No abstract

We present and apply a new method to measure directly weak magnetization in thin films. The polarization of a neutron beam channeling through a thin film structure is measured after exiting the structure edge as a microbeam. We have applied the method to a tri-layer thin film structure acting as a planar waveguide for polarized neutrons. The middle guiding layer is a rare earth based ferrimagnetic material TbCo5 with a low magnetization of about 20 mT. We demonstrate that the channeling method is more sensitive than the specular neutron reflection method.PACS numbers: 3.75. Be, Magnetic nanomaterials are widely used in practical applications and in fundamental physics. Therefore the development of new methods providing enhanced sensitivity for the characterization of their magnetic properties is a challenge. One of the well-established methods for magnetic thin films and nanostructures characterization is the Polarised Neutron Reflectometry (PNR) technique [1]. Its strength is the unique ability to probe the magnitude and the direction of the magnetization vector as a function of the depth in magnetic heterostructures. For collinear spin configurations, the PNR method resolves the depth dependence of the scattering length densities (SLD) ρ(z) which depends on the neutron polarization: ρ ± (z) = ρ 0 (z) ± cB(z). Here ρ 0 (z) and B(z) are the depth profiles of the nuclear SLD and the magnetic induction. By measuring reflectivities of spin-up and spin-down neutrons and fitting them to theoretical values one can extract the SLD depth profile and the depth dependence of the induction in a film. The method is routinely used for the characterization of ferromagnetic systems with rather high magnetization (about 0.3 -2 T). Being a model dependent method it leads to the nonuniqueness of the resulting set of parameters describing the system. As a result, the accuracy of the fitted parameters depends both on the adequacy of a model and on the quality of the experimental data. For systems with weak magnetization (∼ 10 mT), PNR requires very long measurement times (∼ 10 2 hours) in order to achieve sufficient statistics. This is usually not compatible with standard neutron reflectometry experiments.Weak magnetizations, typically less than 100 mT, are characteristic of ferrimagnetic materials or magnetic oxides. Magnetic materials such as Fe 3 O4, CoFe 2 O 4 , BiFeO 3 , GaFeO 3 , Bi 3 Fe 5 O 12 , NiCoMnAl are being increasingly studied because of their specific properties, either their electronic properties [2], their magneto-electric properties [3], their magneto-optical [4] or their magneto caloric properties [5]. The study of these materials is often limited by the weakness of the magnetic interfacial effects that are of interest. While XMCD [6] is a very sensitive technique to probe surface magnetism, it is sometimes rather difficult to apply to complex heterostructures as used in spintronic devices. Also, the XRMS [7] technique cannot access thicker layer structure because the soft x-ray penetration depth is on the order...