One-dimensional bulk ferromagnets: NdAl2 and hcp cobalt

It is shown experimentally that NdAl₂ and hcp cobalt are one-dimensional (1D) bulk ferromagnets. For hcp cobalt this is only under the condition that the sample is magnetically saturated, i.e. that all moments are aligned parallel to the hexagonal c-axis. In 1D magnets the transverse interactions need not to be zero but must be sufficiently weak such that the transverse correlation length does not diverge at the critical temperature. The transverse interactions are then not relevant and the phase transition is driven by the longitudinal interactions. On the other hand, magnetic Bragg scattering relies on finite transverse correlations. For NdAl₂ no conventional magnetic Bragg scattering is observed if all moments are aligned vertical to the scattering plane by a magnetic field. For hcp cobalt the scattering intensity is considerably reduced in this geometry instead of having its maximum. From this observation it can be concluded that the transverse correlation length is practically zero in NdAl₂ but has a finite value in hcp cobalt. The macroscopic magnetization shows normal ferromagnetic saturation.