Time-reversal symmetry breaking and spontaneous Hall effect without magnetic dipole order

Abstract

Spin liquids are magnetically frustrated systems, in which spins are prevented from ordering or freezing, owing to quantum or thermal fluctuations among degenerate states induced by the frustration. Chiral spin liquids are a hypothetical class of spin liquids in which the time-reversal symmetry is macroscopically broken in the absence of an applied magnetic field or any magnetic dipole long-range order. Even though such chiral spin-liquid states were proposed more than two decades ago, an experimental realization and observation of such states has remained a challenge. One of the characteristic order parameters in such systems is a macroscopic average of the scalar spin chirality, a solid angle subtended by three nearby spins. In previous experimental reports, however, the spin chirality was only parasitic to the non-coplanar spin structure associated with a magnetic dipole long-range order or induced by the applied magnetic field, and thus the chiral spin-liquid state has never been found. Here, we report empirical evidence that the time-reversal symmetry can be broken spontaneously on a macroscopic scale in the absence of magnetic dipole long-range order. In particular, we employ the anomalous Hall effect to directly probe the broken time-reversal symmetry for the metallic frustrated magnet Pr(2)Ir(2)O(7). An onset of the Hall effect is observed at zero field in the absence of uniform magnetization, within the experimental accuracy, suggesting an emergence of a chiral spin liquid. The origin of this spontaneous Hall effect is ascribed to chiral spin textures, which are inferred from the magnetic measurements indicating the spin ice-rule formation.

On the magnetic structure of Sr3Ir2O7: an x-ray resonant scattering study

“…A host of particularly interesting physical phenomena may occur at the crossroads of these two energy scales [1,2,3,4]. For the case of Ir 4+ ions, where relativistic effects are significant and for a low-spin d 5 configuration in the presence of an octahedral crystal field, a four-fold degenerate J ef f = 3/2 multiplet and a two-fold degenerate J ef f = 1/2 multiplet are created.…”

Large anomalous Nernst effect at room temperature in a chiral antiferromagnet

“…1a). On the other hand, the recent Berry phase formulation of the transport properties has led to the discovery that a large anomalous Hall effect (AHE) may arise not only in ferromagnets, but in antiferromagnets and spin liquids, in which the magnetization is vanishingly small 10,[16][17][18][19][20][21][22] . As the first case in antiferromagnets, Mn 3 Sn has been experimentally found to exhibit a large AHE 10 .…”

Magnetic and Thermal Properties of the Single Crystalline Pr2Zr2O7 in a [111] Field

“…A new class of spin ice has been recently proposed for Pr and Yb pyrochlores [7][8][9][10][11], in which quantum-mechanical exchange interactions predominately provides the ferromagnetic coupling between neighboring spins; hence, they are called "exchange(-based) spin ice". The classical dipolar interactions are one order of magnitude smaller than exchange interactions due to the small size of the magnetic moment (∼ 3µ B ).…”

“…The resultant strong quantum fluctuations absent in the dipolar spin ice were found to generate exotic quantum magnetism. For example, it has been demonstrated that the Pr-based pyrochlore Pr 2 Ir 2 O 7 shows a novel chiral spin liquid phase [10], which may be explained by quantum melting of spin ice manifold [12,13].…”