Magnetism in 4d perovskite oxides Phillip Barton 05/28/10 MTRL 286G Final Presentation Comparison of 3d and 4d magnetism 3d transition metals Fe, Co, and Ni are ferromagnetic, however no 4d or 5d are ferromagnetic (except reports of nanoparticles) 3d orbitals have a smaller spatial extent than 4d, as shown below schematically with s orbitals. Thus, there is minimal interaction between 3d orbitals which results in a small bandwidth and subsequently a large density of states. This satisfies the Stoner criterion and spontaneous spin polarization occurs to reduce the DOS at the Fermi level. Additionally, 3d electrons are more “correlated” (electron-electron interactions matter) as they are packed into smaller orbitals. 4d has increased spin-orbit interaction, larger crystal field splitting 1s orbital interaction 2s orbital interaction SrRuO3: The only ferromagnetic 4d perovskite Perovskite – Pnma (No. 62) Ru4+ is d4 and experiences an octahedral crystal field Ferromagnetic “bad” metal - TC ~ 165 K Δ DFT LMTO SrRuO3: The only ferromagnetic 4d perovskite Msat does not max out at the expected S=1 spin only 2 μB/Ru as is expected for a d4 ion in a octahedral crystal field even at very low temperatures and high fields Invar effect – zero thermal expansion Due to freezing of octahedra at low temperatures This is evidence for band ferromagnetism Bushmeleva et. al, JMMM 305, 491 (2006). Jin et. al, PNAS 105, 7115 (2008). SrRuO3: The only ferromagnetic 4d perovskite Rhodes Wohlfarth ratio = Msat/μeff = 2.0 for SrRuO3 ; indicates itinerant nature P. Rhodes and E. P. Wohlfarth, PRSL 273, 247 (1963). Perovskites distort in response to relative cation size Rotation A. M. Glazer, Acta Crystallographica Section B 28, 3384 (1972). Tilt Glazer tilt systems describe rotation and tilting Pnma has the tilt system a-b+a-. The +/- indicates in/out of phase while the letter indicates magnitude. The schematic below shows the Pnma tilting pattern in the cubic Perovskite cell. a b Michael Lufaso – SPUDS and TUBERS A. M. Glazer, Acta Crystallographica Section B 28, 3384 (1972). c Tilting and rotation in Pnma In phase tilting of octahedra down the b axis Out of phase tilting of octahedra down the cubic perovskite a axis A. M. Glazer, Acta Crystallographica Section B 28, 3384 (1972). (Ca,Sr,Ba)RuO3: A-site effect on magnetism CaRuO3 is a paramagnetic “bad” metal down to low T. BaRuO3 is ferromagnetic with a TC of 60 K. Base tilts in the end members are 149, 163, and 180° for Ca, Sr, and Ba in ARuO3. Ca1-xSrxRuO3 exhibits a Griffith’s phase that is characterized by deviation from ideal Curie-Weiss at the TC of the parent ferromagnetic compound. Enhanced spinorbit coupling on the Ru4+ ions suppresses FM Ru-O-Ru coupling. Sr1-yBayRuO3 follows the Stoner-Wohlfarth model of band ferromagnetism. Strong ionic character of Ba increases the covalency of Ru-O which increases the bandwidth, lowers the DOS, and disrupts the Stoner FM. Jin et. al, PNAS 105, 7115 (2008). Sr1-xCaxRuO3: A-site effect on magnetism CaRuO3 on verge of a ferromagnetic instability Distortion broadens a singularity in the DOS that occurs at EF for a cubic system Some t2g – eg covalency, but the bands narrow and the t2g – eg gap grows A psuedogap opens up near EF which opposes magnetism Covalency between Ru and O – some of the moment resides on O Mazin and Singh, PRB 56 2556 (1997). Rondinelli et. al, PRB 78, 155107 (2008). Sr1-xCaxRuO3: A-site effect on magnetism Different results that show almost immediate ordering upon substitution Cao et. al, PRB 56 321 (1997). Sr1-xPbxRuO3: A-site effect on magnetism Pb substitution causes distortion due to its lone pairs rather than size difference Pb2+ ionic radius ~ 1.19 for z=6 and 1.49 for z=12. With Ru4+ z=6 ~ 0.620 and Sr2+ z=12 ~ 1.44 it is likely that Pb sits on the A-site. Cheng et. al, PRB 81 134412 (2010). Strange behaviors may be due to impurity phases. Cao et. al, PRB 54, 15144 (1996). References C.-Q. Jin†, J.-S. Zhou§, J. B. Goodenough§, Q. Q. Liu†, J. G. Zhao†, L. X. Yang†, Y. Yu†, R. C. Yu†, T. Katsura¶, A. Shatskiy¶, and E. Ito¶, PNAS 105, 7115 (2008). I. I. Mazin and D. J. Singh, PRB 56, 2556 (1997). A. M. Glazer, Acta Crystallographica Section B 28, 3384 (1972). James M. Rondinelli, Nuala M. Caffrey, Stefano Sanvito, and Nicola A. Spaldin, PRB 78, 155107 (2008). P. Rhodes and E. P. Wohlfarth, Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences 273, 247 (1963). G. Cao, S. McCall, M. Shepard, J. E. Crow, and R. P. Guertin, PRB 56, 321 (1997). S. N. Bushmeleva, V. Y. Pomjakushin, E. V. Pomjakushina, D. V. Sheptyakov, and A. M. Balagurov, Journal of Magnetism and Magnetic Materials 305, 491 (2006). J.-G. Cheng, J.-S. Zhou, and J. B. Goodenough, PRB 81 134412 (2010).