Join the Department of Physics and Astronomy on Wednesday, February 25 in White Hall, Room G09 for the Mohindar Singh Seehra Lecture. Our guest this year is Sara Majetich (Carnegie Mellon University), and her talk is titled "Superparamagnetism in Nanoparticles and Magnetic Tunnel Junctions."

Abstract

A superparamagnet is a single domain where the direction of the magnetic moment changes spontaneously due to thermal fluctuations. Superparamagnets are preferred for most biomedical applications of magnetic particles, while for magnetic data storage the grains must be above the superparamagnetic limit. The collective behavior of superparamagnets will be reviewed, both quasi-static and dynamic. Next, measurements of superparamagnetism for single Fe3O4 nanparticles will be discussed, where the particle acts as the free layer of a magnetic tunnel junction and a conductive atomic force microscopy tip is used to make electrical contact. Since the crystallographic orientation of a spherical surfactant-coated nanoparticle cannot easily be controlled, further measurements were done on lithographically fabricated nanoparticles, mainly of CoFeB, which is the material most commonly used in magnetic tunnel junctions (MTJs). Magnetic field-tuned dynamics analagous to those for the Fe3O4 particles show greatly reduced noise, and telegraphing between two levels that can be correlated to an energy barrier. The dynamics can also be tuned by spin transfer torque by varying the bias voltage, or by voltage control of magnetic anisotropy . While CoFeB is a low anisotropy material, if the sidewalls are oxidized, there can be exchange bias effects for small diameters (≤ 40 nm). Single MTJ experiments show rapid (500 s) training at low fields (500 Oe), but since the sidewall oxide nucleation is inhomogeneous, each MTJ is different. The last part of the talk will discuss applications of electrically controlled superparamagnets. The simplest is a random number generator, which requires a single MTJ. Analog multiplication combines MTJs with CMOS in a way that is more efficient than CMOS alone. Probabilistic logic gates are intriguing because of their potential for reversibility. However, it is still very challenging to make complex structures. An alternative is to use magnetostatic interactions of superparamagnetic MTJs to avoid the need for wiring connections, except for the inputs and outputs to this passive neural network structure. The current status and future directions are described.

Biography

Sara Majetich is a Professor in the Physics Department at Carnegie Mellon University in Pittsburgh, PA, USA, with courtesy appointments in Electrical and Computer Engineering and Materials Science and Engineering. Her research focuses on nanomagnetism. It has included studies of collective magnetic behavior in nanoparticle assemblies through electron microscopy and small angle neutron scattering, and magnetoresistance measurements on single nanoparticles. It also includes investigation of magnetoresistance and current or voltage-induced switching in small magnetic tunnel junctions. She received a US NSF National Young Investigator award, and is a Fellow of the IEEE and the American Physical Society, and has authored over 200 papers. She has been active in the IEEE Magnetics Society, where she has been a Distinguished Lecturer, Education Committee Chair, and Secretary/Treasurer. She is currently the President-elect of the IEEE Magnetics Society.