Phenomenological Model of Magnetocaloric Effect in La<Subscript>0.7</Subscript>Ca<Subscript>0.2</Subscript>Ba<Subscript>0.1</Subscript>MnO<Subscript>3</Subscript> Manganite Around Room Temperature

In this work, we studied in detail the magnetic and magnetocaloric properties of the La_0.7Ca_0.2Ba_0.1MnO₃ compound according to the phenomenological model. Based on this model, the magnetocaloric parameters such as the maximum of the magnetic entropy change ΔS _M and the relative cooling power (RCP) have been determined from the magnetization data as a function of temperature at several magnetic fields. The theoretical predictions are found to closely agree with the experimental measurements, which make our sample a suitable candidate for refrigeration near room temperature. In addition, field dependences of \({{\Delta } S}_{\mathrm {M}}^{\max }\) and RCP can be expressed by the power laws \({\Delta S}_{\mathrm {M}}^{\max }\approx a\)(μ ₀ H) ⁿ and RCP ≈b(μ ₀ H) ^m , where a and b are coefficients and n and m are the field exponents, respectively. Moreover, phenomenological universal curves of entropy change confirm the second-order phase transition.     

Magnetocaloric-Transport Properties Correlation in La<Subscript>0.8</Subscript> Ca<Subscript>0.2</Subscript>MnO<Subscript>3</Subscript>-Doped Manganites

This investigation is interested in studying the relation between magnetocaloric effect and transport properties i La_0.8Ca_0.2MnO₃ manganite compound. The value of the magnetocaloric effect has been determined from the calculation of magnetization as a function of temperature under different external magnetic fields. This study also provides an alternative method to determine the magnetocaloric properties such as magnetic entropy change and heat capacity change on the basis of M(T, H) measurements. On the other hand, based on magnetic and resistivity measurements, the magnetocaloric properties of this compound were investigated using an equation of the form \({\Delta } S \,=\, - \alpha {\int \limits _{0}^{H}} {\left [ {\frac {\delta Ln\left (\rho \right )}{\delta T}} \right ]}_{H} dH\), which relates magnetic order to transport behavior of the compounds. As an important result, the values of MCE and the results of calculation are in good agreement with the experimental ones, which indicates the strong correlation between the electric and magnetic properties in manganites.     

Interface roughness influence on exchange bias effect in La<Subscript>2/3</Subscript>Ca<Subscript>1/3</Subscript>MnO<Subscript>3</Subscript>/La<Subscript>1/3</Subscript>Ca<Subscript>2/3</Subscript>MnO<Subscript>3</Subscript> bilayers

A Monte Carlo simulation study of La_2/3Ca_1/3MnO₃/La_1/3Ca_2/3MnO₃ bilayers exchange bias (EB) properties by using a classical Heisenberg model and Monte Carlo method with Metropolis algorithm is addressed. Samples were built atom-by-atom in order to resemble the real roughness. In this model, several contributions included nearest neighbors exchange interactions; two different interface couplings, magnetocrystalline anisotropy and Zeeman term, were considered. Here, an influence of the relaxation steps on the interface roughness is present. Our study focuses on the influence of interface roughness on hysteresis loops, particularly EB field (H _ex) and coercive force (H _c). Results reveal that H _ex and H _c decrease as the interface roughness increases.     

Metamagnetic Transition and Magnetocaloric Effect in La<Subscript>0.45</Subscript>Dy<Subscript>0.05</Subscript>Ca<Subscript>0.5</Subscript>Mn<Subscript>0.9</Subscript>V<Subscript>0.1</Subscript>O<Subscript>3</Subscript> Compound

La_0.45Dy_0.05Ca_0.5Mn_0.9V_0.1O₃, prepared by solid-state route, was characterized using x-ray diffraction at room temperature. The Rietveld refinement shows that the sample crystallizes in orthorhombic structure with Pbnm space group. A secondary phase LaVO₄ has been also detected. The temperature dependence of the magnetization was investigated to determine the characteristics of the magnetic transition. The sample exhibits a paramagnetic-ferromagnetic transition (PM-FM) at T _C = 81 ± 0.7 K when temperature decreases. The study of the inverse of susceptibility reveals the presence of ferromagnetic clusters in the paramagnetic region. A metamagnetic transition was observed from the M(H) curves and the magnetic entropy change was calculated from magnetization curves at different temperatures in order to evaluate the magnetocaloric effect.     

