Theory and Modelling for Antiferromagnetric Materials-based Spintronic Devices

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Spin-based electronic devices for information storage and memory rely on the efficient control of the spin up and spin down states. Conventionally, the direction of magnetization of a small magnet dictates these spin states and thus ferromagnetic materials are essential for spintronic devices. However, there are a few obstacles for further advancing the spin devices based on the ferromagnetic materials: the electric current density for switching the magnetization direction remains too large and the size reduction of memory elements is fundamentally difficult due to a strong magnetic interaction among nanometer-sized magnetic elements in close vicinity. The present proposal is to explore alternative magnetic materials known as antiferromagnets for spin-based devices. Antiferromagnetic materials are made of two or more sub-lattices in which the spins of each sub-lattice are oriented in one direction, but the total or net magnetization is zero. In spite of many intriguing and superior electric and magnetic properties, the antiferromagnetic materials have not been used as magnetically active elements for spin device applications. If one is able to manipulate spin states in antiferromagnetic materials, one would create a disruption spin-based technology which is faster, denser, and more energy efficient, compared to ferromagnetic based devices. The proposal calls for a comprehensive theoretical investigation on how devices made of the antiferromagnetic materials and their multilayers respond to the time-dependent external electric and magnetic fields. The goal is to evaluate the feasibility of antiferromagnetic based spin devices and to find optimal materials parameters in various structure. The educational components of the proposal include strong graduate student participations in research, training, and visiting industrial research laboratories, as well as for PI to develop a spintronics course related to this research project.In today's spintronics, spins of conduction electrons play a pivotal role in carrying angular momentum information and manipulating the magnetization dynamics of magnetic nanostructures. Antiferromagnetic materials have no net spin or magnetization, but they have two distinct magnetic characteristics: a staggered magnetic moment and a quasi-particle excitation known as antiferromagnetic magnons. Both staggered moment and magnons could carry angular momentum and serve as spin information propagators. This proposal aims at a comprehensive study on the roles of these carriers. In metallic systems, a theory of coupled electron-magnon conduction, which is capable of predicting new magnetotransport properties, will be developed. In insulating materials, the mutual dependence of the direction of staggered moments and the non-equilibrium number of magnons is the main focus of research. The proposal further explores the following novel spin-dependent properties of antiferromagnetic-based multilayered structures: 1) Quantitatively determining spin current from the flow of the electron spins, the staggered magnetic moments, and magnons, as well as their conversion rates across interfaces in various multilayered systems. 2) Investigating the interplay between the staggered moments and magnons for magnetic control. 3) Exploring possible spintronics device concepts based on the antiferromagnetic materials. If successful, the present research could reveal knowledge for staggered magnetic momentum and magnons which may have superior capabilities for enhanced spin information propagation in spintronic applications.