Possibility of controlling the conduction mechanism by choosing a specific doping element in a praseodymium manganite system

Electrical properties of Pr0.7Ca0.3Mn0.9X0.1O3 (X = Co, Ni, Cr and Fe) systems have been investigated using impedance spectroscopy measurements. The reported results confirmed the role of cationic disorder on the transport properties of the doped Pr0.7Ca0.3MnO3 system. For the case of the substitution by Co and Ni and Fe transition metals, the lower temperature side has been marked by the activation of the hopping conductivity over the nearest sites. Moreover, the Shklovskii-Efros-variable range hopping conductivity mechanism has been observed in the case of the substitution by Cr element. In the high temperature range, the evolution of the resistance with temperature confirmed the activation of a hopping process. In such a temperature range, the conduction process of all the studied compounds is dominated by a thermally activated small polaron hopping mechanism. For the Pr0.7Ca0.3Mn0.9Cr0.1O3 compound, AC studies have confirmed that the electrical conductance should be investigated in terms of an activated quantum mechanical tunneling process. At higher frequencies, the Pr0.7Ca0.3Mn0.9Fe0.1O3 compound is characterized by the existence of a high frequency plateau. For the Pr0.7Ca0.3Mn0.9Fe0.1O3 ceramic, the dispersive region of the spectrum has confirmed the activation of the correlated barrier hopping mechanism. Thus, the conductance is found to follow the double Jonscher power law only for the temperature range of [80 K, 200 K]. For the Pr0.7Ca0.3Mn0.9Ni0.1O3 compound, the evolution of the frequency exponent has confirmed the activation of two conduction mechanisms. The non small polaron tunneling mechanism was activated at lower temperatures. Accordingly, the activation of the correlated barrier hopping mechanism was detected for the high temperature range. For Pr0.7Ca0.3Mn0.9Co0.1O3 manganite, the coexistence of two conduction mechanisms (correlated barrier hopping and the non small polaron tunneling) is noticed. The latter's were activated in the whole of the explored temperature range. Using the scaling model, the spectra of both Pr0.7Ca0.3Mn0.9Cr0.1O3 and Pr0.7Ca0.3Mn0.9Ni0.1O3 compounds merge into a single master curve, which confirms the validity of the time temperature superposition principle.

Structural, magnetic, electrical and dielectric properties of Pr0.8Na0.2MnO3 manganite

The orthorhombic Pr_0.8Na_0.2MnO₃ ceramic was prepared in polycrystalline form by a Pechini sol–gel method and its structural, magnetic, electrical and dielectric properties were investigated experimentally. A structural study confirms that the sample is single phase. Magnetic measurements show that the sample is a charge ordered manganite. The sample undergoes two successive magnetic phase transitions with the variation of temperature: a charge ordering transition occurred at T_CO = 212 K followed by a paramagnetic (PM) to ferromagnetic (FM) transition around T_C = 115 K. From an electrical point of view, a saturation region was marked in the conductivity as a function of temperature σ(T) curves at a specific temperature. The dc-conductivity (σ_dc) reaches a maximum value at 240 K. The obtained results are in good agreement with the temperature dependence of the average normalized change (ANC). We found that the conduction mechanism was governed by small polaron hopping (SPH) in the high temperature region and by variable range hopping (VRH) in the low temperature region. Complex impedance analysis indicates the presence of a non-Debye relaxation phenomenon in the system. Also, the compound was modeled by an electrical equivalent circuit. Then, the contribution of the grain boundary in the transport properties was confirmed.

The orthorhombic Pr_0.8Na_0.2MnO₃ ceramic was prepared in polycrystalline form by a Pechini sol–gel method and its structural, magnetic, electrical and dielectric properties were investigated experimentally.

Magnetic and magnetocaloric properties of La0.55Bi0.05Sr0.4CoO3 and their implementation in critical behaviour study and spontaneous magnetization estimation

In this work, we present the results of the magnetic, critical, and magnetocaloric properties of the rhombohedral-structured La_0.55Bi_0.05Sr_0.4CoO₃ cobaltite. Based on the modified Arrott plot, Kouvel–Fisher, and critical isotherm analyses, we obtained the values of critical exponents (β, γ, and δ) as well as Curie temperature (T_C) for the investigated compound. These components were consistent with their corresponding values and they were validated by the Widom scaling law and scaling theory. The obtained critical exponents were close to the theoretical prediction of the mean-field model values, revealing the characteristic of long-range ferromagnetic interactions. The magnetic entropy, heat capacity, and local exponent n(T, μ₀H) of the La_0.55Bi_0.05Sr_0.4CoO₃ compound collapsed to a single universal curve, confirming its universal behaviour. The estimated spontaneous magnetization value extracted through the analysis of the magnetic entropy change was consistent with that deduced through the classical extrapolation of the Arrott curves. Thus, the magnetic entropy change is a valid and useful approach to estimate the spontaneous magnetization of La_0.55Bi_0.05Sr_0.4CoO₃.

In this work, we present the results of the magnetic, critical, and magnetocaloric properties of the rhombohedral-structured La_0.55Bi_0.05Sr_0.4CoO₃ cobaltite.

