Determination of easy axis in a chemical ferromagnet, 4-(p-chlorobenzylideneamino)-TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidin-1-oxyl)

The trapping sites for muon and muonium in ferromagnetic p-Cl-Ph-CH=N-TEMPO [(4-(p-chlorobenzylideneamino)-TEMPO (TEMPO = 2,6,6-tetramethyl-piperidin-1-yloxyl)] and the hyperfine interaction tensors for these sites are obtained using first-principles Unrestricted Hartree-Fock theory. The calculated hyperfine interactions are used to compare the calculated zero field muon spin rotation (μSR) frequencies for different choices for the easy axis and the observed frequency. It has been concluded that the two trapping centers that can best explain the observed μSR frequency are a trapped singlet muonium near the radical oxygen and a trapped muon site near the chlorine. The direction of the easy axis also is determined to be the b-axis of the monoclinic lattice. This direction for the easy axis is confirmed by determining the direction of the distributed magnetization in the molecular solid which leads to a minimum dipole-dipole interaction energy. The consequences of this agreement for the easy axis direction by two independent procedures are discussed.