Layered Manganite Research Articles

Non-Volatile Bipolar TiN/LaMnO3/Pt Memristors with Optimized Performance

LaMnO3+δ (LMO) perovskite is a very interesting candidate for Valence Change Memories due to its flexible stoichiometry, accommodated through the Mn+3/Mn+4 equilibrium, at the origin of significant resistivity changes. Here, the successful combination of a LMO layer, with a top active TiN electrode and a bottom inert Pt electrode, is presented. The manganite layer is integrated on silicon-based substrates in the form of a polycrystalline film. By comparing the memristive behavior of these TiN/LMO/Pt devices with Au/LMO/Pt devices prepared on the same film, the essential role of the active oxygen electrode is put in evidence. TiN/LMO/Pt memristive devices show optimized performance, operating in both sweep and pulse mode, with the capability of cycling more than a hundred times and showing good retention. Furthermore, a simple phenomenological model describing the memristive behavior of the devices is also presented.

Superconductivity at Interfaces in Cuprate-Manganite Superlattices.

One of the unsolved problems for using high-Tc superconducting cuprates for spintronic applications are the short coherence lengths of Cooper pairs in oxides (a few Å), which requires atomically sharp and defect-free interfaces. This research demonstrates the presence of high-Tc superconducting La1.84 Sr0.16 CuO4 in direct proximity to SrLaMnO4 and provides evidence for the sharpness of interfaces between the cuprate and the manganite layers at the atomic scale. These findings shed light on the impact of the chemical potential at the interface of distinct materials on highly sensitive physical properties, such as superconductivity. Additionally, this results show the high stability of ultrathin layers from the same K2 NiF4 -type family, specifically one unit cell of Sr2- x Lax MnO4 and three unit cells of La1.84 Sr0.16 CuO4 . This work advances both the fundamental understanding of the proximity region between superconducting cuprates and manganite phases and the potential use of oxide-based materials in quantum computing.

Tunable resistive nature of LaMnO3 / Nd0.7Sr0.3MnO3 interfaces: Role of swift heavy ion irradiation

In the present communication, high crystalline quality LaMnO3/Nd0.7Sr0.3MnO3/SrTiO3 (LMO/NSMO/STO) structures were fabricated using low cost chemical solution deposition (CSD) technique to perform swift heavy ion (SHI) irradiation using 100 MeV O+7 ions. X–ray diffraction (XRD) θ–2θ, XRD φ–scan and XRD ω–scan results show a single phase nature having finite and ion fluence influenced strain state, four–fold lattice symmetry with epitaxial growth and an effective influence of ion fluence on the crystalline granular structure of LMO/NSMO/STO structure. Atomic force microscopy (AFM) images suggest an effect of ion fluence on the surface morphology through the creation of defects and local annealing effect. Zero field cooled (ZFC) protocol based LMO/NSMO interface resistivity suggests that transport nature of the LMO/NSMO interface of all studied LMO/NSMO/STO structures is strongly governed by lattice strain between LMO//NSMO layers and STO substrate, strain across LMO and NSMO manganite layers, crystalline granular structure, grain size, grain boundary density and grain boundary nature of the LMO and NSMO manganite thin layers. ZFC protocol followed by field cooled cooling (FCC) and field cooled warming (FCW) protocols for the realization of LMO/NSMO interface resistivity behaviors have been understood on the bases of freezing and trapping of tiny high temperature high resistive insulating clusters within the coexisting low temperature low resistive metallic phase fraction. Observed LMO/NSMO interface resistivity behaviors have been understood using the percolation model fits to the recorded experimental LMO/NSMO interface resistivity data in addition to irradiation influences. Obtained fitting parameters have been discussed in detail based on their variations with different employed resistivity measurement protocols for all pristine and irradiated LMO/NSMO/STO structures.

