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Abstract: In this work we present the study of rare earth molybdates RE=(Eu, Tb and Ho) with formula RE2(MoO4)3. Under ambient conditions, Eu- and Tb-containing compounds can be found in the phases α and β0 , while holmium-containing can be found in the phase γ and β0 under these conditions. In the study of the β0 -Tb2(MoO4)3 compression by the CCDD group, the hypothesis of a transition to a new phase, called the phase δ, was considered. Subsequently, in the study of Y2(MoO4)3 synthesised unconventional conditions, non-stoichiometric oxide and molybdate phases with dif fractograms very similar to those of the phase δ were obtained. This opened the door to another possible hypothesis about the β0 → δ transition, that it was a decomposition induced by high pressure. For its verification, the synt hesis of the named compounds was carried out, modifying the solid state synthesis, applying a pressure of 0.66 GPa for compacting powder samples and increasing or decreasing the synthesis temperature, which was different for each case. We carry out a study of the crystalline structures of the most relevant phases involved: chelite-α, β-β0 and γ. Additionally, symmetry relationships provide clarity in understanding the phase transitions. A routine diffractogram was performed for each compound synthesised by using the X-ray diffractometer available at SIDIX (X-ray Facilitiy of the La Laguna Univer sity). Diffraction data collected under pressure were provided by the CCDD (group with which the supervisors of this work are researching). Note that, in addition to the β0 -Tb2(MoO4)3 data, β0 -phase of Ho and Eu molybdates data were also available. Using the ICSD database we simulated the different phases that would be expec ted to be found, and with which a visual identification of the phases was performed. Applying the Le Bail refinement method, the intensities of the full profile were refined as a verification of the existence of the expected phases. The synthesis of europium molybdate was carried out at 500oC, 550oC and 600oC. After the analysis and refinements, different mixtures of phases with struc tural types Sm2O3, MoO3, Eu4Mo7O27, Eu2Mo4O15 and the phase α-Eu2(MoO4)3 were detected. Furthermore, by analysing the pure β0 -Eu2(MoO4)3 phase under pres sure, it was observed that around 2.23 GPa a phase transition occurs, interpreted as a decomposition into the β0 , Eu2O3 and Eu2Mo4O15 phases, while at 5 GPa an amorphisation undergoes. Holmium oxide, β0 -Ho2(MoO4)3, nonstoichiometric Y2Mo4O15 phases were iden tified for the holmium molybdated synthetised at 600oC. Under pressure, as in the case of the europium molybdate, a phase transition occurs around 2.3 GPa in which the β0 -phase, the europium oxide and the Y2Mo4O15 phase are involved and the non-reversible amorphous phase starts at around 5 GPa. In these cases as well as the one studied by the CCDD group on Tb molybdate, when the decompression is carried out, the initial β0 -RE2(MoO4)3 phase is not com pletely recovered, so the phase transition is not reversible, which leads us to think that it is a decomposition induced by pressure. In addition to the experimental work and the analysis and discussion of the results, we would like to highlight the literature review carried out, which is a very important part of this dissertation. On the one hand, the research was contextualised, its interest was explained and an exhaustive description of the crystal structures of the materials studied was made. It was also necessary to: 1) review and introduce some crystallographic terms that were later used in the description of these structures. 2) Describe the experimental techniques used, reviewing their physical foundations. 3) Explain the tools used in the analysis of the data.

