Iron above the curie temperature

A quantitative discussion is given of recent paramagnetic diffuse neutron scattering data on bcc Fe above T_c. It is found that although the spatial spin correlation function has a weak tail of fairly long range the correlation between nearest neighbour spin directions is very small at 1.24T_c. It is concluded that the data are inconsistent with the giant short-range order hypothesis of Prange and Korenman, Capellmann and Sokoloff and another interpretation is given.     

Spin lattice relaxation in TmNi5 above its curie temperature

The hexagonal compound TmNi₅ was investigated by means of ¹⁶⁹Tm Mössbauer spectroscopy in the temperature range 4–350 K. Above T_c=4.5 K the magnetic hyperfine interactions were found to be governed by paramagnetic relaxation. This behaviour was interpreted in terms of a stochastic spin-down model. The spin lattice relaxation time was found to follow an exponential temperature dependence.     

Spin dynamics above the curie temperature studied by neutron scattering

Neutron scattering can in principle give information about magnetic fluctuations over the entire atomic space and time domain. The weakness of the neutron-matter interaction renders this information undistorted by the neutron probe, but at the same time puts intensity limitations on the method. A considerable number of studies on the magnetism of 3d metals have been performed at some of the larger reactor laboratories. In the regions of overlap the experimental results from the different laboratories are consistent, but the interpretations are along different lines. Among the controversial issues are itinerancy versus localization, the degree of order above T_c. In our talk we shall give an introduction to the neutron scattering method, including some of the sophisticated polarized beam methods. In the rest of the talk we shall review recent experimental results and some of the theoretical models used in their interpretation.     

Piezoacoustic impedance fluctuations in gadolinium molybdate in the vicinity of the curie temperature

The temperature dependences of the intensity of the main acoustic resonance peak for two samples of gadolinium molybdate Gd₂(MoO₄)₃ are measured in the temperature range 22–165°C, involving the Curie temperature. It is revealed that low-frequency fluctuations of the amplitude of the resonance current through the samples are observed at temperatures in the vicinity of the Curie point. The amplitudes of the corresponding variations in the voltage at the input of the recording device reach 100 mV. The characteristic times of fluctuations are several tens of seconds.     

Electric hyperfine interactions in ferromagnetic binary alloys above the curie temperature

In the frame of an unscreened point charge model we calculated all possible electric field gradients for first neighbor atom arrangements in disordered binary fcc alloys. With these values and a distribution of isomer shifts we fit experimental spectra of nearly disordered alloys.     

Magnetic susceptibility of the spin polaron states in the s-f exchange model above curie temperature

The static spin susceptibility tensor of magnetic semiconductor in a framework of the s-f exchange model above the Curie temperature is calculated. The Feynman path integral variational method is used to take into account the spin polaron formation. Free energy estimations of the phenomenological theory of Krivoglaz are justified microscopically. The spin polaron effect on ESR frequencies is considered.     

Magnetic interaction and the critical dynamics above the curie point

It is shown that near T_c, where 4πχ ? 1 magnetic interaction completely change the long-wave critical dynamics of the ferromagnets and the new dynamic critical exponent is z_d = (5 - η)/2 - ν^-1 ≈ 1.     

Knight shifts in solid noble metals

The direct contribution to the Knight shift in solid noble metals is evaluated. The entire calculation is accomplished in the framework of the pseudopotential formalism and the effective shape of the Fermi surface is taken into account by using the simple “eight-cone” model by Ziman. The theoretical results are discussed and compared with the available experimental data.     

Planar hall effect in gadolinium thin films near the curie point

The orientational and thermal dependences of planar Hall effect in Gd thin films deposited in an ultra-high vacuum were studied between 100 and 300 K. The critical exponent of magnetization as estimated from these measurements is $\text{β =}\text{1}\text{3}$.     

Knight shifts of metals in the liquid state

A single orthogonalized plane wave calculation has been made, to zero order in the pseudopotential, of the direct contact Knight shift for 30 liquid metals. The average deviation from experimental values is approximately 15%. Larger unexplained divergences occur for Cu and Hg. In addition it is shown that to zero order in the pseudopotential the factorΩP _F in the Knight shift describing the electron density at the nucleus is almost unaffected by thermal expansion.     

Dielectric relaxation of CsHSeO4 above room temperature

Dielectric measurements of CsHSeO₄ show a distinct relaxation at low frequencies at several isotherms (T < 363 K). For example, the relaxation frequency is around 4 kHz at 323 K and increases to higher frequencies (~ 100 kHz) as the temperature increases. The relaxation has an activation energy of 0.8 eV, which is in close agreement with that associated with transport of charge carriers. We suggest that the observed dielectric relaxation could be produced by the H⁺ jump and SeO₄^? 2 reorientation that cause distortion and change the local lattice polarizability, inducing dipoles like HSeO₄^?.     

