Magnetic and Electrical Properties of the ZnGeAs2 : Mn Chalcopyrite

ISSN 1063-7834, Physics of the Solid State, 2007, Vol. 49, No. 11, pp. 2121–2125. © Pleiades Publishing, Ltd., 2006.Original Russian Text © L.I. Koroleva, V.Yu. Pavlov, D.M. Zashchirinskiœ, S.F. Marenkin, S.A. Varnavskiœ, R. Szymczak, V. Dobrovol’skiœ, L. Killinskiœ, 2007, published in FizikaTverdogo Tela, 2007, Vol. 49, No. 11, pp. 2022–2026.MAGNETISMAND FERROELECTRICITYMagnetic and Electrical Propertiesof the ZnGeAs 2 : Mn ChalcopyriteL. I. Koroleva a , V. Yu. Pavlov a , D. M. Zashchirinskiœ a , S. F. Marenkin b ,S. A. Varnavskiœ b , R. Szymczak c , V. Dobrovol’skiœ c , and L. Killinskiœ caLomonosov Moscow State University, Leninskie Gory, Moscow, 119992 Russiae-mail: koroleva@phys.msu.rubKurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,Leninskii pr. 31, 119991 Moscow, RussiacInstitute of Physics, Polish Academy of Sciences, 02668 Warsaw, PolandReceived February 12, 2007Abstract—Doping of the ZnGeAs 2 semiconductor with manganese has produced compositions with spontaneousmagnetization and high Curie temperatures of up to 367 K for the composition 3.5 wt. % Mn. Their magneticproperties are characteristic of spin glasses at temperatures T < T S and magnetic fields H < 11 kOe. Instronger fields, the spin glass state transforms into a phase with a spontaneous magnetization 20–30 timesweaker than that to be expected under ferromagnetic ordering of all Mn ions. This is obviously a singly-connectedferromagnetic phase containing regions with frustrated bonds. The frustrated regions and the spin glassphase have inclusions of noninteracting ferromagnetic clusters, because these regions and the spin glass phaseat low temperatures exhibit a strong increase in the magnetization M, with the dependence M(T) being describedby the Langevin function. Measurements of the electrical resistivity ρ and the Hall effect have revealed that, forT < 30 K, the resistivity ρ of compositions with 1.5 and 3.5 wt. % Mn is higher that at 30 K, which makes superexchangedominant and gives rise to the onset of the spin glass state. The nonuniform distribution of Mn ionsin the spin glass phase accounts for the existence of isolated ferromagnetic clusters, their ferromagnetism beinggenerated by carrier-mediated exchange. As the temperature increases still more, the increase in the mobilityoccurs faster than the decrease in the concentration, thus promoting an enhancement of the carrier-mediatedexchange and growth of the ferromagnetic clusters in size, which at T = T S come in contact. This signifies atransition from a multiply- to a singly-connected ferromagnetic phase, which contains microregions with frustratedbonds.PACS numbers: 75.50.Pp, 75.50.Lk, 72.25.–bDOI: 10.1134/S10637834071101701. INTRODUCTIONThe early 1990s have witnessed emergence of a newarea in the physics of solid state based on the possibilityof transfer of an oriented electron spin from a ferromagnetto a nonmagnetic semiconductor [1, 2]. Part of thisresearch having an application potential was namedspintronics. These studies are of considerable importancefor development of one-electron logical structuresand spin-information systems in informatics (inthis particular case, spin-based informatics, in whichthe electron spin serves as the information storage cell,with one spin providing one bit of information [3]). Insolid-state electronics, spin current transport opens upa novel possibility of using magnetic field to control thecharacteristics of various devices, diodes, triodes etc.;i.e., it provides an additional degree of freedom. Ferromagneticmetals used as emitters of polarized spins arecapable of providing a spin polarization degree of notabove 10%. A substantially higher polarization degree(of up to 100%) was obtained in semiconductor–EuOand semiconductor–chalcogenide-spinel structures, butonly at cryogenic temperatures, an aspect unfavorablefor applications. Besides, one is plagued in this case bya technological difficulty, more specifically, that ofachieving a good electrical contact between a ferromagnetand a semiconductor.A good electrical contact and a high spin-polarizedcurrent can be reached by