Manuel Angst

Three-dimensional magnetic correlations in multiferroic LuFe2O4.

We present single crystal neutron diffraction measurements on multiferroic LuFe(2)O(4). Magnetic reflections are observed below transitions at 240 and 175 K indicating that the magnetic interactions in LuFe(2)O(4) are three-dimensional in character. The magnetic structure is refined as a ferrimagnetic spin configuration below the 240 K transition. Below 175 K a significant broadening of the magnetic peaks is observed along with the buildup of a diffuse component to the magnetic scattering.

Charge order in LuFe2O4: antiferroelectric ground state and coupling to magnetism

X-ray scattering by multiferroic LuFe2O4 is reported. Below 320 K, superstructure reflections indicate an incommensurate charge order with propagation close to (1/3 1/3 3/2). The corresponding charge configuration, also found by electronic structure calculations as most stable, contains polar Fe/O double layers with antiferroelectric stacking. Diffuse scattering at 360 K, with (1/3 1/3 0) propagation, indicates ferroelectric short-range correlations between neighboring double layers. The temperature dependence of the incommensuration indicates that charge order and magnetism are coupled.

Competing ferri- and antiferromagnetic phases in geometrically frustrated LuFe2O4.

We present a detailed study of magnetism in LuFe(2)O(4), combining magnetization measurements with neutron and soft x-ray diffraction. The magnetic phase diagram in the vicinity of T(N) involves a metamagnetic transition separating an antiferro- and a ferrimagnetic phase. For both phases the spin structure is refined by neutron diffraction. Observed diffuse magnetic scattering far above T(N) is explained in terms of near degeneracy of the magnetic phases.

Imaging and manipulation of the competing electronic phases near the Mott metal-insulator transition

The complex interplay between the electron and lattice degrees of freedom produces multiple nearly degenerate electronic states in correlated electron materials. The competition between these degenerate electronic states largely determines the functionalities of the system, but the invoked mechanism remains in debate. By imaging phase domains with electron microscopy and interrogating individual domains in situ via electron transport spectroscopy in double-layered Sr3(Ru1-xMnx)2O7 (x = 0 and 0.2), we show in real-space that the microscopic phase competition and the Mott-type metal-insulator transition are extremely sensitive to applied mechanical stress. The revealed dynamic phase evolution with applied stress provides the first direct evidence for the important role of strain effect in both phase separation and Mott metal-insulator transition due to strong electron-lattice coupling in correlated systems.

Site-selective oxygen isotope effect on the magnetic-field penetration depth in underdoped Y0.6Pr0.4Ba2Cu3O7-δ

We report site-selective oxygen isotope ( 1 6 O/ 1 8 O) effect (OIE) measurements on the in-plane penetration depth λ a b in underdoped Y 0 . 6 Pr 0 . 4 Ba 2 Cu 3 O 7 - δ , using the muon-spin rotation technique. A pronounced OIE on the transition temperature T c as well as on λ a b 2(0) was observed, which mainly arises from the oxygen sites within the superconducting CuO 2 planes (100% within error bar). The values of the corresponding relative isotope shifts were found to be ΔT c /T c =-3.7(4)% and Δλ - 2 ab(0)/λ - 2 ab(0)=-6.2(1.0)%. Our results imply that in this compound the phonon modes involving the movement of planar oxygen are dominantly coupled to the electrons.

Magnetism and cluster glass dynamics in geometrically frustrated LuFe2O4

We report on the magnetic properties of high quality LuFe2O4 single crystals grown by the floating zone method. dc and ac susceptibility measurements and analysis reveal a ferrimagnetic transition at ∼240 K followed by a re-entrant cluster glass transition below 225 K, with an additional magnetic transition around 170 K. Strong frequency dependence of the real (χ′) and imaginary (χ″) parts of the ac susceptibility observed at both these temperatures indicate glassy behavior and we quantitatively fit the data to a cluster glass model, τ=τo(Tf/Tg−1)−zv. Our studies show that these multiple transitions are consistent with the picture of ferrimagnetic clusters in the iron oxide planes with triangular lattice configuration favoring spin frustration and glass dynamics.

Charge Order Superstructure with Integer Iron Valence in Fe2OBO3

Solution-grown single crystals of Fe(2)OBO(3) were characterized by specific heat, Mössbauer spectroscopy, and x-ray diffraction. A peak in the specific heat at 340 K indicates the onset of charge order. Evidence for a doubling of the unit cell at low temperature is presented. Combining structural refinement of diffraction data and Mössbauer spectra, domains with diagonal charge order are established. Bond-valence-sum analysis indicates integer valence states of the Fe ions in the charge ordered phase, suggesting Fe(2)OBO(3) is the clearest example of ionic charge order so far.

Incommensurate Charge Order Phase in Fe2OBO3 due to Geometrical Frustration

The temperature dependence of charge order in Fe2OBO3 was investigated by resistivity and differential scanning calorimetry measurements, Mössbauer spectroscopy, and synchrotron x-ray scattering, revealing an intermediate phase between room temperature and 340 K, characterized by coexisting mobile and immobile carriers, and by incommensurate superstructure modulations with temperature-dependent propagation vector (1/2, 0, tau). The incommensurate modulations arise from specific antiphase boundaries with low energy cost due to geometrical charge frustration.

The charge ordering transition as probed by ultrasound

Charge ordering (CO) phenomena are found in many correlated electron systems but in general are poorly understood. A classical example of CO is found in magnetite, Fe3O4, which undergoes a CO transition (the so‐called Verwey transition) at 120 K. Magnetite is a poor model system for studying CO; however, despite nearly 70 years of study, there is still no generally accepted description of the transition. In recent years several new materials that display CO transitions have been discovered, including Fe2OBO3 and LuFe2O4. These new materials are proving to be far better model systems for studying CO than magnetite. In this talk the Landau theory of CO transitions will be reviewed, and the behavior of the elastic response of LuFe2O4 will be discussed in this context. [Work at ORNL supported by DOE BES Division of Materials Science and Engineering. Work at UT supported by NSF DMR‐0506292.]