On the instability of the giant direct magnetocaloric effect in CoMn0.915Fe0.085Ge at. % metamagnetic compounds

N M Bruno,S Yuce

doi:10.1038/s41598-020-71149-w

Abstract

The giant magnetocaloric effect was quantified in CoMn1-xFexGe (x = 0.085–0.12) nom. at. % polycrystals across the high temperature hexagonal (P63/mmc) to low temperature orthorhombic (Pnma) phase transition via differential scanning calorimetry (DSC) and multiple (thermo) magnetization measurements. It was found that increasing Fe content led to the decrease of both the martensitic transformation temperature and entropy change (Delta S) at the point of the phase transition. Moreover, first-time magnetocaloric measurements resulted in irreproducible entropy change versus temperature diagrams, which was attributed to the release of internal pressure in bulk samples that disintegrated into powder upon transformation. CoMn1-xFexGe demonstrated larger magnetic field-induced entropy changes and giant magnetocaloric effect (MCE) compared to other CoMnGe alloys doped with Si, Sn, Ti, and Ga. However, the observed brittleness and apparent change in volume at the magnetic transition was posited to influence the material’s potential for regenerative applications.

Highlights

The giant magnetocaloric effect was quantified in CoMn1-xFexGe (x = 0.085–0.12) nom. at. % polycrystals across the high temperature hexagonal (P63/mmc) to low temperature orthorhombic (Pnma) phase transition via differential scanning calorimetry (DSC) and multiple magnetization measurements
Some of the most common materials studied for their magnetostructural giant magnetocaloric effect (MCE) are G dSiGe11, Fe2P compounds[12], and NiMn-based intermetallic Heusler alloys, namely metamagnetic shape memory alloys (MMSMAs)[13,14,15,16]
Upon transforming between austenite to martensite, burst-type transformations are observed in isothermal magnetization measurements, which resulted in spurious entropy changes quantified by post-processing data from maiden transformation cycles[69]; upon transforming, bulk samples had disintegrated to powder due to their negative thermal expansion characteristics, releasing microstructural pressure, and exhibited higher, more stable, transformation temperatures, characteristics, and giant MCE

Summary

Introduction

Upon transforming between austenite to martensite, burst-type transformations are observed in isothermal magnetization measurements, which resulted in spurious entropy changes quantified by post-processing data from maiden transformation cycles[69]; upon transforming, bulk samples had disintegrated to powder due to their negative thermal expansion characteristics, releasing microstructural pressure, and exhibited higher, more stable, transformation temperatures, characteristics, and giant MCE. After multiple thermal cycles across the magnetostructural transition, the sample was removed from the DSC in powder form suggesting that the compound released internal pressure from the martensite to austenite transformation.

Results

Conclusion