Thermal imaging of 180o domain contrast in altermagnetic RuO2

Altermagnets host Néel-vector-dependent spin-splitting band structures, offering fertile ground for unconventional condensed-matter phenomena and spintronic applications. Experimental access to their properties, however, is challenging due to the obstacle for imaging 0o and 180o domains with opposite Néel vectors. Here, by exploiting the lock-in thermography to map the domain-dependent thermal contrast originating from the spin-dependent Peltier effect, we achieve real-space imaging of altermagnetic domain signatures of RuO2 in the scale of micrometer. Moreover, we find that magnetic field cooling can enable deterministic control of domain alignment and hence achieve the long-desired uniform 0o or 180o domain determined by the magnetic field direction. These switchable uniform domains restore the characteristic macroscopic magneto-responses of altermagnets, such as spin currents arising from the altermagnetic spin-dependent Seebeck effect. Our findings establish a robust thermal fingerprint of altermagnetism and extend the frontier of spin caloritronics into this emerging class of magnetic materials.