Frustrated Ground State in the metallic Ising 2D Anti-ferromagnet Nd₂Ni₂In

Nd₂Ni₂In is a 2D Ising anti-ferromagnet (T_N = 8 K) with a crystal structure equivalent to the Shastry-Sutherland (SS) lattice, leading to magnetic frustration. This system is well known for its high Hydrogen absorption capabilities, but recently its magnetic properties have been investigated revealing some very interesting anomalies. The magnetization curve in the ordered state obtained on the Nd₂Ni₂In polycrystal is not qualitatively different from a very narrow hysteresis loop of a ferromagnet. Nevertheless, neutron measurements indicate an anti-ferromagnetic structure with Ising Nd moments oriented mutually perpendicular along the direction of the [110] type, i.e. Nd moments in the basal plane.
Data collected at SEQUOIA and CNCS have been used to analyze the crystal field of Nd₂Ni₂In in zero field to shed light on its ground state. The results point towards a structure with Ising spins oriented along the local [110] direction with a magnetic moment of 2.59(13) μ_B/Nd, in excellent agreement with previous measurements. Moreover the analysis on the two magnetic Bragg peaks (2,0,0), (2,1,0) gave an order parameter ß=0.13, which is very close to the critical exponent for the 2D Ising model exactly solved by Onsager ß=1/8.