The growth technique, the micromorphological and microstructural characterization by means of atomic force microscopy (AFM) and secondary ions mass spectrometry (SIMS) as well as the magnetic properties of a novel class of magnetic multilayers, based on radio frequency (RF) sputtered thin amorphous garnet films, are presented. One, three and five thin film multilayers composed by amorphous pure yttrium iron garnet (a:YIG) and amorphous gadolinium gallium garnet (a:GGG) have been grown on GGG single crystal substrates. The multilayer interfaces have been found to be comparable in both, the three and five-layers structure. Low field susceptibility measurements, showed a paramagnetic behaviour for the single layer YIG film. For the three and five layers samples, irreversibility effects were observed, giving evidence of magnetic clusters at the interface YIG/GGG.

The nano-meter controlled iron/iron-oxide multilayer materials have been successfully obtained by the pulse reactive sputtering method with high deposition rate. These multilayer demonstrated a good thermal stability of its structure and magnetic properties up to 500°C when a small amount of Si was doped in the structure, whereas the non-doped multilayer degraded at above 300°C. The difference of the oxidation energy between Fe and Si increases the thermal stability of the interface between Fe and Fe-O layer.

Perovskite-structured LaxSryMnO3 thin-films have been deposited onto LaAlO3 substrates via liquid delivery chemical vapor deposition (LD-CVD) using metal(β-diketonato) precursors, M(thd)x [where M= Ca, Sr, La and Mn, thd = 2,2,6,6-tetramethyl-3,5-heptanedionato and x = 2–3]. Thin films were deposited at temperatures between 500 and 700 °C and subsequently annealed at 1000 °C under O2. These films possess stoichiometries that are: i) vastly different from the La0.67Sr0.33MnO3 compositions commonly reported in the literature and ii) display high temperature, low field responses that may be technologically important. Resistance versus temperature measurements revealed a metal to semiconductor transition at room temperature and above. Hall measurements on a film of La0.35Sr0.24MnO3 displayed a magnetoresistive response (MR) of -10% at 57 °C in a fixed magnetic field of 780 Oe. Based upon our research, the observed film properties are directly related to the deposited film stoichiometry and the best results were observed at Sr / La ratios between 0.30 and 1.0 for A-site deficient LaxSryMnO3 thin-films after thermal annealing.

In this paper we have studied the dia and paramagentic susceptibilities of the holes in ultrathin films of magnetic materials in the presence of a parallel magentic field on the basis of a newly derived dispersion law for such systems. The numerical computations are performed taking Hg1-xMnx Te and Cd1-xMnx Se as examples. Both the susceptibilities increses with decreasing doping and film thickness respectively. It is important to note that not only the paramagnetic-to-diamagnetic susceptibility ratio for the present case deviates from (1/3) in conventional semiconductors, but also that is a critical region, where quenching of the diamagnetic occurs. The theoretical analysis is in agreement with the experimental datas as given elsewhere.

This paper addresses aspects of the theory of the formation and motion of polarons that appear relevant to understanding some metal-to-semiconductor transitions in oxides. First, the physical bases of both the long- and short-range electron-lattice interactions usually considered in polaron theory are described and contrasted with one another. Then the notion of self-trapping and the formal theory of polaron formation are presented. Using a scaling analysis of the nonlinear wave equation that lies at the heart of polaron formation, essential features of polaron formation are readily obtained for both types of electron-lattice interaction operating individually and in tandem. The theory is extended to apply to a carrier bound within a Coulomb potential.

Two distinct types of bound polaron state can exist. A “small” polaron's electronic carrier is confined to a single site. Alternatively, a “large” polaron's electronic carrier is distributed over multiple sites. When separated by an energy barrier, these distinct states can coexist. A “collapse” occurs when a continuous change of physical parameters produces an abrupt change of the groundstate from being large-polaronic to being small-polaronic.

To introduce magnetic effects, the scaling analysis is first applied to the formation of a large magnetic polaron, a charge carrier that moves freely within a large ferromagnetic cluster embedded within an antiferromagnet. The polaron is large enough that the predominant interactions are the exchange interactions of local magnetic moments among themselves and with the charge carrier.

The scaling analysis is then extended to describe the donor-state collapse that is thought to drive the metal-to-insulator transition that occurs in n-type EuO as this ferromagnet is heated toward its paramagnetic state. In this case, the metallic impurity conduction that dominates transport at low-temperatures is suppressed when the ferromagnet's large-radius donor states collapse to small-polaronic states upon approaching the paramagnetic regime. At appropriate doping levels, this transition is associated with a huge negative magneto-resistance.

