Ferroelectricity in spiral magnets

It was recently observed that the ferroelectrics showing the strongest sensitivity to an applied magnetic field are spiral magnets. We present a phenomenological theory of inhomogeneous ferroelectric magnets, which describes their thermodynamics and magnetic field behavior, e.g., dielectric susceptibility anomalies at magnetic transitions and sudden flops of electric polarization in an applied magnetic field. We show that electric polarization can also be induced at domain walls and that magnetic vortices carry electric charge.     

Theory of site-disordered magnets

In realistic spinglasses, such as , and , magnetic atoms are located at random positions. Their couplings are determined by their relative positions. For such systems a field theory is formulated. In certain limits it reduces to the Hopfield model, the Sherrington-Kirkpatrick model, and the Viana-Bray model. The model has a percolation transition, while for RKKY couplings the “concentration scaling” occurs. Within the Gaussian approximation the Ginzburg-Landau expansion is considered in the clusterglass phase, that is to say, for not too small concentrations. Near special points, the prefactor of the cubic term, or the one of the replica-symmetry-breaking quartic term, may go through zero. Around such points new spin glass phases are found. Received: 27 April 1998 / Received in final form: 27 July 1998 / Accepted: 13 August 1998     

Nonlinear dynamics of breathers in the spiral structures of magnets

The structure and properties of pulsating solitons (breathers) in the spiral structures of magnets are analyzed within the sine-Gordon model. The breather core pulsations are shown to be accompanied by local shifts and oscillations of the spiral structure with the formation of “precursors” and “tails” in the moving soliton. The possibilities for the observation and excitation of breathers in the spiral structures of magnets and multiferroics are discussed.     

Origin of ferroelectricity in high-T(c) magnetic ferroelectric CuO

Cupric oxide is a unique magnetic ferroelectric material with a transition temperature significantly higher than the boiling point of liquid nitrogen. However, the mechanism of high-T(c) multiferroicity in CuO remains puzzling. In this Letter, we clarify the mechanism of high-T(c) multiferroicity in CuO by using combined first-principles calculations and an effective hamiltonian model. We find that CuO contains two magnetic sublattices, with strong intrasublattice interactions and weakly frustrated intersublattice interactions. The weak spin frustration leads to incommensurate spin excitations that dramatically enhance the entropy of the multiferroic phase and eventually stabilize that phase in CuO.     

Temperature dependences of ferroelectricity and resistive switching behavior of epitaxial BiFeO_3 thin films

We investigate the resistive switching and ferroelectric polarization properties of high-quality epitaxial BiFeO3 thin films in various temperature ranges. The room temperature current–voltage(I–V) curve exhibits a well-established polarization-modulated memristor behavior. At low temperatures( 253 K), the I–V curve shows an open circuit voltage(OCV), which possibly originates from the dielectric relaxation effects, accompanied with a current hump due to the polarization reversal displacement current. While at relative higher temperatures( 253 K), the I–V behaviors are governed by both space-charge-limited conduction(SCLC) and Ohmic behavior. The polarization reversal is able to trigger the conduction switching from Ohmic to SCLC behavior, leading to the observed ferroelectric resistive switching. At a temperature of 298 K, there occurs a new resistive switching hysteresis at high bias voltages, which may be related to defect-mediated effects.     

Spiral magnetic structure in Heisenberg and non-Heisenberg magnets

The possibility of forming a modulated (spiral) magnetic structure in anisotropic Heisenberg and non-Heisenberg magnets with a frustrated Heisenberg exchange interaction has been investigated. It has been demonstrated that, apart from homogeneous (ferromagnetic and quadrupole) states, the Heisenberg magnets with a strong easy-plane anisotropy can have a spiral magnetic structure. The axis of the spiral coincides with the direction of the external magnetic field. The inclusion of the biquadratic exchange interaction leads to a narrowing of the magnetic field region of the existence of the spiral structure. In the absence of the external magnetic field, the spiral structure can be formed only in the case of easy-axis anisotropy.     

