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)     

Study of the Ru sublattice magnetic structure in the magnetic superconductor RuSr2GdCu2O8

In order to investigate the Ru sublattice magnetic structure, a study of the field dependence of the ^99,101Ru nuclear magnetic resonance (NMR) has been carried out on the magnetic superconductor RuSr₂GdCu₂O₈. It is found that the ^99,101Ru NMR signal intensity increases significantly with applied magnetic field up to ≈3 kOe, beyond which, it progressively decreases. In addition, a shift of the NMR peaks to lower frequency is observed to begin at ≈1.3 kOe. These behaviors are shown to be accompanied by a field-induced Ru moment spin-flop in the ab planes, and are understood in terms of a previously proposed type-I antiferromagnetic ordering for the Ru sublattice. Based on this model, the inter-plane antiferromagnetic exchange coupling is determined to be ≈1.8 kOe along with a reversible in-plane spin-flop which is characterized by a field ≈0.6 kOe.     

Dynamics of the intermediate state and domain walls in an external oscillating magnetic field

We study the dynamics of 90-degree domain walls in the intermediate state of antiferromagnets, the state being realized in a first-order spin-flop transition in an external magnetic field. We show that an additional oscillating external magnetic field leads to a drift in the domain walls and find the dependence of the drift velocity on the amplitude, frequency, and polarization of the oscillating field. Finally, we discuss the possibility of the domain structure drifting as a whole. Zh. éksp. Teor. Fiz. 112, 1374–1385 (October 1997)     

Properties of domain walls and magnetoresistance in low-doped La<Subscript>2−<Emphasis Type="Italic">y</Emphasis></Subscript>Sr<Subscript>y</Subscript>CuO<Subscript>4</Subscript>

The properties of the diagonal stripe structures of the Hubbard model are theoretically studied in relation to the incommensurate spin order and the magnetic effects detected in the dielectric phase of low-doped La_2?y Sr_yCuO₄ (y ≤ 0.05). The mean-field approximation is used to investigate the properties of the solutions with domain walls between antiphase antiferromagnetic domains that are centered on bonds. Such periodic structures with 2l sites in a unit cell are shown to have 2(l ? 1) levels in the lower and upper Hubbard subbands and two levels that are separated into the Hubbard gap and correspond to quasi-one-dimensional states localized on domain walls. The calculation results are employed to check the assumption that the low conduction of the dielectric LSCO phase occurs via the network of domain walls. The maximum relative change in the magnetoresistance during a spin-flop transition in a critical magnetic field is estimated, and the giant magnetoresistance is qualitatively explained.     

First order spin flop transition in yttrium iron garnet (YIG)

We have studied the field dependence of the sublattice magnetization of ferrimagnetic yttrium iron garnet (YIG) using neutron scattering. In contrast to the macroscopic spontaneous magnetization that shows the normal field dependence of a soft ferromagnet (sudden saturation at the demagnetization field and no hysteresis) in neutron scattering a field induced first order spin flop transition with considerable hysteresis is observed at a critical field of H_c∼580 G (external field). Considering that with neutron scattering the antiferromagnetic component of ∼4/5 of the total moment is detected preferentially while in the macroscopic magnetization samples the ferromagnetic component of ∼1/5 exclusively it becomes clear that ferromagnetic and antiferromagnetic component have a completely independent field (and temperature) dependence. This indicates that the two magnetic structures have to be viewed as two weakly coupled order parameters. In the zero field ground state the moment orientations of the two ordering structures are orthogonal. Only for fields H₀>H_c a nearly collinear ferrimagnetic order is established by the field.     

NMR study of KCuF3 at low temperatures

The NMR of F¹⁹ nuclei in KCuF₃ has been measured in the a-type single crystal at 1.7 K. Two types of magnetic domains exist; one occupies most part of the crystal and has easy axis along <110>, and the other occupies the rest of the crystal and has easy axis along <100>. In both domains the moment directions are distributed around the easy axes over a considerable angular range. The spin-flop begins with nearly zero applied field in the <110>;-domain. When the magnetic field is rotated in the c-plane, an angular dependence has been observed for those F¹⁹ nuclei which lie on the c-axis. This dependence arises from the alternate stacking of the ground state wave functions of Cu²⁺ ions.     

