Manipulating Nanoscale Magnets with Ultrafast Heat Conduction

Interview conducted by Stuart MilneJun 15 2015

David Cahill, Willett Professor and Department Head of Materials Science and Engineering at the University of Illinois at Urbana-Champaign, talks to AZoNano about his research into the manipulation of magnetic information with heat and how it can be used to manipulate magnetization at the nanoscale.

Could you please give our readers a brief introduction to your research surrounding the manipulation of magnetic information with heat?

Heat is usually just a problem to be managed. A theme of research in my group is to instead think of heat as a resource and potential solution. To further that vision, we study the basic science of heat conduction in materials. Heat is carried by excitations of atomic vibrations, electrons, and spins. We study how these excitations move through a material, or across an interface between materials, and how the excitations are coupled to each other.

Why is this new phenomena so desirable for manipulating magnetization at the nanoscale?

In a conventional data storage device in a computer, the size of the magnetic information is defined by a magnetic field generated in the “head” that is applied to a thin layer of magnetic material on the “disk”. This technology is approaching the fundamental limits on the size of a bit of information that can be written using this conventional approach. Heat can potentially be confined to smaller volumes and enable higher speed, higher data storage capacity and memories.

Schematic, cutaway view of the geometry used to generate currents of spin from currents of heat. Pulses of laser light heat the left side of the sample and create an intense current of heat passing through the [Co,Ni] ferromagnet. This current of heat creates a separation of electron spins that then diffuse through the Cu heat sink and affect the magnetization of a second ferromagnetic layer, CoFeB, causing the magnetization to tilt and then precess. The total thickness of the sample is approximately 100 nm.

Can you explain how spin currents work and how it enables the manipulation of nanomagnets?

An electron has mass, charge, and “spin”, a characteristic that derives from quantum mechanics that is related to the angular momentum of a spinning top. Spin also defines the magnetic dipole of the electron; an electron often acts much like a microscopic bar magnet with a north and south pole. For a current flowing in a copper wire, the magnetic dipoles of the electrons are randomly oriented as there is no net current of spin. A spin current refers to a flow of electrons where the magnetic dipoles are organized with a preferred direction.

How can the sign and magnitude of the heat-driven spin current be controlled?

We found that we can engineer the sign and magnitude of the heat current by the selection materials used in the device and the geometry of the device.

Where do you see this phenomena being applied?

Practical applications of this field of science, sometimes referred to as “spin caloritronics”, to technology are still many years in to the future. What is envisioned is methods for making high speed, high capacity data storage devices and memories.

What are the next steps in developing this further?

Our interests are in the physics of materials. A route to making the effects larger and therefore more practical is to find materials that enhance the magnitude of the spin current that’s generated by a heat current. This process of “materials discovery” is a critical path forward in many engineering fields. Technology is often held back by the need for new materials. The next steps are to find materials that are more efficient in generating spin currents from heat currents than the materials we have studied to-date.

This development is in contrast to the conventional application of magnetic fields, providing a new, and highly desirable way to manipulate magnetization at the nanoscale. How does this method compare in terms of performance with current methods?

At the moment practical applications are a long way off into the future. It's hard to predict the relative performance that might be achieved.

Where can our readers learn more?

The status of the field of heat conduction at small length scales and time scales is summarized in a recent review article “Nanoscale thermal transport II”, Appl. Phys. Rev. 1, 011305 (2014).

The emerging field of spin caloritronics was reviewed recently in “Spin caloritronics”, S. R. Boona, R. C. Myers, and J. P. Heremans, Energy Environ. Sci. 7, 885 (2014).

The discovery of new materials for spin devices is reviewed in “Spintronics: A Challenge for Materials Science and Solid-State Chemistry”, Angew. Chem. Int. Ed. 46, 668 – 699 (2007).

About David Cahill

David Cahill is the Willett Professor and Department Head of Materials Science and Engineering at the University of Illinois at Urbana-Champaign. He joined the faculty of the U. Illinois after earning his Ph.D. in condensed matter physics from Cornell University, and working as a postdoctoral research associate at the IBM Watson Research Center. His research program focuses on developing a microscopic understanding of thermal transport at the nanoscale; the discovery of materials with enhanced thermal function; the development of new methods of materials analysis using ultrafast optical techniques; and advancing fundamental understanding of interfaces between materials and water. He received the 2015 Touloukian Award of the American Society of Mechanical Engineers and the Peter Mark Memorial Award from the American Vacuum Society (AVS); is a fellow of the AVS, American Physical Society (APS) and Materials Research Society (MRS); and a past-chair of the Division of Materials Physics of the APS.