Hiroshi Katayama-Yoshida

Material Design for Transparent Ferromagnets with ZnO-Based Magnetic Semiconductors

Ferromagnetism of ZnO-based magnetic semiconductors was investigated by ab initio calculations based on the local density approximation. In a system of Mn atom doped ZnO, the ferromagnetic ordering of Mn magnetic moments was induced by hole doping. It was also found that 3d transition metal atoms of V, Cr, Fe, Co and Ni showed the ferromagnetic ordering of their magnetic moments in ZnO without any additional carrier doping treatments. Appearance of the ferromagnetism in these systems suggests possibility for a fabrication of a transparent ferromagnet which will have great impact on industrial applications in magneto optical devices.

First principles materials design for semiconductor spintronics

Materials design of new functional diluted magnetic semiconductors (DMSs) is presented based on first principles calculations. The stability of the ferromagnetic state in ZnO-, ZnS-, ZnSe-, ZnTe-, GaAs- and GaN-based DMSs is investigated systematically and it is suggested that V- or Cr-doped ZnO, ZnS, ZnSe and ZnTe are candidates for high-TC ferromagnetic DMSs. V-, Cr- or Mn-doped GaAs and GaN are also candidates for high-TC ferromagnets. It is also shown that Fe-, Co- or Ni-doped ZnO is ferromagnetic. In particular, the carrier-induced ferromagnetism in ZnO-based DMSs is investigated and it is found that their magnetic states are controllable by changing the carrier density. The origin of the ferromagnetism in the DMSs is also discussed.

Stabilization of Ferromagnetic States by Electron Doping in Fe-, Co- or Ni-Doped ZnO

Detailed guidelines for controlling magnetic states in ZnO-based diluted magnetic semiconductors are given based on ab initio electronic structure calculations within the local spin density approximation using the Korringa-Kohn-Rostoker method. Effects of disorder were taken into account by the coherent potential approximation. It was found that the ferromagnetic state was stabilized by electron doping in the case of Fe-, Co- or Ni-doped ZnO. From the view point of practical applications, it is possible to realize a high-Curie-temperature ferromagnet, because n-type ZnO is easily available.

Material Design of GaN-Based Ferromagnetic Diluted Magnetic Semiconductors

Material design of GaN-based ferromagnetic diluted magnetic semiconductors is given based on ab initio calculations within the local spin density approximation. The electronic structure of 3d-transition-metal-atom-doped GaN was calculated by the Korringa-Kohn-Rostoker method combined with the coherent potential approximation. It was found that the ferromagnetic ground states were readily achievable in V-, Cr- or Mn-doped GaN without any additional carrier doping treatments. A simple explanation on the systematic behavior of the magnetic states in GaN-based diluted magnetic semiconductors is also given.

Ferromagnetism in a transition metal atom doped ZnO

Abstract Ferromagnetism in a 3d transition metal atom doped ZnO was investigated by ab initio electronic structure calculations based on the local density approximation. It was shown that the anti-ferromagnetic state was stable in Mn atom doped ZnO and the ferromagnetic state was stable in the other transition metal, i.e., V, Cr, Fe, Co or Ni, doped ZnO, if no additional carrier dopant was introduced. Carrier induced ferromagnetism in the Mn atom doped ZnO was also investigated. The results showed that the ferromagnetism was induced by hole doping in the Mn atom doped ZnO. The present calculations will provide us with guidelines to produce ferromagnetic magnetic semiconductors and to control their magnetic state.

High curie temperature and nano-scale spinodal decomposition phase in dilute magnetic semiconductors

We show that spinoadal decomposition phase in dilute magnetic semiconductors (DMS) offers the possibility to have high Curie temperatures (TC) even if the magnetic exchange interaction is short ranged. The spinodal decomposition is simulated by applying the Monte Carlo method to the Ising model with realistic (ab initio) chemical pair interactions between magnetic impurities in DMS. Curie temperatures are estimated by the random phase approximation with taking disorder into account. It is found that the spinodal decomposition phase inherently occurs in DMS due to strong attractive interactions between impurities. This phase decomposition supports magnetic network over the dimension of the crystal resulting in a high-TC phase.

Ab initio Study on the Magnetism in ZnO-, ZnS-, ZnSe- and ZnTe-Based Diluted Magnetic Semiconductors

The magnetism and the electronic structure of II-VI compound-based diluted magnetic semiconductors (DMSs) are investigated based on ab initio calculations. The stability of the ferromagnetic state in ZnO-, ZnS-, ZnSe- and ZnTe-based DMSs is investigated systematically and materials design for ferromagnetic DMSs is given. In all host materials, it is found that V- and Cr-doped systems are ferromagnetic and Mn-doped systems are spin-glass state. On the other hand, for Fe-, Co- and Ni-doped systems, the ferromagnetic state is stable in ZnO-based DMS, however, the spin-glasss state is stable in ZnS-, ZnSe- and ZnTe-based DMSs. The carrier-induced ferromagnetism in ZnO-based DMSs is also investigated and it is found that their magnetic states are controllable by changing the carrier density. Analysing the calculated density of states, the mechanism to stabilize the ferromagnetic state in the DMSs is discussed.

Spinodal Decomposition under Layer by Layer Growth Condition and High Curie Temperature Quasi-One-Dimensional Nano-Structure in Dilute Magnetic Semiconductors

We show that spinodal decomposition under layer by layer crystal growth condition leads to characteristic quasi-one-dimensional nano-structures in dilute magnetic semiconductors (DMS). It is found that the DMS systems can form rather large clusters with highly anisotropic shape even for low concentrations. It is suggested that the blocking phenomena in the super-paramagnetism, the magnetic dipole–dipole interaction and the network of the one-dimensional structures should be considered to understand the magnetism in DMS. Based on the present simulations, we propose that the delta-doping method can be effective approach to realize high Curie temperature.

Electronic structure and ferromagnetism of transition-metal-impurity-doped zinc oxide

Abstract The ferromagnetism in ZnO-based diluted magnetic semiconductors (DMSs) is investigated based on the first principles calculations. The electronic structure of a ZnO-based DMS is calculated using the Korringa–Kohn–Rostoker method combined with the coherent potential approximation based on the local density approximation. The stability of the ferromagnetic state compared with that of the spin-glass state is systematically investigated by calculating the total energy difference between them. It is found that the ferromagnetic state is more stable than the spin-glass state in V-, Cr-, Fe-, Co- or Ni-doped ZnO without any additional carrier doping treatments. In the case of the Mn-doped ZnO, the spin-glass state is stable at a carrier concentration of 0%, but the ferromagnetic state is stabilized by the hole doping treatment. Analyzing the calculated density of states, it is proposed that the origin of the stabilization of the ferromagnetism is a double-exchange mechanism.

Physics and control of valence states in ZnO by codoping method

We investigate unipolarity in ZnO based on ab initio electronic band structure calculations. We find that p-type doping using Li, N or As species causes an increase in Madelung energy, n-type doping using B, Al, Ga, In or F species gives rise to a decrease in Madelung energy. In order to solve the unipolarity, to fabricate p-type ZnO, we propose a codoping method using acceptors and reactive donors simultaneously. A codopant pair including Ga-reactive donors and N-acceptors is eminently suitable for use in the codoping. For Li-acceptors, F species is a good candidate for reactive donors. For As-acceptors, Ga species is a preferable codopant.