Patent Application
20190207092
The present invention relates to magnetic random access memory (MRAM) and more particularly to a perpendicular magnetic tunnel junction element having a reference layer that incorporates a layer of Hf for increased perpendicular magnetic anisotropy (PMA) and improved interlayer coupling (Hin).
Magnetic Random Access Memory (MRAM) is a non-volatile data memory technology that stores data using magnetoresistive cells such as Magnetoresistive Tunnel Junction (MTJ) cells. At their most basic level, such MTJ elements include first and second magnetic layers that are separated by a thin, non-magnetic layer such as a tunnel barrier layer, which can be constructed of a material such as Mg—O. The first magnetic layer, which can be referred to as a reference layer, has a magnetization that is fixed in a direction that is perpendicular to that plane of the layer. The second magnetic layer, which can be referred to as a magnetic free layer, has a magnetization that is free to move so that it can be oriented in either of two directions that are both generally perpendicular to the plane of the magnetic free layer. Therefore, the magnetization of the free layer can be either parallel with the magnetization of the reference layer or anti-parallel with the direction of the reference layer (i.e. opposite to the direction of the reference layer).
The electrical resistance through the MTJ element in a direction perpendicular to the planes of the layers changes with the relative orientations of the magnetizations of the magnetic reference layer and magnetic free layer. When the magnetization of the magnetic free layer is oriented in the same direction as the magnetization of the magnetic reference layer, the electrical resistance through the MTJ element is at its lowest electrical resistance state. Conversely, when the magnetization of the magnetic free layer is in a direction that is opposite to that of the magnetic reference layer, the electrical resistance across the MTJ element is at its highest electrical resistance state.
The switching of the MTJ element between high and low resistance states results from electron spin transfer. An electron has a spin orientation. Generally, electrons flowing through a conductive material have random spin orientations with no net spin orientation. However, when electrons flow through a magnetized layer, the spin orientations of the electrons become aligned so that there is a net aligned orientation of electrons flowing through the magnetic layer, and the orientation of this alignment is dependent on the orientation of the magnetization of the magnetic layer through which they travel. When, the orientations of the magnetizations of the free and reference layer are oriented in the same direction, the spin of the electrons in the free layer are in generally the same direction as the orientation of the spin of the electrons in the reference layer. Because these electron spins are in generally the same direction, the electrons can pass relatively easily through the tunnel barrier layer. However, if the orientations of the magnetizations of the free and reference layers are opposite to one another, the spin of electrons in the free layer will be generally opposite to the spin of electrons in the reference layer. In this case, electrons cannot easily pass through the barrier layer, resulting in a higher electrical resistance through the MTJ stack.
Because the MTJ element can be switched between low and high electrical resistance states, it can be used as a memory element to store a bit of data. For example, the low resistance state can be read as an on or “1”, whereas the high resistance state can be read as a “0”. In addition, because the magnetic orientation of the magnetic free layer remains in its switched orientation without any electrical power to the element, it provides a robust, non-volatile data memory bit.
To write a bit of data to the MTJ cell, the magnetic orientation of the magnetic free layer can be switched from a first direction to a second direction that is 180 degrees from the first direction. This can be accomplished, for example, by applying a current through the MTJ element in a direction that is perpendicular to the planes of the layers of the MTJ element. An electrical current applied in one direction will switch the magnetization of the free layer to a first orientation, whereas an electrical current applied in a second direction will switch the magnetic of the free layer to a second, opposite orientation. Once the magnetization of the free layer has been switched by the current, the state of the MTJ element can be read by reading a voltage across the MTJ element, thereby determining whether the MTJ element is in a “1” or “0” bit state. Advantageously, once the switching electrical current has been removed, the magnetic state of the free layer will remain in the switched orientation until such time as another electrical current is applied to again switch the MTJ element. Therefore, the recorded date bit is non-volatile in that it remains intact in the absence of any electrical power.
