Static information storage and retrieval – Systems using particular element – Magnetic thin film
Reexamination Certificate
2000-03-31
2001-09-25
Phan, Trong (Department: 2818)
Static information storage and retrieval
Systems using particular element
Magnetic thin film
C365S173000
Reexamination Certificate
active
06295225
ABSTRACT:
FIELD OF THE INVENTION
The invention is related to the field of tunnel junction devices.
BACKGROUND OF THE INVENTION
The invention relates to a magnetic tunnel junction device provided with a multilayer structure including a pair of electrode layers of a ferromagnetic material and an interposed tunnel barrier layer of insulating material.
Such a device is known from U.S. Pat. No. 5,650,958. The known device has two ferromagnetic electrode layers and an insulating tunnel barrier layer located between and in contact with both ferromagnetic layers. The insulating layer is thin enough to allow quantum mechanical tunneling between the electrode layers. The known tunnel junction device demonstrates a better magnetoresistance response than anisotropic magnetoresistive devices or giant magnetoresistive devices It is known per se from e.g. the article “Generalized Formula for the Electric Tunnel Effect between Similar Electrodes Separated by a Thin Insulating Film”, Journal of Applied Physics, Volume 34, number 6, June 1963, pages 1793-1803, John C. Simmons, that the tunnel resistance of a device, that includes two electrodes separated by a thin insulating film, depends on the thickness of the insulating film and the value of the tunnel barrier height. In order to guarantee electric and magnetic insulations between the two electrodes, a certain minimal thickness of the insulating film is required. The barrier height is physically determined by the combination of electrode material and insulating film material.
The above citations are hereby incorporated herein in whole by reference.
The inventors recognize that in the known tunnel junction device, the tunnel junction resistance is relatively high and thereby has a restricted current-voltage characteristic.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a magnetic tunnel junction device having a relatively low resistance.
According to the invention, this object is achieved in a magnetic tunnel junction device as defined in the opening paragraph, wherein the multi-layer structure further includes an intermediate layer provided between the barrier layer and one of the electrode layers. The intermediate layer includes a conductive material having a work function, with a value at least 25% lower than the value of the work function of the material of the respective electrode layer.
For the sake of clarity it is expressed that the term work function means the electronic work function as defined in McGraw-Hill Encyclopedia of Science and Technology, 1960, page 553. This work function, which is directly related to the electronic properties of a solid material, is usually expressed in electron Volts (eV). This encyclopedia teaches that the work function of a metal is equal to the energy required to raise an electron with the Fermi energy to the energy level corresponding to an electron at rest in vacuum; at absolute zero, the energy of the most energetic electrons in a metal is referred to as the Fermi energy. The barrier layer in the device according to the invention is e.g. an insulating oxide layer, preferably a layer of Al
2
O
3
. The invention is based on the insight that the barrier height between the barrier layer and an electrode layer of a tunnel junction device decreases if an intermediate layer, particularly a metallic layer, of a relatively low work function material is applied between the electrode layer and the barrier layer. Any material having a large positive electronegativity may be used, in principle, as a low work function material. Preferred examples are the metals Cs, Ba, Sr, Y. A possible further metal may be Sc.
An advantage of the device according to the invention is that a low resistance is obtained with maintenance of a normal barrier thickness. Therefore, there is no need to further reduce the thickness of the barrier layer. It is noted that, by way of example, it has appeared that a reduction of the barrier height from 4 eV to 3 eV may yield a resistance reduction by a factor of about 185. A reduction of the barrier height from 2 eV to 1 eV may even yield a resistance reduction by a factor of about 2800.
It is to be noted that, it is known per se from the Journal of Appl. Phys. 79(8), Apr. 15, 1996; “Spin-dependent transmission of free electronics through ultrathin cobalt layers”; H. -J. Drouhin c.s., that the work function of a gold layer can be lowered down from about 4 eV above the Fermi level to 2 eV by deposition of cesium. However, there is not any teaching in that publication that such a work function reduction may be of any advantage in a magnetic tunnel junction device.
In an embodiment of the device according to the invention, the intermediate layer is only provided on one side of the barrier layer.
Although in general an intermediate layer is provided on both sides of the barrier layer, it may be of interest in some applications to use only one intermediate layer. In this embodiment, which is only provided with one intermediate layer, an asymmetrical tunnel barrier is formed. Such a device is virtually a ferromagnetic diode wherein at lower voltages between the electrode layers the tunnel current densities are the same in both current directions between the electrode layers, while at higher voltages the current density in the one direction is higher than in the other direction. A device having such properties is suitable for use in certain memory structures, such as MRAM structures.
The invention further relates to a magnetic field sensor including a transducing element. In the sensor according to the invention, the transducer element is formed by the magnetic tunnel junction device according to the invention. This sensor has all of the advantages of the tunnel junction device according to the invention and is very suitable to detect magnetic field variations. The sensor may also be used as a shielded read head.
The invention further relates to a magnetic head. The magnetic head according to the invention it includes the magnetic field sensor according to the invention and is provided with a magnetic yoke for cooperation with the magnetic tunnel junction device. This magnetic head has all of the advantages of the sensor according to the invention and is very suitable as a read head for reading information, such as audio, video or data information, from a magnetic information carrier.
The invention further relates to a system for reading information from a magnetic information carrier or record carrier. This system according to the invention includes the magnetic field sensor or the magnetic head according to the invention. The information carrier may be a magnetic tape or a disc-shaped carrier, such as a hard disc or a magneto-optical disc.
The invention also relates to a magnetic tunnel junction memory cell. A memory cell is known per se from U.S. Pat. No. 5,650,958. The memory cell according to the invention includes the magnetic tunnel junction device described above according to the invention.
The above-mentioned and other aspects of the invention are apparent from and will be elucidated, by way of non-limitative example, with reference to the embodiments described hereinafter.
REFERENCES:
patent: 5650958 (1997-07-01), Gallagher et al.
patent: 5764567 (1998-06-01), Parkin
patent: 5966012 (1999-10-01), Parkin
John G. Simmons, “Generalized Formula for the Electric Tunnel Effect Between Similar Electrodes Separated by a Thin Insulating Film”, Journal of Applied Physics, vol. 34, No. 6, Jun. 1963, pp. 1793-1803.
McGraw Hill Encyclopedia of Science and Technology, 1960, pp. 553.
H.-J. Drouhin Et Al, Spin-Dependent Transmission of Free Electronics Through Ultrathin Cobalt Layers (Invited), Journal of Appl. Phys. 79(8), Apr. 15, 1996, pp. 4734-4739.
Belk Michael E.
Phan Trong
U.S. Philips Corporation
LandOfFree
Magnetic tunnel junction device having an intermediate layer does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Magnetic tunnel junction device having an intermediate layer, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Magnetic tunnel junction device having an intermediate layer will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2442034