Magnetic field effect on the complex permeability spectra in a Ni–Zn ferrite

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Abstract
Complex permeability spectra μ*(=μ′−iμ′′) in a Ni–Zn ferrite was studied in the frequency range from 10 kHz to 3 GHz under dc magnetic field up to 1000 Oe. In the absence of a dc magnetic field, the μ′ spectrum has a frequency dispersion above 1 MHz; the μ′′ spectrum has a maximum at about 2 MHz. This feature can be described by the superposition of the two types of magnetic resonance, domain wall motion with a resonance frequency ωdwr=3.5 MHz and spin rotation with a resonance frequency ωspinr=8.0 MHz. Under dc magnetic field, low frequency permeability μ′ decreases with increasing static field bias. On the other hand, the μ′′ spectrum is broadened and two distinct peaks appear in the external field of 606 Oe. Under about 900 Oe external field, this ferrite becomes to have single-domain structure and the dispersion of domain wall motion in the permeability spectra disappears. In the above 700 Oe external field, high frequency dispersion of μ* shows ferromagnetic resonance characteristics.