Influence of Interface Mixed Layer on Non-Collinear Exchange Coupling in V-Fe Multilayers

V/Fe multilayers were prepared on naturally oxidized Si(100) substrates at room temperature (RT) by UHV magnetron sputtering. Mixing effects at the Fe-V interfaces were investigated in-situ, directly after deposition, by means of X-ray photoelectron spectroscopy (XPS). The results of systematic in-situ XPS studies of the integral intensity of the Fe-2p peak, as a function of the nominal thickness of the Fe sublayer deposited on vanadium, allowed us to estimate the thickness of the pure iron layer that forms the mixed layer at about 0.4 nm. Assuming the same thickness of the vanadium layer that forms the mixed layer, the estimated thickness of the mixed layer near the Fe-V interface was about 0.8 nm. In the analysis of magnetic hysteresis loops, in addition to the bilinear (J₁) and biquadratic (J₂) coupling constant, the contribution of the cubic exchange constant (J₃) was taken into account, which also contributed significantly to the total energy. Higher order interactions (J₂ and J₃) are particularly important for V spacer thicknesses greater than 7 atomic monolayers. Hydrogen absorption in V/Fe multilayers at RT and a pressure of about 1 bar causes an increase in the biquadratic coupling constant J₂, while the values of J₁ and J₃ are reduced. A comparison of the obtained experimental results and available theoretical models leads to the conclusion that the mechanism of "fluctuating thickness of the non-magnetic spacer" could be responsible for the biquadratic exchange coupling. On the other hand, the "loose spins" model can explain the cubic coupling in the V/Fe multilayers. The modification of the interlayer exchange coupling using hydrogen is fully reversible.