Braided Ising spin-tube physics in a purported kagome magnet

The magnetic insulator Nd$_3$BWO$_9$ has been previously proposed to realize the highly frustrated breathing kagome lattice model. We report a combination of single-crystal neutron scattering studies and numerical simulations that debunk this interpretation. We show that it is the inter-plane couplings that determine the physics. To explain the exotic magnetism, we derive a simple one-dimensional Ising model composed of twisted triangular spin-tubes, i.e., triple braids of Ising spin chains with almost-orthogonal anisotropy frames and competing ferro-antiferromagnetic interactions. This model can account for the ground state, excitations, the numerous field-induced fractional magnetization plateau phases and incommensurate magnetic correlations at elevated temperatures. Nd$_3$BWO$_9$ constitutes a rare example where rich magnetic phenomena can be understood and simulated quantitatively in terms of a simple classical Hamiltonian.