The ferrimagnetic mixed spin- 1 2 and spin-1 Ising system on layered honeycomb lattice * ) A. Bob´ ak, M. Jaˇ sˇ cur, D. Horv´ ath DepartmentofTheoreticalPhysicsandGeophysics,FacultyofScience, P.J. ˇ Saf´ arikUniversity,Moyzesova16,04154Koˇ sice,Slovakia T. Balcerzak SolidStatePhysicsDepartment,Universityof L´od´ z, Pomorska 149/153, 90 236  L´od´ z,Poland Received 8 October 2001 A mixed spin- 1 2 and spin-1 Ising ferrimagnet with a layered honeycomb structure is studied within the framework of an effective-field theory with correlations. The effect of interlayer interactions and a single-ion anisotropy on the existence of the compensation temperature and tricritical point is investigated. In particular, it is pointed out that the tricritical point may be only possible if interlayer interactions are nonzero. PACS : 75.10.Hk, 75.30.Kz, 75.50.Gg Key words : ferrimagnet, mixed spins, tricritical point, compensation temperature 1 Introduction A number of experimental studies have recently appeared in the area of molecular- based magnets [1]. Among these materials, some bimetallic compounds with regu- larly alternated different A and B atoms have exhibited interesting magnetic prop- erties and seem to be rather well described by the use of the mixed spin (Ising or Heisenberg) model. For example, the recently synthetized compounds AFe II Fe III (C 2 O 4 ) 3 [A = N(n-C n H 2n+1 ) 4 ,n = 3–5] have critical temperatures between 35 and 48 K and some of them have a compensation temperatures near 30 K [2]. The series of materials form a layered honeycomb structure, in which cations A + are positioned between layers. It is therefore of great interest to find out how the critical and compensation temperatures are effected by the interlayer interactions, which have been neglected in the previous theoretical treatments [3–5]. In this work, we thus investigate a simple mixed spin- 1 2 and spin-1 Ising ferri- magnetic system on a layered honeycomb lattice which includes an interlayer inter- action and a single-ion anisotropy. We believe that such model may be attractive from both an experimental and a theoretical point of view, because by changing the strength of interlayer interactions, one can pass continuously from the three- to the two-dimensional system. In particular, we show that this system exhibits a tricritical point only above a certain threshold of interlayer interaction, while a compensation point may be possible even if interlayer interactions are zero. * )Presented at 11th Czech and Slovak Conference on Magnetism, Koˇ sice, 20–23 August 2001 Czechoslovak Journal of Physics, Vol. 52 (2002), No. 2 127