Exploring the negative thermal expansion and magnetocaloric effect in Fe2(Hf,Ti) Laves phase materials

Zero Thermal Expansion Effect and Enhanced Magnetocaloric Effect Induced by Fe Vacancies in Fe2Hf0.80Nb0.20 Laves Phase Alloys

“…The magnetic entropy change |Δ S M | illustrated in Figure e,f is calculated from the isofield magnetization curves in Figure c,d based on the Maxwell relation: normalΔ S M false( normalΔ H , T false) = ∫ H 0 H false( ∂ M false( T , H false) ∂ T false) H normald μ 0 H , where we choose μ 0 H 0 = 0 T. The maximal |Δ S M | for the Fe1.95 alloy is 0.50 J/kg K at 170 K with a second peak value of 0.32 J/kg K at 237 K at a magnetic field change of 2 T. For the Fe2.00 alloy, its maximal |Δ S M | is 0.39 J/kg K at 260 K at a magnetic field change of 2 T. The larger magnetization of the Fe1.95 alloy primarily contributes to its 22% larger magnetic entropy change compared to the Fe2.00 alloy. The maximal |Δ S M | of the Fe1.95 alloy is comparable to other Fe-based Laves compounds with a second-order transition, with 0.5 J/kg K in Sc 0.4 Ti 0.6 Fe 2 , with 0.46 J/kg K in Fe 2 Hf 0.85 Ti 0.15 and with 1.3 J/kg K in rare-earth-free Mn 30 Fe 20 Al 50 . It is however smaller than the Fe-based Laves phase materials with a first-order transition, with 1.7 J/kg K in Sc 0.3 Ti 0.7 Fe 2 and 2.3 J/kg K in Fe 2 Hf 0.86 Ta 0.14 .…”

“…Notably, even in stoichiometric Fe 2 (Hf,Ta) compounds, Fe atoms show a preference for the 4f site, as evidenced by the pronounced magnetization change in slightly Fe-deficient Fe 2 (Hf,Ta) alloys. , Unlike the as-cast Fe 2 Hf 0.80 Nb 0.20 alloy in ref , which exhibited a Nb-/Hf-rich secondary phase at grain boundaries, the SEM image and line-scan profile along the yellow line in Figure d confirm the microstructural homogeneity of the single-phase Fe1.95 alloy at a 30 μm length scale. In this study, the immediate deactivation of the power switch likely mitigated the tendency for Nb and Hf atom segregation, since the rapid solidification process can suppress phase segregation, as observed in the melt-spun Fe 2 Hf 0.85 Ti 0.15 alloy …”

“…In this study, the immediate deactivation of the power switch likely mitigated the tendency for Nb and Hf atom segregation, since the rapid solidification process can suppress phase segregation, as observed in the melt-spun Fe 2 Hf 0.85 Ti 0.15 alloy. 12 Figure 2a shows the M−T curves at a magnetic field of 0.1 T for the Fe1.95, Fe2.00, and Fe2.05 alloys. The thermal hysteresis of 1−2 K observed in the M−T curves for all three alloys indicates a first-order magnetic transition.…”

“…It is noteworthy that the phase separation in Fe1.95 and Fe2.00 differs from the phase segregation reported in Fe 2 Hf 0.6 Ti 0.40 , where Ti-rich and Ti-poor phases are observed at room temperature via SEM images. 12 The phase segregation in Fe 2 Hf 0.60 Ti 0.40 is suppressed by the rapid solidification process, but this method is ineffective for the magnetic phase separation in the Fe 1.95 alloy, as indicated by the similarly broad M−T curves shown in Figure S5 in the Supporting Information. Compared to the Fe1.95 alloy, the peak splitting in the Fe2.00 alloy is less significant, indicating a weaker magnetic phase separation, as depicted in Supplementary Figure S4a.…”

“…The hexagonal C14 structure Fe-based Laves phases stand out within the Laves phase family due to their remarkable sensitivity of crystal, magnetism, and electronic properties to compositional variations. − Among them, Fe 2 (Hf,Ta) alloys have garnered considerable attention due to the itinerant-electron metamagnetic transition from ferromagnetic (FM) to antiferromagnetic (AFM) state as the temperature increases. ,− This FM–AFM transition is attributed to a frustration effect, where the ordered magnetic moment of the Fe atom at the 2a site, situated within the middle layer of the Fe atom at the 6h site, vanishes as the temperature exceeds the transition temperature. − A significant research interest has been attracted in Fe-based Laves phase alloys including the mechanism underlying this metamagnetic transition ,− and related rich physical properties such as magnetocaloric effect, − negative thermal expansion (NTE) or zero thermal expansion (ZTE), ,− and magnetoresistance . A large adiabatic temperature change (3.5 K) from the metamagnetic transition was obtained at a magnetic field change of 2 T, which is comparable to the values for the two well-known MCE materials La­(Fe 0.88 Si 0.12 ) 13 H y and Mn x Fe 2– x P 1– y Si y .…”