CROSS-REFERENCE TO RELATED APPLICATION
This application is based upon and claims priority of Japanese Patent Application No. 2000-001861, filed on Jan. 7, 2000, the contents being incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to magnetic elements using ferromagnetic tunnel junction, and magnetic memory devices comprising such elements.
2. Description of the Related Art
MRAM (Magnetic Random Access Memory) is one of memory devices such as DRAM (Dynamic Random Access Memory) and SRAM (Static Random Access Memory) using semiconductor substances. While DRAM and SRAM store data in accordance with presence/absence of electric charges, MRAM stores data in accordance with magnetized directions of magnetic substances. MRAM has many merits, e.g., it is suitable for high speed operation, it shows high radiation resistance, and it shows little deterioration due to repetition of data write operations. For this reason, the study of MRAM has been prosecuted earnestly in recent years.
MRAM has its basic structure in which magnetic memory elements are arranged into a matrix, word and bit lines are disposed in columns and rows for generating magnetic field in a selected element, and terminals are provided for reading out data stored in the selected element. When one of the word lines and one of the bit lines are selected, and electric currents are applied to the selected word and bit lines, magnetic field is generated in the magnetic element at the intersection of the selected word and bit lines. The magnetized direction of the magnetic element can be reversed by the magnetic field. In this manner, two different magnetized states of each magnetic element can be realized. The two magnetized states correspond to bit data of “0” and “1”, respectively.
For the memory structure of each magnetic element of MRAM, usable are so-called MR (Magneto-Resistive) element, GMR (Giant Magneto-Resistive) element, and ferromagnetic tunnel element, any of which changes in its electric resistance in accordance with magnetized directions. To read out data from a magnetic element, an electric current is applied to the element and the electric resistance thereof is measured.
MRAM using MR or GMR element for the memory structure of each magnetic element has been realized. In this type of MRAM, however, the sheet resistance of each magnetic element is measured. Therefore, if the element size is reduced, the resistance to be measured is reduced accordingly, so that the output is reduced. Although reduction in size of such an element is a recent general demand, this type of MRAM has its limit.
Ferromagnetic tunnel element has a tunnel junction structure generally comprising a ferromagnetic layer, an insulating layer, and another ferromagnetic layer laminated in this order. In this structure, the tunnel resistance in case of both the magnetic layers being magnetized in the same direction, differs from that in case of those being magnetized in reverse directions. The amount of change in resistance depends on the polarizability of each magnetic layer. A change in resistance by more than 40-50% is expected if a suitable material is chosen. Besides, the smaller the junction area is, the higher the tunnel resistance is. Thus the element size can be reduced without reducing the output.
For these reasons, use of such a ferromagnetic tunnel junction structure for the memory structure of each magnetic memory element of MRAM is expected to realize a higher packing density in MRAM.
This idea for improving the packing density in MRAM, however, includes the following vital problems.
First, each of the word and bit lines of MRAM must receive an electric current for generating sufficient magnetic field that can reverse the magnetized direction of the target magnetic layer of each memory structure. For this reason, each of the word and bit lines requires a certain degree of size in its cross section. This requirement in size limits the packing density. To avoid this problem, the electric current for generating magnetic field must be made small. This requires selection of a suitable magnetic material whose magnetized direction can be reversed with weaker magnetic field.
Second, if the size of such a memory structure using ferromagnetic tunnel junction is reduced, both the ferromagnetic layers sandwiching the insulating layer may be magnetostatically coupled through leakage fluxes out of the ferromagnetic layers. This lowers the sensitivity of the memory element to external magnetic field.
FIG. 1 shows such a magnetic memory structure. Referring to FIG. 1, ferromagnetic layers 101 and 103 sandwiches an insulating layer 102. In this structure, the ferromagnetic layers 101 and 103 may be magnetically coupled with each other, so that leakage fluxes become very few. This causes a bad sensitivity to external magnetic field.
FIG. 2 shows a more specific structure of a magnetic memory element in this type of MRAM. In this example, a ferromagnetic layer 101 (thickness: 2.0 nm) made of CoFe, an insulating layer 102 (thickness: 1.5 nm) made of Al2O3, and a ferromagnetic layer 103 (thickness: 1.0 nm) made of CoFe are laminated in this order on a magnetic underlayer 104 (thickness: 10 nm) made of IrMn. In this structure, external fluxes may be made from an end of the CoFe layer 103 whose magnetized direction is fixed (hereinafter referred to as fixed layer), to an end of the CoFe layer 101 whose magnetized direction is to be reversed (hereinafter referred to as free layer). This causes very bad sensitivity of the CoFe layer 101 to external magnetic field.
