The origin of cosmic magnetic fields remains one of the most intriguing unsolved mysteries in astrophysics and cosmology. A key issue lies in the generation of initial magnetic fields, which critically determines the initial conditions for subsequent turbulent dynamo processes. Besides the well-known Biermann battery mechanism, the kinetic Weibel instability is also a significant candidate for generating primordial magnetic fields, particularly in weakly collisional or collisionless astrophysical environments such as the intracluster medium (ICM). In laboratory settings, the Weibel instability-induced magnetogenesis typically occurs in laser-produced interpenetrating or counterstreaming plasmas, due to the required anisotropy in the particle distribution function. 
Here, we experimentally demonstrate that the Weibel field can also be excited in freely expanding laser-produced plasmas, where the necessary anisotropy spontaneously arises from a steep temperature gradient. In our experiments, light-element materials like plastic (CH) were used to generate weakly collisional plasmas, while heavy-element materials such as copper (Cu) were employed to produce strongly collisional plasmas. By utilizing advanced three-dimensional synchronous proton radiography technology, we observed that the Weibel field could be excited in the weakly collisional CH plasmas but was suppressed in the strongly collisional Cu plasmas. This indicates that the temperature gradient-driven anisotropy and the associated Weibel instability are highly sensitive to Coulomb collisions. The measured field strength is on the order of 1-10 T, which, when scaled appropriately, could account for the self-generated magnetic fields with strengths up to 0.1 nG in the ICM—significantly exceeding the field strengths predicted by the traditional Biermann battery mechanism ( G). 
In summary, our findings reveal that even without interpenetrating or counterstreaming plasmas, a step temperature gradient can induce anisotropy and excite Weibel magnetic fields. This discovery has profound implications for understanding the long-standing mystery of cosmic magnetogenesis.

magnetic field,Weibel instability,temperature anisotropy