Exchange Bias In Magnetic Thin Films/Multilayers And Its Thermal Stability

Reliability and service life of magnetic-electronic devices or magnetic-sensitive units based onthe magnetic thin films/multilayers are restricted by the thermal stability of the materials. Recently,many studies have been focused on the thermal stability of the exchange bias in the magnetic thinfilms/multilayers. In this thesis, the exchange bias field and its thermal stability ofIr₂₀Mn₈₀/Co₇5Fe₂₅/AlO_x/Co₇₅Fe₂₅magnetic tunnel junction （MTJ） multilayer have been investigatedby XRD, XRR, AFM, HRTEM, STEM and VSM. The effect of Ga⁺ion irradiation on the thermalstability of the magnetic tunnel junction multilayer has also been studied. In addition, the exchangebias and their thermal stability of the novel functional materials, e.g. Ni₅₀Mn₃₇In₁₃film andBiFeO₃/NiFe bilayer, have also been investigated.The thermal stability of the MTJ multilayer can be improved after annealing in a magnetic field.The exchange bias field (H_ex^p) in the pinned ferromagnetic layer increases and the relaxation time （τD）prolongs due to the enhancement of unidirectional anisotropy of antiferromagnetic layer in MTJ afterannealing. The relaxation effect appears in the pinned ferromagnetic layer while holding the films in anegative saturation field; that is, the hysteresis loop shifts to the positive field direction and theexchange bias field H_ex^pmonotonously decreases with the waiting time increasing. The exchange biasfield H_ex^pdecreases with the increase of the temperature T_m. While holding in the negative saturationfield at higher temperature the hysteresis loop shifts to the positive field direction and the exchangebias field H_ex^preduces more rapidly.The exchange bias field H_ex^pincrease markably after Ga⁺ion irradiation with a dose of1×1013ion·cm^-2; however, the H_ex^pdecrease obviously after Ga⁺ion irradiation with doses of6×1013ion·cm^-2and3×1014ion·cm^-2. With the irradiation dose increasing, the microstructure damage will play a majorrole, which leads to the mixing of interfacial atoms. Moreover, large doses of Ga⁺ion irradiation cansignificantly suppress the thermally activated magnetization reversal; and low dose of Ga⁺ionirradiation is conducive to thermally activated magnetization reversal. The exchange bias field H_ex^preduces monotonously with the time holding the film at a negative saturation field. The decrease rateof the exchange bias field H_ex^pis fast at the initial stages; however, it gradually slows downsubsequently.The BiFeO₃/NiFe bilayer sputtered in an electromagnetic field presents an in-plane uniaxialmagnetic anisotropy and show a significant exchange bias effect. The exchange bias field H_exin the BiFeO₃/NiFe bilayer does not show a visible training effect. The forward and recoil loop shiftstowards positive fields while holding the film in a negative saturation field. The H_exdecreasesmonotonously with the increase of the holding time （tsat）, whereas the H_cis almost the same. The H_exwill not alter significantly with the increase of the temperature T_m,which means that the H_exis notsensitive to the temperature, showing a good thermal stability. However, the H_creduce rapidly withthe increase of the temperature T_m. We believe that the good thermal stability may result from thecoupling between ferroelectric and antiferromagnetic moments in BiFeO₃.Exchange bias can be found in the field cooled Ni₅₀Mn₃₇In₁₃film at low temperature. Below thecharacteristic temperature Tf, the alloy presents a super spin glass （SSG） state when cooled in theabsence of a magnetic field; however, it will transform to the super ferromagnetic （SFM） structureafter magnetic field cooled. The SFM clusters in the SFM structure are coupled with theantiferromagnetic matrix, resulting in the exchange bias. The exchange bias field H_exin theNi₅₀Mn₃₇In₁₃film does not show a significant training effect. The forward and recoil loop shiftstowards positive fields while holding the film in a negative field. The H_exdecreases monotonouslywith the increase of the holding time, whereas coercivity is essentially the same. Although the H_exandH_creduce rapidly with the increase of the measuring temperature T_m, the decrease extent of the H_cisweaker than that of the H_ex. The thermal stability of the field cooled Ni₅₀Mn₃₇In₁₃film is poor,indicating that the thermal stability is more affected by the magnetic structure at the interface.