Colossal Magnetoresistive Perovskites Research Articles

X-MCD study of mixed valence manganites

A study of the X-MCD spectra and X-MCD magnetometry, i.e. the X-MCD effect vs. applied field, of the colossal magnetoresistive perovskites La 0.67− y Tb y Ca 0.33MnO 3 is reported. The spectra were measured on polycrystalline samples with y=0.0, 0.1, 0.22 at 10 and 77 K at the K, L 2, L 3 edges of manganese, the L 2, L 3 edges of lanthanum and the L 2, L 3, M 4, M 5 edges of terbium. From the X-MCD signal at the M 4, M 5 edges of terbium a parallel alignment of terbium sublattice magnetisation to the manganese magnetisation and to the applied magnetic field is deduced. The X-MCD magnetometry curves at the Mn K edge and La L edges show a saturation in the applied field, whereas Tb L edge signals do not saturate. The effect is especially pronounced in La 0.45Tb 0.22Ca 0.33MnO 3. The results are discussed in terms of the influence of terbium doping on the magnetic and electronic phase separation and on the strength of the coupling between magnetic moments of Tb and Mn and their relation to the colossal magnetoresistance of the compounds.

Two-phase scenario for the metal-insulator transition in colossal magnetoresistance manganites

Recent experiments indicate the coexistence of localized and itinerant charge carriers close to the metal-insulator transition in the ferromagnetic phase of colossal magnetoresistive manganese perovskites. For a theoretical description of the colossal magnetoresistance transition we propose a two-phase model of competing insulating polaronic and ferromagnetic metallic phases with equal hole densities. We find that the subtle balance between these two phases with distinctly different electronic properties can be readily influenced by varying physical parameters, producing various ``colossal'' effects, such as the large magnetization and conductivity changes in the vicinity of the transition temperature.

Charge correlations in the magnetoresistive oxide La0.7Ca0.3MnO3 (invited)

Neutron scattering has been used to study the nature of the spin dynamics and charge correlations in a single crystal of the colossal magnetoresistive perovskite La0.7Ca0.3MnO3. Diffuse scattering from lattice polarons develops as the Curie temperature is approached from below, along with short range polaron correlations that are consistent with stripe formation. Magnetic fields are found to suppress this polaron formation. The temperature dependence of the polaron correlations follows the same behavior as both the resistivity and the anomalous quasielastic component in the magnetic fluctuation spectrum, indicating that they have a common origin.

NMR of Colossal Magnetoresistive Manganites and Expanded Lattice Intermetallics at High Pressure.

A survey is given of NMR studies of colossal magnetoresistive manganese perovskites (RE,Ca,Sr,Ba)MnO3 (RE = rare earth) and the interstitially modified permanent magnet materials, RE2Fe17A x (A = N, C, H) at ambient pressure and at applied pressures of up to 10 kbar. The different pressure behavior of the Mn hyperfine field found in the metallic and in the insulating manganite compounds is discussed and related to the micro- and macroscopic properties of these magnetically inhomogeneous materials. In the RE2Fe17A x compounds a different pressure behavior of the hyperfine field at the RE sites with different number and type of interstitial atom neighbors is discussed in terms of local compressibility. The influence of the interstitial modification and the applied pressure on the RE hyperfine field and quadrupole splitting is analyzed and the impact of the “chemical pressure” and covalent effects on these quantities is compared. A comparison of the behavior of the magnetic state of the 4f electron shell with pressure and with interstitial modification is made and the relation to the magnetocrystalline anisotropy is discussed.

The surface terminal layer and composition of the colossal magnetoresistance perovskite: La0.65Pb0.35MnO3

The composition in the surface region of the colossal magnetoresistance perovskite manganite materials, La0.65Pb0.35MnO3 has been studied using angle resolved x-ray photoemission spectroscopy. The results are consistent with a Mn–O terminal layer and Pb segregation to the near-surface region. This is similar to the behavior exhibited by La0.65Ca0.35MnO3 and La0.65Sr0.35MnO3. The O–Mn–O terminal layer appears to be close to the Mn3+ valence state.

