Study of muon magnetic hyperfine and nuclear quadrupole interactions and easy axis in the organic ferromagnet p-Cl-Ph-CH = N-TEMPO

The first-principles Hartree-Fock cluster procedure has been used for studying the electronic structure of the organic ferromagnetic p-Cl-Ph-CH = N-TEMPO (4-(p-chlorobenzylideneamino)-2,2,6,6-tetramethylpiperidin-1-yloxyl) radical system with and without positive muon or muonium. Theory is able to provide quantitative information about the trapping sites for muon and muonium and the associated muon hyperfine interaction parameters. Using this information, two sites, one with muonium trapped near oxygen in the electronic singlet state of the muonium-radical pair system involved and the other with muon trapped near chlorine, are identified as likely candidates for the source of the experimentally observed μSR frequency of 3.2 MHz from zero-field measurement. The easy axis is obtained in both cases as the b-axis in the monoclinic lattice in agreement with the results of our magnetic dipole-dipole interaction studies using the magnetic moment distributions in the ferromagnetic state in the absence of muon and muonium. Nuclear quadrupole interaction parameters for nuclei adjacent to the trapped muon and muonium have been evaluated and their role in distinguishing between alternate trapping sites is discussed.