Session L43: Manganites and Cobaltites

Study of correlated electron phase behavior in $\alpha$-Na$_{0.90}$MnO$_2$ single crystals

The layered oxide system, $\alpha$-Na$_x$MnO$_2$, has been studied extensively as a potential battery cathode material, and its structure is not only an attractive one for the battery community, but for the condensed matter physics community as well. It belongs to the class of AMO$_2$ (A = alkali metal, M = transition metal) materials, which have alternating MO$_2$ sheets and alkali ion sheets. The MO$_2$ sheets consist of MO$_6$ edge-sharing octahedra and the alkali ions can generally be intercalated or deintercalated without disrupting the M-O bonds, keeping the layered structure intact. A large Jahn-Teller distortion stabilizes ferro-orbital ordering, locking moments canted out of the $ab$-plane. When $x=1$, $S=2$ Mn$^{3+}$ moments form chains along the crystallographic $b$-axis where antiferromagnetic intrachain coupling ($J_1$) is strong, and interchain coupling ($J_2$) remains weak. Here we present our recent work exploring single crystals of the sodium deficient $x=0.90$ system. Discussion of $\alpha$-Na$_{0.90}$MnO$_2$ will cover the quasi-1D static and dynamic magnetic behavior observed via bulk probes and neutron scattering experiments.   

Modelling domain wall dynamics in hexagoinal manganites

We present model describing low-frequency dynamics of coupled trimerization and ferroelectric domain walls in hexagonal YMnO3. The model parameters are extracted from ab-initio calculations.   

Domain wall ordering and motion in Mn$_3$O$_4$

Mn$_3$O$_4$ is an orbitally ordered, magnetically frustrated spinel with strong spin-lattice coupling, which exhibits a series of low temperature magnetic and structural transitions. Transverse field $\mu$SR has shown that ordered and disordered volumes coexist within this material, while MFM measurements have further shown that the magnetic domain walls themselves order in specific crystallographic directions, with a typical length scale of 100's of nm. In order to directly study these phenomena, we have performed small angle neutron scattering (SANS) and ultra-small angle neutron scattering (USANS) measurements at both zero and applied magnetic field. We will present the results of these measurements and discuss what they show as far as the formation of domains, as well as the motion of the domain walls. We will also discuss the effects of internal disorder on the behavior of the material.   

Microscopic view of the reversibility of charge ordered insulating phase melting in manganites

In colossal magnetoresistive manganites, it has been well known that charge-ordered insulating (COI) phase can be melted by high magnetic field and transits into ferromagnetic metallic (FMM) phase. The reversibility of the field-driven COI to FMM phase transition, however, has not been well studied. In this work, we use magnetic force microscopy (MFM) mounted with both hard and soft magnetic tips to investigate the melting process of the COI phase in epitaxial La0.325Pr0.3Ca0.375MnO3 (LPCMO) thin films and its reappearance after the field is removed. At metal-insulator transition temperature or above, we observe that the COI melting is largely reversible indicating that the electronic phase separation state dominates. The melting of the COI phase becomes irreversible and the FM phase appears to be stable at low temperatures. The results of MFM images are in good agreement with the transport and magnetic measurements.   

Phase competition by design in R$_{0.5}$Ba$_{0.5}$MnO$_{3}$

Phase competition between distinct ground states can arise from interactions on similar energy scales between the spin, charge, lattice, and orbital degrees of freedom. This competition can result in large responses to external perturbations. For example, the colossal magnetoresistance effect in the rare-earth manganites R$_{1-x}$A$_x$MnO$_3$ arises out of competing ferromagnetic metallic and charge/orbital-ordered antiferromagnetic insulating states. Phase competition between polar and magnetic ground states is a promising strategy to realize polarization (magnetization) control with a magnetic (electric) field, which is major goal in multiferroics research. In this regard, the half-doped A-site ordered manganite Sm$_{0.5}$Ba$_{0.5}$MnO$_3$ is of particular interest, because the charge/orbital-ordered antiferromagnetic insulating state in this material is polar. We use a combination of group theoretic methods and first-principles calculations to elucidate the origin of this polar state, and show that epitaxial strain can tune the material to a regime where there is a strong competition between the polar insulating state and the ferromagnetic metallic state. We then explore how to achieve electric and magnetic field control of the order parameters in this system.   

