3 páginas, 3 figuras, 1 tabla.We report the observation of a giant magnetoresistance effect in a low-temperature (LT-)GaAs/MnAs nanoscale ferromagnet hybrid structure. The MnAs nanomagnets are formed by ion implantation of Mn into LT GaAs and subsequent annealing. We have studied the magnetotransport using a vertically biased p+-GaAs/LT-GaAs:MnAs/p+-GaAs structure. A negative magnetoresistance (Δρ/ρ = [ρ(B)−ρ(0)]/ρ(0)) of up to −80% (B = 7 T) is observed at low temperatures (T<20 K), which changes its sign from negative to positive between T = 15 K and T = 20 K. The value of the positive magnetoresistance decreases with increasing temperature from +115% (20 K) to +1.4% (300 K). The magnetoresistance variations with B and T are correlated with the nanomagnet spacing in the structure.This work has been supported by AFOSR/PRET, Center
for Non Stochiometric Semiconductors (Grant No. 49620-
95-1-0394), the Spanish Ministry of Education (J.M.G.), and
the Deutsche Forschungsgemeinschaft (P.J.W.).Peer reviewe

Feng, J. L.

García Martínez, Jorge Manuel

Petroff, Pierre M.

Wellmann, P. J.

Applied Physics Letters

Digital.CSIC

APPLIED PHYSICS LETTERS VOLUME 73, NUMBER 22 30 NOVEMBER 1998DownGiant magnetoresistance in a low-temperature GaAs/MnAs nanoscaleferromagnet hybrid structureP. J. Wellmann,a) J. M. Garcia, J.-L. Feng, and P. M. PetroffMaterials Department, University of California in Santa Barbara, Santa Barbara, California 93106~Received 31 March 1998; accepted for publication 1 October 1998!We report the observation of a giant magnetoresistance effect in a low-temperature ~LT-!GaAs/MnAs nanoscale ferromagnet hybrid structure. The MnAs nanomagnets are formed by ionimplantation of Mn into LT GaAs and subsequent annealing. We have studied the magnetotransportusing a vertically biased p1-GaAs/LT-GaAs:MnAs/p1-GaAs structure. A negative magneto-resistance (Dr/r5@r(B)2r(0)#/r(0)) of up to 280% (B57 T) is observed at low temperatures(T,20 K), which changes its sign from negative to positive between T515 K and T520 K. Thevalue of the positive magnetoresistance decreases with increasing temperature from 1115% ~20 K!to 11.4% ~300 K!. The magnetoresistance variations with B and T are correlated with thenanomagnet spacing in the structure. © 1998 American Institute of Physics.@S0003-6951~98!03148-9#The study of hybrid semiconductor ferromagnet struc-tures based on III–V compounds have gained momentumdue to their possible application in new semiconductor de-vices such as magnetic-field sensors, magneto-optical de-vices, and memories. Molecular beam epitaxy ~MBE! growndiluted magnetic semiconductors like In12xMnxAs andGa12xMnxAs (x50 – 0.09) have been the subject of manyrecent publications1–4 with an emphasis on magnetotransportand magneto-optical properties. For large values of x ~e.g.,x<0.06!, MnAs-like clusters may be formed during anneal-ing of Ga12xMnxAs.5 In a different approach, the formationof nanoscale ferromagnetic MnGa and MnAs crystallites us-ing Mn1 ion implantation into GaAs or LT GaAs (LT5lowtemperature! and subsequent annealing has recently beendemonstrated.6,7 In this letter we present the observation of agiant magnetoresistance effect in the hybrid semiconductornanomagnet system: LT-GaAs:MnAs using the ion implan-tation processing technology.The magnetotransport characteristics have been studiedusing a vertically biased p1-GaAs/LT-GaAs/p1-GaAsstructure grown by MBE ~see the inset of Fig. 1!. On top ofa semi-insulating ~100! GaAs substrate we have deposited 44periods of a GaAs/AlAs