Special features of magnetic susceptibility dispersion in nanocrystalline magnets

It is demonstrated theoretically that the initial (both elastic and viscous) magnetic susceptibility components for nanocrystalline magnets caused by the processes of rotations (in the region of linear response) have resonant rather than relaxation character typical already for the susceptibility component caused by displacements of domain boundaries. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 88–92, June, 2007.     

High-T(c) ferroelectricity emerging from magnetic degeneracy in cupric oxide

Cupric oxide is multiferroic at unusually high temperatures. From density functional calculations we find that the low-T magnetic phase is paraelectric, and the higher-T one is ferroelectric with a size and direction of polarization in good agreement with experiments. By mapping the ab?initio results on to an effective spin model, we show that the system has a manifold of almost degenerate ground states. In the high-T magnetic state noncollinearity and inversion symmetry breaking stabilize each other via the Dzyaloshinskii-Moriya interaction. This leads to an unconventional mechanism for multiferroicity, with the particular property that nonmagnetic impurities enhance the effect.     

Multiscale magnetic domains observed in die-upset melt-spun magnets using magnetic force microscopy

Magnetic force microscopy (MFM) has been used to examine the magnetic reversal process in samples of Nd_13.9(Fe₉₂Co₈)_80.3B_5.5Ga_0.5, one made by hot-pressing and one made by thermomechanical deformation, or die-upsetting. Both samples were imaged in various magnetic states under zero-field conditions. In both the hot-pressed and die-upset samples, the finest scale contrast observable consisted of individual magnetic clusters of approximately 10 grains in volume, on a scale between 150 and 300 nm. In the hot-pressed sample this fine-scale contrast was similar regardless of the magnetic state of the sample and no larger-scale contrast was observed, indicating that exchange interactions do not greatly affect the locality of the magnetic reversal in this sample. The die-upset sample in the thermally demagnetized state showed only fine-scale magnetic contrast. Once the sample was saturated, however, interaction domains formed which extended over many microns, although the fine-scale contrast could still be observed within these larger interaction domains. The increased crystallographic alignment between grains in the die-upset sample as compared with the hot-pressed sample, underlies the hypothesis that intergranular exchange interactions cause the reversal to be localized at the peripheries of previously reversed regions, leading to the creation of large interaction domains.     

Theory of thermal remagnetization of permanent magnets

A self-consistent mean-field theory explaining the thermal remagnetization (TR) of polycrystalline permanent magnets is given. The influence of the environment of a grain is treated by an inclusion approximation, relating the field inside the grain to the local field outside by means of an internal demagnetization factor n. For the switching fields and the fluctuations of the local fields around the mean field, Gaussian distributions of widths σ_s, and σ_f, respectively, are assumed. The isothermal hysteresis curve, the recoil curves, and the TR dependent on the model parameters n, σ_s, and σ_f are calculated. Furthermore, the influence of the initial temperature and the strong dependence of the TR on the demagnetization factor of the sample are studied, and it is shown that for reasonable parameter sets TR effects up to 100% are possible. The theoretical results correspond well with the experimental situation.     

Observation of a bi-domain state and nucleation free switching in mesoscopic ring magnets

We present the results of a study of the magnetic properties of an array of 34-nm thick Co(100) epitaxial ring magnets, with inner and outer diameters of d(in) = 1.3 microm and d(out) = 1.6 microm, respectively. Magnetic measurements and micromagnetic simulations show that a two step switching process occurs at high fields, indicating the existence of two different stable states. In addition to the vortex state, which occurs at intermediate fields, we have identified a new bi-domain state, which we term the onion state, corresponding to opposite circulation of the magnetization in each half of the ring. The onion state is stable at remanence and undergoes a simple and well characterized nucleation free switching.     

