We report the observation of enhanced magnetization in graphene oxide (GO) after thermal annealing. We have also proposed that this enhancement in magnetization is due to the increased density of zigzag edges. We conjecture that on annealing the random epoxy groups in the native GO migrate over the GO surface by acquiring thermal energy and self-assemble to form several long chains of epoxy groups. Subsequently, upon thermal reduction the GO sheet is unzipped along these long chains giving rise to more zigzag edges, resulting in enhancement of magnetization. We also found out that the density of epoxy groups plays an important role in the unzipping process. If the density of epoxy groups is low, then unzipping of GO is not possible. Chemical reduction of GO does not favor unzipping.

We report multiple first order magnetic transitions in TbMn2Si2 as evidenced by the thermal hysteresis in the M-T data and the Arrott plots. Metamagnetic transitions are observed at various temperatures as a result of the antiferromagnetic to ferromagnetic transition of the Mn sublattice. Very interestingly, the compound shows significant exchange bias field of about 600 Oe at 5 K, which is attributed to the formation of small domains or regions with ferromagnetic and antiferromagnetic interactions. Furthermore, a large magnetocaloric effect has been found at relatively low fields at both the transition regions. Maximum magnetic entropy changes (−ΔSM) of 7.2 and 5.4 J kg−1 K−1 have been observed at 68 K and at 48 K, respectively, at 20 kOe.

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