Bilayer Manganite Research Articles

Intralayer and interlayer exchange tuned by magnetic field in the bilayer manganite(La0.4Pr0.6)1.2Sr1.8Mn2O7probed by inelastic neutron scattering

The bilayer manganite ${\text{La}}_{1.2}{\text{Sr}}_{1.8}{\text{Mn}}_{2}{\text{O}}_{7}$ (LSMO) exhibits a phase transition from a paramagnetic insulating (PI) to a ferromagnetic metallic (FM) state with a colossal magnetoresistance (CMR) effect. Upon 60% Pr substitution, both the magnetic order and the PI to FM transition are suppressed. However, application of a magnetic field induces a first-order transition to a FM state with a CMR effect. We report here inelastic neutron scattering from a single crystal of ${({\text{La}}_{0.4}{\text{Pr}}_{0.6})}_{1.2}{\text{Sr}}_{1.8}{\text{Mn}}_{2}{\text{O}}_{7}$ under a magnetic field of 3.7 T, applied along the $c$ axis and along the [110] direction. Our data reveal the in-plane exchange ${J}_{ab}$ close to that of the Pr-free compound LSMO although the interlayer exchange ${J}_{c}$ is much smaller than that found in LSMO. Anisotropic lattice distortion induced by the smaller size of ${\text{Pr}}^{3+}$ ion is considered to explain these results.

Electron spin resonance study of spin correlations in charge-ordered La2−2xSr1+2xMn2O7(x=0.6)

Electron spin resonance (ESR) studies have been performed on a bilayered manganite La2−2xSr1+2xMn2O7(x=0.6) single crystal. The charge ordering (CO) and antiferromagnetic (AFM) transitions can be well identified from the ESR spectra in both the ab plane and c axis directions. A coexistence of paramagnetic resonance signal and low-field AFM resonance signal was observed below the CO temperature TCO. The analysis of the ESR parameters as a function of temperature suggests the existence of weak ferromagnetic (FM) correlations in the paramagnetic state and AFM correlations below TCO. The competition of FM and AFM correlations results in a broad peak in ESR intensity. Our results also show that the intensity of AFM correlations increases with decreasing temperature as the system enters from the CO state into the A-type AFM ground state. Therefore, the evolution and anisotropy of spin correlations can be fully mapped by ESR study.

Electronic properties of bilayered manganiteCa2.5Sr0.5GaMn2O8from first-principles calculations

By means of first-principles calculations using spin-polarized generalized gradient approximation method and ultrasoft pseudopotentials, the electronic and magnetic structure of the recently synthesized orthorhombic phase of bilayer manganite ${\text{Ca}}_{2.5}{\text{Sr}}_{0.5}{\text{GaMn}}_{2}{\text{O}}_{8}$ compound is obtained. Our calculations have shown that the specific antiferromagnetic ordering, in accordance with experimental findings, is the most stable one. The total energies calculated for the several possible magnetic states of the title compound in the orthorhombic phase enable us to estimate spin exchange interactions for nearest ${J}_{nn}=\ensuremath{-}1.60\text{ }\text{meV}$ and next\penalty1000\-\allowhyphens{} nearest neighbors ${J}_{nnn}=\ensuremath{-}0.21\text{ }\text{meV}$. Calculated Curie--Weiss temperature in the mean-field approximation is in excellent agreement with the measured magnetic phase transition temperature.

High field sensitivity at room temperature in p-n junction based bilayered manganite devices

The thickness dependent current-voltage (I-V) properties of the bilayered La0.6Pr0.2Sr0.2MnO3 (LPSMO)∕Nb-SrTiO3 (SNTO) p-n junction devices having two different thicknesses, grown using pulsed laser deposition (PLD) technique, have been studied. The I-V curves of these bilayered junctions show good rectifying behavior and also exhibit large positive magnetoresistance (MR) at room temperature. The p-n junction having LPSMO thickness of 200nm exhibits low saturation voltage (VC) and high positive MR as compared to junction with 100nm p-type LPSMO layer. Distinct feature such as large positive MR with respect to temperature can be understood in terms of thickness dependent modifications in the film-substrate interface.

