Drašar, Č.

Dyck, Jeffrey

Lošt’ák, P.

Luciana, A J

Mitchell, T J

Quayle, P. C.

English

John Carroll University

John Carroll UniversityCarroll CollectedPhysics2007Significant suppression of ferromagnetism byhydrostatic pressure in the diluted magneticsemiconductor Sb2−xVxTe3 with xÏ0.03Jeffrey DyckJohn Carroll University, jdyck@jcu.eduT J. MitchellA J. LucianaP. C. QuayleČ. DrašarSee next page for additional authorsFollow this and additional works at: http://collected.jcu.edu/phys-facpubPart of the Physics CommonsThis Article is brought to you for free and open access by Carroll Collected. It has been accepted for inclusion in Physics by an authorized administratorof Carroll Collected. For more information, please contact connell@jcu.edu.Recommended CitationDyck, Jeffrey; Mitchell, T J.; Luciana, A J.; Quayle, P. C.; Drašar, Č.; and Lošt’ák, P., "Significant suppression of ferromagnetism byhydrostatic pressure in the diluted magnetic semiconductor Sb2−xVxTe3 with xÏ0.03" (2007). Physics. 1.http://collected.jcu.edu/phys-facpub/1AuthorsJeffrey Dyck, T J. Mitchell, A J. Luciana, P. C. Quayle, Č. Drašar, and P. Lošt’ákThis article is available at Carroll Collected: http://collected.jcu.edu/phys-facpub/1Significant suppression of ferromagnetism by hydrostatic pressure in thediluted magnetic semiconductor Sb2−xVxTe3 with xÏ0.03J. S. Dyck,a! T. J. Mitchell, A. J. Luciana, and P. C. QuayleDepartment of Physics, John Carroll University, University Heights, Ohio 44118, USAČ. Drašar and P. LošťákFaculty of Chemical Technology, University of Pardubice, 532 10 Pardubice, Czech Republic!Received 30 July 2007; accepted 31 August 2007; published online 20 September 2007"The authors report on the hydrostatic pressure dependence of the magnetotransport properties offerromagnetic Sb2−xVxTe3 single crystals with x=0.02–0.03. Pressure significantly increases thefree hole concentration in these compounds. In turn, the Curie temperature is suppressed by roughly40%, which goes against many models that would predict an increase in Curie temperature withincreasing carrier concentration. These results indicate that the ferromagnetism in these materials iscarrier mediated and that a full Ruderman-Kittel-Kasuya-Yosida model that takes into account theoscillatory nature of the indirect exchange interaction among localized spins is needed in order toexplain the data. © 2007 American Institute of Physics. #DOI: 10.1063/1.2787881$The tetradymite-type, narrow-gap semiconductors dopedwith small concentrations of transition metal ions have beenidentified as diluted magnetic semiconductors !DMSs".1–4 InSb2−xTMxTe3 single crystals with TM=V,Cr, ferromagneticorder develops at low temperatures !up to %25 K" with aCurie temperature TC that is proportional to x.1,3 The originof the ferromagnetic order in these materials is so far notestablished, but a carrier-mediated interaction among local-ized transition metal !TM" moments is likely given the dilutenature. Theoretical studies have been aimed at predicting TCin DMS materials and understanding the details of the spin-carrier interactions that give rise to long range ferromagneticorder. !See Ref. 5 for a review."Unlike the traditional III-V based diluted magnetic semi-conductors doped with manganese where Mn is an acceptor,6providing both holes as well as localized spins, the TM ionsin Sb2Te3 do not alter the background hole concentration p.DMS systems that allow one to independently vary the mag-netic ion and free carrier concentration independently areattractive to help study the origin of ferromagnetism in thesecompounds. Normally, one is faced with the complication ofcodoping to alter p, introducing a fourth element such as in!Ga,Be"1−xMnxAs !Ref. 7" or !Sb,Bi"2−xVxTe3 !Ref. 8" andin such materials, it is difficult to control x and p simulta-neously over a wide series of samples.This paper is focused on studying the free hole concen-tration dependence of the Curie temperature by varying p ina single sample using pressure as an external variable. Thereare very few experiments in the literature utilizing pressureas a tool to investigate spin-carrier interactions in a ferro-magnetic DMS. In one very recent example, it was observedthat application of high pressures increases TC from7.0 to 8.8 K in In1−xMnxSb thin films.9 Unlike in our experi-ments, pressure did not affect the carrier concentration inIn1−xMnxSb. Therefore, our