Bilayer Manganite Research Articles

Electrical properties of fluorine-intercalated layered manganite: La1.4Sr1.6Mn2O7F2

Fluorinating agent NH4F is employed to intercalate the bi-layered manganite La1.4Sr1.6Mn2O7 to form La1.4Sr1.6Mn2O7F2 using solid state reaction. The insertion of fluorine results in a contraction of ∼0.116Å in the a axis and an expansion of ∼3.152Å in the c axis with respect to the parent compound La1.4Sr1.6Mn2O7. The Mn–O bond lengths are altered due to the electrostatic repulsion between O2− and F−. Rietveld analysis indicates that a longer bond length of Mn–O(1) and shorter bond lengths of Mn–O(2) and Mn–O(3) has been found in La1.4Sr1.6Mn2O7F2 when compared to La1.4Sr1.6Mn2O7. The electrical resistivity of La1.4Sr1.6Mn2O7F2 shows insulating behavior and exhibits remarkably different behavior from La1.4Sr1.6Mn2O7. For the first time, we report the temperature dependence of electrical resistivity of La1.4Sr1.6Mn2O7F2. Magnetic measurements indicate that the magnetic order of local moments is suppressed by fluorine insertion into interstitial sites.

Biskyrmion states and their current-driven motion in a layered manganite

The magnetic skyrmion is a topologically stable spin texture in which the constituent spins point to all the directions wrapping a sphere. Generation and control of nanometric magnetic skyrmions have large potential, for example, reduced power consumption, in spintronics device applications. Here we show the real-space observation of a biskyrmion, as defined by a molecular form of two bound skyrmions with the total topological charge of 2, realized under magnetic field applied normal to a thin plate of a bilayered manganite with centrosymmetric structure. In terms of a Lorentz transmission electron microscopy (TEM), we have observed a distorted-triangle lattice of biskyrmion crystal, each composed of two bound skyrmions with oppositely swirling spins (magnetic helicities). Furthermore, we demonstrate that these biskyrmions can be electrically driven with orders of magnitude lower current density (<10(8) A m(-2)) than that for the conventional ferromagnetic domain walls.

Ultrafast structural and electronic dynamics of the metallic phase in a layered manganite.

The transition between different states in manganites can be driven by various external stimuli. Controlling these transitions with light opens the possibility to investigate the microscopic path through which they evolve. We performed femtosecond (fs) transmission electron microscopy on a bi-layered manganite to study its response to ultrafast photoexcitation. We show that a photoinduced temperature jump launches a pressure wave that provokes coherent oscillations of the lattice parameters, detected via ultrafast electron diffraction. Their impact on the electronic structure are monitored via ultrafast electron energy loss spectroscopy, revealing the dynamics of the different orbitals in response to specific structural distortions.

Reinforcement of double-exchange ferromagnetic coupling by Ru in La1.24Sr1.76Mn2-yRuyO7 manganite system

The effect of Mn-site doping on magnetic and transport properties in the bilayer manganites La1.24Sr1.76Mn2-yRuyO7 (y=0.0, 0.04, 0.08 and 0.15) has been studied. The undoped compound La1.24Sr1.76Mn2O7 exhibits a ferromagnetic metal to paramagnetic insulator transition at TC=130K and the substitution of Ru shifts the transition temperatures to higher temperature values. The increased metal–insulator transition by Ru substitution, obtained from temperature dependence of resistivity measurements, indicates that the Ru substitution enhances the metallic state at low temperature regime and favours the Mn–Ru pairs in the Ru doped samples. Moreover, the activation energy values calculated from the temperature dependence of resistivity curves suggest that the Ru substitution weakens the formation of polarons. The increased magnetoresistance ratio from 108% to 136% by Ru substitution, measured at 5K, points out that the Ru substitution also enhances the inter-grain tunneling magnetoresistance. Thus, the ferromagnetic order and metallic state in La1.24Sr1.76Mn2O7 system have been enhanced by the presence of Ru in the Mn-site. These reinforcements of ferromagnetic metallic state and magnetoresistance have been interpreted by the ferromagnetically coupled high spin states of Ru with Mn ions in the Mn–O–Ru network.

