The magnetic properties and thermal stability Co-TM-Zr (TM = Nb, Ta, Mo, VV, and Ni) amorphous films prepared by rf diode sputtering are investigated. Amorphous films with a homogeneous structure and coercive force H-c of less than 20 Am-1 are obtained at an argon gas pressure of 0.3-1 Pa. The formation range of the amorphous films is broad in the systems containing Ta and Nb, whereas it is limited to the composition range greater than 4-5 at % of Zr in the systems of Mo, W, and Ni. The magnetostriction lambda(s) depends on the concentration ratio of Zr and the TM. Films with zero lambda(s) are obtained at concentration ratios C-Zr/C-TM ranging from 0.3 for Co-Nb-Zr films to 1.5-1.7 for Co-W-Zr films. The crystallization temperature T-x is highest in Co-Ta-Zr films and lowest in Co-Mo-Zr films when lambda(s) is zero and both films have the same saturation magnetic flux density B-s. The anisotropy field H-k is highest in Co-Ni-Zr films and lowest in Co-Nb-Zr films. These results indicate that Co-Ta-Zr and Co-Nb-Zr amorphous films are suitable for use as magnetic head materials because of the Co-Ta-Zr film’s high T-x and B-s, and the Co-Nb-Zr film’s small lambda(s) and low H-k.