磁性纳米颗粒和薄膜中的交换偏置效应

IMR OpenIR

	磁性纳米颗粒和薄膜中的交换偏置效应
	郭盛
学位类型	博士
导师	刘伟 ; 张志东
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料物理与化学
关键词	非传统交换偏置纳米复合物锻炼效应界面效应反铁磁磁结构 Unconvential Exchange Bias Nanocomposite Training Effect Interfacial Effect Antiferromagnetic Structure
摘要	" 本文采用机械合金化及真空退火技术制备了CoCr2O4/Cr2O3、Ni/NiO纳米复合材料。利用磁控溅射方法制备了NiO基、MnO基铁磁/反铁磁的双层、三层磁性纳米薄膜。通过X射线衍射等手段探测样品的物相组成和晶粒大小。用超导量子干涉仪 (SQUID) 对测量样品的磁性能进行了相关的研究。在CoCr2O4（亚铁磁）/Cr2O3（反铁磁）的非传统交换偏置体系中，发现该纳米复合粉末样品经过场冷至低温（5 K）下仍然能获得高达2.5 kOe的负向交换偏置场。同时，亚铁磁CoCr2O4由于反铁磁Cr2O3的强力耦合，其磁性矫顽力显著提高到通常的硬磁材料的矫顽力水平。基于对交换偏置效应来源的重新理解，我们提出了非传统交换偏置产生的两种可能机制，即冷却场磁场通过临近顺磁磁矩对反铁磁表面磁矩结构排列的间接诱导机制和冷却场磁场对反铁磁类磁畴结构排列的直接诱导机制。在Ni（铁磁）/NiO（反铁磁）的纳米复合体系中，我们发现，在５Ｋ的低温下能获得2.2 kOe的负交换偏置场。发现随着低温下锻炼效应的进行，在同一样品中经过不同锻炼次数得到的交换偏置场大小和此时冻结自旋多少存在一个线形关系，但锻炼效应的初次是例外的。这不仅暗示了交换偏置效应来源的本质问题，而且解释了锻炼效应可能存在的两种机制。我们就这两种可能的微观机制进行了讨论和分析。我们发现当铁磁/反铁磁界面处紧邻的铁磁薄膜厚度小于４～５nm时，产生交换偏置的界面能尚处在一个逐步建立的过程中。另外，我们还发现随着铁磁层的不同，相同反铁磁得到不同的交换偏置冻结温度，而且强耦合与弱耦合的交换偏置体系，其场冷-零场冷磁热曲线表现出明显的差异性，这种差异性间接地反映出交换偏置效应带来的单向各向异性和单轴各向异性来源机理的不同。通过磁控溅射的方法，我们制备了一系列硬磁／反铁磁／软磁的三层磁性薄膜结构。在反铁磁奈尔/冻结温度以上，先施加一个足够大的正向/反向饱和磁场，然后采用正向的小磁场作为冷却场，从而实现了上下两层铁磁性薄膜磁矩方向呈平行或反平行排列的磁冷却结构，考察软磁层在这两种冷却场模式下在低温下的交换偏置现象。实验结果表明，在反铁磁中间层比较厚的时候，两层铁磁层磁矩的平行、反平行这两种磁冷却结构并没能对软铁磁层的交换偏置造成任何明显的差异。当反铁磁的厚度小于6 nm的时候，两个铁磁/反铁磁界面处的交换耦合效应将是会相互影响的。确定了反铁磁界面磁结构对于反铁磁体内磁畴的影响穿透深度大概2 ~ 3 nm以内。"
其他摘要	Different kinds of nanocomposites with exchange bias effect were prepared by mechanical alloying. The NiO-based and MnO-based magnetic nanocomposite bilayer or trilayer films were prepared by magnetron sputtering. X-ray diffraction (XRD) of the samples was conducted using Cu--K radiation and the average grain size was determined by means of the Scherrer formula. The magnetic properties of these magnetic samples were investigated by a superconducting quantum interference device (SQUID). In CoCr2O4 / Cr2O3 nanocomposite materials, although the Curie temperature (100 K) of CoCr2O4 ferrimagnetic is far below the Néel temperature (307 K) of antiferromagnet Cr2O3, a large negative exchange bias field of 2.5 kOe is observed at low temperature (5 K). Meanwhile, due to a strong coupling with antiferromagnetic Cr2O3, the magnetic coercive field of the ferrimagnetic CoCr2O4 is significantly enhanced to the value as high as that of a hard magnet. We have proposed two possible exchange-bias machnisms to interpret the unconventional exchange bias. Cooling field may play a decisive role in the creation of the interfacial spin configuration for the unconventional exchange bias, not only by exchange interaction between the induced magnetization of a polarized paramagnet and interfacial spins of an antiferromgnet but also by Zeeman interaction between the domain-state surplus magnetization and the external magnetic field. In the Ni / NiO nanocomposite system, it is founded that a large negative exchange bias field (2.2 kOe) is observed at 5K, and the coercivity of ferromagnetic Ni nanoparticles is greatly improved due to exchange coupling with antiferromagnetic NiO. In such a simple ferromagnetic / antiferromagnetic system with strong magnetic signal, a linear relationship between the value of the exchange bias field and the number of pinning spin after series of exchange bias training upon a specific sample is observed, which can be as a clue to reveal the origin of the exchange bias effect. It is founded that when the thickness of interfacial ferromagnetic thin film is less than 4 ~ 5 nm, the induced exchange-bias interfacial energy is still in the process of the gradual establishment which is consistent with previous study that the exchange-bias effect is a long-range effect. In addition, the blocking temperatures and thermal magnetization behaviors of samples have been changed by different ferromagnetic films. The series of trilayer with (hard magnet)/antiferromagnet/(soft magnet) were prepared by magnetron sputtering. It is shown that the parallel or anti-parallel structure of the two magnetic cooling structures can not result in any significant difference in the exchange bias effect of relatively soft ferromagnetic layer when the thickness of antiferromagnet is thick enough. The interfacial uncompensated spins at the antiferromagnetic / ferromagnetic interface can influent on the antiferromagnetic domain-state configuration near to the interface of antiferromagnet of about 2~3 nm.
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64446
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	郭盛. 磁性纳米颗粒和薄膜中的交换偏置效应[D]. 北京. 中国科学院金属研究所,2012.