Transcript Inelastic Magnetic Neutron Scattering on the Spin

Inelastic Magnetic Neutron
Scattering on the Spin-Singlet
Spin-½ FCC System Ba2YMoO6
Jeremy P. Carlo
Canadian Neutron Beam Centre, National Research Council
G. J. Van Gastel, J. Wagman, J. P. Clancy, B. D. Gaulin
Department of Physics and Astronomy, McMaster University
T. Aharen, J. E. Greedan
Department of Chemistry, McMaster University
G. E. Granroth
Spallation Neutron Source, Oak Ridge National Laboratory
June 2010 American Conference on Neutron Scattering, Ottawa, ON
Ba2YMoO6
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Double Perovskite
Magnetic frustration
Spin-1/2 Mo5+ on edge-sharing tetrahedra
Powder sample
Prior: NMR, mSR, heat capacity, neutron diffraction:
no evidence for magnetic order to 2-3K
likely spin-singlet ground state
• Present: Inelastic neutron scattering measurements
– Numerous phonon branches
– Magnetic scattering at 30 meV, likely 18 meV
• Likely triplet (& 2-triplet) excitations ~18 meV & 30
meV from the singlet ground state
• Magnetic moments on a lattice
– AF exchange interaction
• Hik = -J si  sk with J < 0
• Neel ground state
• Static AF long-range order for T < TN
– What if interactions can’t all be satisfied?
• Geometric Magnetic Frustration
• Huge degeneracy of lowest-energy states
• Typical: triangles, tetrahedra
2D:
3D:
• Frustrated Lattices
– 2D
• Triangular
• Kagome
• Square Lattice J1-J2
– 3D
• Pyrochlore
(network of corner-sharing tetrahedra)
• Another frustrated lattice:
– Double Perovskite A2BB’O6
e.g. Ba2YMoO6
A: divalent cation Ba2+
B: nonmagnetic cation Y3+
B’: magnetic (spin-1/2) cation Mo5+ (4d1)
Magnetic ions: network of edge sharing tetrahedra
• Ground state with large spins (e.g. Ln ~ 4f,
multiple d’s) or close spins
– LRO often wins
• Spin-1/2 moments
– Wide diversity of ground states
• Neel state may win
• Spins freeze randomly at T = Tg
“spin glass”
• No static order, spins fluctuate to T = 0
“spin liquid”
• correlated pairs of spins / dimerization
“spin singlet” ground state
Frustration index:
f = |Qw|/Tc
QM of two coupled spin-1/2 moments:
|S Sz> |sz1 sz2>
|1 1 > = |+ +>
|1 0 > =
1/ (
√2
|+ –> +|– +> )
triplet
|1 -1> = |– –>
|0 0 > = 1/√2( |+ –> - |– +> )
singlet
Ba2YMoO6: previous structural work
• Aharen et al (2010)
• Powder sample
T = 297K
l = 1.33 A
• Neutron diffraction
– Cubic Fm3m, a = 8.3827 A
– no evidence for J-T
– no evidence for Y/Mo B-site mixing
•
89Y
MAS NMR
– ~3% disorder between B and B’
sites
=> well ordered double perovskite!
T = 288K
sim
data
Ba2YMoO6: previous bulk magnetic work
• Susceptibility
– No evidence for order
– Bulk Paramagnetic behavior to 2K
– Curie-Weiss:
• m = 1.73 mB (consistent with spin-1/2)
• Qw = -219(1) K
• Frustration index f = |Qw|/TN > 100
• Magnetic neutron diffraction
– No magnetic Bragg peaks down to 2.8K
• Heat Capacity
– No l-peak:
evidence against LRO
– Very broad peak in mag.
h.c. near 50K
Ba2YMoO6: previous local magnetic work
• mSR
– No rapid relaxation
or precession to 2K:
evidence against LRO, spin freezing
– Dynamic spin fluctuations G(t) ~ e-lt
•
89Y
NMR
– 2 peaks of comparable
intensity
Ba2YMoO6: previous local magnetic work
•
89Y
NMR
– PM Knight shift
– 1/T1 relaxation
– one peak consistent with pm
state
– other consistent with singlet,
gapped state
gap estimate ~ 140K = 12 meV
Comparison to other systems
– Spin-3/2:
• Ba2YRuO6: LRO
• La2LiRuO6: LRO
– Spin-1:
• Ba2YReO6: spin freezing
• La2LiReO6: spin singlet
– Spin-1/2:
• La2LiMoO6: short range order
• Ba2YMoO6: PM / singlet ground state
• Sr2MgReO6: spin freezing
Present Measurements
• Inelastic Neutron Scattering at SNS
– ~6g loose packed powder
– SEQUOIA: Fine Resolution Fermi Chopper Spectrometer
• 6K, 100K @ Ei = 60 meV, 130 meV
• empty sample can subtraction
– ARCS: Wide Angle Range Chopper Spectrometer
• 6K, 300K @ Ei = 60 meV, 100 meV
• empty sample can subtraction
SEQUOIA
ARCS
Conclusions: Ba2YMoO6
• Inelastic neutron scattering data supports the
existence of spin-singlet ground state
– Spectrum dominated by phonons
– Apparent magnetic scattering at 18 meV, 30 meV
• 18 meV feature shows little dispersion
• 30 meV feature more broad
– Evidence for spin-gap for lower energies
– Triplet (and possibly 2-triplet) excitations of singlet
ground state.
• Will need single xtal sample to draw definitive
conclusions!