Photo-induced spin-state change in itinerant correlated-electron system is studied. The model Hamiltonians before and after photon-pumping are derived from the two-orbital Hubbard model with crystalline field splitting. A photon introduced in the low-spin band insulator induces a bound state of the high-spin state and a photoexcited hole. This bound state brings a characteristic peak in the optical absorption spectra in the photo-excited state. The present results well explain the recent experimental results of the ultrafast optical spectroscopy in perovskite cobaltites. 71.10.w, 71.30.+h, 78.20.Bh A number of electronic phases and the phase transition between them are one of the central issues in correlated electron systems. In particular, materials with multi-degrees of freedom, such as charge, spin, orbital and so on, exhibit various exotic phenomena related to the phase transition [1]. Recently developed ultrafast optical techniques open up a new frontier for research of the phase transition [2]. Irradiation of a pump laser pulse into a system in a vicinity of the phase boundary triggers an abrupt change in the electronic structures. This is the so-called photo-induced phase transition (PIPT). In contrast to the conventional phase transitions caused by controlling temperature (T ), doping carriers and so on, photoinduced phase is transient and highly nonequilibrium. Photoinduced phenomena in correlated electron systems offer large possibility of new hidden phases [3,4], which do not realize in the thermal equilibrium state, and prompt several theoretical challenges [5][6][7][8][9][10][11].The spin-state transition is one of the targets in recent PIPT studies. This is a transition between the states with different magnitudes of the spin-angular moment in transition-metal ions. Different spin states are realized owing to a delicate balance of the intra-ion Hund coupling and the crystallinefield splitting. Some examples have been seen in the insulating organometallic complexes, such as the Prussian-blue analog, where magnitudes of the localized spins in Co or Fe ions are switched by photon irradiation [12,13]. Another type of the photo-induced spin-state transition is suggested in the correlated electron systems, the cobalt oxides with a perovskite structure, R 1−x A x CoO 3 and RACo 2 O 6−δ (R: a rare-earth ion, A: an alkaline-earth ion). Possible three spin states in Co 3+ are the low-spin (LS) in the (e g ) 0 (t 2g ) 6 configuration, the intermediate-spin (IS) in (e g ) 1 (t 2g ) 5 , and the high-spin (HS) in (e g ) 2 (t 2g ) 4 [14,15]. Temperature induced spin-state transition from the low-T LS insulating state into the high-T HS or IS metallic one is commonly observed in these cobaltites. The ultrafast optical measurements in the low-T LS insulators show transient metallic spectra which are completely different from the spectra in the high-T phase [16][17][18]. The electron conduction and the spin-state are strongly coupled with each other, in highly contrast to the spin-crossover organometallic comple...

We present a theory of ultrafast photoinduced dynamics in a spin-charge coupled system, motivated by pump-probe experiments in perovskite manganites. A microscopic picture for multiple dynamics in spin and charge degrees is examined. Real-time simulations are carried out by two complimentary methods. Our calculation demonstrates that electron motion governs a short-time scale where charge and spin dynamics are combined strongly, while, in a long-time scale controlled by spin relaxation, the charge sector does not follow the remarkable change in the spin sector. The present results are in contrast to a conventional double-exchange picture in equilibrium states.

Photo-induced spin and charge dynamics in double-exchange model are numerically studied. The Lanczos method and the density-matrix renormalization-group method are applied to onedimensional finite-size clusters. By photon irradiation in a charge ordered (CO) insulator associated with antiferromagnetic (AFM) correlation, both the CO and AFM correlations collapse rapidly, and appearances of new peaks inside of an insulating gap are observed in the optical spectra and the one-particle excitation spectra. Time evolutions of the spin correlation and the in-gap state are correlated with each other, and are governed by the transfer integral of conduction electrons. Results are interpreted by the charge kink/anti-kink picture and their effective motions which depend on the localized spin correlation. Pump-photon density dependence of spin and charge dynamics are also studied. Roles of spin degree of freedom are remarkable in a case of weak photon density. Implications of the numerical results for the pump-probe experiments in perovskite manganites are discussed.

Photo-excited state in correlated electron system with spin-state degree of freedom is studied. We start from the two-orbital extended Hubbard model where energy difference between the two orbitals is introduced. Photo-excited metastable state is examined based on the effective model Hamiltonian derived by the two-orbital Hubbard model. Spin-state change is induced by photoirradiation in the low-spin band insulator near the phase boundary. High-spin state is stabilized by creating a ferromagnetic bound state with photo-doped hole carriers. An optical absorption occurs between the bonding and antibonding orbitals inside of the bound state. Time-evolution for photo-excited states is simulated in the time-dependent mean-field scheme. Pair-annihilations of the photo-doped electron and hole generate the high-spin state in a low-spin band insulator. We propose that this process is directly observed by the time-resolved photoemission experiments.

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Photoirradiation effects in correlated electrons coupled with localized spins are studied based on the extended double-exchange model. In particular, we examine melting of an antiferromagnetic (AFM) charge order insulating state by varying the light intensity. When intense light is irradiated, the AFM insulating characteristics are strengthened, rather than changing into the ferromagnetic metallic characteristics, which are expected from the conventional double-exchange interaction when carriers are introduced by weak light irradiation or chemical doping. This provides a principle for optically manipulating magnetism.

The magnetic properties and microstructures of 2-17 type Sm-Co magnets with high Fe and low Zr content were investigated. The developed magnet achieved maximum energy product, [BH]m of 34.5 MGOe, intrinsic coercivity, Hcj of 21.3 kOe and squareness of 73.3% at 25 °C. Temperature coefficients of remanent magnetic flux density, Br and Hcj were 0.034%/K and 0.28%/K respectively, which values were almost as same as the conventional Sm-Co magnets. Moreover, the developed magnets had high magnetization orientation. For XRD, it was found that Zr was preferentially substituted by Co-Co pair, this made interaction between Co and Co stronger, so that heat resistance was maintained. Magnetic domain structures were observed with a Kerr effect microscope, and then it was observed that the developed magnet had strong pinning force. In the microstructures, the developed magnet had 200∼500 nm cell size with Fe and Cu separated clearly. This led to large gap of domain wall energy which produces strong pinning force. Because the developed magnet had high magnetization orientation and large gap of domain wall energy, we achieved high magnetic properties and high heat resistance on the developed magnet.

Photoinduced spin-charge dynamics in strongly correlated electron systems is studied based on an extended double-exchange model. Solving a time-dependent Schrödinger equation with the Lanczos method, we trace the process of photoinduced melting of an antiferromagnetic charge order and analyze the excitation-density dependence on that. In the case of low density photoexcitation, both charge and spin orders are melted by photocarrier doping. This is interpreted with a conventional double-exchange mechanism. In the case of high density photoexcitation, however, the charge order is melted. The antiferromagnetic spin order is transiently weakened but after turnoff of the photoirradiation it recovers. This phenomenon strikingly differs from the weak photoexcitation case.