Magnetic studies of two-dimensional frustrated systems
Background
The success in understanding the collective behavior of convetional magnets
lead in the past decade to an increased interest in systems that exhibit
a novel type of ordering as a consequence of ``frustration''. In particular,
geometrically frustrated systems (having as a basic building block triangles
of antiferromagnetic bonds) have been the object of extensive theoretical
studies. It was found that frustration gives rise to spectacular and often
unexpected behaviors at low temperatures due to the high degeneracy of
the ground state. Recently, more and more experimental realizations of
such geometrically frustrated systems have been achieved. 2D and 3D materials
have been assembled from triangles and tetrahedra such that neighbouring
geometrical units share either a common edge or a common corner. Such new
materials have stirred a new debate, whether frustration alone is strong
enough to destroy magnetic long range order.
Traditionally, spin glass behavior required both disorder and frustration.
It has been argued that frustration alone can lead to large ground-states
degeneracies, accompanied by extensive ground-state entropies, but cannot
produce a sufficiently ``rough'' free energy landscape necessary for the
development of glassiness. Since the effects of frustration are more pronounced
in lower dimensions, the best candidates for disorder-free glassiness are
two dimensional antiferromagnets on triangular or kagomé lattices.
Such systems have been intensely studied in recent years, glassiness being
found in kagomé systems as well as in other materials, all having
continuous spins and sharing the same basic building block - the triangular
plaquette - for their lattices. Each of these materials has its strengths
and weaknesses as examples of spin glasses.
Recent Advances
Together with our collaborator from the University of Tokyo, we recently
reported, to our knowledge, the first evidence for hybrid organic/inorganic
triangular quantum Heisenberg antiferromagnets with additional Dzyaloshinskii-Moriya
interaction, Cu2(OH)3(CmH2m+1COO),
m=7,9 and 11. Based on dc magnetization and magnetic irreversibility studies,
and on linear and nonlinear ac susceptibility data we showed that these
compounds have an unusual magnetic behavior, presenting, due to the additional
Dzyaloshinskii-Moriya interaction, neither the disordered resonant valence
bond nor the ordered noncollinear Neel ground state. Instead, we proposed
that the interplay between the Heisenberg antiferromagnetic exchange, causing
frustration, and the Dzyaloshinskii-Moriya interaction, leading to spin
canting, allow these systems to evolve, into a new, unusual state with
both canted antiferromagnetic and spin glass-like characteristics. [1,2,3].
Publications
M.A. Girtu, C.M. Wynn, W. Fujita, K. Awaga, and A.J. Epstein, Coexistence
of Glassiness and Canted Antiferromagnetism in Triangular Quantum
Heisenberg Antiferromagnets with Weak Dzyaloshinskii-Moriya Interaction,
Physical Review B, Rapid Communications 57, 11058-11061 (1998).
M.A. Girtu, C.M. Wynn, W. Fujita, K. Awaga, and A.J. Epstein, Canted
Antiferromagnetism and Spin Glasslike Behavior in a Family of 2D Organic/Inorganic
Nanocomposites, Journal of Applied Physics, 7378-7380,
(1998).
Created by Darren Gebler and Mihai Gîrtu. Maintained by John
Rohrbacher. Last updated 6/1/00.