Magnetic properties of A- and B-site cation doped La_0.65Ca_0.35MnO₃ manganites

Abstract

The La_0.65Ca_0.35MnO₃, La_0.65Ca_0.30Pb_0.05MnO₃ and La_0.65Ca_0.30Pb_0.05Mn_0.9Cu_0.1O₃ compounds, prepared by the sol–gel method and sintered in air at 1100 °C for 24 h, have been investigated by Magnetic Properties Measurements System (MPMS) to explore the effects of A- and B-site cation doping. From the measurements, the Curie temperatures (T_C) and the isothermal magnetic entropy changes (−ΔS_M) were determined. The Curie temperature of La_0.65Ca_0.35MnO₃ compound was found to be 273 K. The Curie temperature increases to 286 K for the La_0.65Ca_0.30Pb_0.05MnO₃ compound due to the substitution of Ca by a small amount of Pb. However, replacing 10% of Mn with Cu (La_0.65Ca_0.30Pb_0.05Mn_0.9Cu_0.1O₃) leads to a reduction in the Curie temperature to 223 K. For the field change of 1 T, while the maximum magnetic entropy change for the sample La_0.65Ca_0.35MnO₃ was found to be 4.1 J/kg K it decreases down to 2.6 J/kg K and 3.2 J/kg K for the La_0.65Ca_0.30Pb_0.05MnO₃ and La_0.65Ca_0.30Pb_0.05Mn_0.9Cu_0.1O₃ samples, respectively.

Introduction

Cooling systems, seen in all areas of daily life, are widely used for keeping our foods cool and living spaces at comfort temperatures. Today, process of cooling is mostly realized by means of vapor compression. For very low temperature (<10 K) magnetic cooling systems, based on magnetizing and demagnetizing a paramagnetic material, are becoming to be preferential and widespread. Although the magnetocaloric effect (MCE) has been known for years, significant studies in this field have been carried out during the last couple decades [1], [2], [3], [4], [5]. The discovery, that Gd and some of its alloys exhibit large MCE, has triggered intensive studies on these materials [6], [7], [8], [9]. Besides, recent studies have also shown that manganite compounds in the chemical form of La_1−xA_xMnO₃ where (A is a monovalent ion such as Na, Li, Ag and K or a divalent ion such as Ca, Sr, Ba and Pb) exhibit large MCE around room temperature [10], [11], [12], [13]. It is often expressed that these manganite based compounds have an important potential for the development of magnetic cooling systems in commercial means, because the intrinsic negations seen in Gd and in the other rare earth elements do not show up in these compounds. Additionally, the low production costs, the ease of shaping and preparation and low magnetic hysteresis are among the other superiorities of these materials. Yet, the needed optimization of all parameters in order for these materials to be used as active magnetic cooling element for room temperature applications has not been accomplished.

From the research papers on the effect of monovalent or divalent ion doping of these compounds in La site (A-site) on the magnetic properties it is seen that some magnetic properties are affected positively; some negatively [14], [15], [16]. It is known that the magnetic properties of these compounds are very sensitive to the average valence state of the Mn ions, since the electronic configuration and average ionic radius of the manganese site (B-site) depend on the average valence state of the Mn ions. If the substituted ion (A in La_1−xA_xMnO₃) is divalent, it oxidizes a Mn³⁺ ion to Mn⁴⁺ ion, but if it is a monovalent ion, it oxidizes two Mn³⁺ ions to two Mn⁴⁺ ions. Therefore, Mn³⁺/Mn⁴⁺ ratio changes, depending on the valence state and the concentration of the substitute ion. Mn³⁺ and Mn⁴⁺ ions differ in their ionic radii. La³⁺ and the substitute ion in A-site may also have different ionic radii. Because of these ionic radii differences, the concentration level plays very important role in the formation of crystal structure of the resulting compounds as well as in magnetic properties.In this work, 0.35 Ca and 0.30 Ca + 0.05 Pb have been chosen for the doping in the A-site. Both Ca and Pb ions have 2+ valence state and with the concentration levels given above, Mn³⁺/Mn⁴⁺ ratio is made fixed, in order to explore the effect of the average ionic radius, 〈r_A〉, of the A-site. In addition to the A-site doping, it is possible to simultaneously dope the B-site of these compounds. Some transition metals (Fe, Cu, Ni, Co, etc.) have been used as doping element in the B-site [17], [18], [19]. Cu doping in the B-site has been done in this study because of both the double valuedness of the valence state of Cu ions and differences of its ionic radius in case of 2+ and 3+ valence states. The results in the literature related to the valence state of Cu ions within the structure of B-site doped compounds are diverse.

O. Klyushnikov et al., claim that the oxidation state of Cu is 2+ in the La_0.70Ca_0.30Mn_0.97Cu_0.03O₃ compound, based on their X-ray photoelectron spectroscopy (XPS) results [20]. Pi et al. have investigated the magnetic properties of La_0.825Sr_0.175Mn_1−xCu_xO₃ (0 < x < 0.20) compounds. They concluded that the strength of ferromagnetism of their samples decreases with the increase of Cu content. They claim that the valence state of Cu ions is 2+, hence, Cu substitution increases the content of the Mn⁴⁺ ions and antiferromagnetic superexchange interaction becomes dominant. At the same time, the Mn³⁺–O²⁻–Mn⁴⁺ bonds are destroyed gradually [21]. Zhou et al. have investigated the transport and magnetic properties of La_0.7Ca_0.3Mn_1−xCu_xO₃ compounds. They have found shifting in the T_C values towards lower temperatures and broadening in the width of the magnetic transition region with increasing Cu content. They also observed, through XPS analysis, that some of Cu ions entering into sample is in the form of Cu³⁺, besides Cu²⁺ [22].

The purpose of this work is to investigate the effect of Pb substitution in A-site and Cu substitution in B-site on the magnetic and magnetocaloric properties of La_0.65Ca_0.35MnO₃ compound. It is expected that the substitution of an element having different ionic radii and valence state causes variations in the average ionic radius of both A and B-sites (〈r_A〉, 〈r_B〉). Depending on these variations, Mn–O bond length and Mn–O–Mn bond angle change and ferromagnetic double exchange interaction and antiferromagnetic superexchange interaction between the neighboring Mn ions also change. These changes alter the behavior of the magnetic phase transition of the compounds. So, different element substitution in the parent compound will influence magnetic properties. Further, depending on differences in ionic radii of Mn³⁺ and Mn⁴⁺ ions, if any change occurs in the number of these two ions, it will directly change the Mn⁺³/Mn⁺⁴ ratio and 〈r_B〉, and hence change the magnetic properties of the compounds.