Z-type hexagonal ferrites with strong magnetocrystalline anisotropy are considered for applications at microwave frequencies. In particular, Ba 3Co2Fe24O41 (Co2Z) is expected to exhibit a high permeability and low loss at frequencies >1 GHz. In the present work, we report a systematic study on the effect of lanthanum doping on Co2Z magnetic response. Ba3Co2La xFe24-xO41 (x=0, 0.1, 0.5, and 1.0) was fabricated from precursor powders made by a modified-Pechini route, followed by calcination, compaction and sintering. Samples with preferred grain orientation and porosity of 75-90% were obtained. The Co2Z magnetic properties were found to strongly depend on the microstructure. Likewise, a strong relation was found between the measured permeability and applied frequency. This is ascribed to frequency-dependant domain wall motion and spin rotation.