Magnetoresistance effects in $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{3-\delta }$: Dependence on phase composition and relation to magnetic and charge order

Single crystals of iron(IV) rich oxides $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{3-\delta }$ with controlled oxygen content $(0⩽\delta ⩽0.31)$ have been studied by Mössbauer spectroscopy, magnetometry, magnetotransport measurements, Raman spectroscopy, and infrared ellipsometry in order to relate the large magnetoresistance (MR) effects in this system to phase composition and magnetic and charge order. It is shown that three different types of MR effects occur. In cubic $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_3$ $(\delta =0)$ a large negative MR of 25% at $9\mathrm{T}$ is associated with a hitherto unknown $60\mathrm{K}$ magnetic transition and a subsequent drop in resistivity. The $60\mathrm{K}$ transition appears in addition to the onset of helical ordering at $\sim 130\mathrm{K}$. In crystals with vacancy-ordered tetragonal $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{3-\delta }$ as the majority phase $(\delta \sim 0.15)$ a coincident charge-antiferromagnetic ordering transition near $70\mathrm{K}$ is considered as the origin of a negative giant MR effect of 90% at $9\mathrm{T}$. A positive MR effect is observed in tetragonal and orthorhombic materials with increased oxygen deficiency $(\delta =0.19,0.23)$ which are insulating at low temperatures. Phase mixtures can result in a complex superposition of these different MR phenomena. The MR effects in $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{3-\delta }$ differ from those in manganites as no ferromagnetic states are involved.

Figure 1

(Color online) Illustration of the crystal structures of ${\mathrm{Sr}}_8{\mathrm{Fe}}_8{\mathrm{O}}_{23}$ (top) and ${\mathrm{Sr}}_4{\mathrm{Fe}}_4{\mathrm{O}}_{11}$ (bottom).

Figure 2

(Color online) Room temperature Mössbauer spectra of ground $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{3-\delta }$ crystals. Subspectra: blue—${\mathrm{Fe}}^{4+}$ (single line), red—${\mathrm{Fe}}^{3.5+}$ (doublet with smaller splitting), green—${\mathrm{Fe}}^{3+}$ (doublet with larger splitting).

Figure 3

(Color online) Selected Mössbauer spectra of $5\mathrm{kb}$ oxygen-annealed $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{3.00}$. The data were evaluated with a Voigt based fitting procedure as described in the text. The subspectra of the paramagnetic ${\mathrm{Fe}}^{4+}$ (single line) and ${\mathrm{Fe}}^{3.5+}$ (doublet) sites are drawn.

Figure 4

(Color online) Magnetic hyperfine field distribution $P\left(B_{\mathit{hf}}\right)$ at $90\mathrm{K}$, $110\mathrm{K}$, and $125\mathrm{K}$ obtained from the evaluation of the magnetic hyperfine patterns of $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{3.00}$ by a Voigt based fitting algorithm.

Figure 5

(Color online) Temperature dependence of the area fraction of the magnetically ordered ${\mathrm{Fe}}^{4+}$ sites in $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_3$ (arrow up, blue: heating mode data; arrow down, red: cooling mode data).

Figure 6

(Color online) Temperature dependence of the mean values $⟨B_{\mathit{hf}}⟩$ of the two hyperfine field components in the $B_{\mathit{hf}}$ distribution for $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{3.00}$. The solid line corresponds to a fit of the relation $⟨B_{\mathit{hf}}⟩\left(T\right)∕⟨B_{\mathit{hf}}⟩\left(0\right)={(1-T∕T_N)}^{\beta }$ to the data of the main component.

Figure 7

Mössbauer spectra of $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{2.87}$ at selected temperatures. Solid lines correspond to the best fit to the data. See text for details. The arrow in the $50\mathrm{K}$ spectrum emphasizes the splitting of the ${\mathrm{Fe}}^{3+}$ signal into two components.

Figure 8

(Color online) Decomposition of the $50\mathrm{K}$ Mössbauer spectrum of $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{2.87}$ into subspectra. See text for details. The high velocity component of each sextet is labeled.

Figure 9

(Color online) Temperature dependence of isomer shifts (IS) (top), hyperfine fields $B_{\mathit{hf}}$ (middle), and area fractions (bottom) for the various iron sites of $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{2.87}$. Full symbols correspond to magnetically ordered sites $A\text{–}F$, open symbols to the two paramagnetic components. In the area fraction plot, the full circles correspond to the sum of the ${\mathrm{Fe}}^{3+}$ sites $A$ and $B$.

