Showing 1–13 of 13 results for author: Ku, K C

A Tunable Anomalous Hall Effect in a Non-Ferromagnetic System

Authors: J. Cumings, L. S. Moore, H. T. Chou, K. C. Ku, S. A. Crooker, N. Samarth, D. Goldhaber-Gordon

Abstract: We measure the low-field Hall resistivity of a magnetically-doped two-dimensional electron gas as a function of temperature and electrically-gated carrier density. Comparing these results with the carrier density extracted from Shubnikov-de Haas oscillations reveals an excess Hall resistivity that increases with decreasing temperature. This excess Hall resistivity qualitatively tracks the parama… ▽ More We measure the low-field Hall resistivity of a magnetically-doped two-dimensional electron gas as a function of temperature and electrically-gated carrier density. Comparing these results with the carrier density extracted from Shubnikov-de Haas oscillations reveals an excess Hall resistivity that increases with decreasing temperature. This excess Hall resistivity qualitatively tracks the paramagnetic polarization of the sample, in analogy to the ferromagnetic anomalous Hall effect. The data are consistent with skew-scattering of carriers by disorder near the crossover to localization. △ Less

Submitted 30 December, 2005; originally announced December 2005.

Fabrication and Characterization of Modulation-Doped ZnSe/(Zn,Cd)Se (110) Quantum Wells: A New System for Spin Coherence Studies

Authors: K. C. Ku, S. H. Chun. W. H. Wang, W. Fadgen, D. A. Issadore, N. Samarth, R. J. Epstein, D. D. Awschalom

Abstract: We describe the growth of modulation-doped ZnSe/(Zn,Cd)Se quantum wells on (110) GaAs substrates. Unlike the well-known protocol for the epitaxy of ZnSe-based quantum structures on (001) GaAs, we find that the fabrication of quantum well structures on (110) GaAs requires significantly different growth conditions and sample architecture. We use magnetotransport measurements to confirm the formati… ▽ More We describe the growth of modulation-doped ZnSe/(Zn,Cd)Se quantum wells on (110) GaAs substrates. Unlike the well-known protocol for the epitaxy of ZnSe-based quantum structures on (001) GaAs, we find that the fabrication of quantum well structures on (110) GaAs requires significantly different growth conditions and sample architecture. We use magnetotransport measurements to confirm the formation of a two-dimensional electron gas in these samples, and then measure transverse electron spin relaxation times using time-resolved Faraday rotation. In contrast to expectations based upon known spin relaxation mechanisms, we find surprisingly little difference between the spin lifetimes in these (110)-oriented samples in comparison with (100)-oriented control samples. △ Less

Submitted 30 April, 2005; originally announced May 2005.

Comments: To appear in Journal of Superconductivity (Proceedings of 3rd Conference on Physics and Applications of Spin-dependent Phenomena in Semiconductors)

Exchange Biasing of the Ferromagnetic Semiconductor (Ga,Mn)As by MnO

Authors: K. F. Eid, M. B. Stone, O. Maksimov, T. C. Shih, K. C. Ku, W. Fadgen, C. J. Palmstrom, P. Schiffer, N. Samarth

Abstract: We provide an overview of progress on the exchange biasing of a ferromagnetic semiconductor (Ga1-xMnxAs) by proximity to an antiferromagnetic oxide layer (MnO). We present a detailed characterization study of the antiferromagnetic layer using Rutherford backscattering spectrometry, x-ray photoelectron spectroscopy, transmission electron microscopy, and x-ray reflection. In addition, we describe… ▽ More We provide an overview of progress on the exchange biasing of a ferromagnetic semiconductor (Ga1-xMnxAs) by proximity to an antiferromagnetic oxide layer (MnO). We present a detailed characterization study of the antiferromagnetic layer using Rutherford backscattering spectrometry, x-ray photoelectron spectroscopy, transmission electron microscopy, and x-ray reflection. In addition, we describe the variation of the exchange and coercive fields with temperature and cooling field for multiple samples. △ Less

Submitted 30 April, 2005; originally announced May 2005.