developing an ferromagneticsemiconductor with a Curie point above room temperaturethrough doping with impurities with incompletelyfilled 3d shells. This is why interest of researchersbecame focused on development of ferromagneticsemiconductors through doping semiconductors customarilyemployed in microelectronics, primarily theIII–V compounds, with manganese. The best studiedrepresentative of this class of materials is Ga 1 – x Mn x Aswhich revealed ferromagnetism with a Curie temperatureT C not in excess of 170 K [4, 5].High-temperature ferromagnetism has beenrecently observed in Mn-doped II–IV–V 2 chalcopyrites.These were CdGdP 2 : Mn [6], ZnGeP 2 : Mn [7],2121

2122KOROLEVA et al.M, emu/g43210M, emu/g1.00.80.60.40.2H = 600 OeZFCFCH = 50 Oe0 50 150 250 350T, K100 200 300 400T, KFig. 1. Temperature dependence of the magnetization M forthe composition ZnGeAs 2 with 3.5 wt. % Mn in magneticfields of 50 kOe and 0.6 kOe (inset). The ZFC curve: thesample was zero-field-cooled from 400 to 5 K, after whichits magnetization was measured under heating. The FCcurve: the sample was cooled in a field of 0.6 kOe from 400to 5 K with parallel measurement of its magnetization.and ZnSnAs 2 : Mn [8], compounds with the Curie pointas high as 350 K. We reported on the compoundCdGeAs 2 : Mn, whose Curie temperature T C was stillhigher, 355 K.The II–IV–V 2 ternary semiconductors have beenknown for a long time. These compounds are crystallochemicaland electronic analogs of the III–V semiconductors.Interest in these compounds emerged afterone discovered their unique nonlinear optical properties,namely, the high nonlinear polarizability and birefringence,which opened a way to their use for parametricconversion of laser radiation in the mid-IR range.The most promising for this purpose are high-purityCdGeP 2 , CdGeAs 2 , and ZnGeP 2 crystals. The II–IV–V 2 -type compounds retain the main features characteristicof the III-V materials, namely, the predominantlycovalent bonding, small effective carrier masses, relativelyhigh mobilities of the electrons and holes, andpersistence of the absolute minima and maxima of theconduction and the valence band at Brillouin zone center.We prepared and studied new ZnGeAs 2 : Mn compoundswith the Curie point reaching 367 K, a recordhighvalue for II–IV–V 2 -type compounds. TheZnGeAs 2 compound studied in the present work possessesthe following characteristics: bandgap 0.85 eV;mobility ~10 2 cm 2 /V s (for holes with concentrations of10 18 –5 × 10 19 cm –3 ); effective hole mass 0.4–0.74. Thismakes the II–IV–V 2 : Mn compounds promising forspintronics applications.2. EXPERIMENTAL TECHNIQUEWe report here on a study of the magnetization,electrical resistivity ρ, magnetoresistance ∆ρ/ρ = (ρ H –ρ H = 0 )/ρ H = 0 and the Hall effect of polycrystallineZnGeAs 2 : Mn samples with Mn contents of 1.5 to3.5 wt. %. The magnetization was measured with aSQUID magnetometer. The four-probe technique wasemployed to measure ρ and ∆ρ/ρ. The Hall effect wasstudied by the standard dc method. The contacts to thesamples were secured with current-conducting glue.3. EXPERIMENTAL RESULTSAND DISCUSSIONThe ZnGeAs 2 samples were prepared by directalloying of high-purity powders of ZnAs 2 and Ge takenin stoichiometric ratio. Mn was introduced according tothe hypothetical ZnGeAs 2 –MnGeAs 2 cut. To increasethe Mn solubility, the rate of cooling from 900°C waschosen not lower than 5–10 K/s. X-ray diffraction analysisshowed all samples with the Mn contents of 1.5, 3,and 3.5 wt. % to be single phase, thus identifying themas ZnGeAs 2 . A comparison of the lattice constantsrevealed that the cell volume decreases with increasingMn content, which suggests formation of solid solutionsthrough Mn substitution for Zn. The contents ofthe components were checked by x-ray fluorescence. Astudy of the distribution of elements over the samplelength showed that Zn : Ge : As = 1 : 1 : 2. Within experimentalerror, the Mn was found to be distributed uniformlyover the sample length.Figure 1 displays temperature dependence of themagnetization, M(T), for the composition with 3.5 wt.