Magnetic properties of normal spinels with only a-a interactions

Abstract

2014 The theory of ferrimagnetism accounts for the magnetic properties of the ferrites by strong antiferromagnetic A-B interactions and weak antiferromagnetic A-A and B-B interactions. It was recently discovered that the A-A interaction in Co3O4 was much stronger than anticipated. To ascertain whether the anomalous strong interaction was uniquely a property of Co+3 in an octahedral environment, the study of the A-A interaction has been extended to CoAl2O4, FeAl2O4 and MnAl2O4. Neutron diffraction and magnetic susceptibility measurements have shown that MnAl2O4, and probably CoAl2O4, develop the same antiferromagnetic structure as Co3O4 at low temperature. FeAl2O4 does not develop long range magnetic order, and the difference is attributed to competing A-B interactions due to inversion. The A-A interaction in the aluminates is an order of magnitude weaker than that observed in Co3O4. It now appears that the anomalous strong interaction in Co3O4 is due to indirect exchange coupling through the octahedral Co+3 complex. LE JOURNAL DE PHYSIQUE TOME 25, MAI 1964, Introduction. The magnetic behavior of the ferrimagnetic spinels are in general explained by the theory of Néel [1]. The dominant interaction is negative exchange between ions in the A (tetrahedral) and B (octahedral) sites. The A-A and B-B interactions are negative and weak relative to the A-B interaction. The A-A interaction in Co,O, is unexpectedly large [2]. Co,O, is a normal spinel with Co+ 2 in tetrahedral sites and Co + 3 in B sites [3]. The 3d levels are split into an upper eg doublet and a lower tz, triplet by the octahedral cubic field, and in the ground state the 3d6 electrons of Co + 3 occupy only the levels (in antiparallel pairs) and thus give no net magnetic moment. The moments on the Co + 2 order at 40 OK to form an antiferromagnetic structure in which the spins on the A sites are surrounded by four nearest A neighbors with opposite spins. For z 4 neighbors with spin S = 3/2, and a Néel temperature = 40 OK, the strength of each A-A interaction based on the molecular field approximation [4] is Further information about the A-A interaction in related spinels is needed to determine whether the A-A interaction is larger than has been anticipated, or whether the interaction in Co304 is anomalous and specifically related to the presence of Co ~3 in the octahedral sites. Since the strong A-B coupling usually dominates the magnetic interactions, it would be desirable to study systems in which only diamagnetic ions occupy B sites. This is approximately true for the aluminates of Fe+2 and Co+2. Romeijn [5] prepared CoAl.04 by heating in air, MnAl2O4 by heating in a N2-H2 atmosphere (ratio 2 : 1), and FeAIO by heating in (ratio 2:1:2). All samples were heated at 1200 °C and slowly cooled. From X-ray analysis Romeijn concluded these spinels were normal, i.e., the Al + 3 ions occupied the B sites. However, Greenwald et al. [6] found that CoA’204 and MnA1204 were partially inverse. Their samples were heated at 1 400 °C for 1-2 hours, then either quenched or slowly cooled at a rate of 1° jmin. In CoA1204, the fraction of A sites occupied by Co was 0.69 in the quenched sample and increased to 0.81 when the sample was slowly cooled. In MnÅl2Ü4, the fraction of A sites occupied by iVln was 0.71 -~0.04 and 0.66 ~ 0.04 in quenched and slowly cooled specimens, respectively. The temperature dependence of the susceptibilities of the aluminates is given by Curie-Weiss terms with negative Curie temperatures showing Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01964002505050700