Hard Magnets after Freezing of Spin Dynamics of Soft Magnets in Cobalt(II)–Radical Chain Compounds

Abstract

Abstract Several metal–radical alternating chains ([Co(hfac)2(CnPNN)]) were prepared, and their crystal structures and magnetic properties were systematically studied, where CnPNN stands for a phenylnitronyl nitroxide ligand having a linear Cn alkoxy group at the ortho or para position. X-ray crystal structure analysis was successfully performed for [Co(hfac)2(p-C5PNN)] (p-5). The chain structure is similar to those of the known p-butoxy and o-ethoxy derivatives. Compound p-5 showed a very large coercive field of 51 kOe (4.1 MA m−1) and a saturation magnetization of 9.7 × 103 erg Oe−1 mol−1 at 6 K. o-Propoxy and o-pentoxy derivatives also exhibited large coercive fields of 50–54 kOe at 5–6 K. The activation energies of magnetization reorientation for the present compounds were estimated to be as large as 290–360 K from Arrhenius analysis using ac susceptibility data. In a temperature range of ca. 10–40 K, all of the present compounds behaved as very soft magnets, as indicated by no hysteresis in the ferromagnetic M–H curves. We propose a mechanism for the drastic soft–hard switch. The hard character grows immediately after freezing of the spin dynamics owing to the strong magnetic anisotropy.