Tilt engineering of spontaneous polarization and magnetization above 300 K in a bulk layered perovskite

Crystalline materials that combine electrical polarization and magnetization could be advantageous in applications such as information storage, but these properties are usually considered to have incompatible chemical bonding and electronic requirements. Recent theoretical work on perovskite materials suggested a route for combining both properties. We used crystal chemistry to engineer specific atomic displacements in a layered perovskite, (CaySr1–y)1.15Tb1.85Fe2O7, that change its symmetry and simultaneously generate electrical polarization and magnetization above room temperature. The two resulting properties are magnetoelectrically coupled as they arise from the same displacements

High Bi content GaSbBi alloys

The epitaxial growth, structural, and optical properties of GaSb 1– x Bi x alloys have been investigated. The Bi incorporation into GaSb is varied in the range 0 < x ≤ 9.6% by varying the growth rate (0.31–1.33 μm h−1) at two growth temperatures (250 and 275 °C). The Bi content is inversely proportional to the growth rate, but with higher Bi contents achieved at 250 than at 275 °C. A maximum Bi content of x = 9.6% is achieved with the Bi greater than 99% substitutional. Extrapolating the linear variation of lattice parameter with Bi content in the GaSbBi films enabled a zinc blende GaBi lattice parameter to be estimated of 6.272 Å. The band gap at 300 K of the GaSbBi epitaxial layers decreases linearly with increasing Bi content down to 410 ± 40 meV (3 μm) for x = 9.6%, corresponding to a reduction of ∼35 meV/%Bi. Photoluminescence indicates a band gap of 490 ± 5 meV at 15 K for x = 9.6%

Chemically Controllable Magnetic Transition Temperature and Magneto-Elastic Coupling in MnZnSb Compounds

International audienceMagneto-caloric materials offer the possibility to design environmentally friendlier thermal management devices compared to the widely used gas-based systems. The challenges to develop this solid-state based technology lie in the difficulty of finding materials presenting a large magneto-caloric effect over a broad temperature span together with suitable secondary appli-cation parameters such as low heat capacity and high thermal conductivity. A series of compounds derived from the PbFCl structure is investigated using a combination of computational and experimental methods focusing on the change of cell volume in magnetic and non-magnetic ground states. Scaling analysis of the magnetic properties determines that they are second order phase transition ferromagnets and that the magnetic entropy change is driven by the coupling of magneto-elastic strain in the square-net through the magnetic transition determined from neutron and synchrotron X-ray diffraction. The primary and secondary application related properties are measured experimentally, and the c/a parameter is identified as an accurate proxy to control the magnetic transition. Chemical substitution on the square-net affords tuning of the Curie temperature over a broad temperature span between 252 and 322 K. A predictive machine learning model for the c/aparameter is developed to guide future exploratory synthesis

Sn 5 s 2 lone pairs and the electronic structure of tin sulphides: A photoreflectance, high-energy photoemission, and theoretical investigation

The effects of Sn 5 s lone pairs in the different phases of Sn sulphides are investigated with photoreflectance, hard x-ray photoemission spectroscopy (HAXPES), and density functional theory. Due to the photon energy-dependence of the photoionization cross sections, at high photon energy, the Sn 5 s orbital photoemission has increased intensity relative to that from other orbitals. This enables the Sn 5 s state contribution at the top of the valence band in the different Sn-sulphides, SnS, Sn 2 S 3 , and SnS 2 , to be clearly identified. SnS and Sn 2 S 3 contain Sn(II) cations and the corresponding Sn 5 s lone pairs are at the valence band maximum (VBM), leading to ∼ 1.0 –1.3 eV band gaps and relatively high VBM on an absolute energy scale. In contrast, SnS 2 only contains Sn(IV) cations, no filled lone pairs, and therefore has a ∼ 2.3 eV room-temperature band gap and much lower VBM compared with SnS and Sn 2 S 3 . The direct band gaps of these materials at 20 K are found using photoreflectance to be 1.36, 1.08, and 2.47 eV for SnS, Sn 2 S 3 , and SnS 2 , respectively, which further highlights the effect of having the lone-pair states at the VBM. As well as elucidating the role of the Sn 5 s lone pairs in determining the band gaps and band alignments of the family of Sn-sulphide compounds, this also highlights how HAXPES is an ideal method for probing the lone-pair contribution to the density of states of the emerging class of materials with n s 2 configuration

Discovery of a Low Thermal Conductivity Oxide Guided by Probe Structure Prediction and Machine Learning