Investigation of hole-doping effect on structural, magnetic properties and magnetoresistance via Gd-site substitution by Pb in the layered manganite La0.1Gd0.2−xPbxCa1.2Sr0.6Mn2O7 (0 ≤ x ≤ 0.2)

Investigation of hole-doping effect on structural, magnetic properties and magnetoresistance via Gd-site substitution by Pb in the layered manganite La0.1Gd0.2−xPbxCa1.2Sr0.6Mn2O7 (0 ≤ x ≤ 0.2)

Disentangling the magneto-optic Kerr effect of manganite epitaxial heterostructures

The magneto-optic Kerr effect can probe the process of magnetization reversal in ferromagnetic thin films and can, thus, be used as an alternative to magnetometry. The Kerr effect is wavelength-dependent and the Kerr rotation can reverse sign, vanishing at particular wavelengths. We investigate the epitaxial heterostructures of ferromagnetic manganite, La0.7Sr0.3Mn0.9Ru0.1O3, by using the polar Kerr effect and magnetometry. The manganite layers are separated by or interfaced with a layer of nickelate, NdNiO3. The Kerr rotation hysteresis loops of trilayers, with two manganite layers of different thicknesses separated by a nickelate layer, have intriguing humplike features when measured with light of 400 nm wavelength. By investigating additional reference samples, we disentangle the contributions of the individual layers to the loops: we show that the humps originate from the opposite sense of the Kerr rotation of the two different ferromagnetic layers, combined with the additive behavior of the Kerr signal. The change of sign of the Kerr rotation for the thinner manganite layer is most likely caused by optical interference.

Epitaxial bilayer La0.7Sr0.3MnO3/Ba0.7Sr0.3TiO3 thin films obtained by polymer assisted deposition

In this work bilayer structures, composed of ferromagnetic manganite and ferroelectric titanate layers, were obtained by solution deposition technique. The first step in preparation of the bilayer thin films was deposition of manganite (LaMnO3 or La0.7Sr0.3MnO3) layer by water-based polymer assisted deposition (PAD). Polycrystalline structures were obtained when manganite films were deposited by spin coating on commercial Pt/TiO2/SiO2/Si type substrate, whereas epitaxial films were grown on a single crystal SrTiO3 with (001) orientation substrate. The second ferroelectric titanate (BaTiO3 or Ba0.7Sr0.3TiO3) layer was deposited by spin coating using sol-gel method. The obtained bilayer structures have thickness below 100 nm, and epitaxial growth of the Ba0.7Sr0.3TiO3 film on the La0.7Sr0.3MnO3 surface was obtained, due to the similar lattice parameters between these two crystal structures.

Oxygen controlled perpendicular magnetic anisotropy in LaCoO3−δ/La0.7Sr0.3MnO3/LaCoO3−δ heterostructures

Large data storage at reduced dimension is of a great deal of interest in spintronic devices in which magnetic oxide films lead for realizing the modulation of perpendicular magnetic anisotropy (PMA). Herein, we report the modulation of PMA in oxide heterostructures composed of one manganite layer sandwiched between two cobaltite layers, i.e., LaCoO3−δ/La0.7Sr0.3MnO3/LaCoO3−δ via oxygen vacancies. The PMA is originated by the orbital reconstruction and ferromagnetic exchange interaction at the interface between the CoO6 and MnO6 octahedra layers. The modulation ratio up to a significant value ∼200% of PMA can be realized through the phase transition of LaCoO3−δ layers between perovskite and brownmillerite via deprivation or replenishment of oxygen sites. The antiferromagnetic superexchange interaction at the interface between the oxygen-deficient CoO4 tetrahedra layer and the adjacent MnO6 octahedral layer with lattice distortion may suppress the magnetization and PMA. This work ensures promising outcomes in situ modulation of PMA in oxide films such as ferroelectric polarization or ionic liquid gating via the transport of oxygen vacancies.

