HK1075321B - A method of switching a magnetoresistive memory device and magnetoresistive array - Google Patents

Description

method for switching magnetoresistive memory device and magnetoresistive array

Technical Field

The present invention relates to a semiconductor memory device.

More particularly, the present invention relates to a semiconductor random access memory device using a magnetic field.

Background

Non-volatile memory devices are extremely important components in electronic systems. FLASH memory (FLASH) is now the primary non-volatile memory device in applications. A typical nonvolatile memory device stores information using charges trapped in a floating oxide layer (floating oxide layer). Disadvantages of flash memory include high voltage requirements and long program and erase times. In addition, before memory failure, flash memory has 10⁴～10⁶Poor write duration of one cycle. Furthermore, to maintain reasonable data memory, the scaling (scaling) of the gate oxide is limited by the tunnel barrier encountered by the electrons. Thus, the size to which flash memories can be scaled is limited.

To overcome these drawbacks, research into magnetic memory devices has been conducted. One such device is a magnetoresistive RAM (hereinafter "MRAM"). However, for commercial implementation, MRAM must have comparable memory density to current memory technologies, be scalable to accommodate future upgrades, operate at low voltages, consume low power, and be competitive in read/write speeds.

For MRAM devices, the stability of the non-volatile memory state, the repeatability of the read/write cycles, and the consistency of the memory element-to-element switching field are the three most important aspects of their design characteristics. The memory state in an MRAM is not maintained by electrical energy, but by the direction of the magnetic moment vector. Storing data is accomplished by applying magnetic fields and magnetizing magnetic material in an MRAM device into one of two possible memory states. Data is recovered by detecting the difference in resistance between the two states in an MRAM device. The magnetic field for writing is generated by passing a current through a stripline (trip lines) outside the magnetic structure or the magnetic structure itself.

As the lateral dimensions of MRAM devices decrease, three problems arise. First, for a given shape and film thickness, the switching field increases, requiring a larger magnetic field to switch. Second, the total switching volume is reduced, so that the energy barrier for inversion is reduced. The energy barrier represents the amount of energy required to switch the magnetic moment vector from one state to another. The energy barrier determines the data retention and error rate of the MRAM device and if the barrier is too small, unwanted inversion occurs due to thermal fluctuations (superparamagnetism). The main problem with a small energy barrier is that it is extremely difficult to selectively switch one MRAM device in an array. The switching may be selectively enabled without inadvertently switching other MRAM devices. Finally, because the switching field is generated by shape, the switching field becomes more sensitive to shape changes as MRAM device sizes decrease. As lithographic scaling becomes more difficult at smaller scales, MRAM devices will have difficulty maintaining compact switching profiles.

It would therefore be advantageous to overcome the above-mentioned deficiencies, as well as others inherent in the prior art.

It is therefore an object of the present invention to provide a new and improved method of writing to a magnetoresistive random access memory device.

It is another object of the present invention to provide a new and improved method of writing to a magnetoresistive random access memory device that is highly selectable.

It is a further object of the present invention to provide a new and improved method for a magnetoresistive random access memory device with improved write error rate.

It is a further object of the present invention to provide a preferred new and improved method of writing to a magnetoresistive random access memory device having a switching field that is less shape dependent.

Disclosure of Invention

To achieve the above objects and other objects and advantages, a method of writing to a scalable magnetoresistive memory array is disclosed. The memory array includes a plurality of scalable magnetoresistive memory devices. For simplicity, the present invention will examine how the writing method can be applied to a single MRAM device, although it should be understood that the writing method can be applied to any number of MRAM devices.

The MRAM device used to illustrate the writing method includes a word line and a digit line located adjacent to a magnetoresistive memory element. The magnetoresistive memory element includes pinned magnetic regions (pinned magnetic regions) adjacent to the digit line locations. The tunnel barrier is disposed on the pinned magnetic region. A free magnetic region is then disposed over the tunnel barrier and adjacent to the word line. In a preferred embodiment, the resultant magnetic moment vector of the pinned magnetic region is fixed in an optimal direction. Furthermore, in a preferred embodiment, the free magnetic region comprises synthetic antiferromagnetic (hereinafter "SAF") layer material. The synthetic antiferromagnetic layer material includes N antiferromagnetically coupled ferromagnetic material layers, where the total N is greater than or equal to 2. The N layers determine the magnetic switching volume, which can be adjusted by changing N. In a preferred embodiment, the N ferromagnetic layers are antiferromagnetically coupled by sandwiching an antiferromagnetically coupling spacer layer between each adjacent ferromagnetic layer. In addition, each N layer has a magnetic moment adjusted to provide an optimal write mode.

