Fig. 3: Microwave transmission at the onset of spin-spin interactions.

Experimental (a) and simulated (b) changes in transmission associated with the coupling to DPPH spins measured at B = 125 mT as a function of frequency and temperature. The weak thin line near 3.38 GHz in (a) arises from a spurious mode in the transmission line (horizontal lines in Fig. 2a) that has not been fully eliminated by the data normalization. c Dependence of the spin resonance frequency Ω on the angle ψ that the magnetic field forms with the anisotropy axis of the spin-spin interactions, derived from Eq. (3). The line thickness is proportional to the contribution to the spin resonance, which increases for molecules forming larger angles and therefore, for larger frequencies. The central Ω obtained from the experiments in (a) and from the simulations in (b) is given by the gray open symbols and the solid black line, respectively.