The Magnetoelectronic Materials and Devices Section of the Center for Materials Physics and Technology performs basic and applied research on emerging functional materials and spin-dependent phenomena therein, with an emphasis on understanding and relating the macroscopic behavior to intrinsic and extrinsic atomic scale properties. Our goal is to discover and understand new properties and functionality in these materials to develop advanced technological applications of interest to the Department of the Navy. Particular areas of interest include topologically protected states in condensed matter (e.g. topological insulators, Weyl semimetals); spin transport and spin-related behaviors in both established (e.g. silicon) and emerging (e.g. graphene) materials; single monolayer materials (often called two-dimensional materials) with an emphasis on those beyond graphene (e.g. molybdenum disulphide, tungsten diselenide) and van der Waals heterostructures constructed from these monolayers; and transitioning these efforts to technological applications, including advanced magnetic tunnel junctions for information processing, and novel chemical vapor sensors for detection of chemical and biological threats. The Section has extensive facilities for state-of-the-art synthesis and characterization which enable determination and correlation of the structural, optical, electronic, and magnetic material properties from the nanoscale to the macroscopic. The Section develops innovative scientific and engineering solutions from fundamental physics through the prototyping of devices for Naval applications.

Current Research

Experimental concept of the potentiometric measurement of the topological insulator (TI) surface state spin system, where the projection of the TI spin onto the magnetization of the ferromagnetic contact is measured as a voltage. Topologically Protected States in Condensed Matter

A 3-dimensional topological insulator (TI) is a new quantum state of matter characterized by an insulating bulk and gapless surface states populated by massless Dirac fermions that exhibit many intriguing properties. One of the most striking is that of spin-momentum locking, where the spin of the TI surface states lies in plane and is locked at right angles to the carrier momentum, so that an unpolarized charge current creates a net spin polarization. We interrogate this TI surface state spin system via electrical charge-to-spin and spin-to-charge conversions, optical detection by magneto-optical Kerr effect, and spin transfer torque switching of an adjacent ferromagnet.

2-Dimensional Materials

Graphene heralded the remarkable new properties and science of single monolayer materials. Many other materials share the weakly interacting layered character of graphite - these include semiconductors with an intrinsic band gap such as the transition metal dichalcogenides MX2, where M = molybdenum (Mo) or tungsten (W), and X = sulfur (S), selenium (Se) or tellurium (Te), superconductors such as magnesium diboride and insulators like hexagonal boron nitride. These monolayers exhibit properties very different from the bulk. For example, bulk MoS2 and WSe2 have indirect bandgaps of 1.3 electron-volts and 1 electron-volts respectively, but transform to direct gap character as the thickness decreases from 2 layers to single monolayer, demonstrating the extraordinary potential for new properties and functionality. Theory has predicted similar dramatic property evolution in many other single layer materials, but very few have been examined experimentally. These include metals, insulators, semiconductors, ferromagnets and even potential superconductors.

The goals of this program are to develop the synthesis of single layer samples of these materials, and determine the optical, electronic, chemical and mechanical properties they exhibit. The experimental and theory efforts pursued here will enable us to design and synthesize two-dimensional materials with predefined properties, and provide avenues to engineer and control the fundamental excitations. These scientific goals have relevance to high-speed, low-power electronics, information processing, chemical/biological sensors, photodetection and energy harvesting.

Low temperature and room temperature operation of the homoepitaxial graphene spin valve (left) and a schematic (right) of one of the homoepitaxial fluorinated-graphene/graphene spin valve devices. Distinct steps in the resistance appear at the coercive fields of the ferromagnetic contacts, producing plateaus of higher resistance when the ferromagnetic contact magnetizations are anti-parallel, as indicated by the black arrows. Only a 50% decrease in magnitude is observed from 10 Kelvin to room temperature. T Spintronics

Next generation information systems require new technology since current charged-based devices are reaching their physical limits in terms of size and heat dissipation. This means that today's technology is not suitable for applications where "supercomputer-like" information processing in the field is needed to process real-time data from satellite and sensor inputs. Spin-based devices, enabled by this research, use an additional degree of freedom, the spin state. This allows new technology to do more complex tasks. The results from this research have a significant impact on the Department of Defense missions, specifically for portable electronics where weight and power consumption are key criteria.

van der Waals Heterostructures

The objective of this program is to advance a new paradigm for heterostructures by systematically stacking monolayers of materials that exhibit a layered character in the bulk, determine the new electronic and optical properties these heterostructures exhibit, and develop a fundamental understanding of the dominant interactions. These monolayer building blocks possess exceptionally strong intra-layer bonding and weak inter-layer bonding, and are often referred to as "two-dimensional materials". Graphene is the prototypical example, but there are many more, including the transition metal dichalcogenides (e.g. molybdenum disulphide, tungsten diselenide), hexagonal boron nitride, and others. No chemical bonds exist between the layer planes. The weak interlayer bonding enables a new approach for "materials by design" through van der Waals epitaxy of non-lattice matched two-dimensional materials, an avenue not possible with traditional epitaxial approaches dominated by out-of-plane bonding. This represents a "bottom up" approach towards design and fabrication of new materials that do not exist in nature, and a new class of atomic-scale heterostructures that are expected to exhibit properties and functionality beyond the limits of their bulk counterparts.

Through the fabrication and theory efforts proposed here, this van der Waals epitaxy approach will enable us to design and construct heterostructures with new properties, and provide avenues to engineer and control their fundamental excitations. The expected payoffs include discovery of new material properties, an improved fundamental understanding of behavior in these few layer heterostructures, and the development of new capabilities to predict and fabricate two-dimensional-based heterostructures with desired properties and functionality.

Transitions and Applications

Our basic research efforts produce scientific discoveries and advances which are transitioned into applications oriented programs. The goal of these programs is to develop new material functionality and demonstrate prototypes to meet the future needs of the Navy, Marine Corps and Department of Defense. One of these programs is highlighted here.

  • Advanced Magnetic Tunnel Junction