Uniaxially anisotropic antiferromagnets in a field on a square lattice

Classical uniaxially anisotropic Heisenberg and XY antiferromagnets in a field along the easy axis on a square lattice are analysed, applying ground state considerations and Monte Carlo techniques. The models are known to display antiferromagnetic and spin-flop phases. In the Heisenberg case, a single-ion anisotropy is added to the XXZ antiferromagnet, enhancing or competing with the uniaxial exchange anisotropy. Its effect on the stability of non-collinear structures of biconical type is studied. In the case of the anisotropic XY antiferromagnet, the transition region between the antiferromagnetic and spin-flop phases is found to be dominated by degenerate bidirectional fluctuations. The phase diagram is observed to resemble closely that of the XXZ antiferromagnet without single-ion anisotropy.     

Magnetic-field-induced toroidal moment in the magnetoelectric Cr2O3

The appearance of a toroidal moment is observed in the magnetoelectric Cr₂O₃ in a strong magnetic field above the spin-flop transition field. This conclusion is based on the experimentally established fact that the off-diagonal components of the magnetoelectric susceptibility tensor of Cr₂O₃ contains an antisymmetric part that is dual to the toroidal moment. Therefore it has been shown that the magnetoelectric Cr₂O₃ in the spin-flop phase can be classified as a toroic. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 4, 302–306 (25 February 1999)     

Two-stage spin-flop transitions in the S = 1/2 antiferromagnetic spin chain BaCu(2)Si(2)O(7)

Two-stage spin-flop transitions are observed in the quasi-one-dimensional antiferromagnet BaCu(2)Si(2)O(7). A magnetic field applied along the easy axis induces a spin-flop transition at 2.0 T followed by a second transition at 4.9 T. The magnetic susceptibility indicates the presence of Dzyaloshinskii-Moriya (DM) antisymmetric interactions between the intrachain neighboring spins. We discuss a possible mechanism whereby the geometrical competition between DM and interchain interactions, as discussed for the two-dimensional antiferromagnet La(2)CuO(4), causes the two-stage spin-flop transitions.     

Magnetic properties of the intermetallic compound PrAg

The field and temperature dependences of the magnetization of PrAg are found to be those of an antiferromagnet whose Néel point is ～11°K and which undergoes a spin-flop transition at a critical field of ～5 kOe, as confirmed by high-field neutron diffraction measurements. Preliminary comparisons are made with properties calculated from crystal-field theory.     

Effect of an external electric field on the structure of the magnon spectrum in a finite magnetoelectric crystal

Considering the case of a plate of a centrally antisymmetric tetragonal antiferromagnet for illustration, it is shown that if the tensor of magnetoelectric coupling constants is antisymmetric, then an external electric field causes earlier unknown anomalies to occur in the spectrum of bulk acoustic magnons even in the exchangeless limit.     

Optimization of spin-triplet supercurrent in ferromagnetic Josephson junctions

We have observed long-range spin-triplet supercurrents in Josephson junctions containing ferromagnetic (F) materials, which are generated by noncollinear magnetizations between a central Co/Ru/Co synthetic antiferromagnet and two outer thin F layers. Here we show that the spin-triplet supercurrent is enhanced up to 20 times after our samples are subject to a large in-plane field. This occurs because the synthetic antiferromagnet undergoes a "spin-flop" transition, whereby the two Co layer magnetizations end up nearly perpendicular to the magnetizations of the two thin F layers. We report direct experimental evidence for the spin-flop transition from scanning electron microscopy with polarization analysis and from spin-polarized neutron reflectometry. These results represent a first step toward experimental control of spin-triplet supercurrents.     

Excitations in a spin-flop Heisenberg magnet

Dynamic spin correlations in the spin-flop phase of a Heisenberg antiferromagnet are studied in terms of the quantities that determine the inelastic scattering of neutrons from single spin-wave states. A complete expression for the cross-section is given using linear spin-wave theory. The non-linear features of the spin dynamics which are intrinsic to the spin-flop state are studied by perturbation theory. Two limiting cases are considered, and these correspond to states where the spins are close to an antiferromagnetic configuration (weak field limit) and the almost completely field aligned configuration. Analytic expressions are given for the appropriate collisional self-energies, in the limit of low temperatures, for selected wavevectors in one and three dimensional systems.     