Substrate orientation dependent characteristics of half-metallic and metallic superlattices [La0.7Sr0.3MnO3/LaNiO3]10

We report a detailed study on the orientation dependent growth characteristics, electronic structure, transport, magnetic, and vibrational excitations in atomically flat interfaces of [La0.7Sr0.3MnO3/LaNiO3]10 superlattices (SLs) coherently grown on (001/011/111)-SrTiO3 substrates by the pulsed laser deposition technique. X-ray reflectometry confirms the periodic superlattice stacks from the Kiessig interference fringes and well-defined even interfaces between the nickelate and manganite layers. A complex local atomic environment across the interfaces was noticed, yet trivalent La, divalent Sr, and mixed valent Ni2+/3+ and Mn3+/4+ electronic states prevail at the core level with enhanced relative intensity ratio of the Mn ions in the superlattices grown on (111) oriented SrTiO3 substrates as compared to those grown on (001) and (011) oriented SrTiO3. The temperature (5≤T≤300K) dependence of electrical resistivity ρ(T) analysis reveals 3D variable range hopping model [ρ(T)=ρ0exp⁡(T0/T)(1/4)] with large magnitude of hopping energies (≥40 meV) for the SL-111 system associated with the high energy gap developed by the accumulation of disorderness in the individual constituents of polar layers. Moreover, all SL systems exhibit reduced ferromagnetic ordering temperatures (67≤TC≤110K) with a low-temperature anomaly (11.4≤T∗≤22K) and a substantial enhancement in the effective exchange interaction (Jeff∼3.52meV) having altered ground state-spin configuration S∼1/2 different from S=3/2 of La0.75Sr0.25MnO3. Nevertheless, the SL-011 system exhibits large anisotropy field HK∼18kOe and cubic anisotropy constant K1∼9.3×103J/m3 in comparison to the other two orientations. The second order two-phonon interaction driven by the local polaronic distortion causes significant changes in the vibrational excitations of the investigated system. Nonetheless, most of the Raman modes follow the substrate-induced, highly oriented epitaxial growth pattern except for two modes ν4 (326cm−1) and ν8 (728cm−1), which slightly differ in the case of SL-111 superlattices.

Perpendicular Manganite Magnetic Tunnel Junctions Induced by Interfacial Coupling.

The half-metallic manganite oxide La2/3Sr1/3MnO3 (LSMO) has a very high spin polarization of ∼100%, making it ideal for ferromagnetic electrodes to realize tunneling magnetoresistance (TMR). Because of the in-plane magnetic anisotropy of the ferromagnetic LSMO electrode, which leads to the density limit of memory, realizing perpendicular tunneling in manganite-based magnetic tunnel junctions (MTJ) is critical for future applications. Here, we design and fabricate manganite-based MTJs composed of alternately stacked cobaltite and manganite layers that demonstrate strong perpendicular magnetic anisotropy (PMA) induced by interfacial coupling. Moreover, spin-dependent tunneling behaviors with an out-of-plane magnetic field were observed in the perpendicular MTJs. We found that the direct tunneling effect plays a dominant role in the low bias region during the transport behavior of devices, which is associated with thermionic emission of electrons or oxygen vacancies in the high bias region. Our works of realizing perpendicular tunneling in manganite-based MTJs lead to new approaches for designing and developing all-oxide spintronic devices.

Multi-mode excitation drives disorder during the ultrafast melting of a C4-symmetry-broken phase

Spontaneous C4-symmetry breaking phases are ubiquitous in layered quantum materials, and often compete with other phases such as superconductivity. Preferential suppression of the symmetry broken phases by light has been used to explain non-equilibrium light induced superconductivity, metallicity, and the creation of metastable states. Key to understanding how these phases emerge is understanding how C4 symmetry is restored. A leading approach is based on time-dependent Ginzburg-Landau theory, which explains the coherence response seen in many systems. However, we show that, for the case of the single layered manganite La0.5Sr1.5MnO4, the theory fails. Instead, we find an ultrafast inhomogeneous disordering transition in which the mean-field order parameter no longer reflects the atomic-scale state of the system. Our results suggest that disorder may be common to light-induced phase transitions, and methods beyond the mean-field are necessary for understanding and manipulating photoinduced phases.

