Antiferromagnetic Resonance Frequency Research Articles

Antiferromagnetic resonance and dielectric properties of rare-earth ferroborates in the submillimeter frequency range

The magnetoresonance and dielectric properties of a number of crystals of a new family of multiferroics, namely, rare-earth ferroborates RFe3(BO3)4 (R = Y, Eu, Pr, Tb, Tb0.25Er0.75), are studied in the sub-millimeter frequency range (ν = 3–20 cm−1). Ferroborates with R = Y, Tb, and Eu exhibit permittivity jumps at temperatures of 375, 198, and 58 K, respectively, which are caused by the R32 → P3121 phase transition. Antiferromagnetic resonance (AFMR) modes in the subsystem of Fe3+ ions are detected in the range of anti-ferromagnetic ordering (T < TN = 30–40 K) in all ferroborates that have either an easy-plane (Y, Eu) or easy-axis (Pr, Tb, Tb0.25Er0.75) magnetic structure. The AFMR frequencies are found to depend strongly on the magnetic anisotropy of a rare-earth ion and its exchange interaction with the Fe subsystem, which determine the type of magnetic structure and the sign and magnitude of an effective anisotropy constant. The basic parameters of the magnetic interactions in these ferroborates are found, and the magnetoelectric contribution to AFMR is analyzed.

Antiferromagnetic Resonance in LaMnO3

The external magnetic field and temperature dependencies of the magnetic structure and antiferromagnetic resonance frequencies in LaMnO3 are studied. A recently developed model which includes orbitally dependent superexchange and single-ion anisotropy interactions is used. The temperature dependence is described in the mean-field approximation in good agreement with experiments. H–T phase diagrams in the main field directions (a, b, c) are drawn. The problem of the origin of weak noncollinearity of Mn3+ spins is solved in our work. Also an optimal method for obtaining an expression of the magnetic free energy is represented.

Antiferromagnetic resonance in LaMnO3

The magnetic structure of LaMnO3 and the antiferromagnetic resonance frequencies in it are investigated as functions of magnetic field H and temperature T. A previously developed model that includes the orbitally dependent superexchange and single-ion anisotropy is used. The temperature dependences described in the framework of the mean field approximation are in good agreement with experiment. The H–T phase diagrams are constructed for magnetic field directed along the principal crystallographic directions (a,b,c). The problem of the origin of the weak noncollinearity of the Mn3+ spins is solved. An optimal method of obtaining the expression for the magnetic free energy is proposed.

Magnetic Structure and Antiferromagnetic Resonance Spectrum in Manganites: The Effect of Orbital Structure

This paper presents an investigation into the temperature dependence of magnetic structure and antiferromagnetic resonance frequencies. Taking into account the crystal and orbital structures of pure lanthanum manganite, it is shown that the orbital structure plays a crucial role in forming the magnetic structure. The H-T phase diagrams for the magnetic structure are drawn.

Magnetic and magneto-lattice dynamics in GdMn2O5

The dynamics of magnetoelectric RMn2O5 crystals (R=Eu and Gd) was studied in the frequency and temperature ranges 20–300 GHz and 5–50 K, respectively. The crystals possessed magnetic and ferroelectric long-range order and had close transition temperatures, TN, C≃36 and 30 K for R=Eu and Gd, respectively. Mixed magneto-lattice excitations were observed in GdMn2O5; the excitations were most intense near the transition temperature T≃30 K at frequencies close to the antiferromagnetic resonance frequencies of the Mn subsystem. Along with the antiferromagnetic resonance of the Mn subsystem, the ferromagnetic resonance of the Gd subsystem was observed in GdMn2O5 in an external magnetic field. No such dynamics was characteristic of EuMn2O5.

