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Exchange biasing of the ferromagnetic semiconductor Ga1−xMnxAs

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Appl. Phys. Lett., Vol. 85, No. 9, 30 August 2004

Appl. Phys. Lett., Vol. 85, No. 9, 30 August 2004

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  • 1. APPLIED PHYSICS LETTERS VOLUME 85, NUMBER 9 30 AUGUST 2004Exchange biasing of the ferromagnetic semiconductor Ga1−xMnxAs K. F. Eid, M. B. Stone, K. C. Ku, O. Maksimov, P. Schiffer, and N. Samartha) Department of Physics and Materials Research Institute, Pennsylvania State University, University Park, Pennsylvania 16802 T. C. Shih and C. J. Palmstrøm Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 (Received 10 December 2003; accepted 13 July 2004) We demonstrate the exchange coupling of a ferromagnetic semiconductor ͑Ga1−xMnxAs͒ with an overgrown antiferromagnet ͑MnO͒. Unlike most conventional exchange biased systems, the blocking temperature of the antiferromagnet ͑TB = 48± 2 K͒ and the Curie temperature of the ferromagnet ͑TC = 55.1± 0.2 K͒ 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. © 2004 American Institute of Physics. [DOI: 10.1063/1.1787945] Ferromagnetic semiconductors (FMSs) are the focus of Samples are grown by low temperature molecular beamextensive research for applications in semiconductor epitaxy on (001) semi-insulating, epi-ready GaAs substratesspintronics.1,2 These hybrid materials offer new functionality bonded to a mounting block by indium. After thermal de-because their carrier-mediated ferromagnetism can be ma- sorption of the oxide layer, a high temperature GaAs layer isnipulated by employing established techniques used in semi- grown under standard conditions, followed by a low tem-conductor technology.3 Several device geometries have been perature GaAs layer grown at 250° C. The magnetically ac-demonstrated using the “canonical” FMS Ga1−xMnxAs, in- tive region of the sample consists of a 10-nm-thickcluding magnetic tunnel junctions,4–6 spin polarized light Ga1−xMnxAs layer ͑x = 0.08͒, capped with a thin Mn layeremitting diodes,7 and spin-tunable resonant tunnel diodes.8 It whose thickness is either ϳ4 nm or ϳ8 nm. After the growthis important in this context to develop a means of “exchange of the Ga1−xMnxAs layer, the sample is moved from thebiasing” ferromagnetic semiconductors for device applica- growth chamber into an adjoining high vacuum buffer cham-tions. This entails exchange coupling the FMS to a proximal ber and the As cell is cooled to room temperature. Once theantiferromagnet (AFM), and has been observed in ferromag- As pressure in the growth chamber decreases to an accept-netic (FM) metals, intermetallic alloys, and oxides.9–12 able level, the substrate is reintroduced into the growth The hallmark of an exchange biased system is a hyster- chamber, and the Mn capping layer is grown at room tem-esis loop which is not centered about zero magnetic field, but perature. Alternatively, the sample is transferred in vacuumis shifted either to positive or negative fields depending upon to an adjacent As-free growth chamber for the Mn capping.the orientation of the applied field while cooling the sample Exchange-biased samples have been obtained using boththrough the blocking temperature ͑TB͒ of the AFM layer. schemes. These precautions are to avoid the formation of FMThis is typically accompanied by an increase in the coercive MnAs during the growth of the capping layer.field ͑HC͒ of the FM. In most examples of exchange bias, The epitaxial growth is monitored by in situ reflectionTB Ͻ TN Ӷ TC, where TN is the Néel temperature of the AFM high energy electron diffraction (RHEED) at 12 keV. The Ga1−xMnxAs surface shows a clear reconstructed ͑1 ϫ 2͒and TC is the Curie temperature of the FM. The FMSGa1−xMnxAs is of current interest because TC can be varied RHEED pattern; the Mn surface shows a weaker, yet streaky and unreconstructed pattern whose