Magnetic properties of an easy-plane trigonal NdFe 3 (BO 3 ) 4 antiferromagnet
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SORDER, AND PHASE TRANSITION IN CONDENSED SYSTEMS
Magnetic Properties of an Easy-Plane Trigonal NdFe3(BO3)4 Antiferromagnet D. V. Volkova, A. A. Demidovb, and N. P. Kolmakovab* a Moscow b Bryansk
State University, 119992 Moscow, Russia State Technical University, Bryansk, 241035 Russia *e-mail: [email protected] Received January 9, 2007
Abstract—The field and temperature dependences of magnetization and the temperature dependences of the initial magnetic susceptibility have been theoretically studied for three crystallographic directions in a trigonal NdFe3(BO3)4 antiferromagnetic crystal. The calculations were performed using a molecular field approximation and a crystal field model for the rare-earth subsystem. The obtained theoretical expressions are applied to the interpretation of recent experimental data [1–4] on the magnetic properties of NdFe3(BO3)4. The results of calculations show a good agreement with experiment. The proposed theory adequately describes (i) anomalies of the Schottky type in the temperature dependence of the magnetic susceptibility, (ii) nonlinear curves of magnetization in the basal plane in a magnetic field up to 1 T (showing evidence of the first-order phase transitions) and their evolution with the temperature, and (iii) the field and temperature dependences of magnetization in a magnetic field up to 9 T. PACS numbers: 75.30.-m, 75.70.Ee, 71.70.Ch DOI: 10.1134/S1063776107060076
1. INTRODUCTION Rare-earth (RE) ferroborates of the RFe3(BO3)4 system, where R is an RE element, exhibit a large variety of magnetic properties [5, 6] typical of so-called f–d compounds. This behavior is related to features of the RE ion (with its particular electron structure) magnetized by the f–d interaction, which are manifested on the background of magnetic behavior of the iron subsystem. Some RE ferroborates (e.g., with R = Gd, Nd) exhibit a correlation of their magnetoelastic and magnetoelectrical properties [7, 8], which allow these crystals to be classified as multiferroic compounds. The crystal structure of RFe3(BO3)4 at high temperatures is 7
trigonal and belongs to the R32 ( D 3 ) space group [5, 9, 10]. The main structural elements in this crystal are helical chains formed by FeO6 octahedra, which share edges and are extended along the c axis. The RE ions are built in RO6 prisms and, being separated from each other by BO3 regular triangles, are not bound to common oxygen ions. Three neighboring chains of FeO6 octahedra are linked via BO3 triangles and RO6 prisms. There are no direct Fe–O–Fe bonds lying in a common plane, and the interchain exchange is realized via longer paths: Fe–O–R–O–Fe and Fe–O–B–O–Fe. At the same time, Fe3+ ions belonging to the same chain are coupled via shorter paths. Both intra- and interchain interactions exhibit an antiferromagnetic (AFM) character.
The thermodynamic properties of RE ferroborates have been studied (see, e.g., review [11]), in particular, several investigations were devoted to neodymium ferroborate. Chukalina et al. [12] reported original data (derived
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