Magnetic Structure and Quadrupolar Order Parameter Driven by Geometrical Frustration Effect in NdB₄

This is the authors' abstract. We don't add key points for this paper.

Neutron diffraction experiments have been carried out to characterize the magnetic structures and order parameters in an intermediate phase of NdB₄ showing the successive phase transitions at T₀ = 17.2 K, TN1 = 7.0 K, and TN2 = 4.8 K. We have revealed the antiferromagnetic ordering with the propagation vectors q0=(0,0,0), q0 and qs1=(δ,δ,0.4) (δ ~ 0.14), and q₀ and qs2=(0.2,0,0.4) in phase II (TN1 < T < T₀), phase III (TN2 < T < TN1), and phase IV (T < TN2), respectively. The observed patterns in phase II are successfully explained by postulating a coplanar structure with static magnetic moments in the tetragonal ab-plane. We have found that the magnetic structure in phase II can be uniquely determined to be a linear combination of antiferromagnetic “all-in/all-out”-type (Γ₄) and “vortex”-type (Γ₂) structures, consisting of a Γ₄ main component (77%) with a small amplitude of Γ₂ (23%). Finally, we propose that the quadrupolar interaction holds the key to stabilizing the noncollinear magnetic structure and quadrupolar order. Here, the frustration in the Shastry–Sutherland lattice would play an essential role in suppressing the dominance of the magnetic interaction.