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H. C. Ku Department of Physics, National Tsing Hua University, Hsinchu, Taiwan 300, R.O.C. with: B. N. Lin, P. C. Guan, Y. C. Lin, T. Y. Chiu, M. F. Tai Superconductivity and Anomalous Magnetic Properties of the New RuCa 2 RCu 2 O 8+ (R = Pr-Gd) System
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Introduction Magnetic Superconductivity in RuSr 2 RCu 2 O 8+ Ru-1212 system (R = Sm, Eu, Gd) - tetragonal TlBa 2 CaCu 2 O 7+ -type (1212) structure - weak ferromagnetic metal T m ~ 135 K - high-T c superconductor T c ~ 0-65 K *sensitive to oxygen - hole concentration *coexistence with ferromagnetic order (T c < T m ) Bauernfeind et al., Physica C 254, 151 (1995) Bernhard et al., PRB 59, 14099 (1999) Awana et al., Physica C 357 (2001)
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RuSr 2 GdCu 2 O 8+ Tetragonal 1212-type P4/mmm CuO 5 RuO 6 Gd Sr
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RuSr 2 GdCu 2 O 8+ T m = 133 K, T c = 16-48 K Bernhard et al., PRB 59, 14009 (1999) R(T) v (T)(ZFC) S(T) M m (T)(FC)
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Normal or abnormal ferromagnetic magnetic hysteresis curve M(B a ) for T < T m = 135 K? Current oxygen-deficient sample superconducting T c ~ 10 K superconducting signal? Bernhard et al., PRB 59, 14009 (1999)
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ZFC/FC susceptibility Superconducting diamagnetic signal? Current oxygen-deficient sample superconducting T c ~ 10 K superconducting signal? Cordero et al., PRB 67, 144519 (2003)
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Superconducting diamagnetic signal appears only in very low applied fields due to strong ferromagnetic background M g (T)(FC) Papageorgiou & Braun, PRB 66, 104509 (2002)
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T c only in R = Sm, Eu, Gd of RuSr 2 RCu 2 O 8+ With oxygen effect - hole concentration
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What about the RuCa 2 RCu 2 O 8+ system (R = La, Ce, Pr, Nd, Sm, Eu, Gd)? First try R = Pr : RuCa 2 PrCu 2 O 8+ Question
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Results and Discussion RuCa 2 PrCu 2 O 8+ RuCa 2-x Sr x PrCu 2 O 8+ system (2 x 0) RuCa 2 RCu 2 O 8+ system (R = La, Ce, Pr, Nd, Sm, Eu, Gd) Sample preparation: Direct one-step, oxygen annealed
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Distorted orthorhombic variation of tetragonal 1212 structure: a o ~ b o ~ 2a t c o ~ 2c t
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Abnormal low temperature magnetic behavior at different measuring times oxygen diffusion effect
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Normal ZFC/FC susceptibility and magnetic hysteresis for standard high-T c superconductors without magnetic order
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Magnetization M m (T) in different applied fields Anomalous FC/ZFC behavior
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Normal paramagnetic behavior for T > T m & Mgnetic order (weak-ferromagnetic or spin-glass?) for T m > T > T c
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Weak magnetization relaxation at T= 45 K < T m (itinerant weak ferromagnetism almost spin-glass-like)
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Spin-glass-like weak hysteresis m(B a ) and relaxation m(t) for paramagnetic Ca 1-x Sr x RuO 3 x = 0 & 0.2 Lin et al., JMMM, in press (2004)
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Orthorhombic (distorted) Orthorhombic (pseudo-cubic) a ~ b ~ c/ 2 Lattice distortion of CaRuO 3 compared with SrRuO 3
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Compare the magnetic phase diagram of Ca 1-x Sr x RuO 3 system with RuA 2 RCu 2 O 8 with the same RuO 6 configuration
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Ru 4/5+ 4d 4 /d 3 in RuO 6 - 4d n in t 2g bands due to strong RuO 6 octahedral crystal field splitting cf and weak Hund’ rule exchange coupling J H -strong Ru 4d xy,yz,zx -O 2p x,y,z hybridization -large on-site Coulomb repulsion U dd -Long-range magnetic order (or short-range spin glass) egeg t 2g d Coulomb repulsion U dd Charge transfer On-site Coulomb repulsion energy U dd = E(d n-1 ) + E(d n+1 ) - 2E(d n ) Charge transfer energy = E(d n+1 L) – E(d n )
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Origin of weak, itinerant ferromagnetic order - self-doing with anisotropic Ru 4d xy,yz,zx -O 2p x,y,z hybridization drives the resulting mixed-valent system metallic and ferromagnetic via double exchange interaction - lower T m ~ 50 K for RuCa 2 PrCu 2 O 8 as compared with T m ~ 135 K for RuSr 2 GdCu 2 O 8 (in more robust local moment regime) indicates weaker double exchange coupling strength J de due to RuO 6 distortion
