Electron-Spin-Echo Envelope Modulation Induced by a Very Small Alternating Magnetic Field

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Abstract
The effect of an additional very small alternating magnetic field h̄ on the electron spin echoes is reported. When single crystals of K3Co (CN)6 containing Cr3+, and CaWO4 containing Ce3+, as substitutional impurities were subjected to the extra field h̄, the echo envelope was modulated at the frequency of the extra field ranging from 100 to 2000 kHz. The magnitude of the additional field h̄ ranged from 0.1 to 0.85 G and the depth of the periodic modulation of the echo envelope was directly dependent on the magnitude of h̄. The maxima and minima in the envelope occurred when the period T of the additional field h̄ was related to the time interval between the microwave pulses by τ=nT and τ=(n+½)T, respectively. The results are discussed in view of the paper by M. Dupont and G. A. Woonton [Can. J. Phys. 46, 87 (1968)] which shows that the additional magnetic field h̄ contributes to these effects only at those instants when microwave pulse is also present, provided the field h̄ is homogeneous over the sample. A classical treatment of the problem is presented. The effect of additional field h̄ on the echo amplitude is decided by the magnitude of h̄ relative to microwave field in the cavity and not relative to steady magnetic field H0.