1. Field of the Invention
The present invention relates to a magnetic thin-film memory element for recording information therein by directions of magnetization and regenerating the information therefrom by magneto-resistance, and a magnetic thin-film memory using the magnetic thin-film memory elements.
2. Related Background Art
Magnetic thin-film memories are solid state memories having no moving parts as semiconductor memories, and have advantages over the semiconductor memories; e.g., the information stored therein is not lost even without supply of power; the number of repetitive rewrites is infinite; there is no risk of destroying the recorded contents even if exposed to radiation, and so on. Particularly, thin-film magnetic memories utilizing the giant magneto-resistance (GMR) effect are drawing attention in recent years, because they can produce stronger output than the magnetic thin-film memories using the anisotropic magneto-resistance effect, which have been proposed heretofore.
For example, Journal of the Magnetic Society of Japan, Vol. 20, p 22 (1996) describes a solid state memory consisting of memory elements made of a stack of constituent elements including hard-magnetic film HM/non-magnetic film NM/soft-magnetic film SM/non-magnetic film NM as shown in FIG. 1. Each memory element is provided with sense lines S joined with a metal conductor and a word line W insulated from the sense lines S by insulating film I. Information is written by a magnetic field generated by word-line current and sense-line current.
Specifically, as shown in FIGS. 2A to 2D, memory states "0," "1" are recorded by allowing a current I to flow through the word line W and thereby generating either of magnetic fields of different directions depending upon directions ID of the current I through the word line W so as to cause reversal of magnetization in the hard-magnetic film HM. For example, when the positive current is allowed to flow as shown in FIG. 2A, the rightward magnetic field is generated as shown in FIG. 2B, thereby recording "1" in the hard-magnetic film HM; in contrast, when the negative current is allowed to flow as shown in FIG. 2C, the leftward magnetic field is generated as shown in FIG. 2D, thereby recording "0" in the hard-magnetic film HM.
The information is read by allowing a smaller current I than the recording current to flow through the word line W so as to cause only reversal of magnetization in the soft-magnetic film SM and measuring change in resistance on that occasion, as shown in FIGS. 3A to 3E. When the giant magneto-resistance is utilized, there appears a difference between resistances in the parallel state and in the antiparallel state of magnetization in the soft-magnetic film SM and in the hard-magnetic film HM. Therefore, the memory states of "1" and "0" can be discriminated from each other by change in resistance occurring at that time.
When a pulse signal including a positive pulse and a negative pulse in this order as shown in FIG. 3A is applied, the magnetization varies from rightward to leftward in the soft-magnetic layer. In the memory state "1," the small resistance as shown in FIG. 3B changes to the large resistance as shown in FIG. 3C. In the memory state "0," the large resistance as shown in FIG. 3D changes to the small resistance as shown in FIG. 3E. By detecting this change in resistance, the information recorded in the hard-magnetic film HM can be read irrespective of the magnetization states of the soft-magnetic film SM after recording. This realizes nondestructive reading.
With the magnetic thin-film memory of the above structure, however, the demagnetizing field (self-demagnetizing field) produced inside the magnetic layers cannot be ignored as the area of bit cell becomes smaller. The demagnetizing field changes the magnetization direction from the fixed direction in the record-holding magnetic layer, so as to make the magnetization direction unstable. The magnetic thin-film memory of the above structure thus failed to store information with decreasing sizes of bit cell and had a drawback of not allowing high integration of bit cells accordingly.