Microstructures and magnetic properties of Fe---Pt permanent magnets

We have investigated the magnetic properties of Fe---38.5Pt, Fe---39.5Pt and Fe---50.0Pt (at%) alloys after various heat treatment conditions using a vibrating sample magnetometer, and correlated these properties with the microstructures of the alloys by transmission electron microscopy. The Fe---50Pt alloy shows poor magnetic hardness regardless of the heat treatment conditions. The magnetic hardness of the Fe---39.5Pt alloy shows a maximum value after annealing for 10 h at 873 K, while it monotonically decreases after annealing at 1073 K. The alloy with the highest coercivity was composed of a single phase γ₁ with an average domain size of approximately 10 nm. The electron diffraction results indicate that the alloy is frustrated with accumulated stress, induced by a cubic → tetragonal transformation which occurs without twinning. On the other hand, when stress is relieved by twin formation after prolonged aging, the coercivity decreases. By annealing at 1073 K, the well known polytwin structure evolves. However, only poor hard magnetic properties are observed when this polytwin structure appears. Hence, the highest coercivity is attributed to the formation of nanoscale L1₀ ordered antiphase domains which is expected to be a highly anisotropic single domain magnetic particle.     

The magnetic memory effect in fullerene-containing single molecular magnets

We studied the influence of fullerenes on the properties of molecular magnets of the family of oxocarboxylate clusters of Mn₁₂. A strong asymmetry in the hysteresis loop was observed for the first time. As opposed to the well-known effect of the displacement of a hysteresis loop along the magnetic field axis (the phenomenon of exchange bias), the observed effect of magnetic memory resides in the displacement along the vertical axis.     

NMR and other magnetic resonance techniques in unstable magnets

This paper reviews and compares the use of nuclear magnetic resonance (NMR) and related hyperfine techniques [muon spin rotation (μSR) and, to a lesser extent, other methods] in the study of 4f and 5f magnetism in “unstable magnets”, i.e., intermediate-valent and heavy-fermion materials. In both NMR and μSR the features of interest are the spectral shape, the frequency shiftK (Knight shift in metals) and the spin-lattice relaxation rate 1/T ₁. For temperatures below the characteristic or “Kondo” temperatureT ₀ these experiments given evidence for (1) modification of the transferred hyperfine field [nonlinearK(χ)]. (2) spin fluctuations with a characteristic fluctuation rate ∼k _B T ₀/h, (3) strong energy-gap anisotropy (zeros of the gap along lines on the Fermi surface) in heavy-fermion superconductors, (4) spin-singlet Cooper pairing from the change in muon Knight shift in superconducting UBe₁₃, and (5) very weak static magnetism (10⁻¹–10⁻³ μ_B/f atom) in CeAl₃, CeCu₂Si₂, U_1−x Th _x Be₁₃ (x=0.033), and UPt₃. There is some controversy concerning the interpretation of 1/T ₁ well aboveT ₀ in UBe₁₃; the situation is reviewed.     

Study of the magnetic interaction in nanocrystalline Pr-Fe-Co-Nb-B permanent magnets

The magnetic properties of an isotropic, epoxy resin bonded magnets made from Pr-Fe-Co-Nb-B powder were investigated. The magnetization reversal process and magnetic parameters were examined by measurements of the initial magnetization curve, major and minor hysteresis loops and sets of recoil curves. From the initial magnetization curve and the field dependencies of the reversible and irreversible magnetization components derived from the recoil loops it was found that the magnetization reversal process is the combination of the nucleation of reversed domains and pinning of domain walls at the grain boundaries and the reversible rotation of magnetization vector in single domain grains. The interactions between grains were studied by means of δM plots. The nonlinear behavior of δM curve approve that the short range intergrain exchange coupling interactions are dominant in a field up to the sample coercivity.The interaction domains and fine magnetic structure were revealed as the evidence of exchange coupling between soft α-Fe and hard magnetic Nd₂Fe₁₄B grains.