An MRAM is a promising nonvolatile memory from a viewpoint of high integration and high-speed operation. In the MRAM, a magnetoresistance element that exhibits a “magnetoresistance effect” such as TMR (Tunnel MagnetoResistance) effect is utilized. In the magnetoresistance element, for example, a magnetic tunnel junction (MTJ: Magnetic Tunnel Junction) in which a tunnel barrier layer is sandwiched by two ferromagnetic layers is formed. The two ferromagnetic layers include a pinned layer (magnetization fixed layer) whose magnetization direction is fixed and a free layer (magnetization free layer) whose magnetization direction is reversible. For example, refer to Roy Scheuerlein et al., “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”, 2000 IEEE International Solid-State Circuits Conference, DIGEST OF TECHNICAL PAPERS, pp. 128-129.
It is known that a resistance value (R+ΔR) of the MTJ when the magnetization directions of the pinned layer and the free layer are “anti-parallel” to each other becomes larger than a resistance value (R) when the magnetization directions are “parallel” to each other due to the magnetoresistance effect. The MRAM uses the magnetoresistance element having the MTJ as a memory cell and nonvolatilely stores data by utilizing the change in the resistance value. Data writing to the memory cell is performed by switching the magnetization direction of the free layer.
Conventionally known methods of data writing to the MRAM include an asteroid method (for example, refer to M. Durlam et al., “Nonvolatile RAM based on Magnetic Tunnel Junction Elements”, 2000 IEEE International Solid-State Circuits Conference, DIGEST OF TECHNICAL PAPERS, pp. 130-131). According to the asteroid method, a magnetic switching field necessary for switching the magnetization of the free layer increases in substantially inverse proportion to a size of the memory cell. That is to say, a write current tends to increase with the miniaturization of the memory cell.
As a write method capable of suppressing the increase in the write current with the miniaturization, there is proposed a “spin transfer method” (for example, refer to Yagami and Suzuki, Research Trends in Spin Transfer Magnetization Switching, Journal of The Magnetics Society of Japan, Vol. 28, No. 9, 2004). According to the spin transfer method, a spin-polarized current is injected to a ferromagnetic conductor, and direct interaction between spin of conduction electrons of the current and magnetic moment of the conductor causes the magnetization to be switched (hereinafter referred to as “spin transfer magnetization switching”). The spin transfer magnetization switching will be outlined below with reference to FIG. 1.
In FIG. 1, a magnetoresistance element is provided with a free layer 101, a pinned layer 103 and a tunnel barrier layer 102 that is a nonmagnetic layer sandwiched between the free layer 101 and the pinned layer 103. Here, the pinned layer 103, whose magnetization orientation is fixed, is so formed as to be thicker than the free layer 101 and serves as a spin filter, i.e. a mechanism for generating the spin-polarized current. A state in which the magnetization directions of the free layer 101 and the pinned layer 103 are parallel to each other is related to data “0”, while a state in which they are anti-parallel to each other is related to data “1”.
The spin transfer magnetization switching shown in FIG. 1 is achieved by a CPP (Current Perpendicular to Plane) method, where a write current is injected in a direction perpendicular to the film surface. More specifically, the current flows from the pinned layer 103 to the free layer 101 in a transition from data “0” to data “1”. In this case, electrons having the same spin state as that of the pinned layer 103 being the spin filter move from the free layer 101 to the pinned layer 103. As a result of the spin transfer (transfer of spin angular momentum) effect, the magnetization of the free layer 101 is switched. On the other hand, the current direction is reversed and the current flows from the free layer 101 to the pinned layer 103 in a transition from data “1” to data “0”. In this case, electrons having the same spin state as that of the pinned layer 103 being the spin filter move from the pinned layer 103 to the free layer 101. As a result of the spin transfer effect, the magnetization of the free layer 101 is switched.
In this manner, the data writing is performed by transferring the spin electrons in the spin transfer magnetization switching. It is possible to set the magnetization direction of the free layer 101 depending on the direction of the spin-polarized current perpendicular to the film surface. Here, it is known that the threshold value of the writing (magnetization switching) depends on current density. Therefore, the write current necessary for the magnetization switching decreases with the reduction of the size of the memory cell. Since the write current is decreased with the miniaturization of the memory cell, the spin transfer magnetization switching is important in realizing a large-capacity MRAM.
A magnetoresistance element described in Japanese Laid-Open Patent Application JP-2005-150303 includes a ferromagnetic tunnel junction having a three-layer structure consisting of a first ferromagnetic layer, a tunnel barrier layer and a second ferromagnetic layer. The first ferromagnetic layer is larger in coercivity than the second ferromagnetic layer. Magnetization of an end of the second ferromagnetic layer is fixed in a direction having a component perpendicular to a magnetization easy axis direction of the second ferromagnetic layer.
A magnetic storage device described in Japanese Laid-Open Patent Application JP-2005-191032 has a laminated structure consisting of a magnetization fixed layer, a tunnel insulating layer and a magnetization free layer. The magnetization free layer has a connector section overlapping with the tunnel insulating layer and the magnetization fixed layer, constricted sections adjacent to both ends of the connector section, and a pair of magnetization fixed sections respectively formed adjacent to the constricted sections. The magnetization fixed sections are respectively provided with fixed magnetizations whose directions are opposite to each other. At a time of data writing, a write current penetrating through the connector section, the pair of constricted sections and the pair of magnetization fixed sections of the magnetization free layer flows. A domain wall moves between the pair of constricted sections depending on a direction of the write current.
A magnetic memory element described in Japanese Laid-Open Patent Application JP-2006-73930 has a first magnetic layer, an intermediate layer and a second magnetic layer. Data is recorded as magnetization directions of the first magnetic layer and the second magnetic layer. Magnetic domains whose magnetization directions are anti-parallel to each other and a domain wall separating the magnetic domains are steadily formed in the first magnetic layer. The domain wall moves within the first magnetic layer due to an in-plane current flowing in the first magnetic layer.