Domain structure in centroantisymmetric antiferromagnets

The domain structure of an antiferromagnet whose magnetic-symmetry group contains a center of antisymmetry is studied theoretically. The magnetoelectric effect and weak ferromagnetism are shown to coexist in a domain wall. It is established that when the inhomogeneous magnetic moment interacts with a sufficiently strong magnetic field H ‖ C ₃, a multidomain state with an odd number of 180° domain walls becomes energetically favorable. The critical field for the transition from a single-domain state to a multidomain state is found. It is shown that domain reversal occurs when the magnetic field H is reversed.     

Experimental evidence for a field-induced transition between two types of domain walls in antiferromagnets

The existence of a field-induced transition between two types of domain walls is inferred from the field dependence of the rotational hysteresis, as observed in the spin-flop state of the biaxial antiferromagnet (C₂H₅NH₃)₂CuCl₄. In the spin-flop state a weak-ferromagnetic (WF) moment is present along the difficult axis; the hysteresis is connected with irreversible domain-wall movements, the leverage on the walls being provided by the WF moment in the external field.     

Resonant activation of a synthetic antiferromagnet

The magnetic decay time of a synthetic antiferromagnet comprised of two closely spaced magnetic dipoles is measured in the presence of microwave excitation. The system is known to be highly stable with respect to switching between its two antiparallel ground states under quasistatic magnetic fields. We show that an order of magnitude lower field can switch the pair, provided the field is applied in resonance with the optical eigenmode of the collective spin dynamics in the system. We furthermore show that thermal agitation can play an essential role in spin-flop switching for resonant excitations of near- or subcritical amplitude.     

A new type of surface spin wave in magnetoelectric crystals

It is shown for the case of a two-sublattice model of an antiferromagnet with a linear magnetoelectric effect that a new type of surface spin wave can arise at a boundary between a magnetoelectric and a nonmagnetic metal or between a magnetoelectric and a nonmagnetic insulator. This type of surface magnon arises from hybridization of the exchange and electric-dipole spin-spin interaction mechanisms. Fiz. Tverd. Tela (St. Petersburg) 41, 1044–1049 (June 1999)     

Magnon spectrum features associated with the linear magnetoelectric effect in a finite magnet

Using a mechanically free plate of a centrally antisymmetric tetragonal antiferromagnet of the easy-plane type as an example, it is shown that the linear magnetoelectric effect causes earlier unknown anomalies to occur in the bulk magnon spectrum. The character of these anomalies depends critically on the relationship between the Néel and Debye temperatures of the antiferromagnetic crystal.     

A mean-field theory for strongly disordered non-frustrated antiferromagnets

Motivated by impurity-induced magnetic ordering phenomena in spin-gap materials like TlCuCl₃, we develop a mean-field theory for strongly disordered antiferromagnets, designed to capture the broad distribution of coupling constants in the effective model for the impurity degrees of freedom. Based on our results, we argue that in the presence of random magnetic couplings the conventional first-order spin-flop transition of an anisotropic antiferromagnet is split into two transitions at low temperatures, associated with separate order parameters along and perpendicular to the field axis. We demonstrate the existence of either a bicritcal point or a critical endpoint in the temperature–field phase diagram, with the consequence that signatures of the spin flop are more pronounced at elevated temperature.     

Equilibrium magnetization at the boundary of a magnetoelectric antiferromagnet

Symmetry arguments are used to show that a boundary of a magnetoelectric antiferromagnet has an equilibrium magnetization. This magnetization is coupled to the bulk antiferromagnetic order parameter and can be switched along with it by a combination of E and B fields. As a result, the antiferromagnetic domain state of a magnetoelectric can be used as a nonvolatile switchable state variable in nanoelectronic device applications. Mechanisms affecting the boundary magnetization and its temperature dependence are classified. The boundary magnetization can be especially large if the boundary breaks the equivalence of the antiferromagnetic sublattices.     

