Tuning of dielectric properties and magnetism of SrTiO${}_3$ by site-specific doping of Mn

Combining experiments with first-principles calculations, we show that site-specific doping of Mn into SrTiO${}_3$ has a decisive influence on the dielectric properties of these doped systems. We find that phonon contributions to the dielectric constant invariably decrease sharply on doping at any site. However, a sizable, random dipolar contribution only for Mn at the Sr site arises from a strong off-centric displacement of Mn in spite of Mn being in a non-$d^0$ state; this leads to a large dielectric constant at higher temperatures and gives rise to a relaxor ferroelectric behavior at lower temperatures. We also investigate magnetic properties in detail and critically reevaluate the possibility of a true multiglass state in such systems.

Figure 1

Electron paramagnetic resonance (EPR) spectra of SrMn, SrMnTiMn, and TiMn. To probe the Mn state at the Ti site, we specifically prepared a low-doping TiMn sample with the composition SrTi${}_{0.99}$Mn${}_{0.01}$O${}_3$ to avoid any dipolar broadening due to Mn-Mn interaction, as can be seen at higher concentrations of Mn, e.g., for the SrMnTiMn sample.

Figure 2

Mn $L_{\mathrm{II},\mathrm{III}}$ x-ray absorption spectra of SrMn and TiMn samples along with reference compounds, showing the presence of (a) Mn${}^{2+}$ ions in SrMn sample and (b) Mn${}^{4+}$ ions in TiMn sample.

Figure 3

(a) Dielectric constants of SrMn. Inset shows linear $M$–$H$ measured at 10 K of SrMn. (b) Dielectric constants of SrMnTiMn for various fixed frequencies. Inset shows the corresponding loss $D$. The arrow indicates the direction of increasing frequency in every panel.

Figure 4

Properties of Sr${}_{0.97}$Mn${}_{0.03}$Ti${}_{0.93}$Mn${}_{0.07}$O${}_3$. (a) The field cooled (FC) and zero-field cooled (ZFC) dc magnetization measured with an applied dc magnetic field of 100 Oe with the inset showing the real part of the ac susceptibility data measured for an applied ac magnetic field of 4 Oe for a few frequencies. The arrow indicates the direction of increasing frequency. (b) The difference in ZFC data with intermittent waits ($T_w$) at 25 and 30 K for different times as indicated in the plots.

Figure 5

Dielectric constant of Sr${}_{0.97}$Mn${}_{0.03}$Ti${}_{0.93}$Mn${}_{0.07}$O${}_3$ as a function of temperature for various fixed frequencies in the absence (circles) and in the presence (lines) of a magnetic field of 1 T. The inset shows the specific heat as a function of temperature. The arrow indicates the direction of increasing frequency.

Figure 6

The bulk Raman spectra of SrMnTiMn as shown by the black dashed line. The red solid line shows the Raman spectra from the impurity phase overlapped on top of the bulk signal, which consists of characteristic lines of Mn${}_3$O${}_4$, highlighted by blue circles. The inset shows a detailed XRD scan.

Figure 7

Field-cooled magnetizations ($M$) of various SrMnTiMn samples measured with an applied dc magnetic field of 100 Oe, with the corresponding x-ray diffraction patterns (shown in the inset), focusing on the 100% peak position of hausmannite Mn${}_3$O${}_4$ (2θ = 36°) [30], showing correspondingly varying amounts of Mn${}_3$O${}_4$ impurity phase in these samples.

Figure 8

Calculated total (left and bottom axes) and polar (right and top axes) phonon density of states at the Γ point ($q$ = 0) for various compounds as indicated. Plots are normalized to have the same area in every case.

Figure 9

Undistorted (a) and geometry optimized distorted (b) cuboctahedron of Mn ion in Mn doped at the Sr site of SrTiO${}_3$. Mn and O atoms are drawn as purple (large) and red (small) balls, respectively.