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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
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 < Tm)
Bauernfeind et al., Physica C 254, 151 (1995)
Bernhard et al., PRB 59, 14099 (1999)
Awana et al., Physica C 357 (2001)
RuSr2GdCu2O8+
Tetragonal 1212-type
P4/mmm
Sr
Gd
CuO5
RuO6
RuSr2GdCu2O8+
Tm = 133 K, Tc = 16-48 K
R(T)
S(T)
Bernhard et al.,
PRB 59, 14009 (1999)
v(T)(ZFC)
Mm(T)(FC)
12
9
RuSr2GdCu2O8+
6
3
Magnetization Mm (10 emu/mol)
Normal or abnormal ferromagnetic magnetic
hysteresis curve M(Ba) for T < Tm = 135 K?
3
0.83 /f.u.
0
-3
Tc < 100 K < TC
-6
-9
10 K < Tc?
-12
-1.0 -0.8 -0.6 -0.4 -0.2 0.0
0.2
0.4
0.6
0.8
Applied field Ba(T)
Bernhard et al.,
PRB 59, 14009 (1999)
Current oxygen-deficient sample
superconducting Tc ~ 10 K
superconducting signal?
1.0
ZFC/FC susceptibility
Superconducting diamagnetic signal?
12
3
Susceptibility m (cm /mol)
Tc
RuSr2GdCu2O8+
oxygen-deficient
10
8
1-G FC
6
Tm = 135 K
4
100-G FC
2
100-G ZFC
0
-2
1-G ZFC
0
50
100
150
200
Temperature T (K)
Cordero et al., PRB 67, 144519 (2003)
Current oxygen-deficient sample
superconducting Tc ~ 10 K
superconducting signal?
Superconducting diamagnetic signal appears only in
very low applied fields due to strong ferromagnetic
background
Papageorgiou & Braun,
PRB 66, 104509 (2002)
Mg(T)(FC)
Tc only in R = Sm, Eu, Gd of RuSr2RCu2O8+
With oxygen effect - hole concentration
Superconducting Tc (K)
80
70
60
50
Pr Nd
Sm Eu
RuSr2RCu2O8+
Gd Tb Dy
Y
65 K (03)
Ru-1212 (P4/mmm)
48 K (99)
Tm~135 K
40
33 K (99)
30
20
10
0
1.14
12 K (01)
no Tc(01)
1.12
no Tc(01)
1.10
1.08
1.06
rare earth ionic radius R
1.04
3+
(Å)
1.02
Question
•
What about the RuCa2RCu2O8+ system
(R = La, Ce, Pr, Nd, Sm, Eu, Gd)?
First try R = Pr : RuCa2PrCu2O8+
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
Distorted orthorhombic variation of tetragonal 1212
structure: ao ~ bo ~ 2at co ~ 2ct
(107)(008)
5
(410)
(225)
(1111)
(314)
(1013)
(320)
(1014)
(216)(220)
(118)
(223)
Orthorhombic (Pmmm?)
a = 5.874 Å
b = 5.785 Å
c = 22.460 Å
(117)
500
(115)
1000
0
RuCa2PrCu2O8+
(103)
1500
(004)
Intensity (arb. unit)
2000
10 15 20 25 30 35 40 45 50 55 60 65 70
2 (degree)
Abnormal low temperature magnetic behavior
at different measuring times oxygen diffusion effect
Tc
2
ZFC (D+10)
Tm
3
Susceptiblity m (cm /mol)
4
0
-2
ZFC (D+2)
-4
RuCa2PrCu2O8+
-6
FC (D+2)
-8
FC (D+10)
1. Annealed in O2 12 h (D day)
2. Bulk sample, 100 G
3. Measured at D+2 and D+10 days
-10
0
10
20
30
40
Temperature T(K)
50
60
Normal ZFC/FC susceptibility and magnetic hysteresis
for standard high-Tc superconductors without
magnetic order
Tc = 37 K
0.0
9
La1.85Sr0.15CuO4+
6
T = 10 K < Tc
2
Magnitization Mm (10 emu/mol)
12
3
Susceptibility m (cm /mol)
0.4
-0.4
FC
-0.8
La1.85Sr0.15CuO4
-1.2
Ba = 100 G
ZFC
-1.6
0
10
20
30
40
Temperature T (K)
50
60
3
0
-3
-6
-9
-12
-1.0 -0.8 -0.6 -0.4 -0.2 0.0
0.2
0.4
Applied field Ba (T)
0.6
0.8
1.0
Magnetization Mm(T) in different applied fields
Anomalous FC/ZFC behavior
2
Magnetization Mm(10 emu/mol)
4
Tc
2
Tm
1,10 &100-G/ZFC
0
1-kG/ZFC
-2
1-kG/FC
1-G/FC
-4
RuCa2PrCu2O8+
10-G/FC
-6
100-G/FC
-8
0
10
20
30
40
Temperature T (K)
50
60
Normal paramagnetic behavior for T > Tm &
Mgnetic order (weak-ferromagnetic or spin-glass?)
