Static information storage and retrieval – Systems using particular element – Magnetoresistive
Reexamination Certificate
2002-04-30
2003-12-02
Mai, Son (Department: 2818)
Static information storage and retrieval
Systems using particular element
Magnetoresistive
C365S171000, C365S173000
Reexamination Certificate
active
06657888
ABSTRACT:
TECHNICAL AREA
The disclosed invention relates to memory devices and more particularly to two terminal bistable memory cells which comprise at least two high-spin polarization magnetic material junctions, said junctions being separated from one another by insulator, said insulator typically containing trap sites; said two terminal bistable memory cell demonstrating two stable, low A.C. or D.C. current readable, hysteretic resistance states which are set by the flow of a relatively higher plus or minus D.C. polarity current therethrough. Preferred cells each comprise at least one sequence of: CrO
2
/Cr
2
O
3
/CrO
2
.
BACKGROUND
State of the art electronic devices include, for instance, three terminal Complementary Metal Oxide Semiconductor (CMOS) devices which provide fan-in and fan-out capability, and can perform memory and logic functions. Where only memory is required, however, two terminal electrical devices are applicable. Two terminal electrical memory systems are functionally similar to magnetic dipoles which are set to one of two stable states by application of magnetic fields, but they are set by application of voltage/current and are monitored by applying a current/voltage and reading a voltage/current. Two terminal memory devices include Giant magnetoresistive devices as well as tunnel junction based magnetics.
Two terminal devices can operate based on a tunnel magnetoresistance effect and can be comprise a sequence of:
ferromagnetic/insulator/ferromagnetic;
materials. The resistance across said sequence is determined by the relative magnetic alignment of two ferromagnetic layers. The effect is based in the ability of ferromagnetics to polarize spin in electric currents where Polarization (P) is given by:
P
=(
n+−n−
)/(
n++n−
);
where n+ and n− are the number of conduction electrons with their spin parallel and anti-parallel, respectively. It is noted that optimum magneto-electronic properties correspond to a Polarization of 100%. One important candidate for 100% spin polarization is Chromium Oxide. Articles which show that CrO
2
demonstrates significant magneto-resistance effects are:
“Enhanced Intergrain Tunneling Magnetoresistance in Half-Metallic CrO
2
Films”, Hwang et al., Science 278, (1998);
“Magnetoresistance of Chromium Dioxide Powder Compacts”, Coey et al., Phys. Rev. Lett., 80, (1998);
“Extrinsic Giant Magnetoresistance In Chromium (IV) Oxide, CrO
2
”, Manoahran, Appl. Phys. Lett., 72 (1998).
Additional known references which are relevant to non-volatile memory devices which utilize stray magnetic fields generated by currents to switch resistance states include:
“Double Tunnel Junctions for Magnetic Random Access Memory Devices”, Iomata et al., J. Appl. Phys., Vol. 87, No. 9, (May 2000), which describes fabrication of double tunnel junctions which comprise a central ferromagnetic layer prepared by alternate sputter deposition of Co
80
Pt
20
and Al
2
O
3
. Where said ferromagnetic layer has top and bottom electrodes made of FeCo applied thereto, room temperature Tunnel Magnetoresistance of 20% was achieved.
“Exchange-based Magnetic Tunnel Junctions and Application to Nonvolatile Magnetic Random Access Memory”, Parkin et al., J. Appl. Phys., Vol. 85, No. 8, (April 1999), describes tunnel junctions comprising two ferromagnetic layers separated by an insulating tunnel barrier. Switching between magnetoresistance states is shown as achieved by application of a magnetic field.
“Ultrahigh Density Vertical Magnetoresistive Random Access Memory”, Jian-Gang Zhu et al., J. Appl. Phys., Vol. 87, No. 9, (May 2000), mentions a ring shaped vertical magnetoresistive random access memory comprised of magnetic tunneling junctions.
“Spin Dependent Tunneling Devices Fabricated for Magnetic Random Access Memory Applications Using Latching Mode”, Wang et al., J. App. Phys., Vol. 87, No. 9 (May 2000); describes Spin Dependent Tunneling Junctions comprising: NiFeCo/Al
2
O
3
/CoFe/IrMn formed by rf diode sputtering.
