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Register a new userSingle-axis dependent structural and multiferroic properties of orthorhombic RMnO$_3$ (R = Gd - Lu)
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Controlling material properties by modulating the crystalline structure has been attempted using various techniques, e.g., hydrostatic pressure, chemical pressure, and epitaxy. These techniques succeed to improve properties and achieve desired functionalities by changing the unit cell in all dimensions. In order to obtain a more detailed understanding on the relation between the crystal lattice and material properties, it is desirable to investigate the influence of a smaller number of parameters. Here, we utilize the combination of chemical pressure and epitaxy to modify a single lattice parameter of the multiferroic orthorhombic RMnO$_3$ (R = rare-earth, o-RMnO$_3$) system. By growing a series of o-RMnO$_3$ (R = Gd - Lu) films coherently on (010)-oriented YAlO$_3$ substrates, the influence of chemical pressure is reflected only along the $b$-axis. Thus, a series of o-RMnO$_3$ with $a$ ~ 5.18 {AA}, 5.77 {AA} < $b$ < 5.98 {AA}, and $c$ ~ 7.37 {AA} were obtained. Raman spectra analysis reveals that the change of the $b$-axis parameter induces a shift of the oxygen in the nominally fixed $ca$-plane. Their ferroelectric ground state is independent on the $b$-axis parameter showing polarization of ~ 1 $mu$C cm$^{-2}$ along the $a$-axis for the above-mentioned range, except for $b$ ~ 5.94 {AA} which corresponds to TbMnO$_3$ showing ~ 2 $mu$C cm$^{-2}$. This result implies that multiferroic order of o-RMnO$_3$ is almost robust against the $b$-axis parameter provided that the dimension of the $ca$-plane is fixed to 7.37 {AA} $times$ 5.18 {AA}.
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We report on optical studies of the thin films of multiferroic hexagonal (P.G. 6mm) rare-earth orthoferrites RFeO3 (R=Ho, Er, Lu) grown epitaxially on a (111)-surface of ZrO2(Y2O3) substrate. The optical absorption study in the range of 0.6-5.6 eV shows that the films are transparent below 1.9 eV; above this energy four broad intense absorption bands are distinguished. The absorption spectra are analyzed taking into account the unusual fivefold coordination of the Fe(3+) ion. Temperature dependence of the optical absorption at 4.9 eV shows anomaly at 124 K, which we attribute to magnetic ordering of iron sublattices.
The ground state properties of the pure perovskite compounds PrMnO$_3$ and NdMnO$_3$ were investigated by magnetization, magnetic AC susceptibility and specific heat measurements. A strongly anisotropic behavior has been detected for temperatures below the antiferromagnetic phase transition $T_N$~100K. The susceptibility and the weak spontaneous ferromagnetic moment appear to be different in both compounds due to different anisotropic rare earth contributions. The specific heat shows strong Schottky type contributions at low temperatures, which for NdMnO$_3$ strongly depend on the magnetic field. A spin reorientation phase transition (spin-flop type) induced by a magnetic field along b axis was observed in NdMnO$_3$ at H~110kOe and T=5K. All results can consistently be explained by anisotropic contributions of the rare earth ions: In PrMnO$_3$ the electronic ground state is determined by a low lying quasidoublet split by the crystal field ~19 K. In NdMnO$_3$ the Kramers doublet is split by an exchange Nd-Mn field (~20K).
Multiferroic properties of orthorhombic HoMnO3 (Pbnm space group) are significantly modified by epitaxial compressive strain along the a-axis. We are able to focus on the effect of strain solely along the a-axis by using an YAlO3 (010) substrate, which has only a small lattice mismatch with HoMnO3 along the other in-plane direction (the c-axis). Multiferroic properties of strained and relaxed HoMnO3 thin films are compared with those reported for bulk, and are found to differ widely. A relaxed film exhibits bulk-like properties such as a ferroelectric transition temperature of 25 K and an incommensurate antiferromagnetic order below 39 K, with an ordering wave vector of (0 qb 0) with qb ~ 0.41 at 10 K. A strained film becomes ferroelectric already at 37.5 K and has an incommensurate magnetic order with qb ~ 0.49 at 10 K.
Low-energy magnon excitations in multiferroic BiFeO$_3$ were measured in detail as a function of temperature around several Brillouin zone centers by inelastic neutron scattering experiments on single crystals. Unique features around 1 meV are directly associated with the interplay of the Dzyaloshinskii-Moriya interaction and a small single-ion anisotropy. The temperature dependence of these and the exchange interactions were determined by fitting the measured magnon dispersion with spin-wave calculations. The spectra best fits an easy-axis type magnetic anisotropy and the deduced exchange and anisotropy parameters enable us to determine the anharmonicity of the magnetic cycloid. We then draw a direct connection between the changes in the parameters of spin Hamiltonian with temperature and the physical properties and structural deformations of BiFeO$_3$.
Hexagonal LuFeO$_3$ has drawn a lot of research attention due to its contentious room-temperature multiferroicity. Due to the unstability of hexagonal phase in the bulk form, most experimental studies focused on LuFeO$_3$ thin films which can be stabilized by strain using proper substrates. Here we report on the hexagonal phase stabilization, magnetism, and magnetoelectric coupling of bulk LuFeO$_3$ by partial Sc-substitution of Lu. First, our first-principles calculations show that the hexagonal structure can be stabilized by partial Sc substitution, while the multiferroic properties including the noncollinear magnetic order and geometric ferroelectricity remain robustly unaffected. Therefore, Lu$_{1-x}$Sc$_x$FeO$_3$ can act as a platform to check the multiferroicity of LuFeO$_3$ and related materials in the bulk form. Second, the magnetic characterizations on bulk Lu$_{1-x}$Sc$_x$FeO$_3$ demonstrate a magnetic anomaly (probable antiferromagnetic ordering) above room temperature, $sim425-445$ K, followed by magnetic transitions in low temperatures ($sim167-172$ K). In addition, a magnetoelectric response is observed in the low temperature region. Our study provides useful information on the multiferroic physics of hexagonal $R$FeO$_3$ and related systems.
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