SPAM3 - Spin Polarized Advanced Materials for Magnetic Memories
SPAM3 is supported by

Fondazione CARIPLO

Starting date: 1 February 2009

 Further information:

Dr. Roberto Mantovan
Laboratorio MDM
CNR-IMM, Unita' di Agrate Brianza
Via Olivetti, 2 - 20864
Agrate Brianza(MB) - Italy
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R. Mantovan
R. Colnaghi

Scientific background 


The spin-dependent electronics (spintronics) is characterized by the use of both the electrical charge and the spin of electrons. The operation of spintronic devices is intimately related to magnetic phenomena and to the manipulation of spin-polarized currents. There are several concepts exploiting the electron spin for the development of novel devices, and spintronics is indeed an interesting field for preparing the next generation of micro- and nano-electronics [1]. 



One of the most competitive solution for replacing and/or integrating the currently used non-volatile memories is the magnetoresistive random access memory (MRAM), where the elemental memory cell is the magnetic tunnel junction (MTJ). Essentially, a MTJ stack consists of two ferromagnetic (FM) layers acting as electrodes separated by a thin oxide film, the tunnel barrier. The spin-polarized current-flow in a MTJ is generally manipulated by an external current-induced magnetic field, which controls the relative magnetization orientation of the FM electrodes. The storage element in MTJs is constituted by the soft (also called free) electrode magnetization, being intrinsically non-volatile. The read-out of the data is possible due to the possibility of obtaining different perpendicular-to-plane resistances of the MTJ stack, being maximum RAP (minimum RP) in the antiparallel (parallel) FM electrodes magnetization configurations. The efficiency of the read-out process is defined by the magnitude of the tunnel magnetoresistance (TMR), defined as TMR= (RAP-RP)/RP. MRAM are fast, non-volatile and easy accessible with infinite endurance, a likely candidate for becoming the "universal memory".
The first observation of a TMR was done by Julliere in 1975 in a Fe/Ge/Fe stack prototype device at 4.2 K [2]. A major breakthrough was achieved in 1995 by the Moodera's group that first observed a finite TMR effect at room temperature for a Fe/Al2O3/Co MTJ [3]. Since then, a large number of pubblications reported finite TMR values in a large varity of different multilayer stacks. In 2004 the grops of S. Yuasa and S. S. P. Parkin have reported record TMRs for epitaxial MTJs employing MgO tunnel barriers [4,5], where the MTJs were fabricated by molecular beam epitaxy and magnetron sputtering respectively.



Slonczewski predicted in 1996 the possibility of inducing the magnetization switching in magnetic thin films by using spin-polarized currents [6]. The possibility to electrically manipulate the magnetization direction of thin films is very much attractive for application in spintronics. Consider a typical MTJ stack. The current passing through the hard FM electrode (polarizer) will be fully spin-polarized in the direction determined by the polarizer magnetization direction. In favorable conditions, the tunneling (spin polarized) current could "kick" the soft FM electrode magnetization inducing its reversal thank to the spin transfer torque (STT) induced by the spin polarized electrons. The STT effect can be explained by considering an additional term in the Landau-Lifshitz-Gilbert equation that takes into account the torque inferred to the magnetization of a thin film by the spin polarized injected carriers [7]. Such a STT process could align the magnetization directions of the FM electrodes. If the current is reversed (passing first through the soft FM electrode), the carriers having the spins pointing in the opposite direction with respect to the hard FM electrode magnetization, will be reflected back into the soft electrode then possibly aligning the soft magnetization direction antiparallel to the hard polarizer magnetization.
The STT-based reversal of the magnetization direction in thin films is strongly dependent on the material properties, which critically determine whether the induced torque can overcome the damping effect that is forcing the magnetization in the easy axis direction.


The STT mechanism offers a very attractive way for manipulating the magnetization state of magnetic thin films. The success in writing a MTJ cell with the use of spin-polarized currents rather then externally induced magnetic fields (through wires), is a fundamental step toward the realization of a full electrically-controlled non-volatile magnetic memory [8]. The dimensionality of the FM electrodes and particularly the electrodes lateral size, are crucially determining the chances of observing a STT-driven magnetization reversal. Therefore advanced lithographic methods are demanding for the fabrication of electrically writable MTJs.

[1] S. A. Wolf, D. D. Awschalom, R. A. Buhrman, J. M. Daughton, S. von Molnar, M. L. Roukes, A. Y. Chtchelkanova, and D. M. Treger, Science 294, 1488 (2001).

[2] M. Julliere, Phys. Lett. 54A, 225 (1975).

[3] J. S. Moodera, L. R. Kinder, T. M. Wong, and R. Meservey, Phs. Rev. Lett. 74, 3273 (1995).

[4] S. Youasa, T. Nagahama, A. Fukushima, Y. Suzuki, and K. Ando, Nat. Mat. 3, 868 (2004).

[5] S. S. P. Parkin,C. Kaiser, A. Panchula, P. M. Rice, B. Hughes, M. Samat, and S. Yang, Nature Mat. 3, 862 (2004).

[6] J. C. Slonczewski, J. Magn. Magn. Mat. 159, L1 (1996).

[7] D. C. Ralph and M. D. Stiles, J. Magn. Magn. Mat. 320, 1190 (2008).

[8] C. Chappert, A. Fert, and F. N. Van Dau, Nature 6, 813 (2007).