Muons and magnets

Abstract:

We have performed semi-empirical molecular dynamics calculations of the electronic and molecular structure of muonium (=μ+e-) incorporated into the organic radicals 3-quinolyl nitronyl nitroxide (3-QNNN), para-pyridyl nitronyl nitroxide (p-PYNN), phenyl nitronyl nitroxide (PNN) and para-nitrophenyl nitronyl nitroxide (p-NPNN). These materials are of interest because they show ferromagnetic order at very low temperatures and can be effectively studied using implanted positive muons. Our calculations give evidence for a wide variety of possible muonium binding sites in the conjugated ring systems of the materials as well as in the nitronyl nitroxide group and suggest the formation of local spin triplet as well as singlet states near the muon, a result which is supported by recent experiments.

Abstract:

We present the results of a combined magnetization, muon-spin rotation, transport and magnetotransport study of the n = 3 Ruddlesden-Popper (RP) compound Ca4Mn3O10. This compound adopts a layered structure in which groups of three perovskite layers alternate with single rock-salt layers. The muon-spin rotation data show that there is a sharp magnetic phase transition at 115 K. The resistance and magnetoresistance of the sample show no particular features at this temperature, but the transition affects the energy barriers associated with hopping transport. The magnetoresistance is proportional to the square of the magnetization, and is largest at low temperatures; a 40% drop in resistivity is observed in a magnetic field of 14 T at 61 K, much smaller than that measured in the related n = ∞ RP (perovskite) manganites which exhibit colossal magnetoresistance (CMR).

Abstract:

Experiments on three double-layer (n=2) Ruddlesden-Popper (RP) systems are reported. Doping Sr1.8La1.2Mn2O7 (T-c = 126 K) with Nd to form Sr1.8La1.2-xNdxMn2O7 leads to a reduction in Curie temperature for low doping levels (x = 0.2), and to behavior reminiscent of Sr1.8Nd1.2Mn2O7 for x greater than or equal to 0.7. This suggests that it may be possible to control the temperature of maximum magnetoresistance chemically in these phases. The application of pressure (0 < P/GPa less than or equal to 1.8) is shown to modify the magnetotransport properties of Sr2NdMn2O7 to resemble those of Sr1.9Nd1.1Mn2O7. The changes can be explained by considering the relative strength of ferromagnetic and antiferromagnetic interactions within the material. Finally, the need for careful phase analysis of n = 2 RP materials is demonstrated by the misleading magnetization data recorded for a sample of Sr1.8Sm1.2Mn2O7 containing similar to 2.8% of an n = infinity perovskite phase. (C) 1998 American Institute of Physics. [S0021-8979(98)20911-X].

Abstract:

We report a muon-spin relaxation study of the two-dimensional spin-gap system CaV4O9. We find that the form of the muon-spin relaxation is strongly temperature dependent and attribute this behaviour to the presence of a spin gap. At temperatures below 30 K a root-exponential behaviour is seen for the muon-spin relaxation function, which can be attributed to defect spins. At temperatures above 160 K we see the onset of muon hopping.

Abstract:

Experiments on three double-layer (n=2) Ruddlesden-Popper (RP) systems are reported. Doping Sr1.8La1.2Mn2O7 (Tc = 126 K) with Nd to form Sr1.8La1.2- xNdxMn2O7 leads to a reduction in Curie temperature for low doping levels (x = 0.2), and to behavior reminiscent of Sr1.8Nd1.2Mn2O7 for x≥0.7. This suggests that it may be possible to control the temperature of maximum magnetoresistance chemically in these phases. The application of pressure (0