2025NatAs...9..541I

Low-field magnetars have dipolar magnetic fields of 10¹²-10¹³ G, 10-100 times weaker than the values of magnetic-field strength B ≈ 10¹⁴-10¹⁵ G used to define classical magnetars, yet they produce similar X-ray bursts and outbursts. Using direct numerical simulations of magnetothermal evolution starting from a dynamo-generated magnetic field, we show that the low-field magnetars can be produced as a result of a Tayler-Spruit dynamo inside a proto-neutron star. We find that these simulations naturally explain key characteristics of low-field magnetars: weak ( ≲ 10¹³ G) dipolar magnetic fields, strong small-scale fields and magnetically induced crustal failures producing X-ray bursts. These findings suggest that the formation channel of low- B magnetars is distinct from that for classical magnetars, reflecting potential differences in proto-neutron-star dynamos.