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
RuCa2RCu2O8+ (R = Pr-Gd) System
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-typeP4/mmm
CuO5
RuO6
Gd
Sr
RuSr2GdCu2O8+
Tm = 133 K, Tc = 16-48 K
Bernhard et al., PRB 59, 14009 (1999)
R(T) v(T)(ZFC)
S(T)Mm(T)(FC)
Normal or abnormal ferromagnetic magnetic hysteresis curve M(Ba) for T < Tm = 135 K?
-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0-12
-9
-6
-3
0
3
6
9
12
0.83 /f.u.
Tc < 100 K < T
C
10 K < Tc?
Mag
netiz
atio
n M
m (
103 e
mu/
mol
)
Applied field Ba(T)
RuSr2GdCu
2O
8+
Current oxygen-deficient samplesuperconducting Tc ~ 10 Ksuperconducting signal?Bernhard et al.,
PRB 59, 14009 (1999)
ZFC/FC susceptibilitySuperconducting diamagnetic signal?
0 50 100 150 200-2
0
2
4
6
8
10
12T
c
Tm = 135 K
1-G ZFC
100-G ZFC
100-G FC
1-G FC
RuSr2GdCu
2O
8+oxygen-deficient
Sus
cept
ibili
ty m
(cm
3 /mol
)Temperature T (K)
Current oxygen-deficient samplesuperconducting Tc ~ 10 Ksuperconducting signal?Cordero et al., PRB 67, 144519 (2003)
Superconducting diamagnetic signal appears only invery low applied fields due to strong ferromagnetic background
Mg(T)(FC)
Papageorgiou & Braun, PRB 66, 104509 (2002)
1.14 1.12 1.10 1.08 1.06 1.04 1.020
10
20
30
40
50
60
70
80Pr Nd Sm Eu GdGdGdEu GdGdGdGdSm Eu GdEuEuEu Tb Dy Y
no Tc(01)
65 K (03)
no Tc(01)
33 K (99)
48 K (99)
12 K (01)
Tm~135 K
Ru-1212 (P4/mmm)
RuSr2RCu
2O
8+
S
uperc
on
duct
ing
T c (K
)
rare earth ionic radius R3+ (Å)
Tc only in R = Sm, Eu, Gd of RuSr2RCu2O8+
With oxygen effect - hole concentration
• What about the RuCa2RCu2O8+ system (R = La, Ce, Pr, Nd, Sm, Eu, Gd)?
First try R = Pr : RuCa2PrCu2O8+
Question
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
5 10 15 20 25 30 35 40 45 50 55 60 65 700
500
1000
1500
2000
Orthorhombic (Pmmm?)a = 5.874 Åb = 5.785 Åc = 22.460 Å
(1014)
(1111)
(225)
(216)(1
18)
(115)
(004)
(103)
(410)
(320)
(1013)
(314)
(223)
(220)
(117)
(107)(
008)
RuCa2PrCu
2O
8+
Inte
nsi
ty (
arb
. u
nit)
2 (degree)
Distorted orthorhombic variation of tetragonal 1212
structure: ao ~ bo ~ 2at co ~ 2ct
0 10 20 30 40 50 60-10
-8
-6
-4
-2
0
2
4T
c
Tm
1. Annealed in O2 12 h (D day)
2. Bulk sample, 100 G3. Measured at D+2 and D+10 days
ZFC (D+10)
FC (D+10)
FC (D+2)
ZFC (D+2)
RuCa2PrCu
2O
8+
S
usc
ep
tiblit
y m
(cm
3 /mo
l)
Temperature T(K)
Abnormal low temperature magnetic behavior at different measuring times oxygen diffusion effect
Normal ZFC/FC susceptibility and magnetic hysteresis for standard high-Tc superconductors without magnetic order
0 10 20 30 40 50 60-1.6
-1.2
-0.8
-0.4
0.0
0.4
Tc = 37 K
ZFC
FC
Ba = 100 G
La1.85
Sr0.15
CuO4
Sus
cept
ibili
ty m
(cm
3 /mol
)
Temperature T (K)-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
-12
-9
-6
-3
0
3
6
9
12
T = 10 K < Tc
La1.85
Sr0.15
CuO4+
Magniti
zatio
n
Mm (
102 e
mu/m
ol)
Applied field Ba (T)
Magnetization Mm(T) in different applied fieldsAnomalous FC/ZFC behavior
0 10 20 30 40 50 60-8
-6
-4
-2
0
2
4
1-G/FC
Tc
Tm
1,10 &100-G/ZFC
1-kG/ZFC
1-kG/FC
10-G/FC
100-G/FC
RuCa2PrCu
2O
8+
Ma
gn
etiza
tio
n M m
(102 e
mu
/mo
l)
Temperature T (K)
Normal paramagnetic behavior for T > Tm &Mgnetic order (weak-ferromagnetic or spin-
glass?) for Tm > T > Tc
-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0-10
-8
-6
-4
-2
0
2
4
6
8
100.16
B/f.u.
