Development of Reference Layer Method in Resonant Neutron Reflectometry
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evelopment of Reference Layer Method in Resonant Neutron Reflectometry E. S. Nikovaa, b, *, Yu. A. Salamatova, E. A. Kravtsova, b, V. V. Ustinova, b, V. I. Bodnarchukc, and A. V. Nagornyc, d aInstitute
of Metal Physics, Ural Branch, Russian Academy of Sciences, Yekaterinburg, 620990 Russia b Ural Federal University, Yekaterinburg, 620002 Russia c Frank Laboratory of Neutron Physics, Joint Institute for Nuclear Research, Dubna, Moscow oblast, 141980 Russia d Taras Shevchenko National University, Kyiv, 01601 Ukraine *e-mail: [email protected] Received July 27, 2019; revised August 23, 2019; accepted August 25, 2019
Abstract—A new method of phase recovery in neutron reflectometry is proposed based on the use of the Gd reference layer deposited on the top of an “unknown” layered structure. The idea of the method is to apply the energy dependence of neutron scattering length density to change the reference layer properties. Using three reflectivity curves at different neutron energies, the modulus and phase of the coefficient of reflection from the “unknown” structure can be reconstructed. We discuss the mathematics of phase recovery and present the first experimental results for a multilayered system. Keywords: resonant neutron reflectometry, thin multilayer films, reference layer, phase problem DOI: 10.1134/S1027451020070344
INTRODUCTION Magnetic multilayered nanostructures exhibiting a wide variety of fascinating phenomena have attracted the attention of researchers for several decades. Polarized neutron reflectivity is a powerful and popular technique that allows determining nuclear and magnetization density profiles in planar magnetic nanostructures. In contrast to X-ray reflectometry, which uses resonant (anomalous) scattering near the absorption edges of the elements that make up the nanostructures, neutron reflectometry traditionally does not use resonant effects. This is due to the fact that neutron scattering length densities for the vast majority of isotopes do not depend on energy in the thermal region. However, there are some isotopes, for example113Cd, 157Gd, 149Sm, etc., in which strong resonance effects are observed upon scattering and absorption of thermal neutrons. We propose using these resonant effects in neutron reflectometry to enhance the capacity of resonant neutron reflectometry as compared to that of conventional non-resonant reflectometry. In particular, resonant neutron reflectometry can be applied to solve the phase problem in neutron reflectometry for the certain cases. The fundamental drawback of scattering techniques is that it is not possible to directly measure the phase of the complex reflection coefficient. Only the reflectance, which is the square of the reflection coef-
ficient modulus, can be measured. The lack of phase information in the reflected intensities makes it impossible to unambiguously restore the scattering potential; there is no guarantee that the solution of the inverse scattering problem is the unique. Considerable efforts have been made to overcome this fundamental
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