Production and magnetic characterization of exchange-coupled NiFe/IrMn and IrMn/NiFe films

Nowadays, several advanced magnetic and magnetoelectronic devices rely on interface exchange coupling (EC) between different magnetic phases (for example, magnetoresistive recording read heads, magnetoresistive random access memories, spring magnets, magnetic tunnel junctions, etc). The great challenge for optimizing the performances of such devices is to control the EC strength in order to tune the anisotropy, which represents the key technological parameter for any applications of magnetic materials. In fact, the exchange interaction across the ferromagnetic(FM)/antiferromagnetic(AF) interface gives rise to an additional source of anisotropy (exchange anisotropy), determining the onset of the exchange bias effect and producing significant changes in the coercivity (HC). In this context, we have studied EC in continuous films with AF/FM and FM/AF configurations, grown by dc-magnetron sputtering in Ar atmosphere. As AF phase we have used Ir25Mn75 (IrMn), whilst we have used Ni80Fe20 (Py) as FM. All the samples have been deposited on a 5 nm thick Cu underlayer to favor a fcc (111) orientation and have been capped with a Cu overlayer (5 nm). We have studied and compared the effects of EC on the macroscopic magnetic properties (exchange fied Hex, HC and squareness) of a number of samples grown in different values of a magnetic field applied along the film plane (Hdep= 0, 200, 800 Oe) and having different thickness of the AF layer tAF (3, 6 and 10 nm; the thickness of FM was 5 nm in all the samples). Hysteresis loops have been measured by magneto-optic Kerr effect (MOKE) and SQUID in the 5-300 K temperature range, after cooling in zero-field or in a field Hcool = 100 Oe from T = 400 K. At T = 300 K, the highest Hex (~150 Oe) has been measured in the Si/Cu/NiFe/IrMn/Cu sample with tAF = 10 nm (HC ~ 25 Oe); Hex goes to zero at TB ~ 390 K. In the same configuration, for tAF = 6 nm, TB ~ 370 K, (at T = 300 K, Hex ~ 100 Oe, HC ~ 60 Oe), whereas for tAF = 3 nm TB ~ 230 K (at T = 300 K, HC ~ 20 Oe). In all the samples, both Hex and HC increase with decreasing T especially for T < 100 K and, at T = 5 K, the highest Hex (~ 1000 Oe) is measured for tAF = 3 nm (in this sample, HC ~ 640 Oe). For T < 100 K, Hex and Hc values measured in the Si/Cu/NiFe/IrMn/Cu films are higher than those measured in the samples with the inverted configuration, whereas for T > 100 K, Hex and Hc assume similar values, irrespective of the configuration. This research work has been carried out in the framework of the project FIRB2010 “Tailoring the magnetic anisotropy of nanostructures for enhancing the magnetic stability of magnetoresistive devices” – NANOREST.