Search

The ESR spectrum of a pair of paramagnetic spins $S=\frac{1}{2}$, interacting via dipolar magnetic forces, is calculated in the presence of an incoherent phonon avalanche, which is coupled to the spin pair by linear spin-phonon interaction. The problem is approximately solved by a Green's-function approach. Two cases are discussed, the first for an infinitely narrow phonon spectrum, and the second for finite bandwidth of the avalanche phonons. When the ESR frequency falls out of the avalanche band, the spectrum of the pair is shown to consist of two lines shifted from the no-avalanche resonant field and more closely spaced than in the no-avalanche situation. When the ESR frequency is within the avalanche band, a sharp decrease of the ESR intensity is expected. These results are interpreted in terms of ac Stark shift and of screening of dipolar interaction by the avalanche phonons. Their relationships to electromagnetic power narrowing of ESR lines and to the "magic angle" effect in NMR are discussed. Finally the theoretical results are examined in connection with phonon-avalanche experiments on ${\mathrm{Ce}}^{3+}$-doped lanthanum magnesium nitrate.

The ground state of a three-dimensional crystal with strong spin-phonon interaction is shown to possess a domainlike structure with domains separated by walls. Under the action of an external magnetic field, the walls are shown to acquire dynamical properties. These properties are investigated and the possibility of evidencing experimentally the quantum-mechanical motion of domain walls is briefly discussed, together with the connection between strong-spin phonon models and the dynamic clusters found in displacive materials with double-well atomic potentials.