Ferromagnetic nanoclusters observed by ac and dc magnetic measurements inRuSr2GdCu2O8samples

Abstract

Both ac and dc magnetic measurements have been performed on polycrystalline RuSr 2 GdCu 2 O 8 samples, aimed at studying the magnetic ordering of such a phase. Linear ac susceptibility measurements give clear evidence of the antiferromagnetic transition at a temperature T = T N , which has been already widely observed in such system with different techniques. In addition, nonlinear ͑third-order͒ susceptibility shows that another magnetic structure exists at a temperature near but slightly higher than T N . Shape and sign of this small signal are compatible with the presence of ferromagnetic nanoparticles exhibiting superparamagnetic behavior. Such possibility has been confirmed by magnetization decay measurements performed at temperatures lower than the blocking temperature T B of the superparamagnetic system. A fit of magnetization decay measurements with a model describing the slow dynamic of magnetic nanoparticle systems gives the size distribution of the nanoparticles and a mean volume of the order of V = 2.5ϫ 10 −19 cm 3 , in accord with previous estimates. The scenario of RuSr 2 GdCu 2 O 8 as a phase separated material, already proposed to explain some peculiarities of its magnetic behavior, is here confirmed.

Nonlinear effects in the ac magnetic susceptibility of selected magnetic materials

“…The really interesting point is that no clear evidence of such distribution has been observed in the linear susceptibility measurements. Accurate low field measurements, performed on the as cast sample [9], gave evidence of a structure with two sharp negative peaks instead of a single one thus confirming the hypothesis of the distribution of transition temperatures. As described earlier χ 3 is (moreover), a powerful tool in the accurate determination of T c .…”

“…The measurement of the third order component of the ac susceptibility has been recently performed also on polycrystalline RuSr 2 GdCu 2 O 8 samples [9]. The rutheno-cuprates materials synthesized for the first time around 1995 [10] and belonging to the class of the high T c materials, have attracted great inter- est because of the supposed coexistence of superconductivity and ferromagnetism.…”

A.C. susceptibility as a probe of low-frequency magnetic dynamics

“…Equation 37reduces to equation (35) when β = 1 and to equation (36) when α = 0. We note that though these different expressions for g(τ ) reproduce the various forms of χ(ω), none of them has an obvious physical basis.…”

Spin-glass instability of a short-range random spherical ferromagnet

“…9͒ and in the polycrystalline samples of RuSr 2 GdCu 2 O 8 . 10 In the toroidal polycrystalline samples of La 0.66 Ba 0.34 MnO 3 ͑Ref. 11͒ with demagnetization factor Ϸ 0 the plateau in ͑T͒ same as in Fig.…”