AF Phase Research Articles

Spin-wave and electromagnon dispersions in multiferroicMnWO4as observed by neutron spectroscopy: Isotropic Heisenberg exchange versus anisotropic Dzyaloshinskii-Moriya interaction

High resolution inelastic neutron scattering reveals that the elementary magnetic excitations in multiferroic MnWO4 consist of low energy dispersive electromagnons in addition to the well-known spin-wave excitations. The latter can well be modeled by a Heisenberg Hamiltonian with magnetic exchange coupling extending to the 12th nearest neighbor. They exhibit a spin-wave gap of 0.61(1) meV. Two electromagnon branches appear at lower energies of 0.07(1) meV and 0.45(1) meV at the zone center. They reflect the dynamic magnetoelectric coupling and persist in both, the collinear magnetic and paraelectric AF1 phase, and the spin spiral ferroelectric AF2 phase. These excitations are associated with the Dzyaloshinskii-Moriya exchange interaction, which is significant due to the rather large spin-orbit coupling.

Pressure influence on magnetic properties of Nd2RhIn8

We have studied the pressure effect on the magnetic phase diagram of tetragonal Nd2RhIn8. The magnetization under uniaxial pressure up to 5.3kbar applied along the tetragonal c-axis and the heat capacity under hydrostatic pressure were measured in magnetic fields up to 7 and 14T, respectively. Both types of measurements reveal an increase of TN with increasing pressure; the effect is considerably stronger for the uniaxial pressure. Considering also the anisotropic compressibility in this family of compounds, we conclude that TN increases with decreasing c/a. The phase boundary between the ground-state AF1 phase and the field-induced AF2 phase shifts slightly to higher magnetic fields with increasing pressure. The ground state phase is thus stabilized by applying pressure. The magnetic phase diagram is constructed.

Muon-spin relaxation study of the double perovskite insulators Sr2BOsO6 (B = Fe, Y, ln)

We present the results of zero-field muon-spin relaxation measurements made on the double perovskite insulators Sr2BOsO6 (). Spontaneous muon-spin precession indicative of quasistatic long range magnetic ordering is observed in Sr2FeOsO6 within the AF1 antiferromagnetic phase for temperatures below K. Upon cooling below K the oscillations cease to be resolvable owing to the coexistence of the AF1 and AF2 phases, which leads to a broader range of internal magnetic fields. Using density functional calculations we identify a candidate muon stopping site within the unit cell, which dipole field simulations show to be consistent with the proposed magnetic structure. The possibility of incommensurate magnetic ordering is discussed for temperatures below K and 25 K for Sr2YOsO6 and Sr2InOsO6, respectively.

Thermal and deformation-induced phase transformation behavior of Fe–15Cr–3Mn–3Ni–0.1N–(0.05–0.25)C austenitic and austenitic–martensitic cast stainless steels

The austenite-stabilizing effect of carbon on the martensitic transformation behavior and the temperature dependent tensile properties of five Fe–15Cr–3Mn–3Ni–0.1N–(0.05–0.25C) cast stainless steels with austenitic and austenitic-martensitic microstructures was studied. As the carbon concentration increased in 500ppm increments from 0.05mass-% to 0.25mass-%, the as-quenched martensite fraction as well as Ms, As, and Af phase transformation temperatures decreased. Increased carbon content was also found to increase the austenite stability against the deformation-induced αʼ-martensite formation. During tensile tests in the temperature range of −40–200°C, stress- and strain-induced martensite formation occurred in the investigated steels. Due to the low room temperature stability of austenite in the examined steels, more favorable combinations of tensile properties were obtained at high tensile test temperatures where the deformation-induced martensite transformation of the austenite phase was postponed to high strains or completely suppressed.

Infrared Study of the Spin Reorientation Transition and Its Reversal in the Superconducting State in Underdoped Ba(1-x)K(x)Fe(2)As(2).

