MAGNETIC-PROPERTIES OF THE DILUTED FERRIMAGNET LI0.5FE1.1GA1.4O4 WITHSPATIALLY NONUNIFORM EXCHANGE

The isotherms of the specific magnetization, sigma(T)(H), and temperat ure dependences of the initial susceptibility, chi(o)(T), have been in vestigated in the temperature interval 77-300 It and in magnetic field s from 0 to 17 kOe as well as at T = 4.2 K and H less than or equal to 40 kOe on the diluted ferrimagnet. Li0.5Fe1.1Ga1.4O4. In this specime n, there is a spatial nonuniformity of the short-range indirect exchan ge interaction Fe3+-O-2-Fe3+ due to the composition disorder of nonmag netic ions Ga3+ in the spinel lattice. In order to change the extent a nd/or the character of this disorder, we used different regimes of hea r treating of the initially synthesized polycrystalline samples at 155 0 K: the quenching in air from T = 1550 IC (the Q sample) and the slow cooling to the room temperature together with the furnace (the SC sam ple). As a result of quenching an increase in the Curie point. T-c, fr om similar to 160 K (SC) to 240 K (Q) is observed, while the isotherm behaviour at 4.7 K differs only slightly (by similar to 5% in magnitud e of sigma). For the Q sample at T greater than or equal to 77 K irreg ular curves sigma(T)(H) are observed and the magnetization relaxation follows an exponential (T = 77 It, 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 kO e). Magnetic properties, the effects induced by the sample quenching i ncluded. are discussed within the model of the spatially nonuniform ma gnetic structure which may be visualized as two exchange-interacting s ubsystems: the matrix with a hi,eh concentration of the frustrated exc hange bonds and the clusters, where spins are coupled by a strong exch ange. (C) 1998 Elsevier Science B.V. All rights reserved.