A sensitive tunnel diode oscillator (TDO) technique operating at 4MHz is used to probe the dynamic response of La1−xSrxMnO3 single crystals for x = 0.125, 0.175, 0.28 and 0.33 doping. Systematics of the measured change in reactance (δX) as a function of temperature (30K < T < 320K) and DC magnetic field (0 < H < 6kOe) reveal distinct temperature and field scales associated with the dynamic response of spin. It is notable that these features are far more striking than the corresponding features in static measurements. The results are discussed in the context of structural changes leading to polaron ordering.

From the magnetic phase diagrams established for Ln1−xAExMnO3 manganites with x>0.5, it is shown that the magnetoresistance is only obtained for compositions of smallest average Asite cations <rA>. The magnetic structure study reveals that the small <rA> favors the coexistence of ferromagnetism with G-type antiferromagnetism. It is also shown that ferromagnetism can be induced in the Ln2−xAExMnO4 type electron doped manganites by using compositions with small Ln and AE cations and x values close to 1.90. Finally, manganse site doping by Cr, Co, Ni can be used to weaken the charge-ordering of the Mn4+ rich perovskite manganites (0.5<x≤0.8) and thus to induce CMR properties.

Comprehensive measurements of electron spin resonance and magnetization of La0.7Ca0.3MnO3 in the ferromagnetic as well as paramagnetic phases were carried out. From the quantitative analysis of the ESR signal, attributed to itinerant spins and localized spins in the bottleneck regime, evidences for the inhomogeneous nature of both phases, consisting of clusters of one phase embedded in the other, were found. It is suggested that the microscopic local magnetic structures above and below Tc are qualitatively similar except that the phase below Tc carries long range order.

Infrared reflectance and Raman spectroscopy measurements have been performed through the metal-semiconductor (or -semimetal) transitions of various “colossal magnetoresistance” and charge-ordering materials, including EuB6 and the manganese perovskites. As described in this paper, our results demonstrate that these systems have phase regions characterized by electronic inhomogeneity of various types, including: (1) pure spin polarons, which form in a narrow temperature range above Tc in the “colossal magnetoresistance” system EuB6; (2) magnetoelastic polarons, and an inhomogeneous ferromagnetic phase comprised of both metallic and insulating regions, in the “colossal magnetoresistance” regime of La1−x(Sr,Ca)xMnO3 (x < 0.5); and (3) phase separation behavior involving coexisting ferromagnetic and antiferromagnetic domains in the intermediatetemperature (CO) phase of the charge-ordering system Bi1−xCaxMnO3 (x > 0.5).

We have measured atomic pair distribution functions (PDF) of La1−xCaxMnO3 using high energy x-ray diffraction. This approach yields accurate PDFs with very high real-space resolution. It also avoids potential pitfalls from the more usual neutron measurements that magnetic scattering is present in the measurement, that the neutron scattering length of manganese is negative leading to partial cancellation of PDF peaks, and that inelasticity effects might distort the resulting PDF We have used this to address the following questions which do not have a satisfactory answer: (1) What are the amplitudes and natures of the local Jahn-Teller and polaronic distortions in the CMR region. (2) Is the ground-state of the ferromagnetic metallic phase delocalized or polaronic. (3) As one moves away from the ground-state, by raising temperature or decreasing doping, towards the metal insulator transition, how does the state of the material evolve?

Magnetotransport properties of strained epitaxial Pr2/3Sr1/3MnOz (PSMO) ultrathin films have been studied. The strains are controlled by growing PSMO films on different lattice-mismatched substrates which impose biaxial compressive-, tensile-, and very little strain in the films, respectively. Distinctive magnetoresistance effects in low magnetic field (LFMR) have been observed in films with different types of strain. The films with compressive strain show very large negative LFMR (> 1000 % at 2500 Oe) and significant magnetoresistance hysteresis when a magnetic field is applied perpendicular to the film plane. In a parallel field, the LFMR is much smaller with almost no hysteresis. In contrast, the tensile-strained ultrathin films show positive LFMR at low temperatures in a perpendicular field and a negative LFMR in a parallel field. In comparison, the almost strain-free ultrathin films show very small LFMR (< 2 %) as in single crystal samples at similar temperatures and magnetic fields. In the compressive-strained samples, the large LFMR decreases rapidly with increasing film thickness. These results are interpreted based on spin dependent scattering at the domain walls and domain-rotation.

