Effect of clustering in LaCa(Sr)MnO epitaxial films on their magnetic,electron, and optical properties

This work is aimed at finding a correlation between the magnetic properties, conductivity, and crystal structure of La_0.6Sr_0.2Mn_1.2O₃ and La_2/3Ca_1/3MnO₃ films prepared by pulsed laser (KrF) deposition in order to study the behavior of their M(T) and M(H) magnetizations. X-ray diffraction is used to investigate the cluster crystal structure of the manganites. It is shown that the variation of growth temperature and substrate type, along with laser irradiation, change not only the matrix structure (long-range order in ion arrangement) but also the cluster structure (mesoscopic order). The magnetic, electron, and optical properties revealed experimentally are explained by changes in the atomic and electron subsystems of the cluster structures, which provide magnetic interaction between clusters.     

Effect of the interaction between the magnetic and phonon subsystems on the magnetic properties of a ferromagnet: Model calculations

Thermodynamical analysis and model calculations for an isotropic ferromagnet show that inclusion of the interaction between the magnetic and phonon subsystems (in particular, through a magnetization-dependent Debye temperature) causes renormalization of the equation of the magnetic state and a temperature dependence of the thermodynamic coefficients of the Landau expansion, which are usually assumed to be constant. One of the consequences of this dependence is, for example, an experimentally observed difference between the “true” and paramagnetic Curie temperatures of a ferromagnet.     

Hidden magnetic configuration in epitaxial La(1-x) Sr(x) MnO3 films

We present an unreported magnetic configuration in epitaxial La(1-x) Sr(x) MnO3 (x ～ 0.3) (LSMO) films grown on strontium titanate (STO). X-ray magnetic circular dichroism indicates that the remanent magnetic state of thick LSMO films is opposite to the direction of the applied magnetic field. Spectroscopic and scattering measurements reveal that the average Mn valence varies from mixed Mn(3+)/Mn(4+) to an enriched Mn3+ region near the STO interface, resulting in a compressive lattice along the a, b axis and a possible electronic reconstruction in the Mn e(g) orbital (d(3)z(2)-r(2). This reconstruction may provide a mechanism for coupling the Mn3+ moments antiferromagnetically along the surface normal direction, and in turn may lead to the observed reversed magnetic configuration.     

High pressure effect on the crystal and magnetic structure of Pr0.1Sr0.9MnO3 manganite

The crystal and magnetic structure of Pr_0.1Sr_0.9MnO₃ manganite has been studied by the neutron diffraction at high pressures up to 5 GPa in the temperature range 10?C295 K. At normal pressure and decreasing temperature the appearance of the C-type (T _N = 220 K) and G-type (T _N = 180 K) antiferromagnetic states occurs, which is accompanied by a structural phase transition from the cubic structure (Pm $ \bar 3 $ m space group) to the tetragonal structure (I4/mcm space group). It is shown that the temperature of the transition to the C-type antiferromagnetic phase increases with pressure with the pressure coefficient dT _N/dP = 4.0(5) K/GPa and the temperature of the transition to the G-type antiferromagnetic phase weakly depends on pressure.     

Spin polarization of charge carriers and Andreev reflection in (LaCa)MnO/superconductor point contacts

Spin polarization of charge carriers in La_0.65Ca_0.35MnO₃ (LCMO) is studied using point-contact Andreev spectroscopy. Pb and MgB₂ are used to make superconducting electrodes. In all cases, the transport spin polarization obtained from the conductivity of LCMO/superconductor point contacts does not exceed 80–85%. Different models of the current flow through the superconductor-ferromagnetic metal contact and possible reasons for noncomplete spin polarization of a current in manganites are explored. The level of spin polarization observed in Sharvin contacts (contact area ～10⁴ Ų) is most naturally explained in terms of a model that suggests separation of the crystal into nanosized magnetic phases, only one of which is a ferromagnetic metal with full spin polarization of charge carriers.     

(La0.7Sr0.3MnO3)m(BiFeO3)n超晶格间隔的La0.7Sr0.3MnO3三明治结构制备及表征

采用射频磁控溅射的方法在SrTiO3(001,基片上制备了(La0.7Sr0.3MnO3)m(BiFeO3)n超晶格间隔的La0.7Sr0.3MnO3三明治结构.X射线衍射分析证明(La0.7Sr0.3MnO3)m(BiFeO3)n具有明显的超晶格结构.电流垂直于薄膜表面测得的电阻-温度关系表明.La0.7Sr0.3MnO3)m(BiFeO3)n超晶格薄膜在290 K有金属-绝缘体转变,略低于单层La0.7Sr0.3MnO3薄膜的转变温度.电流在0.01-10 mA范围内,观察到薄膜的峰值电阻随电流增大而减小,峰值变化率远大于单层La0.7Sr0.3MnO3薄膜,且随着超晶格周期厚度的增加而增大.低温下,电流-电压曲线表明其导电机制应主要为空间载子限制,且显示较大的电压偏置,表现出肖特基结的特性.     

