1. Magnetoresistive Random Access Memory (MRAM)
Anton Kapliy
University of Chicago
Exchange Splitting in Ferromagnets
Memory Structure and Read Mode
Spin Transfer Switching
History
1975 – M. Julierre observes weak TMR effect under low temperatures and attributes it to exchange splitting in ferromagnets
1995 – J.S. Moodera uses new materials (MgO) to achieve substantial TMR at room temperatures
2000 – Freescale, IBM, Infineon et al initiate programs to commerialize the technology
present – commercial availability.
Today 1 MB costs > $1, development proceeds
External magnetic field produces Zeeman splitting in the energies of spin-up and spin-down electrons.
In case of ferromagnets, which can maintain their own B-field, this energy splitting can occur without any external field. This so-called exchange splitting can be on the order of an eV, and explains unequal density of states of electrons with different spins near Fermi surface:
Each cell in MRAM is represented by an MTJ. Being in a parallel or antiparallel state defines binary memory states “0” (low R) and “1” (high R).
MRAM cells are organized into a 2-d grid such that each MTJ lies at the intersection of a Word/Digit Line and a Bit Line (see image on the left). This way we can address any cell of interest.
Starting around 2006, MRAM research shifted towards the Spin Transfer Toque technique for changing layer magnetization.
Electrons pass through the fixed layer and get spin-polarized along M1
Electrons next move through the free layer and repolarize along M2
By angular momentum conservation, they apply a torque on electrons in the free layer, so that their spin turns in the direction of M1
With sufficient current, free layer becomes polarized parallel to M1
Since spin transfer method doesn’t require a strong externally-generated B-field, it promises lower power consumption and better scalability.
Read Mode:
Activate the word line (WL)
Clamp bit line (BL) to fixed voltage
Transistor will sink a current (IR) from collector to emitter
Compare that current to reference
Total torque on M2 for different spin torques Note that spin torque is proportional to current
Tunnel Magnetoresistance (TMR)
Classically, there is no conduction across MTJ when we apply bias voltage. However, quantum mechanically, electrons can tunnel through the barrier.
To a good approximation, this process can be described in terms of Julierre’s two-current model:
Different spins tunnel independently: the probability for an electron to tunnel and change its spin is a 2nd order effect.
Electrons near Fermi level account for the majority of tunneling – since they are the most energetic
Tunneling probability is proportional to density of states near Fermi level – according to Fermi’s Golden rule
If two magnetizations are parallel, majority spins (↓) tunnel to majority states and account for most of the conduction. Minority spins (↑) tunnel to minority states and have negligible conduction.
If two magnetizations are antiparallel, majority spins (↓) tunnel to minority states, while minority spins (↑) tunnel to majority states. Thus, both channels are suppressed, resulting in low overall conduction.
Mathematically, we can compute the TMR ratio as follows:
A Magnetic Tunnel Junction (MTJ) consists of a thin insulating layer (~1 nm, corresponding to ~10 atomic monolayers) sandwiched between two ferromagnets (~50 angstroms). One layer has frozen magnetization, and the other is free to change.
Write Mode
Conclusions
The sensing current produced in read mode is not sufficient to change the magnetization state of free layer. Instead, the state is changed as follows:
Send a unidirectional (“hard”) current through the digit line.
Send a bi-directional (“soft”) current through the bit line
Turn the isolation transistor off so that no current flows through the junction during write cycle
Use ferromagnetic material with such coercivity that the switching doesn’t occur when exposed to B-field from one line only.
MRAM has many benefits over alternative technologies:
Non-volatile (draws no power when idle)
No mechanical parts and no wear mechanism
Low power and high density
Much faster than Flash memory
However, it still faces many challenges:
Requires good material uniformity (e.g., for small MTJ resistance variation)
Close packing reduces switching barrier; prone to thermal auto-switching
B-field from neighboring locations starts to interfere as size is reduced
Low adoption, and as a consequence, high prices
Sketch of state switching
References
Reversal gap between states vs switch fields
Physics Review, Volume 168, N2 531 (1968) – Spin-disorder scattering and magnetoresistance of magnetic semiconductors
J.M. De Teresa et al, Role of the Barrier in Magnetic Tunnel Junctions
JOM-e 52(6) (2000) (J.M. Slaughter et al) – Magnetic Tunnel Junction Materials for Electronic Applications
Freescale Semiconductor – website materials (
Industrial Embedded Systems (magazine) – Magnetic Tunnel Junction sensor development for industrial applications
H. Kimura et al - A Study of Multiple-Valued MRAM Using Binary MTJ Devices
Wikipedia.org (to review many relevant concepts)
Plus half-a-dozen other academic and industrial papers/datasheets
Device model from Freescale
Phase space of switch fields
(spin polarization)
Hysteresis curve: Remanence and Coercivity