JPH10209526A - Ferromagnetic spin tunnel effect element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ferromagnetic spin tunnel effect element having a large rate of change in resistance. SOLUTION: A first ferromagnetic body 12 and a second ferromagnetic body 14 are oppositely located via an insulation layer 13 in a ferromagnetic spin tunnel effect element where a tunnel current flowing through the insulation layer 13 changes, depending on the relation of magnetization direction between the first ferromagnetic body 12 and the second ferromagnetic body 14, the first ferromagnetic body 12 or/and second ferromagnetic body 14 are made of a ferromagnetic semimetal. As a result, the rate of change in resistance becomes much greater than that of the conventional ferromagnetic spin tunnel effect element.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an element capable of switching a flowing current at a binary level according to the presence or absence of an external magnetic field of a predetermined value or more, or according to the direction of the external magnetic field. More specifically, an element using the ferromagnetic spin tunnel effect can be used for a magnetic sensor, a pulse type rotation or position sensor, a magnetic memory, a magnetic head for a magnetic memory, and the like.

[0002]

2. Description of the Related Art Conventionally, a ferromagnetic spin tunnel effect device having a structure in which two ferromagnetic materials are sandwiched between extremely thin insulators is known (Japanese Patent Application Laid-Open Nos. Hei 6-244377 and Hei 8-70).
148, JP-A-4-103014). In this device, the magnitude of the tunnel current flowing between two ferromagnetic materials such as Fe, Co, and Ni via an insulator such as Al ₂ O ₃ depends on the magnetization directions of the two ferromagnetic materials. It changes. That is, when the magnetization directions of the two ferromagnetic materials are the same (hereinafter simply referred to as “parallel”), a relatively large current flows, and the magnetization directions are parallel but opposite (hereinafter simply referred to as “parallel”). , "Anti-parallel"), a relatively small current flows. This can be said that the resistance value between two ferromagnetic materials changes according to the direction of magnetization between the two ferromagnetic materials.

[0003]

However, in the above element, the ratio of the amount of increase in resistance from the resistance in parallel when the magnetization is anti-parallel to the resistance in parallel in magnetization (hereinafter referred to as "resistance"). The rate of change is as small as about 20% at the maximum. When the element is used for a sensor, the sensitivity of discriminating the external magnetization direction is low. When the element is used for a switching element, the on / off control performance is poor. There was a problem.

Accordingly, it is an object of the present invention to improve the rate of change in resistance in a ferromagnetic spin tunneling device having the above structure, and to improve the device characteristics as a sensor or a switching device.

[0005]

According to the present invention, a first ferromagnetic material and a second ferromagnetic material are opposed to each other via an insulating layer, and the first ferromagnetic material and the second ferromagnetic material are opposed to each other. In a device using a ferromagnetic spin tunnel effect in which a tunnel current flowing through an insulating layer changes depending on a relationship of a magnetization direction with a magnetic material, a first ferromagnetic material and / or a second ferromagnetic material are combined with a ferromagnetic half-structure. The means of metal was adopted. It is desirable that both the first ferromagnetic material and the second ferromagnetic material are ferromagnetic semimetals, so that the resistance change rate is high. However, even if either one is adopted, the resistance change rate is improved as compared with the conventional element. . Further, as ferromagnetic semimetals, NiMnSb, CoMnSb, FeMnSb, CrO ₂ ,
La _1-x Ca _x MnO ₃ or the like can be employed. In addition, in general, a Heusler alloy having a C1 _b type crystal structure, M
Gold including n can be employed.

[0006]

As described above, since the first ferromagnetic material and / or the second ferromagnetic material is made of a ferromagnetic metalloid, the spin tunneling when the magnetization directions of the two ferromagnetic materials are antiparallel is achieved. The current can be extremely reduced, and the rate of change in resistance can be increased. Ideally, when the magnetization directions are antiparallel, a state in which the spin tunnel current does not flow, that is, the resistance change rate can be improved to infinity.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described based on specific embodiments. FIG. 1 is a cross-sectional view illustrating a structure of a ferromagnetic spin tunnel effect device 100 according to a specific example of the present invention. As shown in FIG.
An electrode 11 made of copper (Cu) and having a thickness of 100 nm is formed thereon, and a ferromagnetic metalloid Ni is formed on the electrode 11.
A first ferromagnetic thin film (first ferromagnetic material) 12 made of MnSb and having a thickness of 100 nm is formed. On the upper surface of the first ferromagnetic thin film 12, an insulating layer 13 of Al ₂ O ₃ having a thickness of 10 nm is formed, and on the upper surface of the insulating layer 13, a ferromagnetic metalloid CrO ₂ is formed. A second ferromagnetic thin film (second ferromagnetic material) 14 having a thickness of 100 nm is formed. Further, an electrode 15 made of copper (Cu) and having a thickness of 100 nm is formed on the upper surface of the second ferromagnetic thin film 14.

