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Register a new userInterplay between spin dynamics and crystal field in multiferroic compound HoMnO$_3$
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In the multiferroic hexagonal manganite HoMnO3, inelastic neutron scattering and synchrotron based THz spectroscopy have been used to investigate the spin waves associated to the Mn order together with Ho crystal field excitations. While the Mn order sets in first below 80 K, a spin reorientation occurs below 37 K, a rare feature in the rare earth manganites. We show that severalHo crystal field excitations are present in the same energy range as the magnons, and that they are all affected by the spin reorientation. Moreover, several anomalous features are observed in the excitations at low temperature. Our analysis and calculations for the Mn spin waves and Ho crystal field excitations support Mn-Ho coupling mechanisms as well as coupling to the lattice affecting the dynamics.
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We demonstrate a new approach for directly measuring the ultrafast energy transfer between elec- trons and magnons, enabling us to track spin dynamics in an antiferromagnet (AFM). In multiferroic HoMnO3, optical photoexcitation creates hot electrons, after which changes in the spin order are probed with a THz pulse tuned to a magnon resonance. This reveals a photoinduced transparency, which builds up over several picoseconds as the spins heat up due to energy transfer from hot elec- trons via phonons. This spin-lattice thermalization time is ?10 times faster than that of typical ferromagnetic (FM) manganites. We qualitatively explain the fundamental differences in spin-lattice thermalization between FM and AFM systems and apply a Boltzmann equation model for treating AFMs. Our work gives new insight into spin-lattice thermalization in AFMs and demonstrates a new approach for directly monitoring the ultrafast dynamics of spin order in these systems.
The Haldane spin-chain (S=1) insulating compound, Sm2BaNiO5, which has been proposed to order antiferromagnetically around (T_N=) 55 K, was investigated for its complex dielectric permittivity, magnetodielectric and pyrocurrent behavior as a function of temperature (T). In order to enable meaningful discussions, the results of ac and dc magnetizatioin and heat-capacity studies are also reported. We emphasize on the following findings: (i) There is a pyrocurrent peak near T_N, but it is shown not to arise from ferroelectricity, but possibly due to thermally stimulated depolarization current, unlike in many other members of this rare-earth series, in which case ferroelectric features were reported at or above T_N; (ii) however, the pyrocurrent measured in the presence of a bias electric field (after cooling in zero electric field) as well as dielectric constant reveal a weak peak with increasing T around 22 K - the temperature around which population of the exchange-split excited state of Kramers doublet has been known to occur. This finding suggests that this compound presents a novel situation in which multiferroicity is induced by an interplay between crystal-field effects and exchange interaction. No multiglass features could be observed down to 2 K unlike in many members of this family.
We report a comprehensive specific heat and inelastic neutron scattering study to explore the possible origin of multiferroicity in HoCrO$_3$. We have performed specific heat measurements in the temperature range 100 mK - 290 K and inelastic neutron scattering measurements were performed in the temperature range 1.5 - 200 K. From the specific heat data we determined hyperfine splitting at 22.5(2) $mu$eV and crystal field transitions at 1.379(5) meV, 10.37(4) meV, 15.49(9) meV and 23.44(9) meV, indicating the existence of strong hyperfine and crystal field interactions in HoCrO$_3$. Further, an effective hyperfine field is determined to be 600(3) T. The quasielastic scattering observed in the inelastic scattering data and a large linear term $gamma=6.3(8)$ mJmol$^{-1}$K$^{-2}$ in the specific heat is attributed to the presence of short range exchange interactions, which is understood to be contributing to the observed ferroelectricity. Further the nuclear and magnetic entropies were computed to be, $sim$$17.2$ Jmol$^{-1}$K$^{-1}$ and $sim$34 Jmol$^{-1}$K$^{-1}$, respectively. The entropy values are in excellent agreement with the limiting theoretical values. An anomaly is observed in peak position of the temperature dependent crystal field spectra around 60 K, at the same temperature an anomaly in the pyroelectric current is reported. From this we could elucidate a direct correlation between the crystal electric field excitations of Ho$^{3+}$ and ferroelectricity in HoCrO$_3$. Our present study along with recent reports confirm that HoCrO$_3$, and $R$CrO$_3$ ($R=$ Rare earth) in general, possess more than one driving force for the ferroelectricity and multiferroicity.
The spectroscopic modes of multiferroic BiFeO$_3$ provide detailed information about the very small anisotropy and Dzyaloshinskii-Moriya (DM) interactions responsible for the long-wavelength, distorted cycloid below $TN = 640$ K. A microscopic model that includes two DM interactions and easy-axis anisotropy predicts both the zero-field spectroscopic modes as well as their splitting and evolution in a magnetic field applied along a cubic axis. While only six modes are optically active in zero field, all modes at the cycloidal wavevector are activated by a magnetic field. The three magnetic domains of the cycloid are degenerate in zero field but one domain has lower energy than the other two in nonzero field. Measurements imply that the higher-energy domains are depopulated above about 6 T and have a maximum critical field of 16 T, below the critical field of 19 T for the lowest-energy domain. Despite the excellent agreement with the measured spectroscopic frequencies, some discrepancies with the measured spectroscopic intensities suggest that other weak interactions may be missing from the model.
We use resonant and non-resonant X-ray diffraction measurements in combination with first-principles electronic structure calculations and Monte Carlo simulations to study the relationship between crystal structure and multiferroic orders in the orthorhombic perovskite manganites, o-$R$MnO$_3$ ($R$ is a rare-earth cation or Y). In particular, we focus on how the internal lattice parameters (Mn-O bond lengths and Mn-O-Mn bond angles) evolve under chemical pressure and epitaxial strain, and the effect of these structural variations on the microscopic exchange interactions and long-range magnetic order. We show that chemical pressure and epitaxial strain are accommodated differently by the crystal lattice of o-$R$MnO$_3$, which is key for understanding the difference in magnetic properties between bulk samples and strained films. Finally, we discuss the effects of these differences in the magnetism on the electric polarization in o-$R$MnO$_3$.
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