superlattice ~2 nm/2 nm! followedby 500 nm of p1-GaAs (Tgrowth5630 °C, p5531018cm23!, 250 nm of LT GaAs (Tgrowth5250 °C, undoped!, and50 nm of p1-GaAs (Tgrowth5350 °C, p5531018 cm23!with carbon as the p dopant. Manganese ions were implantedwith an energy of 180 keV and a dose of 131016 cm22.After rapid thermal annealing at 750 °C in a forming gasatmosphere, nanosize MnAs crystallites are formed in theLT-GaAs layer about 150 nm below the sample surface.7The samples were capped with a silicon nitride layer ~100nm! to prevent the evaporation of As during heat treatment.Mesa structures of p1-GaAs/LT-GaAs/p1-GaAs have beenchemically etched and magnetotransport measurements werea!Current address: Institute for Materials Science 6, University of Erlangen,Martens Str. 7, D-91058 Erlangen, Germany. Electronic mail:peter.wellmann@ww.uni-erlangen.de3290003-6951/98/73(22)/3291/3/$15.00loaded 05 Nov 2010 to 161.111.180.191. Redistribution subject to AIP lcarried out on 2, 10, 90, or 100 devices in series. Thermallyevaporated Au/Zn/Au layers ~10 nm/10 nm/200 nm! an-nealed at 420 °C ~30 s! were used as Ohmic contacts to thep1-GaAs. Magnetotransport measurements were performedin a He cryostat with a variable temperature insert (T52 – 300 K) and a superconducting magnet (Bmax58 T).We have studied three samples. Samples A and B were Mnimplanted and annealed at 750 °C for 180 and 5 s, respec-tively. Sample A contains a low density ('109 cm22) oflarge MnAs nanomagnets (d'50 nm) while sample B con-tains a high density ('1010 cm22) of small MnAs nanomag-nets (d'10 nm) imbedded in the LT-GaAs layer.7 The meanMnAs crystallite diameter was obtained in transmission elec-tron microscope micrographs. Sample C was a controlsample without Mn implantation, and was annealed at750 °C ~180 s!.Figure 1 shows the current–voltage characteristics ofsample A. In this case 90 devices have been measured inseries. At low temperatures (T55 K) rectifying behaviordominates, which is typical for p-doped/intrinsic/p-doped(p – i – p) structures. The rectifying properties disappear athigher temperature because the thermal activation of holesFIG. 1. Temperature dependence of current–voltage characteristics ofsample A @@Mn#5131016 cm22, annealed at 750 °C ~180 s!#. The inset il-lustrates the vertical biased p1-GaAs/LT-GaAs/p1-GaAs structure contain-ing nanosize MnAs ferromagnets in the LT-GaAs layer.1 © 1998 American Institute of Physicsicense or copyright; see http://apl.aip.org/about/rights_and_permissions3292 Appl. Phys. Lett., Vol. 73, No. 22, 30 November 1998 Wellmann et al.Downfrom the deep acceptor manganese into the valence band ofthe ion implanted LT-GaAs layer takes over. At room tem-perature ~300 K! Ohmic behavior dominates.Figure 2 shows for various temperatures the magne-totransport properties of sample A. The magnetoresistance isdefined as Dr/r5@r(B)2r(0)#/r(0) with r being the re-sistivity in ~V cm!. A giant negative magnetoresistive effectis observed at low temperatures (T55 K): Dr/r523.4%(B50.5 T) and Dr/r5280% (B57 T). Between T512 K and T516 K a positive magnetoresistance occursand reaches its maximum value at T520 K: Dr/r515%(B50.5 T) and Dr/r51115% (B57 T). Above T520 Kthe value of the positive magnetoresistance decreases withincreasing temperature to about 1.4% at T5300 K. Experi-ments with the B field applied parallel to the sample surfacebut in opposite direction reproduce the magnetoresistancedata of Fig. 2. Measurements carried out with the B fieldapplied perpendicular to the sample surface instead of paral-lel also