The magnetic resonance of iron-group impurities in dielectric host lattices with high Debye temperatures is shown to be easily detectable by the resulting rise in temperature on resonance of a sample weakly attached to a heat sink at 4 K. The heat causing the temperature rise is provided by spin-lattice relaxation in the sample, which is in a magnetic field and subject to electromagnetic radiation at the electron paramagnetic resonance (EPR) frequency. The experimental technique is very simple, and offers the possibility of studying those impurities with broad EPR lines and short spin-lattice relaxation times that are undetectable by conventional methods. In addition there is the possibility of investigating the subsequent thermal history of the monochromatic phonons released during spin-lattice relaxation. Experimental results for the thermal EPR spectrum of a variety of paramagnetic ions in MgO are presented, and the saturation behaviour of magnetic ions with widely varying strengths of coupling to the lattice is compared. Typically we find temperature rises of 10 mK from 1016 spins/cm3 with relaxation times 0·1 s, independent of the linewidth. These figures lead to an observed sensitivity of 1015 spins/cm3 with relaxation time 10−1 s or 1010 spins/cm3 with relaxation time 10−6 s, on the conservative assumption that 1 mK is the minimum detectable temperature rise.

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