Spin dynamics in perovskites, pyrochlores, and layered manganites

High temperature electron spin resonance (ESR) and magnetic susceptibility (χ) are analyzed for manganites related with colossal magnetoresistance (CMR). The properties of compounds with different crystalline structures: three-dimensional (3D) perovskites, pyrochlore, and La1.2Sr1.8Mn2O7, a two-dimensional layer, are compared. In the paramagnetic regime, and outside the critical regions associated with phase transitions, the temperature dependence of the ESR linewidth presents a universal behavior dominated by the variations of χ(T), ΔHpp(T)=[C/Tχ(T)]ΔHpp(∞). The high temperature limit of the linewidth, ΔHpp(∞), is related to the parameters of the Hamiltonian describing the interactions of the spin system. The role played by magnetic anisotropy, isotropic superexchange, and double exchange is revealed and discussed in the analysis of the experimental data. In CMR and non-CMR pyrochlores, ΔHpp(∞)∝ωp2/J where J is proportional to the Curie–Weiss temperature, including the hybridization mechanism producing CMR. Instead, ΔHpp(∞) of CMR perovskites seems not to be affected by the double-exchange interaction. In contrast with the 3D perovskites, the ESR linewidth and resonance field of La1.2Sr1.8Mn2O7, a bilayer compound, although isotropic at high temperatures, becomes anisotropic for Tc=125 K<T<Tp≈450 K.

Chemical composition effects in the thin films of the colossal magnetoresistive perovskite manganates grown by MOCVD

Application of the single source MOCVD techniques allows the reproducible preparation of the complex oxide solid solutions including 3-4 metal components like perovskite manganate in which a variation of the chemical composition results in drastic variation of the electrical and magnetic properties. The numerous solid solutions (La,R) 1-x A x MnO 3 (R = Pr, Nd, A = Ca, Sr, Na) were prepared on the single crystalline substrates (LaAlO 3 , SrTiO 3 , ZrO 2 (Y 2 O 3 ), MgO) and characterized by X-ray diffraction and HREM. The focus was made on the lattice strain, structural imperfections and microtwinning in the films. The growth of the epitaxial heterostructures including CMR manganates and high T c superconductors was demonstrated.

The Influence of Surface Terminal Layer and Surface Defects on the Electronic Structure of CMR Perovskites: La 0.65 A 0.35 MnO 3 (A = Ca, Sr, Ba)

The electronic structure near to the Fermi level of the colossal magnetoresistance (CMR) perovskite manganite materials. La 0.65 A 0.35 MnO 3 (A = Ca, Sr, Ba), has been studied using both photoemission and inverse photoemission spectroscopy. The electronic structure for all three materials is very similar and consistent with an Mn-O terminal layer regardless of dopant. Small differences in the electronic structure among the materials are, however, observed. The observed band gap is not significant for La 0.65 Ca 0.35 MnO 3 and La 0.65 Ba 0.35 MnO 3 while there is a gap, about 1.5 eV, for La 0.65 Sr 0.35 MnO 3 . There is a shift to higher binding energies of the extensively hybridized Mn-O Δ 5 (e) bands for the surface (the surface on the Mn-O plane with C 4v symmetry) and t 2g bands for the bulk in the valence band spectra with increasing atomic number or atomic radius of dopants, approximately to 5.8, 6.8, and 7.8 eV for La 0.65 Ca 0.35 MnO 3 , La 0.65 Sr 0.35 MnO 3 , and La 0.65 Ba 0.35 MnO 3 . respectively. The O-Mn-O terminal layer in these materials seems to be much more defect free than is the case for La 0.9 Ca 0.1 MnO 3 . where the Ca-O terminal layer appears to be rich in defects.