In a preferred embodiment, N is equal to 2, such that the synthetic antiferromagnetic layer material is a trilayer structure of ferromagnetic layer/antiferromagnetic coupling spacer layer/ferromagnetic layer. Two ferromagnetic layers in the three-layer structure respectively have magnetic moment vectors M₁And M₂And the magnetic moment vectors are oriented antiparallel, typically by coupling of an antiferromagnetically coupled spacer layer. The magnetostatic fields of the layers in the MRAM architecture also produce antiferromagnetic coupling. Thus, the spacer layer does not have to provide any additional antiferromagnetic coupling in addition to eliminating the ferromagnetic coupling between the two magnetic layers. May be found in the book entitled "magnetic resonance Random Access memory for Improved ScalabilityMore information on MRAM devices used to illustrate the writing method is found in co-pending U.S. patent application filed on even date herewith, which is incorporated by reference.

The magnetic moment vectors of the two ferromagnetic layers in an MRAM device may have different thicknesses or materials to provide a magnetic field formed by (M) M₂-M₁) The resulting moment vector and the fractional balance ratio of the sub-layer moment (sub-layer moment fractional balance ratio) are givenThe resultant moment vector of the tri-layer structure is free to rotate with the applied magnetic field. In zero field, the direction of the resultant moment vector will be stable, determined by the magnetic anisotropy, i.e., parallel or anti-parallel to the resultant moment vector of the pinned reference layer. It should be understood that the term "resultant magnetic moment vector" is used for purposes of this description only, and that considering the case of fully balanced magnetic moments, the resultant magnetic moment vector may be zero in the absence of a magnetic field. As described below, only the magnetic moment vectors of the sublayers adjacent to the tunnel barrier determine the state of the memory.

The current flowing through the MRAM device depends on the tunneling magnetoresistance, which is determined by the relative orientation of the magnetic moment vectors of the free and pinned layers directly adjacent to the tunneling barrier. If the moment vectors are parallel, the MRAM device resistance is lower and the bias voltage will cause a greater current to flow through the device. This state is defined as "1". If the moment vectors are anti-parallel, the MRAM device resistance is higher and the applied bias voltage will result in a smaller current flowing through the device. This state is defined as "0". It should be understood that these definitions are arbitrary and vice versa, although the definitions used in this example are for illustration purposes. Thus, in a magnetoresistive memory, data storage is achieved by applying a magnetic field to orient the moment vector in the MRAM device in either a parallel or anti-parallel direction relative to the moment vector in the pinned reference layer.

Methods of writing scalable MRAM devices rely on near balanced (nellybalanced) SAF tri-layer junctionsThe spin-flop (spin-flop) phenomenon of the structure. The term "near-equilibrium" is defined herein as the magnitude of the fractional balance ratio of the magnetic moments of the sublayers in the range 0 ≦ M_brThe | is less than or equal to 0.1. The spin-flop phenomenon reduces the total magnetic energy of the applied field by rotating the moment vectors of the ferromagnetic layers so that they are orthogonal to the nominal (nominally) direction of the applied field, but are still predominantly anti-parallel to each other. Rotation or flipping of the individual ferromagnetic moment vectors in the direction of the applied field with a smaller deflection results in a reduction of the total magnetic energy.

In general, with the flipping phenomenon and timing pulse sequences, two distinct modes can be used to write to an MRAM device; i.e. a direct write mode or a toggle write (toggle) mode. These modes are implemented using the same sequence of timing pulses, as will be described below, but differ in the choice of magnetic sub-layer moment and the magnitude and polarity of the applied magnetic field.

Each writing method has its own advantages. For example, when using the direct write mode, if the state being written is different from the stored state, there is no need to determine the initial state of the MRAM device since only the state is switched. Although the direct write method does not require knowledge of the state of the MRAM device before initiating the write sequence, it requires changing the polarity of the word and digit lines depending on the desired state.

When using the toggle writing method, the initial state of the MRAM device needs to be determined before writing, since the state will be switched each time the same polarity pulse sequence is generated from the word and word lines. Thus, the toggle write mode is enabled by reading the stored memory state and comparing that state to the new state to be written. After the comparison, the MRAM device is written to only if the stored state is different from the new state.

