Muons and magnets

Abstract:

We present the results of a muon-spin rotation/relaxation (μSR) study in zero and longitudinal magnetic fields and temperatures down to 40 mK of para-cyanophenyl nitronyl nitroxide (p-CNPNN). This layered material is expected to show ferromagnetic interactions on the basis of susceptibility measurements. In a zero-field μSR experiment, the spins of muons which are implanted in diamagnetic states will precess in any local internal fields. Hence a study of precession frequencies and/or relaxation rates can be used to follow the magnetic order parameter as a function of temperature. We find strongly temperature dependent muon relaxation below ∼0.17 K similar to that found recently in 1-NAPNN, indicating a magnetic transition at this temperature.

Abstract:

We report the first muon-spin relaxation study of an organic spin-Peierls system, the linear-chain compound MEM(TCNQ)2. Our results show a crossover from a Gaussian relaxation to an exponential relaxation as the temperature is lowered below the spin-Peierls transition. We associate this behaviour with the slowing down of the electronic fluctuations resulting from the opening of a gap in the magnetic excitation spectrum.

Abstract:

Angle dependent magnetoresistance oscillations (AMRO) have been studied for the charge-transfer salt beta ''-(BEDT-TTF)(2)AuBr2. The temperature range 1.5 K-4.2 K and field region 0.5 T-30 T have been explored. A series of AMRO peaks originating from a similar to 40 T elliptical quasi-two-dimensional (Q2D) Fermi surface (FS) pocket have been observed at all fields. The measured orientation of this pocket is in good agreement with band structure calculations. Above a field of similar to 10 T a second series of(Q2D) AMRO peaks emerge, indicating a field-induced reconstruction of the FS. It is proposed that this results from a shifting of the nesting vector of the spin density wave which is believed to be present in this material. The high field series of peaks correspond to a similar to 222 T elliptical FS pocket with major axis aligned along the direction in which band structure calculations predict Q1D Fermi sheets. It is thus attributed to imperfect nesting of the Q1D FS sheets.

Abstract:

A numerical model is used to derive the quantum oscillations in the magnetisation and magnetoresistance of quasi-two-dimensional alpha-phase BEDT-TTF charge-transfer salts in high magnetic fields. Recent experimental results are simulated and the standard Lifshitz-Kosevich formalism is shown to be no longer appropriate.

Abstract:

Shubnikov-de Haas (SdH) experiments involving pressures of up to 15 kbar, temperatures as low as 500 mK and magnetic fields as high as 17 T have been carried out in order to probe the proposed phase transition of alpha-(BEDT-TTF)(2)KHg(SCN)(4) from a spin-density-wave (SDW) state at a pressure of P-c similar to 5 kbar. SdH measurements have also been carried out with the sample at a variety of field orientations. Frequencies of lambda approximate to 181 T, alpha approximate to 671 T, mu approximate to 775 T, nu approximate to 856 T and beta approximate to 4270 T are observed in the Fourier spectra of the ambient pressure oscillations. The lambda, mu, nu and beta oscillations and also the large second harmonic component of the alpha orbit SdH waveform are not observed in the high field (>23T) metallic state, implying that they are a characteristic of the ambient pressure SDW state. Pressure suppresses the lambda, mu and nu frequencies but the a and P orbits are seen at all pressures. The large second harmonic of the waveform of the a orbit SdH is initially suppressed by pressure but reemerges above similar to 10 kbar. The origins of this are discussed.