*

We present magnetic long-range order studied by x-ray scattering and magnetic neutron diffraction (MND) on a single crystal RbEuFe₄As₄ (RbEu1144) suerpconductor. In tetragonal RbEu1144 superconductivity (SC) appears at ~37 K followed by an antiferromagnetic (AFM) transition at T_N ~15 K. Our diffraction studies revealed a long-range, fully coherent, and 3D ordered magnetic phase characterized by a single wavevector, q=(0,0,1/4), which indicates a 4-unit-cell magnetic periodicity along the 4-fold c axis. High-resolution x-ray scattering did not find any detectable structural distortions (e.g. orthorhombic) at low temperatures. Element-selective x-ray resonant scattering (XRS) and MND measurements indicated a helical magnetic order of full Eu²⁺ moments locked into the ab plane, without the presence of any ferromagnetism (FM). The use of charge-magnetic interference phenomena in XRS for measuring hysteresis and determining trapped magnetic field value is presented.