Session K20: Focus Session: Multiferroics II - Hexagonal Systems

Spin-lattice coupling from first principles

The hexagonal manganites are a class of multiferroic materials that are simultaneously ferroelectric and antiferromagnetic, in which many physically interesting and potentially technologically relevant manifestations of spin-lattice coupling have been observed. Chromium spinels such as ZnCr$_2$O$_4$ and CdCr$_2$S$_4$ are antiferromagnetic and ferromagnetic insulators respectively, each displaying a differrent manifestation of a spin-lattice effect. In ZnCr$_2$O$_4$ a large magnetically induced phonon anisotropy has been observed while mode-dependant phonon anomalies have been measured in CdCr$_2$S$_4$. With the continuing advances in theoretical algorithms such as the LSDA+U method and in computational power it is now possible to study structurally and magnetically complex solids such as these using density-functional first-principles methods. Here, I describe a first-principles approach to study the influence of magnetic order on the phonons and dielectric properties of YMnO$_3$, ZnCr$_2$O$_4$, and CdCr$_2$S$_4$, our ongoing investigation of the coupling between the magnetic order and polarization in hexagonal manganites, and the search for ferroelectric behaviour in the simple ferromagnet CdCr$_2$S$_4$.   

Magnetic Phase Diagrams of Hexagonal ErMnO$_{3}$, TmMnO$_{3}$ and HoMnO$_{3}$

Multiferroic rare earth manganites have attracted special attention because of the coexistence of ferroelectric and magnetic orders resulting in higher-order magnetoelectric effects and complex phase diagrams at low temperature. We investigate the magnetic phase diagrams of RMnO$_{3}$ (R = Er, Tm, Ho) and show that all phase boundaries are well determined by sharp changes in their dielectric properties proving the existence of strong spin-lattice coupling. The stability range of the AFM order below the Neel temperature of RMnO$_{3}$ (R=Er, Tm) extends to far higher magnetic fields than previously assumed. For the case when R = Ho, the magnetic phase diagram was found to be extremely complex including new phases with yet unknown magnetic structures. Hence, we extend the phase diagram to higher external magnetic fields up to 14 Tesla for all of the three aforementioned compounds. The detection of these phase boundary lines is through anomalies of the dielectric constant, DC and AC magnetization.   

Molecular beam epitaxy of YMnO$_{3}$ on GaN (0001)

Ferroelectric oxide thin films on semiconductors have attracted considerable attention from the physics and engineering communities for their potential applications in nonvolatile memory, piezoelectric, microwave devices, etc. In this work, we describe the molecular beam epitaxial (MBE) growth of the ferroelectric oxide YMnO$_{3}$ on GaN. YMnO$_{3}$ is an obvious candidate of oxide films on GaN because they both have hexagonal lattice structure and the lattice constant of YMnO$_{3}$ is approximately 2 times that of GaN. YMnO$_{3}$ films are grown on GaN (0001)-on-sapphire templates using MBE. Y and Mn are evaporated using effusion cells. Reactive oxygen was generated by a RF plasma source. The structure of the films as characterized by in-situ RHEED, x-ray diffraction, and atomic force microscopy will be discussed.   

Dependence of the Properties of MBE-grown Multiferroic YMnO$_{3}$/GaN Heterostructures on the Growth Temperature and Post-growth Annealing Processes

We report on the molecular beam epitaxy (MBE) of multiferroic YMnO$_{3}$ on c-plane GaN. Study and understanding of these YMnO$_{3}$/GaN heterostructures are important for potential applications in multifunctional materials and structures. Atomic force microscopy revealed that the YMnO$_{3}$ films grown at different temperatures have significantly different morphologies. X-ray diffraction showed that there is a 30$^{o}$ rotation between the unit cells of YMnO$_{3}$ and GaN. Samples grown at the optimal growth temperature are ferroelectric at room temperature, with a large remnant polarization, and magnetic at low temperatures. The difference between magnetic field-cooled and zero-field-cooled behavior at low temperatures indicates the presence of antiferromagnetic frustration or ferromagnetic behavior. The effects of different growth temperatures and post-growth annealing will be discussed.   

Structural, ferroelectric, and magnetic properties of epitaxial YMnO$_{3}$ thin films grown on wurtzite-structure materials

Thin films of the multiferroic oxide YMnO$_{3}$ have been grown on single crystal substrates with the wurtzite crystal structure, including GaN and ZnO with different polarities. Films have also been grown on the wurtzite derivative 6H-SiC. X-ray diffraction reveals epitaxial, c-axis oriented growth on all of these crystals. The measured ferroelectric polarization is similar to that of single crystal YMnO$_{3}$, and magnetization measurements reveal frustrated antiferromagnetic ordering in the plane of the film. X-ray diffraction also reveals an epitaxial relationship that is associated with a very large ($\sim $10{\%}) in-plane strain for growth on GaN. This unusual observation is explained by first principles calculations of the strain and bonding energies.   

Doping effect on ferroelectric microstructure in YMnO$_{3}$.

YMnO$_{3}$ is one of the typical multiferroic materials showing the coexistence of the antiferromagnetic and ferroelcrric orders at low temperature. In this work, we have investigated doping effect on ferroelectric microstructure in YMnO$_{3}$ by both electron diffraction and real-space imaging techniques. We found that the large ferroelectric domains with several micron meters change into the ferroelectric microdomains with the size of 20-30nm by substituting Y$^{3+}$ ions for Ca$^{2+}$ and Zr$^{4+}$ ions. In addition, diffuse scatterings elongated along both the [110] and [001] directions were found, which were similar to those found in YMn$_{0.825}$Ti$_{0.175}$O$_{3}$ exhibiting the enhanced magnetocapacitance effect.[1] The lattice instability in the Mn triangle lattice induced by the partial substitution at Y$^{3+}$ site should have some influence on the coupling between magnetic and ferroelectric orders. \newline \newline [1] S.Mori et al., Phys.Rev.B (in press).   

