1. Field of the Invention
The present invention generally relates to a tunneling magnetoresistive device, and more particularly, to a tunneling magnetoresistive device including two parallel-coupled ferromagnetic films that form a free layer.
2. Description of the Related Art
Tunneling magnetoresistive (TMR) devices are used in a MRAM (Magnetoresistive Random Access Memory), for example. Each tunneling magnetoresistive device has a tunneling insulating film interposed between two ferromagnetic films. Of the two ferromagnetic films, the ferromagnetic film that has a magnetization direction easily reversed when a magnetic field is applied is the free layer, and the ferromagnetic film that has a magnetization direction not easily reversed is the fixed layer. In a MRAM, for example, data can be written in a nonvolatile manner, depending on the magnetization direction of the free layer. In recent years, attention is drawn to a spin injection technique as a technique for causing a spin reversal in a free layer. According to this technique, spin-polarized carriers are injected so as to reverse the magnetization of a free layer. For example, the spin injection technique is utilized in a MRAM, so that data can be written without a magnetic field. Accordingly, the memory cell area can be made smaller. Also, according to the spin injection technique, the smaller the tunneling magnetoresistive device, the smaller the switching current required for writing data. Accordingly, the memory cells can be made smaller, and the current consumption can be reduced.
Japanese Unexamined Patent Publication No. 2007-294737 discloses a tunneling magnetoresistive device that includes a multilayer-type free layer formed with two ferromagnetic films in which antiparallel interlayer exchange coupling is observed in magnetization. According to Japanese Unexamined Patent Publication No. 2007-294737, higher thermal stability is achieved by the antiparallel coupling between the two ferromagnetic films.
In tunnel magnetoresistive devices, a further reduction in the switching current at the time of spin injection is expected, and higher thermal stability is demanded. However, in the tunneling magnetoresistive device disclosed in Japanese Unexamined Patent Publication No. 2007-294737, the reduction in the switching current and the increase in the thermal stability are insufficient.
It is therefore an object of the present invention to provide a tunneling magnetoresistive device in which the above disadvantage is eliminated.
A more specific object of the present invention is to provide a tunneling magnetoresistive device that can achieve both a sufficient reduction in the switching current and a sufficient increase in the thermal stability.
According to an aspect of the present invention, there is provided a tunneling magnetoresistive device including: a fixed layer that includes a ferromagnetic material; a tunneling insulating film that is provided in contact with the fixed layer; and a free layer that includes a first ferromagnetic film provided in contact with the tunneling insulating film, a second ferromagnetic film with magnetization that is interlayer-exchange-coupled parallel to the first ferromagnetic film, and a conductive film interposed between the first ferromagnetic film and the second ferromagnetic film. In accordance with the present invention, a tunneling magnetoresistive device that can achieve both a reduction in the switching current and an increase in the thermal stability can be provided.
As described above, the present invention can provide a tunneling magnetoresistive device that can achieve both a reduction in the switching current and an increase in the thermal stability.
Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings, in which:
The fixed layer 30 may be a single-layer ferromagnetic film, or may be a multilayer film that has ferromagnetic films interposing a nonmagnetic conductive film. The tunneling insulating film 20 may be a magnesium oxide (MgO) film, for example, or may be some other insulating film. The first ferromagnetic film 12 and the second ferromagnetic film 16 may be CoFeB films each having the body-centered cubic structure containing Co, Fe, and B, which is disclosed in Japanese Unexamined Patent Publication No. 2007-294737.
Next, the reason that higher thermal stability can be achieved with the present invention is described. Thermal stability is the stability required for the free layer 10 not to have its magnetization direction reversed. If the thermal stability is poor, MRAM data is erased in a short time, for example. To restrict the power consumption, it is preferable that the magnetization of the free layer 10 is reversed with a small switching current at the time of spin injection. To do so, it is effective to reduce the magnetization and the volume of the free layer 10. However, the thermal stability becomes poorer at the same time. There is a trade-off relationship between the switching current and the thermal stability. As an index of thermal stability, a thermal stability index Δ is used. The thermal stability index Δ is the index relative to the energy barrier obtained when magnetization is reversed, and is expressed by the following formula 1:
where Eu represents the uniaxial magnetic anisotropy energy, kB represents the Boltzmann's constant, and T presents the temperature. As the thermal stability index Δ is greater, the thermal stability is higher.
