and Rashba spin-orbit effect

The effect of magnetic contacts on spin-dependent electron transport and spin-accumulation in a quantum ring, which is threaded by a magnetic flux, is studied. The quantum ring is made up of four quantum dots, where two of them possess magnetic structure and other ones are subjected to the Rashba spin-orbit coupling. The magnetic quantum dots, referred to as magnetic quantum contacts, are connected to two external leads. Two different configurations of magnetic moments of the quantum contacts are considered; the parallel and the anti-parallel ones. When the magnetic moments are parallel, the degeneracy between the transmission coefficients of spin-up and spin-down electrons is lifted and the system can be adjusted to operate as a spin-filter. In addition, the accumulation of spin-up and spin-down electrons in non-magnetic quantum dots are different in the case of parallel magnetic moments. When the intra-dot Coulomb interaction is taken into account, we find that the electron interactions participate in separation between the accumulations of electrons with different spin directions in non-magnetic quantum dots. Furthermore, the spin-accumulation in non-magnetic quantum dots can be tuned in the both parallel and anti-parallel magnetic moments by adjusting the Rashba spin-orbit strength and the magnetic flux. Thus, the quantum ring with magnetic quantum contacts could be utilized to create tunable local magnetic moments which can be used in designing optimized nanodevices.

Spin-polarized conductance in double quantum dots: Interplay of Kondo, Zeeman, and interference effects

We study the effect of a magnetic field in the Kondo regime of a double-quantum-dot system consisting of a strongly correlated dot (the ``side dot'') coupled to a second, noninteracting dot that also connects two external leads. We show, using the numerical renormalization group, that application of an in-plane magnetic field sets up a subtle interplay between electronic interference, Kondo physics, and Zeeman splitting with nontrivial consequences for spectral and transport properties. The value of the side-dot spectral function at the Fermi level exhibits a nonuniversal field dependence that can be understood using a form of the Friedel sum rule that appropriately accounts for the presence of an energy- and spin-dependent hybridization function. The applied field also accentuates the exchange-mediated interdot coupling, which dominates the ground state at intermediate fields leading to the formation of antiparallel magnetic moments on the dots. By tuning gate voltages and the magnetic field, one can achieve complete spin polarization of the linear conductance between the leads, raising the prospect of applications of the device as a highly tunable spin filter. The system's low-energy properties are qualitatively unchanged by the presence of weak on-site Coulomb repulsion within the second dot.

A first principles study on Mn2NiGa Heusler alloy

Tetragonal distortion of Mn2NiGa Heusler alloy is calculated by first-principles based on density functional theory with projector augmented wave pseudopotential, and the magnetism, electronic structure, elastic constants and phonon frequencies are also calculated and analyzed. The contribution of the spin magnetic moments of Mn atom to the total moment is largest for Mn2NiGa, and the Mn2NiGa alloy shows ferrimagnetism in these two cases, owning to the antiparallel but unbalanced magnetic moments of Mn (A) atom and Mn (B) atom. Analysis of tetragonal distortion shows that there is a local minimum total energy at c/a=0.94 and c/a=1.27, which corresponds to a stable martensitic phase. Elastic constants of Mn2NiGa reveal that cubic structure does not satisfy stability conditions, but tetragonal structure (c/a=0.94 and c/a=1.27) does. The imaginary values of phonon frequencies in cubic structures validate that tetragonal structure (c/a=0.94 and c/a=1.27) of Mn2NiGa is more stable than cubic structure. The phase transition temperature of c/a=1.27 tetragonal structure converting to cubic structure is about 315 K.

Structure and magnetostriction of [formula omitted] alloys

Polycrystalline Ho1−xPrxFe1.9(0≤x≤1) cubic Laves alloys were synthesized by arc-melting and subsequent annealing under high stress. Their structure, magnetic properties and magnetostriction are investigated using X-ray diffraction, vibrating sample magnetometer and standard strain gauge technique, respectively. It was found that Ho1−xPrxFe1.9 single cubic Laves phase cannot be obtained when x>0.2 by a traditional vacuum annealing method. In contrast, the cubic Laves phase can be stabilized over the whole studies range in the samples annealed under high stress. The saturation magnetization for Ho1−xPrxFe1.9 decreases with the increase of x and reaches a minimum at x=0.4, then increases with further increase of x, which indicates the antiparallel magnetic moment alignment between Ho and Pr sublattice. The magnetostriction of Ho1−xPrxFe1.9 does not linearly increase with increasing x, but presents a minimum at x=0.4.

Energetic stability and magnetic coupling in (Cr1−xFex)2O3: Evidence for a ferrimagnetic ilmenite-type superlattice from first principles

Based on density functional theory (DFT) calculations with a Hubbard $U$-term, we explore the possibility to design an artificial ferrimagnet FeCrO${}_{3}$ of ilmenite type out of the two antiferromagnets $\ensuremath{\alpha}$-${\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ and $\ensuremath{\alpha}$-${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$. By varying the concentration of Fe in $\ensuremath{\alpha}$-Cr${}_{2}$O${}_{3}$, we provide a phase diagram of the relative stability of different chemical and magnetic arrangements with respect to the end members. At $50%$ Fe-doped $\ensuremath{\alpha}$-Cr${}_{2}$O${}_{3}$, the ilmenite-like structure with alternating Fe and Cr layers and antiparallel magnetic moments competes energetically with a phase-separated structure containing a mixed Fe-Cr interface layer. The magnetic interaction parameters between Fe($3{d}^{5}$) and Cr($3{d}^{3}$) ions in the digital ferrimagnetic heterostructure, extracted by mapping the DFT total energies to a Heisenberg Hamiltonian, indicate a hematite-like magnetic order with parallel intralayer and antiparallel interlayer alignment.