With infrared spectroscopy we investigated the spin-reorientation transition from an orthorhombic antiferromagnetic (o-AF) to a tetragonal AF (t-AF) phase and the reentrance of the o-AF phase in the superconducting state of underdoped Ba(1-x)K(x)Fe(2)As(2). In agreement with the predicted transition from a single-Q to a double-Q AF structure, we found that a distinct spin density wave develops in the t-AF phase. The pair breaking peak of this spin density wave acquires much more low-energy spectral weight than the one in the o-AF state which indicates that it competes more strongly with superconductivity. We also observed additional phonon modes in the t-AF phase which likely arise from a Brillouin-zone folding that is induced by the double-Q magnetic structure with two Fe sublattices exhibiting different magnitudes of the magnetic moment.

Theory of Electromagnons in CuO.

We develop a theory of electromagnons in CuO by combining a symmetry analysis based on irreducible corepresentations, ab initio calculations, and simulations of spin dynamics using a model Hamiltonian and the Landau-Lifshitz-Gilbert equation. We show that the electromagnon measured in [Jones et al., Nat. Commun. 5, 3787 (2014)] with the electric field along the [101] direction originates from a magnetoelectric coupling mediated by Dzyaloshinskii-Moriya interactions and consists of a rigid rotation of the Cu spins around the axis defined by the electric field. Furthermore we predict the existence of a second electromagnon originating from exchange striction and coupled to electric fields along the [010] direction in the AF2 phase.

Magnetic structures in the magnetic phase diagram ofHo2RhIn8

The magnetic phase diagram of the tetragonal ${\mathrm{Ho}}_{2}{\mathrm{RhIn}}_{8}$ compound has similar features to many related systems, revealing a zero magnetic field AF1 and a field-induced AF2 phases. Details of the magnetic order in the AF2 phase were not reported yet for any of the related compounds. In addition, only the ${\mathrm{Ho}}_{2}{\mathrm{RhIn}}_{8}$ phase diagram contains a small region of the incommensurate zero-field AF3 phase. We have performed a number of neutron diffraction experiments on single crystals of ${\mathrm{Ho}}_{2}{\mathrm{RhIn}}_{8}$ using several diffractometers including experiments in both horizontal and vertical magnetic fields up to 4 T. We present details of the magnetic structures in all magnetic phases of the rich phase diagram of ${\mathrm{Ho}}_{2}{\mathrm{RhIn}}_{8}$. The Ho magnetic moments point along the tetragonal $c$ axis in every phase. The ground-state AF1 phase is characterized by propagation vector $\mathbf{k}=(1/2,0,0)$. The more complex ferrimagnetic AF2 phase is described by four propagation vectors ${\mathbf{k}}_{0}=(0,0,0),{\mathbf{k}}_{1}=(1/2,0,0),{\mathbf{k}}_{2}=(0,1/2,1/2),{\mathbf{k}}_{3}=(1/2,1/2,1/2)$. The magnetic structure in the AF3 phase is incommensurate with ${\mathbf{k}}_{AF3}=(0.5,\ensuremath{\delta},0)$. Our results are consistent with theoretical calculations based on crystal field theory.

Modulated multiferroic phases and electric polarization in Mn1−xRuxWO4+δ

In this work, we investigate the multiferroic phase stability against 4d Ru substitution of 3d Mn in Mn1−xRuxWO4+δ. The Ru substitution results in both Ru3+ and Ru4+ occupations at the Mn sites, while oxygen interstitials in the lattice are expected owing to the charge neutrality requirement. It is suggested that the local electric dipoles as induced by the charge valence variations of Ru cancel with each others, making no contribution to the ferroelectric polarization. The Ru-substitution induced lattice distortion and chemical disorder may be responsible for the enhanced stability of the ferroelectric AF2 phase over the non-ferroelectric AF1 phase, thus leading to the observed polarization enhancement.

