Room-temperature dilute ferromagnetic dislocations in $\mathrm{S}{\mathrm{r}}_{1-x}\mathrm{M}{\mathrm{n}}_x\mathrm{Ti}{\mathrm{O}}_{3-\delta }$

Room-temperature dilute ferromagnetism has been reported for many semiconducting or insulating materials, which are usually in the forms of bulk or thin film. Here, we successfully fabricated dilute ferromagnetic nanowires by using dislocations—one-dimensional lattice defects—embedded between optically transparent, nonmagnetic $\mathrm{SrTi}{\mathrm{O}}_3$ single crystals. At the dislocation cores, we have both locally codoped magnetic $\mathrm{M}{\mathrm{n}}^{2+}$ ions and electron donors. The structure, chemistry, and ferromagnetism of dislocations were studied by atomic-resolution scanning transmission electron microscopy combined with magnetic force microscopy. We discuss the origin of dilute ferromagnetism at the dislocations in terms of the percolation of bound magnetic polarons along the dislocation cores, where antiferromagnetic coupling between the high spin state of $\mathrm{M}{\mathrm{n}}^{2+}$ ions and electron donors leads to the long-range Mn-Mn ferromagnetic exchange interaction.

Figure 1

Schematic views of (a) Mn thin film deposition on 5° mistilt $\mathrm{SrTi}{\mathrm{O}}_3$ bicrystal and (b) Mn pipe diffusion process along the dislocation cores during annealing in a reduced atmosphere. (c) Low-magnification BF-TEM image and electron diffractogram. (d) Atomic-resolution ADF-STEM image obtained from the Mn-doped $\mathrm{SrTi}{\mathrm{O}}_3$ bicrystal. (e) The Mn STEM-EDXS spectrum image obtained from the marked area in (c).

Figure 2

(a) The simultaneously recorded surface topographic and (b) MFM phase images obtained from Mn-doped and undoped $\mathrm{SrTi}{\mathrm{O}}_3$ bicrystals (inset images) under the antiparallel magnetic field condition, where arrowheads indicate the location of the grain boundary. We note that distinct scan noise (in the middle of these images) was externally introduced during the measurement; however, these noises do not affect the interpretation of the MFM signal (we have confirmed similar positive MFM signals along the grain boundary several times). (c) The line profile of the MFM phase image across the $X-X^{\prime }$ direction for Mn-doped $\mathrm{SrTi}{\mathrm{O}}_3$. The scale bars in (a) and (b) are 100 nm.

Figure 3

(a) Elemental composition distribution (at. %) of O, Sr, Ti, and Mn atoms along the $X-X^{\prime }$ direction in Fig. 1. (b) Atomic-resolution ADF-STEM and STEM-EDXS spectrum images of Mn, Sr, and Ti at a dislocation core, where the bottom two images are the composite of Mn and Sr or Ti spectrum images, respectively. (c) The EELS Mn-$L$, O-$K$, and Ti-$L$ edges obtained from a dislocation core (blue) and 5 nm away from the core (green), respectively. The notations of $a_i,b_i$, and $c$ in O-$K$ edge profile indicate the peak positions.

Figure 4

(a)–(c) The histogram tableaux of $N\mathrm{th}$ NN distance between Mn dopants at the Mn doping levels of 1, 3, and 5 at. %. The histograms for the first to fifth NN distances correspond to top to bottom. The ball-and-stick models ($4\times 4\times 100\mathrm{n}{\mathrm{m}}^3$) of (d) doping level of 1 at. % and (e) 3 at. %, where the stick length is less than 0.8 nm.