The S=2 anisotropic triangular lattice alpha-NaMnO2 is studied by neutron inelastic scattering. Antiferromagnetic order occurs at T< or =45 K with opening of a spin gap. The spectral weight of the magnetic dynamics above the gap (Delta approximately equal to 7.5 meV) has been analyzed by the single-mode approximation. Excellent agreement with the experiment is achieved when a dominant exchange interaction (|J|/k(B) approximately 73 K), along the monoclinic b axis and a sizable easy-axis magnetic anisotropy (|D|/k(B) approximately 3 K) are considered. Despite earlier suggestions for two-dimensional spin interactions, the dynamics illustrate strongly coupled antiferromagnetic S=2 chains and cancellation of the interchain exchange due to the lattice topology. alpha-NaMnO2 therefore represents a model system where the geometric frustration is resolved through the lowering of the dimensionality of the spin interactions.
Phase inhomogeneity of otherwise chemically homogenous electronic systems is an essential ingredient leading to fascinating functional properties, such as high-Tc superconductivity in cuprates, colossal magnetoresistance in manganites and giant electrostriction in relaxors. In these materials distinct phases compete and can coexist owing to intertwined ordered parameters. Charge degrees of freedom play a fundamental role, although phase-separated ground states have been envisioned theoretically also for pure spin systems with geometrical frustration that serves as a source of phase competition. Here we report a paradigmatic magnetostructurally inhomogenous ground state of the geometrically frustrated α-NaMnO2 that stems from the system’s aspiration to remove magnetic degeneracy and is possible only due to the existence of near-degenerate crystal structures. Synchrotron X-ray diffraction, nuclear magnetic resonance and muon spin relaxation show that the spin configuration of a monoclinic phase is disrupted by magnetically short-range-ordered nanoscale triclinic regions, thus revealing a novel complex state of matter.
Inhomogeneity in the ground state is an intriguing, emergent phenomenon in magnetism. Recently, it has been observed in the magnetostructural channel of the geometrically frustrated a-NaMnO 2 , for the first time in the absence of active charge degrees of freedom. Here we report an in-depth numerical and local-probe experimental study of the isostructural sister compound CuMnO 2 that emphasizes and provides an explanation for the crucial differences between the two systems. The experimentally verified, much more homogeneous, ground state of the stoichiometric CuMnO 2 is attributed to the reduced magnetoelastic competition between the counteracting magnetic-exchange and elastic-energy contributions. The comparison of the two systems additionally highlights the role of disorder and allows the understanding of the puzzling phenomenon of phase separation in uniform antiferromagnets.A lthough phase separation in a uniform system is a widespread phenomenon in diverse fields of matter 1-3 , ranging from biological systems 4-6 , to soft matter 7,8 , and strongly correlated electron systems 9-15 , in magnetism the microscopic pattering has been, until recently, almost exclusively limited to thin ferromagnetic (FM) films 16,17 . In this case, such a pattering is a trade-off between minimizing the exchange and the dipolar energies. It thus represents one possible manifestation of a general requirement of multiple competing phases that can lead to inhomogeneous states. Lately, it has become increasingly apparent that a similar competition between energetically nearly equivalent phases is also responsible for phase separation in geometrically frustrated spin systems 18-21 that are generically characterized by ground-state degeneracy 22 . However, the balance between the competing phases in these systems is generally much more delicate and, therefore, poorly understood.Recently, the spatially anisotropic triangular antiferromagnet a-NaMnO 2 , with dominant intrachain (J 1 ) and geometrically frustrated interchain (J 2 ) antiferromagnetic (AFM) exchange interactions (inset in Figure 1a), has been highlighted as a paradigm of a phase-separated ground state in the absence of active charge degrees of freedom 18 . Its AFM order that sets in below the Néel temperature T N 5 45 K is accompanied by a simultaneous structural deformation 23 . This was initially suggested as being a phase transition from the high-temperature monoclinic (C2/m) to the low-temperature triclinic (P 1) crystal structure 23 . However, more detailed, recent experiments have shown that the magnetic order fails to drive this improper ferroelastic transition to completion 18 . Instead, an intricate magnetostructurally inhomogeneous state on the nano-scale has been discovered below the T N . Such a state was suggested to be an unforeseen consequence of the subtle interplay between the geometrical frustration and the competing structural phases 18 .In order to fully understand this novel phenomenon, further theoretical studies and experimental investigations of rel...
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