Junho Jeonga, Tina M. Briereb, N. Sahooa,c, T. P. Dasa,*, K. Nishiyamad and K. Nagamined,e
a Department of Physics, State University of New
York at Albany, Albany, NY, 12222, USA
b Institute of Materials Research, Tohoku University, Aoba-ku, Sendai, 984-8577, Japan
c Department of Radiation Oncology, The Albany Medical College, Albany, NY 12208, USA
d Meson Science Laboratory, Institute of Materials Structure Science, KEK, Tsukuba, Ibaraki, 305-0801, Japan
e Muon Science Laboratory, RIKEN, Wako, Saitama, 315-0198, Japan
* Corresponding Author: FAX 1-(518)-442-5260
The molecular crystal p-Cl-Ph-CH=N-TEMPO [abbreviation for 4-(p-chlorobenzylideneamino)-TEMPO (2,2,6,6-tetramethyl-piperidin-1-yloxyl)] is a chemical ferromagnet with Curie temperature about 0.4 K, whose weak ferromagnetism is ascribed [1,2] to an indirect exchange between the unpaired spin on each molecule (mainly localized on the N-O group in TEMPO) and its neighboring molecules, through the interaction between other parts of these molecules involving methyl and methylene groups. An accurate knowledge of the electronic structure and associated spin density distribution is needed for a quantitative understanding of the origin of the ferromagnetism in these materials. We have recently investigated  the electronic structure of the p-Cl-Ph-CH=N-TEMPO molecule by the Unrestricted Hartree-Fock Procedure and also the trapping sites of muon (m) and muonium (Mu) at various sites in this molecule through energy optimization. The magnetic hyperfine properties of the m at the various trapping site for m and Mu have been studied through evaluation of the hyperfine fields due to both intramolecular and intermolecular contributions. From the results for these hyperfine fields, the trapped m and Mu sites which can explain the observed muon spin rotation (mSR) frequency  and the direction of magnetization (easy axis) have been identified, the latter sites being the singlet Mu trapping site at the oxygen of the NO group and m at the chlorine atom. The easy axis found is in agreement with that obtained from magnetic dipole-dipole interaction between the magnetic moments on the NO group on a central molecule and the rest of the molecules in the lattice.
While this result provides a valuable check on the spin distributions in the individual molecules, a test of the electronic charge distribution and its anisotropy at the nuclei from both spherical symmetry and axial symmetry at various nuclei related to their nuclear quadrupole interactions can provide a check on the electronic structure which is complementary to that obtained for the magnetic moment distribution from analysis of mSR data and the easy axis direction.
For this purpose we have studied the nuclear quadrupole interactions [5,6] for the 14N nuclei on the NO group and the bridge nitrogen the 17O nucleus in the NO group and the 35Cl nucleus both in the molecular solid by itself and in the presence of trapped m and Mu. Comparison will be made between our results and available experimental quadrupole coupling constant (e2qQ) and asymmetry parameter (h) data. It is hoped that experimental studies related to the mSR technique  will be possible to apply to test the predictions for e2qQ and h at the various nuclei in the presence of trapped m and Mu.
 M. Yasui, H. Yoshikawa, H. Yamamoto, T. Ishida, T. Nogami
and F. Iwasaki, Mol. Cryst. Liq. Cryst. 279, 77 (1996).
 T. Nogami, T. Ishida, M. Yasui, F. Iwasaki, N. Takeda, M. Ishikawa, T. Kawakami and K. Yamaguchi, Bull. Chem. Soc. Japan. 69, 1841 (1996).
 Brief report by: Junho Jeong, T. M. Briere, N. Sahoo, T. P. Das, Seiko Ohira, K. Nishiyama and K. Nagamine, in Physica B289-290, 132 (2000). Detailed Paper under Preparation.
 Takayuki Ishida, Seiko Ohira, Tomoaki Ise, Kensuke Nakayama, Isao Watanabe, Takashi Nogami and Kanetada Nagamine, Chem. Phys. Lett. 330, 110 (2000).
 T. P. Das and E. L. Hahn “Nuclear Quadrupole Resonance Spectroscopy” Academic Press Inc., March 1958.
 S. Swingle Nunes, S. B. Sulaiman, N. Sahoo, T. P. Das, K. Bharuth Ram, M. Frank, W. Kreische and K. Bonde Nielsen, Hyperfine Interactions 120/121, 151 (1999).
 A. Schenck, “Muon Spin Rotation Spectroscopy, Principles and Application in Solids” Adam Hilger Ltd., Boston, (1985).
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