Resonant Magnetic X-ray Diffraction Study on the Triangular Lattice Antiferromagnet GdPd2Al3

Resonant magnetic x-ray diffraction experiments were carried out on the stacked triangular lattice antiferromagnet GdPd2Al3. The experiments revealed an expected initial collinear c-axis order at T-N1 followed by an additional in-plane order at T-N2, while at the same time we found that the ground state is a helically ordered state of a very long incommensurate period of approximately 700 angstrom. The distribution of K-domains was highly anisotropic, and the domain with the modulation vector normal to the surface of the crystal was ascendant. Low-field magnetization is discussed on the basis of the observed incommensurate magnetic structure.