Neutron Scattering of Frustrated Antiferromagnets Satisfaction without LRO Paramagnetic phase Low Temperature phase Spin glass phase Long range order Spin Peierls like phase Conclusions Collin Broholm Hopkins University and NIST Center for Neutron Rese Supported by the NSF through DMR-9453362 and DMR-0074571 SrCr 9p Ga 12-9p O 19 KCr 3 (OD) 6 (SO 4 ) 2 ZnCr 2 O 4
Neutron Scattering of Frustrated Antiferromagnets. Collin Broholm Johns Hopkins University and NIST Center for Neutron Research. Satisfaction without LRO Paramagnetic phase Low Temperature phase Spin glass phase Long range order Spin Peierls like phase Conclusions . - PowerPoint PPT Presentation
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Neutron Scattering of Frustrated Antiferromagnets
Satisfaction without LRO Paramagnetic phase Low Temperature phase
Spin glass phase Long range order Spin Peierls like phase
Conclusions
Collin BroholmJohns Hopkins University and NIST Center for Neutron Research
Supported by the NSF through DMR-9453362 and DMR-0074571
SrCr9pGa12-9pO19
KCr3(OD)6(SO4)2 ZnCr2O4
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CollaboratorsS.-H. Lee NIST and University of MDM. Adams ISIS Facility, RALG. Aeppli NEC Research InstituteE. Bucher University of KonstanzC. J. Carlile ISIS Facility, RALS.-W. Cheong Bell Labs and Rutgers Univ.M. F. Collins McMaster UniversityR. W. Erwin NISTL. Heller McMaster UniversityB. Hessen Bell Labs/Shell T. J. L. Jones ISIS Facility, RAL T. H. Kim Rutgers UniversityC. Kloc Bell LabsN. Lacevic Johns Hopkins UniversityT. G. Perring ISIS Facility, RAL A. P. Ramirez Bell LabsW. Ratcliff III Rutgers University
A. Taylor ISIS Facility, RAL
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A simple frustrated magnet: La4Cu3MoO12
Masaki Azuma et al. (2000)
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Magnetization and order of composite trimer spins
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Theory of spins with AFM interactions on corner-sharing tetrahedra
SPIN TYPE SPINVALUE
LOW TPHASE
METHOD REFERENCE
Isotropic S=1/2 Spin Liquid Exact Diag. Canals and LacroixPRL'98
Isotropic S= Spin Liquid MC sim. Reimers PRB'92Moessner, ChalkerPRL'98
Anisotropic S= Neel order MC sim. Bramwell, Gingras,ReimersJ. Appl. Phys. '94
What is special about this lattice and this spin system?• Low coordination number• Triangular motif• Infinite set of mean field ground states with zero net spin on all tetrahedra• No barriers between mean field ground states• Q-space degeneracy for spin waves
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SrCr9pGa12-9pO19 : Kagome’ sandwich
Isolated spin dimer
Kagome’-Triangular-Kagome’ sandwich(111) slab of pyrochlore/spinel AFM A. P. Ramirez et al. PRL (1990)
A: Bragg peaksB: Spin wavesC: ResonanceD: Upper band
D
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Spectra at specific Q
Spin waves
Resonance
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Dispersion relation for resonance
ZnCr2O4 single crystals T=1.5 K
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Structure factor for resonance
Extended sharpstructures in reciprocal space
Extended sharpstructures in reciprocal space
Fluctuations satisfy local constraintsFluctuations satisfy local constraints
ZnCr2O4 T=1.4 K meV 6meV 3
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Comparing resonance to PM fluctuations
meV 6meV 3 meV 7.0
• Paramagnetic fluctuations and resonance satisfy same local constraints. • Transition pushes low energy fluctuations into a resonance w/o changing spatial correlations
15 K1.4 K
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Why does tetragonal strain encourage Neel order?
meV06.0d
d
meV04.0d
d
2
0
0||
r
JrJ
r
JrJ
a
ca
Edge sharing n-n exchange in ZnCr2O4 depends strongly on Cr-Cr distance, r :
Cr3+
O2-
AmeV40
d
d /r
JFrom series of Cr-compounds:
r
The effect for a single tetrahedron is to make 4 bonds more AFM and two bondsare less AFM. This relieves frustration!
Tetragonal dist.
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Magnetic order in ZnCr2O4
-Viewed along tetragonal c-axis
•tetrahedra have zero net moment => this is a mean field ground state for cubic ZnCr2O4
•Tetragonal distortion lowers energy of this state compared to other mean field ground states:
meV07.052
1|| JJH
MFS
•In a strongly correlated magnet this shift may yield
MFStNB HTk
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Analysis of magneto-elastic transition in ZnCr2O4
Free energy of the two phases are identical at TC
lHScTsH
ScTl
HsHF
0
From this we derive reduction of internal energy of spin system
meV/Cr21.0
meV/Cr04.02221116
3meV/Cr17.0
sH
acCal
H
ScT
T
F tet, F
cub
TCTetrag. AFM
Cubic paramagnet
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Direct measurement of confirms validity of
analysis sH
From first moment sum-rule for the dynamic spin correlation function we find
0
0
0
2
sin1
,1
2
3
QrQr
QSe
H s
When a single Heisenberg exchange interaction dominates. Inserting magneticscattering data acquired at 15 K and 1.7 K we get
meV)5(35.0 sH
S
S
cBS
H
JH
TkH
where S(Q,) changes
LRO develops froma strongly correlated state
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Analogies with Spin Peirls transition?
There are similarities as well as important distinctions!
Spin-Peirls
ZnCr2O4
Quantum critical above TC yes yesOrder suppressed to | T/CW| <<1 due to low D frustra-
tionChange of lattice symmetry at TC enableslower energy spin state
yes yes
Low energy magnetic spectral weight ispushed into resonance
yes yes
Order of phase transition second firstLow T phase isolated
singletNeelLRO
TC S is significant energy scale no yes
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Low connectivity and triangular motif yields cooperative paramagnet for|T/CW|<<1. The paramagnet consists of small spin clusters with
no net moment, which fluctuate at a rate of order kBT/ h.
Spinels can have entropy driven magneto-elastic transition to Neel order with spin-Peirls analogies.
The ordered phase has a spin-resonance, as expected for under-constrained and weakly connected systems.
Pyrochlore’s can have a soft mode transition to a spin-glass even when there is little or no quenched disorder.
Variations of sub-leading interactions in pyrochlore’s give different types of SRO in different compounds.
Lattice distortions may be a common route to relieving frustration and lowering the free energy of geometrically frustrated magnets.