give qualitatively the same results as shown in Fig. 2~summarized in Table I!.A characteristic feature of the observed magnetoresis-tance effect is that the sign of Dr/r is correlated with the Efield applied across the vertical transport sample for T<20 K ~above T520 K the sign of Dr/r is always positive!.The magnetoresistance gradually changes its sign from nega-tive to positive at T516 K ~Fig. 3! as the E field decreasesfrom 8600 to 1600 V/cm. It should be pointed out that due tothe diode characteristics of our device ~Fig. 1! an initial Efield has to be applied to inject charge carriers into the LT-GaAs layer. An exponential fit of the I(V) curve revealed aninitial E field of 1200 V/cm at T516 K.FIG. 2. Temperature dependence of magnetoresistance Dr/r5@r(B)2r(0)#/r(0) of sample A. The B field has been applied parallel to thesample surface (Bi). The applied electrical field was E58000 V/cm (T55 – 20 K), E56000 V/cm (T525 K), and E5100 V/cm (T5300 K).loaded 05 Nov 2010 to 161.111.180.191. Redistribution subject to AIP lSample B, containing 10 nm MnAs nanomagnets, showsthe same behavior as sample A, containing 50 nm MnAsnanomagnets. However, the observed effects of Dr/r areonly in the order of 1% for sample B versus 100% forsample A ~Table I!. The control sample C with no MnAsnanomagnets imbedded in the LT-GaAs layer shows a smallpositive magnetoresistance Dr/r in the order of 1%, which isattributed to the well-known geometrical magnetoresistanceeffect in semiconductors ~Table I!.Magnetotransport models proposed for dilutedMnxGa12xAs semiconductors (x53% – 9%) ~Refs. 2 and 4!are unlikely to be applicable to our system. These modelsassume a ferromagnetic ordering of the Mn spins (MnGa)mediated by an antiferromagnetic Mn–hole exchange inter-action. In our case the concentration of residual Mn ions onthe lattice site (MnGa) is estimated to be >1019 cm23 and,therefore, more than one order of magnitude less than in thecase of diluted semiconductors. Thus, our system is too di-luted for a hole mediated exchange interaction of the Mnions.Spin-dependent scattering of the charge carriers8 at thesemiconductor–ferromagnet interface, similar to that foundin the granular metallic Co–Cu system,9 is also not believedto be dominant in our samples. Such an effect would favorsample B containing a high density (1010 cm22) of 10 nmMnAs nanomagnets, because the spin information has to becarried from one MnAs nanomagnet to the next in order tocause a resistance change due to spin-polarized scattering ofthe charge carriers at the semiconductor–nanomagnet inter-face. The data summarized in Table I clearly show that themagnetoresistance effect is significantly larger for sample Acontaining a low density (109 cm22) of 50 nm MnAs nano-FIG. 3. E-field dependence of magnetoresistance Dr/r5@r(B)2r(0)#/r(0) of sample A at T516 K. The B field has been applied parallel to thesample surface (Bi).TABLE I. Comparison of magnetoresistance values Dr/r5@r(B)2r(0)#/r(0) for sample A, sample B, andcontrol sample C.dr/r at 0.5 T~5 K!Dr/r at 7 T~5 K!Dr/r at 7 T~80 K!r at B50(T,20 K)Sample A 23.4% (Bi) 280% (Bi) 18% (Bi) 140 MV cm@Mn implanted, 750 °C ~180 s!# 210% (B') 225% (B') 112% (B')Sample B ,20.1% (Bi) 20.4% (Bi) 11.2% (Bi) 6.1 MV cm@Mn implanted, 750 °C ~5 s!#Sample C ,10.1% (Bi) 11.5% (Bi) ,10.1% (Bi) 2.9 MV cm@No Mn, 750 °C ~180 s!