Alexander Schenck

Spontaneous Magnetic Ordering in the Fullerene Charge-Transfer Salt (TDAE)C60

The zero-field muon spin relaxation technique has been used in the direct observation of spontaneous magnetic order below a Curie temperature (Tc) of ∼16.1 kelvin in the fullerene charge-transfer salt (tetrakisdimethylaminoethylene)C60 [(TDAE)C60]. Coherent ordering of the electronic magnetic moments leads to a local field of 68(1) gauss at the muon site at 3.2 kelvin (parentheses indicate the error in the last digit). Substantial spatially inhomogeneous effects are manifested in the distribution of the local fields, whose width amounts to 48(2) gauss at the same temperature. The temperature evolution of the internal magnetic field below the freezing temperature mirrors that of the saturation magnetization, closely following the behavior expected for collective spin wave (magnon) excitations. The transition to a ferromagnetic state with a Tc higher than that of any other organic material is now authenticated.

Electric-field dependence of muonium formation in liquid helium

The influence of electric fields on the formation of muonium in liquid helium (4He,3He, and mixture of4He + 0.2%3He) has been studied. It was found that the relative distribution of muon-electron pairs is anisotropic. The maximum muon density is shifted with respect to the electrons in the direction of the initial muon momentum. Due to the anisotropy the muonium asymmetry in normal liquid helium is enhanced by a factor of 3 in an electric fieldE=1 kV/cm.

Depolarisation studies of positive muons in copper, vanadium, niobium and tantalum single crystals

The behaviour of positive muons ( mu +) has been studied in Cu, V, Nb and Ta single crystals, using the muon spin rotation ( mu SR) technique. The measurements were performed in the temperature regime 5-365K, in transverse external magnetic fields of 50-5500 G. In FCC Cu the mu + diffusional motion can be described in terms of quantum mechanical jump processes of a small polaron among octahedral interstitial sites. An orientation average leads to an activation energy of E=79.2+or-3.1 meV, a pre-exponential factor of D(0)=(1.66+or-0.33)*10-8 cm2 s-1 and a tunnelling matrix element of J=(18.3+or-1.8)*10-6 eV. The mu + total volume expansion was found to be Delta V=3.91 AA3. Furthermore, the electric field gradient (EFG) at the mu + nearest-neighbour nuclei has been determined as q=0.30 AA-3. In BCC V, Nb and Ta the mu + diffusion is trap-limited. As a function of the temperature the mu + probe different classes of trapping sites associated with host metal impurities. At low temperature, before decaying, the mu + always end up in a trapped state which shows that the intrinsic mu + motion between traps is fast and seems to reflect properties of a band diffusion.

Magnetic and structural instabilities in the stripe-phase region of La1.875Ba0.125 - ySryCuO4 (0 ⩽ y ⩽ 0.1)

Zero-field positive muon spin relaxation (ZF-µ+ SR) experiments were performed to investigate the magnetic properties of the low-temperature structural modifications of the La1.875 Ba0.125 - y Sry CuO4 ( y = 0.0, 0.025, 0.050, 0.075, 0.100) series, in which the total hole concentration is close to 1 / 8. Together with high-resolution time-of-flight neutron powder diffraction measurements, the results imply that the interplay among lattice distortions, doping and superconductivity is intimately related to the magnetic correlations of the Cu spins, which are interpreted in terms of a stripe-phase model. Materials with y 0.075 exhibit an incomplete structural transition at T d 2 , from a low-temperature orthorhombic (LTO, Bmab ) to a low-temperature tetragonal (LTT, P 42 / ncm ) phase. Diffraction patterns collected while approaching T d 2 reveal a rapid reduction of the orthorhombicity in the LTO phase with a residual fraction always surviving to low temperatures. Different behaviour is shown by the composition with y = 0.1. Rietveld analysis shows coexistence of the LTO phase with a less distorted low-temperature orthorhombic phase (LTO-2, Pccn ). In the LTT phase, a fraction of muons senses regions with purely static spin correlations due to an incommensurate spin-density wave. The muon spin depolarization suggests that in the spatially separated remnant LTO, hole-rich domains, dynamical correlations of charge density give rise to superconductivity. As the spin fluctuations become static at y 0.075, the London penetration depth, calculated from the transverse-field µ+ SR depolarization rate, , reveals a decrease in the superconducting carrier density. The magnetic freezing temperature, Tf is suppressed and the magnetic phase fraction shrinks as y increases, whereas the superconducting correlations persist in a larger sample volume. The disappearance of long-range magnetic order at y = 0.1 and the growth of a quasi-static component correlate well with the presence of the LTO-2 microstructure which behaves as a buffer phase out of which the LTT domains become dominant in the Ba-rich compositions.

