JP6643612B2 - High frequency oscillator - Google Patents

Description

  The present invention relates to a high-frequency oscillator, and more specifically, to a high-frequency oscillator using self-excited oscillation of a magnetoresistive element.

  Communication in the microwave band (for example, a frequency in the range of 1 GHz to 100 GHz) has become important in recent years, and is widely used in mobile phones, wireless communication, satellite broadcasting, on-vehicle radar, and the like. In its use, the demand for smaller and cheaper high-frequency oscillators is increasing. However, with conventional high-frequency oscillators using semiconductors and LC resonators, it is difficult to reduce the size in the frequency range above a few GHz, so a high-frequency oscillator based on a new principle (or that does not require an LC resonator) is desired. I have.

  As high-frequency oscillation based on a new principle, it has been found that self-excited oscillation in the GHz band occurs when a DC current is applied to a minute magnetoresistive element under an external magnetic field (Non-Patent Document 1). In order to obtain a larger output in an oscillation element using excitation oscillation (hereinafter, spin torque oscillator (STO)), microwave oscillation in a tunnel magnetoresistive element with a magnesium oxide (MgO) barrier with a large magnetoresistance ratio (Non-Patent Document 2) and a proposal of a microwave oscillator using a tunnel magnetoresistive element having a MgO barrier as a component (Patent Document 1) have already been made. Recently, attempts have been made to apply phase synchronization to the STO and use it as an oscillation circuit with a higher Q value.

  According to the conventional STO disclosure, the oscillation frequency of the STO is determined by the material of the magnetoresistive element and is approximately in the range of several GHz to several tens of GHz. It is shown to be. However, the current range in which the STO oscillates stably is limited, and it has not been possible to greatly change the oscillation frequency (for example, a fraction of the oscillation frequency or more) by the current. That is, there is a limit in controlling the oscillation frequency of the STO only by controlling the DC current under the external magnetic field.

JP 2006-295908 A

S. I. Kiselev, et al. “Microwave oscillations of a nanomagnet driven by a spin-polarized current”, Nature, vol 425 (2003) 380, published 25 Sept. 2003 A. M. Deac, et al. “Bias-driven high-power microwave emission from MgO-based tunnel magnetoresistance devices”, Nature Physics 4 (2008) 803, published 10 Aug. 2008

  An object of the present invention is to solve / improve the above-mentioned problem of the conventional technique, and specifically, to greatly change the oscillation frequency in a high-frequency oscillator using STO.

  In one embodiment of the present invention, a first magnetoresistive element, a first current source for self-oscillating the first magnetoresistive element, and a second magnetoresistive element connected in series to the first magnetoresistive element Provided is a high-frequency oscillator including an element, and a second current source for changing a magnetic field generated by a second magnetoresistive element.

  According to one embodiment of the present invention, the state of self-excited oscillation of the first magnetoresistive element is controlled by the magnetic field (leakage magnetic field) generated by the second magnetoresistive element, in addition to the current from the first current source. Thus, in other words, it is possible to provide a high-frequency oscillator capable of greatly changing the oscillation frequency by controlling the current supplied from the two current sources in the absence of an external magnetic field.

  In one embodiment of the present invention, instead of the second magnetoresistance element connected in series to the first magnetoresistance element, at least one second magnetoresistance element connected in parallel to the first magnetoresistance element may be provided. it can.

  According to one embodiment of the present invention, a plurality of second magnetoresistance elements can be arranged adjacent to (around a plane) a first magnetoresistance element, and as a result, the second magnetoresistance element can be arranged. The operating frequency of the STO can be increased to three or four frequencies by discretely changing the magnitude of the leakage magnetic field from the element at a plurality of levels.

  In each of the above aspects of the present invention, the first magnetoresistance element is a tunnel magnetoresistance element including a magnetization free layer, a nonmagnetic intermediate layer, and a magnetization fixed layer, and the second magnetoresistance element is a magnetization free layer. A giant magnetoresistive element including a layer, a non-magnetic intermediate layer, and a magnetization fixed layer can be provided.

