Multicaloric refrigeration enhanced by multisite interactions: Bridging theory and experiment

Project Information

MULTICALORICS

Grant agreement ID: 101025767

DOI

10.3030/101025767

Project closed

EC signature date 27 April 2021

Start date 1 September 2022

End date 31 August 2024

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 160 932,48

EU contribution

€ 160 932,48

Coordinated by

UNIVERSITAT DE BARCELONA
Spain

Periodic Reporting for period 1 - MULTICALORICS (Multicaloric refrigeration enhanced by multisite interactions: Bridging theory and experiment)

Reporting period: 2022-09-01 to 2024-08-31

Summary of the context and overall objectives of the project

Magnetic materials play a fundamental role in many modern solid-state technologies greatly impacting our society. These technologies include critical devices for mobility, ground transportation, energy generation, as well as spintronic devices, such as neuromorphic computing inspired by the human brain and innovative non-volatile memories. An important emerging application is the so-called caloric refrigeration based on magnetic materials. This is a novel cooling technology aimed to replace the harmful greenhouse gases currently prevalent in our society with environmentally friendly solid-state refrigerants to urgently combat the climate change. Our action has focused on a collaborative effort, combining theoretical research conducted by the lead researcher, with experimental work at the host institution, the University of Barcelona, with the overall objective to uncover and understand new and more efficient refrigerant magnetic materials.

Cooling with magnetic materials is typically achieved by applying magnetic fields or mechanical stresses to them. However, caloric refrigeration has yet to become commercially viable due to the reliance on costly neodymium-based permanent magnets needed to generate sufficiently strong magnetic fields, and the fact that some materials show operational lifespan reduced by mechanical fatigue. Our research addresses these challenges by exploring a novel approach: the simultaneous application of magnetic and mechanical stimuli to minimize the required work and maximize the cooling effect, known as the multicaloric effect.

Our theoretical work is grounded in first-principles (ab initio) calculations, which rely exclusively on fundamental laws and, therefore, are instrumental in guiding and supporting the experimental discovery of new multicaloric materials. A key focus of our research has been the development of a novel computational approach that accurately accounts for the coupling between the material’s magnetism with its elasticity and the vibrations of its constituent atoms at finite temperatures—factors that are crucial to the underlying mechanisms of caloric refrigeration. This advanced computational tool is designed to predict materials with cooling power enhanced by multisite interactions, a newly identified mechanism based on complex interactions between atomic-scale magnetic degrees of freedom. We aim to deepen our understanding on the origin of multisite interactions and on how to nanostructure magnetic materials with cooling power boosted by them.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Magnetic materials are composed of atom-scale polarisations known as local moments, which act as little magnets with orientations that can form ordered patterns called magnetic phases/states. Their dynamics and fluctuations, as well as phase transitions between them, underline their functionality for caloric refrigeration. First-order phase transitions, which are discontinuous transformations between different magnetic states, present the largest cooling responses thanks to the fact that small external stimuli suffice to drive an abrupt magnetic change and consequent giant caloric effect. One of the most successful computational approach and workhorse of materials science modelling from first principles is the Density Functional Theory (DFT). In this project we combine DFT with statistical mechanics methods for the local moments to describe magnetic phase transitions and caloric cooling by means of the so-called Disordered Local Moment (DLM) picture, one of the few approaches for finite temperature magnetism from first principles that exists.

The project strongly focused on Mn-based antiperovskite systems, which are magnetic materials with a prominent first-order transition to a triangular magnetic state given rise by multisite interactions. Most importantly, the application of mechanical stress to antiperovskites is not only accompanied by giant caloric cooling, but also by the stabilization of another collinear magnetic phase that should be responsive to a magnetic field. Hence, antiperovskite materials are good multicaloric candidates. We have carefully analysed the electronic properties of antiperovskite materials as well as performed multicaloric experiments on an optimal antiperovskite sample created and characterised by collaborators in the United Kingdom. To this end, we have designed and constructed a novel experimental device that can measure multicaloric effects driven by the simultaneous application of a magnetic field and uniaxial stress. However, an enhanced cooling response driven by the multicaloric effect in this sample has not been observed yet.

Central theoretical work has been centred on the extension of DLM-DFT theory to include the coupling of magnetism with the atom vibrations, a challenging task that posed important fundamental and computational questions. These developments have shown that the size and consequent caloric boost of multisite interactions is highly dependent on the atom vibrations in antiperovskite materials, which has explained previous contradictions between experiment and theory. We plan to apply our new computational tool to predict new antiperovskite materials with optimal properties, as well as other multicaloric materials.

The project also focused on multicaloric effects in La(FexSi1-x)13, one of the most famous magnetic refrigerants, and CrGeTe3, an important material in spintronics. DLM-DFT calculations have successfully unrevealed the origin of cooling responses in La(FexSi1-x)13 in excellent agreement with experiments, explaining the small role of multisite interactions somewhat challenging the current understanding of this material. Moreover, experimental work on CrGeTe3 has shown that its cooling performance can be indeed enhanced via the multicaloric effect. DLM-DFT calculations have demonstrated that multisite interactions are substantial in CrGeTe3 and that a first-order phase transition can be induced by uniaxial stress, promoting its extraordinary multicaloric potential. Our work along these lines has been presented in several scientific events and has produced two publications, another two being in preparation.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Our action focuses on advancing the field of ecological solid-state refrigeration, which aims to replace technologies that contribute significantly to greenhouse gas emissions, thereby helping to mitigate global warming. Two state-of-the-art developments have been achieved:
(1) A unique first principles computational tool to describe finite temperature magnetism on equal footing with atomic vibrations, which is applicable to a wide range of multicomponent transition metal and rare earth magnetic materials with diverse technological applications.
(2) A novel experimental prototype capable of simultaneously applying magnetic and mechanical stimuli to materials.

These innovative computational and experimental developments introduce new capabilities for investigating the thermodynamics of magnetic materials and the phase transitions they undergo when subjected to multiple stimuli. Their broad applicability extends beyond multicaloric effects, offering potential benefits to other technologies that exploit magnetic phenomena. Our advances are thus expected to attract interest from a multidisciplinary community of researchers worldwide.

Representation of a cooling cycle based on the use of magnetic materials under external stimuli.