#icense or copyright; see http://apl.aip.org/about/rights_and_permissions3293Appl. Phys. Lett., Vol. 73, No. 22, 30 November 1998 Wellmann et al.Downmagnets. Therefore, we conclude that spin-dependent scatter-ing cannot explain the observed magnetoresistance effects.The comparison of sample A, sample B, and controlsample C ~Table I! demonstrates that the observed effects arecaused by the MnAs nanosize ferromagnets and that thevalue of Dr/r scales with the size of the nanomagnets. In thesimplified approach of a single-domain particle, the B fieldassociated with a nanomagnet is given by a dipole field dis-tribution and is proportional to the magnetic moment M ofthe MnAs nanomagnet. M is given by M5NMnmBg whereNMn , mB , and g are the number of Mn ions that contribute tothe nanomagnet, the Bohr magneton, and the Lande´ factor,respectively. Therefore, if the magnetoresistance Dr/r scaleswith the nanomagnet size, it also scales with the magnetic-field strength induced by the nanomagnets. Elastic and in-elastic scattering of the charge carriers by the nanomagnetdipole field could be related to the observed magnetoresis-tance effects.Additional evidence for this scattering mechanism isprovided by zero B-field electrical measurements. Below T520 K (B50 T) the resistivity of samples A, B, and C lev-els off at different values ~Table I!. Remarkably, the resistiv-ity is strongly increased in the presence of MnAs nanomag-nets. Sample B, containing 10 nm MnAs nanomagnets showsdouble the resistivity of control sample C without MnAsnanomagnets ~6.1 MV cm vs 2.9 MV cm!. Even more strik-ing, the resistivity of sample A containing 50 nm MnAs na-nomagnets is 140 MV cm vs 2.9 MV cm for control sampleC. We relate this enormous increase in resistivity to the in-creased path of the charge carriers through the device struc-ture, caused by the scattering by the nanomagnet dipole field.The underlying physical effects of the observed negativemagnetoresistance turn out to be rather complicated. How-ever, we will discuss a simplified semiclassical model whichcould give further insight. In this model at large appliedmagnetic fields (B.2 – 3 T) and low temperatures (T<20 K) the charge carriers would be forced on cyclotronorbits that are smaller than the average distance of the MnAsnanomagnets. The charge carriers would drift through theLT-GaAs:MnAs layer without interacting with the MnAs na-nomagnets, giving rise to a negative magnetoresistance. Thecyclotron orbit is given by Rc5vh /vc and vc5eB/mhvh isthe hole velocity and mh and e are the electron charge andthe effective hole mass (mh50.5me ; me5electron mass!,respectively. Due to implantation induced shortcut channelsthrough the substrate we were not able to perform Hall mea-surements in a lateral device structure in order to determinethe hole mobility mh and, hence, vh . Therefore, we willassume that vh is approximately given by the saturation ofthe hole drift velocity vd . Dalal et al.10 have reported that ahole saturation velocity in p-doped GaAs of 1.73107 cm/sfor T,100 K. The measured hole drift velocity vd was 13107 cm/s at an E field of E5104 V/cm and T5135 K. Inthe present samples, the implanted and annealed LT GaAs,which contains 109 cm22 dislocations, we will also assume asimilar hole drift velocity at T<20 K. For vh513107 V/cm and B53 T, we find Rc5140 nm, indicatingthat the cyclotron orbit would be indeed smaller than theaverage distance of the MnAs nanomagnets of 300 nm. Inaddition, the E-field-dependent magnetotransport data ofloaded 05 Nov 2010 to 161.111.180.191. Redistribution subject to AIP lsample A ~Fig. 3! support the existence of a transport regimeat large E fields, where the hole mean-free-path length islarge enough to force the