Device comprising a first and a second ferromagnetic layer...

Static information storage and retrieval – Systems using particular element – Magnetic thin film

Reexamination Certificate

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C365S171000

Reexamination Certificate

active

06252796

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to a device having a first and a second ferromagnetic layer separated by a non-magnetic spacer layer, wherein the resistance is dependent on the orientation of the magnetisation directions of the first and second ferromagnetic layers.
The Giant Magneto-Resistance (GMR) effect and the Tunnel Magneto-Resistance (TMR) effect provide possibilities to realise readout heads for magnetic recording, sensitive magnetic sensors and nonvolatile magnetic memories (MRAM's). Such devices comprise a first and a second ferromagnetic layer separated by a non-magnetic spacer layer. The magnetization of one of the layers is fixed (fixed layer), the other (free layer) can have any orientation with respect to the magnetisation of the fixed layer (for sensors) and for magnetic memories can be switched between two directions (i.e. parallel or anti-parallel to the fixed magnetisation). Due to the GMR (TMR) effect the resistance for both configurations is different, and the state of the memory (‘1’ or ‘0’) can be determined by measuring the resistance of the GMR system. Switching the magnetisation direction of the free layer changes the state of the MRAM. Conventionally an additional conductor is used to switch the magnetisation direction of the free layer. Conventional MRAMs require a relatively large write current. Because of power consumption restrictions this current should be as small as possible. However, on the other hand the memory should be insensitive to disturbing external magnetic fields (to avoid unintended erasing of data). Similar techniques are used in read-out heads for magnetic recording in which the resistance is dependent on the magnetisation direction in the free layer vis-a-vis the fixed layer. In such read-out heads it is advantageous if the magnetisation direction in the fixed layer can be reset relatively easily.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a device as described in the opening paragraph, which requires a reduced current for switching or resetting the magnetisation direction of one or both of the layers.
To this end one or both of the first and second layers (the ‘free’ layer or fixed layer of which the magnetisation direction is to be switched or reset) comprises a multilayer configuration comprising two ferromagnetic sublayers separated by a non-magnetic (preferably well conducting) spacer layer. The two magnetic sublayers are magnetically coupled (e.g. by exchange coupling or magnetostatic coupling) in such manner that their magnetisation directions are anti-parallel. This is similar to a generalised Artificial Anti-Ferromagnet (AAF) configuration. Preferably the coupled magnetic sublayers show uniaxial anisotropy (e.g. due to shape anisotropy and/or induced anisotropy). Since the total magnetic moment of an AAF is very small (theoretically down to zero) the rigidity with respect to external fields is strongly enhanced. Only very large external magnetic fields can switch the magnetic moment of the AAF. Thus the multilayer stack is relatively insensitive to external fields but it can nevertheless be switched (or oriented) relatively easily. To this end the device in accordance with the invention comprises means for directing an in-plane switching or resetting current through the multilayered configuration, the current direction being transverse to the magnetisation directions of the magnetically coupled sub-layers. The in-plane internal current, in contrast to a current in a conductor outside the layer(s) to be switched as is conventional, only has to overcome the anisotropy of a single magnetic (sub)-layer. The distance between the current and the said (sub)-layer(s) is further more very small, which increases substantially the magnetic field at the sub-layer(s) to be switched and consequently reduces the current need. The required current can be as small as the order of 1 mA (e.g. for Permalloy). If the elements are of (&mgr;m-scale or less, it is preferred to use materials and dimensions that can withstand the required current density. However, since the switching or resetting current only has to be applied as a very short pulse, many materials are useful.


REFERENCES:
patent: 5859754 (1999-01-01), Tong et al.
patent: 6124711 (2000-09-01), Tanaka et al.

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