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If two ferromagnets are separated by a thin (about 1 nm) insulator, the resistance of the tunneling current changes with the relative orientation of the two magnetic layers. The resistance is normally higher in the anti-parallel case. This is known as tunnel magnetoresistance (TMR) effect.
It was discovered in 1975 by Julliere [1] , using iron as the ferromagnet and germanium as the insulator.
Room temperature TMR was discovered in 1995 by Moodera et. al. following renewed interest in this field fueled by the discovery of the giant magnetoresistive effect [3,4]. It is now the base for the magnetic random access memory (MRAM). For more technical information see [5].
References
[1] Julliere et. al., Phys. Lett. 54A, 225 (1975)
[2] J. S. Moodera et. al., Phys. Rev. Lett. 25, 1270 (1995)
[3] G. Binasch et. al., Phys. Rev. B 39, 4828 (1989)
[4] M. N. Baibich et. al., Phys. Rev. Lett. 61, 2472 (1988)
[5] Moodera and Mathon, Magn. Magn. Mater. 200, 248 (1999)
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