Comments: To appear in J. Appl. Phys. (invited paper in Proceedings of the 49th Annual Conference on Magnetism & Magnetic Materials); pdf file only

Journal ref: J. Appl. Phys. 97, art. no. 10D304 (2005)

Andreev Reflection and Pair Breaking Effects at the Superconductor/Magnetic Semiconductor Interface

Authors: R. P. Panguluri, K. C. Ku, T. Wojtowicz, X. Liu, J. K. Furdyna, Y. B. Lyanda-Geller, N. Samarth, B. Nadgorny

Abstract: We investigate the applicability of spin polarization measurements using Andreev reflection in a point contact geometry in heavily doped dilute magnetic semiconductors, such as (Ga,Mn)As. While we observe conventional Andreev reflection in non-magnetic (Ga,Be)As epilayers, our measurements indicate that in ferromagnetic (Ga,Mn)As epilayers with comparable hole concentration the conductance spect… ▽ More We investigate the applicability of spin polarization measurements using Andreev reflection in a point contact geometry in heavily doped dilute magnetic semiconductors, such as (Ga,Mn)As. While we observe conventional Andreev reflection in non-magnetic (Ga,Be)As epilayers, our measurements indicate that in ferromagnetic (Ga,Mn)As epilayers with comparable hole concentration the conductance spectra can only be adequately described by a broadened density of states and a reduced superconducting gap. We suggest that these pair-breaking effects stem from inelastic scattering in the metallic impurity band of (Ga,Mn)As and can be explained by introducing a finite quasiparticle lifetime or a higher effective temperature. For (Ga,Mn)As with 8% Mn concentration and 140 K Curie temperature we evaluate the spin polarization to be 83+/-17%. PACS numbers: 72.25.Dc,72.25.Mk,74.45.+c △ Less

Submitted 23 February, 2005; v1 submitted 22 February, 2005; originally announced February 2005.

Optoelectronic control of spin dynamics at near-THz frequencies in magnetically doped quantum wells

Authors: R. C. Myers, K. C. Ku, X. Li, N. Samarth, D. D. Awschalom

Abstract: We use time-resolved Kerr rotation to demonstrate the optical and electronic tuning of both the electronic and local moment (Mn) spin dynamics in electrically gated parabolic quantum wells derived from II-VI diluted magnetic semiconductors. By changing either the electrical bias or the laser energy, the electron spin precession frequency is varied from 0.1 to 0.8 THz at a magnetic field of 3 T a… ▽ More We use time-resolved Kerr rotation to demonstrate the optical and electronic tuning of both the electronic and local moment (Mn) spin dynamics in electrically gated parabolic quantum wells derived from II-VI diluted magnetic semiconductors. By changing either the electrical bias or the laser energy, the electron spin precession frequency is varied from 0.1 to 0.8 THz at a magnetic field of 3 T and at a temperature of 5 K. The corresponding range of the electrically-tuned effective electron g-factor is an order of magnitude larger compared with similar nonmagnetic III-V parabolic quantum wells. Additionally, we demonstrate that such structures allow electrical modulation of local moment dynamics in the solid state, which is manifested as changes in the amplitude and lifetime of the Mn spin precession signal under electrical bias. The large variation of electron and Mn-ion spin dynamics is explained by changes in magnitude of the sp−d exchange overlap. △ Less

Submitted 18 November, 2004; originally announced November 2004.

Comments: 4 pages, 3 figures

Journal ref: Phys. Rev. B 72, 041302(R) (2005)

Exchange Biasing of the Ferromagnetic Semiconductor Ga1-xMnxAs

Authors: K. F. Eid, M. B. Stone, K. C. Ku, P. Schiffer, N. Samarth, T. C. Shih, C. J. Palmstrom

Abstract: We demonstrate the exchange coupling of a ferromagnetic semiconductor (Ga1-xMnxAs) with an overgrown antiferromagnet (MnO). Unlike most conventional exchange biased systems, the blocking temperature of the antiferromagnet (T_B = 48 +- 2 K) and the Curie temperature of the ferromagnet (T_C = 55.1 +- 0.2 K) are comparable. The resulting exchange bias manifests itself as a clear shift in the magnet… ▽ More We demonstrate the exchange coupling of a ferromagnetic semiconductor (Ga1-xMnxAs) with an overgrown antiferromagnet (MnO). Unlike most conventional exchange biased systems, the blocking temperature of the antiferromagnet (T_B = 48 +- 2 K) and the Curie temperature of the ferromagnet (T_C = 55.1 +- 0.2 K) are comparable. The resulting exchange bias manifests itself as a clear shift in the magnetization hysteresis loop when the bilayer is cooled in the presence of an applied magnetic field and an enhancement of the coercive field. △ Less

Submitted 7 May, 2004; v1 submitted 10 December, 2003; originally announced December 2003.

Comments: pdf file only; submitted to Applied Physics Letters

Journal ref: Appl. Phys. Lett. 85, 1556 (2004).