Electric field control of ferroelectric domain structures in MnWO4

Competing interactions make the magnetic structure of MnWO4 highly frustrated, and only the AF2 phase of the three magnetically ordered phases (AF1, AF2, AF3) is ferroelectric. The high frustration may thus allow a possibility to tune the magnetic structure by means of an electric field via magnetoelectric coupling. By using the pyroelectric current method, we measure the remnant ferroelectric polarization in MnWO4 upon application of a poling electric field via two different roadmaps. It is demonstrated that an electric field as low as 10 kV cm−1 is sufficient to enhance the stability of a ferroelectric AF2 phase at the expense of a non-ferroelectric AF1 phase. This work suggests that electric field induced electrostatic energy, although small due to weak magnetically induced electric polarization, may effectively tune ferroelectric domain structures, and thus the magnetic structure of highly frustrated multiferroic materials.

Pressure Influence on Magnetic Properties of TbNiAl

We have investigated the e ect of hydrostatic pressure on magnetic properties of TbNiAl, crystallizing in hexagonal ZrNiAl-type structure. TbNiAl orders antiferromagnetically below TN = 45 K and undergoes further magnetic phase transition to another AF phase at T1 = 23 K. The magnetic eld of Bc ∼= 0.3 T applied along the c axis at 2 K leads to the transition to ferromagnetic order. By applying the hydrostatic pressure, both TN and T1 remain almost una ected whereas Bc shows a strong increase. The hydrostatic pressure stabilizes the antiferromagnetic state which can be related to development of structural parameters.

Doubly degenerate entanglement spectrum and nonzero string order in the spin-1/2 Heisenberg–Ising chain

The ground-state properties and quantum phase transitions (QPTs) in spin-1/2 Heisenberg-Ising alternating chain has been investigated by the iTEBD algorithm. Four different ground-state phases, i.e., a ferromagnetic phase (FM), an antiferromagnetic phase (AF), a stripe phase (SP), and a disordered phase were distinguished. The disordered phase, which has nonzero string orders and the doubly degenerate entanglement spectrum, was observed as Heisenberg coupling JH>0.5. The disordered phase in such a model is found to belong to the same topological phase as the Haldane state. In the disordered phase, every two nearest-neighbor spin-1/2 spins connected by the Ising coupling behave like an integer (S=1) spin. Furthermore, the QPTs from the disordered phase to the AF and SP phases belong to the Ising universality class with central charges c=c¯=1/2.

Multiferroic phase stability in non-stoichiometric MnWO4

We investigate the multiferroic phase stability of MnWO4 in response to the non-stoichiometry of Mn and W, given the Mn:W ratio η. It is observed that the non-stoichiometry does not affect remarkably the ferroelectric transition point (the AF3-AF2 transition point) and the AF2-AF1 transition point, but the non-polar AF1 phase is partially replaced by the ferroelectric AF2 phase. The measured electric polarization is slightly enhanced with increasing stoichiometric deviation ∣η − 1∣. The possible underlying mechanism for these effects is discussed.

Reentrant Surface Anisotropy in the Antiferromagnetic/Ferromagnetic BilayerMn/Co/Cu(001)

We investigated the magnetic anisotropy energy of monatomic surface-step atoms in antiferromagnetic/ferromagnetic (AF/FM) epitaxial Mn/Co bilayers grown on vicinal Cu(001) surfaces. The step-induced anisotropy of the Co/Cu(001) films was quenched upon submonolayer Mn deposition, but a reentrant uniaxial surface anisotropy was observed for Mn thickness (tMn) between 1 and 2 monolayers, which disappears for Mn thickness above 2 monolayers. In the Mn/Co/Cu(001) system, Mn films undergo a tMn-dependent transition from FM to AF in the 1-2 Mn monolayer thickness range, which entails the coexistence of FM and AF Mn phases in the film. The observation of a sizeable uniaxial anisotropy exclusively in the Mn-thickness range of coexistence of the FM and AF phases points out the crucial role of the boundaries between FM and AF regions within the Mn film. A symmetry-breaking mechanism of a magnetic type, rather than a purely geometric one, is therefore proposed as the origin of the reentrant anisotropy.

