measured using optical NV-magnetometer*

The lower critical field of superconductors (H_c1) gives direct access to the absolute value of London penetration depth which is needed to obtain the superfluid density. H_c1 is notoriously difficult to measure due to a variety of obstacles such as demagnetization, several types of surface barriers, imperfections, and anisotropies. Some of the most accurate techniques involve direct measurements of the magnetic field at which first vortices start penetrating the sample, and require outstanding sensitivity and good spatial resolution. Here we employ Nitrogen-Vacancy (NV) centers in diamond as a non-invasive, sensitive optical magnetometer to precisely measure H_c1 in single crystals of Ba(Fe_1-xCo_x)₂As₂ and track it as a function of cobalt concentration (x). We discover an anomalous decrease of superfluid stiffness near x=0.057, the composition at the very edge of the magnetic phase where magnetic order becomes incommensurate. We compare our findings with similar behavior in P-doped BaFe₂As₂.