D. Hohlwein

Magnetic phase control by an electric field.

The quest for higher data density in information storage is motivating investigations into approaches for manipulating magnetization by means other than magnetic fields. This is evidenced by the recent boom in magnetoelectronics and ‘spintronics’, where phenomena such as carrier effects in magnetic semiconductors and high-correlation effects in colossal magnetoresistive compounds are studied for their device potential. The linear magnetoelectric effect—the induction of polarization by a magnetic field and of magnetization by an electric field—provides another route for linking magnetic and electric properties. It was recently discovered that composite materials and magnetic ferroelectrics exhibit magnetoelectric effects that exceed previously known effects by orders of magnitude, with the potential to trigger magnetic or electric phase transitions. Here we report a system whose magnetic phase can be controlled by an external electric field: ferromagnetic ordering in hexagonal HoMnO3 is reversibly switched on and off by the applied field via magnetoelectric interactions. We monitor this process using magneto-optical techniques and reveal its microscopic origin by neutron and X-ray diffraction. From our results, we identify basic requirements for other candidate materials to exhibit magnetoelectric phase control.

Neutron diffraction study of the magnetic structure of α-Mn2O3

The magnetic ordering of α-Mn2O3 has been studied by neutron powder diffraction measurements. An antiferromagnetic ordering occurs with the magnetic unit cell equivalent to the crystalline unit cell. Our results are compared with the results of previous works. The main antiferromagnetic Bragg peaks have different temperature dependence of their intensities, suggesting that the magnetic ordering in α-Mn2O3 cannot be described by a single order parameter. The temperature dependence of magnetization near 0 K for these peaks differs from Bloch’s T3/2 law. The majority of the magnetic peaks disappear at 80 K, which was reported earlier as the Neel temperature. The (100) peak, forbidden in the crystal structure of α-Mn2O3 is present up to 100 K. A new collinear model of the magnetic ordering of α-Mn2O3 at 10 K is presented.

Structural changes of the superconductor YBa2Cu3O7 by cobalt substitution. A high-resolution neutron powder diffraction study

Abstract The structure of cobalt substituted YBa 2 Cu 3− x Co x O 7± z compounds with x =0.05, 0.1, 0.2, 0.3 and 0.65 was determined by high-resolution neutron powder diffraction at 5 and 300 K. The samples are superconducting up to x =0.3. YBa 2 Cu 2.95 Co 0.05 O 7 has orthorhombic structure with space group Pmmm; all compounds with x ⩾0.1 have tetragonal structure with space group P4/mmm. The Rietveld refinements show clearly that the Co-atoms occupy only the Cu(1)-sites at (0, 0, 0), i.e. copper chain sites in the orthorhombic structure, for the whole composition range. The characteristic structural changes as a function of cobalt concentration and temperature are discussed. Along the c -axis the Cu(1) to O(2) distance is decreasing while the Cu(2) to O(2) distance is increasing as in the Co-free, oxygen deficient compounds. A charge transfer from the Cu/Co chains to the Cu planes is discussed. For the nonsuperconducting YBa 2 Cu 2.35 Co 0.65 O 7.23 some magnetic peaks are observed.

A powder diffraction study of the phase transition in LaAlO3

The structure of LaAlO3 has been investigated around the phase transition at Tc ≈ 800 K by neutron powder diffraction in vacuum and by X-ray powder diffraction under nitrogen atmosphere as well as by a very high resolution synchrotron experiment in air. The results were analysed in frame of the Landau theory using the fluctuation-dissipation theorem to relate the susceptibility to the atomic displacement parameters. The room temperature structure is a rhombohedrally distorted perovskite structure, space group R-3c , which undergoes a transition to the ideal perovskite structure, space group Pm-3m , at high temperatures. The order parameter is a rotation of the O6-octahedron described by one x(O)-parameter. This parameter and the spontaneous strain (c/a-√6), as well as the relevant atomic displacement parameter Uop11(O) in the order parameter system, show a critical behaviour in agreement with a second order phase transition. Although the critical exponents of the order parameter and strain show the expected coupling behaviour, there is a striking difference of the transition temperature: the metric becomes cubic roughly 30 K below the proper Tc. This is related to spontaneous formation of domains imposing the average cubic symmetry via internal stresses.

Barium displacements in the superconducting ortho-II phase of YBa2Cu3O6.51: An X-ray structure determination

Abstract From the intensities of superstructure reflections the structure of the ortho-II phase in YBa 2 Cu 3 O 6.51(5) (T c =55.8 K ) was derived by single crystal X-ray diffraction. In the basal plane of the 2 a × b × c supercell the oxygen atoms order in alternating CuOCu and CuCu chains. The correlation lengths of the ortho-II domains are 68 A, 520 A, 65 A in a , b , and c - direction. Due to the oxygen order Ba atoms are displaced in an antiferrodistortive way towards the CuO chains by 0.03 A. In this way the Coulomb energy of the ortho-II structure is reduced. The antiferrodistortive displacements of the Ba atoms could be important for the superconducting pairing mechanism.

Structural and magnetic phase transitions in TbPO4 studied by neutron diffraction

Neutron diffraction experiments on TbPO4 single crystals have been performed in the temperature range from 1.35 to 294 K. We observe two phase transitions: the onset of antiferromagnetic ordering along the tetragonalc-axis at 2.28 K and tilting of the moments away from thec-axis below 2.15 K. The analysis of the measured reflection profiles shows that the tilting is connected with a distortion of the tetragonal zircon structure.

X-ray superstructure determination of ordered oxygen and Cu-displacements in a single domain of YBa2Cu3O6.35

AbstractThe superstructure of ordered oxygen atoms in a single domain of YBa2Cu3O6.35 has been determined by X-ray diffraction. The 45o rotated superstructure cell is orthorhombic—spacegroup Pbam, lattice constants

The defect structure of Y- and N-doped zirconia

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