Next, a method of manufacturing the ferromagnetic spin tunnel effect device 100 will be described with reference to FIG. This element 100 was formed by an ion beam sputtering method. Ar was used as an ion gas at the time of sputtering. The ion gas pressure in the sputtering apparatus was 2.5 × 10 ^-2 Pa, and the acceleration voltage of the ion gun was 1
200 V, ion current is 120 mA, and the distance between target substrates is 120 mm.

First, the substrate 10 is kept at a temperature of 200 ° C.
Sputtering copper (Cu) on the substrate 16 to a thickness of 100
The electrode 11 of nm was formed. Next, NiMnSb was sputtered on the upper surface of the electrode 11 to form a first ferromagnetic thin film 12 having a thickness of 100 nm. Further, Al is sputtered on the upper surface of the first ferromagnetic thin film 12 to form an Al layer 1 having a thickness of 10 nm.
3 was formed. Here, it is left in the air for 30 hours, and the Al layer 1
The insulating layer 13 was formed by oxidizing ₃ and changing it to Al ₂ O ₃ as an insulator.

Next, under the same sputtering conditions as above, CrO ₂ was sputtered on the upper surface of the insulating layer 13 to form a second ferromagnetic thin film 14 having a thickness of 100 nm. Further, copper (Cu) was sputtered on the upper surface of the second ferromagnetic thin film 14 to form an electrode 15 having a thickness of 100 nm. Further, a part of the electrode 11, the second ferromagnetic thin film 14, the insulating layer 13, and a part of the first ferromagnetic thin film 12 were etched by dry etching to expose a part of the electrode 11. Thus, the voltage V is applied between the electrode 15 and the exposed portion 11a of the electrode 11, and the first ferromagnetic thin film 12 is applied by the external magnetic field H.
And the direction of the magnetization M between the second ferromagnetic thin film 14 and
By changing the anti-parallel state, a ferromagnetic spin tunnel effect element capable of changing the magnitude of the flowing current I can be formed. This change in the tunnel current I is detected by converting it into a voltage drop of the resistor R1, and is detected by the transistor Tr.
1 is turned on and off according to the magnitude of the tunnel current I, so that it can be output to the outside as an output signal S1 by the collector voltage of the transistor Tr1.

Next, the operation principle of the ferromagnetic spin tunnel effect device 100 will be described. a) Case where the directions of magnetization are parallel The case where the first ferromagnetic thin film 12 and the second ferromagnetic thin film 14 are magnetized in the same direction will be described. In FIG. 2A, the vertical axis represents the electron energy, and the horizontal axis represents the electron density of states D (E) at the electron energy E. As is well known, the occupancy probability of an electron in a certain state changes with respect to the energy E according to the Fermi-Dirac distribution function F (E). Therefore, the electron density N at energy E
(E) is given by D (E) · F (E).

As well known, at any temperature,
The energy E at which the probability of occupation of electrons is 1/2 is called the Fermi level fe, and the conduction behavior of the electrons in the substance is largely determined by the behavior of the electrons existing within a small width Δ with respect to this Fermi level fe. You.

Next, it is assumed that the first ferromagnetic thin film 12 is magnetized in the positive direction of the reference axis x shown in FIG. The conduction band of the ferromagnetic material is separated into two conduction bands 31 and 32 of up spin and down spin by exchange interaction. FIG. 2 (a)
Shows the state of separation of the conduction bands 31 and 32.
Electrons existing in the conduction band 31 having a low electron energy level are defined as upspin, and electrons existing in the conduction band 32 having a high electron energy level are defined as downspin. That is, an electron whose magnetization M direction of the ferromagnetic material is parallel to the direction of angular momentum of the electron is an up spin, and an electron whose magnetization M direction of the ferromagnetic material is antiparallel to the direction of the angular momentum of the electron is a down spin. Is defined as

The second ferromagnetic thin film 14 is oriented in the positive direction of the reference axis x shown in FIG.
Is magnetized parallel to the direction of. Energy level E
The distribution curve of the electron density of states D (E) with respect to the first ferromagnetic thin film 12 is completely equal to that of the first ferromagnetic thin film 12. Therefore, FIG.
As shown in (1), an energy phase diagram can be drawn in which the conduction band 41 is divided into the conduction band 41 for the up spin and the conduction band 42 for the down spin.