We have performed depth profile analyses of the lattice parameters in epitaxial thin films of La1−xCaxMno3 (LCMO), where x = 0.33 or 0.3, to understand the evolution of strain relaxation processes in these materials. The analyses were done using Grazing Incidence X-ray Scattering (GIXS) on films of different thicnesses on two different substrates, (100) oriented LaAlO3 (LAO), with a lattice mismatch of ∼2% and (110) oriented NGO, with a lattice mismatch of less than 0.1%. Films grown on LAO can exhibit up to three in-plane strained lattice constants, corresponding to a slight orthorhombic distortion of the crystal, as well as near-surface and columnar lattice relaxation. As a function of film thickness, a crossover from a strained film to a mixture of strained and relaxed regions in the film occurs in the range of 700 Å. The structural evolution at this thickness coincides with a change in the resistivity curve near the metalinsulator transition. The in-plane compressive strain has a range of 0.2 – 1.5%, depending on the film thickness for filsm in the range of 400 - 1500 A.

Effects of drastic change of 3-dimensional films' lattice strain by substrate structural transformations on the transport and magnetic properties of tensile strained epitaxial La0.66Sr0.33MnO3 thin films were studied using (001) BaTiO3 as the templates. These films exhibit dramatic jumps in electrical resistance and magnetization at temperatures very close to different crystallographic structural change of BaTiO3 substrate. Application of high magnetic field (50 kOe) suppresses both the electrical transport and magnetization jumps. The observed low-field sharp transitions correspond sudden changes in magnetic anisotropy, caused by the structural transformation of the substrate. The magnetic easy axis lies in the film plane along [100] direction having a strong correlation with the substrate-induced 3-dimensional strain states as determined by the normal and grazing incidence x-ray diffraction. The sudden substrate structural transformations imposes an additional drastic sharp change of the film strain which is found to reorient the magnetic easy axis within the film plane.

Lattice distortions, be they in the form of chemical and hydrostatic pressure in bulk or lattice mismatch between film and substrate, have significant effects on the transport as well as the magnetic properties of colossal magnetoresistance (CMR) materials. We summarize here our results on tensilely and compressively strained La0.7Sr0.3MnO3 (LSMO) thin films that indicate the important role of lattice distortions due to the lattice mismatch between the film and substrate. The strain due to lattice distortions can be used to tune the magnetic domain structure, magnetization, magnetic anisotropy and magnetotransport of LSMO thin films.

We report the temperature dependence of the magnetic anisotropy in both compressive and tensile strained films of Pr0.67Sr0.33MnO3 (PSMO). Compressive strain induced by growth on LaAlO3 (LAO) substrates results in a spontaneous out-of-plane magnetization, while tensile strain (grown on SrTiO3) results in in-plane magnetization. The coefficient of linear proportionality between the magnetic anisotropy energy and the tetragonal strain for both compressive and tensile strained PSMO films is larger than that found previously in strained La0.67Ca0.33MnO3 films. From the data, we estimate a 20 unit cell magnetic domain wall width for PSMO / LAO. Scattering from such a narrow domain wall could produce a potentially significant contribution to the resistivity.

In general, epitaxially grown thin films are biaxially stressed if the lattice mismatch of substrate and film is below 2%. We have used epitaxial strain as an extrinsic source of tensile stress exerted to La0.66Ca0.33 MnO3 thin films deposited on SrTiO3 single crystal substrates. The average stress in the film is a function of film thickness and post deposition annealing. Thickness variation and annealing procedures have been used to explore the stress dependence of the characterisics of magnetization curves of the stressed films. It could be shown that the inflection point of the magnetization curve which determines the transition from Blochwall movements to rotations of the magnetization vector and the Rayleigh constant of the virgin magnetization curve are correlated with the stress of the films in analogy to results obtained for plastically deformed single crystals of conventional ferromagnetic metals.

We report the growth and characterization of Nd0.5Sr0.5MnO3 thin films deposited by the Pulsed Laser Deposition (PLD) technique on [100]-oriented LaAlO3 substrates. X-ray diffraction (XRD) studies show that the films are [101]-oriented, with a strained and quasi-relaxed component, the latter increasing with film thickness. A post-annealing under oxygen leads to a quasi-relaxed film with a metallic behavior. We also observe that transport properties are strongly dependent on the thickness of the films. Variable temperature XRD down to 100 K suggests that this is caused by substrate-induced strain on the films.

Electrical conductivity and magnetic relaxation has been studied in single crystals of Mn doped LaGaO3 with doping concentrations of 0%, 0.5%, 2%, 10% and 50%. The resistivity of the crystals strongly depends on temperature and can be described with the hopping model. Both activation energy and resistivity coefficient grow significantly with decrease in Mn ion concentrations. Dramatic changes are observed in 71Ga NMR spin-lattice relaxation times.

The specific heat of a polycrystalline sample of MnO was measured from T ≈ 1 K to T ≈ 400 K using two different experimental apparatuses at zero applied pressure. Features revealed by the data include a hyperfine contribution due to the Mn nuclei, a T2 temperature dependence at low temperatures due to the type-II antiferromagnetic magnon contribution, and a sharp but well defined antiferromagnetic transition (TN = 117.7095 K) that is clearly second order in nature. The critical exponent, α, deduced from the transition is consistent with a two dimensional Ising model. The specific heat of MnO is also compared with recent results on the type-A antiferromagnet LaMnO3.

