Disentangling Highly Asymmetric Magnetoelectric Effects in Engineered Multiferroic Heterostructures

Abstract

One of the main strategies to control magnetism by voltage is the use of magnetostrictive-piezoelectric hybrid materials, such as ferromagnetic-ferroelectric heterostructures. When such heterostructures are subjected to an electric field, piezostrain-mediated effects, electronic charging, and voltage-driven oxygen migration (magnetoionics) may simultaneously occur, making the interpretation of the magnetoelectric effects not straightforward and often leading to misconceptions. Typically, the strain-mediated magnetoelectric response is symmetric with respect to the sign of the applied voltage because the induced strain (and variations in the magnetization) depends on the square of the ferroelectric polarization. Conversely, asymmetric responses can be obtained from electronic charging and voltage-driven oxygen migration. By engineering a ferromagnetic-ferroelectric hybrid consisting of a magnetically soft 50-nm thick Fe 75 Al 25 (at. %) thin film on top of a 110-oriented Pb(Mg 1/3 Nb 2/3)O 3-32PbTiO 3 ferroelectric crystal, a highly asymmetric magnetoelectric response is obtained and the aforementioned magnetoelectric effects can be disentangled. Specifically, the large thickness of the Fe 75 Al 25 layer allows dismissing any possible charge accumulation effect, whereas no evidence of magnetoionics is observed experimentally, as expected from the high resistance to oxidation of Fe 75 Al 25 , leaving strain as the only mechanism to modulate the asymmetric magnetoelectric response. The origin of this asymmetric strain-induced magnetoelectric effect arises from the asymmetry of the polarization reversal in the particular crystallographic orientation of the ferroelectric substrate. These results are important to optimize the performance of artificial multiferroic heterostructures.

Voltage-driven strain-mediated modulation of exchange bias in Ir20Mn80/Fe80Ga20/Ta/⟨011⟩-oriented PMN-32PT heterostructures

“…3(b,c,d). The overall dependences of HC and MR/MS with voltage resemble those measured along the [100] direction on FeAl sputtered onto <110>-oriented PMN−32PT single crystals, ascribed to electric field-induced piezoelectric substrate deformation 13 . This is expectable since both FeAl and FeGa display similar magnetostrictive behavior 40 .…”

“…By replacing magnetic field actuation (through the use of miniaturized electromagnets) with electric fields (created by external voltage), it is possible to drastically reduce Joule heating effects, thereby minimizing electric power dissipation [1][2][3][4][5][6][7] . Converse magnetoelectric actuation has been shown to be an effective means to tune the coercivity, saturation magnetization or squareness ratio (i.e., effective magnetic anisotropy) of thin films, taking advantage of electric surface charging (or charge carrier modulation in ferromagnetic semiconductors) 8,9 , magneto-ionic effects (voltage-driven ion motion) 10,11 or strain-mediated multiferroic coupling [12][13][14] , when the magnetic films are grown in direct contact with piezoelectric or ferroelectric underlayers or substrates.…”

“…In this work, we investigate voltage-control of exchange bias in IrMn/FeGa/Ta systems grown onto PMN−32PT. This particular ferroelectric substrate is selected because of its large piezoelectric coefficients for a composition near the morphotropic phase boundary and because of the highly asymmetric in-plane voltage-induced strains when out-of-plane poling is applied along the <011> direction 13,37 . We observe that, once the training effect is subtracted, voltage in this system can induce either an increase or a decrease of the exchange bias field with respect to the films with no voltage applied.…”

Strain-gradient effects in nanoscale-engineered magnetoelectric materials

“…Suitable voltages should be selected to achieve maximum reduction in coercivity so that information can be written with low applied magnetic fields (low electric currents and minimizing power dissipation by the Joule heating effect). 78 As flexoelectric effects are inversely proportional to the features' size, i.e., "bits" size, larger effects are anticipated at the nanoscale. This is appealing for high density magnetic recording media.…”

Controlling the magnetoelectric coupling in Ni/BiFe0.95Mn0.05O3 with a dielectric spacer

“…The FE polarization reversal results in the same strain-induced magnetic state at remanence for both the positive and negative voltage branches, which renders this effect unsuitable for non-volatile memory applications. Recent studies have shown that asymmetric polarization curves [27][28] and specific polarization reversal paths related to the FE domain structure [29][30][31][32] provide a possibility to break the symmetry of ME curves in strain coupled heterostructures.…”