Bilayer Manganite Research Articles

Low Temperature Dynamic Polaron Liquid in a Manganite Exhibiting Colossal Magnetoresistance.

Polarons-fermionic charge carriers bearing a strong companion lattice deformation-exhibit a natural tendency for self-localization due to the recursive interaction between electrons and the lattice. While polarons are ubiquitous in insulators, how they evolve in transitions to metallic and superconducting states in quantum materials remains an open question. Here, we use resonant inelastic x-ray scattering to track the electron-lattice coupling in the colossal magneto-resistive bi-layer manganite La_{1.2}Sr_{1.8}Mn_{2}O_{7} across its metal-to-insulator transition. The response in the insulating high-temperature state features harmonic emissions of a dispersionless oxygen phonon at small energy transfer. Upon cooling into the metallic state, we observe a drastic redistribution of spectral weight from the region of these harmonic emissions to a broad high energy continuum. In concert with theoretical calculations, we show that this evolution implies a shift in electron-lattice coupling from static to dynamic lattice distortions that leads to a distinct polaronic ground state in the low temperature metallic phase-a dynamic polaron liquid.

Magnetism in quasi-two-dimensional tri-layer La2.1Sr1.9Mn3O10 manganite

The tri-layer La_{3-3x}Sr_{1+3x}Mn_{3}O_{10} manganites of Ruddlesden–Popper (RP) series are naturally arranged layered structure with alternate stacking of ω-MnO_2 (ω = 3) planes and rock-salt type block layers (La, Sr)_2O_2 along c-axis. The dimensionality of the RP series manganites depends on the number of perovskite layers and significantly affects the magnetic and transport properties of the system. Generally, when a ferromagnetic material undergoes a magnetic phase transition from ferromagnetic to paramagnetic state, the magnetic moment of the system becomes zero above the transition temperature (T _{C} ). However, the tri-layer La_{2.1}Sr_{1.9}Mn_{3}O_{10} shows non-zero magnetic moment above T _{C} and also another transition at higher temperature T ^{*} approx 263 K. The non-zero magnetization above T _{C} emphasizes that the phase transition in tri-layer La_{2.1}Sr_{1.9}Mn_{3}O_{10} not a ferromagnetic to paramagnetic state. We show here the non-zero magnetic moment above T _{C} is due to the quasi-two-dimensional nature of the tri-layer La_{2.1}Sr_{1.9}Mn_{3}O_{10} manganite. The scaling of the magnetic entropy change confirms the second-order phase transition and the critical behavior of phase transition has been studied around T_C to understand the low dimensional magnetism in tri-layer La_{2.1}Sr_{1.9}Mn_{3}O_{10}. We have obtained the critical exponents for tri-layer La_{2.1}Sr_{1.9}Mn_{3}O_{10}, which belong to the short-range two-dimensional (2D)-Ising universality class. The low dimensional magnetism in tri-layer La_{2.1}Sr_{1.9}Mn_{3}O_{10} manganite is also explained with the help of renormalization group theoretical approach for short-range 2D-Ising systems. It has been shown that the layered structure of tri-layer La_{2.1}Sr_{1.9}Mn_{3}O_{10} results in three different types of interactions intra-planer ( J_{ab} ), intra-tri-layer ( J_{c} ) and inter-tri-layer ( J' ) such that J_{ab}> J_{c}>> J' and competition among these give rise to the canted antiferromagnetic spin structure above T _{C} . Based on the similar magnetic interaction in bi-layer manganite, we propose that the tri-layer La_{2.1}Sr_{1.9}Mn_{3}O_{10} should be able to host the skyrmion below T _{C} due to its strong anisotropy and layered structure.

