No abstract available

No abstract available

[en] We present a comprehensive study of the temperature and doping dependence of the 500 meV peak observed at q = (π,0) in resonant inelastic x-ray scattering (RIXS) experiments on La₂CuO₄. The intensity of this peak persists above the Neel temperature (T_N = 320 K), but decreases gradually with increasing temperature, reaching zero at around T = 500 K. The peak energy decreases with temperature in close quantitative accord with the behavior of the two-magnon B_1g Raman peak in La₂CuO₄ and, with suitable rescaling, agrees with the Raman peak shifts in EuBa₂Cu₃O₆ and K₂NiF₄. The overall dispersion of this excitation in the Brillouin zone is found to be in agreement with theoretical calculations for a two-magnon excitation. Upon doping, the peak intensity decreases analogous to the Raman mode intensity and appears to track the doping dependence of the spin-correlation length. Taken together, these observations strongly suggest that the 500 meV mode is magnetic in character and is likely a two-magnon excitation.

[en] It was found that below T/sub N/, the susceptibility diverges logarithmically in parallel T/sub N/--T parallel in good agreement with previous studies by Boyarskii and Starikov. Above T/sub N/, no logarithmic divergence is observed. (auth)

[en] Thermal conductivity (lambda) data for two dense Cr₂O₃ samples are reported. The results, which span the temperature range 270 to 360 K, show an inflection near the Neel transition temperature, 308 K. An analysis shows that the slope of the higher-temperature portion of the lambda^-1-T curve is about what would be expected for ordinary 3-phonon U-process scattering. Above T/sub N/ the mean free path for phonon-spin scattering is constant and equal to approx. =10 nm

[en] We have measured the frequency integrated magnetic critical scattering of neutrons from paramagnetic Dy and Ho. Analysis of these data show the paramagnetic to helical antiferromagnetic phase transitions are characterized by the critical exponents ν = 0.57 ± 0.05 and γ = 1.05 ± 0.07 for Dy and ν = 0.57 ± 0.04 and γ = 1.14 ± 0.10 for Ho

[en] The exponent β that describes the sinusoidally modulate d ↔ paramagnetic phase transition crosses over from a value of 0.39 near T_N to a mean field value away from T_N. Electrical resistivity measurements near T_N are given for the c axis and the critical behaviour is discussed

[en] The possibility of order-order phase transitions with temperature for the magnetic model with coupling to the homogeneous strain, is considered. Ten sequences of two and three phases are found and the corresponding conditions for the model parameters are established and visualized in the phase diagram. (author)

[en] Complete text of publication follows. The mechanism of magnetization reversal in single-domain ferromagnetic particles is of interest in many applications, in most of which losses must be minimized. In cancer therapy by hyperthermia the opposite requirement prevails: the specific loss power should be maximized. Of the mechanisms of dissipation, on the first stage we studied the effect of Neel relaxation on magnetic nanoparticles unable to move or rotate and compared the losses in linearly and circularly polarized field. The results were obtained through exact analytical solutions of the Landau-Lifshitz equation as derived from the Gilbert equation. We used the calculated time-dependent magnetizations to find the energy loss per cycle. In figure (a) the resulting energy loss per cycle is depicted as a function of frequency and the dimensionless damping parameter α. Regarding the merits of linear or circular polarization of the magnetic field, we find opposite preferences for frequencies below and above the Larmor frequency. At lower frequencies, which is relevant to hyperthermia, the energy absorbed by the magnet from linearly polarised field exceeds the absorption from rotating field by orders of magnitude. In the case of high frequencies, which is not lacking technological relevance since the recent interest in materials with high microwave absorption, the relaxation is slow compared to the rate of change of the magnetic field and the loss per cycle becomes frequency dependent. Figure (b) shows the specific absorption power in W/g for a single-domain particle of magnetite. At frequencies relevant to hyperthermia (∼ 10⁵ Hz) with linearly polarized field, respectable losses of kW/g order can be achieved, while the power absorbed from circularly polarized field is two to three orders of magnitude lower

[en] We calculate the spin stiffness of the S = 1/2 frustrated Heisenberg antiferromagnet on finite square lattices by means of the Schwinger - boson approach. COmparison with recent exact numerical results reveals that the observed lack of scaling with lattice size for intermediate to large frustration cannot be taken as an indication of absence of Neel order. This lack of scaling is already apparent for small frustration and is a finite lattice effect. Our results also indicate that the expected behaviour is regained for larger lattices than those considered in numerical studies. (author). 18 refs, 2 figs