material system provides aunique opportunity to probe the impact that hole concentra-tion has on the magnetic phase diagram.Pure Sb2Te3 single crystals, the host structure of theDMS considered here, typically have a background hole con-centration of 1!1020 cm−3 due to the presence of nativeantisite defects. The valence band is sixfold degenerate, anddata from the Shubnikov–de Haas measurements suggest theexistence of a high mobility upper valence band and a lowervalence band with much lower mobility,10 though the lowervalence band is not well characterized. High pressure signifi-cantly alters the electrical transport properties in Sb2Te3 asevidenced by a decrease in electrical resistivity,11 an effectalso seen in vanadium-doped variants to be presented here.The origin of this change may be due to a modification of theband structure, though recent band structure calculations pre-dict very little change in the valence band near the Fermienergy under hydrostatic pressure.12 Another possibility is anincrease in total hole concentration. First-principles calcula-tions found that hydrostatic pressure lowers the formationenergy of antisite defects in Sb2Te3,13 though the kinetics ofdefect formation has not been studied. Regardless of themechanism, our high pressure studies demonstrate a clearconnection between hole concentration and Curie tempera-ture in vanadium-doped Sb2Te3.Bulk, single crystals of Sb2−xVxTe3 with 0.02"x"0.03 were prepared by a modified Bridgman method.Crystal composition and stoichiometry were verified byatomic absorption spectroscopy and electron probe mi-croanalysis. Samples for magnetotransport measurementsunder high pressure were cut using a spark erosion tech-nique. The specimens were loaded into a liquid-clamp pres-sure cell with 3M™ Fluorinert™ electronic liquid FC-77 aspressure transmitting fluid, and pressures were determined insitu using a calibrated InSb gauge. Temperature dependentelectrical resistivity, Hall effect, and magnetoresistance in astandard six-probe configuration were performed in a 6 Tmagnet cryostat system using a digital lock-in amplifier op-erating at 16 Hz. Magnetic field was oriented parallel to thecrystallographic c axis. The Hall coefficient was determinedfrom the slope of the Hall resistivity versus field curve,which was linear above 0.5 T for all samples and tempera-tures !near and below TC, a small nonlinearity at low fieldsresults from the anomalous Hall effect".a"Electronic mail: jdyck@jcu.eduAPPLIED PHYSICS LETTERS 91, 122506 !2007"0003-6951/2007/91"12!/122506/3/$23.00 © 2007 American Institute of Physics91, 122506-1Downloaded 19 Nov 2012 to 76.241.123.224. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsA plot of resistivity # versus temperature for a singlecrystal of Sb2−xVxTe3 with x=0.03 is given in Fig. 1 for arange of pressures up to 1.5 GPa. Over most of the tempera-ture range, # increases with T, characteristic of a degener-ately doped semiconductor. As pressure is increased, the re-sistance is reduced proportionally at all temperatures, whichindicates that the scattering mechanisms do not evolve withpressure. At low temperatures !see inset", a peak developswhich signals a ferromagnetic transition. Qualitatively simi-lar behavior was seen with other samples with lower x, withthe resistivity peak developing at a lower temperature. Ear-lier experiments verified that this peak correlates directlywith the Curie temperature TC as determined by magnetiza-tion data.1 As pressure is increased, this peak smoothly shiftsfrom 14 to 8.5 K at 1.5 GPa, corresponding to a 40% de-crease in TC. Upon releasing the pressure, the original prop-erties are obtained indicating reversibility.Magnetoresistance !MR" data also illustrate a weakeningof the ferromagnetism with pressure. Figure 2!a" displays theresistivity normalized to the zero field value at 4.2 K for arange of pressures in Sb1.97V0.03Te3. Other samples showedthe same behavior. Applied field causes a reduction of thespin disorder scattering resulting in a negative MR, which isdirectly related to the sample magnetism. Increased pressuremoves TC to lower temperatures, and hence the negative MRis less prominent. The coercive field HC was determinedfrom the hysteresis in MR at low fields as displayed in Fig.2!b". Lower vanadium concentration yields a lower HC, andFig. 