Combining different experimental techniques, investigations in hexagonal P 6 3 /mmc Er 2 Fe 17 show remarkable magnetovolume anomalies below the Curie temperature, T C . The spontaneous magnetostriction reaches 1.6 × 10 −2 at 5 K and falls to zero well above T C , owing to short-range magnetic correlations. Moreover, Er 2 Fe 17 exhibits direct and inverse magnetocaloric effects (MCE) with moderate isothermal magnetic entropy S M , and adiabatic temperature T ad changes [ S M ∼ −4.7 J(kgK) −1 and T ad ∼ 2.5 K near the T C , and S M ∼ 1.3 J(kgK)and T ad ∼ −0.6 K at 40 K for H = 80 kOe, respectively, determined from magnetization measurements].The existence of an inverse MCE seems to be related to a crystalline electric field-level crossover in the Er sublattice and the ferrimagnetic arrangement between the magnetic moments of the Er and Fe sublattice. The main trends found experimentally for the temperature dependence of S M and T ad as well as for the atomic magnetic moments are qualitatively well described considering a mean-field Hamiltonian that incorporates both crystalline electric field and exchange interactions. S M (T ) and T ad (T ) curves are essentially zero at ∼150 K, the temperature where the transition from direct to inverse MCE occurs. A possible interplay between the MCE and the magnetovolume anomalies is also discussed.
Alloy ribbons of nominal composition MnNiGe1.05 were produced using the melt-spinning technique. As-quenched (aq) polycrystalline ribbons are single-phase showing the hexagonal Ni2In-type crystal structure. After thermal annealing at 1148 K, the formation of the orthorhombic TiNiSi-type crystal structure by martensitic transformation is favored. However, XRD patterns for different temperatures indicate that the phase transition from hexagonal to orthorhombic structure is incomplete. The starting and finishing temperatures for the direct and reverse martensitic transformation for aq (annealed) samples determined by DSC were MS = 264 (268) K Mf = 235 (255) K, AS = 259 (266) K, and Af = 289 (276) K. Across this structural phase transition the annealed sample undergoes a drop in magnetization giving rise to a narrow temperature dependence of the magnetic entropy change with a peak value on heating (cooling) of 5.8 (4.8) Jkg−1K−1 for a field change of 5 T.
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