CaSi氧化物对Sr1-<italic>x</italic>La<italic>x</italic>Fe11.6-<italic>x</italic>Co<italic>x</italic>O19六角铁氧体晶粒形貌及磁性能影响研究

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陈中艳, 冯则坤, 詹振华, 吴捷. CaSi氧化物对Sr_1-_xLa_xFe_11.6-_xCo_xO₁₉六角铁氧体晶粒形貌及磁性能影响研究 . 无机材料学报, 2014, 29(1): 81-84
CHEN Zhong-Yan, FENG Ze-Kun, ZHAN Zhen-Hua, WU Jie. Influence of CaSi Oxide on Microstructure and Magnetic Properties of Sr_1-_xLa_xFe_11.6-_xCo_xO₁₉ Hexaferrite . Journal of Inorganic Materials, 2014, 29(1): 81-84 复制到剪切板

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CaSi氧化物对Sr_1-_xLa_xFe_11.6-_xCo_xO₁₉六角铁氧体晶粒形貌及磁性能影响研究

陈中艳¹, 冯则坤¹, 詹振华², 吴捷²

1. 华中科技大学光学与电子信息学院, 武汉 430074

2. 广东江粉磁材股份有限公司, 江门 529000

冯则坤, 教授. E-mail:fengzekun@mail.hust.edu.cn

陈中艳(1983-), 女, 博士研究生. E-mail:chenzhongyan88@163.com

基金:广东省教育部产学研合作项目(20110905) Guangdong Province and Ministry of Education Cooperation Projects(20110905)

摘要

基于传统陶瓷工艺制备Sr_1-_xLa_xFe_11.6-_xCo_xO₁₉样品, 重点研究CaSi氧化物对Sr_1-_xLa_xFe_11.6-_xCo_xO₁₉预烧料晶粒形貌及磁性能的影响。采用XRD、SEM和VSM分析样品的结构特征、微观形貌及磁特性。微观形貌分析的结果表明CaSi氧化物可以有效调控预烧料的晶粒形貌, 通过优化CaSi氧化物的含量, 能够得到理想的微观形貌及高的M_r/M_s。当掺杂量x=0.2时, 加入0.15wt% CaCO₃和0.2wt% SiO₂的添加剂, 饱和磁化强度(σ_s)和矫顽力(H_cJ)比未加入CaSi氧化物时分别提高4.76%和6.2%, 样品的M_r/M_s在掺杂范围内均有所提高。

关键词: CaSi氧化物; 六角铁氧体; 微观形貌; 磁性能

中图分类号:TQ174 文献标志码:A 文章编号:1000-324X(2014)01-0081-04

Influence of CaSi Oxide on Microstructure and Magnetic Properties of Sr_1-_xLa_xFe_11.6-_xCo_xO₁₉ Hexaferrite

CHEN Zhong-Yan¹, FENG Ze-Kun¹, ZHAN Zhen-Hua², WU Jie²

1. School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China;

2. JPMF GuangDong Co., Ltd., Jiangmen 529000, China

Abstract

Based on the standard ceramic preparation process, the effects of CaSi oxide on the microstructure and magnetic properties of Sr_1-_xLa_xFe_11.6-_xCo_xO₁₉ were studied. The microstructural, morphology and magnetic properties of the samples were characterized by XRD, SEM and VSM. The SEM results show that the morphology of calcined samples are effective controled by CaSi oxide. So the ideal microstructure and enhancedM_r/M_s can be obtained by optimizing the amount of CaSi oxide. Withx=0.2, 0.15wt% CaCO₃ and 0.2wt% SiO₂ were added to the Sr_1-_xLa_xFe_11.6-_xCo_xO₁₉and the measured σ_s andH_cJ were enhanced by 4.76% and 6.2%, respectively, when compared with non-doped sample. Meanwhile, theM_r/M_s was improved as well in the above mentioned doping range.

