Sept. 14 th 2004 Montauk, Long Island, NY Jason S. Gardner NIST, Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg,

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Presentation transcript:

Sept. 14 th 2004 Montauk, Long Island, NY Jason S. Gardner NIST, Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD Department of Physics, Brookhaven National Laboratory, Upton, NY Neutron Scattering Studies of Magnetically Frustrated Pyrochlores

Pyrochlore Lattice ?

Diffuse Scattering (0,0,l) (h,h,0) Tb 2 Ti 2 O 7 JS Gardner et al., Phys. Rev. B 64, (2001). Y 2 Mo 2 O 7 J S Gardner et al., Phys. Rev. Lett., 83, 211 (1999). Ho 2 Ti 2 O 7 ST Bramwell et al. PRL 87, (2001). S.-H. Lee et al. Nature 418, 856 (2002)

Outline Spin dynamics Spin dynamics 3 Compounds 3 Compounds Antiferromagnetic frustrated systems Antiferromagnetic frustrated systems –Yb 2 Ti 2 O 7, Gd 2 Ti 2 O 7 »Strange dynamics and partial ordering Ferromagnetic frustrated systems Ferromagnetic frustrated systems –Ho 2 Ti 2 O 7 »Spin Ice (Uniquely frozen)

Sept. 14 th 2004 Montauk, Long Island, NY Spin Echo Principle I(Q,t) = I S(Q,  )cos(  t)d  Cosine transform

New Spectrometer

Typical Magnetic Neutron Spin Echo K 22.5K 18.9K 10.0K S(Q,t)/S(Q,0) Time (ns) 50 % M/H (emu/mol) Temperature (K) 22.5 K ZFC FC

Sept. 14 th 2004 Montauk, Long Island, NY Spin Ice

FM Spin Ices – RE 2 Ti 2 O 7 (Ho and Dy) K. Matsuhira et al., J. Phys.: Condens. Matter 12 (2000) L649–L656 and 13 (2001) L737-L746 Ho 2 Ti 2 O 7

FM Spin Ice

Correlation J Snyder et al. PRB (2004) Mag (arb. units) Ho 2 Ti 2 O 7 Dy 2 Ti 2 O 7

Spin Echo Results

New Spectrometer New territory

New Data

Sept. 14 th 2004 Montauk, Long Island, NY Partial Order Gd 2 Ti 2 O 7 -- ½ filled orbital -- Experimental realisation of a Heisenberg spin on a pyrochlore lattice -- found to order???? -- very anisotropic??

Gd 2 Ti 2 O 7 - Partially ordered A. P. Ramirez, et al.. Rev. Lett. 89, (2002) 1.1K 0.7K

Powder diffraction

Magnetic Structure 1k 4k

NPD (Champion et al. Rev. B 64, (2001)) Kagome Planes INTERSTITIAL SITE ∑ = 0 N=1 ∞

0.77 K Transition

50 mK Structure 1k 4k RED circles BLUE balls

0.77 K Transition

What else do we know…. A. P. Ramirez, et al.. Rev. Lett. 89, (2002) (000) (½½½)

Neutron Spin Echo (preliminary results) ¾ ¼ T > 1K Entire spin system is inelastic and T < 1K 25% of the spin system is static REM: NPD revealed 75% of the system is static below the 1 st transition; At all T, NSE fits results in a S(Q,t)/S(q,0) as t  0 of 75%.

Sept. 14 th 2004 Montauk, Long Island, NY More Unusual Magnetism -Yb 2 Ti 2 O 7

Yb 2 Ti 2 O 7

New Data

Conclusions 6 6 Exotic Magnetic Order 6 6 Large moment systems 6 6 Big crystals   NSE ideal time (  ) window

Sept. 14 th 2004 Montauk, Long Island, NY Collaborators G Ehlers (SNS) A Cornelius (UNLV) R Stewart (ILL) S Bramwell, A Wills and T Fennell(UCL) Nick Rosov (NIST)