N.D. Chaudhari

Initial permeability studies on A13+ and Cr3+ substituted Ni–Zn ferrites

The chemical formula of the samples investigated is Ni0.7Zn0.3Alx/CrxFe2-xO4, where x=0.00, 0.05, 0.10, 0.15, 0.20 and 0.25. The samples were obtained by the usual ceramic technology from high-purity oxides. The initial permeability was calculated from the inductance measurements with a torroidal core of 100 turns, using the formula L=0.0046 μiN2h log d2/d1. The initial permeability μi decreases in Ni0.7Zn0.3Alx/CrxFe2-xO4 with increase in Al3+/Cr3+. The decrease in μi is attributed to a decrease of grain size D from 4.9 μm to 4.4 μm with Al3+ and to 1.9 μm with Cr3+ and to variations in the anisotropy constant K1. The main contribution to the variation of permeability with content of Al3+/Cr3+ in the system is the effect each of them has on domain wall motion. The trivalent substituents (Al3+/Cr3+) cause impedance to the domain wall motion, which increases as the content of these ions increases. Al3+ has a stronger effect than Cr3+. The initial permeability components μ′ and μ″ do not exhibit much variation with temperature, except near Tc, where they fall sharply. The maximum of μ″ near Tc has been attributed to a damping effect of domain wall motion.

X-ray and bulk magnetic properties of aluminium substituted ferrites

Abstract The compositions having the general formula Ni 0.55 Zn 0.45 Al t Fe 2- t O 4 and Ni 0.65 Zn 0.35 Al t Fe 2- t O 4 with t = 0.0, 0.1, 0.2, 0.3 have been prepared by the standard ceramic technique and sintered at 1100, 1150, 1200 and 1240°C to attain different microstructures. The lattice parameter a , magnetization M s are seen to decrease with increasing Al 3+ content whereas the coercive force H c shows an increasing trend with Al 3+ content. M s is found to be invariant with the changes in sintering temperature. The coercivity H c and the remanent ratio R = M R / M s are seen to depend on the sintering temperature and hence on the grain size D . The anisotropy field H K A increases with increase of aluminium content. M s and H K A enable calculation of the anisotropy field constant K 1 which decreases with increase of Al 3+ . The compositional, thermal and microstructural variations of hysteresis parameters like M s , H c and R reveal that grains in the compositions investigated are of multidomain (M-D) type. This result is supported by ac susceptibility data.

Effect of sintering conditions and Al3+ addition on wall permeability in Ni1 − xZnxAltFe2 − tO4 ferrites

Ferrite composites having the general formula Ni1 − xZnxAltFe2 − tO4 were prepared using the standard ceramic method. The composites were sintered at four different temperatures to attain different microstructures. The initial permeability μi is found to increase linearly with the grain size for the same composite sintered at different temperatures. Knowledge of the initial permeability μi, magnetization Ms, grain size D, anisotropy field HKA and anisotropy constant K1 led to μi being resolved into wall permeability μW and rotational permeability μr,k. In all the composites the major contribution to μi was due to μw. μi is found to increase with Zn2+ content while it decreases with the addition of Al3+. The initial permeability does not vary significantly with temperature up to the Curie point. Near the Curie point, there is a sharp fall in μi.

Magnetic hysteresis and domain states in Li1+ substituted Ni–Zn ferrites

Compositions of Ni0.32Zn0.68-2xLixFe2+xO4 with x=0.0, 0.025, 0.050, 0.075, 0.100, 0.125, 0.150, 0.175, 0.200 were prepared by oxalate precursors. The Ms−T curves resemble type R. The magnetic moment (nB) values were found to increase with increase of Li1+ content up to x=0.175, and thereafter it decreased. The former explained on the basis of the exchange interactions, whereas the decrease of nB is related to the migration of Ni to tetrahedral (A) sites. There is a decrease in Yafet–Kittle (Y–K) angles at octahedral (B) sites, suggesting that lithium does not favor Hc canting. Actually, Li1+ tends to reduce canting, resulting in the increase of nB coercive force (Hc) and remanance ratio (R) variations have been related to increase of anisotropy constant (Kl) with increasing Li1+ content. The values of R are comparable with the theoretical value, 0.87, which is probably due to the very high density of the samples. Thermal variations of Hc and R reveal that neither parameter changes rapidly with increasing temperature, indicating that the material contains multi-domain (MD) grains in predominance. These results are also supported by a.c. susceptibility studies. The values of wall susceptibility are higher than those of rotational susceptibility, suggesting that the susceptibility is contributed mainly by the domain wall motion.

Relaxation time studies on Ni0.7Zn0.3AlxFe2 − xO4 and Ni0.7Zn0.3CrxFe2 − xO4

The effect of substitutions of Al3+ and Cr3+ on τm in Ni-Zn ferrites prepared using usual ceramic technology has been investigated. The relaxation time τm for the system of ferrites Ni0.7Zn0.3AlxFe2−xO4 and Ni0.7Zn0.3CrxFe2−xO4 increases with the increase in the Al3+ and Cr3+ content. This is attributed to an increase in the resistivity. An increase in τm with Al3+ and Cr3+ suggests that the response of the material to the field variation is slowed down, i.e. these substitutions cause hinderance to the domain wall motion. From the observation that the rate of change of τm with Al3+ substitutions is greater than that with Cr3+ substitutions, it can be concluded that Al3+ causes more hinderance to the domain wall motion than Cr3+ ions.