Session F53: Emergent Properties of Complex Materials Bulk II

Emergent helical texture of electric dipoles

Long-range ordering of magnetic dipoles in bulk materials gives rise to a broad range of magnetic structures, from simple collinear ferromagnets to complex magnetic helicoidal textures stabilized by competing exchange interactions. In contrast, dipolar order in dielectric crystals is typically limited to parallel (ferroelectric) and antiparallel (antiferroelectric) alignments of electric dipoles. In this talk I will report our recent discovery of an emergent, incommensurate helical ordering of electric dipoles by light hole doping of the quadruple perovskite BiMn₇O₁₂. I will show, in close analogy with magnetism, that the electric dipole helicoidal texture is stabilized by competing structural instabilities. Specifically, orbital ordering and lone electron pair stereochemical activity compete, giving rise to phase transitions from a nonchiral cubic structure to an incommensurate electric dipole and orbital helix via an intermediate dipole density wave. This new ground state also supports weak improper ferroelectricity, implying that an electric field might be used to switch the chirality of the electric dipole helix. This remarkable functionality could be utilised, for example, in an electric-field-controllable, optically active device.   

Multiferroic properties and metal-to-insulator transition in hexagonal oxides

ABO₃-type oxides that include the rare-earth ferrites and manganites (RFeO₃, RMnO₃ with R=Lu, Y, etc.) and crystallize in a hexagonal structure with layered trigonal bipyramid BO₅ polyhedrons are often improper ferroelectrics with unique magnetic properties and also exhibit linear magnetoelectricity. In this work, we have investigated the effects of electron doping on LuFeO₃ to stabilize ferro- and ferri-magnetic states similar to that observed in (LuFeO₃)_m/(LuFe₂O₄)₁ superlattices [Nature 537.7621 (2016): 523-527]. We then explicitly achieve these magnetic states with real chemical alloying. Our results indicate that the doping can lead to two magnetic sublattices with spin differentiation in the Fe cations, which breaks the symmetry and alters the 120^o canting of the Fe spins observed in the parent compound. Based on the presence of polar distortions in the structure, electric-field-control of magnetic differentiation, ordering and metal-to-insulator transition may be possible in these systems, which can lead to novel applications.   

Lattice flexibility in Ca3Ru2O7: Control of electrical transport via anisotropic magnetostriction

Ca₃Ru₂O₇ is a correlated and spin-orbit coupled system with an extraordinary anisotropy. It is interesting largely because it exhibits conflicting phenomena that are often utterly inconsistent with traditional precedents, particularly, the quantum oscillations in the nonmetallic state and colossal magnetoresistivity achieved by avoiding a fully spin-polarized state. This work focuses on the relationship between the lattice and transport properties along each crystalline axis and reveals that application of magnetic field along different crystalline axes readily stretches or shrinks the lattice in a uniaxial manner, resulting in distinct electronic states. Furthermore, application of modest pressure drastically amplifies the anisotropic magnetoelastic effect, leading to either an occurrence of a robust metallic state at H||hard axis or a reentrance of the nonmetallic state at H||easy axis. Ca₃Ru₂O₇ presents a rare lattice-dependent magnetotransport mechanism, in which the extraordinary lattice flexibility enables an exquisite control of the electronic state, and the spin polarization plays an unconventional role unfavorable for maximizing conductivity. At the heart of the intriguing physics is the anisotropic magnetostriction that leads to exotic states.   

Magnetoresistance anomaly in a resistive switching system

Application of a strong electric stimulus, voltage or current, to the ferromagnetic oxide (La,Sr)MnO₃ (LSMO) triggers the intrinsic metal-insulator transition producing a volatile switching from a low- to high-resistance state. This resistive switching occurs in a characteristic spatial pattern, the formation of a paramagnetic insulating barrier perpendicular to the current flow, in contrast to the conventional filamentary percolation parallel to the current. We explored the evolution of anisotropic and colossal magnetoresistance in LSMO devices as they undergo resistive switching. We found that the magnetoresistance magnitude can be increased severalfold by initiating the switching and inducing the formation of a paramagnetic barrier inside the device. Moreover, by driving the LSMO device through the resistive switching the sign of magnetoresistance can be flipped from positive to negative and vice versa. Our results demonstrate the potential use of resistive switching in magnetic materials for novel spintronic applications.   

Phase transitions of the ferroelectric (ND4)2FeCl5D2O under a magnetic field

Due to the strong coupling between magnetism and ferroelectricity, (ND₄)₂FeCl₅ .D₂O exhibits several intriguing magnetic and electric phases.  To explain the ferroelectric (FE) phase transitions of (ND₄)₂FeCl₅.D₂O in a magnetic field and to produce the large weights of high-order harmonic components in the cycloid structure that are observed from neutron diffraction experiments, we include higher-order onsite anisotropic spin interactions.  Our theory predicts a new ferroelectric phase sandwiched between the observed FE II and FE III phases in a magnetic field.  This phase may appear with doping.  By emphasizing the importance of the higher-order spin anisotropic interactions, our work provides a framework to understand multiferroic materials with rich phase diagrams.   

