Abstract:
A magnetoresistive Wheatstone-bridge structure includes a magnetoresistive ring structure. The magnetoresistive ring structure includes a first magnetic layer comprising a ferromagnetic material. A second magnetic layer also includes a ferromagnetic material. A non-magnetic spacer is positioned between the first magnetic layer and the second magnetic layer. A vacant open region is positioned in the center region of the magnetoresistive ring structure. A plurality of magnetic states can exist in either the first magnetic layer or second magnetic layer. Furthermore, the magnetoresistive Wheatstone-bridge structure includes a plurality of voltage and current contacts arranged symmetrically upon the magnetoresistive ring structure. The magnetic state of the ring is detected by measuring its resistance.

Description:
PRIORITY INFORMATION 
     This application claims priority from provisional application Ser. No. 60/846,935 filed Sep. 25, 2006, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to the field of magnetic memory devices, and in particular a magnetic memory device that can store multiple bits per ring and can be programmed to carry out different logic functions. 
     The Wheatstone-bridge (WB) configuration has been known for a long time. It is useful mainly in sensors, where four resistors are connected together to make a bridge circuit and a small change in one of the resistors, due to temperature, magnetic field, strain or whatever quantity is being measured, causes a big change in the resistance of the WB circuit. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, there is provided a magnetoresistive ring structure. The magnetoresistive ring structure includes a first magnetic layer comprising a ferromagnetic material. A second magnetic layer also includes a ferromagnetic material. A non-magnetic spacer is positioned between the first magnetic layer and the second magnetic layer. A vacant open region is positioned in the center region of the magnetic ring structure. A plurality of magnetic states can exist in either the first magnetic layer or second magnetic layer. The magnetic state of the ring can be detected by measuring its resistance. 
     According to another aspect of the invention, there is provided a method of forming a magnetoresistive ring structure. The method includes forming a first magnetic layer comprising a ferromagnetic material and forming a second magnetic layer also comprising a ferromagnetic material. Also, the method includes forming a non-magnetic spacer being positioned between the first magnetic layer and the second magnetic layer. A vacant open region is positioned in the center region of the magnetic ring structure. A plurality of magnetic states can exist in either the first magnetic layer or second magnetic layer. The magnetic state of the ring can be detected by measuring its resistance. 
     According to another aspect of the invention, there is provided a magnetoresistive Wheatstone-bridge structure. The magnetoresistive Wheatstone-bridge structure includes a magnetoresistive ring structure. The magnetoresistive ring structure includes a first magnetic layer comprising a ferromagnetic material. A second magnetic layer also includes a ferromagnetic material. A non-magnetic spacer is positioned between the first magnetic layer and the second magnetic layer. A vacant open region is positioned in the center region of the magnetic ring structure. Furthermore, the magnetoresistive Wheatstone-bridge structure includes a plurality of voltage and current contacts arranged on the magnetic ring structure such that the voltage measured across the ring depends sensitively on the magnetic state of the ring. A plurality of magnetic states can exist in either the first magnetic layer or second magnetic layer. The magnetic state of the ring can be detected by measuring its resistance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating the architecture of a single magnetic multilayer ring used in accordance with the invention; 
         FIG. 2A  is a scanning electron micrograph (SEM) corresponding to the single magnetic multilayer ring used in accordance with the invention;  FIG. 2B  shows magnetoresistance curves measured using an asymmetric (non-WB) contact configuration and a symmetric or Wheatstone-bridge (WB) configuration; 
         FIG. 3A  shows SEM images of a non-WB ring structure and WB ring structure; 
         FIG. 3B  are graphs comparing the resistance vs. applied field of the WB ring structure and the non-WB ring structure; 
         FIG. 4A  is a graph illustrating a WB ring structure being cycled multiple times by an applied field;  FIG. 4B  is a graph illustrating the resulting resistance being reproducible; and 
         FIG. 5  is a graph illustrating the states of a nonvolatile memory element using the inventive ring structure. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention provides a Wheatstone-bridge (WB) circuit having a single magnetic multilayer ring with four electrical contacts. The difference between the commonly-known WB circuit and the invention is that a commonly-known WB circuit contains four discrete resistors whereas the invention contains a single magnetic multilayer ring with four electrical contacts. The behavior of the single magnetic multilayer ring bears no relation to the behavior of four discrete magnetic multilayer resistors connected into a conventional WB circuit, because a ring acts as a single magnetic entity that can have special magnetization states such as vortex and onion states, or states containing 360 degree domain walls, related specifically to its ring-shaped geometry. 
     The magnetoresistive response of lithographically-defined multilayered elements is of great current interest as these structures constitute the building blocks for a range of data storage and logic applications.  FIG. 1  shows the basic architecture of a single magnetic multilayer ring structure  2  having two ferromagnetic layers  4 ,  8  (soft and hard) separated by a non-magnetic or insulator spacer  6 . A vacant open region is positioned in the center region of said magnetic ring structure. 
     The ring structure  2  has dimensions comprising a length L and breadth smaller than L, comprising an elliptical arrangement, however, other ring structures having designs such as circular, or rings with corners can be used. The value of L can be in the range of 50 nm to 5 μm and width of the ring structure can be 10-500 nm. In other embodiments of the invention, a layer of gold (Au) or other material can also be deposited on the topmost layer of the multilayer ring structure  2  to reduce corrosion or improve electrical contact. The ring may also comprise an antiferromagnetic layer adjacent to the hard magnetic layer to pin the magnetization of the hard magnetic layer, or other layers such as a synthetic antiferromagnet to reduce magnetostatic interactions between the magnetic layers. 
