Insight into the mechanism of bidirectional magnetoelectric effects unveil cycloidal/uncompensated hybrid antiferromagnetic multiferroics

Abstract

Evaluation of the bidirectional magnetoelectric (ME) effect using a ${\mathrm{BiFeO}}_{3}$ (BFO)/NiFeCuMo bilayer reveals that the electronically controlled ME effect ($E\ensuremath{\rightarrow}M$) and magnetically controlled ME effect ($H\ensuremath{\rightarrow}P$) have individual origins. (i) ME effect ($H\ensuremath{\rightarrow}P$): The ferroelectric polarization, evaluated using the hysteresis loops and positive-up--negative-down method, changed quadratically as a function of the magnetic field. This quadratic change is evidence of a cycloidal magnetic structure like that of bulk BFO. (ii) ME effect ($E\ensuremath{\rightarrow}M$): Magnetization switching was observed via electric-field-induced ferroelectric polarization switching through an ${H}_{\mathrm{EB}}$. In principle, the cycloidal magnetic structure does not exhibit an electrically controlled ME effect owing to the disappearance of ${H}_{\mathrm{EB}}$ because BFO ${(0\phantom{\rule{0.16em}{0ex}}0\phantom{\rule{0.16em}{0ex}}1)}_{\mathrm{pc}}$ is the compensated plane. However, an uncompensated weak ferromagnetic (FM) interface emerged near the BFO/NiFeCuMo by interfacial roughness, strains and defect rather than interfacial oxidation, resulting in ${H}_{\mathrm{EB}}$. An analysis of the bidirectional ME effect mechanism revealed that a hybrid of the cycloidal magnetic structure in the entire film body and uncompensated weak FM emerged near the interface, which developed BFO as an advanced cycloidal/uncompensated antiferromagnetic multiferroic. In addition, the strong ${H}_{\mathrm{EB}}$ realized perfect switching of magnetization polarities at zero electric fields, and the highly oriented ferroelectric domain realized the repetition of the ME effect.