Magnetic and Magnetocaloric Properties of Nano- and Polycrystalline Bulk Manganites La0.7Ba(0.3−x)CaxMnO3 (x ≤ 0.25)

Abstract

Here we report the synthesis and investigation of bulk and nano-sized La0.7Ba0.3−xCaxMnO3 (x = 0, 0.15, 0.2 and 0.25) compounds that are promising candidates for magnetic refrigeration applications. We compare the structural and magnetic properties of bulk and nano-scale polycrystalline La0.7Ba0.3−xCaxMnO3 for potential use in magnetic cooling systems. Solid-state reactions were implemented for bulk materials, while the sol–gel method was used for nano-sized particles. Structurally and morphologically, the samples were investigated by X-ray diffraction (XRD), optical microscopy and transmission electron microscopy (TEM). Oxygen stoichiometry was investigated by iodometry. Bulk compounds exhibit oxygen deficiency, while nano-sized particles show excess oxygen. Critical magnetic behavior was revealed for all samples using the modified Arrott plot (MAP) method and confirmed by the Kouvel–Fisher (KF) method. The bulk polycrystalline compound behavior was better described by the tricritical field model, while the nanocrystalline samples were governed by the mean-field model. Resistivity in bulk material showed a peak at a temperature Tp1 attributed to grain boundary conditions and at Tp2 associated with a Curie temperature of Tc. Parent polycrystalline sample La0.7Ba0.3MnO3 has Tc at 340 K. Substitution of x = 0.15 of Ca brings Tc to 308 K, and x = 0.2 brings it to 279 K. Nanocrystalline samples exhibit a very wide effective temperature range in the magnetocaloric effect, up to 100 K. Bulk compounds exhibit a high and sharp peak in magnetic entropy change, up to 7 J/kgK at 4 T at Tc for x = 0.25. To compare the magnetocaloric performances of the studied compounds, both relative cooling power (RCP) and temperature-averaged entropy change (TEC) figures of merit were used. RCP is comparable for bulk polycrystalline and nano-sized samples of the same substitution level, while TEC shows a large difference between the two systems. The combination of bulk and nanocrystalline materials can contribute to the effectiveness and improvement of magnetocaloric materials.

The magnetocaloric effect properties for potential applications of magnetic refrigerator technology: a review

“…142 For instance, substitution at the A-site includes La 0.7 Ca 0.3 MnO 3 , 117 and La 0.7 Ba 0.05 Ca 0.25 MnO 3 . 77 which exhibit higher −Δ S M values than LaMnO 3 . Based on several studies, it can be concluded that A-site substitution with low-valence ions significantly influences changes in magnetic entropy and T C , often exhibiting extreme values that need to be considered for the effects of substitution in Mn 3+ /Mn 4+ in the material system.…”

“…Generally, size reduction strategies from micro to nanoscale in MCE materials can affect the broadening of the magnetic phase transition, resulting in a broader working temperature range, a decrease in saturation magnetization, and −Δ S M value. 51,65,77–80 These changes are associated with disturbances such as strain fields, atomic disorder, uncompensated surface spins, chemical inhomogeneity, grain size distribution, etc. 65 For example, Zeng et al reduced the size of Gd material to as small as 15 nm, resulting in a reported decrease in the −Δ S M value by up to 50% compared with micro-sized samples.…”

“…Based on several studies, it can be concluded that A-site substitution with low-valence ions significantly influences changes in magnetic entropy and T C , often exhibiting extreme values that need to be considered for the effects of substitution in Mn 3+ /Mn 4+ in the material system. 18,77,117,119,121,143…”