On the Field Dependence of Spin-Lattice Relaxation*

Spin Lattice Relaxation (T₁) and spin-spin electron spin relaxation processes (T₂) are of importance for a variety of applications, like quantum information processing, quantum memory, spin cooling, and dynamic nuclear polarization. We studied the temperature dependence of the relaxation in the high magnetic field regime for a variety of paramagnetic spin systems. High frequency pulsed electron spin resonance at various fields and frequencies in the 3-14 T range was used to measure directly in the time domain. The low-temperature direct single-phonon spin lattice relaxation is found to have a strong field dependence (B²-B⁴) in the high frequency/field regime. A comparison is made between transition metal impurities in crystals, donor-bound electrons in semiconductors, and other systems of interest to quantum information processing. In spite of their small spin-orbit coupling, organic radicals in frozen solutions also show a marked field dependence of T₁. This is of importance to dissolution- and solid state dynamical nuclear polarization (DNP) for NMR and MRI studies. The experimental results are compared with existing theoretical models.