Journal of Magnetism and Magnetic Materials 90 & 91 (1990) 683-684 North-Holland Magnetic structures and their phase transitions in lanthanide cuprates and related compounds K. Yamada, M. Matsuda, K. Kakurai, T.R. Thurston, Y. Endoh, H. Kadowaki '. y. Hidaka 2, T.Murakami 2, R.J. Birgeneau 3, P.M. Gehring 4 and G. Shirane 4 Department of Physics, Tohoku University, Aramaki, Aoba, Sendai 980, Japan 683 Magnetic structures and phase transitions in lanthanide compounds with La2Cu04(T)- and Nd 2CuOiT')-type structures are reviewed. The spin structures can be summarized by two types of structure: La2Cu04 (S.LT) and La 2Ni04-type (SliT). Successive magnetic phase transitions and order of the rare-earth moments are observed in both T- and T'-type compounds. 1. Introduction The discovery of the high 1;, oxide by Bednorz and MUlier [1] has triggered extensive studies of the lanthanide compound with La2Cu04(T)- and Nd2Cu04(T')-type structures. Almost all the carrier undoped compounds studied so far exhibit antiferro- magnetic long range order which is revealed by neutron diffraction measurements. The purpose of this paper is to describe or classify the variety of these antiferromag- netic structures from a simple point of view. 2. Crystal and magnetic structures In T-type, the orientation of the oxygen octahedron surrounding the 3d metal ion determines the symmetry of the structure. This orientation can be determined by two angles Band ep which describe the tilt from the c-axis in tetragonal notation and the rotation about the c-axis, respectively. Different values of these angles correspond to the modifications of the T-type structures as follows; 1) the high temperature tetragonal phase with the space group 14jmmm (B = 0), 2) the ortho- rhombic phase with the space group Cmca (B"* 0 and ep = 0), 3) the orthorhombic phase with the space group Peen (B"* 0 and ep"* 0) and 4) the low temperature tetragonal phase with the space group P42jncm (B"* 0 and ep = 45 0 ). Here ep is measured from the principle axis in the plane. Due to the change of the orientation 1 Institute for Solid State Physics, Tokyo University, Rop- pongi, Tokyo, Japan. 2 NIT Opto-Electronics Laboratories, NIT Corporation, Tokai, Ibaraki 319-11, Japan. 3 Department of Physics, Massachusetts Institute of Technol- ogy, Cambridge, MA 02139, USA. 4 Physics Department, Brookhaven National Laboratory, Up- ton, NY 11973, USA. of the octahedron, T-type compounds exhibit structural phase transition. In contrast, since no such octahedron exists in T'-type, no structural phase transitions have been observed and the crystal symmetry is tetragonal. .A large variety of the magnetic structures in both T- and T'-type is described by two types of basic struc- tures; La 2CU04-[2] and La 2 Ni0 4-type [3] structures. These two structures can be defined because the spin direction S is confined within the 3d metal-oxygen plane. The difference between the two is the relative direction between S and the antiferromagnetic propa- gation vectorv; for La 2Cu04-type, S is perpendicular to T and for La2Ni04-type, S is parallel to T. Two types of the structures are therefore interchangeable by either a 90 0 rotation of all the spins or a 180 0 of rotation of the spins on the alternative planes. The orientation of the oxygen octahedron in T-type also determines the ligand field for the 3d metal mo- ment. Therefore, when the tilt angle B for the oc- tahedron is finite, the antisymmetric interaction stabi- lize the spin canting along the direction perpendicular to the plane for La2Cu04-type. In the case of La2Ni04-type however, since the spin direction is paral- lel to the antisymmetric interaction vector, the spin canting in any direction cannot be stabilized. In the case of La 2Co0 4, we have observed more complicated spin structure and the phase transitions [4], This com- pound exhibits a structural phase transition from Cmca orthorhombic to P42jncm tetragonal phase. As de- scribed above, this transition is ascribed by the change of the tilt direction for the octahedron. Since the plane anisotropy of the Co2+ moment is very strong, the moment follows the reoriention of the octahedron to avoid the canting. As a result, a non-collinear structure as shown in fig. 1 can be modeled which provides a consistent diffraction pattern to the experiment. Re- cently similar successive magnetic phase transitions have been observed in Pr2Ni04 [5,6]. In this case however, 0304-8853/90/$03.50 © 1990 - Elsevier Science Publishers B.V. (North-Holland) and Yamada Science Foundation