Phase Diagram and Soliton Picture of a Spin-Peierls Compound D-F5PNN

We investigate magnetic properties of an S = 1/2, quasi-one dimensional organic antiferromagnet, D-F5PNN using magnetization measurements taken at temperatures as low as 0.5 K. Three distinct phases were observed consisting of uniform, dimerized (D), and incommensurate (I) spin structures in the magnetic field versus temperature plane, where a significant hysteresis appears between DI transitions in the field scan measurements. A combination of magnon (S = 1) and soliton (S = 1/2) excitations have successfully reproduced the observed magnetic susceptibility. In addition, such excitations provide a reasonable interpretation of the temperature dependent electron spin resonance (ESR) spectra. These results indicate that D-F5PNN is an ideal candidate for studying the spin-Peierls transition.