Greg P. Carman
University of California, Los Angeles
Efficient control of small scale magnetism presents a significant problem for future miniature electromagnetic devices. In most macroscale electromagnetic systems we rely on a discovery made by Oersted 200 years ago where an electrical current through a wire creates a distributed magnetic field. While this concept works well at large scale, it suffers significant problems at volumes below 1 mm3. One approach to control nanoscale magnetic states is spin-transfer torque (STT). However, experimental measurements on STT memory devices indicates that 100 fJ is required to reorient a bit of memory with an energy barrier of about 0.5 aJ, i.e., at 0.0005 percent efficiency. Therefore, new nanoscale approaches are needed for future miniature electromagnetic devices. Recently, researchers have explored strain-mediated multiferroic composites to resolve this problem. For this material class, a voltage-induced strain alters the magnetic anisotropy of the magneto-elastic elements. These strain-mediated multiferroics consists of a piezoelectric material coupled to magneto-elastic elements to transfer electrical energy to magnetic energy through a mechanical transduction. The coupling coefficient (energy transferred) in piezoelectric materials (e.g., lead zirconate titanate, PZT) is approximately 0.8 while the coupling coefficient in magneto-elastic materials (e.g., Tb-Dy-Fe, Terfenol-D) is of similar magnitude, 0.8. Thus, the amount of energy to overcome a 0.5 aJ bit barrier is potentially only 0.8 aJ, or an efficiency of about 60 percent, neglecting line losses. This presentation reviews the motivation, history, and recent progress in nanoscale strain-mediated multiferroics. Research descriptions include analytical and experimental work on strain-mediated multiferroic thin films, single magnetic domain structures, and superparamagnetic particles. The results indicate efficiencies orders of magnitude superior to STT approaches and presents a new approach to control magnetism. Discussions of future research opportunities and novel applications are included.
Institut de Ciència de Materials de Barcelona, Spain
Magnetite, Fe3O4, guided early explorers towards unknown frontiers. Since those days, oxides have been the backbone of many scientific and technological developments. When high temperature superconductors were discovered, the subsequent enthusiasm stimulated an impressive development in oxide thin film growth technologies and a deep revision of the understanding of metal oxides and strongly correlated electronic systems. Today, oxides are fueling the discovery and development of unexpected, intriguing, and fascinating new areas of knowledge, such as magnetic ferroelectrics and magnetic monopoles. Ferromagnetic oxides are finding their way as active components in spintronics, either as spin filters for advantageous magnetic tunnel junctions or used to manipulate spins in non-magnetic materials, which could eventually lead to energy-efficient pure spin-current devices. The tiny spin-orbit coupling interaction, responsible for the magnetic anisotropy, has emerged as a toy that allows us the modulation of the transport properties, not only in metallic ferromagnetic systems, but also in antiferromagnetic metals and insulators. This may lead to a new generation of magnetic memory. “Interface is the device” and interfaces between oxides and metals, and interfaces between large band-gap oxides, have led to the discovery of emerging properties such as switchable “on-off” magnetization, by applying suitable electric fields, or magnetism and superconductivity in confined two-dimensional electron gas systems, which challenge our current understanding of oxides. This is the playground in which we fortunately play, learn, and imagine the future while enjoying building a new science out of the good old oxides. In the lecture, we will travel through the new materials and ideas that make this journey possible and so successful.
National Institute for Materials Science, Japan
The hard disk drive industry is making continuous efforts to increase the areal density of magnetic recording. To realize an areal density of higher than 2 Tbit/in2 in the future, both media and readers need technical breakthroughs. Since the bit size will be in the range of 20 nm, the magnetic grains in the recording media must be reduced to less than 6 nm, requiring the use of ferromagnetic materials with high magnetocrystalline anisotropy such as L10 FePt. The shield-to-shield spacing of read sensors must also be smaller than 20 nm with low device resistance (resistance-area product RA 0.5 m2), which is very difficult to achieve using MgO based tunneling magnetoresistance devices. In this talk, we will address the materials challenges to the realization of an ideal media nanostructure using L10 FePt for heat-assisted magnetic recording (HAMR) media and narrow readers for > 2 Tbit/in2 areal density. Recently significant progress has been made in current-perpendicular-to-plane giant magnetoresistive (CPP-GMR) devices using highly spin-polarized Heusler alloy ferromagnetic layers and new spacer materials. The very high magnetoresistance ratios achieved in CPP-GMR are encouraging for future read head applications of CPP-GMR, or its laterally extended version, lateral spin valves. The devices with high magnetoresistive output at low RA may open new applications in addition to disk read heads.
Kyoto University, Japan
Worldwide efforts are underway to create revolutionary and energy-efficient data storage technology such as magnetic random-access memory (MRAM). An understanding of spin dynamics in inhomogeneously magnetized systems is indispensable for further development of nanoscale magnetic memory. This lecture provides a clear picture of inhomogeneously magnetized systems, such as magnetic nanowires with domain walls and disks with magnetic vortices, and presents not only technological developments and key achievements but also the unsolved puzzles and challenges that stimulate researchers in the field. First, the basic concept of an inhomogeneously magnetized system is described by introducing a magnetic vortex structure in a magnetic disk. A magnetic domain wall in a magnetic nanowire is also provided as a typical example. The magnetic field-driven dynamics of these inhomogeneously magnetized systems are described to illustrate their uniqueness. Second, electric-current-induced dynamics of magnetic vortices and domain walls are described. One can flip the core magnetization in a magnetic vortex using electrical current excitation, and move a domain wall by current injection into a wire. The next part focuses on the applications of current-induced magnetization dynamics in devices. The basic operations of two kinds of magnetic memories—magnetic vortex core memory and magnetic domain wall memory—are demonstrated. The lecture describes not only the current understanding about inhomogeneously magnetized systems, but also unexpected features that have emerged. It concludes with prospects for future developments.