La0.67Ca0.33MnO3 thin films on Si (1 0 0) by DC magnetron sputtering technique using nanosized powder compacted target

La_0.67Ca_0.33MnO₃ (LCMO) thin films were successfully fabricated by a DC magnetron sputtering technique on Si (1 0 0) substrates from chemically synthesized compacted powders. Powders of proper stochiometry composites were synthesized by a novel chemical technique [D.R. Sahu, B.K. Roul, P. Pramanik, J.L. Huang, Physica B 369 (2005) 209] and were found to be nanosized (≈40-50 nm). The sinterability of the powders were improved significantly due to their large surface area with a reduction of sintering temperature (up to 500 °C) as compared to the powders prepared by other solid-state reaction route. Bulk LCMO targets were prepared and preliminary structural and magnetic properties of target were investigated for colossal magnetoresistance (CMR) properties. Films deposition parameters like DC power, gas flow rate, deposition time, etc., were critically optimized to achieve desired thickness of film using above LCMO target by DC magnetron sputtering. LCMO films fabricated on Si (1 0 0) substrates showed enhanced magnetoresistance (MR) at low temperature. Maximum MR of about 1000% was observed at 100 K. Paramagnetic to ferromagnetic transitions were observed in films below room temperature and were found at approximately 240 K. However, as compared to bulk target prepared by a chemical route, it was found that Curie temperature (T_c) and MR response of bulk target were higher than the thin films. Preliminary point chemical analysis revealed the deficiency of Ca²⁺ ions in CMR films.     

Microstructure and magnetoresistance of a La0.67Ca0.33MnO3 film produced using the dip-coating method

The microstructure and magnetoresistance properties of polycrystalline bulk La_0.67Ca_0.33MnO₃ and a La_0.67Ca_0.33MnO₃ film produced on a LaAlO₃ (1 0 0) single-crystal substrate using the dip-coating method were investigated. X-ray powder diffraction indicated that both film and bulk samples have perovskite structure. Scanning electron microscope and X-ray results clearly indicated that a La_0.67Ca_0.33MnO₃ film was successfully produced using the dip-coating process. This film showed a metal–insulator transition at 269 K and a maximum magnetoresistance ratio (MR (%)) of 56% at 269 K and 6 T magnetic field.     

Preparation and colossal magnetoresistance in a trilayer La0.67Sr0.33MnO3/La0.75MnO3/La0.67Sr0.33MnO3 device by dc magnetron sputtering

Epitaxial trilayer films of La_0.67Sr_0.33MnO₃ (LSMO)/La_0.75MnO₃ (L0.75MO)/La_0.67Sr_0.33MnO₃ (LSMO) have been prepared on (0 0 1) oriented LaAlO₃ substrates by dc magnetron sputtering. The structure and MR are studied. All as-deposited trilayer films exhibit a semiconductor to metal transition at temperature ranging from 116 to 185 K. The MR is also shown to be dependent on the thickness of the middle oxide layer. A maximum MR value of 32% (ΔR/R₀) has been obtained at 132 K under 0.4 T magnetic field for a LSMO (300 nm)/L0.75MO (70 nm)/LSMO (300 nm) trilayer film. The MR of trilayer film prefers to that of both LSMO and L0.75MO single layer films.     

Impact of granularity on transport properties of mechanically stressed La0.67Ca0.33MnO3 films

(La,Ca)MnO₃ is one of so called collosal magnetoresistive materials and it is of interest to correlate its transport properties to film growth in order to optimize its performance. Two-hundred nanometers thick (100)La_0.67Ca_0.33MnO₃ films were grown by laser ablation on (100)SrTiO₃, (100)LaAlO₃ and (100)(LaAlO₃)_0.3+(Sr₂AlTaO₆)_0.7 substrates. The films were granular in structure with low angle boundaries between the grains. The volume of the unit cell was considerably smaller for films grown on a SrTiO₃ substrate than on LaAlO₃. At temperatures higher than the one where the spins order, the strongest response of resistivity on temperature ρ(T) was measured for those (100)La_0.67Ca_0.33MnO₃ films that had the smallest volume of the unit cell. An increase of the lattice parameter for a film was accompanied by a decrease of the resistivity. The films grown on (LaAlO₃)_0.3+(Sr₂AlTaO₆)_0.7 substrates had the best crystallinity and the largest magnetoresistance.     

