Low resolution TEM micrograph of sample Mg 0.2 Mn 0.5 Ni 0.3 Al 0.1 

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... λ is wave length of Cu- Kα radiation, constant K =0.9. Particle size estimated from XRD was 102.25 nm, 44.50 nm 41.65 nm for y=0.0, 0.05 and 0.10 respectively. Fig. 1 display low resolution TEM image and particle size distribution in Mg 0.2 Mn 0.5 Ni 0.3 Al 0.1 Fe 1.9 O 4 sample. It can be seen that the particles are quite well dispersed and not much agglomerations are present. An interesting aspect of TEM image is the presence of very small particles coating surface of larger ones. These small particles are nearly evenly sized with an average size ranging from 6-12 nm. Particle size of larger ones, which encloses smaller ones is in the range of about 40-45 nm, which is comparable to particle size determined from Scherrer’s formula fo r y=0.05 and 0.10. The particle size estimated from TEM is slightly less than particle size estimated from XRD using Scherrer’s formula. This is because X-ray diffraction gives information of crystalline region only and contribution from amorphous grain surface does not contribute. On other hand TEM gives complete picture of nanoparticles. By analyzing TEM and XRD one can have almost complete picture of particle size, their distribution and morphology. Lattice constant for y=0.0, 0.05 and 0.10 calculated from XRD data was 8.3992 Å, 8.3947 Å and 8.3906 Å respectively. Linear decrease in lattice constant is due to replacement of 3+ 3+ larger Fe ions by smaller Al ions. Experimental density 3+ and X-ray density decreases with Al ions concentration. 3 X-ray density for y=0.0, 0.05 and 0.10 were 5.070 g/cm , 3 3 5.017 g/cm and 5.014 g/cm respectively. Experimental 3 densities for the same samples were 4.807 g/cm , 4.805 3 3 g/cm and 4.801 g/cm . Room temperature Mössbauer spectra of Mg 0.2 Mn 0.5 Ni 0.3 Al y Fe 2-y O 4 (y=0.0, 0.05 and 0.10) are shown in Fig. 2 . Dots in figure represent the experimental data and solid lines through the data points are least-squares fitting. The general trend of decreasing the hyperfine fields value is presumably due to the weakening of A – B exchange 3+ interaction caused by the substitution of non-magnetic Al 3+ ions with the Fe ions. As indicated, the samples exhibit typical relaxation spectra at room temperature and could be analyzed in terms of the superposition of two sextets and a quadrupole split central line. The Mössbauer phenomenon and the hyperfine interactions have characteristic times and the spectrum observed in any situation depends on whether the properties of the nuclear environment or the position of the nucleus are changing relative to these times. These time-dependent effects can influence both the spectral line shapes and the values of the Mössbauer hyperfine parameters. Time-dependent changes in the nuclear environment are often referred to as relaxation processes. In order to observe the Zeeman splitting between the nuclear energy levels, the magnetic field at the nucleus must remain constant over the period of time T necessary for the nucleus to undergo a Larmor precession of approximately one revolution. The Larmor precession period T L for Fe is approximately 4 x 10 s. A magnetic splits is observed in Mössbauer spectrum (with six lines in 57 the case of Fe) when T»T L and a paramagnetic spectrum (with one or two lines) is observed when T«T L . In the intermediate range T~T L (relaxation), complex spectra with 3+ broad ened lines can be observed. Al ions substitution on 3+ B-site results in moving some of the Fe ions, which has a larger magnetic moment (5μ ), from B-site to A-site. For y =0.0, the occupancies of Fe ions at both sites are obtained as nearly equal. On increasing Al content in 3+ Mg 0.2 Mn 0.5 Ni 0.3 Al y Fe 2 O 4 , total area occupied by Fe at A- site was found to increases and that at B-site decreases. This implies that on increasing Al concentration, Al- ions 3+ go to the B-site of spinel structure replacing the Fe ions from B-site to tetrahedral A- site. The Mössbauer spectra of samples show a systematic variation with decrease in particle size. As observed from both XRD and TEM studies, particle size decrease with 3+ increase in Al ions content. Hence variation in the 3+ Mössbauer spectra with Al ions content can be correlated to particle size. Crystallite sizes are so small that thermally induced energy fluctuations can overcome anisotropy energy and change direction of magnetization of a particle from one easy axis to another, a super paramagnetic relaxation is observed in the Mössbauer spectrum and magnetic sextet collapses into doublets. The presence of doublets alone in the Mössbauer spectra of samples can be attributed to super paramagnetic relaxation due to extremely small size of crystallites [11] . The appearance of paramagnetic doublet superimposed on a two six line pattern indicates presence of short range magnetic ordering 3+ induced by fine particle size effects. Al ions has no effect on the isomer shifts values, which indicates that the s- 3+ electron charge distribution of the Fe ions does not change. The isomer shift at A and B sites show that iron is 3+ in the Fe state. Quadrupole interaction has values close to zero for both A and B sites. As in the presence of a strong magnetic interaction, the distribution of quadrupole interactions, which arise from chemical disorder, produces an appreciable broadening of the individual Zeeman lines for both the octahedral and the tetrahedral patterns, but does not produce observable quadrupole line shifts. The variation of ‘μ i ’, with frequency at different temperature for y=0.10 shown in Fig. 3(a) can be explained on basis of Globus model [12] . According to this model, relaxation character ...