単一分子磁石の磁化反転緩和と熱測定

摘要

単一分子磁石（single-molecule magnets, SMM）は常磁性金属錯体の一群で，低温で分子磁化の反転頻度が低くなり，極端なケースでは観測時間スケールで反転が凍結して，磁気的な長距離秩序をもたないにもかかわらず磁気ヒステリシスを示すに至る。一言で表すと，「分子サイズの永久磁石」であり，磁気記録の高密度化や量子コンピューターの演算キュビットとしての応用に興味をもたれ，1990年代以降，精力的に研究されてきた。その多くは複数の遷移金属ィオンが架橋配位子で結ばれた多核錯体で，架橋を通じた交換相互作用で各イオン上のスピンが結合し，大きな合成スピン（分子磁化）を形成している。%Recent studies on heat capacity calorimetry of nanomagnets including single-molecule magnets are reviewed. Single-molecule magnet is the name of paramagnetic molecules showing superparamagnetic behavior and blocking of magnetization reversal despite the absence of magnetic long-range order. Heat capacity calorimetry plays an important role in the characterization of this class of nanomagnets, not only for determination of spin energy levels and magnetic anisotropy but also for confirming the absence of magnetic long-range order. In another view, single-molecule magnets serve as a playground for the demonstration of kinetic selection rule dominating thermal excitations, thanks to an additional control parameter of external magnetic field which can open/close relaxation channels relating to transverse or longitudinal manner of application. Related nanomagnets covering single-chain magnets and antiferromagnetic spin rings are also discussed. The Zeeman-split spin levels of the latter show avoided crossings due to the off-diagonal matrix elements of magnetic anisotropy, and it is revealed that heat capacity calorimetry is useful for understanding such level systems.