Joydipto Bhattacharya

We present a comprehensive first-principles study of the structural, electronic, and magnetic properties of the multiferroic Aurivillius phase Bi5Ti3MnO15. Among ten symmetry-inequivalent configurations arising from distinct distributions of Mn3+ across the perovskite B sites, we find a clear energetic preference for inner-site occupancy, consistent with recent experimental observations in related Aurivillius systems. A detailed investigation of the magnetic ground states of these symmetrically inequivalent configurations shows strong ferromagnetic coupling between nearestneighbor Mn3+ cations, driven by superexchange interactions, while interactions between more distant neighbors are significantly weaker. Using Heisenberg exchange parameters extracted from Green’s function calculations, we estimate a Curie temperature for the energetically most stable structure of approximately 300 K within the mean-field approximation. We further demonstrate that the ferromagnetic ground state in these systems is governed by the splitting of Mn t2g and eg orbitals, which is modulated by the Jahn–Teller distortions of the MnO6 octahedra. Explicit inclusion of Hund’s coupling reveals its important role in determining orbital occupation within the Mn-3d orbital manifold, and influencing the competition between ferromagnetic and antiferromagnetic interactions. These results establish a unified microscopic picture linking Mn site preference, octahedral distortion, and intraorbital exchange to room-temperature ferromagnetism in Aurivillius multiferroics Bi5Ti3MnO15.

Structural phase transitions in organic inorganic metal halide perovskites are driven via the rearrangement of methylammonium cation and distortion in the PbX6 octahedra. Suppression of the structural phase transition in these systems is known to be seen with cation or anion tuning. Along with the compositional tuning, the behavior of strain present in the system can also lead to stabilization of single phase in these systems. In the present investigation, two different samples of CH3NH3PbBr3 perovskite were studied and it is observed that structural phase transition is absent for one of the samples while it is present in the other sample. The non-observance of structural phase transition and stabilization of single phase have been attributed to the perceived tensile strain in the system, contrary to the compressive strain observed in the system showing structural phase transitions. Extended x-ray absorption fine structure measurements revealed an increase in bond lengths in the sample where structural phase transitions are suppressed, compared to the sample exhibiting such transitions. This observation is further supported by theoretical calculations. The stabilization of the cubic phase can enhance device performance and increase overall environmental stability, making these systems more effective for practical applications.

Aurivillius phase materials are valuable ferroelectrics, renowned for their high Curie temperatures (>600 °C) and fatigue resistance, enabling their commercial application in low-power ferroelectric random-access memory (Fe-RAM). Despite their promising attributes, incorporating Aurivillius phases into miniaturized devices presents significant challenges, primarily because their polarization is largely confined to the in-plane direction. This presents a challenge for industry, as conventional vertical devices are designed to operate with out-of-plane (OOP) polarization, making integration with laterally polarized materials difficult. To address this, industrially relevant Direct Liquid Injection Chemical Vapor Deposition (DLI-CVD) processes were developed to enhance OOP polarization and vertical ferroelectric properties in odd-layered Bi4Ti3O12 thin films. By controlling supersaturation and oxygen partial pressure, the growth mechanism is directed to proceed through either two-dimensional nucleation and growth or dislocation-mediated spiral growth. Piezoresponse force microscopy and atomic resolution electron microscopy studies reveal that films exhibiting growth-spiral morphology show enhanced vertical ferroelectric properties. Notably, the minimum vertical switching voltage is reduced from ±20 V to ±5 V. The improvement is attributed to defect-mediated modifications of elastic strain energy, localized electron densities and local symmetry-lowering distortions within the crystal structure. These findings offer valuable insights into the tailored growth of Aurivillius phases, specifically designed for miniaturized, energy-efficient memory and neuromorphic computing applications.

Half-metallic Co-based full Heusler alloys have captured considerable attention of researchers in the realm of spintronic applications, owing to their remarkable characteristics such as exceptionally high spin polarization at the Fermi level, ultra-low Gilbert damping, and a high Curie temperature. In this comprehensive study, employing the density functional theory, we delve into the electronic stability and ballistic spin transport properties of a magnetic tunneling junction (MTJ) comprising a Co₂MnSb/HfIrSb interface. An in-depth investigation of k-dependent spin transmissions uncovers the occurrence of coherent tunneling for the Mn–Mn/Ir interface, particularly when a spacer layer beyond a certain thickness is employed. It has been found that the Co-terminated Co₂MnSb/HfIrSb interface shows perpendicular magnetic anisotropy, while those with Mn–Sb and Mn–Mn termination exhibit in-plane magnetic anisotropy. Furthermore, our spin-dependent transmission calculations demonstrate that the Mn–Mn/Ir interface manifests strain-sensitive transmission properties under both compressive and tensile strain and yields a remarkable three-fold increase in majority spin transmission under tensile strain conditions. We find a tunnel magnetoresistance of ∼500% under a bi-axial strain of −3%, beyond which the tunnel resistance is found to be theoretically infinite. These compelling outcomes place the Co₂MnSb/HfIrSb junction among the highly promising candidates for nanoscale spintronic devices, emphasizing the potential significance of the system in the advancement of the field.