S.M. Yunus

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.

Synthesis and Investigation of the Structural Properties of Al3+ Doped Mg Ferrites

The polycrystalline ferrites of MgAlxFe2-xO4 (0.0≤x≤0.4) were prepared by the conventional solid state ceramic method. The specimens were sintered at 13500C and X-ray diffraction experiments were done at room temperature which showed single phased cubic spinel structure. The lattice parameters were determined from the XRD data using Nelson-Riley extrapolation method and found to decrease with increasing Al concentration obeying Vegard’s law. The cation distribution and oxygen position parameters have also been determined by refining the data using the RIETAN-2000 in the Rietveld method which reveals that the samples possess cubic symmetry corresponding to the space group Fd-3m. The X-ray density and bulk density of each sample were calculated using the lattice parameters. The porosity has been determined from X-ray density (ρx) and bulk density (ρB) and it changes monotonically with Al content.

Cation Distribution And Crystallographic Properties Of The Quaternary Spinel Compound Zn0.4Cu0.6AlxFe2−xO4 (0.0⩽x⩽1.0)

The spinel system Zn(0.4)Cu(0.6)Al(x)Fe(2-x)O4 with x = 0.0, 0.25, 0.50, 0.75 & 1.0 were obtained by solid state reactions in air at 1373 K. X-ray and neutron powder diffraction measurements were performed at room temperature (RT) to characterize the materials. Rietveld refinement of neutron diffraction data showed cubic symmetry corresponding to the space group Fd3m. The distribution of cations over the two sublattices and other crystallographic parameters were determined precisely. The cell parameter decreases and oxygen parameter increases with increasing Al. Sublattices and net magnetic moments at RT were also deduced from the analysis of neutron data. The magnetic structure at RT was found to be ferrimagnetic for x <= 0.75.

Experimental cross section for the 152Sm(n, γ)153Sm reaction at 0.0334 eV

Abstract The neutron capture cross section for the 152Sm(n,  γ)153Sm reaction at an energy of 0.0334 eV was measured for the first time using monochromatic neutrons of a powder diffractometer at the TRIGA Mark II nuclear reactor at Dhaka, Bangladesh. The 197Au(n,  γ)198Au reaction was used to monitor the neutron beam intensity. The radioactivity of the products was determined via high resolution γ-ray spectrometry. The obtained cross section value is 184 ± 22 b, which is consistent with both the ENDF/B-VII and TENDL-2012 data libraries. The measured value at 0.0334 eV and the previous data at 0.0536 eV confirm the reliability of the data in the above libraries.

Crystallographic and magnetic properties of the spinel-type ferrites ZnxCo1-xFe2O4 (0.0 ≤ x ≤ 0.75)

Ultrahigh frequencies (UHF) have applications in signal and power electronics to minimize product sizes, increase production quantity and lower manufacturing cost. In the UHF range of 300 MHz to 3 GHz, ferrimagnetic iron oxides (ferrites) are especially useful because they combine the properties of a magnetic material with that of an electrical insulator. Ferrites have much higher electrical resistivity than metallic ferromagnetic materials, resulting in minimization of the eddy current losses, and total penetration of the electromagnetic (EM) field. Hence ferrites are frequently applied as circuit elements, magnetic storage media like read/write heads, phase shifters and Faraday rotators. The electromagnetic properties of ferrites are affected by operating conditions such as field strength, temperature and frequency. The spinel system ZnxCo1-xFe2O4 (x= 0.0, 0.25, 0.50 and 0.75) has been prepared by the standard solid state sintering method. X-ray and neutron powder diffraction measurements were performed at room temperature. Neutron diffraction data analysis confirms the cubic symmetry corresponding to the space group Fd3m. The distribution of three cations Zn2+, Co2+ and Fe3+ over the spinel lattice and other crystallographic parameters like lattice constant, oxygen position parameter, overall temperature factor and occupancies of different ions in different lattice sites for the samples have been determined from the analysis of neutron diffraction data. The lattice constant increases with increasing Zn content in the system. The magnetic structure was found to be ferrimagnetic for the samples with x <= 0.50. Magnetization measurements show that with the increase of Zn content in the system the value of saturation magnetization first increases and then decreases. The variation of the magnetic moment with Zn substitution has been discussed in terms of the distribution of magnetic and non-magnetic ions over the A and B sub-lattices and their exchange coupling.