Spin wave dispersion of FeBO3 at small wavevectors

Brillouin scattering from thermally excited magnons and ferromagnetic resonance are used to determine the spin wave dispersion of the low-frequency spin wave branch in FeBO₃, a transparent weak ferromagnet. In addition to the dominant exchange and Zeeman contributions, the investigation takes into account magnetic dipole and magnetoelastic interactions. Due to the antisymmetric exchange enhancement the material exhibits a broad spin wave band and a large gap energy at small magnetic fields. Competing directional dependences of the dipole and the exchange energy produce a degeneracy of spin waves with a certain magnitude of the wavevector propagating in different directions. The gap energy is shown to be due to magnetoelastic coupling, whereas the contribution of the anisotropy in the easy plane is negligible atT=300 K.     

Spin waves propagation in Fibonacci quasiperiodic metamagnet superlattices

In this work we present a fractal analysis of the spin wave modes propagating in a Fibonacci quasiperiodic metamagnetic superlattices consisting of a ferromagnetic material (layer A being Fe) and a metamagnetic material (layer B being FeBr₂). The spin wave modes are obtained in the exchange regime, using the transfer matrix technique. Our numerical results show that the power law for this system is independent of the dimensionless in-plane wavevector kxa$k_{x}a$, and that the spectrum present a multifractal characteristic, exhibiting several scale invariant windows for a fixed value of the in-plane wavevector. The results reported here can be experimentally observed by light scattering techniques.     

Abrupt changes in the temperature dependence of the ESR linewidth of La<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript> Ca<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> MnO<Subscript>3</Subscript> single crystals

The temperature dependence of the ESR linewidth in La_{1 ? x} Ca _x MnO₃ single crystals with various dopant concentrations (x = 0.18, 0.2, 0.22, 0.25, and 0.3) has been studied. An abrupt decrease in the ESR linewidth has been observed in the samples with a dopant concentration of x = 0.18 and 0.2 near the respective temperatures T _OO′ ≈ 260 and 240 K of the orthorhombic to pseudocubic structural phase transition. The abrupt decrease in the ESR linewidth by approximately 180 Oe has been also observed in the whole temperature range when the concentration is increased from x = 0.2 to x = 0.22. The formula for the fourth moment of the ESR line has been derived including both crystal fields and isotropic exchange interactions and taking into account the difference between the exchange coupling of a spin to its nearest in-plane and out-of-plane neighbors. The formula has been used to estimate the parameter D of the crystalline field on Mn³⁺ ions.     

On the decay of correlations inSO(n)-symmetric ferromagnets

We prove that for low temperaturesT the spin-spin correlation function of the two-dimensional classicalSO(n)-symmetric Ising ferromagnet decays faster than |x|^–constT providedn2. We also discuss a nearest neighbor continuous spin model, with spins restricted to a finite interval, where we show that the spin-spin correlation function decays exponentially in any number of dimensions.Work supported in part by NSF, Grant PHY76-17191A Sloan Fellow     

Spin waves in static non-periodic magnetic structures

We have studied the propagation of spin waves in a number of static non-periodic magnetic structures. We have established that (1) a ferromagnetic spin wave can ride over a domain wall with little reflection if its wavelength is less than twice the thickness of the wall; (2) in a ferromagnet with a set of parallel but irregularly spaced domain walls the spin wave linewidth is determined by the product of the scattering strength of the walls and the degree of randomness of the wall spacings; and (3) spin waves of rather narrow linewidths can exist in continuously varying irregular spin structures.     

Temperature dependence of SSW linewidth in metallic thin films

Measurements of the standing spin wave linewidth H _n at 17·6 GHz from 6 °K to 400 °K are presented for two permalloy films whose structure is either polycrystalline or epitaxial. It is shown that in both films, H _n is proportional ton ² (n is the mode number). Although H _n is little modified by temperature (20% variation maximum over the entire temperature range), we experimentally state that the slopep of H _n (n ²) is a linear function of the saturation magnetizationM which agrees with a planar inhomogeneity model.The authors wish to thank H. Pascard and F. Hoffmann who have prepared and selected the films as also E. Villain and A. Gatineau for invaluable technical assistance.     