Polaron coherence condensation as the mechanism for colossal magnetoresistance in layered manganites

Angle-resolved photoemission spectroscopy data for the bilayer manganite La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7} show that, upon lowering the temperature below the Curie point, a coherent polaronic metallic groundstate emerges very rapidly with well defined quasiparticles which track remarkably well the electrical conductivity, consistent with macroscopic transport properties. Our data suggest that the mechanism leading to the insulator-to-metal transition in La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7} can be regarded as a polaron coherence condensation process acting in concert with the Double Exchange interaction.

Charge and Orbital-Ordering-Induced Charge-Polarized State in Bilayer Manganite

We have investigated a new type of electronic charge-polarized state in bilayer manganite, Pr(Sr0.1Ca0.9)2Mn2O7 (PSCMO). The PSCMO compound has two charge-orbital ordering (COOO) states with respect to the direction of the orbital stripe, which is clearly identified by addressing the optical anisotropy. Notably, a charge-polarized state is formed in the lower-temperature CO-OO state where the c-axis stacking patterns of the lattice and the charge modulations along the b-axis match with each other. We detail the correlation among the transport, the optical and the structural properties in the charge and orbital-ordering-induced charge-polarized states in bilayer manganite.

Hydrostatic pressure effect on the orbital character and magnetic properties of bilayer manganites

We have investigated theoretically the hydrostatic pressure effect on the electronic and magnetic properties of the doped bilayer manganites ${\mathrm{La}}_{2\ensuremath{-}2x}{\mathrm{Sr}}_{1+2x}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ $(0.30\ensuremath{\leqslant}x\ensuremath{\leqslant}0.48)$. The key observation is the anisotropic variations of lattice parameters of the separate bilayers under the hydrostatic pressure, which in turn leads to the different changes between the intra- and interlayer hopping amplitudes of electrons and an additional energy level splitting of the two ${e}_{g}$ orbits ${x}^{2}\ensuremath{-}{y}^{2}$ and $3{z}^{2}\ensuremath{-}{r}^{2}$ of Mn ions. It was found that these two factors from pressure have opposite effects on the orbital occupation properties of the system and the competition between them can cause very different behaviors depending on the doping concentrations. Our results of magnetic and orbital order variations at different doping levels are consistent with recent experiments.

Optical detection of the rotation of orbital stripes in a charge–orbital ordered bilayer manganite at half-doping

The CE magnetic order of charge–orbital ordering pattern in half-doped manganites is characterized by the formation of orbital stripe as well as the checkerboard-type charge ordering. The quasi-one-dimensionality in the orbital arrangement results in the in-plane anisotropy of physical properties, e.g., optical spectra with light polarization along the orbital stripe exhibits the larger energy-gap state than those along the orbital chain. It has been recently reported that the spontaneous rotation of orbital stripe with variation of temperature ( T ) occurs in a bilayer manganite, Pr ( Sr 0.1 Ca 0.9 ) 2 Mn 2 O 7 . To confirm this peculiar behavior, we performed optical measurements of a single domain sample with polarization dependence. While the in-plane optical anisotropy is identified together with the initial formation of the charge–orbital ordering, it is reversed at a lower T , where the orbital stripe is rotated by 90 ∘ .

Imaging of Polarons in Ferromagnetic Bilayered Manganites by Scanning Tunnelling Microscopy

We report scanning tunnelling microscopy images of polaronic charge ordered patch on the layered ferromagnet La2−2x Sr1+2x Mn2O7, x=0.32. The spectroscopic investigation shows that the material is gapped above and below the critical temperature, where the gap changes from 500 to 800 mV, giving evidence for polaronic transport behaviour.

Electrical and Magnetic Properties of Bilayer Manganites La 1.4Sr 1.6Mn 1.96TE 0.04O 7 (TE = Mn, Fe, Ti, Nb)

Abstract The electrical and magnetic properties of bilayer manganites La 1.4Sr 1.6Mn 1.96TE 0.04O 7 (TE = Mn, Fe, Ti, Nb) were investigated. Doping caused obvious changes in electrical and magnetic behaviors such as decrease of insulator-metal transition and magnetic transition temperatures, increase of peak resistivity, and different magnetoresistance effect. These changes had a significant degree of correlation with the valence of doped ions. From Fe, Ti to Nb doping, the effect was doubly stronger. The results could be well understood by considering the different destructions on double-exchange interaction and different influences on lattice distortion caused by Fe, Ti and Nb doping. The temperature dependence of magnetization measured at high field showed that the influence of doping was greatest near three-dimensional magnetic transition temperature of parent phase.