for Tm > T > Tc
2
Magnitization Mm (10 emu/mol)
10
8
0.16 B/f.u.
RuCa2PrCu2O8+
6
4
2
40 K ~ Tc
(WFM + S?)
T = 60 K > Tm
(paramagnetic)
0
-2
-4
-6
-8
45 K < Tm (weak ferromagnetic?)
-10
-1.0 -0.8 -0.6 -0.4 -0.2
0.0
0.2
0.4
Applied field Ba (T)
0.6
0.8
1.0
Weak magnetization relaxation at T= 45 K < Tm
(itinerant weak ferromagnetism almost spin-glass-like)
Magnetization Mm (emu/mol)
200
ZFC to 45 K
1 kG (FC), 45 K
150
1 kG (ZFC), 45 K
100
RuCa2PrCu2O8+
50
T = 45 K < Tm ~ 50 K
1 kG, 100 K
0
2
10
3
10
Time t (s)
4
10
"0 G"
45 K
WFM
Spin-glass-like weak hysteresis m(Ba) and relaxation m(t)
for paramagnetic Ca1-xSrxRuO3 x = 0 & 0.2
8
0.07 B
6
Ca1-xSrxRuO3
0.03 B
2
0
-2
-4
x=0
T=5K
-6
x = 0.2
-8
-2.0 -1.5
-1.0
-0.5
1.5
ZFC to 5 K
1 kG (FC)
5K
-2
4
Magnetization m (x10 B/Ru)
-2
Magnetization m (x10 B/f.u.)
Lin et al., JMMM, in press (2004)
0.0
0.5
1.0
Applied field Ba (T)
1.5
2.0
1 kG (ZFC), 5 K
1.0
0G
5K
0.5
Ca0.8Sr0.2RuO3
1 kG, 100 K
0.0
2
10
3
10
Time t (s)
4
10
Lattice distortion of CaRuO3 compared with SrRuO3
Orthorhombic
(distorted)
Orthorhombic
(pseudo-cubic)
a ~ b ~ c/2
Compare the magnetic phase diagram of Ca1-xSrxRuO3
system with RuA2RCu2O8 with the same RuO6 configuration
RuSr2PrCu2O8
RuSr2GdCu2O8
RuCa2PrCu2O8
Curie temperature TC (K)
200
150
Ca1-xSrxRuO3
exchange-enhanced
paramagnetic metal (PM-M)
100
50
itinerant ferromagnetic metal
(FM-M)
spin glass
0
0.0
0.2
0.4
0.6
x
0.8
1.0
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
Charge
transfer D
- strong Ru 4dxy,yz,zx-O 2px,y,z hybridization
- large on-site Coulomb repulsion Udd
- Long-range magnetic order
Coulomb
repulsion
Udd
(or short-range spin glass)
On-site Coulomb repulsion energy
Udd = E(dn-1) + E(dn+1) - 2E(dn)
Charge transfer energy
D = E(dn+1L) – E(dn)
eg
d
t2g
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
Complex magnetic flux dynamics for T < Tc < Tm
in magnetization due to interplay between
superconductivity and itinerant ferromagnetic order
2
Magnitization Mm (10 emu/mol)
10
8
RuCa2PrCu2O8+
6
4