Critical scaling and magnetic phase diagram of bi-skyrmion host quasi-two-dimensional La1.37Sr1.63Mn2O7 bi-layer manganite

The critical behavior of bi-skyrmion host centrosymmetric bi-layer manganite La1.37Sr1.63Mn2O7 has been explored and found that it exhibits second-order magnetic phase transition. The critical exponents have been determined using different methods and found to satisfy the Widom and Rushbrooke scaling relations. The critical exponents are compatible with 2D-Ising universality class, which suggests that anisotropy in bi-layer La1.37Sr1.63Mn2O7 due to its layered structure plays a crucial role. Further, we have presented a possible magnetic phase diagram of the bi-skyrmion host bi-layer La1.37Sr1.63Mn2O7.

Electronic, Magnetic Properties and Magnetocaloric Effect of La2SrMn2O7 Bilayer Manganite: An Ab Initio calculations and Monte Carlo Study

The structure of La2SrMn2O7 (LSMO) bilayer manganite has been investigated using the Monte Carlo simulations and ab initio calculations based on the full-potential linearized augmented plane-wave method. The ferromagnetic phase of LSMO is half-metallic with 100% spin polarization, which is important in relation to the colossal magnetoresistance properties of this compound. Thermal magnetization of LSMO bilayer manganite is obtained for several external magnetic fields h = 1, 3, 5 and 7 T. We have given the dM/dT as a function of temperatures for several external magnetic fields. The magnetic transition from ferromagnetic to paramagnetic is found. The second-order phase transition is found at the transition temperature. The temperature dependence of the magnetic entropy changes of temperatures of LSMO bilayer manganite for several external magnetic fields h = 1, 3, 5 and 7 T is also found. The field dependence of relative cooling power of LSMO bilayer manganite is given for several temperatures. Finally, the magnetic hysteresis cycle of LSMO bilayer manganite is obtained for several temperatures T = 160, 190, 213 and 220 K. The superparamagnetic behavior has been found around the transition temperature.

Magnetic exchange coupling in Fe/bilayer-manganites heterostructure

Magnetic exchange couplings among perovskite manganese oxide heterostructures (typified by SrRuO3/La0.3Sr0.7MnO3) have been widely studied as potential components of metal-oxide spintronic devices, while bi-layered manganite related magnetic coupling phenomenon has never been reported. In this work, we find an obvious enhancement of coercivity in Fe/La2−2xSr1+2xMn2O7 (x=0.35, B-LSMO) starting at the transition temperature of B-LSMO single crystal. Systematic analysis on the temperature and external field evolution of magnetization reversal behavior reveals the emergence of antiferromagnetic interfacial exchange coupling between Fe and B-LSMO. Our results shed new light on tuning of magnetic property on layered oxide interface and designing novel spintronic devices.

Investigating the effects of B-site Cr doping on the structural, magnetic and magnetocaloric properties of La1.4Ca1.6Mn2− x Cr x O7 (0 ⩽ x ⩽ 0.4) bilayer manganites

In this work, we describe the preparation of a series of Cr-substituted La1.4Ca1.6Mn2−x Cr x O7 (x = 0.0, 0.1, 0.2, 0.3, 0.4) double-layer perovskite compounds using a solid-state reaction method. The XRD patterns of all the samples are characterized by a tetragonal structure with a bilayer Ruddlesden–Popper phase. Scanning electron microscope micrographs of the doped samples display ambiguous granular variation and distribution, thereby establishing that the Cr content adversely influences the diffusion kinetics during the sintering process. Combined field-cooled moment vs temperature (M–T) and isothermal magnetization measurements establish the presence of a ferromagnetic to paramagnetic transition in all samples. Furthermore, the second-order nature of the transition is identified using the Arrott plots for these compounds. The maximum values of the change in isothermal entropy are found to be 2.98 J kgK−1, 2.39 J kgK−1, 1.48 J kgK−1, 1.29 J kgK−1, and 1.13 J kgK−1 for samples with x = 0.0, 0.1, 0.2, 0.3, and 0.4, respectively, when exposed to a field of 2.5 T. The entropy curves of the pristine sample are bounded within a narrow range of temperatures while the isothermal entropy curves are broadly expanded over a range of temperatures upon Cr-doping. The broad entropy profiles of the Cr-doped compounds versus temperature suggest a relatively high magnetocaloric potential, compared to the parent compound.