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Abstract: We have investigated the multiferroicity and magnetoelectric (ME) coupling in Ho Fe W O 6 . With a noncentrosymmetric polar structure (space group Pna 2 1 ) at room temperature, this compound shows an onset of electric polarization with an antiferromagnetic ordering at the Néel temperature ( T N ) of 17.8 K. The magnetic properties of the polycrystalline samples were studied by DC and AC magnetization and heat capacity measurements. The metamagnetic behavior at low temperatures was found to be directly related to the dielectric properties of the compound. In particular, field-dependent measurements of capacitance show a magnetocapacitance (MC) effect with double-hysteresis loop behavior in direct correspondence with the magnetization. Our x-ray diffraction results show the Pna 2 1 structure down to 8 K and suggest the absence of a structural phase transition across T N . Soft x-ray absorption spectroscopy and soft x-ray magnetic circular dichroism (XMCD) measurements at the Fe L 2 , 3 and Ho M 4 , 5 edges revealed the oxidation state of Fe and Ho cations to be 3 + . Fe L 2 , 3 XMCD further shows that Fe 3 + cations are antiferromagnetically ordered in a noncollinear fashion with spins arranged 90 ∘ with respect to each other. Our findings show that Ho Fe W O 6 is a type-II multiferroic exhibiting a MC effect. The observed MC effect and the change in polarization by the magnetic field, as well as their direct correspondence with magnetization, further support the strong ME coupling in this compound.

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Abstract: A ferromagnetic (FM) thin film deposited on a substrate of Pb ( Mg 1 / 3 Nb 2 / 3 ) O 3 − PbTiO 3 (PMN-PT) is an appealing heterostructure for the electrical control of magnetism, which would enable nonvolatile memories with ultralow-power consumption. Reversible and electrically controlled morphological changes at the surface of PMN-PT suggest that the magnetoelectric effects are more complex than the commonly used “strain-mediated” description. Here we show that changes in substrate morphology intervene in magnetoelectric coupling as a key parameter interplaying with strain. Magnetic-sensitive microscopy techniques are used to study magnetoelectric coupling in Fe/PMN-PT at different length scales, and compare different substrate cuts. The observed rotation of the magnetic anisotropy is connected to the changes in morphology, and mapped in the crack pattern at the mesoscopic scale. Ferroelectric polarization switching induces a magnetic field-free rotation of the magnetic domains at micrometer scale, with a wide distribution of rotation angles. Our results show that the relationship between the rotation of the magnetic easy axis and the rotation of the in-plane component of the electric polarization is not straightforward, as well as the relationship between ferroelectric domains and crack pattern. The understanding and control of this phenomenon is crucial to develop functional devices based on FM/PMN-PT heterostructures.

Open Access

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Abstract: Magnetic vortex cores in polycrystalline Ni discs underwent non-volatile displacements due to voltage-driven ferroelectric domain switching in single-crystal BaTiO3. This behaviour was observed using photoemission electron microscopy to image both the ferromagnetism and ferroelectricity, while varying in-plane sample orientation. The resulting vector maps of disc magnetization match well with micromagnetic simulations, which show that the vortex core is translated by the transit of a ferroelectric domain wall, and thus the inhomogeneous strain with which it is associated. The non-volatility is attributed to pinning inside the discs. Voltage-driven displacement of magnetic vortex cores is novel, and opens the way for studying voltage-driven vortex dynamics.

Open Access

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Abstract: Epitaxial films may be released from growth substrates and transferred to structurally and chemically incompatible substrates, but epitaxial films of transition metal perovskite oxides have not been transferred to electroactive substrates for voltage control of their myriad functional properties. Here we demonstrate good strain transmission at the incoherent interface between a strain-released film of epitaxially grown ferromagnetic La0.7Sr0.3MnO3 and an electroactive substrate of ferroelectric 0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 in a different crystallographic orientation. Our strain-mediated magnetoelectric coupling compares well with respect to epitaxial heterostructures, where the epitaxy responsible for strong coupling can degrade film magnetization via strain and dislocations. Moreover, the electrical switching of magnetic anisotropy is repeatable and non-volatile. High-resolution magnetic vector maps reveal that micromagnetic behaviour is governed by electrically controlled strain and cracks in the film. Our demonstration should inspire others to control the physical/chemical properties in strain-released epitaxial oxide films by using electroactive substrates to impart strain via non-epitaxial interfaces.