30 K
2
0
-2
10 K
initial magnetization curve
-4
-6
-8
-10
-1.0
T < Tc(~40 K)
< Tm(~50 K) (ZFC)
35 K
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
Applied field Ba (T)
0.6
0.8
1.0
FC and ZFC magnetization at 10 K show complex interplay
between superconductivity and itinerant ferromagnetic order
2
Magnitization Mm (10 emu/mol)
10
8
6
RuCa2PrCu2O8+
4
2
(1)
0
ZFC to 10 K
-2
-4
-6
(1)
-8
100-G FC to 10 K
-10
-12
-14
-1.0 -0.8 -0.6 -0.4 -0.2
0.0
0.2
0.4
Applied field Ba (T)
0.6
0.8
1.0
Low field (1 G) time relaxation M(t) at 10 K < Tc < Tm
ZFC to 10 K
Magnetization Mm (emu/mol)
50
1 G, 100 K > Tm
0
1 G (ZFC), 10 K
-50
-100
RuCa2PrCu2O8+
-150
T = 10 K < Tc < Tm
-200
-250
1 G (FC), 10 K
2
10
3
10
Time t (s)
4
10
"0 G"
10 K
1-G and 1-kG time relaxation M(t) at 10 K < Tc < Tm
Magnetization Mm (emu/mol)
ZFC to 10 K
1 kG, 100 K
100
1 kG (ZFC), 10 K
1 G, 100 K
0
1 G (ZFC), 10 K
-100
-200
-300
1 G (FC)
10 K
RuCa2PrCu2O8+
-400
-500
2
10
3
10
Time t (s)
"0 G"
10 K
1 kG (FC)
10 K
4
10
"0 G"
10 K
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
Powder x-ray diffraction patterns of
tetragonal RuSr2PrCu2O8+
(110)(103)
PrSr2RuCu2O8+
(006)(200)
3000
2000
1000
0
5
(101)
(102)
x
x
(220)(206)
Tetragonal (P4/mmm)
a = 3.910 Å
c = 11.726 Å
(116)(213)
4000
(002)
Intensity (arb. unit)
5000
10 15 20 25 30 35 40 45 50 55 60 65 70
2 (degree)
Powder x-ray diffraction patterns for RuCa2-xSrxPrCu2O8+
Intensity (arb. unit)
Ru Ca2-xSrx PrCu2O8+
x=0
0
0
0.2
0.6
0
0.8
0
0
0
5
1.0
2.0
10 15 20 25 30 35 40 45 50 55 60 65 70
2 (degree)
Low temperature spin-glass-like behavior
for tetragonal RuSr2PrCu2O8+
Ref: Awana et al. Physica C357-360, 121 (2001).
100-G
FC
1-kG
7 1-T
Tmerge(1 T)
RuSr2PrCu2O8+
-2
3
Susceptibility m (10 cm /mol)
8
ZFC
6
5
spin glass
4
3
0
10
exchange-enhanced
paramagnetism
20
30
40
Temperature T (K)
50
60
2
Magnitization Mm (10 emu/mol)
Low temperature spin-glass-like behavior
for tetragonal RuSr2PrCu2O8+
6
0.12 /f.u.
RuSr2PrCu2O8+
4
T = 20 K
paramagnetic
2
0
-2
-4
T = 10 K
spin glass?
