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Interplay between localization and magnetism in (III,Mn)V dilute ferromagnetic semiconductors

Zhou, S.; Yuan, Y.; Sawicki, M.; Rushforth, A. W.; Zhao, J.; Dietl, T.; Helm, M.


Although the interplay between hole-mediated ferromagnetism and hole localization has long been recognized as the central issue in dilute ferromagnetic semiconductors (DFSs), its understanding remains in a nascent stage and contradicting approaches are under consideration [1, 2]. Some of the difficulties lie in the sample preparation: the dual role of Mn in III-V compounds providing local spins and holes, the poor control over donor defects (Mn interstitials and As antisites) etc. In this contribution, we examine the influence of localization on the hole-mediated ferromagnetism in (III,Mn)V DFSs by utilizing the well-developed ion-beam technology for microelectronics which can overcome the aforementioned difficulties [3].
First, we have used ion implantation of Mn combined with pulsed laser melting to prepare Ga1−xMnxAs and In1−xMnxAs with Mn concentration from 0.3% to 1.8% covering both sides of the insulator-metal transition [4-8]. The system evolves with x from a paramagnetic phase (probed down to 1.8 K), to a superparamagnetic material, to reach, via a mixed phase consisting of percolating ferromagnetic clusters and superparamagnetic grains, a global ferromagnetism without any superparamagnetism. On the insulating side of the transition, ferromagnetic signatures persist to higher temperatures in In1−xMnxAs compared to Ga1−xMnxAs with the same Mn concentration x. This substantiates theoretical suggestions that stronger p-d coupling results in an enhanced contribution to localization, which reduces hole-mediated ferromagnetism. Second, we use inert Helium ions to precisely compensate holes by donor defects, thereafter to shift the Fermi energy in DFSs while keeping the Mn concentration constant [9-12]. For a broad range of samples including (Ga,Mn)As and (Ga,Mn)(As,P) with various Mn and P concentrations, we observe a monotonous decrease of Curie temperature TC down to zero and a spin-reorientation transition with hole compensation while the conduction is changed from metallic to insulating. The previously questioned existence of TC below 10 K is also confirmed in heavily compensated samples. Our comprehensive results support strongly the heterogeneous model of electronic states at the localization boundary and point to the crucial role of weakly localized holes in mediating efficient spin-spin interactions even on the insulator side of the metal-insulator transition.

[1] M. Sawicki et al., Nat. Phys. 6, 22 (2010).
[2] M. Kobayashi et al., Phys. Rev. B 89, 205204 (2014).
[3] S. Zhou, J. Phys. D: Appl. Phys. 48, 263001 (2015).
[4] S. Zhou et al., Appl. Phys. Express 5, 093007 (2012).
[5] Y. Yuan…, S. Zhou, J. Phys. D: Appl. Phys. 48, 235002 (2015).
[6] S. Prucnal…, S. Zhou, Phys. Rev. B 92, 224407 (2015).
[7] Y. Yuan…, S. Zhou, Phys. Rev. Mater. 1, 054401 (2017).
[8] Y. Yuan…, S. Zhou, J. Phys.: Condens. Matter 30, 095801 (2018).
[9] L. Li…, S. Zhou, J. Phys. D: Appl. Phys. 44 099501 (2011).
[10] S. Zhou et al., Phys. Rev. B 95, 075205 (2016).
[11] M. Lonsky…, S. Zhou, J. Müller, Phys. Rev. B 97, 054413 (2018).
[12] Y. Yuan..., S. Zhou, J. Phys. D: Appl. Phys., in press (2018).

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