An investigation of magnetocaloric effect and its implementation in critical behavior study of La1−xNdxMn2Si2 compounds
Graphical abstract
Introduction
Rare-earth intermetallics compounds receive considerable attention due to their potential use for various applications. Magnetic refrigeration is considered one of the most promising applications of the rare-earth intermetallics [1], [2], [3], [4], [5]. Magnetic refrigeration is based on magnetocaloric effect (MCE), which refers to a reversible change of temperature resulting from a change in the magnetization of a magnetic material. MCE can be observed as an isothermal magnetic entropy change (ΔSM) or an adiabatic temperature change (ΔTad), depending on the thermodynamic conditions. Magnetic refrigeration technology has attracted scientists working on magnetic materials due to its advantages: environment-friendly nature, better adaptability, and higher-energy efficiency near room temperature. The discovery of the giant MCE in Gd5Si2Ge2 has accelerated the study of novel and potential materials with similar properties [6]. Giant MCE has also been found in systems, such as RCo2, MnAs, MnAsSb, LaFe11.4Si1.6, MnFePGe, Heusler alloys, and some manganites [5], [7], [8], [9], [10], [11], [12], which shows first-order ferromagnetic to paramagnetic phase transition. Magnetic entropy change is large in such materials types; however, they exhibit large thermal and field hysteresis on variation in magnetization with temperature and magnetic field, respectively. Furthermore, materials with first-order magnetic phase transition, the value of −ΔSM is highest near the magnetic temperature and falls rapidly with temperature. Thus, these factors limit their usage for MCE applications like low operation frequencies, low cooling power and limited usage over a narrow temperature range [13]. Hence, materials with a large MCE over a broad temperature range are desired. Particularly for ideal Ericcson refrigeration cycle wide range temperature ranges associated to large magnetic entropy change is very beneficial [13]. In consequence, there is a need of magnetic materials with second-order magnetic phase transition, which shows a large reversible magnetocaloric effect at low field.
The knowledge of field dependence of magnetic entropy change (−ΔSM) of a magnetic refrigerant material is important [14]. Understanding the field dependence can provide further clues to improve the performance of refrigerant materials in a lower field rather than those needed in already existing prototypes (generally 10–20 kOe) [14]. The field's dependence of the magnetic entropy change curve helps us to predict the response of a particular material under different extreme conditions, which can be useful for designing new materials for magnetic refrigeration. Therefore, a study of the MCE for a particular material is not only important from a practical application point of view but also it provides a tool to understand the properties of the material. In particular, the details of the magnetic phase transition and critical behavior can be obtained by studying the MCE of the material [14], [15].
Intermetallic ternary RT2X2 compounds where R is a rare-earth element, T is a transitional metal, X is Si or Ge are in the family of rare-earth intermetallics compounds, in which some of them exhibit AF–FM transitions. These compounds have a body-centered tetragonal ThCr2Si2-type crystal structure with space group I4/mmm. In this crystal structure, R, T and X atoms are stacked in layers along the c-axis in the R–X–T–X–R sequence. All of the atoms are located at special positions, R at 2a (0,0,0), T at 4d (0,1/2,1/4) and X at 4e (0,0, z) with z always close to 0.38. The strength of the exchange coupling between the magnetic layers of a multilayered system governs the magnetic properties of the ensemble [16]. The structural, magnetic, and other various properties ranging from heavy fermion behavior, superconductivity, and valence fluctuation to Kondo behavior of RT2X2 compounds are reported in many works [16], [17], [18], [19], [20]. The high sensitivity of exchange parameters to the intralayer Mn–Mn spacing governed by the lattice parameter a leads to complex and very interesting magnetic phase diagrams of these compounds. Roughly, the intralayer Mn–Mn spacing governed by the lattice parameter a for moments in adjacent Mn layers can align parallel along the c-axis at the Curie temperature TCinter when a > 4.06 Å, or antiparallel with a Néel temperature TNinter when a < 4.06 Å [18]. Moreover, one can vary the in-plane Mn–Mn spacing dMn–Mna value within the neighborhood of the critical value by alloying different elements with RMn2Si2. In this study, we present results of the effect of Nd substitution for La on the magnetic and magnetocaloric properties of the La1−xNdxMn2Si2 system, which might have SmMn2Ge2 like behavior, which is a phenomenon with its unusual magnetic behavior [19]. As reported, critical exponent analysis in the vicinity of the magnetic phase transition is a powerful tool to investigate the details of the magnetic interaction responsible for the transition [14], [15]. Hence, the aim of this work is to study the critical phenomena in the La1−xNdxMn2Si2 system by analyzing its critical exponents, which has not been reported to the best of our knowledge on the La1−xNdxMn2Si2 system. Moreover, we investigated the influence of the AF component on the critical exponents in the La1−xNdxMn2Si2 system, which exists around TCinter.
Section snippets
Experimental
The polycrystalline samples La1−xNdxMn2Si2 (x = 0.30, 0.35, 0.40, and 0.45) were synthesized by arc melting the elements in a water-cooled Cu hearth under purified argon gas. The purity of the elements was 99.9% for Nd and La, 99.98% for Mn and 99.99% for Si. The ingots were melted five times to attain homogeneity. The samples' structure was checked by using a Rigaku D-Max 2200 diffractometer with Cu Kα1,2 radiation at room temperature. Magnetization measurements as a function of temperature M(T
Structural characterization
X-ray diffraction patterns of compounds at room temperature indicated that all compounds are single phase and crystallize in the ThCr2Si2-type structure with the space group I4/mmm. The refined unit cell parameters a, c, unit cell volume V, and the intralayer Mn–Mn distances (dMn–Mna) are given in Table 1. Unit cell parameters are decreased as the Nd content increased.
Magnetic characterization
The result of the magnetization measurements of the La1−xNdxMn2Si2 (x = 0.30–0.45) compounds over the temperature range 5–375 K
Conclusion
Substitution of Nd instead of La leads to a decrease in the a cell parameter and the unit cell volume. As a result of variation of Mn–Mn distance with increasing/decreasing temperatures both compounds undergo a transition from an AF to an FM state. The FM state can also be induced by a field-induced transition from an AF state to an FM state. The variations of MCE as inverse and conventional MCE values in these compounds are correlated with the FM–AF magnetic phase transition. We have studied
Acknowledgment
We acknowledge the Duisburg-Essen University the facilities used to make magnetization and we gratefully thank M. Acet for both helpful discussions and experimental support. This work was supported by TUBITAK (Grant number: 109T563).
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