     By designing the ring shape, which can be circular, elliptical, or with corners, one can control the switching behavior and therefore the number of distinct magnetic states that the ring can adopt, and the field required to switch between them. For example, a rhombic shaped ring has an extensive stability range of certain magnetic configurations that include 360° domain walls, and the formation of these states can occur at low applied fields, below 100 Oe. The ring may be set into particular magnetization states by application of a magnetic field, or by the action of an electric current that causes current-induced reversal of the ring. 
       FIG. 2A  shows a scanning electron micrograph (SEM) corresponding to a 4 μm-long, 220 nm-wide, WB arrangement  10  having a multilayer structure comprising NiFe (6 nm)/Cu (5 nm)/Co (5 nm)/Au (4 nm). In particular,  FIG. 2A  represents a current-in-plane (CIP) giant magnetoresistive device.  FIG. 2B  illustrates magnetoresistance curves measured using first, an asymmetric contact configuration having current contacts A-D and voltage contacts E-F, and second, a symmetric or WB configuration having current contacts A-D and voltage contacts C-F, with the contacts labelled as shown in  FIG. 2A . Importantly, in the device of  FIG. 2A  measured using the asymmetric contact configuration, the total resistance change is modest and is typically limited to a few percent, which restricts the signal-to-noise ratio and thus the usefulness of the device. However, for the WB contact configuration, the same device produces a much greater proportional change in resistance. While the maximum relative resistance change for the asymmetric contact arrangement is 2.1% (right hand axis), the WB contact arrangement provides a lower resistance and relative resistance change of 26% (left hand axis). 
     The WB arrangement  10  therefore allows much larger resistance changes to be obtained for rings made out of the multilayer, compared with the asymmetric contact configuration. Other multilayer structures could also be used, for example, those with exchange bias or synthetic antiferromagnetic layer sequences. 
     The WB arrangement  10  provides the advantage that very large signals are obtained from the ring, 100 times or greater than the inherent magnetoresistance of the multilayer which is typically a few percent. Therefore the WB arrangement  10  allows a current-in-plane geometry to be used and still obtain a large signal. The current-in-plane (CIP) geometry is easier to fabricate than current-perpendicular-to-plane (CPP) devices, and the WB arrangement  10  allows large signals to be produced, for example, as large or greater than the resistance changes observed in magnetic tunnel junction devices. 
     When the ring is in a highly symmetrical magnetic state, for example, at remanence after saturation, the voltage CF is very small, but as the soft NiFe layer begins to reverse under the influence of a small applied field, the ‘bridge’ becomes unbalanced and a voltage CF develops. Depending on the placement of the contacts in the WB, and the ring geometry, relative resistance changes between 25% and as high as 185% have been measured in such structures, despite the giant magnetoresistance of the film itself being only 2%. The resistance shows several distinct levels which have been identified as resulting from magnetic configurations containing different combinations of 180 degree and 360 degree domain walls in the hard and soft layers. The use of this structure in magnetoelectronic devices could provide high signal amplitude even for a CIP geometry, which is easier to fabricate than a current-perpendicular-to-plane geometry. 
     The magnetic multilayer stack NiFe (6 nm)/Cu (4 nm)/Co (4 nm)/Au (4 nm) was deposited using DC-triode sputtering and the ring pattern was defined by electron-beam lithography, for example a ring with 4 μm length, 2 μm breadth and a 220 nm width as shown in  FIG. 2A . The final electrical contacts A, B, C, D, E, F comprised Ta (3 nm)/Cu (110 nm)/Au (8 nm). The four point probe measurement technique used an AC (10 μA r.m.s., 1 kHz) current source with lock-in detection to measure the resistance of the ring while a varying magnetic field was applied. 
       FIG. 3A  shows an asymmetrically contacted ring structure  30  and a WB-contacted ring structure  32  and their respective current contacts (I) and voltage contacts (V). The ring structures used in  FIG. 3A  are rhombic in shape. A rhombic shaped ring has an extensive stability range of certain magnetic configurations that include 360° walls, and the formation of these states can occur at low applied fields. 
       FIG. 3B  compares the resistance vs. applied field for the WB ring structure  32  and the asymmetric ring structure  30 . Each plot shows various resistance levels at different fields. The different resistance levels could represent data bits in a memory or logic device. What is important is that the resistance difference between these levels is quite small for the asymmetrically contacted ring structure  30  (right hand axis, about 1% difference) but for the WB ring structure  32 , changes of 35% (left hand axis) are seen. The much greater resistance difference makes the device easier to ‘read’. 
     Moreover, a WB ring structure device can be cycled multiple times by an applied field that varies with time, as shown in  FIG. 4A , and the resulting resistance changes are quite reproducible as shown in  FIG. 4B . 
     The invention can be applied in a nonvolatile multi-bit memory element, using the multiple resistance levels obtained by low-field cycling.  FIG. 5  shows results from the low-field cycling of a ring structure having a multilayer arrangement of NiFe (6 nm)/Cu (5 nm)/Co (5 nm) and a length of 2 μm and width of 200 nm. This cycling can produce four different remanent states (S 1 , S 2 , S 2 R, and S 3 ) which can represent two bits, as shown in  FIG. 5 . A different field cycling enables additional states to be accessed. This ring structure can be used as the data storage element of a magnetic random access memory. It may also be used as a logic gate, in which the ring would first be set into a desired configuration which defines its function, then two inputs are applied (e.g. two magnetic fields, or two currents) and the output (the resistance) is measured. The output will be a logical combination of the two inputs. 
     Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.