Structural, magneto transport and magnetic properties of Ruddlesden–Popper La2-2xSr1+2xMn2O7 (0.42≤x≤0.52) layered manganites

The magneto transport of Ruddlesden–Popper, La2-2xSr1+2xMn2O7 (0.42≤x≤0.52), 2-dimensional bilayered manganites have been investigated in a broad temperature range. The samples have been synthesized using the solid-state reaction method. Rietveld refinement of the X-ray diffraction data indicates the tetragonal structure formation with I4/mmm space group. The resistivity curves of the samples present a general characteristic of metal-insulator (MI) transition at certain temperature (TMI). Besides, for samples with x = 0.48, 0.50, and 0.52, at a certain temperature (TCO) well below TMI the charge ordering is also evident. Furthermore, the samples display a shallow upturn in resistivity below Tmin due to the Kondo like spin scattering effect, weak localization and electron-phonon interaction. The high temperature semiconducting (T>TMI) region, the resistivity curve follows the 3D Mott’s variable hopping transport mechanism. The overall suppression of resistivity accounts for a substantial magnetoresistance by applying a magnetic field and the characteristic change in TMI, TCO, and Tmin are discussed. Temperature-dependent magnetization demonstrates the suppression of ferromagnetism and evident of antiferromagnetic nature with an apparent charge ordering with increasing concentration of Sr2+.

Large magnetocaloric effects in Pr-doped La1.4-xPrxCa1.6Mn2O7 bilayer manganites

Controlled substitution of Praseodymium (Pr) in place of Lanthanum (La) was performed in La1.4-xPrxCa1.6Mn2O7 (x = 0.0, 0.1, 0.2, 0.3 and 0.4) to explore a possible enhancement in the magnetic and magnetocaloric properties of the material. The samples were produced following the solid-state reaction route. Rietveld refined XRD profiling was used that established the bilayer tetragonal structure (I4/mmm) of the Ruddlesden-Popper Phase in all compositions. Significant variations in the unit cell parameter were observed upon incorporation of Pr in place of La. Ferromagnetic to paramagnetic transitions were confirmed from the temperature dependent magnetization. Against the pristine sample, multiple folds increase was achieved in the values of magnetic entropy change (ΔSM) as well as the temperature averaged entropy change (TEC) with controlled Pr-substitution in the doped samples. Highest ΔSM and TEC (ΔTH-C = 10 K) values of 6.16 J kg −1K−1 and 8.329 J kg −1K−1 respectively were obtained in the x ≥ 0.3 samples at 2.5 T applied field. Our findings in this work suggest giant magnetocaloric effects in the Pr-doped samples and support the possibility of further tailoring these materials for applications in magnetic refrigeration technology.

Antiferromagnetic fluctuations and charge carrier localization in ferromagnetic bilayer manganites: electrical resistivity scales exponentially with short-range order controlled by temperature and magnetic field

