Magnetotransport characteristics of strained La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> epitaxial manganite films

The electrical and magnetic characteristics of La_0.7Sr_0.3MnO₃ (LSMO) epitaxial manganite films are investigated by different methods under conditions when the crystal structure is strongly strained as a result of mismatch between the lattice parameters of the LSMO crystal and the substrate. Substrates with lattice parameters larger and smaller than the nominal lattice parameter of the LSMO crystal are used in experiments. It is shown that the behavior of the temperature dependence of the electrical resistance for the films in the low-temperature range does not depend on the strain of the film and agrees well with the results obtained from the calculations with allowance made for the interaction of electrons with magnetic excitations in the framework of the double-exchange model for systems with strongly correlated electronic states. Investigations of the magneto- optical Kerr effect have revealed that an insignificant (0.3%) orthorhombic distortion of the cubic lattice in the plane of the NdGaO₃(110) substrate leads to uniaxial anisotropy of the magnetization of the film, with the easy-magnetization axis lying in the substrate plane. However, LSMO films on substrates (((LaAlO₃)_0.3+(Sr₂AlTaO₆)_0.7)(001)) ensuring minimum strain of the films exhibit a biaxial anisotropy typical of cubic crystals. The study of the ferromagnetic resonance lines at a frequency of 9.76 GHz confirms the results of magnetooptical investigations and indicates that the ferromagnetic phase in the LSMO films is weakly inhomogeneous.     

Raman measurements on thin films of the La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> manganite: a probe of substrate-induced effects

We report on a Raman study of the phonon spectrum of La _0.7Sr_0.3MnO₃ thin films epitaxially grown on LaAlO₃. The spectrum, as a function of film thickness d, does not change over the 1000–100 ? range, whereas a strong hardening of the phonon frequencies of both bending and stretching modes is apparent in ultra-thin films (d<100 ?) where substrate-induced effects are remarkable. This behaviour, which appears to be related with the measured d-dependence of the insulator-to-metal transition temperature, is ascribed to co-operative effects of MnO₆ octahedra rotation and charge-localization. Raman spectroscopy proves to be a simple and powerful tool to monitor subtle structural modifications hardly detectable with conventional diffraction techniques in ultra-thin films.     

Two-phase paramagnetic-ferromagnetic state of La<Subscript>0.7</Subscript>Pb<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> single-crystal lanthanum manganite

Two phases, paramagnetic and ferromagnetic, were shown by the magnetic resonance method to coexist below the temperature T _C in La_0.7Pb_0.3MnO₃ single crystals exhibiting colossal magnetoresistance. The magnetic resonance spectra were studied in the frequency range 10–78 GHz. The specific features in the behavior of the spectral parameters were observed to be the strongest at the temperatures corresponding to the maximum magnetoresistance in the crystals. The concentration ratios of the paramagnetic and ferromagnetic phases in the samples were found to be sensitive to variations in temperature and external magnetic field. This behavior suggests realization of the electronic phase separation mechanism in the system under study.     

Current-induced Conductance Jumps in Mechanically Controllable Junctions of La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> Manganites

The magnetic switching processes in mechanically controllable junctions (MCJ), made from high-quality single crystals of La_0.7Sr_0.3MnO₃, are studied as a function of the DC current (up to 10⁹ A/cm²) passing through the nanoconstriction. The current-voltage (I-V) curves of the MCJ are typical for an electron tunnelling process. By fitting I-V curves to the Simmons model, the barrier width (1 - 1.6) nm and height (0.4 - 1.7) eV of the junctions and their effective tunnel area (1-10) × 10^?11 cm² were estimated. Based on the close relation between transport properties and the magnetization in manganese compounds, we interpret the jumps in the conductance of MCJ, by integer multiples of e²/h, as due to the configuration reorientation of the magnetization of the Mn-ions clusters at the constriction surfaces.     

Energy level alignment at interfaces between organic semiconductors and clean ferromagnetic La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> thin film contacts for spin injection

We have studied the energy level alignment at interfaces between clean ferromagnetic La_0.7Sr_0.3MnO₃ and two archetype organic semiconductors, α-sexithiophene and copper-phthalocyanine, by combined X-ray and ultraviolet photoelectron spectroscopy. We observe the formation of a large interface dipole at both studied interfaces and small hole injection barriers. In addition, our results indicate a small chemical interaction between the organic materials and the La_0.7Sr_0.3MnO₃ surface which leads to a pinning of the Fermi level and the relatively small hole injection barriers.     

