FILMS

RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No.

62/291,282, filed on February 4, 2016, as well as U.S. Provisional Application No.

62/411,916, filed on October 24, 2016. The entire teachings of the above applications are incorporated herein by reference.

GOVERNMENT SUPPORT

[0002] This invention was made with government support under Grant No. W91 INF- 16- 1-0011 awarded by the Defense Advanced Research Projects Agency. This invention was also made with government support under Grant No. DMR - 10002543 awarded by the National Science Foundation. The government has certain rights in the invention.

BACKGROUND

[0003] The magnetic tunnel junction (MTJ) is a circuit component that has been considered as a candidate element for designing the next generation of transistors. An MTJ cell comprises an ultra-thin insulating layer that is used to separate two ferromagnetic metal electrodes. High power amplification can be achieved for a current-field driven MTJ transistor design by using magnesium oxide (MgO) as the insulating layer. This result is due, in part, to the high tunnel magnetoresi stance (TMR) ratios that can be achieved at room temperature injunctions fabricated with crystalline MgO as an oxide layer. The large TMR ratio of a single crystalline MgO can be attributed to different symmetry-related decay rates of the Bloch waves for majority and minority spin channels.

SUMMARY

[0004] Example embodiments provide a circuit comprising first and second magnetic layers separated by an insulating layer. The first and second magnetic layers may be electrical communication with first and second terminals, respectively. The insulating layer may be coupled to opposing surfaces of the first and second magnetic layers, and may transfer an electrical current between the first and second terminals as a function of magnetization of the first magnetic layer relative to magnetization of the second magnetic layer. A magneto-electric film may be electrical communication with a third terminal, and is configured to control the magnetization of the second magnetic layer based on a signal (e.g., a given voltage) applied to the third terminal.

[0005] In further embodiments, a magnetoresi stance between the first and second terminals may be a function of the signal (e.g., voltage value) applied to the third terminal. The magneto-electric film may be further configured to control the magnetization of the second magnetic layer based on a magneto-static field generated by the magneto-electric film as a result of the voltage applied to the third terminal. Further, the magneto-electric film may be configured to selectively reverse the magnetization of the second magnetic layer based on the voltage applied to the third terminal.

[0006] In still further embodiments, the first magnetic layer may include an anti- ferromagnetic sublayer and an amorphous ferromagnetic sublayer. The second magnetic layer may include amorphous ferromagnetic material. The magneto-electric film may include a hexaferrite material, and may be deposited on a silicon substrate.

[0007] A further embodiment may include a magnetic tunnel junction (MTJ) device and a magneto-electric film. The MJT device may include first and second magnetic layers separated by an insulating layer. The first and second magnetic layers may be electrical communication with first and second terminals, respectively. The insulating layer may be coupled to opposing surfaces of the first and second magnetic layers. The magneto-electric film may be in electrical communication with a third terminal, the magneto-electric film being configured to control the tunnel magnetoresi stance of the MTJ device based on a voltage applied to the third terminal.

[0008] A still further embodiment may include a method of controlling a resistance. A magneto-electric film is provided proximate to a magnetic tunnel junction (MTJ) device, the MTJ device having first and second magnetic layers separated by an insulating layer. A signal is applied to the magneto-electric film to generate a magnetic field. The magnetization of the second magnetic layer is changed as a result of the magnetic field, where the resistance across the MTJ device is a function of the magnetization. Further, the magnetization of the second magnetic layer may be reversed based on the signal applied to the third terminal. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention.

[0010] FIG. 1 A is a diagram of a magnetic tunnel junction (MTJ) device that may be implemented in example embodiments.

[0011] FIG. IB is circuit diagram of the MTJ of Fig. 1A

[0012] FIG. 2A is a diagram of a circuit in an example embodiment.

[0013] FIG. 2B is circuit diagram of the circuit of Fig. 2A.

[0014] FIG. 3A is a diagram of the magneto-electric (ME) film implemented in Fig. 2.

[0015] FIG. 3B is a graph illustrating magnetism of the ME film.

[0016] FIG. 4 is a diagram of a circuit in a further embodiment.

[0017] FIGs. 5A-C illustrate a circuit in a still further embodiment.

[0018] FIG. 6 is circuit diagram of the circuit of Figs. 5A-C.

DETAILED DESCRIPTION

[0019] A description of example embodiments of the invention follows.

