Magnets without Direction

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Magnets without Direction
Collin Broholm Johns Hopkins University and NIST
Center for Neutron Research

• Introduction
• Moment Free Magnetism in one dimension
• Higher dimensional MFM
• Frustrated origins of MFM
• Interesting aspects of MFM
• Conclusions

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G. Aeppli P. Bischer Y. Chen J. F. DiTusa D. V.
Ferraris C. D. Frost T. Ito T. Lectka K. Oka
R. Paul D. H. Reich J. Rittner M. B. Stone H.
Takagi M. Treacy G. Xu H. Yardimci I. Zaliznyak
Acknowledgements
Acknowledgements
NIST Center for Neutron Research ISIS Facility,
Rutherford Appleton Laboratory National Science
Foundation DMR 0074571 Civilian Research and
Development Foundation
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Many electrons, few magnetic materials
Ti V Cr Mn Fe Co Ni Cu
Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm
U Np Pu Am Cm
Filled shell in solid
Partially filled shell in solid
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Magnetization of Solid with unfilled Shells
FeBr(C44H28N4) Dilute Fe in organic matrix
Susceptibility data for paramagnetic salt
Spin susceptibility diverges as T 0
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Coulomb Pauli Heisenberg
Coulomb interactions plus Pauli principle split
4-fold spin degeneracy
J
The level scheme is reproduced by Heisenberg
Exchange Hamiltonian
Triplet gnd. State J lt 0
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Interactions orient moments
Ferromagnetic EuO
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Unconventional magnetism in NENP

• Negative Curie Weiss temperature indicates AFM
  interactions
• No phase transition and

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A can of magnetic worms
Magnetic interactions link spins in chains
Ensemble of Quasi-one-dimensional Antiferromagnet
s chains
NENPNi(C2H8N2)2NO2ClO4
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When is the Neel state unstable?
1. Assume Neel order, derive spin wave dispersion
relation 2. Calculate the reduction in
staggered magnetization due to quantum
fluctuations 3. If then Neel
order is an inconsistent assumption
diverges if on planes
in Q-space
A putative D1 Neel state has soft planes and so
is unstable
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Moment Free Magnetism averts infrared catastrophe
Y2BaNiO5
D
Ajiro et al. (1989)
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Unconventional magnetism in PHCC

• Negative Curie Weiss temperature indicates AFM
  interactions
• No phase transition and

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Structure is consistent with spin chains
PHCC C4H12N2Cu2Cl6
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Magnetic Neutron Scattering
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SPINS cold neutron triple axis spectrometer at
NCNR
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Is PHCC quasi-one-dimensional?
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Unconventional magnetism in CuHpCl

• Negative QCW indicates AFM interactions
• No phase transition and
• Spin ladder model consistent with c(T)

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CuHpCl hydrogenous single crystals
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.But there is dispersion to ladder
Q // to chain
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Other means of destabilizing Neel order
Weak connectivity Order in one part of lattice
does not constrain surrounding spins
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A Frustrated Route to Moment Free Magnetism?
1. Assume Neel order, derive spin wave dispersion
relation 2. Calculate the reduction in
staggered magnetization due to quantum
fluctuations 3. If then Neel
order is an inconsistent assumption
diverges if on planes
in Q-space
Frustration can produce local soft modes that
destabilize Neel order
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Neutrons can reveal frustration
The first w -moment of scattering cross section
equals Fourier transform of bond energies
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Magnetic scattering from CuHpCl powder
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Neutrons reveal frustration in CuHpCl
Peak to plateau ratio
Mixed signs for bond energies
Frustration
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Structure of CuHpCl
CuHpCl is hydrogen bonded crystal of
Cu2(C5H12N2)2Cl4
Molecules possess approximate centro
symmetry Exchange interaction within molecule
Jlt1 meV
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Two lattices from H-bond exchange
b
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Building an enigma
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Detailed bond energy distribution
c
(101)
(001)
a
(100)
Point size First moment
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Frustrated three dimensional spin liquid
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Significance of findings so far

• Neutron scattering required to classify quantum
  spin liquids
• Systems thought to be one dimensional may
  represent a richer class of materials
• Experimental realizations of spin liquids were
  sought, not found, in symmetric frustrated
  magnets
• Spin liquids may be more abundant in complex
  geometrically frustrated lattices

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Field induced T0 phase transition
PHCC
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Reentrant behavior close to critical point
3 D long range order
Moment free
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Holes dressed by spin polarons
Ni
Ca2
Y3
O
Xu et al. Science (2000).
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Transport in Ca doped Y2BaNiO5
T. Ito et al. Submitted to PRL (2001)
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Holes in a quantum spin liquid

• Some organic materials can be doped and conduct
  in Field Effect Transistors Schon et al. Science
  (2000)
• Moment free organo-metallic systems should have
  interesting transport properties if they can
  conduct.

PHCC
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Conclusions

• Spin systems with a gap can be mistaken for being
  quasi-one-dimensional
• Two and three dimensional moment free magnetism
  found in PHCC and CuHpCl
• Neutron scattering reveals frustrated bonds in
  the corner-sharing triangular clusters of these
  materials
• Hypothesis Moment free magnetism may be a common
  state of interacting spin systems with triangular
  motif and weak connectivity
• Idea Novel strongly correlated transport may
  occur if the materials can be doped