The effect of electronic doping on the magnetic and superconducting properties of

FeSr2YCu2O6+x and FeSr2Y2-yCeyCu2O8+x

Grant Williams, Jibu Stephen, MacDiarmid Institute, School of Chemical and Physical Sciences, Victoria University, Wellington,

New Zealand

Huyen Nguyen, and Narayanaswamy SureshIndustrial Research Limited, Lower Hutt, New Zealand

La2-xSrxCuO4 example

2p

3d x2-y2

Parent material: La2CuO

4

takes electrons out of the O-2p orbitals, dopes holes.

La3+, Sr2+

Cu2+ (planar) O2- (planar) O2- (apical)

Sr2+ doping on La3+ site leads to hole doping and

Tc,max=40K

Parent compound (x=0): has Cu 3d9 and O 2s22p6.

Superconducting Cuprate Background

~Cu2+

hole in 3dx2 y2

~O2-

HTS Phase Diagram

Hole or electron doping induces a transition from an antiferromagnetic insulator towards a superconducting metal.

Electron doped phase diagram basedon Nd2-xCexCuO4

Superconducting Cuprate Background

FeSr2Y2-yCeyCu2O8+x Structure

Tetragonal and contains fluorite block with structure similar to electron doped R2-xCexCuO4.

Structure similar to superconducting RuSr2Gd2-xCexCu2O10+x.

However, oxygen deficient FeOx layer.

Possibly some Fe on Cu(2) sites.

No evidence for superconductivity.

Fe1222* Ru1222*M. Pissas et al. Phys. Rev. B 52, 10610 (1995)

FeSr2Y2-yCeyCu2O8+x Structure

O

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

O

OO

OO

O

OO

O

O

OO

O

O

O

O

OO

OO O

O O

Oxygen deficient FeOx layer.

One study has x=1 for y=0.5 rather than the fully oxygenated FeO2.

Possibly forms FeO chains similar to those predicted for FeSr2YCu2O6+x.*

Have 4-fold oxygen coordinated Fe.

*T. Mochiku et al. Physica C, 400, 43 (2003)

FeSr2Y2-yCeyCu2O8+x Magnetic Properties

Made using Fe2O3, Sr(NO3)2, Y2O3, CeO2, and CuO2.

Denitrate at 700ºC1050ºC to 1060ºC in air.Long O2 load at 600ºC and slow cool.

Plan experiments with 750ºC Ar annealing to try and get more Fe on the FeOx layer.

Magnetic transition ~23K. 0 20 40 60 80 100 1200.0004

0.0006

0.0008

0.0010

0.0012

0.0014

0.0016

0.0018

0.0020

T (K)

20 Oe

Zero field

cooled.

FeSr2Y1.5Ce0.5Cu2O8+x

FeSr2Y1.5Ce0.5Cu2O8+x

Curie-Weiss fit,

where

Departure below ~90K.

χ0 in the expected range.

Negative θ suggests antiferromagnetic interactions.

Effective moment per unit cell of 2.43μB

No significant change with Ce concentration.

0 100 200 300

0.0002

0.0003

0.0004

0.0005

0.0006

0.0007

0.0008

0.0009

0.0010

0.0011

T (K)

corevvs 0

Ce concentration Peff/μB θ (K) χ0

0.3 2.43 -39 6.6x10-5

0.5 2.46 -38 4.4x10-5

0.7 2.57 -37 5.5x10-5

0.9 2.57 -37 5.8x10-5

0

20

)(3

Tk

P

V

N

H

M eff

FeSr2Y2-yCeyCu2O8+x Magnetic Properties

6T

Low Peff not expected.

Can estimate Peff using for different Fe spin configurations.

Possibly a mixture of Fe3+ spin configurations.

For example 0.1 of Fe3+ S=5/2 and 0.9 of Fe3+ S=1/2 gives Peff=2.43.

)1( JJgPeff

Ion S Peff/μB

Fe3+ 5/2 5.92

Fe3+ 1/2 1.73

Fe4+ 2 4.99

FeSr2Y2-yCeyCu2O8+x Magnetic Properties

Ac magnetization at 13Hz.Magnetic transition ~24K similar to low field dc.Small but not systematic change with Ce concentration.

10 20 30 40 50 60 70

0.000005

0.000006

0.000007

0.000008

0.000009

m' (

emu)

T (K)

0.3Ce 0.5Ce 0.7Ce 0.9Ce

10 20 30 40 50 60 700.0000000

0.0000001

0.0000002

0.0000003

m''

(em

u)

T (K)

0.3Ce 0.5Ce 0.7Ce 0.9Ce

FeSr2Y2-yCeyCu2O8+x Magnetic Properties

FeSr2Y1.1Ce0.9Cu2O8+x

Peak shifts with increasing frequency.

May be due to a spin-glass.

Spin-glasses do not have conventional long range order. Occurs when there is a conflict between the interactions between the moment.

