J. K. Srivastava

Structural, magnetic and transport properties of the spinel ferrites GaxFe1-xNiCrO4

The authors report here on X-ray diffraction, magnetic hysteresis, resistivity and thermoelectric power measurements carried out at different temperatures for the system GexFe1-xNiCrO4(x=0.0, 0.2, 0.4, 0.6, 0.8 and 1.0), which crystallises with a spinel structure. On the basis of the site-preference energy values of the individual cations (as given by Miller, and Smit and Wijn) the cation distribution is expected to be given by (Gax3+Fe1-x3+) (Ni2+Cr3+)-O4, the Ga3+ and Fe3+ ions being on the A sites and the Ni2+ and Cr3+ ions on the B sites, thus providing a selective A-site magnetic dilution with increasing x. The saturation magnetisation measurements indicate a canting of the B-site moments with an average canting angle that increases with x. Whereas the lattice is found to contract with x, the activation energy deduced from the resistivity plots and the Seebeck coefficient is found to increase with x. For a given x the resistivity decreases with temperature (T) and for a given T it increases with x. The Seebeck coefficient is found to be independent of the temperature. These results can be explained by assuming a small-polaron hopping mechanism to be responsible for the conductivity and thermoelectric effect observed in this system.

On the magnetic ordering in the disordered spinels ZnxCo1-xFeCrO4

The AC susceptibility and the low-field magnetisation measurements on the disordered spinel system ZnxCo1-xFeCrO4 (x=0.0, 0.2, 0.4, 0.5, 0.8) are reported. The X-ray analysis shows that the lattice parameter shows a minimum at x=0.5, indicating a redistribution of the cations in the A and B sites on introduction of Zn2+. The low-field AC susceptibility and DC magnesidation indicate two transitions for x=0.0 and 0.2 while only one for x>or=0.4. The high-temperature peak in chi corresponds to a ferrimagnetic transition at TN. The magnetic phase below the lower-temperature peak at Tf could be considered as that of clusters of spins randomly frozen. This state is characterised by strong irreversibility in low DC fields, rapid decay of remanence on increasing the temperature and saturation in rather low applied magnetic fields. For Tf >Tf. For Tf<T<2Tf the interacting groups of spins could be considered to be in a liquid-like state, from which they freeze into random directions at Tf. A suggestive phase diagram is also discussed.

On the re-entrant magnetism in the insulating diluted spinel Co0.5Zn0.5Fe2O4

Disordered spinel ferrites exhibit relaxation behaviour in their Mossbauer spectra. The results of the low-field magnetisation measurements on a representative system show for the first time that the system undergoes a ferrimagnetic transition at TN and then, on cooling, enters into a cluster spin-glass state below Tf with the magnetisation of different clusters randomly frozen. The observed relaxation features in the Mossbauer spectra could be related to the instability in the ferrimagnetic state at higher temperatures.

On the magnetic ordering in the insulating spinel Ga0.8Fe0.2NiCrO4 with the A sublattice diluted to below the percolation threshold

Abstract The results of a.c. susceptibility, d.c. magnetization and Mossbauer measurements on the insulating spinel system Ga 0.8 Fe 0.2 NiCrO 4 are reported. This system has the A sublattice diluted to below the percolation threshold and hence cannot retain any long-range ferrimagnetic order. The a.c. susceptibility shows a peak at T f ≈ 50K. For T #62; 1.5T f the magnetization is highly non-linear and tend to saturate above H #62; 2kOe. The Mossbauer spectra show broad doublets at 77 and 300 K. These observations lead to the picture of gradually evolving spin clusters ferrimagnetically coupled within, as the temperature is lowered, till the intercluster interaction brings about a cluster spin-glass ordering with randomly frozen cluster moments at T f . The absence of the Zeeman splitting in the Fe 57 Mossbauer spectra could be understood in terms of the dynamical effects arising from the thermal excitation of spin clusters.

On anomalous Mössbauer spectra in spinel ferrites

Mossbauer measurements carried out in an external magnetic field rule out the possibility of the domain wall oscillations to be the cause of the anomalous Mossbauer lineshapes observed in spinel ferrites and similar systems.

Cluster spin glass behaviour of mixed spinel ferrite Ga0.8Fe0.2NiCrO4

Mössbauer effect, d. c. magnetisation and a. c. susceptibility measurements in disordered spinel ferrite Ga0.8Fe0.2NiCrO4 (0.3≦T≦300 K, −12≦H (external field) ≦12 kOe) show it to be a cluster spin glass where Gabay-Toulouse phase diagram transitions occur not for individual spins but for groups of correlated spins (clusters) whose effective spins transverse and longitudinal components successively freeze.

Ferri-spin glass nature of spinel ferrites Gax Fe1−x Ni Cr O4

Mössbauer measurements in mixed spinel ferrite Gax Fe1−x Ni Cr O4 (0≤x≤0.8), carried out between 4.2 and 298°K, show the presence of entropic spins in this system. Recent Monte Carlo calculations /4/ have predicted the presence of such spins in a frustrated spin glass lattice.

Pinned and entropic disordering of mixed spinel ferrite Ga0.8Fe0.2NiCrO4

Abstract Mossbauer and magnetic hysteresis measurements ( T =1.4 K and H =-12 to+12 kOe) confirm the existence of pinned and entropic spins in the frustrated disordered spinel ferrite (Ga 3+ 0.8 Fe 3+ 0.2 ) [Ni 2+ Cr 3+ ]O 4 , as suggested by earlier studies.

Cluster spin glass ordering in diluted spinel Zn0.5Co0.5FeCrO4

Abstract The disordered spinel system, Zn 0.5 Co 0.5 FeCrO 4 has been investigated using the low field magnetization and ac susceptibility measurements. From the present results it appears that this system orders into a cluster spin glass state with the magnetic moments of the ferrimagnetic clusters randomly frozen. Compared to the Ni and Co zinc ferrites with the same magnetic dilution, introduction of Cr into the B sites appears to increase the frustration and disorder dramatically. The predicted phase diagrams for the ordering in diluted magnetic spinels do not describe the magnetic behaviour of this system, presumably due to the disorder in the B sublattice in addition to the dilution in the A sublattice.

The spin glass behaviour of disordered spinel ferrite Co2TiO4

AC susceptibility ( chi ) measurements have been carried out in disordered spinel ferrites Co2TiO4, Co2SnO4 and Co1.2Zn0.8TiO4 between 1.7 and 100 K. The last two systems are studied in order to understand the puzzling result for Co2TiO4. The measurements show as many as five peaks in the chi against T curve of this system. In Co2SnO4 only three chi peaks, and in Co1.2Zn0.8TiO4 only one chi peak, are seen. DC magnetisation and neutron diffraction studies by earlier workers have shown Co2TiO4 and Co2SnO4 to be re-entrant systems and Co1.2Zn0.8TiO4 to be a spin glass below about 16.5 K. Furthermore, whereas A site canting is present in Co2TiO4, Co2SnO4 has collinear magnetic structure indicating strong A-B coupling. On the basis of all these results it is concluded that the A and B site spins in Co2TiO4 freeze separately, which is a consequence of the weak A-B coupling in this system. The nature of the variation in the position of the chi peaks with the applied DC field, H, supports this pictures. H is applied collinearly to the AC field.