Scanning Hall probe microscopy of ferromagnetic structures

A state-of-the-art scanning Hall probe microscope (magnetic field sensitivity : spatial resolution: 30 ▪: 0.8 μm—300 ▪: 0.25 μm) has been used to image a range of ferromagnetic media. The technique is non-invasive and yields quantitative profiles of the stray fields near the sample surface as illustrated by images of a standard magnetic reference sample, permalloy nanostructures and colossal magnetoresistive perovskites. Rapid scanning (∼ 1 frame/12 sec) has also been used to study the domain reversal in a thin Ni film.

Magnetic and submillimeter spectroscopy study of phase transitions in La 1− xSr xMnO 3 perovskites: T—x phase diagram

Temperature dependence of magnetization, AC magnetic susceptibility, resistance and submillimeter dielectric permittivity and dynamic conductivity were studied in colossal magnetoresistance perovskites La 1− x Sr x MnO 3, which exhibit a lot of anomalies identified with various magnetic and structural phase transitions. As a result, an overall phase T−x diagram was obtained at temperatures up to 1050 K and 0 ⩽ x ⩽ 0.45.

Very strong magnetic-field dependence of the oxygen isotope shift of the charge-ordering transition inLa0.5Ca0.5MnO3

Oxygen isotope effect on the charge-ordering transition was studied in La0.5Ca0.5MnO3 . Upon replacing 16 O with 18 O, the charge-ordering temperature increases by 9 K under zero magnetic field. With an increase of the magnetic field, the oxygen isotope shift increases rapidly and reaches a magnitude of about 40 K under a magnetic field of 5.4 T. The present results cannot be explained by the existing theories of the charge-ordering transition in manganites. @S0163-1829~99!00601-3# It is well known that the observation of the oxygenisotope effect in conventional superconductors is important for the development of the microscopic theory of superconductivity. Recently, Zhao and co-workers 1‐3 have initiated the studies of oxygen isotope effects in the colossal magnetoresistive manganese perovskites. The observed strong oxygen isotope effects in the manganites give clear evidence for the existence of small lattice polarons in these systems. The isotope effects along with many other experimental results 4‐7 strongly support those theoretical models 8,9 where a strong coupling of conduction electrons to local Jahn-Teller distortions is considered to be important for the basic physics in manganites.

Uniaxial Compressive Property of the Lattice of Colossal Magnetoresistance Perovskite Compound La1-xSrxMnO3

Under a uniaxial pressure of 10-20 MPa, the unusual changes of lattice parameters and remarkable structural distortions in colossal magnetoresistance perovskite La1-xSrxMnO3 (0.15 ⩽ x ⩽ 0.25) have been observed. The structural distortions were found to be in good agreement with Bridgman equation. These phenomena suggest that the lattice of the manganese perovskite is flexible and the versatility of the electronic and magnetic properties in the perovskites may stems from the lattice flexibility.

Investigation of granular films composed of interdispersed La1/3Ca2/3MnO3 particles and metallic Au particles

We have carried out a study of composite, granular films made by depositing LCMO (a colossal magnetoresistive perovskite) and pure, metallic Au onto MgO and LaAlO3 substrates. Elevated temperature caused the Au to segregate into small, micron-sized granules. The films were studied by atomic force microscopy, magnetoresistivity, superconducting quantum interference device magnetization measurements, and magnetically modulated microwave absorption.

Magnetic transition and electronic transport in colossal magnetoresistance perovskites

We present here the results of a careful study of the magnetic transition, as well as, the dc resistivity in the same specimen of a single crystal of ${\mathrm{La}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{MnO}}_{3}.$ We find that the Curie temperature of the specimen is far below the point where the temperature coefficient of the resistivity changes sign.

