MAGNETOELECTRIC COUPLING IN FERROELECTRIC/FERROMAGNETIC HETEROSTRUCTURES: BEYOND VOLUME EFFECTS

University Of Nebraska-Lincoln, Lincoln NE

Investigators

Abstract

This proposal focuses on the direct (changes in ferroelectric polarization with applied magnetic field) and converse (changes in magnetic properties with electric field) magnetoelectric coupling in thin film heterostructures of organic ferroelectrics with transition metal and half-metal ferromagnets and the fabrication, characterization and testing of two prototype devices that exploit these effects. A sensitive low-magnetic-field sensor based on the direct magnetoelectric effect will provide an electrical output proportional to the magnetic field strength, and a non-volatile magnetic memory cell based on the converse magnetoelectric effect, with electric-field-assisted writing and an optical readout of the magnetic state will allow for the well-know advantages of electric rather than magnetic field writing. The large mismatch in stiffness coefficients between the soft polymer ferroelectric and the much stiffer ferromagnets enables the exploration of coupling effects that go beyond the strain driven coupling between magnetostriction and piezoelectricity. The two prototype devices will provide a proof-of-concept that will enable the development of devices that are not dependent on strain, resulting in lower material fatigue, and therefore increasing device longevity. Preliminary results on the direct effect in heterostructures of organic ferroelectrics with ferromagnetic top electrodes show ferroelectric polarization changes that are orders of magnitude too large to ascribe to strain effects alone, but are dependent on the strain gradients at magnetic domain walls changing the polarization via the flexoelectric effect. Studies of the converse effect will focus on the electric field induced effect driven by the spin dependent electric field screening at the surface of a ferromagnet. The ability to apply large electric field is greatly enhanced by the large displacement charge density D at the surface of the polymer ferroelectric. Preliminary results indicate that the high ferroelectric surface charge drives large changes in the magnetic anisotropy in thin Co films. The intellectual merit lies in the elucidation of novel interface coupling effects that, with careful design, have the potential to produce macroscopic changes in ferromagnetic or ferroelectric ordered materials. The ability to accurately attribute and quantify the coupling in these particular heterostructures will lead to a significantly improved ability to design heterostructures with large interfacial coupling effects, as well as the ability to understand how the coupling is driven, an understanding that is applicable to a wide range of interfaces between differently ordered materials. The broader impact lies in the potential for technological benefits from applications in data storage and sensors, as exemplified by the prototype devices mentioned above that will lead to novel methods for magnetic field sensing and for non-volatile magnetic data storage. All three PIs are committed to a wide variety of activities that promote teaching and learning beyond the UNL physics department. These include direct interactions with K-12, such as regular visits to elementary school classrooms, the mentoring of numerous undergraduate students, the development of effective teaching materials for teachers in K-12, the mentoring of student teacher pairs from underrepresented institutions through the MRSEC summer teacher fellowships and public outreach to museums and civic organizations.