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With the rapid development of artificial intelligence and wearable devices, flexible magnetic sensors have attracted much attention in recent years. In this work, large self-biased 0–3 type poly(vinylidene fluoride-trifluoroethylene)/cobalt ferrite (P(VDF-TrFE)/CoFe2O4) flexible magnetoelectric (ME) composite films are proposed for the development of AC magnetic sensors. P(VDF-TrFE)/CoFe2O4 composite films with CoFe2O4 nanoparticle mass contents from 5 to 30% are successfully fabricated through the solution casting method. The results show that the nanoparticles are homogeneously dispersed in the P(VDF-TrFE) matrix, and the composite films have both excellent ferroelectric and ferromagnetic properties. The P(VDF-TrFE)/CoFe2O4 composite films all exhibit a large ME effect. The ME coefficient reaches 47.1 mV·cm−1·Oe−1 for the film with a CoFe2O4 nanoparticle mass content of 20% at resonance frequency. Moreover, the composite films have a large self-biased ME effect with a maximum value of 20.4 mV·cm−1·Oe−1, which is mainly due to the high remnant magnetization of CoFe2O4 nanoparticles. To evaluate the composite films for application in magnetic sensors, the response of the ME output voltage to an AC magnetic field without a bias DC magnetic field was measured. The good linear correlation coefficient, sensitivity and repeatability indicate that the proposed 0–3 type P(VDF-TrFE)/CoFe2O4 composite film is a promising material for flexible AC magnetic sensors.

Flexible magnetic sensors are attracting more and more attention because of their application in wearable devices. In this paper, Metglas/polyvinylidene fluoride (PVDF) bilayer composite with good flexibility was fabricated to evaluate its applicability as a flexible in-plane magnetic sensor. The magnetoelectric (ME) coupling characteristics and sensing performance of the sample were investigated under different test conditions, including different AC and DC magnetic field, and changing the direction of the magnetic field and the bending degree of the sample. The sample shows a large ME coefficient with a value of 176.41 V cm−1 Oe. The sensitivity, linearity and deviation of the sample are 892.96 mV Oe−1, 0.99965 and ±2% for the AC magnetic field, and 157.6 mV Oe−1, 0.99444 and ±5% for the DC magnetic field, respectively, and it shows excellent stability over repetitions. Moreover, the sample was gradually rotated anticlockwise in the magnetic fields. The output voltage of the sample varies with the rotation angle and has a good symmetry in plane, which is described well by a sine function. In addition, the clamping effect of the sample was studied. Even when bent, the sample still maintains an excellent and stable performance. The sensitivity and linearity of the sample with a bent angle of 23.5° are 254.37 mV Oe−1 and 0.99975 for the AC magnetic field, and 28.07 mV Oe−1 and 0.99309 for the DC magnetic field, respectively. The deviation of measurements is small for both the AC and DC magnetic sensors. In summary, the present study shows that the Metglas/PVDF bilayer composite has a good sensing performance and is suitable for = flexible in-plane resonant magnetic sensors.

Magnetic labels are one of the most important components in giant magnetoresistance (GMR) biodetection systems and affect detection signals directly. Many researchers have improved the detection performance of GMR biosensors by optimizing the properties of their magnetic labels, including size, shape, coercivity, and magnetization. In most studies, magnetic labels are usually spherical, and there is little research involving alternative shapes. In this work, we prepared spindlelike Fe3O4 nanoparticles (NPs) by hydrothermal and heating reduction. The as-prepared magnetic NPs were incorporated in the GMR biodetection system. An ultralow limitation of detection concentration of 0.05 ng/ml was achieved, which can be attributed to the shape anisotropy of the spindlelike magnetic particles. A wide linear work range, 0.05–1000 ng/ml, can be achieved by the use of the spindlelike Fe3O4 NPs in GMR biodetection. Moreover, the NPs exhibit good repeatability after multiple measurements, which can be attributed to the NPs’ stable and effective magnetic diffusion field. The use of spindlelike Fe3O4 NPs as magnetic labels in this work provides a new method for improving the sensitivity and repeatability of GMR biodetection.