N. S. Satya Murthy

Magnetic Structure of MnAlGe

The ternary alloy MnAlGe has been studied by diffraction techniques using both neutrons and x rays. The unit cell is found to be tetragonal with a0 = 3.914±0.001 A and c0 = 5.933±0.001 A. The space group is D4h7‐P4/nmm. The intensities in the neutron‐diffraction patterns agree with a ferromagnetic spin alignment of the Mn atoms with a magnetic moment of 1.40±0.12 μB each. The magnetic field dependence of the diffracted neutron intensities reveal a high magnetic anisotropy and a high retentivity.

Magnetic structures in the iron-germanium system

Abstract Three phases in the iron-germanium system have been investigated by neutron diffraction for determining their atomic and magnetic structures. Of these, FeGe 2 is found to be antiferromagnetic with a Neel temperature of 315°K and Fe 1.67 Ge is ferromagnetic having a Curie temperature of 510°K. There is evidence of a distortion of the ideal “filled” NiAS structure in the case of the latter. FeGe is paramagnetic down to 140°K.

Detachable leads for superconducting magnets

A superconducting magnet coil has current supplied thereto through external connection leads. In the persistent current mode the leads are superfluous since current circulation is maintained by a superconducting link across the coil. Accordingly to prevent heat losses by heat conduction through the leads they have joints so that the leads can be detached from the coil by manipulation from outside the cryostat thus enabling the leads to be completely removed when the magnet is in use. The joints in the leads may comprise screw-threaded male and female members or else plug and socket members. In either case provision is made for re-attachment of the leads from outside the cryostat without interruption of the persistent current.

Polarised neutron diffraction study of nickel ferrite

Abstract Polarised neutrons have been used to solve the existing ambiguities in the chemical and magnetic structures of NiFe 2 O 4 . The results show conclusively that it has a completely inverted spinel structure with an oxygen parameter of 0.2573 ±0.0003. The observed magnetic moments at room temperature show that nickel ferrite exhibits the Neel ferrimagnetism with m ( A )Fe = 4.86μ B , m ( B )Fe = 4.73μ B and m ( B )Ni = 2.22μ B leading to a net moment of 2.09μ B . The Debye-Waller temperature correction was determined with the constant B = 0.8 × 10 -16 cm 2 .

A simplified expression for the minimum hotspot current in long, thin-film superconductors

A generalization of the Skocpol-Beasley-Tinkham hotspot theory to include an approximate temperature dependence of the conductive heat transfer term of the heat flow equations clearly indicates that the role of the thermal conductivity of the material of the film is not reflected in the observed temperature dependence of the minimum current Ih required to sustain a hotspot in a long, thin-film superconductor. This observation leads to the derivation of a simplified expression for Ih valid for a wider range of bath temperatures.

Magnon dispersion measurement in MnFe2O4

Abstract Using the polarised neutron diffraction technique, the acoustic magnon dispersion curve in MnFe 2 O 4 has been measured up to 30% of the zone boundary and discussed in terms of a polynomial expression for the magnon energy and the Heisenberg model.

Magnetic moment distributions and form factors in ferromagnetic nickel-ruthenium alloys

Using polarised neutrons, the full three-dimensional magnetic structure amplitudes in the Ni1−c Ruc single crystals forc = 0·027, 0·033 and 0·046 were measured. Moment density maps in various portions of the Wigner-Seitz cell were obtained. It is seen from these maps that unlike Ni-based alloys with 3d impurities, the introduction of Ru to the Ni matrix produces extensive perturbations in the diffuse moment density, giving rise to a netpositive diffuse moment which tends to increase with Ru concentration. The asphericity of the host moment at first increases and then decreases with increasing Ru content. Another significant outcome of the present study is the evidence for the reversal of the sign of the Ru moment, from negative to positive, obtained by comparing the shape of the spherical site form factors of the three-alloy concentrations with the Ni spherical form factor itself. The sign reversal of the impurity moment is confirmed by the form factor analyses. Strong local environmental effects seem to play a major role in this alloy system.

Neutron scattering from paramagnetic chromium halides

Abstract Using the cold neutron inelastic technique, exchange integrals have been measured in CrBr 3 and CrF 3 in their paramagnetic phase.

Ferrimagnetic structure of Mn2Co2C

Abstract The carbide Mn 2 Co 2 C has been found to be ferrimagnetic with a Neel temperature of 535°C by means of neutron diffraction. It has a cubic unit cell with an Mn atom at the cube corner and the Carbon atom at the body centre. The face centre sites are taken up by the other Mn and the two Co atoms in a statistical manner. The Mn moments are aligned anti-parallel to each other while the two Co moments are parallel to the comer Mn moment. The most reasonable values for the moments are: 4.0 μ B for the corner Mn atom, 3.36 μ B for the face centre Mn and 1.25 μ B for each Co atom. The smaller moments of the face centre atoms arise from the influence of the carbon atom. Polarised neutrons were used to choose a structure having cubic symmetry over one of tetragonal symmetry.

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.