The present invention provides a magnetic random access memory element that includes a magnetic reference layer, a magnetic free layer and a non-magnetic barrier layer located between the magnetic reference layer and the magnetic free layer. The magnetic reference layer includes at least one magnetic layer and a layer of Hf.
The layer of Hf in the magnetic reference layer advantageously increases the perpendicular magnetic anisotropy (PMA) and lowers the Hin of the magnetic reference to provide improved resistance to loss of magnetization of the magnetic reference layer. This increases reliability and thermal stability of the magnetic memory element.
High speed data recording requires high write currents to flip the magnetic state of the free layer when writing data to the magnetic data recording element. These high electrical currents can cause instability in the reference layer which in turn will create write errors of the data. The increased PMA and lowered Hin afforded by the presence of the Hf layer in the magnetic reference layer prevents such loss of magnetic stability of the magnetic reference layer, even at such high write current operation conditions.
The layer of Hf can be formed at various locations within the magnetic reference layer. For example, the magnetic reference layer can include a separation layer such as Mo located between magnetic layers of the magnetic reference layer and the layer of Hf can be located next to the barrier layer between a magnetic layer and the barrier layer. The layer of Hf could also be located between magnetic layers within the magnetic reference layer and can also be formed as a part of a bi-layer structure that includes the layer of Hf and a layer of MgO.
These and other features and advantages of the invention will be apparent upon reading of the following detailed description of the embodiments taken in conjunction with the figures in which like reference numeral indicate like elements throughout.
For a fuller understanding of the nature and advantages of this invention, as well as the preferred mode of use, reference should be made to the following detailed description read in conjunction with the accompanying drawings which are not to scale.
The following description is of the best embodiments presently contemplated for carrying out this invention. This description is made for the purpose of illustrating the general principles of this invention and is not meant to limit the inventive concepts claimed herein.
Referring now to
The magnetic reference layer 102 can be part of an anti-parallel magnetic pinning structure 112 that can include a magnetic keeper layer 114, and a non-magnetic, antiparallel coupling layer 116 located between the keeper layer 114 and reference layer 102. The antiparallel coupling layer 116 can be a material such as Ru and can be constructed to have a thickness such that it ferromagnetically antiparallel couples the layers 114, 102. The antiparallel coupling between the layers 114, 102 pins the magnetization 108 of the reference layer 102 in a second direction opposite to the direction of magnetization 118 of the keeper layer 114.
A seed layer 120 may be provided near the bottom of the memory element 100 to initiate a desired crystalline structure in the above deposited layers. A capping layer 122 may be provided near the top of the memory element 100 to protect the underlying layers during manufacture, such as during high temperature annealing. Also, electrodes 124, 126 may be provided at the top and bottom of the memory element 100. The electrodes 124, 126 may be constructed of a non-magnetic, electrically conductive material such as Cu and can provide electrical connection with circuitry 128 that can include a current source and can further include circuitry for reading an electrical resistance across the memory element 100.
The magnetic free layer 104 has a magnetic anisotropy that causes the magnetization 110 of the free layer 104 to remain stable in one of two directions perpendicular to the plane of the free layer 104. In a write mode, the orientation of the magnetization 110 of the free layer 104 can be switched between these two directions by applying an electrical current through the memory element 100 from the circuitry 128. A current in one direction will cause the memory element to flip to a first orientation, and a current in an opposite direction will cause the magnetization to flip to a second, opposite direction. For example, if the magnetization 110 is initially oriented in a downward direction in
On the other hand, if the magnetization 110 of the free layer 104 is initially in an upward direction in
In order to assist the switching of the magnetization 110 of the free layer 104, the memory element 100 may include a spin polarization structure 130 formed above the free layer 104. The spin polarization layer can be separated from the free layer 104 by an exchange coupling layer 132. The spin polarization structure 130 has a magnetic anisotropy that causes it to have a magnetization 134 with a primary component oriented in the in plane direction (e.g. perpendicular to the magnetizations 110, 108 of the free and reference layers 104, 102. The magnetization 134, of the spin polarization layer 130 may either be stationary or can move in a precessional manner as shown in
Reference layer stability is critical to the operation of a magnetic tunnel junction memory element in a magnetic random access memory system. If the reference layer loses its magnetization orientation, the memory element will cease to function correctly, leading to write errors. This becomes even more of an issue at higher switching speeds, wherein higher write currents result in increased instability of the magnetic reference layer. The higher currents used to switch the free layer will induce sufficiently high enough spin torque that may initiate precession or switch the reference layer magnetization. The present invention, embodiments of which are illustrates herein below, provides a structure for increasing reference layer stability to ensure reliability of a magnetic memory element even at high switching speeds with high switching currents.
