Magnetic properties of the diluted ferrimagnet Li 0.5Fe 1.1Ga 1.4O 4 with spatially nonuniform exchange

Abstract

The isotherms of the specific magnetization, σ T ( H), and temperature dependences of the initial susceptibility, χ 0( T), have been investigated in the temperature interval 77–300 K and in magnetic fields from 0 to 17 kOe as well as at T=4.2 K and H⩽40 kOe on the diluted ferrimagnet, Li 0.5Fe 1.1Ga 1.4O 4. In this specimen, there is a spatial nonuniformity of the short-range indirect exchange interaction Fe 3+–O 2–Fe 3+ due to the composition disorder of nonmagnetic ions Ga 3+ in the spinel lattice. In order to change the extent and/or the character of this disorder, we used different regimes of heat treating of the initially synthesized polycrystalline samples at 1550 K: the quenching in air from T=1550 K (the Q sample) and the slow cooling to the room temperature together with the furnace (the SC sample). As a result of quenching an increase in the Curie point, T c, from ∼160 K (SC) to 240 K (Q) is observed, while the isotherm behaviour at 4.2 K differs only slightly (by ∼5% in magnitude of σ). For the Q sample at T⩾77 K irregular curves σ T ( H) are observed and the magnetization relaxation follows an exponential ( T=77 K, H=0.71 kOe) or logarithmic law ( T=123 K, H=0.74 kOe; T=177 K, H=0.73 kOe; T=77 K, H=1.55 kOe). Magnetic properties, the effects induced by the sample quenching included, are discussed within the model of the spatially nonuniform magnetic structure which may be visualized as two exchange-interacting subsystems: the matrix with a high concentration of the frustrated exchange bonds and the clusters, where spins are coupled by a strong exchange.