Spinodal Decomposition and Super-Paramagnetism in Dilute Magnetic Nitride Semiconductors
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0955-I01-01
Spinodal Decomposition and Super-Paramagnetism in Dilute Magnetic Nitride Semiconductors Kazunori Sato, Tetsuya Fukushima, and Hiroshi Katayama-Yoshida The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka, 567-0047, Japan
ABSTRACT We discuss origin of ferromagnetism in dilute magnetic semiconductors (DMS) and propose accurate calculation method for Curie temperature from first-principles. For more realistic materials design, we simulate inhomogeneous impurity distribution due to the spinodal decomposition in DMS from first-principles. Combining these techniques, we discuss superparamagnetism and blocking phenomena in DMS and propose fabrication method of DMS with high blocking temperature. INTRODUCTION Dilute magnetic semiconductors (DMS) are recognized as one of the important materials to realize semiconductor spintronics. Today, it becomes possible to reproduce experimental Curie temperature (TC) of dilute magnetic semiconductors (DMS) such as (Ga, Mn)As and (Zn, Cr)Te accurately from first-principles [1]. In contrast to this success, agreement between the theory and experimental values is not satisfactory in wide band gap DMS such as nitride DMS, and more realistic description of wide band gap DMS is needed. For example, in general, DMS systems have solubility gap in thermal equilibrium and they show phase separation (spinodal decomposition), however in the previous theoretical approaches, homogeneous impurity distribution is supposed [1]. In this paper, we focus on the spinodal decomposition in nitride DMS, such as (Ga, Mn)N, (Ga, Cr)N and (Al, Cr)N, and study how inhomogeneous impurity distribution affects the ferromagnetism. Magnetic and chemical pair interactions are calculated from first-principles and the spinodal decomposition of DMS is simulated by applying the Monte Carlo method. TC of simulated spinodal decomposition phases is calculated within the random phase approximation with taking disorder into account [2]. It will be shown that above the percolation threshold the system is ferromagnetic and TC goes up during the decomposition process in wide gap DMS [3]. For low concentrations, the system in super-paramagnetic (i.e. TC = 0), because small isolated clusters are formed in DMS due to the decomposition. However, the Monte Carlo simulation for the magnetization process of the decomposition phases indicates that super-paramagnetic blocking temperature could be high because the activation energy to flip the magnetization becomes large for the decomposition phases. Finally, we will take into account a layer-by-layer crystal growth condition in our simulations and will show that under this condition, quasi-onedimensional nano-structure of impurities are formed in DMS even for low concentrations [4]. This simulation explains the ferromagnetic behavior of wide gap DMS at high temperature.
HYBRID METHOD FOR CURIE TEMPERATURE CALCULATIONS The magnetic properties of DMS are calculated from first-principles by mapping the abinitio results on a Heis
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