Scientists of FLNP JINR in cooperation with scientific centres of Vietnam, carried out studies of unusual types of functional materials that belong to the so-called geometrically frustrated magnetic materials. These materials demonstrate not only new magnetic states with unusual physical properties, but also new physical phenomena can be observed in them when temperature and pressure change, for example, the appearance of a spin liquid state. The study of the properties of these materials is important not only for the development of modern concepts in the fields of condensed matter physics, but also for the development of an advanced generation of spintronics, nanoelectronics, recording and storage of information.

At the last JINR Prize contest for 2021, the articles cycle “New structural, magnetic, and physical phenomena in geometrically frustrated functional magnetic materials at variation of thermodynamic parameters” has been awarded the second prize in the category of the Experimental Physics Research. The authors of the cycle are D. P. Kozlenko, N. O. Golosova, S. E. Kichanov, E. V. Lukin, A.V. Rutkauskas, B. N. Savenko, O. N. Lis, N. M. Belozerova, Dang Ngoc Toan (Duy Tan University, Vietnam), and Le Hong Khiem (Institute of Physics VAST, Vietnam).

Participants of research: Olga Lis, Boris Savenko, Denis Kozlenko, Sergey Kichanov, Evgeny Lukin, Nadezhda Belozerova

Functional materials with geometrically frustrated magnetic lattices become the objects of the most actual research directions in modern condensed matter physics, materials science, chemistry. This is due to the fact that existence of several energetically equivalent spin configurations on the geometrically frustrated lattices leads to formation of novel magnetic states with unusual physical properties and new physical phenomena, among which are spin-liquid, spin-glass and spin-ice states, quantum critical phenomena, superconductivity, skyrmion states, new types of collective magnetic and lattice excitations, phase transitions, magnetoelectric phenomena, etc.

With an aim of systematic investigation of the features of the magnetic and spin states, several promising classes of functional magnetic materials like CrBr₃, FePS₃, BaYFeO₄, Ca₃Co₂O₆, Ca₃Co_2-xFe_xO₆, Co₃O₄ with geometrically frustrated magnetic lattices were studies in a wide range of thermodynamic parameters (temperature 5 – 300 K and pressure 0 – 10 GPa) using the neutron diffraction method and complementary experimental methods, including X-ray diffraction, macroscopic magnetic properties measurements, Moessbauer and Raman spectroscopy.

The compounds CrBr₃ и FePS₃, which are representatives of a family of low-dimensional van der Waals layered magnetic materials with graphene-like magnetic lattice symmetry, demonstrate the existence of magnetic ordering down to the monolayer limit at sufficiently high temperatures. A rich variety of new physical phenomena have been discovered in these materials under variation of thermodynamic parameters (temperature and pressure), including the insulator-metal transition, spin crossover, and superconductivity. The studies of the structural and magnetic properties of the CrBr3 compound were performed at one of the JINR base facilities – the IBR–2 pulsed reactor, using the neutron diffraction method [1], and revealed the unusual effects. There is anomalous behavior of structural parameters in the temperature range of ferromagnetic ordering (Т_С = 35 K), and a negative thermal expansion of the unit cell volume and quasi two-dimensional van der Waals layers in the temperature range T < T_C (Fig. 1). It should be noted that negative thermal expansion is a relatively rare physical effect found only in a few classes of materials. The coefficient of linear thermal expansion of atomic layers in CrBr₃ in the region T < T_c, α_l = -1.6х10^-5 K^-1, is close to the corresponding value for graphene at low temperatures, which indicates a good compatibility of materials such as CrX₃ and graphene in terms of prospects for development of heterostructures based on them. Its practical use can become an important step towards the development of an advanced generation of spintronics, nanoelectronics, recording and information storage devices.

Fig. 1. a) Neutron diffraction patterns of CrBr3 measured at different temperatures and calculated profiles obtained by the Rietveld method. b) The schematic view of rhombohedral crystal structure of CrBr₃ with symmetry R¯3. On the right van der Waals atomic layers are shown, top and side view. c) Temperature dependences of the lattice parameters and the unit cell volume of CrBr₃, normalised to the corresponding values at room temperature. d) The temperature dependences of distances between Cr magnetic ions inside van der Waals layers (intra-layer) and between layers (inter-layer)

In the FePS₃ compound, the existence of a new structural modification with a monoclinic crystal structure in the pressure range above P = 1 GPa (10000 atmospheres) was found. Structural rearrangement under high pressure leads to a change in the nature of the antiferromagnetic ordering from quasi two-dimensional to three-dimensional, which detected in a two times decrease of the magnetic lattice size [2].

