Magnetization measurement system with giant magnetoresistance zero-field detector

Abstract

Magnetic hysteresis loop measurement [1]-[4] is crucial for the development of advanced magnetic materials. From the magnetization versus magnetizing field (M-H) curve, one can extract the useful magnetic properties, which are important for applications in sensors, transformers, actuators, and miscellaneous electrical devices. The M-H measurements were usually done by using the vibrating sample magnetometer [1] or the SQUID magnetometer [2], for which the magnetizing field are virtually static during the measurement. The induction-coil-based hysteresis loop and susceptibility measurement systems [3], [4] are rapid and low-cost devices capable of determining the alternating-current (AC) magnetic properties, which is important for inferring the frequency-dependent features such as the energy loss due to eddy currents in conducting materials. The convention hysteresis loop measurement device [3] is designed for the magnetic induction versus field (B-H) measurement of toroidal samples. The alternating susceptibility measurement system (AC susceptometer) is capable of determining the frequency dependent initial magnetization and susceptibility. But the induction coil can’t detect the environmental direct-current (DC) field. For the soft magnetic materials, the non-zero environmental magnetic field can significantly alter the measured result, which must be compensated by using a magnetic sensor to detect the unwanted magnetic interference.To achieve higher accuracy in the M-H measurement, we proposed a M-H measurement system combining the signal processing method of B-H measurement and the coil design of the AC susceptometer. The interference from the environmental magnetic field is eliminated by using a giant magnetoresistance (GMR) sensor as the zero field detector, as shown in Fig. 1. The system consists of solenoidal magnetization and pickup coils. The pickup coil consists of reference and sensing coils symmetrically positioned with respect to the excitation coil. The sensing coil detects the magnetization of the sample, whereas the reference coil is wound in the opposite direction to null out the magnetizing signal. The signal output of the integrator is proportional to the magnetization of the sample.The current in the magnetization coil is directly proportional to the magnetizing field strength H. However, the DC level of the magnetizing field at the sample is usually shifted by the environmental field, which can’t be observed by monitoring the current in the coil. To solve the problem, the GMR sensor, which is the GF708 spin-valve sensor [5] from Sensitec GmbH, is used for the zero field detector. The sensor has a very high sensitivity but the dynamic range less than 2 Oe. The output of GMR is used as the trigger for taking the M-H curve to eliminate the disturbance from the environmental DC field, as shown in Fig. 2. The 1-Hz sinusoidal current signal measured by a resistor shows a peak-to-peak amplitude of 0.96 A, corresponding the magnetizing field amplitude of 6.7 mT. When the environmental DC field was completely eliminated in the pickup coil, the observed output signal of the GMR has a zero-field-crossing point defined by the trigger level, shown as in Fig. 2. The trigger point to activate the M-H measurement is at the intersection of the trigger level line and the GMR signal curve for “No external field”. When the DC field (of about 0.77 mT) was applied, the trigger point shifted by 40 ms in comparison with the zero-field curve, while the current waveform in the coil remained unchanged. The new trigger point to activate the M-H measurement is at the intersection between the trigger level line and the GMR signal curve with “External field”. The coercivity and the asymmetry pinning field of the GMR sensor induces a constant time shift in the trigger point, which can be corrected by employing a calibration procedure provided that the sweeping magnetizing field always saturates the output of GMR on both field polarities. With the GMR zero-field detector to set the trigger point, the high-quality M-H curve can be obtained by taking averaging for multiple cycles of the sweeping magnetizing field. In this way, the accuracy of the M-H curve measurement for the soft magnetic materials is successfully improved by eliminating the interference from the environmental field without using a costly magnetic shield chamber.This work is supported by the Ministry of Science and Technology of Taiwan under Grant No. MOST108-2221-E992-083MY2. **