[1] J. C. Slonczewski, J. Magn. Magn. Mater. 159, (1996) L1.
[2] L. Berger, Phys. Review B 54 (1996) 9353.
[3] M. Hosomi et al., 2005 IEDM Technical Digest (2005) p459.
[4] D. Heim and S. S. P. Parkin, U.S. Pat. No. 5,465,185 “Magnetoresistive spin valve sensor with improved pinned ferromagnetic layer and magnetic recording system using the sensor”.
[5] S. S. P. Parkin and D. Heim, Phys. Review. B 44 (1991) 7131.
[6] S. Ikeda et al., Nature Materials, 9 (2010) 721 “A perpendicular-anisotropy CoFeB-MgO magnetic tunnel junction”.
[7] H. Liu et. Al., Appl. Phys. Lett., 97 (2010) 242510 “Ultrafast switching in magnetic tunneling junction based orthogonal spin transfer devices”.
[8] A. D. Kent et. Al., Appl. Phys. Lett., 84 (2004) 3897 “Spin-transfer-induced precessional magnetization reversal”.
[9] A. D. Ken and B. Ozyilmaz, U.S. Pat. No. 7,986,544 B2 “Electronic Devices Based on Current Induced Magnetization Dynamics in Single Magnetic Layers”.
[10] H. Meng et al., Appl. Phys. Lett., 100 (2012) 122405 “Eletcric field effects in low resistance CoFeB-MgO magnetic tunneling junctions with perpendicular anisotropy”
[11] C. Fowley et. Al., IEEE Trans. Mag. 46 (2010) 2116 “Perpendicular Magnetic Anisotropy in CoFeB/Pd Bilayers”.
[12] A. Ken et. al., US 2006/0030058 A1 “High Speed Low Power magnetic Devices Based on Current Induced Spin-Momentum Transfer”
[13] M. Nakayama et. al., J. Appl. Phys. 103 (2008) 07A710, “Spin transfer switching in TbCoFe/CoFeB/MgO/CoFeB/TbCoFe magnetic tunnel junctions with perpendicular magnetic anisotropy”.
[14] C-T Yen and Y-H. Wang, U52011/0241139 A1, “Magnetic Random Access Memory”.
[15] C-T Yen et. al, Appl. Phys. Lett. 93 (2008) 092504, “Reduction in critical current density for spin torque transfer switching with composite free layer”
[16] Meng-Shian Lin and Chih-Huang Lai, J. Appl. Phys. 101, (2007) 09D121, “Perpendicular interlayer coupling through oscillatory Ruderman-Kittel-Kasuya-Yosida interaction between Co/Pt multilayers and Co/TbCo bilayers”
[17] Shaoping Li and C. Potter, U.S. Pat. No. 6,731,473 B2 “Dual Pseudo Spin Valve Heads”
The invention is related to memory cell design for magnetoresistive random access memory (MRAM), more specifically a memory cell comprising two in-plane magnetic stabilization enhancement layers locating on opposite side of the data storage layer of an in-plane anisotropy TMR sensing stack structure. The magnetizations of the stability enhancement layers are normal to the magnetic reference layer of MTJ and point to opposite directions. There is also a switching current spin polarization layer built within the stack to reduce the switching current needed to flip the data storage layer.
Data storage memory is one of the backbones of the modern information technology. Semiconductor memory in the form of DRAM, SRAM and flash memory has dominated the digital world for the last forty years. Comparing to DRAM based on transistor and capacitor above the gate of the transistor, SRAM using the state of a flip-flop with large form factor is more expensive to produce but generally faster and less power consumption. Nevertheless, both DRAM and SRAM are volatile memory, which means they lost the information stored once the power is removed. Flash memory on the other hand is non-volatile memory and cheap to manufacture. However, flash memory has limited endurances of writing cycle and slow write through the read is relatively faster.
MRAM is a relatively a new type of memory technologies. It has the speed of the SRAM, density of the DRAM and it is non-volatile as well. If it is used to replace the DRAM in computer, it will not only give “instant on” but “always-on” status for operation system and restore the system to the point when the system is power off last time. It could provide a single storage solution to replace separate cache (SRAM), memory (DRAM) and permanent storage (HDD or flash-based SSD) on portable device at least. Considering the growth of “cloud computing”, MRAM has a great potential and can be the key dominated technology in digital world.