Besides, magnetic domain structure is a factor of stability of such a magnetic element. In general, a magnetic substance comprises a number of magnetic domains having the same magnetized direction. The boundary between such domains is called magnetic domain wall. In case of a magnetic element of MRAM in which a magnetized direction is changed, such magnetic domain walls may move. This causes noise and deterioration of sensitivity.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide magnetic elements having a relatively simple construction, being less affected by magnetostatic coupling, and capable of realizing improvement of the sensitivity for reversing magnetized direction by a single domain structure. It is another object of the present invention to provide magnetic memory devices comprising such magnetic elements for realizing less power consumption, high-speed operation, and high packing density.
According to an aspect of the present invention, a magnetic element comprises a first ferromagnetic layer, an insulating layer, and a second ferromagnetic layer laminated in this order. At least one of the first and second ferromagnetic layers comprises a lower ferromagnetic layer, a nonmagnetic conductive layer, and an upper ferromagnetic layer laminated in this order.
Preferably, the upper and lower ferromagnetic layers sandwiching the non-magnetic conductive layer are antiferromagnetically coupled with each other.
Preferably, the magnetized direction of at least one of the first and second ferromagnetic layers is fixed by an adjacent antiferromagnetic layer.
Preferably, the non-magnetic conductive layer is made of one of Ru and Cu.
Preferably, the amount of magnetization of the upper ferromagnetic layer differs from that of the lower ferromagnetic layer.
Preferably, the thickness of the upper ferromagnetic layer differs from that of the lower ferromagnetic layer.
According to another aspect of the present invention, a magnetic memory device comprises magnetic elements each of which comprises a first ferromagnetic layer, an insulating layer, and a second ferromagnetic layer laminated in this order. The magnetized direction of one of the first and second ferromagnetic layers is changeable in accordance with data to be stored. At least one of the first and second ferromagnetic layers comprises a lower ferromagnetic layer, a non-magnetic conductive layer, and an upper ferromagnetic layer laminated in this order.
According to the present invention, at least one of the first and second ferromagnetic layers has the structure that upper and lower ferromagnetic layers sandwich a non-magnetic conductive layer. By properly selecting the kind or composition of the material and the thickness of each of the upper and lower ferromagnetic layers, the amount of magnetization of each of them can be so regulated as to reduce the affection by magnetostatic coupling. Changeability of magnetized direction of the first or second ferromagnetic layer in response to external magnetic field can thereby be adjusted into a suitable value. This affords an improvement of sensitivity.
According to the present invention, realized are magnetic elements having a relatively simple construction, being less affected by magnetostatic coupling, and capable of improving the sensitivity for reversing magnetized direction by a single domain structure. Also realized are magnetic memory devices comprising such magnetic elements for less power consumption, high-speed operation, and high packing density.
A magnetic element according to the present invention can be used not only for MRAM but also for a magnetic sensor, e.g., as a sensor device for a magnetic disk. That is, the present invention can apply to either of a magnetic memory device and a magnetic storage device. Here, “magnetic storage device” means, e.g., a data storage device in which data write and read operations in relation to a very small bit area on a magnetic storage medium rotating at a high speed, such as a magnetic disk, are performed through a head that a magnetic sensor is incorporated in and that is mechanically driven to be put close to the storage surface of the medium, while “magnetic memory device” means an electronic device in which data write and read operations are performed entirely in an electronic manner. The magnetic memory device has its basic structure in which a memory cell as a data storage unit is provided at each intersection of bit and word lines, like usual DRAM. In the magnetic memory device, however, usual memory cells as capacitor cells are replaced by magnetic elements, so that data write and read operations in relation to each memory cell are performed electromagnetically. Such a magnetic memory device is currently called MRAM (Magnetic Random Access Memory) or the like and thus it is premised on random access. But, even in case of a device called MRAM, it has fundamentally no need of data refresh operations at short intervals. In spite of the present situation, a magnetic memory device according to the present invention can be used as a read-only memory in which only read operations for data stored in a semipermanent form are possible. Besides, it can also be used like a flash memory in which data stored therein is electrically erasable only in a lump. Thus the present invention is not limited to such devices as dynamic random access memories in which data refresh operations are performed at regular intervals.