Large lattice compression coefficient and uniaxial piezoresistance in CMR perovskite La1−XSrXMaO3 (0.15 ≤ x ≤ 0.25)

Using a method of in-situ pressing X-ray diffraction, a uniaxial pressure-induced unusual change of lattice parameters and a remarkable crystal structural distortion have been observed in the perovskite-type La-Sr-Mn-O system. The pressure coefficient of lattice parameter in [200] orientation, i.e. dlndMn-O-Mn/dP, can reach about 3.76 × 10−4/Map for La0.85Sr0.15MnO3. Under a uniaxial pressure of only 20 ∼ 30 20&30 MPa, a giant piezoresistance and an evident increase of Tp, the onset of the metal-semiconductor-like (M-S) transition have also been obtained. These effects suggest that the structure of colossal magnetoresistance perovskites is very flexible and their transport properties are structurally sensitive.

Magnetic structures in colossal magnetoresistance perovskites (abstract)

Ferromagnetism in colossal magnetoresistance perovskites such as LaxB1−xMnO3 (B=Ba, Sr, Ca) is typically explained using the double exchange (DE) mechanism proposed by Zener. To explain the local structure above Tc one needs to introduce a coupling to lattice degrees of freedom. Here, we show the structure of the resulting magnetoelastic polarons and the temperature dependence of their pair potential. To investigate the “ferro”-magnetic state below Tc we study the ferromagnetic Kondo lattice model in the strong coupling limit. It turns out that the DE mechanism has unexpected properties, when there is more than one itinerant electron. We find that in general, the many-body ground state of the DE model is not globally ferromagnetically ordered. Also, the low-energy excitations of this model are distinct from spin wave excitations in usual Heisenberg ferromagnets, which will result in unusual dynamic magnetic properties.

CMR films structure as a function of growth and processing

Among the family encompassed by the complex metal oxides are the colossal magneto-resistive perovskites with potential applications in advanced high density magnetic data storage and sensor devices based on single or multilayers of these materials fabricated by vapor phase deposition methods. Realization of this potential requires solving basic thin film fabrication problems coupled to understanding the resulting structure-properties relationships. The specific objectives addressed in this study were to determine the dependence of growth and transport properties: (1) reproducibility on the pulsed laser deposition (PLD) target, (2) on sputter deposition chamber gas and substrate, and (3) on a post-deposition anneal in a reducing atmosphere. Toward these ends, we have grown La 0.67Ca 0.33MnO 3 films on perovskite substrates by PLD and RF off-axis sputter deposition from stoichiometric targets at temperatures from 400 to 900°C. The PLD films were grown from a new commercial target and post-deposition annealed in O 2 to reproduce, for comparison, the conditions used previously for the films grown with an in-house fabricated target. The sputter deposited films were grown in both ArO 2 mixed and Ar-only atmospheres, some grown simultaneously on different substrates; one ArO 2 grown film was post-annealed in a reducing atmosphere. Microstructures were studied by scanning probe microscopies and temperature-dependent transport properties by 4-point resistance measurements. Previously we reported on characterization of the T substrate( T s)-dependent microstructure of PLD as-deposited and post-annealed films grown from an in-house fabricated target. Annealed films grown from the new commercial target exhibited improved consistency in T-dependent transport and magnetization properties, independent of growth temperature. None of the as-grown sputter deposited films grown in the ArO 2 mixture exhibited a metallic transition while those grown in Ar-only at T s > 800°C had a range of transition temperatures between 110 and 180 K. The variation in substrate-dependent structure demonstrated again the sensitivity of growth mechanism to template. Finally, the 500°C film grown in the ArO 2 mixture, went from insulating at all T's to exhibiting a 215°C metallic transition after the reducing anneal treatment.

Crossover from charge-localized state to charge-ordered state in Pr2/3Ca1/3MnO3.

