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 CaMn7O12 R. Przeniosło1,2,∗ , I. Sosnowska1 , E. Suard2 , T. Hansen2 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 CaMn7 O12 has been studied by powder neutron diffraction. The magnetic ordering in CaMn7 O12 consists of two phases: α, which is ferrimagnetic and β, which is modulated. 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 characteristic for critical scattering: µ(T ) ∝ (TN − T )β with β 0.31, in agreement with the prediction for a three-dimensional Ising model. Models of possible magnetic orderings in CaMn7 O12 are discussed. PACS: 61.12.-q; 75.25.+z; 75.30.Vn The various physical properties of several manganese perovskite 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 Mn3+ and Mn4+ ions in the material. In this paper we present the results of neutron-diffraction studies of the distorted perovskite-type manganite CaMn7 O12 . Bochu et al. [4] studied the room-temperature crystal structure of CaMn7 O12 and determined the lattice constants ah = 10.464 Å and ch = 6.343 Å in the hexagonal setting of space group R3¯ (this hexagonal setting will be used in the present paper). The crystal structure of CaMn7 O12 corresponds to a distortion of the ideal perovskite structure ABO3 along the 111 direction. The A-type sublattice in the perovskite structure is occupied by Ca2+ and Mn3+ ions in (3a) and (9d) positions, respectively. The B-type sublattice is occupied by Mn3+ and Mn4+ ions in (9e) and (3b) positions, respectively [4]. This charge separation of Mn3+ and Mn4+ ions gives an electrostatically neutral unit cell with a Mn3+ : Mn4+ ratio equal to 6 : 1. The Mn4+ (3b) positions have a 3¯ point symmetry without any distortions of the ideal ∗ Corresponding
author. (Fax: +48-22/628-7252, E-mail:
[email protected])
oxygen octahedron. The Mn3+ positions (9d) and (9e) have a point symmetry of 1¯ with Jahn–Teller distorted oxygen octahedra. 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 materials with the orthorhombically distorted perovskite structure Lax Ca1−x MnO3 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 CaMn7 O12 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 CaMn7 O12 can be described by two phases which will be denoted as α and β [8]. Around TC = 49 K there is a magnetic phase transition: the magnetic 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 performed 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 neutron 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 CaMn7 O12 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 Mn2 O3 (5 vol. %) and Mn3 O4 (1 vol. %). Mn3 O4 is ferrimagnetic with a Curie
S1732
temperature of 42 K (1.89 µB per one Mn3 O4 formula at 4.2 K) [11]. Magnetic Bragg reflections due to the antiferromagnetic ordering of Mn2O3 were also detected. Since the magnetic structure of Mn2 O3 has not been fully established yet we have used earlier powder neutron diffraction measurements of Mn2 O3 in similar experimental conditions [6, 12]. The normalized neutron powder diffraction patterns of Mn2 O3 were then subtracted from the CaMn7 O12 diffraction patterns at each temperature. The Mn3 O4 contribution (one small peak) was also subtracted from the data. After these corrections, the CaMn7 O12 patterns were used in the Rietveld refinements. 2 Results and discussion The analysis of the powder diffraction patterns of CaMn7 O12 measured between 50 K and 100 K has been performed by using the Rietveld method and the program FullProf [10]. A ferrimagnetic collinear arrangement already described in [6] has been used for a description of phase α. The magnetic unit cell equal to the crystallographic unit cell was assumed. The manganese ions are located in horizontal planes (i.e. perpendicular to the c axis) with the z coordinate equal to an integer multiple of 1/6. It was assumed that all Mn3+ ions lying in the same horizontal plane have parallel magnetic moments (positions (9d) and (9e)). The magnetic moments of Mn4+ ions ((3b) positions) are antiparallel to the magnetic moments of Mn3+ ions (9d) located in the same horizontal plane. The magnetic moment directions in each plane are given in Table 1. The most intense magnetic Bragg peaks are indexed as (hk0), so it was assumed that the ordered magnetic moments have only a z component. It was first assumed that the values of the magnetic moments of Mn3+ and Mn4+ ions z coordinate 6/6 5/6 4/6 3/6 2/6 1/6
(Mn3+ ) µ3
(Mn4+ ) µ4
↓ ↓ ↑ ↓ ↓ ↑
↑ ↓ ↑
Table 1. Schematic representation of the directions of the magnetic moments of Mn3+ and Mn4+ ions in CaMn7 O12
Fig. 2. Temperature dependence of the values of the magnetic moments of Mn3+ (solid circles) and Mn4+ (open triangles) ions in the ferrimagnetic ordering of phase α in CaMn7 O12 . These values were obtained from Rietveld analysis of neutron powder diffraction data. The solid lines correspond to the power-law dependence µ(T) ∝ (TN − T)β
are equal to µ3 and µ4 respectively. The use of two different values of the magnetic moments of Mn3+ ions in (9d) and (9e) did not improve the quality of the fit. By way of example the result of the Rietveld refinement at T = 85.3 K is shown in Fig. 1. The main magnetic peaks are well described with the ordering of phase α. Magnetic peaks due to the modulated phase β, which are not described by this ferrimagnetic model, can be seen for 43◦ < 2θ < 60◦ . The refined values of the magnetic moments µ3 (T ) and µ4 (T ) are shown in Fig. 2. The temperature dependence of these magnetic moments was fitted by a power-law dependence characteristic for critical magnetic scattering [13]: µ(T ) ∝ (TN − T )β . The resulting values of the exponent β shown in Fig. 2 are in agreement with the exponent predicted for the three-dimensional Ising model, i.e. β = 0.312 [14]. The refined values of TN are 89.9(4) K and 91.0(1.0) K for µ3 and µ4 respectively, and their difference is not significant. The model of the ferrimagnetic ordering of phase α gives a weak magnetic contribution to the merged nuclear peaks (101) and (110). Due to the very high flux at instrument D20 it was possible to verify that the intensity of these merged nuclear reflections, shown in Fig. 3, decreases with temperature between 50 K and 90 K. In spite of the satisfac-
Fig. 1. The observed neutron-diffraction pattern of CaMn7 O12 obtained at a neutron wavelength λ = 2.40 Å at T = 85.3 K is marked with points. The calculated pattern resulting from Rietveld analysis and the model of ferrimagnetic ordering is shown as a line. Below the graph there is a difference curve and the higher and lower ticks indicate the positions for nuclear and magnetic peaks respectively. The strongest magnetic peaks due to phase α are labeled with the letter M. The letter N labels the strongest nuclear peaks
S1733 Acknowledgements. Thanks are due to P. Cross and F. Thomas for help during the measurements. This work was supported by the European Commission Marie Curie Fellowship which is performed at I.L.L. by one of us (RP) under Contract No. HPMF-CT-2000-01002. This work was partially supported by the Committee for Scientific Research (Poland).
References
Fig. 3. Temperature dependence of the merged nuclear peaks (101) and (110). One can see that the intensity decreases as the temperature approaches 90 K. This is in agreement with the ferromagnetic model of phase α, which gives a small magnetic contribution to the (101) and (110) peaks
tory agreement of the model of ferrimagnetic ordering with the strongest magnetic reflections, the ordering of phase β has not been found yet. The interesting behavior of both α and β phases near the magnetic phase transition at TC = 49 K will be studied in the future.
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