Emergent spin-glass behavior created by chemical doping and self-assembly in transition-metal oxides

Spin glass is a magnetic state with a spin structure incommensurate with lattice and charge. A fundamental understanding of its behavior has a profound impact on many technological problems. Here, we present two novel cases with (1) chemical doping in layered Sr₃(Ru_1-xMn_x)₂O₇ and (2) self-assembly of single-crystalline NiO micro-columns in a single-crystalline NiFe₂O₄ matrix. In addition to the memory effect reflected in the magnetic susceptibility, transmission electron microscopy provides information about dopant distribution and atomic arrangement at the interface. The atomic-level characterization enables us to elucidate the origin of spin-glass (SG) behavior in both systems, which results from spin disorder and frustration. Our results demonstrate effective ways to create novel spin states in transition-metal oxides.   

Magnetic phase diagram of Ba2FeSi2O7: a quasi-two-dimensional square lattice Heisenberg antiferromagnet with a strong uniaxial magnetic anisotropy.

We study the strongly anisotropic quasi-two-dimensional square lattice S = 2 quantum magnet Ba₂FeSi₂O₇ with magnetic field along the anisotropic axis. The compound shows a XY antiferromagnetic (XY AFM) order at T_N∼5.5K at zero field.  We demonstrate from magnetization, magnetocaloric measurement and generalized SU(5) mean field calculation at zero temperature that the XY AFM phase becomes unstable with increasing magnetic field due to level crossings of spin states, and a quantum paramagnetic state appears around 30 T. Another XY AFM state is observed upon further increasing field above 35 T before the magnetization saturates around 45 T. Ultimately, ferroelectricity is also observed in the antiferromagnetic phases by orbital hybridization. We map the temperature/magnetic field phase diagram that consists of two dome shaped AF and multiferroic phases that feature the magnetic Bose-Einstein condensates accompanying ferroelectricity.   

Lanthanum M-type hexaferrite: a magnet, a semiconductor, and a quantum information material

Iron-based hexaferrites are critical-element-free permanent magnet components of magnetic devices. Of particular interest is electron-doped M-type hexaferrite i.e., LaFe₁₂O₁₉ (LaM) in which extra electrons introduced by lanthanum substitution of barium/strontium play a key role in uplifting the magnetocrystalline anisotropy. We investigate the electronic structure of lanthanum hexaferrite using a density functional theory with localized charge density, which reproduces semiconducting behavior and identifies the origin of the very large magnetocrystalline anisotropy. Localized charge transfer from lanthanum to the iron at the crystal's 2a site produces a narrow 3d_z² valence band strongly locking the magnetization along the c axis. The calculated uniaxial magnetic anisotropy energies from fully self-consistent calculations are nearly double the single-shot values, and agree well with available experiments. The chemical similarity of lanthanum to other rare earths suggests that LaM can host other rare earths possessing nontrivial 4f electronic states for, e.g., microwave-optical quantum transduction.   

Dynamically disordered magnetic states at sub-K regime in systems with strong magnetic interactions

Despite high-temperature magnetic susceptibility of Y₂CuTiO₆ exhibiting a large Curie Weiss scale of ~134 K, AC magnetic susceptibility, µSR, and specific heat data show that magnetic ordering and spin freezing are absent down to 50 mK. Heat capacity and magnetization as a function of the applied magnetic field and the temperature exhibit scaling collapses that are reminiscent of the random singlet physics. The suppression of any ordering or freezing scale, if at all present, by more than three orders of magnitude compared to the Curie-Weiss scale, in conjunction with the scaling behaviors, indicates exciting possibilities such as disorder-stabilized long range quantum entangled ground states. Prompted by this, we have explored low temperature thermodynamic and dynamic experimental signatures of several other related systems but with large spins (S > 1/2) and interestingly found similar hugely frustrated magnetic states without ordering or freezing down to the sub-K temperature regime.

 

Work done in collaboration with M. Baenitz, P. J. Baker, S. Bhattacharjee, S. Chattopadhyay, K. Damle, D. Das, R. Das, H. Fabian, T. Herrmannsdoerfer, A. Hossain, L. Hubertus, D. C. Joshi, P. Keerthi S, S. Kundu, P. Mahadevan, A. V. Mahajan, R. Mathieu, J.-C. Orain, S. Pujari, and K. M. Ranjith.