Studies on Low-Field and Room-Temperature Magnetoresistance in La2/3(Ca1−x Sr x )1/3MnO3 Perovskites

The polycrystalline perovskites La_2/3(Ca_1?x Sr _x )_1/3MnO₃ (x=0,0.2,0.4,0.6,0.8,1) were successfully prepared by a modified method of solid-state reactions, in which the ingredient mixture with ethanol as a liquid milling medium to form suspension was milled by high-energy ball milling for 10 h and sintered in air at 1400 °C for 10 h. The microstructure, electrical transport, and low-field magnetoresistance (LFMR) of the perovskites were investigated to study the room-temperature magnetoresistance (RTMR) behavior. The results reveal that the metal-to-insulator transition temperature (T _MI) increased with increasing doping level x, and the peak values of the magnetoresistance (MR) near T _MI dropped with the more Ca²⁺ substituted. A single-phase La_2/3Ca_1/3MnO₃ showed T _MI at 263 K and the peak MR of 23 % in the applied field of 3 kOe near T _MI. The LFMR effect at room temperature could be obtained by controlling the doping Sr²⁺-substituted Ca²⁺ level. When x=0.29, transition temperature (T _MI) was 305.30 K, and the MR effect was recorded up to 12.9 % at 298.55 K and 3 kOe. Finally, the possible mechanism is discussed.     

The demonstration of epitaxial and electronic-magnetic properties for La0.67Sr0.33MnO3−δ films with (1 1 1) orientation

La_0.67Sr_0.33MnO_3−δ films, fabricated on (1 1 1) LaAlO₃ single-crystal substrates using a direct current magnetron sputtering technique, are demonstrated by X-ray diffraction patterns and pole figures to be high quality epitaxial films and there is a perfect matching relationship between the films and the substrates. We observed an obvious difference of the electronic-magnetic transportation properties among films sputtered on (1 1 1), (1 0 0) and (1 1 0) LaAlO₃ substrates, respectively. A mechanism for the difference is discussed briefly.     

Electric resistance and magnetoresistance of 30-nm-Thick La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> films elastically compressed in the (110) plane

We have studied the structure, electric resistance, and magnetoresistance of 30-nm-thick (110)La_0.67Ca_0.33MnO₃ (LCMO) films grown by laser deposition on (001)-oriented LaAlO₃ substrates. The unit cell parameters a and b (along the [100] and [010]LCMO axes, respectively) of these manganite films are significantly (by ∼1.2%) increased as compared to the corresponding values in the pseudocubic unit cell of bulk stoichiometric LCMO crystals. At T < 150 K, the temperature dependence of the resistivity of LCMO films is well described by the relation ρ = ρ₁ + ρ₂ (H) T ^4.5. The value of ρ 2 decreases with increasing magnetic field and is close to the analogous coefficient for manganite films grown on substrates with small lattice misfit.     

Strain effects and phase separation tendency in highly strained La0.7Ba0.3MnO3 thin films on LaAlO3 substrates

The magnetic and transport properties of epitaxial La_0.7Ba_0.3MnO₃ films on LaAlO₃ substrates have been investigated and compared with those on SrTiO₃ substrates. It is found that the ferromagnetic and metallic transition temperature of the highly compressive strained films on LaAlO₃ substrates decreases steadily with decreasing film thickness, while it changes little in the lightly strained films on SrTiO₃ substrates. The properties of the films on LaAlO₃ substrates are more sensitive to a post-annealing procedure. A tendency to phase separation, which induces a difference between the insulator-metal transition temperature T_MI and the Curie temperature T_C, is observed for the films with a medium oxygen annealing process. We argue that the phase separation is due to both the highly compressive strain and oxygen deficiency in the films.     

