A research group led by Prof. LIU Jian at the Ningbo Institute of Materials Technology and Engineering (NIMTE) of the Chinese Academy of Sciences (CAS), has developed a novel approach to reduce the phase-transition hysteresis in magnetocaloric La-Fe-Si alloys by microstructural manipulation. The study was published in Acta Materialia. 

As environmentally friendly and energy-efficient room-temperature refrigerants, various magnetocaloric materials have attracted increasing attention around the world. However, first-order magnetostructural phase transitions of magnetocaloric materials are inevitably accompanied by large hysteresis, which leads to huge energy loss, reduces cooling efficiency, or even shortens the service-lifespan of materials.  

Traditional alloying methods to decrease the hysteresis of magnetocaloric materials inevitably cause a significant reduction of the magnetocaloric effect (MCE). 

To address this issue, the research group at NIMTE employed Ce-doping and hydrogenation to achieve the microstructure manipulation of La-Fe-Si magnetocaloric alloys. The comprehensive study on the microstructure evolution was conducted with high angle annular dark field-scanning transmission electron microscope, three-dimensional atom probe and geometric phase analysis. 

Owing to the release of internal stress triggered by the uneven distribution of hydrogen atoms, an individual grain of the La-Fe-Si alloy was refined into oriented nanocrystals with diameter of 5–50 nm. The nano-crystallization facilitated the nucleation during phase transition, thus reducing the hysteresis loss by up to 98.8% (from 48.3 to 0.6 J kg^-1).  

In addition, the itinerant electron metamagnetic phase transition kept an obvious first-order type, leading to a large adiabatic temperature change of 2.03 K (±0.09 K) under a field change of 1.3 T upon 10⁵ magnetic cycles. This indicates that the optimized La-Fe-Si alloy has a large reversible MCE. 

Moreover, after hydrogenation, the reversible refrigeration capacity of the La-Fe-Si alloy increased from 78.5 to 89.4 J kg^-1 due to the notable reduction of hysteresis loss.  

The study has provided a novel perspective to explore nanocrystalline microstructures, and shed light on the development of hysteresis-free magnetocaloric materials exhibiting first-order transition as well as the large-scale application of magnetic cooling technology.