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

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, (1999) Awana et al., Physica C 357 (2001)

RuSr 2 GdCu 2 O 8+  Tetragonal 1212-type P4/mmm CuO 5 RuO 6 Gd Sr

RuSr 2 GdCu 2 O 8+  T m = 133 K, T c = K Bernhard et al., PRB 59, (1999) R(T)  v (T)(ZFC) S(T) M m (T)(FC)

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, (1999)

ZFC/FC susceptibility Superconducting diamagnetic signal? Current oxygen-deficient sample superconducting T c ~ 10 K superconducting signal? Cordero et al., PRB 67, (2003)

Superconducting diamagnetic signal appears only in very low applied fields due to strong ferromagnetic background M g (T)(FC) Papageorgiou & Braun, PRB 66, (2002)

T c only in R = Sm, Eu, Gd of RuSr 2 RCu 2 O 8+  With oxygen effect - hole concentration

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

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

Distorted orthorhombic variation of tetragonal 1212 structure: a o ~ b o ~  2a t c o ~ 2c t

Abnormal low temperature magnetic behavior at different measuring times oxygen diffusion effect

Normal ZFC/FC susceptibility and magnetic hysteresis for standard high-T c superconductors without magnetic order

Magnetization M m (T) in different applied fields Anomalous FC/ZFC behavior

Normal paramagnetic behavior for T > T m & Mgnetic order (weak-ferromagnetic or spin-glass?) for T m > T > T c

Weak magnetization relaxation at T= 45 K < T m (itinerant weak ferromagnetism almost spin-glass-like)

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)

Orthorhombic (distorted) Orthorhombic (pseudo-cubic) a ~ b ~ c/  2 Lattice distortion of CaRuO 3 compared with SrRuO 3

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

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 )

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

Complex magnetic flux dynamics for T < T c < T m in magnetization due to interplay between superconductivity and itinerant ferromagnetic order

FC and ZFC magnetization at 10 K show complex interplay between superconductivity and itinerant ferromagnetic order

Low field (1 G) time relaxation M(t) at 10 K < T c < T m

1-G and 1-kG time relaxation M(t) at 10 K < T c < T m

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 ~ K *sensitive to oxygen  -- hole concentration), *coexistence with weak magnetic order - anomalous ZFC (Meissner) & FC (field expulsion) behavior due to interplay with ferromagnetic order

Powder x-ray diffraction patterns of tetragonal RuSr 2 PrCu 2 O 8+ 

Powder x-ray diffraction patterns for RuCa 2-x Sr x PrCu 2 O 8+ 

Low temperature spin-glass-like behavior for tetragonal RuSr 2 PrCu 2 O 8+  Ref: Awana et al. Physica C , 121 (2001).

Low temperature spin-glass-like behavior for tetragonal RuSr 2 PrCu 2 O 8+ 

Weak magnetization relaxation at T= 10 K < T m spin-glass-like for tetragonal RuSr 2 PrCu 2 O 8+ 

Field-cooled (FC) susceptibility for RuCa 2-x Sr x PrCu 2 O 8+ 

Magnetic behavior for RuCa 2-x Sr x PrCu 2 O 8+  (x = 0, 0.1)

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  spin-glass-like behavior for tetragonal RuSr 2 PrCu 2 O 8+  (x = 2)

Powder x-ray diffraction patterns for RuCa 2 RCu 2 O 8+  (R = Pr, Gd)

Magnetic behavior for RuCa 2 RCu 2 O 8+  (R = Pr, Gd)

Powder x-ray diffraction patterns for RuCa 2 RCu 2 O 8+ 

Field-cooled (FC) susceptibility for RuCa 2-x Sr x PrCu 2 O 8+   (R = La, Ce, Pr, Nd, Sm, Eu, Gd)

Zero-field-cooled (ZFC) susceptibility for RuCa 2-x Sr x PrCu 2 O 8+  (R = La, Ce, Nd, Sm, Eu, Gd)

RuCa 2 RCu 2 O 8+  system - orthorhombic 1212 phase in R = Pr, Nd, Sm, Eu, Gd - magnetic order T m ~ K for R = Pr-Gd - superconductivity T c ~ 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

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

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 ]

Magnetic behavior  m (T) for bulk and powder samples Diamagnetic signal, bulk superconductivity