Metal-insulator transition in a radiation-disordered La<Subscript>0.85</Subscript>Sr<Subscript>0.15</Subscript>MnO<Subscript>3</Subscript> manganite

The temperature dependences of the electrical resistivity ρ(T) and the ac magnetic susceptibility χ(T, H = 0) are thoroughly investigated for a perovskite-like lanthanum manganite, namely, La_0.85Sr_0.15MnO₃, which is preliminarily exposed to neutron irradiation with a fluence _F = 2 × 10¹⁹ cm^?2 and then annealed at different temperatures ranging from 200 to 1000°C. The results of the electrical resistance measurements demonstrate that neutron irradiation of the samples leads to the disappearance of the low-temperature insulating phase. As the annealing temperature increases, the insulating phase is not restored and the manganite undergoes a transformation into a metallic phase. Analysis of the magnetic properties shows that, under irradiation, the ferromagnet-paramagnet phase transition temperature T_C decreases and the magnetic susceptibility is reduced significantly. With an increase in the annealing temperature, the phase transition temperature T_C and magnetic susceptibility χ(_{T, H} = 0) increase and gradually approach values close to those for an unirradiated sample. This striking difference in the behavior of the electrical and magnetic properties of the radiation-disordered La_0.85Sr_0.15MnO₃ manganite is explained qualitatively.     

Positive magnetoresistance of La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript>/C Composites at room temperature

Two-phase composites xLa_0.7Sr_0.3MnO₃/(100–x)C (x = 5–85 mass %) have been synthesized. The magnetoresistive properties of these materials in magnetic fields from 0 to 15 kOe have been investigated. It has been shown that, at room temperature, the positive isotropic magnetoresistance for samples with x = 50–60 mass % reaches 15%.     

A sonochemical-assisted synthesis and annealing temperature effect of La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> nanoparticles

This paper reports on the sonochemical-assisted synthesis of La_0.7Sr_0.3MnO₃ (LSMO) nanoparticles (NPs) which have a single-crystalline perovskite structure. The average particle size of LSMO NPs was controlled from about 40 to 120 nm by changing the annealing temperatures from 750 to 1050°C. The particle size, electrical resistivity, and ferromagnetic transition temperature of LSMO NPs were strongly dependent on the annealing temperature. A substantial decrease in resistivity and an enhancement in the insulator–metal transition temperature were found on increasing the annealing temperature. Furthermore, the enhancement in magnetization and paramagnetic–ferromagnetic (PM–FM) transition temperatures was observed as the annealing temperature increases.     

Pressure-induced modifications of crystal and magnetic structure of oxygen deficient La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3-d</Subscript> manganites

The crystal and magnetic structures of the oxygen deficient manganites La_0.7Sr_0.3MnO_3-d (d = 0.15, 0.20) have been studied by means of powder neutron diffraction over the 0–5.2 GPa pressure and 10–290 K temperature ranges. La_0.7Sr_0.3MnO_2.85 exhibits a coexistence of rhombohedral and tetragonal (I4/mcm) crystal structures and below T_g ~ 50 K a spin glass state is formed. La_0.7Sr_0.3MnO_2.80 exhibits a tetragonal (I4/mcm) crystal structure. Below T_g ~ 50 K a phase separated magnetic state is formed, involving coexistence of C-type AFM domains with spin glass domains. In both compounds the crystal structure and magnetic states remain stable upon compression. The factors leading to the formation of different magnetic states in La_0.7Sr_0.3MnO_3-d (d = 0.15, 0.20) and their specific high pressure behavior, contrasting with that of the stoichiometric A_0.5Ba_0.5MnO₃ (A = Nd, Sm) compounds showing pressure-induced suppression of the spin glass state and the appearance of the FM state, are analysed.     

Giant magnetorefractive effect in La<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> films

Complex experimental investigations of the structural, optical, and magneto-optical properties (magnetotransmission, magnetoreflection, and transversal Kerr effect, as well as the magnetoresistance, of La_0.7Ca_0.3MnO₃ epitaxial films indicate that magnetoreflection and magnetotransmission in manganite films can reach giant values and depend strongly on the magnetic and charge homogeneity of the films, their thickness, and spectral range under investigation. It has been shown that the optical enhancement of the magnetorefractive effect occurs in thin films as compared to manganite crystals. In the region of the minimum of the reflectance near the first phonon band, the resonance-like magnetorefractive effect has been observed, which is accompanied by change of the sign of the magnetoreflection. A model based on the theory of the magnetorefractive effect has been proposed to qualitatively explain this behavior.     

Effect of the substrate ferroelastic transition on epitaxial La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> films grown on LaAlO<Subscript>3</Subscript>

Thin films of La_0.7Sr_0.3MnO₃ were grown by molecular beam epitaxy on (001)LaAlO₃ crystals. High resolution X-ray diffraction analysis proves the presence of twins in the films at room temperature, showing that the twin structure of the substrate which forms at the ferroelastic transition at T_F = 813 K served as a template for the film microstructure. Magnetic measurements indicate a thermomagnetic irreversibility which is ascribed to the quenched disorder related to twinning and discussed in terms of coexisting ferromagnetic and spin disordered regions connected with the undeformed domain cores and strained domain walls respectively.     

