Static information storage and retrieval – Systems using particular element – Hall effect
Reexamination Certificate
2000-09-18
2002-11-19
Elms, Richard (Department: 2824)
Static information storage and retrieval
Systems using particular element
Hall effect
C365S173000
Reexamination Certificate
active
06483741
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a magnetization drive method, a magnetic functional device, and a magnetic apparatus, which are typically suitable to be used for solid-state magnetic memories.
As compared with semiconductor devices, magnetic devices using magnetic substances have the following merits. First, since components of a magnetic device can be made from metals having high carrier densities and low resistances, such a magnetic device may be suitable for realizing a very fine-structure of the magnetic device, such as a structure having sub-micron dimensions. Second, since a magnetic substance exhibits bi-stable magnetization directions, the magnetization state of the magnetic substance can be stably kept even when external energy is not given to the magnetic substance, with a result that such a magnetic substance is suitable for realizing a non-volatile memory function. Third, if a magnetic field having a sufficient strength is applied externally to a magnetic substance of a magnetic device, a magnetization direction of the magnetic substance can be changed into the field direction for a time being as short as about 1 ns, and accordingly, the magnetic device can realize very fast switching of the magnetization direction.
Magnetic devices using magnetic substances, which have the above-described merits, may be applied to high integration solid-stage memories operated at high speeds to realize energy-saving.
However, the benefit associated with high speed operation of magnetic devices tends to be lost along with the progress of the degree of fineness of fine-structures of the magnetic devices due to the following causes:
One of the causes is that, as described in the earlier application by the present applicant, Japanese Patent Laid-open No. Hei 10-130711, as wires become thinner along with the progress of fineness of a finer-structure of a magnetic device, the amount of a current flowing through the wires tends to be restricted, thereby making it impossible to generate a magnetic field having a sufficient strength. The stronger the magnetic field applied to the magnetic device, the faster the magnetization switching speed thereof. Accordingly, the restriction of the applied magnetic field limits the switching speed of the magnetic device.
Another cause is that as the size of a magnetic substance whose magnetization direction is to be switched becomes small, a damping force applied to damp the movement of a magnetization vector of the magnetic substance becomes small. If the damping force becomes excessively small, the magnetization vector continuously rounds around the field direction, with a result that it takes a lot of time for the magnetization vector to be converged in the field direction.
The reason why the damping force applied to damp the movement of a magnetization vector of a magnetic substance becomes small with a reduction in size of the magnetic substance will be described below. In a magnetic metal thin film used for a solid-state magnetic memory such as a magnetic random access memory (MRAM), during movement of magnetization, an eddy current flow in the direction in which the eddy current obstructs the movement of the magnetization. A Joule heat generated by the eddy current is the dominant part of the loss in movement of magnetization. An eddy current loss density per volume, p(W/m
3
) is approximately proportional to the cross-section of the magnetic thin film, that is, a magnetic substance. That is to say, as the size of a magnetic substance becomes small, the strength of a damping force applied to damp a magnetization vector of the magnetic substance becomes small approximately in proportion to the square of the size of the magnetic substance.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a magnetic functional device including a magnetic substance, which allows magnetization switching at a high speed even if the size of the magnetic substance is made finer on the sub-micron order, a magnetization drive method for the magnetic functional device, and a magnetic apparatus using the magnetic functional device.
First, a general theory concerned with the movement of a magnetization vector will be described.
An equation of a magnetic moment is given, for example, by Landau and Lifshits as follows:
dM/dt=−&ggr;·
(
M×H
)−(&agr;·&ggr;
/M
s
)·
{M×
(
M×H
)} (1)
where M is a magnetization vector, H is a vector of a magnetic field, &ggr; is a gyromagnetic constant, and a is &agr; damping factor. If &agr; is significantly larger than 1, the magnetization vector is slowly converged to the direction of an external magnetic field. For example, when a magnetic field in the −x direction is applied to a magnetization vector directed nearly in the +x direction within an xy plane, if the damping is large, the magnetization vector slowly rounds within the xy plane to be converged in the field direction, to attain the magnetization reversal. This state is shown in FIG.
1
A. As shown in this figure, the magnetization vector rounds in the order of arrows
0
(initial state)→
1
→
2
→
3
→
4
→
5
, to be finally converged in the −x direction by relaxation.
On the contrary, if &agr; is significantly smaller than 1 and thereby the first term of the equation (1) becomes dominant, the change in magnetization vector with time, that is, dM/dt is usually perpendicular to the direction from M to H. Accordingly, the magnetization vector M undergoes precession rounding around the field vector H with an angle from the field vector H kept constant.
In the case of a magnetic substance in which &agr; contributes to movement of a magnetization vector although the value of &agr; is small, when a magnetic field in the −x direction to a magnetization vector directed in the + direction, the magnetization vector undergoes the precession rounding around the x-axis with an open angle from the x-axis gradually increased, and is finally relaxed to the field direction. A first half of a spiral locus of the termination of the magnetization vector is shown in FIG.
1
B.
The magnetization reversal caused by a magnetic field directed in the direction reversed 180° to the magnetization vector in the initial state is generally intermediate between the above-described magnetization reversal with an extremely large damping factor &agr; and the magnetization reversal with an extremely small damping factor &agr;. The time required for magnetization reversal is important as a parameter determining the operational speed of a magnetic device. If the damping factor &agr; is extremely large, the movement of the magnetization vector depicts a locus nearly along the shortest distance; however, the movement is slow and thereby the reversal time is long. On the contrary, if the damping factor a is significantly small, the movement of the magnetization vector is quick; however, it depicts a spiral locus and thereby the time required for the magnetization vector to be converged to the final state is long. The quickest magnetization reversal is obtained with the damping factor set at 1 (&agr;=1). Such a damping state allowing the quickest response is called “a critical damping state”.
For example, in the case of Permalloy (Ni&THgr;Fe alloy) often used for a magnetic functional device such as an MRAM, the critical damping appears when the size of the device is about 1 &mgr;m, and accordingly, the design of most of MRAMs having the size of about 1 &mgr;m, which have been extensively developed at present, is advantageous in making effective use of the material characteristic, that is, the critical damping of Permalloy. The damping, however, is nearly proportional to the square of the size of a magnetic device as described above, so that if a magnetic device has a fine structure, the degree of fineness of which is on the order of sub-micron, such a device causes a problem that the speed of magnetization reversal becomes slow because of the insufficient dampin
Bessho Kazuhiro
Iwasaki Yoh
Miyauchi Teiichi
Phung Anh
Sonnenschein Nath & Rosenthal
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