Effect of strontium deficiency on the transport and magnetic properties of Pr0.7Sr0.3MnO3

Ceramic samples of Pr_0.7Sr_0.3−x□_xMnO₃ with x⩽0.2 have been investigated by various techniques including ⁵⁵Mn nuclear magnetic resonance, SQUID magnetometry, resistivity and magnetoresistance measurements. On increasing x, the samples remain ferromagnetic at a low temperature with a decreasing Curie temperature, but a metal–insulator transition is observed. Besides, a positive magnetoresistance (MR) at very low temperatures with respect to a negative colossal MR at high temperatures is observed in the Pr_0.7Sr_0.2□_0.1MnO₃ sample.     

Displacement-type ferroelectricity with off-center magnetic ions in perovskite Sr(1-x)Ba(x)MnO3

We report a ferroelectric transition driven by the off-centering of magnetic Mn(4+) ions in antiferromagnetic Mott insulators Sr(1-x)Ba(x)MnO(3) with a perovskite structure. As x increases, the perovskite lattice shows the typical soft-mode dynamics, as revealed by the momentum-resolved inelastic x-ray scattering and far-infrared spectroscopy, and the ferroelectricity shows up for x ≥ 0.45. The observed polarization is comparable to that for a prototypical ferroelectric BaTiO(3). We further demonstrate that the magnetic order suppresses the ferroelectric lattice dilation by ～70% and increases the soft-phonon energy by ～50%, indicating the largest magnetoelectric effects yet attained.     

Localized electron states and magnetic properties at the interface of a two-dimensional graphene/MnO(001) system

The results of a density functional theory study of the band structure of two-dimensional (2D) graphene/MnO(001), new materials for spintronics, with an antiferromagnetic type of ordering are presented. The regularities of the change of the valence band electron structure in the 3D MnO → 2D MnO → 2D graphene/MnO(001) series have been studied in a comparison with X-ray photoelectron spectra. The stability of the system has been established, and the energy of chemical binding has been determined using the calculation of the structural energy of 2D graphene/MnO(001). The features of the spin state in the valence band—in particular, at the Fermi level—as well as interatomic interaction in 2D graphene/MnO(001) have been discussed in comparison with 2D and 3D MnO systems with antiferromagnetic ordering. The magnetic moment of the Mn atom in all considered systems has been estimated and compared with the experimental one. The effect of the spin polarization for the oxygen and carbon atoms has been detected. The nature of this effect has been considered.     

Effect of regimes of cooling in a magnetic field on the magnetization of a La0.9Sr0.1MnO3 single crystal

Magnetization measurements were performed on a lanthanum manganite La_0.9Sr_0.1MnO₃ single crystal in the temperature interval 4.2–300 K and magnetic field interval 50 Oe-55 kOe in two sample cooling regimes: 1) cooling down to 4.2 K in a high (55 kOe) magnetic field, and 2) cooling in a “zero” field. It is shown that the temperature dependences of the magnetization M(T) are substantially different in these regimes. Pronounced anomalies of M(T) were observed at temperatures T*=103 K and T _c =145 K. The first anomaly is attributed to a structural transition, while the second one corresponds to a ferromagnet-paramagnet phase transition. The magnetization of a La_0.9Sr_0.1MnO₃ single crystal in the cooling regimes studied shows typical “spin-glass” behavior. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 1, 39–43 (10 July 1998)     

Nature of e(g) electron order in La(1-x)Sr(1+x)MnO(4)

X-ray scattering measurements of the low-temperature structure of La(1-x)Sr(1+x)MnO(4) ( 0.33< or =x< or =0.67) indicate the existence of three distinct regions: a disordered phase (x<0.4), a charge-ordered phase (x> or =0.5), and a mixed phase (0.4< or =x<0.5). For x>0.5, the modulation vector associated with the charge order is incommensurate with the lattice and depends linearly on the concentration of e(g) electrons. The primary superlattice reflections are strongly suppressed along the modulation direction and the higher harmonics are weak, implying the existence of a largely transverse and nearly sinusoidal structural distortion, consistent with a charge-density wave of the e(g) electrons.     

