Significant reduction of critical currents in MRAM designs using dual free layer with perpendicular and in-plane anisotropy

Abstract

Abstract:One essential feature in MRAM cells is the spin torque efficiency, which describes the ratio of the critical switching current to the energy barrier. Within this paper it is reported that the spin torque efficiency can be improved by a factor of 3.2 by the use a of dual free layer device, which consists of one layer with perpendicular crystalline anisotropy and a second layer with in-plane crystalline anisotropy. Detailed simulations solving the spin transport equations simultaneously with the micromagnetics equation were performed in order to understand the origin of the switching current reduction by a factor of 4 for the dual layer structure compared to a single layer structure.The main reason could be attributed to an increased spin accumulation within the free layer due to the dynamical tilting of the magnetization within the in-plane region of the dual free layer.Magnetic random access memory (MRAM) has the potential to become a major player in embedded magnetoresitive non-volatile memory technology due to the fast access and write times. An important design guideline in MRAM development is to reduce the critical switching current while maintaining a high thermal stability. Different designs of spin-transfer torque MRAM (STT-MRAM) cells have been proposed in order to reduce the critical current density. Dual pinned layer were proposed in order to reduce the critical current. Here, the second pinned layer has to point in opposite direction to the first pinned layer and it is separated by a second MgO barrier from the free layer. The critical current was reported to be a factor of 2-3 times smaller than that of a single MgO barrier 1 reaching values of the critical current density of j c = 0.02 TA/m² as reported in Ref[ 2 ]. In this reported structure both pinned layers consist of synthetic antiferromagnetically (SAF) coupled and exchange biased layers leading to layer thicknesses of about 40 nm, which reduces the scalability.

Spin Torque Efficiency and Analytic Error Rate Estimates of Skyrmion Racetrack Memory

“…Hence, this estimate is also valid for thicker films. The spin torque efficiency of a metallic MRAM junction simulated with the same spin diffusion approach is in the similar order,  = E/I c = 0.16 k B T 300 /A [ 47 ]. The spin torque efficiency for experimental CoFeB-MgO based tunnel junctions with diameters below 30 nm are in the range between 1 and 10 k B T/μA as reported in Ref [ 48 ].…”

Self-assembled magnetic heterostructure of Co/DLC films

“…The magnetic heterostructure are PMA/in-plane state formed by hexagonal close packing (hcp) Co layer stacked on Co carbide layer. It is expected to have the highest switching current to the energy barrier ratio as mentioned in previous report [23]. Thus, the self-assembled magnetic heterostructure of Co/DLC films has great potential for developing advanced memory devices.…”

“…The phase change memory (PCM) [3][4][5][6][7][8], resistive random access memory (ReRAM) [9][10][11] or magnetoresistive random access memory (MRAM) [12][13][14][15][16][17][18][19][20][21][22][23][24][25] have the highest possibilities to be a warm storage devices. For example, there is an approach for making energy-efficient deep learning inference hardware by using PCM devices [3].…”

“…For example, the electric-field control of magnetic anisotropy energy (MAE) is studied in modulating and for the optimal performance of the memory device in the BaTiO 3 /CoFe/(Hf, Ta, W, Re, Os, Ir, Pt, or Au) heterostructure [17]. The magnetic heterostructure are perpendicular magnetic anisotropy (PMA)/in-plane state in simulation can be improved to a factor of 4 of the critical switching current to the energy barrier [23] which is much higher than 2-3 from previous reports [24,25].…”

Impact of Spin-Transfer Torque on the Write-Error Rate of a Voltage-Torque-Based Magnetoresistive Random-Access Memory

“…Joule heating. In order to decrease the write energy further much effort has been devoted to decreasing the critical current density for STT-switching [12][13][14].…”