Magnetism in 4d perovskite oxides Phillip Barton 05/28/10 MTRL 286G Final Presentation

Magnetism in 4d perovskite oxides

Phillip Barton05/28/10

MTRL 286GFinal 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)

Ferromagnetic “bad” metal - TC ~ 165 K

Δ

Ru4+ is d4 and experiences an octahedral crystal field

DFT LMTO


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

This is evidence for band ferromagnetism

Jin et. al, PNAS 105, 7115 (2008).

Invar effect – zero thermal expansion

Due to freezing of octahedra at low temperatures

Bushmeleva et. al, JMMM 305, 491 (2006).


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 Tilt

A. M. Glazer, Acta Crystallographica Section B 28, 3384 (1972).

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.

Michael Lufaso – SPUDS and TUBERS

a b 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

(Ca,Sr,Ba)RuO3: A-site effect on magnetism

Jin et. al, PNAS 105, 7115 (2008).

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 spin-orbit 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.

Sr1-xCaxRuO3: A-site effect on magnetism

Mazin and Singh, PRB 56 2556 (1997).

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

Rondinelli et. al, PRB 78, 155107 (2008).

Sr1-xCaxRuO3: A-site effect on magnetism

Cao et. al, PRB 56 321 (1997).

Different results that show almost immediate ordering upon substitution

Sr1-xPbxRuO3: A-site effect on magnetism

Cheng et. al, PRB 81 134412 (2010).

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.

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).


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).

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Magnetism in 4d perovskite oxides Phillip Barton 05/28/10 MTRL 286G Final Presentation