Magic-angle-spinning NMR on solid biological systems. Analysis Of the origin of the spectral linewidths

Magic-angle-spinning (MAS) high-power ¹H-decoupled ¹³C and ³¹P NMR has been applied to solid biological materials to obtain information about the mechanisms that determine the spectral linewidths. The line broadening in MAS ³¹P NMR spectra of solid tobacco mosaic virus (TMV) has been investigated by selective saturation and T₂ measurements. About 90 Hz stems from homogeneous effects, whereas the inhomogeneous contribution is approximately 100 Hz. The inhomogeneous line broadening is assigned to macroscopic inhomogeneities in the sample and not to variations in the nucleotide bases along the RNA strand in TMV. It is concluded that sample preparation is of vital importance for obtaining well-resolved spectra. Under optimal preparation techniques the isotropic values of the chemical shift of the different ³¹P sites have been determined to obtain information about the secondary structure of the viral RNA. The chemical shift anisotropy has been determined from the relative intensities of the spinning side bands in the spectra. The chemical shift information is used to make a tentative assignment of the resonance in terms of the three structurally distinguishable phosphate groups in TMV. The origin of the linewidths in MAS NMR has been examined further by ¹³C NMR of approximately 10% ¹³C-enriched coat protein of cowpea chlorotic mottle virus, using selective excitation and saturation techniques, as well as measurements of the relaxation times T_1γ and T₂. The CO resonance in the spectrum is composed of an inhomogeneous and homogeneous part with a total linewidth of 700 Hz. The homogeneous linewidth, contributing with 200 Hz, is found to arise from slow molecular motions in the solid on a millisecond timescale.     

Spin physics in solid helium: Experimental results and applications

We have observed optical pumping signals from Cs atoms trapped in solid⁴He. While the longitudinal electronic spin relaxation timeT ₁ is found to be in the range of 1–2 s, the transverse relaxation timeT ₂, as inferred from magnetic resonance linewidths has a lower bound of 150 s, and is determined by magnetic field inhomogeneities. We present a quantitative discussion of how paramagnetic species trapped in solid He might be used in a highly sensitive search for permanent atomic electric dipole moments.     

Failure of reflection positivity in the quantum Heisenberg ferromagnet

We show that reflection positivity does not hold in the finite-volume spin 1/2 quantum Heisenberg ferromagnet in two cases: (i) for any dimension or degree ofS ¹-invariant anisotropy, above a volume independent temperatureT ₀, and (ii) for the isotropic model in one dimension, below a volume dependent temperatureT ₁.     

Magnetic properties of a quasi-one-dimensional organic ferromagnet

A quasi-one-dimensional alternating-spin Heisenberg model is used to describe the charge-transfer organic ferromagnet, which is studied by the Green's function method. The magnetic properties of the charge-transfer organic ferromagnet in different temperature regions are obtained. A formula for the critical temperature T_c is obtained and is found to be related to the spin of donors, to the intrachain exchange interaction, and to the spatial anisotropy parameter. This conclusion can explain T_c of the organic ferromagnet. The spatial anisotropy parameter of the organic ferromagnet [FeCp₂¹][TCNE] we obtained is consistent with the estimated value of Narayan et al. The Curie-Weiss temperature Θ for the organic ferromagnet [CrCp₂¹][TCNE] we obtained is Θ = 15.6 K which is near the experimental result of θ = 22±1 K.     

Spin fluctuations in the stacked-triangular antiferromagnet YMnO<Subscript>3</Subscript>

The spectrum of spin fluctuations in the stacked-triangular antiferromagnet YMnO₃ was studied above the Néel temperature using both unpolarized and polarized inelastic neutron scattering. We find an in-plane and an out-of-plane excitation. The in-plane mode has two components just above T _N : a resolution-limited central peak and a Debye-like contribution. The quasi-elastic fluctuations have a line width that increases with q as Dq ^z and the dynamical exponent z = 2.3. The out-of-plane fluctuations have a gap at the magnetic zone center and do not show any appreciable q dependence at small wave vectors.     

