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The static magnetization and dynamic susceptibility responses of the cluster system within a Griffiths phase of the magnetocaloric compound Tb5−xLaxSi2Ge2 (x = 0.075) have been investigated. A novel cluster-glass state within the Griffiths phase is formed at a characteristic freezing temperature where short-range ferromagnetic correlations set in the paramagnetic regime. Ferromagneticlike correlations are built up at around 155 K, which suddenly become frozen at a lower temperature ∼140 K, thus in analogy with a reentrant spin glass behavior. The ac susceptibility near the freezing temperature follows a critical slowing down process characterized by τ0 = 10−13s and dynamic exponents zν ∼ 6 and β ∼ 0.4, similar to well-known spin glass systems. The nonlinear ac susceptibility analysis shows clearly the existence of a transition associated to the reentrant behavior. The origin of the intermediate cluster-glass phase inside the Griffiths phase is proposed to be the result of a combination of short-ranged RKKY intralayer positive exchange interactions between rare-earth Tb3+ ions and antiferromagnetic exchange between adjacent interlayers involving Si and Ge atoms in connection to the Tb3+ atoms Work at the University of Zaragoza is supported by the Spanish Ministerio de Ciencia, Innovación y Universidades MAT2017-82970-C2-2-R and Spanish DGA (Grant No.E26). Work at the University of Cantabria is supported by the Spanish Ministerio de Ciencia, Innovación y Universidades MAT2017-83631-C3-R.

The static magnetization and dynamic susceptibility responses of the cluster system within a Griffiths phase of the magnetocaloric compound Tb5−xLaxSi2Ge2 (x=0.075) have been investigated. A novel cluster-glass state within the Griffiths phase is formed at a characteristic freezing temperature where short-range ferromagnetic correlations set in the paramagnetic regime. Ferromagneticlike correlations are built up at around 155 K, which suddenly become frozen at a lower temperature ∼140K, thus in analogy with a reentrant spin glass behavior. The ac susceptibility near the freezing temperature follows a critical slowing down process characterized by τ0=10−13s and dynamic exponents zν∼6 and β∼0.4, similar to well-known spin glass systems. The nonlinear ac susceptibility analysis shows clearly the existence of a transition associated to the reentrant behavior. The origin of the intermediate cluster-glass phase inside the Griffiths phase is proposed to be the result of a combination of short-ranged RKKY intralayer positive exchange interactions between rare-earth Tb3+ ions and antiferromagnetic exchange between adjacent interlayers involving Si and Ge atoms in connection to the Tb3+ atoms., Work at the University of Zaragoza is supported by the Spanish Ministerio de Ciencia, Innovación y Universidades MAT2017-82970-C2-2-R and Spanish DGA (Grant No. E26). Work at the University of Cantabria is supported by the Spanish Ministerio de Ciencia, Innovación y Universidades MAT2017-83631-C3-R. J.H.B. would like to thank CICECO-Aveiro Institute of Materials, POCI-01-0145-FEDER-007679 (FCT Ref. UID/CTM/50011/2013), financed by national funds through the FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement.

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Spin-phonon coupling is investigated in epitaxially strained Sr1-xBaxMnO3 thin films with perovskite structure by means of microwave (MW) and infrared (IR) spectroscopy. In this work we focus on the Sr0.6Ba0.4MnO3 composition grown on (LaAlO3)0.3(Sr2AlTaO6)0.7 substrate. The MW complex electromagnetic response shows a decrease in the real part and a clear anomaly in the imaginary part around 150 K. Moreover, it coincides with a 17% hardening of the lowest-frequency polar phonon seen in IR reflectance spectra. In order to further elucidate this phenomenon, low-energy muon-spin spectroscopy was carried out, signaling the emergence of antiferromagnetic order with Néel temperature (TN) around 150 K. Thus, our results confirm that epitaxial Sr0.6Ba0.4MnO3 thin films display strong spin-phonon coupling below TN, which may stimulate further research on tuning the magnetoelectric coupling by controlling the epitaxial strain and chemical pressure in the Sr1-xBaxMnO3 system.

