Giant Exchange Bias in Antiferromagnetic Mixed-Valence MOFs

Regulating magnetism of a metal-organic framework (MOF) is highly desirable in magnetoelectric devices. Single-crystalline MOFs of [(CH₃)₂NH₂]₂[Fe^IIIFe_2-<i>x</i>^IICo_<i>x</i>^II(HCOO)₉] (<i>x</i> = 0, 0.2, 0.3, 0.6) with an unprecedented (4¹²·6³)₁(4⁹·6⁶)₂ topology were synthesized. These MOFs exhibit distinct antiferromagnetism, and the Néel temperature (<i>T</i>_N) changes from 32.8 to 29.0 K with increasing <i>x</i>. After field cooling from above <i>T</i>_N to 2 K, these MOFs demonstrate giant exchange bias (EB) featured by a significant vertical and simultaneous horizontal shift in an almost linear curve of magnetization (<i>M</i>) versus the field (<i>H</i>). As the temperature rises, the <i>M</i>-<i>H</i> curve becomes hysteretic, with significantly increased coercivity and a diminished EB field. In combination with the Monte Carlo simulations, this EB effect is attributed to a large enough (negligible) anisotropy of Fe^II (Fe^III) ions and antiferromagnetic exchange coupling between Fe^II and Fe^III ions. This work provides a clue to designing a linear magnetic field sensor by using MOFs.