The evolution of spin-and orbital-ordered states has been investigated for a series of insulating perovskites RMnO3 (R=La,Pr,Nd,...). RMnO3 with a large GdFeO3-type distortion is regarded as a frustrated spin system having ferromagnetic nearest-neighbor and antiferromagnetic (AF) nextnearest-neighbor (NNN) interactions within a MnO2 plane. The staggered orbital order associated with the GdFeO3-type distortion induces the anisotropic NNN interaction, and yields unique sinusoidal and up-up-down-down AF ordered states in the distorted perovskites with e 1 g configuration. and e 1 g t 6 2g configurations, respectively, the e g orbital is doubly degenerate and the t 2g orbital degree of freedom is quenched. It is widely recognized that the layered-type (A-type) antiferromagnetic (AF) structure in LaMnO 3 is understood from the view point of the anisotropic superexchange (SE) interaction under the directional order of orbital [1,2]. On the other hand, the spin structure in nickelates (R =La) is distinct from the A-type AF; the socalled "up-up-down-down"-type one where two Ni sites of "up" spins are followed by two sites of "down" spins along the principal axes in the cubic unit cell. Origin of this unusual magnetic order has been a long-standing question, as well as its relations to metal-insulator transition, orbital order (OO), and charge disproportionation [3][4][5]. Recently, a similar spin structure, i.e. the up-up-down-down order in a MnO 2 plane (E-type AF order in the Wollan-Koehler notation [6]), is found in a manganite, HoMnO 3 [7] with a significantly distorted perovskite structure. This has to be a bridge between the well-understood A-type AF in manganites and the unique magnetic ground state in nickelates.In this Letter, we examine systematically the magnetic and orbital structures in a series of RMnO 3 as a function of the ionic radius (r R ) of R. The most significant effect on the crystal structure by decreasing r R is an enhancement of the cooperative rotation of the MnO 6 octahedra (the GdFeO 3 -type distortion) characterized by the decrease of Mn-O-Mn bond angle φ. Let us first summarize in Fig. 1 the orbital (a) and spin (b) ordering temperatures (T OO and T N , respectively) on Mn sites of RMnO 3 as a function of φ, which is based on both the present and former studies [7][8][9][10]. Here, we adopt the φ at room temperature [11]. The T OO monotonically increases with decreasing r R , while the magnetic transition occurs from the A-type AF to the E-type one through the incommensurate structure. We argue that the combination of OO and next-nearest-neighbor (NNN) SE interaction brings about a nontrivial effect on the magnetic ground state in the systems with the orbital degeneracy and the large GdFeO 3 -type distortion. Microscopic calculation shows that the magnetism in this system is mapped onto the frustrated spin model which well reproduces the phase diagram of RMnO 3 .A series of RMnO 3 (R=La−Dy) crystals were grown by the floating zone method. We made powder x-ray diffraction measurements on th...

A basic concept in solid-state physics is that when some kind of symmetry in a solid is spontaneously broken, collective excitations will arise. For example, phonons are the collective excitations corresponding to lattice vibrations in a crystal, and magnons correspond to spin waves in a magnetically ordered compound. Modulations in the relative shape of the electronic clouds in an orbitally ordered state could in principle give rise to orbital waves, or 'orbitons', but this type of elementary excitation has yet to be observed experimentally. Systems in which the electrons are strongly correlated-such as high-temperature superconductors and manganites exhibiting colossal magnetoresistivity-are promising candidates for supporting orbital waves, because they contain transition-metal ions in which the orbital degree of freedom is important. Orbitally ordered states have been found in several transition-metal compounds, and orbitons have been predicted theoretically for LaMnO3. Here we report experimental evidence for orbitons in LaMnO3, using Raman scattering measurements. We perform a model calculation of orbiton resonances which provides a good fit to the experimental data.

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