Conical magnetic structures in multiferroic SrScxFe12-x O19 hexaferrites derived from powder neutron diffraction

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The new powder diffractometer D1B of the Institut Laue Langevin

D1B is a medium resolution high flux powder diffractometer located at the Institut Laue Langevin, ILL. D1B a suitable instrument for studying a large variety of polycrystalline materials. D1B runs since 1998 as a CRG (collaborating research group) instrument, being exploited by the CNRS (Centre National de la Recherche Scientifique, France) and CSIC (Consejo Superior de Investigaciones Cientificas, Spain). In 2008 the Spanish CRG started an updating program which included a new detector and a radial oscillating collimator (ROC). The detector, which has a sensitive height of 100mm, covers an angular range of 128°. Its 1280 gold wires provide a neutron detection point every 0.1°. The ROC is made of 198 gadolinium- based absorbing collimation blades, regular placed every 0.67°. Here the present characteristics of D1B are reviewed and the different experimental performances will be presented

Optimizing the Curie temperature of pseudo-binary RxR'2-xFe17 (R,R' = rare earth) for magnetic refrigeration

Several pseudo-binary RxR'2-xFe17 alloys (with R = Y, Ce, Pr, Gd and Dy) were synthesized with rhombohedral Th2Zn17-type crystal structure determined from x-ray and neutron powder diffraction. The choice of compositions was done with the aim of tuning the Curie temperature (TC) in the 270 ± 20 K temperature range, in order to obtain the maximum magneto-caloric effect around room temperature. The investigated compounds exhibit broad isothermal magnetic entropy changes, ΔSM(T), with moderate values of the refrigerant capacity, even though the values of ΔSMPeak are relatively low compared with those of the R2Fe17 compounds with R = Pr or Nd. The reduction on the ΔSMPeak is explained in terms of the diminution in the saturation magnetization value. Furthermore, the ΔSM(T) curves exhibit a similar caret-like behavior, suggesting that the magneto-caloric effect is mainly governed by the Fe-sublattice. A single master curve for ΔSM/ΔSMPeak(T) under different values of the magnetic field change are obtained for each compound by rescaling of the temperature axis.España MICINN MAT2011-27573-C04Basque Government IT-347-07CONACYT CB-2010-01-156932Slovak R&D Agency VVCE-0058-0

Elucidating Individual Magnetic Contributions in Bi-Magnetic Fe3O4/Mn3O4 Core/Shell Nanoparticles by Polarized Powder Neutron Diffraction

Heterogeneous bi-magnetic nanostructured systems have had a sustained interest during the last decades owing to their unique magnetic properties and the wide range of derived potential applications. However, elucidating the details of their magnetic properties can be rather complex. Here, a comprehensive study of FeO/MnO core/shell nanoparticles using polarized neutron powder diffraction, which allows disentangling the magnetic contributions of each of the components, is presented. The results show that while at low fields the FeO and MnO magnetic moments averaged over the unit cell are antiferromagnetically coupled, at high fields, they orient parallel to each other. This magnetic reorientation of the MnO shell moments is associated with a gradual evolution with the applied field of the local magnetic susceptibility from anisotropic to isotropic. Additionally, the magnetic coherence length of the FeO cores shows some unusual field dependence due to the competition between the antiferromagnetic interface interaction and the Zeeman energies. The results demonstrate the great potential of the quantitative analysis of polarized neutron powder diffraction for the study of complex multiphase magnetic materials

Exploring the Different Degrees of Magnetic Disorder in TbxR1−xCu2 Nanoparticle Alloys

Recently, potential technological interest has been revealed for the production of magnetocaloric alloys using Rare-Earth intermetallics. In this work, three series of TbxR1−xCu2 (R ≡ Gd, La, Y) alloys have been produced in bulk and nanoparticle sizes via arc melting and high energy ball milling. Rietveld refinements of the X-ray and Neutron diffraction patterns indicate that the crystalline structure in all alloys is consistent with TbCu2 orthorhombic Imma bulk crystalline structure. The analyses of the DC-magnetisation (MDC) and AC-susceptibility (χAC) show that three distinct degrees of disorder have been achieved by the combination of both the Tb3+ replacement (dilution) and the nanoscaling. These disordered states are characterised by transitions which are evident to MDC, χAC and specific heat. There exists an evolution from the most ordered Superantiferromagnetic arrangement of the Tb0.5La0.5Cu2 NPs with Néel temperature, TN∼ 27 K, and freezing temperature, Tf∼ 7 K, to the less ordered weakly interacting Superparamagnetism of the Tb0.1Y0.9Cu2 nanoparticles (TN absent, and TB∼ 3 K). The Super Spin Glass Tb0.5Gd0.5Cu2 nanoparticles (TN absent, and Tf∼ 20 K) are considered an intermediate disposition in between those two extremes, according to their enhanced random-bond contribution to frustration.This work has been supported by the Spanish MAT2017-83631-C3-R grant. E.M.J.’s work was supported by “Beca de Colaboración”, BDNS: 311327 granted by Ministerio de Educación, Cultura y Deporte and “Beca Concepción Arenal” BDNS: 406333 granted by the Gobierno de Cantabria and the Universidad de Cantabria. MRF work was supported by FPI (BES-2012-058722)

