Critical behavior at high magnetic fields in LaPr(Ca,Sr)MnO manganites
Introduction
Perovskite manganites with a general formula of R1-xAxMnO3 (where R represents a trivalent rare earth and A divalent alkaline earth element) attracted a lot of interest in materials chemistry and physics communities [[1], [2], [3]]. The reason is that these materials exhibit interesting physical properties around their phase transitions, such as colossal magnetoresistance [4,5] and magnetocaloric effects [6,7]. These properties come from the interplay between the spin, orbital, charge and lattice degrees of freedom [8,9].
Phase transitions are modeled by fundamental rules which contribute to understand condensed matter systems. They are classified according to the nature of the transition into first or second order [10,11]. Contrary to first order magnetic phase transition (FOMPT), the second order magnetic phase transition (SOMPT) can be unambiguously described in terms of a critical behavior, which addresses the thermodynamic properties of the system near its critical temperature [12,13]. Because the width of ferromagnetic to paramagnetic (FM-PM) transition region in FOMPT materials is narrow, their applications in technologies is limited. Furthermore, large hysteretic losses of FOMT materials are detrimental to the refrigerant capacity in refrigeration applications. To reduce these restrictions, it is desirable to widen the FM-PM transition region by transforming the FOMPT into a SOMPT. The process can be achieved by quenched disorder [14,15], which yields a “rounding” of a discontinuous FOMPT in to a continuous SOMPT. In manganites, this can be accomplished through chemical doping on the La/Ca and/or Mn sites [[16], [17], [18]]. Moreover, it was found that for the manganites exhibiting a crossover of the nature of magnetic phase transition, the values of critical parameters strongly depend on the magnetic field ranges selected for studying the critical behavior [19,20]. A previous study of the critical behavior in La1-xSrxCoO3 (0.2 x 0.3) [21] has shown that the critical exponent β matches with a mean-field-like value, while γ corresponds to the Heisenberg model. A detailed analysis of the critical behavior in the La0.67Sr0.33CoO3 compound has proved that all the exponent values correlate with the three-dimensional Heisenberg model [22]. The study of the critical behavior in half-doped La0.5Sr0.5CoO3 [23] showed that the exponent δ is near the mean-field value, whereas the γ value is close to three-dimensional Ising model. Contrary, Mukherjee et al. [24] have reported that all the critical exponents values for the same sample follow the Heisenberg class. Khan et al. [25] found that the critical exponents deviate from Heisenberg towards a mean field value.
To get more insight into the interplay between critical behavior and magnetocaloric properties, we address in this paper the magnetic properties of La0.225Pr0.4(Ca1−xSrx)0.375MnO3 compounds, with 0 ≤ x ≤ 0.10. Since the SOMPT region in the undoped sample is relatively small even under high magnetic fields, we chose to study the critical exponents only for the samples with x = 0.05 and 0.10. Our results show that La0.225Pr0.4(Ca1−xSrx)0.375MnO3 alloys exhibit a crossover from the first order to the second order FM-PM phase transition with the application of a magnetic field. Using the critical exponents obtained under high magnetic fields, the magnetic entropy change, , can be predicted, and the entopic peak reaches 19.7 J kg−1 K−1 for the sample x = 0.05 and 20.6 J kg−1 K−1 for x = 0.10.
Section snippets
Experimental
Polycrystalline samples La0.225Pr0.4(Ca1−xSrx)0.375MnO3, with 0 ≤ x ≤ 0.10, were synthesized by a citrate–nitrate decomposition method [26]. Magnetization measurements as a function of temperature and applied magnetic field were made with a vibrating sample magnetometer (VSM) in a Physical Properties Measuring System (PPMS), manufactured by Quantum Design Inc. A quartz rod used for sample mounting.
Results and discussions
Fig. 1 shows isothermal magnetization curves, , of La0.225Pr0.4(Ca1−xSrx)0.375MnO3 with x = 0, 0.05, and 0.10. The temperature dependence of the magnetization of the studied samples was previously published [27]. A paramagnetic to ferromagnetic transition occurs, with transition temperatures, , within the range 230–240 K. The magnetization tends to saturate at high fields and lower temperatures, as expected for a standard ferromagnet.
According to a scaling hypothesis, critical exponents
Conclusion
The PM-FM transition in La0.225Pr0.4(Ca1−xSrx)0.375MnO3 compounds, with , exhibit a discontinuous FOMPT rounded to a continuous SOMPT. This crossover in the nature of the transition is triggered by the intensity of the applied magnetic field. Under high magnetic fields, where a SOMPT is observed, and the critical exponents of the samples with x = 0.05 and x = 0.10 have been estimated. The critical parameters values yield a reliable prediction of the magnetic entropy change under high
Author statement
LG made the measurements, MH performed the analysis. Both authors contributed equally to the writing of the manuscript.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
MH was supported by a post-doctoral grant from the Brazilian agency Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ), project E-26/202.347/2019. LG was also supported by FAPERJ, Projects E-26/202.820/2018 and E-26/010.101136/2018, and by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), project 305021/2017–6.
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