Modulation of ultrafast laser-induced magnetization precession in BiFeO<Subscript>3</Subscript>-coated La<Subscript>0.67</Subscript>Sr<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> thin films

The ultrafast laser-excited magnetization dynamics of ferromagnetic (FM) La_0.67Sr_0.33MnO₃ (LSMO) thin films with BiFeO₃ (BFO) coating layers grown by laser molecular beam epitaxy are investigated using the optical pump-probe technique. Uniform magnetization precessions are observed in the films under an applied external magnetic field by measuring the time-resolved magneto-optical Kerr effect. The magnetization precession frequencies of the LSMO thin films with the BFO coating layers are lower than those of uncoated LSMO films, which is attributed to the suppression of the anisotropy field induced by the exchange interaction at the interface between the antiferromagnetic order of BFO and the FM order of LSMO.     

Magnetotransport properties of nano-constriction array in La<Subscript>0.67</Subscript>Sr<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> film

Nano-constriction array in La_0.67Sr_0.33MnO₃ film was fabricated by using ion beam etching masked by a monolayer of packed and ordered array of SiO₂ microspheres. Nano-constrictions of around 50 nm in width were fabricated. The low field magnetoresistance (LFMR) exhibited in the samples were observed to be current dependent and the I-V characteristics of the film were found to be nonlinear. These observations were attributed to the co-existence of the ferromagnetic regions and the nano-constricted region of weakened ferromagnetic coupling where Mn³⁺-O-Mn⁴⁺ bond were distorted due to the ion beam bombardment. The spin polarized bias current would strengthen local ferromagnetic coupling when passing through this nano-constricted regions. This current effect is relatively large comparing to the external magnetic field to the drop of resistance.     

Magnetorefractive effect in La<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> in the infrared spectral range

The reflection and magnetic reflection spectra, magnetic resistance, electrical properties, and equatorial Kerr effect in La_0.7Ca_0.3MnO₃ crystals have been complexly investigated. The measurements have been performed in wide temperature and spectral ranges in magnetic fields up to 3.5 kOe. It has been found that magnetic reflection is a high-frequency response in the infrared spectral range to the colossal magnetore-sistance near the Curie temperature. Correlation between the field and temperature dependences of the magnetic reflection and colossal magnetoresistance has been revealed. The previously developed theory of the magnetorefractive effect for metallic systems makes it possible to explain the experimental data at the qualitative level. Both demerits of the theory of the magnetorefractive effect in application to the magnets and possible additional mechanisms responsible for the magnetic reflection are discussed.     

Peculiarities of magnetic phase separation in anion-deficient La<Subscript>0.70</Subscript>Sr<Subscript>0.30</Subscript>MnO<Subscript>2.85</Subscript> manganite

The temperature and field dependences of the specific magnetic moment of the anion-deficient La_0.70Sr_0.30MnO_2.85 manganite have been measured. It is established, that the magnetic state of the sample studied is a cluster spin glass and it is the result of frustration of exchange Mn³⁺-O-Mn³⁺ interactions due to the redistribution of oxygen vacancies. The increase of the magnetic field leads to an increase in the degree of polarization of local spins of manganese. It is established using the magnetic criterion that a phase transition into the paramagnetic state for the anion-deficient La_0.70Sr_0.30MnO_2.85 manganite is a thermodynamic second order phase transition. The causes and mechanism of the magnetic phase separation are discussed.     

Hysteresis of magnetoresistance in granular La<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> at low temperatures

The magnetoresistance of granular La_0.7Ca_0.3MnO₃ is studied experimentally over wide ranges of temperatures and magnetic fields. The emphasis is on anomalously large hysteresis of magnetoresistance at low temperatures (T = 4.2 K). The observed ρ(H) dependence can be qualitatively explained by spin-dependent tunneling of electrons through the dielectric boundaries of conducting granules characterized by a wide spread in the magnetic-moment magnitudes.     

La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> thin films on SrTiO<Subscript>3</Subscript> and CaTiO<Subscript>3</Subscript> buffered Si substrates: structural,static, and dynamic magnetic properties