The spin polarization P (E) in the energy level E is spin-up electron density N ₊ (E), the electron density of spin-down N _- (E) be defined as follows to the Can be.

[Number 1] _{P (E) = (N +} (E) -N - (E)) / (N + (E) + N - (E)) ... (1)

The ferromagnetic metalloid is one metal spin polarization P (E) is at the Fermi level fe defined by formula (1), N ₊ is the N _- if sufficiently large in comparison with the , The spin polarization is sufficiently close to 1. In a ferromagnetic semimetal, the up spin becomes a large number of electrons, and there is no dawn spin state near the Fermi level fe. Therefore, the relationship between the two conduction bands 31 and 32 and the Fermi level fe is shown in FIG.
It becomes as shown in.

Assume that a voltage is applied so that the electrode 15 side is positive and the electrode 11 side is negative. When electrons tunnel from the first ferromagnetic thin film 12 to the side of the second ferromagnetic thin film 14 through the insulating layer 13, the spin angular momentum is conserved, and therefore the spin direction is also conserved. The energy level that can be taken by the up spin in the ferromagnetic thin film 12 must also be present in the second ferromagnetic thin film 14. When the directions of the magnetizations M of the first ferromagnetic thin film 12 and the second ferromagnetic thin film 14 are parallel, as shown in FIG. 2A, the Fermi level fe is present in the up-spin conduction bands 31 and 41. Since it is present, the up-spin electrons at the Fermi level fe in the first ferromagnetic thin film 12 can transition to the same energy level in the second ferromagnetic thin film 14 which is filled at a rate of 1/2. Both the first ferromagnetic thin film 12 and the second ferromagnetic thin film 14 have an occupancy probability of 1 at the Fermi level fe.
/ 2, and the number of up-spin electrons can be tunneled by a number proportional to the square of the number of electrons, and the insulating layer 13 has a low resistivity.
Can be tunneled. Thereby, the resistance value when the magnetization directions are parallel can be made extremely small.

B) The case where the magnetization directions are antiparallel The case where the first ferromagnetic thin film 12 and the second ferromagnetic thin film 14 are magnetized in opposite directions will be described. First ferromagnetic thin film 12
Is magnetized in the positive direction of the reference axis x shown in FIG.
This is the same as the state of magnetization of the first ferromagnetic thin film 12 when the direction of magnetization is parallel, and thus the energy state diagram of the conduction bands 31 and 32 of the first ferromagnetic thin film 12 at this time. Can be drawn exactly the same as shown in FIG.

On the other hand, it is assumed that the second ferromagnetic thin film 14 is magnetized in the negative direction of the reference axis x shown in FIG. Also at this time, the degeneracy of the energy level is released by the exchange interaction, and the second ferromagnetic thin film 14
FIG. 2B shows an energy state diagram of the conduction band 42 having a spin parallel to the direction of the magnetization M and the conduction band 41 having an antiparallel spin.

However, it is inconvenient for the first ferromagnetic thin film 12 and the second ferromagnetic thin film 14 to have different directions of electron spins in different directions. That is, the positive direction of the reference axis x is determined as the unified reference direction. Then, in the second ferromagnetic thin film 14, spins antiparallel to the direction of the magnetization M become up spins, and spins parallel to the direction of the magnetization M become down spins. In this way, as shown in FIG. 2B, an energy state diagram of a common up spin on the right side and a common down spin on the left side with respect to the energy axis is obtained.

When electrons tunnel from the first ferromagnetic thin film 12 to the second ferromagnetic thin film 14 via the insulating layer 13, the spin direction is preserved. When the electron tunnels, the energy level that can be taken by the up spin in the first ferromagnetic thin film 12 must also exist in the second ferromagnetic thin film 14. However, when the directions of the magnetizations M of the first ferromagnetic thin film 12 and the second ferromagnetic thin film 14 are antiparallel, as shown in FIG. Although the state exists at the Fermi level fe of obi 31,
In the ferromagnetic thin film 14, no state exists in the Fermi level fe of the conduction band 41 of the up-spin electrons, and the up-spin electrons can exist only at an energy higher than the Fermi level fe. Therefore, the up-spin electrons cannot tunnel through the insulating layer 13 from the first ferromagnetic thin film 12 to the second ferromagnetic thin film 14. Thereby, when the direction of magnetization is antiparallel, the resistance value can be increased.

