Outline

Geometric Magnetic Frustration in Double Perovskite OxidesA2BBO6

Jeremy P. Carlo

Department of PhysicsVillanova University

June 2014 Oxides for Energy Meeting, Philadelphia, PAOutlineMagnetism in MaterialsGeometric FrustrationThe Tools:Neutron ScatteringMuon Spin RelaxationFrustration in Double PerovskitesResults and Conclusions

2OutlineMagnetism in MaterialsGeometric FrustrationThe Tools:Neutron ScatteringMuon Spin RelaxationFrustration in Double PerovskitesResults and Conclusions

3Magnetism in materialsWhy transition metals / lanthanides / actinides?Need unpaired electrons in valence shells: 1 orbitalp: 3 orbitalsd: 5 orbitalsf: 7 orbitals

4Magnetism in materialsSimplest model: assume moments dont interact with each other.High temps: spins fluctuate rapidly and randomly, but can be influenced by an applied magnetic field H: U = -mH

M = H = susceptibilityParamagnetism ( > 0) Diamagnetism ( < 0)

Temp dependence:(T) = C / TCurie Paramagnetism

Real materials: moments do interactExchange Interaction: U = J S1 S2

Then,(T) = C / (T - CW) Curie-Weiss behavior

5Magnetism in materialskBT > J: thermal fluctuations dominatekBT < J: interaction energy dominates

Expect: Torder |CW|

Spins may collectively align, leading to a spontaneous nonzero magnetizationFerromagnetism (FM)(J, CW > 0)

Or they can anti-align: large local magnetic fields in the material, but zero overall magnetic momentAntiferromagnetism (AF)(J, CW < 0)

(T) = C / (T - CW) U = J S1 S26OutlineMagnetism in MaterialsGeometric FrustrationThe Tools:Neutron ScatteringMuon Spin RelaxationFrustration in Double PerovskitesResults and Conclusions

http://leadershipfreak.files.wordpress.com/2009/12/frustration.jpg7Geometric FrustrationFrustration: Geometric arrangement of magnetic ions prevents allInteractions from being simultaneously satisfied.If all interactions cannot be simultaneously satisfiedthe onset of magnetic order is inhibited.

f = |QCW| / Torder frustration index

CW ~ Weiss temperatureTorder ~ actual magnetic ordering temp

MFT: f should be 1

8Geometric FrustrationIn 2-D, associated with AF coupling on triangular lattices

edge-sharing triangles: triangular lattice

corner-sharing triangles: Kagome lattice

Usually quasi-2D systems composed of weakly-interacting layers

http://en.wikipedia.org/wiki/File:Herbertsmithite-163165.jpgHerbertsmithiteZnCu3(OH6)Cl2

9Geometric FrustrationIn 3-D, associated with AF couplingon tetrahedral architectures

corner-sharing tetrahedra:pyrochlore lattice A2B2O7

edge-sharing tetrahedra:FCC lattice

10Geometric FrustrationWhat happens in frustrated systems?Huge degeneracy of ground states!

Sometimes magnetic LRO at sufficiently low T 100)32Neutron Scattering Studies of Ba2YMoO6Ba2YMoO6: Mo5+ 4d1

Maintains ideal cubic structure; CW = -219K but no order found down to 2K: f > 100!

XRDT = 297Kl = 1.33 A

Susceptibility

Neutron diffractionT. Aharen et al. PRB 201033Neutron Scattering Studies of Ba2YMoO6Heat capacity shows a broad peak

And NMR shows two signals,one showing the developmentof a gap at low temperatures

But mSR shows nothing.

T. Aharen et al. PRB 20103434Neutron Scattering Studies of Ba2YMoO6Resolution comes from inelastic neutron scattering.

Whats happening? At low temps, neighboring moments pair up, to form singlets.

But no long range order!

SEQUOIA BeamlineSpallation Neutron SourceOak Ridge National LaboratoryJ. P. Carlo et al, PRB 20113535Neutron Scattering Studies of Ba2YRuO6Ba2YRuO6:Ru5+4d3

Much more conventional behavior?

Heat capacity

qW = -571K

T. Aharen et al. PRB 200936Neutron Scattering Studies of Ba2YRuO6Clear signs of antiferromagnetic order, but with f ~ 11-15.

[100] magnetic Bragg peakJ. P. Carlo et al. PRB 2013.37Neutron Scattering Studies of Ba2YRuO6But the inelastic scattering dependence is much more exotic!

J. P. Carlo et al. PRB 2013.38Neutron Scattering Studies of Ba2YRuO6The ordered state is associated with a gap.Interesting: Egap kBTorderBut why should such a gap exist?Suggestive of exotic physics: relativistic spin-orbit coupling!

J. P. Carlo et al. PRB 2013.39Muon Spin Relaxation studies of Ba2CaOsO6 + Ba2YReO6Ba2YReO6 ~ Re5+, 5d2 ~spin glass ~ 50KBa2CaOsO6 ~Os6+, 5d2 transition @50K, but is it similar to Ba2YReO6?Isoelectronic, isostructural, similar S-O coupling?

C. M. Thompsonet al. AcceptedTo JPCM (2014).40mSR measurements of Ba2CaOsO6mSR, TRIUMF (Vancouver, BC)

Muon spin precession