Origin of the multiferroic-like properties of Er2CoMnO6

We report on the magnetoelectric properties of Er2CoMnO6. This compound adopts the structure of a double perovskite with a strong monoclinic distortion. Our specimen exhibits a nearly perfect Co-Mn order. It undergoes a ferromagnetic transition at TC~70 K due to the Co2+-O-Mn4+ ferromagnetic superexchange interaction. Below 30 K, the Er3+ moments start to order antiferromagnetically to the Co/Mn sublattice. Pyroelectric measurements reveal electrical polarization at low temperature but its strong dependence on the heating rate indicates the lack of a spontaneous ferroelectricity. Instead, electric polarization is derived from thermally stimulated depolarization currents

Cation distribution of cobalt ferrite electrosynthesized nanoparticles. A methodological comparison

The present work seeks to analyse the structural and magnetic properties of cobalt ferrite nanoparticles obtained by electrochemical synthesis by high-resolution transmission electronic microscopy (HRTEM), X-ray absorption spectroscopy (XAS), Mössbauer spectroscopy (MS), neutron diffraction (ND) and SQUID magnetometer. The cationic distribution is analyzed by different techniques. The inversion degree determined by the most accurate measurements was 0.73(1), and the formula for the nanoparticles therefore was (↑Co0.27Fe0.73)[↓Co0.73Fe1.27]O4. The magnetic moment found from DC and Mössbauer spectroscopy measurements was 3.8(3) μB, and the coercivity was 7870 Oe at 100 K.This work is supported by the MINECO/FEDER Project MAT2015- 67557-C2-2-P. The authors are grateful to the Institut Laue Langevin and the Spanish CRG D1B for the neutron beam-time allocated (experiment codes 5-31-2259 and CRG-1940; https://doi.org/10. 5291/ILL-DATA.5-31-2259) and to the SpLine CRG beamline staff at ESRF for assistance during XAS experiments

119Sn Mössbauer Spectroscopy for assessing the local stress and defect state towards the tuning of Ni-Mn-Sn alloys

[EN] The influence of defects and local stresses on the magnetic properties and martensitic transformation in Ni50Mn35Sn15 is studied at macroscopic and atomic scale levels. We show that both the structural and magnetic properties of the alloy are very sensitive to slight microstructural distortions. Even though no atomic disorder is induced by milling, the antiphase boundaries linked to dislocations promote the antiferromagnetic coupling of Mn, resulting in a significant decrease in the saturation magnetization. On the other hand, the temperature range of the transformation is considerably affected by the mechanically induced local stresses, which in turn does not affect the equilibrium temperature between the austenitic and martensitic phases. Finally, we demonstrate that the recovery of the martensitic transformation is directly related to the intensity of the non-magnetic component revealed by 119Sn Mössbauer spectroscopy. This result opens the possibility of quantifying the whole contribution of defects and the local stresses on the martensitic transformation in Ni-Mn-Sn alloys.This work was supported for the Basque Government Grant No. IT-1005-16 by the Spanish Ministry of Economy and Competitiveness under the project MAT2015-65165-C2- R (MINECO/FEDER) and GIC1585. I. Unzueta also wants to acknowledge the Basque Government Grant No. PRE2014-1-214. ILL and SpINS are acknowledged for beam time allocation. J. Lopez-Garcıa acknowledges ILL for his Ph.D. contract

Magnetocaloric effect enhancement driven by intrinsic defects in Ni-Mn-Sn- Co alloys

