D. P. Kozlenko
S. E. Kichanov, E. V. Lukin, B. N. Savenko, N. M. Belozerova, N. O. Golosova, A. V. Rutkauskas, O. Lis

Recently, there has been considerable progress in the investigation of materials under extreme conditions. The impact of high pressure often results in new physical phenomena in materials, including pressure-induced superconductivity, various changes in magnetic states, dielectric-metal transitions, spin crossover, structural and electronic phase transitions. Furthermore, investigations at high pressures provide unique opportunities to study the microscopic mechanisms of physical phenomena in functional materials as a result of analyzing the response of various properties to changes in structural parameters during crystal lattice compression. Also, at high pressures and temperatures, the synthesis of new metastable forms of materials with unusual properties is possible.

The search, development and investigation of the properties of new materials and structures are urgent tasks in condensed matter physics and physical materials science. Interest in the research of such materials as complex transition metal oxides, magnetic intermetallics, van der Waals magnets and others that exhibit a wide variety of physical properties, such as magnetic, ferroelectric, piezoelectric, magnetocaloric, is due to the possibility of their wide practical use.

The neutron diffraction technique has a number of significant advantages in the investigation of the crystal and magnetic structure of such materials under extreme conditions as compared to other techniques concerning the peculiarities of the neutron interaction with matter. Unlike X-ray and synchrotron radiation, neutrons can be used to study both the crystal structure, including those that contain light atoms and the magnetic structure of the materials investigated. In addition, as an electrically neutral particle, the neutron is highly penetrating. Therefore, neutron scattering techniques allow to obtain volumetric characteristics of the materials under investigation even with the use of complex devices surrounding the sample, such as cryostats, heaters, high-pressure chambers, burners and electromagnets.

A whole range of experimental techniques for implementing unique experiments at high pressures, including a combination of neutron diffraction, X-ray diffraction and Raman spectroscopy is developed at the Frank Laboratory of Neutron Physics of JINR. The recently developed DN-6 diffractometer allows to carry out neutron diffraction investigations under simultaneous high pressures in the range up to 50 GPa and low temperatures down to 5 K. This facility is currently one of the best in the world for such experiments. The DN-12 diffractometer complements the possibilities of carrying out experiments in a more limited pressure range up to 10 GPa. At both facilities, a range of interesting results for the research of structural and magnetic properties under high pressures have recently been obtained.

The recent results include investigation of the magnetic and electronic properties of magnetite using neutron diffraction and synchrotron Mössbauer spectroscopy on the iron isotope ⁵⁷Fe in the pressure range of 0–40 GPa and temperature of 10–300 K [1].

The mineral magnetite Fe₃O₄ is one of the first magnetic materials having been used by mankind since ancient times and today, it is also widely used in the development of modern technology. Magnetite shows a range of unusual physical phenomena that have been actively studied for over a hundred years. An anomalous behavior of the physical properties of magnetite has been recently discovered at high pressures in the range of a structural phase transition occurring at 20-25 GPa. As a result of neutron diffraction, experiments carried out on the DN-6 diffractometer, ferrimagnetic ordering in the high-pressure phase that occurs at a temperature of T_NP ~ 420 K has been discovered and the characteristics of the magnetic structure have been determined. In concern with extra experimental data obtained using the synchrotron Mössbauer spectroscopy, the structural, magnetic and electronic phase diagram of magnetite has been established in the investigated range of thermodynamic parameters (Fig. 1).

Fig.1. Left: neutron diffraction spectra of magnetite measured at pressures up to 33 GPa on the DN-6 diffractometer in a high-pressure chamber with diamond anvils and processed using Rietveld refinement. Right: magnetic structure of the orthorhombic high-pressure phase of magnetite (top) and structural, magnetic and electronic phase diagram of magnetite (bottom).

The structural aspects of the development of magnetic states in nanostructured manganites La_0.63Sr_0.37MnO₃ and La_0.72Sr_0.28MnO₃ under high pressures have been studied on the DN-6 diffractometer [2–3]. Whilst the bulk analogues of these compounds are ferromagnets, the occurrence of the antiferromagnetic phase [2] is observed in their nanostructured form, already at atmospheric pressure and an increase in the share of this magnetic phase with a simultaneous suppression of the initial ferromagnetic phase (Fig. 2) has been discovered with an increase in pressure.  Based on the experimental data obtained, a structural model describing the magnetic phase separation in nanostructured manganites has been suggested [3]. Depending on the synthesis conditions, a manganese oxide nanoparticle is a complex core-shell structure composed of a ferromagnetic core with a rhombohedral crystal structure, surrounded by a layer of an antiferromagnetic phase with an A-type magnetic ordering and an orthorhombic crystal structure (Fig. 2b). As the pressure increases, the size of the ferromagnetic core decreases and the fraction of the antiferromagnetic phase increases [2]. This is accompanied by a sharp decrease in the Curie temperature and a slight increase in the Neel temperature of the magnetic component of the nanostructured system.