  According to one embodiment of the present invention, a large oscillation output can be obtained as an STO by a tunnel magnetoresistance element having a large magnetoresistance ratio, and at the same time, a large leakage magnetic field is generated by a giant magnetoresistance element capable of flowing a large current. / It is possible to greatly change the oscillation frequency of the STO by changing

FIG. 2 is a diagram illustrating a configuration (Example 1) of a high-frequency oscillator according to an embodiment of the present invention. It is a conceptual diagram of the oscillation of the first magnetoresistive element 1 of one embodiment of the present invention. It is a conceptual diagram of the oscillation of the first magnetoresistive element 1 of one embodiment of the present invention. It is a conceptual diagram of the magnetization reversal of the 2nd magnetoresistive element 2 of one Embodiment of this invention. FIG. 9 is a diagram illustrating another configuration (Example 2) of the high-frequency oscillator according to one embodiment of the present invention. FIG. 6 is a diagram illustrating calculation results of frequency characteristics of the high-frequency oscillator according to the embodiment (Example 2) of the present invention illustrated in FIG. 5. FIG. 9 is a diagram illustrating another configuration (Example 3) of the high-frequency oscillator according to one embodiment of the present invention. FIG. 14 is a diagram illustrating another configuration (Example 4) of the high-frequency oscillator according to one embodiment of the present invention. FIG. 14 is a diagram illustrating another configuration (Example 5) of the high-frequency oscillator according to one embodiment of the present invention.

  An embodiment (example) of the present invention will be described below with reference to FIGS.

  FIG. 1 shows an example of a high-frequency oscillator 100 including a first magnetoresistive element 1 serving as a self-excited oscillation and a second magnetoresistive element 2 for generating a leakage magnetic field. The example of FIG. 1 is characterized in that a first magnetoresistance element 1 and a second magnetoresistance element 2 are connected in series. The first magnetoresistance element 1 includes a magnetization free layer 11, a tunnel barrier layer (nonmagnetic intermediate layer) 12, and a magnetization fixed layer 13. The second magnetoresistive element 2 includes a magnetization free layer 21, a non-magnetic intermediate layer 22, and a magnetization fixed layer 23. A current source 3 for exciting self-excited oscillation is connected to the first magnetoresistive element 1, and a current source 4 for changing the magnetization arrangement (direction) is connected to the second magnetoresistive element 2. .

  Since it is desirable for the first magnetoresistance element 1 to obtain a high output as STO, it is desirable to use a tunnel magnetoresistance element having a high magnetoresistance ratio (hereinafter referred to as MR ratio). If the withstand voltage of the first magnetoresistive element 1 is sufficiently large with respect to the current required to reverse the second magnetoresistive element 2, the current source 4 to the second magnetoresistive element 2 is omitted. It is possible to However, at present, there is no such tunnel magnetic resistance element having a high withstand voltage. Therefore, it is desirable to separately provide a power supply 4 for the second magnetic resistance element 2 as shown in FIG. Further, the second magnetoresistance element 2 is for generating a leakage magnetic field, and does not need to have a high magnetoresistance ratio. Therefore, it is desirable to use a giant magnetoresistive element that can pass a larger current than a tunnel magnetoresistive element.

  Therefore, as the first magnetoresistive element 1, for example, a tunnel magnetoresistive element having FeB as the magnetization free layer 11, MgO as the tunnel barrier layer 12, and CoFeB as the magnetization fixed layer 13 can be used. As the second magnetoresistive element 2, for example, a giant magnetoresistive element having FePt as the magnetization free layer 21, Au as the non-magnetic intermediate layer 22, and FePt as the magnetization fixed layer 23 can be used. Note that the materials of these layers are merely examples, and other materials and combinations thereof can be used as long as the first and second magnetoresistive elements 1 and 2 described below can operate. it can.

  Here, the outline of the operation of the first magnetoresistive element 1 and the second magnetoresistive element 2 of FIG. 1 will be described with reference to FIGS. The operations of the two magnetoresistive elements 1 and 2 are basically the same in Examples 2 to 5 described below. FIG. 2 is a conceptual diagram of the oscillation in the first magnetoresistance element 1. In FIG. 2, both the magnetization free layer 11 and the magnetization fixed layer 13 are formed of in-plane magnetization films. 2A, a direct current is supplied from the current source 3 in a direction from the magnetization free layer 11 to the magnetization fixed layer 13 (from the top to the bottom of the drawing). In this case, there is no change in the magnetization state in both layers, and the magnetization direction 15 of the magnetization free layer 11 and the magnetization direction 16 of the magnetization fixed layer 13 are maintained in the plane (horizontal) direction of the layer.

