Noncollinear Magnetism Research Articles

Frustration Induced Noncollinear Magnetic Order Phase in One-Dimensional Heisenberg Chain with Alternating Antiferromagnetic and Ferromagnetic Next Nearest Neighbor Interactions

By using the coupled cluster method, the numerical exact diagonalization method, and the numerical density matrix renormalization group method, we investigated the properties of the one-dimensional Heisenberg chain with alternating antiferromagnetic and ferromagnetic next nearest neighbor interactions. In the classical limit, the ground state is in the collinear Neel state if a<1/2, while for a>1/2, there is an noncollinear canted state. For the quantum case, we found that, although the classical Neel state is absent, the canted state exists if the frustration parameter a exceeds a critical point ac1. The precise critical point ac1 can be determined by using the coupled cluster method and the numerical exact diagonalization method separately. The results of the coupled cluster method and the exact diagonalization method both disclose that the type of phase transition occurring at ac1 changes from a classical second-order transition to a quantum first-order transition due to quantum fluctuation. Although there is another critical point ac2 in a finite system at which the ground state evolves from the canted state to the collinear Neel plus ferromagnetic state, that state is absent because ac2 tends to infinity in the thermodynamic limit.

Charge and spin currents in ferromagnetic Josephson junctions

We determine, using a self consistent method, the charge and spin currents in ballistic Josephson junctions consisting of ferromagnetic ($F$) layers sandwiched between superconducting ($S$) electrodes ($SFS$-type junctions). When there are two $F$ layers, we also consider the experimentally relevant configuration where a normal ($N$) nonmagnetic spacer layer separates them. We study the current-phase relationships as functions of geometrical parameters that are accessible experimentally including the angles that characterize the relative orientation of the magnetization in the $F$ layers. Our self-consistent method ensures that the proper charge conservation laws are satisfied. As we vary the phase difference $\Delta\varphi$ between the two outer $S$ electrodes, multiple harmonics in the current phase relations emerge, their extent depends on the interface scattering strength and the relative $F$ layer widths and magnetization orientations. By manipulating the relative $F$ layer magnetization orientations, we find that the charge supercurrent can reverse directions or vanish altogether. These findings are discussed in the context of the generation and long-range nature of triplet pair correlation. We also investigate the spin currents and associated spin transfer torques throughout the junction. For noncollinear relative magnetizations, the non-conserved spin currents in a given $F$ region gives rise to net torques that can switch directions at particular magnetic configurations or $\Delta\varphi$ values. The details of the spin current behavior are shown to depend strongly on the degree of magnetic inhomogeneity in the system.

ChemInform Abstract: Basics and Prospective of Magnetic Heusler Compounds

AbstractReview: 61 refs.

Nematic versus ferromagnetic spin filtering of triplet Cooper pairs in superconducting spintronics

We consider two types of magnetic Josephson junctions~(JJ). They are formed by two singlet superconductors~S and magnetic layers between them so that the JJ is a heterostructure of the S$_{\text{m}}$/n/S$_{\text{m}}$ type, where~S$_{\text{m}}$ includes two magnetic layers with non-collinear magnetization vectors. One layer is represented by a weak ferromagnet and another one---the spin filter---is either conducting strong ferromagnet (nematic or N\nobreakdash-type JJ) or magnetic tunnel barrier with spin-dependent transparency (magnetic or M\nobreakdash-type JJ). Due to spin filtering only fully polarized triplet component penetrates the normal n~wire and provides the Josephson coupling between the superconductors~S. Although both filters let to pass triplet Cooper pairs with total spin~$\mathbf{S}$ parallel to the filter axes, the behavior of nematic and magnetic JJs is completely different. Whereas in the nematic case the charge and spin currents,~$I_{\text{Q}}$ and~$I_{\text{sp}}$, do not depend on mutual orientation of the filter axes, both currents vanish in magnetic~JJ in case of antiparallel filter axes, and change sign under reversing the filter direction. The obtained expressions for~$I_{\text{Q}}$ and~$I_{\text{sp}}$ show clearly a duality between the superconducting phase~$\varphi$ and the angle~$\alpha$ between the exchange fields in the weak magnetic layers.

Electronic topological transition and noncollinear magnetism in compressed hcp Co

Recent experiments showed that Co undergoes a phase transition from ferromagnetic hcp phase to non-magnetic fcc one around 100 GPa. Since the transition is of first order, a certain region of co-existence of the two phases is present. By means of \textit{ab initio} calculations, we found that the hcp phase itself undergoes a series of electronic topological transitions (ETTs), which affects both elastic and magnetic properties of the material. Most importantly, we propose that the sequence of ETTs lead to the stabilisation of a non-collinear spin arrangement in highly compressed hcp Co. Details of this non-collinear magnetic state and the interatomic exchange parameters that are connected to it, are presented here.

