Complex magnetotransport in LaSr 2Mn 2O 7

A detailed study of the magnetic and electrical transport properties of well characterized samples of the layered manganite LaSr ₂Mn ₂O ₇ by a battery of magnetization studies reveal the complex interplay of ferromagnetic and antiferromagnetic interactions in this phase. The preponderantly ferromagnetic (in terms of sign) interactions that set in at around 490 K give way to antiferromagnetism at 230 K as evidenced by low-field studies. Interestingly, the antiferromagnetic transition is coupled with a sharp rise in the resistivity of the samples allowing us to speculate that in this phase, notionally comprising equal amounts of Mn ^III and Mn _IV, the localized holes could be ordered on different lattice sites at low temperatures. The simultaneous presence of the two kinds of interactions in this layered phase give rise to complex electrical transport properties, including a re-entrant insulator at low temperatures with no magnetic signature in the transition. The last feature we ascribe to disorder-induced localization, important in these quasi-2D materials.