L. P. Le

Searching for spontaneous magnetic order in an organic ferromagnet. μSR studies of β-phase p-NPNN

Abstract Zero-field muon spin rotation (ZF-μSR) studies in the recently synthesized organic ferromagnet β-phase p-NPNN are reported. Our measurements, using a microscopic magnetic probe, provide the first direct observation of spontaneous magnetic order in p-NPNN. The behaviour of the system is close to that of an isotropic three-dimensional (3D) Heisenberg spin system.

MAGNETIC BEHAVIOR IN LI-DOPED LA2CUO4

We report muon-spin-relaxation studies in the Li-doped cuprates La{sub 2}Cu{sub 1{minus}{ital x}}Li{sub {ital x}}O{sub 4} for {ital x}=0.01, 0.05, 0.10, 0.45, and 0.50. For low Li concentrations ({ital x}{le}0.10) we find a rapid suppression of {ital T}{sub {ital N}} as {ital x} increases, but little change in the magnitude and temperature dependence of the antiferromagnetic order parameter. This indicates that Li doping effectively destroys antiferromagnetism (similar to Sr doping, but different from Zn doping) without strongly affecting either the on-site Cu moments or the shape of the spin-wave excitation spectrum. For high Li concentrations we find that the majority of the sample volume is nonmagnetic, suggesting possible singlet-state formation. {copyright} {ital 1996 The American Physical Society.}

Magnetic behavior in Li-doped La{sub {bold 2}}CuO{sub {bold 4}}

We report muon-spin-relaxation studies in the Li-doped cuprates La{sub 2}Cu{sub 1{minus}{ital x}}Li{sub {ital x}}O{sub 4} for {ital x}=0.01, 0.05, 0.10, 0.45, and 0.50. For low Li concentrations ({ital x}{le}0.10) we find a rapid suppression of {ital T}{sub {ital N}} as {ital x} increases, but little change in the magnitude and temperature dependence of the antiferromagnetic order parameter. This indicates that Li doping effectively destroys antiferromagnetism (similar to Sr doping, but different from Zn doping) without strongly affecting either the on-site Cu moments or the shape of the spin-wave excitation spectrum. For high Li concentrations we find that the majority of the sample volume is nonmagnetic, suggesting possible singlet-state formation. {copyright} {ital 1996 The American Physical Society.}

Muon-spin-rotation studies of HoNi2B2C.

Muon-spin-rotation and relaxation studies on single-crystalline HoNi{sub 2}B{sub 2}C show two magnetic phase transitions at {ital T}{sub {ital N}1}=6.0 K and at {ital T}{sub {ital N}2}=5.0 K. While the low-temperature phase displays a commensurate antiferromagnetic structure, the higher-temperature phase exhibits an incommensurately sinusoidal modulation of the spin amplitude. The nature of the incommensurate state, along with the causes of the reentrant superconductivity near 5 K, are discussed. {copyright} {ital 1996 The American Physical Society.}

μ+SR measurements on pure and copper doped samples of the 1D antiferromagnet Ni(C2H8N2)2NO2(ClO4) (NENP)

Abstract Relaxation reflecting the freezing/slowing down of spins is seen in pure and Cu-doped NENP below ≈ 30 K. This relaxation increases sharply for both materials near 2.5 K and saturates below ≈ 1 K. The applied field dependence of the μ + SR spectra at 20 mK indicates that the internal fields corresponding to these spins are still fluctuating and are probably too large to be attributed to loose chain ends or impurities.

Correlations betweenT c andn s/m * (carrier density/ effective mass) in high-T c and organic superconductors

We update our muon spin relaxation studies of the magnetic field penetration depth which show the correlations betweenTc and the relaxation rate σ αns/m* (carrier density/effective mass) of hole-doped high-Tc cuprate superconductors (La2, Sr3)CuO4, YBa2Cu3O7 (Y1−xPrx)Ba2Cu3O7, and other double and triple layer systems. These studies are extended to the organic superconductor (BEDT-TTF)2Cu(NCS)2.

Search for magnetic fields due to anyons in high-T c superconductors

We have searched for anomalous internal magnetic fields in highTc materials which are predicted to occur in anyon and flux phase models of superconductivity. The magnitude, anisotropy and temperature dependence of the observed fields inc-axis oriented samples of sintered YBa2CuO3O7 and of thick-film Bi2Sr2CaCu2O8 are consistent with a conventional nuclear dipolar origin. An upper limit of ≲0.08 mT is set for any anomalous magnetic fields along thec-axis atμ+ sites in bulk CuO2 superconductors.

Magnetic penetration depth in V3Si and LiTi2O4 measured by μSR

Muon spin relaxation (μSR) studies have been performed in the normal spinel LiTi2O4 and the A-15 superconductor V3Si to measure the magnetic penetration depth λ. The relaxation rateσ(T) ∝ 1/λ2 in field-cooled measurements shows a sharp increase belowTc followed by saturation at low temperatures in both systems. This feature implies an isotropic energy gap without anomalous zeros, and most likelys-wave pairing. The low temperature penetration depth λ(T → 0) is determined to be 2100Å for LiTi2O4 and 1300Å for V3Si respectively. Assuming a clean limit relationσ ∝ λ−2 ∝ns/m*, we derive the Fermi temperatureTF ∝ns/2/3m* from the relaxation rateσ and the Sommerfeld constantγ asTF ∝ σ3/4γ−1/4. Unlike conventional superconductors, both LiTi2O4 and V3Si have a large ratio ofTc/TF ≈ 0.01, only slightly smaller than those ratios in more exotic superconductors.

μ+SR study of two-dimensional antiferromagnets, delafossite-type compounds

Abstractμ+SR experiments were performed on delafossite-type compounds, CuCrO2, AgCrO2, CuFeO2, which are model compounds of triangular lattice antiferromagnets. The initial asymmetries are much smaller than the expected value, implying muonium formation. The time spectra are composed of slow andfast relaxation components. We attributed the components to signals from μ+ stopped at the center of O2− ions andmuonium far from nuclear dipole moments, respectively. The asymmetries decrease belowTN but no precession spectra were observed. Relaxation rates of slow andfast relaxation components show maxima atTN.

μSR measurements of spin fluctuations in CePt2Sn2

The heavy fermion compound CePt2Sn2 is characterized by an enormouslinear specific heat coefficient γ > 3J/mol K2. Through longitudinal field measurements of the muon 1/T1 relaxation rate, we were able to allow the formation of the heavy fermion state. We estimated a coherence temperature of ∼ 20 K. Below 1 K, the electronic moments slowed down, however, static magnetic order was not observed above 20 mK; instead clear evidence was found for the presence of slow spin fluctuations down to our lowest attainable temperatures.