MRAM memory cell with a reference layer and method for fabricating
The invention relates to a method for fabricating a reference layer for MRAM memory cells and an MRAM memory cell equipped with a reference layer of this type. A reference layer of this type comprises two magnetically coupled layers having a different Curie temperature. When cooling from a temperature above the Curie temperature T C 1 of the first layer in an external magnetic field, the magnetization of the second layer is oriented by a second-order phase transition along the field direction of the external magnetic field. Upon further cooling below the Curie temperature T C 2 of the second layer, the latter is oriented antiparallel with respect to the first layer as a result of the antiferromagnetic coupling between the two layers.
1. A method for fabricating a reference layer for MRAM memory cells, comprising:
providing a layer system for the reference layer, the layer system exclusively consisting of:
a first layer of a material having a first Curie temperature, wherein the first layer has a saturation field strength and can be permanently magnetized by an external magnetic field;
a second magnetic layer of a material having a second Curie temperature which is significantly lower than the first Curie temperature, wherein the second layer can be magnetized by antiferromagnetic coupling with only the first layer; and
a very thin intermediate coupling layer between the first and second layers;
forming an artificial antiferromagnet exlusively from the first layer, the second layer and the intermediate coupling layer by the steps of:
generating an external magnetic field having a field strength;
cooling the layer system from a temperature above the first Curie temperature to below the first Curie temperature by action of the external magnetic field, the field strength of the external magnetic field being greater than the saturation field strength of the first layer, so that magnetization of the first layer is oriented by a second-order phase transition along the field direction of the external magnetic field; and
subsequently cooling the layer system below the second Curie temperature, magnetization of the second layer being oriented antiparallel with respect to the magnetization of the first layer on account of antiferromagnetic coupling between the first and second layers, said antiferromagnetic coupling being imparted by the intermediate coupling layer.
2. The fabrication method of claim 1 , further including setting the net magnetization of the layer system through the choice of a saturation flux, in particular of the layer cross section in each case of the first and second layers.
3. The fabrication method of claim 1 , further including setting the net magnetization of the layer system to zero by the respectively identical net magnetization of the first layer and the second layer.
4. The fabrication of claim 1 , further including setting the net magnetization of the layer system to be not equal to zero through selection of the second layer such that the layer cross section thereof is smaller than that of the first layer.
5. The fabrication of claim 1 , wherein subsequently cooling the layer system below the second Curie temperature further includes applying an external magnetic field, whose field direction is opposite to the magnetization direction of the first layer, upon passing through the second Curie temperature.
6. The fabrication method of claim 1 , wherein the material of the first layer is chosen from a group comprising (Co,Fe,Mn) 80 (Si,B) 20 ;(Co,Fe) 83 (Si,B) 17 ; and
Tb 20 Fe 40 Co 40 , and the material of the second layer is chosen from the group comprising (Co,Fe,Mo) 73 (Si,B) 27 ; (Ni,Fe) 78 (Si,B,C) 22 ; and Tb 20 Feg 80 .
7. The fabrication method of claim 1 , wherein the material of the intermediate coupling layer is chosen from the group comprising ruthenium, copper, and gold.