Giant Electric Field Tuning of Magnetism in Novel Multiferroic FeGaB/Lead Zinc Niobate–Lead Titanate (PZN‐PT) Heterostructures

Multiferroic composite materials consisting of both a magnetic phase and a ferroelectric phase are of great current interest, as they offer the possibility ofmagnetoelectric (ME) coupling, that is, electric field manipulation of magnetic properties (converse ME effect) or vice versa (direct ME effect), and have led to many novel multiferroic devices. One important series of such multiferroic devices is constituted by electrostatically tunable microwave multiferroic signal processing devices, including tunable resonators, phase shifters, and tunable filters. Compared to conventional tunable microwave magnetic devices, which are tuned by magnetic fields, these electrostatically tunable microwave multiferroic devices are much more energy efficient, less noisy, compact, and lightweight. ME effects can be realized in multiferroic composites through a strain/stress-mediated interaction, which enables effective energy transfer between electric and magnetic fields and leads to important new functionalities and devices. Strong ME coupling is critical for multiferroic devices; however, it has been difficult to achieve at microwave frequencies, leading to a very limited tunability in electrostatically tunable microwave multiferroic devices. The demonstrated tunable range of most of these devices has been very limited, with a frequency tunability of Df< 150MHz and a low tunable magnetic field of DH< 50Oe (1 Oe 79.6 A m ). This is mainly due to the large loss tangents at microwave frequencies of the two constituent phases, that is, the ferroelectric phase and, particularly, the magnetic phase, which is less resistive. The ME coupling strength in multiferroic composites is determined by many factors, such as the properties of the two constituent phases, the interface between them, the mode of ME coupling, and the orientation of the magnetic and electric fields. As a result, layered multiferroic heterostructures with magnetic thin films provide great opportunities for achieving strong ME coupling at microwave frequencies, owing to minimized charge leakage paths and low loss tangents associated with magnetic thin films. It is also desirable for the magnetic phase in the multiferroic composites to have a narrow ferromagnetic resonance (FMR) linewidth and a large piezomagnetic coefficient (dl/dH), that is, a large saturation magnetostriction constant (ls) and a low saturation magnetic field (Hs). However, suchmagnetic materials have not been readily available. Very recently, we have reported a new class of metallic magnetic FeGaB films that has a high ls of ca. 70 ppm, a lowHs of ca. 20Oe, and a narrow FMR linewidth of ca. 16Oe at X-band (ca. 9.6GHz). The maximum piezomagnetic coefficient of the FeGaB films is about 7 ppm Oe , which is much higher than those of other well-known magnetostrictive materials used in multiferroic composites, such as Terfenol-D (Tb-Dy-Fe), Galfenol (Fe-Ga), and Metglas (FeBSiC), as shown in Figure 1. The combination of narrow FMR linewidth and high piezomagnetic coefficient makes these FeGaB films excellent candidates for the magnetic material in microwave multiferroic composites. Single-crystal ferroelectrics such as lead magnesium niobate–lead titanate (PMN-PT) and lead zinc niobate–lead titanate (PZN-PT) having giant piezoelectric coefficients and low loss tangents are desired for microwave multiferroic composites as well. In particular, (011)-cut PMN-PT and PZN-PT single-crystal slabs have anisotropic piezoelectric coefficients d31 and d32 when poled along their [011] crystalline direction. For example, (011)-cut PZN-PT single crystals with 6% lead titanate have high anisotropic piezoelectric coefficients d311⁄4 3000 pC N 1 and d321⁄4 1100 pC N . The giant anisotropic piezoelectric coefficients of the PZN-PTsingle crystal provide great opportunities for generating a large in-plane magnetic anisotropic field and