VP15-08 Magnetocaloric effect for pyramid and flower-shaped CoO nanoparticles
Creators: Yuri Hotta
Tokito Tachibana and Satoru Kobayashi
Publisher: name: IEEE
Year: 2025

Magnetic refrigeration systems based on the magnetocaloric effect (MCE) have received much attention due to possible higher energy efficiency and environmentally friendly features. In particular, magnetic nanoparticles have a possibility of including the MCE materials into electric devices and better heat exchange with a surrounding matrix owing to their large surface area. So far, many kinds of magnetic nanoparticles have been surveyed [1], with varying particle size, chemical compositions, etc., the effect of particle shape on MCE was less studied though MCE-related magnetic properties such as effective anisotropy, surface effect, can be tuned. In this study, we focused on the effect of particle morphology on MCE for pyramid and flower CoO nanoparticles (insets in Fig.1). We examined four CoO samples: hexagonal pyramid CoO with size of 26 nm (P26) and 32 nm (P32), face-centered-cubic flower CoO with size of 24 nm (F24) and 29 nm (F29). A flower nanoparticle for F24 and F29 consists of randomly oriented small grains with size of 5.2 and 8.5 nm, respectively, whereas a pyramid nanoparticle is typically a single crystal. Magnetization under magnetic fields was measured in the temperature range of 2-300 K to evaluate a magnetic entropy change, which is the measure of MCE. Whereas CoO is antiferromagnetic in nature, all the samples exhibit ferromagnetism, which develops at low temperatures. Magnetic entropy changes calculated from magnetization curves exhibit a maximum at low temperatures below 10 K (Fig. 1). Although the maximum value is weakly dependent on particle size for pyramid nanoparticles, it drastically increases with decreasing particle size for flower nanoparticles and attains 0.49 J/K/kg at T=3 K for F24. These results indicate that the surface spin freezing dominates MCE for CoO nanoparticles, but magnetic coupling between small grains can give an additional effect on MCE for flower-shaped nanoparticles.References: [1] V. Franco, J.S. Blazquez, J.J. Ipus, L.M. Moreno-Ramirez, A. Conde, Prog. Mater. Sci. vol. 93, 112 (2018).
Magnetic refrigeration systems based on the magnetocaloric effect (MCE) have received much attention due to possible higher energy efficiency and environmentally friendly features. In particular, magnetic nanoparticles have a possibility of including the MCE materials into electric devices and better heat exchange with a surrounding matrix owing to their large surface area. So far, many kinds of magnetic nanoparticles have been surveyed [1], with varying particle size, chemical compositions, etc., the effect of particle shape on MCE was less studied though MCE-related magnetic properties such as effective anisotropy, surface effect, can be tuned. In this study, we focused on the effect of particle morphology on MCE for pyramid and flower CoO nanoparticles (insets in Fig.1). We examined four CoO samples: hexagonal pyramid CoO with size of 26 nm (P26) and 32 nm (P32), face-centered-cubic flower CoO with size of 24 nm (F24) and 29 nm (F29). A flower nanoparticle for F24 and F29 consists of randomly oriented small grains with size of 5.2 and 8.5 nm, respectively, whereas a pyramid nanoparticle is typically a single crystal. Magnetization under magnetic fields was measured in the temperature range of 2-300 K to evaluate a magnetic entropy change, which is the measure of MCE. Whereas CoO is antiferromagnetic in nature, all the samples exhibit ferromagnetism, which develops at low temperatures. Magnetic entropy changes calculated from magnetization curves exhibit a maximum at low temperatures below 10 K (Fig. 1). Although the maximum value is weakly dependent on particle size for pyramid nanoparticles, it drastically increases with decreasing particle size for flower nanoparticles and attains 0.49 J/K/kg at T=3 K for F24. These results indicate that the surface spin freezing dominates MCE for CoO nanoparticles, but magnetic coupling between small grains can give an additional effect on MCE for flower-shaped nanoparticles.References: [1] V. Franco, J.S. Blazquez, J.J. Ipus, L.M. Moreno-Ramirez, A. Conde, Prog. Mater. Sci. vol. 93, 112 (2018).