Nearly 50-nm thick La_0.7Sr_0.3MnO₃ (LSMO) films were grown on Si substrates using molecular beam epitaxy on (001) Si substrates over-layered by a 20 nm thick SrTiO₃ (STO) or by a 20 nm thick CaTiO₃ (CTO) film. In addition, a reference LSMO film was directly deposited on a (001) STO substrate by pulsed laser deposition. For all the samples, X-ray diffraction revealed an excellent epitaxy of the LSMO film and small mosaicity around (001), with in-plane [100] and [010] cubic axes. The LSMO/CTO films are in-plane compressed while the LSMO/STO ones are in-plane extended. The temperature dependence of their static magnetic properties was studied using a SQUID, showing a Curie temperature overpassing 315 K for all the samples. Hysteresis loops performed at room temperature (294 K) with the help of a vibrating sample magnetometer (VSM) are also discussed. At 294 K Micro-strip ferromagnetic resonance (MS-FMR) was used to investigate the dynamic magnetic properties. It allows concluding to a strong anisotropy perpendicular to the films and to a weak fourfold in-plane anisotropy with easy axes along the [110] and [1[`1]0 1\bar{1}0 ] directions. Their values strongly depend on the studied sample and are presumably related to the strains suffered by the films.     

Magnetic state of the structural separated anion-deficient La<Subscript>0.70</Subscript>Sr<Subscript>0.30</Subscript>MnO<Subscript>2.85</Subscript> manganite

The results of neutron diffraction studies of the La_0.70Sr_0.30MnO_2.85 compound and its behavior in an external magnetic field are stated. It is established that in the 4–300 K temperature range, two structural perovskite phases coexist in the sample, which differ in symmetry (groups R[`3]cR\bar 3c and I4/mcm). The reason for the phase separation is the clustering of oxygen vacancies. The temperature (4–300 K) and field (0–140 kOe) dependences of the specific magnetic moment are measured. It is found that in zero external field, the magnetic state of La_0.70Sr_0.30MnO_2.85 is a cluster spin glass, which is the result of frustration of Mn³⁺-O-Mn³⁺ exchange interactions. An increase in external magnetic field up to 10 kOe leads to fragmentation of ferromagnetic clusters and then to an increase in the degree of polarization of local spins of manganese and the emergence of long-range ferromagnetic order. With increasing magnetic field up to 140 kOe, the magnetic ordering temperature reaches 160 K. The causes of the structural and magnetic phase separation of this composition and formation mechanism of its spin-glass magnetic state are analyzed.     

Particle size effect on magnetotransport properties of nanocrystalline Nd<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript>

Nanocrystalline samples with an average particle size of 40 and 52 nm have been synthesized by citrate-complex auto-ignition method. Magnetic properties of the samples show para- to ferromagnetic transition at around 135 K. The electron magnetic resonance (EMR) study on these samples indicates the presence of coexistence of two magnetic phases below 290 K. Electrical resistivity follows variable range hopping (VRH) mechanism in the paramagnetic regime. The magnetoresistance (MR) data has been analysed by spin dependent hopping between the localized spin clusters together with the phase-separation phenomenon. These clusters are assumed to be formed by distribution of canted spins and defects all over the nanoparticle. In addition, the hopping barrier depends on the magnetic moment orientation of the clusters. The magnetic moments of the clusters are narrowly oriented in ferro- and are randomly oriented in paramagnetic phase. The ferromagnetic phase contributes to the total MR at low applied magnetic fields whereas the paramagnetic phase contributes at relatively high fields in both the samples. The average cluster size in ferromagnetic phase is bigger than that in paramagnetic phase. It is also observed that the cluster size, in ferromagnetic phase, in 52 nm sample is bigger than that in the 40 nm sample. However, the average cluster size in paramagnetic phase is almost same in both the samples.     

Magnetic properties of nanoparticle La<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> under applied hydrostatic pressure

Pressure effects on magnetic properties of two La_0.7Ca_0.3MnO₃ nanoparticle samples with different mean particle sizes were investigated. Both the samples were prepared by the glycine-nitrate method: sample S—as-prepared (10 nm), and sample S₉₀₀—subsequently annealed at 900 °C for 2 h (50 nm). Magnetization measurements revealed remarkable differences in magnetic properties with the applied pressure up to 0.75 GPa: (i) for S sample, both transition temperatures, para-to-ferromagnetic T _C = 120 K and spin-glass-like transition T _f = 102 K, decrease with the pressure with the respective pressure coefficients dT _C/dP = −2.9 K/GPa and dT _f/dP = −4.4 K/GPa; (ii) for S₉₀₀ sample, para-to-ferromagnetic transition temperature T _C = 261 K increases with pressure with the pressure coefficient dT _C/dP = 14.8 K/GPa. At the same time, saturation magnetization M _S recorded at 10 K decreases/increases with pressure for S/S₉₀₀ sample, respectively. Explanation of these unusual pressure effects on the magnetism of sample S is proposed within the scenario of the combined contributions of two types of disorders present in the system: surface disorder introduced by the particle shell, and structural disorder of the particle core caused by the prominent Jahn–Teller distortion. Both disorders tend to vanish with the annealing of the system (i.e., with the nanoparticle growth), and so the behavior of the sample S₉₀₀ is similar to that previously observed for the bulk counterpart.