The Magnetic Structure of the Rare-Earth Intermetallic Compound La 3 Co 29 Si 4 B 10
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The Magnetic Structure of the Rare-Earth Intermetallic Compound La3Co29Si4B10 Heng Zhang1 M. Hofmann2 S. J. Kennedy 3 and S. J. Campbell1, 2 1
School of Physical, Environmental and Mathematical Sciences, University College, The University of New South Wales, Australian Defence Force Academy, Canberra, ACT 2600, Australia 2 Technische Universität München, ZWE, FRM-II, 85747 Garching, Germany 3 Bragg Institute, Australian Nuclear Science and Technology Organization, Private Mail Bag 1, Menai, NSW 2234, Australia ABSTRACT A quaternary rare-earth intermetallic compound La3Co29Si4B10 has been synthesized and the crystal and magnetic structures investigated by neutron diffraction over the temperature range 7300 K. Rietveld refinements of the neutron diffraction patterns demonstrate that La3Co29Si4B10 is tetragonal and isostructural with Nd3Ni29Si4B10. The magnetic scattering indicates that the moments of the Co sublattice are collinear and lie in the basal plane. The mean magnetic moment for the Co atoms is µ ~ 0.5 µB, or 13.6µB/F.U., in agreement with magnetization measurements. INTRODUCTION Binary and ternary rare earth transition-metal intermetallic compounds have been extensively studied in the past two decades. Investigation of higher order compounds, such as the quaternary series, provides further scope for preparing novel materials with useful magnetic properties. The recent discovery of the R3T29Si4B10 series of compounds (R= rare earth, T = Ni, Co) is a further step in the continuing search for novel rare-earth magnetic compounds [1]. The exchange couplings between the magnetic moment components are the key factors governing the magnetic properties of rare-earth intermetallic compounds and, in the case of the R3T29Si4B10 series, DC magnetisation measurements [2] have been successfully described by a molecular mean field analysis [3]. However, the Nd3Ni29Si4B10 tetragonal structure is complicated with two R, seven T, one Si and three B crystalline sites [4]. This results in a total of 92 atoms per unit cell and neutron diffraction studies are required for determination of the magnetic structure [e.g. 5]. The La3Co29Si4B10 compound was chosen for initial analysis as the non-magnetic element La enables the magnetic structure and anisotropy of the Co sublattice to be derived. Determination of the magnetic structure of the Co sub-lattice then provides a basis for determining the magnetic structure of the entire set of R3Co29Si4B10 compounds. EXPERIMENTAL The specimen of nominal composition La3Co29Si4B10 was prepared by arc melting and the ingot annealed at 900 °C for 1 week. Variable-temperature (7-300 K) powder neutron diffraction experiments (wave length λ=0.16673 nm) were carried out on MRPD, reactor HIFAR, Lucas Heights with additional experiments at 180 K and 7 K on the E6 diffractometer, BER II reactor, Hahn-Meitner-Institut, Berlin. The longer wave length (λ=0.2380 nm) of E6 allows better access for magnetic diffraction at low scattering vectors compared with MRPD.
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Figure 1 Neutron diffract
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