Can occur due to frozen in structural disorder or random vacancies.

Can lead to a distribution in the interactions between the moments as well as competing ferromagnetic and antiferromagnetic interactions.

The lower right figures show an example of a spin-glass and ferromagnetic order.

See: http://en.wikipedia.org/wiki/Spin_glass

10 20 30 40 50 60

0.000005

0.000006

0.000007

0.000008

m' (

emu)

T (K)

M'13Hz M'57Hz M'252Hz M'997Hz

FeSr2Y2-yCeyCu2O8+x Magnetic Properties

Can model with the Vogel Fulcher equation,

T0 is a phenomenological parameter. Attributed to inter-cluster interactions in a cluster-glass model.

All samples show similar Ln(1/f) vs Tf behaviour.

For x=0.9Ce could fit to f0=1011Hz, Ea/kB=149K, T0/kB=16K.

f0 is in the range expected for a spin-glass. However Ea/kB is slightly higher than normally observed.

-Looks like a spin-glass.

23.5 24.0 24.5 25.0

-7

-6

-5

-4

-3

-2

Ln

(1/f

)

Tf(K)

23 24 25 26 27 28

-7

-6

-5

-4

-3

-2

Ln(1

/f)

Tf (K)

0.3Ce 0.5Ce 0.9CeFeSr2Y1.1Ce0.9Cu2O8+x

FeSr2Y2-yCeyCu2O8+x Magnetic Properties

))(/exp( 00 TTkEff fBa

FeSr2Y2-yCeyCu2O8+x Magnetic Properties

O

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

O

OO

OO

O

OO

O

O

OO

O

O

O

O

OO

OO O

O O

O

O

O

Origin of the spin-glass behaviour?

Possibly additional oxygen in the FeOx layer with oxygen disorder?

Requires more structural analysis.O

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

Fe

O

OO

OO

O

OO

O

O

OO

O

O

O

O

OO

OO O

O O

x=0.5Large increase in the resistance as the temperature isreduced.

At low temperatures can model in terms of 3D variable range hopping due to electrons hopping in localized states about the Fermi level.

Observed in amorphous semiconductors.

-Fe1222 is probably semiconducting from low CuO2

doping. May be from Fe on the Cu(2) site and/or oxygen disorder that leads to site disorder.

10 100

0.1

1

10

100

1000

R ()

T (K)

0.2 0.3 0.4 0.5 0.6 0.7-4

-2

0

2

4

6

8

10

Ln(R

)

1/T1/4 (K1/4)

)/exp( 4/10 TB

FeSr2Y2-yCeyCu2O8+x Resistance

x=0.5Large magnetoresistence at 10K. 300K magnetoresistence is too small to measure.

Small magnetoresistances of <~-1% might be expected from a reduction in spin-flip scattering.

Large magnetoresistances observed in spin-polarized materials (e.g. Sr2FeMoO6). Unlikely in this case.

Only appears below the spin-glass temperature.

-8 -6 -4 -2 0 2 4 6 8-0.14

-0.12

-0.10

-0.08

-0.06

-0.04

-0.02

0.00

(R(B

)-R

(0)/

R(0

)

B (T)

Decreasing field Increasing field

FeSr2Y2-yCeyCu2O8+x Magnetoresistance

Large thermopower at high temperatures.

Low temperature thermopower can be approximated by S α T1/2.

Possibly due to variable range hopping thermopower?

FeSr2Y2-yCeyCu2O8+x Thermopower

0 50 100 150 200 250 3000

5

10

15

20

25

30

35

40

45

S (V

/K)

T (K)

0.3Ce 0.9Ce 0.7Ce VRH

FeSr2YCu2O6+x Structure

Fe1212: has oxygen deficient O(1). One study has FeO1.75. Some Fe on Cu(2) site.

Superconducting Tc~60K.

Structurally similar tosuperconducting RuSr2GdCu2O8

with Tc~45K and magnetic transition ~132K. Still see in RuSr2EuCu2O8 with Sn where Tc~32K and Tm~40K.

Fe1212* Ru1212

*H. Fuji et al. Physica C, 415, 85 (2004)

Samples made from oxides and nitrates.Denitrate at 700ºC.1000ºC in air, 750ºC in N2, 600ºC to 350ºC in O2 at 1 bar.

As made not superconducting. Need N2 process to get Fe from Cu(2) site to Cu(1) site.

Superconducting with Tc=64K.Suggests that not much Fe on Cu(2) site and in superconducting CuO2 plane.

Fe in CuO2 plane would lead to pair breaking and a reduction in Tc. This is ~11K/%Fe to ~18K/%Fe in CuO2 planes.

If TcMax~90 K similar to YBa2Cu3O7-x then have <~4.6% Fe in the CuO2 planes.