DC magnetization studies of nano- and micro-particles of bilayered manganite LaSr2Mn2O7

Abstract Systematic studies of the magnetic properties of LaSr2Mn2O7 as a function of crystalline grain size provide information on how the crystalline grain size affects the magnetic and charge ordering in this compound. The half-doped bilayered manganite LaSr2Mn2O7 (x = 0.5) in its bulk form has the CE-type anti-ferromagnetic (CE-AFM) charge ordering phase transition. In this work, we prepare LaSr2Mn2O7 ceramic samples using the Pechini sol–gel method in order to produce different grain sizes, and the effects of different crystalline grain sizes between 200 and 1000 nm on the magnetic properties of the compound, obtained by a SQUID magnetometer, are investigated. The DC magnetization (DCM) measurements for all samples indicate that the crystalline grain size has no considerable effect on the charge ordering transition temperature, TCO (just a few degrees shift to lower temperatures as the grain sizes become bigger). The width of the peak observed in these measurements (transition to the charge ordering phase) becomes broader, and susceptibility measurement at the zero field cooling (ZFC) and filed cooling (FC) increases as the grain size becomes systematically smaller. The results obtained from the magnetic hysteresis curves confirm the ground state phase of the system is anti-ferromagnetic and the Arrott plots obtained manifest existence of the first and second order magnetic phase transition in all samples. In addition, in the sample with a grain size of 200 nm, enhancement of the magnetic properties, which is accompanied with the formation of FM phase on the surface of grain or particle, is observed.

Publisher’s Note: Antiferromagnetic domain structure in bilayer manganite [Phys. Rev. B88, 075134 (2013)

Publisher’s Note: Antiferromagnetic domain structure in bilayer manganite [Phys. Rev. B88, 075134 (2013)

Anomalous Magnetic Properties of the Bilayered LaSr2Mn2−z Co z O7 (z=0–0.15) Manganite

The Co-doped bilayered LaSr2Mn2O7 manganite at low Co concentrations (0–0.15) was synthesized by the sol–gel process. The X-ray diffraction (XRD) technique confirms phase formation for all the samples under investigation. The results of ac magnetic susceptibility measurements indicate the effect of Co doping on the magnetic ordering phases. The indications of charge ordering (CO) transitions were observed in all the prepared compounds. The CO magnetic phase transition temperature was observed systematically shift to lower temperatures as the Co concentration increases. There was an anomalous oscillating magnetic behavior in all samples with a few peaks before the CO temperature in the paramagnetic (PM) region so that with an increasing Co doping, the number of peaks and amplitude were decreased. Also, the ac susceptibility measurements were performed in the presence of an applied dc magnetic field to further study of this oscillating behavior.

Colossal piezoresistance effect in Sm0.55(Sr0.5Ca0.5)0.45MnO3 single crystal

We have investigated the effect of uniaxial pressure (P) on electrical resistivity along the ab plane and c axis in a single crystal of Sm0.55(Sr0.5Ca0.5)0.45MnO3. A huge piezoresistance (PR ∼107% at P = 90 MPa) and a remarkable increase (at the rate of ∼79 K/GPa) of metal-insulator transition temperature (TMI) have been observed for P applied along the c axis, while TMI decreases at the rate of ∼77 K/GPa for P perpendicular to the c axis. These values of PR and dTMI/dP are much larger than those observed in other perovskite and bilayer manganites. Such colossal PR and large value of dTMI/dP may be utilized for various technological applications.

90° Rotation of orbital stripes in bilayer manganite PrCa2Mn2O7 studied by in situ transmission electron microscopy

Abstract We present an in situ transmission electron microscopy study on the half-doped bilayer manganite PrCa2Mn2O7 to reveal the rotation process of the orbital stripes. Between the reported initial and final ordering phases, we identified an intermediate state with two sets of satellite spots to bridge the 90° rotation of the orbital stripes. Furthermore, we determined that the rotation of the orbital stripes does not always occur. Some restricted conditions for the orbital rotation to occur were found and reasons are discussed.

A Study of Structural and Physical Properties of Heavily Co-doped LaSr2Mn2O7 Bi-layered Manganite

In this paper, we report the influence of cobalt doping on the structure and magnetic properties of bilayered-perovskite LaSr2Mn2−xCoxO7 (x=0.25,0.3). Parent and doped samples were synthesized through the sol-gel method under optimized conditions. X-ray diffraction studies coupled with the Rietveld refinement of xrd data, show that both doped and parent samples crystallize in a Sr3Ti2O7-type layered perovskite structure. Temperature dependent magnetic behavior of the parent sample exhibits an antiferromagnetic and a charge-ordering state around 220 K. Magnetic field dependence of magnetization M(H) for doped samples at 20 K show well-defined M–H loops confirming the formation of intrinsic ferromagnetic ordering due to the Co doping. Additionally, we studied the ac–magnetic susceptibility and magnetization relaxation process in our samples. Dynamic scaling analysis hints an existence of glassy magnetic states in doped samples. Electrical and magnetic properties of our samples further demonstrate that Co doping suppresses charge ordering in bilayered LaSr2Mn2O7.

