Magnetoelectric (ME) sensors for the weak magnetic field measurement have attracted a lot of attention because of their high sensitivity and easy integration. However, the ME sensor performs well only at its working-point under a direct current (DC) bias field (Hbias) and mechanical resonance frequency (fres). The measurement of DC to ultralow frequency (0–100 Hz) weak magnetic fields has increasing demands, such as in geomagnetic anomaly fields, geological and mineral exploration, magnetocardiography, and magnetoencephalography. Unfortunately, fres of ME sensors is on the order of several tens of kilohertz, which is far higher than the ultralow frequency desired. Moreover, if the operation frequency deviates from fres, the sensitivity will deteriorate rapidly. In this study, a working-point perturbation method was used to measure the weak magnetic fields at 0–100 Hz with a high magnetic field resolution. (1) The perturbation of fres using an ultralow frequency (fac) magnetic field results in two modulation peaks with frequencies of fres ± fac. The frequency and resolution of the measured alternating current magnetic field can be obtained by varying fac and the modulation depth. A resolution around 1 nT for fac > 10 Hz and a lowest operation frequency of 0.1 Hz were achieved using our measurement system. (2) A high field resolution of 3 nT (better than the frequency perturbation method with a resolution of 16 nT at 0.1 Hz) can be achieved by the perturbation of Hbias at fres because the ME sensor is still working at the quasi-working-point and helped by lock-in amplifier technology.

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The antiferromagnetic (AFM) interlayer coupling effective field in the ferromagnetic/non-magnetic/ferromagnetic (FM/NM/FM) sandwich structures, as a driving force, can dramatically enhance the ferromagnetic resonance (FMR) frequency. Changing the non-magnetic spacer thickness is an effective way to control the interlayer coupling type and intensity, as well as the FMR frequency. In the study, FeCoB/Ru/FeCoB sandwich trilayers with Ru thickness (t Ru ) from 1 to 16 Å were prepared by a compositional gradient sputtering (CGS) method. It was revealed that a stress induced anisotropy is present in FeCoB films due to the B composition gradient in the samples. A t Ru dependent oscillation of interlayer coupling from FM to AFM with two periods was observed. An AFM coupling occurs in the range of 2 Å ≤ t Ru ≤ 8 Å and over 16 Å, while a FM coupling is present in the range of t Ru < 2 Å and 9 Å ≤ t Ru ≤ 14.5 Å. It is interesting that an ultrahigh optical mode (OM) FMR frequency in excess of 20 GHz was obtained in the sample with t Ru = 2.5 Å under an AFM coupling. The dynamic coupling mechanism in trilayers was simulated, and the corresponding coupling type at different t Ru was verified by Layadi's rigid model. This study provides a controllable way to prepare and investigate the ultrahigh FMR films.

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