C) Spin Polarization Ratio and Resistance The spin polarization ratio indicates the degree of deviation of upspin and downspin of the outermost electron contributing to conduction as defined by the equation (1). The ferromagnetic semimetal has a value sufficiently close to 1 at the Fermi level fe, and as shown above, only one of the spin states has an outermost electron,
The energy diagram of the conduction band is also drawn as shown in FIG. As described above, the resistance is small when the magnetization directions are parallel and large when the magnetization directions are antiparallel.

On the other hand, the ferromagnetic thin film used in the conventional ferromagnetic spin tunneling device has a spin polarization of 0.1.
It is a substance of about 2 to 0.5. That is, the outermost shell electrons contributing to conduction exist in a state where both the up-spin electrons and the down-spin electrons are in a comparable order and one of them is large. FIG. 3A shows an energy state diagram of the conduction band when the magnetization directions are parallel in this case, and FIG. 3B shows an energy state diagram when the magnetization directions are antiparallel.

In the case where the directions of magnetization are parallel, when the energy phase diagram is shown in FIG. 3A, tunnel conduction of electrons can be explained by the same concept as in FIG. 2A. However, in this case, the up-spin conduction bands 31 and 4 in the first ferromagnetic thin film 12 and the second ferromagnetic thin film 14 are used.
Since the Fermi level fe exists in the conduction bands 32 and 42 of 1 and down spin, the up spin and down spin contribute to the tunnel conduction in proportion to the square of each electron number, and therefore, the resistivity Will also be lower.

On the other hand, the case where the magnetization directions are antiparallel is
When the energy phase diagram is shown in FIG. 3B, the Fermi level fe indicates the up-spin conduction bands 31 and 41 and the down-spin conduction bands 32, in the first ferromagnetic thin film 12 and the second ferromagnetic thin film 14, respectively. There is a Fermi level fe in 42. Further, in the conduction bands 31 and 41, the ratio of the density of states at the Fermi level fe is considered to be about the ratio of the number of electrons. Therefore, the tunnel of the upspin in the conduction band 31 is different from the case of FIG. The number of electrons in the conduction band 31 is considered to be proportional to the product of the number of electrons in the conduction band 31 and the number of electrons in the conduction band 41. Therefore, in this case, the tunnel current is as shown in FIG.
Although they are smaller than in the case of (a), the difference between them is not so large. Therefore, unlike in the case of FIG. 2B, there are electrons that can conduct tunnel conduction, so that the resistance value does not increase so much. Does not grow.

The number of electrons transitioning between the same Fermi levels is proportional to the product of the number of electrons at that level and the number of empty states at the transition destination level, and the number of empty states is also proportional to the number of electrons at that Fermi level. Therefore, the number of electrons of the up spin that contributes to the tunnel current is the product of the number of electrons of the up spin existing in the conduction band 31 and the conduction band 41, and the number of electrons of the down spin is
It is proportional to the product of the number of downspin electrons existing in the conduction band 32 and the conduction band 42. Therefore, the resistance change rate MR of the ferromagnetic spin tunnel effect element can be expressed as follows using the polarization rate.

[0027]

## EQU2 ## where MR = (R _ap −R _p ) / R _p = 2P ₁ P ₂ / (1−P ₁ P ₂ ) (2) where R _ap is the resistance when anti-parallel, and R _p is when parallel. Resistance,
P ₁ is the spin polarization of the first ferromagnetic material, and P ₂ is the spin polarization of the second ferromagnetic material. Therefore, considering this equation, it can be seen that the resistance change rate increases when the spin polarization of both ferromagnetic materials approaches 1. Therefore, the resistance change rate can be made infinite by using a ferromagnetic semimetal, which is a substance having a spin polarization of 1 as the ferromagnetic substance.

In the above embodiment, the first ferromagnetic thin film 1
The ferromagnetic spin tunnel effect element was formed by using both the second and second ferromagnetic thin films 14 as ferromagnetic semimetals. However, one of the first ferromagnetic thin film 12 and the second ferromagnetic thin film was used as a ferromagnetic semimetal, One can be formed of a ferromagnetic material. In this case, as is clear from equation (2), the resistance change rate is smaller than when both ferromagnetic thin films are formed of ferromagnetic metalloids. The rate of change in resistance can be increased as compared with the case where the body is formed. On the other hand, as a ferromagnetic thin film, a ferromagnetic material having a small coercive force, a material containing two or more of Fe, Ni, and Co elements and containing 40 at% or less of Co element, and a ferromagnetic material having a large coercive force, such as Co, Co-Sm, Co-Cr-Fe, Co-Pt, Co-Pt-Ni, Co-Pt-V
For example, a material containing 20 at% or more of a Co element can be used.

