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The magnetic properties and the low-temperature magnetic structures of the orthorhombic perovskite HoMnO3 (space group Pnma) have been studied on polycrystalline samples by magnetization, specific heat, and neutron diffraction measurements. By cooling, HoMnO3 exhibits three singularities at T N = 41 K, T ≈ 26 K, and T ≈ 6.5 K, suggesting a rich magnetic phase diagram. The neutron diffraction data show that below T N = 41 K, the Mn3+ magnetic moments become ordered in an antiferromagnetic arrangement, adopting a modulated sinusoidal magnetic structure characterized by the wave vector k = (k x ,0,0) (k x = 0.40 at 41 K) and defined by the magnetic mode (C x ,0,0). When the temperature is decreased, the propagation vector varies and at T ≈ 29 K a transition to a commensurate magnetic structure defined by k = (1/2,0,0) takes place. Below T ≈ 22 K, a small ordered magnetic moment appears on the Ho3+ cations, strongly increasing below 9 K and reaching 7.3(1) μB at T = 1.8 K. The magnetic structure of the Ho3+ moments is defined by a (A x ,0,C z ) mode. The (H,T) phase diagram has been mapped out and the different magnetic structures interpreted on the basis of competing superexchange interactions.

We present a rationalization of the Raman spectra of orthorhombic and rhombohedral, stoichiometric and doped, manganese perovskites. In particular, we study RMnO 3 (RϭLa, Pr, Nd, Tb, Ho, Er, Y, and Ca͒ and the different phases of Ca-or Sr-doped RMnO 3 compounds as well as cation deficient RMnO 3 . The spectra of manganites can be understood as combinations of two kinds of spectra corresponding to two structural configurations of MnO 6 octahedra and independently of the average structure obtained by diffraction techniques. One type of spectra corresponds to the orthorhombic Pbnm space group for octahedra with cooperative or dynamic Jahn-Teller distortions, with stretching modes as the main features and whose frequencies correlate to Mn-O distances. The other spectrum is associated to regular but tilted octahedra whose modes can be described in the rhombohedral R3c structure, where only bending and tilt modes are observed. The main peaks of compounds with regular MnO 6 octahedra, such as CaMnO 3 , highly Ca-doped LaMnO 3 , or the metallic phases of Ca-or Sr-doped LaMnO 3 , are bending and tilt MnO 6 octahedra modes which correlate to R-O(1) bonds and Mn-O-Mn angles, respectively. In low and optimally doped manganites, the intensity and width of the broad bands are related to the amplitude of the dynamic fluctuations produced by polaron hopping in the paramagnetic insulating regime. The activation energy, which is proportional to the polaron binding energy, is the measure of this amplitude. This study permits to detect and confirm the coexistence, in several compounds, of a paramagnetic matrix with lattice polaron together with regions without dynamic or static octahedron distortions, identical to the ferromagnetic metallic phase. We show that Raman spectroscopy is an excellent tool to obtain information on the local structure of the different microphases or macrophases present simultaneously in many manganites.

The magnetic structure of the orthorhombic perovskite YMnO3 has been investigated. A study on a polycrystalline sample based on neutron diffraction data and magnetization measurements has shown that YMnO3 becomes magnetically ordered below TN = 42 K. In the space group Pnma, the sinusoidal magnetic structure is defined by a (Cx,0,0) mode and characterized by the propagation vector k = (kx,0,0). The kx-component increases from 0.420(4), immediately below the ordering temperature, to 0.435(2) at T = 28.7 K. Below 28 K the kx-component remains unchanged. The sinusoidal spin arrangement remains stable down to 1.7 K; at this temperature the amplitude of the sinusoid is Ak = 3.89(6) µB. YMnO3 is the most distorted perovskite of the RMnO3 series (R = rare earths); the observed sinusoidal magnetic structure is in contrast with those exhibited by the less-distorted members (i.e. LaMnO3), which are commensurate-type antiferromagnetic structures.

Hexagonal, nonperovskite HoMnO3 oxide, containing a triangular arrangement of Mn3+ cations, has been prepared in polycrystalline form by the thermal decomposition of metal citrates. The crystal structure has been refined from neutron powder diffraction data. Magnetic and specific-heat measurements anticipate a complex phase diagram:  HoMnO3 becomes magnetically ordered at T N ≈ 72 K, and another two magnetic transitions take place at lower temperatures. Neutron powder diffraction measurements demonstrate that, below the ordering temperature, the moments of the Mn3+ cations adopt a triangular spin arrangement, the magnetic moments lying in the basal plane and parallel to the [100] axis. At T = 44.6 K, the moments suddenly reorientate within the basal plane and become aligned perpendicularly to the initial direction. Below T = 25.4 K, an ordered magnetic moment is observed on the Ho atoms at the 4b sites of the crystal structure, whereas those of the 4a site remain in a paramagnetic state. The Ho atoms adopt an antiferromagnetic structure with the moments parallel to the c axis. At 1.7 K, the ordered moment on the Mn3+ cations is 3.05(2) μB, and that on the Ho3+ cations is 2.97(3) MB.

We search for general patterns that explain the low field magnetoresistance at low temperatures in the system A(2-x)A'xFeMoO6. The observed linear dependence of the low field magnetoresistance with the saturation magnetization for the series is related to the antisite disorder at the Fe and Mo sites. This is explained in terms of a spin dependent crossing of intragranular barriers originated from the presence of antiferromagnetic SrFeO3 patches that naturally develop when antisite disorder occurs in the double perovskite. The presence of a moderate level of antisite disorder is at the very root of low field magnetoresistance although effects such as disorder distribution, connectivity, or morphology add their contribution.

The title compounds (R = La, Pr, Nd, Sm, Eu, Tb, Ho, Er) have been prepared in polycrystalline form by a citrate technique and, excepting the Sm and Eu phases, structurally studied by high-resolution neutron powder diffraction. All the materials are isostructural (space group Pbam, Z = 4) and contain infinite chains of octahedra sharing edges, linked together by and units. The size of the three kinds of coordination polyhedron regularly decreases as R cations become smaller. A bond-valence study allowed us to detect the presence of important tensile and compressive stresses in the crystal structure of , which are progressively released along the series as the rare-earth size decreases. The magnetic properties strongly depend on the nature of R, going from the spin-glass behaviour observed at low temperature in to the field-induced transitions exhibited by . A cusp in the susceptibility curves suggests an antiferromagnetic ordering at low temperatures, which is masked in the compounds containing strongly paramagnetic rare earths (Tb, Ho, Er). At high temperatures the paramagnetic moments are consistent in all cases with the presence of high-spin and cations.