Ferromagnetic Quantum Dots on Semiconductor Nanowires D. G. Ramlan et. al., Nano Letters 6, 50-54 (2006) Rouin Farshchi EE235 3/7/07 1

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. 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 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 87 016601 (2001) [2] Y. B. Xu et. al., PRB 58 890 (1998) 3

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 91 087203 (2003) 4

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

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 25.42 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, 50-54 (2006) 6

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, 50-54 (2006) 7

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, 50-54 (2006) 8

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 308-313 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, 50-54 (2006) 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 Farshchi ee235, 3/7/07