Yu. V. Nikitenko

V. D. Zhaketov, S. V. Kozhevnikov, A. V. Petrenko, E. D. Kolupaev

Polarized neutron reflectometry (PNR) is a technique for studying the surface of matter and thin-film nanoscale structures. In a neutron reflectometry experiment, a collimated neutron beam falls on a sample at a grazing angle and experiences specular reflection; neutron scattering from surfaces and from external or internal interfaces in multilayer thin films during grazing incidence is studied. At present, the technique has been widely developed and current neutron sources are usually equipped with several reflectometers. PNR is an indispensable technique for the investigation of magnetic nanosystems. The neutron, although an electrically neutral particle, has a magnetic moment. The technique is based on the interaction between the magnetic moment of the neutron and the magnetic moments inside the structure. Therefore, when using polarized neutrons beams, the capability of studying the distribution of magnetization perpendicular to the plane of the structure is added to the structural characteristics determined by the spatial distribution of the nuclear density of the matter. The advantage of PNR as compared to other experimental techniques is that the technique allows one to determine the spatial distribution of magnetization, while other techniques allow to determine either only the macroscopic magnetic parameters of the structure (SQUID magnetometry) or only the distribution of nonmagnetic parameters (X-ray techniques). At the same time, PNR is a non-destructive experimental technique. A classical PNR schematic is shown in Fig. 1. At glancing angles of the neutron beam on the sample θ=1÷30 mrad and a neutron wavelength of several angstroms, the parameters typical for neutron reflectometry, the value of the transmitted wave vector is 10^-3÷10^-1 Å^-1 that provides a spatial resolution of 1÷100 nm to define the distribution of magnetization perpendicular to the plane of the structure. The REMUR reflectometer that is one of the high-aperture reflectometers in the world is positioned on beamline 8 of the IBR-2 high-flux pulsed reactor of FLNP JINR. The main purpose of this reflectometer is to study the structural and magnetic properties of layered nanostructures.

Fig. 1. Schematic of a reflectometry experiment with polarized neutrons on the REMUR reflectometer positioned on beamline 8 of the IBR-2 high-flux pulsed reactor. A polarized neutron beam is produced using a supermirror polarizer, adiabatic radio-frequency spin-flippers and a fan mirror analyzer is used for the analysis of the polarization of the reflected beam. Neutrons are registered using a 3He position-sensitive gas detector.

Great interest in PNR, first of all, refers to recent discoveries of new impacts in the physics of nanomaterials that underlie the functioning of the units of modern computer technology. The capabilities of the non-destructive PNR technique allow us to obtain data on the internal structure of new nanomaterials that are inaccessible to other techniques. In recent years, increased attention has been drawn to the investigation of magnetism in magnetic layers and thin films. Understanding the role of novel interaction mechanisms will result not only in the understanding of the fundamental laws of nanomagnetism, interest in multilayer magnetic layered structures is also due to their numerous applications in magnetic and spin electronics: such as, in highly sensitive magnetic field sensors, magnetic registering and storage devices, others. One of the recent promising research areas includes thin films with a nontrivial magnetic ordering, such as, heterostructures with helicoidal layers of dysprosium and holmium magnets. 2D non-specular neutron scattering spectrum for the Al₂O₃//Nb(40nm)/Dy(200nm)/V(15nm) structure, measured at a temperature T=100 K in a magnetic field H=1 kOe [1] is shown in Figure 2a. The horizontal bright line at Qz=0.2 Å^-1 corresponds to a magnetic helicoid with a period of d_he≈31 Å.

Fig. 2. (a) 2D non-specular neutron scattering spectrum for the Al2O3//Nb(40nm)/Dy(200nm)/V(15nm) structure, measured at a temperature of T=100 K in a magnetic field H=1 kOe applied along the axis of easy magnetization, the horizontal bright line at Qz=0.2 Å-1 corresponds to a magnetic helicoid with a period of dhe≈31 Å. (b) Reflection coefficients with spin flip (-+) and without spin flip (++ and --) for the Cu(32nm)/V(40nm)/Fe(1nm)/MgO structure.

