Sept. 14th 2004 Montauk, Long Island, NY

Jason S. Gardner 

NIST, Center for Neutron Research,   National Institute of Standards  and Technology,                    Gaithersburg, MD 20899-8562  

Department of Physics,Brookhaven National Laboratory,Upton, NY 11973-5000.

Neutron Scattering Studies of

Magnetically Frustrated Pyrochlores

Pyrochlore Lattice

?

Diffuse Scattering

(0,0,l)

(h,h

,0)

Tb2Ti2O7

JS Gardner et al., Phys. Rev. B 64, 224416 (2001).

0

200

400

600

800

1000

1200

1400

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Net

Inte

nsi

ty (

coun

ts/2

min

)

Q ( Å -1)

Y2Mo2O7

J S Gardner et al., Phys. Rev. Lett., 83, 211 (1999).

Ho2Ti2O7

ST Bramwell et al. PRL 87, 047205 (2001).

S.-H. Lee et al. Nature 418, 856 (2002)

Outline Spin dynamicsSpin dynamics

3 Compounds3 Compounds

Antiferromagnetic frustrated Antiferromagnetic frustrated systemssystems–YbYb22TiTi22OO77, Gd, Gd22TiTi22OO77

»Strange dynamics and partial ordering

Ferromagnetic frustrated Ferromagnetic frustrated systemssystems–HoHo22TiTi22OO77

»Spin Ice (Uniquely frozen)

Sept. 14th 2004 Montauk, Long Island, NY

Spin Echo Principle

I(Q,t) = S(Q,)cos(t)dCosine transform

New Spectrometer

Typical Magnetic Neutron Spin Echo

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0.01 0.1 1

25.0K22.5K

18.9K10.0K

S(Q

,t)/S

(Q,0

)

Time (ns)

50 %

0.009

0.01

0.011

0.012

0.013

0.014

0.015

5 10 15 20 25 30 35 40

M/H

(em

u/m

ol)

Temperature (K)

22.5 KZFC

FC

Sept. 14th 2004 Montauk, Long Island, NY

Spin Ice

FM Spin Ices – RE2Ti2O7 (Ho and Dy)

K. Matsuhira et al., J. Phys.: Condens. Matter 12 (2000) L649–L656 and 13 (2001) L737-L746

Ho2Ti2O7

FM Spin Ice

Correlation

J Snyder et al. PRB 69 064414 (2004)

Mag

(ar

b. u

nits

)

Ho2Ti2O7

Dy2Ti2O7

Spin Echo Results

New Spectrometer

New territory

New Data

Sept. 14th 2004 Montauk, Long Island, NY

Partial Order

Gd2Ti2O7 -- ½ filled orbital -- Experimental realisation of a Heisenberg spin on a pyrochlore lattice-- found to order????-- very anisotropic??

Gd2Ti2O7 - Partially ordered

A. P. Ramirez, et al.. Rev. Lett. 89, 067202 (2002)

1.1K

0.7K

Powder diffraction

Magnetic Structure

A B

C D

1k 4k

NPD (Champion et al. Rev. B 64, 140407 (2001))

Kagome Planes

INTERSTITIAL SITE

∑ = 0N=1

∞

0.77 K Transition

50 mK StructureA B

C D

1k 4k

RED circlesBLUE balls

0.77 K Transition

What else do we know….A. P. Ramirez, et al.. Rev. Lett. 89, 067202 (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 1st transition; At all T, NSE fits results in a S(Q,t)/S(q,0)

as t 0 of 75%.

Sept. 14th 2004 Montauk, Long Island, NY

More Unusual Magnetism

-Yb-Yb22TiTi22OO77

Yb2Ti2O7

New Data

Conclusions

Exotic Magnetic Order

Large moment systems

Big crystals

NSE ideal time () window

Sept. 14th 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)