Unconventional magnetic field response of the hyperhoneycomb Kitaev magnet β-Li2IrO3

We present a unified description of the response of the hyperhoneycomb Kitaev magnet β-Li2IrO3 to applied magnetic fields along the orthorhombic directions a, b, and c. This description is based on the minimal nearest-neighbor J-K-Γ model and builds on the idea that the incommensurate counter-rotating order observed experimentally at zero field can be treated as a long-distance twisting of a nearby commensurate order with six spin sublattices. The results reveal that the behavior of the system for H∥a, H∥b, and H∥c share a number of qualitative features, including (i) a strong intertwining of the modulated, counter-rotating order with a set of uniform orders; (ii) the disappearance of the modulated order at a critical field H∗, whose value is strongly anisotropic with Hb∗<Hc∗≪Ha∗; (iii) the presence of a robust zigzag phase above H∗; and (iv) the fulfillment of the Bragg peak intensity sum rule. It is noteworthy that the disappearance of the modulated order for H∥c proceeds via a "metamagnetic"first-order transition which does not restore all broken symmetries. This implies the existence of a second finite-T phase transition at higher magnetic fields. We also demonstrate that quantum fluctuations give rise to a significant reduction of the local moments for all directions of the field. The results for the total magnetization for H∥b are consistent with available data and confirm a previous assertion that the system is very close to the highly frustrated K-Γ line in parameter space. Our predictions for the magnetic response for fields along a and c await experimental verification.