Erasmus Langer
Siegfried Selberherr
Oskar Baumgartner
Markus Bina
Hajdin Ceric
Johann Cervenka
Raffaele Coppeta
Lado Filipovic
Lidija Filipovic
Wolfgang Gös
Klaus-Tibor Grasser
Hossein Karamitaheri
Hans Kosina
Hiwa Mahmoudi
Alexander Makarov
Mahdi Moradinasab
Mihail Nedjalkov
Neophytos Neophytou
Roberto Orio
Dmitry Osintsev
Mahdi Pourfath
Florian Rudolf
Franz Schanovsky
Anderson Singulani
Zlatan Stanojevic
Viktor Sverdlov
Stanislav Tyaginov
Michael Waltl
Josef Weinbub
Yannick Wimmer
Thomas Windbacher
Wolfhard Zisser

Alexander Makarov
Alexander Makarov was born in Volgograd, Russia, in 1985. He studied information technology at the Volgograd State Technical University, where he received the BSc degree in 2006 and MSc degree in 2008. He joined the Institute for Microelectronics in October 2009, where he is currently working on his doctoral degree. His scientific interests include Monte Carlo simulations, micromagnetic modeling, and non-volatile memory device modeling.

Structural Optimization of MTJs with a Composite Free Layer

New types of spintronics devices utilizing magnetization switching by current, such as Spin-Torque Transfer RAM (STT-MRAM) and spin-torque oscillators, have been intensely developed on MgO based Magnetic Tunnel Junctions (MTJs) with a large magneto-resistance ratio. MTJ is a sandwich of two magnetic layers separated by a thin nonmagnetic spacer. While the magnetization of the pinned layer is fixed due to the fabrication process, the magnetization direction of the free layer can be switched between the two states parallel and anti-parallel to the fixed magnetization direction.
Reducing the magnitude of the switching current density in MTJ structures is of considerable importance for the success of STT-MRAM. A MTJ with a composite free layer (C-MTJ) was earlier proposed (figure 1 left). The free magnetic layer of such a structure consists of two equivalent parts of half-ellipses separated by a narrow nonmagnetic spacer. The C-MTJs demonstrate a substantial decrease of the switching time and switching current as compared to the standard MTJ with the monolithic free layer.
We further performed the structural optimization of C-MTJs by means of extensive micromagnetic simulations and proposed a new structure of the composite free layer, C2-MTJ. In a C2-MTJ the free layer consists of the two ellipses with the major axes a/2 and b (a>2b) inscribed into a rectangle a x b (figure 1 right). This structure is easier to fabricate compared to the previous generation of C-MTJs. We investigated the switching statistics depending on the geometry. We found that the new C2-MTJ switches as fast as the previous one, which is twice as fast as the structure with a monolithic free layer, without loss of thermal stability. The narrow switching time distribution characteristic of C-MTJs is also preserved in C2-MTJs. Therefore, the newly proposed C2-MTJ offers greater potential for performance optimization of STT-MRAM devices.

Figure 1. Schematic illustration of penta-layer MTJs with composite free layer C-MTJ (left) and C2-MTJ (right).

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