Hydrogen in ultrathin vanadium layers
Abstract: The influence of elastic boundary conditions on the thermodynamics of hydrogen in vanadium were investigated using resistometry, x-ray diffraction and nuclear reaction analysis. To provide varying lattice strain states Mo/V (110) and Fe/V (001) superlattices with thickness L of individual layers in the range 3-16 monolayers (ML) were used. In addition the thickness ratio LFe/LV in the latter samples was varied. The results were compared with earlier work on Mo/V (001) superlattices.In the Mo/V (110) superlattices the H-H interaction was found to be repulsive in contrast to the (001) oriented samples and bulk V. This was also the case for Fe/V (001). For the thickness ratios LFe/LV = 3/10 and 3/16 it was found that the hydrogen-depleted interface region in V was populated at lower average concentration than in the symmetric samples (LFeLV = 1). This was related to the decreased Fe thickness.For Fe(3ML)/V(12-15 ML) samples the effects of hydrogenation on the magnetic exchange coupling between the Fe layers were investigated using neutron reflectometry. For 12 and 14 ML of V the coupling was switched continuously from antiferromagnetic (AFM) to ferromagnetic (FM) upon introduction of small amounts of hydrogen, whereas the initially FM-coupled 15-ML sampleexhibited an antiferromagnetic "resonance" at H/V ≈ 0.02. This was found to be due to changes in the vanadium Fermi surface. An intermediate state of noncollinear coupling was also observed in the 14-ML sample.
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