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 ${\mathrm{Rb}}_2$ ${\mathrm{Cu}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Co}}_{\mathrm{x}}$ ${\mathrm{F}}_4$(0≤x≤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, ${\mathrm{Rb}}_2$ ${\mathrm{Cu}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Co}}_{\mathrm{x}}$ ${\mathrm{F}}_4$ 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.