Frustrated Triangular Magnetic Structures of Mn3ZnN: Applications in Thermal Expansion

One of the specific subjects in frustrated magnetic systems is the phenomenon coupled with noncollinear magnetism, such as zero or negative thermal expansion (ZTE or NTE) in antiperovskite compounds. The first-principles calculations and neutron powder diffraction (NPD) are used to reveal the control of the noncollinear Γ5g antiferromagnetic (AFM) structure and corresponding thermal expansion properties in Mn3Zn0.875X0.125N (X = Mn, Ge, and Sn). Based on the optimal exchange-correlation functional, our results demonstrate that X (X = Mn, Ge, and Sn) doping at Zn site could stabilize the noncollinear Γ5g AFM structure and produce magnetovolume effect (MVE). The predictions of Γ5g AFM ground state and MVE is further verified by the NPD results of Mn3Zn0.83Mn0.15N0.99. Intriguingly, this special magnetic structure with strong spin–lattice coupling can be tunable to achieve ZTE behavior. On the basis of these results we suggest that frustrated magnetic systems with noncollinear Γ5g AFM structure of Mn atoms i...

Octonary Resistance States in La0.7Sr0.3MnO3/BaTiO3/La0.7Sr0.3MnO3 Multiferroic Tunnel Junctions

General drawbacks of current electronic/spintronic devices are high power consumption and low density storage. A multiferroic tunnel junction (MFTJ), employing a ferroelectric barrier layer sandwiched between two ferromagnetic layers, presents four resistance states in a single device and therefore provides an alternative way to achieve high density memories. Here, an MFTJ device with eight nonvolatile resistance states by further integrating the design of noncollinear magnetization alignments between the ferromagnetic layers is demonstrated. Through the angle‐resolved tunneling magnetoresistance investigations on La0.7Sr0.3MnO3/BaTiO3/La0.7Sr0.3MnO3 junctions, it is found that, besides collinear parallel/antiparallel magnetic configurations, the MFTJ shows at least two other stable noncollinear (45° and 90°) magnetic configurations. Combining the tunneling electroresistance effect caused by the ferroelectricity reversal of the BaTiO3 barrier, an octonary memory device is obtained, representing potential applications in high density nonvolatile storage in the future.

Field-induced spin reorientation in[Fe/Cr]nmultilayers studied by nuclear resonance reflectivity

We present depth-resolved nuclear resonance reflectivity studies of the magnetization evolution in ${[^{57}\mathrm{Fe}(3\phantom{\rule{0.16em}{0ex}}\mathrm{nm})/\mathrm{Cr}\phantom{\rule{0.16em}{0ex}}(1.2\phantom{\rule{0.16em}{0ex}}\mathrm{nm})]}_{10}$ multilayer under applied external field. The measurements have been performed at the station BL09XU of SPring-8 at different values of the external field (0--1500 Oe). We apply the joint fit of the delayed reflectivity curves and the time spectra of the nuclear resonance reflectivity measured at different grazing angles for enhancement of the depth resolution and reliability of results. We show that the azimuth angle, which is used in all papers devoted to the magnetization profile determination, has a more complicated physical sense due to the partially coherent averaging of the scattering amplitudes from magnetic lateral domains. We describe how to select the true azimuth angle from the determined ``effective azimuth angle.'' Finally we obtain the noncollinear twisted magnetization depth profiles where the spin-flop state appears sequentially in different $^{57}\mathrm{Fe}$ layers at increasing applied field.

Analytical description of ballistic spin currents and torques in magnetic tunnel junctions

In this work we demonstrate explicit analytical expressions for both charge and spin currents which constitute the 2x2 spinor in magnetic tunnel junctions with noncollinear magnetizations under applied voltage. The calculations have been performed within the free electron model in the framework of the Keldysh formalism and WKB approximation. We demonstrate that spin/charge currents and spin transfer torques are all explicitly expressed through only three irreducible quantities, without further approximations. The conditions and mechanisms of deviation from the conventional sine angular dependence of both spin currents and torques are shown and discussed. It is shown in the thick barrier approximation that all tunneling transport quantities can be expressed in an extremely simplified form via Slonczewski spin polarizations and our effective spin averaged interfacial transmission probabilities and effective out-of-plane polarizations at both interfaces. It is proven that the latter plays a key role in the emergence of perpendicular spin torque as well as in the angular dependence character of all spin and charge transport considered. It is demonstrated directly also that for any applied voltage, the parallel component of spin current at the FM/I interface is expressed via collinear longitudinal spin current components. Finally, spin transfer torque behavior is analyzed in a view of transverse characteristic length scales for spin transport.