Multiferroicity of RMn$_{1-x}$Ga$_{x}$O$_{3}$ (R = Ho, Y) and Ho$_{1-x}$Y$_{x}$MnO$_{3}$

RMn$_{1-x}$Ga$_{x}$O$_{3}$ (R = Ho or Y) and Ho$_{1-x}$Y$_{x}$MnO$_{3}$ single-crystals have been prepared. The experimental results revealed that the $c$ axis decreases with increasing temperature with a larger $\left| {\mbox{d}c/\mbox{d}T} \right|$ above $T_{C}$ than below it. This shows that the cooperative MnO$_{5}$ site rotations responsible for the ferrielectricity expend energy to induce the ferroic R$^{3+}$-ion displacements along the $c$ axis. Ga doping raises the ferrielectric Curie temperature $T_{C}$ and the Mn-spin reorientation temperature $T_{SR}$ while lowering $T_{N}$ of the Mn spins and the Ho magnetic ordering temperature $T_{2}$. The data show (i) an important coupling between the Mn$^{3+}$-ion and Ho$^{3+}$-ion spins; (ii) a $T_{SR}$ that is driven by the cooperative MnO$_{5}$ site rotation and R$^{3+}$-ion displacements that control the $c$ lattice parameter. Y doping favors the formation of $P6'_{3}$\textit{cm'} magnetic phase below $T_{N}$, and enhances the temperature region of $P6'_{3}$\textit{cm'} phase. Therefore, $T_{SR}$ for the transition from $P6'_{3}$\textit{c'm to P6'}$_{3}$\textit{cm'} phase increases with increasing $x$, but $T_{SR}$ disappears for $x \quad >$ 0.8 samples because the $P6'_{3}$\textit{cm'} phase already occupies the whole temperature region below $T_{N}$. The thermal conductivity data also support an enhanced spin-lattice interaction above $T_{N}$ in the geometrically frustrated (GF) Mn-spin system.   

Multiferroicity in the Mixture of Orthorhombic and Hexagonal RMnO$_3$ (R=rare earths)

Orthorhombic perovskite RMnO$_3$ (R = Tb, Dr) shows an incommensurate magnetic/lattice modulation below $\sim $40 K and a lock-in transition below $\sim $25 k. The system becomes ferroelectric at the lock-in transition. On the other hand, hexagonal RMnO$_3$ (R = Ho-Lu, Y) exhibits ferroelectricity with a much higher transition temperature ($\approx $1000 K) and a magnetic ordering transition at $\sim $100 k. We will report the results of our comprehensive study of what happens when these two types of ferroelectric 113 compounds are mixed.   

Magneto-thermal conductivity in multiferroic HoMnO$_3$

It is well known that multiferroic HoMnO$_3$ exhibits a rich phase H-T diagram. We have discovered that thermal conductivity of hexagonal HoMnO$_3$ shows an unconventional diversity of anomalies at low temperatures under external magnetic fields. In order to understand the origin of these anomalies, we have performed comprehensive characterization experiments on the multiferroic compound, including measurements of dielectric constant, DC and AC magnetic susceptibility, and polarization as functions of temperature and applied magnetic field.   

Magnetoelectric and magnetoelastic effects in a triangular lattice antiferromagnet CuFeO$_{2}$

Magnetoelectric and magnetoelastic effects related to a phase transition into noncollinear magnetic phase have been investigated for single crystals of CuFeO$_{2}$ with a frustrated triangular lattice. CuFeO$_{2}$ exhibits several long-wavelength magnetic structures related to the spin frustration, and it is found that finite electric polarization, namely inversion symmetry breaking, occurs with noncollinear but not at collinear magnetic phases. This result demonstrates that the noncollinear spin structure is a key role to induce electric polarization, and suggests that frustrated magnets which often favor noncollinear configurations can be plausible candidates for magnetoelectrics with strong magnetoelectric interaction.   

Landau Theory of the Magnetic Phases of Hexagonal Rare Earth Manganites

A group theoretical analysis is presented on the magnetic structure of rare earth atoms in the multiferroic hexagonal manganites RMnO$_{3}$ (R=Ho, Er, Tm, Yb, Sc, Y) which exhibit the coexistence of ferroelectricity (T$_{c }\sim $900 K) and antiferromagnetism (T$_{N }\sim $100 K). Using Landau theory, a phenomenological model of the free energy based on four one-dimensional magnetic order parameters has been developed. Coupling of the various order parameters leads to complex magnetic field -- temperature phase diagrams in qualitative agreement with experimental data. \newline \newline References: \newline [1] S. H. Curnoe and I. Munawar, Magnetic Phases of Rare Earth Hexagonal Manganites, Proceedings of The International Conference on Strongly Correlated Electron Systems SCES '05, July 26$^{th}$ - 30$^{th}$, 2005, Vienna, Austria, (accepted on July 29, 2005 to be published in Physica B)\newline [2] Manfred Fiebig. (2005) Revival of the magnetoelectric effect, Journal of Physics D: Applied Physics, 38, R123-R152