Effect of Fe substitution on magnetocaloric effect in La0.7Sr0.3Mn1−xFexO3 (0.05≤x≤0.20)

We have studied the effect of Fe substitution on magnetic and magnetocaloric properties in La_0.7Sr_0.3Mn_1−xFe_xO₃ (x=0.05, 0.07, 0.10, 0.15, and 0.20) over a wide temperature range (T=10-400 K). It is shown that substitution by Fe gradually decreases the ferromagnetic Curie temperature (T_C) and saturation magnetization up to x=0.15 but a dramatic change occurs for x=0.2. The x=0.2 sample can be considered as a phase separated compound in which both short-range ordered ferromagnetic and antiferromagnetic phases coexist. The magnetic entropy change (−ΔS_m) was estimated from isothermal magnetization curves and it decreases with increase of Fe content from 4.4 J kg⁻¹ K⁻¹ at 343 K (x=0.05) to 1.3 J kg⁻¹ K⁻¹ at 105 K (x=0.2), under ΔH=5 T. The La_0.7Sr_0.3Mn_0.93Fe_0.07O₃ sample shows negligible hysteresis loss, operating temperature range over 60 K around room temperature with refrigerant capacity of 225 J kg⁻¹, and magnetic entropy of 4 J kg⁻¹ K⁻¹ which will be an interesting compound for application in room temperature refrigeration.     

Structural, magnetic and magnetotransport behavior of La0.7SrxCa0.3−xMnO3 compounds

A systematic investigation of the structural, magnetic and electrical properties of a series of nanocrystalline La_0.7Sr_xCa_0.3−xMnO₃ materials, prepared by high energy ball milling method and then annealed at 900 °C has been undertaken. The analysis of the XRD data using the Win-metric software shows an increase in the unit cell volume with increasing Sr ion concentration. The La_0.7Sr_xCa_0.3−xMnO₃ compounds undergo a structural orthorhombic-to-monoclinic transition at x=0.15. Electric and magnetic measurements show that both the Curie temperature and the insulator-to-metal transition temperature increase from 259 K and 253 K correspondingly for La_0.7Ca_0.3MnO₃ (x=0) to 353 K and 282 K, respectively, for La_0.7Sr_0.3MnO₃ (x=0.3). It is argued that the larger radius of Sr²⁺ ion than that of Ca²⁺ is the reason to strengthen the double-exchange interaction and to give rise to the observed increase of transition temperatures. Using the phenomenological equation for conductivity under a percolation approach, which depends on the phase segregation of ferromagnetic metallic clusters and paramagnetic insulating regions, we fitted the resistivity versus temperature data measured in the range of 50-320 K and found that the activation barrier decreased with the raising Sr²⁺ ion concentration.     

Magnetic properties and Griffiths singularity in La0.45Sr0.55 Mn1−xCoxO3

The magnetic properties and the Griffiths singularity were investigated in Mn-site doped manganites of La_0.45Sr_0.55Mn_1−xCo_xO₃ (x=0, 0.05, 0.10 and 0.15) in this work. The parent sample La_0.45Sr_0.55MnO₃ undergoes a paramagnetic-ferromagnetic transition at T_C=290 K and a ferromagnetic-antiferromagnetic transition at T_N=191 K. The doping of Co ions enhances the ferromagnetism and suppresses the antiferromagnetism. The enhanced ferromagnetism results from the fact that the Co doping enhances the Mn³⁺-Mn⁴⁺ double-exchange interaction and induces the Co²⁺-Mn⁴⁺ ferromagnetic superexchange interaction. Detailed investigation on the magnetic behavior above T_C exhibits that the Griffiths singularity takes place in this series of Mn-site doped compounds. The correlated disorder induced by the Co ionic doping, together with the phase competition from the ferromagnetic and the antiferromagnetic interactions among Mn ions, is responsible for the Griffiths singularity.     

Structural and magnetic phase diagram and room temperature CMR effect of La1−xAgxMnO3

The study of the structural and magnetic phase diagram of the manganites La_1−xAg_xMnO₃ shows similarity with the La_1−x′Sr_x′MnO₃ series, involving a metallic ferromagnetic domain at relatively high temperature (≈300 K). The Ag-system differs from the Sr-one by a much smaller homogeneity range (x≤1/6) and the absence of charge ordering. But the most important feature of the Ag-manganites deals with the exceptionally high magnetoresistance (−25%) at room temperature under 1.2 T, that appears for the composition x=1/6. The latter is interpreted as the coincidence of the optimal double exchange condition (Mn³⁺:Mn⁴⁺=2) with T_max=300 K (maximum of the ρ(T) curve in zero field).     

