Texture-induced enhancement of the magnetocaloric response in melt-spun DyNi2 ribbons

"The magnetocaloric properties of melt-spun ribbons of the Laves phase DyNi2 have been investigated. The as-quenched ribbons crystallize in a single-phase MgCu2-type crystal structure (C15; space group Fd (3) over barm) exhibiting a saturation magnetization and Curie temperature of M-S = 157 +/- 2 A m(2) kg(-1) and T-C = 21.5 +/- 1 K, respectively. For a magnetic field change of 2 T, ribbons show a maximum value of the isothermal magnetic entropy change vertical bar Delta S-M(peak)vertical bar = 13.5 J kg(-1) K-1, and a refrigerant capacity RC = 209 J kg(-1). Both values are superior to those found for bulk polycrystalline DyNi2 alloys (25% and 49%, respectively). In particular, the RC is comparable or larger than that reported for other potential magnetic refrigerants operating at low temperatures, making DyNi2 ribbons promising materials for use in low-temperature magnetic refrigeration applications.

Enhanced refrigerant capacity in two-phase nanocrystalline/amorphous NdPrFe17 melt-spun ribbons

"he magnetocaloric properties of NdPrFe17 melt-spun ribbons composed of nanocrystallites surrounded by an intergranular amorphous phase have been studied. The nanocomposite shows two successive second-order magnetic phase transitions (303 and 332 K), thus giving rise to a remarkable broadening (approximate to 84 K) of the full-width at the half-maximum of the magnetic entropy change curve, Delta S-M(T), with a consequent enhancement of the refrigerant capacity RC. For a magnetic field change of 2 T, vertical bar Delta S-M(peak)vertical bar = 2.1 J kg(-1) K-1 and RC = 175 J kg(-1). Therefore, the reversible magnetocaloric response together with the one-step preparation process makes these nanostructured Fe-rich alloy ribbons particularly attractive for room temperature magnetic refrigeration.

Magnetovolume and magnetocaloric effects in Er2Fe17

Combining different experimental techniques, investigations in hexagonal P63/mmc Er2Fe17 show remarkable magnetovolume anomalies below the Curie temperature, TC. The spontaneous magnetostriction reaches 1.6×10−2 at 5 K and falls to zero well above TC, owing to short-range magnetic correlations. Moreover, Er2Fe17 exhibits direct and inverse magnetocaloric effects (MCE) with moderate isothermal magnetic entropy ΔSM, and diabatic temperature ΔTad changes [ΔSM∼−4.7 J(kgK)−1 and ΔTad∼2.5 K near the TC, and ΔSM∼1.3 J(kgK)−1 and ΔTad∼−0.6 K at 40 K for ΔH=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 ΔSM and ΔTad 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. ΔSM(T) and ΔTad(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.Financial support from Spanish MICINN (MAT2011-27573-C04-02) and from the Basque Government (IT-347- 07) is acknowledged. J.L.S.Ll. acknowledges the support received from CONACYT, Mexico, under the project CB2010-01-156932, and Laboratorio Nacional de Investigaciones en Nanociencias y Nanotecnología (LINAN, IPICyT). J.A.R.V. acknowledges the support from the research project MAT2007-61621. We thank ILL and CRG-D1B for allocating neutron beamtime, and ESRF for synchrotron beamtime. The SCTs at the University of Oviedo and the technical support received from M.Sc. G. J. Labrada-Delgado and B. A. Rivera-Escoto (DMA, IPICyT) are also acknowledged

Entangled core/shell magnetic structure driven by surface magnetic symmetry-breaking in Cr2O3 nanoparticles

Bulk Cr2O3 is an antiferromagnetic (AFM) oxide that exhibits the magnetoelectric effect at room temperature, with neither spontaneous magnetization nor net electric polarization. These physical properties stem from a subtle competition between exchange and crystal field interactions. In this article, we exploit the symmetry breaking at the surface of Cr2O3 nanoparticles for unbalancing this delicate physical equilibrium. The emerging weak ferromagnetic signal we observe persists up to near room temperature (≈ 270 K) at which the antiferromagnetic order disappears. In addition, an exchange-bias effect, that rapidly decreases on heating from low temperature up to 30 K, is resistant to thermal disorder above 200 K. Our findings point to the possible formation of an entangled core/shell magnetic structure, where pinned uncompensated spins at the shell are randomly distributed in a low-temperature spin-glass ordering, with low net magnetic moment and an ordering temperature governed by the AFM Néel temperature.Work at University of Oviedo was financially supported by research projects MCIU-19-RTI2018-094683-B-C52 (MCIU/AEI/FEDER, UE) and AYUD/2021/51822 (FICyT, Principality of Asturias). Thanks are due to Elettra-Sincrotrone Trieste (Italy) and to Institut Laue-Langevin (France) for allocating beam time. We are grateful to the Scientific-Technical Services of the University Oviedo for providing assistance in transmission microscopy image acquisition and processing. Work at USF supported partially through US Department of Energy, Office of Basic Energy Sciences, Division of Materials Science and Engineering under Award # DE-FG02-07ER46438. H. S. acknowledges support from the Bizkaia Talent Program, Basque Country (Spain). X. M. acknowledges support from the Grant Agency of the Czech Republic Grant no. 14-37427.Peer reviewe

Enhanced refrigerant capacity and magnetic entropy flattening using a two-amorphous FeZrB(Cu) composite

"The temperature dependence of the isothermal magnetic entropy change, Delta S-M, and the magnetic field dependence of the refrigerant capacity, RC, have been investigated in a composite system xA + (1 - x) B, based on Fe87Zr6B6Cu1 (A) and Fe90Zr8B2 (B) amorphous ribbons. Under a magnetic field change of 2 T, the maximum improvement of the full-width at half maximum of Delta S-M(T) curve (47% and 29%) and the RC (18% and 23%), in comparison with those of the individual alloys (A and B), is observed for x approximate to 0.5. Moreover, a flattening over 80K in the Delta S-M(T) curve around room temperature range is observed, which is a key feature for an Ericsson magnetic refrigeration cycle.