Journal of the Korean Physical Society, Vol. 55, No. 1, July 2009, pp. 304∼308 Magneto-transport Properties of GaMnAs:Si Ferromagnetic Semiconductors Hyungchan Kim, Hakjoon Lee, S. J. Chung and Sanghoon Lee ∗ Physics Department, Korea University, Seoul 136-701 Y. J. Cho, X. Liu and J. K. Furdyna Physics Department, University of Notre Dame, IN 46545, U.S.A. (Received 26 August 2008) The magnetic properties of a series of GaMnAs:Si ferromagnetic semiconductor films, in which the Mn concentration ranges from 7% to 10%, were investigated by using magneto-transport mea- surements. The temperature dependence of the resistivity revealed a systematic increase in the Curie temperature (Tc) with increasing Mn concentration in the series. Since the Tc of the undoped GaMnAs ferromagnetic semiconductor decreases with increasing Mn concentration above 6%, the observation of a systematic increase of Tc with increasing Mn concentration in our GaMnAs:Si se- ries indicates the effectiveness of our counter doping for the incorporation of a a large amount of 7% Mn in the system. The field scan of the planar Hall effect (PHE) showed a typical two-step switching behavior at low temperatures, indicating the presence of a strong cubic anisotropy. The switching fields, however, systematically decreased with increasing Mn concentration in the series. The angular dependences of the switching fields were fitted by using the magnetic free energy and Cowburn’s model to obtained the domain pinning energy, which showed systematically smaller val- ues as the Mn concentration of the sample was increased. The temperature dependences of the pinning energies indicated a change in the uniaxial anisotropy from the [ 110] to the [110] direction with increasing Mn concentration in the series. PACS numbers: 75.60.-d, 75.47.-m, 75.50.Pp, 75.70.-i Keywords: Ferromagnetic semiconductros, Hall effect, Magnetic anisotropy, Doping I. INTRODUCTION Recently, spintronic devices, in which both the charge and the spin properties of the electrons are simultane- ously utilized [1], have received a great deal of atten- tion in the solid-states research community. A ferromag- netic semiconductor is considered to be an ideal mate- rial system for realizing such spintronic devices because it possesses both the characteristics of a magnet and a semiconductor. The best-known material system is a GaMnAs ferromagnetic semiconductor, in which the fer- romagnetism is induced by mediating carriers generated from incorporation of Mn [2, 3]. Owing to the carrier- mediation origin of the magnetism, the ferromagnetic properties of the material can be controlled by changing the carrier density in the material system. For example, it was demonstrated that light illumination [4] and/or application of a gate voltage [5] could change the car- rier density in the material and consequently was able to tune the ferromagnetic properties. Unfortunately, the manipulation of the magnetic properties by such exter- nal means is only applicable in the temperature range ∗ E-mail: slee3@korea.ac.kr; Fax: +82-2-927-3292 far below room temperature, hindering applications to practical spintronic devices. This raised the important issue of increasing Curie temperature (T c ) in this mate- rial system. It is natural to attempt growth of GaMnAs with a high Mn incorporation to increase the T c of the material. However, the T c of GaMnAs was limited to 110 K at 6% Mn and decreased with further incorporation of Mn in the system [6,7]. An extensive structural study done by using particle-induced X-ray emission (PIXE) revealed that the existence of Mn in interstitial positions (Mn I ), which acts as a compensating donor and also antiferro- magnetically couples with substitutional Mn(Mn Ga ), is the major reason for the low T c in this material [8]. Later, a fundamental thermodynamic limit for accommodating the hole concentration was found to exist in these sys- tems. Thus, the incorporation of above high 7% Mn into GaMnAs leads to the formation of Mn I , donors and/or electrically inactive precipitate [9,10]. In this researchs, we have strategically applied a counter-doping technique to increase the Mn concentra- tion in the GaMnAs layer. While the magnetic ion Mn act as a p-type dopant for GaAs, the Si act as an n-type dopant for GaAs. Thus, the presence of Si compensates parts of the holes generated by Mn in GaMnAs, which -304-