1. Field of the Invention
This invention, in general, relates to the field of magnetoresistive spin-valve (SV) elements. More specifically, the present invention relates to current perpendicular-to-the-plane (CPP) spin-valve device configurations, applicable as a magnetic non-volatile mass-memory called (MRAM) and/or as a magnetic transducer or “head” for reading information signals recorded on a magnetic medium.
2. Description of Prior Art
The related art, e.g., U.S. Pat. No. 5,159,513, discloses a magnetic transducer which is sometimes referred to as a spin-valve magnetoresistive sensor. A spin-valve element includes at least two ferromagnetic layers having different magnetic field sensitivity. The ferromagnetic layer having the higher sensitivity is referred to as the “free layer” and the other ferromagnetic layer is referred to as the “pinned layer.” The resistance of the spin valve element is a function of the angle between the magnetizations of the two layers (cosine of the angle) and is independent of the current direction with respect to the magnetization directions (which is traditionally called the giant magnetoresistive effect (GMR)).
In MRAM's, binary information is stored either by the direction of the magnetization in the free layer or in the pinned layer. The stored information is read out by applying a field and sensing the sign of the change of the resistance of the spin-valve element. See, for instance, “Characteristics of AP Bias in Spin Valve Memory Elements,” J. Zhu and Y. Zheng, IEEE Trans. Magn. Vol. 34, 1063-1065 (1998); “Pseudo Spin Valve MRAM Cells with Sub-Micrometer Critical Dimension,” B. A. Everitt et al, IEEE Trans. Magn. Vol. 34, 1060-1062 (1998), which is incorporated herein by reference. Binary information can also be read by directly sensing the resistance of the spin-valve element. A spin-valve element can also be applied to a magnetic field sensor such as a read head in magnetic recording by detecting a magnetic field through the resistance change of the element. It has been shown that a spin-valve head is capable of reading data from a magnetic surface at great linear densities.
The GMR effect is larger when measured with a current perpendicular to the layer structure than when measured with a current in a plane direction. See, for example, “Perpendicular Giant Magnetoresistance of Microstructured Fe/Cr Magnetic multilayers from 4.2 to 300 K,” M. A, M. Gijs, S. K. J. Lenczovski and J. B. Giesbers, Physical Review Letters, Vol. 70, 3343-3346 (1993), both of which are incorporated herein by reference. The former is called CPP (current perpendicular-to-the-plane) and the latter, CIP (current-in-plane). Therefore, it is desirable to make use of CPP rather than CIP if the circumstances allow. However, the drawback in the ordinary CPP structure is the fact that only a very low resistance change is obtainable compared to the CIP structure. Consequently, the very low resistance change makes it necessary to apply a very large current to obtain a reasonable output signal level.
U.S. Pat. No. 6,560,077 discloses a confined current path (CCP) CPP spin-valve device including a spin-valve element including a substrate and a layered structure formed on the substrate. The layered structure has a first thin film layer of ferromagnetic material and a second thin film layer of ferromagnetic material. The first thin film layer and the second thin film layer are separated by a thin non-magnetic layer structure. One of the first and second thin film layers is a free layer and the other of the first and the second thin film layers is a pinned layer. The non-magnetic layer structure has a current confining (CC) layer structure having at least one CCP and means for producing a current flow through the spin-valve element between the free and the pinned layer through the plurality of confined current paths. At least one confined current path of the plurality of confined current paths is formed within every flux path of a width of the exchange length of the free layer except at the edge of the free layer.
FIG. 1 depicts the CPP spin-valve element disclosed by U.S. Pat. No. 6,560,077. The spin-valve element includes magnetic layer structures 11 and 12, either one of which is a free layer structure and the other is a pinned layer structure with a non-magnetic conducting spacer layer structure 13 and a CC-layer structure 14 in between. Each layer structure has a width w and a height h. The CC-layer structure confines the current path going from one side of the CC-layer structure to the other side of the CC-layer structure in order to increase the total resistance of the spin-valve element. This enables a high output voltage when a reasonable amount of current is applied to the spin-valve element perpendicular to the element. The CC-layer structure 14 includes an insulator with a conducting part or a plurality of conducting parts. In case the conducting part is of a hole, the magnetic layer structure 11 and the conducting spacer layer structure 13 are directly connected to each other through the hole(s). The CC-layer structure 14 can be of a mosaic structure composed of at least two parts having significantly different conductivities. The spin-valve element is connected to a current source which applies a current from one side to the other through the leads 15 and 15′. The leads may be either of a magnetic or of a nonmagnetic material. When applied to magnetic recording read heads it is especially beneficial for avoiding the additional magnetic noise if the distance between the adjacent conducting parts is made smaller than the, so called, “exchange length” of the free layer material, which is usually in the range of between several tens of nanometers to several hundred nanometers.
U.S. Pat. No. 5,715,121 discloses a CPP spin-valve element which comprises a plurality of magnetic films and one or more nonmagnetic films. The magnetic films and the one or more nonmagnetic films are layered so that each nonmagnetic film is arranged between a pair of the magnetic films. The nonmagnetic film includes an electrical insulator film and at least one electrical conductor positioned in the electrical insulator film in order to confine the current path to a smaller cross-sectional area than that of the element. Hence, the spin valve element provides a large resistance and a resistance change which renders it practicable.
FIG. 2 depicts the CPP spin-valve element disclosed by U.S. Pat. No. 5,715,121. The spin valve element includes magnetic layer structures 21 and 22, either one of which is a free layer structure and the other is a pinned layer structure with a non-magnetic CC-layer structure 24 in between. The CC-layer structure 24 can be of a mosaic structure including at least two parts having significantly different conductivities. The spin-valve element is connected to a current source in order to apply a current from one side to the other through the leads 25 and 25′, which may be of a magnetic or of a nonmagnetic material, each layer structure having a width w and a height h.
Both U.S. Pat. Nos. 5,715,121 and 6,560,077 are incorporated herein by reference. See also “High-resolution focused ion beams”, by Jon Orloff, Review of Scientific Instruments, Vol. 64, 1105-1130 (1993), which is incorporated herein by reference.
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