Magnetic phase diagram of superantiferromagnetic TbCu₂nanoparticles

Abstract

The structural state and static and dynamic magnetic properties of TbCu2 nanoparticles are reported. The nanoparticles were produced by mechanical milling under inert atmosphere with low milling times ≤ 15 hours. The randomly dispersed nanoparticles as detected by TEM retain the bulk symmetry with an orthorhombic Imma lattice and Tb and Cu in 4c positions. Rietveld refinements confirm that the milling produces a controlled reduction of particle sizes reaching down to ≃ 6 nm and an increase of the microstrain ≃ 0.6 %. The DC-susceptibility shows a reduction of the Néel transition (from 49 K to 43 K) and a progressive increase of the spin glass peak (from 9 to 15 K) in the zero field cooled magnetization with size reduction. The exchange anisotropy is very weak (bias field of ≃ 30 Oe) and is due to the presence of a disordered (thin) shell coupled to the antiferromagnetic core. The dynamic susceptibility evidences a critical slowing down in the spin disordered state with a tendency to increase of zv and β exponents when the size becomes smaller (zv ≃ 6.6 and β ≃ 0.85). A Rietveld analysis of neutron diffraction patterns 1.8 ≤ T ≤ 60 K including the magnetic structure determination reveals that there is a reduction of the expected moment (≃ 80 %) which must be connected to the presence of the disordered particle shell. The core magnetic structure retains the bulk antiferromagnetic arrangement. The overall interpretation is based on a superantiferromagnetic behavior which at low temperatures coexists with a canting of surface moments. We propose a novel magnetic phase diagram as a function of the particle size.

Surfactant-assisted production of TbCu2 nanoparticles

“…Longer milling times result in smaller D, irrespective of the surfactant effect, and with further milling, it is expected that there would be an asymptotic value (Echevarria-Bonet et al 2015). In the present case, the milling times are really short (t ≤ 5 h), saving sample handling risk and costs.…”

“…Therefore, in this article, we are investigating the effect of production with surfactants on the MNPs focusing in particular on especially the modifications in the crystallographic structure, the MNPs size distribution and, concomitantly, the Néel temperature and the magnetic disorder. The results will be compared to the observations in bare superantiferromagnetic MNPs of TbCu 2 analysed comprehensively in Refs EchevarriaBonet et al (2013) and Echevarria-Bonet et al (2015). In particular, the modifications will be understood to affect the magnetic relaxation of the TbCu 2 nanoparticles.…”

“…Recently, there are consistent investigations on MNPs of rare Earth (R)-based alloys (Zhou and Bakker 1995;Li et al 2013;Morales et al 2004;Wang et al 2007;de Paula et al 2016;Sánchez-Valdés et al 2014;Rojas et al 2007;EchevarriaBonet et al 2013;Echevarria-Bonet et al 2015). These studies are much less widespread than the usual investigations of Fe, Ni, Co oxide MNPs (Tartaj et al 2003;Estrader et al 2013;RinaldiMontes et al 2015), although some of these R-based alloys allow for technological transfer as permanent magnets or magnetocaloric materials (Wang et al 2007;de Paula et al 2016;Sánchez-Valdés et al 2014).…”

“…The basic attraction is conspicuous as Rbased alloys are dependent on the magnetic exchange coupling between localized magnetic moments (related to the deeply buried 4f-electrons) and display a rich variety of magnetic structures by changes of the R atoms and/or of the non-magnetic metal (Luong and Franse 1995). We have been lately investigating TbCu 2 alloys, which constitute a paradigmatic member of the extended RX 2 family (Echevarria-Bonet et al 2013;Echevarria-Bonet et al 2015). The TbCu 2 MNPs consist of an antiferromagnetic (AFM) core in which the magnetic moment is localized on the Tb 3+ ions (J = 6) atoms.…”

“…Such disorder is observed in the thermal variation of magnetization to a low-temperature peak (T B ≈ 11 K) which is well below from that peak characteristic of the Néel transition (T N ≈ 50 K). Overall, their behaviour can be understood as superantiferromagnetic (SAFM) MNPs (Echevarria-Bonet et al 2013;Echevarria-Bonet et al 2015). The concept behind i s s i m i l a r t o t h e s u p e r s p i n g l a s s a n d superferromagnetic behaviours found in other varied systems containing MNPs such as α-Fe 2 O 3 (Hansen et al 2000), Fe/γ-Fe 2 O 3 (Chandra et al 2012), δ-(Fe 0.67 Mn 0.33 )OOH (Peddis et al 2012), FeAg (Alba Venero et al 2013) and γ-Fe 2 O 3 (Andersson et al 2015), to cite a few.…”