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.     

Investigation of Magnetic,Magnetocaloric, and Critical Properties of La<Subscript>0.5</Subscript>Ba<Subscript>0.5</Subscript>MnO<Subscript>3</Subscript> Manganite

We present an extensive study of the magnetic properties of a novel La_0.5Ba_0.5MnO₃ perovskite material prepared by the hydrothermal method. The explored sample was structurally studied by the x-ray diffraction (XRD) method which confirms the formation of a pure cubic phase of a perovskite structure with Pm3m space group. The magnetic properties were probed by employing temperature M (T) and external magnetic field M (μ_oH) dependence of magnetization measurements. A magnetic phase transition from ferromagnetic to paramagnetic phase occurs at 339 K in this sample. The maximum magnetic entropy change (\(\left | {{\Delta } S}_{M}^{\max } \right |\)) took a value of 1.4 J kg^??1 K^??1 at the applied magnetic field of 4.0 T for the explored sample and has also been found to occur at Curie temperature (T_C). This large entropy change might be instigated from the abrupt reduction of magnetization at T_C. The magnetocaloric effect (MCE) is maximum at T_C as represented by M (μ_oH) isotherms. The relative cooling power (RCP) is 243.2 J kg^??1 at μ_oH =?4.0 T. Moreover, the critical properties near T_C have been probed from magnetic data. The critical exponents δ, β, and γ with values 3.82, 0.42, and 1.2 are close to the values predicted by the 3D Ising model. Additionally, the authenticity of the critical exponents has been confirmed by the scaling equation of state and all data fall on two separate branches, one for T < T_C and the other for T > T_C, signifying that the critical exponents obtained in this work are accurate.     

Photoinduced characteristics in La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> film

A systematic investigation of photoinduced properties is carried out in La_0.67Ca_0.33MnO₃ film prepared on LaAlO₃ (100) substrate by magnetron sputtering method. At T < 270 K, the resistivity of film induced by laser increases because of the demagnetization effect of manganites. The photoinduced relaxation character of film indicates that the time constant increases with increasing temperature, which is attributed to the growing thermal fluctuation. After laser irradiation, the resistivity returns to the original value and the relaxation time seems to be independent of temperature. In insulating state, laser irradiation induces the reduction in resistivity of film due to the excitation of small polarons.     

Phenomenological model of the magnetocaloric effect and its correlation with critical behavior near room temperature in La<Subscript>0.7</Subscript>Ca<Subscript>0.2</Subscript>Sr<Subscript>0.1</Subscript>MnO<Subscript>3</Subscript> manganite

In this paper, we investigate the field dependence of the magnetocaloric properties of La_0.7Ca_0.2Sr_0.1MnO₃ powder sample using a phenomenological model. Our compound was elaborated by the conventional solid state reaction. The model parameters were determined from the magnetization data and were used to give better fits to magnetic transition and to calculate the magnetocaloric quantities. The magnetocaloric parameters such as the maximum of the magnetic entropy change \(\Delta S_M^{max}\) and the relative cooling power RCP, have been determined from the calculation of the magnetization as a function of temperature under several magnetic applied field. Thus, from the magnetocaloric results, such as RCP?≈?b(μ₀H)^1+1/δ and T_peak???T_C ≈ b (µ₀H)^1/Δ, the critical exponents values related to the magnetic transition have been determined. The estimated results are close to those expected by the tricritical mean-field model. Furthermore, the values of the ferromagnetic transition temperature T_C, as well as the critical exponents β, γ and δ obtained by the theoretical model, are compared with those obtained by other various techniques (such as the modified Arrott plots, the Kouvel–Fisher method and the critical isotherm analysis). A good agreement has been found in the vicinity of the Curie temperature.     