symmetry is suggestivefrom ϳ10 to ϳ150 K by varying parameters such as Mn of the stabilization of a cubic phase of Mn. The thickness ofconcentration and sample thickness;13 in principle, this al- the Ga1−xMnxAs layer is calculated from RHEED oscilla-lows studies of exchange bias in a FM where TB can be tions, while the thickness of the Mn overlayer is estimatedabove or below TC. from RHEED oscillations of MnAs whose growth rate is We are aware of only one reported attempt to exchange mainly determined by the sticking coefficient of Mn. The Mnbias a FMS, using MnTe/ Ga1−xMnxAs bilayers.14 Although concentration in the ferromagnetic layer is estimated at xthe FMS showed an increased HC, there was no conclusive ϳ 0.08 based upon a calibration as a function of the Mn cellevidence for any exchange coupling between the FMS and temperature ͑T = 785° C͒.16 Magnetization measurements arethe AFM. We demonstrate here unambiguous exchange bias performed using a superconducting quantum interference de-in a Ga1−xMnxAs epilayer on which a thin layer of Mn has vice (SQUID) magnetometer, where samples are either fieldbeen deposited. Rutherford backscattering scattering (RBS) cooled or zero field cooled from room temperature to themeasurements show that—upon removal from the vacuum measuring temperature in a finite or zero magnetic field, re-chamber—the layer of Mn oxidizes to form MnO (an AFM spectively.with TN = 118 K 15). The postgrowth removal of the samples from the mount- ing block requires a modest annealing cycle for a few min-a) Electronic mail: nsamarth@phys.psu.edu utes at ϳ200° C during which the top Mn layer oxidizes to0003-6951/2004/85(9)/1556/3/$22.00 1556 © 2004 American Institute of Physics Downloaded 27 Aug 2004 to 129.169.10.56. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
  • 2. Appl. Phys. Lett., Vol. 85, No. 9, 30 August 2004 Eid et al. 1557FIG. 1. Hysteresis loops of a Ga1−xMnxAs ͑t = 10 nm͒ / MnO͑t ϳ 4 nm͒ bi-layer measured at T = 10 K, after cooling in the presence of different mag-netic fields: (a) H = 2500 Oe, (b) H = −2500 Oe, and (c) H = 0. In (d), weshow the hysteresis loop measured at T = 10 K for an uncapped Ga1−xMnxAs͑t = 15 nm͒ control sample, after field cooling in H = 1000 Oe. The diamag-netic and/or paramagnetic background has been subtracted from these (and FIG. 2. Panels (a)–(c) provide a summary of the temperature dependence ofall subsequently presented) hysteresis loops. the hysteresis loops and the hysteresis loop parameters for the sample de- scribed in Fig. 1. The hysteresis loop at any given temperature is measured after cooling the sample from T = 100 K to the measurement temperature inform MnO. The nature of the Mn layer is probed by RBS a field of H = 2500 Oe. Panels (a) and (b) show the field cooled hysteresisanalysis using 2.3 and 1.4 MeV He+ ions with both normal loop at T = 5 K and T = 30 K, respectively. Panel (c) shows the temperatureand glancing angle detector geometries, corresponding to dependence of the exchange field, HE = ͉͑HC− + HC+͒ / 2͉, and the coercive field, HC = ͉͑HC− − HC+͒ / 2͉ for T Ͻ TN. The inset panel (d) shows thescattering angles of 165° and 108°, respectively. Both ran- temperature-dependent magnetization at H = 20 Oe, after field cooling in Hdom and ͗001͘ channeling measurements are conducted. The = 10 kOe.channeling measurements show that the Mn overlayer iscompletely oxidized with stoichiometry close to MnOx (xϳ 1 to 1.5); the thickness of this MnO layer is consistent origin. As the temperature approaches TC, the hysteresis loopwith the estimate from RHEED oscillations. collapses into a single nonhysteretic curve of zero coercivity Figure 1(a) and 1(b) depict two hysteresis loops for the (not shown), indicating that the Ga1−xMnxAs layer has be-Ga1−xMnxAs ͑10 nm͒ / MnO ͑4 nm͒ bilayer, measured at T come paramagnetic. We do not observe any training effects= 10 K after cooling the sample in a magnetic field of H for a hysteresis loop performed multiple