This paper finally addresses small-polaronic hopping transport in p-type LaMnO3. Attention is focused on the effects of compensating holes with electrons generated by oxygen vacancies. The Curie temperature is reported to be insensitive to this compensation. The low-temperature ferromagnetism is even unaffected when the hole density is reduced enough to eliminate metallic conductivity. These results imply that the ferromagnetism is not carrier-induced. Furthermore, the strong sensitivity of the high-temperature Seebeck coefficient to compensation suggests that the carriers hop amongst only a small subset of Mn sites. These cation sites may be associated with the divalent cation dopants. The observation of an n-type Hall effect is consistent with the notion that the hopping is a type of impurity conduction. Indeed, Hall effect sign anomalies are predicted and observed for the hopping of holes in disordered solids. In this view the transition from a ferromagnetic-metal to a paramagnetic-semiconductor in doped LaMnO3 is similar to that of EuO, in that both transitions are associated with the collapse of carriers from extended states into small-polaronic impurity states as the temperature approaches the Curie temperature.

We study some compounds of the perovskite (or pseudo-cubic perovskite) series AMO3, where M is a transition metal and A is Ca, Sr, or Nd, by LSDA self-consistent electronic structure calculations with the LMTO method. Transport and magnetic properties, as well as Fermi surfaces are calculated. These materials exhibit sharp density of states features in the vicinity of the Fermi level that strongly affect their transport and magnetic properties and make them very sensitive to structural deformation and stoichiometry. Calculated total energies are very close for anti-ferromagnetic and ferromagnetic solutions. This explains qualitatively the magnetoresistive anomalies shown by this family of compounds.

We have performed X-ray absorption (XAS) and angle-resolved photoemission (ARPES) on single crystals of both the layered and cubic colossal magnetoresistive manganites to determine the electronic structure and the relevant energy scales in the problem: the intra-atomic exchange energy J (∼ 2.7 eV), the Jahn-Teller energy gain EJ-T (<.25 eV), the one-electron bandwidth W (>3 eV for layered compounds) and the lattice relaxation or polaron binding energy EB (.65 eV ferromagnetic phase and.8 eV paramagnetic phase). Lattice polarons are deemed important especially in the paramagnetic but also to a degree in the ferromagnetic phase. Due to the energy scale mismatch, the Jahn-Teller effect is unlikely to be the cause for these lattice polarons, at least for the layered samples.

Different tunneling mechanisms in conventional and half-metallic ferromagnetic tunnel junctions are analyzed within the same general method. Direct tunneling is compared with impurity-assisted, surface state assisted, and inelastic contributions to a tunneling magnetoresistance (TMR). Theoretically calculated direct tunneling in iron group systems leads to about a 30% change in resistance, which is close to experimentally observed values. It is shown that the larger observed values of the TMR might be a result of tunneling involving surface polarized states. We find that tunneling via resonant defect states in the barrier radically decreases the TMR (down to 4% with Fe-based electrodes), and a resonant tunnel diode structure would give a TMR of about 8%. With regards to inelastic tunneling, magnons and phonons exhibit opposite effects: one-magnon emission generally results in spin mixing and, consequently, reduces the TMR, whereas phonons are shown to enhance the TMR. The inclusion of both magnons and phonons reasonably explains an unusual bias dependence of the TMR.

The model presented here is applied qualitatively to half-metallics with 100% spin polarization, where one-magnon processes are suppressed and the change in resistance in the absence of spin-mixing on impurities may be arbitrarily large. Even in the case of imperfect magnetic configurations, the resistance change can be a few 1000 percent. Examples of half-metallic systems are CrO2/TiO2 and CrO2/RuO2, and an account of their peculiar band structures is presented. The implications and relation of these systems to CMR materials, which are nearly half-metallic, are discussed.

We have constructed artificial superlattices with a combination of magnetic/magnetic and ferroelectric/ferromagnetic materials using a laser ablation technique. An ideal hetero-epitaxy can be obtained due to the similar crystal structure of the perovskite type di/ferroelectric BaTiO3, Pb(Zr,Ti)O3 (so-called PZT), SrTiO3 and ferro/antiferromagnetic LaFeO3, LaCrO3, (La,Sr)MnO3. First of all, we have controlled ferromagnetic order on LaFeO3/LaCrO3 superlattices formed on SrTiO3(l11) substrate. Such a spin structure(ferromagnetic order) can't be got in bulk condition. In the heterostructured ferromagnetic /ferroelectric devices, (La,Sr)MnO3/PZT there are remarkable and interesting phenomena. The electric properties of the ferromagnetic material can be controlled by the piezoelectric effect via distortion of the crystal structure.