The perovskite (Sr,Ba)MnO3 system is an ideal candidate for tailoring electrical and magnetoelectric properties through the accurate control of Ba content and epitaxial strain due to the strong coupling between polar instability, spin order, and lattice. Here, first, the polar order is proved to be induced in Sr1−xBaxMnO3 thin films through lattice expansion either by epitaxial strain or chemical pressure, which correlates with the evolution of the dielectric properties. Second, due to strong spin–phonon coupling, a large magnetoelectric response is found in the (Sr,Ba)MnO3 system, in which the dielectric constant drops up to 50% when the antiferromagnetic order emerges, larger than most magnetoelectric oxides. More important, this coupling between magnetism and dielectric properties can be tuned from ≈18% to ≈50% by appropriately selecting Ba content and epitaxial strain. Third, a clear trend of increasing the band gap energy on increasing the unit cell volume either by epitaxial strain or chemical pressure is found, which opens the way for engineering the semiconducting properties of (Sr,Ba)MnO3 system at will. Thus, this work proves the possibility to design the electrical response and the magnetoelectric coupling in (Sr,Ba)MnO3 system., Financial support from Spanish Ministerio de Economía y Competitividad through project MAT2014-51982-C2 and from regional Gobierno de Aragón through project E26 including FEDER funding is also acknowledged. The low-energy μSR measurements were performed at the Swiss Muon Source SμS, Paul Scherrer Institut, Villigen, Switzerland. R.G. and C.M. were funded by the European Union under the Seventh Framework Programme under a contract for an Integrated Infrastructure Initiative Reference 312483-ESTEEM2. R.G. was also supported by the ERC StG “STEMOX” 239739.

Resumen del trabajo presentado a la 20th International Conference on Solid Compounds of Transition Elements, celebrada del 11 al 15 de abril de 2016 en Zaragoza (España)., The discovery of the giant magnetocaloric effect in Gd5Si2Ge2 has attracted a strong interest in the study of other R5(SiGe)4-type (R= rare earth) compounds in order to understand the relationship between their structural and magnetic properties. The extraordinary sensitivity of the electronic and crystal structures to external parameters such as the application of hydrostatic pressure have led to a systematic investigation over the last years in order to comprehend the microscopic processes taking place in the 5:4 materials. Experimental observations of a Griffiths-like phase (GP) in the paramagnetic (PM) phase at a characteristic temperature TG above the long-range order temperature (either TC or TN) were reported later in these compounds. The appearance of this regime has pointed to arise from the strong interplay between structure and magnetism present in these materials. According to a recent work, this behavior has been found to be restricted to 5:4 compounds which present the monoclinic Gd5Si2Ge2-type (M) or the orthorhombic Sm5Ge4-type (O(II)) of crystal structure at room temperature. The short-range magnetic correlations observed in the M-PM and O(II)-PM phases, an essential ingredient for the existence of Griffiths-like singularities, indicated that the system retains memory of the O(I) phase (pure state) signaled by an anomaly at TG. In this work we report on a detailed study of the pressure effect on the Griffiths-like phase in several compounds of the R5(Si1-xGex)4 ( R = Tb, Gd and Dy) system stable in the M - PM and O (II) – PM phases at room temperature. The selected compounds for this study are representative of O(II)-AFM (R = Gd), O(I)-FM (R = Tb) and O(I)- FM (R = Dy) ground states, respectively presenting GP in both AFM and FM states. For all the analyzed cases, low field - dc susceptibility measurements show that the GP persists at pressures up to 10 kbar. The value of TG, associated with the onset of short-range ferromagnetic correlations, increases linearly with pressure. However, large differences in the rate of change dTG/dP have been observed among them: The rate of the Tb - based compound is an order of magnitude greater than that of the Dy-based compound. Such a different pressure dependence of the Griffiths phase in these compounds could arise from the particular character of the clusters and the competition between the different structural and magnetic phases.

Epitaxial films of SrMnO3 and bilayers of SrMnO3/La0.67Sr0.33MnO3 have been deposited by pulsed laser deposition on different substrates, namely, LaAlO3 (001), (LaAlO3)0.3(Sr2AlTaO6)0.7 (001), and SrTiO3 (001), allowing us to perform an exhaustive study of the dependence of antiferromagnetic order and exchange bias field on epitaxial strain. The Néel temperatures (TN) of the SrMnO3 films have been determined by low-energy muon spin spectroscopy. In agreement with theoretical predictions, TN is reduced as the epitaxial strain increases. From the comparison with first-principles calculations, a crossover from G-type to C-type antiferromagnetic orders is proposed at a critical tensile strain of around 1.6±0.1%. The exchange bias (coercive) field, obtained for the bilayers, increases (decreases) by increasing the epitaxial strain in the SrMnO3 layer, following an exponential dependence with temperature. Our experimental results can be explained by the existence of a spin-glass (SG) state at the interface between the SrMnO3 and La0.67Sr0.33MnO3 films. This SG state is due to the competition between the different exchange interactions present in the bilayer and favored by increasing the strain in the SrMnO3 layer.

