arXiv subjects

G. Allodi

Publications and source records attributed to G. Allodi.

27 records · Page 2Linked to original sources

arXiv2007

Muon Spin Spectroscopy evidence of the violation of Anderson condition in magnetite

We present new muon spectroscopy data on a Fe$_3$O$_4$ single crystal, revealing different spin precession patterns in five distinct temperature ranges. A careful analysis of the local field and its straightforward modeling obtains surprisingly good agreement with experiments only if a very specific model of localized charges violating Anderson condition, and a correlated muon local dynamics are implemented. Muon evidence for fluctuations just above the Verwey temperature, precursor of the low temperature charge localized state, is provided.

cond-mat.str-el↗

arXiv2007

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

We 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 4 nm-thick manganite films reveal a dramatic reduction (about 185 K) of the Curie temperature TC and a lower saturation low-temperature magnetization M0. A sizeable TC reduction (about 60 K) is observed even when an inert SrTiO3 layer is inserted between the gold film and the 4 nm-thick manganite layer, suggesting that this effect might have an electrostatic origin.

cond-mat.mtrl-sci↗

arXiv2005

55Mn NMR and magnetization studies of La0.67Sr0.33MnO3 thin films

55Mn nuclear magnetic resonance and magnetization studies of the series of La0.67Sr0.33MnO3 thin films have been performed at low temperature. Two distinct lines were observed, at 322 MHz and 380 MHz, corresponding to two different phases, the former located at the interface, with localized charges, and the latter corresponding to the film bulk, with itinerant carriers (as it was also found in Ca manganite films). The spin-echo amplitude was measured as a function of a dc magnetic field applied either in the film plane or perpendicular to it. The field dependence of both the main NMR signal intensity and frequency shift is quite consistent with that calculated in a simple single domain model. The best fit to the model shows that magnetization rotation processes play a dominant role when the applied field exceeds the effective anisotropy field. Distinctly different magnetic anisotropies are deduced from the interface NMR signal.

cond-mat.mtrl-sci↗

arXiv2004

Effect of two gaps on the flux lattice internal field distribution: evidence of two length scales from muSR in Mg1-xAlxB2

We have measured the transverse field muon spin precession in the flux lattice (FL) state of the two gap superconductor MgB2 and of the electron doped compounds Mg1-xAlxB2 in magnetic fields up to 2.8T. We show the effect of the two gaps on the internal field distribution in the FL, from which we determine two coherence length parameters and the doping dependence of the London penetration depth. This is an independent determination of the complex vortex structure already suggested by the STM observation of large vortices in a MgB2 single crystal. Our data agrees quantitatively with STM and we thus validate a new phenomenological model for the internal fields.

cond-mat.supr-con↗

arXiv2004

Nanoscopic coexistence of magnetism and superconductivity in YBa2Cu3O6+x detected by MuSR

We performed zero and transverse field MuSR experiments on a large number of YBa2Cu3O6+x samples. We detect coexistence of antiferro-type (AF) short range magnetism with superconductivity below T_f~10 K in compositions 0.37<x<0.39. Most muons experience local AF fields, even when SQUID detects a full SC 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.

cond-mat.supr-con↗

arXiv2001

Evidence of ultra-slow polaron dynamics in low-doped manganites from 139La NMR-NQR and muon spin rotation

We report a 139La NMR investigation of low-doped insulating manganite samples (LaMnO(3+y) and La(1-x)Ca(x)MnO(3+y)) 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 muSR data demonstrates that the wipe out of the 139La 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.

cond-mat.str-el↗

arXiv2001

Staggered magnetization, critical behavior and weak ferromagnetic propert ies of LaMnO$_3$ by muon spin rotation

We present a study of a microtwinned single crystal of LaMnO$_3$ 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 ($β=0.36(2)$) is extracted and discussed. The muon spin-spin 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.

cond-mat↗

arXiv2000

Critical chain length and superconductivity emergence in oxygen-equalized pairs of YBa2Cu3O6.30

The oxygen-order dependent emergence of superconductivity in YBa2Cu3O6+x is studied, for the first time in a comparative way, on pair samples having the same oxygen content and thermal history, but different Cu(1)Ox chain arrangements deriving from their intercalated and deintercalated nature. Structural and electronic non-equivalence of pairs samples is detected in the critical region and found to be related, on microscopic scale, to a different average chain length, which, on being experimentally determined by nuclear quadrupole resonance (NQR), sheds new light on the concept of critical chain length for hole doping efficiency.

cond-mat.supr-con↗

arXiv1999

Field-induced segregation of ferromagnetic nano-domains in Pr$_{0.5}$Sr$_{0.5}$MnO$_3$, detected by $^{55}$Mn NMR

The antiferromagnetic manganite Pr$_{0.5}$Sr$_{0.5}$MnO$_3$ was investigated at low temperature by means of magnetometry and $^{55}$Mn NMR. A field-induced transition to a ferromagnetic state is detected by magnetization measurements at a threshold field of a few tesla. NMR shows that the ferromagnetic phase develops from zero field by the nucleation of microscopic ferromagnetic domains, consisting of an inhomogeneous mixture of tilted and fully aligned parts. At the threshold the NMR spectrum changes discontinuously into that of a homogeneous, fully aligned, ferromagnetic state, suggesting a percolative origin for the ferromagnetic transition.

cond-mat.str-el↗