J. Alaria

Charge-transfer ferromagnetism in oxide nanoparticles

A new model is proposed for ferromagnetism associated with defects in the bulk or at the surface of nanoparticles. The basic idea is that a narrow, structured local density of states Ns(E) is associated with the defects, but the Fermi level (which may lie above or below a mobility edge) will not normally coincide with a peak in Ns(E). However, if there is a local charge reservoir, such as a dopant cation coexisting in two different charge states or a charge-transfer complex at the surface, then it may be possible for electron transfer to raise the Fermi level to a peak in the local density of states, leading to Stoner splitting of Ns(E). Spontaneous Stoner ferromagnetism can arise in percolating defect-rich regions, such as the nanoparticle surface. The charge-transfer ferromagnetism model may be applicable to a wide range of nanoparticles and thin films of dilute magnetic oxides have previously been regarded as dilute magnetic semiconductors.

Absence of ferromagnetism in Al-doped Zn0.9Co0.10O diluted magnetic semiconductors

A 0.5% Al-doped Zn0.895Co0.100O polycrystalline powder was synthesized by the co-precipitation method. Raman spectroscopy revealed that divalent cobalt ions were substituted for Zn2+ ions into the ZnO matrix and that Al ions activate additional modes which are nonspecific to the dopant. These additional modes disappear after annealing at 1373K for 1h under Ar flow. We suggest that the electrical dopant becomes active in substitutional sites after annealing. Resistance measurements confirm that free carriers were created in our sample. Nevertheless, the sample shows the same magnetic properties: a mixture of paramagnetism and antiferromagnetism.

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

Tilting toward two properties Opposing electronic and symmetry constraints can make it difficult to combine some pairs of material properties in a single crystalline material. Magnetization and electrical polarization are such a pair, but their combination could be useful for applications such as magnetoelectric information storage. Pitcher et al. now show that careful design of chemical substitutions in a layered perovskite are both electrically polar and weakly ferromagnetic at temperatures up to 330 K. Science, this issue p. 420 Chemical substitutions produce atomic displacements in a crystal that lead to both electrical polarization and magnetization. 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.

Pure paramagnetic behavior in Mn-doped ZnO semiconductors

Polycrystaline Mn-doped zinc oxides (Zn1−xMnxO) were synthesized by coprecipitation method with x varying between 0.01 and 0.1. Raman spectroscopy indicates the appearance of an additional mode which is an indicator for the incorporation of Mn ions into the ZnO host matrix. The magnetic properties have been studied by electron paramagnetic resonance (EPR) spectroscopy. The Mn-related EPR spectra exhibit the expected pattern for isolated Mn ions. Temperature dependence of the reciprocal integrated EPR signal follows a Curie law indicating a typical paramagnetic behavior for x⩽0.05.

Improved electrical mobility in highly epitaxial La:BaSnO3 films on SmScO3(110) substrates

Heteroepitaxial growth of BaSnO3 and Ba1−xLaxSnO3 (x = 7%) lanthanum doped barium stannate thin films on different perovskite single crystal (SrTiO3 (001) and SmScO3 (110)) substrates has been achieved by pulsed laser deposition under optimized deposition conditions. X-ray diffraction measurements indicate that the films on either of these substrates are relaxed due to the large mismatch and present a high degree of crystallinity with narrow rocking curves and smooth surface morphology while analytical quantification by proton induced X-ray emission confirms the stoichiometric La transfer from a polyphasic target, producing films with measured La contents above the bulk solubility limit. The films show degenerate semiconducting behavior on both substrates, with the observed room temperature resistivities, Hall mobilities, and carrier concentrations of 4.4 mΩ cm, 10.11 cm2 V−1 s−1, and 1.38 × 1020 cm−3 on SmScO3 and 7.8 mΩ cm, 5.8 cm2 V−1 s−1, and 1.36 × 1020 cm−3 on SrTiO3 ruling out any extrinsic contribution from the substrate. The superior electrical properties observed on the SmScO3 substrate are attributed to reduction in dislocation density from the lower lattice mismatch.

