JP2016063062A - Half-metal ferromagnetic junction structure, five-layer magnetic tunnel junction element arranged by use thereof, and magnetic memory device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a half-metal ferromagnetic junction structure which enables the modulation of the spin polarization of a half-metal ferromagnetic layer, and a multiferroic structure.SOLUTION: A combination of a metal ferromagnetic layer 1 as a free layer, a tunnel barrier layer 2 and a half-metal ferromagnetic layer 3 as a pin layer constitutes a three-layer tunnel junction (MTJ) structure S1. To the three-layer MTJ structure S1, a ferroelectric layer 4 having an intrinsic polarization P is bonded on the side of the half-metal ferromagnetic layer 3 and further, a conductive layer 5 is bonded under the ferroelectric layer 4, thereby modulating the spin polarization of the half-metal ferromagnetic layer 3. A combination of the half-metal ferromagnetic layer 3, the ferroelectric layer 4 and the conductive layer 5 constitutes a half-metal ferromagnetic junction structure S2. A combination of the three-layer MTJ structure S1 and the half-metal ferromagnetic junction structure S2, which share the half-metal ferromagnetic layer 3, constitutes a five-layer MTJ element S3.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a half-metal ferromagnetic junction structure including a half-metal ferromagnetic (HMF) layer, a five-layer magnetic tunnel junction (MTJ) element using the same, and a magnetic memory (MRAM) device.

Generally, in the ferromagnetic layer, as shown in (A) of FIG. 1, up-spin and down-spin have occupied up to the Fermi level E _F, as a result, caused the difference in the number of occupied state by the spin polarization, The spin polarizability P is
_{P = (D ↑ (E F} ) -D ↓ (E F)) / (D ↑ (E F) + D ↓ (E F))
Given in. On the other hand, in a full Heusler alloy layer made of a half-metal ferromagnet (HMF) layer such as Co ₂ MnSi, the up spin has a metallic band structure as shown in FIG. spin has a semiconductor band structure present energy gap at the Fermi level E _F vicinity. As a result, the spin polarizability P is
P = D _↑ (E _F ) / D _↑ (E _F )
= 1
Which is theoretically 100%.

  Therefore, when the above-mentioned half-metal ferromagnetic layer is applied to a magnetic tunnel junction (MTJ) element, a large tunnel magnetoresistance (TMR) effect can be expected.

  One cell of a conventional magnetic memory (MRAM) device includes one three-layer MTJ element and one transistor. The three-layer MTJ element has a three-layer structure including two magnetic layers sandwiching the tunnel barrier layer, and the upper magnetic layer of the tunnel barrier layer is called a free layer because the magnetization direction is a free direction. The lower magnetic layer of the tunnel barrier layer is called a pinned layer because the magnetization is fixed in a specific direction. In this case, depending on whether the magnetization direction of the free layer is parallel or antiparallel to the magnetization direction of the pinned layer, the three-layer MTJ element is in a low magnetoresistance state (for example, corresponding to the memory state “0”) or a high magnetoresistance state (for example, the memory layer). Corresponding to state “1”).

  Therefore, the degree of integration of the MRAM device described above can be improved by applying a half metal ferromagnetic layer to the pinned layer (see: Patent Documents 1 and 2).

  Further, in the MRAM device, by adopting a spin transfer torque (STT) method that reverses the magnetization direction of the free layer by flowing a polarized spin current in the write operation to the three-layer MTJ element, low power consumption and This can contribute to improvement of the degree of integration.

  Furthermore, in the MRAM device, the perpendicular magnetization method utilizing the perpendicular magnetization of the magnetic layer is adopted, and the magnetization direction of the free layer can be reversed with a small current, thereby contributing to the improvement of the degree of integration. it can.

  Thus, an improvement in integration can be expected by the STT-perpendicular magnetization type MRAM device using the half-metal ferromagnetic layer.

JP 2001-257395 A JP 2011-141934 A

  However, it is considered that the density of states and the band structure of the half-metal ferromagnetic layer do not change depending on the component, and therefore the spin polarizability of the half-metal ferromagnetic layer is considered to be fixed. As a result, the MRAM device including the three-layer MTJ element to which the above-described conventional half-metal ferromagnetic layer is applied has a problem that the binary memory cell structure is maintained and the degree of integration is limited.

