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Register a new userUnconventional spin fluctuations in the hexagonal antiferromagnet YMnO$_3$
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We used inelastic neutron scattering to show that well below its N{e}el temperature, $T_{rm N}$, the two-dimensional (2D) XY nearly-triangular antiferromagnet YMnO$_{3}$ has a prominent {it central peak} associated with 2D antiferromagnetic fluctuations with a characteristic life time of 0.55(5) ps, coexisting with the conventional long-lived spin-waves. Existence of the two time scales suggests competition between the N{e}el phase favored by weak interplane interactions, and the Kosterlitz-Thouless phase intrinsic to the 2D XY spin system.
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Hexagonal YMnO$_3$ is well known for the co-occurrence of ferroelectricity and antiferromagnetism at low temperatures. Using temperature-dependent spectroscopic ellipsometry at an $a$-plane oriented single crystal, we show how the dielectric function is affected by the magnetic order transition at the Neel temperature. We focus especially on the pronounced charge transfer transitions around (1.6-1.7)eV which are strongly connected to Mn 3$d$ electrons. If described with a Bose-Einstein model, the temperature dependency of their energy and broadening is characterized by effective phonon energies not larger than 8meV. We argue that this is a hint for the occurrence of a soft phonon mode related to the antiferromagnetic phase transition. This is observed in both tensor components of the dielectric function, parallel and perpendicular to the crystallographic $c$-axis. Furthermore, a suitable parametrization for the uniaxial dielectric function is presented for the NIR-VUV spectral range. The broad transitions at energies higher than a critical point-like bandgap do not show a clear temperature dependence. We also observe some weak discrete absorption features around the strong charge transfer transitions with energies matching well to low-temperature photoluminescence signals.
We report the magnetic susceptibility, specific heat and dielectric constant on high purity polycrystalline samples of three hexagonal manganites: YMnO_3, LuMnO_3 and ScMnO_3. These materials can exhibit a ferroelectric transition at very high temperatures (T_{FE} > 700K). At lower temperatures there is magnetic ordering of the frustrated Mn^{3+} spins (S=2) on a triangular Mn lattice (YMnO_3:T_N=71K; LuMnO$_3:T_N=90K and ScMnO_3:T_N=130K). The transition is characterized by a sharp kink in the magnetic susceptibility at T_N below which it continues to increase due to the frustration on the triangular lattice. The specific heat shows one clear continuous phase transition at T_N, which is independent of external magnetic field up to 9T with an entropy content as expected for Mn^{3+} ions. The temperature dependent dielectric constant displays a distinct anomaly at T_N.
Muon spin rotation (muSR) experiments reveal unconventional spin freezing and dynamics in the two-dimensional (2D) triangular lattice antiferromagnet NiGa2S4. Long-lived disordered Ni-spin freezing (correlation time > 10-6 s at 2 K) sets in below T_f = 8.5 +- 0.5 K with a mean-field-like temperature dependence. The observed exponential temperature dependence of the muon spin relaxation above T_f is strong evidence for 2D critical spin fluctuations. Slow Ni spin fluctuations coexist with quasistatic magnetism at low temperatures but are rapidly suppressed for fields > 10 mT, in marked contrast with the field-independent specific heat. The muSR and bulk susceptibility data indicate a well-defined 2D phase transition at T_f, below which NiGa2S4 is neither a conventional magnet nor a singlet spin liquid.
Improper ferroelectrics are described by two order parameters: a primary one, driving a transition to long-range distortive, magnetic or otherwise non-electric order, and the electric polarization, which is induced by the primary order parameter as a secondary, complementary effect. Using low-temperature scanning probe microscopy, we show that improper ferroelectric domains in YMnO$_3$ can be locally switched by electric field poling. However, subsequent temperature changes restore the as-grown domain structure as determined by the primary lattice distortion. The backswitching is explained by uncompensated bound charges occuring at the newly written domain walls due to the lack of mobile screening charges at low temperature. Thus, the polarization of improper ferroelectrics is in many ways subject to the same electrostatics as in their proper counterparts, yet complemented by additional functionalities arising from the primary order parameter. Tailoring the complex interplay between primary order parameter, polarization, and electrostatics is therefore likely to result in novel functionalities specific to improper ferroelectrics.
We report on multiple fundamental qualitative improvements in the growth of improper ferroelectric hexagonal YMnO$_3$ (YMO) thin films and heterostructures by pulsed laser deposition (PLD). By a combination of pre-growth substrate annealing and low-energy-fluence PLD, we obtain a two-dimensional growth mode of the YMO films on yttria-stabilized zirconia (YSZ) with ultralow roughness and devoid of misoriented nanocrystalline inclusions. By inserting a sacrificial manganite layer capped with conducting indium-tin oxide between the substrate and the final film, the latter is grown in a fully lattice-relaxed mode and, thus, without any misfit dislocations while maintaining the extraordinary flatness of the films grown directly on pre-annealed YSZ. This provides a template for the fabrication of heterostructures based on hexagonal manganites as promising class of multiferroics with improper room-temperature ferroelectricity and the implementation of these into technologically relevant epitaxial metal|ferroelectric-type multilayers.
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