T. R. Adams

Magnetic ordering in (Y1−xPrx)Ba2Cu3O7 as evidenced by muon spin relaxation

Using the zero‐field‐muon‐spin‐relaxation (μSR) technique clear evidence has been found for antiferromagnetic ordering of Cu moments within the CuO planes of (Y1−xPrx)Ba2Cu3O7. The Neel temperatures are approximately 285, 220, 35, 30, and 20 K for x=1, 0.8, 0.6, 0.58, and 0.54, respectively. For x=0.50 we observe a fast‐relaxing component of the muon polarization in addition to a long‐time tail, reminiscent of spin‐glass behavior. This region of the phase diagram (0.5≤x≤0.54) corresponds to the existence of both superconductivity and magnetism. The fully developed local magnetic field for x>0.54 is found to be ∼16 mT, but decreases to ∼12 mT at T=17 K for the x=1 sample, presumably due to the onset of Pr‐ion ordering. Magnetic ordering also occurs in PrBa2Cu3O6; the Neel temperature is ∼325 K.

Muon-spin-relaxation study of magnetism in ErBa2Cu3O6.2

The copper magnetism of ErBa2Cu3O6.2 is examined by transverse‐field (TF) and zero‐field (ZF) muon‐spin relaxation (μSR). These data indicate two magnetic phases with TN1≂330 K and TN2∼65 K. The second phase is signaled by deviation of the ZF‐μSR frequencies from a standard magnetization curve and an abrupt change in the TF‐μSR relaxation rate. A relaxation feature indicates a muon depolarization mechanism with a T3/2 dependence in the low‐temperature phase. Observed fields are compared to those calculated for proposed magnetic structures.

Stability of muon-oxygen bond sites in RBa2Cu3O7

Relative energies of muon probe sites in the chain region of RBa2Cu3O7 (RBCO) are investigated using a molecular quantum chemistry calculation for (Oμ)− embedded in a cluster of point charges to simulate local charge distributions in RBCO. Partial Cu−O chain covalency results in a O-μ...O muon site between the chain and bridging oxygens. However, Cu-μ “hydride”-like sites are suggested by results for nominally ionic clusters.

μSR investigation of magnetism and superconductivity in (Y1−xPrx)Ba2Cu3O7

Abstract Muon spin rotation and relaxation techniques have been used to study the superconductivity and magnetism in (Y 1− x P x )Ba 2 Cu 3 O 7 (0⩽ x ⩽1) and PrBa 2 Cu 3 O 6 . Clear evidence for magnetic ordering of the Cu moments within the Cu-O planes is seen. Additionally, a lower magnetic transition is observed which, based upon previous work, has been associated with the ordering of Pr moments on the Y sublattice of the YBa 2 Cu 3 O 7 structure. For x = 1, the upper Neel temperature T N1 is ∼270 K and the magnitude of the fully developed local magnetic field is ∼16 mT. Below the lower Neel temperature T N2 = 17 K, the magnitude of the static field is reduced to ∼ 12mT. For 0.4⩽ x ⩽0.54, there appears to be a coexistence region of long range magnetism and superconductivity.

Development of a μLCR facility at LAMPF

It has long been recognized thatμLCR could profitably be done with the high intensity surface beam at LAMPF [1]. A spectrometer has been built that is matched to the LAMPF beam characteristics. The polarization information is obtained from a downstream array of counters while side counters, containing no polarization signal, monitor theμ+ beam. Degraders select higher energy e+, thereby reducing rates and required counter segmentation while maintaining information content. We apply a ramped longitudinal field in addition to the static one to average over instabilities in theμ+ beam. This field scan allows direct interpretation of data and does not require a prior estimate of the resonance structure of a sample. Flux coils monitor the applied ramp field and eddy-current induced fields. High average rate (2×107μ+/s). good stability, and the versatile field scan permitted useful data to be collected from Cu, Al(Cu), Al, Si(Al), and polycrystalline Si targets.

Transverse-and zero-field μSR investigation of magnetism and superconductivity in (Y1−xPr x )Ba2Cu3O7

Zero-field muon-spin-rotation (μSR) measurements on (Y1−xPrx)Ba2Cu3O7 [x=1.0, 0.8, 0.6, and 0.54] show evidence for antiferromagnetic ordering of the Cu moments within the Cu−O planes, with Néel temperatures 285, 220, 35. 30 and 20 K, respectively. Forx=1.0 the local muon magnetic field is ≈16 mT, but decreases to ≈12 mT at 17K, due to additional magnetic ordering. The zero-field data, in conjunction with transport data, allow construction of a complete phase diagram for this system. Transverse-field (1 kOe) μSR data forx=0.2 (Tc=75 K) show that the muon depolarization is determined primarily by the Cu nuclear moments forT>Tc, and by the vortex state forT<Tc. Fitting the superconducting-state data to a BCS model yields an extrapolated zero-temperature magnetic penetration depth of 2170 Å.

Electric-field gradient at oxygen sites in RBCO: Effects from a positive muon

A limited quantum chemistry approach has been used to examine changes to the electric-field gradient at oxygen sites in rare-earth barium copper oxides (RBCO) due to a bonded muon (or hydrogen). Results are reported for an all electron, extended basis set calculation for an OMu− molecule which is embedded in various point charge clusters to simulate the RBCO crystal environment. Differences between the gradients for O2− and OMu− are examined for five likely muon bond sites. In each case the muon significantly modifies the calculated gradient.

Anisotropy inc-axis oriented YBa2Cu3O7−δ

Muon spin relaxation (μSR) data taken at LAMPF on ac-axis oriented fine powder sample of YBa2Cu3O7−δ (YBCO) embedded in epoxy are analyzed for relaxation rate anisotropy. Clear differences beyond simple magnetic field penetration depth anisotropy are observed forB‖c andBc. The low-temperature anisotropy ratio is consistent with oriented ceramic data. Small crystallite size and anisotropic flux pinning characteristics are suggested as the fundamental cause of the additional effects.