As shown in the formula 2 below, the uniaxial magnetic anisotropy energy Eu is represented by the sum of the shape magnetic anisotropy energy Eushape and the remaining magnetic anisotropy energy Eufilm. Here, the remaining magnetic anisotropy energy Eufilm is equivalent to all the magnetic anisotropy energy other than the shape magnetic anisotropy energy Eushape such as magnetic crystalline anisotropy energy and induced magnetic anisotropy energy.
E
u
=E
u
film
+E
u
shape [Formula 2]
Table 1 shows the shape magnetic anisotropy energy Eushape observed in a case where the free layer 10 is a single-layer ferromagnetic film, a case where the free layer 10 is a multiplayer formed with the first ferromagnetic film 12 and the second ferromagnetic film 16, in which parallel interlayer exchange coupling in magnetization (parallel coupling), and a case where the free layer 10 is a multilayer in which antiparallel interlayer exchange coupling in magnetization (antiparallel coupling) is observed between the first ferromagnetic film 12 and the second ferromagnetic film 16. In Table 1, N represents the difference between the demagnetizing field coefficient of the long axis direction and the demagnetizing field coefficient of the short axis direction of the cells in the free layer 10, M1 represents the magnetization of the first ferromagnetic film 12 or the single-layer ferromagnetic film, M2 represents the magnetization of the second ferromagnetic film 16, V1 represents the volume of the first ferromagnetic film 12 or the single-layer ferromagnetic film, V2 represents the volume of the second ferromagnetic film 16, d1 represents the film thickness of the first ferromagnetic film 12 or the single-layer ferromagnetic film, and d2 represents the film thickness of the second ferromagnetic film 16.
The switching current is the current for causing a reverse in the first ferromagnetic film 12. Therefore, the switching current depends on the magnetization M1, and can be made smaller by reducing the magnetization M1. Where the magnetization M1 is a fixed value in two structures, the structure having the greater shape magnetic anisotropy energy Eushape also has the greater uniaxial magnetic anisotropy energy Eu and the greater thermal stability index Δ. In other words, a desirable switching current and excellent thermal stability can be achieved at the same time.
As shown in Table 1, in the case of antiparallel coupling, the shape magnetic anisotropy energy Eushape is proportional to the square of (M1d1−M2d2)/(d1+d1). In the case of parallel coupling, on the other hand, the shape magnetic anisotropy energy Eushape is proportional to the square of (M1d1+M2d2)/(d1+d1). Accordingly, the structure in which the first ferromagnetic film 12 and the second ferromagnetic film 16 are in a parallel-coupled state has a greater shape magnetic anisotropy energy Eushape than that of the structure in which the first ferromagnetic film 12 and the second ferromagnetic film 16 are in an antiparallel-coupled state. Thus, it is most probable that both a desirable switching current and excellent thermal stability can be achieved simultaneously in the case of parallel coupling.
As described above, the shape magnetic anisotropy energy Eushape affects the thermal stability index Δ, when the shape magnetic anisotropy energy Eushape is dominant in the uniaxial magnetic anisotropy energy Eu. Therefore, it is preferable that the shape magnetic anisotropy energy Eushape is greater than the remaining magnetic anisotropy energy Eufilm.
Based on the conventional technical knowledge, increases both in the switching current and thermal stability are predicted where parallel interlayer exchange coupling is observed between the magnetization of the first ferromagnetic film 12 and the magnetization of the second ferromagnetic film 16 in the free layer 10. This is because the free layer 10 having two parallel-coupled ferromagnetic films interposing the conductive film 14 is considered to behave like a free layer that is virtually a single thick ferromagnetic film having the two ferromagnetic films in direct contact with each other without the conductive film 14. Where the film thickness of a free layer formed with a single ferromagnetic film is increased, the switching current and the thermal stability also become greater at the same time. To counter this problem, the present invention employs the free layer 10 in which parallel interlayer exchange coupling is observed between the magnetization of the first ferromagnetic film 12 and the magnetization of the second ferromagnetic film 16, so as to reduce the switching current and improve the thermal stability at the same time, as described above. In the following, embodiments of the present invention are described.
