The magnetoelectric (ME) effect, a coupling effect between magnetic and electric orders, has been widely investigated, both from a fundamental science perspective and an applications point of view. Magnetoelectric composites with one piezoelectric phase and one magnetostrictive phase can be magneto-electrically coupled via elastic strain mediation. Bulk magnetoelectric composites have been intensively studied as magnetic sensors due their significant magnetic-to-electric signal transforming efficiency, which promises high magnetic field sensitivity. In contrast, electric field-controlled magnetization in magnetoelectric thin films is more attractive for information recording and novel electrically-tunable microwave magnetic devices.

For the present work, we prepared a series of magnetoelectric structures capable of modulating the magnetization with an electric field -- all of which display unprecedented magnetic coercive field tunability. These structures show promise for a number of applications, including magnetic memory and spintronics. First, we generated self-assembled BiFeO3-CoFe2O4 (BFO-CFO) nanostructures of varying architectural structures on differently-oriented perovskite substrates. We were able to control aspect ratio through both thickness control and by manipulating growth thermodynamics. The relationship between magnetic shape and strain anisotropy was systematically analyzed using both in-plane and out-of-plane magnetic easy axis data. The BFO-CFO self-assembled structures may be useful for applications, including longitudinal and perpendicular magnetic memory; additionally they can serve as a prototype for analyzing the magnetoelectric effect-based magnetoresistive random-access memory. BFO-CFO grown on piezoelectric Pb(Mg,Nb)O3-PbTiO3 (PMN-PT) shows a large magnetoelectric coupling coeffcient.

Second, we sought to clarify the relationship between ferroelectric/ferroelastic phase transformation and the magnetoelectric effect in CFO films on PMN-PT heterostructures. Elastic strain is an essential component of electro-mechanical-magnetic coupling. Most prior studies that used piezoelectric materials as a strain source assumed that these materials shared a linear relationship (d31 or d33) with the electric field, which is true only with small electric field signals. In contrast, the largest strain is produced during phase transformation in piezoelectric single crystals. In this work, we systematically investigated electric field induced phase transformation in PMN-PT single crystals with different compositions. A signficant finding that emerged from this study is that a large in-plane uniaxial strain can be controlled by an electric field, and this strain can be used to control the magnetic easy axis distribution in the in-plane. The electric field is along the out-of-plane direction, which is perpendicular to the uniaxial strain and the surface of the sample, and thus can be easily incorporated into real device design.

Finally, we identified very large magnetic coercive field tunability in the CFO/PMN-PT monolithic structures -- in fact, more than ten times larger than previously reported magnetoelectric heterostructures. We used a <011> oriented PMN-PT substrate, where a large uniaxial strain can be induced by an electric field. Importantly, since the two in-plane directions have the same dimensions, the uniaxial strain can induce a significant magnetic anisotropy distribution change in the two in-plane directions. A unprecedented magnetic coercive field change of up to 580 Oe has been observed, which shows great potential for applications in both magnetic memory and microwave magnetic devices.