The velocity axis was calibrated by simultaneous laser interferometry. Samples and the detector foil were cooled to ~5 K with liquid He during the experiments. Least-squares fits were performed in Matlab using 'lsqcurvefit' for a magnetic Hamiltonian and a magnetic plus perturbing quadrupolar Hamiltonian (resulting in the component of electric field gradient in the direction of the magnetic field). All fits were performed after subtracting a polynomial background (to account for the time-windowing effect inherent in SR-MS ) and kept a constant FWHM for all Lorentzian-broadened peaks of 0.8 mm/s with fitting parameters of 1) intensity, 2) B and, in some cases, 3) Vzz. Standard error of fit parameters were calculated using the "nlparci" function in Matlab.