The effect of electronic doping on the magnetic and superconducting properties of FeSr 2 YCu 2 O 6+x and FeSr 2 Y 2-y Ce y Cu 2 O 8+x Grant Williams, Jibu Stephen, MacDiarmid Institute, School of Chemical and Physical Sciences, Victoria University, Wellington, New Zealand Huyen Nguyen, and Narayanaswamy Suresh Industrial Research Limited, Lower Hutt, New Zealand
La 2-x Sr x CuO 4 example 2p takes electrons out of the O-2p orbitals , dopes holes. La 3+ , Sr 2+ Cu 2+ (planar) O 2- (planar) O 2- (apical) Sr 2+ doping on La 3+ site leads to hole doping and T c,max =40K Parent compound (x=0): has Cu 3d 9 and O 2s 2 2p 6 . Superconducting Cuprate Background ~O 2- 3d x 2 -y 2 ~Cu 2+ hole in
HTS Phase Diagram Hole or electron doping induces a transition from an antiferromagnetic insulator towards a s uperconducting metal . Electron doped phase diagram based on Nd 2-x Ce x CuO 4 Superconducting Cuprate Background
FeSr 2 Y 2-y Ce y Cu 2 O 8+x Structure Tetragonal and contains fluorite block with structure similar to electron doped R 2-x Ce x CuO 4 . Structure similar to superconducting RuSr 2 Gd 2-x Ce x Cu 2 O 10+x . However, oxygen deficient FeO x layer. Possibly some Fe on Cu(2) sites. No evidence for superconductivity. Fe1222* Ru1222 *M. Pissas et al. Phys. Rev. B 52 , 10610 (1995)
FeSr 2 Y 2-y Ce y Cu 2 O 8+x Structure Oxygen deficient FeO x layer. One study has x=1 for y=0.5 rather than the fully oxygenated FeO 2 . Possibly forms FeO chains similar to those predicted for FeSr 2 YCu 2 O 6+x .* Have 4-fold oxygen coordinated Fe. *T. Mochiku et al. Physica C, 400 , 43 (2003) 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 O O O O O O O O O O O O O O O O O O O O O O
FeSr 2 Y 2-y Ce y Cu 2 O 8+x Magnetic Properties Made using Fe 2 O 3 , Sr(NO 3 ) 2 , Y 2 O 3 , CeO 2 , and CuO 2 . Denitrate at 700ºC 1050ºC to 1060ºC in air. Long O 2 load at 600ºC and slow cool. Plan experiments with 750ºC Ar annealing to try and get more Fe on the FeO x layer. Magnetic transition ~23K. 20 Oe Zero field cooled . FeSr 2 Y 1.5 Ce 0.5 Cu 2 O 8+x
FeSr 2 Y 1.5 Ce 0.5 Cu 2 O 8+x Curie-Weiss fit, where Departure below ~90K. χ 0 in the expected range. Negative θ suggests antiferromagnetic interactions. Effective moment per unit cell of 2.43 μ B No significant change with Ce concentration. FeSr 2 Y 2-y Ce y Cu 2 O 8+x Magnetic Properties 6T Ce concentration P eff /μ B θ (K) χ 0 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
Low P eff not expected. Can estimate P eff using for different Fe spin configurations. Possibly a mixture of Fe 3+ spin configurations. For example 0.1 of Fe 3+ S=5/2 and 0.9 of Fe 3+ S=1/2 gives P eff =2.43. FeSr 2 Y 2-y Ce y Cu 2 O 8+x Magnetic Properties Ion S P eff /μ B Fe 3+ 5/2 5.92 Fe 3+ 1/2 1.73 Fe 4+ 2 4.99
Ac magnetization at 13Hz. Magnetic transition ~24K similar to low field dc. Small but not systematic change with Ce concentration. FeSr 2 Y 2-y Ce y Cu 2 O 8+x Magnetic Properties
FeSr 2 Y 1.1 Ce 0.9 Cu 2 O 8+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 FeSr 2 Y 2-y Ce y Cu 2 O 8+x Magnetic Properties
Can model with the Vogel Fulcher equation, T 0 is a phenomenological parameter. Attributed to inter-cluster interactions in a cluster-glass model. All samples show similar Ln( 1/f) vs T f behaviour. For x=0.9Ce could fit to f 0 =10 11 Hz, E a /k B =149K, T 0 /k B =16K. f 0 is in the range expected for a spin-glass. However E a /k B is slightly higher than normally observed. -Looks like a spin-glass. FeSr 2 Y 1.1 Ce 0.9 Cu 2 O 8+x FeSr 2 Y 2-y Ce y Cu 2 O 8+x Magnetic Properties
FeSr 2 Y 2-y Ce y Cu 2 O 8+x Magnetic Properties Origin of the spin-glass behaviour? Possibly additional oxygen in the FeO x 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 O O O O O O O O O O O O O O O O O O O O O O O O O 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 O O O O O O O O O O O O O O O O O O O O O O
x=0.5 Large increase in the resistance as the temperature is reduced. 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 CuO 2 doping. May be from Fe on the Cu(2) site and/or oxygen disorder that leads to site disorder. FeSr 2 Y 2-y Ce y Cu 2 O 8+x Resistance
x=0.5 Large 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. Sr 2 FeMoO 6 ). Unlikely in this case. Only appears below the spin-glass temperature. FeSr 2 Y 2-y Ce y Cu 2 O 8+x Magnetoresistance
