Ubiquity of amplitude-modulated magnetic ordering in the $H − T$ phase diagram of the frustrated non-Fermi-liquid YbAgGe

International audience; YbAgGe contains a magnetic geometrically frustrated kagome-like lattice that also features significant local single-ion anisotropy. The electronic state is established by hybridization of $4f$ and conduction electrons leading to heavy electronic masses. The competition between these various interactions leads to nontrivial behavior under external magnetic field, including a sequence of magnetic phase transitions, non-Fermi-liquid states, and possibly a quantum critical point. We present a series of neutron diffraction experiments performed in the mK temperature range and under magnetic fields up to 8 T in the hexagonal plane, revealing the microscopic nature of the first four subsequent magnetic states of this phase diagram. The magnetic phases are associated with the propagation vectors $k_1$ = (${1\over 3}$ 0 ${1\over 3}$) for $H$ < 2 T, $k_2$ = (0 0 0.32) for 2 T < $H$ < 3 T, $k_1$ = (${1\over 3}$ 0 ${1\over 3}$) for 3 T < $H$ < 4.5 T, and $k_3$ = (0.195 0.195 0.38) for 4.5 T < $H$ < 7 T. Our structural refinements reveal a strong modulation of the magnetic moment amplitude in all phases. We observe that the ordered moments of the three magnetically different Yb sites become increasingly different in field, which complies with the principle local anisotropy directions relative to the field direction. While the ordered moments are aligned predominantly in the hexagonal plane, we also find a significant out-of-plane component and a ferromagnetic contribution above 2 T. We discuss possible scenarios that may evolve around the phase boundary at 4.5 T, which is associated with putative quantum criticality as identified by various bulk probes. We propose further steps that are required to better understand the microscopic interactions in this material.