钙钛矿锰氧化物中的巨磁阻抗效应研究
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摘要
可以用来制造高灵敏度传感器、大容量快速读写磁记录设备以及其他各种磁电子器件的具备灵敏磁响应特性的材料,一直以来是人们探索和追求的目标。自从霍尔效应以及非磁性金属的内禀磁电阻现象被发现以来,人们就开始试着将其应用到传感器等领域。然而,这种由于运动中的电子在外加磁场下受到洛仑兹力发生偏转而产生的磁电现象,其灵敏度一般比较低,在实际应用中,往往效果并不好。而随着时代的发展,传统半导体晶体管尺寸已经缩减至几个纳米,即将步入介观体系物理,受到一系列量子干涉效应的影响而使经典物理定律失效。因而开发新型的磁电效应以保持传感器、磁读写等设备领域的高速发展趋势,成为了时代的要求。
1988年,巨磁电阻效应在Fe/Cr多层膜中被发现。在外加磁场的作用下,Cr两侧的Fe层可以克服反铁磁性耦合,实现磁矩平行排列,从而大大降低了系统的电阻值。这一革命性的发现迅速给磁记录、磁读取以及磁传感器设备带来了飞跃。而将物质的电性与磁性相联系的磁电子学,也成了新兴的热门学科。
钙钛矿锰氧化物材料因其双交换作用而在居里温度以下具有铁磁性和金属导电性,是结构、自旋、磁性、电性的强关联体系。1993年,这一体系的庞磁电阻效应被发现,其在居里温度附近变化率甚至远远超过了巨磁电阻效应,因而引起了人们的广泛兴趣。然而,庞磁电阻效应依然具有一些局限性,较大变化率只出现在居里温度附近,且需要几个特斯拉的强外加磁场才能使其电阻率发生明显变化。这无疑会大大限制其应用前景。进入新世纪以来,这一体系的巨磁阻抗效应开始进入人们的视野中来。
巨磁阻抗效应最早在金属软磁材料中被发现,其原理为外加磁场导致的横向／圆周磁导率的变化通过趋肤深度来影响样品的交流输运特性。在几十个奥斯特的低外加磁场下,巨磁阻抗效应就能获得-50%左右的交流磁响应变化率。这无疑使其具有非常巨大的应用价值。与金属软磁材料类似,钙钛矿锰氧化物在居里温度以下也具有很高的饱和磁化强度和较低的矫顽力,这使得在这一体系中获得巨磁阻抗效应成为可能。2000年,胡季帆与秦宏伟首先在钙钛矿锰氧化物材料La0.67Ba0.33MnO3的多晶块状样品中发现了室温巨磁阻抗效应,其磁响应灵敏度远大于直流磁电阻效应。随后,其他钙钛矿锰氧化物材料中的巨磁阻抗效应陆续被报道。与金属软磁材料的巨磁阻抗效应大多在一百奥斯特以内就趋于饱和不同,钙钛矿锰氧化物体系的巨磁阻抗效应在几百个奥斯特的外加磁场下,可以持续地获得很高的磁响应灵敏度,这无疑使其具有独特的应用价值,值得人们去继续探索和研究。
由于磁性原理和导电特性的不同,钙钛矿锰氧化物材料巨磁阻抗效应的表现也与金属软磁材料中的有所不同。因此,有必要对钙钛矿锰氧化物材料巨磁阻抗效应的原理及现象进行深入的研究比较。与此同时,通过探索不同的测量手段,获得更大的巨磁阻抗效应变化率以提高其应用价值,也有着十分重要的意义。而为了应对磁传感器和磁读写记录装置的小型化要求,对钙钛矿锰氧化物纳米颗粒及薄膜材料的巨磁阻抗效应的研究也十分必要。
本论文对钙钛矿锰氧化物材料巨磁阻抗效应的研究主要包括以下结果：
1、室温下La0.65Ba0.35MnO3溶胶-凝胶块状样品的巨磁阻抗效应。
我们采用溶胶-凝胶法制备了La0.65Ba0.35MnO3块状样品,并分别用四探针法和绕线法进行了巨磁阻抗效应的测量。使用四探针法测量,可以在H=500Oe外加磁场下获得最大为△R/R0=53.9%的交流磁电阻、△X/X0=-36.0%的磁电抗以及AZ/Z0=-27.8%的巨磁阻抗效应值。经过与横向磁导率随外加磁场的变化规律的对比,我们认为钙钛矿锰氧化物的巨磁阻抗效应与横向磁导率在外加磁场作用下的变化密切相关,横向磁导率的变化会通过改变交流输运的趋肤深度来影响样品的阻抗的大小。而对同一La0.65Ba0.35MnO3溶胶-凝胶块状样品使用绕线法测量的结果表明,绕线法比起传统的四探针法可以获得更大的巨磁阻抗效应值,室温H=500Oe外加磁场下,使用绕线法获得的最大交流磁电阻可达△R/R0=-95.0%,最大磁电抗可达△X/X0=-80.7%,最大巨磁阻抗效应值可达△Z/Z0=-80.6%。这是目前为止在钙钛矿锰氧化物体系中所报道的最灵敏的巨磁阻抗效应之一。使用绕线法测量的交流阻抗与绕线样品沿长度方向的纵向磁导率有关。通过公式推导以及磁导率随外加磁场变化率的测量对比,我们认为使用绕线法获得的阻抗值与纵向磁导率之间成近似的线性关系,因此巨磁阻抗效应值与纵向磁导率的变化率之间也是近似的线性关系。这使得绕线法可以更充分的将外加磁场作用下磁导率的变化率转化为巨磁阻抗效应值。从而获得更加灵敏的交流磁响应。
2、样品尺度及测量参数对巨磁阻抗效应值大小的影响。
我们使用四探针法测量了不同厚度的La0.7Sr0.3MnO3溶胶-凝胶块状样品的巨磁阻抗效应。结果表明,具有更大厚度的样品,倾向于在更低的频率下表现出更明显的趋肤效应以及巨磁阻抗效应的最大值。而不同匝数下使用绕线法测量的La0.65Ba0.35MnO3样品的巨磁阻抗效应的结果表明,使用更大的绕线匝数或者绕线密度可以获得更高的巨磁阻抗效应值。这些研究可以帮助我们探索如何获得更灵敏的巨磁阻抗效应以提高其应用价值。