Non-adiabatic small-polaron and 3D-Mott's variable range conductions above 100 K in Pb-substituted double-layered manganites LaSm0.4Ca1.6-xPbxMn2O7

In this study, the role of Pb substitution on the microstructural and magneto-transport properties of the double layered manganites LaSm0.4Ca1.6-xPbxMn2O7 (x = 0, 0.1 and 0.2), prepared by the solid-state reaction method, was investigated. On the samples, X-Ray Diffraction, Scanning Electron Microscopy and resistivity as a function of temperature measurements were performed. XRD measurements revealed that the samples crystallized in a tetragonal phase with an I4/mmm space group and that the lattice parameters increased with increasing Pb content. SEM measurements indicated the granular character and the average grain sizes ranged between 0.5 and 4 μm for all the samples. The temperature dependence of resistivity of all the samples shows insulating-like behavior for the studied range of temperature (T > 100 K). The maximum magnetoresistance (MR%) value was found to be 31.95% at 100 K for the sample with x = 0, while it was 11% and 8% for the samples with x = 0.1 and 0.2, respectively. The resistivity curves are found to be well described by the Adiabatic Small Polaron Model in the range and Mott's 3D Variable Range Hopping model in the range. The density of states near the Fermi level (DOS), mean hopping distance, and mean hopping energy values were computed in the range, and the role of Pb substitution on these parameters was discussed as well.

Magnetic field dependence of the copper charge density wave order in a YBa2Cu3O7/Nd0.65(Ca0.7Sr0.3)0.35MnO3 superlattice

For a ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}/{\mathrm{Nd}}_{0.65}{({\mathrm{Ca}}_{0.7}{\mathrm{Sr}}_{0.3})}_{0.35}{\mathrm{MnO}}_{3}$ (YBCO/NCSMO) superlattice, we studied with resonant elastic x-ray scattering (REXS) at the Cu ${L}_{3}$ edge how the copper sublattice charge density wave (Cu-CDW) order in YBCO is affected by a large magnetic field up to 6.9 T that weakens the CE-type antiferromagnetic (AF) and the charge/orbital (Mn-CO) orders of the manganite in favor of a ferromagnetic state. While a field of only 2 T induces a strong ferromagnetic moment in the manganite, we find that the Bragg peak of the Cu-CDW hardly changes up to 6 T. Moreover, as the magnetic field is further increased to 6.9 T, the Cu-CDW Bragg peak gets suddenly enhanced and broadened, whereas the ferromagnetic moment of the manganite is already saturated. The observed uncorrelated magnetic field dependence of the charge orders in the cuprate and manganite layers suggests that these orders are not directly coupled across the interface. We rather interpret our data in terms of an indirect coupling via the domain boundaries of the Mn-CO and the related disorder and lattice strain. This interpretation is supported by additional studies of the magnetoelectric response, which provide evidence for a crossover in the dynamics of the Mn-CO in the range between 6 and 7 T, from a low-field state with pinned domains to a high-field state with more mobile and flexible domain boundaries. We attribute the concomitant enhancement and broadening of the Cu-CDW Bragg peak to this crossover.

Semiconducting nature and magnetoresistance behaviour of ZnO / La0.3Ca0.7MnO3 / SrTiO3 heterostructures

In this report, ZnO /La0.3Ca0.7MnO3 /SrTiO3 (ZnO /LCMO /STO) heterostructures, with various thicknesses of LCMO, were grown successfully using chemical solution deposition (CSD) method. X–ray diffraction (XRD) measurement confirms the presence of LCMO and ZnO phases only without any other unwanted phases. To understand the charge conduction mechanisms across ZnO /LCMO interface as well as for LCMO manganite thin layer, temperature dependent resistivity measurements have been investigated under different applied magnetic fields. The measured ρ (T) data of all the heterostructures in current perpendicular to plane (CPP; for ZnO /LCMO interface studies) mode and current in plane (CIP; for LCMO thin layer studies) mode show the semiconducting behaviour throughout the temperature and magnetic field range studied. The semiconducting state of all the heterostructures have been studied using Mott type variable range hopping (VRH) mechanism to understand the field dependence of charge carrier localization length. Observed negative magnetoresistance (MR) has been discussed in detail with its dependence on applied magnetic field, temperature and manganite layer thickness.

Proximity-Induced Novel Ferromagnetism Accompanied with Resolute Metallicity in NdNiO3 Heterostructure.

