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.     

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.     

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 .     

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.     

Magnetic cluster state in anion-deficient La<Subscript>0.70</Subscript>Sr<Subscript>0.30</Subscript>MnO<Subscript>2.85</Subscript> manganite

It is established that the magnetic state of the anion-deficient La_0.70Sr_0.30MnO_2.85 manganite represents the spin-glass state of the cluster. The magnetic field at the beginning (H < 10 kOe) leads to the fragmentation of ferromagnetic clusters, then (H > 10 kOe) leads to the transition to a ferromagnetic state of an antiferromagnetic matrix and to increase in the degree of polarization of local spins of manganese. It is determined that the freezing temperature of magnetic moments varies as T _f = 65 − 6H ^0.21. The causes and mechanism of the magnetic phase separation are discussed.     

Microstructure and Transport Properties in Tin Boned La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> Composites

A novel boned perovskite manganese oxide magnetoresistant material was prepared using La_0.7Sr_0.3MnO₃ (LSMO) as the precursor powders and metal tin (Sn) as the binder. The microstructure and phase characteristics, low-field transport properties were studied. Sn segregated at the grain boundaries of LSMO grains. The insulator–metal (I–M) transition and enhanced LFMR are only observed with a low content of Sn, due to grain boundary disorder and spin polarized tunneling between grain boundaries. The Sn addition induced resistivity decreasing dramatically. In the high temperature PM region, the resistivities for samples with low Sn content follow the adiabatic small-polaron-hopping model.     

Prediction of Magnetocaloric Effect by a Phenomenological Model and Critical Behavior for La<Subscript>0.78</Subscript>Dy<Subscript>0.02</Subscript>Ca<Subscript>0.2</Subscript>MnO<Subscript>3</Subscript> Compound

The La_0.78Dy_0.02Ca_0.2MnO₃ (LDCMO) compound prepared via high-energy ball-milling (BM) presents a ferromagnetic-to-paramagnetic transition (FM-PM) and undergoes a second-order phase transition (SOFT). Based on a phenomenological model, magnetocaloric properties of the LDCMO compound have been studied. Thanks to this model, we can predict the values of the magnetic entropy change ΔS, the full width at half-maximum δ T _FWHM, the relative cooling power (RCP), and the magnetic specific heat change ΔC _p for our compound. The significant results under 2 T indicate that our compound could be considered as a candidate for use in magnetic refrigeration at low temperatures. In order to further understand the FM-PM transition, the associated critical behavior has been investigated by magnetization isotherms. The critical exponents estimated by the modified Arrott plot, the Kouvel–Fisher plot, and the critical isotherm technique are very close to those corresponding to the 3D-Ising standard model (β = 0.312 ± 0.07, γ = 1.28 ± 0.02, and δ = 4.80).Those results revealed a long-range ferromagnetic interaction between spins.     

Substrate effect on the properties of La<Subscript>0.775</Subscript>Sr<Subscript>0.225</Subscript>MnO<Subscript>3</Subscript> films

La_0.775Sr_0.225MnO₃ films have been produced by screen printing on various substrates (Al₂O₃, BaTi_0.85Zr_0.11Sn_0.04O₃, Ba_0.996Y_0.004TiO₃, Ba_0.996Y_0.004TiO₃ + 0.04%Mn, and Ba_0.996Y_0.004Ti_0.65Sn_0.35O₃), and their electrical properties have been studied in comparison with those of bulk materials. The structural properties of the substrates are shown to influence the electrical properties of the films.     

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.     

Dilution effects on X-ray photoelectron spectra of La<Subscript>0.8</Subscript>Sr<Subscript>0.2</Subscript>MnO<Subscript>3</Subscript> with SiO<Subscript>2</Subscript>

Diluted samples of La_0.8Sr_0.2MnO₃ with SiO₂ were prepared by calcination of the MCM-41 molecular sieve soaked in precursor solutions with various concentrations (0.1, 0.05 and 0.005 mol/l). The diluted samples from the 0.1, 0.05 and 0.005 mol/l solutions showed ferromagnetic behavior in temperature dependent magnetic susceptibility below 300, 250 and 100 K, respectively. The electron binding energy of the La 3d_5/2 main peak of the diluted samples from the 0.1, 0.05 and 0.005 mol/l solutions were 835.7, 836.0 and 837.4 eV, respectively, and were larger than that of La_0.8Sr_0.2MnO₃ bulk sample (834.2 eV). Reduction in intensity of the La 3d charge-transfer satellite was more pronounced for more diluted sample. The Mn 2p_3/2 main peak of all the diluted samples remained at 642.2 eV, which is larger by 0.6 eV than that of the bulk sample (641.6 eV). The charge-transfer peak energy in the optical absorption spectra was the same (2.3 eV) for all the diluted samples, and the energy was larger by 0.4 eV than the bulk one (1.9 eV). The dilution caused localization of charge carriers within the La_0.8Sr_0.2MnO₃ lattices isolated by the insulator. Suppression of charge flow between the isolated La_0.8Sr_0.2MnO₃ lattices resulted in the enlargement of the La 3d electron binding energy and the decrease of the La 3d charge-transfer satellite peak intensity, and also the lowering of the ferromagnetic ordering temperature. © Springer Science + Business Media, Inc.     

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.     

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.     

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.     

