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

Non-spherical magnetic moment in MnAlGe

The magnetic structure factors of MnAlGe (space group P4/nmm) measured with polarised neutrons have been expressed in terms of the magnetic moment of the Mn atom (site symmetry tetrahedral with tetragonal distortion), the Bessel transforms &#9001;jn&#9002; of the Mn radial functions and the fractional occupancies of the moment density in the various crystal field orbitals. The measured structure factors were least-squares fitted with the theoretical expression involving &#9001;jn&#9002; appropriate to the Mn0, Mn+ and Mn2+ atoms. The best fit was got using Mn0 transforms, yielding 1.45&#956;B as the Mn magnetic moment. The fractional occupancies of the moment density in the crystal field orbitals A1g, B1g Eg and B2g were obtained. This analysis shows the magnetic moment to be highly non-spherical with a large fractional occupancy (38%) in the A1g orbital directed along the tetragonal axis while the fractional occupancies of B1g and B2g are found to be 31% and 30% respectively. The fractional occupancy of the moment in the Eg orbital directed towards the Ge and Al atoms is very low (1%). The spatially averaged moment density of Mn in MnAlGe is more diffuse than that of Mn I and Mn II in isostructural Mn2Sb

POLARIZED NEUTRON DIFFRACTION IN FERRITES

La technique de la diffraction des neutrons polarisés a trouvé de larges applications dans l'étude des facteurs de forme magnétique et de la distribution des densités de moments dans les cristaux magnétiquement ordonnés. Dans cet exposé on présente après un rappel sur la technique utilisée, les résultats des études effectuées à Trombay sur des monocristaux de Fe3O4 naturels et de ferrite MnFe3O4. On termine par une brève mention des travaux effectués sur les poudres.The technique of polarized neutron diffraction has found wide application in the investigation of magnetic form factors and magnetic moment density distribution in magnetically ordered crystals. Results of studies at Trombay on natural Fe3O4 and synthetic MnFe3O4 ferrite single crystals are presented in this talk after outlining the technique. A brief mention of powder work is made in the end

Neutron beam research in magnetism: The Indian scene

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MAGNETIC STRUCTURE AND EXCHANGE INTEGRAL MEASUREMENT IN TlMnF3

Une étude par diffraction neutronique dans le composé TlMnF3 polycristallin de 300 °K à 4,2 °K indique qu'il a une structure cubique du type pérovskite (a0 = 4,248 Å). Il subit une transition antiferromagnétique au-dessous de 76,5 °K à une structure du type G sans aucune déformation cristallographique. La maille magnétique TlMnF3 est cubique simple. Chaque ion magnétique (Mn2+) est lié à ses proches voisins par une interaction d'échange du type σ, qui est l'interaction dominante dans ce cristal. En se basant sur cette hypothèse on a évalué cette intégrale d'échange en phase paramagnétique en utilisant la technique de temps-de-vol de neutrons et en se servant de la théorie de de Gennes. On trouve J égal à 3,0 ± 0,3 °K. Ceci est en bon accord avec les calculs théoriques.Neutron diffraction patterns of polycrystalline TlMnF3 from 300 °K to 4.2 °K indicate that it has a cubic perovskite structure (a0 = 4.248 Å) which undergoes an antiferromagnetic transition below 76.5 °K to a structure of the G-type without any crystallographic distortion. The magnetic moment of the Mn2+ ion in this compound is 4.90 µB at 42 °K. The dominant exchange interaction among the magnetic ions in TIMnF3 which form a simple cubic lattice is a σ-type superexchange among the nearest neighbours. Under this assumption, the exchange integral has been evaluated by paramagnetic neutron scattering using the moments method of de Gennes. J is found to be 3° ± 0.3 °K which is in very good agreement with theoretical calculations

Self-heating-induced phenomena in long reentrant ferromagnetic thin film superconductors

We predict several novel self-heating-induced features of the current-voltage characteristics at bath temperatures below the lower critical temperature as well as the temperature dependence of the various critical currents of long reentrant ferromagnetic thin film superconductors on the basis of existing theories and available experimental results. Further we point out how the experimental investigation of the above can lead to a better understanding of reentrant superconductivity and magnetic Kapitza resistance

Thermal neutron polarisation

The basic principle for the production of polarised thermal neutrons is discussed and the choice of various crystal monochromators surveyed. Brief mention of broad-spectrum polarisers is made. The application of polarised neutrons to the study of magnetisation density distributions in magnetic crystals, the dynamic concept of polarisation, principle and use of polarisation analysis, the neutron spin-echo technique are discussed