Theoretical Investigation of Muon and Muonium Trapping and Hyperfine Interactions in the Chemical Ferromagnet p-NPNN (b-phase)

J. Jeong1, T. M. Briere3, N. Sahoo1,2, T.P. Das1, S. Ohira4, K. Nishiyama3, K. Nagamine3,4

1   Department of Physics, State University of New York at Albany, Albany, New York, 12222, USA
2   Department of Radiation Oncology, Albany Medical College, Albany, New York, 12208, USA
3  Meson Science Laboratory, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki, 305, Japan
4   Muon Science Laboratory, Institute for Physical and Chemical Research (RIKEN), Wako Shi, Saitama, 351-01, Japan

As part of our program for studying the hyperfine fields and easy axes in organic ferromagnets, we have investigated the trapping sites for muon (m+) and singlet and triplet muonium (Mu) sites in the b-phase of para –nitrophenyl nitronyl nitroxide (p-NPNN). Trapping sites are observed for m+ near both nitrogen and oxygen sites of the two NO groups in each NPNN molecule. Triplet Mu is found to be trapped near the two oxygen sites of the NO2 group while singlet Mu is trapped at both these two latter sites as well as at the nitrogen and oxygen sites of the two NO groups. The isotropic hyperfine constants and dipolar hyperfine tensors are being investigated at the various trapping sites and will be utilized together with the hyperfine field of 150 Gauss from mSR measurements [1,2] to assign the trapping sites associated with the experimentally observed hyperfine field and also to determine the easy axis direction.  The suggestion that a much larger observed hyperfine field of about 30 kGauss (mSR frequency of about 400MHz) [2] is associated with a trapped triplet Mu will also be tested using our calculated hyperfine fields at the muon in the trapped triplet Mu sites. Additionally comparison will be made between our theoretical predictions for m+ and Mu trapping sites and hyperfine interactions with those from earlier theoretical work [3] by different procedure.

[1]  L. P. Le et al., Chem. Phys. Lett. 206, 405 (1993)
[2]  S. P. Bhundell et al., Europhys. Lett. 31, 573 (1995)
[3]  M. I. J. Probert and A. J. Fisher, J. Phys. Condens. Matter. 9, 3635 (1995)

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