Ground State in a Half-Doped Manganite Distinguished by Neutron Spectroscopy

We have measured the spin-wave spectrum of the half-doped bilayer manganite Pr(Ca,Sr)(2)Mn(2)O(7) in its spin, charge, and orbital ordered phase. The measurements, which extend throughout the Brillouin zone and cover the entire one-magnon spectrum, are compared critically with spin-wave calculations for different models of the electronic ground state. The data are described very well by the Goodenough model, which has weakly interacting ferromagnetic zig-zag chains in the CE-type arrangement. A model that allows ferromagnetic dimers to form within the zigzags is inconsistent with the data. The analysis conclusively rules out the strongly bound dimer (Zener polaron) model.

Nonmonotonic Fermi surface evolution and its correlation with stripe ordering in bilayer manganites

Using angle-resolved photoemission spectroscopy, we have measured La${}_{2\ensuremath{-}2x}$Sr${}_{1+2x}$Mn${}_{2}$O${}_{7}$ over a wide doping range to study the correlation between Fermi surface nesting and stripes of charge and orbital degrees of freedom. We found that the Fermi surface nesting deviates from band calculations with a nonmonotonic behavior, and that one type of stripe is exclusively linked to long flat portions of nested Fermi surface, while the other type prefers to be commensurate with the real-space lattice but also may be driven away from this by the Fermi surface. Complementarily, for certain doping levels pressure from the stripe ordering also may drive the Fermi surface away from its preferred trend.

Interplay between intrinsic and stacking-fault magnetic domains in bi-layered manganites

We present a low temperature x-ray photoemission electron microscopy study of the bi-layered manganite compound La1.2Sr1.8Mn2O7 (BL-LSMO) to investigate the influence of stacking faults, which are structurally and magnetically different from the bi-layered host. In BL-LSMO, small magnetic moment persists to T*=300 K, well above the Curie temperature of 120 K (TC). Our magnetic images show that 3D stacking faults are responsible for the T* transition. Furthermore, close to the TC, stacking faults are well coupled to the bi-layered host with latter magnetic domains controlling the spin direction of the stacking faults. Contrary to recent reports, we find that stacking faults do not seed magnetic domains in the host via an exchange spring mechanism and the intrinsic TC of the BL-LSMO is not lower than 120 K.

Pressure-enhanced ferromagnetism and metallicity in La1.24Sr1.76Mn2O7 bilayered manganite system

We report the low temperature magnetization, electrical resistance, and magnetoresistance of La1.24Sr1.76Mn2O7 bilayer manganite system under hydrostatic pressure. At ambient pressure, the compound shows a sharp ferromagnetic transition (TC) accompanied by a metal–insulator transition (TMI) at 130 K. We observe that the TC and TMI increase with hydrostatic pressure at a rate of dTC/dP = 2.08 K/kbar and dTMI/dP = 2 K/kbar, respectively. Also, we observe an appreciable increase of magnetic moments at low temperatures with increasing pressure. The high temperature regime of temperature dependence of resistivity curves was fitted with the Emin–Holstein’s polaron hopping model and the calculated activation energy values suggest that the applied pressure weakens the formation of Jahn–Teller polarons. The magnetoresistance ratio (MRR) was measured at TC and at 4.2 K upon an external magnetic field of 5 T. The observed MRR at TC is about 210 % and the applied pressure increases the MRR significantly. These results can be interpreted by the pressure-enhanced overlap between the orbitals of Mn–O–Mn, which facilitates the charge transfer and hence enhances the ferromagnetism and metallicity.

Floating zone crystal growth and magnetic properties of bilayer manganites Pr(Sr1−xCax)2Mn2O7

Single crystals of a bilayer manganite series Pr(Sr1−xCax)2Mn2O7 (x=0, 0.4, 0.9, 1) have been grown using traveling solvent floating zone technique under various oxygen partial pressure. It was found that the crystal quality is very sensitive to oxygen partial pressure applied during growth and highly Ca doped crystals could only be obtained at elevated oxygen pressure. Whereas a secondary phase was observed for PrCa2Mn2O7 grown at oxygen partial pressure below 1bar, a pure phase can be achieved at 8bar of oxygen. This was evidenced studying composition mapping by X-ray fluorescence spectroscopy. X-ray Laue diffraction and X-ray powder diffraction have been carried out on the samples for the quality check of the grown crystals. Susceptibility measurements indicate that all samples are antiferromagnetic at low temperature. They also reveal the existence of two anomalies at ∼320K and 370K, which coincide with the charge ordering/orbital ordering transitions described in the literature.

