In the present work, the structure, magnetic, and dielectric characteristics of Nd2NiMnO6 and Sm2NiMnO6 have been investigated with comparison to those for La2NiMnO6. The magnetic and dielectric properties of the present ceramics vary with decreasing lanthanide ionic radius (RLn). Both Nd2NiMnO6 and Sm2NiMnO6 are ferromagnetism with the monoclinic symmetry (space group P21/n). The Curie temperature TC of Nd2NiMnO6 and Sm2NiMnO6 is 194 K and 156 K, respectively, and it decreases with decreasing 〈Ni-O-Mn〉 bond angle which depends on RLn. Meanwhile, the dielectric constant ε′ monotonically decreases with decreasing RLn. The similar variation tendency of magnetic and dielectric characteristics with RLn indicates that the structural origins for the magnetic and dielectric response should be closely linked with each other.

RFeO3 orthoferrites, where R is a rare-earth ion of the lanthanide series, are attracting attention mostly because of their promising fast spin dynamics. The magnetic properties of these materials seem to crucially depend on whether the magnetizations of the R and Fe ions’ weak ferromagnetic (WFM) components are parallel or antiparallel to each other. Here, we report an extensive investigation of a high-quality DyFeO3 single crystal in which the induced Dy3+ magnetization (FDy) has a natural tendency to be antiparallel to Fe3+ sublattice magnetization (FFe) within a large temperature window. Moreover, we find that specific variations of temperature and applied magnetic fields allow us to make FDy parallel to FFe, or force a spin-flip transition in FFe, among other effects. We found three different magnetic states that respond to temperature and magnetic fields, i.e. linear versus constant or, alternatively, presenting either behavior depending on the history of the sample. An original magnetic field-versus-temperature phase diagram is constructed to indicate the region of stability of the different magnetic phases, and to reveal the precise conditions yielding sudden spin switching and reversals. Knowledge of such a phase diagram is of potential importance to applications in spintronics and magnetic devices.

First-principles calculations are performed on magnetic multidomain structures in the SmFeO 3 rare-earth orthoferrite compound. We focus on the magnetic symmetry breaking at (001)-oriented anti-phase domain walls, treating magnetism in the simplest (collinear) approximation without any relativistic (spin-orbit coupling) effects. We found that the number of FeO 2 layers inside the domains determines the electrical nature of the whole system: multidomains with odd mumber of layers are paraelectric, while multidomains with even number of layers possess an electric polarization aligned along b-axis and a resulting multiferroic Pmc2 1 ground state. Our ab initio data and model for ferroelectricity induced by spin order reveal that this polarization is of the improper type, and originates from an exchange striction mechanism that drives a polar displacement of the oxygen ions located at the magnetic domain walls. Additional calculations ratify that this effect is general among magnetic perovskites with an orthorhombic SmFeO 3-like structure.

Collective rotations and tilts of oxygen polyhedra play a crucial role in the physical properties of complex oxides such as magnetism and conductivity. Such rotations can be tuned by preparing thin films in which dimensionality, strain, and interface effects come into play. However, little is known of the tilt and rotational distortions in films a few unit cells thick including the question of if coherent tilt patterns survive at all in this ultrathin limit. Here, a series of films of perovskite LaNiO3 is studied and it is shown that the phonon mode related to oxygen octahedral tilts can be followed by Raman spectroscopy down to a film thickness of three pseudocubic perovskite unit cells (∼1.2 nm). To push the limits of resolution to the ultrathin regime, a statistical analysis method is introduced to separate the Raman signals of the film and substrate. Most interestingly, these analyses reveal a pronounced hardening of the tilt vibrational mode in the thinnest films. A comparison between the experimental results, first principles simulations of the atomic structure, and the standing wave model, which accounts for size effects on the phononic properties, reveals that in the ultrathin regime, the Raman spectra are a hybrid entity of both the bulk and surface phononic behavior. These results showcase Raman spectroscopy as a powerful tool to probe the behavior of perovskite films down to the ultrathin limit.