Magnetic and structural properties of Fe5SiB2 under pressure

In recent years, the search for new materials suitable for preparation of rare-earth-free permanent magnets has led to a renewed interest in the magnetic properties of Fe compounds with an anisotropic crystal structure, which may promise sizeable magnetic anisotropy and high Curie temperature. The Fe₅SiB₂ phase was discovered in 1960 by B. Aronsson and I. Engström. It possess a high Curie temperature of 845 K and a high value of saturated magnetization of about 9 μ_B / f.u.. The compound crystallizes in the tetragonal crystal structure of Cr₅B₃ type with lattice parameters of about a = 5.43 Å and c = 10.33 Å [1-5]. Unfortunately, the resulting magnetic anisotropy is not sufficient to present high coercivity. The magnetic properties were previously studied by chemical substitution which can lead to significant changes of saturated magnetization and magnetic anisotropy [2,4,5]. Pure Fe₅SiB₂ has uniaxial magnetocrystaline anisotropy (at room temperature) and undergoes a spin-reorientation transition at temperatures below 200 K to easy-plane anisotropy. At present, the compound does not exhibit properties suitable for application though it is a very interesting compound for studying the magneto-structural relations and pressure/volume effects on magnetic properties. The previous experimental and theoretical studies of the effect of chemical substitutions, particularly volume variations and fix spin moment calculations, motivated us to connect this approach by following the effects of applied pressure on magnetic and structural properties. In this work we present the evolution of the unit cell parameters a and c under pressure up to 6 GPa and at temperatures down to 10 K at ambient pressure as well as the evolution of saturated magnetization under hydrostatic pressure up to 1 GPa. The pressure evolution of the spin reorientation occurring at low temperature is also studied. The obtained value of the bulk modulus of 124 GPa allows us to compare the effect of pressure with previously studied effects of chemical substitution. The determined V-P diagram is presented in figure 1. 

Figure 1. V-P diagram determined using XRD under pressure at room temperature. The obtained data were fitted by the Birch-Murnaghen equation of state (green line).

Acknowledgments. The work of J. K. was supported by the Operational Programme Research, Development and Education financed by European Structural and Investment Funds and the Czech Ministry of Education, Youth and Sports (Project No. CZ.02.2.69/0.0/0.0/16-027/0008215).

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[4] M. Werwiński, et al., Phys. Rev. B 93, 174412 (2016).
[5] D. Hedlund, et al., Phys. Rev. B 96, 094433 (2017).