>Hole concentration vs. Mn fraction in a diluted (Ga,Mn)As ferromagnetic semiconductor
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Hole concentration vs. Mn fraction in a diluted (Ga,Mn)As ferromagnetic semiconductor Raimundo R. dos Santos1, L. E. Oliveira2 and J. d´Albuquerque e Castro1 1 Inst. de Física, Univ. Fed. do Rio de Janeiro, CP 68.528, Rio de Janeiro–RJ, 21945-970, Brazil 2 Instituto de Física, Unicamp, CP 6165, Campinas-SP, 13083-970, Brazil ABSTRACT The dependence of the hole density on that of Mn sites in Ga1-xMnxAs is studied within a mean-field approach to the hole-mediated ferromagnetism in III-V Mn-based semiconductor compounds. We parametrize the hole concentration, p, as a function of the fraction of Mn sites, x, in terms of the product m* (Jpd)2 (where m* is the hole effective mass and Jpd is the Kondo-like hole/local-moment coupling), and the critical temperature Tc. By fitting m* (Jpd)2 to experimental data for Tc(x), we establish the dependence of p on x, which is interpreted in terms of a reentrant metal-insulator transition taking place in the hole gas.
INTRODUCTION The discovery in the early 1990's of ferromagnetism in III-V materials alloyed with transition elements like Mn [1,2] has increased the interest in the study of the electronic, optical and transport properties of diluted magnetic semiconductors (DMS). Ferromagnetic semiconductors bring about the possibility of controlling both spin and charge degrees of freedom. The combination of this feature with the capability of growing low-dimensional structures has opened up new perspectives for the production of spintronic devices. Applications would include non-volatile memory systems [3] and quantum computing [4]. A great deal of attention has been focused on Ga1-xMnxAs alloys, which exhibit very interesting magnetic and transport properties. When Mn atoms replace Ga in GaAs, their five 3d electrons remain localized in a core state, giving rise to S = 5/2 local moments. In addition, Mn atoms have one less electron in the 4s level than Ga, so they act as acceptors generating hole states in the material; for large enough doping, these states merge to form an impurity band. It should be pointed out that the equilibrium solubility of Mn atoms in GaAs is quite low [5], being only of the order of 1019 cm-3. However, with help of molecular-beam epitaxy techniques at low temperatures, it has been possible to produce homogeneous samples of Ga1-xMnxAs with x as high as 0.071. Magnetization measurements in these systems show that for 0.015 ≤ x ≤ 0.071 they become ferromagnetic, with doping-dependent critical temperatures Tc(x) reaching a maximum of 110 K for x = 0.053 [6]. The origin of ferromagnetic order in these materials is not fully understood, but there exists consensus on the fact that it results from the exchange coupling between the localized Mn moments mediated by the holes. The strength of the of the coupling is expected to depend on the hole concentration p, which eventually determines the Curie temperature Tc(x). In principle, one would expect that each Mn would provide one hole, leading to a density of holes equal to that of the magnetic ions. However, this has
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