Field induced crossover in critical behaviour and direct measurement of the magnetocaloric properties of La0.4Pr0.3Ca0.1Sr0.2MnO3

Abstract

La_0.4Pr_0.3Ca_0.1Sr_0.2MnO₃ has been investigated as a potential candidate for room temperature magnetic refrigeration. Results from X-ray powder diffraction reveal an orthorhombic structure with Pnma space group. The electronic and chemical properties have been confirmed by X-ray photoelectron spectroscopy and ion-beam analysis. A second-order paramagnetic to ferromagnetic transition was observed near room temperature (289 K), with a mean-field like critical behaviour at low field and a tricritical mean-field like behaviour at high field. The field induced crossover in critical behaviour is a consequence of the system being close to a first-order magnetic transition in combination with a magnetic field induced suppression of local lattice distortions. The lattice distortions consist of interconnected and weakly distorted pairs of Mn-ions, where each pair shares an electron and a hole, dispersed by large Jahn-Teller distortions at Mn³⁺ lattice sites. A comparatively high value of the isothermal entropy-change (3.08 J/kg-K at 2 T) is observed and the direct measurements of the adiabatic temperature change reveal a temperature change of 1.5 K for a magnetic field change of 1.9 T.

Figure 1

(a) Rietveld refined XRPD pattern and XPS spectra of (b) Mn-2p, (c) Mn-3s and (d) valence band of the LP3 compound.

Figure 2

(a) Raw ToF-ERDA data from sample LP3. (b) ⁴He RBS data with overlaid SIMNRA calculated spectrum for sample LP3. (c) ¹²C³⁺ RBS data for the La + Pr signal edge, verifying that the expected ratio reproduces the measured data while calculations using only La or Pr do not. (d) PIXE data for sample LP3 and a comparison with spectra presented in Ref.. The spectra have been normalized to the L_α-signal from La at 4.65 keV.

Figure 3

(a) Magnetization and inverse susceptibility as a function of temperature. (b) Isothermal entropy-change as a function of temperature for different ${\mu }_0{H}_f$ values. (c) Magnetization versus magnetic field, the inset shows a blow-up of the low field region. (d) $\mathrm{\Delta }{T}_{\mathit{ad}}$ versus temperature for different ${\mu }_0{H}_f$ values between 0.1 and 1.9 T in steps of 0.2 T. The dashed upper curves correspond to extrapolated values for ${\mu }_0{H}_f=$ 3 T (red) and 5 T (blue). The dotted horizontal line at $\mathrm{\Delta }{T}_{\mathit{ad}}=2$ K, indicates the minimum requirement for magnetic cooling.

Figure 4

(a) Arrott plots at different temperatures. (b) variation of exponent $n^{\prime }$ with magnetic field and temperature.

Figure 5

Variation of $-\mathrm{\Delta }{S}_M^{\mathit{max}}$ and magnetization with intrinsic field at $T_C$. The red (blue) curves indicate fits to Eqs. (8) and (11) in the high (low) field region.

Figure 6

$M{\left|\epsilon \right|}^{-\beta }$ versus $H_i{\left|\epsilon \right|}^{-\left(\beta +\gamma \right)}$. Here M is in Am²/kg and H_i in A/m. A temperature range $T_C\pm 5$ K has been used for the scaling analysis.

Figure 7

Temperature variation of relative slope in modified Arrot plots with respect to the slope of the curve at T_c.