Accordingly, an object of the present invention is to provide a half-metal ferromagnetic junction structure capable of modulating the spin polarizability of a half-metal ferromagnetic layer, and an MTJ element using the same.
It is another object of the present invention to provide a memory cell structure capable of realizing multilevel storage of four or more values in an MRAM device including an MTJ element, thereby improving the integration degree of the MRAM device.

In order to solve the above-described problems, a half metal ferromagnetic junction structure according to the present invention includes a half metal ferromagnetic layer, a ferroelectric layer provided under the half metal ferromagnetic layer, and a ferroelectric. And a conductive layer provided under the body layer, and modulates the spin polarizability of the half-metal ferromagnetic layer by the polarization of the ferroelectric layer. In this case, the polarization of the ferroelectric layer induces minority spin carriers at the interface with the ferroelectric layer in the half-metal ferromagnetic layer.
The five-layer MTJ element according to the present invention includes a metal ferromagnetic layer, a tunnel barrier layer provided below the metal ferromagnetic layer, and a half metal ferromagnetic layer provided below the tunnel barrier layer. And a ferroelectric layer provided under the half metal ferromagnetic layer and a conductive layer provided under the ferroelectric layer, and the half metal ferromagnetic layer is formed by polarization of the ferroelectric layer. This modulates the spin polarizability. In this case, the polarization of the ferroelectric layer induces minority spin carriers at the interface with the ferroelectric layer in the half-metal ferromagnetic layer.

  Furthermore, an MRAM device according to the present invention includes the above-described five-layer MTJ element, a bit line connected to the metal ferromagnetic layer, and a sense line connected to the conductive layer, Multivalue storage information of four or more values is stored by a combination of the magnetization direction and the polarization direction and magnitude of the ferroelectric layer.

Further, the ferroelectric layer can be replaced with a multiferroic layer having both characteristics of ferroelectricity and antiferromagnetism. When a multiferroic layer is used, the unidirectional anisotropy of magnetization of the half metal ferromagnetic layer is introduced. As a result, the perpendicular magnetization of the half metal ferromagnetic layer is fixed, and the half metal ferromagnetic layer acts efficiently as a magnetization fixed layer (pinned layer).

  The spin polarizability of the half-metal ferromagnetic layer can be modulated. In addition, since the multi-value storage memory cell structure having four or more values of the MRAM device including the five-layer MTJ element can be realized, the integration degree of the MRAM device can be improved.

(A) is a graph which shows the density of states of a metal ferromagnetic layer, (B) is a graph which shows the density of states of a half metal ferromagnetic layer. 1 is a diagram showing a perpendicular magnetization type MRAM device including an embodiment of a half-metal ferromagnetic junction structure according to the present invention. FIG. FIG. 3 is a band diagram in which the magnetization direction of the metal ferromagnetic layer of the five-layer MTJ element of FIG. 2 is antiparallel and the polarization direction of the ferroelectric layer is downward. FIG. 3 is a band diagram in which the magnetization direction of the metal ferromagnetic layer of the five-layer MTJ element of FIG. 2 is parallel and the polarization direction of the ferroelectric layer is downward. FIG. 3 is a band diagram in which the magnetization direction of the metal ferromagnetic layer of the five-layer MTJ element of FIG. 2 is antiparallel and the polarization direction of the ferroelectric layer is upward. FIG. 3 is a band diagram in which the magnetization direction of the metal ferromagnetic layer of the five-layer MTJ element of FIG. 2 is parallel and the polarization direction of the ferroelectric layer is upward. 3 is a graph showing conductance of a tunnel barrier layer of the 5-layer MTJ element of FIG. FIG. 3 is a diagram illustrating a modification of the perpendicular magnetization type MRAM device of FIG. 2. FIG. 9 is a magnetic field-magnetization (HM) characteristic diagram of the half-metal ferromagnetic layer of FIG. 8.

  FIG. 2 is a diagram showing a perpendicular magnetization type MRAM device including an embodiment of a half-metal ferromagnetic junction structure according to the present invention.

