Probing deuterium-induced magnetic phase transitions in TbCo alloys with in-situ polarized neutron reflectometry

Abstract

Hydrogen-based magneto-ionics is a promising approach for rapid magnetoelectric control of spintronic devices. Most investigations so far into the magneto-ionic manipulation of rare-earth transition-metal alloys have used electrochemical methods for evaluating the magnetoelectric properties, but this technique makes it difficult to discriminate between the effects of competing ionic species. In this work, we use atmospheric loading to evaluate the effect of an isotope of hydrogen, deuterium, on the magnetic properties of TbCo films using in-situ polarized neutron reflectometry. With this approach, we are able to simultaneously measure the magnetization, thickness expansion and deuterium concentration of TbCo films. We quantitatively observe the deuterium concentrations at which the paramagnetic phase transition occurs for a Tb-rich film, and the weakening of out-of-plane magnetic anisotropy for a Co-rich film. For the Tb-rich film the expansion of the film thickness is the primary mechanism identified for the paramagnetic phase transition, while for the Co-rich film no thickness expansion is observed. We also find that an oxidized interface is insensitive to deuterium loading, but remains exchange coupled to the rest of the film and can be indirectly manipulated by loading of deuterium in the alloy. We expect these results to be directly translatable to that of hydrogen.

Figures (7)

• Figure 1: Normalized hysteresis loops for samples a) Tb$_{14}$Co$_{86}$ measured in a polar geometry and b) Tb$_{35}$Co$_{65}$ measured in a longitudinal geometry. Dashed lines are intended as a guide to the eye.
• Figure 2: a, b) X-ray reflectivity data and electronic SLD profile model used to fit the reflectivity for the Tb$_{35}$Co$_{65}$ sample. c, d) corresponding data and model for the same sample as measured by PNR. Inset in c) is a schematic representation of the sample layer stack for the PNR model. Between the Pd and TbCo layer there is a small interfacial layer which has a Co-dominated magnetization that is coupled antiparallel to $m_{\mathrm{net}}$ in the main TbCo layer.
• Figure 3: a) Change in polarized neutron reflectivity with D loading for three D concentrations. Data is offset for visual clarity. b) The corresponding nuclear SLD (top) and magnetization (bottom) profiles for each D concentration.
• Figure 4: Fitting results extracted from the deuterium-loaded reflectometry profiles. a) The thickness expansion of the Tb$_{35}$Co$_{65}$ layer. b) The change in the net magnetic moment ($m_{\mathrm{net}}$) of the Tb$_{35}$Co$_{65}$ layer. c) Magnetic moment of the interfacial oxidized layer. Dashed lines are included as a guide to the eye.
• Figure 5: a) Change in neutron reflectivity with D loading for the Tb$_{14}$Co$_{86}$ sample under vacuum, and at a D$_\textnormal{2}$ pressure of 100 mBar (measured concentration of 14 at.% D.) b) The corresponding nuclear and magnetic SLD profiles.