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Inelastic Magnetic Neutron Scattering on the Spin-Singlet Spin- ½ FCC System Ba 2 YMoO 6 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

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Ba 2 YMoO 6 Double Perovskite Magnetic frustration Spin- 1 / 2 Mo 5+ on edge-sharing tetrahedra Powder sample Prior: NMR, SR, 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

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Magnetic moments on a lattice – AF exchange interaction H ik = -J s i s k with J < 0 Neel ground state Static AF long-range order for T < T N – What if interactions can’t all be satisfied? Geometric Magnetic Frustration Huge degeneracy of lowest-energy states Typical: triangles, tetrahedra 2D: 3D:

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Frustrated Lattices – 2D Triangular Kagome Square Lattice J 1 -J 2 – 3D Pyrochlore (network of corner-sharing tetrahedra)

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Another frustrated lattice: – Double Perovskite A 2 BB’O 6 e.g. Ba 2 YMoO 6 A: divalent cation Ba 2+ B: nonmagnetic cation Y 3+ B’: magnetic (spin- 1 / 2 ) cation Mo 5+ (4d 1 ) Magnetic ions: network of edge sharing tetrahedra

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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 = T g “spin glass” No static order, spins fluctuate to T = 0 “spin liquid” correlated pairs of spins / dimerization “spin singlet” ground state Frustration index: f = | w |/T c

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QM of two coupled spin- 1 / 2 moments: |S S z > |s z1 s z2 > |1 1 > = |+ +> |1 0 > = 1 / √2 ( |+ –> +|– +> ) |1 -1> = |– –> |0 0 > = 1 / √ 2 ( |+ –> - |– +> ) triplet singlet

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Ba 2 YMoO 6 : previous structural work Aharen et al (2010) Powder sample Neutron diffraction – Cubic Fm3m, a = A – no evidence for J-T – no evidence for Y/Mo B-site mixing 89 Y MAS NMR – ~3% disorder between B and B’ sites => well ordered double perovskite! T = 297K = 1.33 A T = 288K sim data

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Ba 2 YMoO 6 : previous bulk magnetic work Susceptibility – No evidence for order – Bulk Paramagnetic behavior to 2K – Curie-Weiss: = 1.73 B (consistent with spin- 1 / 2 ) w = -219(1) K Frustration index f = | w |/T N > 100 Magnetic neutron diffraction – No magnetic Bragg peaks down to 2.8K Heat Capacity – No -peak: evidence against LRO – Very broad peak in mag. h.c. near 50K

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Ba 2 YMoO 6 : previous local magnetic work SR – No rapid relaxation or precession to 2K: evidence against LRO, spin freezing – Dynamic spin fluctuations G(t) ~ e - t 89 Y NMR – 2 peaks of comparable intensity

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Ba 2 YMoO 6 : previous local magnetic work 89 Y NMR – PM Knight shift – 1/T 1 relaxation – one peak consistent with pm state – other consistent with singlet, gapped state gap estimate ~ 140K = 12 meV

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Comparison to other systems – Spin- 3 / 2 : Ba 2 YRuO 6 : LRO La 2 LiRuO 6 : LRO – Spin-1: Ba2YReO6: spin freezing La2LiReO6: spin singlet – Spin- 1 / 2 : La 2 LiMoO 6 : short range order Ba 2 YMoO 6 : PM / singlet ground state Sr 2 MgReO 6 : spin freezing

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Present Measurements Inelastic Neutron Scattering at SNS – ~6g loose packed powder – SEQUOIA: Fine Resolution Fermi Chopper Spectrometer 6K, Ei = 60 meV, 130 meV empty sample can subtraction – ARCS: Wide Angle Range Chopper Spectrometer 6K, Ei = 60 meV, 100 meV empty sample can subtraction

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SEQUOIA ARCS

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Conclusions: Ba 2 YMoO 6 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!

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