Large thermopower at high temperatures. Low temperature thermopower can be approximated by S α T 1/2 . Possibly due to variable range hopping thermopower? FeSr 2 Y 2-y Ce y Cu 2 O 8+x Thermopower
FeSr 2 YCu 2 O 6+x Structure Fe1212: has oxygen deficient O(1). One study has FeO 1.75 . Some Fe on Cu(2) site. Superconducting T c ~60K. Structurally similar to superconducting RuSr 2 GdCu 2 O 8 with T c ~45K and magnetic transition ~132K. Still see in RuSr 2 EuCu 2 O 8 with Sn where T c ~32K and T m ~40K. Fe1212* Ru1212 *H. Fuji et al. Physica C, 415 , 85 (2004)
Samples made from oxides and nitrates. Denitrate at 700 ºC. 1000ºC in air, 750ºC in N 2 , 600ºC to 350ºC in O 2 at 1 bar. As made not superconducting. Need N 2 process to get Fe from Cu(2) site to Cu(1) site. Superconducting with T c =64K. Suggests that not much Fe on Cu(2) site and in superconducting CuO 2 plane. Fe in CuO 2 plane would lead to pair breaking and a reduction in T c . This is ~11K/%Fe to ~18K/%Fe in CuO 2 planes. If T cMax ~90 K similar to YBa 2 Cu 3 O 7-x then have <~4.6% Fe in the CuO 2 planes. FeSr 2 YCu 2 O 6+x Superconductivity
Can electron dope by La and hole dope by Ca. T c forms a curve with similarities to HTS phase diagram. Probably also effect of more Fe on Cu(2) site. FeSr 2 YCu 2 O 6+x Doping and Superconductivity
CuO 2 plane doping state? See S(300K) from Fe1212 decrease with increased hole doping. Suggests that hole doping onto CuO 2 plane is occurring. Doped holes per Cu from ~0.10 (underdoped) to ~0.16 (optimally doped). Might expect 0.2La and 0.2Ca sample to be superconducting. T c possibly also partly suppressed by Fe pair breaking. FeSr 2 YCu 2 O 6+x Doping and Superconductivity *S. D. Obertelli et al. Phys. Rev. B 46 , 14928 (1992)
See Curie-Weiss temperature dependence. where 6 T FeSr 2 YCu 2 O 6+x Doping and Superconductivity
FeSr 2 YCu 2 O 6+x Doping and Effective Moment Find that the effective moment per unit cell changes when doping La or Ca. Higher than found in Fe1222 of ~2.5 μ B Also lower than expected for Fe 3+ and Fe 4+ high spin configurations. Perhaps -54% Fe 4+ S=2 + 46% Fe 3+ S=1/2? Perhaps Fe 4+ fraction decreasing when going from La 3+ to Ca 2+ doping and reduction in oxygen content? Ion S P eff /μ B Fe 3+ 5/2 5.92 Fe 3+ 1/2 1.73 Fe 4+ 2 4.99
FeSr 2 YCu 2 O6 +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 FeO x layer, or a change in the Fe 3+ and Fe 4+ ratio, or a change in the lattice parameters.
FeSr 2 YCu 2 O 6+x Doping and Static Susceptibility χ 0 is in the range expected for cuprates for 0.2La and 0.2Ca where Sum is ~4x10 -5 for YBa 2 Cu 3 O 7 . Large enhancement for superconducting samples. Seen in nearly ferromagnetic metals where strong electron-electron interactions lead to an enhancement of χ s (Stoner enhancement), where U is a measure of the onsite interaction strength. Seen in Ni 3 Ga, Pd and believed to occur in Ru1212 from the RuO 2 layer. Could also be an electron effective mass effect.
FeSr 1.8 La 0.2 YCu 2 O 6+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 due to superconductivity in a fraction of the sample? More magnetization measurements are required at low temperature. 0.2La 20 Oe
Summary FeSr 2 Y 2-y Ce y Cu 2 O 8+x ● Not superconducting. Need high pressures to increase oxygen content in FeO x layer. ● Negative Curie Weiss temperature suggests antiferromagnetic interactions. ● Low effective moment. Possibly a mixture of high and low Fe 3+ spin configurations? ● Effective moment independent of Ce doping. ● Spin-glass with T f ~24K. ● Probably semiconducting with 3D variable range hopping. ● Large magnetoresistence of up to -13% at 8T and 10K. FeSr 2 YCu 2 O 6+x ● Superconducting, T c ~64K in pure sample. ● Electron and hole doping reduces T c . Possible additional effect of Fe pair breaking in the CuO 2 plane. ● Negative Curie Weiss temperature suggests antiferromagnetic interactions. Changes with electron or hole doping. ● Effective moment too small for high spin Fe 3+ . Mixture of low spin Fe 3+ and Fe 4+ ? ● Large T-independent component when fitting the susceptibility for 0.1La, pure and 0.1Ca. Possibly from FeO x layer near a magnetic transition?
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