Systematic Investigation of a New Family of Transition-Metal Phosphates Exhibiting Field-Dependent Magnetization Steps

We recently reported a low-dimensional compd., RbNa3Fe7(PO4)6, that contains electronically confined magnetic nanostructures (Angew.Chem. Int. Ed. 2007, 46, 5344-5347). The field dependent magnetization plot of RbNa3Fe7(PO4)3 shows stepped variations at intervals of approx. 0.54T in the 2K data. These magnetic anomalies are said due to geometrically frustrated magnetic lattices and the uniaxial magnetic properties of the high-spin Fe2(d6) ions residing in the trigonal bipyramidal sites. High d. ac susceptibility has also revealed a double-peak feature in the vs. T plot. To investigate the origin of these magnetization steps, we have carried out a systematic study of a series of this fascinating compd., ANa3M7(PO4)3, where A = K, Rb, Cs and M = Mn2, Fe2, Co2. This magnetically interesting system has offered a rare opportunity for an in-depth investigation of electronic interactions within these differing spin systems. We have surveyed the magnetic properties of the series, and found that the Co and Fe derivs. exhibit a ferromagnetic transition at temps. around 15K with an addnl. antiferromagnetic transition at 6K. The Mn derivs., however, exhibit an antiferromagnetic transition at temps. below 6K. The obsd. difference could be the result of the high spin (d5) ions lacking an axial Jahn Teller distortion. Magnetization plotted as a function of temp. for the Fe derivs. shows an increase in the magnetization with decreasing size of the A-site cation. In this presentation we will show structure-property correlation studies of this series, including magnetic characterization and heat capacity studies.