Large magnetoresistance in low temperature metallic region of manganite compounds (La0.7−2xEux)(Ca0.3Srx)MnO3 (0.05≤x≤0.15)

Magnetoresistance (MR) and magnetization (dc and ac) measurements have been carried out on the manganites, (La_0.7−2xEu_x)(Ca_0.3Sr_x)MnO₃ (0.05≤x≤0.15), in the temperature range of 5-320 K. At 5 K, an unusually large MR of almost 98% is observed in the x=0.15 sample, nearly up to fields of 4-5 T. This large high-field MR occurs in the metallic region, far below the insulator-metal transition temperature, and does not vary linearly with applied field. The unusual magnetoresistance is explained in the light of various possibilities such as phase segregation, cluster spin-glass behavior, etc.     

Magnetocaloric properties of (La0.67−xGdx)Sr0.33MnO3 polycrystalline nanoparticles

In this paper, magnetic property and magnetocaloric effect (MCE) in nanoparticles perovskite manganites of the type (La_0.67−xGd_x)Sr_0.33MnO₃ (x=0.10, 0.15, 0.20) synthesized by using an amorphous molecular alloy as precursor have been reported. From the magnetic measurements as function of temperature and magnetic applied field, we have discovered that the Curie temperature (T_C) of the prepared samples is found to be strongly dependent on Gd content. The Curie temperature of samples is 358.4, 343.2, and 285.9 K for x=0.1, 0.15, and 0.2, respectively. A large magnetocaloric effect close to T_C has been observed with a maximum of magnetoentropy change in all the samples, ∣ΔS_M∣_max of 1.96 and 4.90 J/kg K at 2 and 5 T, respectively, for a substitution rate of 0.15. In addition, the maximum magnetic entropy change observed for samples with different concentration of Gd, exhibits a linear dependence with the applied high magnetic field. These results suggest that (La_0.67−x Gd_x)Sr_0.33MnO₃ (x=0.10, 0.15, 0.20) compounds could be a suitable candidate as working substance in magnetic refrigeration near room temperature.     

Synthesis and physical characterization of superconductivity-magnetism crossover compound RuSr2EuCeCu2O10−δ

We carefully studied the nonsuperconducting sample of the magneto-superconducting RuSr₂(Eu_1−xCe_x)Cu₂O_10−δ series with composition RuSr₂EuCeCu₂O_10−δ. This compound seems to exhibit a complex magnetic state as revealed by host of techniques like resistivity, thermopower, magnetic susceptibility, and MR measurements. The studied compound exhibited ferromagnetic like M(H) loops at 5, 20, and 50 K, and semiconductor like electrical conduction down to 5 K, with −MR^7 T of up to 4% at low temperatures. The −MR^7 T decreases fast above 150 K and monotonically becomes close to zero above say 230 K. Below, 150 K −MR^7 T decreases to around 3% monotonically down to 75 K, with further increase to 4% at around 30 K and lastly having a slight decrease below this temperature. The thermopower S(T) behavior closely followed the −MR^7 T steps in terms of d(S/T)/dT slopes. Further, both MR^7 T steps and d(S/T)/dT slopes are found in close vicinity to various magnetic ordering temperatures (T_mag) of this compound.     

Electrical and magnetic properties of non-stoichiometric (La0.8Sr0.2)1−xMnO3 perovskites

The structure, transport and magnetic properties of (La_0.8Sr_0.2)_1−xMnO₃ (0≤x≤0.30) polycrystalline perovskite manganites have been investigated. For all the samples the Curie temperatures, T_c, remain nearly unchanged (329±3 K). Resistivity versus temperature curves for the samples show a double-peak behavior. A significant magnetoresistance (MR) effect and different temperature dependences of the MR ratios of the samples are observed. The shapes of the MR-T curves of the samples can be adjusted by changing x. For the x=0.30 sample, a nearly constant MR ratio of (9.5±0.5)% is obtained over the temperature range from 205 to 328 K.     