Employing X‐ray magnetic circular dichroism (XMCD), angle‐resolved photoemission spectroscopy (ARPES), and momentum‐resolved density fluctuation (MRDF) theory, the magnetic and electronic properties of ultrathin NdNiO3 (NNO) film in proximity to ferromagnetic (FM) La0.67Sr0.33MnO3 (LSMO) layer are investigated. The experimental data shows the direct magnetic coupling between the nickelate film and the manganite layer which causes an unusual ferromagnetic (FM) phase in NNO. Moreover, it is shown the metal–insulator transition in the NNO layer, identified by an abrupt suppression of ARPES spectral weight near the Fermi level (E F), is absent. This observation suggests that the insulating AFM ground state is quenched in proximity to the FM layer. Combining the experimental data (XMCD and AREPS) with the momentum‐resolved density fluctuation calculation (MRDF) reveals a direct link between the MIT and the magnetic orders in NNO systems. This work demonstrates that the proximity layer order can be broadly used to modify physical properties and enrich the phase diagram of RENiO3 (RE = rare‐earth element).

Stress and disorder effect in the physical properties of artificially engineered multiferroic superlattices

La0·8Ba0·2MnO3, Ba0·25Sr0·75TiO3 and BaTiO3 superlattices were grown to study the influence of structural disorder on the physical properties of multiferroic multilayers. Controlling the lattice mismatch of the superlattices allowed growing structures with different growth mechanisms. The manganite layers in the samples were used as “sensor layers”, that respond to the structural changes in the superlattices, induced by changing the thickness and nature of the ferroelectric layers. Stress has a weak influence on the magnetic properties of these systems. Transport properties are characterized by a high temperature thermally activated regime and a low temperature variable hopping one. The strain and structural disorder in the samples increases the localization energy of the current carriers for both regimes. Important interface effects can be achieved controlling the strain and disorder in the interfaces, allowing tuning the metal-insulator transition temperature. These results help to further understand the role of interface effects in the development of manganite based ferromagnetic/ferroelectric multilayered systems.

Studies on Colossal Magnetoresistance Behaviour of Pr0.6Sr0.4MnO3/Pr0.5Ca0.5MnO3 Heterostructure Films

Heterostructure bilayer thin films of Pr0.6Sr0.4MnO3/Pr0.5Ca0.5MnO3 were prepared by pulsed laser deposition, and the structural, magnetic and magnetotransport properties were studied, and the results were compared to that of Pr0.6Sr0.4MnO3 and Pr0.5Ca0.5MnO3 single-layer thin films. Structural analysis of the heterostructure reveals c-axis orientation for both the manganite layers. The heterostructure exhibits a higher metal-insulator transition temperature and a larger colossal magnetoresistance near room temperature than that of the single-layer counterparts. The electrical conduction above the metal-insulator transition could be explained within the realm of the adiabatic small polaron hopping mechanism, with lesser activation energies compared to the single-layer films. Magnetization measurements portray the presence of an unsaturated symmetric hysteresis loop for the heterostructure with the Curie temperature of ~270 K corresponding to the ferromagnetic ordering emanating from Pr0.6Sr0.4MnO3. These results could be qualitatively understood considering the combined effect of strain and ferromagnetic proximity of Pr0.6Sr0.4MnO3 with charge-ordered Pr0.5Ca0.5MnO3 manganite.

Epitaxial integration of improper ferroelectric hexagonal YMnO3 thin films in heterostructures

We report on multiple fundamental qualitative improvements in the growth of improper ferroelectric hexagonal YMnO$_3$ (YMO) thin films and heterostructures by pulsed laser deposition (PLD). By a combination of pre-growth substrate annealing and low-energy-fluence PLD, we obtain a two-dimensional growth mode of the YMO films on yttria-stabilized zirconia (YSZ) with ultralow roughness and devoid of misoriented nanocrystalline inclusions. By inserting a sacrificial manganite layer capped with conducting indium-tin oxide between the substrate and the final film, the latter is grown in a fully lattice-relaxed mode and, thus, without any misfit dislocations while maintaining the extraordinary flatness of the films grown directly on pre-annealed YSZ. This provides a template for the fabrication of heterostructures based on hexagonal manganites as promising class of multiferroics with improper room-temperature ferroelectricity and the implementation of these into technologically relevant epitaxial metal|ferroelectric-type multilayers.

