Determination of the substitution scheme of gallium and magnetic features of Nd2Fe14−xGaxB

Abstract

The unsolved question of the location of the gallium atoms within the Nd2Fe14B structure is addressed. We have performed a high-resolution neutron diffraction investigation on Nd2Fe14−xGaxB powder samples evidencing a preferential substitution of Ga in the Fe (4c), Fe (8j1), and Fe (16k2) sites of the structure. The crystal structure has been investigated using the Rietveld method and the evolution of the shorter iron–iron interatomic distances has been linked to the observed increase of the Curie temperature when the Ga concentration increases. The combined use of neutron diffraction investigation and of scanning electron microscope analysis reveals that the solubility limit of Ga is equal to 1.8 atoms per formula unit. The effects of Ga for Fe substitution on the structural and magnetic properties of the Nd2Fe14−xGaxB compounds have been studied. The magnetic moment values of each atom and the saturation magnetization are clearly sensitive to the Ga for Fe substitution but also to the temperature. A significant modification of the Nd atoms near neighborhood is observed via the evolution of the interatomic distances and the influence of the Ga concentration on the spin reorientation temperature is discussed. The angle of tilt of the magnetization away from the c axis has been determined below the spin reorientation temperature for both Nd2Fe13GaB and Nd2Fe12.2Ga1.8B phases. Finally, a comparison with the influence of Ga on the structural properties of R2Fe17 compounds is presented.

Neutron diffraction investigation of the crystal and magnetic structure of the new ThCo4B compound

“…At low temperature, the magnetic moment is also found to be aligned along the c axis of the structure. The magnitude of the magnetic moments on the Co 2c site is very close to that observed on elemental Co (1.7 µ B /Co atom), or that earlier reported for YCo 5 (1.8 µ B /Co atom) [28,29] or that found on the same position in YCo 4 B phases [19][20][21]. In the case of the Co 6i site, the situation is very peculiar since no significant magnetic moment is observed even at 2 K. In the isotypic RCo 4 B phases [30,31] a small but significant magnetic moment of about 0.7 µ B /atom is measured on this site.…”

“…This is a clear indication that, at room temperature, the EMD of ThCo 4 B is along the c axis of the crystal structure. Hence, the refinement of the magnetic structure has been carried out assuming that the Co magnetic moments are aligned along the c axis of the unit cell, in agreement with the RT alignment in YCo 4 B [19][20][21]25]. Refinement of (0, 0, 0) (0, 0, 0) Th 1b (x, y, z) (0, 0, 0.5) (0, 0, 0.5) Co 2c (x, y, z)…”

“…The CeCo 4 B type structure can be derived from the CaCu 5 structure and has two different crystallographic sites for cerium (1a and 1b), two other sites for cobalt (2c and 6i) and one site for the boron atom (2d), figure 1. This structure type [18,19,25].…”

“…was first reported by Kuz'ma and Bilonizhko [18] who observed that a limited substitution of boron for cobalt is possible in the RCo 5 structure, leading to a series of compounds of formula R n+1 Co 3n+5 B 2n . Since then several studies have been focused on the investigation of the structural and magnetic properties of these phases either by neutron diffraction [19][20][21], nuclear magnetic resonance (NMR) spectroscopy [22][23][24], magnetic measurements [25] or band structure calculations [26,27]. To our best knowledge the formation of the ThCo 4 B phase has not been reported until now.…”