Muons and magnets

Abstract:

We study effects on magnetotransport produced by changing the size of the trivalent ions R3+ in the layered polycrystalline materials Sr2RMn2O7 (R = Pr, Nd, Ho, Y) and by varying the mixed-valence ratio Mn3+/Mn4+ in the series Sr2-xNd1+xMn2O7 (x = 0, 0.1, 0.2, 0.5). The results of our magnetic and electrical measurements are related to the measured structural characteristics of each compound. For the Sr2RMn2O7 series, colossal magnetoresistance (CMR) is observed below 150 K for R = Pr and R = Nd but not for samples containing R = Ho or R = Y ions, which have smaller ionic radii. For R = Ho and R = Y, the absence of CMR is correlated with the paramagnetic and insulating properties of the samples. Within the series Sr2-xNd1+xMn2O7, the CMR effect is also observed for temperatures below 150 K and increases as the temperature decreases. In this series the samples show a mixture of spin-glass and antiferromagnetic behaviour (x = 0, 0.1) or no long-range magnetic order (x = 0.2, 0.5). The largest CMR at high temperatures (above 100 K) and high magnetic fields (14 T) is observed for the x = 0.2 sample, which also shows the largest magnetization per formula unit. The observed CMR in the compounds studied here is attributed to magnetic cluster formation.

Abstract:

We have performed detailed studies of the angle- and temperature-dependent resistive upper critical fields in the layered organic superconductor κ-(BEDT-TTF)2Cu(NCS)2. With the magnetic field lying in the conducting planes, our measurements show an upper critical field which comfortably exceeds the Pauli-paramagnetic limit in this material. We find no azimuthal angle dependence of the critical field, in spite of recent evidence that this material has gap nodes characteristic of d-wave superconductivity. We propose that the large critical fields may be due to a Fulde-Ferrell-Larkin-Ovchinnikov state which can exist in exactly in-plane fields because of the nature of the Fermi surface of κ-(BEDT-TTF)2Cu(NCS)2. © 1999 IOP Publishing Ltd.

Abstract:

Muon-spin rotation (μSR) measurements have been used to study the vortex lattice and its instabilities in the organic superconductors κET2Cu(SCN)2 and βET2IBr2. We ascribe the field- and temperature-dependent destruction of the vortex lattice to the large anisotropy found in these materials.

Abstract:

We experimentally demonstrate a mechanism for a new kind of magnetic resonance, the Fermi-surface traversal resonance (FTR). This is caused by the periodic traversal of carriers across quasi-one-dimensional (Q1D) sections of Fermi surface (FS) in an external magnetic field. Owing to the warping of the Q1D Fermi sheets, the real space velocities of the carriers oscillate as they cross the FS, generating resonances in the high frequency conductivity of the material. The results contain information about the FS, including the direction and harmonic content of the warping components. Using a rotating resonant-cavity system, FTRs have been observed in alpha-(BEDT-TTF)(2)KHg(SCN)(4).