Nonlinear magnetoelectric effects in flexible composite ferromagnetic - Piezopolymer structures

Nonlinear magnetoelectric (ME) effects in a flexible composite planar structure, containing mechanically coupled layers of amorphous FeBSiC ferromagnet and PVDF piezoelectric polymer have been experimentally investigated. Under the action of a weak harmonic magnetic field h with frequency f = 50-1000 Hz and tangential bias magnetic field H = 1-80 Oe, the structure generated a voltage of the same frequency. The efficiency of linear ME conversion reached 3.4 V/(cm·Oe) for the optimum bias field Hm ≈ 15 Oe. On increasing the excitation field up to h ∼7 Oe, the structure generated second and third harmonics with efficiencies of ∼25 mV/(cm·Oe2) and ∼2.5 mV/(cm·Oe3), respectively. The amplitudes of the harmonics were not monotonous functions on the bias field H and grew with the increase in the alternating field h. Under the action of two alternating fields with different frequencies f1 and f2, the structure generated ac voltages with frequencies equal to the sum and difference frequencies f1 ± f2. The efficiency of magnetic fields mixing reached a maximum of ∼30 mV/(cm·Oe2) in the absence of the bias field. The effects of harmonics generation and magnetic fields mixing arise due to the nonlinear dependence of the ferromagnet's magnetostriction λ on the bias field H. The efficiency of the nonlinear processes is proportional to the derivatives of the magnetostriction over magnetic field. The nonlinear ME effects in the ferromagnet-piezopolymer flexible structures can be used to design high-sensitivity dual ac/dc magnetic field sensors and energy harvesting devices.