A series of 400 nm thick metal-insulator films of compositions (Co50Fe50)x(Al2O3(100-x) (7 ≤ x ≤ 52; x is in vol.%) are deposited on glass substrates using dual electron beam evaporation technique. The films are nanocrystalline with crystallite sizes of 1-3 nm. Resistivity of the films varies as a function of (I/T)0.5 showing a tunneling type behaviour. The films show isotropic and negative magnetoresistance (GMR). A film of composition (Co50Fe50)82.5(Al2O3)17.5 show maximum tunneling magnetoresistance (TMR) of 7.2% at room temperature and in a magnetic field of 8.2 kOe.

The evolution of structural, electrical and magnetic properties with the isovalent chemical substitution of Ca2+ into the Sr2+ sites in new series of two-dimensional La1.2(Srl.8−XCax)Mn2O7 compounds (x = 0 ∼ 1.8) and three-dimensional La0.6(Sr0.4−xCax)MnO3 compounds (x = 0 ∼ 0.4) are investigated. The highest magnetoresistance (MR) ratios [ρ(0) - ρ(H) / ρ(0)] of 52 % (H = 1.5 T) at 102 K and 13 % (H = 1.5 T) at 210 K were observed for the x = 0.4 samples in La1.2(Sr1.8−xCax Mn2O7 and La0.6(Sr0.4−xCax)MnO3, respectively. The Curie temperatures (Tc) decreased from 135 K to 102 K and 370 K to 270 K for x = 0 to 0.4 in La1.2(Sr1.8−xCax)Mn2O7 and La0.6(Sr0.4−xCax)MnO3, respectively. The compositional dependence of the structural variation has been found in La0.6(Sr0.4−xCax)MnO3. Our results confirm that the dimensionality as well as ionic size plays an important role in controlling the colossal magnetoresistance in manganites.

Magnetic perovskites of the general form La0.7Sr0.3Mn1−yFeyO3 (y = 0, 0.05, 0.10, and 0.15) have successfully been synthesized using deposition by aqueous acetate solution (DAAS). Crystalline, iron-doped, lanthanum strontium manganite (Fe-doped LSMO) powders are obtained by preparing an aqueous solution of metal acetate precursors in the proper stoichiometry, drying the solution to generate a glassy gel, consolidating the gel, and then firing the gel for short periods of time (<2hrs). This novel technique has the potential for depositing large area thin films with high throughput and low cost. Powder samples of La0.7Sr0.3Mn1−yFeyO3 prepared by DAAS and annealed for 100 minutes at 1200°C are of high purity, are single phase, and exhibit excellent electrical and magnetic characteristics. Powders annealed at 1200°C or greater exhibit sharp metalinsulator transitions. Increasing the iron dopant concentration in these powders from 0% to 15% decreases the metal-insulator transition temperature of these samples from ∼360K to about 140K. The resistivity of these powders also increases with increasing substitution of the lattice B-site with iron, as does the unit cell volume of the lattice. Preparation of an iron doped LSMO powder that exhibits a maximum magnetoresistance at 305K was successfully carried out via a careful selection of iron content and anneal temperature. This compound, La0.7Sr0.3Mn0.93Fe0.07O3, exhibits a magnetoresistance of 40% at 305K and an applied field of 5 Tesla.

We show that the synthesis conditions have a dramatic influence on the resistivity behavior of calcium and sodium doped LaMnO3. Several samples prepared by lowtemperature techniques exhibit a double-peaked curve of resistance versus temperature. The model of spin-polarized intergranular tunneling provides a good approach to discuss our experimental results. Grain size and crystallinity are proved to be essential parameters, which are strongly influenced by the preparation process (e.g. the precursors nature and the thermal treatment).

Using both a “classical” pulsed laser deposition process and a laser-MBE process, superlattices in the Pr0.5Sr0.5MnO3 system were grown from PrMnO3 and SrMnO3 targets. A-site ordering of perovskite-like manganite was achieved using both deposition processes. However the electric and magnetic properties of the superlattices are shown to be strongly dependent on the growth process.

The structure and magnetic properties of sol-gel-synthesized, nanocrystalline KσMnO2 were investigated. The nanoparticles were determined by x-ray diffraction and high-resolution transmission electron microscopy to be single-crystal rods of the cryptomelane phase of MnO2, with a typical particle size of 6 nm × 20 nm. The field and temperature dependence of the magnetization indicates superparamagnetic behavior, with a blocking temperature of 15K. The dependence of the magnetic properties on particle size, surface layers and mixed valency is discussed.