Ferromagnetic nanoclusters observed by ac and dc magnetic measurements in ${\mathrm{RuSr}}_2{\mathrm{GdCu}}_2{\mathrm{O}}_8$ samples

Both ac and dc magnetic measurements have been performed on polycrystalline ${\mathrm{RuSr}}_2{\mathrm{GdCu}}_2{\mathrm{O}}_8$ 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 $T=T_{\mathrm{N}}$, which has been already widely observed in such system with different techniques. In addition, nonlinear (third-order) susceptibility shows that another magnetic structure exists at a temperature near but slightly higher than $T_{\mathrm{N}}$. 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 $T_{\mathrm{B}}$ 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 $V=2.5\times {10}^{-19}{\mathrm{cm}}^3$, in accord with previous estimates. The scenario of ${\mathrm{RuSr}}_2{\mathrm{GdCu}}_2{\mathrm{O}}_8$ as a phase separated material, already proposed to explain some peculiarities of its magnetic behavior, is here confirmed.

Figure 1

Real part of the linear susceptibility ${\chi }_0^{\prime }$ as a function of the temperature measured at the magnetic transition $(110\mathrm{K}–150\mathrm{K})$ on the sample $A2$ in the frequency range $7\mathrm{Hz}–2230\mathrm{Hz}$. The amplitude of the ac field is $20\mathrm{Oe}\mathrm{rms}$. In the inset, ${\chi }_0^{\prime }$ is shown for all the samples in the same temperature range at a selected frequency $\left(333\mathrm{Hz}\right)$.

Figure 2

Real part of the nonlinear susceptibility ${\chi }_2^{\prime }$ as a function of the temperature measured on the three samples at the magnetic transition temperature $(110\mathrm{K}–150\mathrm{K})$. The frequency is $\nu =333\mathrm{Hz}$, the amplitude of the ac field is $20\mathrm{Oe}\mathrm{rms}$.

Figure 3

Real part of the nonlinear susceptibility ${\chi }_2^{\prime }$ as a function of the temperature measured on the sample $A2$ in the frequency range $7\mathrm{Hz}–333\mathrm{Hz}$. The amplitude of the ac field is $20\mathrm{Oe}\mathrm{rms}$. In the inset, ${\chi }_2^{\prime }$ measured at different amplitudes of the ac field and at $\nu =333\mathrm{Hz}$ is shown.

Figure 4

ZFC and FC curves vs temperature for the three samples. The applied magnetic field is $H=5.5\mathrm{Oe}$. In the inset, the ZFC curves are presented for each sample together with the real part of the linear signal of susceptibility ${\chi }_0^{\prime }$.

Figure 5

Magnetization decays ($M∕M_{\mathrm{i}}$ vs time) measured on sample $A3$ with the procedure described in the text at six different temperatures.

Figure 6

Magnetization decays ($M∕M_{\mathrm{i}}$ vs time) measured on the three samples with the procedure described in the text at $T=80\mathrm{K}$.