A sample (single-layer sample) having a single-layer ferromagnetic film as the free layer 10, a sample (a parallel-coupled sample; this sample is the first embodiment) having the first ferromagnetic film 12 and the second ferromagnetic film 16 coupled parallel to each other, and a sample (an antiparallel-coupled sample) having the first ferromagnetic film 12 and the second ferromagnetic film 16 coupled antiparallel to each other are formed.
Next, the technique for measuring the thermal stability index Δ is described. A current is swept between the free layer 10 and the fixed layer 30 of a produced sample, and the magnetoresistance of the tunneling magnetoresistive device is measured. As shown in
where P represents the switching probability, Ic represents the switching current, IC0 represents the intrinsic switching current before subjected to thermal agitation, tP represents the pulse current width, τP-AP represents the time required for the tunneling magnetoresistive device to switch from a parallel state to an antiparallel state, and τ0 represents the reciprocal of the attempt frequency. With the use of the formula 3, the intrinsic switching current IC0 and the thermal stability index Δ can be determined from the switching current distributions.
Table 2 shows the intrinsic switching current density JC0 (IC0 per junction area) and the thermal stability index Δ of each of the samples calculated with the use of the formula 3 based on the switching current distributions in the regions A and B. Both Jc0 and Δ in the table 2 represent mean values of parallel P to antiparallel AP switching (regions A) and from antiparallel AP to parallel P switching (regions B). Table 2 also shows mean values of the coercive force Hc. As can be seen from Table 2, the intrinsic switching current density JC0 is substantially the same among the samples. Although the coercive force Hc of the antiparallel-coupled sample is greater than the coercive force Hc of the parallel-coupled sample, the thermal stability index Δ of the parallel-coupled sample is greater than the thermal stability index Δ of the antiparallel-coupled sample. Accordingly, the parallel-coupled sample can achieve both a more desirable switching current and higher thermal resistance stability than the antiparallel-coupled sample and the single-layer sample.
Samples that differ from the parallel-coupled sample of the first embodiment shown in
As shown in Table 3, the intrinsic switching current density JC0 is substantially the same between the two samples. Where the film thickness (4 nm) of the second ferromagnetic film 16 is greater than the film thickness (2 nm) of the first ferromagnetic film 12, the coercive force Hc and the thermal stability index Δ are both greater than the coercive force Hc and the thermal stability index Δ obtained in the case where the film thickness of the second ferromagnetic film 16 is smaller (1 nm). Where the free layer 10 is a parallel-coupled layer, and the second ferromagnetic film 16 is thicker than the first ferromagnetic film 12, even higher thermal stability can be achieved.
In the following, the reason that higher stability can be achieved where the film thickness of the second ferromagnetic film 16 is equal to or greater than the film thickness of the first ferromagnetic film 12 is described. As shown in Table 1, where the free layer 10 is a parallel-coupled layer, the shape magnetic anisotropy energy Eushape is proportional to the square of (M1d1+M2d2)/(d1+d1). To reduce the switching current, it is preferable to reduce the product M1d1 of the magnetization and the thickness of the first ferromagnetic film 12. More specifically, it is preferable to reduce the product M1d1 of the magnetization and the thickness of the first ferromagnetic film 12, and increase the product M2d2 of the magnetization and the thickness of the second ferromagnetic film 16 (or increase the magnetization and the thickness of the second ferromagnetic film 16). By doing so, the switching current can be reduced, and the thermal stability index Δ can be improved. The film thicknesses of the first ferromagnetic film 12 and the second ferromagnetic film 16 are relative to their volumes. Therefore, it is preferable that the product of the magnetization and the volume of the second ferromagnetic film 16 is larger than the product of the magnetization and the volume of the first ferromagnetic film 12. It is more preferable that the product of the magnetization and the volume of the second ferromagnetic film 16 is twice or more as large as the product of the magnetization and the volume of the first ferromagnetic film 12.
Next, the film thickness of the conductive film 14 of a device in which the first ferromagnetic film 12 and the second ferromagnetic film 16 of the free layer 10 are coupled parallel to each other is described.
