Researchers of the Frank Laboratory of Neutron Physics JINR are developing and testing a new method for studying interfaces in thin-film heterostructures – i.e. isotope-identifying neutron reflectometry. The method is carried out on a pulsed neutron source spectrometer. This unique practice can have great prospects for the development of nanoelectronics, superconducting spintronics, and quantum computing.

V. D. Zhaketov

FLNP JINR researchers have carried out the study of interfaces in nanostrucrures composed of metals with superconducting and magnetic properties at the REMUR spectrometer (beamline 8 of the IBR-2). In the beginning, a well-known technique of polarised neutron reflectometry was used for that purpose. The method of neutron reflectometry is based on the detection of a collimated beam of thermal neutrons reflected from a heterostructure. Specialists register the total energy of neutron interaction with elements of the medium distributed through the depth of the structure. However, this method also has a drawback. It allows clearly studying magnetic properties of the structure, but it is impossible to detect the distribution of individual chemical elements. Specialists have developed additional registration channels in order to simultaneously obtain all data on both the distribution of magnetic properties and the distribution of chemical elements and their isotopes. It has led to the creation of a new joint universal method, i.e. the method of isotope-identifying neutron reflectometry.

According to a researcher of the FLNP Sector of Neutron Optics, Candidate of Physics and Mathematics Vladimir Zhaketov, who takes part in this research, before the creation of the method of isotope-identifying neutron reflectometry specialists had to study the distribution of elements of structure samples using additional techniques, as a rule, in other scientific centres. “The data obtained by different methods were often incorrectly correlated with each other,” the scientist noted. “That was the reason why the method of polarised neutron reflectometry should be universal. In this case, specialists can obtain all information about the structure in one experiment. For this purpose, we have developed the method of isotope-identifying neutron reflectometry.”

Fig. 1. a) Ionisation chamber (1) installed in goniometer (2) of the REMUR spectrometer; b) schematic of the ionisation chamber: 1 — neutron beam; 2 — input and output windows; 3 — cathode; 4 — grid; 5 — mesh frame; 6 — collector (anode)

It was proposed to detect not only polarised neutrons but also secondary radiation during the absorption of neutrons by various isotopes: charged particles, gamma rays, nuclear fission fragments. Additional channels make it possible, along with the determination of the average (over the surface) density profile by neutron reflectometry, to obtain the distribution profiles of individual elements across the depth of the structure.

The first measurement of secondary radiation (gamma rays), when neutrons were reflected from a structure containing layers of gadolinium with a large cross section of the (n, γ) reaction, was carried out in 1994 in the USA at a steady state neutron source with a constant neutron wavelength. First experiments on the detection of secondary radiation from layered structures were carried out in 1998‒2000. The advantage lies in the fact that the IBR-2 is a pulsed source, on which the time of-flight method allows obtaining data for various neutron energies in one measurement. In the 2010s, this direction was resumed at JINR. Since 2014, the REMUR reflectometer has been undergoing modernisation at JINR in order to provide various channels for detecting secondary radiation. In 2015 – 2016, researchers began working on the drawings of equipment, and in 2018 – 2019, they began installation on the spectrometer. The project is carried out in collaboration between two departments of FLNP: Department of Neutron Investigations of Condensed Matter (Yu. Nikitenko, A. Petrenko, V. Aksenov, V. Zhaketov) and Division of Nuclear Physics (Yu. Kopach, N. Gundorin, Yu. Gledenov, K. Khramko, E.Sansarbayar). To date, the main work on the development of the method of isotope-identifying neutron reflectometry on the REMUR spectrometer has been completed. The channels for detection of secondary radiation were implemented and tested: charged particles, gamma rays and neutrons that scatter with a spin flip. As a result of the work, scientific articles have been published, the project is in the testing stage, and the method is already being used for real scientific tasks.

Fig. 2. a) 2D neutron intensity distribution on the detector in the NzNt plane: 1 — refracted beam; 2 — reflected beam; b) 2D intensity distribution for tritons (1) and alpha particles (2) in the ionization chamber depending on the signal amplitudes (channel number) from the anode (NAa) and cathode (NAc)

The IBR-2 reactor is currently switched to a temporary shutdown mode until the autumn of 2023. At present, researchers are processing data and designing software for future experiments. There are already ideas for the next steps in the development of the new method.

Vladimir Zhaketov called the study of superconducting magnetic heterostructures the flagship task in the development of the method. “In case of bulk matter, the magnetic properties of superconductors and ferromagnets are opposite. However, as for nanoscale heterostructures with layer thickness of units-tens of nanometres, these phenomena begin to coexist. It causes many new interesting phenomena that can be used for nanoelectronics, superconducting spintronics, the development of quantum computing,” he explained.

Among various ways to improve the method, the expert mentioned the modernisation of different spectrometer nodes. It will allow extending the range of specified elements including their isotopes (for example, the distribution of two gadolinium isotopes ¹⁵⁵Gd and ¹⁵⁷Gd has already been determined in the sample). In addition, specialists will develop the data processing procedure. It is also possible to improve the method resolution, which now reaches up to 1 nm across the depth of the structure. “It is possible to go further up to the atomic level with the resolution of about 1 angstrom,” the scientist said. He added that one of the directions planned for the development is to study actinide heterostructures. “It is known that actinides and their compounds have unusual ferromagnetic and superconducting properties that have been little studied; superconducting and ferromagnetic properties of thin actinide layers have not been practically studied. At this point, our method is more suitable than ever, as interacting with neutrons, actinides emit gamma quanta and fission fragments, and we can register them,” he added.