Fig. 2. a) Neutron diffraction patterns of La0.72Sr0.28MnO3 nanomanganite obtained at low temperature, standard pressure and high pressure of 4.5 GPa. Processing has been carried out using Rietveld refinement. The diffraction peaks corresponding to the antiferromagnetic and ferromagnetic phases of the nanostructured manganite are marked AFM and FM, respectively

Fig. 2.b. Graphical representation of magnetic phase separation in manganite nanoparticles, in which a ferromagnetic FM core is surrounded by an antiferromagnetic AFM phase. When a high pressure is applied, the fraction of the ferromagnetic phase decreases and that of the antiferromagnetic phase increases

The impact of high pressure on the structures of intermetallides of rare earth metals and cobalt R-Co has been studied [4-7]. 

Neutron diffraction investigations of the atomic and magnetic structure of RCo2 compounds with temperature variations in the range of 10–300 K and pressure in the range of 0–5 GPa have shown that the conventional concept of band electron metamagnetism used does not allow one to adequately describe the behavior of the magnetic properties of the entire class of these materials. Thus, in TbCo₂ [4], HoCo₂ [5] and DyCo₂ [6] compounds with a relatively high Curie temperature T_C, there is a high degree of correlation between the rare-earth and cobalt magnetic sublattices that is manifested by a consistent decrease in the sublattice Curie temperatures concerning the pressure-induced collapse of magnetic moments on the Co sublattice. In the ErCo₂ compound (Fig. 3a) [7] with a lower T_C, a pressure-induced collapse of magnetic moments and a marked decrease in the Curie temperature of the Co sublattice have been observed against the background of a weakly varying Curie temperature of the Er sublattice. The highest absolute value of the pressure coefficient of the Curie temperature dT_N/dP = -9 K/GPa has been observed in TbCo₂, the lowest dT_N/dP = -2 K/GPa - in HoCo₂ for the pressure range P > 2 GPa (Fig. 3b). It is interesting to note that in all the investigated compounds, the baric coefficient of change in the ordered magnetic moment of Co ions has remained approximately the same, dM_Co/dP » 0.1 µB/GP.

Fig. 3a. Neutron diffraction spectra of intermetallide ErCo2 [7] obtained at various pressures and temperatures. The experimental data and the profile calculated using Rietveld refinement are presented

Fig. 3b. Baric dependences of the Curie temperature and ordered magnetic moment on the cobalt sublattice in RCo2

Publications:

1. D.P.Kozlenko, L.S.Dubrovinsky, S.E.Kichanov, et al., Magnetic and electronic properties of magnetite across the high-pressure anomaly. Scientific reports, 9(1), 1-9 (2019). Doi:10.1038/s41598-019-41184-3
2. Belozerova N.M., Kichanov S.E., Jirák Z., et al., High pressure effects on the crystal and magnetic structure of nanostructured manganites La_0.63Sr_0.37MnO₃ and La_0.72Sr_0.28MnO₃. J. Alloys Compd., 646, 998-1003 (2015). Doi:10.1016/j.jallcom.2015.06.154
3. Belozerova N.M., Kichanov S.E., Kozlenko D.P., et al., Core-Shell Magnetic Structure of La_1–xSr_xMnO_3+δ Nanocrystallites. IEEE Transactions on Magnetics, 53(11), 1-5 (2017). Doi: 10.1109/TMAG.2017.2700394
4. Burzo E., Vlaic P., Kozlenko D.P., et al., Magnetic properties of TbCo₂ compound at high pressures. J. Alloys Compd., 551, 702-710 (2013). Doi:10.1016/j.jallcom.2012.10.178
5. Burzo E., Vlaic P., Kozlenko D.P., et al., Magnetic properties, electronic structures and pressure effects of Ho_xY_1−xCo₂ compounds. J. Alloys Compd., 584, 393-401 (2014). Doi:10.1016/j.jallcom.2013.09.076
6. Burzo E., Vlaic P., Kozlenko D.P., et al., Crystal structure and magnetic behaviour of DyCo₂ compound at high pressures. J. Alloys Compd., 724, 1184-1191 (2017). Doi:10.1016/j.jallcom.2017.07.078
7.  Kozlenko D.P., Burzo E., Vlaic P., et al., Sequential cobalt magnetization collapse in ErCo₂: beyond the limits of itinerant electron metamagnetism. Scientific reports, 5(1), 1-6 (2015). Doi:10.1038/srep08620