International audienceWe report the aperiodic titanate Ba10Y6Ti4O27 with a room-temperature thermal conductivity that equals the lowest reported for an oxide. The structure is characterised by discontinuous occupancy modulation of each of the sites and can be considered as a quasicrystal. The resulting localisation of lattice vibrations suppresses phonon transport of heat. This new lead material for low-thermal-conductivity oxides is metastable and located within a quaternary phase field that has been previously explored. Its isolation thus requires a precisely defined synthetic protocol. The necessary narrowing of the search space for experimental investigation was achieved by evaluation of titanate crystal chemistry, prediction of unexplored structural motifs that would favour synthetically accessible new compositions, and assessment of their properties with machine-learning models

Magnetic semiconductors for spin electronics

THESIS 9346The development of the relatively new field of spin electronics is based on the possibility of manipulating the spin of the electron through a structure. If one aspect is to develop new device architecture to inject, transmit, and detect spin polarised current, the materials involved in the fabrication of these structures plays a major role. In fact, if most of the standard electronic devices have been successfully translated in spin-based electronic devices, the magnetic bipolar transistor has not been demonstrated. The major obstacle for the construction of such device is the lack of a ferromagnetic semiconductor that can be bipolar. Other materials such as half metals have been studied extensively as potential spin polariser. The experiments described in this thesis are aimed to synthesise and characterise compoimds that have a strong potential to be integrated in spin electronic structures. It includes materials with a high Curie temperature, metallic conduction and a relatively high spin polarisation and ferromagnetic semiconductor. The fundamental properties of these materials have been studied thoroughly as well as the magnetic properties of diluted magnetic oxide

Persistence of ferroelectricity close to unit-cell thickness in structurally disordered Aurivillius phases

Multiferroics intertwine ferroelectric and ferromagnetic properties, allowing for novel ways of manipulating data and storing information. To optimize the unique Bi6TixFeyMnzO18 (B6TFMO), multiferroic, ultrathin (<7 nm) epitaxial films were synthesized by direct liquid injection chemical vapor deposition (DLI-CVD). Epitaxial growth is, however, confounded by the volatility of bismuth, particularly when utilizing a postgrowth anneal at 850 °C. This results in microstructural defects, intergrowths of differing Aurivillius phases, and formation of impurities. Improved single-step DLI-CVD processes were subsequently developed at 710 and 700 °C, enabling lowering of crystallization temperature by 150 °C and significantly enhancing film quality and sample purity. Ferroelectricity is confirmed in 5 nm (1 unit-cell thick) B6TFMO films, with tensile epitaxial strain enhancing the piezoresponse. In-plane ferroelectric switching is demonstrated at 1.5 unit-cell thickness. The persistence of stable ferroelectricity near unit-cell thickness in B6TFMO, both in-plane and out-of-plane, is significant and initiates possibilities for miniaturizing novel multiferroic-based devices

Growth of M-type hexaferrite thin films with conical magnetic structure

Thin films of the M-type hexaferrite, BaFe10.2Sc1.8O19, have been grown on Al2O3 (00.1) substrates by pulsed laser deposition. Post-deposition annealing improves the structural quality and produces completely relaxed thin films. The post-annealed films show magnetic behavior corresponding to a conical magnetic structure, which is required to establish the magnetoelectric effect in hexaferrites. The magnetic phase diagram has been obtained from hard magnetization curves. Finite-size effects due to the restricted length scale of the magnetic helix explain differences in magnetic properties between thin films and the bulk

Magnetic, electronic, and thermal properties of buckled kagome Fe3Ge2Sb

The magnetic, electronic, and thermal properties of Fe3Ge2Sb single crystals, a derivative of the hexagonal FeGe structure with a buckled Fe kagome net and Sb-Sb dimers are reported. Electronic structure calculations show most of the kagome-derived bands remain intact with the Fe buckling, with the exception of a van Hove singularity near the Fermi level, which is selectively destroyed. This selective destruction of the van Hove singularity is associated with the lack of a charge order transition in Fe3Ge2Sb compared to FeGe. Magnetization measurements show two antiferromagnetic transition at 290 K and 16 K. The low-temperature transition is attributed to spin canting and is associated with a metamagnetic transition observed in the isothermal magnetization below the transition temperature. Electrical and thermal transport measurements show metallic behavior, and more significant magnetic scattering associated with the metamagnetic transition is observed in the magnetothermal conductivity compared to the magnetoresistance. This is consistent with a modification of the long-period magnetic structure that modifies preferentially small angle scattering thus having a strong impact on thermal transport properties. We conclude that buckled Fe3Ge2Sb exhibits similar properties to unbuckled hexagonal FeGe with the exception of the lack of a charge density wave transition in Fe3Ge2Sb, likely due to the selective destruction of the van Hove singularity near EF, making the family of compounds Fe3Ge3-xSbx a good host to study various physical effects in kagome metals, especially the electronic and structural stabilization of charge ordered states