Masayoshi Seike

New Class of Diluted Ferromagnetic Semiconductors based on CaO without Transition Metal Elements

We propose a new class of diluted magnetic semiconductors based on CaO without transition metal elements. The electronic structure and the magnetic properties of B-, C- or N-doped CaO are calculated by using the Korringa-Kohn-Rostoker method within the local spin density approximation. The substitutional and magnetic disorder is taken into account by the coherent potential approximation. It is found that B, C and N impurities show finite local magnetic moments in CaO at the oxygen-substitutional site. Moreover, these C- and N-doped CaO show the room-temperature ferromagnetism with half-metallic density of states.

Self-Organized Nanostructures and High Blocking Temperatures in MgO-Based d0 Ferromagnets

We propose a new mechanism explaining the magnetic properties of MgO-based d0 ferromagnets determined from multi-scale simulations. Chemical pair interactions between N atoms in Mg(O,N) and Mg vacancies (VMg) in (Mg,VMg)O were calculated using a generalized gradient approximation and the VASP code. Monte Carlo simulations of the crystal growth were performed, using the Ising model, to predict the favored configurations of dopant distribution. It was found that self-organized nanowires can be formed both in Mg(O,N) and (Mg,VMg)O under layer-by-layer crystal growth, which suggests high blocking temperatures can be obtained in these d0 ferromagnets by spinodal nanodecomposition.

New Route to Fabricate Ferromagnetic Semiconductors without Transition Metal Elements

We propose a new route for ferromagnetic diluted magnetic semiconductors by controlling the impurity-band width (W) and the electron-correlation energy (U) in the partially occupied impurity band in the condition of highly correlated electron system (U>W). Based upon first-principles calculations of K2(S,Si) and K2(S,Ge), we demonstrate that transparent, half-metallic and room-temperature ferromagnetic DMS could be designed even without transition metal elements. The results show that it is possible to fabricate the room-temperature ferromagnets in K2(S,Si) and K2(S,Ge) around 8 at%-impurity concentration.

The Magnetic Properties of Hole-Doped MgO

We present a first-principles study on a new class of diluted magnetic semiconductors based on MgO with no magnetic elements. It was found that hole-doping by Mg vacancies or Li impurities leads to a spin-polarized ground state, and Curie temperatures can reach room temperature at sufficient concentrations (in the range of 15 to 30 at. %) under a homogeneously distributed condition. However, an inhomogeneous dopant distribution in MgO is the favored configuration, which indicates that spinodal decomposition leads to the room-temperature blocking temperature at smaller impurity concentrations than those estimated for room-temperature ferromagnetism under the homogeneous distribution condition.

Materials design of 4d-transition-metal-doped transparent and half-metallic ferromagnets with K2S-based diluted magnetic semiconductors

New functional 4d-transition-metal (4d-TM)-doped K2S diluted magnetic semiconductors (DMSs) with transparent and half-metallic ferromagnetism are designed based upon the first principles calculations. We have systematically investigated the magnetism in 4d-TM-doped K2S DMSs. K2S is a transparent semiconductor with anti-CaF2 crystal structure and has large lattice spacing due to its large ionic radius of K+. It is found that Zr-, Nb-, Tc-, Ru- and Rh-doped K2S show the half-metallic and high-spin ferromagnetism and that Zr- or Nb-doped K2S are promising candidate for high-TC (TC ~350 K for 5 at% doping of 4d-TM) ferromagnetic DMSs with transparecy and large magneto-optical effect. Mo-doped K2S shows the spin-glass state since anti-ferromagnetic super-exchange interaction dominates the system without ferromagnetic double-exchange interaction.

Design of Transparent, Half-Metallic Ferromagnetic

Curie temperatures are evaluated using first principles in (K,Zr)2S and (K,Nb)2S diluted magnetic semiconductors which have been designed as half-metallic and transparent materials with a large magnetoptical effect. From the total energy differences between the ferromagnetic state and the spin-glass state, estimations of Curie temperatures are achieved by mapping to the Heisenberg model within the mean-field approximation. According to the results, it is possible to fabricate transparent, half-metallic and room-temperature ferromagnets with Zr- and Nb-concentrations of around 5 at%. Effects of additional carrier doping for (K,Mo)2S are also evaluated and it is found that (K,Mo)2S with hole doping (~5 at%) exhibits transparency, is half-metallic and has room-temperature ferromagnetism.