Magnetic and Electrical Properties Induced by the Substitution of Divalent by Monovalent in the La<Subscript>0.6</Subscript>Ca<Subscript>0.4</Subscript>MnO<Subscript>3</Subscript> Compound

The La_0.6Ca_0.4?x Ag _x MnO₃ samples with x = 0 and 0.10 were prepared by sol–gel methods. Structural and electrical measurements were performed to examine the effect of the silver substitution in the calcium sites on the physical properties. Magnetization versus temperature studies have shown that all samples exhibit a magnetic transition from ferromagnetic to paramagnetic phase when temperature is increased. The second transition in the resistivity in the La_0.6Ca_0.4MnO₃ compound can be attributed to an abnormality characteristic of charge ordering (CO) effect. The electrical resistivity was described by a phenomenological percolation model. Ten percent of Ag substitution in the Ca site exhibits a magnetoresistance value about 75 % near room temperature at the applied magnetic field of 8 T.     

Influence of Na Addition on Magnetic and Magnetocaloric Effects of La<Subscript>0.67</Subscript>Pb<Subscript>0.13</Subscript>Na<Subscript>0.2</Subscript>MnO<Subscript>3</Subscript> Ceramics

Structural, magnetic, magnetocaloric, and electrical properties are reported for mixed-valence manganite La_0.67Pb_0.13Na_0.2MnO₃. X-ray diffraction reveals that the sample crystallizes in the rhombohedric structure with the R-3c space group. The magnetic properties of the polycrystalline La_0.67Pb_0.13Na_0.2MnO₃ compound are discussed in detail, based on the susceptibility, magnetization, and isotherm. The sample presents a ferromagnetic property with T _C= 275 K and a Griffiths phase at T _G= 325 K which gives the existence of ferromagnetic clusters in the paramagnetic domain. A large deviation is usually observed between field cooled (FC) and zero field cooled (ZFC). M(T) is a low temperature below the blocking temperature. At 40 K, a spin-glass or a cluster-glass state is seen to arise from a ferromagnetic state. This is caused by the competition between the antiferromagnetic and ferromagnetic interactions. The electrical properties show the presence of a metal–semiconductor transition at T _M?Sc. To understand the dependence of disorder with the transport mechanism, we used the phenomenological equation for resistivity under a percolation approach, which is dependent on the phase segregation of a paramagnetic semiconductor and ferromagnetic metallic regions.     

Low-field Magnetoresistance,Specific Heat and Magnetocaloric Effect in Sr Substituted Pr<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript>

We investigate the effect of ionic size variation on the electrical and thermodynamic properties in a series of Pr_0.7Ca_0.3−x Sr _x MnO₃ (PCSMO) samples. The increase in Sr content results in an increase of the unit cell volume, as a bigger Sr²⁺ ion replaces the smaller Ca²⁺ ions. Resistivity measurements show that the increase in the Sr content also results in the induction of a metal–insulator transition (T _MI), which increases with increasing Sr content. The activation energy (E _a), calculated from the resistivity data, decreases with increasing Sr content confirming the metallic character. The effect of the magnetic field on resistivity and specific heat has also been studied.     

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.     

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.     

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.     

Effect of A-site dopant on the piezoresistive characteristics of La<Subscript>0.8</Subscript>Sr<Subscript>0.2</Subscript>MnO<Subscript>3</Subscript> ceramics

This letter reports the effect of the A-site doping (2, 4, and 6 mol% Bi and Li) on the piezoresistance phenomenon in La_0.8Sr_0.2MnO₃ (LSMO) polycrystalline ceramics. The fractional change in resistivity was found to be nominal for Li-modified samples but samples modified with 4 and 6 mol% Bi₂O₃ exhibited 1.8% and 2.3% fractional change in resistivity at 19.2 MPa which is significantly higher than that for pure LSMO. The enhancement in piezoresistive phenomenon is attributed to distortion of Mn–O bond due to substitution of smaller ion on to La-site. All the samples showed a sudden increase in resistivity with applied stress in the range of 0.5–2 MPa and the behavior was found to saturate as the magnitude of applied stress increases. Magnetic measurements as a function of field and temperature were conducted to confirm the A-site substitution.     