Field-induced evolution of magnetic ordering in the quantum spin system (CuBr)Sr2Nb3O10with a⅓magnetization plateau

The field-induced evolution of the magnetic ordering in (CuBr)Sr${}_{2}$Nb${}_{3}$O${}_{10}$ with a $⅓$ magnetization plateau has been investigated by neutron diffraction under magnetic fields up to 10 T. With an increasing magnetic field, the zero-field helical antiferromagnetic (AFM) phase, AF1, with \ensuremath{\kappa} $=$ [0 $⅜$ $\textonehalf{}$] is replaced by a simple ferromagnetic phase with \ensuremath{\kappa} $=$ [0 0 0], the formation of which is, however, retarded by the appearance of a second AFM, AF2, with \ensuremath{\kappa} $=$ [0 $⅓\ensuremath{\sim}0.46$]. Upon further increasing of the magnetic field, the AF2 phase disappears and only the ferromagnetic phase persists. The results clearly show that the magnetization plateau, induced by the competition between field-induced ferromagnetic, F, and AF2 phases, is coincidentally situated at $M$ $=$ $⅓$ ${M}_{S}$ of the dc magnetization curve. The AF1 and AF2 phases have strongly differing magnetic propagation vectors and are therefore not directly related.

Stabilized helical spin order and multiferroic phase coexistence in MnWO4: Consequence of4dRu substitution of Mn

Most earlier work on the effect of chemical substitution in multiferroic MnWO${}_{4}$ has focused on the 3$d$ transition metal substitution of Mn. In this paper, we investigate the Ru substitution of Mn in polycrystalline Mn${}_{1\ensuremath{-}x}$Ru${}_{x}$${}_{/2}$WO${}_{4}$ in order to unveil the consequence of 4$d$ transition metal substitution in terms of magnetic transitions and ferroelectricity. It is found that the Ru substitution substantially reshuffles the magnetic frustration and stabilizes the incommensurate helical spin-order phase (AF2) by partially suppressing the collinear spin-order phase (AF1 phase). The coexistence of the AF2 and AF1 phases at low temperature is suggested. Consequently, the ferroelectric polarization is remarkably enhanced, accompanied with significant response of the polarization to magnetic field. It is argued that the structural distortion and enhanced spin-orbital coupling associated with the Ru substitution may be responsible for this ferroelectricity enhancement.

Multiferroicity on the Zigzag-Chain Antiferromagnet MnWO4 in High Magnetic Fields

The electric polarization and magnetization of the zigzag-chain antiferromagnet MnWO 4 were measured in high pulsed magnetic fields of up to 53 T for the three principal directions x , y , and z , where x corresponds to the easy axis and y is the b * -direction. In addition to the AF1, AF2, and HF phases already known, two magnetic phases, IV and V, successively appear in high magnetic fields for B ∥ x at 4.2 K. We found that phase IV is ferroelectric with the electric polarization P parallel to the b * -axis, similar to the AF2 phase. It is also confirmed that phase IV is continuously connected to the previously reported X phase for the B ∥ y axis by changing the field direction. Interestingly, the successive field-induced phase transitions AF2–HF–IV in B ∥ x at a low temperature revealed that the signs of P ∥ b * for AF2 and phase IV are always opposite each other, in spite of the intervening paraelectric HF phase extending to over 20 T. This result implies that an unusual memory effect is present in th...

Influence of boron doping on magnetic properties and microwave characteristics of MnIr/FeCoB multilayers

A detailed investigation of the influence of B doping on the magnetic properties and high frequency characteristics of FeCoB/MnIr multilayered thin films was carried out. Exchange bias was found to decrease monotonically with B doping possibly due to less favorable of AF phase of MnIr in the samples with rich B concentration. However, with small amount of doping up to 9% of B concentration, the MnIr fcc (111) peaks become more prominent and the real part of permeability enhanced. Our results suggest no correlation between the structural phase of MnIr and the exchange bias. Also, the influences of B doping on several static and dynamic parameters, such as rotational magnetic anisotropy field, coercivity, saturation magnetization, resonance frequency, and effective damping factor are presented and discussed. Our results support the claim that the coercivity enhancement and the arising of rotational anisotropy may have the same physical origin.

