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Measurement of Spin-Lattice Relaxation Times at Millikelvin Temperatures

We present the first measurements of spin-lattice relaxation times with the energy being transferred from the lattice to the spins. On a semi-adiabatically suspended bolometer, consisting of a paramagnetic sample and an absorber, the lattice is heated up by absorbing single α-particles, and the relaxation of the magnetisation is measured with a SQUID of special design. With decreasing temperatures the relaxation time first increases as τ1 ∼ 1/T, but then it decreases strongly and at 30 mK it is reduced by orders of magnitude compared with τ1. This result is in agreement with a theoretical model which takes into account the heat capacities of the lattice, the resonant phonons and the spins and thermal resistances between these capacities. Under the condition of the bottleneck effect the relaxation time is found to be proportional to T3 at low temperatures.