Theoretical study of neutron scattering by magnetic excitations in KCoF3

The cross-section of inelastic neutron scattering by magnetic excitations in KCoF₃ has been calculated as a function of temperature, to predict experiment. Also, the temperature-dependent magnetic excitation energies along [100] have been calculated and compared with observed results.     

Theoretical study of magnetic susceptibility of orbitally unquenched compound KCoF3

The effect of residual orbital magnetic moment of Co²⁺ in KCoF₃ on the magnetic susceptibility has been studied. For the calculation for both the ordered state and the paramagnetic state we have applied the correlated effective field approximation developed by Lines. An excellent value of the Néel temperature is obtained and, except near the Néel temperature, the calculated susceptibility agrees well with the experimental results over the whole temperature range.     

A mechanism of magnon-phonon interaction in KCoF3

A new mechanism is proposed for the magnon-phonon interaction in KCoF₃. The interaction Hamiltonian is derived by treating the vibration-modulated crystal field and the spin-orbit interaction as perturbations. The mechanism can well explain the experimental results of the inelastic neutron scattering and the Raman scattering.     

The magnetic behaviour of CePd3 at low temperatures

The susceptibility of samples of CePd_3±x (x=0.0, 0.1) shows an upturn at low temperatures. Magnetization measurements in high magnetic fields strongly indicate that this behaviour is intrinsic to CePd₃ and not due to the spurious presence of magnetic impurities.     

Critical behaviour of the linear birefringence in 3d-Heisenberg antiferromagnets : RbMnF3, KNiF3 and KCoF3

The linear birefringence of stress induced single antiferromagnetic T-domains of RbMnF₃, KNiF₃ and KCoF₃ has been measured in the vicinity of T_N. We deduce the critical behaviour in such 3d-Heisenberg systems, showing a symmetry breaking at the transition involving a unique wave vector of the order parameter in the Brillouin zone. The critical exponent $\text{β}$ of the linear birefringence has been found to be larger than 2β, in agreement with the theoretical predictions of G. Gehring.     

Exchange bias and anisotropy in the Fe/KCoF3 structure

A new type of ferromagnet/antiferromagnet structure (Fe/KCoF₃) was deposited by molecular beam epitaxy. Unidirectional and uniaxial anisotropies of 5.1 and 3.4 kA/m were measured at 23 K using ferromagnetic resonance. Magnetization measurements at 5 K showed a hysteresis loop shift of 6 kA/m due to exchange bias. Significant enhancement of four-fold anisotropy was found at low temperatures in the samples with polycrystalline KCoF₃ structure.     

First order phonon and magnon Raman scattering in KCoF3

At T=2°K, a magnon and a phonon Raman line is observed respectively at 38.5 and 56.7 cm^?1. The last one arises from an acoustic phonon at R point in the paramagnetic Brillouin zone, coupled to the magnon in the magnetically ordered phase.     

Neutron diffraction study of the magnetic structure of NiCO3 at various temperatures

Neutron diffraction experiments performed on a powdered sample of NiCO₃ show that from 4.5K up to T_a=21K, the antiferromagnetic vector is perpendicular to the trigonal axis. At T_b=25K and up to about 44K, short range magnetic ordering is observed. These results are in good agreement with recent magnetization measurements performed in our laboratory on the same powdered sample.     

Ionic conductivity of the perovskites NaMgF3, KMgF3, KMgK3 and KZnF3 at high temperatures

We have carried out a study of the ionic conductivity of NaMgF₃, KMgF₃ and KZnF₃ up to temperatures close to the melting point. Our results, in contrast to previous reports in the literature, show no abnormal ionic conductivity at high temperatures. Care in interpretation of results is required because of surface electronic conduction.     

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.     

Thermal and magnetic properties of DyB62 at low temperatures

Temperature dependences of heat capacity C_P(T) and magnetization M(T) of an icosahedral dysprosium boride (DyB₆₂) single crystal have been experimentally investigated in the temperature range of 2-300 K. The magnetic susceptibility χ(T) of DyB₆₂ follows Curie-Weiss law with a paramagnetic Curie temperature of −3.7 K, which implies that the antiferromagnetic interactions are dominant in this material and suggests the possibility of magnetic ordering at low temperatures. This conjecture is supported by the temperature dependence of heat capacity C_P(T), which decreases upon heating from 2 to 7 K. The heat capacity of DyB₆₂ at 2 K is analyzed as a sum of magnetic, Debye, two-level system and soft atomic potential components.     

Infrared study in high magnetic fields of the crystal-field excitations in PrMnO3

Pr³⁺ ion crystal field (CF) excitations in PrMnO₃ single crystals have been studied by infrared transmission, in the 1800–8000 cm⁻¹ range, as a function of temperature and applied magnetic field up to 13 T. No noticeable frequency shifts which might occur below T_N∼100 K, as a result of the antiferromagnetic transition, are observed in the Pr³⁺ CF levels. A set of CF parameters that fit the experimental levels as well as the low temperature Pr³⁺ magnetic moment in PrMnO₃ has been determined.     

