A polarised neutron study of magnetic moment density in Cu<SUB>2</SUB>MnAl

A polarised neutron study of the ferromagnetic Heusler alloy Cu2Mn0.863Al1.057 has been made. It has been concluded that the magnetic moment density is primarily situated on the Mn ions. On assigning the Mn-moment value, the observed magnetic form factor is found to be in good agreement with the Mn2+ free ion form factor calculated by Watson and Freeman. A slight asphericity has been observed in the moment density. It is estimated that there are about 3% excess 3d-electrons in the Eg states compared to spherical distribution. There is evidence of a very small positive polarisation of the Cu atoms. No appreciable conduction electron polarisation is found

Sign reversal in the exchange bias and the collapse of hysteresis width across the magnetic compensation temperature in a single crystal of Nd0.75Ho0.25Al2

In the admixed Nd0.75Ho0.25Al2 system, magnetic moments of Nd and Ho occupying the same crystallographic site are antiferromagnetically coupled and the chosen stoichiometry displays a magnetic compensation behavior (Tcomp of about 24 K) in all orientations. In the vicinity of Tcomp, the conduction electron polarization (CEP) assumes the role of a soft ferromagnet exchange coupled to a pseudo-antiferromagnet comprising Nd/Ho moments, resulting in an asymmetry in the hysteretic (M-H) loop, i.e., the notion of an exchange bias field (Hexch). Across Tcomp, the CEP contribution reverses sign, and in consonance, the asymmetry in the M-H loop also undergoes a phase reversal. Interestingly, the width of the M-H loop shows a divergence, followed by a collapse on approaching Tcomp from either end. The observed behavior confirms a long standing prediction based on a phenomenological model for ferrimagnetic systems. The field induced changes across Tcomp leave an imprint of a quasi-phase transition in the heat capacity data. Magneto-resistance (\Delta R / R vs T) has an oscillatory response, in which the changes across Tc and Tcomp can be recognized.Comment: 14 text pages + 5 figure

Use of symmetry in non-muffin-tin multiple scattering theory

Use of symmetry in the framework of multiple scattering theory and with non-muffin-tin potentials is shown for a model molecular cluster. After a discussion of cellular symmetrised functions, as treated earlier by Williams and Morgan (1974), a procedure is detailed which fully exploits the results of cellular problem, to obtain the symmetrised molecular functions. Elements of secular matrix are given in a form appropriate for computations

Induced magnetization density in the 5f system UA1/sub 2/

A polarized-neutron diffractometer was used to study the induced magnetization density in the unit cell of the 5f system UA1/sub 2/. Measurements have been made on single crystals at 4.2/sup 0/K with an applied magnetic field of 42.5 KOe (induced magnetic moment 0.0344 ..mu../sub B/ per mole). Neutron wavelengths of 1.067 and 0.785 A and different crystal thicknesses have been used to correct for and eliminate any effects from extinction. It is found that the major part of the magnetization density appears as a contribution localized at the U site, exhibiting a form factor similar to that of the f/sup 3/ free ion, and extrapolating in the forward direction to the value given by the bulk susceptibility. However, there are two interesting aspects of the density. First, the form factor points differ significantly from a smooth curve. This indicates asphericity in the 5f electron distribution arising from the crystalline environment of the U atoms. Second, and more unusual, finite magnetic scattering amplitudes have been observed for a series of reflections to which the centrosymmetric density at the U site cannot contribute. We consider this as strong evidence of noncentrosymmetric bonding involving the 5f-6d electrons

APPARENT LOW A-SITE MOMENT IN Fe3O4

En utilisant la technique des neutrons polarisés, on a mesuré jusqu'à des valeurs sin θ/λ = 0,83 Å -1 les amplitudes de structure magnétique de 49 réflexions indépendantes dans la zone de la magnétite naturelle Fe3O4. La densité de moment magnétique intégrée sur des surfaces circulaires au voisinage des sites A dans le plan (110) conduit à une valeur de moment localisé plus faible (3,82 µB) que celle prévue pour les ions Fe3+. Les points représentatifs des facteurs de forme magnétique des sites A, obtenus à partir de l'analyse des réflexions sur les sites A seulement, se placent près des valeurs de facteur de forme de l'ion libre Fe3+ en prenant cette valeur du moment ; il subsiste toutefois quelque ambiguïté quant au facteur de forme (220). Les facteurs de forme des sites B sont déduits des réflexions restantes. La réduction apparente des moments localisés en site A est considérée comme la conséquence de la covalence dans le cristal. On soutient que le transfert de moments prédominant a lieu par l'intermédiaire des orbitales moléculaires T2. Une étude de diffraction aux neutrons non polarisés sur le même échantillon en poudre conduit à des valeurs de moment un peu plus élevées pour les sites A et B que les résultats ci-dessus.Magnetic structure amplitudes of 49 independent reflections in zone in natural magnetite (Fe3O4) have been measured up to sin θ/λ = 0.83 Å -1 using the polarised neutron technique. The magnetic moment density integrated over circular areas about the A-site in the (110) plane projection leads to a lower value for the localised moment (3.82 µB) than expected for the Fe3+ ions. The A-site magnetic form factor points obtained from ail analysis of A-site-only reflections scale closely to Fe3+ free ion form factor with this value of moment though there is some ambiguity about (220) form factor. The B-site form factors are deduced from the remaining reflections. The apparent reduction of the localised A-site moment is considered to be a consequence of covalency in the crystal. It is argued that predominant transfer of moment takes place through T2 molecular orbitals. An unpolarised neutron powder diffraction study of the same specimen leads to little higher moment values for the A and B-sites than from the above analysis

Neutron interferometry in India

A spectrometer for carrying out perfect crystal diffractometry and neutron interferometry is proposed to be set up at the new 100 MW research reactor Dhruva, as part of a programme to expand and upgrade neutron beam based research activities at the Bhabha Atomic Research Centre (BARC). At present a facility has been set up at the CIRUS reactor where interference oscillations have been recorded using a symmetric LLL silicon interferometer. The setup at CIRUS will serve as a test bed for development of interferometry in BARC

Field and sample history dependence of the compensation temperature in Sm<SUB>0.97</SUB>Gd<SUB>0.03</SUB>Al<SUB>2</SUB>

We present magnetization data on three polycrystalline specimens of Sm0.97Gd0.03Al2: (1) as-cast (grainy texture), (2) powder, and (3) re-melted fast-quenched (plate). The data are presented for nominally zero- (ZFC) and high-field-cooling (HFC) histories. A zero cross-over in magnetization curve at some temperature T=T0 was seen in ZFC data on grainy and powder samples, but not in the plate sample. At fields surpassing magnetocrystalline anisotropy, a 4f magnetic moment flip was still evidenced by HFC data in all samples at a compensation temperature Tcomp, which must necessarily be treated as distinct from T0 (T0 may not even exist). Proper understanding of Tcomp should take account of thermomagnetic history effects

New features relating to magnetic compensation behaviour in zero magnetization spin ferromagnetic alloy Sm_0.98Gd_0.02Al₂

Isothermal magnetic hysteresis studies on the zero-magnetization spin ferromagnetic alloy, Sm0.98Gd0.02Al2, revealed a development of asymmetry in the shape of the M-H loops which are shifted along the field axis near the xompensation temperature (Tcomp). The loop recoeded at Tcomp was essentially a collapsed loop with a negligible apparent coercivity. A measure of shift in the loops along the H axis; referred as 'exchange bias' (He), when plotted against temperature shows a change in sign from positive to negative near Tcomp and is understood to arise from a turn around mechanism of the conduction electron polarization direction