MRAM devices are constructed such that the magnetic anisotropy axis is ideally at a 45 deg. angle to the digit and digit lines. Thus, at time t₀Magnetic moment vector M₁And M₂Oriented in the optimum direction, with the direction of the word and digit linesForming an angle of 45 degrees. As an example of a writing method, to switch the state of the MRAM device using direct or toggle writing, the following current pulse sequence is used. At time t₁Increasing word current, M₁And M₂Start rotating in either a clockwise or counterclockwise direction depending on the direction of the word current, so that they are aligned nominally orthogonal to the field direction due to the spin-flip effect. At time t₂The digital current is switched on. The direction of flow of the digital current is such that M₁And M₂Further rotation in the same direction as that induced by the digit line magnetic field. At this time, the word line current and digit line current are turned on, and M₁And M₂Nominally orthogonal to the net magnetic field direction, at 45 ° to the current line.

It is important to understand that when only one current is switched on, the magnetic field will cause M to flow₁And M₂Nominally aligned in a direction parallel to the word lines or digit lines. However, if two currents are switched on, M₁And M₂Nominally aligned orthogonally at a 45 angle to the word and digit lines.

At time t₃The word line current is switched off so that M is rotated only by the digit line magnetic field₁And M₂. At this time, M is usually present₁And M₂Has been rotated past its unstable point of hard-axis (hard-axis). At time t₄Cutting off the digital current, M₁And M₂Aligned along the optimum anisotropy axis. At this time, M₁And M₂Has been rotated 180 deg., and the MRAM device has been switched. Thus, by sequentially switching word and digit currents on and off, M of the MRAM device can be made₁And M₂Rotated 180 deg. to switch the state of the device.

Drawings

The foregoing and other more specific objects and advantages of the present invention will be readily apparent to those skilled in the art from the following detailed description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings of which:

FIG. 1 is a simplified cross-sectional view showing a magnetoresistive random access memory device;

FIG. 2 is a simplified plan view of a magnetoresistive random access memory device having word and word lines;

FIG. 3 is a graph that simulates the combination of magnetic field magnitudes that produce a direct or toggle write mode in a magnetoresistive random access memory device;

FIG. 4 shows a timing diagram of the word current and the digit current when both are turned on;

FIG. 5 shows the rotation of the magnetic moment vectors of a magnetoresistive random access memory device when writing a '1' to a '0' for a toggle write mode;

FIG. 6 shows the rotation of the magnetic moment vectors of a magnetoresistive random access memory device when writing a '0' to a '1' for a toggle write mode;

FIG. 7 illustrates the rotation of the magnetic moment vectors of a magnetoresistive random access memory device when writing a '1' to a '0' for the direct write mode;

FIG. 8 shows the rotation of the magnetic moment vectors of a magnetoresistive random access memory device when writing a '0' to a state that has been '0' for the direct write mode;

FIG. 9 shows a timing diagram of the word current and the digit current when only the digit current is turned on;

FIG. 10 illustrates the rotation of the magnetic moment vectors of a magnetoresistive random access memory device when only a digital current is turned on.

Detailed Description

Referring now to fig. 1, fig. 1 shows a simplified cross-sectional view of an MRAM array 3 according to the present invention. In this figure only a single magnetoresistive memory device 10 is shown, but it will be appreciated that the MRAM array 3 comprises a plurality of MRAM devices 10, only one such device being shown here to simplify the description of the writing method.

MRAM device 10 is sandwiched between word line 20 and digit line 30. Word line 20 and digit line 30 comprise conductive materials that are capable of passing a current. In this figure, word line 20 is at the top of MRAM device 10 and digit line 30 is at the bottom of MRAM device 10 and is oriented at a 90 angle to word line 20 (see FIG. 2).

MRAM device 10 includes a first magnetic region 15, a tunnel barrier 16, and a second magnetic region 17, where tunnel barrier 16 is sandwiched between first magnetic region 15 and second magnetic region 17. In a preferred embodiment, the magnetic region 15 comprises a tri-layer structure 18 having an anti-ferromagnetic coupling spacer layer 65 sandwiched between two ferromagnetic layers 45 and 55. The anti-ferromagnetic coupling spacer layer 65 has a thickness 86 and the ferromagnetic layers 45 and 55 have thicknesses 41 and 51, respectively. In addition, magnetic region 17 has a tri-layer structure 19 with an anti-ferromagnetic coupling spacer layer 66 sandwiched between two ferromagnetic layers 46 and 56. The anti-ferromagnetic coupling spacer layer 66 has a thickness 87 and the ferromagnetic layers 46 and 56 have thicknesses 42 and 52, respectively.