Order and dynamics of intrinsic nanoscale inhomogeneities in manganites

Neutron elastic, inelastic, and high-energy x-ray scattering techniques are used to explore the nature of the polaron order and dynamics in the colossal magnetoresistive (CMR) system ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$. Polaron correlations are known to develop within a narrow temperature regime as the Curie temperature is approached from low temperatures, with a nanoscale correlation length that is only weakly temperature dependent. The static nature of these short-range polaron correlations indicates the presence of a glasslike state, very similar to the observations for the bilayer manganite in the metallic-ferromagnetic doping region. In addition to this elastic component, inelastic scattering measurements reveal dynamic correlations with a comparable correlation length and with an energy distribution that is quasielastic. The elastic component disappears at a higher temperature ${T}^{*}$, above which the correlations are purely dynamic. These observations are identical to the polaron dynamics found in the bilayer manganite system in the CMR regime, demonstrating that they are a general phenomenon in the manganites.

Observation of 300K high energy magnetodielectric contrast in the bilayer manganite (La0.4Pr0.6)1.2Sr1.8Mn2O7

Large high energy magnetodielectric effects are observed in the bilayer manganite (La0.4Pr0.6)1.2Sr1.8Mn2O7, a direct consequence of exploiting magnetoresistance changes associated with the field driven spin-glass insulator to ferromagnetic metal transition and its high temperature remnant. The low temperature magnetodielectric contrast is as large as ∼100% near 0.8eV at 10T and over 10 000% in selected phonon regions. The 300K magnetodielectric contrast is ∼20% near 1.1eV at 30T. The results are potentially useful for magnetic memory applications away from the dc limit.

Suppressed magnetization at the surfaces and interfaces of ferromagnetic metallicmanganites

What happens to ferromagnetism at the surfaces and interfaces of manganites? With thecompetition between charge, spin, and orbital degrees of freedom, it is not surprising thatthe surface behaviour may be profoundly different to that of the bulk. Usinga powerful combination of two surface probes, tunnelling and polarized x-rayinteractions, this paper reviews our work on the nature of the electronic andmagnetic states at manganite surfaces and interfaces. The general observation is thatferromagnetism is not the lowest energy state at the surface or interface, which resultsin a suppression or even loss of ferromagnetic order at the surface. Two caseswill be discussed ranging from the surface of the quasi-2D bilayer manganite(La2−2xSr1+2xMn2O7) to the 3Dperovskite (La2/3Sr1/3MnO3)/SrTiO3 interface. For the bilayer manganite, which is ferromagnetic and conductingin the bulk, these probes present clear evidence for an intrinsic insulatingnon-ferromagnetic surface layer atop adjacent subsurface layers that display thefull bulk magnetization. This abrupt intrinsic magnetic interface is attributed tothe weak inter-bilayer coupling native to these quasi-two-dimensional materials.This is in marked contrast to the situation for the non-layered manganite system(La2/3Sr1/3MnO3/SrTiO3), whose magnetization near the interface is less than half the bulk value at lowtemperatures and decreases with increasing temperature at a faster rate than that for thebulk.

Steplike Lattice Deformation of Single Crystalline(La0.4Pr0.6)1.2Sr1.8Mn2O7Bilayered Manganite

We report a steplike lattice transformation of single crystalline (La0.4Pr0.6)1.2Sr1.8Mn2O7 bilayered manganite accompanied by both magnetization and magnetoresistive jumps, and examine the ultrasharp nature of the field-induced first-order transition from a paramagnetic insulator to a ferromagnetic metal phase accompanied by a huge decrease in resistance. Our findings support that the abrupt magnetostriction is closely related to an orbital frustration existing in the inhomogeneous paramagnetic insulating phase rather than a martensitic scenario between competing two phases.

