Transition metal oxides are of great potential for technological applications since they show a wide range of extraordinary properties like multiferroic behavior, colossal magnetoresistance, and high-Tc superconductivity. The physics of these systems is governed by an intricate interplay of competing interactions among the charge, spin, and orbital degrees of freedom which may result in coexistence or competition of various types of ordered ground states. In order to map the hierarchy of these interactions the combined use of a broad range of experimental techniques is required. HZB houses a wide choice of x-ray and neutron based techniques that are ideally suited for the study of structural, electronic, and magnetic properties of materials. On the firm ground of state-of-the-art sample preparation and characterization we carry out neutron scattering, advanced photon spectroscopy, and resonant x-ray scattering experiments in extreme sample environments which provide combinations of very low temperatures, extremely high magnetic fields, high pressures and other external stimuli. Understanding the response of the investigated materials to variation of any external parameter is the great challenge in the study of multifunctional oxides.
In the following we present a few examples of research carried out within the last few years.
For a full list of publications of our research group klick here.
Observation of a Devil’s Staircase in the Novel Spin-Valve System SrCo6O11
Using resonant soft-x-ray scattering as a function of both temperature and magnetic field, we reveal a large number of almost degenerate magnetic orders in SrCo6O11. The Ising-like spins in this frustrated material in fact exhibit a so-called magnetic devil’s staircase. It is demonstrated how a magnetic field induces transitions between different microscopic spin configurations, which is responsible for the magnetoresistance of SrCo6O11. This material therefore constitutes a unique combination of a magnetic devil’s staircase and spin-valve effects, yielding a novel type of magnetoresistance system.
T Matsuda, S Partzsch, T Tsuyama, E Schierle, E Weschke, J Geck, T Saito, S Ishiwata, Y Tokura, and H Wadati, Phys. Rev. Lett. 114, 236403 (2015)
See also the associated highlight report
Competing Jahn-Teller distortions and ferrimagnetic ordering in the geometrically frustrated system Ni1−xCuxCr2O4
Competing Jahn-Teller distortions combined with geometrical frustration give rise to a rich phase diagram as a function of x(Cu) and temperature in the spinel system Ni1−xCuxCr2O4. The Jahn-Teller distortion of the end members acts in opposite ways, with an elongation of the NiO4 tetrahedra resulting in a structural transition at TS1 = 317K in NiCr2O4, but a flattening in the CuO4 tetrahedra at TS1 = 846K in CuCr2O4. In both cases the symmetry is lowered from cubic (Fd-3m) to tetragonal (I41/amd) on cooling. In order to follow the influence of Jahn-Teller active Ni2+ and Cu2+ ions on the structural and magnetic properties of chromium spinels, we have investigated a series of samples of Ni1−xCuxCr2O4 by x-ray and neutron powder diffraction.
M Reehuis, M Tovar, DM Többens, P Pattison, A Hoser, and B Lake, Physical Review B 91, 024407 (2015)
See also the associated highlight report
Competing Exchange Interactions on the Verge of a Metal-Insulator Transition in the Two-Dimensional Spiral Magnet Sr3Fe2O7
We have performed a neutron scattering study of the magnetic order and dynamics of the bilayer perovskite Sr3Fe2O7, which exhibits a temperature-driven metal-insulator transition at 340 K. We show that the Fe4+ moments adopt incommensurate spiral order below TN = 115 K and provide a comprehensive description of the corresponding spin-wave excitations. The observed magnetic order and excitation spectra can be well understood in terms of an effective spin Hamiltonian with interactions ranging up to third-nearest-neighbor pairs. The results indicate that the helical magnetism in Sr3Fe2O7 results from competition between ferromagnetic double-exchange and antiferromagnetic superexchange interactions whose strengths become comparable near the metal-insulator transition. They thus confirm a decades-old theoretical prediction and provide a firm experimental basis for models of magnetic correlations in strongly correlated metals.
J-H Kim, A Jain, M Reehuis, G Khaliullin, DC Peets, C Ulrich, JT Park, E Faulhaber, A Hoser, HC Walker, DT Adroja, AC Walters, DS Inosov, A Maljuk, B Keimer, Physical Review Letters 113, 147206 (2014)
Stability of spin-driven ferroelectricity in the thin-film limit: Coupling of magnetic and electric order in multiferroic TbMnO3 films
We demonstrate spin-spiral-induced ferroelectricity in epitaxial TbMnO3 films grown on YAlO3 substrates down to a film thickness of 6nm. The ferroelectric polarization is identified by optical second-harmonic generation. Using x-ray resonant magnetic scattering we directly prove the existence of a noncollinear magnetic structure in the ferroelectric phase and thus bulk-like multiferroicity. The electric-field-induced reversal of the magnetic domains along with the reversal of the ferroelectric polarization evidences the rigid coupling of magnetic and ferroelectric order and hence a “giant” magnetoelectric effect in the films.
A Glavic, C Becher, J Voigt, E Schierle, E Weschke, M Fiebig, T Brückel, Phys. Rev. B 88, 054401 (2013)
Spatial map of the circular XRMS dichroism measured at (0 τMn 0) at the Mn L edge of a TbMnO3 film of 100 nm. The sample was cooled with the x-ray beam at the “burn point” position (green ellipse approximates beam size) and measured at 11 K. The arrows indicate the electric-polarization direction associated with the respective cycloidal domain. © APS
Lattice Instability and Competing Spin Structures in the Double Perovskite Insulator Sr2FeOsO6
The semiconductor Sr2FeOsO6, depending on temperature, adopts two types of spin structures that differ in the spin sequence of ferrimagnetic iron-osmium layers along the tetragonal c axis. Neutron powder diffraction experiments 57Fe Mössbauer spectra, and density functional theory calculations suggest that this behavior arises because a lattice instability resulting in alternating iron-osmium distances fine-tunes the balance of competing exchange interactions. Thus, Sr2FeOsO6 is an example of a double perovskite, in which the electronic phases are controlled by the interplay of spin, orbital, and lattice degrees of freedom.
AK Paul, M Reehuis, V Ksenofontov, B Yan, A Hoser, DM Többens, PM Abdala, P Adler, M Jansen, C Felser Physical Review Letters 111 (2013)