Current-Phase Relation of Superconductor-Ferromagnet-Superconductor Junctions with a Composite Interlayer

We report on microwave measurements of hybrid superconductor-ferromagnet-superconductor (SFF’S’) junctions with a composite oxide ferromagnetic bilayer with non-collinear magnetizations. Current-phase relation (CPR) of the junction superconducting current has been evaluated as a function of magnetic field. A significant part of the 2nd harmonic in CPR, about 50%, was measured.

Dual Control of Giant Field-like Spin Torque in Spin Filter Tunnel Junctions.

We predict a giant field-like spin torque, , in spin-filter (SF) barrier tunnel junctions in sharp contrast to existing junctions based on nonmagnetic passive barriers. We demonstrate that has linear bias behavior, is independent of the SF thickness, and has odd parity with respect to the SF’s exchange splitting. Thus, it can be selectively controlled via external bias or external magnetic field which gives rise to sign reversal of via magnetic field switching. The underlying mechanism is the interlayer exchange coupling between the noncollinear magnetizations of the SF and free ferromagnetic electrode via the nonmagnetic insulating (I) spacer giving rise to giant spin-dependent reflection at the SF/I interface. These findings suggest that the proposed field-like-spin-torque MRAM may provide promising dual functionalities for both ‘reading’ and ‘writing’ processes which require lower critical current densities and faster writing and reading speeds.

Spin rectification for collinear and noncollinear magnetization and external magnetic field configurations

Spin rectification in a single crystal Fe/Au/Fe sandwich is electrically detected for collinear and noncollinear magnetization and external magnetic field configurations. The line shape, linewidth, and signal polarity are analyzed. The spin rectification theory has been much extended by taking the magnetocrystalline anisotropy and shape anisotropy into account, which explains noncollinear resonances and agrees very well with experimental data. Thus, a comprehensive understanding of spin rectification in ferromagnetic metal is demonstrated in this work.

Stability of the complex generalized Hartree-Fock equations

For molecules with complex and competing magnetic interactions, it is often the case that the lowest energy Hartree-Fock solution may only be obtained by removing the spin and time-reversal symmetry constraints of the exact non-relativistic Hamiltonian. To do so results in the complex generalized Hartree-Fock (GHF) method. However, with the loss of variational constraints comes the greater possibility of converging to higher energy minima. Here, we report the implementation of stability test of the complex GHF equations, along with an orbital update scheme should an instability be found. We apply the methodology to finding the local minima of several spin-frustrated hydrogen rings, as well as the non-collinear molecular magnet Cr3, illustrating the utility of the broken symmetry GHF method and some of its lesser-known nuances.

Basics and prospective of magnetic Heusler compounds

Heusler compounds are a remarkable class of materials with more than 1000 members and a wide range of extraordinary multi-functionalities including halfmetallic high-temperature ferri- and ferromagnets, multi-ferroics, shape memory alloys, and tunable topological insulators with a high potential for spintronics, energy technologies, and magneto-caloric applications. The tunability of this class of materials is exceptional and nearly every functionality can be designed. Co2-Heusler compounds show high spin polarization in tunnel junction devices and spin-resolved photoemission. Manganese-rich Heusler compounds attract much interest in the context of spin transfer torque, spin Hall effect, and rare earth free hard magnets. Most Mn2-Heusler compounds crystallize in the inverse structure and are characterized by antiparallel coupling of magnetic moments on Mn atoms; the ferrimagnetic order and the lack of inversion symmetry lead to the emergence of new properties that are absent in ferromagnetic centrosymmetric Heusler structures, such as non-collinear magnetism, topological Hall effect, and skyrmions. Tetragonal Heusler compounds with large magneto crystalline anisotropy can be easily designed by positioning the Fermi energy at the van Hove singularity in one of the spin channels. Here, we give a comprehensive overview and a prospective on the magnetic properties of Heusler materials.