The Mössbauer effect and magnetic phase transitions

The Mössbauer effect provides a direct method for identifying the spin axis in magnetic crystals and observing magnetic phase transitions. The order of the transition may be inferred from the Mössbauer spectrum. Phase changes can occur as a function of temperature (e.g. when the anisotropy fieldB _A changes sign) or as a function of applied magnetic field. In an antiferromagnet a field ?(2B _E B _A)^1/2 along the spin axis whereB _E is the exchange field causes the spin-flop transition which is normally first order (sharp) whereas the transition to the paramagnetic phase which occurs at higher fields?2B _E is second order (continuous). In quasi-one-dimensional crystals Mössbauer spectra show that the spin-flop transition is first order locally but occurs over a range of fields throughout the crystal, so that the first order character is masked in a conventional magnetization measurement. In fields applied at a finite angle>B _A/2B _E to the spin axis the transition becomes second order, i.e. a continuous rotation of the spins occurs. In canted antiferromagnets (or weak ferromagnets) the spin-flop transition is also continuous; in addition a “screw” re-orientation may be induced by fields applied perpendicular to the spin axis and arises from antisymmetric exchange. For crystals with lowT _N the hyperfine field changes when a magnetic field is applied and has a minimum at a phase transition; this may be used to map out the magnetic phase diagram.     

Anisotropic contributions to the transferred hyperfine field studied using a field-induced spin-reorientation

We report here a comparison between a field-driven spin-flop (TbMn₆Sn_5.46In_0.54) and a temperature-driven spin reorientation (TbMn₆Sn_6–x Ga _x ) in order to demonstrate that the anisotropic contribution to B _hf at the Sn sites can be obtained through the moment reorientation and is independent of the driving force. We show that a complete 90° spin reorientation can be achieved at 300 K in an applied field of 0.57(3) T and that the changes in hyperfine field due to the anisotropic contribution exceed 45% at one of the Sn sites. Quantitative values for the anisotropic constant at the three Sn sites are obtained.     

Magnetoelectric coupling in antiferroelectric and magnetic laminate composites

We researched the properties of magnetoelectric composites between antiferroelectric Pb_0.94La_0.04(Zr_0.55Sn_0.3Ti_0.15)O₃ and magnetic Terfenol-D. The magnetic field enhances the electric field-induced strain and polarization of the composite. The magnetic moment induced by the electric field increases at the electromechanical resonance frequency because the antiferroelectric ceramics exhibit ferroelectric behaviors under a high electric field. The induced magnetic moment increases with decreasing thickness ratio and shows a hysteresis loop with the bias magnetic field. Due to the antiferroelectric characteristics, it also shows a hysteresis loop with the bias electric field, which could be used in the magnetic switch controlled by electricity.     

Incommensurate modulated magnetic structure in orthorhombic EuPdSb

Orthorhombic EuPdSb is known to undergo two magnetic transitions, at 12 K and at T _N≃ 18 K, and in phase III (T < 12 K), single crystal magnetisation data have shown that the spin structure is collinear antiferromagnetic, with magnetic moments along the crystal a axis. From a ¹⁵¹Eu M?ssbauer absorption study, we show that, at any temperature within phase III, all the moments have equal sizes, and that in phase II (12 K< T <18 K) the magnetic structure is modulated and incommensurate with the lattice spacings. The modulation is close to a pure sine-wave just below T _N = 18 K, and it squares up as temperature is lowered. We measured the thermal variations of the first and third harmonics of the moment modulation, and we could determine the first and third harmonics of the exchange coupling. We furthermore show that the antiferromagnetic-incommensurate transition at 12 K is strongly first order, with a hysteresis of 0.05 K, and that the incommensurate-paramagnetic transition at 18 K is weakly first order. Finally, we present an explanation of the spin-flop transition observed in the single crystal magnetisation data in phase III when || in terms of an anisotropic molecular field tensor. Received 17 January 2001 and Received in final form 20 March 2001     

The effects of random fields on the critical and bicritical behavior of Mn x Zn1−x F2

