Anisotropic temperature-field phase diagram of single crystalline β−Li2IrO3 : Magnetization, specific heat, and Li7

Abstract

Detailed magnetization, specific heat, and 7 Li nuclear magnetic resonance (NMR) measurements on single crystals of the hyperhoneycomb Kitaev magnet β-Li2IrO3 are reported. At high temperatures, anisotropy of the magnetization is reflected by the different Curie-Weiss temperatures for different field directions, in agreement with the combination of a ferromagnetic Kitaev interaction (K) and a negative off-diagonal anisotropy (Γ) as two leading terms in the spin Hamiltonian. At low temperatures, magnetic fields applied along a or c have only a weak effect on the system and reduce the Néel temperature from 38 K at 0 T to about 35.5 K at 14 T, with no field-induced transitions observed up to 58 T on a powder sample. In contrast, the field applied along b causes a drastic reduction in the TN that vanishes around Hc = 2.8 T giving way to a crossover toward a quantum paramagnetic state. 7 Li NMR measurements in this field-induced state reveal a gradual line broadening and a continuous evolution of the line shift with temperature, suggesting the development of local magnetic fields. The spin-lattice relaxation rate shows a peak around the crossover temperature 40 K and follows power-law behavior below this temperature. arXiv:1906.09089v2 [cond-mat.str-el]

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

“…Importantly, our semiclassical results show further that, for H c, the magnitude of the magnetization jump at H * c is significantly reduced by quantum fluctuations, almost to the point that there is no visible change, including the overall slopes of the curves below and above H * c . This renders the detection of this feature in magnetization measurements more challenging and probably explains the absence of the kink in recent measurements [24]. The detection is even more challenging for powder samples given that m c m b .…”

“…Remarkably, all experimental data reported so far for Kitaev materials show that their response to the magnetic field depends very strongly on its direction. This is true for the layered compounds Na 2 IrO 3 [10], α-Li 2 IrO 3 [46], and α-RuCl 3 [29][30][31]47], as well as for the three-dimensional (3D) iridates β-Li 2 IrO 3 [21,24] and γ-Li 2 IrO 3 [19,48]. Here we revisit the case of the hyper-honeycomb β-Li 2 IrO 3 and show that its strongly anisotropic response signifies a large separation of energy scales between the relevant microscopic interactions, and can thus be used to extract information about the relative strength of these interactions in a direct way.…”

“…These components are also present below H * b , but are too small to be detected at zero field [49,50]. For H a and H c, the system shows a much weaker response, with the IC order remaining robust and the magnetization being linear up to the maximum fields measured (see supplemental material in [21] and also [24]).…”

Reentrant incommensurate order and anomalous magnetic torque in the Kitaev magnet β−Li2IrO3

“…However, most of these materials exhibit complex long-range magnetic orders at sufficiently low temperatures, indicating that other subdominant interactions between magnetic moments are present and may also have nontrivial bond-dependent character. Both experiment [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] and theory [27][28][29][30][31][32][33] have shown that these orders are fragile and can be efficiently suppressed by external magnetic field. It was also found that the competition between the external field and anisotropic bond-dependent exchange interactions gives rise to highly anisotropic magnetization processes and a variety of complex orders at intermediate fields [27][28][29][30][31][32][33].…”

“…Therefore, the purpose of this paper is to understand the recent torque measurements in β-Li 2 IrO 3 [26] and γ -Li 2 IrO 3 [12,13]. These two materials appear to have closely related local energetics [35], and we will therefore focus entirely on the hyperhoneycomb β-Li 2 IrO 3 [23][24][25][26]36,37], whose microscopic minimal model, the nearest-neighbor (NN) J-Kmodel, has been better understood [30,31,[38][39][40].…”