Background
It has long been recognized that active materials provide unique opportunities to meet challenges in communications, sensors, guidance systems, antennas, reconfigurable electronics as well as biological-chemical-physical detection and response systems. They have nonlinear and tunable properties, and they also provide coupling between electrical, magnetic, optical, mechanical, thermal and other properties. For example, the superior dielectric properties of ferroelectric materials have led to their application as filters in RF devices. Their nonlinear properties (χ²) hold promise for RF to optical conversion. Multiferroic materials enable negative refractive index materials, the extraordinary potential of which are only now being recognized. Ferromagnetic Heusler alloys are attractive for semiconductor and metallic spintronic devices such as magnetic tunnel junction devices for magnetic field sensing.

For all its promise, however, the current generation of active materials either has a limited response (e.g., limited opto-electric coefficient) or large hysteresis. These severely limit their applicability. There are also reasons from fundamental physics why certain couplings – for example ferromagnetism and ferroelectricity – are inherently limited in single-phase materials. The current multiphase materials suffer from hysteresis which is debilitating in many ways: it limits the frequency of operation, gives rise to undesirable heating, requires large power and shortens their fatigue life. Unfortunately, various attempts through incremental improvements of existing materials, highly strained epitaxy, super-lattice formation and phase-segregation have met with limited success. In addition, past attempts to reduce the hysteresis in transforming materials by manipulating defects have failed. For all these reasons, there is a compelling need to develop new classes of materials that display large response, couple different electromagnetic properties and have exceptionally low hysteresis.

Strategy
This project pursues a strategy that looks for materials that undergo structural phase transformations with extremely low hysteresis between two phases with an unusual combination of electro-magnetic-optical properties. This strategy brings together the idea that electronic structure and electro-magnetic-optical properties of materials depend extremely sensitively on the lattice parameters and crystal structure with the idea that one can have large first order phase transformations with little hysteresis. The former implies that unprecedented combinations of physical properties are possible at a phase transformation with a significant structural change. The latter relies on a new understanding of the origins of hysteresis explained in more detail below. The strategy is applicable to a wide range of material systems. Further, it is complementary to the other established approaches such as compositionally graded materials, super-lattices and phase-segregated composites.

This builds on three recent advances: (i) an understanding that the electronic structure and electromagnetic properties of materials depend extremely sensitively on crystal structure and lattice parameters. Thus, reversible structural phase transformations with a significant change of structure enable coupling of unusual properties, (ii) hysteresis in structural phase transformations is dramatically influenced by interfacial compatibility. This has been recently demonstrated by ternary modifications of the shape-memory alloy NiTi. By changing composition, the lattice parameters have been tuned to satisfy the conditions for an exactly compatible habit plane, and the hysteresis has been found to drop ten-fold, and (iii) the emergence of an understanding that a delicate interplay between local chemical disorder and non-local elastic and electromagnetic interactions gives rise to frustrated systems with fine-scale incipient phase mixtures at special compositions and display extremely low coercive fields. The morphotropic phase boundary in a relaxor perovskite is a prime example.

Approach
The approach is the theory-guided tuning of composition for high reversibility and low hysteresis, in the context of interesting combinations of properties that would not be possible in single-phase materials. Specifically, we use state-of-the-art density functional theory (DFT) methods for prediction of electromagnetic properties, together with DFT-informed atomistic, Monte-Carlo and nonlinear continuum methods for prediction of special conditions for low hysteresis. These guide high throughput methods of synthesis to identify interesting regions of composition, which in turn are fine-tuned using additional thin film and bulk synthesis methods

Materials Systems
We envision entirely new classes of materials, with no existing analogs. They have exceptional promise for electro-optical conversion, sensing, actuation, energy conversion and power generation. We currently focus initially on four systems – ferroelectrics and ferromagnetic perovskites with morphotropic boundaries, fuel-cell protonic conductors, ferromagnetic Heusler alloys and Cu-based super shape memory alloys – chosen to balance impact on applications and scientific feasibility. The management plan builds in flexibility to redirect resources to materials of exceptional promise that emerge as the research progresses.

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