3、颗粒边界效应对巨磁阻抗效应的影响。
在实验中,我们广泛采用了溶胶-凝胶法制备钙钛矿锰氧化物粉末。这些样品的平均颗粒尺寸普遍小于1μm。其中La0.7Sr0.3MnO3以及La0.75Ba0.25MnO3溶胶-凝胶样品都在直流情况下观测到了典型的颗粒边界效应。然而在交流下,颗粒边界的存在对于La0.7Sr0.3MnO3样品的室温巨磁阻抗效应并没有明显影响。对于平均颗粒大小为1μm的La0.75Ba0.25MnO3样品,其在直流及交流的低频情况由于颗粒边界的影响而出现的双电阻峰,随着交流频率的增加,逐渐被因为趋肤效应而隆起的电阻峰所掩盖。高频下,颗粒边界效应在交流电阻上的影响并不明显。然而颗粒边界效应却在电抗X上表现明显。极低频下,样品中可以观测到极小的负电抗,与颗粒边界的电容性有关,而高频下,电抗变为正值,表现出电感特性。在f=20MHz的高频下,可以观测到因颗粒边界的存在而在低温隆起的X峰,证明颗粒边界区域中含有弱的铁磁金属性。因而我们可得出结论,在交流情况下,颗粒边界并不是简单的隧穿绝缘相,而是混合了弱金属导电性、弱铁磁性、绝缘性和顺磁性等复杂性质的综合体系。随着样品的颗粒大小、边界区域大小的不同,以及交流频率、温度等实验条件的不同,颗粒边界区域可能会表现出不同的性质。另外,由于颗粒边界效应的影响,不同温度下La0.75Ba0.25MnO3样品的巨磁阻抗效应的最大值出现在了低于居里温度的室温附近,这无疑为其应用前景提供了很高的价值。
4、钙钛矿锰氧化物薄膜的交流磁输运特性
我们使用激光脉冲沉积技术,分别在LaAlO3(100)衬底上生长了La0.67Sr0.33MnO3薄膜,在Si(100)衬底上制备了La0.75Sr0.25MnO3多晶薄膜。两种薄膜样品在室温下均具有铁磁金属相,并且观测到了本征的直流磁电阻现象。然而,在交流情况下,两者的导电性都受到了绝缘性衬底的影响,在高频下表现出了一定的电容导电特性。其中LaAlO3衬底上生长的La0.67Sr0.33MnO3薄膜由于厚度较小,受衬底的影响更深,在高频下失去了金属导电性,因而无法观测到巨磁阻抗现象,仅仅因为直流磁电阻效应的影响,观测到了交流阻抗随外加磁场的线性变化。而Si(100)衬底上制备的La0.75Sr0.25MnO3多晶薄膜,其颗粒边界的存在导致样品在极低频下就表现出了一定的电容导电特性。然而由于薄膜厚度较厚,以及衬底影响较小,La0.75Sr0.25sMnO3多晶薄膜上依然可以观察到明显的巨磁阻抗现象,H=500Oe外加磁场下,巨磁阻抗效应值的大小为△Z/Z0=-8.1%。这—结果比起样品的直流磁电阻效应要灵敏的多,但是依然比块状样品的巨磁阻抗效应值要小。通过对两个薄膜样品的交流磁输运特性的研究对比,我们认为,是否存在颗粒边界并非在钙钛矿锰氧化物薄膜样品上获得巨磁阻抗效应的决定性条件,薄膜厚度对巨磁阻抗效应是否存在有着决定性的影响。为了实现钙钛矿锰氧化物巨磁阻抗效应在小型化传感器或者磁记录设备领域的应用,微米级厚度的薄膜是比较可行的方案。
High sensitive magnetic sensors and large magnetic memory devices require materials with sensitive magnetic response properties. Such materials have been explored by researchers ever since the discovery of the Hall Effect and the intrinsic magnetoresistance effects, which are based on the Lorentz force. The Hall Effect materials have been widely applied on the magnetic sensors, such as Hall Effect displacement transducer, Hall Effect compass and Hall Effect tachometric transducer. However, the sensitivity of the Hall Effect materials is still too low to perform in some applications. Meanwhile, the dimension of semi-conductor devices has been reduced to several nanometers, a series of quantum interference effects in the mesoscopic physics could disable the classical physics laws. Therefore, it is necessary to develop new magnetic induction materials.
