Ferromagnetic Manganites Research Articles

Complex dynamics of magnetic-flux oscillations in bulk lead-doped lanthanum manganite

Magnetic-flux oscillations in a core made from lead-doped lanthanum manganite have been investigated. The core was located in crossed dc and radio-frequency ac magnetic fields. At a sufficiently large amplitude of the rf field, magnetic-flux jumps were observed and regions of ultrasharp oscillations of the magnetic flux appeared, which correspond to a bifurcation of a soft generation of a limit cycle. Flattened segments appeared in the oscillograms. The phenomena observed are qualitatively explained based on an analysis of processes of the dynamic magnetization reversal of the core along a minor hysteresis loop. The nonlinear phenomena, which indicate a complex dynamics of the magnetic-flux oscillations, exist in a moderately conducting ferromagnetic manganite only in a certain range of frequencies of the ac field, where the depth of a skin layer is comparable with the thickness of the core.

Polaron formation in the optimally doped ferromagnetic manganitesLa0.7Sr0.3MnO3andLa0.7Ba0.3MnO3

The nature of the polarons in the optimally doped colossal magnetoresistive materials ${\text{La}}_{0.7}{\text{Ba}}_{0.3}{\text{MnO}}_{3}$ (LBMO) and ${\text{La}}_{0.7}{\text{Sr}}_{0.3}{\text{MnO}}_{3}$ (LSMO) is studied by elastic and inelastic neutron scattering. In both materials, dynamic nanoscale polaron correlations develop abruptly in the ferromagnetic state. However, the polarons are not able to lock-in to the lattice and order, in contrast to the behavior of ${\text{La}}_{0.7}{\text{Ca}}_{0.3}{\text{MnO}}_{3}$. Therefore ferromagnetic order in LBMO and LSMO survives their formation, explaining the conventional second-order nature of the ferromagnetic-paramagnetic transition. Nevertheless, the results demonstrate that the fundamental mechanism of polaron formation is a universal feature of these ferromagnetic perovskite manganites.

The effect of chemical distribution on the magnetocaloric effect: A case study in second-order phase transition manganites

Abstract Ferromagnetic manganites of general formula ABMnO 3 (where A is a trivalent rare-earth ion and B is a divalent dopant) are candidates for magnetic cooling applications, since they can present either second- or first-order magnetic phase transitions, and the perovskite structure allows for substantial chemical substitution and tuning of the magnetocaloric properties. The consequent chemical distribution from substitution affects the magnetic and magnetocaloric properties of the compound. The change of relative cooling power with chemical disorder is discussed by the use of the molecular mean-field model. We present experimental results of the ferromagnetic, second-order phase transition La 0.70 - x ( Er, Eu ) x Sr 0.30 MnO 3 system.

SrTiO 3 on piezoelectric PMN-PT(001) for application of variable strain

SrTiO 3 (STO) is the most frequently used substrate material for complex oxide films. In this work, STO is explored as a buffer layer on piezoelectric pseudocubic Pb(Mg1/3Nb2/3)0.72Ti0.28O3(001) (PMN-PT) substrates, which serve to reversibly strain thin films. The STO buffer layer reduces the in-plane lattice parameter and allows for a better lattice matching to a broader range of thin film materials. STO films (30 nm) have been grown with epitaxial orientation on PMN-PT with an in-plane lattice parameter close to that of bulk STO. The substrate’s rhombohedral domain structure has been imaged by atomic force microscopy. The related ferroelectric domain structure has been investigated by piezoresponse force microscopy. Within a domain, STO grows with a rather low roughness (rms<0.2 nm). The transfer of the piezoelectric substrate strain to the STO film and its variation with an applied electric field are studied using x-ray diffraction. The strain dependence of the electrical resistance is measured for a ferromagnetic manganite film grown on top of the STO. Both experiments confirm qualitatively that the STO buffer transfers the substrate strain into a functional film deposited on top.