Electric resistance and magnetoresistance of a two-layer epitaxial heterostructure (30 nm)La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript>/(30 nm)La<Subscript>0.67</Subscript>Ba<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript>

Two-layer epitaxial heterostructures (30 nm)La_0.67Ca_0.33MnO₃/(30 nm)La_0.67Ba_0.33MnO₃ (LCMO/LBMO) have been grown by laser deposition on single crystal (001)LaAlO₃ (LAO) substrates. In this system, the upper (LCMO) layer occurs under the action of tensile stresses in the substrate plane, whereas the lower (LBMO) layer exhibits biaxial compression. The formation of a 30-nm-thick LCMO film on the surface of the 30-nm-thick LBMO layer leads to an increase in the level of mechanical stresses in the latter layer. The maximum electric resistivity ρ of the (30 nm)LCMO/(30 nm)LBMO/LAO structure was observed at a temperature 25–30 K below that corresponding to the maximum of the ρ(T) curve for a single (30 nm)LBMO film on the same LAO substrate.     

Interfacial capacitance in epitaxial heterostructures La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript>/SrTiO<Subscript>3</Subscript>/La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO

Epitaxial trilayer heterostructures of the type La_0.67Ca_0.33MnO₃/SrTiO₃/La_0.67Ca_0.33MnO₃ were grown by laser ablation on (001)[(LaAlO₃)_0.3+(Sr₂AlTaO₆)_0.7] substrates. The real part of the dielectric permittivity ε and the loss factor tan δ of a 1100-nm-thick SrTiO₃ interlayer were studied in the temperature interval T=4.2–300 K in a nonbiased state and at a bias voltage of ±2.5 V applied to the manganite electrodes. Using the temperature dependence ε(T) measured for the SrTiO₃ layer grown between the manganite electrodes, we have estimated the capacitance of La_0.67Ca_0.33MnO₃/SrTiO₃ interfaces (C₁≈2 μF/cm²) related to the electric field penetrating from the interlayer into La_0.67Ca_0.33MnO₃.     

Magnetoresistance of La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> films grown on substrates with orthorhombically distorted unit cells

We have studied the structure and magnetoresistance of 40-nm-thick epitaxial La_0.67Ca_0.33MnO₃ (LCMO) films grown by laser deposition on (001)-oriented NdGaO₃ (NGO) substrates. The manganite layers were oriented so that the b axis was perpendicular to the substrate plane and occurred under the action of inhomogeneous biaxial mechanical stresses. The negative magnetoresistance of the LCMO films in the vicinity of the ferromagnetic spin ordering was about 71% (μ₀ H = 1 T). The observed azimuthal anisotropy of the magnetotransport properties of 40-nm-thick LCMO/(001)NGO films can be explained within the framework of a model of anisotropic magnetoresistance taking into account the presence of the preferred orientation of the spontaneous magnetization.     

Ferroic metal-oxide films grown by polymer assisted deposition

Polymer assisted deposition is a versatile technique to grow simple and complex metal-oxide thin films. In this paper we report the structural and electrical properties of ferroic materials, namely La_0.67M_0.33MnO₃ (M = Sr and Ca) and Ba_1 − xSr_xTiO₃ (x = 0.3, 0.5, and 0.7) prepared using this process. The films were prepared on single crystalline LaAlO₃ substrates. The films were highly c-axis oriented and epitaxial in nature. The ferromagnetic La_0.67Sr_0.33MnO₃ and La_0.67Ca_0.33MnO₃ films show intrinsic transport properties with maximum magnetoresistance values (at applied field of 5 T) of − 50% and − 88%, respectively. The highest dielectric constant (∼ 1010) and tunability (∼ 69%) of Ba_1 − xSr_xTiO₃ film occurs at x = 0.3 for films, which is at the phase boundary of tetragonal and cubic.     

