Muons and magnets

Abstract:

We report structural, magnetic, and magnetotransport properties of the two-dimensional chromium sulfide Cr2S2.92, which contains both Cr3+ and Cr2+ ions. Below its magnetic-ordering temperature (TN=118 K), Cr2S2.92 exhibits a weak spontaneous magnetization with a maximum around 90 K and a value of 0.013(4) μB per Cr ion at 5 K. The resistivity and magnetoresistance exhibit pronounced local maxima around TN (MR=48% in a field of 14 T). The magnetotransport properties are discussed in terms of magnetic-polaron formation.

Abstract:

It is shown by magnetometry and microSR spectroscopy that short-range magnetic interactions between the Mn cations in the nonmetallic K(2)NiF(4)-like phase La(0.8)Sr(1.2)Mn(0.6)Rh(0.4)O(4) become significant below approximately 200 K. Negative magnetoresistance (rho/rho(0) approximately 0.5 in 14 T at 108 K) is apparent below this temperature. Neutron diffraction has shown that an applied magnetic field of 5 T is sufficient to induce saturated (3.38(7)mu(B) per Mn) long-range ferromagnetic ordering of the atomic moments at 2 K, and that the induced ordering persists up to a temperature of 50 K in 5 T. Spin glass behavior is observed below 20 K in the absence of an applied field. The induced magnetic ordering is attributed to the subtle changes in band structure brought about by the external field, and to the controlling influence of Rh(3+) over the relative strength of competing magnetic exchange interactions.

Abstract:

The muon spin rotation (mu SR) technique is primarily a powerful probe of the magnetic properties of materials; but it is also possible to measure charge-density related effects using muon quadrupolar level crossing resonance (QLCR). We present the first study of a charge density wave (CDW) using mu SR in TTF-TCNQ. The CDW develops below the metal-insulator transition at 54 K and we study the evolution of the CDW as a function of temperature using the QLCR resonances of the nitrogen atoms on the TCNQ molecules as the probe.

Abstract:

We present a model of a quasi-one-dimensional (Q1D) conductor with weak dispersion in the direction perpendicular to the chains in a magnetic field. Finite-energy electric dipole transitions between the eigenstates of the system constitute Fermi-surface traversal resonance, a Q1D analogue of cyclotron resonance. We extend the model to describe a (TMTSF)(2)X-like material, with dispersion in two directions perpendicular to the chains, and find that for certain orientations of the magnetic field large degeneracies occur between the magnetic-field induced states. These are angles at which maxima are observed experimentally in the d.c. conductivity, and thus we explain one class of angle-dependent magnetoresistance oscillations (AMRO) in terms of zero-energy electric dipole transitions between magnetic-field induced states.