[0020] FIG. 1 A is a block diagram of a magnetic tunnel junction (MTJ) device 100 that may be implemented in example embodiments. The device 100 includes a first magnetic layer 1 10 coupled to a first terminal VI, and a second magnetic layer 120 coupled to a second terminal V2. The magnetic layers 110, 120 are separated by an insulating layer 140, which may comprise an oxide material (e.g., magnesium oxide (MgO)). MTJs such as the device 100 can take advantage of the up and down spins of electrons, as well as the giant magnetoresi stance (GMR) effect that can be achieved by the switching in magnetization of ferromagnetic metal electrodes. One of the applications of GMR is a "spin valve" in which the current flow is controlled by an external magnetic field H 160 (also referred to as an "H field").

[0021] The device 100 exhibits the MTJ effect as described below. Electrons transported in the second layer 120 are magnetically polarized depending on the direction of M2, the magnetization of the second layer 120. Electron spins are polarized along the same direction as M2 in this layer 120, which may be an amorphous magnetic metal (referred to as a soft magnetic material or a "free layer"). The first magnetic layer 110, in contrast, may be an amorphous magnetic metal sub-layer where the magnetization, Ml, is exchange-coupled to an anti -ferromagnetic sub-layer. Thus, the magnetization direction is "locked" along one fixed direction by the anti-ferromagnet, as it would require extremely high external magnetic fields to overcome the exchange field coupling the two layers. In effect, the two sub-layers in in the first magenetic layer 110 are represented by one layer with the understanding that the magnetization the layer 110 is fixed in direction.

[0022] As the spin polarized electrons are tunneled across the thin (e.g., 20-30 angstroms (A)) insulating oxide layer 140, the electron carrier is transported in the first magnetic layer 110, whereby the magnetization direction, Ml, is either parallel or anti-parallel to M2. M2 may be biased to be either parallel or anti-parallel to Ml via an external, variable H field 160. The field magnitude of H may not be sufficient to affect the fixed direction of Ml . As a result, electron carriers whose spin direction opposes magnetization in the first magnetic layer 110 are scattered more than those in phase. As a result, the electrical resistance is maximum (R.) for Ml and M2 opposing each other in direction and minimum (R ₊ ) when in parallel. The circuit equivalent of the MTJ device 100, including this variable resistance, is shown in Fig. IB. Thus, the direction of H 160 applied to the second magnetic layer 120 determines the value of resistance R between the first and second terminals of the device 100. The ratio of resistance variability of the device 100 may be referred to as the tunnel magnetoresi stance ratio (TMR) or

D _ D

TMR = ⁺

A TMR of approximately 2 or greater is useful for substantially controlling current through the device 100, and can be sufficient for use in a transistor device.

[0023] Example embodiments utilize the above-described phenomena of magneto- electricity, via the variable H field 160, to control current flow in a two-terminal MTJ device such as the device 100. The current flow can be modulated or changed by introducing a third terminal with a signal (e.g., a voltage, or a modulated signal) applied to a magneto-electric (ME) film to generate and control the H field 160, without the need for high power current generators or permanent magnets. A resulting circuit may exhibit properties comparable to those of a conventional semiconductor transistor. An example of such a circuit is illustrated in Fig. 2, described below.

[0024] Fig. 2A is a diagram of a circuit 200 in an example embodiment. The circuit 200 includes an MTJ device 202, which may be comparable to the device 100 described above with reference to Fig. 1 A, including a first magnetic layer 210 coupled to a first terminal VI, a second magnetic layer 220 coupled to a second terminal V2, and an insulating layer 240 (e.g., an oxide material such as magnesium oxide (MgO)). Further, an ME film 250 is coupled to a third terminal V3, and is positioned proximate to the second magnetic layer 220. The ME film 250 and can selectively generate a H field 260 as a function of a voltage applied to the third terminal V3.

[0025] Fig. 3 A illustrates the ME film 250 in isolation. The ME film 250 may be composed of a hexaferrite material, and may be deposited on a sapphire or silicon wafer buffered by a conductive oxide buffer layer (not shown). Due to the magneto-electric effect, the application of a voltage or electric field causes the induction of magnetic polarization (also referred to as magnetization), M, in the ME film 250. Conversely, the application of a H field 260 implies the induction of electric polarization. A change in a magnetization (G) over a corresponding change in magnetic field H (Oe) exhibited by an example FIE film is illustrated in the graph of Fig. 3B.