4d

A new class of 4d transition-metal-doped anti-CaF2 I2-VI diluted magnetic semiconductors (DMSs) is presented based on first-principles calculations. We have systematically investigated the stability of the ferromagnetic state in K2O-, K2S-, K2Se-, K2Te-, Li2S-, Na2S- and Rb2S-based DMSs. Anti-CaF2 I2-VI compounds have a large lattice spacing due to their large ionic radius of cations and many of them are wide-bandgap semiconductors. From the total energy differences between the ferromagnetic state and the spin-glass state, ferromagnetic solutions are derived and it is found that Zr-, Tc- and Ru-doped K2O, Zr-, Nb-, Tc- and Ru-doped K2S, and Zr-, Nb-, and Ru-doped K2Se, K2Te, and Rb2S are good candidates for transparent, half-metallic and room-temperature ferromagnetic DMSs with a large magnetoptical effect.

-Transition-Metal-Doped K2S with High Curie Temperature

We present a computational materials design for defect-induced ferrimagnetic MnO. The magnetic properties of MnO containing Mn vacancies were investigated using first-principle calculations. For these electronic structure calculations, we employed a pseudo-self-interaction-corrected local density approximation (PSIC-LDA). We used the Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) to create a random distribution of atoms at the assigned sites. Having described the magnetic properties with a classic Heisenberg model, we calculated the effective exchange coupling constants by applying the magnetic force theorem to two magnetic sites embedded in the CPA medium. We estimated the Curie temperatures from the calculated exchange interactions. This study found that the Mn vacancies induced ferrimagnetic ground states in MnO, and that the Curie temperature could reach room temperature at Mn vacancy concentrations above 20%. These findings suggest a new route for designing ferrimagnetic materials from anti-ferromagnetic host materials.

Material Design of Transparent, Half-Metallic and Room-Temperature Ferromagnets in I2-VI Semiconductors with 4d Transition Metal Element

This chapter describes the current status of the computational nanomaterial design: the room-temperature spintronics applications in 3d-transition-metal-doped dilute magnetic semiconductors (DMS), d 0 ferromagnetism in nontransition-metal and defect-doped DMS, and rare-earth-metal-doped semiconductors including the efficient (linear and circular-polarized) light-emitting diodes based upon the codoping methods. On the basis of ab initio electronic structure calculations, we propose the ferromagnetic mechanisms and magnetic-control methods, and design the spintronics materials by controlling the magnetic interactions upon the impurities codoping and the electric-field gating. The multihierarchical simulations of ab initio calculations for magnetism and the Monte Carlo simulations provide accurate estimation of Curie temperatures ( T C ) of DMS. Using the self-organized spinodal nanodecomposition in two-dimensional (2D) and three-dimensional (3D) crystal growth conditions, we design 2D Konbu phases and 3D Dairiseki phases, which are promising candidates for the room-temperature spintronics applications with the high-blocking temperature in the nanomagnets and high-efficiency luminescent centers. We can expect high- T C Dairiseki phase in DMS by the 3D spinodal nanodecomposition, where the ferromagnetic percolation path increases dramatically upon the spinodal nanodecomposition if the concentration of the transition metal is above the percolation limit. The comparisons of our design and predictions with the available experimental data are presented.

Computational materials design of defect-induced ferrimagnetic MnO.

We propose new‐type diluted ferromagnetic semiconductors (DMSs) based on K2S without any transition metal elements. These materials show transparent and half‐metallic ferromagnetism if the deep‐impurity‐band width (W) induced by doping of Si of Ge impurities, and the electron‐correlation energy (U) satisfy the Stoner’s condition of highly‐correlated electron system (U > W). Based on our first‐principles calculations, we demonstrate that these materials satisfy the Stoner’s condition, and could be new candidates for transparent and half‐metallic DMSs.