Exploring a low temperature glassy state, exchange bias effect, and high magnetic anisotropy in Co2C nanoparticles

Nirmal Roy; Md Arif Ali; Arpita Sen; D T Adroja; Prasenjit Sen; S S Banerjee

doi:10.1088/1361-648X/ac0f9f

Exploring a low temperature glassy state, exchange bias effect, and high magnetic anisotropy in Co₂C nanoparticles

J Phys Condens Matter. 2021 Jul 19;33(37). doi: 10.1088/1361-648X/ac0f9f.

Authors

Nirmal Roy¹, Md Arif Ali¹, Arpita Sen², D T Adroja^{3

4}, Prasenjit Sen², S S Banerjee¹

Affiliations

¹ Indian Institute of Technology Kanpur, Kanpur, Uttar Pradesh 208016, India.
² Harish-Chandra Research Institute, HBNI, Chhatnag Road, Jhunsi, Allahabad, Uttar Pradesh 211019, India.
³ ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot Oxon OX11 0QX, United Kingdom.
⁴ Highly Correlated Matter Research Group, Physics Department, University of Johannesburg, P.O. Box 524, Auckland Park 2006, South Africa.

PMID: 34186520
DOI: 10.1088/1361-648X/ac0f9f

Abstract

It is interesting to explore the connections between the exchange bias effect (EBE) and magnetic anisotropy (MA). It is often found that materials exhibiting a strong EBE also have enhanced MA. Here we explore 40 nm diameter Co₂C nanoparticles (NPs) that exhibit ferromagnetism with a blocking temperature exceeding 300 K. We report the first observation of EBE in these Co₂C NPs below 50 K. The effect arises from the exchange coupling of frozen ferromagnetic spins with a freely rotatable spin component. The dynamics of the freely rotatable component freezes in a temperature range between 5 K to 20 K resulting in low-temperature coexistence of a glassy behavior along with ferromagnetism. In fact, Co₂C displays a unique separation of onset temperatures of spin freezing (∼20 K), vanishing of EBE (∼50 K), and magnetic blocking (⩾450 K). Our calculations show that Co₂C NPs have a core-shell structure. Our study suggests that modifying chemical co-ordination in the shell is one of the effective routes to manipulating MA compared to manipulating EBE.

Keywords: exchange bias; magnetic anisotropy; magnetism; nanoparticles; spin glass.