Dependence of spin fluctuations on magnetic field in weak itinerant-electron ferromagnets

An analytical expression that quantifies the suppression of exchange-enhanced local spin-density fluctuations with magnetic field for temperatures in the vicinity of, but still away from, the Curie point in weak itinerant-electron magnets has been derived within the framework of the spin fluctuation model. Our recent magnetization data on the archetype weak itinerantelectron ferromagnet Ni3Al testify to the validity of the theoretical predictions based on the presently derived expression

Effect of site disorder on the magnetic properties of weak itinerant ferromagnet Ni<SUB>75</SUB>Al<SUB>25</SUB>

Detailed study of Ni75Al25 samples with varying degree of site disorder reveals that site disorder promotes magnetic excitations such as spin waves and local spin-density fluctuations and thereby reduces both spin-wave stiffness and Curie temperature. Irreversibility lines in theT-H phase diagram of the weak itinerant ferromagnet Ni75Al25 have been determined for the first time and the effect of site disorder on them has been ascertained

Low-lying magnetic excitations in Ni<SUB>3</SUB>Al and their suppression by a magnetic field

Results of high-resolution magnetization (M) measurements performed on well-characterized polycrystalline Ni3Al sample over wide ranges of temperature and external magnetic field are presented and discussed in the light of existing theoretical models. Contrary to the earlier claims that either Stoner single-particle excitations or nonpropagating spin fluctuations solely determine the temperature dependence of spontaneous magnetization M(T,0), at low temperatures, we find that propagating transverse spin-density fluctuations (spin waves) almost entirely account for the thermal demagnetization of both M(T,0) and "in-field" magnetization M(T,H), at temperatures T&#8804; 0.28TC (TC=Curie point ). The spin-wave stiffness possesses a field-independent value of 69.6(14) meV &#197;2 which conforms well with those determined earlier from small-angle and inelastic neutron-scattering experiments. In the temperature range 0.32TC&#8804; T&#8804; 0.92TC, enhanced nonpropagating spin-density fluctuations (SF) give a contribution to M(T,0) and M(T,H) that completely overshadows the one arising from spin waves. In accordance with the predictions of a modified spin-fluctuation theory, proposed by the authors recently, the thermally excited SF's get strongly suppressed by magnetic field H while the zero-point SF's are relatively insensitive to H

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Extensive magnetization measurements have been performed over wide ranges of temperature and magnetic field on polycrystalline Ni75Al25 samples 'prepared' in different states of site disorder and thoroughly characterized by X-ray diffraction (XRD), X-ray fluorescence and inductively coupled plasma optical emission spectroscopy. Detailed analysis of the magnetization (XRD) data permits an accurate determination of the spontaneous magnetization, M0, zero-field differential susceptibility, &#967;0, and spin wave stiffness, D0, at 0 K, Curie temperature, TC, density of states (DOS) at the Fermi level, N(EF) (the atomic long-range order parameter, S, which is a direct measure of the site disorder present). The effect of site disorder on the ground-state as well as the finite-temperature magnetic properties of Ni75Al25 is clearly brought out by the observed variations of M0, &#967;0, D0, TC and N(EF) with S. The main findings are as follows. Site disorder smears out the sharp features in the DOS curve near the Fermi level, EF, reduces N(EF) and thereby promotes both propagating transverse spin fluctuations (spin waves) and non-propagating zero-point as well as thermally excited spin fluctuations. The reduction in N(EF), in turn, results in diminished (enhanced) values of M0, D0 and TC(&#967;0) with increased site disorder. Site disorder only affects the magnitude of the suppression of spin waves and thermally excited spin fluctuations by the magnetic field (H) but does not alter the functional form of the suppression with field. This functional dependence on field is &#8730;H for spin waves at low temperatures and slows down from a linear variation (~H)at intermediate temperatures to a &#8730;H variation at temperatures close to TC for the thermally excited spin fluctuations. Another important observation is that all the physical quantities of interest such as M0, &#967;0, D0, TC and N(EF) exhibit a sizable change in their magnitudes when point defects such as vacancies play an important role in enhancing the degree of site disorder

Exchange-enhanced Pauli spin paramagnetism in nanocrystalline Ni₃Al

The results of a detailed comparative bulk magnetization study of nanocrystalline (average crystallite size &#x2243;25 nm) and polycrystalline samples of Ni3Al reported here clearly demonstrate that unlike crystalline Ni3Al, which is a well-known weak itinerant-electron ferromagnet, no long-range ferromagnetic order exists in the nanocrystalline counterpart at any temperature &gt;~5 K. Instead, nanocrystalline Ni3Al exhibits exchange-enhanced Pauli spin paramagnetism