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Complex magnetic flux dynamics for T < T c < T m in magnetization due to interplay between superconductivity and itinerant ferromagnetic order
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FC and ZFC magnetization at 10 K show complex interplay between superconductivity and itinerant ferromagnetic order
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Low field (1 G) time relaxation M(t) at 10 K < T c < T m
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1-G and 1-kG time relaxation M(t) at 10 K < T c < T m
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RuCa 2 PrCu 2 O 8+ - distorted orthorhombic variation of tetragonal 1212 structure: a o ~ b o ~ 2a t c o ~ 2c t - Ru 4/5+ itinerant weak ferromagnetic metal close to spin-glass with T m ~ 50 K - superconductor T c ~ 37-40 K *sensitive to oxygen -- hole concentration), *coexistence with weak magnetic order - anomalous ZFC (Meissner) & FC (field expulsion) behavior due to interplay with ferromagnetic order
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Powder x-ray diffraction patterns of tetragonal RuSr 2 PrCu 2 O 8+
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Powder x-ray diffraction patterns for RuCa 2-x Sr x PrCu 2 O 8+
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Low temperature spin-glass-like behavior for tetragonal RuSr 2 PrCu 2 O 8+ Ref: Awana et al. Physica C357-360, 121 (2001).
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Low temperature spin-glass-like behavior for tetragonal RuSr 2 PrCu 2 O 8+
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Weak magnetization relaxation at T= 10 K < T m spin-glass-like for tetragonal RuSr 2 PrCu 2 O 8+
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Field-cooled (FC) susceptibility for RuCa 2-x Sr x PrCu 2 O 8+
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Magnetic behavior for RuCa 2-x Sr x PrCu 2 O 8+ (x = 0, 0.1)
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RuCa 2-x Sr x PrCu 2 O 8+ system - orthorhombic 1212 structure only near RuCa 2 PrCu 2 O 8+ (x ~ 0) - multiphase samples for 0 < x < 2 - diamagnetic, superconducting signal observed only for x 0.2 - spin-glass-like behavior for tetragonal RuSr 2 PrCu 2 O 8+ (x = 2)
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Powder x-ray diffraction patterns for RuCa 2 RCu 2 O 8+ (R = Pr, Gd)
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Magnetic behavior for RuCa 2 RCu 2 O 8+ (R = Pr, Gd)
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Powder x-ray diffraction patterns for RuCa 2 RCu 2 O 8+
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Field-cooled (FC) susceptibility for RuCa 2-x Sr x PrCu 2 O 8+ (R = La, Ce, Pr, Nd, Sm, Eu, Gd)
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Zero-field-cooled (ZFC) susceptibility for RuCa 2-x Sr x PrCu 2 O 8+ (R = La, Ce, Nd, Sm, Eu, Gd)
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RuCa 2 RCu 2 O 8+ system - orthorhombic 1212 phase in R = Pr, Nd, Sm, Eu, Gd - magnetic order T m ~ 45-50 K for R = Pr-Gd - superconductivity T c ~ 25-40 K for R = Pr-Gd with anomalous ZFC and FC behavior due to magnetic order - weak paramagnetic spin-glass-like behavior signal for R = La, Ce
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Conclusions Superconductivity are observed in the RuCa 2 RCu 2 O 8+d (R = Pr, Nd, Sm, Eu, Gd) system with the distorted orthorhombic 1212-type structure Structural and anomalous magnetic properties are strongly correlated with the RuO 6 distortion and Ru 4d-O2p hybridization with weak, anisotropic double-exchange coupling strength due to distortion
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Unit for magnetic measurements Total magnetic moment m (emu) [Gaussian system] Volume magnetization M v (emu/cm 3 G) Mass magnetization M g (emu/g = Gcm 3 /g) Molar magnetization M m (emu/mol = Gcm 3 /mol) (M m M g molar formula unit weight) Magnetic moment per formula unit m ( B /f.u.) (m M m /(N o B ) Volume magnetic susceptibility v M v /B a (emu/cm 3 G = 1) Mass magnetic susceptibility g M g /B a (emu/gG = cm 3 /g) Molar magnetic susceptibility m (emu/molG = cm 3 /mol) ( m g molar formula unit weight) [good for SI system with g M g / 0 B a ]
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Magnetic behavior m (T) for bulk and powder samples Diamagnetic signal, bulk superconductivity
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