Geometry-Dependent Magnetoelectric and Exchange Bias Effects of the Nano L-T Mode Bar Structure Magnetoelectric Sensor

The geometry-dependent magnetoelectric (ME) and exchange bias (EB) effects of the nano ME sensor were investigated. The sensor consisted of the Longitudinal-Transverse (L-T) mode bi-layer bar structure comprising the ferromagnetic (FM) and ferroelectric (FE) materials and the anti-ferromagnetic (AFM) material. The bi-layer ME coefficient was derived from constitutive equations and Newton's second law. The trade-off between peak ME coefficient and optimal thickness ratio was realized. At the frequency × structure length = 0.1 and 1200, minimum and maximum peak ME coefficients of the Terfenol-D/PZT bi-layer were around 1756 and 5617 mV/Oe·cm, respectively, with 0.43 and 0.19 optimal thickness ratios, respectively. Unfortunately, the bi-layer could not distinguish the opposite magnetic field directions due to their similar output voltages. PtMn and Cr₂O₃, the AFM, were introduced to produce the EB effect. The simulation results showed the exchange field starting at a minimum PtMn thickness of 6 nm. Nevertheless, Cr₂O₃ did not induce the exchange field due to its low anisotropy constant. The tri-layer ME sensor consisting of PZT (4.22 nm)/Terfenol-D (18 nm)/PtMn (6 nm) was demonstrated in sensing 2 Tbit/in² magnetic bits. The average exchange field of 5100 Oe produced the output voltage difference of 12.96 mV, sufficient for most nanoscale magnetic sensing applications.