Magnetic order in the two-dimensional randomly mixed ferromagnet-antiferromagnet Rb2Cu1-xCoxF4.

Abstract

With the use of the ac and dc susceptibility and the specific heat, a systematic experimental study is performed of the magnetic order in the mixed two-dimensional ferromagnet-antiferromagnet ${\mathrm{Rb}}_{2}$${\mathrm{Cu}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Co}}_{\mathrm{x}}$${\mathrm{F}}_{4}$(0\ensuremath{\le}x\ensuremath{\le}1). The concentration versus temperature (x,T) diagram contains at the Co-rich side an axial antiferromagnetic phase, and at the Cu-rich side planar, oblique, and axial ferromagnetic phases. For 0.18<x<0.40, ${\mathrm{Rb}}_{2}$${\mathrm{Cu}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Co}}_{\mathrm{x}}$${\mathrm{F}}_{4}$ is a spin glass, materializing the Edwards-Anderson nearest-neighbor random-bond Ising spin glass in two dimensions. Spin-glass freezing near 3 K is evident from a frequency-dependent cusp in the ac susceptibility, the absence of an anomaly in the specific heat, and the thermomagnetic-history dependence of the dc susceptibility. The data are consistent with a zero-temperature spin-glass transition. In cooling to the oblique ferromagnetic phase, axial and transverse spin components are found to order at distinct critical temperatures. The axial ferromagnetic phase exhibits extraordinarily slow domain-wall dynamics.