ESR-Linienbreite der Ionen der Eisengruppe in Lösungen

Electron spin resonance linewidths of ions belonging to the first transition group with quenched orbital angular momentum are calculated using a modified relaxationmatrix theory including third and fourth order perturbation terms. We base our calculation on a Hamiltonian, which depends on electron spin, nuclear spin, orbital angular momentum, rotation of the whole complex, and vibration of the ligands. Quadrupol effects, intermolecular electron-electron and electron-nucleus interactions are neglected. The results show that the well-known formula of the transverse relaxation time derived by using the spin-Hamiltonian is correct, if first the contribution of the rotational spin orbit process is taken in consideration and second the rotational correlation timeτ _c is replaced byτ _v =(1/τ _c +1/τ(0))^?1. 1/τ(0) describes the linewidth of the lowest energy value of the electrostatic energy of unpaired electrons in the ligand field. The linewidth arises from the normal modes of the complex; the calculation gives τ(0)=l0^?11...10^?12 sec.     

Study of a random Heisenberg ferromagnet by the Molecular CPA method

The spin wave Green function of a random dilute Heisenberg ferromagnet is calculated by the Molecular CPA method for the bond model of the f.c.c. lattice. It is found that the ferromagnetic spin order atT=0 becomes unstable, if the concentration of the non-zero bondC _A decreases below the critical valueC _A ⁰ =0.1702, which is some-what larger than the percolation concentrationC _A ^P =0.125. In the regionC _A <C _A ⁰ , aδ function spectrum appears atE=0 corresponding to the free rotations of the total spins of the finite clusters. The peculiar structure of the spin wave state density near the critical concentration is analysed in detail and a speculation is made for the topological structure of the spin clusters.     

Thermopower of spin glasses

We calculate the thermopowerS _d(T) of spin glasses on the basis of thesd exchange model with additional interactions between the impurity spins by means of time dependent perturbation theory. Similarly to the Kondo effect, the observed giant thermopower can only be explained if one also takes into account the spin independent interactionV between the magnetic impurities and the conduction electrons. We obtain a Kondo termS _d ⁽¹⁾ (T) (which reduces for vanishing spin interactions to the thermopower of Kondo systems) and in addition a resonance termS _d ⁽²⁾ (T) of opposite sign which vanishes for vanishing interactions. The superposition of both terms leads to an additional extremum of the total thermopower and to a change of sign at a temperatureT ₀ which depends on the Kondo temperature and on the magnetic excitations of the spin glass and therewith on the freezing temperatureT _f. We obtain at low temperatures for oscillating and relaxing spin glass modesS _d ⁽¹⁾ T² andS _d ⁽²⁾ T. At high temperaturesS _d ⁽¹⁾ (T) decreases as |lnT|^–3 andS _d ⁽²⁾ asT ^–1. A close relationship between the temperature dependent resistivity in Born approximation andS _d ⁽¹⁾ (T) is pointed out.SFB 125 Aachen-Jülich-Köln     

Antiferromagnetic Resonance in GdB<Subscript>6</Subscript>

The electron spin resonance has been measured for the first time both in the paramagnetic phase of the metallic GdB₆ antiferromagnet (T_N = 15.5K) and in the antiferromagnetic state (T < T_N). In the paramagnetic phase below T* ~ 70 K, the material is found to exhibit a pronounced increase in the resonance linewidth and a shift in the g-factor, which is proportional to the linewidth Δg(T) ~ ΔH(T). Such behavior is not characteristic of antiferromagnetic metals and seems to be due to the effects related to displacements of Gd³⁺ ions from the centrosymmetric positions in the boron cage. The transition to the antiferromagnetic phase is accompanied by an abrupt change in the position of resonance (from μ₀H₀ ≈ 1.9 T to μ₀H₀ ≈ 3.9 T at ν = 60 GHz), after which a smooth evolution of the spectrum occurs, resulting eventually in the formation of the spectrum consisting of four resonance lines. The magnetic field dependence of the frequency of the resonant modes ω₀(H₀) obtained in the range of 28–69 GHz is well interpreted within the model of ESR in an antiferromagnet with the easy anisotropy axis ω/γ = (H ₀ ² +2H_AH_E)^1/2, where H_E is the exchange field and H_A is the anisotropy field. This provides an estimate for the anisotropy field, H_A ≈ 800 Oe. This value can result from the dipole?dipole interaction related to the mutual displacement of Gd³⁺ ions, which occurs at the antiferromagnetic transition.     