The compound La2−2xSr1+2xMn2O7, x = 0.30–0.40, consists of bilayers of ferromagnetic metallic MnO2 sheets that are separated by insulating layers. The materials show colossal magnetoresistance—a reduction in resistivity of up to two orders of magnitude in a field of 7 T—at their three-dimensional ordering temperatures, TC = 90–126 K, and are the layered analogues of the widely studied pseudo-cubic perovskite manganites, R1−xAxMnO3 (R = rare earth, A = Ca, Sr, Ba, Pb). Two distinct short-range orderings—antiferromagnetic fluctuations and correlated polarons, which are related to the magnetic and the lattice degrees of freedom respectively—have previously been discovered in La2−2xSr1+2xMn2O7, x = 0.40, and have each been qualitatively connected to the resistivity. Here, in a comprehensive study as a function of both temperature and magnetic field for the different hole-concentrations per Mn site of x = 0.30 and 0.35, we show that antiferromagnetic fluctuations also appear at temperatures just above TC, and that the intensities of both the antiferromagnetic fluctuations and polaron correlations closely track the resistivity. In particular, for x = 0.35 we show that there is a simple scaling relation between the intensities of the antiferromagnetic fluctuations and the in-plane resistivity that applies for the temperatures and magnetic fields used in the experiments. The results show that antiferromagnetic fluctuations are a common feature ofLa2−2xSr1+2xMn2O7 with ferromagnetic bilayers, and that there is a close connection between the antiferromagnetic fluctuations and polarons in these materials.

Structural, magnetic, and magnetocaloric properties of bilayer manganite La1.38Sr1.62Mn2O7

In this study, we investigated the structural, magnetic phase transition, and magnetocaloric properties of bilayer perovskite manganite La1.38Sr1.62Mn2O7 based on X-ray diffraction, electron paramagnetic resonance, and temperature-/magnetic field-dependent magnetization measurements. The structural characterization results showed the prepared sample had a tetragonal structure with the space group I4/mmm. The Curie temperature was determined as 114 K in the magnetization studies and a second-order paramagnetic-ferromagnetic transition was confirmed by the Arrott plot, which showed that the slopes were positive for all the curves. According to the variation in the electron paramagnetic resonance spectrum, we detected obvious electronic phase separation across a broad temperature range from 220 to 80 K in this magnetic material, thereby indicating that the paramagnetic and ferromagnetic phases coexist above as well as below the Curie temperature. Based on a plot of the isothermal magnetization versus the magnetic applied field, we deduced the maximum magnetic entropy change, which only reached 1.89 J/kg.K under an applied magnetic field of 7.0 T. These theoretical investigations indicated that in addition to the magnetoelastic couplings and electron interaction, electronic phase separation and anisotropic exchange interactions also affect the magnetic entropy changes in this bilayer manganite.

Chemical pressure effects on crystal and magnetic structures of bilayer manganites PrA2Mn2O7 (A = Sr or Ca)

The crystal and magnetic structures of the bilayer manganites PrSr2Mn2O7 (PSMO) and PrCa2Mn2O7 (PCMO) have been studied by neutron powder diffraction. It was found that PSMO crystallizes in space group I4/mmm, while PCMO adopts space group Cmc21 at room temperature. The difference in the structure arises from chemical pressure induced by the Ca substitution for Sr on the A sites, which causes different Jahn-Teller distortions. In PSMO, the MnO6 octahedra suffer a small elongated distortion, while those in PCMO adopt strong compressed distortion along the axial direction. In addition, the octahedra in PCMO show a+b0c0 rotation and a0b+c+ tilting in the Glazer notation in comparison to PSMO. As a result, these two compounds adopt very different magnetic structures: The magnetic structure of PSMO is an A-type magnetic structure (Im'm'm) with propagation vector k = (0, 0, 1) and magnetic moments in the ab plane. In contrast, a C-type antiferromagnetic magnetic structure (Cm'c2′1) with the multiple propagation vectors (k = (0, 12, 12) and (0, 12, 0)) and magnetic moments mainly along the b axis is found in PCMO. The critical exponent of the magnetic phase transition is around 0.345 for PSMO and 0.235 for PCMO, indicating 3D and 2D XY transitions, respectively. The strong Jahn-Teller distortion induced by the chemical pressure is believed to suppress the double exchange and favour super-exchange in PCMO, leading to the dramatic difference in the magnetic structure.