2!c" shows that pressure significantly reduces HC for allvalues of x.Hall effect measurements were carried out to uncoverthe dual role that carrier concentration plays in affecting boththe resistivity and Curie temperature. Because the Hall coef-ficient is independent of magnetic field, a single band modelwith only one type of hole, rather than two, is favored here.This enables a straightforward determination of hole concen-tration from p=1/ !eRH", where RH is the Hall coefficient.The influence of pressure on the carrier concentration inSb2−xVxTe3 is illustrated in Fig. 3!a". Our data show a clearincreasing trend in p with applied pressure for all samples.Further, the Hall mobility $H=Ro /# is very insensitive topressure #see Fig. 3!b"$. By comparing to the pressure depen-dence of the electrical resistivity in Fig. 1 and noting that#=1/ pe$, one can clearly see that the reduction in # iscaused by an increase in p. The fact that mobility is roughlyconstant with pressure also suggests that the hole effectivemass is not changing, further supporting a single bandpicture.It is clear that pressure is suppressing the Curie tempera-ture in Sb2−xVxTe3 single crystals with x"0.03, and we pos-tulate that the primary cause is an increase in the hole con-centration. A mean-field model is often invoked to describeTC of DMS and predicts that TC%S!S+1"mnip1/3, where S isthe vanadium spin !S=1", m is the hole effective mass, and niFIG. 1. Pressure dependence of the resistivity vs temperature forSb1.97V0.03Te3 single crystal. !Inset" Normalized resistivity at low tempera-ture which shows that pressure decreases the Curie temperature.FIG. 2. !a" Pressure dependence of normalized magnetoresistance vs mag-netic field at 4.2 K for Sb1.97V0.03Te3 single crystal. !b" Atmospheric pres-sure magnetoresistance hysteresis loop; coercive field HC is indicated bydashed lines. !c" Pressure dependence of the coercive field at 4.2 K.FIG. 3. !a" Free hole concentration and !b" Hall mobility at 4.2 K and roomtemperature as a function of pressure for Sb2−xVxTe3 single crystals.122506-2 Dyck et al. Appl. Phys. Lett. 91, 122506 "2007!Downloaded 19 Nov 2012 to 76.241.123.224. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsis the concentration of magnetic ion impurities.14,15 Ourtransport results above suggest a single band model with pincreasing by roughly a factor of 2 at the highest pressures,while m stays constant, implying that TC should increase.However, this model is not valid unless p /ni&1 !Refs. 16and 17" and carrier mobility is small ensuring that the sign ofthe Ruderman-Kittel-Kasuya-Yosida !RKKY" interaction isonly ferromagnetic because the first zero in the oscillatingRKKY term !see below" occurs at a distance much longerthan the mean free path.18 Adopting a model of a sixfolddegenerate valence band !i.e., total hole concentration di-vided among six equivalent carrier pockets", p /ni increasesfrom %0.1 to %0.2 with increasing pressure. In this caseRKKY oscillations could lead to frustrated indirect couplingbetween local moments and to a suppression of ferromag-netism with increased p.The RKKY exchange Hamiltonian between the vana-dium spins at sites i and j is expressed by H=&i,jJ!rij"Si ·S j, with J!r"%mJ02kF4F!2kFr"e−r/!. Here, rij isthe distance between i and j, ! is the mean free path ofcarriers, J0 is the local Zener coupling between vanadiumlocal moments and hole spins, kF= !3'2p"1/3 is the Fermiwave number in a parabolic band approximation, and F!x"=x−4#sin!x"−x cos!x"$. In Fig. 4, we plot F!2kFrij" for asingle carrier pocket versus rij for p of 1.5!1019 and2.8!1020 cm−3, representing the range of hole concentra-tions accessed by tuning pressure. The average separationbetween vanadium spins rav= !ni"−1/3 for x=0.03 is shownwith a dashed line, illustrating that the first zero in F ap-proaches rav as p increases. Further, we note that mean freepaths calculated from the carrier mobilities at 4 K had valuesof 8±1 nm, encompassing several sign changes in J!r".While a full calculation of the Curie temperature is beyondthe scope of this paper, we explore a simplified model. Curietemperature will be proportional to kF4&rarF!2kFr"e−r/!,where ar is the number of rth nearest vanadium ion neigh-bors. A calculation based on vanadium ion concentration dis-tributed uniformly over a simple cubic lattice includingterms out to rij(12 nm is shown in the inset of Fig. 4. Thesum is positive at a hole concentration