Structure and magnetic properties of Fe1.95-xNixP1-ySiy alloys

Abstract

Hexagonal Fe2P is an iron binary known to present a particularly large magnetocrystalline anisotropy potentially interesting for permanent magnets. However, its Curie temperature is far too low for applications. Here, we investigate the structural and magnetic phase diagrams of Fe1.95-xNixP1-ySiy quaternary alloys, looking for a possibility to combine high Curie temperature and uniaxial magnetic anisotropy. We systematically explore the stability range of the Fe2P-type hexagonal structure in Fe1.95-xNixP1-ySiy quaternaries with 0.2 ≤ y ≤ 0.4. While Fe2P1-ySiy ternary alloys crystallize in an orthorhombic structure from y ~0.1, we show that Ni substitutions restore the hexagonal structure. In contrast to Fe2-xNixP ternaries for which x = 0.1 substitution increased the Curie temperature by ~130 K, Ni substitutions are found to systematically decrease the Curie temperature in the present Fe1.95-xNixP1-ySiy quaternaries as a result of a c/a expansion weakening the interlayer magnetic interaction between the 3f and 3g sites stacked along the c axis. X-ray diffraction patterns on powders oriented in magnetic field indicate a c axis uniaxial anisotropy in Fe1.75Ni0.2P0.8Si0.2, and magnetization curves measured along different direction reveal a sizable magnetic anisotropy at room temperature. However, further band filling by Ni for Fe substitutions decrease the saturation magnetization, magnetic anisotropy and Curie temperature.