Charge order melting and magnetic transition in Nd0.5(1+x)Ca0.5(1−x)Mn(1−x)CrxO3 system

The magnetic property of double doped manganite Nd_0.5(1+x)Ca_0.5(1−x)Mn_(1−x)Cr_xO₃ with a fixed ratio of Mn³⁺:Mn⁴⁺=1:1 has been investigated. For the undoped sample, it undergoes one transition from charge disordering to charge ordering (CO) associated with paramagnetic (PM)-antiferromagnetic (AFM) phase transition at T<250 K. The long range AFM ordering seems to form at 35 K, rather than previously reported 150 K. At low temperature, an asymmetrical M-H hysteresis loop occurs due to weak AFM coupling. For the doped samples, the substitution of Cr³⁺ for Mn³⁺ ions causes the increase of magnetization and the rise of T_c. As the Cr³⁺ concentration increases, the CO domain gradually becomes smaller and the CO melting process emerges. At low temperature, the FM superexchange interaction between Mn³⁺ and Cr³⁺ ions causes a magnetic upturn, namely, the second FM phase transition.     

Thermal conductivity of La0.67(CaxSr1−x)0.33MnO3 (x=0, 0.5, 1) and La0.6Y0.07Ca0.33MnO3 pellets between 10 and 300 K

Thermal conductivity (λ) of nanocrystalline La_0.67(Ca_xSr_1−x)_0.33MnO₃ (x=0, 0.5, 1) and La_0.6Y_0.07Ca_0.33MnO₃ pellets prepared by a novel ‘pyrophoric’ method have been studied between the temperature range 10 and 300 K. Our data show that the magnitude of thermal conductivity is strongly influenced by the ion substitutions at La-site. The analysis of the thermal conductivity data indicates that the thermal transport is governed largely by phonons scattering in these systems and the electronic contribution is as small as 0.2-1% of total thermal conductivity (λ_total). At low temperatures (<90 K) 2D like lattice defects contribute to the phonon scattering dominantly and its strength increases with increasing Sr content and also with partial substitution of La by Y. Depending upon the composition of the samples, the magnon thermal conductivity contributes 2-15% of λ_total close to T_C. In the paramagnetic regime the unusual increase in λ_total keeps signature of large dynamic lattice distortion.     

Hydrothermal synthesis and Magnetocaloric effect of La0.5Ca0.3Sr0.2MnO3

The hydrothermal synthesis and magnetic entropy change for the perovskite manganite La_0.5Ca_0.3Sr_0.2MnO₃ have been studied. The La_0.5Ca_0.3Sr_0.2MnO₃ can be produced as phase-pure, crystalline powders in one step from solutions of metal salts in aqueous potassium hydroxide solution at a temperature of 513 K in 72 h. Scanning electron microscopy shows that the materials are made up of cuboid-shaped particles in typical dimension of 4.0×2.5×1.6 μm. Heat treatment can improve the magnetocaloric effect for the hydrothermal sample. The maximum magnetic entropy change ΔS_M for the as-prepared sample is 0.88 J kg⁻¹ K⁻¹ at 315 K for a magnetic field change of 2.0 T. It increases to 1.52 J kg⁻¹ K⁻¹, near its Curie temperature (317 K) by annealing the sample at 1473 K for 6 h. The hydrothermal synthesis method is a feasible route to prepare high-quality perovskite material for magnetic refrigeration application.     

Low temperature specific heat of bi-layered manganites La2−2xSr1+2xMn2O7 (x=0.3 and 0.5)

The specific heat (C) of bi-layered manganites La_2−2xSr_1+2xMn₂O₇ (x=0.3 and 0.5) is investigated for the ground state of low temperature excitations. A T^3/2 dependent term in the low temperature specific heat (LTSH) is identified at zero magnetic field and suppressed by magnetic fields for x=0.3 sample, which is consistent with a ferromagnetic metallic ground state. For x=0.5 sample, a T² term is observed and is consistent with a two-dimensional (2D) antiferromagnetic insulator. However, it is almost independent of magnetic field within the range of measured temperature (0.6-10 K) and magnetic field (6 T).     

