Spin Transfer Torque and associated Magnetic Random Access Memory. Module 4A Debanjan Bhowmik

Transcription

1 Spin Transfer Torque and associated Magnetic Random Access Memory Module 4A Debanjan Bhowmik

2 Magnetic Tunnel Junction Difference in density states on either side of tunnel junction leads to magnetoresistance (TMR).

3 Different structures for MTJ a) Simplest structure of MTJ b) In-plane MTJ structure with synthetic Antiferromagnetic structure, Pinned by AFM layer c) Out of plane MTJ structure with synthetic antiferromagnetic Reference layer FL: Free Layer, RL: Reference Layer, PL: Pinned layer RL and PL are coupled through the thin layer of Ruthenium (RKKY coupling). Based on the thickness such coupling can be ferromagnetic or antiferromagnetic.

4 Design of MRAM cell BL: Bit line WL: Word line A particular combination of bit and world line is selected to sense the resistance of a MTJ. But stray current through other cells is a problem. A gating element is needed.

5 Design of MRAM cell One transistor- One Magnetic Tunnel Junction (1T-1MTJ cell) BL: Bit line WWL: Write Word Line RWL: Read Word Line During read operation, RWL and BL are activated. During write operation, WWL and BL are activated. Field flowing through the lines switch the magnetization. a) Cross-section of a cell b) Schematic of the cell c) Array of cells

6 Joule heating to generate magnetic field Oersted field: H = m 0 J d 2 Magnetic field generated for given amount of current H field Current Disadvantages: 1. Joule heating due to flow of so much current 2. The technology is not scalable, thicker wires are needed, making the feature size reduction harder. 3. Transistor unable to supply so much current. Spin current (Spin transfer torque, spin orbit torque, etc.) comes to the rescue!!!

7 Definition of Spin Current Q M Q: Charge Magnetization (M): moment (m) per unit volume Charge Current J J s : Spin Current, T corresponds to spin flipping.

8 Spin Transfer Torque Fixed Layer Free Layer Free Layer Current coming out of the fixed layer (F2) is spin polarized in direction of magnetizaton of M 2. This spin current acts on the magnetization of free layer F1 (M 1 ) and provides spin transfer Torque (Sonczweski torque) given by: Fixed Layer η ( ) η : Srength of Slonczweski torque

9 Landau Lifschitz Gilbert (LLG) equation with modified Spin Transfer Torque term 1 Precession term, due to magnetic field Damping term, due to magnetic field Anti- damping / Slonczweski term due to Spin transfer torque Field- like term due to Spin transfer torque Based on the polarity of M 2 with respect to M 1 Slonczweski term can add or subtract to the Damping term due to magnetic field.

10 Slonczweski torque and field like torque term Spin Current. Sadamichi Maekawa et al. Oxford (2011)

11 Spin transfer torque driven Magnetization dynamics Easy axis Magnetic field: M 1 Anisotropy field Free layer Spin transfer torque: Tunnel barrier Electron Flow (J c ) M 2 Fixed layer t= thickness of ferromagnet, M s : saturation magnetization, θ= spin polarization

12 Spin transfer torque driven Magnetization dynamics H k = 1000 G, M s = 800 emu/cc, damping factor (α)= 0.2 J c = 0.5 x 10 7 A/cm 2 J c = 1 x 10 7 A/cm 2

13 Spin transfer torque driven Magnetization dynamics J c = 1.5 x 10 7 A/cm 2 J c = 2 x 10 7 A/cm 2

14 Spin transfer torque driven Magnetization dynamics J c = 2.5 x 10 7 A/cm 2 1. There is a critical current density above which the magnet switches. 2. Larger the current (spin transfer torque) shorter is the switching time.

15 Switching time versus current density As spin transfer torque increases, switching time goes down -> Conservation of angular momentum

16 Switching current density vs Damping Spin transfer torque competes against damping torque due to magnetic field.

17 Switching current density vs Damping The spin transfer torque competes against the damping torque due to magnetic field, resulting in the ringing effect while the magnetization moves out of Its previous equilibrium position.