The Giant Magnetoresistance (GMR) effect was discovered on the Fe/Cr multilayer films. With the application of magnetic fields, the Fe layers could overcome the anti-ferromagnetic coupling, and parallel the magnetic moments, thus reduce the resistance of the multilayer film. Such GMR effects have been applied in tremendous storage capacities magnetic memory devices.
Perovskite manganites such as La1-xAxMnO3(A=Ca, Sr, Ba, Pb) could exhibit ferromagnetic because of the double-exchange interaction, There are complex interplay among charge, spin, and lattice within the manganites, which could lead to a rich spectrum of unique physical properties, such as the colossal magnetoresistance (CMR) effect discovered in1993. The CMR effect could exhibit even large field induced change of resistance than the GMR effect near Curie temperature (TC). However, the CMR effect is only very strong near Tc under large applied magnetic fields of several Teslas, but usually very weak at room temperature under low fields, which limits the potential applications. In2000, Hu and Qin demonbstrate that the Giant magnetoimpedance effect at radio frequencies on the bulk manganites and caught much attention due to the sensitive response to low magnetic fields. The GMI effects was firstly found in soft magnetic metallic materials, such as amorphous Co-Fe-Si-B or Fe-based nanocrystalline wires and ribbons, and mainly connected with the field induced change of transverse permeability via the penetration depth. Similar to the metallic materials, the manganite materials have high Ms and low Hc, which make it possible to achieve GMI effect on manganite materials. In2000, the GMI effect on manganites was reported on La0.67Ba0.33Mn03polycrystalline bulk samples. Later on, more reports about the GMI effect on manganites were published. For the manganites, the GMI effect could provide much more sensitivity to the applied magnetic fields than the CMR effect. Compared with the metallic materials, the manganite materials could approach the saturation of GMI effect under larger fields. These properties make the GMI effect on manganites have a potential application on some magnetic sensors.
