Muons and magnets

Abstract:

We report results of the first muon-spin rotation measurements performed on the low carrier density ferromagnet EuB6. The ferromagnetic state is reached via two magnetic transitions at Tm=15.5 K and T c=12.6 K. Two distinct components are resolved in the muon data, one oscillatory and one non-oscillatory, which arise from different types of magnetic environment, and we have followed the temperature dependence of these components in detail. These results provide evidence for magnetic phase separation and can be interpreted in terms of the gradual coalescing of magnetic polarons.

Abstract:

Historically most materials in magnetic applications are based on inorganic materials. Recently, however, organic and molecular materials have begun to show increasing promise. Purely organic ferromagnets, based upon nitronyl nitroxide radicals, show long range magnetic order at very low temperatures in the region of 1 K, while sulfur based radicals show weak ferromagnetism at temperatures up to 36 K. It is also possible to prepare molecule based magnets in which transition metal ions are used to provide the magnetic moment, but organic groups mediate the interactions. This strategy has produced magnetic materials with a large variety of structures, including chains, layered systems and three-dimensional networks, some of which show ordering at room temperature and some of which have very high coercivity. Even if long range magnetic order is not achieved, the spin crossover effect may be observed, which has important applications. Further magnetic materials may be obtained by constructing charge transfer salts, which can produce metallic molecular magnets. Another development is single-molecule magnets, formed by preparing small magnetic clusters. These materials can show macroscopic quantum tunnelling of the magnetization and may have uses as memory devices or in quantum computation applications.

Abstract:

Zero-field muon spin relaxation (μ+SR) measurements on materials from the series [ZIIICp*2][M IIMIII(ox)3] show precession signals at several frequencies, characteristic of quasistatic magnetic fields at up to three distinct muon sites.

Abstract:

The angle dependences of the magnetoresistance of two different isotopic substitutions (deuterated and undeuterated) of the layered organic superconductor κ-(ET)2Cu(NCS)2 are presented (where ET is the organic molecule bis(ethylenedithio)-tetrathiafulvalene). The angle-dependent magnetoresistance oscillations (AMRO) arising from the quasi-one-dimensional and quasi-two-dimensional Fermi surfaces in this material are easily confused. By using the Boltzmann transport equation extensive simulations of the AMRO are made that reveal the subtle differences between the different species of oscillation. No significant differences are observed in the electronic parameters derived from quantum oscillations and AMRO for the two isotopic substitutions. The interlayer transfer integrals are determined for both isotopic substitutions and a slight difference is observed which may account for the negative isotope effect previously reported. The success of the semiclassical simulations suggests that non-Fermi liquid effects are not required to explain the interlayer transport in this system.

Abstract:

The Hofstadter butterfly is the energy spectrum of an infinite square lattice, plotted as a function of the magnetic field. We illustrate a method of calculating similar spectra for finite lattices in a magnetic field, using methods that consider the appropriate molecular orbitals, and find that the spectra resemble the Hofstadter butterfly. We relate the bonding and antibonding orbitals used to describe small systems to the Landau levels of the infinite system. This approach provides an unusual, but instructive, method of introducing the physics of Landau levels from the basic quantum mechanics of small systems. © 2004 American Association of Physics Teachers.