Low temperature pressure dependence of the muon hyperfine fields in iron,cobalt and nickel

The pressure dependence of the μ⁺ local magnetic fields in polycrystalline Fe and Ni and a Co single crystal has been measured at 77 K, up to 0.7 GPa, using a He gas high pressure setup. The pressure derivatives dlnB_μ/dP in units of mT/GPa are +4.4±1.0 (Fe), -0.7±1.1 (Co) and +0.63±0.10 (Ni). From these values the hyperfine field volume derivatives are deduced. Using these values together with previously determined room temperature derivatives the thermal expansion part of the temperature dependence of the hyperfine field can be calculated. The remaining explicit temperature dependence below 300 K, which deviates markedly from the temperature dependence of the bulk magnetization, is discussed.     

Muon depolarization by the spin density wave hyperfine fields of lead

Muon depolarization arising from hyperfine fields created by the twelve SDWs in Pb is studied. Since the SDW wave vectors, ={210}, are commensurate with the lattice, there are no hyperfine fields at the octahedral interstitial sites (where muons ordinarily reside). Instead, the SDWs have antinodes at the tetrahedral interstitial sites and generate ⁺ hyperfine fields estimated to be 3000 G. Recent data show that the depolarization rate in Pb falls sharply towards zero at 35K (on account of ⁺ diffusion), but then slowly recovers to half its low temperature value near 200 K. Such recovery of the depolarization rate can be attributed to growth of the thermally activated occupation probability for muons at tetrahedral sites, where their spins undergo (motionally narrowed) transient precession caused by the static 3000 G fields. SDW depolarization can be distinguished from recovery caused by deep traps. A 10 G longitudinal field will quench the latter but not the former. Such an experiment could provide a crucial confirmation of the exotic magnetism of Pb.     

High pressure induced shifts in the superconducting transition temperature of sputtered films

Both negative and positive shifts in the superconducting transition temperature with negligibly small changes in the resistance ratio were observed in the NbGe and V₃Si films, respectively, under pressure up to ～ 22 kbar. The results are discussed along with the roles of defects recently proposed in these materials.     

151 Eu hyperfine fields,isomer shifts and moments in Eu-based EuT 2 X 2 intermetallic compounds

The closed-shell, well-screened europium moment should make its compounds an easy and robust system in which to study the correlation between the local moment and the observed hyperfine field (B_hf). Having explored why one might expect B_hf to provide a reasonable measure of the local moment, and why the isomer shift (δ) might allow for a first-order correction to include the effects of the local chemical environment, we proceed with an anlysis of the extensive ¹⁵¹Eu Mössbauer data on the EuT₂X₂ compound family. We find that while in some limited cases a useful correlation may exist, in general there are far too many as yet unknown contributions to B_hf for it to provide a meaningful estimate of the moment.     

Glass-like behaviour of spin systems due to relaxation freezing in crystal and hyperfine fields

A mechanism of glass-like behaviour in solid paramagnets is suggested, based on exponential slowing down the spin relaxation via upper energy levels caused by crystalline or hyperfine splitting of the ground state of a paramagnetic ion. Some recent experimental data can be described by this model.     

Laser-induced shifts and splittings of hyperfine structure lines in electron spin resonance spectra of 35Cl atoms

Laser-induced shifts and splittings of the hyperfine structure (hfs) lines in the electron spin resonance (ESR) spectra of ³⁵Cl atoms have been studied using both dressed-atom theory and semiclassical dispersion theory. In resonant cases, the calculated results by the dressed-atom theory of the ac Stark effect indicate that laser-induced hfs ESR shifts of ³⁵Cl atoms in the ground state 3p^{5 2}P⁰ _3/2 can be more than 1 MHz W^?1 cm², and a single ESR hfs line is split into four lines under the action of a laser beam with intensity 10 W cm^?2. In non-resonant cases, the two theories give similar results for the shifts. The laser-induced broadening of the ESR hfs lines is also calculated and is less than the shifts. The laser-induced shifts in ESR can be observable within the resolution of ESR experiments.     

Mössbauer isomer shifts,hyperfine interactions,and magnetic hyperfine anomalies in compounds of iridium

The dependence of the isomer shift of the 73 keVγ-rays of¹⁹³Ir on the oxidation state was studied in a variety of compounds of trivalent and tetravalent iridium as well as in IrF₅ and IrF₆. The observed shifts can be attributed to the sielding effect of the varying number of 5d electrons, but there may be additional direct contributions fromσ-bonding electrons. In the hexahalogen complexes with Kramers-degenerate electronic groundstates magnetically ordered phases were found and studied at temperatures between 0.5 and 4.2 K. Between IrF₆ and tetravalent [IrCl₆]^2? and [IrBr₆]^2? complexes magnetic hyperfine anomalies of up to 10% were observed, which support the presented interpretation of the hyperfine fields in terms of core polarization, orbital and spin-dipolar contributions.     

