Magnetic memory unit and magnetic memory array

Details Magnetic memory unit and magnetic memory array Magnetic memory unit and magnetic memory array

2004-01-09

2004-10-26

Nelms, David (Department: 2818)

Active solid-state devices (e.g., transistors, solid-state diode

Field effect device

Having insulated electrode

C257S421000, C257SE27006

Reexamination Certificate

active

06809361

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a magnetic memory unit and a magnetic memory array.
BACKGROUND OF THE INVENTION
The development in communications and information technology has led in recent years to continually rising requirements made of the capacity and the rapidity of memory modules. So-called nonvolatile mass storage devices such as, e.g., hard disks, magnetic tapes or optical storage devices are distinguished by a high storage capacity and by a low price per megabyte of storage capacity.
However, conventional silicon microelectronics will reach its limits as miniaturization advances further. In particular the development of increasingly smaller and more densely arranged nonvolatile memory elements, which are often based on transistors, will be subject to fundamental physical problems in the next ten years. If structural dimensions fall below 80 nm, the components are influenced by quantum effects in a disturbing manner, and they are dominated by quantum effects below dimensions of about 30 nm. The increasing integration density of the components on a chip also leads to a dramatic increase in the waste heat.
One known possible successor technology to follow conventional semiconductor electronics is carbon nanotubes. An overview of this technology is given for example in Harris, P. J. F (1999) “Carbon Nanotubes and Related Structures—New Materials for the Twenty-First Century”, Cambridge University Press, Cambridge. A nanotube is a single-walled or multi-walled, tubular carbon compound. In the case of multi-walled nanotubes, at least one inner nanotube is surrounded coaxially by an outer nanotube. Single-walled nanotubes typically have diameters of approximately 1 nanometer; the length of a nanotube may amount to hundreds of nanometers. The ends of a nanotube are often terminated with in each case half a Fulleren molecule part.
The extended &pgr; electron system and the geometrical structure of nanotubes bring about a good electrical conductivity, for which reason nanotubes are suitable for the construction of circuits with dimensions in the nanometer range. Dekker, C. et al. (1999) “Carbon Nanotubes as Molecular Quantum Wires”, Physics Today 5/99: 22-28, discloses that the electrical conductivity of carbon nanotubes can significantly exceed that of metals of the same dimensioning. On account of the good electrical conductivity of nanotubes, the latter are suitable for a large number of applications, for example for electrical connection technology in integrated circuits, for components in microelectronics and also for electron emitters.
However, carbon nanotubes also have interesting properties with regard to magnetoelectronic applications which utilize the spin properties of electrical charge carriers. Tsukagoshi, K. et al. (1999) “Coherent transport of electron spin in a ferromagnetically contacted carbon nanotube” Nature 401: 572-574, discloses that carbon nanotubes transport the spin of conduction electrons over considerable dimensions without a spin flip-over process, if conduction electrons are injected into a carbon nanotube. If conduction electrons with polarized spins are injected into, a carbon nanotube for example proceeding from a ferromagnetic conductor, then the spin orientation of the conduction electron is maintained in the carbon nanotube over a path length of approximately 250 nm. This characteristic path length is referred to as spin coherence length. Moreover, in carbon nanotubes, there is comparatively little scattering of conduction electrons at phonons and lattice defects; the elastic scattering length is approximately 60 nm.
An interesting concept for fabricating miniaturized nonvolatile memories is so-called MRAMs (magnetic random access memory). The functioning of MRAMs is based on the giant magnetoresistance effect (XMR effect), the principles of which are described in Reiss, G. et al. (1998) “Riesenmagnetowiderstand—Transfer in die Anwendung” [“Giant magnetoresistance—transfer to application”] Physikalische Blätter 4/98: 339-341. XMR effects can be observed at structures in which two ferromagnetic layers are separated by a thin nonferromagnetic intermediate layer. If the magnetization directions of the two ferromagnetic layers are antiparallel with respect to one another, then majority spin carriers become minority spin carriers during spin-retaining passage from the first ferromagnetic layer across the intermediate layer through into the second ferromagnetic layer. The term majority charge carriers denotes conduction electrons with a spin orientation which corresponds to the ferromagnetic preferred direction. In other words, the spin preferred direction in the second ferromagnetic layer, which is adopted by the majority spin carriers of the second ferromagnetic layer, is antiparallel with respect to the spin preferred direction in the first ferromagnetic layer, which is adopted by the majority spin carriers of the first ferromagnetic layer. This results in a significantly increased electrical resistance given mutually antiparallel orientation of the two ferromagnetic layers compared with the case of a parallel magnetization of the two ferromagnetic layers. The reason for the dependence of the electrical resistance on the orientation of the magnetization vectors of the two layers with respect to one another is the spin-dependent tunnel effect. The electric current flow between the two layers is determined by the availability of free states in the conduction bands of the second layer. In the case of mutually parallel magnetization directions, the electron spins, which, in the first ferromagnetic layer, are for example preferably in a “spin up” state, find a high number of free states in the corresponding band for “spin up” electrons in the second ferromagnetic layer, so that a considerable tunneling current flows. In the case of antiparallel orientation with respect to one another, the energy bands are shifted relative to one another in such a way that at the Fermi edge there are not sufficiently many free states into which the electrons with “spin up” could tunnel. The tunneling current is low in this case and the layer arrangement is at high impedance. A parallel orientation of the magnetizations which is constrained by means of an external magnetic field, for example, therefore leads to the XMR effect, i.e. to a great reduction of the electrical resistance.
It must be emphasized that the intermediate region between the two ferromagnetic layers may also be a nonmagnetic layer, for example an electrically insulating tunnel barrier. The term “tunneling magnetoresistance” (TMR effect) is used in this case. MRAMs exploit the TMR effect; in other words, the quantum mechanical tunnel effect is combined with the giant magnetoresistance effect. MRAMs are distinguished by the nonvolatility of the stored information items and thus open up new market segments, precisely also in the case of portable products (digital photographic cameras, intelligent smart cards).
The functioning of an MRAM memory element is described below with reference to FIG.
1
. The MRAM memory unit
100
shown in
FIG. 1
has a soft-magnetic electrode
101
, a hard-magnetic electrode
102
and a tunnel layer
103
arranged in between.
A hard-magnetic (or a magnetically hard) material is understood hereinafter to be a material in which the hysteresis curve, i.e. the relationship between the magnetization and the magnetic field strength, encloses a larger area than in the case of a soft-magnetic (or a magnetically soft) material, whose hysteresis curve encloses a smaller area. In general, both the remanence and the coercive force are greater in the case of a hard-magnetic material than in the case of soft-magnetic materials. In other words, reversing the magnetization of a soft-magnetic material requires a low work, whereas reversing the magnetization of a hard-magnetic material requires a high work. Since reversing the magnetization of the hard-magnetic electrode
102
requires a large work or a strong external magnetic field, in the case of the hard-magne

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