  In FIG. 2B, a relatively small DC current is supplied from the current source 3 in the opposite direction to that in FIG. 2A, that is, in the direction from the magnetization fixed layer 13 to the magnetization free layer 11 (from bottom to top in the figure). Shed. In this case, the magnetization direction 15 of the magnetization free layer 11 is in a state where it can be precessed within the ellipse (easy magnetization direction) 17, and a small self-excited oscillation occurs under an external magnetic field. (c) and (d) show a case where the DC current is increased to a medium current and a large current in the same current direction as in (b). As the current increases, medium oscillation (c) and large oscillation (d) can be generated. As described above, when both the magnetization free layer 11 and the magnetization fixed layer 13 are formed of in-plane magnetization films, the first magnetoresistive element 1 (the magnetization free layer 11) depends on the direction and magnitude of the DC current from the current source 3. ) Can control the presence or absence of oscillation and its magnitude. In the present invention, as will be described in detail later, a leakage magnetic field generated by the second magnetoresistive element 2 is used as an external magnetic field.

  FIG. 3 is a conceptual diagram of oscillation when the magnetization free layer 11 is formed of an in-plane magnetization film and the magnetization fixed layer 13 is formed of a perpendicular magnetization film. In both (a) and (b), the magnetization direction 16 of the magnetization fixed layer 13 is vertically upward. 3A, a direct current is supplied from the current source 3 in a direction from the magnetization free layer 11 to the magnetization fixed layer 13 (from the top to the bottom of the drawing). In this case, the magnetization direction 15 of the magnetization free layer 11 can be precessed within the ellipse (easy magnetization direction) 17, and self-excited oscillation occurs under an external magnetic field.

  In FIG. 3B, a direct current is supplied from the current source 3 in a direction opposite to that in FIG. 3A, that is, in a direction from the magnetization fixed layer 13 to the magnetization free layer 11 (from bottom to top in the figure). In this case, similarly, the magnetization direction 15 of the magnetization free layer 11 can be precessed within the ellipse (easy magnetization direction) 17, and self-excited oscillation occurs under an external magnetic field. As described above, when the magnetization free layer 11 is formed of the in-plane magnetization film and the magnetization fixed layer 13 is formed of the perpendicular magnetization film, oscillation occurs regardless of whether the direction of the DC current from the current source 3 is up or down. The magnitude of oscillation can be changed depending on the magnitude.

  FIG. 4 is a conceptual diagram of the magnetization reversal of the second magnetoresistance element 2 according to one embodiment of the present invention. In (a), first, the magnetization direction 25 of the magnetization free layer 21 and the magnetization direction 26 of the magnetization fixed layer 23 are in an antiparallel state. In this state, a DC current flows from the current source 4 in a direction from the magnetization free layer 21 to the magnetization fixed layer 23 (from the top to the bottom in the drawing). Then, as shown in (b), the magnetization direction 25 of the magnetization free layer 21 and the magnetization direction 26 of the magnetization fixed layer 23 are in parallel.

  Next, as shown in (c), a DC current flows from the current source 4 in the direction from the magnetization fixed layer 23 to the magnetization free layer 21 (from the bottom to the top in the figure). Then, as shown in (d), the magnetization direction 25 of the magnetization free layer 21 and the magnetization direction 26 of the magnetization fixed layer 23 become antiparallel. Thus, the direction of magnetization can be reversed according to the direction of the DC current flowing through the second magnetoresistive element 2. The second magnetoresistive element 2 has two stable states in which no current flows and a state in which the magnetization directions in (b) are parallel and a state in which the magnetization directions in (d) are antiparallel. It is configured so that the leakage magnetic field has a difference in the two states. That is, switching of the leakage magnetic field is controlled by changing the magnetization states of (b) and (d) described above.

  FIG. 5 is a diagram showing another configuration (Example 2) of the high-frequency oscillator 100 according to one embodiment of the present invention. The configuration of FIG. 5 is different from the configuration of the high-frequency oscillator 100 of FIG. 1 in that the positions of the magnetization free layer 11 and the magnetization fixed layer 13 of the first magnetoresistive element 1 are vertically inverted. Other configurations are the same for both. In the configuration of FIG. 5, by bringing the magnetization free layer 11 of the first magnetoresistance element 1 close to the second magnetoresistance element 2, the efficiency of giving the leakage magnetic field of the second magnetoresistance element 2 to the magnetization free layer 11 is improved. Can be higher.

