Muons and magnets

Abstract:

We have used polarised neutron reflection to directly determine the vector orientation of the individual Fe layer average magnetisations in an antiferromagnetically coupled 50 ÅFe/15 ÅCr/50 ÅFe sandwich structure prepared by sputtering. The spin dependent neutron wave in the multilayer structure is represented by a two component spinor and the resulting reflected intensity is found to be highly sensitive to variations in the orientations of the layer-dependent magnetisation vectors, permitting the preferred orientation of the average magnetisation in each Fe layer to be studied. The saturated state is found to correspond to a uniform ferromagnetic alignment of the Fe layer magnetisations. At low field, we are able to assess the degree to which the magnetic alignment is fully antiparallel, and conclude that our results are consistent with the development of a multidomain structure. © 1993.

Abstract:

We discuss the similarities which exist between the reflection of neutrons from ferromagnetic media and the reflection of light from media rendered gyrotropic by the application of a magnetic field, and show that the vector character of the neutron polarisation can be fully exploited in spin-analysed neutron reflection measurements (termed 'neutron ellipsometry' because of the similarity to optical ellipsometry) to give valuable magnetometric information for ultrathin films and multilayers, and with certain advantages over conventional polarised neutron reflection measurements. We show that for the case of a single ultrathin magnetic layer with uniform magnetisation, the component of the in-plane magnetisation perpendicular to the incident neutron polarisation determines both the spin conserving and spin flipping reflectivities in the region of total reflection, while a more complex regime obtains in the partially reflecting region. Making neutron ellipsometric measurements in both reflection regions thus allows the magnitude and orientation of the in-plane magnetisation vector to be directly determined. This is illustrated by numerical calculations for a single ultrathin iron layer supported by a silver substrate and with a silver cap. © 1993.

Abstract:

We have combined polarised neutron reflection (PNR), X-ray reflection, the extraordinary Hall effect (EHE) and magnetoresistance (MR) in a detailed multi-technique study of the magnetic properties of a 200 Å polycrystalline Co film supported by a GaAs(001) substrate. X-ray reflection and PNR data are found to be well fitted by the same values of layer thickness so providing an additional check on the structural parameters yielded by the fits to the PNR data. Measurements of the reflected neutron polarisation for the same 200 Å Co film are consistent with a layer averaged moment of 1.6μB and a magnetisation profile in which the moment per Co atom reaches 1.7μB in the interior of the film and falls to 0.8μB in a 20 Å thick layer at the GaAs interface. A perpendicular saturation field of 1.39 T is obtained from EHE measurements extended to 20 T on the same sample, from which we estimate a small perpendicular anisotropy field which opposes the demagnetising field. This result indicates that the magnetisation profile is not associated with a large interface anisotropy energy. © 1992.