Magnetic entropy table-like shape and enhancement of refrigerant capacity in La1.4Ca1.6Mn2O7–La1.3Eu0.1Ca1.6Mn2O7 composite

In this work, we have investigated the structural, magnetic and magnetocaloric properties of La_1.4Ca_1.6Mn₂O₇ (A) and La_1.3Eu_0.1Ca_1.6Mn₂O₇ (B) oxides. These compounds are synthesized by a solid-state reaction route and indexed with respect to Sr₃Ti₂O₇-type perovskite with the I4/mmm space group. The substitution of La by 10% Eu enhances the value of magnetization and reduces the Curie temperature (T_C). It is also shown that these compounds undergo a first-order ferromagnetic–paramagnetic phase transition around their respective T_C. The investigated samples show large magnetic entropy change (ΔS_M) produced by the sharp change of magnetization at their Curie temperatures. An asymmetric broadening of the maximum of ΔS_M with increasing field is observed in both samples. This behaviour is due to the presence of metamagnetic transition. The ΔS_M(T) is calculated for A_x/B_1−x composites with 0 ≤ x ≤ 1. The optimum ΔS_M(T) of the composite with x = 0.48 approaches a nearly constant value showing a table-like behaviour under 5 T. To test these calculations experimentally, the composite with nominal composition A_0.48/B_0.52 is prepared by mixing both individual samples A and B. Magnetic measurements show that the composite exhibits two successive magnetic transitions and possesses a large MCE characterized by two ΔS_M(T) peaks. A table-like magnetocaloric effect is observed and the result is found to be in good agreement with the calculations. The obtained ΔS_M(T) is ≈4.07 J kg⁻¹ K⁻¹ in a field change of 0–5 T in a wide temperature span over ΔT_FWHM ∼ 68.17 K, resulting in a large refrigerant capacity value of ≈232.85 J kg⁻¹. The MCE in the A_0.48/B_0.52 has demonstrated that the use of composite increases the efficiency of magnetic cooling with μ₀H = 5 T by 23.16%. The large ΔT_FWHM and RC values together with the table-like (−ΔS_M)^max feature suggest that the A_0.48/B_0.52 composite can meet the requirements of several magnetic cooling composites based on the Ericsson-cycle. In addition, we show that the magnetic field dependence of MCE enables a clear analysis of the order of phase transition. The exponent N presents a maximum of N > 2 for A, B and A_0.48/B_0.52 samples confirming a first-order paramagnetic–ferromagnetic transition according to the quantitative criterion. The negative slope observed in the Arrott plots of the three compounds corroborates this criterion.

In this work, we have investigated the structural, magnetic and magnetocaloric properties of La_1.4Ca_1.6Mn₂O₇ (A) and La_1.3Eu_0.1Ca_1.6Mn₂O₇ (B) oxides.

Analysis based on scaling relations of critical behaviour at PM-FM phase transition and universal curve of magnetocaloric effect in selected Ag-doped manganites

The critical behaviour of Pr0.5Sr0.5-x Ag x MnO3 (0 ≤ x ≤ 0.2) samples around the paramagnetic-ferromagnetic phase transition is studied based on isothermal magnetization measurements. The assessments based on Banerjee's criteria reveal the samples undergoing a second-order magnetic phase transition. Various techniques such as modified Arrott plot, Kouvel-Fisher method, and critical isotherm analysis were used to determine the values of the ferromagnetic transition temperature T C, as well as the critical exponents of β, γ and δ. The values of critical exponents, derived from the magnetization data using the Kouvel-Fisher method, are found to be (β = 0.43 ± 0.002, 0.363 ± 0.068 and 0.328 ± 0.012), (γ = 1.296 ± 0.007, 1.33 ± 0.0054 and 1.236 ± 0.012) for x = 0.0, 0.1 and 0.2, respectively. This implies that the Pr0.5Sr0.5-x Ag x MnO3 with 0 ≤ x ≤ 0.2 systems does not belong to a single universality class and indicates that the presence of magnetic disorder in the system must be taken into account to fully describe the microscopic interaction of these manganites. With these values, magnetic-field dependences of magnetization at temperatures around T C can be well described following a single equation of state for our samples. From magnetic entropy change (ΔS M), it was possible to evaluate the critical exponents of the magnetic phase transitions. Their values are in good agreement with those obtained from the critical exponents using a modified Arrott plot (MAP). We used the scaling hypotheses to scale the magnetic entropy change and heat capacity changes to a universal curve respectively for Pr0.5Sr0.5-x Ag x MnO3 samples.

Effect of erbium concentration on the structural, optical and electrical properties of a Bi4Ti3O12 system

We have studied the influence of erbium doping on the optical and electrical properties of a Bi₄Ti₃O₁₂ compound.

Critical behavior and magnetocaloric study in La0.6Sr0.4CoO3 cobaltite prepared by a sol–gel process

The structure, magnetic properties, critical exponents and magnetocaloric effect (MCE) of La_0.6Sr_0.4CoO₃ cobaltite are studied.