  FIG. 6 is a diagram showing calculation results of frequency characteristics of the high-frequency oscillator according to the embodiment (Example 2) of the present invention shown in FIG. FIG. 6 is a result obtained by estimating, by numerical calculation, how much the oscillation frequency of the first magnetoresistive element 1 which is the STO changes due to the leakage magnetic field of the second magnetoresistive element 2. In FIG. 6A, the magnetization directions of the magnetization free layer 11 of the first magnetoresistance element 1 and the two magnetic layers of the second magnetoresistance element 2 due to the leakage magnetic field applied to the magnetization free layer 11 are shown for simplicity. Are indicated only by the arrows 25 and 26 indicating. When the two magnetic layers of the second magnetoresistive element 2 are parallel, the largest leakage magnetic field is generated, and when they are antiparallel, they cancel each other out and the leakage magnetic field becomes small.

The parameters used in this numerical calculation are as follows. Here, FeB is assumed as the material of the magnetization free layer 11.

・ Magnetization of magnetization free layer: 1500emu / cc
・ Damping of magnetization free layer: 0.01
・ Magnetorotation ratio of the magnetization free layer: 17.6 × 10 ⁶ rad / (Oes)
・ Thickness of magnetization free layer: 2nm
・ Spin polarization of spin torque: 0.7
・ Asymmetry of spin torque: square of spin polarization of spin torque ・ Current density: 5 × 10 ⁶ A / cm ²
・ The magnitude of leakage magnetic field: ± 600 Oe

  According to the graphs A and B of FIG. 6B, when the external magnetic field is zero, when the leakage magnetic field of the second magnetoresistive element 2 changes from +600 Oe of A to −600 Oe of B, one embodiment according to the present invention. It can be seen that the oscillation frequency of the high-frequency oscillator 100 greatly changes from 14 GHz to 10 GHz. In the example shown in FIG. 5, changes in the case where the leakage magnetic field from the second magnetoresistive element 2 is large (parallel state) and in the case where it is small (antiparallel state) should be considered. It can be estimated to be about half of the above. The magnitude of the leakage magnetic field (about 600 Oe) can be realized by using an antiferromagnetically coupled perpendicular magnetization film such as CoPt / Ru / CoPt.

  FIG. 7 is a diagram illustrating another configuration (Example 3) of the high-frequency oscillator 100 according to an embodiment of the present invention. In FIG. 7, the first magnetoresistive element 1 and the second magnetoresistive element 2 are joined in parallel. Other configurations are the same as those in FIG. The first magnetoresistive element 1 is connected to a current source 3 for exciting self-excited oscillation, and the second magnetoresistive element 2 is connected to a current source 4 for changing its magnetization arrangement. As described above with reference to FIG. 4, the second magnetoresistive element 2 has two stable states in a state where no current flows, and is configured such that a difference occurs in a leakage magnetic field in these two states. . Further, as a method of changing the magnetization state of the second magnetoresistive element 2, a DC current from the current source 4 passing through the second magnetoresistive element 2 is used.

  By arranging the first magnetoresistive element 1 and the second magnetoresistive element 2 in parallel, the leakage from the second magnetoresistive element 2 to the first magnetoresistive element 1 is smaller than in the configuration of the first embodiment shown in FIG. It is expected that the efficiency of applying the magnetic field will be reduced. However, by providing a plurality of second magnetoresistive elements 2, that is, by providing a third magnetoresistive element or a fourth magnetoresistive element for providing a leakage magnetic field, the magnitude of the leakage magnetic field is discretely changed. It is also possible to increase the operating frequency of the STO to three or four frequencies.

  FIG. 8 is a diagram illustrating another configuration (Example 4) of the high-frequency oscillator 100 according to an embodiment of the present invention. In FIG. 8, the first magnetoresistive element 1 and the second magnetoresistive element 2 are connected in series similarly to the configuration of the first embodiment of FIG. Unlike the configuration of the first embodiment shown in FIG. 1, in the second magnetoresistive element 2, the magnetization free layer 21 is disposed below the magnetization fixed layer 23. A current source 3 for exciting self-pulsation is connected to the first magnetoresistive element 1. As described above with reference to FIG. 4, the second magnetoresistive element 2 has two stable states in a state where no current flows, and is configured such that a difference occurs in a leakage magnetic field in these two states. .

