Delving into the Correlation between Magnetic and Lattice Degrees of Freedom from Magnetocaloric and Magnetovolume Effects in Lu2Fe17 Ribbons

Nowadays, R₂Fe₁₇ (R = rare earth) materials with zero (ZTE) or negative (NTE) thermal expansion are of significant interest in advanced applications, especially for intermediate performance low-cost magnets. Lu₂Fe₁₇ ribbon flakes were fabricated by means of the melt-spinning technique, while a bulk sample was synthesized by arc melting and long-term annealing as a reference alloy. Both the as-cast ribbons and the bulk sample adopt a Th₂Ni₁₇-type hexagonal crystal structure. The anomalous temperature dependence of the lattice parameters in the ribbons confirms the existence of strong magnetovolume effects, characterized by NTE and ZTE along the c and a crystallographic axis, respectively. In addition, magnetic measurements show two magnetic phase transitions, from paramagnetic to helimagnetic and from helimagnetic to a fan structure, with transition temperatures differing between ribbon (T_HEL = 276 K and T_FAN = 252 K) and bulk (273 and 257 K) samples, respectively. These differences can be attributed to variations in the exchange interactions caused by slightly different interatomic distances between the Fe atoms. The isothermal entropy change versus temperature curves, ΔS_M(T), measured under low magnetic field values (up to 150 mT), provide clear evidence of the existence of a double peak, thus confirming the two successive magnetic phase transitions that occur in Lu₂Fe₁₇ ribbon flakes.