charge carriers on cyclotron orbits.Increasing the electrical field above '5000 V/cm ~at this Efield the magnetoresistance changes from a positive to anegative sign! implies an increase of the hole mean-free-pathlength lh to .300 nm, which is the average distance of theMnAs nanomagnets. The latter argument, the E-field depen-dent increase of lh , holds only if vh is not in saturation atE,5000 V/cm.Above T520 K, independent of the applied E field, apositive magnetoresistance is always observed. The tempera-ture dependence of the IV characteristics of sample A ~Fig.1! shows a decrease of the rectifying behavior above T520 K, indicating an onset of the thermal activation of holesfrom the deep acceptor Mn into the valence band, causing adecrease of the mean-free-path length due charge carrierscattering by ionized Mn impurities. Above T520 K thepositive magnetoresistance can be related to the well-knowngeometrical magnetoresistance effect. In the absence of aHall-field, Dr/r is given by (mhB)2 with mh5vdE . With thedata given by Dalal et al.10 (vd513107 cm/s at E5104 V/cm!, Dr/r is approximately 150% at B57 T. Thelatter value for Dr/r coincides pretty well with our measure-ments in Fig. 2: Dr/r(20 K)51100% and Dr/r(25 K)5150%. However, below T520 K the negative magnetore-sistance due to a decreased interaction of the charge carrierswith the MnAs nanomagnets is believed to overcompensatethe contribution of the positive geometrical magnetoresis-tance effect.To clarify whether the semiclassical approach as sug-gested above is capable of explaining the giant magnetore-sistance effect in our samples, a semiclassical calculation forvarious charge carriers trajectories through the LT-GaAslayer containing the MnAs nanomagnets would be helpful.This work has been supported by AFOSR/PRET, Centerfor Non Stochiometric Semiconductors ~Grant No. 49620-95-1-0394!, the Spanish Ministry of Education ~J.M.G.!, andthe Deutsche Forschungsgemeinschaft ~P.J.W.!.1 H. Munekata, H. Ohno, S. von Molnar, A. Segmu¨ller, L. L. Chang, and L.Esaki, Phys. Rev. Lett. 63, 1894 ~1989!.2 H. Ohno, A. Shen, F. Matsakura, A. Oiwa, A. Endo, S. Katsumoto, and Y.Iye, Appl. Phys. Lett. 69, 363 ~1996!; F. Matsukura, H. Ohno, A. Shen,and Y. Sugawara, Phys. Rev. B 57, R2037 ~1998!.3 T. Hayashi, M. Tanaka, T. Nishinaga, and H. Shimada, J. Appl. Phys. 81,4865 ~1997!.4 A. Van Esch, L. Van Bockstal, J. De Boeck, G. Verbanck, A. S. vanSteenbergen, P. J. Wellmann, B. Grietens, R. Bogarts, F. Herlach, and G.Borghs, Phys. Rev. B 56, 13103 ~1997!.5 J. De Boeck, R. Osterholt, A. Van Esch, H. Bender, C. Bruynseraede, C.Van Hoof, and G. Borghs, Appl. Phys. Lett. 68, 2744 ~1996!.6 J. Shi, J. M. Kikkawa, R. Proksch, T. Schaeffer, D. D. Awschalom, G.Medeiros-Ribeiro, and P. M. Petroff, Nature ~London! 377, 707 ~1995!.7 P. J. Wellmann, J. M. Garcia, J.-L. Feng, and P. M. Petroff, Appl. Phys.Lett. 71, 2532 ~1997!.8 H. Akinaga, J. De Boeck, G. Borghs, S. Miyanishi, A. Asamitsu, W. VanRoy, Y. Tomioka, and L. H. Kuo, Appl. Phys. Lett. 72, 3368 ~1998!.9 A. E. Berkowitz, J. R. Mitchell, M. J. Carey, A. P. Young, S. Zhang, F. E.Spada, F. T. Parker, A. Hutten, and G. Thomas, Phys. Rev. Lett. 68, 3745~1992!; J. Q. Xiao, J. S. Jiang, and C. L. Chien, ibid. 68, 3749 ~1992!.10 V. L. Dalal, A. B. Dreeben, and A. Triano, J. Appl. Phys. 42, 2864~1971!.icense or copyright; see http://apl.aip.org/about/rights_and_permissions

Giant magnetoresistance in a low-temperature GaAs/MnAs nanoscale ferromagnet hybrid structure

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