Magnetic resonance in dilute quasi-one-dimensional antiferromagnet CsNi1−x MgxBr3

The effect of alloying with nonmagnetic Mg²⁺ ions on the low-frequency branch of resonance of a noncollinear quasi-one-dimensional CsNiBr₃ antiferromagnet is investigated experimentally. It is found that a weak dilution (x=2 to 4%) leads to a considerable (up to 15%) reduction of the resonant gap and of the spin-flop field. The results agree with the theory of Korenblit and Schender, according to which the small parameter of perturbation of the initial system is $x\sqrt {J/J'}$ rather than the impurity concentration x; i.e., a quasi-one-dimensional amplification coefficient exists, which is equal in this case to approximately six.     

Size effects in thin antiferromagnetic layers and “ferromagnet-nonmagnetic metal” multilayer magnetic structures

The specific features of the spin-flop and spin-flip transitions in thin antiferromagnetic layers and “ ferromagnet-nonmagnetic metal” multilayer magnetic structures are considered. The dependence of the magnetic fields corresponding to these phase transitions on the thickness of the antiferromagnet or on the number of layers in the multilayer is determined.     

Parametric magnetoelectric effect in an AC magnetic field

Electric polarization is predicted to occur under the conditions of parametric instability of vibrations of magnetization in a longitudinal high-frequency magnetic field (parametric magnetoelectric effect). The requirements on materials are indicated favoring the observation of this effect. An example of such materials is the easy-plane antiferromagnet Cr₂TeO₆.     

Exchangeless magnetoelectric magnons: A new class of mixed hybrid dipole waves

A layer of an easy-axis antiferromagnet with antisymmetry center is considered. It is shown that the combination of magneto- and electric-dipole mechanisms of the indirect spin–spin interactions gives rise to the previously unknown class of hybrid dipole waves such as exchangeless magnetoelectric magnons and to the spin–spin and spin–dipole resonances accompanying them. The conditions for the formation of these waves are determined.     

Interplay of iron and rare-earth magnetic order in rare-earth iron pnictide superconductors under magnetic field

The magnetic properties of iron pnictide superconductors with magnetic rare-earth ions under strong magnetic field are investigated based on the cluster self-consistent field method. Starting from an effective Heisenberg model, we present the evolution of magnetic structures on magnetic field in RFeAsO(R = Ce, Pr, Nd, Sm, Gd, and Tb) and RFe_2As_2(R =Eu) compounds. It is found that spin-flop transition occurs in both rare-earth and iron layers under magnetic field, in good agreement with the experimental results. The interplay between rare-earth and iron spins plays a key role in the magneticfield-driven magnetic phase transition, which suggests that the rare-earth layers can modulate the magnetic behaviors of iron layers. In addition, the factors that affect the critical magnetic field for spin-flop transition are also discussed.     

Lumped-equivalent circuit model for multi-stage cascaded magnetoelectric dual-tunable bandpass filter

A lumped-equivalent circuit model of a novel magnetoelectric tunable bandpass filter, which is realized in the form of multi-stage cascading between a plurality of magnetoelectric laminates, is established in this paper for convenient analysis.The multi-stage cascaded filter is degraded to the coupling microstrip filter with only one magnetoelectric laminate and then compared with the existing experiment results. The comparison reveals that the insertion loss curves predicted by the degraded circuit model are in good agreement with the experiment results and the predicted results of the electromagnetic field simulation, thus the validity of the model is verified. The model is then degraded to the two-stage cascaded magnetoelectric filter with two magnetoelectric laminates. It is revealed that if the applied external bias magnetic or electric fields on the two magnetoelectric laminates are identical, then the passband of the filter will drift under the changed external field; that is to say, the filter has the characteristics of external magnetic field tunability and electric field tunability. If the applied external bias magnetic or electric fields on two magnetoelectric laminates are different, then the passband will disappear so that the switching characteristic is achieved. When the same magnetic fields are applied to the laminates, the passband bandwidth of the two-stage cascaded magnetoelectric filter with two magnetoelectric laminates becomes nearly doubled in comparison with the passband filter which contains only one magnetoelectric laminate. The bandpass effect is also improved obviously. This research will provide a theoretical basis for the design, preparation, and application of a new high performance magnetoelectric tunable microwave device.