Pressure-induced colossal piezoresistance effect and the collapse of the polaronic state in the bilayer manganite (La0.4Pr0.6)1.2Sr1.8Mn2O7

We have investigated the effect of hydrostatic pressure as a function of temperature on the resistivity of a single crystal of the bilayer manganite (La0.4Pr0.6)1.2Sr1.8Mn2O7. Whereas a strong insulating behaviour is observed at all temperatures at ambient pressure, a clear transition into a metallic-like behaviour is induced when the sample is subjected to a pressure (P) of ∼1.0 GPa at T < 70 K. A huge negative piezoresistance ∼106 in the low temperature region at moderate pressures is observed. When the pressure is increased further (5.5 GPa), the high temperature polaronic state disappears and a metallic behaviour is observed. The insulator to metal transition temperature exponentially increases with pressure and the distinct peak in the resistivity that is observed at 1.0 GPa almost vanishes for P > 7.0 GPa. A modification in the orbital occupation of the eg electron between 3dx2−y2 and 3dz2−r2 states, as proposed earlier, leading to a ferromagnetic double-exchange phenomenon, can qualitatively account for our data.

Micro-fabrication process for small transport devices of layered manganite

Devices have been fabricated based on the bilayer manganite La1.4Sr1.6Mn2O7, which in the bulk state orders magnetically below 90 K, at which point both in-plane and c-axis bulk resistivity decrease by 2-3 orders of magnitude. We provide an optimized procedure to fabricate devices to electrical transport in- and out of plane. Fabricated mesoscopic devices have dimensions comparable to a typical magnetic domain, allowing us to study structures going from a single domain to several domains.

Electronic Confinement and Ordering Instabilities in Colossal Magnetoresistive Bilayer Manganites

We present angle-resolved photoemission studies of (La{1-z}Pr{z}){2-2x}Sr{1+2x}Mn{2}O{7} with x=0.4 and z=0.1, 0.2, and 0.4 along with density functional theory calculations and x-ray scattering data. Our results show that the bilayer splitting in the ferromagnetic metallic phase of these materials is small, if not completely absent. The charge carriers are therefore confined to a single MnO{2} layer, which in turn results in a strongly nested Fermi surface. In addition to this, the spectral function also displays clear signatures of an electronic ordering instability well below the Fermi level. The increase of the corresponding interaction strength with z and its magnitude of ∼400 meV make the coupling to a bare phonon highly unlikely. Instead we conclude that fluctuating order, involving electronic and lattice degrees of freedom, causes the observed renormalization of the spectral features.

Response of Acoustic Phonons to Charge and Orbital Order in the 50% Doped Bilayer ManganiteLaSr2Mn2O7

We report an inelastic neutron scattering study of acoustic phonons in the charge and orbitally ordered bilayer manganite LaSr(2)Mn(2)O(7). For excitation energies less than 15 meV, we observe an abrupt increase (decrease) of the phonon energies (linewidths) of a transverse acoustic phonon branch at q = (h, h, 0), h ≤ 0.3, upon entering the low temperature charge and orbital ordered state (T(COO) = 225 K). This indicates a reduced electron-phonon coupling due to a decrease of electronic states at the Fermi level leading to a partial removal of the Fermi surface below T(COO) and provides direct experimental evidence for a link between electron-phonon coupling and charge order in manganites.

Bilayer manganites reveal polarons in the midst of a metallic breakdown

The exact nature of the low temperature electronic phase of the manganite materials family, and hence the origin of their colossal magnetoresistant (CMR) effect, is still under heavy debate. By combining new photoemission and tunneling data, we show that in La{2-2x}Sr{1+2x}Mn2O7 the polaronic degrees of freedom win out across the CMR region of the phase diagram. This means that the generic ground state is that of a system in which strong electron-lattice interactions result in vanishing coherent quasi-particle spectral weight at the Fermi level for all locations in k-space. The incoherence of the charge carriers offers a unifying explanation for the anomalous charge-carrier dynamics seen in transport, optics and electron spectroscopic data. The stacking number N is the key factor for true metallic behavior, as an intergrowth-driven breakdown of the polaronic domination to give a metal possessing a traditional Fermi surface is seen in the bilayer system.