Capping-induced suppression of annealing in Ga(1-x)Mn(x)As epilayers

Authors: M. B. Stone, K. C. Ku, S. J. Potashnik, B. L. Sheu, N. Samarth, P. Schiffer

Abstract: We have studied the effects of capping ferromagnetic Ga(1-x)Mn(x)As epilayers with a thin layer of undoped GaAs, and we find that even a few monolayers of GaAs have a significant effect on the ferromagnetic properties. In particular, the presence of a capping layer only 10 monolayers thick completely suppresses the enhancement of the ferromagnetism associated with low temperature annealing. This… ▽ More We have studied the effects of capping ferromagnetic Ga(1-x)Mn(x)As epilayers with a thin layer of undoped GaAs, and we find that even a few monolayers of GaAs have a significant effect on the ferromagnetic properties. In particular, the presence of a capping layer only 10 monolayers thick completely suppresses the enhancement of the ferromagnetism associated with low temperature annealing. This result, which demonstrates that the surface of a Ga(1-x)Mn(x)As epilayer strongly affects the defect structure, has important implications for the incorporation of Ga(1-x)Mn(x)As into device heterostructures. △ Less

Submitted 10 July, 2003; originally announced July 2003.

Comments: 13 pages with figures attatched

Coercive Field and Magnetization Deficit in Ga(1-x)Mn(x)As Epilayers

Authors: S. J. Potashnik, K. C. Ku, S. H. Chun, R. F. Wang, M. B. Stone, N. Samarth, P. Schiffer

Abstract: We have studied the field dependence of the magnetization in epilayers of the diluted magnetic semiconductor Ga(1-x)Mn(x)As for 0.0135 < x < 0.083. Measurements of the low temperature magnetization in fields up to 3 T show a significant deficit in the total moment below that expected for full saturation of all the Mn spins. These results suggest that the spin state of the non-ferromagnetic Mn sp… ▽ More We have studied the field dependence of the magnetization in epilayers of the diluted magnetic semiconductor Ga(1-x)Mn(x)As for 0.0135 < x < 0.083. Measurements of the low temperature magnetization in fields up to 3 T show a significant deficit in the total moment below that expected for full saturation of all the Mn spins. These results suggest that the spin state of the non-ferromagnetic Mn spins is energetically well separated from the ferromagnetism of the bulk of the spins. We have also studied the coercive field (Hc) as a function of temperature and Mn concentration, finding that Hc decreases with increasing Mn concentration as predicted theoretically. △ Less

Submitted 11 December, 2002; originally announced December 2002.

Comments: 15 total pages -- 5 text, 1 table, 4 figues. Accepted for publication in MMM 2002 conference proceedings (APL)

Journal ref: J. Appl. Phys. 93, 6784 (2003)

Highly enhanced Curie temperatures in low temperature annealed (Ga,Mn)As epilayers

Authors: K. C. Ku, S. J. Potashnik, R. F. Wang, M. J. Seong, E. Johnston-Halperin, R. C. Meyers, S. H. Chun, A. Mascarenhas, A. C. Gossard, D. D. Awschalom, P. Schiffer, N. Samarth

Abstract: We report Curie temperatures up to 150 K in annealed Ga1-xMnxAs epilayers grown with a relatively low As:Ga beam equivalent pressure ratio. A variety of measurements (magnetization, Hall effect, magnetic circular dichroism and Raman scattering) show that the higher ferromagnetic transition temperature results from an enhanced free hole density. The data also indicate that, in addition to the car… ▽ More We report Curie temperatures up to 150 K in annealed Ga1-xMnxAs epilayers grown with a relatively low As:Ga beam equivalent pressure ratio. A variety of measurements (magnetization, Hall effect, magnetic circular dichroism and Raman scattering) show that the higher ferromagnetic transition temperature results from an enhanced free hole density. The data also indicate that, in addition to the carrier concentration, the sample thickness limits the maximum attainable Curie temperature in this material - suggesting that the free surface of Ga1-xMnxAs epilayers is important in determining their physical properties. △ Less

Submitted 19 October, 2002; originally announced October 2002.

Comments: pdf file only

Journal ref: Appl. Phys. Lett. 82, 2302 (2003)

Spin-polarized Tunneling in Hybrid Metal-Semiconductor Magnetic Tunnel Junctions

Authors: S. H. Chun, S. J. Potashnik, K. C. Ku, P. Schiffer, N. Samarth

Abstract: We demonstrate efficient spin-polarized tunneling between a ferromagnetic metal and a ferromagnetic semiconductor with highly mismatched conductivities. This is indicated by a large tunneling magnetoresistance (up to 30%) at low temperatures in epitaxial magnetic tunnel junctions composed of a ferromagnetic metal (MnAs) and a ferromagnetic semiconductor (GaMnAs) separated by a nonmagnetic semico… ▽ More We demonstrate efficient spin-polarized tunneling between a ferromagnetic metal and a ferromagnetic semiconductor with highly mismatched conductivities. This is indicated by a large tunneling magnetoresistance (up to 30%) at low temperatures in epitaxial magnetic tunnel junctions composed of a ferromagnetic metal (MnAs) and a ferromagnetic semiconductor (GaMnAs) separated by a nonmagnetic semiconductor (AlAs). Analysis of the current-voltage characteristics yields detailed information about the asymmetric tunnel barrier. The low temperature conductance-voltage characteristics show a zero bias anomaly and a V^1/2 dependence of the conductance, indicating a correlation gap in the density of states of GaMnAs. These experiments suggest that MnAs/AlAs heterostructures offer well characterized tunnel junctions for high efficiency spin injection into GaAs. △ Less

Submitted 6 July, 2002; originally announced July 2002.