First, as shown in
To counter this problem, samples B each having the structure shown in
As shown in
As described in the first embodiment, where the free layer 10 is formed on the tunneling insulating film 20, the preferred film thickness of the conductive film 14 varies with the material of the first ferromagnetic film 12. Therefore, where a Ru film is used as the conductive film 14, and a CoFeB film is used as the first ferromagnetic film 12, it is preferable that the film thickness t of the conductive film 14 is in the range of 1.3 nm to 1.7 nm, so as to realize parallel coupling between the first ferromagnetic film 12 and the second ferromagnetic film 16. It is preferable that the second ferromagnetic film 16 is also a CeFeB film. The film thickness t suitable for realizing the parallel coupling between the first ferromagnetic film 12 and the second ferromagnetic film 16 is hardly affected by the film thicknesses of the first ferromagnetic film 12 and the second ferromagnetic film 16, because of the above mentioned reasons.
The same experiments are carried out in a different manner. A parallel-coupled sample and an antiparallel-coupled sample are formed independently of each other. In the parallel-coupled sample, the film thickness of the conductive film 14 is 1.5 nm. In the antiparallel-coupled sample, the film thickness of the conductive film 14 is 1.1 nm. The other aspects of the structures of those samples are the same as those of the parallel-coupled sample and the antiparallel-coupled sample of the first embodiment.
Measurement by a magnetization reversal technique is carried out as follows.
The theoretical formula of the switching probability PSW is expressed by the following formula 4:
P
SW=1−exp{(−tp/t0)×exp(−Δ×(1−H/H00)2)} [Formula 4]
where tp represents the ratio between the mean value of Hc and the magnetic field sweep rate v, and t0 represents the reciprocal of the attempt frequency. Based on the results shown in
Measurement by a spin-injection magnetization reversal technique is carried out in the following manner.
P
SW=1−exp[(−tp/t0)×exp(−Δeff)] [Formula 5]
where Δeff represents the effective thermal stability. According to this formula, the effective thermal stability Δeff in the case of an current Ic and a magnetic field H is determined. The effective thermal stability Δeff is expressed by the following formula 6 and formula 7:
Δeff=Δeff(I)×(1−H/HC0)2 [Formula 6]
Δeff(I)=Δ×(1−IC/IC0) [Formula 7]
The pulse current Ic is set at ±0.5 mA, ±0.6 mA, ±0.7 mA, and ±0.8 mA, and the magnetic field is varied at ten points including positive points and negative points for each of the current values. The effective thermal stability Δeff is then measured.
As shown in Table 4, with the use of different samples and different evaluation technique from those of the first embodiment, it is confirmed that the parallel-coupled sample has a greater thermal stability index Δ than the antiparallel-coupled sample.
As a modification of the second embodiment, samples in which the film thickness of the second ferromagnetic film 16 is 1 nm, 2 nm, and 4 nm are formed. As in the fourth embodiment, the intrinsic switching current IC0, the thermal stability index Δ, and the coercive force at absolute zero temperature HC0 are measured by a spin-injection magnetization reversal technique. Also, the thermal stability index Δ and the coercive force at absolute zero temperature HC0 of the same samples are measured by a magnetic field reversal technique as in the fourth embodiment. The results of the measurements are shown in Table 5. As shown in Table 5, where the second ferromagnetic film is thicker, a greater thermal stability index Δ can be achieved with the use of a different sample and different evaluation technique from the second embodiment. As can be seen from Table 5, it is preferable that the product of the magnetization and the volume of the second ferromagnetic film 16 is equal to or larger than the product of the magnetization and the volume of the first ferromagnetic film 12.
A sixth embodiment of the present invention is an example in which the film thickness of the second ferromagnetic film 16 of the second embodiment is further increased.
In
As described above, in accordance with the second embodiment, it is preferable that the second ferromagnetic film 16 is thick, or the product of the magnetization and the volume of the second ferromagnetic film 16 is large. In the sixth embodiment, on the other hand, it has become apparent that the magnetization of the first ferromagnetic film 12 returns to the original state after a magnetization reversal, if the second ferromagnetic film 16 is too thick or the product of the magnetization and the volume is too large. In view of these facts, it is preferable that the product of the magnetization and the volume of the second ferromagnetic film 16 is smaller than three times the product of the magnetization and the volume of the first ferromagnetic film 12.
Although a few preferred embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.
The present application is based on Japanese Patent Application Nos. 2008-222753 filed on Aug. 29, 2008 and 2009-158982 filed on Jul. 3, 2009, the entire disclosure of which is hereby incorporated by reference.
Number | Date | Country | Kind |
---|---|---|---|
2008-222753 | Aug 2008 | JP | national |
2009-158982 | Jul 2009 | JP | national |