Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Magnetic field
Reexamination Certificate
2001-06-21
2003-12-09
Flynn, Nathan J. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Magnetic field
C257S414000, C257S421000
Reexamination Certificate
active
06661071
ABSTRACT:
The invention relates to a magnetic memory comprising an array of magnetic memory elements, each memory element including at least one layer of a magnetic material, said memory being provided with a shielding layer against magnetic fields.
Such a magnetic memory is disclosed in U.S. Pat. No. 5,902,690.
Magnetic memories may take the place of SRAM, DRAM, FLASH and non-volatile memories, such as EPROM and EEPROM owing to the short read and write times, the non-volatile memory and the comparatively low power dissipation. The operation of the magnetic memory elements is based on a magnetoresistance effect, which means that a magnetic field determines the direction of magnetization of the magnetic material, and, when an electric current is sent through the material, the electric resistance depends upon the orientation of the magnetization of the magnetic material. By means of a magnetic field, the direction of magnetization can be switched to two states.
One magnetization state corresponds to a comparatively low resistance, a 0, the other state corresponds to a comparatively high resistance, a 1. By directing the magnetization in each memory element by means of a local magnetic field, the memory can be written. External magnetic fields can erase the memory when the magnetic field changes the orientation of the direction of magnetization. Shielding against magnetic disturbing fields is necessary.
In the known memory, the material of the shielding layer is an electrically non-conducting ferrite.
A drawback of the known shielding layer resides in that comparatively strong magnetic disturbing fields of, for example, several tens of kA/m cannot be stopped completely because the ferrite shielding layer becomes magnetically saturated. Once the shielding layer is magnetically saturated, the magnetic field penetrates through the shielding layer, as a result of which the stored magnetization direction in the memory elements can flip over and the non-volatile memory may be erased. Weak magnetic fields of several kA/m are sufficient to erase memory elements.
In certain applications, for example smart cards, the magnetic memories must be protected against strong magnetic disturbing fields of at least 80 kA/m. Shielding against such strong, external magnetic fields is impossible with the known shielding layer.
Another drawback resides in that the known shielding layer must be thicker as the magnetic disturbing field is stronger. Magnetic memories are often provided on Si semiconductor substrates. In the semiconductor technology, thin-film technologies are employed to apply layers. Shielding layers whose thickness is above 10 &mgr;m are very expensive owing to the long deposition time.
It is an object of the invention to provide a magnetic memory of the type described in the opening paragraph, which is shielded against comparatively strong magnetic disturbing fields of several hundred kA/m.
In the magnetic memory in accordance with the invention, this object is achieved in that the shielding layer is split into mutually separated regions.
A magnetic layer becomes saturated as a result of a magnetic field of sufficient strength. As the shielding layer in the memory in accordance with the invention is split into mutually separated regions, the magnetic field can fan out between the regions. In the magnetic regions of the shielding layer, the magnetic field lines are drawn into the material. The density of the magnetic field lines in these regions is reduced with respect to a continuous layer, as a result of which saturation of the magnetization occurs less rapidly, so that a stronger magnetic field can be shielded than in the case of a continuous shielding layer of the same layer thickness.
Preferably, each memory element comprises, in addition to a first layer of a magnetic material, a second layer of a magnetic material, which is separated from said first layer of magnetic material by a non-magnetic material, because magnetoresistance effects occur in such memory elements which exceed the magnetoresistance effect that occurs in a memory element comprising a single layer of magnetic material.
Spin valves and magnetic tunnel junctions comprise such a layer packet and have the advantage that they can function as a magnetic memory element when the magnetic fields are weak. In its most basic form, a spin valve is a three-layer structure composed of a soft magnetic layer wherein the magnetization changes as a result of a comparatively weak magnetic field, a harder magnetic layer and a noble metal sandwiched therebetween. In a magnetic tunnel junction, the two magnetic layers are separated by an electrical insulation layer. As the magnetoresistance effect in spin valves and magnetic tunnel junctions is comparatively large, they can very suitably be used as a magnetic memory element.
The position of the regions of the shielding layer preferably is such that the perpendicular projection of a region on the memory elements comprises at least one memory element. When the dimensions of the memory elements lie in the submicron range, it is advantageous for the perpendicular projection of a region of the shielding layer on the memory elements to comprise a plurality of memory elements, so that the dimensions of the shielding regions can be readily realized and, in general, the shielding effect is also favorably influenced.
In order to provide sufficient shielding against a magnetic disturbing field at the location of the memory elements, two conditions must be met.
First, the magnetization of the material of the shielding layer must remain below the saturation value. Above the saturation value, the field penetrates the material. If the distance between the memory elements and the shielding layer is small, for example 0.5 or less, with respect to the dimensions
w
of the region, then the magnetization
M
is given by M=H
appl
&khgr;/(1+&khgr;t/w), wherein
H
appl
is the external magnetic disturbing field,
&khgr;
is the magnetic susceptibility and
t
is the thickness of the shielding layer. The magnetic regions may be of arbitrary shape. In general, the regions will be, for example, rectangular, square, round or hexagonal. The dimensions w of a region then correspond to, respectively, a length and a width, a diameter or a diagonal. Sufficient shielding requires, in the first place, that the following equation is met t/w>H
appl
/M
s
−1/&khgr;, wherein
M
s
is the saturation value of the magnetization. If this equation is not met, the shielding layer is subject to saturation.
At this stage, it can be readily understood that, owing to the fact that the shielding layer of thickness
t
is split into regions, the dimensions
w
of a region are much smaller than the dimensions w′ of a continuous shielding layer extending throughout the surface of the magnetic memory, so that the magnetic disturbing fields can be approximately a factor of w′/w larger before saturation of the magnetization takes place and the magnetic fields are penetrated.
Consequently, as the shielding layer is divided into regions, it is also possible, with a view to shielding against a magnetic disturbing field, to choose the thickness of the shielding layer to be smaller than that of an undivided shielding layer.
Second, it is necessary for the regions of the shielding layer to sufficiently attenuate the magnetic disturbing field, so that the direction of magnetization in the memory elements cannot flip over. Below saturation, the magnetic field at the location of the magnetic memory element is given by H=H
appl
/(1+&khgr;t/w). The magnetic disturbing field H
appl
is attenuated to a much lower magnetic field H at the location of the magnetic memory element, if the following equation is met t/w>H
appl
/(H&khgr;)−1/&khgr;.
Consequently, in order to sufficiently shield each memory element, the following equations must be met t/w>H
appl
/M
s
−1/&khgr; and t/w>H
appl
/(H&khgr;)−1/&khgr;. Consequently, it is favorable for the ratio t/w to be as large as possible.
An example: T
Lenssen Kars-Michiel Hubert
Ruigrok Jacobus Josephus Maria
Biren Steven R.
Flynn Nathan J.
Quinto Kevin
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