Unidirectionally biased Permalloy: A polarized-neutron-reflection experiment

The depth profile of the magnetization of an exchange-coupled Permalloy-${\mathrm{Fe}}_{50}$ ${\mathrm{Mn}}_{50}$ bilayer is determined nondestructively from polarized-neutron reflectometry. The interfacial exchange coupling between the ferromagnetic Permalloy layer and the antiferromagnetic ${\mathrm{Fe}}_{50}$ ${\mathrm{Mn}}_{50}$ layer gives rise to an unusual unidirectional anisotropy of the Permalloy layer, whose magnitude is much smaller than would be expected from the bulk exchange constants of these materials. A variety of models have been proposed to account for this result, some of which require inhomogeneous magnetization distributions within the Permalloy layer or at the Permalloy-${\mathrm{Fe}}_{50}$ ${\mathrm{Mn}}_{50}$ interface. These experiments reveal that, for a structure of the form Si(111)/${\mathrm{Ni}}_{81}$ ${\mathrm{Fe}}_{19}$(400 Å)/${\mathrm{Fe}}_{50}$ ${\mathrm{Mn}}_{50}$(400 Å)/Ta(300 Å), the depth profile of the magnetization is uniform within experimental error. In particular, for fields sufficiently large to magnetize the Permalloy layer at one or other extreme of the hysteresis loop, the layer-by-layer profile of the magnetization is identical, ruling out the possibility of planar domain-wall formation in the Permalloy layer. The hysteresis loop is derived from polarized-neutron reflectometry data as a function of in-plane magnetic field and is the same as that found using conventional magnetometry. The actual layer thicknesses of the structure were separately determined by neutron reflectometry for the structure magnetized with the magnetic moment perpendicular to the neutron-scattering plane.