In ${\mathrm{Pr}}_{\frac{2}{3}}{\mathrm{Ca}}_{\frac{1}{3}}\mathrm{Mn}{\mathrm{O}}_{3}$, we have found a crossover from a charge-localized state to a charge-ordered (CO) state. Linear-thermal-expansion and magnetostriction measurements up to 14 T show that for temperatures below ${T}_{p}\ensuremath{\sim}400$ K a charge localization is present as in the case of ${\mathrm{La}}_{\frac{2}{3}}{\mathrm{Ca}}_{\frac{1}{3}}\mathrm{Mn}{\mathrm{O}}_{3}$. Below ${T}_{\mathrm{CO}}\ensuremath{\approx}210$ K, a charge ordering, as found in some ${R}_{0.5}{A}_{0.5}\mathrm{Mn}{\mathrm{O}}_{3}$ ($R\ensuremath{-}A=\mathrm{L}\mathrm{a}\ensuremath{-}\mathrm{C}\mathrm{A},\phantom{\rule{0ex}{0ex}}\mathrm{P}\mathrm{r}\ensuremath{-}\mathrm{C}\mathrm{a},\phantom{\rule{0ex}{0ex}}\mathrm{P}\mathrm{r}\ensuremath{-}\mathrm{S}\mathrm{r},\phantom{\rule{0ex}{0ex}}\mathrm{N}\mathrm{d}\ensuremath{-}\mathrm{S}\mathrm{r}$) compounds, is observed. A magnetic field suppresses the CO state below ${T}_{\mathrm{CO}}$ giving rise to a first-order structural transition. A continuous change in volume is measured between ${T}_{\mathrm{CO}}<T<{T}_{p}$. The electrical behavior is similar to the structural behavior, and is the result of the strong spin-charge-lattice coupling present in the colossal-magnetoresistance perovskites.

Double degeneracy and Jahn-Teller effects in colossal-magnetoresistance perovskites.

Jahn-Teller (JT) electron-phonon coupling effects in the colossal magnetoresistance perovskite compounds $La_{1-x}A_xMnO_3$ are investigated. Electron-electron correlations between two degenerate Mn $e_g$ orbitals are studied in the Gutzwiller approximation. The static JT distortion and antiadiabatic polaron effects are studied in a modified Lang-Firsov approximation. We find that (i) the electron or hole character of the charge carrier depends on the static JT distortion, and (ii) due to the two-component nature of the JT coupling, fluctuations in the JT distortion direction contribute to the charge transport in similar fashion as the local spins.

Microstructural Study of CMR Films as a Function of Growth Temperature, As-Deposited and Annealed

AbstractThe properties encompassed by the family of complex metal oxides span the spectrum from superconductors to insulating ferroelectrics. Included in this family are the new colossal magnetoresistive perovskites with potential applications in advanced high density magnetic data storage devices based on single or multilayer thin films units of these materials fabricated by vapor phase deposition (PVD) methods. The realization of this potential requires solving basic thin film materials problems requiring understanding and controlling the growth of these materials. Toward this end, we have grown La0.7Ca0.3MnO3 and La0.7Sr0.3MnO3 on LaAlO3 single crystal substrates by pulsed laser and RF sputter deposition at temperatures from 500° C to 900° C and annealed at over 900° C for about 10 hours. The evolution of the microstructure of these films was studied by scanning probe microscopies and transmission electron microscopy (TEM).The results of SPM characterization showed that at the lower end of the growth temperature range, the as-grown films were polygranular with grain size increasing with temperature. The 500° C as-grown films appeared to be amorphous while the 750° C film grains were layered with terrace steps often one unit cell high. In contrast, films grown at 900° C consisted of coalesced islands with some 3-D surface crystals. After annealing, all films had coalesced into very large extended layered islands. The change in microstructure was reflected in a decreased resistivity of coalesced films over their unannealed granular precursors. Previous reported work on the growth of La0.84 Sr0.16MnO3 and La0.8Sr0 2CoO3 grown demonstrated the sensitivity of the microstructure to substrate and deposition conditions. Films grown on an “accidental” vicinal surface grew by a step flow mechanism.