G. Guidi

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.

Phase diagram of low doping manganites

We present a study performed on several LaMnO3+x/2 compositions. Detailed analysis around x=0.09 detects phase separation between a higher transition, Tc, and a lower one TN, with coexisting fluctuating and magnetically ordered clusters (the Neel point, TN, being the onset of an inhomogeneous non-collinear antiferromagnetic order detected by neutrons). The ordered clusters give rise to an extremely fast, static Kubo–Toyabe muon depolarization, while mixed static and dynamic local fields are detected inside the fluctuating clusters. Below TN precession frequency lines are resolved, despite the huge broadening introduced by localized Mn4+ ordered moments. A phase diagram is obtained and briefly discussed in the context of electronic phase separation.

μSR and NMR observation of antiferromagnetic ordering in Bi2Sr2YCu2O8+δ

Abstract We present data from muon spin rotation (μSR) and NMR experiments on Bi 2 Sr 2 Y 1− x Ca x Cu 2 O 8+δ samples with 0 x 2 Sr 2 YCu 2 O 8+δ around a well defined local magnetic field (≈3 mT), in zero applied field (ZF), indicates the presence of a spontaneous magnetization up to 295 K. This is also supported by the complete disappearance in the same sample of the powder quadrupole Cu NMR spectrum, characteristic of the Bi 2 Sr 2 CaCu 2 O 8 material. Furthermore, a second sizeable fraction of muons experiencing a null local field in ZF suggests the existence of a muon site symmetrical with respect to magnetization sublattices, and therefore of an antiferromagnetic type of order.

Effects of oxygen stoichiometry on the copper NMR and NQR in YBa2Cu3O7−y

Abstract We present Cu NMR and NQR measurements performed in the normal phase on YBa 2 Cu 3 O 7− y samples of different oxygen content. An attribution of the signals to the inequivalent copper sites in the lattice, and the charge distribution around these ions are discussed.

Effect of Two Gaps on the Flux-Lattice Internal Field Distribution: Evidence of Two Length Scales in Mg~1~-~xAl~xB~2 from muSR

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 Mg(1-x)AlxB2 in magnetic fields up to 2.8 T. 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 agree quantitatively with STM and we thus validate a new phenomenological model for the internal fields.

Relation between Tc and London penetration depth in YBa2(Cu1−xZnx)3O7 by μSR

Abstract We report on NQR and μSR experiments on YBa 2 (Cu 1− x Zn x ) 3 O 7 aimed at investigating the role of carrier effective mass in determining T c . Our findings indicate Zn localization in Cu planes and a nonlinear dependence of T c on the ratio m ∗ c / m ∗ ab .

Phase separation in antiferromagnetic YBa2Cu3O\it 6+x

We present a detailed study of the ZF‐μSR precession signals in AF YBa2Cu3O6+x for a series of 0.06\,\leq\,x\,\leq\,0.39 ceramic samples. Careful analysis of the temperature and oxygen content dependence shows a complex behaviour of the magnetic field at the muon site. The two main novelties are that a) the muon does not diffuse freely above 250 K, as previously stated, and that b) mobile holes phase separate, inducing a finite size reduction of the AF order parameter.

Study of the plastic phases of norbornene and bicyclooctene by means of muonic radicals

We have observed the formation of muonium adduct radicals in norbornene and bicyclooctene. The behavior of the hyperfine constants and of the μSR linewidths have been investigated as a function of temperature; they have been correlated with the reorientational dynamics of the molecules, which have already been partially determined by means of Raman, NMR, and neutron scattering data. Our measurements have also been interpreted in terms of the electronic configuration of the radicals. In the case of norbornene at the plastic–brittle transition, an abrupt change of τc of 13 orders of magnitude was determined.

Zeeman perturbed nuclear quadrupole resonance investigation of orbitally ordered KCuF3

Abstract We investigated the electric field gradient tensor at the Cu nuclei by Zeeman perturbed nuclear quadrupole resonance in orbitally ordered KCuF 3 . Our data show the presence of two inequivalent crystallographic sites with distinct orientations of the copper wave function, compatibly with the proposed orbital order for this compound. An unusual behaviour of the paramagnetic NQR signal at the Neel transition is reported.

STUDIES OF RANGE AND STRAGGLING OF MUONS IN METALS BY THE PROJECTED RANGE IMAGING (PRI) TECHNIQUE

Abstract A new μSR method for directly imaging the implantation depth distribution of positive muons in metals (Projected Range Imaging: PRI) is presented and the results obtained from several experiments are reported. Strong magnetic field gradients inside a metallic film are produced by driving an electric current in it: spin polarized muons stopped in the film have Larmor precession frequencies proportional to the implantation depth. Generation of field gradients, advantages and features of the method are discussed. For muon energy in the MeV range the results are compared with Monte Carlo simulations and with experimental results obtained by using the conventional moderator curve method. Applications to range and straggling studies with epithermal muons are also envisaged. These future extensions will allow a better understanding of the energy loss mechanisms in the previously inaccessible very low energy range.