Spintronic Memories

Magnetic Domain Wall Motion memory Magnetic race-track memory Stuart Parkin, “Magnetic race-track – a novel storage class spintronic memory”, Intern J. Mod. Physics B 22 (2008) 117 Spin torque transfer MRAM Should we consider storage-class memories in ERD? Does is belong to PIDS or to ERD? MRAM has been transferred from ERD to PIDS in 2003

Selection criteria for new technology entries candidates

“Minimum Requirement” Criteria The ‘minimum requirements’ criteria for a new technology to be considered as a candidate for ERD chapter is sufficient research activity, e.g. the technology is explored by several research groups, or There is an extensive research activity of one group There are at least 2 publications in peer-reviewed journals

General ‘Loose’ Criteria Potential for scaling Roadmap Driver On-chip integrated solutions Potential for embedded applications Outstanding research issues exist Guidelines for the research community and government funding agencies Not in production Innovation phase

Spin torque transfer MRAM

Conventional MRAM FM free layer W FM pinned layer MRAM element is operated by magnetic field generated from current lines in the proximity of MTJ FM free layer W FM pinned layer “Wireless communication” Proximity effects – crosstalk Scaling issue, Heat, reliability etc.

MRAM Scaling issue T. Kawahara et al, “2Mb SPRAM (Spin-Transfer Torque RAM)…”, IEEE J. Solid-State Circ. 43 (2008) 109 Hitachi group Scaling issue: Conventional MRAM needs a larger current for smaller dimensions

Spin-torque switching Injected spin-polarized electrons interact with the magnetic moment of a free layer and transfer their angular momentum If sufficient current is applied, the exerted spin torque switches the free layer either parallel or anti-parallel to the pinned layer depending on the direction of flow of the current Attractive for memory array applications, does not have the magnetic half-select problem smaller switching current Spin torque transfer RAM (ST-RAM) MTJ for spin-torque switching

MTJ for spin-torque switching MTJ is operated by spin polarized current passing through MTJ Injection efficiency W ~Nat ~L2 Scaling promise: spin-torque MRAM needs a smaller current for smaller dimensions

MRAM and ST-MRAM Scaling Reciprocal Scaling Relations

MRAM and ST-MRAM Scaling

Switching time vs. current

Outstanding research issues Theory: Critical current issue Ic needs to be decreased From 107 A/cm2 to 105 A/cm2 New material structures MTJ current needs to be increased New MTJ design Injection efficiency “Although the presence of spin torque has been unambiguously observed, its quantitative behavior in MTJ, especially its bias dependence has yet to be understood in detail” J. C. Sankey et al., Nature Physics 4 (2008) 67 IBM group

New concepts are needed Nano-current-channel (NCC) injection FeSiO layer with columnar NCC structure Current can pass through NCC only Provide magnetic nucleation points and induce the free layer switching through the growth of the nucleation points

General ‘Loose’ Criteria Discussion: Spin torque transfer MRAM Does is belong to PIDS or to ERD? MRAM has been transferred from ERD to PIDS in 2003 Potential for scaling Roadmap Driver On-chip integrated solutions Potential for embedded applications Outstanding research issues exist Guidelines for the research community and government funding agencies Not in production Innovation phase

Magnetic Domain Wall Motion memory

Magnetic Domain Wall Motion memory Current-driven magnetic domain wall (DW) motion DWM DWM occurs in a submicron-size ferromagnetic stripe Charge carriers become polarized by the interaction between conduction electrons and local magnetic moments Exert torque on the magnetic moments within DW Sensed by TMR or GMR device

Magnetic Race-track Memory A proposal for a novel storage-class memory, in which magnetic domains are used to store information in a “magnetic race-track” Shift register scheme A solid state memory with storage capacity same/better than HDD Improved performance and reliability The magnetic race track is comprised of tall columns of magnetic material arranged perpendicularly to the Si surface The domains are moved up and down by current pulses ~ns pulses Sensing by magnetic tunnel junction device

DWM at ~107 has been demonstrated

Domain wall velocity as a function of domain wall width Benakli et al., JAP 103 (2008) Seagate Group

Planar Configuration t=5 nm W L W, nm N, bit n, bit/cm2 R I V L, cm P, W/cm2 J, A/cm2 100 2.0E+05 5.0E+09 4.0E+06 5.0E-03 20000 1 4.8E+06 1.0E+09 10 2.0E+06 5.0E+11 4.0E+07 5.0E-04 3.3E+06

W, cm N, bit n, bit/cm2 R I V L, cm P, W/cm2 J, A/cm2 1.00E-05 2.10E+01 4.77E+09 4.00E+02 5.00E-06 0.00 1.00E-04 5 1.00E+06 1.00E-06 2.01E+02 4.98E+11 4.00E+03 5.00E-07 0.0001 3 W, cm N, bit n, bit/cm2 R I V L, um P, W/cm2 J, A/cm2 1.00E-05 2.10E+01 4.77E+09 4.00E+02 5.00E-05 0.02 1 476 1.00E+07 1.00E-06 2.01E+02 4.98E+11 4.00E+03 5.00E-06 333 W, nm N, bit n, bit/cm2 R I V L, um P, W/cm2 J, A/cm2 100 2.10E+01 4.77E+09 4.00E+02 5.00E-03 2 1 4.76E+06 1.00E+09 10 2.01E+02 4.98E+11 4.00E+03 5.00E-04 3.33E+06

Main Issue Due to the high current densities, strong heating occurs DW transformations have been shown to originate not only from spin torque effects but also from thermal excitations For applications, it is a key requirement to devise ways for efficient cooling

There is a considerable interest in DWM IBM Samsung Hitachi Seagate Canon

Outstanding research issues The capacity of spin-polarized current to move a domain wall was experimental established, but The mechanisms responsible for that motion remain under debate Current density needs to be decreased!