Pressure-Induced Decrease in the Curie Temperature of Gd 2 Fe 17 : LSDA+U Calculations
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DISORDER, AND PHASE TRANSITION IN CONDENSED SYSTEM
Pressure-Induced Decrease in the Curie Temperature of Gd2Fe17: LSDA+U Calculations P. A. Igosheva, E. E. Kokorinab, M. V. Medvedevb, and I. A. Nekrasovb,* a
Institute of Metal Physics, Ural Branch, Russian Academy of Sciences, Yekaterinburg, 620108 Russia for Electrophysics, Ural Branch, Russian Academy of Sciences, Yekaterinburg, 620016 Russia *e-mail: [email protected]
b Institute
Received April 7, 2018
Abstract—To explain the magnetic properties of advanced ferromagnetic intermetallic compounds of the R2Fe17 (R is a rare-earth element) class, experimentalists often use the hypothesis of competition between ferromagnetic exchange and antiferromagnetic exchange between four types of the nearest iron atoms in nonequivalent lattice sites. For the rhombohedral Gd2Fe17 ferromagnet, we calculate the magnetic moments of iron and gadolinium ions, the parameters of exchange between Fe atoms, and Curie temperature TC at a zero pressure and during hydrostatic lattice compression. The magnetic moment of the unit cell of Gd2Fe17 is shown to decrease under pressure, and this decrease is almost completely associated with a decrease in the magnetic moments of Fe rather than Gd ions, the pressure dependence of the magnetic moments of which is weaker by an order of magnitude. In contrast to the hypothesis regarding the competition of exchange interactions between different kinds of Fe atoms, the parameters of exchange between the nearest iron atoms in different crystallographic sites are found to be positive ferromagnetic (at a zero pressure and during compression), and a ferromagnetic character of interaction is shown to remain unchanged under pressure even for Fe atoms in the so-called dumbbell sites with the nearest interatomic distances. The Curie temperature TC of Gd2Fe17 is shown to decrease with increasing pressure. The changes in the exchange parameters and the magnetic moments of Gd2Fe17 during compression are found to be mainly related to a change in the position of energy spectrum branches with respect to each other and the Fermi level eF rather than to a change in the overlapping of wavefunctions, which play a minor role. DOI: 10.1134/S1063776118080186
1. INTRODUCTION The R2Fe17 (R is rare-earth element) intermetallic compounds, which have the highest iron content among the iron and rare-earth metal intermetallics, are being extensively studied experimentally and theoretically [1]. From an experimental standpoint, these compounds are interesting due to invar-like anomalies of thermal expansion (e.g., in Y2Fe17 [2]) and the prospects of achieving a strong magnetic anisotropy in R2Fe17-based interstitial alloys (e.g., Ny (y ≈ 2.6) in Sm2Fe17 [3]). From a theoretical standpoint, these iron intermetallics are of interest as a model object to study the magnitude and the sign of the iron ion–iron ion exchange interaction parameters as functions of the interatomic distances in complex multilattice magnetic systems. The authors of [2] were likely to be the first to sup
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