Many ultrathin films of transition metal oxides exhibit nonmetallic behavior, in contrast to their metallic bulk counterpart, thus displaying a metal-insulator transition (MIT) as the film thickness is reduced. The nature of this MIT has been a long-standing issue in the epitaxial oxide research community. Here, we report the processing dependence of the critical thickness ($t_{\mathrm{c}}$) of MIT and the origin of the insulating phase in $\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{S}{\mathrm{r}}_{1/3}\mathrm{Mn}{\mathrm{O}}_3$ (LSMO) films. A $t_{\mathrm{c}}$ of 3 unit cells (u.c.) is achieved by minimizing oxygen vacancies under optimal growth conditions, diminishing the epitaxial strain with a tunable buffer layer and suppressing surface strain by film capping. The electrical transport measurements demonstrate that the nonmetallic behavior in LSMO thin films is an unavoidable result of localization initiated by inherent disorder but amplified by the reduction in dimensionality.

The signature of correlated electron materials (CEMs) is the coupling between spin, charge, orbital and lattice resulting in exotic functionality. This complexity is directly responsible for their tunability. We demonstrate here that the broken symmetry, through cubic to orthorhombic distortion in the lattice structure in a prototype manganite single crystal, La _0.69 Ca _0.31 MnO ₃ , leads to an anisotropic magneto-elastic response to an external field, and consequently to remarkable magneto-transport behavior. An anomalous anisotropic magnetoresistance (AMR) effect occurs close to the metal-insulator transition (MIT) in the system, showing a direct correlation with the anisotropic field-tuned MIT in the system and can be understood by means of a simple phenomenological model. A small crystalline anisotropy stimulates a “colossal” AMR near the MIT phase boundary of the system, thus revealing the intimate interplay between magneto- and electronic-crystalline couplings.

Epitaxial thin films of ordered double perovskite ${\text{La}}_2{\text{NiMnO}}_6$ have been studied by a combination of high-resolution scanning transmission electron microscopy, quantitative electron energy loss spectroscopy, x-ray absorption spectroscopy, and x-ray magnetic circular dichroism (XMCD) spectroscopy. Our results show the nominal oxidation states of Ni and Mn ions to be ${\text{Ni}}^{2+}$ and ${\text{Mn}}^{4+}$ thus the ferromagnetism in ground state is mainly due to ${\text{Ni}}^{2+}{\text{-O-Mn}}^{4+}$ superexchange interactions. In addition, short-range ferromagnetic correlations are observed above the Curie temperature $\left(T_C\sim 280\text{K}\right)$ from XMCD measurement, which are likely induced by antisite defects against long-range ordering of the Ni/Mn sublattice. The XMCD results also demonstrate that the ${\text{Ni}}^{2+}$ and ${\text{Mn}}^{4+}$ ions are ferromagnetically aligned but exhibit large differences in the spin and orbital contributions to their effective magnetic moments.

Thickness-driven phase transitions have been widely observed in many correlated transition metal oxides materials. One of the important topics is the thickness-driven metal to insulator transition in half-metal La_2/3Sr_1/3MnO₃ (LSMO) thin films, which has attracted great attention in the past few decades. In this article, we review research on the nature of the metal-to-insulator (MIT) transition in LSMO ultrathin films. We discuss in detail the proposed mechanisms, the progress made up to date, and the key issues existing in understanding the related MIT. We also discuss MIT in other correlated oxide materials as a comparison that also has some implications for understanding the origin of MIT.

Neutron scattering studies reveal an unusual $E$-type antiferromagnetic (AFM) structure in Mn-substituted single crystals of ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ ($x=0.16$). The material behaves as quasi-two-dimensional (2D) antiferromagnet with in-plane $\left(ab\right)$ long-range ordering and exhibits only single-bilayer (5–6 $\AA$) ferromagnetic correlations along the $c$ direction below $T_N=78$ K. However, the critical behavior of the staggered magnetization, the AFM order parameter, does not reflect the expected behavior of a 2D magnetic phase transition. Such an unusual magnetic structure deviates from what would be expected from the conventional spin-lattice coupling scenario in this class of materials.

Significance: Electric polarization due to polar structural distortion is a central characteristic of an ferroelectric (FE) material, which is normally unattainable in a metallic system because itinerant electrons screen the internal electric field. A possible exception for a polar metal is if the itinerant electrons are decoupled with FE distortion, as proposed by Anderson and Blount. Here, we reveal an unusual FE phase transition in recently discovered polar metal LiOsO ₃ , different from the typical transition in insulating FEs, with dramatic temperature-dependent electron−phonon coupling. Our results provide a look at models for FE transitions with the interplay between FE dipoles and itinerant electrons, suggesting that an improvement of the Anderson and Blount model is needed.