Condensed Matter Physics Seminar
Effects of Nonstoichiometry and Chemical Substitutions on Structural and Magnetic Properties of SrRuO3
Northern Illinois University
SrRuO3 has been known for half a century as a strongly-correlated ferromagnetic metal and proposed for use as an electrode material in micro- and nano-electronic circuits. We have discovered that annealing of stoichiometric SrRuO3 perovskites in high-pressure oxygen produces SrRu1-vO3 compounds with vacancies on the Ru-sites. The creation of Ru vacancies rapidly suppresses ordered moment per Ru from 1.6 to 0.8 &\mu;B and the ferromagnetic ordering temperature, TC, from 163 K to 45 K with increase of v ≈ 0.09, whereas the resistivity increases. Subtle structural changes that accompany creation of Ru-site vacancies are different from the typical properties of transition metal perovskites, for which an increased formal oxidation state of the B-site cations normally leads to decreased B-O interatomic distances and contraction of the unit cell volume. The reduced charge screening caused by the Ru-vacancies offsets expected decrease of the average interatomic distance Ru-O and rotation of the RuO6 octahedra as Sr atoms relax toward Ru-vacancies increases observed volume. Local probes of Ru with XANES, NMR, and EELS find no valence change while isomer shifts of 99Ru from Mossbauer experiments reveal a small change from +4 toward +5 as the TC decreases. By using “design rules” of the synthesis and magnetic interactions for perovskites we have established that Cr substitution into Ru site of SrRuO3 increases TC to 188 K at the solubility limit of 12%. The 53Cr and 99,101Ru NMR studies have revealed that the electronic configurations of Cr and Ru are 3d3 (t2g3) and 4d4 (t2g31), respectively. Compared with SrRuO3, the spin-down electron in Ru 4d shell is more itinerant in SrRu0.875Cr0.125O3 which is consistent with a broadened conduction band and a possible minority-band Cr(d3)-O2--Ru(d4) double-exchange interaction. The exchange interaction involves the Cr3+ into the ferromagnetic ordering and enhances the ordering temperature. This picture is supported by a steady decrease of the ordered moment per Ru/Cr. Most importantly we have found that the ~ 5% substituted samples display the smallest coersive field and the lowest resistivity; i.e., the very attractive properties for electronic applications. Neutron powder diffraction analysis revealed that below Tc = 163 K the b and c lattice parameters and the unit cell volume are virtually temperature independent for the stoichiometric material. We show that this previously reported Invar effect below the ferromagnetic ordering temperature originates from freezing of the octahedral tilting about the  axis. The Invar effect is much less pronounced in the Ru-deficient samples with decreased Tc. We show that the Invar effect gradually disappears also for the Cr-substituted samples with increased Tc.
Thursday, April 21, 2005
Gant Science Complex
(The seminar is followed by coffee/tea, cookies, and an informal discussions.)