Transcript 4d and 5d magnetism

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).