Microstrain Sensitivity of Orbital and Electronic Phase Separation in ${\mathrm{SrCrO}}_3$

An orbital ordering transition and electronic phase coexistence have been discovered in ${\mathrm{SrCrO}}_3$. This cubic, orbitally-degenerate perovskite transforms to a tetragonal phase with partial orbital order. The tetragonal phase is antiferromagnetic below 35–40 K, whereas the cubic phase remains paramagnetic at low temperatures. The orbital ordering temperature (35–70 K) and coexistence of the two electronic phases are very sensitive to lattice strain. X-ray measurements show a preferential conversion of the most strained regions in the cubic phase. This reveals that small fluctuations in microstrain are sufficient to drive long range separation of competing electronic phases even in undoped cubic oxides.

Figure 1

Electrical resistivity and magnetization results for ${\mathrm{SrCrO}}_3$. SQUID magnetization measurements in a 500 Oe field and four probe resistance measurements on a sintered bar were made using commercial instruments. The main plot shows resistivity and zero field cooled (ZFC) and field cooled (FC) magnetization/field data for the 10 GPa sample 13. The divergence between ZFC and FC magnetizations is likely to reflect the cooperative freezing scenario reported in phase separated manganites [7]. The lower inset shows the effect of cycling the 8 GPa sample between 5 and 70 K. Warming/cooling data for cycles 1, 2, and 4 are shown as broken/continuous lines. The decrease in low temperature magnetization on cycling evidences a reduction in the proportion of the tetragonal phase. The upper inset shows the $d_{xy}{}^1$ orbital order and antiferromagnetic spin order in the tetragonal phase of ${\mathrm{SrCrO}}_3$.

Figure 2

Powder diffraction profiles for ${\mathrm{SrCrO}}_3$; (a) The temperature evolutions of the cubic (200) synchrotron x-ray diffraction peak for the 8 and 10 GPa samples, showing the growth of additional reflections from the tetragonal phase at low temperatures. (b) Fit to the 10 K synchrotron x-ray diffraction profile of the 10 GPa sample with cubic/tetragonal reflections indicated by lower/upper tick marks. (c) Two phase fit to the 10 K neutron diffraction profile of the 10 GPa sample showing the magnetic superstructure positions. The inset shows the fit of the antiferromagnetic model in Fig. 1 to the intensity difference between 10 and 100 K neutron data. Neutron data were collected on instrument D20 at ILL, Grenoble with wavelength $\lambda =1.8889\AA$.

Figure 3

Temperature variation of the tetragonal phase fraction ($p_T$) in samples of ${\mathrm{SrCrO}}_3$ prepared at 6, 8, and 10 GPa. Data for the latter are shown while warming during cycles 1 and 2.

Figure 4

Thermal evolutions of the cubic and tetragonal phases (“$C$” and “$T$” subscripts) of the 8 GPa sample of ${\mathrm{SrCrO}}_3$, from fits to synchrotron x-ray diffraction profiles. (a) Lattice parameters $a$ and $c$ and cell volumes $V$. The apparently large thermal expansion of the tetragonal phase evidences large strains in the residual domains approaching the transition. (b) Microstrains for the cubic ($s_C$) and tetragonal (shown as $s_T/10$) phases are plotted together with the total microstrain ($s_{\mathrm{Total}}=[p_{T}s_T{}^2+(1-p_T)s_C{}^2{]}^{1/2}$). Inset shows $s_C$ for the 6 and 10 GPa samples.