[0026] Referring back to Fig. 2A, the change in magnetization M in the ME film 250 by applying a voltage, as described above, causes the H field 260. The H field 260, in turn, biases the MTJ circuit 202 by altering the magnetization M2 of the second magnetic layer 220 as a result of the magneto-static effect. As predicted by the magneto- static effect, an instantaneous M within a film induces magnetic "poles" such that an internal demagnetizing magnetic field is excited or induced. However, if the direction of M is in the plane of the ME film 250, the demagnetizing field, Hd , is approximately 0, but the magneto-static field H 260 external to the film 250 is not. Applying Maxwell's boundary conditions on the magnetic flux density field (B field), the H field 260, external to the ME film 250, can be expressed as:

H ~ 4πΑΜ

where ΔΜ is the change in magnetization induced in the ME film 250 due to the application of a voltage across the film plane or an E field in the film plane. Thus, applying the E field in the film plane and reversing its polarity is equivalent to reversing the H field 260, which can be used to control the MTJ effect. The H field in example embodiments may be in the order of + 10 gauss (CGS), which is greater than the coercive field of a soft magnetic amorphous film (~ 1 Oe) making up the second magnetic layer 220.

[0027] Based on the effects described above, the flow of current through the MTJ device 202 can be controlled via the application of a small voltage introduced via the third terminal V3 coupled to the ME film 260. The ME film 250 may be positioned in electrical communication with the second magnetic layer 220, whereby the second magnetic layer 220 represents one terminal of the ME film 250 (terminal V2), and the other terminal is opposite the ME film (terminal V3). The H field 260 generated by the ME film 250 is transposed to the second magnetic layer 220. Hence, when the circuit 200 is viewed as a transistor, the terminal V2 represents a grounded source and the terminal V3 represents the gate terminal. The drain terminal (terminal VI) is connected to the first magnetic layer 210. Thus, by applying varying positive or negative voltages to the gate, positive and/or negative ( + ) H fields are generated, resulting in R _± resistance drops from the first magnetic layer 210 to the second magnetic layer 220. A circuit diagram illustrating a circuit equivalent to the circuit 200 is shown in Fig. 2B, where R _± denotes the variable resistance of the MTJ device 202 as a function of the applied voltage at terminal V3. In a particular example as shown

(dependent on the configuration of the circuit 200), when the voltage at V3 is less than 0, the MTJ device 202 exhibits a maximum resistance R. as a result of the magnetizations Ml, M2 having opposite polarity. Conversely, when the voltage at V3 is greater than 0, the MTJ device 202 exhibits a minimum resistance R ₊ as a result of the magnetizations Ml, M2 having matching polarity.

[0028] The power gain, G, provided b the circuit 200 may be calculated as follows:

G , where

TMR≡ Tunnel magnetore si stance ratio - 1-2

V _D s ~1 volt (voltage between drain and source (first and second layers))

VGS ~ 0.01-1 volt (voltage between gate and source (first layer and edge of ME film))

E = VGS W (electric field applied across ME film) W ~ 1 micron

Example embodiments of the circuit 200 may achieve a power gain of 30, given VGS =0.1 volt.

[0029] Fig. 4 is a diagram of a circuit 400 in a further embodiment. The circuit 400 may include some or all of the features of the circuit 200 described above, but is provided to illustrated an example of layers ti- U that may be implemented in the MTJ 402 of the circuit 400. The MTJ device 402 and the ME film 450 may be deposited on a suitable substrate (e.g., silicon or sapphire). In an example embodiment, the lateral dimension of each components may not exceed 1 micron in order to keep voltages relatively small across the ME film 450. The first magnetic layer may be a composite multi-layer of anti -ferromagnetic material (Ir ₂₂ Mn ₇₈ ) (ti, 300A) and an amorphous ferromagnetic material (Co4oFe4oBi5Si ₅ ) (ti, 50A) in order to fix the direction of Mi in the first magnetic layer. The insulating layer separating the two magnetic layers may be an oxide material (MgO) (t ₃ ~30A). The second magnetic layer may be composed of an amorphous ferromagnetic material (Co4oFe4oBi5Si ₅ ) (t ₄ ~ 1000A). The ME film 450, deposited adjacent or proximate to the second magnetic layer, may be a hexaferrite material (SrFe ₈ Ti ₂ Co ₂ Oi ₉ ) (~ 1000A, at a width of ~1 micron or less). A buffer layer of ITO may be deposited between substrate 490 and ME film 450. Alternative suitable buffer layers may include MgO, A1 ₂ 0 ₃ , MgAl ₂ 0 _4, or any other nonmagnetic spinel structures. Annealing the amorphous film at -300C (well below the growth conditions for ME films) may improve the quality of the amorphous magnetic layers and induces a uniaxial symmetry axis and low coercive field (well below the magneto-static field H) to approximately 0.5 Oe.