Quasi-Particle Relaxation and Quantum Femtosecond Magnetism in Non-Equilibrium Phases of Insulating Manganites

We study the role of spin quantum fluctuations driven by photoelectrons during 100fs photo-excitation of colossal magneto-resistive manganites in anti-ferromagnetic (AFM) charge-ordered insulating states with Jahn-Teller distortions. Our mean-field calculation of composite fermion excitations demonstrates that spin fluctuations reduce the energy gap by quasi-instantaneously deforming the AFM background, thus opening a conductive electronic pathway via FM correlation. We obtain two quasi-particle bands with distinct spin-charge dynamics and dependence on lattice distortions. To connect with fs-resolved spectroscopy experiments, we note the emergence of fs magnetization in the low-temperature magneto-optical signal, with threshold dependence on laser intensity characteristic of a photo-induced phase transition. Simultaneously, the differential reflectivity shows bi-exponential relaxation, with fs component, small at low intensity, exceeding ps component above threshold for fs AFM-to-FM switching. This suggests the emergence of a non-equilibrium metallic FM phase prior to establishment of a new lattice structure, linked with quantum magnetism via spin/charge/lattice couplings for weak magnetic fields.   

Magnetic Anisotropy and Magnetic Phase Diagram in the Ferromagnetic States of La$_{\mathrm{1-}}_{x}$Sr$_{x}$MnO$_{\mathrm{3}}$ for 0.1 \textless $x$ \textless 0.2

Magnetic textures such as magnetic vortices, magnetic bubbles and magnetic stripe domains have been found in magnetic materials such as helical magnets without the inversion symmetry by neutron scattering and Lorentz microscopy (LM) observation. On the contrary, it has been reported that compounds having the inversion symmetry such as La$_{\mathrm{0.875}}$Sr$_{\mathrm{0.125}}$MnO$_{\mathrm{3}}$ exhibited magnetic textures in the ferromagnetic phase. Thus, we investigated formation processes of magnetic textures as functions of temperature and strength of magnetic fields by using LM and small-angle electron diffraction experiments. \textit{In-situ} LM observations revealed that magnetic bubbles are formed by applying magnetic field perpendicular to thin plate having periodic magnetic stripe domains in the ferromagnetic metallic phase of La$_{\mathrm{1-}}_{x}$Sr$_{x}$MnO$_{\mathrm{3\thinspace }}$for 0.1 \textless $x$ \textless 0.2. In addition, we constructed phase diagram of magnetic textures such as magnetic bubbles[1]. It is revealed that magnetic bubbles in La$_{\mathrm{1-}}_{x}$Sr$_{x}$MnO$_{\mathrm{3}}$ should formed by high magnetocrystalline anisotropy in the orthorhombic and monoclinic structures. [1]A. Kotani, \textit{et al.} \textit{Phys. Rev. B} \ 94, 02407 (2016).   

Emergent Phase Coherence of Charge Density Waves in Hole-Doped Manganites

Charge density waves (CDW) are prevalent electronic instabilities that compete, and even coexist, with high Tc superconductivity in cuprates or ferroic order in manganites. Mesoscopic probes including diffraction and transport have provided rich insights into average CDW correlations and their interactions with other electronic phases. However, atomic scale disorder and nanoscale inhomogeneity require real space, phase sensitive characterization of the CDW order parameter and its evolution across temperatures. Here we use scanning transmission electron microscopy (STEM) to map lattice displacements associated with CDWs in a holed-doped manganite. We directly reveal phase disorder in the complex order parameter at room temperature and characterize emergent phase coherence at cryogenic temperatures. Further, while the modulations appear incommensurate in globally averaged electron diffraction, we establish that PLDs are locally commensurate, albeit with temperature dependent order parameter variations. These results demonstrate that phase sensitive STEM of modulated lattices across temperatures permits direct visualization of the phenomenology and evolution of ordered states.   

Electronic origin of spin phonon coupling effect in transition-metal perovskite materials

Spin-phonon couping in transition metal ABO$_3$ perovskites can be identified by the softening of the low-lying phonon modes including the polar ones, when the spin configuration is changed from being antiferromagnetic (AFM) to being ferromagnetic (FM). We studied the spin-phonon coupling effect by computing the changes of the superexchange energies as functions of typical soft modes' amplitudes in SrMnO$_3$ as an example. The superexchange interactions are computed by a recently developed extended Kugel-Khomskii model based on maximally localized Wannier functions. The spin-phonon coupling effect in SrMnO$_3$ is generally attributed to the suppressed superexchange interaction by all the soft modes under investigation. However, the spin-phonon coupling strength varies significantly among all the different soft modes. The individual superexchange interaction involves the hopping process between the $d$-like state of neighboring metal ions with strong hybridized oxygen $p$ character. As a result, the phonon modes, such as Slater and antiferrodistortive mode which modify the relative position of metal ion and oxygen octahedral cage, are found to be more effectively important in spin-phonon coupling effect. Furthermore, the electronic configuration also plays a crucial role.   

Exotic magnetic structures in high-pressure synthesized perovskites.

We present a neutron powder diffraction study of the crystal and magnetic structures of the high-pressure stabilized perovskite phases of TlMnO3, ScCrO3, InCrO3 and TlCrO3. These compounds exhibit original magnetic structures compared to other members of their respective manganite and orthochromite families with TlMnO3 also displaying unusual orbital ordering pattern. For both systems, we rationalise the structures through a combination of group theory and first principle calculations. We also highlight the dominant mechanism controlling the spin direction as being the single ion anisotropy.   