A capping layer 208 may be provided at the top of the magnetic tunnel junction element 200 to protect the underlying layers such as the magnetic free layer 204 during manufacture. The capping layer 208 can include a layer of MgO and could include various other layers as well. In addition, one or more seed layers 210 can be provided at the bottom of the magnetic tunnel junction element 200. The seed layer 210 can be a material chosen to enhance a desired crystalline structure in above deposited layers for improved magnetic performance.
The reference layer 202 can be a part of an anti-parallel coupled, synthetic antiferromagnetic (SAF) structure 212 that includes a first magnetic layer (SAF1) 214 and a second magnetic layer structure (SAF2) (reference layer structure 202) which are both anti-parallel exchange coupled across an anti-parallel exchange coupling layer 216. The anti-parallel coupled exchange coupling layer 216 can be a material such as Ru, and has a thickness that is chosen to maintain an anti-parallel exchange coupling between the magnetic structures (SAF1) 214 and SAF2 202. For example, the layer 216 could be a layer of Ru having a thickness of 4-6 Angstroms. The anti-parallel coupling of the SAF1 and SAF2 layers 214, 202 causes the layers 214, 202 to have magnetizations that are pinned in directions opposite to one another. This is indicated by arrow 218 for the SAF1 layer 214 and by arrows 220, for the SAF2 structure 202. The antiparallel exchange coupling of these layers 214, 220 helps to maintain these pinned magnetizations 218, 220.
However, as discussed above, at high speed data recording, which requires high write currents, spin torque on the reference layer can cause the reference layer structure 202 to lose its pinned magnetization. In order to improve reference layer magnetic stability it is desirable to increase the perpendicular magnetic anisotropy (PMA) of the reference layer structure 202 and also to lower the internal field Hin of the reference layer structure 202.
The present invention provides a structure which advantageously achieves these goals of increasing PMA and lowering Hin to assure reference layer stability, even at high write currents. An example of a structure that achieves these goals is described with reference to
In order to further increase perpendicular magnetic anisotropy and reduce Hin, the reference layer structure 202 includes thin layer of Hf 228, which greatly increases the perpendicular magnetic anisotropy to ensure magnetic stability of the reference layer structure 202 even at high temperatures. In the embodiment described with reference to
In the structure 202, the magnetic layer 224 closest to the barrier layer 206 has the greatest impact on magnetic tunnel junction ratio (TMR). Therefore, this magnetic layer 224 could be considered to be the reference layer. However, in the structure shown in
With reference now to
In the embodiment 300 of
With reference now to
In addition to the Hf layer 412 of the bi-layer structure 412, 414, an additional layer of Hf 416 is located between the third magnetic layer 408 and the barrier layer 206. The layer of Hf 416 can be as thin as 1 Angstrom to provide PMA enhancing effects without negatively impacting tunneling magnetoresistance (TMR) values.
The above embodiments described with reference to
The addition of Hf into the reference layer to increase perpendicular magnetic anisotropy and Hin also allow the magnetic memory element to be constructed with increased tunneling magnetoresistance (TMR) such as by allowing the reference layer structure to be constructed thicker while still preventing reference layer instability.
While various embodiments have been described above, it should be understood that they have been presented by way of example only and not limitation. Other embodiments falling within the scope of the invention may also become apparent to those skilled in the art. Thus, the breadth and scope of the inventions should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.