We have studied the fc-dependent electronic structure of the layered colossal magnetoresistive manganite La1.2Sr1.8Mn2O7 using high-resolution angle-resolved photoemission spectroscopy. We found dispersive energy bands as a function of the crystal momentum k near the Fermi level (EF). We have also performed local spin density approximation (LSDA)+U band-structure calculations on the current system. The overall experimental dispersion relation is basically in agreement with the band-structure calculations yet close to EF there is a significant deviation from the predicted dispersions. Instead of clear Fermi-surface (FS) crossings, we observe a depression of the features as the FS is approached as if there is a “pseudo” gap in the excitation spectrum. The pseudogap continuously opens with temperature and does not show further significant opening above Tc, corresponding to the metal-insulator transition. Those unusual aspects of the spectra has been discussed from the viewpoint of the strong electron-lattice coupling model.

Metallic manganite oxides, La1-xDxMnO3 (D=Sr, Ca, etc.), display “colossal” magnetoresistance (CMR) near their magnetic phase transition temperatures (Tc) when subject to a Tesla-scale magnetic field. This phenomenal effect is the result of the strong interplay inherent in this class of materials among electronic structure, magnetic ordering, and lattice dynamics. Though fundamentally interesting, the CMR effect achieved only at large fields poses severe technological challenges to potential applications in magnetoelectronic devices, where low field sensitivity is crucial. Among the objectives of our research effort involving manganite materials is to reduce the field scale of MR by designing and fabricating tunnel junctions and other structures rich in magnetic domain walls. The junction electrodes were made of doped manganite epitaxial films, and the insulating barrier of SrTiO3. The interfacial expitaxy has been imaged by using high-resolution transmission electron microscopy (TEM). We have used self-aligned lithographic process to pattern the junctions to micron scale in size. Large MR values close to 250% at low fields of a few tens of Oe have been observed. The mechanism of the spin-dependent transport is due to the spin-polarized tunneling between the half-metallic electrodes, in which the spins of the conduction electrons are nearly fully polarized. We will present results of field and temperature dependence of MR in these structures and discuss the electronic structure of the manganite inferred from tunneling measurement. Results of large MR at low fields due to the grain-boundary effect will also be presented.

Large magnetoresistance values are obtained on tunnel junctions epitaxially deposited by pulsed-laser deposition and consisting of ferromagnetic manganite La0.67Sr0.33MnO3 electrodes separated by various tunnel barriers: SrTiO3, PrBaCu2.8Ga0.2O7 and CeO2. The magnetoresistance can be decomposed into a low-field and a high-field contribution. The latter is attributed to the presence of canted interfacial manganite phases, as confirmed by the temperature behaviour of the resistance. A low-field magnetoresistance ratio of 450% below 100 Oe is obtained on a sample with a SrTiO3 barrier, indicating a spin polarization value in excess of 0.83 for the manganite.

We report the fabrication of ferromagnet-insulator-ferromagnet junction devices using a ramp-edge geometry based on (La0.7Sr0.3)MnO3 ferromagnetic electrodes and a SrTiO3 insulator. The multilayer thin films were deposited using pulsed laser deposition and the devices were patterned using photolithography and ion milling. As expected from the spin-dependent tunneling, the junction magnetoresistance depends on the relative orientation of the magnetization in the electrodes. The maximum junction magnetoresistance (JMR) of 30 % is observed below 300 Oe at low temperatures (T < 100 K).

Epitaxial growth of oxide heterostructures, which may be utilized in spin valve applications, has been demonstrated. The heterostructures consist of two ferromagnetic layers separated by a nonmagnetic metallic interlayer. The ferromagnetic material used is the manganese perovskite oxide, La0.7Sr0.3MnO3, while the metallic oxide interlayer is La0.5Sr0.5CoO3. X-ray diffraction spectra demonstrate the high structural quality of the heterostructures. The magnetization of the heterostructure as a function of magnetic field measured at room temperature yields a double hysteresis loop that is characteristic of this type of spin valve structure. The behavior of this double hysteresis loop is also examined as a function of the metallic interlayer thickness.

Trilayer YBa2Cu3O7-δ/(SrTiO3, CeO2)/La0.67Sr0.33MnO3-δ devices have been fabricated for the study of supercurrent suppression due to the injection of spin-polarized quasiparticle current. The critical current for a YBa2Cu3O7-δ/100 Å SrTiO3/La0.67Sr0.33MnO3-δ device was found to decrease from 118 mA to 12.6 mA, for an injection current of 60 mA. The effect of film microstructure on the critical current suppression was investigated. Defects in the SrTiO3 and CeO2 layers were found to control the device properties.

