Magnetic and Electrical Properties of the ZnGeAs2 : Mn Chalcopyrite

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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
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