Anomalous thermal expansion and enhanced magnetocaloric effect in <001>-textured Mn_xFe_5–xSi₃ alloys

The development of zero and negative thermal expansion (i.e., ZTE and NTE) materials is of crucial importance to the control of undesirable thermal expansion for high-precision devices. In the present work, ZTE and NTE were obtained in directionally-solidified Mn_xFe_5–xSi₃ alloys with a strong <001> texture, in striking contrast to positive thermal expansion in their isotropic counterparts. Magnetometry and in-situ X-ray diffraction (XRD) measurements were performed to uncover the origin of the anomalous thermal expansion. Magnetic measurements indicate a strong easy-plane magnetocrystalline anisotropy in the textured samples, where the magnetic moments are aligned within the ab plane of the hexagonal structure. Temperature-dependent XRD on the x = 1 sample reveals a ZTE character in the ab plane that is coupled to a ferromagnetic transition. As a result, the macroscopic ZTE (~ 0.22 × 10^–6 K⁻¹) in the x = 1 sample can be attributed to the microscopic magneto volume effect within the ab plane, which is realized by the introduction of the <001>-textured microstructure. Besides, the competition between antiferromagnetic and ferromagnetic exchange coupling leads to NTE in textured x = 1.5 and 2 samples. Additionally, textured x = 1 sample displays enhanced magnetocaloric properties as compared to the conventional counterparts with randomly-oriented grains. Consequently, this work demonstrates a new strategy toward the exploration of anomalous thermal expansion properties as well as the enhancement of magnetocaloric properties for materials with a strong magnetocrystalline anisotropy.