Modulational instability of nonlinear spin waves in easy-axis antiferromagnetic chains. II. Influence of sample shape on intrinsic localized modes and dynamic spin defects

An indirect method of producing intrinsic localized modes (ILM's) is to drive an unstable uniform mode to large amplitude. In this investigation the intimate connection between the shape-dependent demagnetization factor, the modulation instability, and ILM behavior is established. The stability of the antiferromagnetic resonance (AFMR) against breakup into such ILM's is shown to depend strongly on the frequency difference between the linear AFMR and the long-wavelength limit of the spin-wave dispersion curve. This difference, which stems from the long-range dipole-dipole interactions, depends on the demagnetization factor and hence the sample shape. Here it is demonstrated initially with linear perturbation analysis and later with molecular-dynamics simulations that the instability characteristics and ILM production properties depend strongly on the sign and magnitude of the frequency difference. The simulations show that when the AFMR frequency is coincident with the spin-wave band, a spatially coherent train of ILM's appears, but this coherence is lost at long times. When the AFMR frequency is inside the spin-wave band, the Suhl instability populates the degenerate spin waves, whose subsequent modulational instability leads to traveling and decelerating ILM's. When the AFMR frequency is below the spin-wave band, a large-amplitude threshold for the instability appears. Above this threshold ILM's and localized dynamic spin flops occur, confined to specific lattice regions.

High-frequency properties of KNiPO4 in alternating magnetic and electric fields

The dependences of the antiferromagnetic resonance frequencies on the constant magnetic field H and constant electric field E are calculated for a KNiPO4 crystal with spontaneous electric polarization and antiferromagnetic order. It is demonstrated that the KNiPO4 crystal is characterized by an exchange-enhanced effect of the electric field E on the antiferromagnetic resonance frequencies. This effect is not revealed in the magnetoelectric materials studied earlier. It is established that oscillations of both magnetization and electric polarization exhibit resonance response at antiferromagnetic resonance frequencies. The expressions for these responses in alternating magnetic and electric fields are presented.

High-field ESR spectroscopy of the spin dynamics inLa1−xSrxMnO3(x&lt;~0.175)

High-field electron-spin-resonance (ESR) experiments have been carried out in single crystals of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ in the concentration range $0&lt;~x&lt;~0.175.$ Different quasioptical arrangements have been utilized for frequencies $40&lt;~\ensuremath{\nu}&lt;~700 \mathrm{GHz}$ and for magnetic fields $B&lt;~12 \mathrm{T}.$ A splitting of the antiferromagnetic resonance (AFMR) mode is observed in the magnetic field for the parent compound ${\mathrm{LaMnO}}_{3},$ in agreement with the antiferromagnetic structure of this material. Abrupt changes in the AFMR frequencies have been observed around $x\ensuremath{\simeq}0.025,$ and were attributed to a transition between a pure antiferromagnetic and a canted state. For increasing Sr doping the observed AFMR modes are split even in a zero field, which can be naturally explained using a concept of a canted magnetic structure for $x&lt;0.1.$ In ${\mathrm{La}}_{0.825}{\mathrm{Sr}}_{0.175}{\mathrm{MnO}}_{3}$ the ESR spectra are consistent with the ferromagnetic and metallic state. The lines of ferromagnetic resonance and ferromagnetic antiresonance can be clearly observed. For intermediate concentrations $0.1&lt;~x&lt;~0.15$ complicated ESR spectra were observed, which can be well explained by a single ferromagnetic resonance mode and taking electrodynamic effects into account.

Nonlinear surface polaritons in semimagnetic semiconductor

The paper develops the theory of s-polarized nonlinear surface polaritons in the semimagnetic semiconductor placed in the external static magnetic field. A model of a uniaxial nonlinear medium for the semimagnetic semiconductor is considered. Rigorous criteria of existence of different new types of nonlinear surface polaritons are formulated. It is shown that the new polaritons may exhibit monotonic and nonmonotonic fall of the field in a nonlinear medium. We suggest the method of estimation of the antiferromagnetic resonance frequency in the semimagnetic semiconductors due to the nonlinear surface polariton excitation by the attenuated total reflection method.

Determining the Antiferromagnetic Resonance Frequency in Semimagnetic Semiconductors by Means of Surface Polaritons

In the articles surface polaritons are investigated in semimagnetic semiconductors in a phase of antiferromagnetic ordering. The influence of the external static magnetic field and the electron concentration on the dispersion properties of the surface polaritons is determined. It is shown that the surface polaritons can have a negative dispersion, independent of the direction of propagation in the Voigt geometry. A new method based on excitation of the surface polaritons in the semimagnetic semiconductors is proposed for determination of the antiferromagnetic resonance frequency.

