Incommensurate spin-density-wave antiferromagnetism in CeRu2Al2 B

The newly discovered Ising-type ferromagnet CeRu2Al2B exhibits an additional phase transition at TN=14.2 K before entering the ferromagnetic ground state at TC=12.8 K. We clarify the nature of this transition through high resolution neutron diffraction measurements. The data reveal the presence of a longitudinal incommensurate spin-density wave (SDW) in the temperature range of TC<T<TN. The propagation vector q∼(0,0,0.148) is nearly temperature independent in this region and discontinuously locks into q=0 at TC. Mean-field calculations of an effective Ising model indicate that the modulated SDW phase is stabilized by a strong competition between ferromagnetic and antiferromagnetic exchange interactions. This makes CeRu2Al2B a particularly attractive model system to study the global phase diagram of ferromagnetic heavy-fermion metals under the influence of magnetic frustration.