Low-field magnetoresistance in La<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> manganite compounds prepared by the spray drying technique

Calcium-substituted lanthanum manganite compounds were synthesized by the spray drying technique. This method—whose main advantages are versatility, high reproducibility and scalability—yields small grain materials of high homogeneity and displaying low-field magnetoresistance effects. We report about the physical and chemical characterizations of these samples in order to investigate the potential interest of spray drying for the production of materials for low-field magnetoresistance applications. We have studied the dependence of the low-field magnetoresistance on the temperature and duration of the thermal treatment applied to the pelletized powders. The issue of the shape anisotropy (demagnetisation effects) influence on the magnetoresistance properties has also been dealt with.     

Electron Spin Resonance Study of Polycrystalline La<Subscript>0.4</Subscript>Bi<Subscript>0.1</Subscript>Ca<Subscript>0.5</Subscript>MnO<Subscript>3</Subscript>

We have inspected the magnetic properties of polycrystalline La_0.4Bi_0.1Ca_0.5MnO₃ using electron spin resonance (ESR) in the temperature range 150–280 K. The temperature dependence of magnetization indicates that the Curie temperature is T _C= 225 K. ESR spectra revealed that the sample is not completely paramagnetic above its Curie temperature through the presence of ferromagnetic interactions in the temperature range 225–270 K which can be attributed to the presence of Griffiths phase in this temperature range. The sample becomes completely paramagnetic above 270 K. The presence of Griffiths phase can be attributed to the disorder induced by the 6 s ² lone pair electrons of Bi³⁺ ions.     

Ac and dc magnetotransport properties of the phase-separated La<Subscript>0.6</Subscript>Y<Subscript>0.1</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> manganite

The physical properties of the La_0.6Y_0.1Ca_0.3MnO₃ compound have been investigated, focusing on the magnetoresistance phenomenon studied by both dc and ac electrical transport measurements. X-ray diffraction and scanning electron microscopy analysis of ceramic samples prepared by the sol–gel method revealed that specimens are single phase and have average grain size of ∼0.5 μm. Magnetization and 4-probe dc electrical resistivity ρ(T,H) experiments showed that a ferromagnetic transition at T _C ∼ 170 K is closely related to a metal-insulator (MI) transition occurring at essentially the same temperature T _MI . The magnetoresistance effect was found to be more pronounced at low applied fields (H ≤ 2.5 T) and temperatures close to the MI transition. The ac electrical transport was investigated by impedance spectroscopy Z(f,T,H) under applied magnetic field H up to 1 T. The Z(f,T,H) data exhibited two well-defined relaxation processes that exhibit different behaviors depending on the temperature and applied magnetic field. Pronounced effects were observed close to T _C and were associated with the coexistence of clusters with different electronic and magnetic properties. In addition, the appreciable decrease of the electrical permittivity ε′(T,H) is consistent with changes in the concentration of e _g mobile holes, a feature much more pronounced close to T _C .     

Microstructural characterisation by X-ray scattering of perovskite-type La<Subscript>0.8</Subscript>Sr<Subscript>0.2</Subscript>MnO<Subscript>3±δ</Subscript> thin films prepared by a dip-coating process

The La_0.8Sr_0.2MnO₃ (LSM) cathode materials are widely used in solid oxide fuel cells (SOFCs) as electronic conductors. In such materials, the reduction of oxygen is located at the triple contact boundaries: air/cathode LSM/electrolyte which is generally Yttria Stabilised Zirconia (YSZ). In order to improve the chemical reactions at these air/cathode LSM/electrolyte interfaces, the triple phase boundary length has to be optimised. In this aim, we have first synthesised the La_0.8Sr_0.2MnO₃ phase by a sol–gel route and, second, LSM thin films have been deposited on various polished substrates by using a dip-coating process. The structure and microstructure of the resulting LSM thin layers have been investigated by using well suited complementary techniques such as X-ray reflectometry, grazing incidence small angle X-ray scattering, X-ray diffraction and scanning electronic microscopy. The structural and microstructural parameters of LSM thin films have been managed and studied as a function of synthesis parameters such as initial metallic salt concentration, time and temperature of annealing. The higher the metallic salt concentration, the higher the thickness of the film, the smaller the film density. The as-prepared layers are amorphous and the single crystallised perovskite form is obtained for low temperature heat treatments. Therefore, the annealed coatings are constituted by randomly oriented LSM nanocrystals, which organise in a more or less dense close-packed microstructure according to the initial metallic salt concentration.     

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.