In the above embodiment, the insulating layer 13
Is formed of Al ₂ O ₃ which is a metal oxide, but can be formed of phthalocyanine which is an organic material. When the insulating layer 13 is formed of an organic material, the insulating property is high and stable, so that the characteristics of the ferromagnetic tunnel effect element are improved.

The manufacturing method in the case of using an organic material for the insulating layer 13 is almost the same as that of the above-described embodiment. However, as in the case of forming the insulating layer of a metal oxide, a metal serving as a conductor is sputtered and the metal is sputtered in the air. The insulating layer is formed by being oxidized by leaving the substrate in a vacuum. However, the step of leaving the substrate in the air is unnecessary since the organic material is an organic material which is an insulator and is directly vacuum-deposited on the ferromagnetic layer.

After forming the first ferromagnetic thin film 12 of the first ferromagnetic material of FIG. 1 in the above embodiment, an ultrafine needle similar to that of a scanning microscope is formed of a ferromagnetic metalloid in a vacuum state. Then, the needle may be used as a second ferromagnetic material and opposed to the first ferromagnetic thin film 12 at an interval where a tunnel current can flow. Also in this configuration, both or one of the first ferromagnetic body and the second ferromagnetic body may be made of a ferromagnetic metalloid.

First ferromagnetic thin film 12 and second ferromagnetic thin film 14
By using ferromagnetic materials with different holding powers,
When the external magnetic field is changed in the range of −H to + H (H> 0), a negative parallel state, an anti-parallel state, and a positive parallel state can be realized with respect to the magnetization directions of the two due to the difference between the hysteresis curves of the two. . As a result, the resistance value changes to a minimum value, a maximum value, and a minimum value.
N-poles and S-poles are periodically arranged around the rotating body, and a change in the magnetization is detected as a pulse signal, so that it can be used as a rotation sensor or a position sensor.

Further, the first ferromagnetic thin film 12 is stored with binary data corresponding to the magnetization directions "0" and "1", and the second ferromagnetic thin film 14 is magnetized in a fixed direction. By detecting a change in the current flowing through the first ferromagnetic thin film 12, the present invention can be applied to a storage device that can read data stored in the first ferromagnetic thin film 12. That is, a memory device can be realized by arranging a large number of the elements having the configuration shown in FIG.

[Brief description of the drawings]

FIG. 1 is a sectional view of a ferromagnetic spin tunnel effect device according to a specific example of the present invention.

FIG. 2 is an energy state diagram for explaining a ferromagnetic tunnel effect in the ferromagnetic spin tunnel effect element of the present embodiment.

FIG. 3 is an energy state diagram for explaining a ferromagnetic tunnel effect in a conventional ferromagnetic spin tunnel effect element.

[Explanation of symbols]

 11, 15 ... electrode 12 ... first ferromagnetic thin film (first ferromagnetic material) 13 ... insulating layer 14 ... second ferromagnetic thin film (second ferromagnetic material)

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yasukun Taga 41-Cho, Yokomichi, Nagakute-cho, Aichi-gun, Aichi Prefecture Inside Toyota Central Research Institute, Inc. (72) Inventor Hiroshi Tadano Hiroshi Tadano, Nagakute-machi, Aichi-gun, Aichi Prefecture 41, Yokomichi, Toyota Central Research Laboratory Co., Ltd. (72) Inventor Toru Kachi, 41, Chuchu Yokomichi Oji, Nagakute-cho, Aichi-gun, Aichi Prefecture, Japan Toyota Central Research Laboratory Co., Ltd. (72) Inventor Yuichi Tanaka Aichi Prefecture 41 Toyota Central Research Laboratory, Nagakute-cho, Aichi-gun, Toyota-Chuo R & D Co., Ltd. (72) Inventor Kazuyoshi Tomita 41-Cho Toyota-Chuo Laboratory, Nagakute-cho, Aichi-gun, Aichi, Japan

Claims (1)

[Claims] A first ferromagnetic material and a second ferromagnetic material which face each other via an insulating layer, and wherein the insulating property is determined by a relationship of a magnetization direction between the first ferromagnetic material and the second ferromagnetic material. An element using a ferromagnetic spin tunnel effect in which a tunnel current flowing through a layer changes, wherein the first ferromagnetic material and / or the second ferromagnetic material is a ferromagnetic semimetal. Spin tunnel effect element.