Spin-Disorder State Near Nonmagnetic Impurities In The Frustrated Antiferromagnet Ymno3

The effects of nonmagnetic Al impurities on a magnetic bath YMnO3 are studied by magnetization, heat capacity, neutron diffraction, and NMR measurements. Suppression of the Neel order with an increase in the doping rate is evidenced by the heat capacity and M-T curve, also showing vacancy-induced magnetization. Theory predicts that the vacancies in triangular antiferromagnets release the local frustration partially by a readjustment of the spin directions, which results in a net magnetic moment of 0.04S around the impurity site, where S is the single spin moment. Interestingly, our NMR results show that the amplitude of the impurity moment is similar to S, which is one order of magnitude larger than a previous prediction. The direction of the impurity moment and neighboring spins generating a hyperfine field at a nonmagnetic impurity site are random. Above T-N, the release of frustration due to a spin-vacancy is evidenced by the enhancement of the antiferromagnetic correlation of spins near impurity sites. Based on careful consideration about the structural distortion, we suggest that this huge spin-disorder originates from the magnetic correlation between impurity moments.