Polarized neutron channeling as a tool for the investigations of weakly magnetic thin films

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...

Neutron waveguides in neutron optics: Green’s functions formalism with Dirichlet boundary conditions

“…In our case, our interest has been the analysis of confined propagation modes and their characterization. So far, neutron propagation has been described mostly by means of classical optics approximations but this propagation has not been extended, to the best of our knowledge, to the formalism of Green's functions to characterize these waveguides (and thus, propagation modes were not described but they were experimentally observed in [7][8][9][10][11][12][13][14][15]).…”

“…Additionally, in 1994, Feng et al experimentally demonstrated confined propagation of modes for slow neutrons in thin titanium films [8]. For further significant works on confined thermal neutron propagation in thin planar waveguides, also in the framework of neutron optics, see the other references in [8][9][10][11][12][13][14]. Other important studies are those of Kumakhov et al [15] and Chen et al [16], both in 1992, establishing experimentally the focusing of slow neutrons by using polycapillary glass fibre.…”

Polarized neutron channeling in weakly magnetic films

“…In [58] more detailed analysis of the neutron transmission and reflection at the boundary of magnetically non-collinear films was done. We can write the energy conservation law for spin transition "++" from the external applied magnetic field into the magnetic film inside the waveguide for the fixed neutron wavelength: (11) It is shown in [8] that we can neglect by the value 21 23 21 23 arg( ),arg( ),arg( ),arg( ) O r r r r       .…”

“…Thin magnetic films containing rare-earth elements are promising materials for the development of new methods of magnetic data storage and ultra-fast switching [7], and have a low magnetization inaccessible for PNR. Recently we demonstrated the new method of Polarized Neutron Channeling (PNC) in planar waveguides to extract magnetization value of a few 10 G [8,9] in TbCo 5 films with saturation magnetization around 200 G [10]. In this work we present new results on the TbCo 11 films with the higher saturation magnetization, and compare the performance of fixed wavelength and the time-offlight polarized neutron reflectometers for PNC studies.…”

Neutron resonances in planar waveguides

“…The parameter of the neutron wavefunction density decay termed as the neutron channeling length was measured experimentally in [19,20]. In [21] we proposed the method of Polarized Neutron Channeling for the investigations of weakly magnetic thin films. In this case we register the neutron microbeam but the investigated system is a weakly magnetic layer inside a planar waveguide.…”