Ferromagnetic resonance study of Y3−z Ca z Fe5−x−y Co x Ge y O12 films

Conclusions It has been found that the character of the magnetic anisotropy changes markedly when the Co + Ge YIG films are doped with calcium. For the induced uniaxial and in-plane anisotropy we observe an increasing departure from the predictions based on the two-parameter model and for the cubic anisotropy the constantK ₁ becomes dependent on the growth direction. As a source for the latter effect the octahedral or tetrahedral preference of Co ions depending on the growth direction may be considered. The measured linewidth proportional to frequency indicates the presence of a relaxation mechanism which is, probably, connected with octahedral Co³⁺ ions.The authors express their thanks to Dr. P. Görnert and M. Neviva for preparation of the LPE films and to Dr. P. Novák for valuable discussions.Dedicated to Jan Kaczér DrSc on the occasion of his 65th birthday.     

Magnetic field effect on the critical EPR-dynamics of the cubic ferromagnets CdCr2Se4 and CdCr2S4

We present measurements of the critical behaviour of the EPR linewidths at frequencies 9.21 and 35.5 GHz. In the exchange critical region above T_c (4πx ? 1) the strong field dependence of the linewidths is observed, even when the field variation of susceptibility x is small. This phenomenon is explained by the spin diffusion effect on the linewidth. The spin diffusion coefficients D for CdCr₂Se₄ and CdCr₂S₄ are determined from the linewidths data. The temperature dependence D is found to be consistent with the predictions of the dynamical scaling theory.     

Muon spin relaxation studies in frustrated and/or low-dimensional spin systems

An overview is given on muon spin relaxation (SR) measurements in frustrated and/or low dimensional spin systems. In the frustrated Kagomé lattice system SrCr₈Ga₄O₁₉, we observed dynamic spin fluctuations of 30 GHz, without any static frozen component even atT=0.1 K, much below the susceptibility-cusp temperatureT _g=3.5 K. This is in clear contrast with the case in dilute-alloy spin glassesCuMn andAuFe, where static order develops belowT _g. We also present the dimensionality dependence of the sub-lattice magnetization curves in 2-d Heisenberg systems, the remarkable suppression of the ordering temperature in a 1-d system Sr₂CuO₃, the observation of activation type spin dynamics in a 1-d Ising ferromagnet (DMeFc)(TCNE) aboveT _c, slow spin fluctuations ( 60 MHz) in Haldane-gap systems at low temperatures, and some results from organic 1-d and 2-d magnetic systems.     

On the high spin expansion in the SYM theory

We study the form of the high spin expansion of the minimal anomalous dimension for long operators belonging to the sl(2) sector of SYM. Keeping fixed the ratio j between the twist and the logarithm of the spin, the minimal anomalous dimension expands as γ(g,j,s)=f(g,j)lns+f⁽⁰⁾(g,j)+O(1/lns). This particular double scaling limit is efficiently described, including the desired accuracy O((lns)⁰), in terms of a linear integral equation. By its use, we are able to evaluate both at weak and strong coupling the subleading scaling function f⁽⁰⁾(g,j) as a series in j, up to the order j⁵. Thanks to these results, the possible extension of the liaison with the O(6) non-linear sigma model may be tackled on a solid ground.     

Observation of quasi-static spin dynamics in the giant anisotropic ferromagnet PrRu2Si2 by μSR spectroscopy

We report zero field and longitudinal fieldSR measurements on single crystals of the ferromagnet PrRu₂Si₂ (T _c15K). BetweenT=T _c and 250K we observe a quasi-static spin dynamics of the Pr³⁺ ion sub-lattice. BelowT _c, depending on the orientation of the initial muon beam polarisation relative to thec axis of the compound, we find either an oscillating signal or an exponential depolarisation function. These latter data allow us to get information related to the muon in PrRu₂Si₂.