Magnetotransport characteristics of biaxially strained 25-nm-thick La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> films

We have studied the structure and the electro-and magnetotransport properties of 25-nm-thick epitaxial La_0.67Ca_0.33MnO₃ (LCMO) films mechanically strained during nucleation and growth on (001)-oriented LaAlO₃ substrates. The unit cell parameters of such LCMO films measured parallel and perpendicular to the substrate plane are significantly different (a _‖ = 3.790 Å, a _⊥ = 3.948 Å). The magnetocrystalline anisotropy and phase separation induced by the unit cell distortion in the film lead to the appearance of clearly pronounced hysteresis loops on the plots of electric resistance versus magnetic field.     

Electric resistance of La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> films biaxially strained during growth on lattice-mismatched substrates

We have studied the electric resistance of 20-nm-thick La_0.67Ca_0.33MnO₃ films coherently grown on single crystal substrates with considerable (negative) and almost zero lattice mismatch. The unit cell volume in the growing film depends on the substrate lattice parameter. At T<200 K and μ₀H=0, the resistance of manganite films on (001)LaAlO₃ substrates was several orders of magnitude greater than the value for an analogous film grown on (001)La_0.29Sr_0.71Al_0.65Ta_0.35O₃. The observed decrease in resistance of the elastically strained (biaxial compression) manganite films is related to a superstoichiometric (≈45%) relative concentration of Mn⁴⁺ ions in the film volume.     

Low-temperature magnetoresistance of biaxially strained 24-nm-thick La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> films

The lateral unit cell parameter in nanodimensional La_0.67Ca_0.33MnO₃ (LCMO) films grown on (001)-oriented LaAlO₃ substrates is significantly (approximately 4%) smaller than the value measured along the normal to the substrate plane. At T < 140 K, the temperature dependence of the resistivity ρ of LCMO films follows the relation ρ − ρ (T = 4.2 K) ≈ρ₂(H)T ^4.5, where ρ₂ is independent of the temperature but decreases with increasing magnetic field H. It is shown that this decrease is related both to a decay of the spin waves in ferromagnetic domains and to the transformation of antiferromagnetic phase inclusions into ferromagnetic ones.     

Preparation and properties of La1 − xAgyMnO3 + δ thin epitaxial films

We report here the preparation and properties of La_1 − xAg_yMnO_3 + δ thin epitaxial films. The original two-step preparation procedure was developed. At first, La_1 − xMnO_3 + δ were grown epitaxially by metal-organic chemical vapor deposition on the single-crystal substrates (001) and (110) SrTiO₃, (001) LaAlO₃, (111) and (001) ZrO₂(Y₂O₃). Treatment by the vapor of the metallic silver in the oxygen atmosphere (at 1 bar and 20 bar) was the second step resulting in the selective absorption of silver by La_1 − xMnO_3 + δ phase. The value of y depended on the process conditions and revealed different kinetics of the silver absorption for (001) and (110) orientation of La_1 − xMnO_3 + δ films. The films prepared were characterized by X-ray diffraction, scanning electron microscopy with energy-dispersion X-ray analysis, high resolution transmission electron microscopy, X-ray photoelectron spectroscopy, electrical resistivity and magnetoresistance measurements in a four-probe configuration. We have found that metal-insulator transition temperature (T_p) in the series La_1 − xAg_xMnO_3 + δ possessed a maximum of 380 K at x = 0.15. Thus, T_p of La_1 − xAg_xMnO_3 + δ films was significantly higher than ever reported in the literature for the La_1 − xAg_xMnO_3 + δ ceramics. La_1 − xAg_xMnO_3 + δ films demonstrated the important role of the ferromagnetic fluctuations above Curie temperature T_c resulting in the sign change of the resistivity curve temperature slope dR / dT and a significant shift of T_p well above T_c. The maximum of the magnetoresistance on the temperature scale was close to dR / dT maximum. The intrinsic magnetoresistance values as high as 22% at 310 K and 50% at 280 K were measured in the magnetic field of 1 T in the series of La_1 − xAg_yMnO_3 + δ epitaxial films.     