In FIG. 2, for example, a metal ferromagnetic layer 1 having a thickness of about 1 to 2 nm as a free layer made of FeCoB or the like, a tunnel barrier layer 2 having a thickness of about 1 nm made of an insulator such as MgO, and Co ₂ MnSi ( A half-metal ferromagnetic layer 3 having a thickness of about 1 nm as a pinned layer made of CMS) or the like constitutes a three-layer MTJ structure S1. On the other hand, on the half-metal ferromagnetic layer 3 side of the three-layer MTJ structure S1, a ferroelectric layer 4 made of BaTiO ₃ or the like having spontaneous polarization P or the like and a thickness of about 1 nm made of a conductor such as Ta or the like. The conductive layers 5 are bonded to modulate the spin polarizability of the half-metal ferromagnetic layer 3. In this case, the half metal ferromagnetic layer 3, the ferroelectric layer 4 and the conductive layer 5 constitute a half metal ferromagnetic junction structure S2.

    The three-layer MTJ structure S1 and the half-metal ferromagnetic junction structure S2 that share the half-metal ferromagnetic layer 3 together constitute a five-layer MTJ element S3. In FIG. 2, the opposite side of the metal ferromagnetic layer 1 to the tunnel barrier layer 2 is connected to the bit line BL. Furthermore, the opposite side of the conductive layer 5 to the ferroelectric layer 4 is connected to the source line SL. Further, a switching element SW1 made of a MOS transistor controlled by the potential of the word line WL1 is connected between the half metal ferromagnetic layer 3 and the sense line SL. Further, a switching element SW2 made of a MOS transistor controlled by the potential of the word line WL2 is connected between the half metal ferromagnetic layer 3 and the bit line BL. In the MRAM device, a plurality of word lines WL1 and a plurality of word lines WL2 are provided in the row direction, while a plurality of bit lines BL and a plurality of sense lines SL are provided in the column direction. One cell is provided at each intersection. The switching elements SW1, SW2 and the word lines WL1, WL2 are manufactured by CMOS technology, and the metal ferromagnetic layer 1, the tunnel barrier layer 2, the half metal ferromagnetic layer 3, the ferroelectric layer 4, and the conductive layer 5 are multilayer films. A nano heterostructure is manufactured using a manufacturing apparatus, and the bit line BL and the sense line SL are manufactured by Cu wiring technology or the like.

Direction of the spontaneous magnetization M ₁ of a metal ferromagnetic layer 1 is variable, the write operation to turn on the switching element SW1 by the potential of the word line WL1, between the bit line BL and sense lines SL by using the STT It is controlled by passing a relatively large write current that is spin-polarized. On the other hand, the read operation of the memory cell is also performed by detecting the read current flowing between the bit line BL and the sense line SL by turning on the switching element SW1.

  The write operation for determining the direction and magnitude of the spontaneous polarization P of the ferroelectric layer 4 is performed by turning on the switching element SW2 by the potential of the word line WL2, and at this time, the write operation is strongly performed by the sense line SL and the bit line BL. Spontaneous polarization P is generated by applying a voltage to the dielectric layer 4. The direction and magnitude of the remanent polarization of the spontaneous polarization P can be controlled by the polarity and magnitude of the applied voltage, the application method, and the like.

Next, the modulation of the spin polarizability of the half-metal ferromagnetic layer 3 in FIG. 2 will be described with reference to the band diagrams in FIGS. 3, 4, 5, and 6. In FIG. 2, there are the following four combinations of the vertical direction of the magnetization direction M ₁ of the metal ferromagnetic layer 1 and the vertical direction of the polarization direction P of the ferroelectric layer 4. Incidentally, the magnetization direction M3 of the half-metal ferromagnetic layer ₃ is always upward.
AP- state (Figure 3): the magnetization direction _{M 1} is directed downward _(M 1, _{M 3} is antiparallel state of the MTJ) and the polarization direction P is facing down.
P-state (FIG. 4): Magnetization direction M ₁ is in an upward state (MT ₁ and M _{3 of} MTJ are in a parallel state), and polarization direction P is in a downward state.
AP + state (FIG. 5): Magnetization direction M ₁ is in a downward state (MTJ M ₁ and M ₃ are antiparallel states) and polarization direction P is in an upward state.
P + state (FIG. 6): Magnetization direction M ₁ is in an upward state (MT ₁ and M _{3 of} MTJ are in a parallel state), and polarization direction P is in an upward state.

