p erovskite oxides Phillip Barton 052810 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 ID: 280480
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Slide1
Magnetism in 4d perovskite oxides
Phillip Barton05/28/10MTRL 286GFinal PresentationSlide2
Comparison of 3d and 4d magnetism3d 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 interactionSlide3
SrRuO3: The only ferromagnetic 4d perovskite
Perovskite – Pnma
(No. 62)
Ferromagnetic “bad”
metal - TC ~ 165 K
Δ
Ru
4+
is d
4
and experiences an octahedral crystal field
DFT LMTOSlide4
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 fieldsThis is evidence for band ferromagnetismJin 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).Slide5
SrRuO3: The only ferromagnetic 4d perovskite
Rhodes Wohlfarth ratio = Msat
/
μ
eff = 2.0 for SrRuO3 ; indicates itinerant natureP. Rhodes and E. P. Wohlfarth, PRSL 273, 247 (1963). Slide6
Perovskites distort in response to relative cation size
Rotation
Tilt
A. M. Glazer,
Acta Crystallographica Section B 28, 3384 (1972).Slide7
Glazer tilt systems describe rotation and tiltingPnma 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
A. M. Glazer,
Acta
Crystallographica Section B 28, 3384 (1972
).Slide8
Tilting and rotation in PnmaA. M. Glazer,
Acta Crystallographica Section B 28, 3384 (1972).
In phase tilting of
octahedra
down the b axis
Out of phase tilting of octahedra
down the cubic perovskite a axisSlide9
(Ca,Sr,Ba)RuO3: A-site effect on magnetism
Jin et. al, PNAS 105, 7115 (2008).
CaRuO
3
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 ARuO
3.Ca1-x
SrxRuO3 exhibits a Griffith’s phase
that is characterized by deviation from ideal Curie-Weiss at the T
C of the parent ferromagnetic compound. Enhanced spin-orbit coupling on the Ru4+ ions suppresses FM
Ru-O-Ru coupling. Sr
1-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.Slide10
Sr1-xCaxRuO
3: A-site effect on magnetismMazin and Singh, PRB 56
2556 (1997).
CaRuO
3 on verge of a ferromagnetic instabilityDistortion broadens a singularity in the DOS that occurs at EF for a cubic systemSome t2g – eg covalency, but the bands narrow and the t2g
– eg
gap growsA psuedogap opens up near EF which opposes magnetismCovalency
between Ru and O – some of the moment resides on O
Rondinelli
et. al, PRB 78
, 155107 (2008).Slide11
Sr1-xCaxRuO
3: A-site effect on magnetismCao et. al, PRB 56 321 (1997).
Different results that show almost immediate ordering upon substitutionSlide12
Sr1-xPbxRuO
3: A-site effect on magnetismCheng 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).Slide13
ReferencesC.-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).