Observation of single ion transitions in the ordered states of FeCl2

Electron spin resonance (ESR) experiments using a far infrared laser and a superconducting magnet have been performed on a single crystal sample of the metamagnet FeCl2. A new ESR signal has been observed at a much higher frequency than the ordinary antiferromagnetic and ferromagnetic resonance frequencies. The new ESR signal originates from a transition within the lowest triplet state of single Fe2+ ions in the presence of exchange fields.

Spin dynamics and onset of Suhl instability in bcc solid3He in the nuclear-ordered U2D2 phase

Pulsed NMR experiments have been performed on U2D2 solid3He with a single domain in high fields where the Larmor frequency was much larger than the zero-field antiferromagnetic resonance frequency. The free induction signal decayed rapidly under certain conditions. The rapid decay is attributed to the onset of the instability of the uniform precession. We propose a model for the instability due to the self-induced emission from the upper-mode magnon to the lower-mode magnon which is similar to the Suhl instability in electronic ferromagnets. A large negative frequency shift was observed during the rapid decay, which is consistently explained by the model. Under stable conditions of the spin motion outside of the instability region, we observed the tipping-angle-dependent frequency shift and multiple spin echoes, both of which agree well with Namaizawa's theory.

Antiferromagnetic resonance in over wide ranges of frequency and magnetic field

Antiferromagnetic resonance (AFMR) experiments on a well characterized single-crystal sample of have been performed using a spectrometer with wide ranges of both frequency and magnetic field. All the AFMR modes predicted from the theory have been observed. We find a significant contribution of the non-linear term in the expression of the AFMR frequency to the temperature dependence of the resonance point in the high-field low-frequency region.

Oblique-incidence far-infrared reflectivity study of the uniaxial antiferromagnet FeF2.

Theoretical and experimental results are presented for high-resolution frequency scans of 45\ifmmode^\circ\else\textdegree\fi{} oblique-incidence far-infrared reflectivity spectra of ${\mathrm{FeF}}_{2}$ in the vicinity of the antiferromagnetic resonance frequency. The magnetic field is vertical and the plane of incidence horizontal. Calculations are presented and illustrated numerically for an arbitrary angle \ensuremath{\theta} between the crystal c axis and the magnetic field ${\mathit{H}}_{0}$. The main point of principle is that the equilibrium spin configuration and therefore the magnetic susceptibility depend on \ensuremath{\theta}. The reflectivity R is nonreciprocal, i.e., R(-${\mathit{H}}_{0}$)\ensuremath{\ne}R(${\mathit{H}}_{0}$). For \ensuremath{\theta}=0\ifmmode^\circ\else\textdegree\fi{} magnetic features are seen only in s polarization (E field vertical), but otherwise they are seen in both polarizations. Except for \ensuremath{\theta}=0\ifmmode^\circ\else\textdegree\fi{} and 90\ifmmode^\circ\else\textdegree\fi{} weak s to p and p to s reflections are predicted. We discuss the instrumental developments required for the high resolution that is necessary for this spectroscopy. Experimental results for \ensuremath{\theta}=45\ifmmode^\circ\else\textdegree\fi{} are presented (\ensuremath{\theta}=0\ifmmode^\circ\else\textdegree\fi{} and 90\ifmmode^\circ\else\textdegree\fi{} are in a previous publication). The whole range of spectra, including the s to p and p to s reflections, are accounted for with the same set of parameters as were used previously to fit the \ensuremath{\theta}=0\ifmmode^\circ\else\textdegree\fi{} and 90\ifmmode^\circ\else\textdegree\fi{} results.

Antiferromagnetic resonance in Rb2MnCl4

Antiferromagnetic resonance (AFMR) experiments on a single-crystal sample of Rb2MnCl4 have been performed using a spectrometer with wide frequency and magnetic field ranges. All the AFMR modes predicted for a uniaxial antiferromagnet have been observed. We find a significant contribution of the non-linear term in the expression of the AFMR frequency to the temperature dependence of the resonance point in the high-field low-frequency region.

