| The enthusiasm and intensive interest to molecular based magnetic enduredduring the last decade due to its extraordinary properties. For example, its low density,electrical insulation, and low temperature fabrication make it a good candidature tothe material manufacturing; the existence of one or several compensation temperatureis very useful in electronic, computer technologies, and thermomagnetic recording;and a series of novel magnetic properties such as enhanced surface magnetic moment,surface magnetic anisotropy, and giant magnetoresistance have been found in suchmaterials. Specifically, the celebrated ferrimagnetic material AMIIFeIII(C2O4)3(A=N(n-C3H7)4, M=Mn,Fe) has attracted intensive attention since first synthesizeddue to its great properties. As a result, the various aspects regarding mixed spin-2andspin-5/2Ising model with single ion anisotropies, which can describeAMIIFeIII(C2O4)3effectively, have been intensively studied within various theoreticalmethods.As far as we know, researchers usually pay more attention to the commonsecond-order phase transition in magnetic material and Ising model, while studiesfocusing on the first order phase transition are still lacking. In fact, first order phasetransition is also important during which many interesting phenomena occur such asthe discontinuity in the order parameter, the jump in dynamic and magnetic quantities,and the existence of latent heat. Thus the potential application of first-order magnetictransition in molecular based magnets should be great. Even though first ordertransition exists in a variety of systems, the detailed conditions for it to occur havenot been clarified.Moreover, the widely used methods associated with Ising first-order transitionare limited to MFT and effective-field theory (EFT), which is not necessarily reliabledue to the coarse approximation. Although the MFT and EFT can generally predictthe valid magnetic behavior of Ising model (except that the approximation will lead to higher second order phase transition point), we noticed that, in some special case, theEFT or MFT can lead to qualitatively wrong results. For example, in the study ofmonolayer mixed spin-2and spin-5/2honeycomb system with one anisotropy, there isno compensation temperature to be found by MC simulation; however, it was foundin large parameter range using the EFT method. In addition, even when the parameteris in the region where the ground state (zero temperature) is paramagnetic, the MFTand EFT can lead to the existence of first order transition. Therefore, MC simulationshould be employed to clarify the real situation.In this paper, we carry out Monte Carlo simulation on a mixed spin-2andspin-5/2honeycomb lattice to study the possible first order phase transition inmolecular based magnet AMIIFeIII(C2O4)3. As an orientation and starting point, themean-field results are also presented for comparison. We start at the ground statediagram and find that first order transition would occur only at a small region abovethe boundary of ferrimagnetic and paramagnetic phase. Around the critical points atthe ground state diagram, the magnetization will decrease from ground state to a statethat is the mixture of all the states around the points at very low temperature. At theregion where the ground state is paramagnetic phase, no phase transition would occureven though the MC simulation will lead to a seemingly reentrant due to finite sizeeffect. The comparison with MFT shows that MFT results are not necessarily reliableand should be re-examined. |
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