As the origin of the magnetism and transport properties for the manganites and metallic materials are quite different, it is necessary to investigate the study the correlation between the GMI effect and field induced permeability change for the manganites. Meanwhile, to develop new measurement methods which could obtain larger GMI effect, could also improve the application prospect for manganites. Miniature magnetic memory devices require small sample size, where the manganite film samples should also be considered.
In this dissertation, we investigated the GMI effects for the manganites, and the following results are included:
1. GMI effect for sol-gel Lao.65Ba0.35MnO3at room temperature.
We prepared the La0.65Ba0.35MnO3manganite samples by sol-gel method. GMI measurements were carried out with both four-terminal method and coil method. With the four-terminal method, the maximum of ac magnetoresistance△R/R0=-53.9%, magnetoreactance△X/X0=-36.0%and GMI effect△Z/Z0=-27.8%could be obtained under H=500Oe. By comparing the GMI effect and field induced permeability change ratio, we conformed that the GMI effect on manganites are connected to there field induced change of transverse permeability via the penetration depth. The results measured with the coil method on the same sol-gel La0.65Bao.35MnO3manganite indicated that with the coil method we could obtain much larger GMI effect on the same sample. Under H=500Oe, the maximum of ac magnetoresistance△R/R0=-95.0%, magnetoreactance△X/X0=-80.7%and GMI effect△Z/Z0=-80.6%could be obtained, which are the largest magnetoimpedance effect among all GMI reports for manganites at radio frequency range so far. When measuring with the coil method, it is the longitudinal permeability that is connected with the GMI effect. By studying the classical electrodynamics equations, and comparing the GMI results and field induced longitudinal permeability change ratio, we can infer that the there is approximative linear relation between the GMI results and the longitudinal permeability change ratio. Thus larger GMI effect could be obtained with the coil method through the longitudinal permeability change.
2. Influence of sample thickness and measurement geometry on GMI effect.
We measured the GMI effects of sol-gel Lao.7Sro.3Mn03manganite samples with different thickness. The results indicated that the thicker sample prefers to have a stronger skin effect and GMI effect at low frequencies. Meanwhile, the results obtained by coil method with different number of turns shows that larger GMI effect could be gained with more turns. Such results could help us seeking larger GMI effects.
3. The effects of grain boundaries on GMI effect.
For the sol-gel La0.7Sr0.3MnO3manganite with average grain size of about100nm, a classic low field magnetoresistance effect was observed at room temperature. However, under ac case, there is no obviously effect of the grain boundaries on the GMI effect at room temperature. As for the La0.65Bao.35MnO3sol-gel manganite with average grain size of about1um, the existence of grain boundaries could bring a secondary resistance-temperature peak at low temperature below Tc under dc case and low frequency range of ac case. With increasing ac frequency, skin effect gradually enhances, the peak associated with the grain boundaries are less pronounced. The effects of grain boundaries on ac resistance are not obviously. At a very low frequency of100Hz, we observed a very small negative reactance of about-10mcΩ, which is connected with the capacitance of insulator state of the grain boundaries. At high frequencies, an X peak is pronounced at200K, which is connected with the inductance behavior of the grain boundaries. Thus we can conclude that the grain boundaries should not be simply considered as tunneling insulator state, but a mixture of weak ferromagnetic metal and paramagnetic insulating state, and exhibits different behaviors as the frequency or temperature changes. Meanwhile, with the affect of grain boundaries, the maximum of GMI effect take place near room temperature below Tc, which brings a special advantage in potential applications.
4. GMI effects on manganite films.
We prepared the La0.67Sr0.33MnO3film on LaAlO3(100) substrate and La0.75Sr0.25MnO3polycrystalline film on Si (100) substrate using by PLD, respectively. Both of the films exhibit ferromagnetic metal at room temperature, and intrinsic CMR effects. Under ac case, both the films are affected by the capacitive substrate. The La0.67Sr0.33MnO3film with thinner thickness could not exhibit the GMI effect due to the stronger affect from the substrate. However, GMI effect with△Z/Z0=-8.1%could be observed on the thicker La0.75Sr0.25MnO3polycrystalline film with less effect from the substrate in spite of the grain boundary effect. It can be concluded that the existence of grain boundaries is not a decisive factor for gaining GMI effect on manganite films. To fit the requirement of applications on Miniature magnetic memory devices, thicker film with thickness larger than1μm should be considered.