Dielectric relaxation in NH4HSO4 above room temperature

The dispersion curves of the dielectric response of NH₄HSO₄ show that the corrected imaginary part of permittivity, εʺ, and its real part ε′ versus frequency reveal a dielectric relaxation around 9.1 × 10⁵ Hz at 31 °C, which shifts to higher frequencies (∼ 10⁶ Hz) as the temperatures increases. The relaxation frequency shows an activated relaxation process over the temperature range 31–83 °C with activation energy E_a = 0.14 eV, which is close to that derived from the dc conductivity. We suggest that the observed dielectric relaxation could be produced by the H⁺ jump and SO₄⁻ reorientation that cause distortion and change the local lattice polarizability inducing dipoles like HSO₄⁻.     

Ferromagnetic resonance in nickel around the curie temperature

The ferromagnetic resonance behaviour of nickel up to and above T_c is investigated. It is shown that up to the Curie point, which is 357.8°C, the resonance can be described by the Gilbert equation with the parameters g = 2.20 ±0.02 and λ = (4.0±0.5) × 10⁹s^?1. Above T_c, however, it is an open question if the Gilbert equation describes the resonance adequately. No sign of a decreasing linewidth above 365°C is seen. On the contrary, the resonance broadens rapidly and fades away.The experiment is performed in a new way with the cavity kept at room temperature and the temperature of the sample is monitored during the run of the spectrum.     

Knight shifts around vacancies in the 2D Heisenberg model

The local response to a uniform field around vacancies in the two-dimensional spin-1/2 Heisenberg antiferromagnet is determined by numerical quantum Monte Carlo simulations as a function of temperature. It is possible to separate the Knight shifts into uniform and staggered contributions on the lattice which are analyzed and understood in detail. The contributions show interesting long- and short-range behavior that may be of relevance in NMR and susceptibility measurements. For more than one impurity, remarkable nonlinear enhancement and cancellation effects take place. We predict that the Curie impurity susceptibility will be observable for a random impurity concentration even in the thermodynamic limit.     

Cross-correlation effects involving curie spin relaxation in methyl groups

Cross-correlation effects arising in methyl protons due to the simultaneous presence of dipole-dipole, chemical shift anisotropy, and Curie spin relaxation mechanisms in paramagnetic systems are analyzed. We assess the potential of obtaining structural constraints from the cross-correlation of Curie spin relaxation with dipolar relaxation mechanisms among methyl proton spins. By theoretical analysis and numerical simulations we characterize the transfer functions describing the interconversion processes of different ranks of multispin order. The time dependence of these processes contains a new type of structural information, the orientation of the methyl C(3)-axis with respect to the electron center. Experimental confirmation is found for selected methyl groups in low spin Fe(3+) sperm whale myoglobin.     

Isotope shifts and hyperfine structure in the transitions of gadolinium

High-resolution resonance ionization mass spectrometry has been used to measure isotope shifts and hyperfine structure in all (J = 2-6) and the transitions of gadolinium (Gd I). Gadolinium atoms in an atomic beam were excited with a tunable single-frequency laser in the wavelength range of 422-429 nm. Resonant excitation was followed by photoionization with the 363.8 nm line of an argon ion laser and resulting ions were mass separated and detected with a quadrupole mass spectrometer. Isotope shifts for all stable gadolinium isotopes in these transitions have been measured for the first time. Additionally, the hyperfine structure constants of the upper states have been derived for the isotopes ^{155, 157} Gd and are compared with previous work. Using prior experimental values for the mean nuclear charge radii, derived from the combination of muonic atoms and electron scattering data, field shift and specific mass shift coefficients for the investigated transitions have been determined and nuclear charge parameters for the minor isotopes ^{152, 154} Gd have been calculated. Received 18 November 1999     

Knight shifts in binary liquid alloys of equi-valent metals

The N.M.R. Knight shift depends upon the conduction electron density at the nucleus under observation. An interpretation of its absolute magnitude in liquid metals cannot be undertaken in terms of a pseudopotential formulation, but relative changes of shift with some parameter such as temperature, volume or alloy composition can be investigated. Knight shift changes with concentration have been measured for a series of binary liquid alloys. Since application of a pseudopotential treatment is simplest when the two constituents have the same valency, the alloys chosen have mostly been of this type, e.g. In-Ga, In-Tl, Sn-Pb, Sb-Bi. Although theory predicts the correct qualitative behaviour of the shift changes, the precise theoretical predictions are very sensitive to details of both the pseudopotentials and the liquid structure factor. In the case of In-Sn and In-Pb, where the shifts of both constituents decrease with increasing indium content for In-Sn, but increase for In-Pb, the difference between the pseudopotentials of tin and lead does not appear to be sufficient to explain the observed differences in changes of shift.