times at a particular= ± 2500 Oe. Out of a total of 12 samples, similar data are temperature after a single field cooling of the sample.reproduced for three other samples, with MnO layer thick- Figure 2(c) summarizes the temperature dependence ofness of both 4 and 8 nm. The “horizontal” offset of the hys- the hysteresis loop parameters HE and HC. From this figure,teresis loops from the origin in a direction opposite to that of we estimate TB = 48± 2 K. The temperature dependence ofthe cooling field is a clear signature of exchange coupling in the saturation magnetization [Fig. 2(d)] indicates that TCthe bilayer. Control measurements are used to rule out ex- = 55.1± 0.2 K; both the value of TC and the magnitude of thetrinsic effects: Figure 1(c) shows that cooling the sample in saturation magnetization are similar for the uncapped andzero applied magnetic field results in a slightly biased hys- capped samples, indicating that the Mn overlayer does notteresis loop. The presence of some bias suggests that a small affect the FM state of the Ga1−xMnxAs layer except throughremnant magnetic field was present in the SQUID while the exchange biasing. Fig. 2(c) shows that HE undergoes acooling the sample. Such a field partially magnetizes the FM monotonic decrease with temperature until vanishing at TB,layer, which in turn sets the bias in the AFM layer. The while HC passes through a broad peak near TB before becom-sheared appearance of the magnetization curve suggests that ing zero at TC. The absence of a second peak in HE at TC andthere are multiple domains in the FM and that the sample of any exchange bias in the paramagnetic state further sug-was very close to being zero-field and zero-magnetization gest that TC Ͼ TB.18cooled. Figure 1(d) shows that the exchange bias shift is Figure 3 shows the dependence of HE and HC on theabsent in a sample of similar thickness and grown under magnitude of the cooling field. Cooling fields as small asidentical conditions, but without the AFM overlayer. There is H = 25 Oe produce a significant exchange bias, and both HEalso a notable increase of the coercive field for the and HC vary only slightly with the magnitude of cooling fieldMn-capped sample compared to typical values for uncapped up to H ӷ HE, HC. These data indicate that once exchangesamples,17 which is consistent with the expected effects of bias has been established, it becomes insensitive to the mag-exchange biasing.9 nitude of the cooling field. Establishing exchange bias re- Figure 2(a) and 2(b) compare the field-cooled hysteresis quires only a field large enough to saturate the ferromagneticloops at 5 and 30 K, respectively. In these plots, the sample layer at temperatures above TB. Although the coupling en-is cooled from T Ͼ TC to the measuring temperature in a field ergy in this system ͑ϳ3 ϫ 10−3 erg/ cm2͒ is small comparedof H = 2500 Oe. With increasing temperature, the hysteresis to that in typical exchange bias systems, we observe rela-loops become narrower, and their centers move closer to the tively large exchange bias fields due to the relatively low Downloaded 27 Aug 2004 to 129.169.10.56. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
  • 3. 1558 Appl. Phys. Lett., Vol. 85, No. 9, 30 August 2004 Eid et al. Nonetheless, the demonstrated feasibility of exchange bias- ing of this FMS and the identification of the accompanying difficulties provides an important starting point for additional experiments. This research has been supported by the DARPA-SPINS program under Grant Nos. N00014-99-1093, -99-1-1005, -00-1-0951, and -01-1-0830, by ONR N00014-99-1-0071 and by NSF DMR 01-01318. We thank J. Bass for useful discussions. 1 H. Ohno, in Semiconductor Spintronics and Quantum Computation, edited by D. D. Awschalom, D. Loss, and N. Samarth, (Springer, Berlin, 2002), p. 1. 