P Wagner

Influence of the cooperative Jahn-Teller effect on the transport- and magnetic properties of La 7/8 Sr 1/8 MnO 3 single crystals

The low-doped magnetic perovskite

Transport properties at the insulator–superconductor phase boundary of La2−xSrxCuO4 thin films

{\mathrm{La}}_{7/8}{\mathrm{Sr}}_{1/8}{\mathrm{MnO}}_{3}

Influence of the morphology on the magneto-transport properties of laser-ablated ultrathin La0.7Ba0.3MnO3 films

undergoes within the paramagnetic-semiconducting phase a first-order structural transition due to antiferrodistorsive ordering of Jahn-Teller deformed

Normal state resistivity of underdoped YBa2Cu3O7-d films and La2-XSrXCuO4 ultra -thin films in fields up to 60 T

{\mathrm{MnO}}_{6}

Orbital Ordering and the Cooperative Jahn–Teller Effect in Single Crystals of the Magnetic Perovskite La7/8Sr1/8MnO3

octahedra. This allows us to study not only the influence of the spin configuration on the magnetotransport properties [colossal magnetoresistance (CMR) effect] but also the role of orbital order and disorder. The orbital ordering transition (at 269 K in zero magnetic field) causes a doubling of the resistivity (regardless of the CMR effect in applied magnetic fields) and a drop of the paramagnetic susceptibility. The latter might be interpreted in terms of a shrinking of spin polarons. External magnetic fields shift the ordering transition to lower temperatures according to the field-induced decrease of the carrier localization. The magnetic-field\char21{}temperature phase boundary line was investigated by means of magnetoresistance (up to 12 T) and pulsed-fields magnetization measurements up to 50 T. The pronounced magnetization anomalies, associated with the phase transition, vanish for fields exceeding 20 T. This behavior has been attributed to a field-induced crossover from antiferrodistorsive order to a nondistorsive/ferromagnetic orbital configuration.

Colossal Negative Magnetoresistivity of Nd0.5Sr0.5MnO3 Films in Fields up to 50 T

Abstract The longitudinal resistivity and the transverse Hall resistivity of La1.94Sr0.06CuO4 and La1.97Sr0.03CuO4 epitaxial thin films are investigated. The zero-field resistivity ρ(T) of La1.94Sr0.06CuO4 shows a minimum at 80 K and a resistive midpoint of the superconducting transition at Tcmid≈7.5 K. A magnetic field of 8 T suppresses the superconducting state and induces a semiconducting normal state with ρ(T) increasing with decreasing temperature. The resistivity of La1.97Sr0.03CuO4 has a minimum at 100 K and no superconducting transition is observed down to 1.5 K. The semiconducting low-temperature behaviour can be described by Shlovskii–Efros hopping, while the metallic-like regime corresponds to 1D quantum transport of charge stripes. The field dependence of the Hall resistivity ρxy(B) has an opposite behaviour for the two doping levels: the ρxy(B) slope of La1.94Sr0.06CuO4 decreases with decreasing temperature, while it increases in the case of La1.97Sr0.03CuO4. Within the superconducting transition regime of La1.94Sr0.06CuO4 (below 15 K) we observe a sign inversion of ρxy(B) in the limit of weak magnetic fields.

Stripe formation and disorder induced stripe fragmentation

We investigate the thickness dependent properties of manganite films characterized by colossal negative magnetoresistance. Ultrathin, wedge-type films (0–120 A) of La0.7Ba0.3MnO3 were deposited by laser ablation onto SrTiO3 and LaAlO3 substrates. The films were patterned into strips of different thickness and magneto-transport measurements were performed at temperatures between 5 and 290 K and in magnetic fields up to 5 T. Atomic force- and transmission electron microscopy were done to correlate the microstructure with the transport data. The resistivity of the films increases slightly with decreasing thickness due to substrate-induced compressive strain. Below 50 A, the resistivity rises abruptly indicating a crossover to discontinuous and finally island-like film growth as confirmed by the microstructural techniques. At thicknesses slightly above the threshold for percolative conduction (≈30 A), an enhanced low-field magnetoresistance was observed as a signature of spin-dependent tunneling.

Hopping conductivity of magnetic polarons in epitaxial Pr0.5Sr0.5MnO3 films

Abstract We present the normal-state resistance of La 2-x Sr x CuO 4 thin films under epitaxial strain, measured below T c by applying pulsed fields up to 60 T. We compare these data with earlier data on YBa 2 Cu 3 O x thin films in the underdoped regime. The data are analyzed in terms of the recently proposed 1D quantum transport model and charge-stripe models. The high field data have been used to identify the new regimes and dimensional crossovers caused by the formation of stripes and their interplay with disorder.

Scaling of the transport properties in the Y 1 − y Pr y Ba 2 Cu 3 O x system

In the low-doped magnetic perovskite La7/8Sr1/8MnO3, a strong coupling exists between crystal structure and magnetic and electronic properties. At room temperature, the material is in a paramagnetic-semiconducting state, showing a dynamic Jahn–Teller (DJT) ordering of the Mn3+ ions. Below TJT=269 K however, the crystal goes into a cooperative Jahn–Teller (CJT) state, exhibiting antiferrodistorsive orbital ordering of the MnO6 octahedra. This leads to an increase in resistivity by a factor of 2 and to a drop of the paramagnetic susceptibility by about 20% [Physica B 155 (1989) 362; Phys. Rev. Lett. 71 (1993) 2331]. Around TC=188 K, a ferromagnetic transition occurs while the CJT effect becomes gradually weaker. Below TC the sample is metallic-like and shows a considerable colossal negative magnetoresistance (CMR) effect. Finally, the material goes into a charge-ordered, insulating state below TCO=147 K where the CJT effect completely vanishes. When crossing the phase boundary from the DJT to the CJT state, the resistivity shows a jump-like increase by a factor of roughly 2.15. This results from a frustration of the charge transport at the onset of orbital ordering. By assuming that each Mn4+ ion is surrounded only by Mn3+ nearest-neighbors and by taking into account only charge transport along the principal axes, the square-diagonals and the cube-diagonals, we are able to calculate a jump ratio of 2.26, corresponding well to the experimental value. The CJT transition also causes a sudden decrease in the magnetic moment, which has been studied by performing field-dependent SQUID measurements around TJT. Since the crystal is in a paramagnetic state around TJT, the magnetization M scales with the Brillouin function β(J), where J is the spin of small ferromagnetic clusters. Fitting the magnetization data with the Brillouin function results in a CJT-induced shrinking of magnetic clusters from roughly four Mn ions to three Mn ions. These cluster sizes can also be understood in terms of the orbital structure of the material in both phases.

Anomalous hall effect in thin films of Pr0.5Sr0.5MnO3

The low carrier mobility found in Mn perovskites implies that the dominant conductivity mechanism is related to Mott hopping. As a modification to the original model, we take into account that the hopping barrier is influenced by the relative spin orientation at the two Mn ions involved in an elemental hopping process. From this, we deduce a scaling behavior of the colossal negative magnetoresistivity according to the Brillouin function in the ferromagnetic-quasi-metallic state and proportional to a squared Brillouin function in the paramagnetic-semiconducting phase. Both predictions could be verified by pulsed magnetic field measurements up to 50 T.