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Journal of Magnetism and Magnetic Materials 304 (2006) e337–e339

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Vertical spin transport in MnAs/GaMnAs heterostructures

S.H. Chuna,b,, J.P. Yua, Y.S. Kimc, H.K. Choic,d, J.H. Bakc,d, Y.D. Parkc,d, Z.G. Khimc

aDepartment of Physic and Institute of Fundamental Physics, Sejong University, Seoul 143-747, Republic of KoreabFuture Technology Research Division, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea

cSchool of Physics, Seoul National University NS50, Seoul 151-747, Republic of KoreadCenter for Strongly Correlated Materials Research, Seoul National University NS53, Seoul 151-747, Republic of Korea

Available online 28 February 2006

Abstract

We have studied the effect of barrier strength on the tunneling magnetoresistance of MnAs/GaMnAs heterostructures with double

AlAs barriers. The epitaxial structures were grown by low-temperature molecular beam epitaxy. The vertical magnetotransport

properties were studied for various GaAs spacer thicknesses. We find that the junction resistivities of double barrier samples increase

exponentially as the barrier strength increases, implying that direct tunneling process governs the transport properties. In contrast, the

tunneling magnetoresistance depends primarily on the number of interfaces rather than on the barrier strength.

r 2006 Elsevier B.V. All rights reserved.

PACS: 72.25.Mk; 73.40.Gk

Keywords: GaMnAs; MnAs; Heterostructures; Spin injection; Double tunnel junction

Efficient injection of spin-polarized currents frommetallic ferromagnets into semiconductors is one of thekey elements for semiconductor spintronic devices operat-ing at room temperature. The difficulty comes from thelarge conductivity mismatch between metals and semicon-ductors, and it is suggested that a large interface resistancemight help solving the problem [1,2]. In fact, Schottkybarriers or tunnel barriers have been successfully used inthe demonstration of spin injection by optical method inlight-emitting diodes (LEDs) [3,4]. However, a definiteevidence of metal–semiconductor spin injection probed byelectrical measurement is still lacking. A partial success wasaccomplished by one of the authors in MnAs/GaMnAshybrid magnetic tunnel junctions (MTJs) separated byAlAs tunnel barriers [5]. GaMnAs is the most studiedferromagnetic semiconductor [6], and MnAs is a metallicferromagnet that can be grown epitaxially on the top ofzinc-blende semiconductors by low-temperature molecular

- see front matter r 2006 Elsevier B.V. All rights reserved.

/j.jmmm.2006.02.048

onding author. Department of Physic and Institute of

al Physics, Sejong University, Seoul 143-747, Republic of

+822 3408 3398; fax: +82 2 461 9356.

ddress: schun@sejong.ac.kr (S.H. Chun).

beam epitaxy (MBE) [7]. In that experiment, more than30% tunneling magnetoresistance (TMR) was observeddespite four orders of magnitude difference in conductivity.The next step was to inject spins from ferromagnetic metalsinto nonmagnetic semiconductors, which is the subject ofthis report. Here, we present the result of vertical transportmeasurements on MnAs/AlAs/GaAs/AlAs/GaMnAs dou-ble tunnel junctions (DTJs). Similar experiments onGaMnAs–GaMnAs DTJs claimed that the large TMRcame from sequential tunneling with negligible spinrelaxation in the GaAs spacer [8]. The results of ourMnAs–GaMnAs DTJs, however, imply direct tunneling asthe primary process governing the magnetotransport.All samples are prepared in a Riber MBE 32 deposition

chamber with standard effusion cells containing elementalsources of Ga(7N), Mn(5N), Al(7N), and As(7N) [9].Before each growth, molybdenum sample holders arechemically etched and epi-ready p+-GaAs(0 0 1) substratesare indium-mounted. Chemically etched sample holdersinsure an absolute range of temperature readings to becomparable between a run to run. After the growth of a200 nm-thick p-GaAs buffer layer at 600 1C, a sequence of100 nm-thick Ga1xMnxAs (xE3%), 1.5 nm-thick AlAs,

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S.H. Chun et al. / Journal of Magnetism and Magnetic Materials 304 (2006) e337–e339e338

GaAs (thickness dGaAs ¼ 2, 7, 10, and 20 nm), 5 nm-thickAlAs, and 40 nm-thick MnAs are grown at 250 1C. Theferromagnetic transition temperatures of GaMnAs andMnAs determined by a superconducting quantum inter-ference device are 60 and 320K, respectively. For verticaltransport measurements, 400 mm-diameter mesas are de-fined by photolithography and wet etching. The current–voltage characteristics along the whole structure aremeasured in a closed-cycle He refrigerator with an in-plane magnetic field ranging up to 2 kG. Most data aretaken by a standard DC source-measure unit, and low-frequency AC transport measurements using a lock-inamplifier are conducted for some samples to increasesignal-to-noise ratio.

