A metal-radical polymer [Co(hfac)2.BPNN] showed a very large coercive field of 52 kOe (4.1 MA m-1) at 6 K, indicating that it is the hardest magnet ever reported. Above 10 K, a soft character appeared, owing to the fast dynamics of magnetization reorientation.
A long-range-ordered state of CoBPNN was characterized by muSR. The temperature at the depolarization rate divergence (40 K) is defined as a magnetic transition temperature (T(N)). The phase is unique below T(N), but gradual freezing of the domain-wall motion finally leads to the magnetically hard character.
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.
A coordination polymer [Co(hfac)2·L] (L = o-ethoxyphenyl nitronyl nitroxide) was prepared, structurally characterized, and revealed to show a record coercive field of 54 kOe at 6 K. The magnetic easy axis was found to be perpendicular to the chain direction. Interchain dipolar coupling is important for the bulk and/or single-chain properties in this series.
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