90 ISSN 1063-7834, Physics of the Solid State, 2019, Vol. 61, No. 2, pp. 90–93. © Pleiades Publishing, Ltd., 2019. Russian Text © I.S. Tereshina, G.A. Politova, V.A. Chetyrbotskii, E.A. Tereshina-Chitrova, M.A. Paukov, A.V. Andreev, 2019, published in Fizika Tverdogo Tela, 2019, Vol. 61, No. 2, pp. 230–233. Effect of Hydrogenation on Magnetostriction and Magnetocaloric Effect in Gadolinium Single Crystal I. S. Tereshina a, *, G. A. Politova b, c , V. A. Chetyrbotskii a , E. A. Tereshina-Chitrova d, e , M. A. Paukov d, f , and A. V. Andreev e a Moscow State University, Moscow, 119991 Russia b Baikov Institute of Institute of Metallurgy and Materials Science, Russian Academy of Sciences, Moscow, 119334 Russia c St. Petersburg Polytechnic University, St. Petersburg, 195251 Russia d Charles University, Prague, 116 36 Czech Republic e Institute of Physics, Czech Academy of Sciences, Prague, 182 21 Czech Republic f Immanuel Kant Baltic Federal University, Kaliningrad, 236016 Russia *e-mail: irina_tereshina@mail.ru Received September 10, 2018 Abstract—The gadolinium single crystal obtained by the Czochralski method was hydrogenated to the com- position GdH 0.15 , which corresponds to a metal–hydrogen solid solution (α phase). The magnetostriction and magnetocaloric effect were measured for both the initial and hydrogenated samples. It is found that the hydrogen atoms in the hexagonal lattice of gadolinium can affect the magnitude and sign of the magnetostric- tion constants and cause the anisotropy of the magnetocaloric effect. The main mechanisms responsible for the observed effects are discussed. DOI: 10.1134/S1063783419020306 1. INTRODUCTION Rare-earth metals (REMs) have aroused consider- able interest among researchers for quite a long time [1], since the study of the properties of these metals has not only scientific but also practical importance. Rare-earth metals and their alloys and compounds are widely used in many fields of modern production, namely, in the atomic energy and metallurgy; oil refin- ing; glass, ceramic, and optical industries; electronics; and medicine [2–4]. Among all the rare-earth metals, researchers pay particular attention to gadolinium [5– 7]. It is a functional magnetic metal (for example, serves as a working fluid of a magnetic refrigerator) with a Curie temperature close to room temperature (T C = 293 K). Below T C , gadolinium (the only one among magnetic rare-earth metals) has a ferromag- netic ordering, which persists upon cooling down to the lowest temperatures. At T = 220 K, gadolinium exhibits a spin-reorientation transition (SRT) of the type of “axis of easy magnetization–light cone.” In refrigerators, gadolinium works in hydrogen- containing media. That is why the study of the effect of hydrogenation on the magnetic properties of gado- linium is critical. Earlier, we studied the effect of hydrogen and nitrogen on the Curie temperature, the magnetocaloric effect (MCE) in the T C region and on the mechanical properties for structurally inhomoge- neous gadolinium samples obtained after distillation [8–12]. The purpose of this work is to study the anom- alies of the magnetostriction and magnetocaloric effect in a gadolinium single crystal before and after hydrogenation near two magnetic phase transitions: “order–order” (in the SRT region) and “order–disor- der” (in the region of the Curie temperature). 2. EXPERIMENTAL The gadolinium single crystal was grown under argon by the Czochralski method, using an arc fur- nace, a water-cooled copper crucible, and a tungsten rod (as a nucleant) for this purpose. Confirmation of the single crystal state of the obtained sample, as well as its orientation along the main crystallographic directions, was carried out using the Laue method. Plates with dimensions of 8 × 4 × 2 mm were cut out from the resulting crystal to study the magnetoelastic and magnetothermal properties along the c axis ([0001]) and b axis ([ ]). The sample was hydrogenated using a Sievert’s type installation under low pressure conditions (up to 0.1 MPa) in order to avoid destruction of the single crystal. The amount of absorbed hydrogen was deter- mined by measuring the pressure difference in the chamber before and after completion of the reaction. The accuracy of determining the concentration of 10 10 METALS