1. Ferromagnetic Quantum Dots on Semiconductor Nanowires
D. G. Ramlan et. al., Nano Letters 6, (2006)
Rouin Farshchi
EE235
3/7/07 1

2. Spintronics Two key requirements:
GMR heads in
hard-drives
(IBM)
Two key requirements:
1- Injection of spin-polarized current
2- Room temperature operation
7.9mm x 10mm 16 Mb
MRAM chip, IBM
GMR Read Heads: C. Tsang. Zhu et. al., IEEE Trans. Magn (1994)
Reviews: S. Wolf et. al., Science (2001)
S. Wolf et. al., IBM J. Res & Dev (2006) 2

3. FM/SC Spin-injection P = (I+ - I-) / (I+ + I-) 3
Spin-injection into SC would allow for
Gate manipulation of spins  spin transistors
Spin injection efficiency: ~P
Inject spin-polarized electrons from
FM into n-type GaAs and allow recombination
with holes from p-type GaAs in LED structure.
Measure degree of circular
polarization 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].
[1] H. J. Zhu et. al., PRL (2001)
[2] Y. B. Xu et. al., PRB (1998) 3

4. MnAs on III-V’s 4 Desirable properties of MnAs grown 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
A. K. Das et. al., PRL (2003) 4

5. MnAs dots 5 Epitaxially grown MnAs dots on sulfur passivated GaAs:
Low surface energy due to passivation leads
to nanoscale MnAs dots with zinc-blend structure
 Dot formation relaxes lattice mismatch to 0.7%
 Exhibit near room temp TC.
K. Ono et. al., JAP 91, 8088 (2002) 5

6. MnAs QD’s on InAs NW’s 100 nm 6 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 mmole/min,
TMIn at a rate of 1.07 mmole/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 mmole/min and increasing
AsH3 flow-rate to mmole/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)
D. G. Ramlan et. al., Nano Letters 6, (2006) 6

7. TEM 7 -InAs NW can be indexed to hexagonal structure,
with lattice spacing d0002 = 3.50 A.
-MnAs QD can be indexed to hexagonal structure
(a-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.
D. G. Ramlan et. al., Nano Letters 6, (2006) 7

8. MFM-magnetic switching
Can QD’s acts as nano-bits?
-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.
 Switching of QD between two stable states
Images are 400 x 185 nm2
D. G. Ramlan et. al., Nano Letters 6, (2006) 8

9. MFM-Curie Temperature
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 K, in good agreement
with bulk a-MnAs (TC=318 K).
Images are 300 x 300 nm2
D. G. Ramlan et. al., Nano Letters 6, (2006) 9

10. 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, (2006) 10

11. thank you
Rouin Farshchi
ee235, 3/7/07