SIN FINANCIACIÓN No data (2015) UEM

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).-- et al., Magnetization studies have been performed on a polycrystalline sample of Dy5Si3Ge as a function of an applied magnetic field (up to 50 kOe) and hydrostatic pressure (up to 10 kbar) in the 5-300 K temperature range. The anomalous behavior of the magnetic susceptibility indicates that a Griffiths-like phase exists at low magnetic fields and pressures up to 10 kbar. We present evidence that the high-temperature second-order ferromagnetic transition can be coupled with the low-temperature first-order crystallographic transformation into a single first-order magnetic-crystallographic transformation using a magnetic field and hydrostatic pressure as tuning parameters. The effect of pressure on the Griffiths-like phase is reported and analyzed in the framework of the complex competition between the interslab and intraslab magnetic interactions. © 2013 American Physical Society., Work at the University of Zaragoza is supported by the Spanish Ministry of Science (MAT2011-27553-C02, MAT2011-27573-C04) and Spanish DGA (Grant No. E26). Work at the Ames Laboratory is supported by the Office of Basic Energy Sciences, Materials Sciences and Engineering Division of the Office of Science of the US Department of Energy. Ames Laboratory is operated by Iowa State University of Science and Technology for the US Department of Energy under Contract No. DE-AC02-07CH11358.

et al., Magnetic and magnetocaloric properties of single-crystalline Er 5Si 4 have been investigated as a function of the applied magnetic field (up to 50 kOe) and the hydrostatic pressure (up to 10 kbar) in the 5-300 K temperature range along the three main crystallographic directions. The magnetization isotherms show a highly anisotropic behavior with the easy-magnetization direction along the b axis for the low-pressure monoclinic and high-pressure orthorhombic structures, in good agreement with previous neutron scattering experiments. Below T C, the approach to the saturation shows a steplike behavior when the magnetic field is applied along the hard directions. The steps are sharper as the pressure increases. At constant magnetic field change, increasing the pressure induces a highly anisotropic enhancement of the magnetic entropy change. An enhancement of 20% is observed along the easy axis b, where the magnetic entropy change is maximum. The different evolution of the magnitude and temperature dependence of the magnetocaloric effect along the three crystallographic directions with pressure is discussed. © 2012 American Physical Society., Work at the University of Zaragoza was supported by the Spanish Ministry of Science (MAT2008-06567-C02) and Spanish DGA (Grant No. E26). N. Marcano acknowledges Spanish CSIC (JAE-doc program) for financial support. Work at the Ames Laboratory was supported by the Office of Basic Energy Sciences,Materials Sciences and EngineeringDivision of theOffice of Science of theUSDepartment of Energy. Ames Laboratory is operated by Iowa StateUniversity of Science and Technology for the US Department of Energy under Contract No. DE-AC02-07CH11358.

Resumen del trabajo presentado al 75th Annual Meeting of the DPG and DPG Spring Meeting, celebrado en Dresden (Alemania) del 13 al 18 de marzo de 2011., Different experimental techniques allowed us to identify a new magnetic configuration within the paramagnetic phase of ferrimagnet ErCo2. In this new magnetic ordering, coined as parimagnetism, the Co net magnetic moment is aligned antiparallel to the Er and to the applied field at temperatures well above T𝐶 and below certain flipping temperature. Magnetic short-range correlations of Co atoms within the parimagnetic state have been also established by means of small angle neutron scattering measurements. Thermopower and 𝜇SR spectroscopy measurements give also evidence about the existence of these strong correlated magnetic entities. Recently analysis carried on X-ray magnetic circular dichorism (XMCD) spectra taken in HoCo2 and DyCo2 have showed also the antiparallel arrangement of Co net magnetic moment respect to the rare earth in both compounds. The XMCD measurements allow us to propose a new magnetic phase diagram, for ErCo2, HoCo2 and DyCo2. These results suggest that parimagnetism is a more general phenomenon among this ferromagnetic family.

We present an overview of unconventional metallic states arising close to magnetic quantum critical points with a focus on d-electron systems. The applicability and potential breakdowns of traditional self-consistent field theories of such materials are discussed as well as related phenomena in other systems.

7 páginas, 6 figuras, 1 tabla.-- PACS number(s): 75.50.Gg, 71.20.Lp, 81.30.Bx, 76.75.+i, Zero-field muon spin relaxation spectroscopy (μSR) measurements in the paramagnetic phase of ErCo2 ferrimagnetic system are reported. Previous experimental studies using macro- and microscopic techniques led to the identification of magnetic short-range correlations between the Co atoms in the paramagnetic state of this material. The analysis of μSR spectra recorded on ErCo2 well above Tc indicates two dynamical magnetic processes characterized by remarkably different relaxation rates: a slow-relaxing signal, which quantitatively coincides with previous μSR studies on ErAl2, and fast-relaxing signal, which can be ascribed to the Co short-range correlated regions observed above Tc. Moreover, the μSR technique provides information on the thermal dependence of Co correlated regions at true zero field. These findings shed light on the dynamical nature of the clustered phase developing in the paramagnetic state of ErCo2., Financial support from the MAT08-01077 and Aragonese IMANA projects as well as from European FEDER funds is acknowledged. C.M.B. acknowledges a Spanish MICINN grant.