[0030] Figs. 5A-C illustrate a circuit 500 in a still further embodiment. Fig. 5A provides a lateral view of the physical layout of the circuit 500 as deposited as successive layers on a substrate 580. In particular, an MTJ cell 502 may be fabricated on top of an amorphous layer of Si0 ₂ on the substrate (e.g., silicon) 580, and an ME film 550 may be deposited above the MTJ cell 502, either directly or separated by a buffering layer. A right stack 535 includes electrodes (e.g., Au/Cr) to a source terminal 507 and a drain terminal 506, separated by layers of Si0 ₂ , and is adjacent to a gate terminal 508. The MTJ 502 may include layers as described above with reference to Figs. 1-4. The drain terminal 506 may connect to the first magnetic layer of the MTJ 502, while the source terminal 507 may connect to the second magnetic layer. Further, the gate 508 and ground 505 terminals may connect to opposite ends of the ME film 550, enabling application of a voltage across the ME film 550. By applying a voltage in the range of 100 mV to 1 V across the ME film 550, a magnetic field H _c is induced in the plane of the film. If the induced field is sufficiently large, it can be used as a coercive force to push the magnetization of the second magnetic layer in the opposite direction.

[0031] Such an operation is illustrated in Figs. 5B and 5B. In Fig. 5B, as a result of applying a negative voltage at the gate 508, the HE film 550 generates an H field 560 causing the second magnetic layer to exhibit a magnetization opposite that of the first magnetic layer. Accordingly, the resistance across the MTJ device 502 is at a maximum value (R.). In contrast, as shown in Fig. 5C, upon applying a positive voltage at the gate 508, the HE film 550 generates an H field 560 causing the second magnetic layer to exhibit a magnetization opposite that of the first magnetic layer. Accordingly, the resistance across the MTJ device 502 is at a maximum value (R ₊ ).

[0032] The circuit 500 provides particular advantages for low power applications. In an example embodiment, the TMR of the circuit may be 180%, with an ON current of approximately 600 μΑ. With a gate voltage of 100 mV, the power amplification at the output of the circuit 500 can be approximately 53X.

[0033] The single stack configuration of the MTJ 502 of the circuit 500 may simplify the fabrication process. However, the ME film 550 deposited using the furnace annealing process may exhibit large surface roughness, and, therefore, may benefit from a separation of ~5 μπι between the active device and metal terminals. In such a case, the minimum required width of the ME film would be approximately 12 μπι. However, if annealing is performed using a Rapid Thermal Processing (RTP), then the separation can be reduced to 2 μπι, thereby decreasing the dimensions of the ME film down to 6 μπι. An increase of 1 μπι in the width of the ME film 550 may result in a 1 V increase in the required gate voltage. Thus, minimizing the width of the ME film 550 is beneficial.

[0034] The fabrication process for the circuit 500 on the substrate 580 may include the following features. M-type hexaferrite SrCo ₂ Ti ₂ Fe80i9 targets may be prepared by ceramic processing; silicon wafers can be oxidized in a wet oxidation oven to obtain a uniform Si0 ₂ thickness of 500 nm for isolation. A conductive oxide layer (Indium-Tin Oxide or ITO) of 500 nm thickness can be deposited using Pulse Laser Deposition (PLD) with the substrate heated to 400 °C. The base pressure in the PLD deposition chamber is maintained at 9 ^χ 10 ^" Torr and the films can be deposited in a high purity oxygen environment of 10 mTorr; the repetition rate of the laser is set to 10 Hz. The ITO film serves as a buffer layer; the ITO not only reduces the lattice mismatch between the ME film and silicon, but it also lowers the required voltages to induce the ME effect on the Si substrate at room temperature. The SrCo ₂ Ti ₂ Fe80i9 film is then deposited on top of the ITO layer using PLD after heating the substrate to 600 °C. The repetition rate for the laser may also be gradually increased from 1 Hz to 10 Hz during deposition to improve the growth of ME film. Finally, the sample may be inserted into a heated furnace at 1050 °C to be annealed.

