Non-itinerant-type ferromagnetism and the magnetocaloric response of quinary all-d-metal ribbon: Ni₃₅Mn_34.5Co₁₄Fe₁Ti_15.5

Abstract

Most of the ‘ferromagnetic shape memory’ (FSM) Heusler alloys, which are primarily studied in bulk form in the literature, exhibit p-d type hybridization. This research thoroughly investigates a multidirectional study of a strongly d-d hybridized quinary melt spun annealed ribbon having composition Ni35Mn34.5Co14Fe1Ti15.5 (NMCFT-1). This off-stoichiometric, polycrystalline FSM, fabricated using the melt-spin technique, exhibits a highly textured microstructure, double magnetic transitions, and super mechanical features mitigating brittleness. It crystallizes in a perfectly B2-type disorder austenite (Pm-3m, space group number 225) phase at room temperature (RT). It is hypothesized that ‘geometric frustration’ is the causative factor for this disorder. ‘Curie temperature of austenite phase’ (T_C^A) between ‘paramagnetic’ → ‘ferromagnetic’ state is found to be ~ 364.57 K, whereas, ‘martensite transformation temperature’ from ‘weak magnetic martensite’ state to ‘ferromagnetic austenite’ state is ~ 174.74 K. Calculated moments (effective moment, µ_eff = 5.12 µ_B; low-temperature saturation moment, µ_S (or M_S (0)) = 5.08 µ_B) yield Rhodes-Wohlfarth Ratio (RWR) ~ 1, indicating existence of the non-itinerant nature of 3d electrons, whereas the ferromagnetism and the linear dependency of 'M_s^2 (T)’ on T2 around T_C^A indicates presence of long-range (RKKY type) interaction. More importantly, the ‘maximum magnetic entropy change’ (〖∆S〗_m) obtained across the ‘FOMT’ (‘first-order magneto-structural transition’) and ‘SOMT’ (‘second-order magnetic transition’) are, +18.2 J.kg-1K-1 at 6 T and -8.8 J.kg-1K-1 at 2 T, respectively, while, a very high ‘working temperature span’ (∆TFWHM) of 28.561 K and 6.922 K are found for the same condition. The sample exhibits a significant ‘relative cooling power (RCP)’ of 402.98 J.kg-1 at a magnetic field of 6 T across FOMT and 60.19 J.kg-1 at a 2 T field across SOMT, respectively, along with excellent mechanical features: a Vickers’s hardness (HV) of 411.80 HV (~ 4.04 GPa). While Chen's super hard model fails to predict the ribbon's HV value, Miao's hard model does, indicating that the ribbon is hard but not super hard. It also paves the way for additional investigations into innovative FSMs like this.