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Memory-loss in magnetic random access memories


In numerous electronic devices, as mobiles or computers, the information can be stored in sets of nano-magnets which form a magnetic random access memory (MRAM). In this type of random access memory, states 0 and 1 (the bits) correspond to two opposite directions of magnetization of the nano-magnets. To reverse the magnetization, we normally use small magnetic fields. Since very recently, we also know how to manipulate the magnetization with electric currents (STT-MRAM). The working principle is the following one : when an electric current goes through a magnetic material, the magnetic material forces the alignment of the magnetization of the electrons (called spin), which carry the electric current. We then say that electrons are spin-polarized. The injection of spin polarized electrons in a material of opposite magnetization can force the reversal of the magnetization. This mechanism (so-called Spin Transfer Torque, STT) is not very well understood and is at the heart of numerous researches of fundamental physics.

In this context, we highlighted a rather unexpected phenomenon. The injection of an electric current from a non-magnetic material (Gold) into a magnetic material (the semiconductor GaMnAsP) produces a random reversal of the magnetization. From the point of view of applications, it should be rather annoying to have a magnetic memory which randomly loses its information. However, it is particularly interesting from the fundamental point of view. The random character gives access to the energies and characteristic times involved in the mechanisms of reversal and thus allows a better understanding of the underlying physics. Thanks to a statistical analysis, we were able to show that the injection of an electric current (not polarized) weakened strongly the magnetic state of the magnetic material and favored a random reversal of the magnetization. This should suppose no concerns for mobiles or computers, since STT-MRAM are currently builed with other kinds of magnetic materials (metals) for which the mechanisms of reversal of magnetization are different.

 


Random reversal of the magnetization produced by a continuous electric current.

Image (black and white) of magnetic domains obtained by Kerr microscopy superimposed to an image (colors) of the experimental set-up. The circulation of a continuous current (red arrows) between the semicircular golden electrode (Au) and the fine golden electrode produces a random reversal of the magnetization and the formation of the observed circular magnetic domains in the magnetic material (GaMnAsP).

 

Contact : Vincent Jeudy

Reference :
Stochastic Current-Induced Magnetization Switching in a Single Semiconducting Ferromagnetic Layer
J. Gorchon, J. Curiale, A. Lemaître, N. Moisan, M. Cubukcu, G. Malinowski, C. Ulysse, G. Faini, H. J. von Bardeleben, and V. Jeudy
Phys. Rev. Lett. 112, 026601 (2014).