Magnetic anisotropy of the alkali iridate Na2IrO3 at high magnetic fields: Evidence for strong ferromagnetic Kitaev correlations

Abstract

The magnetic field response of the Mott-insulating honeycomb iridate Na2IrO3 is investigated using torque magnetometry measurements in magnetic fields up to 60 tesla. A peak-dip structure is observed in the torque response at magnetic fields corresponding to an energy scale close to the zigzag ordering (≈ 15K) temperature. Using exact diagonalization calculations, we show that such a distinctive signature in the torque response constrains the effective spin models for these classes of Kitaev materials to ones with dominant ferromagnetic Kitaev interactions, while alternative models with dominant antiferromagnetic Kitaev interactions are excluded. We further show that at high magnetic fields, long range spin correlation functions decay rapidly, signaling a transition to a long-sought-after field-induced quantum spin liquid beyond the peak-dip structure. Kitaev systems are thus revealed to be excellent candidates for field-induced quantum spin liquids, similar physics having been suggested in another Kitaev material α−RuCl3.

Competition between magnetic order and charge localization in Na2IrO3 thin crystal devices

“…First-principles calculations [4] have found that under high pressure, Na 2 IrO 3 goes through successive structural and magnetic phase transitions, some of them zigzag magnetic ordered and some nonmagnetic, all with low energy excitations that can be well described by j eff = 1/2 states and in particular having some phases (with a structure corresponding to space group P1) [4] resembling a gapped spin liquid Kitaev state. Similarly, magnetic torque measurements have found that at magnetic fields up to 60 T, long-range spin correlation functions decay rapidly pointing to a field-induced quantum spin liquid [6]. In the absence of high pressure or magnetic fields, recent inelastic x-ray scattering measurements have reported a proximate spin liquid regime above the long-range ordering temperature, where fractional excitations are emer-gent, revealed by spin-spin correlations restricted to nearest neighbor sites, among other factors [7].…”

“…In real materials such as Na 2 IrO 3 , the existence of Heisenberg and off diagonal interactions benefit the appearance of magnetically ordered ground states over a spin liquid. Recent studies have explored the effect of external pressure and high magnetic fields on the ground states of Na 2 IrO 3 [4][5][6]. First-principles calculations [4] have found that under high pressure, Na 2 IrO 3 goes through successive structural and magnetic phase transitions, some of them zigzag magnetic ordered and some nonmagnetic, all with low energy excitations that can be well described by j eff = 1/2 states and in particular having some phases (with a structure corresponding to space group P1) [4] resembling a gapped spin liquid Kitaev state.…”

“…Older and newer studies on Na 2 IrO 3 have been limited to probes that require bulk crystals [1,[4][5][6][7][8][9], where the insulating character of Na 2 IrO 3 has hampered its integration to an electronic device, advantageous for the study and manipulation of quasiparticle excitations, of interest in topological quantum computing [10,11]. Although electrical resistivity measurements have been reported for Na 2 IrO 3 , they have been limited to bulk crystals [2] or heteroepitaxial thin films [12], where results are highly dependent on the substrate used and the conditions of growth.…”

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].…”

Honeycomb rare-earth magnets with anisotropic exchange interactions

“…It is well-known that the anisotropic exchange couplings could appear in the temperature dependence of the thermodynamic quantities such as the specific heat, spin susceptibility [17] and magnetotropic coefficients [18,19]. Especially for the spin susceptibility and magnetic torque, magnetic fields along different directions induce magnetization of different magnitudes, leading to the anisotropic spin susceptibility [20][21][22][23][24][25] and the angular dependence of the magnetic torque [26][27][28][29], and providing a natural detection of the intrinsic spin anisotropy in the system. To go beyond the thermody-namic properties, we further consider the electron spin resonance (ESR) measurement [30,31] of the system.…”