We have carried out a comprehensive study of the static and dynamic spin-spin correlations of Mn _x Zn_1–x F₂ in a magnetic field. Samples withx=0.75 andx=0.5 have been studied. This system exhibits behavior closely related, if not identical, to that of the Random Field Ising Model (RFIM). An additional feature of Mn _x Zn_1–x F₂ is that it exhibits an easily accessible bicritical point; thus one can study the changeover from the RFIM to the uniformXY model with a transverse random field. Quite generally, the instantaneous spin-spin correlations in a field are described by a combination of Lorentzian, Lorentzian-squared and delta function terms the latter corresponds to the long range order (LRO) component. In the Ising phase one finds history dependent behavior as discussed previously. In theXY phase, except very near the spin-flop boundary, one finds ergodic behavior withXY LRO and Lorentzian squared Ising fluctuations. Rather complicated instability effects are found all along the spin-flop boundary. Further, when one establishes LRO in theXY phase and lowers the field through the spin-flop value, one obtains a LRO Ising state in thex=0.75 sample whereas one obtains the field-cooled domain state in thex=0.50 sample. This dramatic difference in behavior is not understood. Our results on the RFIM aspects of the problem are consistent with our previous studies. The transition is dominated by the metastability effects with an underlying equilibrium transition which is either first order or weakly second order (0). The underlying transition manifests itself directly in measurements of the dynamic response nearT _N (H). From the data above the metastability boundary we deduce for the static correlation length exponentv=1.4±0.3 in good agreement with theory. We find for the RFIM crossover exponent _RF=1.5±0.2 where the errors represent the spread in values obtained from different techniques. Finally, we have determined in detail the field-temperature phase diagram of thex=0.5 sample including the critical behavior along the spin-flop line; the latter transition appears to be second order for an extended region.     

Magnetic anisotropy and low-frequency dielectric response of weak ferromagnetic phase in κ-(BEDT-TTF)2Cu[N(CN)2]Cl, where BEDT-TTF is Bis(ethylenedithio)tetrathiafulvalene

We report a detailed characterization of the magnetism and AC transport in single crystals of the organic conductor -(BEDT-TTF)₂Cu[N(CN)₂]Cl by means of magnetic anisotropy measurements and low-frequency dielectric spectroscopy. Magnetic anisotropy obeys Curie-Weiss law with negative Curie-Weiss temperature in the temperature range 300 K-70 K. An antiferromagnetic transition with concomitant canted antiferromagnetic state is established at 22 K. A large hysteresis in the spin-flop transition and magnetic field reversal of the weak ferromagnetic magnetization are documented for the first time. A broad dielectric relaxation mode of moderate strength () emerges at 32 K, and weakens with temperature. The mean relaxation time, much larger than that expected for single-particle excitations, is thermally activated in a manner similar to the DC conductivity and saturates below 22 K. These features suggest the origin of the broad relaxation as an intrinsic property of the weak ferromagnetic ground state. We propose a charged domain wall in a random ferromagnetic domain structure as the relaxation entity. We argue that the observed features might be well described if Dzyaloshinsky-Moriya interaction is taken into account. A Debye relaxation with similar temperature dependence was also observed and seems to be related to an additional ferromagnetic-like, most probably, field-induced phase. We tentatively associate this phase, whose tiny contribution was sample dependent, with a Cu²⁺ magnetic subsystem. Received 15 June 1998 and Received in final form 1 February 1999     

Gap-behavior in the vicinity of a saturation transverse field

We have studied the behavior of the energy gap of the 1D AF spin- XXZ model in a transverse magnetic field (h) using the exact diagonalization technique. The ground state phase diagram consists of two spin-flop and paramagnetic phases. Using a modified finite-size scaling approach, we have computed the critical exponent of the energy gap in the vicinity of the critical transverse field h_c(Δ). Our numerical results confirm that the continuous phase transition from the spin-flop phase to the paramagnetic one is in the universality class of the Ising model in the transverse field (ITF). By applying conformal estimates of a small perturbation (h≪1), we have also justified our numerical results.     