[EN] The influence of mechanically-induced defects on the magnetostructural properties is analyzed in a Ni-Co-Mn-Sn alloy subjected to soft milling and subsequent annealing treatments. It is found that, opposite to what occurs in Ni-Mn-Sn ternary alloys, the annealing treatment affects the magnetic properties in a different way in martensite and in austenite. In particular, the saturation magnetization significantly increases in martensite after annealing whereas just a very slight variation is observed in austenite. This leads to the interesting fact that the presence of microstructural defects, far for worsening, makes the magnetocaloric effect to be higher in the as-milled state than after annealing. This behavior is explained as the result of the combination of the effect of defects on the Mn-Mn distance, the effect of Co on the magnetic exchange coupling between Mn atoms, and the effect of defects on the vibrational entropy change at the martensitic transformation.This work has been carried out with the financial support of the Spanish “Ministerio de Economía y Competitividad” (Projects number MAT2015-65165-C2-R) and of the Basque Government (Grant No. IT-1005-16). We also acknowledge ILL and SpINS for beam time allocation: experiments 5-24-591 (https://doi.org/10.5291/ILL-DATA.5-24-591) and CRG-2352. J. López-García acknowledges ILL for his Ph. D. contract and I. Unzueta also wants to thank the Basque Government Grant No. PRE-2014-1-214

Mechanically induced disorder and crystallization process in Ni-Mn-In ball-milled alloys

[EN] High mechanical deformation has been induced in a Ni-Mn-In metamagnetic shape memory alloy by means of ball milling. The evolution of both the martensitic transformation and the magnetic properties associated to the microstructural variations has been characterized. The as-milled nanometric particles display an amorphous structure with a frustrated magnetic state compatible with a canonical spin-glass. On heating, an abrupt crystallization process occurs around 500 K leading to a cubic B2 structure, which, in turn, does not show martensitic transformation. Modified Arrott plots point to competing long- and short-range magnetic couplings in the B2 structure. On further heating, a relaxation process takes place above 700 K concurrently with a B2-L21 atomic ordering, giving rise to an anomalous two-step thermal expansion. The combined effect of both processes makes possible the subsequent occurrence of a martensitic transformation, which takes place at the same temperature than in the bulk. The large relative-cooling-power linked to the magnetocaloric effect at the martensitic transformation in the annealed powder makes it interesting for practical applications of magnetic refrigeration at nanoscale.This work has been carried out with the financial support of the Spanish “Ministerio de Economía y Competitividad” (Projects number MAT2012-37923-C02 and MAT2015-65165-C2-R). We also acknowledge ILL and SpINS for beam time allocation (experiment CRG-2158). RCF acknowledges a Postdoctoral fellowship from the Univeridad Pública de Navarra (grant number: 1081/2015). JARV acknowledges CSIC for a JAEdoc contract. J. Pons is acknowledged for TEM observations

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

Effect of high-energy ball-milling on the magnetostructural properties of a Ni45Co5Mn35Sn15 alloy

[EN] The effect of high-energy ball-milling on the magnetostructural properties of a Ni45Co5Mn35Sn15 alloy in austenitic phase at room temperature has been analyzed by neutron and high-resolution X-ray diffraction. The ball milling promotes a mechanically-induced martensitic transformation as well as the appearance of amorphous-like non-transforming regions, following a double stage; for short milling times (below 30 min), a strong size reduction and martensite induction occur. On the opposite, for longer times, the increase of strains predominates and consequently a larger amount of non-transforming regions appears. The effect of the microstructural defects brought by milling (as dislocations) on both the enthalpy change at the martensitic transformation and the high field magnetization of the austenite has been quantitatively estimated and correlated to the internal strains. Contrary to what occurs in ternary Ni-Mn-Sn alloys, the mechanically induced defects do not change the ferromagnetic coupling between Mn atoms, but just cause a net reduction on the magnetic moments.This work has been carried out with the financial support of the Spanish “Ministerio de Economía y Competitividad” (Projects number MAT2015-65165-C2-R) “Agencia Estatal de Investigación (AEI), Ministerio de Ciencia, Innovación y Universidades” (Projects number RTI2018-094683-B-C54 (MCIU/AEI/FEDER, UE)), Navarra Government (Project number PC017-018 AMELEC) and Basque Government Grant No. IT-1005-16. We acknowledge ILL and ALBA for the beam time allocations: (http://doi.org/10.5291/ILL-DATA. INTER-411), CRG-2352, and ALBA BL04_MPSD beamline at ALBA Synchrotron with the collaboration of ALBA staff. PLR has received funding from “la Caixa” and "Caja Navarra" Foundations, under agreement LCF/PR/PR13/51080004