Kenichi Hatakeyama

Frequency dispersion and temperature variation of complex permeability of Ni‐Zn ferrite composite materials

Permeability spectra in Ni‐Zn ferrite composite materials were studied at the volume loading of ferrite above 30% and at temperatures from 100 to 400 K. The permeability decreased with decreases in the volume loading of ferrite. This decrease was much larger than that expected from the empirical mixing law. This was attributed to the demagnetizing field, generated by the magnetic poles on the surface of the ferrite particles. Simultaneously, the demagnetizing field increased spin resonance frequency. For the sintered ferrite, the primary peak of the permeability was just below the Curie temperature. The peak becomes obscure and disappeared as the volume loading decreased. The temperature dependence of the spin resonance frequency was lower in the ferrite composite material than that in the sintered ferrite. These features were also discussed from the view point of the demagnetizing field.

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

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.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.

Experimental study on direction dependency of reflection coefficients of microwave electromagnetic anechoic chamber

For measurements of reflection coefficients in an anechoic chamber by the free-space voltage standing wave ratio (VSWR) method, the composite antenna technique, using a combination of two antennas, is proposed. Reliable data on reflection coefficients can be obtained by this technique, with respect to reflected waves from arbitrary directions. Using the measurement results, the direction dependency of reflection coefficients is studied in relation to specular reflection and to the directivity of a transmitting antenna. >