【作者】 周广宏；

【导师】 王寅岗；

【作者基本信息】 南京航空航天大学 ， 材料加工工程， 2012， 博士

【摘要】 基于磁性薄膜/多层膜的磁电子器件或磁敏感单元的可靠性和使用寿命受到材料热磁稳定性的制约。近年来，国内外众多学者针对磁性薄膜/多层膜中的交换偏置及其热磁稳定性开展了卓有成效的研究工作。本文通过XRD、XRR、AFM、HRTEM、STEM以及VSM等分析测试手段，系统研究了基于CoFe/IrMn的磁性隧道结多层膜中的交换偏置及其热磁稳定性，探讨了Ga⁺离子辐照对磁性隧道结中的交换偏置场热稳定性的影响。此外，还对新型功能材料BiFeO₃/NiFe双层膜和Ni₅₀Mn₃₇In₁₃薄膜中的交换偏置及其热磁稳定性作了初步探索。研究结果表明：磁场退火处理能够提高磁性隧道结多层膜的热稳定性。通过磁场退火处理可增大多层膜中反铁磁层的单轴各向异性，使得多层膜中的交换偏置场H_ex^p增大，弛豫时间τD变大。在负饱和场中等待的过程中，被钉扎层的磁滞回线向正场方向移动，交换偏置场H_ex^p随等待时间单调减小；随着测量温度T_m的升高，多层膜中的交换偏置场H_ex^p单调下降；在多个温度下进行负饱和场等待过程中，磁滞回线均向正场方向移动，交换偏置场H_ex^p不仅单调减小，且随温度升高减小的速度加快。经1×1013ion·cm^-2剂量Ga⁺离子辐照后，磁性隧道结多层膜中的交换偏置场H_ex^p明显变大，而经6×1013ion·cm^-2剂量和3×1014ion·cm^-2剂量的Ga⁺辐照后，H_ex^p则显著减小。随着辐照剂量的增大，磁性多层膜的结构损伤效应将起着主要的影响。另外，大剂量的Ga⁺离子辐照可明显抑制热激活磁化反转；而低剂量的Ga⁺离子辐照有利于热激活磁化反转。Ga⁺离子辐照后，随着在负饱和场中停留时间的增加，H_ex^p呈单调减小趋势，且在负饱和场中停留的初期，H_ex^p的减小速率较快，但随后逐渐减慢。磁场诱导生长的BiFeO₃/NiFe双层膜呈现出显著的面内磁单向各向异性，并产生交换偏置效应。BiFeO₃/NiFe双层膜的交换偏置场H_ex未表现出显著的磁练习效应。在负场等待过程中，BiFeO₃/NiFe双层膜磁滞回线的前支和后支曲线都随着在负饱和磁场中等待时间的增加向正场方向偏移。交换偏置场H_ex的大小随着等待时间的增加而减小，矫顽力基本不变。交换偏置场H_ex的大小随测量温度T_m的升高变化不明显，也就是说交换偏置场H_ex大小对温度不敏感，呈现出良好的热稳定性；但矫顽力H_c随T_m的升高而显著减小。良好的热稳定性可能来源于铁电性和反铁磁性间的耦合作用。场冷后的Ni₅₀Mn₃₇In₁₃薄膜在低温下表现出了一定的交换偏置现象。在特征温度Tf以下，合金处于超自旋玻璃SSG态，场冷后出现了从自旋玻璃态SSG向磁有序的超铁磁SFM结构的变化，SFM结构中的SFM团簇与反铁磁基体构成了铁磁/反铁磁耦合，从而导致了交换偏置的产生。Ni₅₀Mn₃₇In₁₃薄膜交换偏置场H_ex未表现出明显的磁练习效应。在负场等待过程中，Ni₅₀Mn₃₇In₁₃薄膜磁滞回线的前支和后支曲线都随着在负场中等待时间的增加向正场方向偏移，交换偏置场H_ex的大小随着等待时间的增加而减小，矫顽力基本不变。交换偏置场H_ex大小随测量温度T_m的升高急剧减小，矫顽力H_c大小随T_m的升高也呈快速减小趋势，但和交换偏置场H_ex相比，矫顽力H_c减小趋势则稍弱。场冷后的Ni₅₀Mn₃₇In₁₃薄膜中交换偏置场较差的热磁稳定性说明交换偏置的热稳定性更多地受界面磁结构的影响。更多还原

【Abstract】 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.更多还原