Experimental Aspects of Neutron Diffraction William Ratcliff NCNR.

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Presentation transcript:

Experimental Aspects of Neutron Diffraction William Ratcliff NCNR

1932 Neutrons Discovered by Chadwick

1942 Fermi’s 30 th pile goes critical

ORNL Graphite Reactor

R r

Reciprocal Space aaaa

Here there be dragons…

Laue Condition

Here there be dragons…

Ewald Sphere

So… See scattering where reciprocal lattice point lies on Ewald Sphere But, whither magnetism???

1932, Néel predicts Antiferromagnetic order

Intermission

How it starts?

Bulk Data Comes In Physical Review B 78, (2008)

Neutrons to the Rescue Physical Review B 78, (2008)

Guess and Check (Refinement) Predict Intensities from model Compare predicted intensities to data Update model

Choices

Goodness of Fit or

Optimization Approaches Simulated Annealing Marquardt-Levenberg Genetic Algorithm

How I proceed Think about the problem Powder diffraction Think some more Try Representational Analysis Single crystal diffraction Think a lot!!! Polarized diffraction Spherical polarimetry Think some more…

YMn 2 O 5 Powder xtal Xtal+spherical polarimetry Xtal+more representation analysis

Powder Diffraction Advantages You get the big picture Can get the propagation vector Avoids the muss and fuss of extinction It’s often Good Enough TM Disadvantages Can be hard to truly index k— is it [3 4 0] or [0 0 5]? You average over all symmetry equivalent k at any particular Bragg angle You lose information in the powder averaging No domain info No multi-k info Can be very hard to determine phase

Single Crystal Diffraction Advantages Can fully determine k Can investigate domain populations Can apply probes (magnetic field, E-field, pressure, etc.) along a particular direction to see effect on magnetic ordering Disadvantages Extinction Absorption depends on shape Reciprocal space is large… Crystal growth is hard…

Questions?