Relieving geometrical frustration through doping in the Dy1−x Ca x BaCo4O7 swedenborgites

Abstract

The geometrically frustrated antiferromagnet DyBaCo4O7 is investigated through a combination of x-ray diffraction, magnetization and dielectric measurements. Systematic doping in the series Dy1−x Ca x BaCo4O7 causes a lifting of the geometrical frustration resulting in a structural transition from a trigonal P31c to an orthorhombic Pbn2 1 symmetry at x = 0.4. This structural transition can also be accessed as a function of temperature, and all our orthorhombic specimens exhibit this transition at elevated temperatures. The temperature at which this structural transition occurs is observed to scale linearly with the mean ionic radius of the R site ion. However, CaBaCo4O7 which has an equal number of Co2+ and Co3+ ions clearly violates this quasilinear relationship, indicating that charge ordering could also play a critical role in stabilizing the orthorhombic distortion in this system. Using thermoremanent magnetization measurements to circumvent the problem of the large paramagnetic background arising from Dy3+ ions, we chart out the phase diagram of the Dy1−x Ca x BaCo4O7 series.