Cycloidal Magnetic Ordering in Noncentrosymmetric EuIrGe3

Successive magnetic phase transitions at T-N = 12.2 K, T'(N) = 7.0 K, and T*(N)= 5.0 K in EuIrGe3, an intermetallic compound with a body centered tetragonal lattice belonging to a polar space group 14mm, has been investigated by neutron diffraction and resonant X-ray diffraction. It is shown that EuIrGe3 exhibits an incommensurate longitudinal sinusoidal order with q similar to (0, 0, 0.792) and m(q) parallel to c-axis in the high temperature phase (T'(N) < T < T-N), which changes to a cycloidal order with q = (delta', 0, 0.8) (delta' similar to 0.017) and m(q) parallel to ac-plane in the intermediate phase (T'(N) < T < T'(N)). In the low temperature phase (T < T*(N)), the cycloidal plane rotates by 45 degrees to have q = (delta, delta, 0.8) (delta similar to 0.012). It is also pointed out that the X-ray scattering amplitude from odd-parity magnetic quadrupole due to the polar environment interfere with that from normal even-parity magnetic dipole in the magnetic ordered phase.