Typically, the antiferromagnetically-coupling spacer layers 65 and 66 include at least one of the elements Ru, Os, Re, Cr, Rh, Cu, or combinations thereof. In addition, the ferromagnetic layers 45, 55, 46, and 56 include at least one of the elements Ni, Fe, Mn, Co, or combinations thereof. And it should be understood that magnetic regions 15 and 17 may comprise other synthetic antiferromagnetic layer material structures in addition to the tri-layer structure, which is used in this embodiment for illustrative purposes only. For example, one such synthetic antiferromagnetic layer material structure may include a five-layer stack of a ferromagnetic layer/antiferromagnetic coupling spacer layer/ferromagnetic layer structure.

Ferromagnetic layers 45 and 55 each have a magnetic moment vector 57 and 53, with magnetic moment vectors 57 and 53 held anti-parallel, typically by coupling through an anti-ferromagnetic coupling spacer layer 65. In addition, magnetic region 15 has a resultant magnetic moment vector 40 and magnetic region 17 has a resultant magnetic moment vector 50. Resultant magnetic moment vectors 40 and 50 are both oriented along an anisotropy easy-axis (easy-axis) at an angle, preferably 45 degrees, with respect to word line 20 and digit line 30 (see fig. 2). In addition, magnetic region 15 is a free ferromagnetic region, meaning that resultant magnetic moment vector 40 is free to rotate under the application of a magnetic field. Magnetic region 17 is a pinned ferromagnetic region, meaning that resultant magnetic moment vector 50 is not free to rotate with the appropriate application of a magnetic field and serves as a reference layer.

Although an antiferromagnetic coupling layer is shown between two ferromagnetic layers in each tri-layer structure 18, it should be understood that the ferromagnetic layers may be antiferromagnetically coupled by other means, such as a magnetostatic field or other properties. For example, when the aspect ratio of the cell is reduced to 5 or less, ferromagnetic layers are coupled antiparallel by a magnetostatic flux closure (magnetostatic flux closure).

In a preferred embodiment, MRAM device 10 has a tri-layer structure 18 with a length/width ratio in the range of 1 to 5 for non-circular planes. However, the plane of the present description is circular (see fig. 2). In a preferred embodiment, MRAM device 10 is circular in shape to minimize the effect on the switching field due to shape anisotropy, and also because the device is more easily scaled laterally to smaller dimensions using photolithographic processing. It should be understood, however, that MRAM device 10 may have other shapes, such as square, oval, rectangular, or diamond, but the circular shape is shown for simplicity and to improve performance.

Furthermore, during fabrication of MRAM array 3, the various successive layers (i.e., 30, 55, 65, etc.) are sequentially deposited or otherwise formed, and each MRAM device 10 may be defined by selective deposition, photolithographic processing, etching, etc. by any of the techniques known in the semiconductor fabrication art. During deposition of at least the ferromagnetic layers 45 and 55, a magnetic field is provided to set the optimum easy axis (induced anisotropy) for this bilayer. The magnetic field provided produces an optimum anisotropy axis for moment vectors 53 and 57. The best axis is selected to be at a 45 angle between word line 20 and digit line 30, as will be described below.

Referring now to fig. 2, fig. 2 shows a simplified plan view of an MRAM array 3 according to the present invention. To simplify the description of MRAM device 10, all directions will be referenced to the illustrated x-and y-coordinate system 100 as well as clockwise rotation direction 94 and counter-clockwise rotation direction 96. To further simplify the description, assume again that N is equal to 2, such that MRAM device 10 includes a tri-layer structure in region 15 having magnetic moment vectors 53 and 57 and resultant magnetic moment vector 40. In addition, only the magnetic moment vector of region 15 is shown, as the magnetic moment vector of region 15 will be switched.

To illustrate how the writing method works, it is assumed that the directions of the optimal anisotropy axes of magnetic moment vectors 53 and 57 are at 45 ° angles with respect to the negative x-and negative y-directions and at 45 ° angles with respect to the positive x-and positive y-directions. By way of example, the direction of the moment vector 53 shown in FIG. 2 is at a 45 angle relative to the negative x-and negative y-directions. Since moment vector 57 is generally oriented anti-parallel to moment vector 53, its direction is at a 45 angle to the positive x-and positive y-directions. This initial orientation will be used to illustrate an example of a writing method, which will be described below.

In the preferred embodiment, word current 60 is defined as positive if flowing in the positive x-direction and digital current 70 is defined as positive if flowing in the positive y-direction. The purpose of word line 20 and digit line 30 is to generate a magnetic field within MRAM device 10. The positive word current 60 will result in a toroidal word magnetic field H_W80, a positive digital current 70 will result in a toroidal digital magnetic field H_D90. Since word line 20 is above MRAM device 10, in the plane of the element, for a positive word current 60, H will be driven in the positive y-direction_W80 to MRAM device 10. Also, since digit line 30 is below MRAM device 10, in the plane of the element, for a positive digit current 70, H will be in the positive x-direction_D90 is applied to MRAM device 10. It should be understood that the definitions for positive and negative currents are arbitrary and are defined herein for illustrative purposes. The effect of reversing the current flow is to cause the direction of the magnetic field induced in MRAM device 10 to change. The behavior of the magnetic field caused by the current is well known to the person skilled in the art and will not be described in detail here.

Referring now to fig. 3, fig. 3 simulates the switching behavior of an SAF tri-layer structure. The simulation includes two single domain magnetic layers having approximately the same magnetic moment with intrinsic anisotropy (nearly balanced SAF), anti-ferromagnetically coupled, and whose magnetization dynamics are described by the Landau-Lifshitz equation. The x-axis is the word line magnetic field amplitude in Oersted and the y-axis is the digit line magnetic field amplitude in Oersted. The magnetic field is applied in a pulse sequence 100 as shown in fig. 4, wherein the pulse sequence 100 comprises a word current 60 and a digital current 70 as a function of time.

As shown in fig. 3, there are three operating regions. There is no switching in region 92. For MRAM operation in region 95, the direct write method is effective. When using the direct write method, there is no need to determine the initial state of the MRAM device, since the state is only switched if the state being written is different from the stored state. The selection of the written state is determined by the direction of current flow in both word line 20 and digit line 30. For example, if a '1' is to be written, the direction of current flow in both lines will be positive. If a '1' has been stored in the element and a write of a '1' is made, the final state of the MRAM device will continue to be a '1'. Further, if '0' is stored and writing of '1' is performed with a positive current; the final state of the MRAM device will be '1'. A similar effect is obtained when writing a '0' by using a negative current in both the word and word lines. Thus, regardless of its initial state, either state can be programmed to the desired '1' or '0' with the appropriate polarity of the current pulse. In the present description, the operation in the area 95 is defined as "direct write mode".

The toggle writing method is effective for MRAM operation in region 97. When using the toggle writing method, the initial state of the MRAM device needs to be determined before writing, since the state is switched each time the MRAM device is written to, regardless of the current direction, as long as current pulses of the same polarity are selected for both word line 20 and digit line 30. For example, if a '1' is initially stored, the state of the device will be switched to '0' after a positive current pulse sequence has flowed through the word and digit lines. Repeating the positive current pulse sequence on the stored '0' state will write it back to '1'. Thus, to be able to write a memory element to a desired state, the initial state of MRAM device 10 must first be read and compared to the state to be written. Sensing and comparing may require additional logic circuitry including buffers for storing information and comparators for comparing memory states. Thus, MRAM device 10 is only written to when the stored state is different from the state to be written. One advantage of this approach is that power consumption is reduced since only different bits are switched. Another advantage of using the toggle writing method is that only a unipolar voltage is required, so that a smaller N-channel transistor can be used to drive the MRAM device. In this description, the operation in the area 97 is defined as "toggle writing mode".

Both writing methods involve providing currents in word line 20 and digit line 30 that cause magnetic moment vectors 53 and 57 to be oriented in one of two optimal directions as previously described. To fully illustrate these two switching modes, a specific example describing the variation of the moment vectors 53, 57 and 40 over time is now given.

Referring now to fig. 5, fig. 5 illustrates a toggle write mode for writing a '1' to a '0' using pulse sequence 100. At time t in the figure₀With moment vectors 53 and 57 oriented in the optimal direction shown in fig. 2. This orientation will be defined as '1'.

At time t₁Turning on a positive word current 60 which causes H_w80 point in the positive y-direction. Positive H_w80 will cause the nearly balanced, inversely aligned MRAM trilayer to "flip" (FLOP) and become oriented at approximately 90 deg. to the applied magnetic field direction. The limited antiferromagnetic exchange interaction between ferromagnetic layers 45 and 55 will allow magnetic moment vectors 53 and 57 to momentarily deflect a small angle toward the magnetic field direction, and resultant magnetic moment vector 40 will bisect the angle between magnetic moment vectors 53 and 57 and will intersect H_w80 are aligned. Thus, moment vector 53 rotates in clockwise direction 94. Since resultant magnetic moment vector 40 is the vector sum of moment vectors 53 and 57, moment vector 57 also rotates in clockwise direction 94.

At time t₂Turning on a positive digital current 70, which results in a positive H_D90. Resultant magnetic moment vector 40 is thus simultaneously represented by H_w80 points in the positive y-direction from H_D90 refers toToward the positive x-direction, the effect is to rotate effective moment vector 40 further in clockwise direction 94 until it is generally oriented at a 45 angle between the positive x-and positive y-directions. Thus, moment vectors 53 and 57 also rotate further in clockwise direction 94.

At time t₃Word current 60 is turned off so that only H is present at this time_D90 direct resultant magnetic moment vector 40 when resultant magnetic moment vector 40 will be oriented in the positive x-direction. Both moment vectors 53 and 57 will now generally be oriented at an angle that passes through the point of instability of their anisotropy axis.

At time t₄Digital current 70 is turned off and thus the magnetic field force does not act on resultant magnetic moment vector 40. Thus, moment vectors 53 and 57 will be oriented in their closest optimal directions to minimize the anisotropy energy. In this case, the optimal direction for moment vector 53 is at a 45 angle relative to the positive y-direction and the positive x-direction. The optimal direction also corresponds to the time t₀The initial direction of moment vector 53 is at an angle of 180 deg. and is defined as '0'. Consequently, MRAM device 10 has been switched to a '0'. It should be understood that MRAM device 10 may also be switched by rotating moment vectors 53, 57, and 40 in counterclockwise direction 96 using negative currents in both word line 20 and digit line 30, although shown for illustrative purposes only.

Turning now to fig. 6, fig. 6 illustrates a toggle write mode for writing a '0' to a '1' using pulse sequence 100. Shown at respective times t₀，t₁，t₂，t₃And t₄Time moment vectors 53 and 57, and resultant magnetic moment vector 40, as previously described, represent the ability to switch MRAM device 10 from a '0' to a '1' state using the same current and magnetic field directions. The state of MRAM device 10 is thus written to by toggling the write mode, which corresponds to region 97 in fig. 3.

For the direct write mode, given that the magnitude of moment vector 53 is greater than moment vector 57, moment vector 40 points in the same direction as moment vector 53, but has a smaller magnitude in the zero field. This unbalanced moment causes the dipole energy, which tends to align the total moment with the applied field, to break the symmetry of the nearly balanced SAF. Thus, for a given current polarity, switching can occur in only one direction.

Referring now to fig. 7, fig. 7 shows an example of writing a '1' to a '0' using a pulse sequence 100 using a direct write mode. Here again, the memory state is initially '1', with the moment vector 53 pointing at 45 to the negative x-and negative y-directions and the moment vector 57 pointing at 45 to the positive x-and positive y-directions. Writing is performed in a similar manner as the toggle write mode described above, with the pulse sequence described above with positive word current 60 and positive digital current 70. Note that at time t₁The magnetic moment is again "flipped", but the final angle deviates by 90 ° due to the unbalanced magnetic moment and anisotropy. At time t₄MRAM device 10 has then been switched to the '0' state, and resultant magnetic moment 40 is oriented at the desired 45 ° angle to the positive x-and positive y-directions. Similar results are obtained when writing a '0' to a '1', except that negative word current 60 and negative word current 70 are used.

Referring now to fig. 8, fig. 8 shows an example of writing using the direct write mode when the new state is the same as the stored state. In this example, a '0' has been stored in MRAM device 10, and current pulse sequence 100 is now repeated to store a '0'. At time t₁Magnetic moment vectors 53 and 57 attempt to "flip," but act to attenuate the rotation because the unbalanced magnetic moment must oppose the applied field. Thus, there is an additional energy barrier to rotate out of the opposite state. At time t₂The main moment 53 is nearly aligned with the positive x-axis, making an angle of less than 45 ° with its initial anisotropy direction. At time t₃The magnetic field is oriented along the positive x-axis. Instead of rotating further clockwise, the system now lowers its energy by changing the SAF magnetic moment symmetrically with respect to the applied field. The passive moment 57 crosses the x-axis and the main moment 53 returns to near its original orientation and the system is stabilized. Thus, at time t₄When the magnetic field is removed, the state stored in MRAM device 10 will remain a '0'. The sequence illustratesThe mechanism of the direct write mode is shown in fig. 3 as area 95. Therefore, in general, to write '0', a positive current needs to flow in both the word line 60 and the digit line 70, and conversely, to write '1', a negative current needs to flow in both the word line 60 and the digit line 70.

If a larger field is applied, eventually, the energy decreases in relation to the switching, and the shear value (scissor) exceeds the additional energy barrier created by the dipole energy of the unbalanced moment that is blocking the occurrence of the triggering event. At this point, a triggering event will occur and the switch is as depicted in area 97.

The region 95 where the direct write mode is applied can be expanded, i.e. the toggle mode region 97 can be moved to a higher magnetic field if at time t₃And t₄Identical or as close to identical as possible. In this case, when word current 60 is turned on, the magnetic field direction is initially 45 ° with respect to the bit anisotropy axis, and then, when digit current 70 is turned on, it moves to a direction parallel to the bit anisotropy axis. This example is similar to a typical magnetic field application sequence. However, here the word current 60 and the word current 70 are switched off substantially simultaneously, so that the magnetic field direction no longer rotates any more. Therefore, the applied field must be large enough so that by turning on word current 60 and digital current 70, resultant magnetic moment vector 40 has been moved past its point of instability of the magnetization-hard axis. Now, since the magnetic field direction is rotated only 45 °, instead of the previous 90 °, the toggle write mode event is less likely to occur. With substantially uniform decay time t₃And t₄Has the advantage of a field rise time t₁And t₂Without any additional limitations. Thus, the magnetic fields may be switched on in any order, or may also be made substantially uniform.

Since only at time t₂And time t₃The MRAM device between which word current 60 and word current 70 are turned on will switch states, and thus the foregoing writing method is highly selective. This characteristic is illustrated in fig. 9 and 10. Fig. 9 shows pulse sequence 100 when word current 60 is turned off and digital current 70 is turned on. FIG. 10 shows the states of MRAM device 10The corresponding behavior of (c). At time t₀Magnetic moment vectors 53 and 57 and resultant magnetic moment vector 40 are oriented as shown in FIG. 2. In the pulse train 100, at time t₁Digital current 70 is turned on. At this time, H_D90 will cause resultant magnetic moment vector 40 to point in the positive x-direction.

Because word current 60 never turns on, resultant magnetic moment vectors 53 and 57 never rotate past their anisotropy hard-axis instability points. As a result, when digital current 70 is turned off at time t3, moment vectors 53 and 57 will reorient themselves in the closest optimal direction, in this case time t₀In the initial direction. Thus, the state of MRAM device 10 is not switched. It should be appreciated that the same result would occur if word current 60 were turned on at a similar time as described above and digital current 70 were not turned on. This feature ensures that only one MRAM device in the array is switched while the other devices retain their original state. Thus, unintentional switching is avoided and the bit error rate is minimized.

Many variations and modifications of the embodiments described herein will be apparent to those skilled in the art. Such changes and modifications are intended to be included within the scope of this invention and assessed only by a fair interpretation of the following claims without departing from the spirit thereof.

The present invention has been fully described above in such clear and concise terms as to enable those skilled in the art to understand and practice the invention as recited in the claims:

Claims (10)

