Magnetic Transition and Large Magnetocaloric Effect Associated with Surface Spin Disorder in Co and Co_coreAg_shell Nanoparticles

Abstract

We report a reversible, large magnetocaloric effect in the vicinity of a low-temperature magnetic transition in Co and CocoreAgshell nanoparticles synthesized using a wet chemical method. The as-synthesized assembly of the particles shows a sharp low-temperature peak in the zero-field-cooled (ZFC) magnetization well below the blocking transition temperature, and this feature is associated with the surface spin disorder. Co nanoparticles show a large increase in the magnetic entropy at around 15 K with a peak value of nearly 2.25 J/K·kg for an applied field of 30 kOe. A similar trend is also observed in the silver-coated Co particles. These are some of the largest MCE values observed in nanoparticles to date. The features are ascribed to the low-temperature spin-glass-like freezing transition associated with the surface spins in the shell region that is distinct from the behavior of core spins. Our studies reveal that manipulating the surface anisotropy in core−shell nanoparticles has the potential to lead to a large MCE effect and thus prove to be useful for magnetic refrigeration.

Field dependence of the magnetocaloric effect in core-shell nanoparticles

“…The change in composition of the particles and nature of the shell can be used to tailor the magnitude and position of the peak, 15 but the present results indicate that it does not alter the characteristics of the phase transition, as evidenced by the collapse of the ⌬S M curves.…”

“…11,12 More recently, it has been shown that the adequate control of the magnetic anisotropy of a nanostructured system can originate behaviors that are qualitatively different from those of the bulk materials. 13 There have been studies of the influence on the MCE of the control of the anisotropy of the material, especially through surface modification processes, either by ball-milling zinc ferrite particles, 14 which lead to disorder on the surface of the particles causing some of the ions to change from spinel to the inverse spinel structure, or by systems of core-shell nanoparticles, 15 where the total magnetization of each particle may be considered as consisting of two parts: the core magnetic moment ͑which is ordered and behaves as a superspin͒ and the shell magnetization ͑which is mostly disordered and somewhat similar to spin-glass systems͒. The exchange coupling between the surface and core spins can lead to short-range order of the surface spins below a certain temperature with the surface anisotropy also competing in this process.…”

“…This surface anisotropy that these particles exhibit at low temperatures causes the ⌬S M values obtained for these core-shell nanoparticles at their associated spin freezing transition to be the largest reported on any nanoparticle system so far. 15 This transition, detected in the magnetization curves as an abrupt change in temperature dependence of magnetization, can be perceived as an orderdisorder transition. While the critical characteristics of this phase transition cannot be studied using standard procedures such as the Kouvel-Fisher method, the analysis of the field dependence of MCE proposed recently by us is believed to be a useful tool for this purpose.…”

“…The details of the magnetic characterization of these samples, including the collection of the family of isothermal magnetization curves for extracting the MCE, have been reported in Ref. 15.…”

Enhanced cryogenic magnetocaloric effect in Eu8Ga16Ge30 clathrate nanocrystals

“…In the case of fine particles, it has been suggested that the ordering of surface spins can contribute to ÀDS M . 27 The effect of surface spins is mostly realized at low temperatures and increases as the particle size of a material decreases. One might therefore attribute this to the enhancement of ÀDS M observed at low temperatures in the present case.…”