-6
-1.0 -0.8 -0.6 -0.4 -0.2
0.0
0.2
0.4
Applied field Ba (T)
0.6
0.8
1.0
Weak magnetization relaxation at T= 10 K < Tm
spin-glass-like for tetragonal RuSr2PrCu2O8+
Magnetization Mm (emu/mol)
70
ZFC to 10 K
1 kG (FC), 10 K
60
1 kG (ZFC), 10 K
50
40
30
20
10
0
RuSr2PrCu2O8+
1 kG, 100 K
T = 10 K < Tmerge ~ 17 K
spin glass
2
10
3
10
Time t (s)
4
10
"0 G"
10 K
small
Field-cooled (FC) susceptibility for RuCa2-xSrxPrCu2O8+
3
Susceptibility m (cm /mol)
4
2
0
x = 0.0
x = 0.1
x = 0.2
x = 0.6
x = 1.0
x = 2.0
-2
-4
100 G FC
-6
RuCa2-xSrxPrCu2O8+
-8
0
10
20
30
40
Temperature T (K)
50
60
Magnetic behavior for RuCa2-xSrxPrCu2O8+(x = 0, 0.1)
2
x = 0.1 ZFC
3
Susceptibility m (cm /mol)
4
0
x = 0 ZFC
-2
x = 0.1 FC
-4
Ba = 100 G
-6
-8
x = 0 FC
0
10
RuCa2-xSrxPrCu2O8+
20
30
40
Temperature T (K)
50
60
RuCa2-xSrxPrCu2O8+ system
- orthorhombic 1212 structure only near RuCa2PrCu2O8+
(x ~ 0)
- multiphase samples for 0 < x < 2
- diamagnetic, superconducting signal observed only
for x  0.2
- spin-glass-like behavior for tetragonal RuSr2PrCu2O8+
(x = 2)
(030)
(221)
(117)
(124)
(118)
(115)
(320)
RuCa2RCu2O8+
(103)
(004)
Intensity (arb. unit)
(017)
Powder x-ray diffraction patterns for
RuCa2RCu2O8+ (R = Pr, Gd)
R = Pr
0
R = Gd
0
5
10 15 20 25 30 35 40 45 50 55 60 65 70
2 (degree)
3
Molar susceptibility m (cm /mol)
Magnetic behavior for RuCa2RCu2O8+(R = Pr, Gd)
4
2
Gd ZFC
0
Pr ZFC
-2
-4
Gd FC
RuCa2RCu2O8+
-6
-8
Pr FC
0
Ba = 100 G
10
20
30
40
Temperature T (K)
50
60
Powder x-ray diffraction patterns for RuCa2RCu2O8+
Intensity (arb. unit)
RuCa2RCu2O8+
0
La
0
Ce
Pr
0
0
0
Nd
Sm
Eu
Gd
0
0
5
10 15 20 25 30 35 40 45 50 55 60 65 70
2 (degree)
3
Molar susceptibility m (cm /mol)
Field-cooled (FC) susceptibility for RuCa2-xSrxPrCu2O8+
(R = La, Ce, Pr, Nd, Sm, Eu, Gd)
15
Eu
10
100 G FC
Sm
5
La,Ce
0
Gd
-5 Pr
RuCa2RCu2O8+
-10
Nd
-15
0
10
20
30
40
Temperature T (K)
50
60
Zero-field-cooled (ZFC) susceptibility
for RuCa2-xSrxPrCu2O8+(R = La, Ce, Nd, Sm, Eu, Gd)
3
Molar susceptibility m (cm /mol)
12
RuCa2RCu2O8+
9
100 G ZFC
Sm
6
Eu
3
Gd
Nd
0
La, Ce
0
10
20
30
40
Temperature T (K)
50
60
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
Conclusions
• Superconductivity are observed in the
RuCa2RCu2O8+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
RuO6 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 Mv (emu/cm3  G)
Mass magnetization Mg (emu/g = G•cm3/g)
Molar magnetization Mm (emu/mol = G•cm3/mol)
(Mm  Mg • molar formula unit weight)
Magnetic moment per formula unit m (B/f.u.)
(m  Mm/(No • B)
Volume magnetic susceptibility v  Mv/Ba (emu/cm3•G = 1)
Mass magnetic susceptibility g  Mg/Ba (emu/g•G = cm3/g)
Molar magnetic susceptibility m (emu/mol•G = cm3/mol)
(m  g • molar formula unit weight)
[good for SI system with g  Mg/0Ba]
3
Molar susceptibility m (cm /mol)
Magnetic behavior m(T) for bulk and powder samples
Diamagnetic signal, bulk superconductivity
4
RuCa2PrCu2O8+
2
Tc (onset)?
Tm (onset)?
0
ZFC(P)
-2
-4
ZFC(B)
FC(P)
1. Annealed in O2 12 h (D day)
2. Ba = 100 G at D+2 day
3. B (bulk), P (powder)
4. FC (field-cooled)
ZFC (zero-field-cooled)
-6
FC(B)
-8
-10
0
10
20
30
40
Temperature T (K)
50
60