% Mn in a magnetic field H = 50 kOe. The M(T) curveobtained for T > 60 K is seen to be characteristic of aferromagnet. For T < 60 K, however, magnetization isobserved to grow strongly with decreasing temperature,an effect that can be interpreted as an additional contributiondue to the superparamagnetic phase. The inset toFig. 1 shows the M(T) plot measured in a weak magneticfield of 600 Oe. The M(T) curves obtained in astrong (50 kOe) and a weak (0.6 kOe) field are immediatelyseen to differ substantially. When measuring inthe weak field, the magnetization is observed to dropstrongly as the temperature falls below T S = 86 K, withM decreasing 4.5 times. At T k = 10 K, this drop stops,and here one sees a difference between the magnetizationsof the sample cooled from T > T C in this weakmagnetic field (FC curve) and the one that was zerofieldcooled (ZFC curve) The FC curve exhibits a risewith further decrease in the temperature. Shown inFig. 2 are M(T) curves in the temperature intervalincluding T S , which were measured in magnetic fieldsof 3, 6, 8, 9, and 11 kOe. Examining this figure showsthat as H increases, the decrease in the magnetization atT = T S becomes weaker to disappear altogether at11 kOe, and the value of T S shifts toward lower temper-PHYSICS OF THE SOLID STATE Vol. 49 No. 11 2007

MAGNETIC AND ELECTRICAL PROPERTIES 21232.52.0H = 11 kOe986M, emu/g0.10M, emu/g1.53–0.1ZFCFCZFC–0.2 00.2H, kOe1.00 40 80 120T, KFig. 2. Temperature dependence of the magnetization M forthe composition ZnGeAs 2 with 3.5 wt. % Mn in differentmagnetic fields in the temperature range from 5 to 100 K.Each curve was measured after the sample had been cooledto 5 K with no magnetic field.atures. Indeed, as the field increases from 600 Oe to9 kOe, T S decreases from 86 to 49 K. This is paralleledby the rise in the M(T) curves observed to occur for T

2124KOROLEVA et al.41431212M, emu/g21T = 5 K25 K50 Kρ · 10 2 , Ω cm1081110R h · 10 2 , cm 3 /C0 10 20 30 40 50H, kOeFig. 4. Magnetization isotherms of the ZnGeAs 2 samplewith 3.5 wt. % Mn at different temperatures. Each curvewas measured after the sample had been cooled to 5 K in azero magnetic field.60 50 100150 200 250 300T, K9Fig. 5. Temperature dependence of the electrical resistivityρ and the normal Hall coefficient R H for the ZnGeAs 2 samplewith 1.5 wt. % Mn.romagnetic phase depends on its configuration, whichvaries with temperature. Therefore, for the Curie pointwas taken the temperature obtained by extrapolation ofthe steepest part of the M(T) curve measured in themaximum field reachable (50 kOe) to its intercept withthe temperature axis. The Curie temperature was foundto be 367 K. This is the highest Curie temperaturefound thus far in the II–IV–V 2 : Mn systems. This providedsubstantiation for the theoretical suggestion [10]that the Curie temperatures of Zn-containing chalcopyritesshould be higher than those of the correspondingCd-based ones.The Hall electric field of the samples under studywas found to depend linearly on magnetic field, whichevidences the absence of anomalous Hall effect. Measurementsof the electrical resistivity and of the Halleffect revealed that all the samples support hole conduction,with the hole concentration p ~ 10 19 –10 20 cm –3and mobility varying from 0.25 to 2.5 cm 2 V –1 s –1 . Figures5 and 6 plot the temperature dependences of theelectrical resistivity ρ, normal Hall coefficient R H andmobility. We readily see that the ρ(T) relation of the1.5 wt. % Mn composition follows a pattern characteristicof semiconductors, while the p(T) curve has apurely metallic signature, with p varying very little,from 6.4 × 10 19 cm –3 at T = 50 K to 5.2 × 10 19 cm –3 at300 K. The mobility grows faster, from 1.25 cm 2 V –1 s –1 at20 K to 2.6 cm 2 V –1 s –1 at 300 K. In the 3.5 wt. % Mncomposition, ρ(T) passes through a minimum at T ~30 K, and the mobility increases by an order of magnitudein the region 10 ≤ T ≤ 300 K, whereas p varies from2.5 × 10 20 cm –3 at 50 K to 8 × 10 19 cm –3 at 300 K. Themagnetoresistance is small; it does not exceed 4% atH = 8 kOe.The origin of ferromagnetism in dilute magneticsemiconductors was studied by first-principles electronicstructure calculations [11, 12]. It is maintained[11, 12] that the effective exchange interaction in thesecompounds derives primarily from competitionbetween double exchange and superexchange interactions.It is known that semiconductors with chalcopyritestructure are stabilized by internal defects acting assources of holes which form stable complexes with Mnρ · 10 3 , Ω cm34302622180 50 1003.53.02.52.01.51.00.50150 200 250 300T, KFig. 6. Temperature dependence of the electrical resistivityρ and the hole mobility for the ZnGeAs 2 sample with3.5 wt. % Mn.µ, cm 2 /V sPHYSICS OF THE SOLID STATE Vol. 49 No. 11 2007