0 50 100 150 200 250 300-0.0010

-0.0008

-0.0006

-0.0004

-0.0002

0.0000

m (

em

u)

T (K)

20 Oe ZFC 20 Oe FC

FeSr2YCu2O6+x Superconductivity

Can electron dope by La and hole dope by Ca.

Tc forms a curve with similarities to HTS phase diagram.

Probably also effect of more Fe on Cu(2) site.

-0.2 -0.1 0.0 0.1 0.2-10

0

10

20

30

40

50

60

70

CaLa

Tc

(K)

Doping

FeSr2YCu2O6+x Doping and Superconductivity

CuO2 plane doping state?

See S(300K) from Fe1212 decrease with increased hole doping.Suggests that hole doping onto CuO2 plane is occurring. Doped holes per Cu from ~0.10 (underdoped) to ~0.16 (optimally doped).

-0.2 -0.1 0.0 0.1 0.2

10

CaLa

S(3

00K

) (

V/K

)

Doping

Might expect 0.2La and 0.2Ca sample to be superconducting.

Tc possibly also partly suppressed by Fe pair breaking.

FeSr2YCu2O6+x Doping and Superconductivity

*S. D. Obertelli et al. Phys. Rev. B 46, 14928 (1992)

See Curie-Weiss temperature dependence.

where

0 50 100 150 200 250 300 3500

50

100

150

200

250

105

Temperature (K)

0

20

)(3

Tk

P

V

N

H

M eff

corevvs 0

6 T

FeSr2YCu2O6+x Doping and Superconductivity

FeSr2YCu2O6+x Doping and Effective Moment

Find that the effective moment per unit cellchanges when doping La or Ca.

Higher than found in Fe1222 of ~2.5μB

Also lower than expected for Fe3+ and Fe4+ high spin configurations.

Perhaps-54% Fe4+ S=2 + 46% Fe3+ S=1/2?

Perhaps Fe4+ fraction decreasing when going from La3+ to Ca2+ doping and reduction in oxygen content? -0.2 -0.1 0.0 0.1 0.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

4.0

4.2

La Ca

Pef

f/B

Doping

Ion S Peff/μB

Fe3+ 5/2 5.92

Fe3+ 1/2 1.73

Fe4+ 2 4.99

FeSr2YCu2O6+x Doping and Curie Weiss Temperature

Curie Weiss temperature is negative. Suggests antiferromagnetic interactions?

Find that the Curie Weiss temperature decreases with doping.

Could be due to doping in FeOx layer, or a change in the Fe3+ and Fe4+ ratio, or a change in the lattice parameters.

-0.2 -0.1 0.0 0.1 0.2-70

-65

-60

-55

-50

-45

-40

CaLa

(K

)Doping

FeSr2YCu2O6+x Doping and Static Susceptibility

χ0 is in the range expected for cuprates for 0.2La and 0.2Ca where

Sum is ~4x10-5 for YBa2Cu3O7.

Large enhancement for superconducting samples.

Seen in nearly ferromagnetic metals where strong electron-electron interactions lead to an enhancement of χs (Stoner enhancement),

-0.2 -0.1 0.0 0.1 0.20

5

10

15

20

CaLa

105

0

Doping

corevvs 0

NUs

s

10

where U is a measure of the onsite interaction strength.

Seen in Ni3Ga, Pd and believed to occur in Ru1212 from the RuO2 layer.

Could also be an electron effective mass effect.

FeSr1.8La0.2YCu2O6+x Magnetic Order?

Is there a magnetic transition?

The 0.2La sample is not superconducting.

There appears to be a magnetic transition at ~12K. Possibly from magnetic order?

There is also a peak at ~43K. Could be dueto superconductivity in a fraction of the sample?

More magnetization measurements are required at low temperature.

0 20 40 60 80 100 120

100

150

200

250

300

Zero Field Cooled

Field Cooled

105

Temperature (K)

0.2La20 Oe

Summary

FeSr2Y2-yCeyCu2O8+x

●Not superconducting. Need high pressures to increase oxygen content in FeOx layer. ●Negative Curie Weiss temperature suggests antiferromagnetic interactions.●Low effective moment. Possibly a mixture of high and low Fe3+ spin configurations?● Effective moment independent of Ce doping.● Spin-glass with Tf~24K.● Probably semiconducting with 3D variable range hopping.● Large magnetoresistence of up to -13% at 8T and 10K. FeSr2YCu2O6+x

●Superconducting, Tc~64K in pure sample.●Electron and hole doping reduces Tc. Possible additional effect of Fe pair breaking in the CuO2 plane. ●Negative Curie Weiss temperature suggests antiferromagnetic interactions. Changes with electron or hole doping.●Effective moment too small for high spin Fe3+. Mixture of low spin Fe3+ and Fe4+?●Large T-independent component when fitting the susceptibility for 0.1La, pure and 0.1Ca. Possibly from FeOx layer near a magnetic transition?