Electric‐Field‐Tunable Low Loss Multiferroic Ferrimagnetic–Ferroelectric Heterostructures

Abstract

Multiferroic materials have attracted major attention in recent yearsduetotheuniquepossibilitytotune themagneticproperties with a modest electric field and vice versa. [1‐3] Up to now, however, no single phase material has been put forward that demonstrates a practical capacity for such tuning at room temperature. [4] This, in turn, has resulted in a shift in focus from single phase multiferroics to multiferroic heterostructures. [5] Besides the new materials aspect of such structures, there are numerous device benefits. These include fast wide-bandwidth tuning with low insertion loss in the microwave and millimeter wave frequency range, planar integratibility, and negligible power consumption. These benefits speak to the strong potential for practical information storage, logic device, radar, satellite, and telecommunication applications. Both multilayer thin-film stacks as well as composite nanopillar films can be produced by various methods to satisfy such needs. [6‐8] The layer-by-layer fabrication of multilayer structures is generally more straightforward than the complicated growth process needed for nanopillar films. Previous workers have studied a variety of layered stacks with a broad range of ferromagnetic or ferrimagnetic (FM) and ferroelectric (FE)/ piezoelectric (PE) components. [9‐13] Up to now, the main emphasis has been on FM‐PE layers and the tuning of the magnetic response through the electric-field-induced stress on the FM layer. In our view, however, electric-field-tunable hybrid magneto-electric modes in the monolithic FM‐FE structures can yield a better tuning of the magnetic response with electric fields and a better compatibility with various device applications. In principle, a micrometer-thick FM‐FE monolithic heterostructure, in combination with 10nm or so embedded biasing electrodes, for example, can offer (1) a good quasi-lattice-to-lattice contact with the layers to give rise to the best possible magneto-electric coupling and (2) an appreciable magnetic tuning at small applied voltages in the range of 15‐25V or so. However, the challenge to fabricate heteroepitaxial FM oxide‐ metal electrode‐FE oxide stacks with the key physical properties of each layer properly maintained and with extremely smooth interfaces has restricted the realization of such stacks so far. There have only been a few reports on thick mechanically assembled structures that (1) do not offer the lattice-to-lattice contact between the layers and (2) need about two orders of magnitude higher voltages than the monolithic structures. [14,15] In this report, we present the realization of a significant tuning of the magnetic response with extremely small applied voltages in a multiferroic FM‐FE heterostructure. The structure comprised a pulsed-laser-deposited near-single-crystal yttrium iron garnet (YIG) layer with very low microwave loss, an oriented barium strontium titanate (BSTO) layer with good electric-field-tunable relative dielectric permittivity er, and embedded platinum (Pt) electrodes on a single-crystal gadolinium gallium garnet (GGG) substrate. The results indicate an order of magnitude higher magneto-electric coupling than for the pasted nonmonolithic structures. Scheme 1 shows a concept diagram of the cube-on-cube unit cell match-up for an all oriented GGG-YIG-Pt-BSTO-Pt stack. The cubic YIG lattice, with a lattice parameter of 12.38A ˚ , matches to the GGG lattice within 0.03%. This close match, along with the compatible oxygen coordinations, serves to promote epitaxial YIG film growth. The lattice parameter for platinum is about 3.93A˚ . As apparent from the diagram, each YIG unit cell can reasonably accommodate nine face-centered-cubic (FCC) Pt unit cells with a mismatch of about 5%. Moving up the stack, the FCC BSTO lattice parameter is 3.95A˚ , for a mismatch of about 0.5% with the Pt. All told, this overall assembly is expected to yield a reasonable heteroepitaxial layered structure. For ease of visualization, the concept diagram shown here is for (100) plane layers. Similar match-ups apply to the actual structure developed here with (111) plane heteroepitaxy.