In situ μ⁺SR measurements on the hydrogen desorption reaction of magnesium hydride

In order to clarify the reason why the hydrogen desorption temperature (T_d) of MgH₂ is lowered by milling, we have studied the change in a local nuclear magnetic field with temperature by means of μ⁺SR. We have found a very clear oscillation in the ZF-spectrum at 2 K for the ''milled'' and ''milled with Nb₂O₅'' samples, while such oscillation is weaker for the ''as prepared'' MgH₂. It was also found that the oscillation signal is stable up to 250 K and is assigned mainly due to the formation of a H-μ-H system. At temperatures above ambient T, we also found that the ZF-μ⁺SR spectrum exhibits a static Kubo-Toyabe behavior due to the nuclear magnetic field of ¹H. Furthermore, it was clarified that rapid H diffusion starts well below T_d only in the milled samples, leading to the conclusion that the consequent enhanced diffusion rate in MgH₂ is essential to accelerate the desorption reaction and to decrease T_d