Fermionic representation for the ferromagnetic Kondo lattice model: Diagrammatic study of spin-charge coupling effects on magnon excitations

A purely fermionic representation is introduced for the ferromagnetic Kondo lattice model which allows conventional diagrammatic tools to be employed to study correlation effects. Quantum 1/S corrections to magnon excitations are investigated using a systematic inverse-degeneracy expansion scheme which incorporates correlation effects in the form of self-energy and vertex corrections, while explicitly preserving the continuous spin-rotation symmetry. Magnon self-energy is studied in the full range of interaction strength, and shown to result in strong magnon damping and anomalous softening for zone boundary modes, which accounts for several zone-boundary anomalies observed in recent spin-wave measurements of ferromagnetic manganites.

Modeling of the microwave characteristics of layered superconductor/ferromagnetic structures

Microwave (MW) characteristics of tangentially magnetized double layer structures consisting of a high temperature superconducting (HTS) and a conductive ferromagnetic (FM) manganite films (with the parameters similar to those of HTS YBa 2Cu 3O 7 (YBCO) and FM La 0.7Sr 0.3MnO 3 (LSMO) films) were modeled with respect to the linear polarized planar transverse electromagnetic wave incident perpendicularly to the FM film surface. The surface impedance of such a structure is characterized with the peculiarities caused by the HTS transition and the ferromagnetic resonance (FMR). The MW losses caused by the conductivity of the FM manganite film in double layer FM/HTS structures decrease drastically at temperatures T lower than the critical one T C and FMR losses become dominant: in this case the peculiarity of the surface impedance caused by FMR can be observed clearly in the structure. A strong magnetic field dependence of the surface resistance can be observed in manganite/HTS structures at T < T C due to the FMR effect.

Giant positive magnetoresistance in ultrathin films of mixed phase manganites

Magnetic tunnel junctions (MTJs) based on fully spin polarized ferromagnetic manganites have generated a lot of interest due to their enhanced field sensitivity at low temperatures. However, the tunneling magnetoresistance (TMR) drops rapidly with increasing temperature due to the reduction of spin polarization at the manganite-insulator interface. We have devised a method for creating intrinsic tunnel barriers by tuning the phase competition in manganites using substrate induced strain. Ultrathin films (7.5nm) of the mixed phase manganite (La0.5Pr0.5)0.67Ca0.33MnO3 (LPCMO) grown on the substrate (110) NdGaO3 using pulsed laser deposition show positive magnetoresistance (MR) of about 30% at magnetic fields less than 1T. Unlike the fabricated MTJ devices, this MR effect has its maximum value close to the insulator to metal transition temperature and reduces with decreasing temperature. To find out the mechanism leading to this positive MR, the effect of three orientations of the magnetic field on the LPCMO thin films were studied: (1) perpendicular to the plane of the film, (2) parallel to the plane of the film and applied current, and (3) parallel to the plane of the film but perpendicular to the applied current. The effect of field orientation suggests that a possible mechanism for the positive MR is tunneling magnetoresistance due to the spin conserving tunneling process across the insulating regions separating the ferromagnetic metallic regions. The voltage dependence of the MR also supports this mechanism. Our results suggest a novel method for obtaining enhanced TMR in manganite based MTJs by creating strain induced intrinsic tunnel barriers.

Magnetoresistance of an all-manganite spin valve: A thin antiferromagnetic insulator sandwiched between two ferromagnetic metallic electrodes

We study the magnetic and transport properties of an all-manganite spin valve consisting of a thin antiferromagnetic and insulating manganite sandwiched between two ferromagnetic and metallic manganite electrodes. When the ferromagnetic electrodes are in a parallel configuration, the double-exchange mechanism in the middle manganite slab is enhanced and the whole heterostructure becomes metallic. In the antiparallel alignment of the electrodes, the antiferromagnetic order in the middle layer is more robust and the resistance of the heterostructure is larger than in the parallel configuration. The strong dependence of the electronic structure of the middle manganite on the relative orientation of the magnetization in the leads turns out in a large tunneling magnetoresistance. We also find that the application of a magnetic field to the heterostructure in the parallel metallic configuration increases the electrical conductance, producing a large magnetoresistance. We discuss our conclusions in the context of recent experiments performed in manganite heterostructures.

