Spin re-orientation in heavy fermion system $\alpha -YbAl_{1-x} Fe_{x} B_{4} $

Non centro-symmetric $\alpha -YbAlB_{4} $ has a heavy Fermi liquid ground state and shares many characteristics with centro-symmetric $\beta -YbAlB_{4} $. Both isomorphs display intermediate valence, associated with a fluctuation scale of $T_{0} =$ 200 K and a Kondo lattice scale of $T^{\ast }=$ 8 K[1]. Unlike $\beta -YbAlB_{4} $, $\alpha -YbAlB_{4} $ is at the boundary of a transition from a Fermi liquid metallic state to an antiferromagnetic (AFM) insulating state, driven by Fe substitution of Al [2]. Magnetization and specific heat measurements reveal two different antiferromagnetic phases with $T_{N} =$ 9 K and $T_{N} =$ 2 K for Fe concentration above and below x$=$0.07. We report single crystal neutron scattering experiments on Fe doped YbAlB$_{4}$ with x$=$0.035 and x$=$0.125. While the ordering wave vector is identical,$\vec{{k}}=(1,0,0)$, the spin orientation switches from $c$ to $a$ with increasing Fe concentration. This suggests different anisotropic hybridization between 4f and conduction electrons that we confirmed by determining the crystal field levels. [1] M. Okawa et al., PRL 104, 247201 (2010) [2] K. Kuga, S. Nakatsuji PRB 86, 224413 (2012)