Surface spin-flop 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 angstrom)/Cr( 11 angstrom)](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 psi = 0 degrees to 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 psi < 5 degrees, while a smooth increase of M versus H is found for psi > 5 degrees. A dramatic increase in the full width at half maximum of the magnetic susceptibility is observed for psi >= 5 degrees. The phase diagram obtained from micromagnetic calculations displays a first-order transition to a surface spin-flop phase for low psi values, while the transition becomes continuous for psi greater than a critical angle, psi(max) approximate to 4.75 degrees. This is in fair agreement with the experimentally observed results.