New magnetic states in copper metaborate CuB<Subscript>2</Subscript>O<Subscript>4</Subscript>

The static and resonance properties of copper metaborate CuB₂O₄ were experimentally studied in a magnetic field applied in the crystal tetragonal plane. The field-induced second-order phase transition to a weakly ferromagnetic state was observed in the temperature range 10–20 K. The low-field state is characterized by the absence of spontaneous moment, and it represents, presumably, a long-period helicoid. At temperatures below 2 K, two sequential first-order phase transitions were observed. They were accompanied by jumps in resonance absorption with a hysteresis upon changing field-scan direction. These transitions can be caused by the transformation of the incommensurate spin structure into the helicoidal states with periods commensurate with the lattice translation period.     

Etude de la structure magnetique et de la transition de typr “spin-flop” de Rb2FeF5

The nature of the magnetic interactions in the chain compound Rb₂FeF₅ has been investigated using neutron diffraction and magnetic measurements under high applied fields. Rb₂FeF₅ orders antiferromagnetically at T_N = 8.0 ± 0.5 K; the magnetic structure is of the A_Z + G_X mode and the moment of the Fe³⁺ ion extrapoled to 0K is 3.5 ± 0.2 μ_B, this low value being due to zero-point spin reduction. Within a chain the Fe³⁺ ions are antiferromagnetically coupled with an exchange constant of J/k = ?8.8 K. A spin-flop behavior has been observed and interpreted on the basis of the molecular field theory. The critical field was found to be H_C = 65 kOe at 1.7 K.     

The quantum compass chain in a transverse magnetic field

We study the magnetic behaviors of a spin-1/2 quantum compass chain (QCC) in a transverse magnetic field, by means of the analytical spinless fermion approach and numerical Lanczos method. In the absence of the magnetic field, the phase diagram is divided into four gapped regions. To determine what happens by applying a transverse magnetic field, using the spinless fermion approach, critical fields are obtained as a function of exchanges. Our analytical results show, the field-induced effects depend on in which one of the four regions the system is. In two regions of the phase diagram, the Ising-type phase transition happens in a finite field. In another region, we have identified two quantum phase transitions (QPT)s in the ground state magnetic phase diagram. These quantum phase transitions belong to the universality class of the commensurate-incommensurate phase transition. We also present a detailed numerical analysis of the low energy spectrum and the ground state magnetic phase diagram. In particular, we show that the intermediate state (h _c1 < h < h _c2) is gapful, describing the spin-flop phase.     

Real-space renormalization group study of a two-dimensional nonlinear massless field

The magnetic susceptibility of CuF₂·2H₂O has been measured as a function of magnetic field from 1.5 to 10 K. The spin-flop transition was observed and its value extrapolated to zero temperature is H_SF(0) = 30.5 kOe. This critical field is in very good agreement with data obtained from zero field measurements.     

La2CuO4在平行CuO2面磁场中的磁转变

本文在平均场近似下,给出一个关于在平行CuO₂面的磁场作用下,La₂CuO₄的磁转变理论。理论给出的临界磁场随温度T的变化,在定性上与实验相符。但是,从本文理论看,不存在多重临界点。简单讨论了理论和实验不符合的原因。 关键词：     

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.     

The field induced magnetization reorientation in weak ferromagnets—III: The case of non-equivalent single ion anisotropy

The phase transitions in a single-ion anisotropy type weak ferromagnet at 0°K are studied using a two sublattice model and molecular field theory. In an external field applied along the antiferromagnetic axis, there exist three stable phases, denoted as weak ferromagnetic (WF), spin-flop (SF), and metamagnetic (MM). The character of the WF to SF transition changes from first to second order as the angle between a sublattice uniaxial anisotropy axis and the antiferromagnetic axis is increased. The tricritical field for this phase transition is proportional to the one-half power of the uniaxial anisotropy field, when the latter is much smaller than the exchange field. The transitions to and from the MM phase are always of first order. The former can result from either a WF or a SF phase instability threshold being reached. The latter always results in a transition to the WF phase. All three phases can, under specified conditions, coexist for a range of applied field values. In this case, an instability of the WF phase always results in a first order transition to the SF phase.