Magnetic properties of the (Mo2/3R1/3)(2)AlC (R=Ho, Dy) i-MAX phases studied by x-ray magnetic circular dichroism and neutron diffraction

We report on the magnetic properties of single crystals of Ho- and Dy-based (Mo2/3R1/3)(2)AlC i-MAX phases. In these nanolamellar compounds, where planes of R and Mo arranged in a skewed triangular lattice are separated by planes of Al and C, geometrical frustration and magnetic exchange interactions lead to complex magnetic properties. Temperature-dependent bulk magnetization, specific heat, and resistivity measurements reveal two magnetic phase transitions in Dy i-MAX (15 and 12 K) and only one in Ho i-MAX (8.5 K). Strong magnetic anisotropy and metamagnetic transitions with a step at 1/3 of saturation moment along the crystal a axis are observed in field-dependent bulk magnetization curves. X-ray magnetic circular dichroism measurements unveil induced moments on Mo and Al, and a quantitative estimation of the orbital and spin moments of Mo based on magneto-optical sum rules suggests an unusual interaction between the R 4f and the Mo 4d magnetic moments. Magnetic structures are derived from neutron diffraction measurements, revealing a zero-field incommensurate amplitude modulated order in both compounds, followed by an antiferromagnetic equal-moments structure at lower temperature for Dy i-MAX. The bulk magnetization 1/3 step is found to be linked to the flip of one R moment out of three within the planes. Detailed phase diagrams for Ho and Dy i-MAX are derived from these measurements.