Effect of Mn-site vacancies on the magnetic entropy change and the Curie temperature of La0.67Ca0.33Mn1−xO3 perovskite

Single-phase polycrystalline samples of La_0.67Ca_0.33Mn_1−xO₃ (x=0.00, 0.02, 0.04, 0.06) have been prepared using the sol-gel method. The structure, magnetocaloric properties and the Curie temperature of the samples with different Mn vacancy concentrations have been investigated. The experimental results show that vacancy doping at the Mn-sites has a significant influence on the magnetic properties of La_0.67Ca_0.33Mn_1−xO₃. The Curie temperature decreases monotonically with increasing the Mn-site vacancy concentration x. A remarkable enhancement of the magnetic entropy change has been obtained in the La_0.67Ca_0.33Mn_0.98O₃ sample. The entropy change reaches |ΔS_M|=3.10 J kg⁻¹ K⁻¹ at its Curie temperature (264 K) under an applied magnetic field H=10 kOe, which is almost the same value as that of pure Gd.     

Magnetic and Microwave Absorbing Properties of Electrospun Ba(1−x)LaxFe12O19 Nanofibers

Ba_(1−x)La_xFe₁₂O₁₉ (0.00≤x≤0.10) nanofibers were fabricated via the electrospinning technique followed by heat treatment at different temperatures for 2 h. Various characterization methods including scanning electron microscopy (SEM), X-ray diffraction (XRD), vibrating sample magnetometer (VSM), and microwave vector network analyzer were employed to investigate the morphologies, crystalline phases, magnetic properties, and complex electromagnetic parameters of nanofibers. The SEM images indicate that samples with various values of x are of a continuous fiber-like morphology with an average diameter of 110±20 nm. The XRD patterns show that the main phase is M-type barium hexaferrite without other impurity phases when calcined at 1100 °C. The VSM results show that coercive force (H_c) decreases first and then increases, while saturation magnetization (M_s) reveals an increase at first and then decreases with La³⁺ ions content increase. Both the magnetic and dielectric losses are significantly enhanced by partial substitution of La³⁺ for Ba²⁺ in the M-type barium hexaferrites. The microwave absorption performance of Ba_0.95La_0.05Fe₁₂O₁₉ nanofibers gets significant improvement: The bandwidth below −10 dB expands from 0 GHz to 12.6 GHz, and the peak value of reflection loss decreases from −9.65 dB to −23.02 dB with the layer thickness of 2.0 mm.     

Synthesis, structural and transport properties of nanocrystalline La1−xBaxMnO3 (0.0≤x≤0.3) powders

Nanocrystalline La_1−xBa_xMnO₃ (0.0≤x≤0.3) manganites have been prepared by a simple and instantaneous solution combustion method, which is a low temperature initiated synthetic route to obtain fine-grained powders with relatively high surface area. The phase purity and crystal structure of the combustion products are carried out by powder X-ray diffraction. The as-made nanopowders are in cubic phase. On calcination to 900 °C, barium doped manganites retain cubic phase, whereas barium free manganite transformed to rhombohedral phase. The scanning electron microscope (SEM) results revealed that the combustion-derived compounds are agglomerated with fine primary particles. The doped manganites have surface area in the range 24-44 m²/g. The surface area of the manganites increases with barium content, whereas it decreases on calcination. Both undoped and doped lanthanum manganites show two active IR vibrational modes at 400 and 600 cm⁻¹. The low temperature resistivity measurements have been carried out by four-probe method down to 77 K. All the samples exhibit metal-insulator behaviour and metal-insulator transition temperature (T_M-I) in the range 184-228 K and it is interesting to note that, as the barium content increases the T_M-I shifts to lower temperature side. The maximum T_M-I of 228 K is observed for La_0.9Ba_0.1MnO₃ sample.     

Electronic transport and magnetic studies of La1−xCax−0.08Sr0.04Ba0.04MnO3

Lanthanum based mixed valence manganite system La_1−xCa_x−0.08Sr_0.04Ba_0.04MnO₃ (LCSBMO; x=0.15, 0.24 and 0.33) synthesized through the sol-gel route is systematically investigated in this paper. The electronic transport and magnetic susceptibility properties are analyzed and compared, apart from the study of unit cell structure, microstructure and composition. Second order phase transition is observed in all the samples and significant difference is observed between the insulator to metal transition temperature (T_MI) and paramagnetic (PM) to ferromagnetic (FM) transition temperature (T_C). In contrast to the insulating FM behaviour usually observed in La_1−xCa_xMnO₃ (LCMO) for x=0.15, a clear insulator to metal transition is observed for LCSBMO for the same percentage of lanthanum. The temperature dependent resistivity of polycrystalline pellets, when obeying the well studied law ρ=ρ_o+ρ₂T² for T<T_MI, is observed to differ significantly in the values of ρ_o and ρ₂, with the electrical conductivity increasing with x. The variable range hopping model has been found to fit resistivity data better than the small polaron model for T>T_MI. AC magnetic susceptibility study of the polycrystalline powders of the manganite system shows the highest PM to FM transition of 285 K for x=0.33.     