Local electronic structure and ferromagnetic interaction in La(Co,Ni)O$_{3}$

Perovskite-related transition-metal oxides exhibit properties ranging from insulating to superconducting as well as unusual magnetic phases, and cobaltates, in particular, have been known for their propensity for spin-state transitions. Nonmagnetic LaCoO$_{3}$ and paramagnetic LaNiO$_{3}$ are parent compounds for the La(Co$_{1-x}$Ni$_{x})$O$_{3}$ (LCNO) family, which, for intermediate Ni content x, exhibits ferromagnetism. The local electronic structure and the ferromagnetic interaction in LCNO have been studied by x-ray absorption (XAS) and x-ray magnetic circular dichroism (XMCD). XAS indicates a mixed-valence state for both Co and Ni, with both valences changing systematically with increasing x. Simultaneously, a spin-state redistribution towards HS (Co site) and LS (Ni site) occurs, and temperature-dependent spin-state transitions are increasingly suppressed. XMCD identifies the element-specific contributions to the magnetic moment and interactions. A simple model based on a double-exchange-like mechanism between Co$^{3+}$ HS and Ni$^{3+\, }$HS can qualitatively account for the evolution of ferromagnetism in the LCNO series.   

Giant magnetovolume effect in a cubic perovskite Sr$_{1-x}$Ba$_x$CoO$_3$ with competing magnetic orders

Perovskite oxides with unusually high valence transition metal ions have been of great interest because of the unique magnetism inherent to the strong p-d hybridization. This is exemplified by the unusual helimagnetism in SrFeO$_3$ and room-temperature ferromagnetism in SrCoO$_3$. However, search for novel functions in these oxides has been hampered by the difficulty in growing crystals. In this work, we synthesized single crystals of a cubic perovskite Sr$_{1-x}$Ba$_x$CoO$_3$ by high-pressure and chemical oxidation technique to study the effect of controlling the p-d hybridization on magnetism. By magnetization measurements, we established the phase diagram [H. Sakai et al.], where a novel helimagnetic phase appears at around x = 0.35. To discuss this result from the viewpoint of the lattice change, we applied pressure on the heilmagnetic compound with x = 0.4 and found the helimagnetic to ferromagnetic transition. This novel pressure effect can be regarded as a giant magnetovolume effect, the origin of which will be discussed with showingthe first-principles calculations[S. Yokoyama et al.].   

Large magnetovolume effect induced by ferromagnetic-antiferromagnetic competition in a cobaltite perovskite

Materials that show negative thermal expansion (NTE) have significant industrial merit because they can be used to fabricate composites whose dimensions remain invariant upon heating. In some materials, NTE is concomitant with the spontaneous magnetization, known as the magnetovolume effect (MVE). Here we report a new class of MVE material; namely, a layered perovskite PrBaCo$_{2}$O$_{5.5+}_{x} $(0 $\le \quad x \le $ 0.41)$_{,\, }$in which strong NTE ($\beta \quad \approx $ -3.3$\times$ 10$^{-5}$ K$^{-1}$ at $x =$ 0.24) is triggered by embedding ferromagnetic (F) clusters into the antiferromagnetic (AF) matrix. The strongest MVE is found near the boundary between F and AF phases in the phase diagram, indicating the essential role of competing interaction between the F-clusters and the AF-matrix. Furthermore, the MVE is not limited to the PrBaCo$_{2}$O$_{5.5+}_{x}$ but is also observed in the NdBaCo$_{2}$O$_{5.5+}_{x}$. The present study provides a new approach to obtaining MVE and offers a path to the design of NTE materials.   

Electronic structure, interactions, and spin-orbit coupling at the CoO$_2$-terminated surfaces of delafossite oxide metals

The ABO$_2$ family of delafossite oxide metals has recently found renewed prominence due to their remarkable transport properties. The Pd- and Pt-based cobaltates are the most conductive oxides known, with room-temperature resistivities lower per carrier even than copper metal [1,2]. This is underpinned by extremely broad bandwidths of the bulk electronic structure around the Fermi level, dominated by Pd/Pt-derived carriers that behave remarkably like free electrons [2]. Here, we will discuss how the polar CoO$_2$-terminated surface of PtCoO$_2$ hosts a markedly different electronic structure to that of the bulk. From angle-resolved photoemission spectroscopy, we will demonstrate that this transition-metal oxide surface layer supports massive Co-derived surface states. We will show how these exhibit a pronounced interplay of many-body interactions and spin-orbit coupling, and will discuss how these together shape the low-energy surface electronic structure of this compound. [1] Hicks et al., Phys. Rev. Lett. 109 (2012) 116401; [2] Kushwaha et al., Science Adv. 1 (2015) e1500692.