Continuing, it is generally accepted that spin polarized current density larger than about 10
7
A/cm
2
is necessary to produce sufficient torque on a magnetic nanoparticle and change its orientation, thus that several Milliamps are required to flip regions in system fabricated by electron-lithography of a typical size of 100×100 nm area. It is also noted that asymetric results occur when positive and negative currents are applied. Articles which provide insight to non-volatile memory devices which operate based on spin transfer from electrons to set hysteretic resistance states are:
“Magnetization Precession by Hot Spin Injection”, Weber et al., Science, 291, (2001) discloses experimental results which demonstrate that electron spins experience a torque when going through a ferromagnetic material. Following Newton's Third Law the electrons produce an opposite effect on the magnetic material and can modify its magnetic orientation;
“A New Twist for Magnetics”, Ralph, Science, Vol. 291, (February 2001), which describes that electric currents can manipulate magnets not only by the well known effect of the translation motion of electrons, say through a wire etc., but that the spin of electrons can be beneficially used as well. This article describes that electrons first passed through a spin filter so that a flow thereof is populated predominately by electrons of the same spin, can specifically affect magnetic states on a less than 1 micron dimension scale, whereas magnetic field effects which are based on stray field effects decay slowly with distance, thereby limiting packing density of dipoles which can be specifically controlled thereby, without affecting nearby dipoles. The use of electron spin then makes possible smaller memory cells, and for devices with dimensions of less than 100 nm, electron spin becomes the dominate effect;
“Excitation of Spin Waves by an Electric Current”, Slonczewski, J. Magn. Magn. Matter, 195, (1999);
“Emission of Spin Waves by a Magnetic Multilayer Traversed by a Current”, Berger, Phys. Rev. B 54, (1996);
“Excitation of a Magnetic Multilayer by an Electric Current”, Tsoi et al., Phys. Rev. Lett 80, (1998);
“Current-Induced Magnetization Reversal in Magnetic Nanowires”, Wegrowe et al., Europhys. Lett. 45, (1999);
“Current-Induced Switching of Domains in Magnetic Multilayer Devices”, Myers et al., Science 285, (1999);
“Spin-Polarized Current Switching of a Co Thin Film Nanomagnet”, Albert et al., Appl. Phys. Lett. 77, (2000);
“Spin-Polarized Current Induced Switching in Co/Cu/Co Pillars”, Grollier et al., Appl. Phys. Lett. 78, (2001).
“Current-Driven Switching of Magnetic Layers”, Heide et al., Phys. Rev. B, Vol. 63, (2001);
“Current-Driven Magnetic Switching in Manganite Trilayer Junctions”, Sun, J. of Magnetism and Magnetic Materials, 202 (1999);
“Current Hysteresis Due to Changes in Magnetization of Magnetic Tunnel Junctions by Spin-Polarization Current”, Baranov, Europhys. Lett. 53 (5) (2001).
A great many papers report research into materials which provide bi-stable memories. The following are representative:
“Current-Driven Insulator-Conductor Transition and Nonvolatile Memory in Chromium-Doped SrTiO
3
Single Crystals”, Watanabe et al., J. App. Phys., Vol 78, No. 23, (June 2001), which describes non-volatile memory comprised of Chromium doped SrTiO
3
single crystals in which D.C. current flow induced reversible insulator-conduction transition of up to five orders of magnitude, and
“Reproducible Switching Effect in Thin Oxide Films for Memory Applications”, Beck et al., Appl. Phys. Lett. 77 (2000).
With the present invention in mind, a Search of Patents provided:
U.S. Pat. No. 6,069,820 to Inomata et al. describes a Spin Dependent Conduction Device. This Patent describes a sequence of:
Tunnel Junction—Ferromagnetic Layer—Tunnel Junction
wherein a ferromagnetic material is sandwiched between tunneling junctions.
U.S. Pat. No. 5,841,692 to Gallagher et al. describes a magnetic tunneling junction device with antiferromagnetically coupled pinned layer.
U.S. Pat. No. 5,650,
Doudin Bernard
Liou Sy-Hwang
Sokolov Andrei
Yang Cheol-Soo
Yuan Lu
Board of Regents of the University of Nebraska
Mai Son
Welch James D.
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