Surface spin-flop transition in a uniaxial antiferromagnetic Fe/Cr superlattice induced by a magnetic field of arbitrary direction
We studied the transition between the antiferromagnetic and the surface spin-flop phases of a uniaxial antiferromagnetic [Fe(14 Å)/Cr(11 Å]$_{\rm x20}$ superlattice. For external fields applied parallel to the in-plane easy axis, the layer-by-layer configuration, calculated in the framework of a mean-field one-dimensional model, was benchmarked against published polarized neutron reflectivity data. For an in-plane field $H$ applied at an angle $ψ\ne 0$ with the easy axis, magnetometry shows that the magnetization $M$ vanishes at H=0, then increases slowly with increasing $H$. At a critical value of $H$, a finite jump in $M(H)$ is observed for $ψ<5^{\rm o}$, while a smooth increase of $M$ $vs$ $H$ is found for $ψ>5^{\rm o}$. A dramatic increase in the full width at half maximum of the magnetic susceptibility is observed for $ψ\ge 5^{\rm o}$. The phase diagram obtained from micromagnetic calculations displays a first-order transition to a surface spin-flop phase for low $ψ$ values, while the transition becomes continuous for $ψ$ greater than a critical angle, $ψ_{\rm max} \approx 4.75^{\rm o}$. This is in fair agreement with the experimentally observed results.