Noncollinear magnetism in post‐perovskites from first principles: Comparison between CaRhO3 and NaNiF3 (Phys. Status Solidi B 4/2015)

Noncollinear magnetism in post‐perovskites from first principles: Comparison between CaRhO₃ and NaNiF₃ (Phys. Status Solidi B 4/2015)

Triplet superconductivity in oxide ferromagnetic interlayer of mesa-structure

We present experimental data on Nb-Au/La0.7Sr0.3MnO3/SrRuO3/YBa2Cu3O7-δ mesa- structure with in plane linear size 10÷50 μm. The mesa-structures were patterned from the epitaxial heterostructures fabricated by pulsed laser ablation and magnetron sputtering. Superconducting critical current was observed for mesa-structures with the interlayer thicknesses up to 50 nm. In the mesa-structures with just one, either La0.7Sr0.3MnO3 or SrRuO3 interlayer with a thickness larger than 10 nm no superconducting current was observed. The registered superconducting current for the mesa-structures with a thinner interlayer is attributed to pinholes. Obtained results are discussed in terms of superconducting long-range triplet generation at interfaces of superconductor and a composite ferromagnet consisting of ferromagnetic materials with non-collinear magnetization.

Constrained density functional for noncollinear magnetism

Energies of arbitrary small- and large-angle noncollinear excited magnetic configurations are computed using a highly accurate constrained density functional theory approach. Numerical convergence and accuracy are controlled by the choice of Lagrange multipliers ${\ensuremath{\lambda}}_{I}$ entering the constraining conditions. The penalty part ${E}_{p}$ of the constrained energy functional at its minimum is shown to be inversely proportional to ${\ensuremath{\lambda}}_{I}$, enabling a simple, robust, and accurate iterative procedure to be followed to find a convergent solution. The method is implemented as a part of ab initio vasp package, and applied to the investigation of noncollinear B2-like and $\ensuremath{\langle}001\ensuremath{\rangle}$ double-layer antiferromagnetic configurations of bcc iron, ${\mathrm{Fe}}_{2}$ dimer, and amorphous iron. Forces acting on atoms depend on the orientations of magnetic moments, and the proposed approach enables constrained self-consistent noncollinear magnetic and structural relaxation of large atomic systems to be carried out.

Noncollinear magnetism in post‐perovskites from first principles: Comparison between CaRhO3 and NaNiF3

Based on first‐principles calculations, we study the noncollinear magnetism in the post‐perovskites (pPv) phase of NaNiF and CaRhO crystals. We find that the magnetic canting is one of the most promising properties of pPv systems, which is allowed by symmetry in all the pPv crystals with a magnetically active B‐site. In the pPv phase of NaNiF, which has a antiferromagnetic ground state with a ferromagnetic canting, we obtain a magnetic canting amplitude of about 0.1 atom, which is much larger than in the one obtained in CaRhO (canting amplitude of 0.04 ). We also computed the exchange constants (), the single‐ion anisotropy (SIA) parameters and the anti‐symmetric magnetic coupling described by the Dzyaloshinsky–Moriya (DM) interaction in order to scrutinize the origin of the magnetic canting. We find that the canting in NaNiF is mainly due to the DM interaction, while in CaRhO, both DM and SIA contribute to the magnetic canting. Our calculations thus confirm the noncollinear magnetic ground‐state solution experimentally observed in both compounds and the calculated magnetic exchange interactions also confirm the quasi‐2D magnetic behavior reported in pPv.

Non-collinear magnetism with analytic Bond-Order Potentials

The theory of analytic Bond-Order Potentials as applied to non-collinear magnetic structures of transition metals is extended to take into account explicit rotations of Hamiltonian and local moment matrix elements between locally and globally defined spin-coordinate systems. Expressions for the gradients of the energy with respect to the Hamiltonian matrix elements, the interatomic forces and the magnetic torques are derived. The method is applied to simulations of the rotation of magnetic moments in α iron, as well as α and β manganese, based on d-valent orthogonal tight-binding parametrizations of the electronic structure. A new weighted-average terminator is introduced to improve the convergence of the Bond-Order Potential energies and torques with respect to tight-binding reference values, although the general behavior is qualitatively correct for low-moment expansions.

Gilbert damping in noncollinear ferromagnets.

The precession and damping of a collinear magnetization displaced from its equilibrium are well described by the Landau-Lifshitz-Gilbert equation. The theoretical and experimental complexity of noncollinear magnetizations is such that it is not known how the damping is modified by the noncollinearity. We use first-principles scattering theory to investigate transverse domain walls (DWs) of the important ferromagnetic alloy Ni80Fe20 and show that the damping depends not only on the magnetization texture but also on the specific dynamic modes of Bloch and Néel DWs in ways that were not theoretically predicted. Even in the highly disordered Ni80Fe20 alloy, the damping is found to be remarkably nonlocal.