Muons and magnets

Abstract:

Polycrystalline samples of Fe Mo4 Ge3 have been synthesized by the reduction of an oxide mixture at 1248 K and characterized by a combination of diffraction, muon spin relaxation (μ+ SR), Mössbauer spectroscopy, magnetometry, transport, and heat-capacity measurements. The compound adopts a tetragonal W5 Si3 structure (space group I4 mcm); the iron and molybdenum atoms are disordered over two crystallographic sites, 16k and either 4a or 4b. The synthesis conditions determine which fourfold site is selected; occupation of either leads to the presence of one-dimensional chains of transition metals in the structure. In both cases, the electrical resistivity below 200 K is ∼175 μΩ cm. The dc magnetization rapidly rises below 35 K (Fe Mo on 16k and 4b sites) or 16 K (16k and 4a sites), and a magnetization of 1 μB or 0.8 μB per Fe atom is observed in 4 T at 2 K. The ac susceptibility and the heat capacity both suggest that these are glasslike magnetic transitions, although the transition shows a more complex temperature dependence (with two maxima in χ″) when the 4b sites are partially occupied by iron. No long-range magnetic order is thought to be present at 5 K in either structural form; this has been proven by neutron diffraction and μ+ SR for the case when Fe and Mo occupy the 16k and 4b sites. © 2008 The American Physical Society.

Abstract:

We have used zero-field muon-spin rotation to study the local magnetic properties of SrRu O3, Y2 Ru2 O7, and La2 Ru O5. While SrRu O3 and Y2 Ru2 O7 give rise to coherent precession signals, in La2 Ru O5 we find no signature of long range magnetic order below the structural transition, but observe relaxation consistent with that expected due to a dimerization into spin-singlet states. In Y2 Ru2 O7 we observe two precession signals with different temperature dependences, consistent with the nature of the noncollinear ordering being temperature dependent. © 2008 The American Physical Society.

Abstract:

Zero-field muon-spin relaxation measurements of the frustrated triangular quantum magnet AgNiO2 are consistent with a model of charge disproportionation that has been advanced to explain the structural and magnetic properties of this compound. Below an ordering temperature of TN=19.9(2) K we observe six distinct muon precession frequencies, due to the magnetic order, which can be accounted for with a model describing the probable muon sites. The precession frequencies show an unusual temperature evolution which is suggestive of the separate evolution of two opposing magnetic sublattices.

Abstract:

We have developed an analytical model of angle-dependent magnetoresistance oscillations (AMROs) in a quasi-two-dimensional metal in which magnetic breakdown occurs. The model takes account of all the contributions from quasiparticles undergoing both magnetic breakdown and Bragg reflection at each junction and allows extremely efficient simulation of data which can be compared with recent experimental results on the organic metal κ- (BEDT-TTF)2 Cu (NCS)2. AMROs resulting from both closed and open orbits emerge naturally at low field, and the model enables the transition to breakdown-AMROs with increasing field to be described in detail. © 2008 The American Physical Society.