Influence of He-ion-irradiation on thin NiMn/NiFe exchange bias films

He ion irradiation has been demonstrated to be a versatile tool not only for interfacial mixing in the case of Co/Pt mulilayers [1] but also for low temperature L10-ordering of FePt alloys [2]. L10 ordered NiMn films are due to the high blocking temperature and if exchange coupled to a ferromagnetic material due to the large shift filed a promising candidate for magnetic sensor applications. However, for the creation of the L10 phase an annealing is required. The corresponding thermal budget should be as low as possible to impede the detrimental effect of fast Mn diffusion. In order to reduce the thermal budget for A1 - L10 phase transformation the potential of ion induced vacancy formation has been exploited.
Magnetron sputtered film stacks of 5nm Ta/50(15)nm NiMn/20nm Fe20Ni80 /5nm Ta deposited at Si/SiO2 substrates were subsequently irradiated with He+ ions (30 keV, 10^15-3*10^16cm-2, RT or 250°C). The transition from the paramagnetic NiMn phase (fcc) to the chemically ordered, antiferromagnetic tetragonal L10 phase during annealing (100-500°C, Vacuum) was studied by in-situ X-ray diffraction using synchrotron radiation. A small L10 fraction (< 15%) is already available after deposition. The transformation to a dominating L10 (LRO-parameter S > 0.5) ordered NiMn film takes place between 250-300°C irrespective of the irradiation. Annealing at elevated temperatures (T > 400°C) leads to a loss of L10 ordering due to a complete intermixing of the NiMn and the Permalloy films. A benefit of ion iradiation is the improvement of the crystallinity and the degree of <111> texture in the NiMn films already after low fluence (10^15 cm-2) implantation.

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