Static information storage and retrieval – Systems using particular element – Superconductive
Reexamination Certificate
2000-10-18
2002-07-02
Nelms, David (Department: 2818)
Static information storage and retrieval
Systems using particular element
Superconductive
C365S174000
Reexamination Certificate
active
06414870
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a microquenched superconductor valve device and, more particularly, to an improved valve device having a bilayer ferromagnetic film.
2. Description of the Related Art
A magnetoquenched superconducting valve is a superconductor switching device with applications in digital and analog superconducting electronics. As explained in more detail below, locally strong fringe fields from a ferromagnetic film microfabricated on top of a superconducting strip create a weak link in the superconductor and suppress the critical current (I
c
). The magnitude of the magnetic fringe field is controlled by manipulating the orientation of the ferromagnetic film and in this way the weak link can be created or removed, i.e., the device can be turned “on” and “off.” The intrinsic remanence of the magnetization provides a natural memory function, and the device can be used as a nonvolatile storage cell. Current pulses in contiguous write wires can be used to control the magnetization orientation of the ferromagnetic and thereby control the switching state of the device. The magnitude of the control current can be made smaller than the critical current of the superconductor and thus the device can have current gain and can be used as a current amplifier.
The basic operation and the fundamental capabilities of the magnetoquenched semiconductor valve are described, for example, in U.S. Pat. No. 5,930,165 (Johnson et al), which is hereby incorporated by reference. However, to briefly summarize this description, reference will be made to
FIG. 1
which shows an exploded perspective view of such a valve device, which is generally denoted
10
. A superconducting bridge
10
is fabricated on a substrate
11
so as to extend parallel to the y axis and has a width W
s
and critical current I
c
(t), where t is the reduced temperature T/T
c
and T
c
is the critical temperature. A supply current is indicated by arrow
13
. A thin ferromagnetic film
14
is fabricated on top of semiconductor
12
and is isolated therefrom by a very thin insulating layer
16
, such as a tunnel barrier. Orthogonal write wires
17
and
19
provide first and second control currents indicated by arrows
18
and
20
and are formed by a conductor
22
and corresponding insulator
24
. Ferromagnetic film
14
has two anisotropy axes such that the magnetization {right arrow over (M)} is stable along either the x or y axis.
The device of
FIG. 1
works in the following way. When the magnetization is oriented along the x axis, dipolar fringe fields originating from the ends of film
14
are substantially parallel to the plane of superconductor
12
and have a weak magnitude in the vicinity thereof. Thus there is little or no effect on the superconducting properties of superconductor
12
. This is called the unquenched state of superconductor
12
, equivalently the “off” state of the device. However, when the magnetization of film
14
is oriented along the y axis, the dipolar magnetic fringe fields {right arrow over (B)} that originate at the edge of film
14
have a large magnitude in the vicinity of superconductor
12
. In particular, there is a large component B
z
perpendicular to the plane of superconductor
12
. Much of the fringe field is shielded by Meissner currents in the superconductor
12
, and most of the field is deflected to the side of the superconductor. However, the component B
z
is sufficiently large that it exceeds the critical field in a small, local region directly beneath the edge of film
14
and a “weak link” of normal metal is thereby created. The weak link causes substantial suppression of the critical current I
c
(t) of the superconducting bridge
12
. This is called the quenched state of the superconductor
12
, equivalently the “on” state of the switch.
The operation of the switch
10
relies on the remanent magnetization M
r
of the ferromagnetic film
14
, and therefore is intrinsically both “latching” and “nonvolatile.” Energy is only used to set or reset the magnetization state of ferromagnetic film
14
, and the quiescent power is zero. An upper bound of the switching speed of the device is the order of 1 nsec, and it may switch as rapidly as 1 psec. Integrated device operation invokes the application of current pulses to integrated write wires
17
and
19
shown in
FIG. 1. A
current pulse of appropriate amplitude and duration applied to the write wire
19
oriented parallel to the y axis orients the magnetization along the x axis, and the device is put into the unquenched or “off” state. A current pulse of appropriate amplitude and duration applied to the write wire
17
oriented parallel to the x axis orients the magnetization along the y axis, and the device is put into the quenched or “on” state.
SUMMARY OF THE INVENTION
According to the invention, a magnetoquenched superconducting valve device is provided which provides important advantages over the basic valve device described above. Generally speaking, whereas the basic device uses a single ferromagnetic film or layer (corresponding to layer
14
of
FIG. 1
) and two magnetic anisotropy axes to provide fringe fields, the valve device of the invention uses a bilayer of ferromagnetic films as the ferromagnetic element that provides the fringe magnetic field. With this construction, a single magnetic anisotropy axis can be employed and this provides several associated advantages. More particularly, the areal dimensions of the device all can be reduced or shrunk, thereby permitting greater packing densities. Further, a single write wire can be used to control the magnetization orientation, thereby resulting in a simpler cell architecture.
An object of the invention is to provide an improved magnetoquenched superconductor valve device characterized by a single magnetic anisotropy axis. As indicated above, such a device needs only a single integrated write wire to control the device state. In contrast, the original or basic device requires two perpendicular write wires to control the device state. The write wire of the modified device of the invention can be fabricated from a superconducting material, thereby affording low power operation.
A further object of the invention is to provide an improved valve device with a single magnetic anisotropy axis which can be used as an integrated memory cell and which can be fabricated in a two dimensional array of cells, using a standard half-select process for addressing individual cells.
Yet another object of the invention is to provide an improved valve device with a single magnetic anisotropy axis which can be fabricated with smaller dimensions, and which can, therefore, be integrated with higher packing densities.
A further object of the invention is to provide an improved valve device that includes a superconducting ground plane which acts to increase the write fields (thereby decreasing the write current for a given, required write field), and to decrease the effects of the self-fields of the superconducting current in the bridge.
An additional object of the invention is to provide a modified ferromagnetic element comprising two ferromagnetic layers that can be incorporated in Hybrid Hall Effect (HHE) memory cells, such that each of the two ferromagnetic layers in the element stores a single bit of information. In this way, each HHE cell can store two bits of data. This object can, in principle, be generalized to n ferromagnetic layers storing n bits of information per cell.
In accordance with the invention, a magnetoquenched superconductor is provided which comprises: a substrate; a superconductor element disposed on the substrate; a first ferromagnetic film disposed on the superconductor element; and a second ferromagnetic film disposed on the first ferromagnetic film, said first and second films being magnetically coupled and having magnetizations which are switchable between a parallel relation wherein the films produce additive magnetic fringe fields that form a weak link in a portion of the superconductor eleme
Clinton Thomas W.
Johnson Mark B.
Karasek John J.
Ketner Philip E.
Le Thong
Nelms David
The United States of America as represented by the Secretary of
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