2FIG. 3. Hysteresis loop parameters at T = 10 K, as a function of the cooling S. A. Wolf, D. D. Awschalom, R. A. Buhrman, J. M. Daughton, S. vonfield for the sample described in Fig. 1. Measurements are taken after cool- Molnar, M. L. Roukes, A. Y. Chtchelkanova, and D. M. Treger, Scienceing from T = 100 K to T = 10 K in the respective magnetic fields. Note that 294, 1488 (2001). 3the field axis is split at H = 1.1 kOe and is depicted on different scales for H. Ohno, A. Shen, F. Matsukura, A. Oiwa, A. Endo, S. Katsumoto, and Y.magnetic fields less than or greater than this value. Iye, Appl. Phys. Lett. 69, 363 (1996). 4 M. Tanaka and Y. Higo, Phys. Rev. Lett. 87, 026602 (2001). 5 S. H. Chun, S. J. Potashnik, K. C. Ku, P. Schiffer, and N. Samarth, Phys.magnetization of the FM layer. Samples with an 8-nm-thick Rev. B 66, 100408(R)(2002). 6MnO layer have a similar value of HE as the ones with a R. Mattana, J.-M. George, H. Jaffres, F. Nguyen Van Dau, A. Fert, B.4 nm layer, suggesting that the critical thickness of MnO Lepine, A. Guivarc’h, and G. Jezequel, Phys. Rev. Lett. 90, 166601required for exchange biasing is less than 4 nm.10 This small (2003). 7 Y. Ohno, D. K. Young, B. Beschoten, F. Matsukura, H. Ohno, and D. D.value of the critical thickness is consistent with the large Awschalom, Nature (London) 402, 790 (1999).anisotropy constant K reported for bulk MnO ͑K = 6 8 H. Ohno, N. Akiba, F. Matsukura, A. Shen, K. Ohtani, and Y. Ohno, Appl.ϫ 103 ergs/ cm3͒.19 Phys. Lett. 73, 363 (1998). 9 Finally, we comment on the reproducibility of our re- W. H. Meiklejohn and C. P. Bean, Phys. Rev. 102, 1413 (1956). 10 J. Nogués and I. Schuller, J. Magn. Magn. Mater. 192, 203 (1999).sults: we attribute the low yield of successfully exchange- 11 X. W. Wu and C. L. Chien, Phys. Rev. Lett. 81, 2795 (1998).biased samples to the extreme reactivity of Mn with GaAs.20 12 P. J. van der Zaag, R. M. Wolf, A. R. Ball, C. Bordel, L. F. Feiner, and R.This may create a detrimental interfacial layer when the Jungblut, J. Magn. Magn. Mater. 148, 346 (1995). 13samples are annealed during removal from the mounting K. C. Ku, S. J. Potashnik, R. F. Wang, S. H. Chun, P. Schiffer, N. Samarth,block. To test this hypothesis, we intentionally annealed a M. J. Seong, A. Mascarenhas, E. Johnston-Halperin, R. C. Myers, A. C. Gossard, and D. D. Awschalom, Appl. Phys. Lett. 82, 2302 (2003).sample that shows exchange bias for 10 min at 220° C; these 14 J. K. Furdyna, X. Liu, Y. Sasaki, S. J. Potashnik, and P. Schiffer, J. Appl.conditions—only slightly more severe than those used for Phys. 91, 7490 (2002).sample removal—completely eliminate the exchange bias. 15 M. S. Jagadeesh and M. S. Seehra, Phys. Rev. B 23, 1185 (1981). 16RBS analysis on this annealed sample still shows a MnO S. J. Potashnik, K. C. Ku, R. Mahendiran, S. H. Chun, R. F. Wang, N.overlayer with similar characteristics to the exchange biased Samarth, and P. Schiffer, Phys. Rev. B 66, 012408 (2002). 17 S. J. Potashnik, K. C. Ku, R. F. Wang, M. B. Stone, N. Samarth, P.sample; however, we find a slight increase in the channeling Schiffer, and S. H. Chun, J. Appl. Phys. 93, 6784 (2003).minimum yield and also a small enhancement of the Ga 18 The absence of any appreciable background magnetization beyond TC inchanneling interface peak by ϳ1 ϫ 1015 atoms/ cm2. This either of the samples measured demonstrates the absence of observablesuggests the formation of a Mn– GaAs interfacial layer, con- MnAs clusters or any ferromagnetism derived from the Mn capping lay-sistent with studies of thermally induced reactions between er;this is verified by carrying out magnetization measurements above the Curie temperature of MnAs ͑320 K͒.Mn and GaAs.20 Such interfacial reactivity will likely play 19 F. Keffer and W. O’Sullivan, Phys. Rev. 108, 637 (1957).an important role in attempted exchange biasing 20 J. L. Hilton, B. D. Schultz, S. McKernan, and C. J. Palmstrøm, Appl.Ga1−xMnxAs using other AFMs such as MnTe and FeMn. Phys. Lett. 84, 3145 (2004). Downloaded 27 Aug 2004 to 129.169.10.56. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp