Critical behavior at high magnetic fields in LaPr(Ca,Sr)MnO manganites

Highlights

• The critical behavior of LaPr(Ca,Sr)MnO manganites was investigated.

• Under high magnetic fields the PM-FM transition becomes second order, and the critical exponents could be determined.

• The exponent values obtained yield an estimate the magnetic entropy change.

Abstract

In this work, we demonstrate that La_0.225Pr_0.4(Ca_1−xSr_x)_0.375MnO₃ manganites with $0\le x\le 0.10$, exhibit a discontinuous first order ferromagnetic-paramagnetic transition at temperatures ~ 230–240 K, which evolves to a continuous second order ferromagnetic-paramagnetic transition according to the intensity of the applied magnetic field. Under high magnetic fields and through a magnetization scaling analysis, we have determined the critical exponents of the sample with x = 0.05 - β = 0.15, γ = 0.77, and δ = 6.24, and x = 0.10 - β = 0.165, γ = 0.62, and δ = 4.69. The exponents values obey the scaling laws but do not belong to any existing universality class. This analysis of critical behavior can be used to predict the magnetic entropy change under high magnetic fields.

Introduction

Perovskite manganites with a general formula of R_1-xA_xMnO3 (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 La_1-xSr_xCoO₃ (0.2 $\le$x $\le$ 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 La_0.67Sr_0.33CoO₃ 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 La_0.5Sr_0.5CoO₃ [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 La_0.225Pr_0.4(Ca_1−xSr_x)_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 La_0.225Pr_0.4(Ca_1−xSr_x)_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, $-\Delta S_M$, 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.