Muons and magnets

Abstract:

We present numerical calculations of a tight-binding model applied to a finite square lattice in the presence of a perpendicular magnetic field. The persistent current associated with each eigenstate is calculated, the chirality of which is determined by whether the eigenstate exists within the bulk or localised to the edges of the lattice. This treatment allows us to extract oscillations in the magnetization, which are analogous to de Haas-van Alphen oscillations. We consider the influence of short range disorder and long range potential modulations on these systems.

Abstract:

We report millimetre-wave electron spin resonance (ESR) measurements on single crystals of the high-spin molecules Cr-10(OMe)(20)(O2CCMe3)(10) and Cr12O9(OH)(3)(O2CCMe3)(15) within a temperature range of 1.4 K to 50 K and in magnetic fields of up to 5 Tesla. In our experiments it is possible to vary the orientation of the magnetic field with respect to the crystal axes, and thus to study the ESR lineshapes as a function of both temperature and angle. Our results confirm that Cr-10(OMe)(20)(O2CCMe3)(10) behaves as a single-molecule magnet with S = 15 and D = -0.03 K, while Cr12O9(OH)(3)(O2CCMe3)(15) has S = 6 and D similar to 0.1 K. A comparison of the experimental spectra with numerical simulations gives good agreement at low temperatures. At higher temperatures, we observe a narrowing of the ESR spectrum that is not explained by simple models.

Abstract:

Spin-polarized muons can be implanted in various molecular magnetic materials in order to measure static and dynamic magnetic field distributions at a local level. The positively-charged muon is an unstable, radioactive particle which has spin-1/2, a lifetime of 2.2 mu s, about one-ninth of the proton mass and a magnetic moment of approximately 1/200 mu(B). Both pulsed and continuous beams of muons can be produced with almost 100% spin polarization and significant intensity at various accelerator facilities. The subsequent decay of the muon into a positron allows the extraction of the muon-spin autocorrelation function which can be related to the magnetic field distribution inside a sample. This experimental technique has found particular application to the problem of hydrogen in semiconductors, as well as the study of the vortex lattice in both high-temperature and organic superconductors. Nevertheless, it has been most widely employed in the field of magnetism. We describe how our experiments using spin-polarized muons have been used to provide information about organic ferromagnets, molecular magnets, spin-chains and single molecule magnets.

Abstract:

Muon-spin rotation (mu SR) measurements have been used to study the superconducting vortex properties of layered organic superconductors based on molecular donors such as BEDT-TTF. The mu SR is particularly sensitive to the degree of local ordering of pancake vortices and can detect when the pancake layers become decoupled by intrinsic or defect-driven decoupling mechanisms, or by thermally driven motion. Further novel features of the vortex system occur when the field is tilted away from a crystal axis. Knowledge of the vortex phase behaviour allows appropriate parameter regions to be selected for reliable determination of the superconducting penetration depth lambda and studies of the temperature dependence of lambda have shown a T-linear term at low fields that is suppressed with increasing field. Systematic studies of lambda across the range of organic superconductors have revealed a strong correlation between lambda and T-c. In contrast to the linear scaling T-c proportional to lambda(-2) seen in high T-c cuprates, the organics show an overall correlation better described as T-c proportional to lambda(-3). One interpretation is that the superconducting carriers are only a small fraction of the total carrier concentration in these low-T-c superconductors. Understanding this result may give us some important clues about the nature of the superconductivity in the organics.