40 K ~ Tc
(WFM + S?) T = 60 K > T
m
(paramagnetic)
45 K < Tm (weak ferromagnetic?)
RuCa2PrCu
2O
8+
M
ag
nitiz
atio
n
M m (
102 e
mu
/mo
l)
Applied field Ba (T)
Weak magnetization relaxation at T= 45 K < Tm
(itinerant weak ferromagnetism almost spin-glass-like)
0
50
100
150
200ZFC to 45 K
T = 45 K < Tm ~ 50 K
104103102
"0 G"45 KWFM
1 kG, 100 K
1 kG (FC), 45 K
1 kG (ZFC), 45 K
RuCa2PrCu
2O
8+
M
agne
tizat
ion
Mm (
emu/
mol
)
Time t (s)
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)
EuMnO orthorhombic Pbnm3
R
Ru
O1
O2
with larger distortion
EuMnO orthorhombic Pbnm3
R
Ru
O1
O2
with larger distortion
Orthorhombic (distorted)
Orthorhombic(pseudo-cubic)
a ~ b ~ c/2
Lattice distortion of CaRuO3 compared with SrRuO3
Compare the magnetic phase diagram of Ca1-
xSrxRuO3 system with RuA2RCu2O8 with the same RuO6 configuration
Ru4/5+ 4d4 /d3 in RuO6
- 4dn in t2g bands due to strong RuO6
octahedral crystal field splitting cf 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)
eg
t2g
d
Coulomb repulsion Udd
Charge transfer
On-site Coulomb repulsion energyUdd = E(dn-1) + E(dn+1) - 2E(dn)
Charge transfer energy = E(dn+1L) – E(dn)
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
-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0-10
-8
-6
-4
-2
0
2
4
6
8
10
initial magnetization curve
35 K
T < Tc(~40 K)
< Tm(~50 K) (ZFC)
30 K
10 K
RuCa2PrCu
2O
8+
Ma
gn
itiz
atio
n
M m (
102 e
mu
/mo
l)
Applied field Ba (T)
FC and ZFC magnetization at 10 K show complex interplay between superconductivity and itinerant
ferromagnetic order
-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0-14
-12
-10
-8
-6
-4
-2
0
2
4
6
8
10
(1)
(1)
100-G FC to 10 K
ZFC to 10 K
RuCa2PrCu
2O
8+
M
ag
niti
zatio
n
M m (
102 e
mu
/mo
l)
Applied field Ba (T)
Low field (1 G) time relaxation M(t) at 10 K < Tc < Tm
-250
-200
-150
-100
-50
0
50ZFC to 10 K
T = 10 K < Tc < T
m
104103102
"0 G"10 K
1 G, 100 K > Tm
1 G (FC), 10 K
1 G (ZFC), 10 K
RuCa2PrCu
2O
8+
M
ag
ne
tiza
tion
M m
(e
mu
/mo
l)
Time t (s)
1-G and 1-kG time relaxation M(t) at 10 K < Tc
< Tm
-500
-400
-300
-200
-100
0
100
1 G (FC)10 K
"0 G" 10 K
"0 G" 10 K
1 G, 100 K
1 G (ZFC), 10 K
104103102
1 kG, 100 K
1 kG (FC)10 K
1 kG (ZFC), 10 K
ZFC to 10 K
RuCa2PrCu
2O
8+
Ma
gn
etiz
atio
n
M m (
em
u/m
ol)
Time t (s)
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 oftetragonal RuSr2PrCu2O8+
5 10 15 20 25 30 35 40 45 50 55 60 65 700
1000
2000
3000
4000
5000
Tetragonal (P4/mmm)a = 3.910 Åc = 11.726 Å
x
x
(110
)(1
03
)(1
02)
(101)
(00
2)
(22
0)(
20
6)
(116
)(213
)
(006)(
20
0)
Inte
nsity
(arb
. u
nit)
2 (degree)
PrSr2RuCu
2O
8+
5 10 15 20 25 30 35 40 45 50 55 60 65 70
Inte
nsi
ty (
arb
. u
nit)
2 (degree)
1.0
2.0
0.8
0.6
0.2
0
0
0
0
0
0
Ru Ca2-x
Srx PrCu
2O
8+
x = 0
Powder x-ray diffraction patterns for RuCa2-xSrxPrCu2O8+
Low temperature spin-glass-like behavior for tetragonal RuSr2PrCu2O8+
Ref: Awana et al. Physica C357-360, 121 (2001).