One of the most interesting research areas of multilayer magnetic structures is the investigation of proximity effects in heterostructures with superconducting and ferromagnetic properties. The magnetic properties of superconductors and ferromagnets are antagonistic. If in a ferromagnet the magnetic moments of atoms are aligned collinearly to the external magnetic field, then the superconductor completely displaces the magnetic field, since superconducting electron pairs have antiparallel spin ordering. On the other hand, proximity effects are known in superconductivity. The classical proximity effect is the penetration of superconducting correlations into a ferromagnet when they come into contact. The investigation of low-dimensional heterostructures with alternating ferromagnetic and superconducting layers is of particular interest. Such systems can be used for the manufacture of superconducting spin valves and Josephson qubits on π-contacts. These systems have been widely studied on the REMUR reflectometer [2, 3]. For instance, the neutron reflection coefficient for the Cu(32nm)/V(40nm)/Fe(1nm)/MgO structure, in which Fe is a ferromagnet and V is a superconductor, is shown in Figure 2b. The dependences of neutron reflection coefficients with and without spin flip at T=10 K and H=20 Oe are presented. In this paper, the technique of amplified standing neutron waves (resonance amplification of the neutron wave field) in the total reflection mode has been used that has allowed to carry out precision investigations of weakly perturbed magnetic structures and isotopic substitution.

Since 2014, the REMUR reflectometer has been modernized aimed at developing various beamlines for the registration of secondary radiation. To date, the main work on the development of the technique for isotope-identifying neutron reflectometry on the REMUR spectrometer has been completed. The beamlines for the registration of secondary radiation: charged particles, gamma quanta and neutrons that have experienced a spin flip have been developed and tested. The units of the registration beamline of gamma radiation are shown in Figure 3a; a semiconductor high-purity germanium detector (HPGe) is used for registering gamma quanta [4]. The neutron reflection coefficients and the coefficients of secondary radiation (gamma rays) for the Cu(10nm)/V(65nm)/Gd(5nm)/V(5nm)/Cu(100nm)/glass and Cu(10nm)/V( 55nm)/Gd(5nm)/V(15nm)/Cu(100nm)/glass structures are presented in Figure 3b. A gamma signal from ¹⁵⁵Gd and ¹⁵⁷Gd isotopes has been registered. At present, several dozens of isotopes and magnetic components are available for measurements. It is demonstrated that the technique allows to study the spatial profile (distribution) of a wide range of isotopes and magnetic components with a resolution of 1 nm in layered structures, as well as to investigate, among others, proximity phenomena that occur at the interface between two environments. In particular, it applies to the interface between a superconductor and a ferromagnet. Secondary radiation should be registered to determine the profile of the neutron interaction potential with individual units. Isotope-identifying neutron reflectometry significantly expands the possibilities of studying multilayer magnetic heterostructures.

Fig. 3. (a) Components of the beamline for registering gamma radiation: 1 – neutron beam collimator; 2 - place for positioning the sample; 3 - protection of the place for positioning the sample; 4 – gamma detector with cryostat. (b) Long-wavelength dependences of the neutron reflection coefficient (1,2) and the secondary radiation coefficient (gamma rays) (3,4) for the Cu(10nm)/V(65nm)/Gd(5nm)/V(5nm)/Cu(100nm)/glass (1.3) and Cu(10nm)/V(55nm)/Gd(5nm)/V(15nm)/Cu(100nm)/glass(2.4) structures.

Publications

[1] Devyaterikov D.I., Proglyado V.V., Zhaketov V.D. et al., Influence of Dimensional Effects on the Curie Temperature of Dy and Ho Thin Films. Physics of Metals and Metallography, 122(5), 465-471 (2021). Doi:10.1134/S0031918X21050033;

[2] Zhaketov V.D., Nikitenko Y.V., Khaidukov Y.N.  et al., Magnetic and Superconducting Properties of the Heterogeneous Layered Structures V/Fe0. 7V0. 3/V/Fe0. 7V0. 3/Nb and Nb/Ni0. 65 (0.81) Cu0. 35 (0.19). Journal of Experimental and Theoretical Physics, 129(2), 258-276 (2019). Doi: 10.1134/S1063776119070136;

[3] Khaydukov Yu.N., Aksenov V.L., Nikitenko Y.V. et al., Magnetic proximity effects in V/Fe superconductor/ferromagnet single bilayer revealed by waveguide-enhanced polarized neutron reflectometry. Journal of superconductivity and novel magnetism, 24, 961-968 (2011). Doi:10.1007/s10948-010-1041-0;

[4] Zhaketov V.D., Hramco K., Petrenko A.V., et al. Polarized Neutron Reflectrometer with the Recording of Neutrons and Gamma Quanta. Journal of Surface Investigation: X-ray, Synchrotron and Neutron Techniques, 15(3), 549-562 (2021). Doi:10.1134/S1027451021030356.