MRAM storage the informative bit “1” or “0” into the two magnetic states in the so-called magnetic storage layer. The different states in the storage layer gives two distinctive voltage outputs from the whole memory cell, normally a patterned TMR or GMR stack structures. The TMR or GMR stack structures provide a read out mechanism sharing the same well-understood physics as current magnetic reader used in conventional hard disk drive.
There are two kinds of the existing MRAM technologies based on the write process: one kind, which can be labeled as the conventional magnetic field switched (toggle) MRAM, uses the magnetic field induced by the current in the remote write line to change the magnetization orientation in the data stored magnetic layer from one direction (for example “1”) to another direction (for example “0”). This kind of MRAM has more complicated cell structure and needs relative high write current (in the order of mA). It also has poor scalability beyond 65 nm because the write current in the write line needs to continue increase to ensure reliable switching the magnetization of a dimension shrinking magnetic stored layer because of the smaller the physical dimension of the storage layer, the higher the coercivity it normally has for the same materials. Nevertheless, the only commercially available MRAM so far is still based on this conventional writing scheme. The other class of the MRAM is called spin-transfer torque (STT) switching MRAM. It is believed that the STT-RAM has much better scalability due to its simple memory cell structure. While the data read out mechanism is still based on TMR effect, the data write is governed by physics of spin-transfer effect [1, 2]. Despite of intensive efforts and investment, even with the early demonstrated by Sony in late 2005 [3], no commercial products are available on the market so far. One of the biggest challenges of STT-RAM is its reliability, which depends largely on the value and statistical distribution of the critical current density needed to flip the magnetic storage layers within the every patterned TMR stack used in the MRAM memory structures. Currently, the value of the critical current density is still in the range of 106 A/cm2. To allow such a large current density through the dielectric barrier layer such as AlOx and MgO in the TMR stack, the thickness of the barrier has to be relatively thin, which not only limits the magnetoresist (MR) ratio value but also cause potential risk of the barrier breakdown. As such, a large portion of efforts in the STT-RAM is focused on lower the critical current density while still maintaining the thermal stability of the magnetic data storage layer. Another challenge is related partially to the engineering challenge due to the imperfection of memory cell structure patterning (patterned TMR element) such as edge magnetic moment damage and size variation, as well as uniformity of the barrier thickness during the deposition and magnetic uniformity in the data storage layer and spin polarized magnetic layer (also called reference layer). This non-uniformity leads to variation of the size, edge roughness, magnetic uniformity and barrier thickness for patterned TMR elements, which ultimately cause the statistic variation of critical current density needed for each patterned cell.
The success of the STT-RAM largely depends on the breakthrough on the material used in STT-RAM, which give a fair balance between the barrier thickness (related to broken down voltage and TMR ratio), critical current density and thermal stability of the magnetic storage layer. Currently, Based on the anisotropy of the data storage layer, the STT-RAM can be classified into in-plane anisotropy cell and perpendicular cell. The in-plane anisotropy cell has much high magnetoresistance value (MR value) than that of the perpendicular cell but suffers from the thermal stability issue when the size of the cell is reduced, particularly when the magnetization of the storage layer (SL) is parallel to the fixed reference magnetic layer (RL), the magnetostatic coupling between the SL and RL will low the energy barrier and cause large noise or even SL flips.
In this invention, we propose a stabilization scheme to enhance the thermal stability of in-plane MRAM cell with spin-polarization layer, which could also low the critical current needed to flip the data storage layer.
The present invention of the proposed memory cells for MRAM to enhance the thermal stability while maintaining low switching current, which comprises an in-plane anisotropy magnetic tunneling junction (MTJ), two within stack magnetic stabilization layers whose magnetization point to opposite direction and all normal to the that of the reference layer of the MTJ as well as spin polarization layer for switching current.
The following description is provided in the context of particular designs, applications and the details, to enable any person skilled in the art to make and use the invention. However, for those skilled in the art, it is apparent that various modifications to the embodiments shown can be practiced with the generic principles defined here, and without departing the spirit and scope of this invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles, features and teachings disclosed here.
