Dynamic polarization echoes in metallic powders

Dynamic polarization echoes in normal and superconducting metal powders have been investigated. The dynamic echoes are found to be caused by the anharmonicity of the mechanical oscillators excited by rf fields in static magnetic fields, but not by the parametric coupling of the applied rf fields to the oscillator modes. Most of the experimental data showed a large signal effect even at the lowest applied pulse amplitudes consistent with adequate signal to noise. Effects of rf pulse interval and amplitudes, static magnetic field, temperature, and the angle between rf and static magnetic fields, $\psi$, on the echo amplitude are consistently explained by the calculation in the large signal regime, where the higher-order interactions between anharmonically oscillating modes are effective. Dependence of the echo amplitude on the angle $\psi$ revealed that the mechanical oscillation consists of different oscillation modes. The damping constant of the two pulse echo affected by surrounding media, rf frequency, static magnetic field, and temperature is well understood based upon the energy transmission from an individual oscillating particle to the media and internal loss such as energy absorption due to conduction electrons, thermal phonons, and crystalline imperfections.