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

In order to clarify the reason why the hydrogen desorption temperature (Td) 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 Td 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 Td.