Field-induced magnetic transitions in Ca10(Pt3As8)((Fe1-xPtx)2As2)5 compounds

We report a high magnetic field study up to 55 T of the parent and the nearly optimally doped iron-pnictide superconductor Ca₁₀(Pt₃As₈)((Fe_1-xPt_x)2As₂)5 [x =0 and 0.078(6)] using magnetic torque, tunnel diode oscillator technique, and transport measurements. We determine the superconducting phase diagram, revealing an anisotropy of the irreversibility field up to a factor of 10 near T_c and signatures of multiband superconductivity. Unexpectedly, we find a prominent anomaly in magnetic torque close to 22 T, when the magnetic field is applied perpendicular to the (ab) planes, which becomes significantly more pronounced as the temperature is lowered to 0.33 K. We suggest that this field-induced transition, observed both in the magnetically ordered parent compound and a nonordered superconducting sample, is a signature of a spin-flop-like transition associated not with long-range order but driven by antiferromagnetic fluctuations of magnetic moments aligned preferentially out of the conducting planes at low temperatures.