C-axis oriented La0.7Sr0.3MnO3.δ (LSMO) films were fabricated on the top of SrTiO3/YBa2Cu3O7 grown on MgO(001) substrates. From x-ray φ-scan and planar transmission electron microscopy measurements, the LSMO layer in the LSMO/SrTiO3/YBa2Cu3O7/MgO heterostructure is found to have coherent in-plane grain boundaries with a predominance of 45° rotations (between [100] and [110] grains) in addition to the cube-on-cube epitaxial relationship. Also, epitaxial LSMO/Bi4Ti3O12/LaAl03 (001) and c-axis textured LSMO/Bi4Ti3O12/SiO2/Si(001) with random in-plane grain boundaries are introduced as the counterparts for comparison. The resistivity and magnetoresistance (MR) of LSMO layer were measured and compared in these three different heterostructures. The low field MR at low temperature shows a dramatic dependence on the nature of the grain boundary. An attempt is made to interpret these results on the basis of correlation between the magnetic properties and grain structures.

Ambient observation of magnetic structures by magnetic force microscopy (MFM) in La0.67Sr0.33MnO3 films has not yet been clearly correlated with stresses induced by the kinetic or thermodynamic growth processes or the compressive (LaAlO3) or tensile (SrTiO3) nature of the substrate lattice-mismatch. Although domain-like magnetic structures have been seen in some as-grown films on LAO and related to substrate-induced stress and film thickness, no magnetic structure has been seen for films on STO and other films grown under different kinetic conditions on LAO. In this study we have identified a set of pulsed-laser deposition conditions with the substrate temperature as a variable to determine the relationship between growth and stress-induced magnetic structures. Results from scanning tunneling, atomic force, and MFM microscopies, magnetization, and coercivity measurements will be presented.

We report the direct measurement of elastic strain effect on the electrical and magnetic properties of single domain epitaxial SrRuO3 thin films, using a lift-off technique. The as-grown films on vicinal (001) SrTiO3 substrates are subjected to elastic biaxial compressive strain within the plane and tensile strain normal to the plane. In contrast, the lift-off films prepared by chemical etching of SrTiO3 substrates, are completely strain free with bulk like lattice. Our measurements indicate that the elastic strain can significantly affect the electrical and magnetic properties of epitaxial ferromagnetic SrRuO3 thin films. For the strained films, the Curie temperature (Tc) was suppressed to 150K and the saturation magnetic moment (Ms) was decreased to 1.15μB/Ru atom as compared to a Tc of 160K and Ms of 1.45μB/Ru atom for the strain free films. These property changes are attributed to the structural distortion due to the elastic strain in the as-grown epitaxial thin films. Our results provide direct evidence of the crucial role of lattice strain in determining the properties of the perovskite epitaxial thin films.

We review resistivity, x-ray-absorption fine-structure (XAFS) and muon spin relaxation (μSR) data which provide clear evidence for localized holes causing polaron distortion and unusual spin dynamics below Tc in “colossal magnetoresistive” (CMR) La1-xCaxMnO3. Resistivity measurements for x=0.33 under an applied field H have shown that ln[ρ(H,T)] α -M, where M is the magnetization. The XAFS data show a similar functional dependence for the polaron distortions on M. The data from these two measurements are interpreted in terms of some fraction of the available holes x remaining localized and some increasing fraction becoming delocalized with increasing M. Finally, this polaron-induced spatial inhomogeneity yields anomalously slow, spatially inhomogeneous spin dynamics below Tc, as shown in the μSR data. These experiments individually probe the charge, lattice and spin degrees of freedom in this CMR system and suggest that the polarons retain some identity even at temperatures significantly below Tc.

The effects of disorder on the transport and magnetic properties of pulsed laser deposited La0.7Ca0.3MnO3 thin films were studied. Ion irradiation with 10 MeV I and 6 MeV Si ions was used to produce controlled levels of defects ranging from 0.006 to 0.024 displacements per atom. The peak resistance temperature of the I-irradiated films decreased from 264K for the undamaged film to OK for the 0.016 dpa film. The magnetic ordering temperature decreased from 270K (undamaged) to 130K (0.011 dpa), and remained nearly constant at 130K for higher damage levels. This demonstrates a decoupling of the magnetic and metal-insulator transitions. A characteristic relationship between the peak resistance temperature and activation energy of resistance with the form Tp = 285(1-Δρ/115)0.13, was observed for all films. This characteristic relationship indicates that the range of resistivity and magnetoresistivity values for observed La0.7Ca0.3MnO3 can be explained in terms of disorder-limited polaron hopping.