The BaYFeO₄ compound is a multiferroic material, in which the long–range magnetic order and ferroelectric polarisation coexist simultaneously. A reason for high interest in the study of multiferroics is the potential to control their electrical properties using a magnetic field and vice versa – magnetic properties control using an electric field. In BaYFeO₄, spontaneous ferroelectric polarisation has a spin-induced nature associated with the symmetry of the magnetic states. The symmetry features of magnetically ordered states and the magnetic properties of this compound were studied in details (Fig. 2) using a combination of neutron diffraction, measurements of macroscopic magnetic properties, and Moessbauer spectroscopy [3]. The existence of modulated states with a long-range magnetic order, such as spin density waves and cycloids, as well as a magnetically disordered spin-glass phase, occurred below T* = 17 K, was revealed. It was shown that BaYFeO₄ is a rare example of a multiferroic, where the disordered spin glass phase plays an important role in the formation of the magnetoelectric properties.

Fig. 2. a) The view of orthorhombic crystal structure of BaYFeO₄ with Pnma symmetry. b) The neutron diffraction patterns of BaYFeO₄ measured at various temperatures, and corresponding profiles calculated by the Rietveld method. c) The dependence of the change in the magnetic entropy on the magnitude of the external magnetic field at different temperatures

One of the interesting representatives of complex cobalt oxides is the compound Ca₃Co₂O₆. The presence of alternating octahedra CoO₆ containing Co³⁺ ions in the low-spin state LS (S = 0) and CoO₆ trigonal prisms containing Co³⁺ ions in the high-spin state HS (S = 2) leads to the formation of quasi-one-dimensional spin-chain magnetic structure on a frustrated magnetic triangular lattice. At normal pressure, an antiferromagnetic state of the spin-density wave type is formed in Ca₃Co₂O₆, the periodicity of the magnetic lattice is incommensurate to the crystal lattice, the propagation vector is q_sdw = (0, 0, 1.01), and the Neel temperature is T_N = 25 K. Also, in the low temperature region, appearance of a disordered magnetic phase was observed with a characteristic magnetic correlation length of 128 Å. With a pressure increase above 2 GPa, a suppression of the incommensurate antiferromagnetic phase, and appearance of a new commensurate antiferromagnetic phase with a propagation vector q_caf = (0.5, -0.5, 1) and T_NC = 26 K was observed, Fig. 3 [4]. A similar effect was found also at the substitution of cobalt with iron ions: with an increase in the concentration of iron in the Ca₃Co_2-xFe_xO₆ compounds, the suppression of incommensurate magnetic states was found [5]. Within the framework of existing theoretical models, the changes in intra-chain and inter-chain magnetic interactions and their roles in the mechanisms of formation of magnetic ordered states and properties were analysed. The baric coefficient of the Néel temperature was calculated in the mean field approximation based on the one-dimensional Ising model, and a good agreement with the experimental value was obtained.

Fig. 3. a) Neutron diffraction patterns of Ca₃Co₂O₆ obtained at various temperatures and pressures, and calculated profiles obtained using the Rietveld method. b) The representation of rhombohedral crystal structure of Ca₃Co₂O₆ with R3 ̅c symmetry, and the magnetic structure of the new high-pressure phase. c) Relative variation of intra-chain (J₁) and inter-chain (J₂₃) magnetic interactions as a function of pressure.

In the Co₃O₄ spinel, an existence of spatial ordering of the Co³⁺ ions in the low-spin state LS (S = 0) in an octahedral oxygen environment and the Co²⁺ ions in the high-spin state HS (S = 3/2) in a tetrahedral oxygen environment leads to the formation of a frustrated diamond-type magnetic lattice, on which a unique state of a spiral spin-liquid state may appear. At ambient pressure, the formation of a complex magnetic state was observed, including the coexistence of a collinear antiferromagnetic phase and a magnetically disordered phase, presumably of the spiral spin-liquid type. With a pressure increase up to 9 GPa, the suppression of the disordered magnetic phase, and significant increase of the magnetic ordering temperature T_N by 1.5 times, from 30 to 51 K, were observed. The important role of super-exchange interactions by means of two anions of type A-O-O-A in the formation of magnetic properties has been revealed [6].

References:

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