Effect of H on the crystalline and magnetic structures of the YCo 3 − H ( D ) system.

I. ${\mathrm{YCo}}_3$ from neutron powder diffraction and first-principles calculations

Xiang-Yuan Cui, Jian Liu, Peter A. Georgiev, Ian Morrison, D. Keith Ross, Mark A. Roberts, Ken A. Andersen, Mark Telling, and Dave Fort

Phys. Rev. B 76, 184443 – Published 30 November 2007

ABSTRACT

This paper reports investigations into the influence of hydrogen on the magnetic properties of the ${\mathrm{YCo}}_3\text{−}H$ system. We report results on the magnetic structure and magnetic transitions of ${\mathrm{YCo}}_3$ using a combination of neutron powder diffraction measurements and first-principles full potential augmented plane wave $+$ local orbital calculations under the generalized gradient approximation. The ferromagnetic and ferrimagnetic structures are examined on an equal footing. However, we identify that, no matter which structure is used as the starting point, the neutron diffraction data always refines down to the ferrimagnetic structure with the ${\mathrm{Co}}_2$ atoms having antiparallel spins. In the ab initio calculations, the inclusion of spin-orbit coupling is found to be important in the prediction of the correct magnetic ground state. Here, the results suggest that, for zero external field and sufficiently low temperatures, the spin arrangement of ${\mathrm{YCo}}_3$ is ferrimagnetic rather than ferromagnetic as previously believed. The fixed spin moment calculation technique has been employed to understand the two successive field-induced magnetic transitions observed in previous magnetization measurements under increasing ultrahigh magnetic fields. We find that the magnetic transitions start from the ferrimagnetic phase $(0.61{\mu }_B∕\mathrm{Co})$ and terminate with the ferromagnetic phase $(1.16{\mu }_B∕\mathrm{Co})$, while the spin on the ${\mathrm{Co}}_2$ atoms progressively changes from antiparallel ferrimagnetic to paramagnetic and then to ferromagnetic. Our neutron diffraction measurements, ab initio calculations, and the high field magnetization measurements are thus entirely self-consistent.

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• Received 25 May 2007

https://sci-hub.tw/https://journals.aps.org/prb/abstract/10.1103/PhysRevB.76.184443

Effect of H on the crystalline and magnetic structures of the YCo3-H(D) system. II. YCo3-H(D)x from x-ray and neutron powder diffraction

Received 25 May 2007; published 30 November 2007

Abstract

The crystalline and magnetic structures of the YCo3-H(D) system have been investigated by means of x-ray and neutron diffraction with the objective of understanding the complex magnetic changes that are observed in this system as hydrogen is added. Synchrotron x-ray diffraction (XRD) patterns were first refined to yield the lattice parameters and coordination of Y and Co atoms in the metal and two β -hydride phases while XRD was used for the γ phase. In situ neutron powder diffraction measurements of YCo3Dx were then made in all four phases to determine the deuterium site occupancies and magnetic structures. The site occupancies were also rationalized using the Westlake geometric model. The highest hydrogen concentration measured was YCo3H4.6 . Using the Westlake model, we conclude that the saturated hydrogen content would be YCo3H5 . Our results reported here and in Part I [Phys. Rev. B 76, 184443 (2007)] have enabled us to rationalize the changes in the magnetic structures in terms of changes in the cobalt-cobalt distance caused by the addition of hydrogen. In particular, in the antiferromagnetic γ phase, we observe Co atomic displacements that enable the structure to adopt a particular antiferromagnetic structure in a manner that is reminiscent of a Peierls distortion as observed in transitions from the conducting to nonconducting hydrides on addition of hydrogen in YH3 .

https://sci-hub.tw/10.1103/PhysRevB.76.184444