Suppressed Magnetization at the Surfaces and Interfaces of Ferromagnetic Metallic Manganites

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Magnetocaloric manganites: Progress and challenges

AbstractMagnetic refrigeration based on the magnetocaloric effect (MCE) becomes a topic of internationally current interest. This enabling technology provides several advantages (e.g. high energy efficiency, small volume, non‐pollution) over a conventional gas compression technique. It depends heavily upon the magnitude of MCE of magnetic substances that are used as working refrigerants. Therefore, current research is focused on a search for new materials that are low cost and exhibit small magnetic field‐induced large MCEs. This paper aims to review the recent developments of a new class of magnetocaloric material, namely, ferromagnetic manganites (R1‐xMxMnO3, where R = La, Nd, Pr and M = Ca, Sr, Ba, etc.). The good magnetocaloric properties of ferromagnetic manganites in addition to their flexibility and cost‐effectives make them very promising for active magnetic refrigeration (AMR) applications in the temperature range of 30–375 K. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Magnons in Ferromagnetic Metallic Manganites

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Quenched disorder suppression of the first-order magnetic phase transition in manganites

The role of quenched disorder on the magnetic phase transition of ferromagnetic manganites is investigated through the field and pressure dependence of magnetization around ${T}_{C}$. Our results demonstrate that moderate-small chemical disorder promotes second-order behavior. This can be rationalized on the basis of a simple thermodynamic model in which the magnetic order parameter is coupled to the lattice, this coupling being sensitive to chemical disorder. A plausible microscopic scenario is also discussed.

Unified description of spin-dependent transport in granular ferromagnetic manganites

We have presented direct experimental evidence that the description of spin-dependent transport in granular ferromagnets at low temperature is incomplete unless both intergrain tunneling and nontunneling transport are incorporated in a unified treatment. Depending on the microstructure of the system, charge-carrier localization inside the grain and the lowering of intergrain potential barrier by electric field will play a crucial role behind the magnetoresistive properties in granular ferromagnets.

Suppressed Magnetization at the Surfaces and Interfaces of Ferromagnetic Metallic Manganites

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Magnetic interactions in ferromagnetic manganite nanotubes of different diameters

In this work we present a magnetic study of La 0.67Sr 0.33MnO 3 (LSMO) and La 0.67Ca 0.33MnO 3 (LCMO) nanotubes with nominal external diameters ( ϕ) of 100, 200, 600 and 800 nm. The 800 nm diameter nanotubes have walls of around 50 nm thickness in all the cases. The walls are constituted by an assembly of nanoparticles with a non-Gaussian size distribution presenting a maximum at 24 ± 6 nm (LSMO) and 25 ± 8 nm (LCMO). We carried out isothermal remanent magnetization (IRM) and dc demagnetization (DCD) experiments. We determined that the crystallites are single magnetic domains with a magnetic dead layer on the surface which avoids exchange interactions among grains. We conclude that the dominating interactions are of dipolar type of the same magnitude for all the samples.

Magnons in ferromagnetic metallic manganites

Ferromagnetic (FM) manganites, a group of likely half-metallic oxides, are of specialinterest not only because they are a testing ground for the classical double-exchangeinteraction mechanism for the ‘colossal’ magnetoresistance, but also because they exhibitan extraordinary arena of emergent phenomena. These emergent phenomena are related tothe complexity associated with strong interplay between charge, spin, orbital,and lattice. In this review, we focus on the use of inelastic neutron scatteringto study the spin dynamics, mainly the magnon excitations in this class of FMmetallic materials. In particular, we discuss the unusual magnon softening anddamping near the Brillouin zone boundary in relatively narrow-band compoundswith strong Jahn–Teller lattice distortion and charge–orbital correlations. Theanomalous behaviours of magnons in these compounds indicate the likelihood ofcooperative excitations involving spin and lattice as well as orbital degrees of freedom.

