Ce3−xMgxCo9 : Transformation of a Pauli Paramagnet into a Strong Permanent Magne

Abstract

We report on the synthesis of single crystalline and polycrystalline samples of Ce3−xMgxCo9 solid solution (0 ≤ x 1.4) and characterization of their structural and magnetic properties. The crystal structure remains rhombohedral in the whole composition range and Mg partially replaces Ce in the 6c site of the CeCo3 structure. Ferromagnetism is induced by Mg substitutions starting as low as x = 0.18 and reaching a Curie temperature as high as 450 K for x = 1.35. Measurements on single crystals with x = 1.34 and TC = 440 K indicate an axial magnetic anisotropy with the anisotropy field of 6 T and a magnetization of 6 µB/f.u. at 300 K. Coercicity is observed in the polycrystalline samples consistent with the observed axial magnetic anisotropy. Our discovery of ferromagnetism with large axial magnetic anisotropy induced by substituting a rare-earth element by Mg is a very promising result in the search of inexpensive permanent-magnet materials and suggests other nonmagnetic phases, similar to CeCo3, may also conceal nearby ferromagnetic phases.

Borderline Magnetism: How Adding Mg to Paramagnetic CeCo3 Makes a 450-K Ferromagnet with Large Magnetic Anisotropy

“…Motivated by a recent experimental study [25] we have carried out first principles calculations to understand the remarkable transformation of paramagnetic CeCo 3 by Mg alloying into a material with magnetic properties (including large magnetization, Curie point, and magnetic anisotropy) comparable to those of a potential permanent magnet. We find this transformation to result from a combination of Stoner physics and the magnetic anisotropy of the Ce and Co atoms.…”

“…The calculations were performed by using the all electron density functional code WIEN2K [29] at the experimental lattice parameters [25], which are listed in Table I. The internal coordinates were relaxed until forces on all the atoms were less than 1 mRyd/Bohr.…”

“…Also the Co magnetic moment calculated at experimental lattice parameters within LDA (0.33 µ B ) and GGA (0.99 µ B ) functionals are very different. Experimentally CeCo 3 is often characterized as Pauli paramagnetic [25,42], although there is older literature [43] finding evidence for magnetic character in CeCo 3 . This suggests that CeCo 3 is at or near a magnetic instability, which is better represented in this case by the LDA approach, with its much smaller magnetic energy (−1.9 meV/Co).…”

“…Unless otherwise stated, all the calculations presented in the following sections are therefore performed using the LDA at the experimental lattice constants from Ref. [25].…”

“…For simplicity, in this calculation we omit spinorbit coupling and the Hubbard U which will play an important role in detailed comparison with experiment later. 2.20 [25] The plot depicts the calculated DOS in a narrow window around the Fermi level (E F ) (main plot) and for several eV around E F in the inset. From the inset we see two main features -the large Ce peak (in blue), from the unoccupied localized 4f states, centered around 1 eV above E F , and the Co DOS, mainly from −4 eV to +1 eV (in red).…”