Nanosize effect: Enhanced compensation temperature and existence of magnetodielectric coupling in $\mathrm{SmFe}{\mathrm{O}}_3$

In transition metal oxides, quantum confinement arising from a large surface to volume ratio often gives rise to novel physicochemical properties at nanoscale. Their size-dependent properties have potential applications in diverse areas, including therapeutics, imaging, electronic devices, communication systems, sensors, and catalysis. We have analyzed the structural, magnetic, dielectric, and thermal properties of weakly ferromagnetic $\mathrm{SmFe}{\mathrm{O}}_3$ nanoparticles of sizes of about 55 and 500 nm. The nanometer-size particles exhibit several distinct features that are neither observed in their larger-size variants nor reported previously for the single crystals. In particular, for the 55-nm particle, we observe a sixfold enhancement of compensation temperature, an unusual rise in susceptibility in the temperature range 550 to 630 K due to spin pinning, and a coupled antiferromagnetic-ferroelectric transition, directly observed in the dielectric constant.

Figure 1

TEM, HRTEM, and EDS analyses of SFO-1 and -2. (a) Low-resolution micrograph of SFO-1, (${\mathrm{a}}_1$)–(${\mathrm{a}}_3$) HRTEM micrographs of SFO-1 representing twinning domains, presence of defects, and core-shell behavior, respectively, (${\mathrm{a}}_4$) and (${\mathrm{a}}_5$) show Sm and Fe atomic columns within the $\mathrm{SmFe}{\mathrm{O}}_3$ lattice for SFO-1, (${\mathrm{b}}_1$)–(${\mathrm{b}}_4$) show the elemental mapping and EDS spectrum, respectively, for SFO-1. (c) Low-resolution micrograph of SFO-2, (${\mathrm{c}}_1$)–(${\mathrm{c}}_3$) HRTEM micrographs of SFO-2 representing distorted twinning domains and presence of defects, respectively, (${\mathrm{c}}_4$) and (${\mathrm{c}}_5$) show the atomic column arrangement for Sm and Fe atoms within the lattice, (${\mathrm{d}}_1$)–(${\mathrm{d}}_4$) represent the elemental mapping and EDS spectrum of SFO-2, respectively, and (e) and (f) show the particle size distribution for SFO-1 and SFO-2 as obtained from TEM.

Figure 2

Illustrating the Sm-O and Fe-O bond lengths. (a) $\mathrm{Sm}{\mathrm{O}}_{12}$ dodecahedron of sample SFO-1 (nano), (b) $\mathrm{Fe}{\mathrm{O}}_6$ octahedron for sample SFO-1 (nano), (c) $\mathrm{Sm}{\mathrm{O}}_{12}$ dodecahedron of sample SFO-2 (micro), and (d) $\mathrm{Fe}{\mathrm{O}}_6$ octahedron for sample SFO-2 (micro).

Figure 3

Electron density maps obtained by Fourier transformation of Rietveld refined data. The ED plots for samples SFO-1 and SFO-2 illustrating the change in Fe-O1-Fe angle and Fe-O2 bond lengths corresponding to nanometer- and micrometer-sized particles.

Figure 4

High- and low-temperature-dependent susceptibility curves showing various transition temperatures in SFO nano- and microparticles: $\chi \text{−}T$ curve for temperature 300 to 750 K for samples (a) SFO-1 and (b) SFO-2, respectively, at magnetic field 0.1 T depicting $T_{\mathrm{SR}}$ and $T_{\mathrm{N}}$ (derivative of ${\chi }_{\mathrm{FW}}$ in inset) for both the samples and an anomaly at $\sim 630\mathrm{K}$ in case of SFO- 1. $\chi \text{−}T$ curve for temperature 2 to 300 K for samples (c) SFO-1 and (d) SFO-2, respectively, at magnetic field 1 mT showing compensation temperature $T^{*}\sim 22\mathrm{K}$ for SFO-1 and $\sim 4\mathrm{K}$ for SFO-2.

Figure 5

Magnetic hysteresis measurements for samples SFO-1 and SFO-2 at various temperatutres: (a) 2, (b) 70, (c) 310, and (d) 650 K. Figures (e) to (h) show zoomed-in view of thee corresponding magnetization plots.

Figure 6

(a) Coregression of the activation (conductivity, σ) and relaxation (loss tangent, tan $\delta$) behaviors and its near similarity above and below the spin-reorientation temperature in the SFO-1 $(\sim 55\mathrm{nm})$ sample marks $T_{\mathrm{SR}}$ as the turn-back double minima. (b) For SFO-2 $(\sim 500\mathrm{nm})$, Jonscher-form-fitted conductivity isotherms at benchmark temperatures across the $T_{\mathrm{SR}}$, along with the locus traced out by their dc/ac crossover $({\omega }_c={\omega }_h;{\sigma }_c=2{\sigma }_{dc})$. (c) Jonscher-coefficient [$A\left(T\right)$, top] and power-law exponent [$s\left(T\right)$, bottom] determined from the empirical fits to the conductivity isotherms mark the spin-reorientation transition, via switching of the conduction behavior from classical correlated barrier hopping (CBH) below $T_{\mathrm{SR}}$ to quantum small polaron tunneling (SPT) above. The behavior breaks found at $T_{\mathrm{SR}}$ in $A\left(T\right)$ and $s\left(T\right)$ (shown), and also in Arrhenic ${\sigma }_{dc}\left(T^{-1}\right)$ (not shown) correspond to discontinuities in the dispersion of the dc/ac crossover $\mathrm{\Delta }\left(d\ln {\omega }_c/dT\right){|}_{T_{\mathrm{SR}}}\sim 14\%$ [open circles in (b)] and in the activation energy $\mathrm{\Delta }{E}_a{|}_{T_{\mathrm{SR}}}\sim 12\%$.

Figure 7

Anomalies in the dielectric constant indicate respectively the emergence of magneto-electrically “induced-FE” phase upon the AFM ordering ($T_{\mathrm{C}}=T_{\mathrm{N}}$, SFO-1, $\sim 55\mathrm{nm}$, top) and the “incipient-FE” signature above the AFM ordering ($T_{\mathrm{a}}>T_{\mathrm{N}}$, SFO-2, $\sim 500\mathrm{nm}$, bottom). The contrasting nature of the anomalies is consistent with the single/multiple FE domains, reckoned as hosted by the nano-/micro-sized grains of the two specimens, respectively.