Magnetic field dependence of the antiferromagnetic phase transitions in Co-doped YbRh₂Si₂

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We present first specific-heat data of the alloy Yb(Rh(1-x)Coₓ)₂Si₂ at intermediate Co-contents x=0.18, 0.27, and 0.68. The results already point to a complex magnetic phase diagram as a function of composition. Co-doping of YbRh₂Si₂ (Tₙˣ⁼⁰=72 mK) stabilizes the magnetic phase due to the volume decrease of the crystallographic unit cell. The magnetic phase transitions are clearly visible as pronounced anomalies in C⁴ᶠ(T)/T and can be suppressed by applying a magnetic field. Going from x=0.18 to x=0.27 we observe a change from two mean-field (MF) like magnetic transitions at Tₙ⁰.¹⁸=1.1 K and Tₗ⁰.¹⁸=0.65 K to one sharp λ-type transition at Tₙ⁰.²⁷=1.3 K. Preliminary measurements under magnetic field do not confirm the field-induced first-order transition suggested in the literature. For x=0.68 we find two transitions at Tₙ⁰.⁶⁸=1.14 K and Tₗ⁰.⁶⁸=1.06 K.