Magnetic storage device

Abstract

A magnetic storage device comprises an array of magnetic memory cells ( 50 ). Each cell ( 50 ) has, in electrical series connection, a magnetic tunnel junction (MTJ) ( 30 ) and a Zener diode ( 40 ). The MTJ ( 30 ) comprises, in sequence, a fixed ferromagnetic layer (FMF) ( 32 ), a non-magnetic spacer layer ( 33 ), a tunnel barrier layer ( 34 ), a further spacer layer ( 35 ), and a soft ferromagnetic layer (FMS) ( 36 ) that can change the orientation of its magnetic moment. The material type and thickness of each layer in the MTJ ( 30 ) is selected so that the cell ( 50 ) can be written by applying a voltage across the cell, which sets the orientation of the magnetic moments of the FMF ( 32 ) and FMS ( 36 ) relative to one another. The switching is effected by means of an induced exchange interaction between the FMS and FMF mediated by the tunneling of spin-polarised electrons in the MTJ ( 30 ). The cell ( 50 ) therefore has low power consumption during write operations allowing for fast writing and dense integration of cells ( 50 ) in an array. The mechanism used to control the array to write and sense the information stored in the cells ( 50 ) is simplified.

1 . A magnetic tunnel junction device comprising first and second stacks of layers of magnetic material, each stack comprising at least one layer, the stacks being separated by a third stack of layers of non-magnetic material, the third stack comprising at least one layer of electrically insulating material, with contacts being made to the first and second stack to apply a voltage across the device, the magnetic materials and insulating material(s) each being of a type and the said layers each having a thickness such that the orientation of the magnetic moments of said first and second stack relative to one another are changeable by applying a voltage across the device, characterised in that said orientation can be switched to a first state by applying a first voltage across the device and that said orientation can be switched to a second state applying a second voltage across the device, whereby after either switching the said orientation is maintained when a third voltage is applied to the device the said third voltage being in between the first and second voltage.  
     
     
         2 . A device as claimed in  claim 1  in which at least one of the said first and second stacks is separated from the third stack by a further layer of non-magnetic conducting material.  
     
     
         3 . A device as claimed in  claim 1  or  claim 2  in which the third stack comprises two layers of insulating material separated from one another by a layer of nonmagnetic conductive material.  
     
     
         4 . A device as claimed in any preceding claim in which at least one of the layers of magnetic material in the said first or second stack has a substantially elliptical shape.  
     
     
         5 . A device as claimed in claims  1 ,  2  or  3  in which at least one of the layers of magnetic material in the said first or second stack has a substantially hexagonal or octagonal shape.  
     
     
         6 . A device as claimed in any preceding claim in which the easy axes of the magnetisation of the said first and second stack are aligned with one another such that the orientation of the magnetic moments of said first and second stack relative to one another are changeable between substantially parallel and substantially anti-parallel states.  
     
     
         7 . A device as claimed in  claim 6  with the magnetic materials and insulating material(s) each being of a type and the said layers each having a thickness thus causing an asymmetric current response such that the changing from substantially parallel to substantially anti-parallel state is allowed to be effected by applying a forward or backward voltage to the device, wherein the change effected by applying a forward voltage is opposite to the change effected by applying a backward voltage.  
     
     
         8 . A device as claimed in any preceding claim comprising sensing means connected to the said contacts which allows the orientation of the magnetic moments of said first and second stack relative to one another to be sensed by applying a fourth voltage across the device and measuring the resistance of the device.  
     
     
         9 . A device as claimed in  claim 8  in which the fourth voltage applied to the device in order to sense the state of the device is smaller in absolute terms than a voltage applied to the device in order to change the state of the device.  
     
     
         10 . A device as claimed in any preceding claim in which the said third stack comprises at least one layer of manganese oxide (MgO).  
     
     
         11 . A device as claimed in  claim 10  in which the easy axes of the magnetisation of the said first and second stack are aligned with one another such that the device can be switched between two states in which the orientation of the magnetic moments of said first and second stack can be changed from a substantially parallel alignment to substantially anti-parallel alignment by applying one of the said first and second voltages to the device and said magnetic moments can be switched from a substantially anti-parallel alignment to a substantially parallel alignment by applying the other of the said first and second voltages to the device.  
     
     
         12 . A magnetic memory cell device comprising a magnetic tunnel junction device as claimed in any preceding claim which comprises a diode in electrical series connection with the magnetic tunnel junction device.  
     
     
         13 . A magnetic memory cell device as in  claim 12  wherein the diode is a Zener diode.  
     
     
         14 . A device as claimed in  claim 12  or  claim 13  having means allowing to effect one of the said voltages required to switch the relative orientation of the magnetic moments of the said first and second stacks in the magnetic tunnel junction device by allowing to apply a further voltage across the magnetic memory cell device said further voltage effecting a voltage across the diode which is a reverse voltage on the diode, said reverse voltage being greater than the breakdown voltage of the diode.  
     
     
         15 . An array of magnetic memory cell devices comprising a first plurality of conducting leads, a second plurality of conducting leads, each lead in the said second plurality crossing over each lead in the said first plurality, a plurality of magnetic memory cell devices as claimed in  claim 12  to  claim 14 , each magnetic memory cell device being located at an intersection region between one of the first plurality of leads and one of the second plurality of leads, the array having means to apply a voltage to the leads in the first and second plurality such that a voltage drop across a specific memory cell device can be effected, the voltage drop causing the said memory cell device to be written by setting the orientation of said magnetic moments relative to one another.  
     
