Patent ID: 8027110

Claim:
A magnetic field measurement apparatus that measures an in-plane high frequency magnetic field intensity generated from a microwave generation mechanism provided in a microwave-assisted magnetic head, the magnetic field measurement apparatus comprising: a fixing mechanism that fixes the microwave-assisted magnetic head that is a subject to be measured; a high frequency current drive system that applies a high frequency current to the microwave generation mechanism provided in the microwave-assisted magnetic head; an in-plane high frequency magnetic field intensity measurement element that measures the in-plane high frequency magnetic field intensity generated from the microwave generation mechanism, the in-plane high frequency magnetic field intensity measurement element being one of a microstrip line and a coplanar wave guide that functions to obtain a production output voltage by electromagnetic induction from the in-plane high frequency magnetic field generated from the microwave generation mechanism; a measurement circuit system connected to the one of the microstrip line and the coplanar wave guide, which is the in-plane high frequency magnetic field intensity measurement element; and a three-dimensionally movable stage on which is mounted the one of the microstrip line and the coplanar wave guide, which is the in-plane high frequency magnetic field intensity measurement element, wherein the measurement circuit system includes an amplifier, a mixer circuit and a band-pass filter that are connected in order on an output end side of the in-plane high frequency magnetic field, a frequency immediately before being inputted in the band-pass filter is down-converted by the mixer circuit to a frequency so that a band width of the band-pass filter can be used, the band-pass filter uses a narrow band of ±0.5-±10 KHz (1 KHz-20 KHz as a bandwidth) centering a fundamental frequency selected from 5-20 MHz that is down-converted by the mixer circuit as a center peak passing frequency, and the measurement circuit system is configured to obtain 3 dB or greater of a signal-to-noise ratio (SNR) that is a ratio of S with N; where S represents the reproduction voltage (reproduction output) of the high frequency reproduction signal induced by the in-plane high frequency magnetic field intensity measurement element, and N represents a total noise voltage of a circuit noise including a noise generated by the in-plane high frequency magnetic field intensity measurement element.