With reference of the
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The magnetic stabilization layer 202 and 211 has their magnetizations pointing at opposite direction and being normal to the magnetization of magnetic layers in SAF layer 205. The magnetization of the spin polarization 209 also points to opposite to that of the stabilization layer 211 with the moment of the layer 211 is noticeably larger than that of layer 209. Overall, the design of the materials of layers 202, 209 and 211 follows the rule that the data storage data 207 sees balanced magnetic torque from layer 202, 209 and 211 when it slightly rotates from its stable positions. One of the way to achieve the design rule is to balance the overall distance between the data storage layer 207 to layer 209, 211 and 202 and keep overall the net moment of these three layers, considering the orientation of the magnetization of each layer, is zero or very close to zero so that they can form a flux close loop with edge magnetic charge canceling each other. The layer 210 separates the layer 211 from the spin polarization layer 209 and can be made of metallic layer with short pin diffusion length. The thickness of layer 210 need to large enough to destroy the spin memory of the electrons obtained from the magnetic layer 211.
The layer 202 and 211 can be made of the hard magnetic materials such as CoCr, CoPt, CoCrPt or bilayer or multilayer comprising soft magnetic layer and hard magnetic layer such as CoPt/CoFe, CoCrPt/NiFe etc. For layer 209, it is preferable to be made of bilayer or multilayer comprising soft magnetic layer and hard magnetic layer such as CoPt/CoFeB, CoCrPt/CoFeB etc because the layer 209 is a fixed spin polarization layer for write current 213.
As said previously, non-magnetic layer 208 is made of MgO, TiOx, AlOx or the combination of dielectric with metal such as Cu, Al, Ag such as MgO/Cu with significant low value of resistance-area product RA compared to the barrier 206. When the write current 213 through layer 209 get polarized, the polarized write current 213 will preserve this polarized state when move into the data storage layer 207. Based on theory [1,2,8], the magnetization of data storage layer 207 will be switched direction. This reduces the critical current needed to flip the data storage layer 207 comparing to a based MTJ at the same conditions.
Layers 208, 209 and 210 build up the separating layer between the layer 211 and data storage layer 207.
The magnetic stabilization layer 302 and 312 has their magnetizations pointing at opposite direction and being normal to the magnetization of magnetic layers in SAF layer 305.
The magnetization directions of the magnetic layers for the SAF spin polarization layer 309 points also normally to the magnetization of magnetic layers in SAF layer 305. SAF polarizer stabilizing layer 310 is above the SAF spin polarization layer and it can be made of either permanent magnetic layer such as CoPt or CoCr-based hard magnetic layer or antiferromagnetic layer such as IrMn or PtMn, whose Neel temperature is significantly different from the one of the layer 304. Regardless of the materials used for layer 310, the design rule is that the magnetic moment from layer 309 and layer 310 on both sides of the Ru layer in SAF layer 309 should be equal or very closely to be equal. As such, the magnetic layers, including layer 310, on both sides of the Ru layer of SAF layer 309 will form a close flux loop and give zero combined edge magnetic charges.
The layer 311 separates the layer 312 from the layer 310 and can be made of metallic layer with short pin diffusion length. The thickness of layer 311 need to large enough to destroy the spin memory of the electrons obtained from the magnetic layer 312.
The layer 302 and 312 can be made of the hard magnetic materials such as CoCr, CoPt, CoCrPt or bilayer or multilayer comprising soft magnetic layer and hard magnetic layer such as CoPt/CoFe, CoCrPt/NiFe etc. The coercivity of layer 302 and layer 312 need widely different so that they can be set by external magnetic field independently.
As said previously, non-magnetic layer 308 is made of MgO, TiOx, AlOx or the combination of dielectric with metal such as Cu, Al, Ag such as MgO/Cu with significant low value of resistance-area product RA compared to the barrier 306. When the write current 314 through layer 309 get polarized, the polarized write current 314 will preserve this polarized state when move into the data storage layer 307. Based on theory [1,2,8], the magnetization of data storage layer 307 will be switched direction. This reduces the critical current needed to flip the data storage layer 307 comparing to a based MTJ at the same conditions.
Layers 308, 309, 310 and 311 build up the separating layer between the layer 312 and data storage layer 307.