【作者】 罗杰；

【导师】 张悦；

【作者基本信息】 华中科技大学 ， 微电子学与固体电子学， 2020， 硕士

【摘要】 各向异性磁电阻（AMR）传感器凭借其在低频噪声、工艺、成本和磁场方向探测等方面的突出优势占有着广阔的应用市场。目前,以坡莫合金为磁敏感材料的AMR传感器的量程只有10 Oe,限制了其测量更大的磁场。另外,一般AMR电阻条需要具有单轴各向异性,具有四重对称磁各向异性等其它磁各向异性的材料即使具有较强的AMR,一般也无法用于磁传感。然而,基于多铁效应,有望通过外加电场实现四重对称磁各向异性向单轴各向异性的转变,并能够改善其磁场测试范围,噪声等特性,因而拓展可用于AMR传感器的材料范围。但四重对称磁各向异性材料的AMR及其电场调控目前还未见报道。基于此,本文基于磁性薄膜材料基本理论和实验技术,系统研究了具有四重对称磁各向异性薄膜材料的磁性和AMR的电场调控。理论上,本文基于Stoner-Wohlfarth（SW）模型研究了单轴各向异性和四重对称磁各向异性共存薄膜的AMR效应,其中单轴各向异性通过外加电场引入。本文系统分析了从单纯四重对称磁各向异性向单纯单轴各向异性转变,对薄膜磁滞回线,磁电阻-磁场强度关系,以及磁电阻-磁场方向关系等特性的影响。发现随着单轴各向异性的增强,薄膜从四重对称磁各向异性向单轴各向异性过渡,磁电阻-磁场强度曲线展宽,并且其磁电阻极值点向零磁场靠近。当单轴各向异性能不小于四重对称磁各向异性能时,薄膜的磁性与AMR特性与只具有单轴各向异性的情况高度类似。实验上,基于Ni₈₀Co₂₀/PMN-PT（110）多铁异质结,我们系统研究了电场调控Ni₈₀Co₂₀薄膜的AMR。在衬底被极化前,薄膜具有四重对称磁各向异性。经过强电场极化后形成较强的单轴各向异性,在衬底的[1–10]方向形成易磁化轴,在[001]方向则形成难磁化轴,并且该单轴各向异性具有非易失性,在撤掉极化衬底的电场后能长时间稳定保持。随着极化电场增强,薄膜的磁场响应范围从100 Oe拓宽到300 Oe左右。上述实验结果与理论分析结果相近。本文研究表明,对于具有四重对称磁各向异性的磁性薄膜,通过外加电场可以诱导产生稳定的强单轴各向异性,并且其AMR特性和单纯的单轴各向异性的AMR接近,因而为开发具有较宽磁场测试范围的AMR传感器奠定了基础。更多还原

【Abstract】 The anisotropic magnetoresistance（AMR）sensor has played an important role in the market of magnetic-field sensors due to its outstanding advantage in low-frequency noise,fabrication technique,cost and the capability for measuring the direction of magnetic field.At present,AMR sensors are always made from permalloy,and its measurement range is around 10 Oe,which limits its ability for measuring a larger magnetic field.On the other hand,a uniaxial magnetic anisotropy energy is necessary for the magnetic film used in the AMR sensor,and the ferromagnetic materials with other sorts of magnetic anisotropy,such as the four-fold symmetric magnetic anisotropy cannot be used even though they also have strong AMR.However,recent investigations reveal the possibility for converting a four-fold symmetric magnetic anisotropy energy into a uniaxial magnetic anisotropy one under an electric field based on the multiferroic heterostructure.This may expand the sorts of materials that can be exploited in AMR sensors with improved performance like measurement range and noise.However,the electric-field control of magnetism and AMR of magnetic materials with a four-fold symmetric magnetic anisotropic energy has not been reported yet.In both theory and experiment,this dissertation reported systematic work about electric-field control of magnetism and AMR of magnetic materials with a four-fold symmetric magnetic anisotropic energy.In theory,the AMR effect of the magnetic film with the coexistence of a uniaxial anisotropy energy and a four-fold symmetric anisotropy energy has been investigated based on the Stoner-Wohlfarth（SW）model.Here,the uniaxial anisotropy energy is introduced by an external electric field.The hysteresis loop,the relationship between magnetoresistance-magnetic field strength,and that between magnetoresistance-magnetic field direction have been analyzed systematically.Under the transition from a four-fold magnetic anisotropy energy to a uniaxial magnetic anisotropy energy,the response range of AMR to external magnetic field is widened,and the extreme point in the AMR-magnetic field strength curve approaches zero magnetic field.When the uniaxial anisotropy constant is not smaller than that of the four-fold symmetric magnetic anisotropy constant,the magnetism and AMR become very similar to that with pure uniaxial anisotropy energy.In experiment,the electric-field control of the AMR of the Ni₈₀Co₂₀film deposited on a PMN-PT（110）substrate has been investigated in detail.Before the substrate is polarized,the film exhibits typical four-fold symmetric magnetic anisotropy.After the substrate is polarized under a strong electric field,a strong uniaxial anisotropy field is generated,and the easy axis and hard axis are along the[1–10]and[001]direction,respectively.This uniaxial anisotropy energy is non-volatile and stable after the electric field is removed.With the increase of electric field strength,the range for measuring magnetic field increases from 100Oe to about 300 Oe.Finally,the experimental results are close to the results of theoretical analysis.The investigation reported in this dissertation shows that a stable strong uniaxial anisotropy energy can be induced under an external electric field in a magnetic film with an initial four-fold symmetrical magnetic anisotropy.This paves a way to develop AMR sensors with the capability for measuring strong magnetic field.更多还原