Magnetic random access memory having perpendicular composite reference layer

1. A spin transfer torque magnetic random access memory (STT-MRAM) device comprising a plurality of memory elements, each of said memory elements including a magnetic tunnel junction (MTJ) structure in between a non-magnetic seed layer and a non-magnetic cap layer, said MTJ structure comprising:

a magnetic free layer structure and a magnetic reference layer structure with an insulating tunnel junction layer interposed therebetween; and

a magnetic fixed layer separated from said magnetic reference layer structure by an anti-ferromagnetic coupling layer,

wherein said magnetic reference layer structure includes a first magnetic reference layer formed adjacent to said insulating tunnel junction layer and a second magnetic reference layer separated from said first magnetic reference layer by an intermediate magnetic reference layer, said first, second, and intermediate magnetic reference layers having a first invariable magnetization direction substantially perpendicular to layer planes thereof, said magnetic fixed layer having a second invariable magnetization direction that is substantially perpendicular to a layer plane thereof and is opposite to said first invariable magnetization direction.

2. The STT-MRAM device according to claim 1 , wherein said intermediate magnetic reference layer is made of an alloy of iron and tantalum or an alloy of cobalt, iron, boron, and tantalum.

3. The STT-MRAM device according to claim 1 , wherein said second magnetic reference layer is made of cobalt, iron, or an alloy of cobalt and iron.

4. The STT-MRAM device according to claim 1 , wherein said first magnetic reference layer is made of a material comprising cobalt, iron, and boron, and has a thickness in the range of about 0.8 nm to about 1.2 nm, said second magnetic reference layer is made of a material comprising cobalt, iron, and boron, and has a thickness in the range of about 0.6 nm to about 1.5 nm.

5. The STT-MRAM device according to claim 1 , wherein said magnetic free layer structure includes a first magnetic free layer formed adjacent to said insulating tunnel junction layer and a second magnetic free layer separated from said first magnetic free layer by a second non-magnetic perpendicular enhancement layer, said first and said second magnetic free layers having respectively a first and a second variable magnetization directions substantially perpendicular to layer planes thereof.

6. The STT-MRAM device according to claim 5 , wherein said second non-magnetic perpendicular enhancement layer is made of magnesium oxide.

7. The STT-MRAM device according to claim 5 , wherein each of said first magnetic reference layer and said first magnetic free layer is made of a material comprising cobalt, iron, and boron.

8. The STT-MRAM device according to claim 1 , wherein said magnetic free layer structure includes a magnetic free layer formed adjacent to said insulating tunnel junction layer and a magnetic dead layer separated from said magnetic free layer by a second non-magnetic perpendicular enhancement layer, said magnetic free layer having a variable magnetization direction substantially perpendicular to a layer plane thereof, said magnetic dead layer comprising at least one ferromagnetic element but having no net magnetic moment in absence of an external magnetic field.

9. The STT-MRAM device according to claim 8 , wherein said magnetic dead layer is made of a material comprising cobalt, iron, and boron, and has a thickness less than about 0.7 nm.

10. The STT-MRAM device according to claim 8 , wherein said second non-magnetic perpendicular enhancement layer is made of magnesium oxide.

11. The STT-MRAM device according to claim 8 , wherein each of said first magnetic reference layer and said first magnetic free layer is made of a material comprising cobalt, iron, and boron.