3 and 4, a negative charge is induced at the ferroelectric layer 4 side interface in the half-metal ferromagnetic layer 3 by the polarization direction P of the ferroelectric layer 4. Band bending is excited in the down-spin band structure of the half-metal ferromagnetic layer 3 by the electric field due to the negative charge. However, _↓ the state density D at the Fermi level E _F of the spin-down half-metal ferromagnetic layer _{3, 3} is independent of the polarization direction P of the ferroelectric layer 4 still is zero. That is, the modulation by the downward polarization direction P of the ferroelectric layer 4 does not change the spin polarization of the half-metal ferromagnetic layer 3. Therefore, based on the Julliere model, the down-spin state density D _{↓, 3} = 0 at the Fermi level of the metal ferromagnetic layer 1 (assuming the spin polarizability P ₁ = 0.6) and the half metal ferromagnetic layer 3 As shown in FIG. 7, the conductance G of the AP-state and P-state of the tunnel barrier layer 2 calculated based on the above-mentioned values does not depend on the spin polarizability modulation degree k, and has a constant value of 0.2 and 0. 8

On the other hand, in FIG. 5 and FIG. 6, positive charges as minority spin carriers are induced at the ferroelectric layer 4 side interface in the half metal ferromagnetic layer 3 by the polarization direction P of the ferroelectric layer 4. Band bending is induced in the down-spin band structure of the half-metal ferromagnetic layer 3 by the electric field due to the positive charge. In this case, the intensity depending on the polarization direction P of the dielectric layer 4, the spin polarization of the modulation degree k of the Fermi level E _F of the down spin at the interface between the ferroelectric layer 4 of the half-metal ferromagnetic layer 3 The density of states D _{↓, 3} = k is expressed. That is, the spin polarizability of the half-metal ferromagnetic layer 3 is modulated by the upward polarization direction P of the ferroelectric layer 4. Accordingly, based on the Julliere model, the down-spin state density D _{↓, 3} = k at the Fermi level of the metal ferromagnetic layer 1 (assuming the spin polarizability P ₁ = 0.6) and the half metal ferromagnetic layer 3 As shown in FIG. 7, the conductance G of the AP + state and the P + state of the tunnel barrier layer 2 calculated based on the above increases depending on the spin polarizability modulation degree k. The spin polarizability modulation k is a value corresponding to the direction and magnitude of the polarization P of the ferroelectric layer 4 and is defined as k = 0 when the above-described four conductances G are minimized.

As shown in FIG. 7, for example, when k = 0.4, the quaternary tunnel barrier layer 2 of the quaternary tunnel barrier layer 2 depends on the magnetization direction M ₁ of the metal ferromagnetic layer ₁ and the polarization direction P of the ferroelectric layer 4. It can be seen that conductance G is realized. Therefore, a quaternary memory state can be realized by assigning 00, 01, 10, and 11 to the P + state, P- state, AP + state, and AP- state when k = 0.4.

  Since the polarization P of the ferroelectric layer 4 can change not only the direction of remanent polarization but also the magnitude, the degree of spin polarizability modulation of the half-metal ferromagnetic layer 3 in the P + state and the AP + state depends on this magnitude. k can be varied. Thereby, for example, different conductances G can be realized by setting to a value other than k = 0.4, so that the above-described four or more memory states can be realized.

FIG. 8 is a diagram showing a modification of the perpendicular magnetization type MRAM device of FIG. In FIG. 8, the ferroelectric layer 4 of FIG. 2 is replaced with a multiferroic layer 4 ′ made of, for example, BiFeO ₃ having both characteristics of ferroelectricity and antiferromagnetism, and a half-metal ferromagnetic junction structure S2 Was replaced with a half-metal ferromagnetic junction structure S2 ′.

