Static information storage and retrieval – Systems using particular element – Magnetic thin film
Reexamination Certificate
2000-05-17
2002-04-30
Lebentritt, Michael S. (Department: 2824)
Static information storage and retrieval
Systems using particular element
Magnetic thin film
C365S171000
Reexamination Certificate
active
06381171
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to magnetic elements utilizing spin-dependent tunneling effects and also to magnetic read heads, magnetic sensor devices, magnetic storage device, and magnetic memory devices using the elements.
2. Description of the Related Art
Currently available spin-dependent transport elements utilizing magnetism include known giant magnetoresistive (GMR) elements due to spin-dependent scattering at an interface between a magnetic metal layer and a non-magnetic metal layer. This is an artificial superlattice which is structured from alternate lamination of magnetic metal layers and nonmagnetic metal layers of thicknesses on the order of magnitude of several Angstroms to several tens of Angstroms, wherein magnetic moments of opposed magnetic metal layers with a nonmagnetic layer sandwiched therebetween are coupled together in a magnetically antiparallel state at zero externally applied magnetic field. When applying an external magnetic field to this artificial super lattice for establishment of one directional alignment of the magnetic moments of the magnetic metal layer, its resistivity decreases significantly, resulting in observation of a giant magnetoresistive effect as large as twenty to sixty percent. This advantage of such artificial superlattice does not come without accompanying penalties which follow: the requisite number of layers laminated over one another should be increased in order to obtain the significant magnetoresistive effect required; and the resulting saturated magnetic field stays significant on the order of magnitude of Tesla or greater, which serves as a serious bar to practical implementation thereof.
In recent years, a so-called “spin valve” GMR film has been developed, which is less than metallic artificial super lattices both in lamination number and in intensity of saturated magnetic field. The spin valve GMR film is designed to employ a tri-layered structure that consists of magnetic metal layers with an intervening nonmagnetic metal layer sandwiched between them. The magnetization of one of these magnetic metal layers is fixed or “pinned” in a desired direction while permitting only the remaining magnetic metal layer to invert or “switch” its magnetization upon application of an external magnetic field thereto, thereby forcing a relative angle between the magnetization directions of such two magnetic metal layers to change or vary accordingly. The tri-layered spin valve film exhibits a magnetoresistance effect of about 10% or less.
Another type of tunnel magnetoresistance (TMR) element adaptable for use as the spin-dependent transport device is known, which is based on the spin-dependent tunneling effect that is different in mechanism from spin-dependent scattering phenomena. This element includes a tri-layered structure consisting of two magnetic metal layers with an intermediate dielectric layer sandwiched therebetween, wherein one of the magnetic metal layers stacked has smaller coercive force than the other for producing a tunnel current flow upon applying of a voltage between the opposite magnetic metal layers. At this time, when inverting or switching the direction of the magnetization of the magnetic metal layer having small coercivity, spinning motion of such magnetic metal layer less in coercivity, a significant difference will be found between the value of a tunnel current obtainable when the relative orientation of the magnetizations are parallel to each other and that obtained when these are anti-parallel to each other. This large tunnel current difference leads to the capability of obtaining increased magnetoresistive effect of more than 10% at room temperature.
Ferromagnetic double tunnel junction elements are also known which have a five-layer laminated structure consisting of three—i.e. upper, intermediate, and lower—magnetic metal layers with a dielectric layer sandwiched between the upper and intermediate metal layers and also with another dielectric layer between the intermediate and lower metal layer. Some ferromagnetic double tunnel junction elements have been proposed by the inventors as named herein, which element has a multilayer structure with its intermediate ferromagnetic metal layer being constituted from a layer of fine particles made of ferromagnetic materials, as disclosed by the present inventors,
Jpn. J. Appl. Phys
., 36 L1380 (1997), and also Japanese Patent Laid-Open No. 308313/1998. These ferromagnetic double tunnel junction elements are featured in that any possible TMR reducibility due to biassing stays less.
Several ways of applying the above-noted GMR and TMR elements to magnetic read/write heads and magnetic random access memories and the like have also been studied until today—those magnetic heads employing spin valve GMR elements have already been put to practical use.
In the context of this document, the term “magnetic random access memory (MRAM) ” is defined to mean any solid-state memory device capable of randomly rewriting information to be recorded andalso storing and reading same by utilization of magnetization directions of ferromagnetic materials as information carrier. Typically an MRAM is configured from an array of memory cells each having a ferromagnetic thin film, nonmagnetic thin film, dielectric film or a multilayer structure thereof, and driver circuitry operatively associated with the memory cells via a plurality of electrical leads including read/write signal transmission lines connected thereto.
Recording data into MRAMs may be performed by causing the magnetization direction of a ferromagnetic material making up a memory cell to invert or “switch” in response to a current magnetic field occurring due to a current flow in a write line coupled thereto, and then determining whether the switched magnetization direction is parallel or antiparallel to a prespecified reference direction in a way such that the former case corresponds to a binary data bit of logical value “1” whereas the latter corresponds to a data bit of “0.” In MRAMs, electrical power consumption is principally zero during storing data once written thereinto, which permits MRAMs to function as nonvolatile memory devices capable of retaining information for an extended time period even when power is removed.
On the other hand, reading data recorded in MRAMs is carried out by utilization of a specific phenomenon that the memory cell's electrical resistivity is variable depending upon either a relative angle between the magnetization direction of a ferromagnetic material constituting a cell and a sense current or, alternatively, a relative angle between the magnetization directions of a plurality of ferromagnetic layers. This phenomenon is called the magnetoresistive (MR) effect among those skilled in the art to which the invention pertains. Known magnetoresistive effect is an anisotropic magnetoresistive (AMR) effect in which an electrical resistance varies in value depending on whether a relative angle between a current and magnetization is parallel or vertical to each other. Other MR effects are also known, including giant magnetoresistive (GMR) effect and tunnel magnetoresistive (TMR) effect, wherein the former is such that electrical resistivity varies depending on whether magnetizations of multiple ferromagnetic layers with a non-magnetic conductor sandwiched therebetween are parallel or antiparallel in alignment to each other, whereas the latter is that tunnel resistivity varies depending on whether magnetization of a plurality of ferromagnetic layers with an electrically insulative material sandwiched therebetween are parallel or antiparallel in alignment direction to each other.
With prior known memory cells exhibiting the AMR and GMR effects (referred to hereinafter as “AMR cell” and “GMR” cell respectively), the direction of a sense current flow typically stays parallel to the film surface of a ferromagnetic material used. Almost all of currently employed materials exhibiting the AMR and GMR effects are good conductive, the shee
Inomata Koichiro
Nakajima Kentaro
Saito Yoshiaki
Kabushiki Kaisha Toshiba
Lebentritt Michael S.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Phung Anh
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