Ferromagnetic correlations and mixed Ru valence in the magnetic superconductorRuSr2(Eu,Gd)Cu

Abstract

We present a characterization through magnetization measurements as a function of field and temperature of the magnetic superconductor RuSr 2 RCu 2 O 8 (RϭEu or Gd͒. The effective magnetic moment was carefully determined from measurements up to 900 K. For both compounds we have obtained almost the same value, e f f ϳ3.15(4) ␤ . This value confirms recent x-ray absorption experiments where a mixed valence state (40% Ru 4ϩ -60% Ru 5ϩ ) was proposed. We have found that the predominant interaction among Ru ions is ferromagnetic ͓⌰ϭ100(3) K and ⌰ϭ87(3) K for the Eu and Gd compounds, respectively͔, but the high-field susceptibility curve displays the characteristics of an antiferromagnetic transition. The possible type of order of the Ru moments is discussed in view of these and previous experiments. We propose a type-I ordering ͑antiferromagnetically coupled ferromagnetic RuO 2 planes͒ and give reasons supporting this choice.

Magneto-Superconductivity of Rutheno-Cuprates

“…This value is lees than for Ru 5+ low spin state ordering. In Gd/Ru-1212 compound, based on various magnetization data the Ru 5+ state is reported with an effective saturation moment of nearly 1µ B [8,11], which ironically differs with more recent magnetic analysis [17]. Ru-1222 Superconductivity is seen in terms of diamagnetic transition at below T d , and T c (R=0) at slightly higher temperature.…”

Effect of Ca substitution on crystal structure and superconducting properties of ferromagnetic superconductor RuSr2−Ca Gd1.4Ce0.6Cu2O10−δ

“…The first one measures the physical properties of a standard sample using different techniques, which should supply complementary information sufficient to support or discard a specific model. Significant results have been obtained in this way, although sometimes there have been contradictions in results between different groups [8][9][10][11][12]. A second approach is the investigation of chemically altered compounds, where by proper chemical substitutions one can control relevant parameters that affect the sample's properties and, in this way, get a better understanding of the mechanisms involved in it [13][14][15][16][17][18][19][20].…”

Double exchange model for RuSr2(Eu,Gd)Cu2O8

“…The first measurements in Gd samples indicated ferromagnetic (FM) ordering of the Ru moments, 1 but neutron diffraction experiments found superlattice reflections consistent with an antiferromagnetic (AF) order with nearest-neighbor spins antiparallel in all three directions. 3 The Ru contribution to the magnetic susceptibility at temperature T > T M can be very well described by χ = C/(T − Θ), with Θ = 100 ± 3 K. 4 The fact that Θ > 0 seems difficult to reconcile with AF order at low temperatures.From previous intensive research on the similar system YBa 2 Cu 3 O 6+x , 5 it seems clear that each of the superconducting CuO 2 planes has doping slightly less than 0.1 holes per Cu ion in order to lead to the observed superconducting critical temperature T S . Since Eu or Gd are expected to be in the oxidation state +3, assuming as usual that the inactive SrO layers are neutral, charge balance imposes that the RuO 2 planes have an electron doping n 0.2 per Ru ion.…”

“…The first measurements in Gd samples indicated ferromagnetic (FM) ordering of the Ru moments, 1 but neutron diffraction experiments found superlattice reflections consistent with an antiferromagnetic (AF) order with nearest-neighbor spins antiparallel in all three directions. 3 The Ru contribution to the magnetic susceptibility at temperature T > T M can be very well described by χ = C/(T − Θ), with Θ = 100 ± 3 K. 4 The fact that Θ > 0 seems difficult to reconcile with AF order at low temperatures.…”

“…In spite of the difficulties with the sluggish statistics of the ferromagnetic polarons mentioned above, one can see an abrupt increase at low fields, due to the alignment of these polarons, followed by a linear increase with a smaller slope, in agreement with experiment. 4 .…”