A method for producing MnZn ferrite comprising Fe, Mn and Zn as main components, and Ca, Si and Co, and at least one selected from the group consisting of Ta, Nb and Zr as sub-components, comprising a step of molding a raw material powder for the MnZn ferrite to obtain a green body, and a step of sintering the green body; the sintering step comprising a temperature-elevating step, a high-temperature-keeping step, and a cooling step; the cooling step including a slow cooling step of cooling in a temperature range of 1100 C. to 1250 C. at a cooling speed of 0 C./hour to 20 C./hour for 1 hours to 20 hours, and a cooling speed before and after the slow cooling step being higher than 20 C./hour; the MnZn ferrite having a volume resistivity of 8.5 .Math.m or more at room temperature, an average crystal grain size of 7 m to 15 m, and core loss of 420 kW/m.sup.3 or less between 23 C. and 140 C. at a frequency of 100 kHz and an exciting magnetic flux density of 200 mT.

A ferrite composition is provided containing Ba, Co, and Ir and having a Z-type hexaferrite phase and a Y-type hexaferrite phase. The ferrite composition has the formula Ba.sub.3Co.sub.(2-x)Ir.sub.xFe.sub.(24-2x)O.sub.41 where x=0.05-0.20. The composition has equal or substantially equal values of permeability and permittivity while retaining low magnetic and dielectric loss factors. The composition is suitable for ultrahigh frequency applications such as high frequency and microwave antennas.