The electrical transition temperature and magnetoresistance prediction of LaSr2Mn2O7 bilayered manganite

In this paper, the resistivity of bilayer manganite LaSr2Mn2O7 around the metal–insulator-like transition was systematically calculated as a function of temperature and applied magnetic field. A typical mathematical function (Gauss function) was used for fitting the experimental data. The theoretical values simulated in this work such as – metal–insulator transition temperature (Tp) and maximum resistivity (ρmax) showed promising agreement with the experimental data.

Effect of copper on the magnetism of half doped bilayer manganite

We have studied the effect of substituting copper at the manganese site on the structure, resistivity and magnetism of the bilayer manganite LaSr2Mn2−xCuxO7 (0≤x≤0.3). All samples form in tetragonal structure with I4/mmm space group. Copper was established to be present in the +2 state by the ESCA spectra. Although no structural transition was observed with this substitution, the electrical transport and magnetic properties change significantly. The parent composition consists of an antiferromagnetic phase ordering at 210K (=TN(x)) and a ferromagnetic phase with its spins ordering at 135K (=TC(x)). Charge ordering is not observed. These transition temperatures agree with those of the majority La0.96Sr2.04Mn2O7 and minority La1.2Sr1.8Mn2O7 phases observed by Battle et al. The transport data is also more consistent with two manganites conducting in parallel. A freezing transition appears just above the TN(x) of each composition x≤0.15. The transition temperatures of both phases decrease with increasing copper concentration due to the replacement of the strong intersheet antiferromagnetic (AFM) superexchange Mnn+–Mnn+(n=3,4) with a much weaker Cu2+–Mn3+interaction. But in the ferromagnetic (FM) composition this substitution replaces the FM intersheet interaction with an AFM interaction and thereby produces heterogeneity in the intersheet interactions leading to a canonical spin glass transition in the minority phase of the x=0.2 composition.

Ferromagnetic domain behavior and phase transition in bilayer manganites investigated at the nanoscale

Understanding the underlying mechanism and phenomenology of colossal magnetoresistance in manganites has largely focused on atomic and nanoscale physics such as double exchange, phase separation, and charge order. Here we consider a more macroscopic view of manganite materials physics, reporting on the ferromagnetic domain behavior in a bilayer manganite sample with a nominal composition of La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ with $x=0.38$, studied using in-situ Lorentz transmission electron microscopy. The role of magnetocrystalline anisotropy on the structure of domain walls was elucidated. On cooling, magnetic domain contrast was seen to appear first at the Curie temperature within the $a-b$ plane. With further reduction in temperature, the change in area fraction of magnetic domains was used to estimate the critical exponent describing the ferromagntic phase transition. The ferromagnetic phase transition was accompanied by a distinctive nanoscale granular contrast close to the Curie temperature, which we infer to be related to the presence of ferromagnetic nanoclusters in a paramagnetic matrix, which has not yet been reported in bilayer manganites.

Lorentz transmission electron microscopy on nanometric magnetic bubbles and skyrmions in bilayered manganites La1.2Sr1.8(Mn1−yRuy)2O7 with controlled magnetic anisotropy

We have investigated nanometric magnetic textures in thin (<150 nm) plates of Ru-doped bilayered manganites La1.2Sr1.8(Mn1−yRuy)2O7. Ru substitution for Mn site changes the magnetic anisotropy from in-plane to out-of-plane easy axis type without any significant change of global magnetic and crystal structures. The combination of conventional and Lorentz transmission electron microscopy observations confirms the emergence of magnetic bubbles and skyrmions in the absence of magnetic field. With the changing Ru concentration, systematic changes in the type of magnetic bubbles are observed. A tiny residual magnetic field also affects the generation and the type-change of magnetic bubbles.

