At 300 K, Y$_{ 0.9}$Gd$_{ 0.1}$Fe$_{ 2}$H$_{ x}$ hydrides crystallize sequentially with increasing H concentration in various structures related to a lowering of the cubic MgCu$_{ 2}$ type structure of the parent alloy: cubic C1, monoclinic M1, cubic C2, monoclinic M2, cubic C3, orthorhombic O. Above 300 K, they undergo a first-order transition at a T$_{ O-D}$ temperature driven by order-disorder of hydrogen atoms into interstitial sites. Their magnetic, structural and magnetocaloric properties have been investigated through magnetic measurements, and high-resolution synchrotron diffraction experiments. The magnetization at 5 K decreases slightly from 4 to 3.8 ${ \mu}_{ B}$ for x = 3 to 3.9 H f.u., then with a larger slope for higher H content. A discontinuous decrease of the magnetic transition temperature is observed: M1 and C2 hydrides are ferrimagnetic with T$_{ C}$ near 300 K, M2 hydride displays a sharp ferromagnetic-antiferromagnetic transition at T$_{ FM-AFM}$ =144 K, whereas C3 and O hydrides present only a sharp increase of the magnetization below 15 K and a weak magnetization up to RT. Negative magnetic entropy variations (${ \Delta}$S$_{ M}$) are measured near T$_{ C}$ for the M1 and C2 phases, near T$_{ FM-AFM}$ for the M2 phase, whereas positive ${ \Delta}$S$_{ M}$ peaks due to inverse MCE effect are found near T$_{ O-D}$. A structural and magnetic phase diagram is proposed.
Categories: cond-mat.mtrl-sci