MEMOIRS OF SHONAN INSTITUTE OF TECHNOLOGY Vol. 39, No. 1, 2005 * ** Study of Electronic Structures and Thermoelectric Properties of Half Metal Yukio YUTOH* and Sunao SUGIHARA** Electronic structures and magnetic structures of one of half metal material, Sr 2 FeMoO 6 were studied using the ab initio calculation of molecular orbital method and they were compared with the experimental results. As for calculation, magnetic structures drastically changed with the position of oxygen atom. The Crystal 98 and the DV-Xa were used to find out a total energy by the former and covalent bonding by the latter. Furthermore, the substance was explained in terms of ferrimagnetic with Fe, 3.866 m B and Mo, 0.408 m B which were similar to the experimental values of 3.9 m B and 0.37 m B. Seebeck coefficient was 0.010 0.015 mv/k and electrical resistivity, the order of 10 4 W m and the thermal conductivity was 2 to 3 W/m K according to our experiment. Sr 2 FeMoO 6 indicated to be metallic increasing with temperature, which agreed with electronic structure of down spin crossing the Fermi energy level and without energy gap. 1. 1.1 20 (E f ) (DOS) Fig. 1 up spin DOS E f down spin La Sr Mn O Sr 2 FeMoO 6 1) Mn 2) (Ga,Mn)As 3) NiMnSb 4) CrAs 5) 2 3 (Giant Magneto Resistance: GMR) (Tunneling Magneto Resistance: TMR) 1.2 Sr 2 FeMoO 6, Sr 2 FeWO 6, Ca 2 FeMoO 6, Ba 2 FeMoO 6, Sr 2 FeSmO 6 Fe Mo * ** 16 10 26 Fig. 1. DOS for half metallic structure. 63
39 1 Fig. 2. Crystal structure of Sr 2 FeMoO 6. Fig. 3. Ferrimagnetism structure. Fe 3d (Ferrimagnetism) GMR TMR Sr 2 FeMoO 6 B Fe Mo 2 (Fig. 2) Fe Mo 100% 100% Fe 2.5 6),7) Fe 2 Mo 6, Fe 3 Mo 6 2 6) Cubic Tetragonal 2 400 K Tetragonal 8) 1.3 (Ferrimagnetism) A, B 2 2 1948 Fig. 3 (Fe 3 O 5 ) 1.4 - Fig. 4 p n Seebeck Peltier (T c ) (T h ) DT V V a(t h T c ) (1) a [V/K] Q Q pi (2) p p at (3) Fig. 4 (10 25 m 3 ) Z 64
Fig. 4. Principle of Thermoelectric Module. Z a 2 /kr (4) a (V/K) k (W/m K) r (W m) (a) a (b) k (c) r 1.5 1993 9) ZnO 10),11), Y 2 O 12) 3, Ca(Mn,In)O 13) 3, NaCo 2 O 14) 4 NaCo 2 O 4 0.4 10 3 K Zn Ca 0.1 0.2 10 3 /K NaCo 2 O 4 1 10 W/m K Na NaCo 2 O 4 Na 16) 1.6 DT E f (adt) DT E f a α S (5) e S e (g) S k B ln g (6) k B 65
39 1 Sr 2 FeMoO 6 CRYSTAL98 DV-Xa Sr 2 FeMoO 6 Sr 2 FeMoO 6 2. 2.1 Sr 2 FeMoO 6 Fig. 2 Sr 2 FeMoO 6 FM- 3M cubic a 7.8973 Å Fe(0,0,0), Mo(0,0,0.5), Sr(0.25,0.25, 0.25), O(u,0,0) u 0.251 CRYSTAL98 Sr 2 FeMoO 6 17) a u 2.2 Discrete Variational (DV)-Xa DV-Xa 18) 3 V xc () 1 3α () 1 4π ρ 13 / r a (7) Fig. 5. Cluster model of (Sr 7 Fe 4 Mo 4 O 60 ) 74. a 0.7 φi*( 1) φj*( 2)( 1/ r12) φj( 1) φi( 2) dν2 νex() 1 φ *( 1) φ ( 1) f i i 4 (7) V(1) V z r eff () 1 ν ν 1 ν n n Z n 1 2 φ () 1 C χ () 1 l il i i (8) (9) (10) c i i i ρ( 2) dν2 r 12 V xc 66
Table 1. Characteristics of structure optimization. Before After a (Å) 7.8973 7.9673 u 0.251 0.254 Fig. 6. Center of cluster. XPS (X-ray Photoelectron Spectroscopy; X EELS Electron Energy Loss Spectroscopy; XAFS X-ray Absorption Fine Structure; X XANES X- ray Absorption Near Edge Structure; X DV-Xa Sr 2 FeMoO 6 DV-Xa DV-Xa 3. Fig. 7. Density of states before or after substitution; 0 ev is Fermi level. DV-Xa c i DV-Xa Xa a a 3.1 CRYSTAL98 Sr 2 FeMoO 6 1),20) 22) Table 1 2 3% 1% Fe Mo 3.866 m B, 0.408 m B Fe Mo 19) 67
39 1 Fe Mo 3.9 m B 0.37 m B Sr 2 FeMoO 6 Fig. 8(a) Fe Mo a Fe, Mo Fig. 8(b) Fe Mo O u u O Mo Fe u Fe, Mo Fig. 8(c) O u Mo O a O u Mo O Mo Sr 2 FeMoO 6 Fe Mo O Fe, Mo, O 3 Fe O, Mo O O u O u Mo O (6) g (5) (6) A, 3.2 DV-Xa DV-Xa Fig. 5 Sr Fig. 8. Dependence of magnetic moment of Fe and Mo on a-axis and coordinate u of O. DV-Xa (DOS) Fig. 7 DOS Sr 2 FeMoO 6 up spin 2.57 ev down spin 68
Fig. 9. Effective charges for Sr 2 FeMoO 6. Atomic positions correspond to Fig. 6. Fig. 11. X-ray photoelectron spectroscopy for calculation and experiment. Fig. 10. Covalent bonding for Sr 2 FeMoO 6. 0 Fig. 9 Fe, Mo, Sr, O 2.04, 2.59, 1.83, 1.31 Mo 2.59 Mo 5.5 DV-Xa Fe Mo 4.385 m B, 0.673 m B CRYS- TAL98 CRYSTAL98 Fig. 10 Fe O Mo O 0.196 0.141 Sr O Sr 2 FeMoO 6 Fe O Mo O DV-Xa XPS XPS Fig. 11 0eV Ef 3.3 Sr 2 FeMoO 6 XRD SrCO 3 Fe 2 O 3 MoO 3 24 Ar H 2 900 C 2 Ar H 2 1000 C 24 XRD 5 5 15 mm XRD Fig. 12 Sr 2 FeMoO 6 Fig. 11 XPS Fig. 13 Sr 2 FeMoO 6 (a) (r) a n a r a 69
39 1 4. Fig. 13. Fig. 14. Fig. 12. XRD pattern for Sr 2 FeMoO 6. Seebeck coeffcient and electrical resistivity against temperature. Thermal conductivity against temperature. r 10 5 (W m) r down spin Fig. 14 k 2 3 (W/mK) FeSi 2 10 6 10 7 (/K) BiTe 3 4 Sr 2 FeMoO 6 a Sr 2 FeMoO 6 Fe, Mo Sr 2 FeMoO 6 Fe O Mo O 0.196 0.141 Sr O Sr 2 FeMoO 6 Fe O Mo O CRYSTAL98 O u Mo a XPS XPS Fig. 11, DV-Xa, Frozen phonon 70
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