In FIG. 8, the half metal ferromagnetic junction structure S2 ′ can induce the modulation of the spin polarizability of the half metal ferromagnetic layer 3 in the same manner as the half metal ferromagnetic junction structure S2 described above. In addition, the effect of stabilizing the magnetization of the half-metal ferromagnetic layer 3 due to the exchange bias effect is exhibited by adopting the half-metal ferromagnetic junction structure S2 ′. Production of the half-metal ferromagnetic junction structure S2 ′ is performed by growing a multiferroic layer 4 ′ made of, for example, BiFeO ₃ and having a thickness of about 10 nm on the conductive layer 5 on the substrate using the composite film forming apparatus described above. Further, for example, perpendicular magnetization is realized by growing a half-metal ferromagnetic layer 3 made of Co ₂ FeSi and having a thickness of about 1 nm on the multiferroic layer 4 ′. Thereafter, the NeFe temperature of BiFeO ₃ is heated to 380 ° C. or higher, and the BiFeO ₃ is cooled by a magnetic field perpendicular to the surface of the multiferroic layer 4 ′. Accordingly, the exchange bias effect is caused by the antiferromagnetism of the multiferroic layer 4 ′, and the one-way anisotropic magnetization of the half-metal ferromagnetic layer 3 is shown in the magnetic field-magnetization (HM) characteristic diagram of FIG. Guide sex. As a result, the half metal ferromagnetic layer 3 efficiently acts as a fixed layer (pinned layer) of the five-layer MTJ element S3.

  In the above-described embodiment, the five-layer MTJ element adopts the perpendicular magnetization method, but by selecting the material of the metal ferromagnetic layer 1 and the half metal ferromagnetic layer 3 as appropriate, the in-plane magnetization method is used. Can be adopted.

  It should be noted that the present invention can be applied to any modifications within the obvious range of the above-described embodiment.

  The present invention can be used not only for magnetic memory devices but also for magnetic heads, magnetic sensors, and the like.

1: Metal ferromagnetic layer 2: Tunnel barrier layer 3: Half metal ferromagnetic layer 4: Ferroelectric layer 4 ′: Multiferroic layer 5: Conductive layer S1: Three-layer MTJ structure S2, S2 ′: Half metal Ferromagnetic junction structure S3: 5-layer MTJ element BL: bit line
SL: sense lines WL1, WL2: word lines

Claims (8)

A half-metal ferromagnetic layer;
A ferroelectric layer provided under the half-metal ferromagnetic layer;
A conductive layer provided under the ferroelectric layer,
A half-metal ferromagnetic junction structure that modulates the spin polarizability of the half-metal ferromagnetic layer by the polarization of the ferroelectric layer.   The half-metal ferromagnetic junction structure according to claim 1, further comprising means for controlling the direction and magnitude of polarization of the ferroelectric layer. A half-metal ferromagnetic layer;
A multiferroic layer provided under the half-metal ferromagnetic layer and having ferroelectricity and antiferromagnetism;
A conductive layer provided under the multiferroic layer,
A half-metal ferromagnetic structure in which a spin polarizability of the half-metal ferromagnetic layer is modulated by polarization of the multiferroic layer.   The half-metal ferromagnetic structure according to claim 3, further comprising means for controlling a polarization direction and a magnitude of the multiferroic layer. A metal ferromagnetic layer;
A tunnel barrier layer provided under the metal ferromagnetic layer;
A half-metal ferromagnetic layer provided under the tunnel barrier layer;
A ferroelectric layer provided under the half-metal ferromagnetic layer;
A conductive layer provided under the ferroelectric layer,
A five-layer magnetic tunnel junction element that modulates the spin polarizability of the half-metal ferromagnetic layer by the polarization of the ferroelectric layer. A metal ferromagnetic layer;
A tunnel barrier layer provided under the metal ferromagnetic layer;
A half-metal ferromagnetic layer provided under the tunnel barrier layer;
A multiferroic layer provided under the half-metal ferromagnetic layer and having ferroelectricity and antiferromagnetism;
A conductive layer provided under the multiferroic layer,
A five-layer magnetic tunnel junction element that modulates the spin polarizability of the half-metal ferromagnetic layer by the polarization of the multiferroic layer. The five-layer magnetic tunnel junction device according to claim 5 or 6,
A bit line connected to the metal ferromagnetic layer;
A sense line connected to the conductive layer;
A magnetic memory device for storing multilevel storage information of four or more values by a combination of a magnetization direction of the metal ferromagnetic layer and a polarization direction and magnitude of the ferroelectric layer or the multiferroic layer. further,
First and second word lines;
A first switching element connected between the half-metal ferromagnetic layer and the sense line and controlled by the potential of the first word line;
The magnetic memory device according to claim 7, further comprising: a second switching element connected between the half-metal ferromagnetic layer and the bit line and controlled by a potential of the second word line.

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