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

Zinc blende and wurtzite CoO polymorph nanoparticles : rational synthesis and commensurate and incommensurate magnetic order

On the nanoscale, CoO can have different polymorph crystal structures, zinc blende and wurtzite, apart from rock salt, which is the stable one in bulk. However, the magnetic structures of the zinc blende and wurtzite phases remain virtually unexplored. Here we discuss some of the main parameters controlling the growth of the CoO wurtzite and zinc blende polymorphs by thermal decomposition of cobalt (II) acetylacetonate. In addition, we present a detailed neutron diffraction study of oxygen deficient CoO (CoO) nanoparticles with zinc blende (∼15 nm) and wurtzite (∼30 nm) crystal structures to unravel their magnetic order and its temperature evolution. The magnetic order of the zinc blende nanoparticles is antiferromagnetic and appears at the Néel temperature T ∼ 203 K. It corresponds to the 3rd type of magnetic ordering in a face-centered cubic lattice with magnetic moments aligned along a cube edge. The magnetic structure in the wurtzite nanoparticles turned out to be rather complex with two perpendicular components. One component is incommensurate, of the longitudinal spin wave type, with the magnetic moments confined in the ab-plane. In the perpendicular direction, this magnetic order is uncorrelated, forming quasi-two-dimensional magnetic layers. The component of the magnetic moment, aligned along the hexagonal axis, is commensurate and corresponds to the antiferromagnetic order known as the 2nd type in a wurtzite structure. The Néel temperature of wurtzite phase is estimated to be ∼109 K. The temperature dependence of the magnetic reflections confirms the reduced dimensionality of the incommensurate magnetic order. Incommensurate magnetic structures in nanoparticles are an unusual phenomenon and in the case of wurtzite CoO it is probably caused by structural defects (e.g., vacancies, strains and stacking faults)

Magneto-caloric effect in the pseudo-binary intermetallic YPrFe17 compound

We have synthesized the intermetallic YPrFe17 compound by arc-melting. X-ray and neutron powder diffraction show that the crystal structure is rhombohedral with View the MathML source space group (Th2Zn17-type). The investigated compound exhibits a broad isothermal magnetic entropy change {\Delta}SM(T) associated with the ferro-to-paramagnetic phase transition (TC \approx 290 K). The |{\Delta}SM| (\approx 2.3 J kg-1 K-1) and the relative cooling power (\approx 100 J kg-1) have been calculated for applied magnetic field changes up to 1.5 T. A single master curve for {\Delta}SM under different values of the magnetic field change can be obtained by a rescaling of the temperature axis. The results are compared and discussed in terms of the magneto-caloric effect in the isostructural R2Fe17 (R = Y, Pr and Nd) binary intermetallic alloys.Comment: Preprint, 5 pages (postprint), 4 figures, regular pape

Incommensurate and multiple-q\boldsymbol{q} magnetic misfit order in the frustrated quantum spin ladder material antlerite, Cu3_3SO4_4(OH)4_4

In frustrated magnetic systems, the competition amongst interactions can introduce extremely high degeneracy and prevent the system from readily selecting a unique ground state. In such cases, the magnetic order is often exquisitely sensitive to the balance among the interactions, allowing tuning among novel magnetically ordered phases. In antlerite, Cu$_3$SO$_4$(OH)$_4$, Cu$^{2+}$ ($S=1/2$) quantum spins populate three-leg zigzag ladders in a highly frustrated quasi-one-dimensional structural motif. We demonstrate that at zero applied field, in addition to its recently reported low-temperature phase of coupled ferromagnetic and antiferromagnetic spin chains, this mineral hosts an incommensurate helical+cycloidal state, an idle-spin state, and a multiple-$q$ phase which is the magnetic analog of misfit crystal structures. The antiferromagnetic order on the central leg is reentrant. The high tunability of the magnetism in antlerite makes it a particularly promising platform for pursuing exotic magnetic order.Comment: 18.3 pages, 16 Figures, follow-up paper to arXiv:2203.1534

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