1. Field of the Invention
The present invention relates to a magnetoresistive element using a method of reducing magnetic field inverting magnetization thereinafter, referred to as switching field of a magnetic film, a memory element having the magnetoresistive element, and a memory using the memory element.
2. Related Background Art
In recent years, semiconductor memories as solid-state memories are adopted in many information devices, and are of various types such as a DRAM, FeRAM, and flash EEPROM. The characteristics of the semiconductor memories have merits and demerits. There is no memory which satisfies all specifications required by current information devices. For example, the DRAM achieves high recording density and large rewritable count, but is volatile and loses its information upon power-off. The flash EEPROM is nonvolatile, but takes a long erase time and is not suitable for high-speed information processing.
Under the present circumstances of semiconductor memories, a magnetic memory (MRAM: Magnetic Random Access Memory) using a magnetoresistive element is promising as a memory which satisfies all specifications required by many information devices in terms of the recording time, read time, recording density, rewritable count, power consumption, and the like. In particular, an MRAM using a spin-dependent tunneling magnetoresistive (TMR) effect is advantageous in high-density recording or high-speed read because a large read signal can be obtained. Recent research reports verify the feasibility of MRAMs.
The basic structure of a magnetoresistive film used as an MRAM element is a sandwich structure in which magnetic layers are formed adjacent to each other via a nonmagnetic layer. Known examples of the material of the nonmagnetic film are Cu and Al2O3. A magnetoresistive film using a conductor such as Cu in a nonmagnetic layer is called a GMR film (Giant MagnetoResistive film). A magnetoresistive film using an insulator such as Al2O3 is called a spin-dependent TMR film (Tunneling MagnetoResistive film). In general, the TMR film exhibits a larger magnetoresistance effect than the GMR film.
When the magnetization directions of two magnetic layers are parallel to each other, as shown in FIG. 13A, the resistance of the magnetoresistive film is relatively low. When these magnetization directions are antiparallel, as shown in FIG. 13B, the resistance is relatively high. One of the magnetic layers is formed as a recording layer, and the other layer is as a read layer. Information can be read out by utilizing the above property. For example, a magnetic layer 13 on a nonmagnetic layer 12 is formed as a recording layer, and a magnetic layer 14 below the nonmagnetic layer 12 is as a read layer. The rightward magnetization direction of the recording layer is defined as xe2x80x9c1xe2x80x9d, and the leftward direction is as xe2x80x9c0xe2x80x9d. If the magnetization directions of the two magnetic layers are rightward, as shown in FIG. 14A, the resistance of the magnetoresistive film is relatively low. If the magnetization direction of the read layer is rightward and that of the recording layer is leftward, as shown in FIG. 14B, the resistance is relatively high. If the magnetization direction of the read layer is leftward and that of the recording layer is rightward, as shown in FIG. 14C, the resistance is relatively high. If the magnetization directions of the two magnetic layers are leftward, as shown in FIG. 14D, the resistance is relatively low. That is, when the magnetization direction of the read layer is pinned rightward, xe2x80x9c0xe2x80x9d is recorded in the recording layer for a high resistance, and xe2x80x9c1xe2x80x9d is recorded for a low resistance. Alternatively, when the magnetization direction of the read layer is pinned leftward, xe2x80x9c1xe2x80x9d is recorded in the recording layer for a high resistance, and xe2x80x9c0xe2x80x9d is recorded for a low resistance.