Magnetic ordering in the T* phase La1.2Tb0.8CuO4

We report a muon spin relaxation study of the magnetic properties of the La1.2Tb0.8CuO4 phase with the T* structure. Random magnetic order is revealed between 280 and 170 K by the zero field data. A spontaneous muon precession then appears below 170 K, arising from antiferromagnetic long range order of the Cu2+ spins. Evidence exists below 20 K for ordering of the Tb3+ ions. We find that the T* phase adopts the same magnetic structure as the (T/O) phase La2CuO4.

Indication for a delocalized precursor μ+ state in CeAl3

The low temperature (≲ 0.6 K) ZF-μSR signal in the heavy electron compound CeAl3 displays a three component structure with one component revealing the precession ofμ+ in a static internal field of 220 G. Surprisingly the phase of this signal is shifted by the order of 90° which indicates the presence of a precursor state. The precursor state manifests itself by a quickly decaying nonoscillating component. A simple two state model with zero average field in the precursor state and 220 G in the final state can describe the results except for the magnitude of the phase shift.

Muon Knight shift investigations in LaAg1-xInx

Muon Knight shift measurements are performed in the pseudobinary intermetallic compound LaAg1-xInx (CsCl structure). Negative, temperature-dependent Knight shift values are found. This observation is discussed in terms of a negative transferred hyperfine field between the La 5d states and the muon, whose magnitude varies from -3 kG mu B-1 to -10 kG mu B-1 for 0<or=x<or=0.3.

Spin-freezing in the two-dimensional spin-gap systems SrCu2-xMgx(BO3)2 (x = 0, 0.04, 0.12)

The magnetic properties of the two-dimensional dimer spin-gap system SrCu 2 (BO 3 ) 2 were investigated by the μ + SR technique. The relatively slow fluctuations of spin-dimers slow down with decreasing temperature and an unusual spin-freezing process is unraveled at T f <3.75 K, well within the spin-gap temperature range (T SG 20 K). This quasi-static phase displays a Gaussian field distribution with a remarkable stability with applied longitudinal fields. In support of the criticality of the SrCu 2 (BO 3 ) 2 spin-gap ground state towards an antiferromagnetic transition, Knight-shift measurements suggest that implanted muons may liberate spin density at T< T SG that undergoes spin-freezing at very low temperatures. On the other hand, non-magnetic impurity-doping of the copper sublattice does not suppress the spin-gap ground state and does not lead to magnetic ordering effects of static nature.

Spin-freezing in the layered perovskites La2CoxCu1−xO4 + δ

The muon-spin relaxation (μ + SR) technique was used to investigate the spin glass phase of La 2 Co 0.75 Cu 0.25 O 4.16 (T f 30 K), an oxygen-rich Mott-Hubbard insulator, isostructural with the high-T c La 2 CuO 4+δ superconductor. On approaching T f from above, the μ + -spin dynamics are described by a stretched exponential decay with rapid growth of the correlation times, reminiscent of the spin freezing process in Ising spin glasses

Spin dynamics in CuGeO3 studied by muon spin rotation

We report a muon spin rotation (μSR) study of the magnetic properties of the Cu2+ quasi-one-dimensional CuGeO3 system and its lightly-doped derivative Cu0.97Zn0.03GeO3. Susceptibility measurements on CuGeO3 show a sudden change in the vicinity of 14 K that has been interpreted before as a magnetic transition to a spin-Peierls state. μSR shows no evidence of spin freezing below 14 K, implying that the transition is to a magnetic state with no static (random or ordered) electronic moments. A modest slowing down of the electronic spin dynamics is also identified at this temperature. Similarly, no evidence of a transition to a static magnetic state is found for Cu0.97Zn0.03GeO3 whose susceptibility shows hysteretic behaviour between zero-field and field cooled measurements at 4 K, previously ascribed to spinglass-like behaviour. Given the nature of the muon spin as a local magnetic probe, the present results necessitate a re-interpretation of the origin of the susceptibility anomaly observed in the doped system.