G. Allodi

Nanoscopic coexistence of magnetism and superconductivity in YBa2Cu3O6+x detected by muon spin rotation.

We performed zero and transverse field muon spin rotation experiments on a large number of YBa2Cu3O6+x samples. We detect the coexistence of antiferromagnetic (AF) short range magnetism with superconductivity below T(f) < or = 10 K in compositions 0.37 < or = x < or = 0.39. Most muons experience local AF fields, even when a SQUID detects a full superconducting volume fraction, which points to a local minimal interference organization of short AF stripes embedded in the superconductor. A detailed phase diagram is produced and the consequences of the minimal interference are discussed.

Preparation and characterization of LaMnO3 thin films grown by pulsed laser deposition

We have grown LaMnO3 thin films on (001) LaAlO3 substrates by pulsed laser deposition. X-ray diffraction confirms that the films are only slightly relaxed and are oriented “square on square” relative to the substrate. The measured Raman spectra closely resemble that observed in bulk LaMnO3, which indicates no relevant distortions of the MnO6 octahedra induced by the epitaxial strain. Therefore, no detectable changes in the lattice dynamics occurred in our LaMnO3 strained films relative to the bulk case. Mn55 nuclear magnetic resonance identifies the presence of localized Mn4+ states. Superconducting quantum interference device magnetization measures TN=131(3)K and a saturation moment μ=1.09μB∕Mn, revealing a small concentration of Mn4+ and placing our films within the antiferromagnetic insulating phase.

Correlated trends of coexisting magnetism and superconductivity in optimally electron-doped oxypnictides.

We report on the recovery of the short-range static magnetic order and on the concomitant degradation of the superconducting state in optimally F-doped SmFe(1-x)Ru(x)AsO(0.85)F(0.15) for 0.1≤x≲0.5. The two reduced order parameters coexist within nanometer-size domains in the FeAs layers and eventually disappear around a common critical threshold x(c)~0.6. Superconductivity and magnetism are shown to be closely related to two distinct well-defined local electronic environments of the FeAs layers. The two transition temperatures, controlled by the isoelectronic and diamagnetic Ru substitution, scale with the volume fraction of the corresponding environments. This fact indicates that superconductivity is assisted by magnetic fluctuations, which are frozen whenever a short-range static order appears, and totally vanish above the magnetic dilution threshold x(c).

Ultraslow Polaron Dynamics in Low-Doped Manganites fromL139aNMR-NQR and Muon Spin Rotation

We report a (139)La NMR investigation of low-doped insulating manganite samples (LaMnO(3+delta) and La(1-y)Ca(y)MnO(3+delta)) as a function of temperature. A volume fraction with fast nuclear relaxations was revealed by the inhomogeneous loss of the NMR signal over a broad temperature interval. Comparison with muon spin rotation data demonstrates that the wipeout of the (139)La signal is mainly due to slowly fluctuating electric field gradients. This provides strong evidence for the slow diffusion of lattice excitations, identified with Jahn-Teller small polarons.

Proximity effects induced by a gold layer on La0.67Sr0.33MnO3 thin films

The authors report about La0.67Sr0.33MnO3 single crystal manganite thin films in interaction with a gold capping layer. With respect to uncoated manganite layers of the same thickness, Au-capped 4nm thick manganite films reveal a dramatic reduction (≃185K) of the Curie temperature TC and a lower saturation low temperature magnetization M0. A sizable TC reduction (≃60K) is observed even when an inert SrTiO3 layer is inserted between the gold film and the 4nm thick manganite layer, suggesting that this effect might have an electrostatic origin.

HyReSpect: A broadband fast-averaging spectrometer for nuclear magnetic resonance of magnetic materials

We announce the successful development of a homemade frequency-swept nuclear magnetic resonance (NMR) spectrometer entirely designed and built at the University of Parma, optimized for the study of magnetic materials but also offering good performance as a general-purpose instrument for solid-state NMR. The spectrometer features heterodyne-based pulser and receiver with four-quadrant phase shifting and quadrature detection; a 150 MHz digital signal processor as a digital pulser for timing and control functions, capable of triggering events with a resolution of 6.6 ns; a two-channel 12 bit 25MS∕s digitizer hosted by a personal computer; and a graphical user interface control program running under Linux, which also integrates external field and temperature controls. The receiver exhibits a flat response from 8 up to 670 MHz, a frequency span suitable for the investigation of magnetic transition metal compounds (V, Co, Mn, Cu), and intrinsic dead time of less than 2μs, as required with the fast-relaxing NMR ...

Staggered magnetization, critical behavior and weak ferromagnetic properties of LaMnO3 by muon spin rotation.

We present a study of a microtwinned single crystal of LaMnO3 by means of implanted muons. Two muon stopping sites are identified from the symmetry of the internal field in the ordered phase. The temperature dependence of these fields yields the behavior of the staggered magnetization from which a static critical exponent (� = 0.36(2)) is extracted and discussed. The muon spinspin relaxation rate shows a critical slowing down (contrary to preliminary findings) with a critical exponent n = 0.7(1), witnessing the Ising nature of the dynamic fluctuations. The muon precession frequencies vs. applied magnetic field reveal the saturation of the weak ferromagnetic domain structure originated by the Dzialoshinski-Moriya antisymmetric exchange.

Field-induced segregation of ferromagnetic nanodomains in Pr 0.5 Sr 0.5 MnO 3 detected by 55 Mn NMR

The antiferromagnetic manganite

Field-induced segregation of ferromagnetic nano-domains in Pr

{\mathrm{Pr}}_{0.5}{\mathrm{Sr}}_{0.5}\ensuremath{-}{\mathrm{MnO}}_{3}

_{0.5}

was investigated at low temperature by means of magnetometry and