Response of the electric resistance of 40-nm-thick La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> films to an increase in the lattice mismatch between film and substrate

We have studied the structure and resistivity of 40-nm-thick La_0.67Ca_0.33MnO₃ (LCMO) films coherently grown on (001)-oriented La_0.29Sr_0.71Al_0.65Ta_0.35O₃ single crystal substrates bearing epitaxial interlayers of strontium titanate with a thickness of 7 or 70 nm. As the effective mismatch between the crystal lattice parameters of the film and substrate increases, the ρ value exhibits a sharp growth, while the maximum of the ρ(T) curve shifts by ∼40 K toward lower temperatures. At T < 150 K, the temperature dependence of the resistivity of the LCMO films obeys the relation ρ ∼ ρ₁ T ^4.5, where the coefficient ρ₁ decreases with increasing applied magnetic field strength and with decreasing lattice mismatch between the manganite film and the substrate.     

Magnetocaloric properties in Ln0.67Ba0.33Mn1 − xFexO3 (Ln = La or Pr) manganites

We have investigated the magnetocaloric properties of Ln_0.67Ba_0.33Mn_1 − xFe_xO₃ (Ln = La or Pr) manganites with x = 0 and 0.05. All compounds present a maximum and large magnetocaloric effect near the Curie temperature (T_C). The associated maximum value of the magnetic entropy change, at 5 T applied change in the magnetic field, is 4.37 J.kg^− 1.K^− 1 for Pr_0.67Ba_0.33MnO₃ manganite with a T_C value of 205 K. The corresponding relative cooling power (RCP) reaches 230 J.kg^− 1. All the samples present similar RCP values that are relatively high and are promising materials to be used in ecologically friendly magnetic refrigeration technology. Iron doping reduces both T_C and ΔS_M^max and spreads the temperature working range with an almost constant RCP and can then be used to tune the working conditions of a refrigerator device.     

Microstructure, electrical and magnetic properties of polycrystalline La0.85K0.15MnO3 manganites prepared by different synthesis routes

Influence of different synthesis techniques (solid state reaction, sol–gel and co-precipitation) on the structure, microstructure, magnetic and electrical properties of polycrystalline La_0.85K_0.15MnO₃ (LKMO) sintered at 900 °C is investigated. All the as-synthesized compounds were confirmed as single phase and hexagonal structure at room temperature. The nano-crystallite size and average grain size were increased from the sample synthesized through solid state, sol–gel and co-precipitation techniques. The electrical and magneto-transport properties of polycrystalline LKMO was relied on the synthesis method. Significant decreases in metal–insulator transition temperature (T_p) with the increment of resistivity were observed for co-precipitation synthesized sample when comparing with solid state and sol–gel synthesized samples. Magnetization was decreased while ferro-paramagnetic transition temperature (T _c) was shifted toward lower temperature from solid state synthesized sample to co-precipitation synthesized sample. Furthermore, co-precipitation synthesized sample achieved the highest negative magnetoresistance at room temperature.     

Magnetic coupling in La0.67Ca0.33MnO3/La0.67Sr0.33CoO3/La0.67Ca0.33MnO3 sandwiches

La_0.67Ca_0.33MnO₃ (LCMO)/La_0.67Sr_0.33CoO₃ (LSCO)/LCMO trilayer films are fabricated on single-crystal substrates NdGaO₃ (110) and the interlayer coupling are investigated. Compared with LCMO single layer, sandwiches showed the enhanced metal-insulator transition temperature of LCMO layers. The magnetoresistance is dependent on spacer thickness and the peak value dramatically decreases when LSCO layer is thick enough because of shorting by the LSCO layer. The magnetic coercivity H_C shows a nonmonotonic behavior with changing spacer layer thickness and the waist-like hysteresis indicates that there is an indirect exchange coupling between the top and bottom LCMO layers across the spacer layer.