1. Field of the Invention
The present invention relates generally to magnetic devices and magnetic recording media using ferromagnetic films and to devices employing ferroelectric films. The present invention is particularly directed to improvements on the dimensions of the elements and magnetic particles so as to achieve uniform coercive force and coercive field characteristics and to contribute to miniaturization of the devices.
2. Description of the Related Art
Various devices utilizing the giant magnetoresistive (GMR) effect and the tunneling magnetoresistive (TMR) effect have been known in the art. Examples of such devices include magnetic recording elements and magnetic read heads.
FIG. 1 shows a basic structure of a magnetic random access memory. An example of the magnetic random access memory can be found in Wang et al., IEEE Trans. Magn. 33 (1997), 4498. Referring to FIG. 1, the magnetic random access memory is basically constituted from memory elements which are either GMR elements or TMR elements, word lines, and bit lines, which also function as sense lines. The word lines are orthogonal to the bit lines, and the memory elements are held between the word lines and the bit lines at the intersections thereof. Note that in FIG. 1, reference symbol W denotes the width of each memory element in the direction parallel to the bit lines and reference symbol L denotes the length of each memory element in the direction parallel to the word lines.
Referring now to FIG. 2, a first end of the memory element is connected to the bit line, and a second end of the memory element is connected via a lead to a logic circuit that selects a memory cell. In the example shown in FIG. 2, a field effect transistor (FET) constituted from a silicon substrate, a drain D, a source S and a gate G is connected to the second end of the memory element via a plug (interconnecting lead). Note that FIG. 2 illustrates an example using a TMR element. The TMR element is constituted from a free layer composed of CoFe, NiFe, or the like, a barrier layer composed of Al2O3 or the like, a reference layer composed of CoFe or the like, a non-magnetic layer composed of Ru or the like, a fixed layer composed of CoFe or the like, and an antiferromagnetic layer composed of PtMn or the like. These layers are arranged in that order when viewed from the bit line side. A GMR element has basically the same multilayer structure as that of the above-described TMR element and only differs from the TMR element in structural details such as the absence of barrier layer, etc.
The combination of the anisotropic magnetic field in the soft magnetic free layer and the demagnetization field determined by the size of the free layer defines the magnetic field necessary for rotating the magnetization direction of the free layer, i.e., the coercive force Hc.
FIG. 3 is an enlarged perspective view of the free layer, the reference layer, and the fixed layer of the memory element. In the drawing, the bold arrow in each layer indicates the magnetization direction of that layer. As shown in the drawing, the x axis extends along the long side of rectangular layers and the y axis extends orthogonal to the x axis. Reference symbol W denotes the width of the memory element in the y direction, and reference symbol L denotes the length of the memory element in the x direction. As shown in FIG. 3, easy axes of the free layer and the fixed layer are substantially parallel to each other. The magnetization direction of the reference layer is antiparallel to those of the free layer and the fixed layer.
The magnetization of the fixed layer is fixed by the antiferromagnetic layer. Given, for example, that bit information “1” is represented by the magnetization direction of the free layer being oriented in a direction parallel to the magnetization direction of the reference layer and that bit information “0” is represented by the magnetization direction of the free layer being oriented in a direction antiparallel to the magnetization direction of the reference layer, the magnetization direction of the free layer rotates due to the magnetic field, induced by a bit line current and a word line current, that exceeds the above-described coercive force Hc. Magnetic recording is performed through such a rotation.
FIGS. 4A to 4C illustrate example structures of shield-type magnetic read heads including read elements each disposed in the gap between a pair of shields (for example, refer to C. Tsang et al., IEEE Trans. Magn. 30 (1994), 3801). The shield-type magnetic read head includes a read element, namely, a GMR element or a TMR element, a lower shield S1, and an upper shield S2. In the drawings, hard magnet layers for controlling magnetic domains, disposed adjacent to the read element, a write head integrally formed above the read head, and so on are omitted for the sake of simplicity of explanation. The GMR or TMR heads shown in FIGS. 4A to 4C are classified into three types according to the direction of the sense current Is. FIG. 4A illustrates, for example, a horizontal current-in-plane (CIP) GMR head in which an electric current flows in the track direction. FIG. 4B illustrates, for example, a vertical CIP GMR head in which an electric current flows in the height direction of the element. FIG. 4C illustrates, for example, a current-perpendicular-to-plane (CPP) GMR or TMR head in which a sense current Is flows in the thickness direction. The view of FIG. 4C is made partially transparent for the sake of simplicity of explanation. In the FIGS. 4A to 4C, arrows in strip-shaped recording media represent the recording magnetization direction.