Pressure-induced variation of the magnetic structure of the surface of La0.6Sr0.4MnO3 granules

The transport properties of textured films and tunneling junctions of La_0.6Sr_0.4MnO₃, defined by the surface state of the granule, are studied in low magnetic fields (below 100 Oe) and at pressures of up to 10 kbar. Tunneling junctions of two types are investigated, namely, mechanical break junctions and La_0.6Sr_0.4MnO₃-insulator-superconductor junctions. Although only one electrode represents the magnetic material in the latter case, all samples exhibit a low-field magnetoresistive effect. Hydrostatic compression suppresses the magnetoresistive effect to considerably change the transport properties of ceramic and tunnel samples. The reasons for such behavior are discussed in connection with the model of spin-polarized inelastic tunneling of charge carriers through a potential barrier formed both by the intergranular region and by the surface of contacting granules. Reasons are given for the fact that it is most probable that the magnetic state of the barrier and its height vary under the effect of pressure because of the transition of the surface of granules to the metallic state.     

Effect of oxygen non-stoichiometry on magnetic and electrical transport properties of La0.67Sr0.33MnO3

The effect of oxygen deficiency and oxygen excess on the magnetic and electrical transport properties of La_0.67Sr_0.33MnO₃ has been investigated. The thermal and isothermal magnetization measurement results show that the Curie temperature and saturation magnetization of oxygen deficient sample (defined as A) are higher than those oxygen excess sample (defined as B). The electrical resistivity of A is lower than that of B in studied temperature range. The magnetoresistance (MR) of B is larger than that of A in the temperature range from 280 to 360 K, which agrees with the magnetic field needed full spin polarization at room temperature. The colossal MR (CMR) around transition temperature from ferromagnetic metal to paramagnetic insulator (T_MI) for A is larger than that for B, which arises from assistance of stronger lattice deformation for A.     

Effect of thickness on magnetic phase coexistence and electrical transport in Nd0.51Sr0.49MnO3 films

We present the impact of the film thickness on the coexistence of various magnetic phases and its link to the magnetoresistance of Nd_0.51Sr_0.49MnO₃ thin films. These epitaxial films are deposited on LaAlO₃ (001) substrates by DC magnetron sputtering. Films with thicknesses of approximately 30 nm are found to be under full compressive strain while those with thicknesses ∼100 nm and beyond exhibit the presence of both strained and relaxed phases, as evidenced from X-ray diffraction studies. Both films exhibit multiple magnetic transitions controlled by strong electron correlations and phase coexistence. These films also display insulator–metal transitions (IMT) and colossal magnetoresistance (CMR) under moderate magnetic fields. Among the two set of films, only the 30-nm films show a weak signature of charge ordering at T≈50 K. Even at temperatures much lower than the IMT, the 30-nm films show huge magnetoresistance (MR) ∼80%. This suggests presence of softened charge-ordered insulating (COI) clusters that are transformed into ferromagnetic metallic (FMM) ones by the external magnetic field. In the 100-nm films, the corresponding MR is suppressed to less than 20%. Our study demonstrates that the softening of the COI phase is induced by the combined effect of the in-plane compressive strain and a slight reduction in Sr concentration.     

Effect of isotopic composition and microstructure on the crystalline and magnetic phase states in R0.5Sr0.5MnO3

The results of structural neutron experiments on determining crystal and magnetic phase states of perovskite-like manganites R_0.5Sr_0.5MnO₃ (R = ¹⁵²Sm, Nd_0.772Tb_0.228, and Nd_0.544Tb_0.456) are reported. Experiments are carried out for revealing microscopic factors responsible for the giant oxygen isotope effect that was discovered recently in Sm_1?x Sr _x MnO₃ for x ≈ 0.5. It is shown that separation into two crystal phases P ₁ and P ₂ with the same spatial symmetry but different types of Jahn-Teller distortions in MnO₆ octahedra and magnetic ordering of Mn atoms takes place in all studied compounds at low temperatures. Structural analysis has been carried out successfully owing to exceptionally large differences in the unit cell parameters of the coexisting phases. The P ₁ phase is ferromagnetic and MnO₆ octahedra are distorted only slightly. The P ₂ phase is antiferromagnetic (A-type ordering) and MnO₆ octahedra are strongly compressed in the apical direction. The relative volumes occupied by the P ₁ and P ₂ phases depend on the mean radius of the A cation, and the replacement of ¹⁶O by ¹⁸O results in their redistribution in favor of the P ₂ phase. The results unambiguously point to the percolation nature of the metal-insulator transition in a Sm-containing compound upon isotopic substitution of oxygen due to a sharp decrease (from 65 to 13%) in the fraction of ferromagnetic phase P ₁. In all investigated compounds, the ordered magnetic moment of manganese Mn in the P ₁ and P ₂ phases varies from 1.7μ_B to 3.5μ_B. The data on the evolution of the miscrostructure parameters during a phase transition to the stratified state indicate that the initial spread in the A cation radii, as well as the internal microstrains, produce a critical effect on the formation of mesoscopic phase separation.     

