New methods of researching complex magnetic materials

Prof. Dr. Le Hong Khiem with his team at the Dubna Joint Institute for Nuclear Research

Research methods for electromagnetic multiphase materials

Multiferroics exhibit a strong electromagnetic correlation, attracting the research interest of scientists around the world not only because of their high practical application value but also because of the complex physical phenomena occurring within them. In recent years, the investigation and improvement of electromagnetic multiphase properties of classical multiferroics materials as well as the search for new multiferroics materials have been of interest to research.

New multiferroics materials with ferroelectromagnetic properties BaYFeO4 were discovered in recent years. In the anomalous crystal structure of this material system, the FeO6 octahedral and square pyramid FeO5 are linked together into sequences of Fe4O8 tetramers arranged along the b crystal axis. Besides, the spinel crystal structure of Mn3O4 oxide can to some extent be seen as a simple structural homologue of BaYFeO4. The combination of magnetic magnitude and electromagnetic effect promises the practical application of this material system.

However, information on the magnetic phase structure characteristics of Mn3O4 and BaYFeO4 as well as the crystal structure of BaYFeO4 under high pressure conditions is still unclear. Besides, studies on the influence of chemical doping on the electromagnetic properties of BaYFeO4 materials have not been conducted.

Neutron diffraction is the most efficient experimental method for determining the crystal structure and magnetic structure of complex oxides over a wide range of variations of external conditions such as temperature, pressure, external magnetic fields. Compared to other experimental methods, it is one of the few research methods that allows to accurately locate light elements (H, Li, O) or elements with numerical atoms close together in the crystal structure. It is clear that neutron diffraction is an appropriate research method for characterizing the properties of multiferroics Mn3O4 and BaYFeO4 materials.

Experimental proof

With the ability to simultaneously and directly investigate the change of the magnetic phase structure and structural characteristics of the material when changing structural parameters such as bond length, bond angle, displacement of ions by neutron diffraction under high pressure,  the research team of Prof. TS. Dr. Le Hong Khiem cooperated with the Dubna Joint Institute for Nuclear Research to carry out the project: "Study on the structural state and magnetic order state of BaYFeO4 and Mn3O4 oxide complex electromagnetic materials according to changes in temperature parameters and chemical composition" (code:  QTRU01.02/20-21). The study aims to provide important information about the formation mechanism of the electrical order state, the magnetic order state, and the nature of the magneto-electrical correlation within multiferroics materials.

The scientists conducted a detailed survey of the crystal structure and magnetic phase properties of BaYFeO4 and Mn3O4 under the influence of high pressure up to 10 GPa and in the temperature range of 5 - 300 K using neutron diffraction. The research team has obtained many new results on the structural state and magnetic order state of BaYFeO4 and Mn3O4 oxide complex electromagnetic materials according to changes in temperature parameters and chemical composition. In addition, the team investigated the influence of doping of other transition metals on the structural and magnetic properties of BaYFeO4 materials. From the experimental data obtained, the researchers established the mechanism for forming magnetic order states in materials as well as confirmed the role of each factor in the formation of their physical properties, and specifically established the P-T phase diagram of the study material.

Prof. Dr. Le Hong Khiem shared: The research has discovered physical effects that help the search for applications of new materials in the future. With its high practical applicability and complex physical effects, the direction of studying the nature of physical phenomena as well as the search for strong electromagnetic multiphase materials is one of the important problems for modern solid state physics. His and his colleagues' research has obtained important information about the crystal structure as well as the magnetic order state of BaYFeO4 multiferroics materials as well as Mn3O4 structural homologous materials in a wide range of variation in thermodynamic parameters and the effect of chemical doping on BaYFeO4 material properties. From the empirical data obtained, the relationship between structural parameters and magnetic phase features and ferroelectric states will be established. The results obtained have contributed to the overall understanding of the properties of electromagnetic multiphase materials.

The neutron diffraction diagram of a sample of multiferroic material BaYFeO4 at temperature change from 15 K to 100 K measured at scattering angles of 2θ equal to 45.5°(a) and 90°(b), and processed by Rietveld method. The diffraction diagram in the range 4.1-4.5 Å(c) shows the redistribution of magnetic field strength at the junction between the spin-density wave phase and the helix phase. The lines below are the calculation positions of the nuclear peaks of the orthorhombic phase Pnma

(a) Neutron diffraction diagram of Mn3O4 materials at 2 GPa and low temperature; (b) Low-temperature neutron diffraction diagram of Mn3O4 at different pressures; and (c) Diffraction diagram at P = 2 GPa and temperatures 30, 75 and 300 K. Solid lines represent the results of processing using the Rietveld method. The lines show the position of calculation of diffraction peaks of structural phases

Translated by Phuong Ha
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