Dimensional crossover and the magnetic transition in electron-doped manganites

We introduce a microscopic model for electron doped manganites that explains the mechanism of the observed transition from $G$-type antiferromagnetic ($G$-AF) to $C$-type antiferromagnetic ($C$-AF) order under increasing doping by double exchange mechanism. The model unravels the crucial role played by $e_g$ orbital degrees of freedom and explains the observed metal-to-insulator transition by a dimensional crossover at the magnetic phase transition. The specific heat and the spin canting angle found for the $G$-AF phase agree with the experimental findings. As a surprising outcome of the theory we find that spin canting is suppressed in the $C$-AF phase, in agreement with the experiment, due to the Fermi surface topology.

The quantum spin-1/2 frustrated Heisenberg model on a stacked square lattice: a spin wave theory study

The ground state phase diagram and staggered magnetization of the quantum spin-1/2 Heisenberg models in the presence of nearest-neighbor (J1) andnext-nearest-neighbor (J2) interactions (J1–J2 model) on a stacked square lattice, where we introduce an interlayer coupling throughnearest-neighbor bonds of strength , are investigated by using linear spin wave theory. We analyze the order parametermA for the ordered phases: antiferromagnetic (AF,J1 > 0), ferromagnetic(F, J1 < 0) and collinear (CAF—collinear antiferromagnetic and CF—collinearferromagnetic) as a function of the frustration parameter (α = J2/|J1|) and for several values of the interlayer parameter (). We obtain the ground state phase diagram in theλ–α plane. We found different ordered states depending on the values ofλ andα and the signof J1: AF (F)and CAF (CF), and a disordered state: the quantum paramagnetic state (QP). For an intermediate regionα1c(λ) < α < α2c(λ) we observe a QP phasefor the case of J1 > 0 (AF)that disappears for λ below some critical value . For α < α1c(λ) and α > α2c(λ), andbelow λ1, we have the AF and CAF semi-classically ordered states, respectively. Atα = α1c(λ) a second-order transition between the AF and QP states occurs and atα = α2c(λ) a first-order transition between the AF and CAF phases takes place. The boundariesbetween these ordered phases merge at the critical end point—, where αc = 1/2. Above this CEP there is again a direct first-order transition betweenthe AF and CAF phases, with a behavior governed by the pointαc independentlyof λ ≥ λ1. ForJ1 < 0 (F)and λ > 0 the QP intermediate region was not observed; only a small region ofthe disordered state (QP) was predicted in the two-dimensional limit (λ = 0).

Multiferroic Phase Control in MnWO4 Doped with Fe, Co, and Zn: A Comparative Study

MnWO4 is a bulk multiferroic showing three different magnetic phases at low temperature. The intermediate AF2 phase (7.8 K < T < 12.6 K) is also ferroelectric (multiferroic) due to the inversion symmetry breaking helical Mn-spin order and the strong coupling of the spins to the lattice. The complex phase sequence and the multiferroic properties are a consequence of a delicate balance of competing magnetic exchange interactions, anisotropy, and the resulting frustration. It is therefore of interest to tune and control these interactions by substituting the magnetic Mn ions with other magnetic or nonmagnetic ions. We have investigated the effects of chemical substitutions (Co, Zn, and Fe for Mn) on the multiferroic phases. In Mn1-xFexWO4 the helical (multiferroic) phase is quickly suppressed by less than 5% Fe. In contrast, Co substitution in MnWO4 of only 2% is sufficient to stabilize the helical (multiferroic) phase as the ground state. The substitution of nonmagnetic Zn for Mn also stabilizes the multiferroic phase. The multiferroic phase is surprisingly stable with regard to the dilution of the magnetic system of more than 50% which indicates the long-range nature of the magnetic exchange interactions.