[0035] Using the process described above, the thickness of the ME film 550 may be approximately 1 μπι. Thin films for single crystal Fe-Si/MgO (001)/Fe-Si junctions can be prepared using molecular beam epitaxy (MBE). The top and bottom ferromagnetic Fe-Si electrodes can be grown up to lOOnm and the MgO barrier thickness up to 3.2nm (which can be epitaxially grown using e-beam evaporation of MgO source). A layer of antiferromagnetic material such as Ir ₂₂ Mn ₇₈ (10 nm) may be placed in contact with the top electrode to pin the electrode in one direction. A layer of Ru is then deposited as a capping layer that is used to increase the magnetore si stive (MR) ratio. The Ru layer also separates the pinned layer from the fixed ferromagnetic layer so resulting in a synthetic antiferromagnet (SAF). The strong exchange between the two magnetic layers in the SAF stack pins the magnetization of the fixed layer in one direction and also prevents it from switching upon application of an external magnetic field. The deposition of the films can be performed on much larger scale (e.g., a 2 cm x 2 cm substrate), and the MTJ junctions can then be patterned into smaller dimensions (e.g., 3 μπι x 2 μπι) using suitable microfabrication techniques.

[0036] In alternative embodiments, the MTJ cell 502 may be fabricated on top of the ME film 550, which is deposited on the amorphous layer of Si0 ₂ on the substrate 580. In such an embodiment, the order of layers of the MTJ 502 may be reversed, with the second magnetic layer at bottom and adjacent to the ME film 550.

[0037] Fig. 6 is a circuit diagram illustrating a circuit 600, equivalent to the circuit 500 of Figs. 5A-C. An external load resistance R _L may be connected in series with the circuit 600 such that the device 500 resembles a single transistor amplifier. The input power due to the voltage applied to the ME film using the electrode VGS is given by:

where RG is the resistance of the ME film. The voltage drop across the external load resistor RL may be denoted by ΔΥ; thus,

where V _D D is the supply voltage, R _P (RAP) denotes the MTJ resistance in the parallel (or antiparallel) orientation.

[0038] The change in output ower ΔΡο may be calculated as:

Because the power gain is given by G _p =—^- and assuming thati^ =— - = , the power

^R in 2 2

gain can be simplified for maximum ΔΡ as:

where TMR is the tunnel magnetore si stance ratio given 100% M rotation, VDD = 1 V and VGS ranges between 100 mV to 1 V depending on the film quality and properties of the deposited ME film.

[0039] The parallel and antiparallel resistance values may be a function of the oxide thicknesses; also, the tunneling current I _D s (from the drain to the source junction of the MTJ) can then be found using an equivalent circuit model. The TMR generally decreases with an increase in the DC bias voltage applied to the MTJ. This behavior may be due to the influence of the applied E field on the shape of the barrier and is an intrinsic effect of the MTJ (i.e., it is not due to the inelastic tunneling through a non-ideal interface and the barrier). An increase of the bias voltage increases the overall conductance of the junctions (due to randomization of the tunneling electron spins) and therefore it decreases the TMR. In this case, the figure of merit is the bias voltage required for reducing the maximum TMR by a factor of 2 (half-voltage or Vi _/2 ). As reported in, MTJ junctions using A1 ₂ 0 ₃ exhibit a TMR decrease to half of its zero bias value at roughly 400 mV; in MTJs with Fe-MgO-Fe tunnel junctions, this half-voltage is found to be 1250 mV for a positive bias. [0040] Example embodiments, such as the circuits 200, 400, 500 can provide a number of advantages in circuit applications. For example, the circuits may be fabricated without the use of semiconductor materials. The current flow through the circuits can be modulated via a relatively small voltage applied to the ME film to generate and control the H field, without the need for high power current generators or permanent magnets. A resulting circuit may exhibit properties comparable to those of a conventional semiconductor transistor.

[0041] The circuit 500 as described above, in particular, provides considerable advantages in terms of fabrication complexity, power gain, on/off currents and resistance range. The TMR for a MTJ junction by iron silicon alloy electrodes with 2.3 nm MgO is estimated to be 180%. For an example embodiment of the circuit 500, the ON current for the device (at a 1 V supply) may be approximately 600 μΑ and for a gate voltage of 100 mV across a 1 μπι thick ME film, the power gain at the output of the MTJ based transistor is nearly 53. Further power amplification may be achieved if the junction area is reduced below 3 μπι x 2 μπι.

[0042] While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.