Spin Torque Nano-Oscillators: A Novel Spintronic Device for Microwave Applications
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Date: 04-26-2006
Start Time:
4:00pm
End Time: 5:00pm
Speaker: Tom Silva
From:
N.I.S.T.
Location: Interschool Lab, 7th floor, Schapiro/CEPSR
Hosted by:
Center for Integrated Science
Abstract:
Our research group at NIST is developing spin torque nano-oscillators
(STNOs) as microwave microwave sources and mixers. STNOs employ
lithographically defined point contacts with a 40 nm diameter to inject
high density current into magnetic multilayers. Current-driven transfer
of angular momentum between the magnetic layers in such a structure
results in spontaneous precession of the magnetization once the current
density exceeds the stability threshold. The magnetization precession
generates microwave electrical signals as a result of giant
magnetoresistance that occurs in such magnetic multilayer structures.
Devices exhibit high Q-factors in excess of 10,000 at 25 GHz. They are
tunable by varying both applied magnetic field and dc current. STNOs
are essentially a nano-scale magnetic analog to voltage-controlled
oscillators (VCOs). We have demonstrated much of the functionality
associated with VCOs using STNOs, including frequency modulation [1]
and injection locking [2]. Most recently, we have shown that two STNOs
can be mutually phase locked to each other, thereby resulting in a
coherent increase in output power [3]. I will describe the basic
principle of STNO operation, including a review of the fundamentals of
spin dynamics in ferromagnetic systems. I will then review the subject
of injection locking, which is a universal property of regenerative
oscillators that employ a negative-loss element to compensate for
internal losses of the oscillator. Finally, I will show how current
injection locking and mutual phase locking of STNOs are further
examples of the injection locking principle.
[1] M.R. Pufall, W.H. Rippard, S. Kaka et al., Applied Physics Letters *86* (8), 082506 (2005).
[2] W.H. Rippard, M.R. Pufall, S. Kaka et al., Physical Review Letters *95* (6), 067203 (2005).
[3] S. Kaka, M.R. Pufall, W.H. Rippard et al., Nature *437*, 389 (2005).