Single crystal (ZnY) (Ba2Zn2Fe12O22) was used in an investigation of the variation of the magnetic susceptibility with microwave magnetic field. The large anisotropy field in these materials makes them attractive for applications in microwave devices. Measurements were made in the frequency range of 8.6 to 37.7 kMc. The microwave magnetic field in the cavities extended up to 35 oe. Measurements taken at a constant frequency indicate that there is an appreciable shift in the field required for resonance as a function of power. This shift in the resonance field is superimposed on the usual decline of the main resonance. Measurements indicate that the shift effect of the resonance field decreases as the linewidth and frequency are increased. In the frequency range of 8.6 to 16.6 kMc, the critical field increases with frequency. Above this frequency, the critical field tends to decrease. AtX‐band frequencies, the critical field compares favorably with that of YIG material.