0 10 20 30 40 50 603
4
5
6
7
8
RuSr2PrCu
2O
8+
Tmerge
(1 T)
exchange-enhancedparamagnetism
spin glass
FC
ZFC
1-T1-kG100-G
S
usc
ep
tibili
ty m
(1
0-2cm
3 /mo
l)
Temperature T (K)
Low temperature spin-glass-like behavior for tetragonal RuSr2PrCu2O8+
-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
-6
-4
-2
0
2
4
60.12 /f.u.
T = 10 Kspin glass?
T = 20 Kparamagnetic
RuSr2PrCu
2O
8+
M
ag
niti
zatio
n
M m (
102 e
mu
/mo
l)
Applied field Ba (T)
0
10
20
30
40
50
60
70
T = 10 K < Tmerge
~ 17 Kspin glass
"0 G"10 Ksmall
1 kG, 100 K
1 kG (FC), 10 K
1 kG (ZFC), 10 K
ZFC to 10 K
104103102
RuSr2PrCu
2O
8+
M
ag
ne
tiza
tion
M m
(e
mu
/mo
l)
Time t (s)
Weak magnetization relaxation at T= 10 K < Tm
spin-glass-like for tetragonal RuSr2PrCu2O8+
Field-cooled (FC) susceptibility for RuCa2-xSrxPrCu2O8+
0 10 20 30 40 50 60-8
-6
-4
-2
0
2
4
x = 0.0
100 G FC
RuCa2-x
SrxPrCu
2O
8+
x = 0.1 x = 0.2 x = 0.6 x = 1.0
Su
sce
ptib
ility
m (
cm3 /m
ol)
Temperature T (K)
x = 2.0
0 10 20 30 40 50 60-8
-6
-4
-2
0
2
4
x = 0.1 ZFC
Ba = 100 G
x = 0 ZFC
x = 0.1 FC
x = 0 FCRuCa
2-xSr
xPrCu
2O
8+
S
usc
ep
tibili
ty m
(cm
3 /mo
l)
Temperature T (K)
Magnetic behavior for RuCa2-xSrxPrCu2O8+(x = 0, 0.1)
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)
5 10 15 20 25 30 35 40 45 50 55 60 65 70
(124
) (320
)
(030
)(2
21)
(118
)
(017
)(1
17)
(115
)
(103
)(0
04)
R = Pr
R = Gd0
0
RuCa2RCu
2O
8+
Inte
nsity
(ar
b. u
nit)
2 (degree)
Powder x-ray diffraction patterns for RuCa2RCu2O8+ (R = Pr, Gd)
0 10 20 30 40 50 60-8
-6
-4
-2
0
2
4
Pr ZFC
Pr FC
Gd FC
Gd ZFC
Ba = 100 G
RuCa2RCu
2O
8+
M
ola
r su
sce
ptib
ility
m (
cm3 /m
ol)
Temperature T (K)
Magnetic behavior for RuCa2RCu2O8+(R = Pr, Gd)
5 10 15 20 25 30 35 40 45 50 55 60 65 70
0
0
0
0
0
0
0
Ce
Gd
La
EuSmNdPr
RuCa2RCu
2O
8+
Inte
nsi
ty (
arb
. u
nit)
2 (degree)
Powder x-ray diffraction patterns for RuCa2RCu2O8+
0 10 20 30 40 50 60-15
-10
-5
0
5
10
15
La,Ce
Nd
Pr
Gd
Sm
Eu 100 G FC
RuCa2RCu
2O
8+
M
olar
sus
cept
ibili
ty
m (
cm3 /m
ol)
Temperature T (K)
Field-cooled (FC) susceptibility for RuCa2-xSrxPrCu2O8+
(R = La, Ce, Pr, Nd, Sm, Eu, Gd)
0 10 20 30 40 50 60
0
3
6
9
12
Nd
La, Ce
Eu
Gd
Sm
RuCa2RCu
2O
8+
100 G ZFC
M
ola
r su
sce
ptib
ility
m (
cm3 /m
ol)
Temperature T (K)
Zero-field-cooled (ZFC) susceptibility for RuCa2-xSrxPrCu2O8+(R = La, Ce, Nd, Sm, Eu,
Gd)
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]
0 10 20 30 40 50 60-10
-8
-6
-4
-2
0
2
4
Tc (onset)?
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)
Tm (onset)?
FC(P)
ZFC(P)
FC(B)
ZFC(B)
RuCa2PrCu
2O
8+
M
olar
sus
cept
ibili
ty
m (
cm3 /m
ol)
Temperature T (K)
Magnetic behavior m(T) for bulk and powder samples
Diamagnetic signal, bulk superconductivity