Designing switchable polarization and magnetization at room temperature in an oxide

Ferroelectric and ferromagnetic materials exhibit long-range order of atomic-scale electric or magnetic dipoles that can be switched by applying an appropriate electric or magnetic field, respectively. Both switching phenomena form the basis of non-volatile random access memory, but in the ferroelectric case, this involves destructive electrical reading and in the magnetic case, a high writing energy is required. In principle, low-power and high-density information storage that combines fast electrical writing and magnetic reading can be realized with magnetoelectric multiferroic materials. These materials not only simultaneously display ferroelectricity and ferromagnetism, but also enable magnetic moments to be induced by an external electric field, or electric polarization by a magnetic field. However, synthesizing bulk materials with both long-range orders at room temperature in a single crystalline structure is challenging because conventional ferroelectricity requires closed-shell d0 or s2 cations, whereas ferromagnetic order requires open-shell dn configurations with unpaired electrons. These opposing requirements pose considerable difficulties for atomic-scale design strategies such as magnetic ion substitution into ferroelectrics. One material that exhibits both ferroelectric and magnetic order is BiFeO3, but its cycloidal magnetic structure precludes bulk magnetization and linear magnetoelectric coupling. A solid solution of a ferroelectric and a spin-glass perovskite combines switchable polarization with glassy magnetization, although it lacks long-range magnetic order. Crystal engineering of a layered perovskite has recently resulted in room-temperature polar ferromagnets, but the electrical polarization has not been switchable. Here we combine ferroelectricity and ferromagnetism at room temperature in a bulk perovskite oxide, by constructing a percolating network of magnetic ions with strong superexchange interactions within a structural scaffold exhibiting polar lattice symmetries at a morphotropic phase boundary (the compositional boundary between two polar phases with different polarization directions, exemplified by the PbZrO3–PbTiO3 system) that both enhances polarization switching and permits canting of the ordered magnetic moments. We expect this strategy to allow the generation of a range of tunable multiferroic materials.

Conventional and inverse magnetocaloric effects in La0.45Sr0.55MnO3 nanoparticles

The magnetocaloric effect of La0.45Sr0.55MnO3 nanoparticles was studied by dc magnetization measurements. A sample with mean particle size of about 140 nm exhibits both a conventional magnetocaloric effect around the Curie temperature (≈ 295 K) and a large inverse magnetocaloric effect around the antiferromagnetic-ferromagnetic transition temperature (≈ 200 K). The change of magnetic entropy increases monotonically with applied magnetic field and reaches the values of 5.51 J/kg K and − 2.35 J/kg K at 200 K and 295 K, respectively, in an applied field of 5 T. The antiferromagnetic-ferromagnetic transition is absent in a 36 nm size sample, which shows only a broad ferromagnetic transition around 340 K and a small change in magnetic entropy near room temperature. The results are discussed in terms of the entropy difference between the A-type antiferromagnetic ground state of La0.45Sr0.55MnO3 and the low moment ferromagnetic state. By comparing the results obtained on nanoparticles and bulk La0.45Sr0.55MnO3, ...

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 A. 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%.

Interface control by chemical and dimensional matching in an oxide heterostructure

Interfaces between different materials underpin both new scientific phenomena, such as the emergent behaviour at oxide interfaces, and key technologies, such as that of the transistor. Control of the interfaces between materials with the same crystal structures but different chemical compositions is possible in many materials classes, but less progress has been made for oxide materials with different crystal structures. We show that dynamical self-organization during growth can create a coherent interface between the perovskite and fluorite oxide structures, which are based on different structural motifs, if an appropriate choice of cations is made to enable this restructuring. The integration of calculation with experimental observation reveals that the interface differs from both the bulk components and identifies the chemical bonding requirements to connect distinct oxide structures.

Bi-induced band gap reduction in epitaxial InSbBi alloys

The properties of molecular beam epitaxy-grown InSb 1− x Bi x alloys are investigated. Rutherford backscattering spectrometry shows that the Bi content increases from 0.6% for growth at 350 °C to 2.4% at 200 °C. X-ray diffraction indicates Bi-induced lattice dilation and suggests a zinc-blende InBi lattice parameter of 6.626 A. Scanning electron microscopy reveals surface InSbBi nanostructures on the InSbBi films for the lowest growth temperatures, Bi droplets at intermediate temperatures, and smooth surfaces for the highest temperature. The room temperature optical absorption edge was found to change from 172 meV (7.2 μm) for InSb to ∼88 meV (14.1 μm) for InSb 0.976Bi0.024, a reduction of ∼35 meV/%Bi.