1988年,巨磁电阻效应在Fe/Cr多层膜中被发现。在外加磁场的作用下,Cr两侧的Fe层可以克服反铁磁性耦合,实现磁矩平行排列,从而大大降低了系统的电阻值。这一革命性的发现迅速给磁记录、磁读取以及磁传感器设备带来了飞跃。而将物质的电性与磁性相联系的磁电子学,也成了新兴的热门学科。
钙钛矿锰氧化物材料因其双交换作用而在居里温度以下具有铁磁性和金属导电性,是结构、自旋、磁性、电性的强关联体系。1993年,这一体系的庞磁电阻效应被发现,其在居里温度附近变化率甚至远远超过了巨磁电阻效应,因而引起了人们的广泛兴趣。然而,庞磁电阻效应依然具有一些局限性,较大变化率只出现在居里温度附近,且需要几个特斯拉的强外加磁场才能使其电阻率发生明显变化。这无疑会大大限制其应用前景。进入新世纪以来,这一体系的巨磁阻抗效应开始进入人们的视野中来。
巨磁阻抗效应最早在金属软磁材料中被发现,其原理为外加磁场导致的横向／圆周磁导率的变化通过趋肤深度来影响样品的交流输运特性。在几十个奥斯特的低外加磁场下,巨磁阻抗效应就能获得-50%左右的交流磁响应变化率。这无疑使其具有非常巨大的应用价值。与金属软磁材料类似,钙钛矿锰氧化物在居里温度以下也具有很高的饱和磁化强度和较低的矫顽力,这使得在这一体系中获得巨磁阻抗效应成为可能。2000年,胡季帆与秦宏伟首先在钙钛矿锰氧化物材料La0.67Ba0.33MnO3的多晶块状样品中发现了室温巨磁阻抗效应,其磁响应灵敏度远大于直流磁电阻效应。随后,其他钙钛矿锰氧化物材料中的巨磁阻抗效应陆续被报道。与金属软磁材料的巨磁阻抗效应大多在一百奥斯特以内就趋于饱和不同,钙钛矿锰氧化物体系的巨磁阻抗效应在几百个奥斯特的外加磁场下,可以持续地获得很高的磁响应灵敏度,这无疑使其具有独特的应用价值,值得人们去继续探索和研究。
由于磁性原理和导电特性的不同,钙钛矿锰氧化物材料巨磁阻抗效应的表现也与金属软磁材料中的有所不同。因此,有必要对钙钛矿锰氧化物材料巨磁阻抗效应的原理及现象进行深入的研究比较。与此同时,通过探索不同的测量手段,获得更大的巨磁阻抗效应变化率以提高其应用价值,也有着十分重要的意义。而为了应对磁传感器和磁读写记录装置的小型化要求,对钙钛矿锰氧化物纳米颗粒及薄膜材料的巨磁阻抗效应的研究也十分必要。
本论文对钙钛矿锰氧化物材料巨磁阻抗效应的研究主要包括以下结果：
1、室温下La0.65Ba0.35MnO3溶胶-凝胶块状样品的巨磁阻抗效应。
我们采用溶胶-凝胶法制备了La0.65Ba0.35MnO3块状样品,并分别用四探针法和绕线法进行了巨磁阻抗效应的测量。使用四探针法测量,可以在H=500Oe外加磁场下获得最大为△R/R0=53.9%的交流磁电阻、△X/X0=-36.0%的磁电抗以及AZ/Z0=-27.8%的巨磁阻抗效应值。经过与横向磁导率随外加磁场的变化规律的对比,我们认为钙钛矿锰氧化物的巨磁阻抗效应与横向磁导率在外加磁场作用下的变化密切相关,横向磁导率的变化会通过改变交流输运的趋肤深度来影响样品的阻抗的大小。而对同一La0.65Ba0.35MnO3溶胶-凝胶块状样品使用绕线法测量的结果表明,绕线法比起传统的四探针法可以获得更大的巨磁阻抗效应值,室温H=500Oe外加磁场下,使用绕线法获得的最大交流磁电阻可达△R/R0=-95.0%,最大磁电抗可达△X/X0=-80.7%,最大巨磁阻抗效应值可达△Z/Z0=-80.6%。这是目前为止在钙钛矿锰氧化物体系中所报道的最灵敏的巨磁阻抗效应之一。使用绕线法测量的交流阻抗与绕线样品沿长度方向的纵向磁导率有关。通过公式推导以及磁导率随外加磁场变化率的测量对比,我们认为使用绕线法获得的阻抗值与纵向磁导率之间成近似的线性关系,因此巨磁阻抗效应值与纵向磁导率的变化率之间也是近似的线性关系。这使得绕线法可以更充分的将外加磁场作用下磁导率的变化率转化为巨磁阻抗效应值。从而获得更加灵敏的交流磁响应。
2、样品尺度及测量参数对巨磁阻抗效应值大小的影响。
我们使用四探针法测量了不同厚度的La0.7Sr0.3MnO3溶胶-凝胶块状样品的巨磁阻抗效应。结果表明,具有更大厚度的样品,倾向于在更低的频率下表现出更明显的趋肤效应以及巨磁阻抗效应的最大值。而不同匝数下使用绕线法测量的La0.65Ba0.35MnO3样品的巨磁阻抗效应的结果表明,使用更大的绕线匝数或者绕线密度可以获得更高的巨磁阻抗效应值。这些研究可以帮助我们探索如何获得更灵敏的巨磁阻抗效应以提高其应用价值。
3、颗粒边界效应对巨磁阻抗效应的影响。
在实验中,我们广泛采用了溶胶-凝胶法制备钙钛矿锰氧化物粉末。这些样品的平均颗粒尺寸普遍小于1μm。其中La0.7Sr0.3MnO3以及La0.75Ba0.25MnO3溶胶-凝胶样品都在直流情况下观测到了典型的颗粒边界效应。然而在交流下,颗粒边界的存在对于La0.7Sr0.3MnO3样品的室温巨磁阻抗效应并没有明显影响。对于平均颗粒大小为1μm的La0.75Ba0.25MnO3样品,其在直流及交流的低频情况由于颗粒边界的影响而出现的双电阻峰,随着交流频率的增加,逐渐被因为趋肤效应而隆起的电阻峰所掩盖。高频下,颗粒边界效应在交流电阻上的影响并不明显。然而颗粒边界效应却在电抗X上表现明显。极低频下,样品中可以观测到极小的负电抗,与颗粒边界的电容性有关,而高频下,电抗变为正值,表现出电感特性。在f=20MHz的高频下,可以观测到因颗粒边界的存在而在低温隆起的X峰,证明颗粒边界区域中含有弱的铁磁金属性。因而我们可得出结论,在交流情况下,颗粒边界并不是简单的隧穿绝缘相,而是混合了弱金属导电性、弱铁磁性、绝缘性和顺磁性等复杂性质的综合体系。随着样品的颗粒大小、边界区域大小的不同,以及交流频率、温度等实验条件的不同,颗粒边界区域可能会表现出不同的性质。另外,由于颗粒边界效应的影响,不同温度下La0.75Ba0.25MnO3样品的巨磁阻抗效应的最大值出现在了低于居里温度的室温附近,这无疑为其应用前景提供了很高的价值。