per carrier pocket of1.5!1019 cm−3, indicating that ferromagnetism is predicted,and increase to 2.8!1019 cm−3 results in a roughly 30%decrease—a notable agreement with our reduction in TC ob-served experimentally. The pressure dependence of J0 istaken to be constant here but is expected to increase mod-estly as has been seen in In1−xMnxSb.9 More detailed calcu-lations are warranted for this system, as was recently donefor Ga1−xMnxAs taking into account the discrete nature of theunderlying lattice and disorder in the ion distribution.16 Wenote that our data are in qualitative agreement with thiswork, which predicts that ferromagnetism is suppressed forp /ni in the range 0.1–0.25.In conclusion, we have measured the transport propertiesas a function of magnetic field and pressure on Sb2−xVxTe3single crystals over a range of ion concentrationx=0.02–0.03. Electrical resistivity and Curie temperaturedecrease significantly with applied pressure. Our Hall dataindicate that the reduction in # is due to an increase in freehole concentration, and effective mass and scattering mecha-nisms are unaffected. Further, transport data are consistentwith a single valence band picture, greatly simplifying com-parisons to free-carrier-mediated models of ferromagnetism.Our data strongly suggest that the ferromagnetic order in thismaterial is hole mediated and that a full RKKY theory whichincludes the oscillatory nature of the indirect ion–ion inter-action is needed in order to explain the data.We wish to acknowledge financial support from Re-search Corporation Award CC6035 !J.S.D." and Ministry ofEducation Youth and sports of Czech Republic under theproject MSM 0021627501 !C.D. and P.L.".1J. S. Dyck, P. Hajek, P. Lostak, and C. Uher, Phys. Rev. B 65, 115212!2002".2V. A. Kulbachinskii, A. Yu. Kaminskii, K. Kindo, Y. Narumi, K. Suga, P.Lostak, and P. Svanda, Physica B 311, 292 !2002".3J. S. Dyck, C. Drasar, P. Lostak, and C. Uher, Phys. Rev. B 71, 115214!2005".4J. Choi, H. W. Lee, B. S. Kim, S. Choi, J. Choi, J. H. Song, and S. L. Cho,J. Appl. Phys. 97, 10D324 !2005".5T. Jungwirth, J. Sinova, J. Masek, J. Kucera, and A. H. MacDonald, Rev.Mod. Phys. 78, 809 !2006".6H. Ohno, A. Shen, F. Matsukura, A. Oiwa, A. Endo, S. Katsumoto, and Y.Iye, Appl. Phys. Lett. 69, 363 !1996".7S. U. Yuldashev, H. Im, V. S. Yalishev, C. S. Park, T. W. Kang, S. Lee, Y.Sasaki, X. Liu, and J. K. Furdyna, Appl. Phys. Lett. 82, 1206 !2003".8Z. H. Zhou, C. Uher, M. Zabcik, and P. Lostak, Appl. Phys. Lett. 88,192502 !2006".9M. Csontos, G. Mihaly, B. Janko, T. Wojtoicz, X. Liu, and J. K. Furdyna,Nat. Mater. 4, 447 !2005".10V. A. Kulbachinskii, N. Miura, H. Nakagawa, C. Drasar, and P. Lostak, J.Phys.: Condens. Matter 11, 5273 !1999".11L. G. Khvostantsiv, A. I. Orlov, N. Kh. Abrikosov, and L. D. Ivanova,Phys. Status Solidi A 58, 37 !1980".12P. Larson, Phys. Rev. B 74, 205113 !2006".13T. Thonhouser, G. S. Jeon, G. D. Mahan, and J. O. Sofo, Phys. Rev. B 68,205207 !2003".14T. Dietl, H. Ohno, F. Matsukura, J. Cibert, and D. Ferrand, Science 287,1019 !2000".15T. Junwirth, W. A. Atkinson, B. H. Lee, and A. H. MacDonald, Phys. Rev.B 59, 9818 !1999".16D. J. Priour and S. Das Sarma, Phys. Rev. Lett. 97, 127201 !2006".17T. Dietl, A. Haury, and Y. Merle d’Aubigne, Phys. Rev. B 55, R3347!1997"; J. Schliemann, J. Konig, and A. H. MacDonald, ibid. 64, 165201!2001".18F. Matsukura, H. Ohno, A. Shen, and Y. Sugawara, Phys. Rev. B 57,R2037 !1998".FIG. 4. Dependence of RKKY oscillatory function F!2kFr" as a functionion–ion separation r for two values of the carrier concentration. The dashedline represents the average separation between vanadium ions for x=0.03.!Inset" Sign of the magnetic interaction !ferromagnetic is positive" as afunction of carrier concentration. The impact of pressure on carrier concen-tration leads to a suppression of ferromagnetism within this model.122506-3 Dyck et al. Appl. Phys. Lett. 91, 122506 "2007!Downloaded 19 Nov 2012 to 76.241.123.224. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

Significant suppression of ferromagnetism by hydrostatic pressure in the diluted magnetic semiconductor Sb2−xVxTe3 with xÏ0.03

https://collected.jcu.edu/cgi/viewcontent.cgi?article=1000&amp;context=phys-facpub

Significant suppression of ferromagnetism by hydrostatic pressure in the diluted magnetic semiconductor Sb2−xVxTe3 with xÏ0.03

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