Emergent magnetic state in (111)-oriented quasi-two-dimensional spinel oxides

We report on the emergent magnetic state of (111)-oriented CoCr2O4 ultrathin films sandwiched by Al2O3 in the quantum confined geometry. At the two-dimensional crossover, polarized neutron reflectometry reveals an anomalous enhancement of the total magnetization compared to the bulk value. Synchrotron x-ray magnetic circular dichroism (XMCD) demonstrates the appearance of long-range ferromagnetic ordering of spins on both Co and Cr sublattices. Brillouin function analyses further corroborates that the observed phenomena are due to the strongly altered magnetic frustration, manifested by the onset of a Yafet-Kittel type ordering as the new ground state in the ultrathin limit, which is unattainable in the bulk

Orthorhombic distortion drives orbital ordering in an antiferromagnetic 3d1d^1 Mott insulator

The orbital, which represents the shape of the electron cloud, very often strongly influences the manifestation of various exotic phenomena, e.g., magnetism, metal-insulator transition, colossal magnetoresistance, unconventional superconductivity etc. in solid-state systems. The observation of the antiferromagnetism in $RE$TiO$_3$ ($RE$=rare earth) series has been puzzling since the celebrated Kugel-Khomskii model of spin-orbital super exchange predicts ferromagnetism in an orbitally degenerate $d^1$ systems. Further, the existence of the orbitally ordered vs. orbital liquid phase in both antiferromagnetic and paramagnetic phase have been unsettled issues thus far. To address these long-standing questions, we investigate single crystalline film of PrTiO$_3$. Our synchrotron X-ray diffraction measurements confirm the retention of bulk-like orthorhombic ($D_{2h}$) symmetry in the thin film geometry. We observe similar X-ray linear dichroism signal in both paramagnetic and antiferromagnetic phase, which can be accounted by ferro orbital ordering (FOO). While the presence of $D_{2h}$ crystal field does not guarantee lifting of orbital degeneracy always, we find it to be strong enough in these rare-earth titanates, leading to the FOO state. Thus, our work demonstrates the orthorhombic distortion is the driving force for the orbital ordering of antiferromagnetic $RE$TiO$_3$.Comment: 37 pages and 17 figure

Glass-like ordering and spatial inhomogeneity of magnetic structure in Ba3FeRu2O9 : The role of Fe/Ru-site disorder

Several doped 6H hexagonal ruthenates, having the general formula Ba3MRu2O9, have been studied over a significant period of time in order to understand the unusual magnetism of ruthenium metal. However, among them, the M=Fe compound appears different since it is observed that unlike others, the 3d Fe ions and 4d Ru ions can easily exchange their crystallographic positions and as a result many possible magnetic interactions become realizable. The present study involving several experimental methods on this compound establish that the magnetic structure of Ba3FeRu2O9 is indeed very different from all other 6H ruthenates. Local structural study reveals that the possible Fe/Ru-site disorder further extends to create local chemical inhomogeneity, affecting the high temperature magnetism of this material. There is a gradual decrease of 57Fe M\"ossbauer spectral intensity with decreasing temperature (below 100 K), which reveals that there is a large spread in the magnetic ordering temperatures, corresponding to many spatially inhomogeneous regions. However, finally at about 25 K, the whole compound is found to take up a global glass-like magnetic ordering.Comment: 24 page, 7 figure

Electron correlation mediated site-selective charge compensation in polar/non-polar heterointerface

One of the boundary conditions of the classical electromagnetic theory demands continuous electric potential across any boundary, which may not be naturally satisfied in atomically engineered heterostructures. Such polarity mismatch in oxide heterointerfaces is compensated through some (electronic/chemical/structural) reconstructions, leading to a myriad of emergent phenomena. The question we are posing is whether conventional semiconductor band bending framework is sufficient to comprehend compensation mechanisms in oxide heterostructures since, unlike semiconductors, complex oxides host strong electron correlations whose effects are indispensable. To address this, we investigate the interface between a prototypical insulating double perovskite Nd$_2$NiMnO$_6$ and a wide-bandgap insulator SrTiO$_3$. This polar/non-polar interface offers a similar scenario as the famous LaAlO$_3$/SrTiO$_3$ system but with an exception - two transition metal sites with two individual correlated energy scales. By combining several experimental techniques and density functional theory, we establish a site-selective charge compensation process that occurs explicitly at the Mn site of the film, leaving the Ni sites inert. This surprising selectivity, which cannot be accounted by existing polar compensation mechanisms, is directly attributed to the TM cations' relative correlation energy scales. This discovery presents that site-specific charge compensation can be a designers tool for tailoring emergent phenomena in oxide heterostructures.Comment: 20 pages, 4 figures, 63 reference

Dependence of magnetism on GdFeO3 distortion in the t(2g) system ARuO(3) (A = Sr, Ca)

We have examined the stability of the ferromagnetic (FM) state in CaRuO3 and SrRuO3 as a function of the GdFeO3 distortion. Model calculations predict the dependence of the FM transition temperature (T-c) on the rotation angle theta to vary as cos(2)(2 theta) for e(g)-electron systems. However, here, we find an initial increase and then the expected decrease. Furthermore, a much faster decrease is found than predicted for e(g)-electron systems. Considering the specific case of CaRuO3, a larger deviation of the Ru-O-Ru angle from 180 degrees in CaRuO3 as compared to SrRuO3 should result in a more reduced bandwidth, thereby making the former more correlated. The absence of long-range magnetic order in the more correlated CaRuO3 is traced to the strong collapse of various exchange interaction strengths that arises primarily from the volume reduction and increased distortion of the RuO6 octahedra network that accompanies the presence of a smaller ion at the A site

Invalidity of a localized spin mechanism for SrRuO₃ and CaRuO₃

We have calculated the electronic structure for CaRuO₃ and SrRuO₃ within first principles electronic structure calculations. We find that the experimentally observed ferromagnetic state is favoured in the case of SrRuO₃, while our calculations also suggest that no state with long range magnetic order will be stabilised for CaRuO₃. Mapping the total energies for different magnetic configurations to an extended Heisenberg model, we find that a localized spin model is found to be invalid to describe the magnetic state of these systems