4、钙钛矿锰氧化物薄膜的交流磁输运特性
我们使用激光脉冲沉积技术,分别在LaAlO3(100)衬底上生长了La0.67Sr0.33MnO3薄膜,在Si(100)衬底上制备了La0.75Sr0.25MnO3多晶薄膜。两种薄膜样品在室温下均具有铁磁金属相,并且观测到了本征的直流磁电阻现象。然而,在交流情况下,两者的导电性都受到了绝缘性衬底的影响,在高频下表现出了一定的电容导电特性。其中LaAlO3衬底上生长的La0.67Sr0.33MnO3薄膜由于厚度较小,受衬底的影响更深,在高频下失去了金属导电性,因而无法观测到巨磁阻抗现象,仅仅因为直流磁电阻效应的影响,观测到了交流阻抗随外加磁场的线性变化。而Si(100)衬底上制备的La0.75Sr0.25MnO3多晶薄膜,其颗粒边界的存在导致样品在极低频下就表现出了一定的电容导电特性。然而由于薄膜厚度较厚,以及衬底影响较小,La0.75Sr0.25sMnO3多晶薄膜上依然可以观察到明显的巨磁阻抗现象,H=500Oe外加磁场下,巨磁阻抗效应值的大小为△Z/Z0=-8.1%。这—结果比起样品的直流磁电阻效应要灵敏的多,但是依然比块状样品的巨磁阻抗效应值要小。通过对两个薄膜样品的交流磁输运特性的研究对比,我们认为,是否存在颗粒边界并非在钙钛矿锰氧化物薄膜样品上获得巨磁阻抗效应的决定性条件,薄膜厚度对巨磁阻抗效应是否存在有着决定性的影响。为了实现钙钛矿锰氧化物巨磁阻抗效应在小型化传感器或者磁记录设备领域的应用,微米级厚度的薄膜是比较可行的方案。
High sensitive magnetic sensors and large magnetic memory devices require materials with sensitive magnetic response properties. Such materials have been explored by researchers ever since the discovery of the Hall Effect and the intrinsic magnetoresistance effects, which are based on the Lorentz force. The Hall Effect materials have been widely applied on the magnetic sensors, such as Hall Effect displacement transducer, Hall Effect compass and Hall Effect tachometric transducer. However, the sensitivity of the Hall Effect materials is still too low to perform in some applications. Meanwhile, the dimension of semi-conductor devices has been reduced to several nanometers, a series of quantum interference effects in the mesoscopic physics could disable the classical physics laws. Therefore, it is necessary to develop new magnetic induction materials.
The Giant Magnetoresistance (GMR) effect was discovered on the Fe/Cr multilayer films. With the application of magnetic fields, the Fe layers could overcome the anti-ferromagnetic coupling, and parallel the magnetic moments, thus reduce the resistance of the multilayer film. Such GMR effects have been applied in tremendous storage capacities magnetic memory devices.