Variable supercells in layered bismuth manganite controlled by oxygen pressure

Recently, layered materials have been extensively studied and attracted unprecedented attention. Layered oxides with superior properties have potential applications for nanoscale devices. Here, we report variable supercells in layered bismuth manganite controlled by oxygen pressure and reveal the strong piezoelectric responses from the layered supercells with tunable physical properties, including an approximately three-times difference in the magnetization range and 10% difference in the optical transmittance. The tunability of the sublattice from Bi3O3 to Bi2O2 in supercells is realized by a precise oxygen pressure control of 10 mTorr and 100 mTorr, respectively. The results presented here show a tantalizing opportunity to discover emerging multifunctional oxides for future nanoelectronics.

Decoupling spin-orbital correlations in a layered manganite amidst ultrafast hybridized charge-transfer band excitation

In the mixed-valence manganites, a near-infrared laser typically melts the orbital and spin order simultaneously, corresponding to the photoinduced $d^{1}d^{0}$ $\xrightarrow{}$ $d^{0}d^{1}$ excitations in the Mott-Hubbard bands of manganese. Here, we use ultrafast methods -- both femtosecond resonant x-ray diffraction and optical reflectivity -- to demonstrate that the orbital response in the layered manganite Nd$_{1-x}$Sr$_{1+x}$MnO$_{4}$ ($\it{x}$ = 2/3) does not follow this scheme. At the photoexcitation saturation fluence, the orbital order is only diminished by a few percent in the transient state. Instead of the typical $d^{1}d^{0}$ $\xrightarrow{}$ $d^{0}d^{1}$ transition, a near-infrared pump in this compound promotes a fundamentally distinct mechanism of charge transfer, the $d^{0}$ $ \xrightarrow{}$ $d^{1}L$, where $\it{L}$ denotes a hole in the oxygen band. This novel finding may pave a new avenue for selectively manipulating specific types of order in complex materials of this class.

Elastic strain control of electronic structure, and magnetic properties of [Pr1−xCaxMnO3/SrTiO3]15 superlattices

We report the growth, electronic structure, and in-plane magnetic properties of pulsed laser deposition grown 2D superlattice structures [Pr0.7Ca0.3MnO3/SrTiO3]15 and [Pr0.5Ca0.5MnO3/SrTiO3]15 on (001) oriented SrTiO3 and LaAlO3 single crystal substrates. The x-ray reflectivity measurements reveal well-defined interfaces between the manganite and titanate layers along with the existence of Kiessig fringes, providing the evidence for the smooth periodic superlattice structure. The reciprocal space mapping provides signature of tetragonal distortion in all the superlattices. The electronic structure determined from the x-ray photoelectron spectroscopy reveals divalent Sr and Ca, tetravalent Ti, and mixed valent Mn with a pronounce shift of binding energy peaks toward the higher energy side in the superlattices grown on (001) oriented LaAlO3 as compared to those grown on SrTiO3. These superlattices exhibit highly anisotropic ferromagnetic character. We used the law of approach to saturation to determine the anisotropy field (HK) and cubic anisotropy constant (K1) for all the investigated superlattices. This analysis yields the highest HK∼9 kOe and K1∼8×105 erg/cc for the [Pr0.7Ca0.3MnO3/SrTiO3]15 superlattice system. Furthermore, significant enhancement of the overall magnetic moment and a decrease in TC (<100 K) was observed in the case of LaAlO3 grown superlattice, which indicates a substantial role of residual elastic strain on the magnetic ordering. Our results indicate that the strain induced elongation of MnO6 octahedra leads to finite possibility of non-orthogonal overlapping of orbitals in the presence of large crystal field splitting of eg levels, which, in turn, causes suppression of the ferromagnetic double exchange interaction.