Comments: 14 pages, submitted to Phys. Rev. B

Journal ref: Phys. Rev. B 66, 100408(R) (2002)

Saturated Ferromagnetism and Magnetization Deficit in Optimally Annealed (Ga,Mn)As Epilayers

Authors: S. J. Potashnik, K. C. Ku, R. Mahendiran, S. H. Chun, R. F. Wang, N. Samarth, P. Schiffer

Abstract: We examine the Mn concentration dependence of the electronic and magnetic properties of optimally annealed Ga1-xMnxAs epilayers for 1.35% < x < 8.3%. The Curie temperature (Tc), conductivity, and exchange energy increase with Mn concentration up to x ~ 0.05, but are almost constant for larger x, with Tc ~ 110 K. The ferromagnetic moment per Mn ion decreases monotonically with increasing x, imply… ▽ More We examine the Mn concentration dependence of the electronic and magnetic properties of optimally annealed Ga1-xMnxAs epilayers for 1.35% < x < 8.3%. The Curie temperature (Tc), conductivity, and exchange energy increase with Mn concentration up to x ~ 0.05, but are almost constant for larger x, with Tc ~ 110 K. The ferromagnetic moment per Mn ion decreases monotonically with increasing x, implying that an increasing fraction of the Mn spins do not participate in the ferromagnetism. By contrast, the derived domain wall thickness, an important parameter for device design, remains surprisingly constant. △ Less

Submitted 11 April, 2002; originally announced April 2002.

Comments: 8 pages, 4 figures, submitted for Rapid Communication in Phys Rev B

Journal ref: Phys. Rev. B 66, 012408 (2002)

Effects of annealing time on defect-controlled ferromagnetism in Ga1-xMnxAs

Authors: S. J. Potashnik, K. C. Ku, S. H. Chun, J. J. Berry, N. Samarth, P. Schiffer

Abstract: We have studied the evolution of the magnetic, electronic, and structural properties of annealed epilayers of Ga1-xMnxAs grown by low temperature molecular beam epitaxy. Annealing at the optimal temperature of 250 C for less than 2 hours significantly enhances the conductivity and ferromagnetism, but continuing the annealing for longer times suppresses both. These data indicate that such anneali… ▽ More We have studied the evolution of the magnetic, electronic, and structural properties of annealed epilayers of Ga1-xMnxAs grown by low temperature molecular beam epitaxy. Annealing at the optimal temperature of 250 C for less than 2 hours significantly enhances the conductivity and ferromagnetism, but continuing the annealing for longer times suppresses both. These data indicate that such annealing induces the defects in Ga1-xMnxAs to evolve through at least two different processes, and they point to a complex interplay between the different defects and ferromagnetism in this material. △ Less

Submitted 28 May, 2001; originally announced May 2001.

Comments: 14 pages, pdf only, submitted to Applied Physics Letters

Journal ref: Appl. Phys. Lett. 79, 1495 (2001)

Two-Carrier Transport in Epitaxially Grown MnAs

Authors: J. J. Berry, S. J. Potashnik, S. H. Chun, K. C. Ku, P. Schiffer, N. Samarth

Abstract: Magneto-transport measurements of ferromagnetic MnAs epilayers grown by molecular beam epitaxy reveal the presence of both positive and negative charge carriers. Electrical transport at high temperatures is dominated by holes, and at low temperatures by electrons. We also observe distinct changes in the magnetoresistance associated with the transition between the electron- and hole-dominated tra… ▽ More Magneto-transport measurements of ferromagnetic MnAs epilayers grown by molecular beam epitaxy reveal the presence of both positive and negative charge carriers. Electrical transport at high temperatures is dominated by holes, and at low temperatures by electrons. We also observe distinct changes in the magnetoresistance associated with the transition between the electron- and hole-dominated transport regimes. These results are of direct relevance to MnAs/semiconductor hybrid heterostructures and their exploitation in electronic and optical spin injection experiments. △ Less

Submitted 25 May, 2001; originally announced May 2001.

Comments: 10 pages + 3 figures; to be published In Phys. Rev. B

Journal ref: Phys. Rev. B. 64, 052408 (2001)