Perovskite manganites such as La1-xAxMnO3(A=Ca, Sr, Ba, Pb) could exhibit ferromagnetic because of the double-exchange interaction, There are complex interplay among charge, spin, and lattice within the manganites, which could lead to a rich spectrum of unique physical properties, such as the colossal magnetoresistance (CMR) effect discovered in1993. The CMR effect could exhibit even large field induced change of resistance than the GMR effect near Curie temperature (TC). However, the CMR effect is only very strong near Tc under large applied magnetic fields of several Teslas, but usually very weak at room temperature under low fields, which limits the potential applications. In2000, Hu and Qin demonbstrate that the Giant magnetoimpedance effect at radio frequencies on the bulk manganites and caught much attention due to the sensitive response to low magnetic fields. The GMI effects was firstly found in soft magnetic metallic materials, such as amorphous Co-Fe-Si-B or Fe-based nanocrystalline wires and ribbons, and mainly connected with the field induced change of transverse permeability via the penetration depth. Similar to the metallic materials, the manganite materials have high Ms and low Hc, which make it possible to achieve GMI effect on manganite materials. In2000, the GMI effect on manganites was reported on La0.67Ba0.33Mn03polycrystalline bulk samples. Later on, more reports about the GMI effect on manganites were published. For the manganites, the GMI effect could provide much more sensitivity to the applied magnetic fields than the CMR effect. Compared with the metallic materials, the manganite materials could approach the saturation of GMI effect under larger fields. These properties make the GMI effect on manganites have a potential application on some magnetic sensors.
As the origin of the magnetism and transport properties for the manganites and metallic materials are quite different, it is necessary to investigate the study the correlation between the GMI effect and field induced permeability change for the manganites. Meanwhile, to develop new measurement methods which could obtain larger GMI effect, could also improve the application prospect for manganites. Miniature magnetic memory devices require small sample size, where the manganite film samples should also be considered.
