EE PhD Oral Examination - Larkhoon Leem, Wednesday, March 3rd, 2010, 3:00pm

Larkhoon Leem tryon16 at
Tue Mar 2 11:06:03 PST 2010

Magnetic Coupled Spin-Torque Device for Non-volatile Logic Applications

Stanford University Ph.D. Thesis Defense

Larkhoon Leem (tryon16 at
Research Advisor: Professor James S. Harris
Department of Electrical Engineering

March 3, 2010 @ 3:00pm
(Refreshements served at 2:45pm)
Allen 101X Auditorium


    Power consumption has become the key constraint in chip design,
since the MOSFET threshold voltage (VT) and hence the supply voltage
(Vdd) can no longer be scaled. This trend calls for new device
concepts such as Spintronics devices that are fundamentally different
from CMOS. However, the MOSFET-type Spintronics transistor has not
been demonstrated due to the technical difficulties in transporting
and detecting spin information. In this talk, I present an alternative
Spintronics logic device, Magnetic Coupled Spin Torque Device (MCSTD),
which is free from spin-injection, transport and detection problems.
It leverages spin torque transfer effect and magnetic dipole coupling
between spin-torque devices to modulate its magnetization reversal
energy barrier. Its device switching speed, signal inversion and
signal level restoration capabilities will be discussed. For device to
device level spin communication, MCSTD uses a novel interconnection
technique that efficiently converts spin (or magneto-resistance)
information to current amplitude difference information, which is then
converted back to spin information at the subsequent gates. In
micro-magnetic simulations, MCSTD-based NAND, NOR, XOR gates and a
three-stage ring oscillator have been demonstrated to estimate
realistic device speed and power consumption. The fabrication of 20nm
gap MCSTDs has been successfully completed in two different types of
spin-torque devices, i.e., Magnetic Tunnel Junction (MTJ) and
Spin-Valve (SV). They demonstrated the switching voltage modulation
depending on the magnetic moments of the input spin-torque devices.
The amount of voltage shifts ranged between 40~100mV, which is well
above thermal fluctuations. In addition, non-volatility of MCSTD opens
up very unique potential applications in future power management
techniques and smart sensor technologies. For example, MCSTDs can
replace SRAMs and pass gate transistors in reconfigurable logics such
as Field Programmable Gate Array (FPGA). Instant-on/off nature of
MCSTD enables low overhead system-level power gating scheme for
embedded devices. Also, MCSTD can be used as a magnetic sensor with
in-situ logic operations for error-resilient DNA microarray sensors.

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