The anisotropy paradox of Co2+ ions in lithium ferrite

Abstract

Microwave power absorption and hysteresis loops were studied on a series of ferrite materials prepared from a standard LiTi spinel host with Co2+ additions in amounts up to 0.05 ions per formula unit. Experimental results over this concentration range indicate that the rapid relaxation of Co2+ causes X-band spin wave linewidths to increase by more than an order of magnitude. The usual strong effects of Co2+ positive magnetic anisotropy, however, were not found in either ferrimagnetic resonance linewidths or hysteresis loop parameters. To account for the absence of anisotropy effects in the presence of strong relaxation effects, Co2+ is considered to act as a paramagnetic ion through decoupling from the exchange fields of the iron sublattices. This new theory of exchange isolation is based on selective clustering of dilutant ions that results from minimization of magnetic exchange, lattice electrostatic, and crystal field stabilization energies.