Fig. 1(a) shows the low-bias junction resistivity as afunction of GaAs spacer thickness. The exponentialdependence implies that the transport is primarily due todirect tunneling across the whole AlAs/GaAs/AlAs com-posite barrier. According to sequential tunneling or asimilar mechanism, the resistivity should show negligibledependence on dGaAs. The current–voltage characteristicsshown in Fig. 1(b) help in figuring out the shape of this

0 2 4 6 8 10 12 14 16 18 20 22

1

10

100

1000

Res

isti

vity

(Ω

Ω

cm

2 )

dGaAs (nm)(a)

-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3

0.0

0.1

0.2

0.3

0.4

dGaAs = 10 nm

dGaAs = 20 nm

Co

nd

uct

ivit

y (Ω

-1 c

m-2

)

Bias Voltage (V)(b)

Fig. 1. (a) Junction resistivity of DTJs as a function of GaAs spacer

thickness. (b) Conductivity curves of thick GaAs spacer samples.

composite tunnel barrier. We observe a similar asymmetryin the conductivity curves as those of single-barrier hybridMTJs [5]. The difference is that DTJs are more conductingunder the reverse bias, which implies that the barrier heightexperienced by MnAs is larger than that experienced byGaMnAs (we define forward bias when the potential ofMnAs is higher than that of GaMnAs). It is plausibleconsidering the thicker AlAs barrier near MnAs. Thecomplexity of the composite barrier, however, prohibitsfurther analysis.Our main interest is on the TMR exhibited by DTJs. As

shown in Fig. 2(a), the field-dependent current under fixed-bias voltage shows about 1% TMR at low temperature.The TMR values decrease sharply as we increase the bias.Compared to more than 30% TMR of single-barrier MTJs,

-1 0 10.98

1.00

1.02

dGaAs = 2 nm

0.8%

dGaAs = 7 nm

R(H

)/R

(H=0

)

Magnetic Field (kOe)(a)

0 5 10 15 200.0

0.5

1.0

TM

R (

%)

dGaAs (nm)(b)

Fig. 2. (a) Normalized tunneling magnetoresistance of DTJs as a function

of in-plane magnetic field. (b) Dependence of tunneling magnetoresistance

on GaAs spacer thickness.

ARTICLE IN PRESSS.H. Chun et al. / Journal of Magnetism and Magnetic Materials 304 (2006) e337–e339 e339

this small TMR represents the role of interfaces in the spininjection. We note that the junction resistivities are muchlarger than that of single-barrier MTJ with similar barrierstrength (Fig. 1(a)). The introduction of two AlAs/GaAsinterfaces increases the overall junction resistivity, andmost of the spin information is lost due to the spin-flipscattering at the interfaces. This interpretation is supportedby the fact that the thickness of GaAs spacer, which affectsthe junction resistivity significantly, does not change theTMR as shown in Fig. 2(b). For dGaAsp10 nm, the TMRvalues remain essentially the same within the experimentaluncertainty. Therefore, we can conclude that the number ofinterfaces determines the magnitude of TMR in MnAs–GaMnAs DTJs.

For more efficient spin injection, there are severalparameters to be optimized. We fixed the thicknesses ofupper and lower AlAs in this experiment. Although thevalues are chosen based on the results from single-barrierMTJs [5,10], the optimum values for DTJs can be different.Especially, it is known that the TMR of GaMnAs–GaM-nAs MTJ depends strongly on the AlAs thickness [10].

The increase of DTJ resistivity compared to single-barrier MTJ resistivity of similar barrier strength impliesthat the interface quality should be improved. Due to thelow-temperature MBE growth, we expect higher density ofdefects such as As anti-sites. More elaborate growthtechniques like migration-enhanced epitaxy can be analternative way to improve the spin injection efficiency.

This work was supported by Korea Research Founda-tion Grant (KRF-2004-003-C00070), by KIST Vision 21program, and by the Korea Science and EngineeringFoundation (KOSEF) through the Center for StronglyCorrelated Materials Research (CSCMR). We would alsolike to thank H. C. Kim at KBSI for magnetizationmeasurements.

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