1. A method of switching a magnetoresistive memory device, comprising the steps of:

providing a magnetoresistive memory element adjacent to a first conductor and a second conductor, wherein the magnetoresistive memory element comprises a first magnetic region and a second magnetic region separated by a tunnel barrier, at least one of the first magnetic region and the second magnetic region comprising N antiferromagnetically coupled layers of ferromagnetic material, wherein N is an integer at least equal to 2, and wherein each layer has a magnetic moment adjusted to provide a write mode, and each of the first and second magnetic regions has a magnetic moment vector adjacent to the tunnel barrier and oriented in an initial optimal direction at time t0, and wherein the initial optimal direction is at an angle of about 45 degrees with respect to the first conductor and the second conductor;

at time t₁Switching on a first current flowing through the first conductor;

at time t₂Switching on a second current flowing through the second conductor;

at time t₃Cutting off a first current flowing through the first conductor; and

at time t₄Cutting off the second current flowing through the second conductor, wherein the time t is set₀、t₁、t₂、t₃And t₄Is t₀＜t₁＜t₂＜t₃＜t₄And one of the moment vectors proximate the tunnel barrier is oriented in a direction different from the initial optimal direction, and wherein the current in each of the first and second conductors is pulsed with the same polarity to write a state and the current in each of the first and second conductors is pulsed with the same polarity to invert the state.

2. The method of claim 1, wherein:

the first magnetic region is a pinned magnetic region and the second magnetic region is a free magnetic region, the free magnetic region including N layers of antiferromagnetically coupled ferromagnetic material, the N layers defining a volume, and wherein a fractional balance ratio of magnetic moments of sublayers of one of the first and second magnetic regions is about 0 ≦ M_brThe | is less than or equal to 0.1.

3. A magnetoresistive array, comprising:

a first conductor;

a second conductor;

a magnetoresistive device adjacent to the first and second conductors, the magnetoresistive device comprising:

a first magnetic region;

a tunnel barrier; and

a second magnetic region separated from the first magnetic region by a tunnel barrier, the second magnetic region comprising a multilayer structureThe structure has a magnetic moment oriented along an anisotropy easy axis at an angle of about 45 degrees to the first and second conductors, and the second magnetic region has a magnetic moment fractional balance ratio (M)_br) In an amount of about 0 ≦ M_brThe | is less than or equal to 0.1.

4. A magnetoresistive array as claimed in claim 3 wherein information is stored by: the bit location to be written is switched from the first logic state to the second logic state with a first current having a first polarity on the first conductor and a second current having the first polarity on the second conductor, and is switched from the second logic state to the first logic state with a third current having the first polarity on the first conductor and a fourth current having the first polarity on the second conductor, wherein the first conductor is a digit line and the second conductor is a word line.

5. The magnetoresistive array of claim 4 further comprising circuitry to read an initial state of the magnetoresistive array and compare the initial state to a new state to be stored in the magnetoresistive array, and program the magnetoresistive array in response to the comparison only if the new state is different from the initial state.

6. The magnetoresistive array of claim 3 wherein:

the first conductor is a word line;

the second conductor is a digit line;

the magnetoresistive device is adjacent to the digit line and the word line;

the first magnetic region is a pinned magnetic region;

the second magnetic region is a free magnetic region;

the multilayer structure includes N antiferromagnetically coupled ferromagnetic material layers, where N is an integer greater than or equal to 2, and where the N ferromagnetic material layers define a volume, each of the N ferromagnetic material layers having a magnetic moment that is adjusted to provide a write mode, and where a sublayer magnetic moment fraction of one of the free and pinned magnetic regions is flatThe balance ratio is about 0 ≦ M_br| ≦ 0.1, and the free magnetic region has a near tunnel barrier and an edge at t₀A magnetic moment vector of the initial optimal directional orientation of the moment; and is

The magnetoresistive array is configured to store information by: at time t₁Providing a word line current pulse to the word line and at time t₃Cutting off the word line current pulse and at time t₂In addition, a digit line current pulse is supplied to the digit line and at time t₄Cutting off the digit line current pulse, where t₀＜t₁＜t₂＜t₃＜t₄This results in that at time t₄The free magnetic region near the tunnel barrier has a magnetic moment vector oriented differently than the initial optimal direction.

7. The magnetoresistive array of claim 3 wherein:

the first conductor is a word line;

the second conductor is a digit line; and is

The magnetoresistive device is sandwiched between a word line and a digit line.

8. The magnetoresistive array of claim 3, wherein the magnetic moment is a resultant magnetic moment of a plurality of magnetic moments of the multilayer structure.

9. The magnetoresistive array of claim 3 wherein the multilayer structure is a synthetic antiferromagnetic layer material.

10. The magnetoresistive array of claim 3 wherein the multilayer structure comprises N antiferromagnetically coupled ferromagnetic material layers, where N is an integer greater than or equal to 2.