Muons and magnets

Abstract:

We have investigated the molecular dynamics in the nematic liquid crystal 5CB using the ALC mu SR technique. Our measurements are consistent with a change in the amplitude of director fluctuations at the nematic-isotropic transition and we develop a model to describe this. (C) 2000 Elsevier Science B.V. All rights reserved.

Abstract:

We have measured the muon-spin rt laxation (mu SR) in La0.6Sr0.4MnO3 and in its layered sister system La1.2Sr1.8Mn2O7 to study the influence of dimensionality on their magnetic properties. Both compounds show ferromagnetic ordering with decreasing temperature. Above their Curie temperatures we followed the spin fluctuations. While the electron spin fluctuations in the three-dimensional La0.6Sr0.4MnO3 slow down critically as the transition temperature is approached they remain finite in the layered material La1.2Sr1.8Mn2O7. We can fit them to 2D critical behaviour with a lower T-C. In addition, we observe a spin precession signal in the ferromagnetically ordered state in La1.2Sr1.8Mn2O7; the temperature dependence of the muon precession frequency is in agreement with a three-dimensional character of the ordering. (C) 2000 Published by Elsevier Science B.V. Ail rights reserved.

Abstract:

Significant muon spin-lattice relaxation is found in a number of non-magnetic semimetals and metals. Measured in longitudinal magnetic field, the relaxation rate is independent of field up to several kilogauss and generally increases monotonically with temperature. This suggests a form of Korringa relaxation, originating in the hyperfine interaction between the implanted muons and the conduction electrons. Bearing in mind that NMR data on Korringa relaxation refers chiefly to the host nuclei, the muon offers a probe of conduction-electron encounter at an interstitial site, linking the topic to the nature of defect screening in metals, to relaxation by spin-density fluctuations in magnetic materials and to spin- and charge-exchange on paramagnetic muonium centres in semiconductors. Data are presented for C (graphite), Bi, Pb and Cd and compared with the Korringa predictions using known values of the muon Knight shift. Control experiments are described on Zn and Cu, both pure and deliberately doped with magnetic impurity. For graphite, an interpretation is given in terms of charge-exchange on a molecular radical formed by the chemical reaction of interstitial muonium. (C) 2000 Elsevier Science B.V. All rights reserved.

Abstract:

We present the results of mu SR experiments on a variety of molecular magnetic materials, either purely organic or combinations of transition metal ions and organic groups, which have been recently prepared. In a purely organic metamagnet, tanol suberate, we have observed a spin precession signal with a temperature dependence which has provided evidence of the two-dimensional nature of the antiferromagnetic ground state. In a family of dicyanamide-based molecular magnets with ordering temperatures of up to 21 K, and in a ferrimagnetic cobalt hydroxide, mu SR has been used to study the temperature dependence of the spin fluctuations. (C) 2000 Elsevier Science B.V. All rights reserved.

Abstract:

We have observed strong muon-spin relaxation (CBR) in the spin-Peierls compound MEM(TCNQ)(2) at temperatures down to 39 mK. We attribute this relaxation to the creation of defect spins by the muon. Furthermore, we observe a slowing down of spin fluctuations as the spin-Peierls energy gap opens, and we relate this effect to the size of the energy gap. (C) 2000 Elsevier Science B.V. All rights reserved.