Wide range magnetoresistance and high temperature coefficient of resistance in La<Subscript>0.7</Subscript>Sr<Subscript>0.3−x</Subscript>Ag<Subscript>x</Subscript>MnO<Subscript>3</Subscript> system

We report observation of composition and process parameter dependant wide range magnetoresistance and high temperature coefficient of resistance in La_0.7Sr_0.3−xAg_xMnO₃ (0 ≤ x ≤ 0.3) system. The polycrystalline samples synthesized through solid state reaction method exhibited rhombohedral structure with systematic change in lattice parameters. The samples with x ≤ 0.2 showed metallic behavior below room temperature. A broad metal-insulator transition is observed for x = 0.3, which becomes sharp for the samples sintered at high temperature. The broad transition resulted in magnetoresistance (MR) value around 35% over a wide temperature range whereas the sharp transition results in MR value as high as 85%. The temperature coefficient of resistance value of 11% is seen as a consequence of sharp transition. The magnetic transition was sharp for the samples sintered at high temperature.     

Features of the magnetic properties of La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> Manganite films obtained by an extraction-pyrolysis method

We have experimentally studied the magnetic properties of manganite films obtained for first time using an extraction-pyrolysis technique. It is established that the characteristics of samples significantly depend on the conditions of final annealing. The annealing at temperatures T _a < 970 K is accompanied by strong thermomagnetic effects, and the resulting films possess properties similar to those of spin glasses. When the annealing temperature is increased to T _a ≥ 1020 K, the films exhibit magnetic properties typical of ferromagnetic systems.     

Interface-dependent resistance switching in Nd<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> ceramics

Interface-dependent electric-pulse-induced resistance switching effect (EPIR) in Nd_0.7Sr_0.3MnO₃ ceramics was studied. The results reveal that the EPIR effect originates from the interface between the electrodes and the bulk, and the EPIR ratio as well as the high and low resistance states can be strongly influenced by applying a large electrical field on the sample for different intervals. Also, the pulse parameters have great effect on the stability of EPIR and the optimal pulse width, pulse amplitude and read bias are obtained. Based on the space charge limited current mechanism together with the theory of interfacial charge-trapped state, the interface-dependent resistance switching effect is discussed.     

Magnetic,magnetocaloric properties,and critical behavior in a layered perovskite La<Subscript>1.4</Subscript>(Sr<Subscript>0.95</Subscript>Ca<Subscript>0.05</Subscript>)<Subscript>1.6</Subscript>Mn<Subscript>2</Subscript>O<Subscript>7</Subscript>

We report the results of magnetic, magnetocaloric properties, and critical behavior investigation of the double-layered perovskite manganite La_1.4(Sr_0.95Ca_0.05)_1.6Mn₂O₇. The compounds exhibits a paramagnetic (PM) to ferromagnetic (FM) transition at the Curie temperature T _C = 248 K, a Neel transition at T _N = 180 K, and a spin glass behavior below 150 K. To probe the magnetic interactions responsible for the magnetic transitions, we performed a critical exponent analysis in the vicinity of the FM–PM transition range. Magnetic entropy change (??S _M) was estimated from isothermal magnetization data. The critical exponents β and γ, determined by analyzing the Arrott plots, are found to be T _C = 248 K, β = 0.594, γ = 1.048, and δ = 2.764. These values for the critical exponents are close to the mean-field values. In order to estimate the spontaneous magnetization M _S(T) at a given temperature, we use a process based on the analysis, in the mean-field theory, of the magnetic entropy change (??S _M) versus the magnetization data. An excellent agreement is found between the spontaneous magnetization determined from the entropy change [(??S _M) vs. M ²] and the classical extrapolation from the Arrott curves (µ₀H/M vs. M ²), thus confirming that the magnetic entropy is a valid approach to estimate the spontaneous magnetization in this system and in other compounds as well.     

Anomalous magnetic susceptibility in Nd<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>MnO<Subscript>3</Subscript> manganite single crystals

Chaotic oscillations of the low-frequency magnetic susceptibility χ in a Nd_0.5Sr_0.5MnO₃ (NSMO) manganite single crystal have been observed in the temperature interval of the coexistence of the ferro- and antiferromagnetic phases of the given composition. It is established that the amplitude of χ oscillations depends on the constant bias magnetic field. The appearance of susceptibility oscillations in NSMO single crystals is attributed to the formation of magnetic transition regions, which can exist at the boundaries between ferro- and antiferromagnetic phases.     

Effect of Ho-doping on structural,electrical and magnetic properties of La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> ceramics prepared by Plasma-Activated Sintering

Different components of La_0.7?x Ho _x Sr_0.3MnO₃ (LHSMO, x = 0, 0.1, 0.2, 0.3) ceramics were fabricated by Plasma-Activated Sintering (PAS), so as to study the correlation between the contents of Ho³⁺ and the structural, electrical, magnetic properties. XRD and SEM confirmed that LHSMO ceramics prepared by PAS exhibited high-purity phase and dense microstructure. The measurement of electrical resistivity showed that the resistivity of LHSMO ceramics increased, and the metal–insulator transition temperature decreased with the increasing Ho-doping content. The resistivity data were then fitted using various empirical equations, and the conduction mechanism of LHSMO ceramics was found to be in accord with the electron–magnon scattering process in the low-temperature region and the small polaron hopping model in the high-temperature region. Lastly, we calculated the values of magnetoresistance of the LHSMO ceramics, which increased with increasing Ho-doping content, from 3.5% for x = 0 to 14.6% for x = 0.3. Therefore, the doping of Ho³⁺ into La_0.7Sr_0.3MnO₃ can effectively enhance the low-field magnetoresistance effect.