     
         16 . A method of providing a magnetic tunnel junction device comprising providing a magnetic tunnel junction comprising first and second stacks of layers of magnetic material, each stack comprising at least one layer, the stacks being separated by a third stack of layers of non-magnetic material, the third stack comprising at least one layer of electrically insulating material, with contacts being made to said first and second stack to apply a voltage across the magnetic tunnel junction the type and thickness of said magnetic and insulating materials being selected such that the orientation of the magnetic moments of said first and second stack relative to one another can be changed by applying a voltage across the device, characterised in that said orientation can be switched to a first state by applying a first voltage across the device and that said orientation can be switched to a second state applying a second voltage across the device, whereby after either switching the said orientation is maintained when a third voltage is applied to the device the said third voltage being in between the first and second voltage.  
     
     
         17 . A method as claimed in  claim 16  comprising providing two layers of insulating material in the said third stack, which two layers of insulating material are separated from one another by a layer of non-magnetic conductive material.  
     
     
         18 . A method as claimed in  claim 16  or  claim 17  in which at least one of the layers of magnetic material in the said first or second stack is provided having a substantially elliptical shape.  
     
     
         19 . A method as claimed in  claims 16  to  17  in which at least one of the layers of magnetic material in the said first or second stack is provided having a substantially hexagonal or octagonal shape.  
     
     
         20 . A method as claimed in  claims 16  to  19  comprising providing a layer of nonmagnetic conductive material between the said third stack and at least one of the said first and second stacks.  
     
     
         21 . A method as claimed in  claim 16  to  claim 20  in which the orientation of the magnetic moments of said first and second stack relative to one another can be changed between substantially parallel and substantially anti-parallel states.  
     
     
         22 . A method as claimed in  claim 21  in which changing from substantially parallel to substantially anti-parallel state is effected by applying a forward or backward voltage to the device, wherein the change effected by applying a forward voltage is opposite the change effected by applying a backward voltage.  
     
     
         23 . A method as claimed in  claims 16  to  22  in which the orientation of the magnetic moments of said first and second stack relative to one another can be sensed by applying a fourth voltage across the magnetic tunnel junction device and measuring the resistance of the device.  
     
     
         24 . A method as claimed in  claim 23  in which the fourth voltage applied to the device in order to sense the orientation of the magnetic moments of the said first and second stack relative to one another is smaller in absolute terms than the voltage applied to the device in order to change the relative alignment of said magnetic moments.  
     
     
         25 . A method as claimed in any preceding claim in which the said third stack is provided with at least one layer of manganese oxide (MgO).  
     
     
         26 . A method as claimed in  claim 25  in which the device can be switched between two states in which the orientation of the magnetic moments of said first and second stack can be changed from a substantially parallel alignment to substantially anti-parallel alignment by applying on of the said first and second voltages to the device and said magnetic moments can be switched from a substantially anti-parallel alignment to a substantially parallel alignment by applying the other of the said first and second voltages to the device.  
     
     
         27 . A method of providing magnetic memory cell device comprising providing a magnetic tunnel junction device as claimed in any of  claims 16  to  25  which comprises providing a diode in electrical series connection with the magnetic tunnel junction device.  
     
     
         28 . A method as claimed in  claim 27  wherein the diode is a Zener diode.  
     
     
         29 . A method as claimed in  claim 27  or  claim 28  in which one of the said voltages required to switch the relative orientation of the magnetic moments of said first and second stack in the magnetic tunnel junction device is effected by applying a further voltage across the magnetic memory cell device said further voltage effecting a voltage across the diode which is a reverse voltage on the diode, said reverse voltage being greater than the breakdown voltage of the diode.  
     
     
         30 . A method of providing an array of magnetic memory cell devices comprising providing a first plurality of conducting leads, a second plurality of conducting leads, each lead in the said second plurality crossing over each lead in the said first plurality, a plurality of magnetic memory cell devices provided as claimed in  claim 27  to  claim 29 , each magnetic memory cell device being located at an intersection region between one of the first plurality of leads and one of the second plurality of leads, and providing means to apply a voltage to the leads in the first and second plurality such that a voltage drop across a specific memory cell device can be effected, the voltage drop causing a magnetic field in the device through tunnelling of spin-polarised electrons which effects the device to be written by setting the orientation of the said magnetic moments relative to one another.  
     
     
         31 . A magnetic tunnel junction device substantially as herein before described with reference to FIG. 2 to FIG. 5 of the accompanying drawings.  
     
     
         32 . A magnetic memory cell device substantially as herein before described with reference to FIG. 2 to FIG. 5 of the accompanying drawings.  
     
     
         33 . A magnetic memory cell array substantially as herein before described with reference to FIG. 2 to FIG. 5 of the accompanying drawings.  
     
     
         34 . A method of providing a magnetic tunnel junction device substantially as herein before described with reference to FIG. 2 to FIG. 5 of the accompanying drawings.  
     
     
         35 . A method of providing a magnetic memory cell device substantially as herein before described with reference to FIG. 2 to FIG. 5 of the accompanying drawings.  
     
     
         36 . A method of providing a magnetic memory cell array substantially as herein before described with reference to FIG. 2 to FIG. 5 of the accompanying drawings.