Magnetotransport properties of nanocrystalline Pr0.65(Ca1−ySry)0.35MnO3 (y∼0.4,0.3): Influence of phase coexistence

The influence of the coexistence of charge ordered insulating and ferromagnetic metallic phases on magnetotransport properties of nanocrystalline Pr0.65(Ca1−ySry)0.35MnO3 (y∼0.4,0.3) has been studied. Large magnetoresistance at low magnetic field has been observed. The sharp increase of low field magnetoresistance with increase of temperature below the insulator to metal transition temperature is in complete contrast to the magnetoresistive properties of ferromagnetic manganite nanoparticles. The anomalous magnetoresistive behavior is attributed to the destabilization of the charge ordered insulating state by the magnetic field.

Electronic properties of domain walls inLa2∕3Sr1∕3MnO3: Magnetotransport measurements on a nanopatterned device

Domain walls (DWs) in strongly correlated ferroics are expected to exhibit rich physical properties due to the competition between ground states that exists in these systems. A typical example is provided by ferromagnetic mixed-valence manganites for which insulating DWs in an otherwise metallic phase have been predicted. Through magnetotransport experiments on a nanopatterned device we have determined the electronic properties of DWs in ${\mathrm{La}}_{2∕3}{\mathrm{Sr}}_{1∕3}\mathrm{Mn}{\mathrm{O}}_{3}$. We find a DW resistance-area (RA) product of $\ensuremath{\sim}2.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}\phantom{\rule{0.3em}{0ex}}\ensuremath{\Omega}\phantom{\rule{0.2em}{0ex}}{\mathrm{m}}^{2}$ at low temperature and bias, which is several orders of magnitude larger than the values reported for $3d$ ferromagnets. However, the current-voltage characteristics are highly linear, which indicates that the DWs are not phase separated but metallic. Remarkably, the DWRA is also found to increase upon increasing the injected current, presumably reflecting some deformation of the wall by spin transfer.

Colossal electroresistance without colossal magnetoresistance in La0.9Sr0.1MnO3

The authors report on colossal electroresistance (ER) in the ferromagnetic insulator manganite La0.9Sr0.1MnO3. The single crystal samples exhibit a transition into a low resistive state above a certain threshold current. Pulsed measurements demonstrate that this transition is not a consequence of heating. ER behaves similarly to magnetoresistance (MR) above the orbital order temperature TOO. Below TOO the MR is only 20% while ER enhances the conductivity by several orders of magnitude. Magnetic field and electric current have opposite effects on the conductivity, therefore, it seems that only the injected carriers are able to modify the ordering of eg Mn orbitals.

Spin waves throughout the Brillouin zone and magnetic exchange coupling in the ferromagnetic metallic manganitesLa1−xCaxMnO3(x=0.25,0.30)

Using time-of-flight and triple-axis inelastic neutron spectroscopy, we determine spin wave excitations throughout the Brillouin zone for ferromagnetic manganites La$_{1-x}$Ca$_x$MnO$_3$ ($x=0.25,0.3$) in their low temperature metallic states. While spin wave excitations in the long wavelength limit (spin stiffness $D$) have similar values for both compounds, the excitations near the Brillouin zone boundary of La$_{0.7}$Ca$_{0.3}$MnO$_3$ are considerable softened in all symmetry directions compared to that of La$_{0.75}$Ca$_{0.25}$MnO$_3$. A Heisenberg model with the nearest neighbor and the fourth neighbor exchange interactions can describe the overall dispersion curves fairly well. We compare the data with various theoretical models describing the spin excitations of ferromagnetic manganites.