Superconductivity and Anomalous Magnetic Properties of the New RuCa2RCu2O8 ? (R = Pr-Gd) System

1
Superconductivity and Anomalous Magnetic
Properties of the New RuCa2RCu2O8? (R Pr-Gd)
System
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
2
Introduction

• Magnetic Superconductivity in RuSr2RCu2O8?
• Ru-1212 system (R Sm, Eu, Gd)
• - tetragonal TlBa2CaCu2O7? -type (1212)
  structure
• - weak ferromagnetic metal Tm 135 K
• high-Tc superconductor Tc 0-65 K
• sensitive to oxygen ? - hole concentration
• coexistence with ferromagnetic order (Tc lt Tm)
  
• 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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RuSr2GdCu2O8d Tetragonal 1212-type P4/mmm
Sr
Gd
CuO5
RuO6
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RuSr2GdCu2O8d Tm 133 K, Tc 16-48 K
Bernhard et al., PRB 59, 14009 (1999)
R(T)
??v(T)(ZFC)
Mm(T)(FC)
S(T)
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Normal or abnormal ferromagnetic magnetic
hysteresis curve M(Ba) for T lt Tm 135 K?
Current oxygen-deficient sample superconducting
Tc 10 K superconducting signal?
Bernhard et al., PRB 59, 14009 (1999)
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ZFC/FC susceptibilitySuperconducting diamagnetic
signal?
Current oxygen-deficient sample superconducting
Tc 10 K superconducting signal?
Cordero et al., PRB 67, 144519 (2003)
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Superconducting diamagnetic signal appears only
invery low applied fields due to strong
ferromagnetic background
Papageorgiou Braun, PRB 66, 104509 (2002)
Mg(T)(FC)
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Tc only in R Sm, Eu, Gd of RuSr2RCu2O8d With
oxygen effect - hole concentration
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Question

• What about the RuCa2RCu2O8? system
• (R La, Ce, Pr, Nd, Sm, Eu, Gd)?
• First try R Pr RuCa2PrCu2O8?

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Results and Discussion

• RuCa2PrCu2O8?
• RuCa2-xSrxPrCu2O8? system (2 ? x ? 0)
• RuCa2RCu2O8? 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 ao bo ?2at co 2ct
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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-Tc superconductors
without magnetic order
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Magnetization Mm(T) in different applied
fields Anomalous FC/ZFC behavior
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Normal paramagnetic behavior for T gt Tm Mgnetic
order (weak-ferromagnetic or spin-glass?) for Tm
gt T gt Tc
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Weak magnetization relaxation at T 45 K lt
Tm(itinerant weak ferromagnetism almost
spin-glass-like)
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Spin-glass-like weak hysteresis m(Ba) and
relaxation m(t) for paramagnetic Ca1-xSrxRuO3 x
0 0.2Lin et al., JMMM, in press (2004)
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Lattice distortion of CaRuO3 compared with SrRuO3
Orthorhombic (pseudo-cubic) a b c/?2
Orthorhombic (distorted)
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Compare the magnetic phase diagram of
Ca1-xSrxRuO3 system with RuA2RCu2O8 with the same
RuO6 configuration
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• Ru4/5 4d4 /d3 in RuO6
• - 4dn in t2g bands due to strong RuO6
  octahedral crystal field splitting Dcf and weak
  Hund rule exchange coupling JH
• strong Ru 4dxy,yz,zx-O 2px,y,z hybridization
• large on-site Coulomb repulsion Udd
• Long-range magnetic order
• (or short-range spin glass)

Charge transfer D
Coulomb repulsion Udd
eg
On-site Coulomb repulsion energy Udd E(dn-1)
E(dn1) - 2E(dn) Charge transfer energy D
E(dn1L) E(dn)
d
t2g
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Origin of weak, itinerant ferromagnetic order

• - self-doing with anisotropic Ru 4dxy,yz,zx-O
  2px,y,z
• hybridization drives the resulting
  mixed-valent
• system metallic and ferromagnetic via double
• exchange interaction
• - lower Tm 50 K for RuCa2PrCu2O8 as compared
  with Tm 135 K for RuSr2GdCu2O8 (in more robust
  local moment regime) indicates weaker double
  exchange coupling strength Jde due to RuO6
  distortion

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Complex magnetic flux dynamics for T lt Tc lt Tm
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 lt Tc
lt Tm
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1-G and 1-kG time relaxation M(t) at 10 K lt Tc lt
Tm
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• RuCa2PrCu2O8?
• distorted orthorhombic variation of tetragonal
  1212
• structure ao bo ?2at co 2ct
• Ru4/5 itinerant weak ferromagnetic metal close
  to
• spin-glass with Tm 50 K
• superconductor Tc 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 oftetragonal
RuSr2PrCu2O8d
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Powder x-ray diffraction patterns for
RuCa2-xSrxPrCu2O8d
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Low temperature spin-glass-like behavior for
tetragonal RuSr2PrCu2O8dRef Awana et al.
Physica C357-360, 121 (2001).
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Low temperature spin-glass-like behavior for
tetragonal RuSr2PrCu2O8d
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Weak magnetization relaxation at T 10 K lt
Tmspin-glass-like for tetragonal RuSr2PrCu2O8d
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Field-cooled (FC) susceptibility for
RuCa2-xSrxPrCu2O8d
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Magnetic behavior for RuCa2-xSrxPrCu2O8d (x 0,
0.1)
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• RuCa2-xSrxPrCu2O8? system
• orthorhombic 1212 structure only near
  RuCa2PrCu2O8?
• (x 0)
• multiphase samples for 0 lt x lt 2
• diamagnetic, superconducting signal observed
  only
• for x ? 0.2
• spin-glass-like behavior for tetragonal
  RuSr2PrCu2O8?
• (x 2)

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Powder x-ray diffraction patterns for
RuCa2RCu2O8d (R Pr, Gd)
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Magnetic behavior for RuCa2RCu2O8d (R Pr, Gd)
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Powder x-ray diffraction patterns for
RuCa2RCu2O8d
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Field-cooled (FC) susceptibility for
RuCa2-xSrxPrCu2O8d (R La, Ce, Pr, Nd, Sm, Eu,
Gd)
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Zero-field-cooled (ZFC) susceptibility for
RuCa2-xSrxPrCu2O8d (R La, Ce, Nd, Sm, Eu, Gd)
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• RuCa2RCu2O8? system
• orthorhombic 1212 phase in R Pr, Nd, Sm, Eu,
  Gd
• magnetic order Tm 45-50 K for R Pr-Gd
• superconductivity Tc 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
• RuCa2RCu2O8d (R Pr, Nd, Sm, Eu, Gd)
• system with the distorted orthorhombic
• 1212-type structure
• Structural and anomalous magnetic
• properties are strongly correlated with the
• RuO6 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 Mv (emu/cm3 ? G) Mass
magnetization Mg (emu/g Gcm3/g) Molar
magnetization Mm (emu/mol Gcm3/mol) (Mm ? Mg
molar formula unit weight) Magnetic moment per
formula unit m (mB/f.u.) (m ? Mm/(No
mB) Volume magnetic susceptibility ?v ? Mv/Ba
(emu/cm3G 1) Mass magnetic susceptibility ?g
? Mg/Ba (emu/gG cm3/g) Molar magnetic
susceptibility ?m (emu/molG cm3/mol) (?m ?
?g molar formula unit weight) good for SI
system with ?g ? Mg/?0Ba
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Magnetic behavior cm(T) for bulk and powder
samples Diamagnetic signal, bulk
superconductivity