An antiferromagnetically exchange-coupled structure for use in various types of magnetic devices, such as magnetic tunnel junctions and spin-valve giant magnetoresistance recording heads, includes an antiferromagnetic layer formed of an alloy of osmium and manganese, wherein the osmium is present in the range of approximately 10 to 30 atomic %. The antiferromagnetic layer is deposited on a non-reactive underlayer, preferably one formed of a noble metal, such as platinum, palladium or alloys thereof. The antiferromagnetic material provides a strong exchange biasing for the ferromagnetic layer that is deposited on the antiferromagnetic layer. Iridium may be added to the osmium-manganese alloy, wherein the total of osmium and iridium is in the range of the approximately 10 to 30 atomic %, to increase the blocking temperature of the antiferromagnetic material. A template layer of permalloy (nickel-iron alloy) may be formed between the underlayer and the antiferromagnetic layer to improve the growth of the osmium-manganese alloy. The resulting antiferromagnetically exchange-coupled structure exhibits very high thermal stability, i.e., the magnetoresistance of magnetic tunnel junction devices is retained even during relatively high annealing process temperatures. This allows magnetic tunnel junction devices using the structure to be used as memory cells in magnetic random access memory arrays that are formed on substrates with electronic circuitry formed by conventional high-temperature CMOS processes and which require high temperature anneals of the completed memory chips.