Change in optical anisotropy with rotation of orbital stripe in a half-doped bilayer manganite

We report an optical study of a half-doped bilayer manganite, $\mathrm{Pr}{({\mathrm{Sr}}_{0.1}{\mathrm{Ca}}_{0.9})}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$, which shows spontaneous rotation of the orbital stripe with variation of temperature $(T)$ in a charge-orbital ordered phase. While the in-plane optical anisotropy is identified together with the initial formation of the charge-orbital ordering, it is reversed at a lower $T$, where the orbital stripe is rotated by 90\ifmmode^\circ\else\textdegree\fi{}. The change in optical anisotropy leads to anomalous $T$ dependence of electrodynamic parameters while keeping the orthorhombic lattice axes intact.

Enhanced Charge Gap in the Bilayer ManganiteLa2−2xSr1+2xMn2O7nearx=0.4

We have investigated the optical conductivity spectra of La2-2xSr1+2xMn2O7 (0.3</=x</=0.5) systematically and found that for x near 0.4 the charge gap shows enhancement of up to approximately 0.3 eV just above the long-range magnetic ordering temperature. From comparison of this charge gap with other experimental results, we conclude that the peculiar x dependence of the charge gap cannot be understood in terms of the charge and lattice correlations only. We suggest that the unusual enhancement originates from the cooperative coupling between the short-range charge or lattice correlations and the quasi-one-dimensional energy band near the Fermi level.

A local metallic state in globally insulating La1.24Sr1.76Mn2O7 well above the metal–insulator transition

Angle-resolved photoemission spectroscopy was used to investigate the evolution of the electronic structure across the metal-insulator transition in bi-layer manganite $La_{1.24}Sr_{1.76}Mn_2O_7$. We found that this system is a metal for $T<T_C$, a local metal but global insulator for $T<T_C<T^*$, and a global insulator for $T>T^*$. These results indicate the critical role of electronic phase separation and percolation effects for the metal-insulator transition in $La_{1.24}Sr_{1.76}Mn_2O_7$.

Anomalous pressure effect on the remanent lattice striction of a(La,Pr)1.2Sr1.8Mn2O7bilayered manganite single crystal

We have studied the pressure effect on magnetostriction, both in the $ab$ plane and along the $c$ axis of a ${(\mathrm{La},\mathrm{Pr})}_{1.2}{\mathrm{Sr}}_{1.8}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ bilayered manganite single crystal over the temperature region where the field-induced ferromagnetic metal (FMM) transition takes place. For comparison, we have also examined the pressure dependence of magnetization curves at the corresponding temperatures. The applied pressure reduces the critical field of the FMM transition and it enhances the remanent magnetostriction. An anomalous pressure effect on the remanent lattice relaxation is observed and is similar to the pressure effect on the remanent magnetization along the $c$ axis. These findings are understood from the viewpoint that the double-exchange interaction-driven FMM state is strengthened by the application of pressure.

Bulk electronic properties of the bilayered manganiteLa1.2Sr1.8Mn2O7from hard-x-ray photoemission

We have performed hard-x-ray photoemission experiments at photon energy close to $6\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ on the ferromagnetic bilayered manganite ${\mathrm{La}}_{1.2}{\mathrm{Sr}}_{1.8}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ across its transition temperature ${T}_{C}$. Differences in both core level and valence band spectra have been observed as a function of temperature: We correlate the energy shift of the O $1s$ peak with a charge-transfer associated with the structural change taking place at the transition. A prominent shoulder on the low binding energy side of the Mn $2p$ core level that is observed below ${T}_{C}$ is tentatively ascribed to increased nonlocal screening in the ordered ferromagnetic phase.

Two- and three-dimensional magnetic ordering in the bilayer manganiteCa2.5Sr0.5GaMn2O8

Neutron diffraction study as a function of temperature on the bilayered manganite ${\mathrm{Ca}}_{2.5}{\mathrm{Sr}}_{0.5}{\mathrm{GaMn}}_{2}{\mathrm{O}}_{8}$ has revealed antiferromagnetic correlations between the [010] oriented Mn spins in the $a\text{\ensuremath{-}}c$ plane. Three-dimensional (3D) short-range and two-dimensional (2D) long-range order over the temperature ranges $180--170\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and $165--160\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, respectively, have been observed. Below $\ensuremath{\sim}150\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, a pure 3D long-range ordering has been observed. Finally a phase diagram is proposed. A qualitative connection is made between the 2D and 3D magnetic ordering to the transport and magnetotransport properties reported for this system.