L. Drouglazet, E. Alecian, A. Sousa, P. I. Cristofari, E. Artigau, J. Bouvier, A. Carmona, N. J. Cook, C. Dougados, G. Duchêne, C. P. Folsom, H. Nowacki, K. Perraut, S. H. P. Alencar, L. Amard, M. Audard, S. Cabrit, J. -F. Donati, K. Grankin, N. Grosso, O. Kochukhov, Á. Kóspál, V. J. M. Le Gouellec, L. Manchon, G. Pantolmos, P. Petit, L. Petitdemange, R. Devaraj, H. Shang, M. Takami

Magnetic fields play a crucial role throughout stellar evolution, regulating angular momentum, channelling accretion, and launching jets and outflows. While the magnetic properties of Classical T Tauri Stars (CTTS) are well characterised, those of their progenitors, Class I and Flat-Spectrum (FS) protostars, remain poorly constrained due to observational challenges linked to their embedded nature. We aim to detect and characterise large-scale magnetic fields in a sample of Class I and FS protostars, which are expected to host strong dynamo-generated fields. Using SPIRou, a high-resolution near-infrared spectropolarimeter, we analysed polarised spectra and applied the Least Squares Deconvolution (LSD) technique to extract magnetic signatures and measure longitudinal fields from Stokes V profiles. We report new detections of large-scale magnetic fields in 5 FS protostars. Including the previously known magnetic FS protostar V347 Aur, 40% of our sample (15 objects) is confirmed to be magnetic. These stars exhibit clear Zeeman signatures, with longitudinal field strengths ranging from ~80 to ~200 G. The remaining targets show no detectable Stokes V signature, with upper limits on dipolar fields between 500 G and >5 kG. These results indicate that Class I and FS protostars can host large-scale magnetic fields, possibly weaker than in CTTS, supporting the idea that magnetic processes are already active during the main accretion phase and may influence star-disk interactions from the earliest stages.