Magnetic domain tuning and the emergence of bubble domains in the bilayer manganiteLa2−2xSr1+2xMn2O7(x=0.32)

We report a magnetic force microscopy study of the magnetic domain evolution in the layered manganite La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ (with $x=0.32$). This strongly correlated electron compound is known to exhibit a wide range of magnetic phases, including a recently uncovered biskyrmion phase. We observe a continuous transition from dendritic to stripe-like domains, followed by the formation of magnetic bubbles due to a field- and temperature dependent competition between in-plane and out-of-plane spin alignments. The magnetic bubble phase appears at comparable field- and temperature ranges as the biskyrmion phase, suggesting a close relation between both phases. Based on our real-space images we construct a temperature-field phase diagram for this composition.

Fermi Surface of Three-Dimensional La(1-x)Sr(x)MnO3 Explored by Soft-X-Ray ARPES: Rhombohedral Lattice Distortion and its Effect on Magnetoresistance.

Electronic structure of the three-dimensional colossal magnetoresistive perovskite La(1-x)Sr(x)MnO3 has been established using soft-x-ray angle-resolved photoemission spectroscopy with its intrinsically sharp definition of three-dimensional electron momentum. The experimental results show much weaker polaronic coupling compared to the bilayer manganites and are consistent with the theoretical band structure including the empirical Hubbard parameter U. The experimental Fermi surface unveils the canonical topology of alternating three-dimensional electron spheres and hole cubes, with their shadow contours manifesting the rhombohedral lattice distortion. This picture has been confirmed by one-step photoemission calculations including displacement of the apical oxygen atoms. The rhombohedral distortion is neutral to the Jahn-Teller effect and thus polaronic coupling, but affects the double-exchange electron hopping and thus the colossal magnetoresistance effect.

Slow Thermodynamics in the Phase Separated State of the Bilayered Manganite (La0.4Pr0.6)1.2Sr1.8Mn2O7

Bilayered perovskite (La0.4Pr0.6)1.2Sr1.8Mn2O7 exhibits a field-induced paramagnetic-insulator to ferromagnetic- metal transition, and a phase-separated state is realized after the field is switched off. In order to clarify the thermodynamic properties of the phase-separated state which is a common feature in many mangan- ites, we performed specific heat measurement with field along the magnetization easy axis. We have revealed that the remanent specifc heat might exhibit a stretched exponential decay and the relaxation time is remarkably long. These results indicate the importance of magnetic frustration originating from the competition between double-exchange interaction and superexchange interaction.

Orbital Order and Electronic Correlations in a Planar Model for Manganites

Understanding orbital ordered (OO) Mott insulating states lies at the heart of a consistent resolution of the colossal magneto-resistance (CMR) observed in manganites, where its melting induces a low-$T$ insulator-metal transition upon hole doping for $0.25 \le x\le 0.45$. Motivated thereby, we study the OO states in a planar model for bilayer manganites using dynamical-mean-field-theory (DMFT) and finite-size diagonalisation methods. We derive the OO ground states in manganites, for $x=0,\frac{1}{2},\frac{2}{3},\frac{3}{4}$ in agreement with observations, including the charge-orbital-magnetic ordered stripe phases for $x>\frac{1}{2}$. These OO states are shown to be associated with an alloy ordering of the $d_{3x^{2}-r^{2}}/d_{3y^{2}-r^{2}}$ orbitals on each $Mn^{3+}$ site.

A Minimal tight-binding model for ferromagnetic canted bilayer manganites.

Half-metallicity in materials has been a subject of extensive research due to its potential for applications in spintronics. Ferromagnetic manganites have been seen as a good candidate, and aside from a small minority-spin pocket observed in La2−2xSr1+2xMn2O7 (x = 0.38), transport measurements show that ferromagnetic manganites essentially behave like half metals. Here we develop robust tight-binding models to describe the electronic band structure of the majority as well as minority spin states of ferromagnetic, spin-canted antiferromagnetic, and fully antiferromagnetic bilayer manganites. Both the bilayer coupling between the MnO2 planes and the mixing of the |x2 − y2 > and |3z2 − r2 > Mn 3d orbitals play an important role in the subtle behavior of the bilayer splitting. Effects of kz dispersion are included.