Magnetocaloric effect and critical behavior in Mn2-imidazole-[Nb(CN)8] molecular magnetic sponge

• Magnetocaloric effect (MCE) for wet and dry phase of the sponge is reported.
• Dependence of MCE on magnetic field at Tc is consistent with critical exponents.
• MCE vs.Tc-2/3 plot for the family of compounds and the modified phases is linear.
• Spin contribution to MCE is estimated using a molecular field approximation.

A comprehensive study of magnetocaloric effect (MCE) and critical behavior in the {Mn2(imH)2(H2O)4[Nb(CN)8]·4H2O}n molecular magnet is reported. The compound is an example of a magnetic sponge, where structural changes provoked by dehydration process lead to the increase of Tc critical temperature from 25 K for the as-synthesized sample (1) up to 60 K for the anhydrous one (2). MCE and critical behavior were investigated by magnetization measurements. The maximum value of magnetic entropy change ΔS, determined by the magnetization measurements for 1 is 6.70 J mol−1 K−1 (8.95 J kg−1 K−1) at µ0ΔH=5 T, while for 2 it is equal to 4.02 J mol−1 K−1 (7.73 J kg−1 K−1) at the same magnetic field change. The field dependence of MCE at Tc for 1 and 2 was consistent with critical exponents, which allowed to classify both phases to 3D Heisenberg universality class. The Tc-2/3 dependence of the maximum entropy change has been tested using data of 1 and 2 together with MCE data previously reported for other members of the ferrimagnetic Mn2-l-[Nb(CN)8] (L=imidazole, pyridazine and pyrazole) series. Experimental MCE results have been compared with the spin contribution to the magnetic entropy change estimated using a molecular field approximation.