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