Linewidth Characteristics of Ce3+-Doped YIG

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Abstract
The linewidth characteristics of single‐crystal Ce3+‐doped YIG have been measured as a function of temperature and microwave frequency. The linewidth‐vs‐temperature curve exhibits a maximum at about 120°K. The temperature of this linewidth maximum is essentially the same both at 12 000 Mc/sec and 56 022 Mc/ sec. Of more direct interest in determining the linewidth mechanism is the observation that the linewidth increases linearly with microwave frequency both above and below the linewidth maximum. These results can be interpreted either in terms of a longitudinal or transverse relaxation mechanism. For the longitudinal mechanism, the microwave frequency ω and relaxation time τ enter the expression for the linewidth as ωτ/(1+ω2τ2), and the linear dependence of linewidth on frequency can only be accounted for by assuming ω2τ2≪1. In our experiment, this requires that the relaxation time be less than about 10−12 sec. For the transverse mechanism, the linewidth is proportional to ωτ in agreement with experiment, but for this mechanism to be operative τ must be sufficiently short so that there is overlap between adjacent energy levels above the ground state. With present infrared data, it is not possible to ascertain if this condition is satisfied.

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