As the element is downsized for a higher recording density of an MRAM, the MRAM using an in-plane magnetization film becomes more difficult to hold information under the influence of a demagnetizing field or magnetization curling at the end face. To avoid this problem, for example, a magnetic layer is formed into a rectangle. This method cannot downsize the element, so an increase in recording density cannot be expected. U.S. Pat. No. 6,219,275 has proposed the use of a perpendicular magnetization film to avoid the above problem. According to this method, the magnetizing field does not increase even with a smaller element size. A smaller-size magnetoresistive film can be realized, compared to an MRAM using an in-plane magnetization film. Similar to a magnetoresistive film using an in-plane magnetization film, a magnetoresistive film using a perpendicular magnetization film exhibits a relatively low resistance when the magnetization directions of two magnetic layers are parallel to each other, and a relatively high resistance when these magnetization directions are antiparallel. As shown in FIGS. 15A to 15D, a magnetic layer 23 on a nonmagnetic layer 22 is formed as a recording layer, and a magnetic layer 21 below the nonmagnetic layer 22 is as a read layer. The upward magnetization direction of the recording layer is defined as xe2x80x9c1xe2x80x9d, and the downward direction is as xe2x80x9c0xe2x80x9d. As FIGS. 14A to 14D showed, it can compose as a memory element.
Main examples of the perpendicular magnetization film are an alloy film or artificial lattice film made of at least one element selected from the group consisting of rear-earth metals such as Gd, Dy, and Tb and at least one element selected from the group consisting of transition metals such as Co, Fe, and Ni, an artificial lattice film made of a transition metal and noble metal such as Co/Pt, and an alloy film having crystallomagnetic anisotropy in a direction perpendicular to the film surface, such as CoCr. In general, the switching field of a perpendicular magnetization film is larger than that having longitudinal magnetic anisotropy by a transition metal. For example, the switching field of a permalloy as an in-plane magnetization film is about several hundred A/m. The switching field of a Co/Pt artificial lattice film as a perpendicular magnetization film is as very high as about several ten kA/m. An alloy film of a rear-earth metal and transition metal exhibits different apparent magnetization intensities depending on the film composition because the sub-lattice magnetization of the rear-earth metal and that of the transition metal orient antiparallel to each other. Hence, the switching field of this alloy film changes depending on the composition. A GdFe alloy film shows a relatively small switching field among alloy films of rear-earth metals and transition metals. In general, the GdFe alloy film has a switching field of about several thousand A/m around the critical composition at which the squareness ratio of the magnetization curve starts decreasing from 1.
When a sensor, memory, or the like is formed from a magnetoresistive film using a perpendicular magnetization film, the sensor, memory, or the like cannot operate unless a large magnetic field is applied owing to the above-described reason. For example, in the sensor, a stray field must be concentrated on the magnetic layer of the magnetoresistive film. In the memory, a large magnetic field must be generated. A magnetic field applied to a memory is generally generated by supplying a current through a conductor. Especially in a memory used in a portable terminal, supply of a large current is undesirably flowed under restrictions on the power supply capacity. Thus, a conductor for generating a magnetic field must be wound around a memory element formed from a magnetoresistive film. This measure complicates a structure or electrical circuit around the magnetoresistive film, and is difficult to form. This results in low yield and very high cost.
The present invention has been made in consideration of the above situation, and has as its object to provide a magnetoresistive film which reduces the switching field of a perpendicular magnetization film and is easy to form without decreasing the yield or greatly increasing the cost, and a memory requiring only small power consumption.
The above object is achieved by a magnetoresistive film comprising a nonmagnetic film, and a structure in which magnetic films are formed on two sides of the nonmagnetic film, wherein at least one of the magnetic films includes a perpendicular magnetization film, and a magnetic film whose easy axis of magnetization is inclined from a direction perpendicular to a film surface is formed at a position where the magnetic film contacts the perpendicular magnetization film but does not contact the nonmagnetic film.
The above object is also achieved by a memory having a memory element with the magnetoresistive film, comprising means for applying a magnetic field to the magnetoresistive film in a direction perpendicular to a film surface, and means for detecting a resistance of the magnetoresistive film.
The above object is also achieved by the memory wherein a plurality of magnetoresistive films are arranged, and the memory further comprises means for selectively recording information on a desired magnetoresistive film, and means for selectively reading out information recorded on a desired magnetoresistive film.