Defects and the structure of a manganite La0.85Sr0.15MnO3 crystal

Defects in a ferromagnetic crystal of manganite La_0.85Sr_0.15MnO₃ were created by irradiation with fast neutrons (E > 0.1 MeV). Fast neutrons produce defect clusters in a crystal lattice. The volume fraction of the clusters in the crystal after irradiation to a dose F = 2 × 10¹⁹ cm^?2 (T _irr = 340 K) was ?40%. The structural and magnetic states of the modified manganite were studied using thermal-neutron diffraction and magnetic measurements. It was revealed that neutron irradiation of a crystal suppresses the cooperative Jahn-Teller effect and the initial charge modes and decreases the temperature of ferromagnetic ordering. Under irradiation with fast neutrons, the crystalline structure of the manganite changes from the orthorhombic O′ to the pseudocubic O* phase. Arguments are advanced in favor of the specific features of the irradiated-manganite structural state being determined by long-wavelength strains induced in the crystal by antisite defects.     

Effect of the substrate crystalline structure on the magnetic, electrical, and crystallographic characteristics of La0.35Nd0.35Sr0.3MnO3 epitaxial films

A study has been made of the electrical resistivity ρ, magnetoresistance Δρ/ρ, and magnetization of La_0.35Nd_0.35Sr_0.3MnO₃ epitaxial films on ZrO₂(Y₂O₃), SrTiO₃, LaAlO₃, and MgO substrates. The first film can exist in four equivalent crystallographic orientations in the sample plane, while the other three have only one orientation. The maxima of ρ and Δρ/ρ of the first film are broadened considerably in the vicinity of the Curie point T _C compared to the three others, the magnitude of ρ itself being larger by 1.5 orders of magnitude, and a large negative magnetoresistance (|Δρ/ρ| ～ 10% in a field of 8.4 kOe) is observed at temperatures 80≤T≤200 K. In all films, the magnetic moment per molecule at 5 K is ～46% smaller than the pure spin value, due to the existence of magnetically disordered regions. The larger value of ρ of the film deposited on ZrO₂(Y₂O₃) is due to the electrical resistance of the boundaries separating regions with different crystallographic orientations, and the magnetoresistance is associated with polarized carriers tunneling through the boundaries coinciding with domain walls. The low-temperature magnetoresistance in fields above technical saturation is caused by the strong p-d exchange coupling within spin-ordered regions.     

Synthesis,X-ray,magnetic and electrical studies of substituted (Pr,Sr)MnO3 perovskites

A systematic study of the structural, magnetic and electrical properties of the manganites Pr_1-x Sr _x MnO₃(0≤ x ≤ 0.5) has been carried out. X-ray diffraction investigation shows a structural change with composition, from orthorhombic (0 ≤ x ≤ 0.2) to rhombohedral (0.25 ≤ x ≤ 0.5). The magnetic properties of Pr_1-x Sr _x MnO₃ samples could be explained on the basis of a double exchange mechanism between pairs of Mn³⁺ and Mn⁴⁺ ions. These properties are strongly dependent on the ratio of Mn³⁺/Mn⁴⁺. The maximum of the ferromagnetic transition temperature T_c is reached at x ≈ 0.35 corresponding to a value 1.85 of this ratio. The investigation of the electrical properties shows a semiconductor to metal transition as a function of temperature (0.25≤x≤0.4) with a metallic-like behaviour above a critical temperature T_p . A semiconducting-like one is observed for all the range of temperature (50–300 K) for (0 ≤ x ≤ 0.2 and x = 0.5). The evolution of activated energies with the carrier concentration has been investigated.     

Effect of the -doping on the electron transport in a structure modulated by the magnetic barriers

The effect of the δ-doping on the electron transport has been theoretically studied in a structure modulated by the magnetic barriers. The results show that the transmission probability and the electron conductance can be dramatically suppressed by the weight of the δ-doping. However, the spin-injection efficiencies are obviously enhanced. In addition, the transmission probability and the spin-polarization both show a periodic profile with the increase of L₂. These interesting features will be more helpful for developing new types of devices.