Theoretical study on pressure dependence of isomer shift and hyperfine field

The pressure dependence of the isomer shift and the hyperfine field are theoretically investigated. By expanding the Gibbs free energy of the system under proper circumstances, the explicit form of the pressure dependence of the isomer shift is obtained. At low temperatures, by applying some well-known relations, the pressure dependence of the hyperfine field is approximately derived. Finally, the theoretical results are compared with the experimental data.     

Magnetic hyperfine field changes at different Fe-sites below spin reorientation temperature in polycrystalline untextured Nd2Fe14B

In order to estimate the magnetic hyperfine fields at different sites, an analysis of the Mössbauer spectra of polycrystalline untextured Nd₂Fe₁₄B, recorded at temperatures above and below the spin reorientation temperature T_s(≈150 K), has been made. Here the signs of the electric field gradients (efg) and the principal axes of efg components have been constrained according to the available crystal structure information below and above T_s. It has been observed that the magnetic hyperfine field changes are similar to the recently reported magnetic moment changes at different sites in an isostructural alloy Tm₂Fe₁₄B.     

The low temperature hyperfine structure of quartz under uniaxial pressure

The uniaxial pressure dependence of the muonium hyperfine tensor was measured at 80 K. Uniaxial pressures (0 and 0.2 GPa) were applied along the a‐axis of quartz, and the hyperfine tensor was measured by applying various transverse fields between 0 and 7 G. The structure appears to be the result of the three chemically equivalent sites in quartz. When pressure is applied perpendicular to the crystallographic c‐axis of quartz, the symmetry of these sites is broken such that two chemically inequivalent sites are observed. The resulting hyperfine structure is discussed and qualitative explanations are proposed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Changes in structure and magnetic hyperfine fields of Finemet alloys, induced by transition elements substitution

Amorphous as well as two-phase nanocrystalline Finemet-type soft magnetic alloys substituted by transition elements (Ni, Co, V) have been studied by means of Mössbauer spectroscopy. The main subject of interest was the distribution of magnetic hyperfine field (MHF) in Ni substituted alloys. The increase of mean MHF was stated as a result of crystallization. It was found that changes of transition metal concentration cause different kind of hyperfine field evolution, attributed both to the alteration in grain structure and other effects related to crystallization processes.     

Isotope shifts and hyperfine structure of natural dysprosium

Isotype shifts and hyperfine structure measurements for five transitions in natural dysprosium have been performed using a CW tunable dye laser and a collimated atomic beam. The consistency of experimental isotope shifts is tested and values of the change in rms radius of the nuclear charge distribution are calculated.     

First-principles studies of the magnetic hyperfine field in Fe multilayers

A detailed theoretical study of the magnetic moments and magnetic hyperfine fields in several Fe multilayers (Fe fcc(001)/5X fcc(001), X=Cu and Ag, and Fe bcc(001)/5X fcc(001), X=Ag and Au) as well as in bulk Fe is presented. The calculations have been performed using the spin-polarized, relativistic linear muffin-tin orbital (SPR-LMTO) method of band structure calculation. Therefore, not only the contribution to the hyperfine fields due to the conventional Fermi contact interaction but also due to the spin dipolar and orbital contributions induced by the crystal field and by spin-orbit coupling are accounted for. To decompose the hyperfine field of non-s-electrons into these contributions it has been assumed that they are proportional to the corresponding so-called magnetic dipole moment and the orbital magnetic moment, respectively. In contrast to previous results for pure metals and alloys not only the orbital but also the spin dipolar hyperfine field was found to be non-negligible. The anisotropy of the hyperfine field determined by calculations for in-plane and perpendicular orientation of the magnetisation was found to be very pronounced and closely connected with the corresponding anisotropy of the magnetic dipole moment and the orbital moment.     

Mössbauer studies of hyperfine fields in disordered Fe2CrAl

Heusler-like alloy Fe₂CrAl was prepared and studied. Structure determination was done by X-ray. The structure was found to conform to the B₂ type. Magnetic hyperfine fields in this sample were studied by the Mössbauer effect. The Mössbauer spectra were recorded over a range of temperature from 40 to 296 K. The Mössbauer spectra showed the co-existence of a paramagnetic part with a magnetic hyperfine portion at all recorded temperatures. Even with the distribution in the magnetic hyperfine field, the average hyperfine field follows the (T/T _c)^3/2 law. The paramagnetic part of the hyperfine field is explained in terms of the clustering of Cr atoms.     

Calculations of pressure induced widths and shifts in microwave spectra

Calculations of the width and shift of pressure induced spectral lines in gases at low pressures in the microwave region of the electromagnetic spectrum are presented. These calculations are based on an extension of the Anderson-Tsao-Curnutte theory and are compared with those based on the quantum mechanical model developed by Di Giacomo and Cattani.