Magnetic excitations of HoAl2 in a magnetic field

The ground state spin wave excitation energies of single crystalline HoAl₂ have been studied at T= 4.2 K in external magnetic fields up to 7 T by means of inelastic neutron scattering. The results have been interpreted in terms of a cubic crystalline electric field using the parameters determined by magnetization measurements and an exchange interaction with the exchange parameters taken from the zero field measurements.     

Surface-layer effect in MnTiO3 ceramics at low temperatures

MnTiO₃ ceramics were prepared via the traditional solid-state reaction route. The low-temperature (100–330 K) dielectric properties of MnTiO₃ have been systematically investigated in the frequency range from 100 Hz to 5 MHz. Our results showed that MnTiO₃ exhibits intrinsic dilectric response in a wide temperature range up to 200 K. A relaxation appears near room temperature. This relaxation can be enhanced by annealing treatment in oxygen and weakened in nitrogen. Reducing the sample thickness gradually leads to the disappearance of the relaxation. Based on these experimental facts, the relaxation was ascribed to be a Maxwell–Wagner relaxation due to surface-layer effect.     

Thermal expansion of La3S4 at low temperatures

The thermal expansion of the superconductor La₃S₄ has been investigated on a single crystal by a capacitive method between 1.6 and 14 K. The relative length change shows a well pronounced second order phase transformation near 8.05 K, which is caused by the superconducting transition. From the calculated linear thermal expansion coefficient we determine the pressure coefficient of the transition temperature and of the thermodynamic critical field as well as the thermodynamic critical field itself. Furthermore we estimate the electronic, phononic and total Grüneisen parameters. These parameters have been used to discuss on the basis of the McMillan equation the reason for the increase of the transition temperature under pressure.     

Elementary excitations for the Anderson model at finite temperatures

The elementary excitation spectrums for the Anderson model at finite temperatures are calculated by using the Bethe-ansatz solution. The formulation is based on the method of Yang and Yang, which was developed for the one-dimensional boson systems with the -function type interaction. We obtain the temperature dependence of the spin and the charge excitation spectrums. When the impurity level lies deeply from the Fermi level and the Coulomb interaction is suitably large, the resonant peak structure develops in the low energy region of the spin excitation spectrum and the hump structure grows around the impurity level of the charge excitation spectrum with decreasing temperature. Received: 21 January 1998 / Accepted: 17 March 1998     

Co2+ spin-lattice relaxation narrowing of 19F NMR in KCoF3

The temperature dependence of the ¹⁹F NMR linewidth ΔH in KCoF₃ has been measured over the entire paramagnetic solid state region. The dramatic decrease in the hyperfine-broadened, exchange narrowed ΔH that occurs above 200 K is interpreted as arising from fast Co²⁺ single-ion, spin-lattice relaxation. A model theory of the temperature dependence of ΔH is given which incorporates the interplay of exchange and spin lattice relaxation effects on the decay of the spin autocorrelation function.     

Experimental studies of the magnetic excitations in the 1D Ising-antiferromagnet CsCoCl3

The electronic Raman and infrared absorption spectrum within the ⁴T_1g ground term of the one dimensional antiferromagnet CsCoCl₃ is investigated. At 2K three magnon lines are observed whereas above the magnetic phase transition at 8K the number of lines is increased to twelve. No drastic change of the magnon spectrum is observed in passing the Neél temperature at 21.5K. The magnon spectrum could still be observed at 150K. Theoretically the one-exciton spectrum is described within the molecular field approximation, using an exchange coupling between nearest neighbours along the spin chains only. Satisfactory agreement is obtained between measured and calculated energies.     

Second order theory of excitations in trapped bose condensates at finite temperatures

We present a finite temperature field theory for collective excitations of trapped Bose condensates which includes the dynamics of the thermal cloud. In spherical traps we show that excitations couple strongly to a small number of modes, giving resonance structure in their frequency spectra. Where possible, we derive energy shifts and lifetimes of excitations. For the l = 0 mode we show that the simple picture of a decay rate fails, which should be observable in suitable experiments. It also suggests a possible explanation for the anomalous behavior of the m = 0 mode observed in anisotropic traps.     

The magnetic structures of HoTiO3 and ErTiO3

The magnetic structures of rare-earth titanium perovskites, ErTiO₃ and HoTiO₃, have been determined at 4.2 K by neutron diffraction. The Er³⁺ moment of (8.5 ± 0.5) μ_B lies along [001] and is colinear with the titanium moment of (-0.7 ± 0.3) μ_B. The Ho³⁺ moment of (8.1 ± 0.5) μ_B is inclined at an angle of 24° to the bc plane and 32° to the ab pla so as to produce an antiferromagnetic ordering of the x component and a ferromagnetic ordering of the y and the z components. The titanium moment of (-0.55 ± 0.3) μ_B lies in the bc plane but its precise direction has not been determined.