  Further, as a method of changing the magnetization state of the second magnetoresistive element 2, use of a spin orbit torque caused by a current flowing through the nonmagnetic metal terminal 6 in contact with the magnetization free layer 21 of the second magnetoresistive element 2. Features. Here, Ta or Pt is desirable as the nonmagnetic metal. A current source 4 for changing the magnetization arrangement of the second magnetoresistive element 2 is connected so that a current flows through the nonmagnetic metal terminal 6. In such an arrangement, there is an advantage that it is not necessary to supply a current for inverting the magnetization of the second magnetoresistive element 2 to the first magnetoresistive element 1 which is an STO.

  FIG. 9 is a diagram showing another configuration (Example 5) of the high-frequency oscillator 100 according to one embodiment of the present invention. In FIG. 9, the first magnetoresistive element 1 and the second magnetoresistive element 2 are connected in series, similarly to the configuration of the first embodiment of FIG. Unlike the configuration of the first embodiment shown in FIG. 1, in the second magnetoresistive element 2, the magnetization free layer 21 is disposed below the magnetization fixed layer 23. The second magnetoresistive element 2 has two stable states in a state where no current flows, and is configured such that there is a difference in a leakage magnetic field in these two states. Further, as a method of changing the magnetization state of the second magnetoresistance element 2, a magnetic field generated by passing a direct current through the electric wire 7 provided near the magnetization free layer 21 of the second magnetoresistance element 2 is used. It is characterized by the following. When such an arrangement is adopted, as in the case of the configuration example (Embodiment 4) in FIG. 8, the first magnetoresistance element 1 which is an STO is used to perform the magnetization reversal of the second magnetoresistance element 2. There is an advantage that it is not necessary to supply a current.

  Embodiments of the present invention have been described with reference to the drawings. However, the present invention is not limited to these embodiments. Furthermore, the present invention can be implemented in various modified, modified, and modified embodiments based on the knowledge of those skilled in the art without departing from the spirit thereof.

  The high-frequency oscillator according to the present invention can be used as a local oscillator of a communication device in a microwave band that can be used in mobile phones, wireless communication, satellite broadcasting, in-vehicle radar, and the like.

1: first magnetic resistance element 2: second magnetic resistance element 3, 4: current source 6: non-magnetic metal terminal (layer)
7: conductive (metal) layer 11, 21: magnetization free layer 12, 22: tunnel barrier layer (non-magnetic intermediate layer)
13, 23: fixed magnetization layer 15, 16, 25, 26: magnetization direction 17: easy magnetization direction 100: high frequency oscillator

Claims (5)

A first magnetoresistive element,
A first current source for causing the first magnetoresistive element to self-oscillate,
A second magnetoresistance element connected in series to the first magnetoresistance element,
A second current source for changing the magnetic field generated by the second magnetoresistive element,
A nonmagnetic metal layer having a surface joined to the second magnetoresistive element and two ends connected to the second current source;
The second current source is a high-frequency oscillator that changes the magnitude of a magnetic field generated by the second magnetoresistive element by changing the magnitude of a direct current flowing between two ends of the nonmagnetic metal layer. A first magnetoresistive element,
A first current source for causing the first magnetoresistive element to self-oscillate,
A second magnetoresistance element connected in series to the first magnetoresistance element,
A second current source for changing the magnetic field generated by the second magnetoresistive element,
A conductor provided near the second magnetoresistive element and having two ends connected to the second current source;
The second current source is a high-frequency oscillator that changes the magnitude of a magnetic field generated by the second magnetoresistive element by changing the magnitude of a direct current flowing between two ends of the conductor. The first magnetoresistive element is composed of a tunneling magnetoresistive element including a magnetization fixed layer, a nonmagnetic intermediate layer, and a magnetization free layer that are sequentially stacked,
2. The high-frequency oscillator according to claim 1, wherein the second magnetoresistive element includes a giant magnetoresistive element including a magnetization free layer, a non-magnetic intermediate layer, and a magnetization fixed layer which are sequentially stacked. 3. The first magnetoresistive element is composed of a tunneling magnetoresistive element including a magnetization fixed layer, a nonmagnetic intermediate layer, and a magnetization free layer that are sequentially stacked,
3. The high-frequency oscillator according to claim 2, wherein the second magnetoresistive element comprises a giant magnetoresistive element including a magnetization free layer, a non-magnetic intermediate layer, and a magnetization fixed layer, which are sequentially stacked. The magnetization free layer of the second magnetoresistance element is bonded to the surface of the nonmagnetic metal layer, and the magnetization of the second magnetoresistance element is controlled by a spin orbit torque generated by the direct current flowing through the nonmagnetic metal layer. 4. The high frequency oscillator according to claim 3, wherein the state changes .

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