Figure 10

Mössbauer spectra of $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{2.95}$ at selected temperatures. The solid line corresponds to a best fit assuming a superposition of the spectra of the cubic and tetragonal phase.

Figure 11

(Color online) Mössbauer spectra of $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{2.69}$ at selected temperatures. The subspectra arising from the majority $O$ phase are also drawn (blue: ${\mathrm{Fe}}^{4+}$, red: ${\mathrm{Fe}}^{3+}$ signal). The other lines correspond to the hyperfine sextets of the minority ${\mathrm{Sr}}_2{\mathrm{Fe}}_2{\mathrm{O}}_5$ component. The ${\mathrm{Fe}}^{3+}$ component of the $O$ phase which is paramagnetic at $250\mathrm{K}$ and magnetically ordered at 4 and $200\mathrm{K}$ is indicated by an arrow.

Figure 12

(Color online) Mössbauer spectra of $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{2.81}$ at selected temperatures. The presence of the $O$ phase is most clearly apparent from the ${\mathrm{Fe}}^{3+}$ sextet with large quadrupole interaction (red subspectrum) and the magnetically disordered ${\mathrm{Fe}}^{4+}$ (blue subspectrum) sites in the $12\mathrm{K}$ spectrum.

Figure 13

Magnetic susceptibility of $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{3.00}$, $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{2.95}$, $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{2.85}$, $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{2.81}$, and $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{2.77}$ single crystals, measured in field cooling and subsequent field heating runs at $B=1\mathrm{T}$. The curves for the latter four samples were shifted by the amounts indicated in the legend.

Figure 14

(Color online) Resistivity in zero-field cooling and heating runs (black) compared to field cooling and heating runs with a $9\mathrm{T}$ field (red) for (a) $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{3.00}$, (b) $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{2.95}$, (c) $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{2.85}$, (d) $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{2.81}$, and (e) $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{2.77}$. 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 (see Ref. 29).

Figure 15

Temperature dependence of the magnetoresistance MR at $9\mathrm{T}$ for the five compositions. MR values were calculated according to $\mathrm{MR}\left(9\mathrm{T}\right)=[\rho \left(9\mathrm{T}\right)-\rho \left(0\right)]∕\rho \left(0\right)$ from the isothermal field scans at the temperatures indicated by dots.

Figure 16

(Color online) Field dependence of the magnetoresistance $\mathrm{MR}\left(B\right)=[\rho \left(B\right)-\rho \left(0\right)]∕\rho \left(0\right)$ for the five compositions as derived from isothermal field scans at the temperatures where the respective MR effects are most pronounced: (a) large negative MR for $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{3.00}$ at $T=58\mathrm{K}$ (in black) and for $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{2.95}$ at $T=65\mathrm{K}$ (in red), (b) negative GMR effect for $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{2.85}$ at $T=68\mathrm{K}$, and (c) positive MR effect for $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{2.81}$ (black) and $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{2.77}$ (red), both at $T=10\mathrm{K}$.

Figure 17

(Color online) Derivatives $d\ln \chi ∕dT$ (blue) and $d\ln \rho ∕dT$ (black) of the magnetic susceptibilities and resistivities of $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{3.00}$.

Figure 18

(Color online) Ellipsometric far-infrared (a) and Raman spectra (b) of cubic $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{3.00}$ at selected temperatures. In order to facilitate comparison, offsets have been added to the Raman spectra.

Figure 19

(Color online) Temperature dependence of the Raman spectra (a) polarized Raman spectra of $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{2.85}$ in $ZZ$ and (b) unpolarized spectra of $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{2.69}$. The incident light is the green line of an Ar laser at $\lambda =514.5\mathrm{nm}$. The $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{2.85}$ sample was cut along the ⟨100⟩ direction, $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{2.69}$ was polycrystalline. The asterisk in panel (b) stands for a plasma line. The dotted lines that extend in both panels are guides to the eye to observe the shifts of some common modes.

Figure 20

(Color online) Comparison of the Raman and far-infrared ellipsometry spectra for the charge-order composition $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{2.85}$ at low temperature and room temperature (a) Raman spectra in parallel $\left(ZZ\right)$ and cross $\left(XZ\right)$ polarization, (b) far-infrared ellipsometry spectra: optical conductivity $\sigma$ and real part of the dielectric permittivity ${\epsilon }_1$.