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

4

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.

5

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)

6

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.

7

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

8

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

9

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