M.A. Asgar

Neutron diffraction study of magnetic ordering in the spinel oxide CoxMn1−xAl2xFe2−2xO4

Abstract Four samples of the mixed spinel system Co x Mn 1− x Al 2 x Fe 2−2 x O 4 ( x =0.2, 0.4, 0.6, 0.8) were synthesized by ceramic sintering method. The magnetic ordering in the system has been investigated by neutron diffraction measurements in the temperature range 15–773 K. It appears that the system possesses a long-range ferrimagnetic ordering at x ≤0.6 along with some randomly canted spin components, while at x =0.8 a semi-spin glass behavior is observed. The latter composition exhibits three transitions: at temperatures larger than 100 K, there is formation of magnetic spin clusters showing super-paramagnetic type behavior before the true paramagnetic region is reached at about 200 K; a ferrimagnetic long-range ordering is observed in the temperature range 100–50 K; and finally below 50 K there is freezing of the magnetic spin clusters.

Preparation of Zn substituted Ni-Fe-Cr ferrites and study of the crystal structure by neutron diffraction

The spinel type solid solutions of ZnxNi1−xFeCrO4 (x=0.2, 0.4, 0.6 and 0.8) were obtained by solid state sintering technique in air at 1523 K. X-ray and neutron powder diffraction experiments have been carried out on the samples at 295 K in order to characterize the materials. Rietveld refinement of the neutron diffraction data reveals that all the samples of the series possess cubic symmetry corresponding to the space group Fd3m. The distribution of Zn ions on the A and B sites as affected by the variation of x and also the distribution of Cr ions on the A and B sites have been determined. The Fe ions are distributed over both the A and B sites for the whole compositional range investigated. Only a small portion of the Ni ions are found to occupy the A site for x=0.2, while all the Ni ions enter the B site for x≥0.4. The fractional coordinates of oxygen u, and the lattice parameter a, increase with increasing Zn content. The moment distributions in the two sublattices for different composition have been determined.

Investigation of the magnetic structure of the spinel compound ZnxCu1−xFeCrO4 by neutron diffraction

Abstract Temperature-dependent neutron diffraction measurements have been performed on the spinel system Zn x Cu 1− x FeCrO 4 prepared by the ceramic sintering method. The system shows significant deviations from the usual ferrimagnetic behavior. The ferrimagnetic order is perturbed due to the presence of non-collinear spins mainly at the B site. For compositions with larger x values, an unstable ferromagnetic transition takes place before reaching the paramagnetic state. The system may be said to be in a perturbed ferrimagnetic state at x ⩽0.4, where ferrimagnetic ordering is perturbed by the presence of locally canted spin components. The presence of small fluctuating magnetic clusters superimposed on the ferrimagnetic long range order has been revealed by the features shown by neutron data from the compositions with x ⩾0.6. For x =0.6, the lowest-Q Bragg reflection is significantly broadened by the diffuse scattering from these clusters. A semi-spin glass state is proposed for the composition with x =0.6, while a drastic reduction of the sublattice magnetization and a decrease of the diffuse signal is in favor of a cluster spin glass-like phase for the specimen with x =0.8.

The study of magnetic ordering in the spinel system ZnxNi1−xFeCrO4 by neutron diffraction

Abstract Neutron diffraction measurements have been performed in the temperature range 600 K⩾ T ⩾10 K on the spinel system ZnxNi1−xFeCrO4 (x=0.2, 0.4, 0.6 and 0.8) synthesized in the ceramic sintering method. The moment distributions in the two sublattices and their ordering as affected by compositional change and temperature variation have been determined. Ferrimagnetic transition temperatures and spin compensation temperatures for all the specimens have been determined. The system shows significant deviations from the usual ferrimagnetic behavior, which increases with the increase of diamagnetic substitutions, being more prominent in the B sublattice. The ferrimagnetic ordering is perturbed due to the presence of non-collinear spins mainly in the B site. Presence of small fluctuating magnetic clusters for x⩾0.6 is evident from the diffuse nature of the scattered intensity. A qualitative explanation of the observed features is put forward in the light of competing inter and intra sublattice exchange interactions. A randomly canted ferrimagnetic ordering is suggestive for the system at x⩽0.4, while a semi-spin glass like transition is favorable for x⩾0.6 due to large reduction in moment and evolution of diffuse signal from short-range clusters at low temperatures.

Probing of magnetic spin clusters in the spinel oxide compound Co0.8Mn0.2Al1.6Fe0.4O4

Abstract The competition of magnetic spin clusters with the ferrimagnetic long range order in the disordered spinel compound Co0.8Mn0.2Al1.6Fe0.4O4 has been probed through computer analysis of neutron data. A computer program has been developed for this analysis assuming a model for calculating the intensity of the (111) Bragg diffraction peak which contains a diffuse signal from magnetic spin clusters. The behavior of the clusters as a function of temperature has been studied from the temperature response of the parameters which contribute to the neutron intensity of the (111) Bragg peak.

Magnetization and magnetocrystalline anisotropy of Ni-Mo single crystal alloys

Abstract Magnetizations of the binary alloy system Ni 1− x Mo x [ x (wt%) = 4, 6, 8, 10] single crystals have been measured at 4.2, 77 and 300 K, respectively, using the Vibrating Sample Magnetometer (VSM) and the Superconducting Quantum Interference Device (SQUID). The determination of magnetocrystalline anisotropy constants have been done by the integral technique using the M versus H curves in the three principal crystallographic directions [1 1 1], [1 1 0] and [1 0 0]. The effect of alloying Nickel with Molybdenum on magnetization is found to be not linear with increasing Mo concentration and the non-linearity increases more rapidly at higher Mo concentration. The ferromagnetic ordering vanishes around 8 at% Mo. The results of magnetocrystalline anisotropy constants agree with the unpublished previous data of J.E. Scott (Ph.D. Thesis, University of Sheffield, UK, 1969) determined by magnetic torque measurements on this alloy system.