Appl. Phys. A 74 [Suppl.], S1731–S1733 (2002) / Digital Object Identifier (DOI) 10.1007/s003390101266 Applied Physics A Materials Science & Processing Magnetic order parameter in the perovskite system CaMn 7 O 12 R. Przenioslo 1,2, ∗ , I. Sosnowska 1 , E. Suard 2 , T. Hansen 2 1 Institute of Experimental Physics, Warsaw University, Ho˙ za 69, 00 681 Warsaw, Poland 2 Institut Laue-Langevin, 6 rue Jules Horowitz, BP-156X 38042 Grenoble Cedex 9, France Received: 6 July 2001/Accepted: 24 October 2001 –  Springer-Verlag 2002 Abstract. The magnetic ordering in the distorted perovskite system CaMn 7 O 12 has been studied by powder neutron diffraction. The magnetic ordering in CaMn 7 O 12 consists of two phases: α, which is ferrimagnetic and β, which is modu- lated. The magnetic peaks due to both phases disappear at the same temperature near 90 K. The temperature dependence of the value of the magnetic moments μ(T ) in phase α can be described for 55 K < T < 90 K with a power-law characteris- tic for critical scattering: μ(T ) ∝ (T N - T ) β with β ≃ 0.31, in agreement with the prediction for a three-dimensional Ising model. Models of possible magnetic orderings in CaMn 7 O 12 are discussed. PACS: 61.12.-q; 75.25.+z; 75.30.Vn The various physical properties of several manganese per- ovskite materials have been under very active investigation recently because of the interesting interplay between their magnetic electronic and structural properties [1–3]. Most of these phenomena are related to magnetic, charge and orbital orderings of interpenetrating sublattices of Mn 3+ and Mn 4+ ions in the material. In this paper we present the results of neutron-diffraction studies of the distorted perovskite-type manganite CaMn 7 O 12 . Bochu et al. [4] studied the room-temperature crystal structure of CaMn 7 O 12 and determined the lattice constants a h = 10.464 Å and c h = 6.343 Å in the hexagonal setting of space group R ¯ 3 (this hexagonal setting will be used in the present paper). The crystal structure of CaMn 7 O 12 corres- ponds to a distortion of the ideal perovskite structure ABO 3 along the 〈111〉 direction. The A-type sublattice in the per- ovskite structure is occupied by Ca 2+ and Mn 3+ ions in (3a) and (9d) positions, respectively. The B-type sublattice is occupied by Mn 3+ and Mn 4+ ions in (9e) and (3b) pos- itions, respectively [4]. This charge separation of Mn 3+ and Mn 4+ ions gives an electrostatically neutral unit cell with a Mn 3+ : Mn 4+ ratio equal to 6 : 1. The Mn 4+ (3b) positions have a ¯ 3 point symmetry without any distortions of the ideal ∗ Corresponding author. (Fax: +48-22/628-7252, E-mail: radek@fuw.edu.pl) oxygen octahedron. The Mn 3+ positions (9d) and (9e) have a point symmetry of ¯ 1 with Jahn–Teller distorted oxygen oc- tahedra. The Mn–O–Mn angles corresponding to the B–O–B type (180 ◦ in the ideal perovskite structure) are near to 140 ◦ , while the Mn–O–Mn angles corresponding to the A–O–B type (90 ◦ in the ideal perovskite structure) are near to 110 ◦ . One should note that in colossal magnetoresistance materi- als with the orthorhombically distorted perovskite structure La x Ca 1-x MnO 3 the Mn ions are located in the B sublattice only and the Mn–O–Mn angles are about 160 ◦ . Earlier neutron powder diffraction studies of CaMn 7 O 12 have shown that this material is paramagnetic above 90 K and it has a modulated magnetic ordering below 90 K [5–7]. The magnetic ordering in CaMn 7 O 12 can be described by two phases which will be denoted as α and β [8]. Around T C = 49 K there is a magnetic phase transition: the mag- netic peaks due to phase β become broadened and they correspond to a coherence length ξ of about 150 Å, while both below 45 K and above 50 K they are resolution-limited, i.e. ξ> 800 Å [5, 7, 8]. The main aim of this paper is to describe the magnetic order parameter related to the phase α. 1 Experimental The neutron powder diffraction measurements have been per- formed at the high-flux diffractometer D20, ILL Grenoble [9], operating at a wavelength of 2.42 Å for temperatures between 100 K and 2 K. The measurements have been performed in a large angular range from 5 ◦ to 165 ◦ . Thanks to the high neu- tron flux it was possible to measure the intensity of magnetic reflections more precisely than in earlier experiments [5–7], and the positions of many weak magnetic satellite peaks have been determined. The neutron powder diffraction pattern of CaMn 7 O 12 obtained in the paramagnetic phase at 100 K was successfully analyzed by the Rietveld method in terms of the structural model [4] by using the program FullProf [10]. As was already found [6], the observed neutron-diffraction patterns contained Bragg peaks due to Mn 2 O 3 (5 vol. %) and Mn 3 O 4 (1 vol. %). Mn 3 O 4 is ferrimagnetic with a Curie