FIG. 5 illustrates another example structure including a TMR element. The TMR element is constituted from a free layer, a barrier layer, a reference layer, a nonmagnetic layer, a fixed layer, and an antiferromagnetic layer arranged along the z axis in the drawing. In short, the TMR element in FIG. 5 has substantially the same layer structure as that shown in FIG. 2. However, in FIG. 5, hard magnets #1 and #2 are formed at the two sides of the TMR element, and the nonmagnetic layers NM are formed between the TMR element and the hard magnets #1 and #2. Hard magnet layers for controlling the magnetic domains must be disposed at the two sides of the TMR element to orient the magnetization direction of the free layer in the x axis direction.
FIG. 6 is an enlarged perspective view of the free layer, the reference layer, and the fixed layer of the above TMR element. The hard magnet layers for controlling the magnetic domains, a nonmagnetic layer, a base layer, and a protection layer are omitted from the drawing. In the drawing, a bold arrow in each layer indicates the magnetization direction of that layer. As shown in FIG. 6, the x axis extends along the long side of the rectangular layers and the y axis extends orthogonal to the x axis. Reference symbol W denotes the width of the element in the y direction, and reference symbol L denotes the length of the element in the x direction. As shown in FIG. 6, whereas the easy axis of the free layer extends substantially in the x-axis direction, the easy axes of the reference layer and the fixed layer are orthogonal to the easy axis of the free layer. The magnetization directions of the reference layer and the fixed layer are antiparallel to each other. The magnetization direction of the fixed layer is pinned by the antiferromagnetic layer.
FIGS. 7A and 7B are schematic plan views showing the shape of the above-described memory element and the read element. The memory element and read element are formed into a rectangular shape, as shown in FIG. 7A, or into an elliptic shape, as shown in FIG. 7B. In FIG. 7A, reference symbol W denotes the breadth of the element, and reference symbol L denotes the longitudinal length of the element. In FIG. 7B, reference symbol W denotes the length of the short axis and reference symbol L denotes the length of the long axis. In FIG. 7B, the element is formed into an elliptic shape to make the demagnetizing field as uniform as possible inside the element.
Known magnetic recording elements and magnetic read elements, however, suffer from the following technical bottlenecks:
(1) Since the variation in the coercive force Hc among magnetic memory elements is large, practical production of the magnetic memory element is difficult; and
(2) For magnetic read elements, as the element size is reduced, a decrease in the sensitivity occurs due to the hard magnet layers for controlling magnetic domains, and thus magnetic read heads for higher-density media are difficult to design.
First, regarding point (1) above, in order to put magnetic random access memories having a storage capacity comparable to that of current widespread flash RAMs or DRAMs into practical use, all 106 to 109 magnetic memory elements must exhibit a uniform coercive force Hc. If the variation in Hc is 50% or more, the magnetization rotating current may differ by as much as 200% or more between some elements. In practice, this precludes selective recording.
As the size of magnetic memory elements decreases, the ratio of the demagnetizing field in coercive force Hc increases. Since the demagnetizing field is heavily dependent on the size and shape of the element, the variation in the coercive force Hc tends to increase in inverse proportion to the size of the elements. For the purpose of illustration, the dependency of the coercive force Hc on the element size was examined using a magnetic memory element having a free layer made of CoFe 2 nm in thickness. The results are shown in FIGS. 8 and 9.
In the graph of FIG. 8, characteristics of square and rectangular elements, i.e., box-shaped elements, as shown in FIG. 7A, when the aspect ratio W:L is varied are shown. The abscissa represents 1/W (unit: 1/μm), and the ordinate represents the coercive force Hc (unit: Oe=103/4π A/m).
In the graph of FIG. 9, characteristics of elliptic elements, as shown in FIG. 7B, are shown. The aspect ratio W:L is varied. The abscissa and the ordinate are the same as in FIG. 8.
In these graphs of FIGS. 8 and 9, solid lines connecting circular symbols represent the characteristics of an element having an aspect ratio W:L=1:1, long dotted lines connecting square symbols represent the characteristics of an element having an aspect ratio W:L=1:2, and short dotted lines connecting rhombus symbols represent the characteristics of an element having an aspect ratio W:L=1:3. An inclined straight line extending from the origin represents the characteristic according to the theoretical formula: Hc=2πMs(t/W), wherein Ms denotes the intensity of the magnetization, and t denotes the thickness.
As shown in these graphs, theoretically, the demagnetizing field is supposed to increase in inverse proportion to the length W of the short side of the element. However, this is not the case in practice. Particularly when the aspect ratio W:L is low, deviation from the theoretical value is significant. Since the number of the magnetic memory elements is large, as described above, it is preferred that the coercive force Hc be constant although some degree of variation in the aspect ratio may be observed.