Nuclear-spin-relaxation mechanisms of nuclear-ordered bcc solid 3He in the low-field phase.

cw-NMR experiments have been performed on nuclear-ordered bcc solid $^{3}\mathrm{He}$ in the low-field phase. We measured the cw-NMR spectrum and its linewidth as a function of external magnetic fields and temperatures for single-crystal samples having various crystal orientations relative to the external field. From the data on the external magnetic field dependence of the linewidth, we found clear evidence of the field region where the three-magnon process is forbidden. We compared our experimental data with the recent theoretical calculation by Ohmi and Tsubota on the lifetime of a uniform spin precession via the three- and four-magnon processes. The agreement is satisfactory. We estimated the size and distribution of the magnetic domains by using an NMR imaging technique. The size of a magnetic domain was found to be comparable to that of a whole crystal. We found some evidence that a crystal in the ordered phase was distorted (on order of ${10}^{\mathrm{\ensuremath{-}}3}$) uniaxially due to magnetostriction. In some crystal samples, we observed a peculiar field dependence of the antiferromagnetic resonance frequency ${\mathrm{\ensuremath{\Omega}}}_{0}$.

Properties of anisotropic spin-density-wave and triplet superconducting states in quasi-one-dimensional conductors

A weak-coupling itinerant electron theory for spin-density-wave phases in quasi-one-dimensional conductors is presented. The phase diagram shows easy-axis states and a spin-flop transition. There are two finite-frequency antiferromagnetic resonance frequencies, forwhich explicit expressions are derived. Analogous results are found for spin anisotropic triplet superconducting phases. Especially, there is an unusual “spin-flop” transition between states of different spin structures, and there are two ESR resonance frequencies.

Theoretical explanation of the antiferromagnetic anisotropy in organic superconductors: A probe for the spin-density-wave amplitude and nesting vector

The experimentally observed anisotropy, in the spin-density-wave (SDW) state of tetramethyl-tetraselenafulvalene and tetramethyltetrathiafulvalene salts ${(\mathrm{TMT}C\mathrm{F})}_{2}X$ ($C=\mathrm{Se} or \mathrm{S}$ and $X=\mathrm{As}{\mathrm{F}}_{6},\mathrm{Cl}{\mathrm{O}}_{4},\dots{}$), is quantitatively explained by pure dipolar anistropy in sulfur compounds and a competition between dipolar and spin-orbit coupling in selenium salts. In sulfur compounds, dipolar anisotropy leads to a hard axis close to $\mathbf{a}$. In selenium compounds, the competing spin-orbit interactions favor the stacking axis $\mathbf{a}$ (perpendicular to the molecular plane) and permute the hard and intermediate axes, as experimentally observed. We prove that the variations of the nesting vector $\mathbf{Q}$, recently confirmed by proton NMR, are responsible for the large differences observed in the anisotropy axes for different compounds with the same TMTCF cation. We calculate the eigenstates of the anisotropy tensor in terms of $\mathbf{Q}$ and deduce from the experimental anisotropy axes and antiferromagnetic resonance (AFMR) frequencies the amplitude $\ensuremath{\delta}$ and $\mathbf{Q}$ vector of the SDW for all the salts that have been experimentally investigated. Although the electrical properties of Se and S compounds strongly differ, their SDW states are strikingly similar with $\ensuremath{\delta}$ of order 10% and $0.1{b}^{*}l{Q}_{b}l0.4{b}^{*}$.

On the Possible Exchange Parameters of bcc Solid3He

Applying the spin wave theory to bcc solid 3He and taking into account interactions between spin waves, the free energy at finite temperatures, antiferromagnetic resonance frequency in the ordered phase and magnetic field strength where the transition from the uudd state to the pseudoferromagnetic state occurs, are calculated. If the exchange parameters are assumed to be K P = K F = -0.2471 mK and j t = -0.1565 mK, large positive high temperature expansion coefficient e 3 determined from specific heat and pressure measurements by Fukuyama et al. and Sawada et al. can be explained. The temperature dependence of free energy between 0.3 and 0.7 mK determined by Osheroff et al. is also explained as a combined effect of the interactions between spin waves and thermal excitations of spin waves with various wave lengths.

New effective-medium approach for randomly diluted uniaxial antiferromagnets: Application toFe1−xZnxF2

A new model for uniaxial anisotropy in randomly diluted Heisenberg antiferromagnets, which is particularly appropriate for systems with high anisotropy, is presented. A generalized coherent-potential approximation has been developed to describe spin waves in those random systems where each spin is subjected to a local anisotropy field dependent on the magnetic (non-magnetic) occupation of its neighborhood. The results are in very good agreement with recent experimental data for the antiferromagnetic resonance frequency in ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Zn}}_{x}{\mathrm{F}}_{2}$.