Ferromagnetic Quantum Dots on
Semiconductor Nanowires
Rouin Farshchi
EE235
3/7/07
D. G. Ramlan et. al., Nano Letters 6, 50-54 (2006)
1
Spintronics
Two key requirements:1- Injection of spin-polarized current2- Room temperature operation
GMR heads in hard-drives
(IBM)
7.9mm x 10mm 16 Mb MRAM chip, IBM
GMR Read Heads: C. Tsang. Zhu et. al., IEEE Trans. Magn. 30 3801 (1994)Reviews: S. Wolf et. al., Science 294 1488 (2001) S. Wolf et. al., IBM J. Res & Dev. 50 102 (2006)
2
FM/SC Spin-injection
[1] H. J. Zhu et. al., PRL 87 016601 (2001)[2] Y. B. Xu et. al., PRB 58 890 (1998)
Spin-injection into SC would allow for Gate manipulation of spins spin transistors
Spin injection efficiency: ~P
Inject spin-polarized electrons fromFM into n-type GaAs and allow recombinationwith holes from p-type GaAs in LED structure.
Measure degree of circularpolarization of electroluminescence.
P = (I+ - I-) / (I+ + I-)
• For Fe as FM: P ~ 2% [1]
Fe is known to form a magnetically dead layer at GaAs interface [2].
3
MnAs on III-V’s
A. K. Das et. al., PRL 91 087203 (2003)
Desirable properties of MnAsgrown on III-As films:
1- MnAs is ferromagnetic at room temperature
2- Forms a chemically stable interface.
3- Can be grown epitaxially (MBE, MOCVD) on GaAs despite 30% lattice mismatch
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MnAs dots
K. Ono et. al., JAP 91, 8088 (2002)
Epitaxially grown MnAs dots on sulfur passivated GaAs:
Low surface energy due to passivation leadsto nanoscale MnAs dots with zinc-blend structure
Dot formation relaxes lattice mismatch to 0.7%
Exhibit near room temp TC.
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MnAs QD’s on InAs NW’s
D. G. Ramlan et. al., Nano Letters 6, 50-54 (2006)
MnAs quantum dots on InAs nanowires:
InAs Nanowire growth:-Au catalyst nanoparticles deposited on Si(100) substrate-Nanowire growth occurs in MOCVD chamber under flow of AsH3 at a rate of 6.13 mole/min, TMIn at a rate of 1.07 mole/min, and H2 at 400C
MnAs Quantum dot growth:-TMIn flow is stopped, T increases to 480C, while flowing AsH3 and H2.-MnAs QD formation is initiated by introducing TCMn at a rate of 0.28 mole/min and increasing AsH3 flow-rate to 25.42 mole/min.100 nm
MnAs QD formation driven by large (~20%) lattice mismatch between MnAs and InAs. Mn is a mobile species on InAs nanowire, facilitating qrowth of strain-relieving 3D islands (Volmer-Weber growth)
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TEM
D. G. Ramlan et. al., Nano Letters 6, 50-54 (2006)
-InAs NW can be indexed to hexagonal structure, with lattice spacing d0002 = 3.50 A.
-MnAs QD can be indexed to hexagonal structure (-MnAs) with lattice spacing d0002 = 2.86 A, representing 18% lattice mismatch in [0001]
Hexagonal form of MnAs stabilized, so expect QD’s to be ferromagnetic at room T.
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MFM-magnetic switching
D. G. Ramlan et. al., Nano Letters 6, 50-54 (2006)
Images are 400 x 185 nm2
-AFM scan locates MnAs QD
-MFM cantiliver operating in tapping mode experiences phase shift due to force gradients of magnetic field from QD
-MnAs QD’s are known to have small coercivities.
QD is stable when subjected to H=40 Oe in direction opposite to remnant magnetization
QD magnetization “switches” when applied field is increased to H=60 Oe.
Can QD’s acts as nano-bits?
Switching of QD between two stable states
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MFM-Curie Temperature
D. G. Ramlan et. al., Nano Letters 6, 50-54 (2006)
Variable temp MFM for determination of TC:
-constant phase contrast up to 308 K (b,c)
-phase contrast disappears abruptly at 313 K
TC lies between 308-313 K, in good agreement with bulk -MnAs (TC=318 K).
Images are 300 x 300 nm2
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Conclusions
• MnAs grown on GaAs is a promising FM/SC structure for spin-injection and has been incorporated into spintronic devices such as spin-valves[1].
• MnAs QD’s grown on InAs NW’s represent a new FM/SC structure with possible application in high-density memory storage.
•Spin-Injection studies from MnAs QD’s into InAs NW’s would be required to determine their potential for use nano-scale spintronic devices.
D. Saha et. al., APL 89, 142504 (2006) 10
thank you
Rouin Farshchiee235, 3/7/07