In this dissertation, we investigated the GMI effects for the manganites, and the following results are included:
1. GMI effect for sol-gel Lao.65Ba0.35MnO3at room temperature.
We prepared the La0.65Ba0.35MnO3manganite samples by sol-gel method. GMI measurements were carried out with both four-terminal method and coil method. With the four-terminal method, the maximum of ac magnetoresistance△R/R0=-53.9%, magnetoreactance△X/X0=-36.0%and GMI effect△Z/Z0=-27.8%could be obtained under H=500Oe. By comparing the GMI effect and field induced permeability change ratio, we conformed that the GMI effect on manganites are connected to there field induced change of transverse permeability via the penetration depth. The results measured with the coil method on the same sol-gel La0.65Bao.35MnO3manganite indicated that with the coil method we could obtain much larger GMI effect on the same sample. Under H=500Oe, the maximum of ac magnetoresistance△R/R0=-95.0%, magnetoreactance△X/X0=-80.7%and GMI effect△Z/Z0=-80.6%could be obtained, which are the largest magnetoimpedance effect among all GMI reports for manganites at radio frequency range so far. When measuring with the coil method, it is the longitudinal permeability that is connected with the GMI effect. By studying the classical electrodynamics equations, and comparing the GMI results and field induced longitudinal permeability change ratio, we can infer that the there is approximative linear relation between the GMI results and the longitudinal permeability change ratio. Thus larger GMI effect could be obtained with the coil method through the longitudinal permeability change.
2. Influence of sample thickness and measurement geometry on GMI effect.
We measured the GMI effects of sol-gel Lao.7Sro.3Mn03manganite samples with different thickness. The results indicated that the thicker sample prefers to have a stronger skin effect and GMI effect at low frequencies. Meanwhile, the results obtained by coil method with different number of turns shows that larger GMI effect could be gained with more turns. Such results could help us seeking larger GMI effects.
3. The effects of grain boundaries on GMI effect.
For the sol-gel La0.7Sr0.3MnO3manganite with average grain size of about100nm, a classic low field magnetoresistance effect was observed at room temperature. However, under ac case, there is no obviously effect of the grain boundaries on the GMI effect at room temperature. As for the La0.65Bao.35MnO3sol-gel manganite with average grain size of about1um, the existence of grain boundaries could bring a secondary resistance-temperature peak at low temperature below Tc under dc case and low frequency range of ac case. With increasing ac frequency, skin effect gradually enhances, the peak associated with the grain boundaries are less pronounced. The effects of grain boundaries on ac resistance are not obviously. At a very low frequency of100Hz, we observed a very small negative reactance of about-10mcΩ, which is connected with the capacitance of insulator state of the grain boundaries. At high frequencies, an X peak is pronounced at200K, which is connected with the inductance behavior of the grain boundaries. Thus we can conclude that the grain boundaries should not be simply considered as tunneling insulator state, but a mixture of weak ferromagnetic metal and paramagnetic insulating state, and exhibits different behaviors as the frequency or temperature changes. Meanwhile, with the affect of grain boundaries, the maximum of GMI effect take place near room temperature below Tc, which brings a special advantage in potential applications.
4. GMI effects on manganite films.
We prepared the La0.67Sr0.33MnO3film on LaAlO3(100) substrate and La0.75Sr0.25MnO3polycrystalline film on Si (100) substrate using by PLD, respectively. Both of the films exhibit ferromagnetic metal at room temperature, and intrinsic CMR effects. Under ac case, both the films are affected by the capacitive substrate. The La0.67Sr0.33MnO3film with thinner thickness could not exhibit the GMI effect due to the stronger affect from the substrate. However, GMI effect with△Z/Z0=-8.1%could be observed on the thicker La0.75Sr0.25MnO3polycrystalline film with less effect from the substrate in spite of the grain boundary effect. It can be concluded that the existence of grain boundaries is not a decisive factor for gaining GMI effect on manganite films. To fit the requirement of applications on Miniature magnetic memory devices, thicker film with thickness larger than1μm should be considered.
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