Study of magnetic structures by neutron diffraction and investigation of other magnetic effects in some spinel ferrites

Abstract

Spinel oxides show varied crystalline and magnetic structure due to the distribution of magnetic and nonmagnetic atom in the sublattices and the relative strengths of inter and intra sublattice interactions. In the present work three systems i) ZnxNil_xFeCr04 (x=0.2, 0.4, 0.6 & 0.8), ii) Zn0.4Coo.6AlxFe2-x0(x4=O.O,0.25, 0.50, 0.75 & 1.0) and iii) MnGaxFel_xCr04(x=O.O,0.25, 0.50, 0.75 & 1.0) have been chosen to see the effect of composition in respect of atomic size, moments of the magnetic atoms and non magnetic dilution on the magnetic structure. The samples have been prepared by the standard solid state sintering method. The distribution of magnetic and non magnetic atoms in the sublattices have been determined from the neutron diffraction data using the Rietveld refinement technique. Neutron powder diffraction (NPD) experiments have been carried out on all the samples over a wide range of temperature covering T>Tc down to 10K. Rietveld refinement of the NPD data reveal that all the samples posses cubic symmetry corresponding to the space group Fd3m. The magnetic moment distribution of the sublattices and their transition temperatures has been determined from neutron diffraction data. Also from the technical magnetization, as studied by vibrating sample magnetometer (VSM), the saturation magnetization, coercive force, transition temperature have been determined and the LCR Bridge technique has been used for measuring complex permeability. In the system ZnxNil_xFeCr04,all the Zn ions occupy the tetrahedral site only for x=0.2. For x>0.2, Zn ions also enter into the octahedral site and their occupation in this site gradually increases with increasing x. On the other hand, Cr ions invariably occupy the B site for x";0.6, while only a small fraction (6%) of Cr enter into A site for x=0.8. Fe ions are distributed over both A and B sites for the whole compositional range investigated. Only a small portion (12.5%) 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 x2:0.4.The fractional coordinates of oxygen u, and the lattice parameter ao, increases with increasing Zn content. The moment distributions in the two sublattices and their ordering, as affected by compositional change and temperature variation, have been determined. A deviation from collinearity has been observed, which increases with the increase of diamagnetic iii substitution, being more prominent in the B sublattice. This is interpreted as due to the weakening of JAB interaction. 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;::O.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 suggested for the system at x~O.4,while a semi-spin glass like transition is more likely for x;::O.6due to large reduction in moment and evolution of diffuse signal from short range clusters at low temperatures. The crystal and magnetic structure of the spinel system Zn0.4Coo.6AlxeF2-x04have been determined in the temperature range 800K~T~17K by neutron diffraction technique as in the earlier series. The distribution of magnetic and nonmagnetic ions over the two sublattices are such that all the Zn ions occupy the A site, while AI ions occupy the B site. Co and Fe ions are distributed over both the sites. The distribution of magnetic moments in the A and B sublattices and their ordering as functions of temperature and composition have been obtained which show that the B sublattice magnetization deviates from the usual Brillouin function. There is an increasing loss in B sublattice magnetization with increasing x causing gradual destabilization of the ferrimagnetic order. A ferromagnetic transition is found to take place before reaching the paramagnetic state at higher x values. The net magnetization becomes zero at some temperatures for compositions x~O.75, indicating a state of spin compensation. The presence of diffuse signal at low temperature causing significant broadening of the (Ill) Bragg peak indicates the formation of magnetic spin clusters in the system. Diffuse signal slightly fluctuates with temperature, indicating freezing of the clusters at some temperatures. Progressive loss of B site moment combined with the superposition of magnetic short range clusters over the magnetic long range order has induced a complicated magnetic structure in the system. The features observed in neutron results suggest several transitions for the system including a ferromagnetic transition at x;::O.75 before the paramagnetic state is reached, a spin compensation state at some temperatures when the A and B sublattice moments become equal in magnitude and a iv (- I semi-spin glass like state at x~0.75, as evident from the very high reduction in ordered B sublattice moment and freezing effect of short range clusters. As in the previous two series, the spinel system MnGaxFel_xCr04 with x=O.O, 0.25, 0.50,0.75 and 1.0 have been studied at room temperature for its ion distributions in the sublattices by neutron diffraction. For all the compositions of the series Mn and Fe ions are distributed over both the octahedral and tetrahedral sites. The Ga ions invariably occupy the B site for all values of x, and some Cr ions (44%) enter the A site . at x=1.0, while for the compositions with x:>:0.75all the Cr+3 ions occupy the B site. DC magnetization measurements have been carried out using VSM for all the samples in the three different series. Saturation magnetization and coercive force have been determined from the magnetization curves. Curie temperature has been obtained for the different samples both by neutron diffraction and from the temperature dependence of magnetization. The complex permeability for all the specimens have been determined using LCR Bridge at room temperature from which the real and imaginary parts of the complex permeability, loss factor and relative quality factor have been calculated.