Spontaneous and induced magnetic phase transitions in Tb0.9Er0.1Ni5

Magnetic and neutron powder diffraction measurements have been carried out on Tb_0.9Er_0.1Ni₅ intermetallic compound. The intermetallide crystallizes in the hexagonal CaCu₅-type structure and possesses a long-range magnetic order at temperatures below 22 K. A fan-like magnetic structure is described by two propagation vectors: k₁ = 0 and k₂ = 2π/c(0, 0, 0.036), at 20 K. The total Tb-ion magnetic moment has the ferromagnetic and modulated components. The latter is a transverse spin wave. When the sample is cooled at a temperature below 8 K, the module of the k₂ vector does not change and is equal to k₂ = 2π/c(0, 0, 0.027). An "incommensurate – lock-in" magnetic transition takes place at 8 K. The k₂ vector exhibits a temperature hysteresis of about (5–6) K, whereas the intensities of Bragg reflections and satellites do not show up appreciable changes. When an external magnetic field is applied to the sample, the satellites and the module of the k₂ vector decrease, while the Bragg intensities increase. The sample becomes a ferromagnetic at a field of ∼2 kOe, and the Tb-ion magnetic moment is equal to 8.3 μ_B. A general notion of the Tb_0.9Er_0.1Ni₅ magnetic state evolution with an external field is given using the field dependence of the background intensity in diffraction patterns. First principle calculations for TbNi₅ and Tb_0.9Er_0.1Ni₅ are performed including the 4f states into the orbital basis and accounting for strong electronic correlations and spin-orbital coupling. This allowed obtaining both spin and orbital moments of the effective Tb-ion moment and estimating also the value of Tb-Tb exchange interaction.