Magnetic and magnetostrictive properties in high-pressure synthesized Dy 1− xPr xFe 1.9(0 ≤ x ≤ 1) cubic Laves alloys

Polycrystalline alloys Dy 1− x Pr x Fe 1.9 (0 ≤ x ≤ 1) were synthesized by arc-melting and subsequent high-pressure annealing. Their crystal structure, magnetic properties and magnetostriction have been investigated. X-ray diffraction results show that the system exhibits almost single cubic Laves phase with MgCu 2-type structure over the whole range. The lattice parameter of the cubic Laves phase increases linearly with increasing Pr concentration, while the Curie temperature goes the opposite way. The saturation magnetization for Dy 1− x Pr x Fe 1.9 decreases with the increase of x and reaches a minimum at x = 0.4, then it continues to increase with further increase of x, which reflects the antiparallel magnetic moment of Dy and Pr. The magnetostriction λ || − λ ⊥ first increases and then decreases within the range of 0.0 ≤ x ≤ 0.4, and increases monotonically with further increasing x.

Low Magnetization and Anisotropy in the Antiferromagnetic State of Undoped Iron Pnictides

We examine the magnetic phase diagram of iron pnictides using a five-band model. For the intermediate values of the interaction expected to hold in the iron pnictides, we find a metallic low moment state characterized by antiparallel orbital magnetic moments. The anisotropy of the interorbital hopping amplitudes is the key to understanding this low moment state. This state accounts for the small magnetization measured in undoped iron pnictides and leads to the strong exchange anisotropy found in neutron experiments. Orbital ordering is concomitant with magnetism and produces the large zx orbital weight seen at Γ in photoemission experiments.

First-principle prediction of magnetic shape memory effect of Heusler alloy Mn2NiGe

Tetragonal distortions,crystal structure,magnetism,electronic structure and pressure response of Hg2CuTi-type Mn2NiGe are calculated by the first-principles method based on the density functional theory. The calculations show that 1)in the process of transform from cubic to a tetragonal in structure,Mn2NiGe alloys exhibit a stable martensitic phase at c/ a≈1.34; 2)in austenite and martensite,Mn atoms are the main contributors to the magnetism in Mn2NiGe,and Mn2NiGe alloys show ferrimagnetism due to antiparallel but unbalanced magnetic moments of Mn(A) atom and Mn(B) atom; 3)in austenite and martensite,the direct d-d exchange interaction between Mn(A) atom and Mn(B) atom is weak because of small overlap of d-projected DOS of Mn(A) atom and Mn(B) atom nearby the Fermi level. This is the reason why the Mn2NiGe alloys show ferrimagnetism. We can predict the magnetic shape memory effect (MSME) in Mn2NiGe alloys based on the the results of calculation above.

Magnetic properties and neutron diffraction study of (NixMnl-x)3[Cr(CN)6]2 molecule-based magnets

Neutron diffraction measurements were performed on (NixMn1-x)3[Cr(CN)6]2·zD2O (x = 0, 0.38 and 1) powders above and below the Curie temperature TC. The crystal structure, including D-sites, was fully described. Neutron diffraction experiments revealed magnetic contribution for all compounds. Magnetization measurements and elastic neutron diffraction of Mn3[Cr(CN)6]2 indicate magnetic structure consisting of Mn and Cr sublattices with antiparallel magnetic moments μMN = 3.790 μB and μCr = -1.375 μB leading to overall ferromagnetic ordering below the Curie temperature TC = 64.5 K. Very weak magnetic contribution was observed on investigated Ni3[Cr(CN)6]2, and (Ni0.38Mn0.62)3[Cr(CN)6]2 powders at 1.6 K and at 60 K respectively. Our results are compared and discussed with magnetization measurements of M(T) and heat capacity measurements C(T) in zero magnetic field and magnetic fields up to μ0H = 5 T.

Half-metallic ferrimagnetism in the [formula omitted] and [formula omitted] alloys adopting the zincblende and wurtzite structures from first-principles

Employing first-principles calculations we study the structural, electronic and magnetic properties of the [ Sc 1 - x V x ] C and [ Sc 1 - x V x ] Si alloys assuming that they crystallize in the zincblende and wurtzite structures. Both structures are degenerated with respect to the total energy. For all concentrations the alloys in these lattice structures are half-metallic with the gap located in the spin-down band. The total spin moment follows the Slater–Pauling behavior varying linearly between the - 1 μ B of the perfect ScC and ScSi alloys and the + 1 μ B of the perfect VC and VSi alloys. For the intermediate concentrations V and Sc atoms have antiparallel spin magnetic moments and the compounds are half-metallic ferrimagnets. At the critical concentration, both [ Sc 0.5 V 0.5 ] C and [ Sc 0.5 V 0.5 ] Si alloys present zero total spin-magnetic moment but the C-based alloy shows a semiconducting behavior contrary to the Si-based alloys which is a half-metallic antiferromagnet.