MAGNETIC AND ELECTRICAL PROPERTIES 2125[13]. It should be pointed out that double exchangeinvolves carrier transfer among ions residing in differentvalence states, in this particular case, between Mn 2+and Mn 3+ . As shown earlier, in the system under studyMn substitutes for Zn, so that the above complexes areactually (Zn, V C , Mn), where V C are vacancies. It is notknown, however, how vacancies affect the distributionof the Mn 2+ and Mn 3+ ions. Therefore we are going toreplace subsequently the term “double exchange” witha broader concept of “carrier-mediated exchange”. Asseen from Figs. 5 and 6, for T < 30 K, the electricalresistivity of the compositions with 1.5 and 3.5 wt. %Mn is higher than that at 30 K, a factor that apparentlyaccounts for the superexchange becoming dominant.As shown in [14], spin glass is the ground state in thiscase. As the temperature increases, mobility increaseswith a rate faster than that of the small concentrationdrop, thus favoring carrier-mediated exchange, whichinitiates the transition from the spin glass to the ferromagneticstate. One could suggest also another scenariobased on the energies of superexchange and carriermediatedexchange being close, which is not in conflictwith the observed magnetic and transport properties.Because the Mn ions dissolving in ZnGeAs 2 becomerandomly distributed, one may conceive formation atlow temperatures in the spin glass matrix of ferromagneticclusters, whose ferromagnetism rests upon thehigher carrier-mediated exchange energy. As the temperatureincreases, these clusters grow in volume due tothe increasing hole mobility, until at a certain temperaturethese clusters come in contact. This signals thetransition from a multiply connected ferromagneticphase made up of isolated ferromagnetic clusters to thesingly connected ferromagnetic phase, within whichmicroregions with frustrated bonds are embedded.REFERENCES1. H. Ohno, Science (Washington) 281, 951 (1998).2. G. A. Prinz, Science (Washington) 282, 1660 (1998).3. F. Matsukura, H. Ohno, A. Shen, and Y. Sugawara, Phys.Rev. B: Condens. Matter 57, R2037 (1998).4. K. M. Edmonds, K. Y. Wang, R. P. Campion, A. C. Neumann,N. R. S. Farley, B. L. Gallagher, and C. T. Foxon,Appl. Phys. Lett. 81, 4991 (2002).5. K. M. Edmonds, P. Boguslawski, K. Y. Wang, R. P. Campion,S. N. Novikov, N. R. S. Farley, B. L. Gallagher,C. T. Foxon, M. Sawicki, T. Dietl, M. BuongiornoNardelli, and J. Bernholc, Phys. Rev. Lett. 92, 03720(2004).6. G. A. Medvedkin, T. Ishibashi, T. Nishi, K. Hayata,Y. Hasegawa, and K. Sato, Jpn. J. Appl. Phys. 39, L949(2000).7. G. A. Medvedkin, K. Hirose, T. Ishibashi, T. Nishi,V. G. Voevodin, and K. Sato, J. Cryst. Growth 236, 609(2002).8. S. Choi, G.-B. Cha, S. C. Hong, S. Cho, Y. Kim,J. B. Ketterson, S.-Y. Jeong, and G.-C. Yi, Solid StateCommun. 122, 165 (2002).9. R. V. Demin, L. I. Koroleva, S. F. Marenkin, S. G. Mikhaœlov,V. M. Novotortsev, V. T. Kalinnikov, T. G. Aminov,R. Szymczak, G. Szymczak, and M. Baran, Pis’maZh. Tekh. Fiz. 30 (21), 81 (2004) [Tech. Phys. Lett. 30(11), 924 (2004)].10. P. R. Kent and T. C. Schulthess, in Proceedings of the27th International Conference on Physics of Semiconductors(ICPS-27), Flagstaff, AZ, 2005, Ed. by J. Menendezand Ch. G. van de Walle (Flagstaff, 2005),p. 1369.11. H. Akai, Phys. Rev. Lett. 81, 3002 (1998).12. H. Akai, T. Kamatani, and S. Watanabe, J. Phys. Soc.Jpn. 69 (Suppl. A), 112 (2000).13. P. Mahadevan and A. Zunger, Phys. Rev. Lett. 88,047205 (2002).14. Y.-J. Zhao, W. T. Geng, A. J. Freeman, and T. Oguchi,Phys. Rev. B: Condens. Matter 63, R201202, (2001).Translated by G. SkrebtsovSPELL: OKPHYSICS OF THE SOLID STATE Vol. 49 No. 11 2007