Room temperature magnetoresistance in Ln2/3A1/3MnO3 manganites

The investigation of the manganites La_2/3−xPr_xSr_1/3MnO₃, La_2/3Sr_1/3−xCa_xMnO₃ and La_2/3+xCa_1/3−2xAg_xMnO₃, which all exhibit Mn³⁺:Mn⁴⁺=2, shows that it is possible to reach high magnetoresistance at room temperature, up to 21% under 1.2 T. These materials are compared to La_5/6Ag_1/6MnO₃ which corresponds to the same Mn³⁺:Mn⁴⁺ ratio and exhibits a magnetoresistance of 25% in this field. An interesting feature deals with the value of the insulator-metal transition temperature T_IM, often higher than T_C, especially for Ag-based compounds. It is suggested that the latter results either from a better oxygenation of the surface of the grains or from a migration of silver toward the surface.     

Magnetocaloric effects in (Gd1−xTbx)Co2

The structures and magnetocaloric effects of (Gd_1−xTb_x)Co₂ (x=0, 0.25, 0.4, 0.5, 0.6, 0.7, 0.8, and 1) pseudobinary compounds were investigated by X-ray powder diffraction and magnetic properties measurement. The results show that the T_c of the alloy is near room temperature when X=0.6. The magnetic entropy changes of the compounds increase from 1.7 to 3.6 J/kg K with increasing the content of Tb under an applied field up to 2 T. All the compounds exhibit second order magnetic change. As a result, the values of their ΔS_M are lower than that of some large magnetocaloric effect materials.     

On the coexistence of spin and lattice polarons in the La0.67−xEuxCa0.33MnO3 CMR system

The electrical and magnetic transport properties of the La_0.67−xEu_xCa_0.33MnO₃ system exhibit lowering of insulator to metal and paramagnetic to ferromagnetic transition temperature (T_C) with the increase of Eu concentration in addition to possessing CMR property. The temperature variation of electrical resistivity and magnetic susceptibility for x=0.21 is found to have two distinct regions in the paramagnetic state for T>T_P; one with the localization of lattice polaron in the high-temperature region (T>1.5T_P) satisfying the dynamics of variable range hopping (VRH) model and the other being the combination of the spin and lattice polarons in the region T_P<T<1.5T_P. The resistivity variation with temperature and magnetic field, the cusp in the resistivity peak and CMR phenomenon are interpreted in terms of coexistence of spin and lattice small polarons in the intermediate region (T_P<T<1.5T_P). The spin polaron energy in the La_0.46Eu_0.21Ca_0.33MnO₃ system is estimated to be 106.73±0.90 meV and this energy decreases with the increase of external magnetic field. The MR ratio is maximal with a value of 99.99% around the transition temperature and this maximum persists till T→0 K, at the field of 8 T.     

Magnetostriction, Curie temperature and microstructure of Tb0.29(Dy1−xPrx)0.71Fe1.97 oriented alloys

The Tb_0.29(Dy_1−xPr_x)_0.71Fe_1.97 (x=0, 0.1, 0.2 and 0.3) alloys were prepared by directional solidification method. The orientation, magnetostriction λ, Curie temperature T_c and microstructure of alloys were characterized by XRD, standard resistant strain gauge technique, VSM and SEM-EDS. The results reveal that the alloys have a preferred orientation of 〈1 1 0〉 and 〈1 1 3〉 direction when x>0. With the increase in Pr content, the T_c of alloys decreases gradually and the non-cubic phase appears, resulting in the decline of λ dramatically, from 1935.2×10⁻⁶ for x=0 to 695.9×10⁻⁶ for x=0.3 at a compressive stress of 6 MPa and a magnetic field of H=240 kA m⁻¹.