Secondly, regarding point (2) above, in order to read higher-density recording media, the size of the read element installed in the magnetic read head must be reduced. Such a reduction in size increases the relative thickness of the domain walls at the ends of the free layer of the read element.
FIG. 10 is a graph showing the resistance/magnetic field characteristics of read elements each having a NiFe free layer with a track width L of 0.1 μm and 2 nm in thickness. No hard magnet layers for controlling the magnetic domains are provided for the read elements. The abscissa indicates the magnetic field (unit: Oe=103/4π A/m) in the y axis direction in FIG. 6, and the ordinate indicates the resistance R (arbitrary units). In the graph, the solid lines connecting circular symbols represent the characteristics of read elements having an aspect ratio W:L=1:1, the long dotted lines connecting square symbols represent the characteristics of read elements having an aspect ratio W:L=1:2, and the short dotted lines connecting rhombus symbols represent the characteristic of read elements having an aspect ratio of 1:3.
As the thickness of the magnetic walls increases, hysteresis appears in the resistance/magnetic field curves of the read elements. In other words, when the aspect ratio is large, the curve around zero magnetic field is a straight line; however, the curve clearly exhibits hysteresis as the aspect ratio decreases.
The aspect ratio is preferably as small as is feasibly possible so as to read higher density media. However, read elements must maintain a particular magnetization state when no signals are provided and must respond linearly to an external magnetic field. In other words, read elements with a large coercive force Hc and hysteresis are not preferred. In order to eliminate hysteresis, hard magnet layers for controlling the magnetic domains are provided at the two sides of the element so as to forcibly orient the magnetization direction of the free layer in the track direction by the biasing magnetic field from these hard magnet layers. However, application of the biasing magnetic field from the hard magnet layers inhibits the rotation of the magnetization of the free layer, thereby drastically decreasing the sensitivity to the external magnetic field. In other words, small elements suffer from a decrease in the sensitivity because of the presence of the hard magnet layers.
As for the problem of point (1) above, defect-free bulk memories can be manufactured by making the coercive force Hc uniform over all the elements. In contrast, as for the problem of point (2) above, the coercive force Hc needs to be eliminated so as to decrease the biasing magnetic field of the hard magnet layers as much as possible and to manufacture magnetic read heads having high sensitivity. In other words, the object in point (1) is to maintain the coercive force Hc in the hysteresis curve in the easy axis direction of the free layer at a particular level; and the object in point (2) is to reduce the coercive force to zero in the hysteresis curve in the hard axis direction of the free layer. These objects appear contradictory but can be achieved simultaneously if the magnetization of the free layer can rotate simultaneously. The means for achieving these objects is the same.
The dependency of the coercive force characteristic on the size and shape of the element can be confirmed through investigation of the magnetization distribution immediately before rotation.
FIGS. 11A, 11B, 12A, and 12B show example magnetization distributions immediately before the rotation of the magnetization. The distributions are estimated by carrying out a micromagnetics simulation. FIGS. 11A and 11B show the distributions in rectangular elements, and FIGS. 12A and 12B show the distributions in elliptic elements. In the drawings, arrows inside the frames represent magnetization distributions, and streamlines above the frames schematically illustrate the general directions of the magnetization.
A large rectangular element of, for example, 1/W=1.5 μm−1, has a vortex distribution, as shown in FIG. 11A. A small rectangular element of, for example, 1/W=3.0 μm−1, has a distribution resembling the shape of letter S (hereinafter, the “S distribution”), as shown in FIG. 11B.
A large elliptic element of, for example, 1/W=1.5 μm−1, has a vortex distribution, as shown in FIG. 12A. A small elliptic element of, for example, 1/W=3.0 μm−1, has a distribution resembling the shape of letter C (hereinafter, the “C distribution”), as shown in FIG. 12B.
Note that the C and S distributions can be considered as a low-level buckling magnetic wall. Whereas buckling magnetic wall has many undulations, the number of undulations in the C and S distributions is low. The state of the magnetic wall in the element is determined according to the balance between the demagnetizing field energy, the anisotropic energy, the exchange coupling energy, and the Zeeman energy.
As the size of the element is reduced, the relative strength of the demagnetizing field energy increases, resulting in changes in magnetization distribution immediately before the rotation of the magnetization. Presumably, in most cases, the vortex distribution has a relatively large coercive force Hc, and the S and C distributions have a relatively small coercive force Hc.
It should be noted that the above description can be applied not only to devices using ferromagnetic layers but also to devices using ferroelectric layers. When applied to devices using ferroelectric layers, the demagnetizing field corresponds to the depolarization field, and the coercive force Hc corresponds to the coercive field Ec.