Magnetic phase transitions in layered NdMn2Ge2-xSix

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Australian Institute of Physics
The discovery of a giant magnetocaloric effect (GMCE) near room temperature in Gd5Si2Ge2 has led to much attention being paid to layered structures in order to understand the MCE behaviour of such materials [1]. NdMn2Ge2-xSix offers interesting prospects for enhanced magnetocaloric behaviour as the tetragonal, layered structure allows the structural and magnetic states to be controlled via inter- and intra-planar separations of the Mn atoms. We have investigated the structural and magnetic behaviour of NdMn2Ge2-xSix (x=0-2.0) by magnetic measurements, X-ray and neutron diffraction (Wombat, OPAL) over the temperature range 6-465 K. Replacement of Ge by Si leads to contraction of the unit cell with lattice constant a of NdMn2Ge2-xSix passing through two critical values a crit1 and a crit2 in RMn2X2 [2]: a crit1=4.06 Å at x1∼.0 and a crit2=4.02 Å at x∼1.8. This leads to significant modifications of the magnetic states of NdMn2Ge2-xSix. For example, at room temperature both NdMn2Ge1.6Si0.4 and NdMn2Ge1.2Si0.8 are found to exhibit canted ferromagnetism (Fmc) while NdMn2Ge0.8Si1.2 and NdMn2Ge0.4Si1.6 show canted antiferromagnetism (AFmc). By comparison, NdMn2Si2.0 exhibits interlayer antiferromagnetism (AFil) at 300 K [3]. We have established that Fmc and AFmc co-exist for NdMn2Ge1.2Si0.8 between TCNd (9∼0 K) and TNinter (∼180K) while NdMn2Ge0.4Si1.6 has a GMCE value of -∆SMmax=18.4 J kg -1 K -1 around TCNd=36 K for a field change ∆B = 5 T. The overall magnetic behaviours of NdMn2Ge2-xSix compounds are governed by the strong dependence of the magnetic couplings on the Mn-Mn spacing within the ab-plane. A detailed magnetic phase diagram for the NdMn2Ge2-xSix system has been constructed over the entire temperature and composition ranges.
Phase transformations, Layers, Neodymium, Manganese, Germanium, Atoms, Lead, Ambient temperature
Wang, J. L., Campbell, S. J., Cadogan, J. M. Studer, A. J., Zeng, R., & Dou, S. X. (2010). Magnetic phase transitions in layered NdMn2Ge2-xSix. Poster presented to the 34th Annual Condensed Matter and Materials Meeting 2010, Waiheke Island Resort, Waiheke, Auckland, New Zealand 2 - 5 February 2010. Retrieved from: