The maximization of Hex/HC ratio (Hex being the exchange bias field and HC the coercive field) and Hex thermal stability are key aspects to improve the performances of Thermally Assisted Magnetic Random Access Memory (TA-MRAM). These properties can be enhanced on a IrMn/Co based exchange biased system by adding a (Pt(or Pd)/Co)3 out-of-plane magnetic layer coupled with the IrMn layer.
Exchange bias coupling is exploited as reference layer for technological applications such as spin valve read heads and MRAM devices. In TA-MRAMs, this property is employed also in the storage layer, thus its optimization and control in temperature is a key aspect. In our study, we showed how the value of exchange bias in IrMn/Co exchange biased systems is increased if the IrMn layer is sandwiched between the in-plane magnetized Co layer and an additional layer with out-of-plane magnetization, i.e. (Pt/Co)3 or (Pd/Co)3 multilayers.
Figure 1: Schematic view of the proposed trilayer system, in which the antiferromagnetic layer AF is sandwiched between two ferromagnetic layers F1 and F2 of orthogonal magnetizations
Figure 2: Temperature dependence of the exchange bias field Hex (a-c) and of the coercive field HC (d-f) of bilayer (full squares) and trilayer (open squares) structures for different thicknesses of the IrMn antiferromagnetic layer
Together with an increased value of Hex at room temperature and a reduction of critical IrMn thickness, these trilayer structures show an increase of blocking temperature and concave Hex(T) curvature compared to the initial bilayer structure, in case of thin IrMn layer. These characteristics, very promising for TA-MRAM storage layer, are attributed to two coexisting effects. Firstly, a canting of Mn spins due to coupling of IrMn with the out-of-plane layer. The second proposed contribution is an indirect coupling of IrMn grains mediated through the
strongly coupled grains of the out-of-plane magnetized layer.
These characteristics have been further increased by adding few Angstroms of Cu layer at the IrMn/Co interface. This additional dusting layer reduces the spin frustration at the interface, enhancing the exchange coupling.
This work was partially funded by the European Commission through the Adv ERC Project HYMAGINE No. 246942. The work was part of the PhD thesis of Giovanni Vinai in collaboration with Crocus Technology.
J. Moritz, G. Vinai and B. Dieny, IEEE Magn. Lett. 3, 4000244 (2012)
G. Vinai, J. Moritz, S. Bandiera, I.L. Prejbeanu and B. Dieny, J. Phys. D: Appl. Phys. 46,
G. Vinai, J. Moritz, S. Bandiera, I. L. Prejbeanu, and B. Dieny, Applied Physics Letters 104, 162401 (2014)
Last update : 04/28 2015 (1109)