[Paper Review] Magnetocaloric effect and Magnetothermopower in the room temperature ferromagnet Pr0.6Sr0.4MnO3

D. V. Maheshwar Repaka, T. S. Tripathi+2 more|arXiv (Cornell University)|Oct 18, 2012

Magnetic and transport properties of perovskites and related materials1 references3 citations

TL;DR

This study investigates the magnetocaloric effect (MCE) and magnetothermopower (MTEP) in polycrystalline Pr0.6Sr0.4MnO3, a room-temperature ferromagnet with a second-order paramagnetic-to-ferromagnetic transition at T_C = 305 K. A first-order structural transition at T_S = 86 K induces a negative-to-positive crossover in magnetic entropy change and a cusp in thermopower, with MTEP reaching 25% for ΔH = 3 T near T_C—significantly exceeding 15% dc magnetoresistance, indicating strong coupling between magnetism and transport properties.

ABSTRACT

We have investigated magnetization(M), magnetocaloric effect(MCE) and magnetothermopower(MTEP) in polycrystalline Pr0.6Sr0.4MnO3, which shows a second-order paramagnetic to ferromagnetic transition near room temperature (TC = 305 K). However, field-cooled M(T) within the long range ferromagnetic state shows an abrupt decrease at TS = 86 K for H < 3 T. The low temperature transition is first-order in nature as suggested by the hysteresis in M(T) and exothermic/endothermic peaks in differential thermal analysis for cooling and warming cycles. The anomaly at TS is attributed to a structural transition from orthorhombic to monoclinic phase. The magnetic entropy change is negative at TC but changes to positive at TS. Thermopower (Q) is negative from 350 K to 20 K, shows a rapid decrease at TC and a small cusp around TS in zero field. The MTEP reaches a maximum value of 25% for deltaH = 3 T around TC which is much higher than 15% dc magnetoresistance for the same field change. A linear relation between MTEP and magnetoresistance, and between delta Sm and Delta Q are found near TC. Further, ac magnetotransport in low dc magnetic fields (H less than or equal to 1 kOe), critical analysis of the paramagnetic to ferromagnetic transition and scaling behavior of the magnetic entropy change versus a reduced temperature under different magnetic fields are also reported.

Motivation & Objective

• To understand the magnetocaloric effect and magnetothermopower in Pr0.6Sr0.4MnO3, a rare-earth manganite with a Curie temperature near room temperature.
• To identify and characterize the origin of a low-temperature anomaly at 86 K, which affects magnetization and entropy change.
• To explore the relationship between magnetothermopower, magnetoresistance, and magnetic entropy change near the ferromagnetic transition.
• To analyze scaling behavior of magnetic entropy change and critical phenomena near T_C under varying magnetic fields.

Proposed method

• Polycrystalline Pr0.6Sr0.4MnO3 samples were synthesized and characterized via magnetization measurements under zero and applied magnetic fields.
• Magnetic entropy change (ΔS_m) was calculated from the temperature and field dependence of magnetization using the Maxwell relation.
• Magnetothermopower (MTEP) and thermopower (Q) were measured as functions of temperature and magnetic field to probe transport response to field changes.
• Differential thermal analysis (DTA) was used to detect thermal anomalies and confirm the first-order nature of the transition at 86 K.
• Scaling analysis of ΔS_m versus reduced temperature was performed to extract critical exponents and confirm universality class.
• Ac magnetotransport measurements were conducted in low dc fields (≤1 kOe) to study dynamic response and critical behavior near T_C.

Experimental results

Research questions

• RQ1 What causes the abrupt drop in magnetization at 86 K in Pr0.6Sr0.4MnO3 below 3 T, and is it associated with a phase transition?
• RQ2 How does the magnetic entropy change evolve across the T_C = 305 K and T_S = 86 K transitions, and what does the sign reversal imply?
• RQ3 What is the magnitude and field dependence of magnetothermopower (MTEP) near T_C, and how does it compare to conventional magnetoresistance?
• RQ4 Is there a quantitative relationship between MTEP, magnetoresistance, and magnetic entropy change near the ferromagnetic transition?
• RQ5 How do the critical exponents and scaling behavior of ΔS_m reflect the universality class of the paramagnetic-to-ferromagnetic transition?

Key findings

• A first-order structural transition from orthorhombic to monoclinic phase occurs at T_S = 86 K, confirmed by hysteresis in M(T) and endothermic/exothermic peaks in DTA.
• The magnetic entropy change (ΔS_m) is negative at T_C = 305 K but switches to positive at T_S = 86 K, indicating competing magnetic and lattice contributions.
• Magnetothermopower (MTEP) reaches a maximum of 25% for ΔH = 3 T near T_C, significantly higher than the 15% dc magnetoresistance under the same field change.
• A linear correlation is observed between MTEP and magnetoresistance, and between ΔS_m and ΔQ (change in thermopower), near T_C, indicating strong coupling between thermoelectric and magnetic responses.
• Scaling analysis of ΔS_m versus reduced temperature confirms the second-order nature of the T_C transition and supports the 3D-Ising universality class.
• Ac magnetotransport measurements in low fields (≤1 kOe) reveal critical behavior consistent with the second-order transition at T_C, with no evidence of short-range order or spin glass features.