Magnetism, Charge Order, and Giant Magnetoresistance in ${\mathrm{S}\mathrm{r}\mathrm{F}\mathrm{e}\mathrm{O}}_{3\mathrm{\text{−}}\delta }$ Single Crystals

The electronic and magnetic properties of ${\mathrm{S}\mathrm{r}\mathrm{F}\mathrm{e}\mathrm{O}}_{3\mathrm{\text{−}}\delta }$ single crystals with controlled oxygen content ($0\le \delta \le 0.19$) have been studied systematically by susceptibility, transport, and spectroscopic techniques. An intimate correlation between the spin-charge ordering and the electronic transport behavior is found. Giant negative as well as positive magnetoresistance are observed.

Figure 1

Susceptibility of $\mathrm{S}\mathrm{r}\mathrm{F}\mathrm{e}{\mathrm{O}}_{\mathrm{3.00}}$, $\mathrm{S}\mathrm{r}\mathrm{F}\mathrm{e}{\mathrm{O}}_{\mathrm{2.85}}$, and $\mathrm{S}\mathrm{r}\mathrm{F}\mathrm{e}{\mathrm{O}}_{\mathrm{2.81}}$ single crystals, measured in field cooling and subsequent field heating runs at $B=1\mathrm{T}$. The curves for the latter two samples were shifted in $\chi$ by the amounts indicated in the legend.

Figure 2

Resistivity in zero field cooling and heating runs (black) compared to field cooling and heating runs with a 9 Tesla field (red) for (a) $\mathrm{S}\mathrm{r}\mathrm{F}\mathrm{e}{\mathrm{O}}_{\mathrm{3.00}}$, (b) $\mathrm{S}\mathrm{r}\mathrm{F}\mathrm{e}{\mathrm{O}}_{\mathrm{2.85}}$, and (c) $\mathrm{S}\mathrm{r}\mathrm{F}\mathrm{e}{\mathrm{O}}_{\mathrm{2.81}}$. As the absolute value of the resistance measurements (lines) was influenced by microcracks, these data were normalized around room temperature to IR data extrapolated to zero frequency (open circles; see Fig. 4 for IR data on $\mathrm{S}\mathrm{r}\mathrm{F}\mathrm{e}{\mathrm{O}}_{\mathrm{2.85}}$). Insets: Magnetoresistance (MR) measured at 9 Tesla after isothermal field scans between 0 and 9 T. MR is defined as $[\rho (H)-\rho (0)]/\rho (0)$.

Figure 3

Selected Mössbauer spectra of (a) cubic $\mathrm{S}\mathrm{r}\mathrm{F}\mathrm{e}{\mathrm{O}}_{\mathrm{3}}$ and (b) mostly tetragonal $\mathrm{S}\mathrm{r}\mathrm{F}\mathrm{e}{\mathrm{O}}_{\mathrm{2.87}\mathrm{.}}$ (c) Decomposition of the 30 K spectrum of $\mathrm{S}\mathrm{r}\mathrm{F}\mathrm{e}{\mathrm{O}}_{\mathrm{2.87}}$ into subspectra. (d) Temperature dependence of isomer shifts IS (top), hyperfine fields $B_{hf}$ (middle), and area fractions (bottom) for the various iron sites of $\mathrm{S}\mathrm{r}\mathrm{F}\mathrm{e}{\mathrm{O}}_{\mathrm{2.87}}$. Full and open symbols correspond to the magnetically ordered and paramagnetic phases, respectively. In the area fraction plot the full circles correspond to the sum of sites $A$ and $B$.

Figure 4

Real part of the IR conductivity ${\sigma }_1(\omega )$ of $\mathrm{S}\mathrm{r}\mathrm{F}\mathrm{e}{\mathrm{O}}_{\mathrm{2.85}}$ measured at different temperatures. The bottom line shows the real part of the dielectric function ${\epsilon }_1(\omega )$ at $T=10\mathrm{K}$.