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Abstract: We present the synthesis of a novel binary metal oxide material: Ba7Mn4O15. The crystal structure has been investigated by high-resolution powder synchrotron X-ray diffraction in the temperature range of 100–300 K as well as by powder neutron diffraction at 10 and 80 K. This material represents an isostructural barium-substituted analogue of the layered material Sr7Mn4O15 that forms its own structural class. However, we find that Ba7Mn4O15 adopts a distinct magnetic ordering, resulting in a magnetoelectric ground state below 50 K. The likely magnetoelectric coupling mechanisms have been inferred from performing a careful symmetry-adapted refinement against the powder neutron diffraction experiments, as well as by making a comparison with the nonmagnetoelectric ground state of Sr7Mn4O15.

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Abstract: The authors describe and compare two complementary techniques that are habitually used to image ferromagnetic and ferroelectric materials with sub-micron spatial resolutions (typically 50 nm, at best 10 nm). The first technique is variable-temperature photoemission electron microscopy with magnetic/antiferromagnetic/polar contrast from circularly/linearly polarized incident X-rays (XPEEM). The second technique is magnetic force microscopy (MFM). Focusing mainly on the authors’ own work, but not exclusively, published/unpublished XPEEM and MFM images of ferroic domains and complex magnetic textures (involving vortices and phase separation) are presented. Highlights include the use of two XPEEM images to create 2D vector maps of in-plane (IP) magnetization, and the use of imaging to detect electrically driven local reversals of magnetization. The brief and simple descriptions of XPEEM and MFM should be useful for beginners seeking to employ these techniques in order to understand and harness ferroic materials.

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Abstract: Matter-light interaction is at the center of diverse research fields from quantum optics to condensed matter physics, opening new fields like laser physics. A magnetic exciton is one such rare example found in magnetic insulators. However, it is relatively rare to observe that external variables control matter-light interaction. Here, we report that the broken inversion symmetry of multiferroicity can act as an external knob enabling the magnetic exciton in van der Waals antiferromagnet NiI2. We further discover that this magnetic exciton arises from a transition between Zhang-Rice-triplet and Zhang-Rice-singlet's fundamentally quantum entangled states. This quantum entanglement produces an ultra-sharp optical exciton peak at 1.384 eV with a 5 meV linewidth. Our work demonstrates that NiI2 is two-dimensional magnetically ordered with an intrinsically quantum entangled ground state.

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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.