Keyword: CaSi oxide; hexaferrite; microstructure; magnetic properties

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随着汽车向安全、舒适、自动化、轻量化方向发展, 汽车用动力控制产品越来越多, 永磁电机用量大幅度增加, M型六角铁氧体是一种性价比高, 广泛应用于电机的永磁材料, 为了满足电机小型化的需求, 对永磁铁氧体提出了更高的性能要求。Grössinger等^{[ 1, 2, 3]}的研究表明, La、La-Co、Ca-La-Co等稀土元素的掺杂取代在不降低饱和磁化强度的情况下能大幅度提高六角铁氧体的磁晶各向异性, 因而能获得综合磁性能高的永磁铁氧体。但实验证明, 随着LaCo掺杂量的增加, 预烧样品的矩形比 M_r/ M_s会降低, M_r /M_s是反映材料综合指标的重要参数, 具有较低 M_r /M_s的样品无法应用到退磁场强的环境, 大大限制了产品的应用范围。

为了探究掺杂取代带来 M_r/ M_s下降的原因, 部分学者进行了相关研究。根据Kools等^{[ 1]}的报道, 随着LaCo掺杂量的增加, 烧结样品会出现杂相和晶粒粒度分布加宽的双峰现象, 引起磁性能的降低。针对烧结样品的粒度分布加宽现象, Kools^{[ 4]}用RIGGI(Reaction-induced grain growth impediment)作用机理来解释。该机理指出, 掺杂元素在进入晶粒的过程中会抑制晶粒的生长, 溶入过程结束晶粒重新开始生长, 当存在不同程度的掺杂取代时, 晶粒的尺寸存在差异, 除了各向异性场, 晶粒的尺寸及分布对 H_cJ也有重要影响^{[ 5, 6, 7]}, 不同尺寸的晶粒具有不同的 H_cJ, 因而抗退磁能力不同, 最终导致 M_r/ M_s和综合磁性能下降。

镧钴掺杂通过提高磁晶各向异性场来提高磁性能, 但同时掺杂元素进入晶粒的过程也影响了晶粒均匀性, 因而得到均匀细小的晶粒是进一步提高磁性能的关键。Töpfer等^{[ 8, 9]}学者对二次添加时CaO/SiO₂的最佳比例、烧结工艺的优化进行了研究, 二次工艺的调整能在一定程度上提高磁性能, 但无法消除一次预烧过程中由于LaCo元素不同程度的掺杂取代而导致的晶粒度尺寸分布加宽的现象。本研究在一次配方中加入CaSi氧化物, 通过CaSi氧化物对掺杂的促进作用实现预烧料晶粒形貌的调控, 从而得到理想的微观形貌和高的磁性能。

1 实验

采用传统陶瓷工艺制备Sr_1- _xLa _xFe_11.6- _xCo _xO₁₉( x=0~0.3 mol)样品。将原料Fe₂O₃、SrCO₃、Co₃O₄、La₂O₃在行星球磨机中均匀混合2 h, 115℃下烘干处理后在马弗炉中预烧, 预烧温度为1220℃, 保温1.5 h, 得到的预烧料留样做相组成分析和磁特性测试。为了进一步分析预烧料形貌对 B_r和 H_cJ的影响, 将得到的预烧料进行初破碎(平均粒度约为5 μm), 然后进一步细磨得到平均粒度为0.6 μm左右的浆料, 湿压成型得到φ25 mm的圆饼, 以3 ℃/min的升温速率升温到1160 ℃烧结, 保温时间1.5 h, 样品随炉冷却, 打磨处理后测试 B_r和 H_cJ等指标。作为对比, 另一组实验采用相同的配方及工艺, 在一次配料中加入0.15wt% CaCO₃和0.2wt% SiO₂作为添加剂。用荷兰Panalytical B.V.公司的χ° Pert PRO 型 X射线衍射仪(XRD)分析预烧料的相结构, 用VEGA 3 SBU型扫描电镜(SEM)分析预烧料的微观形貌, 用美国Lake Shore 7407 振动样品磁强计(VSM)测试预烧料的磁特性, 测试所加最大外加磁场为15 kOe (1Oe=(1000/4π)A/m)。用TYU-2000H系列永磁铁氧体 B- H测量仪测量烧结样品的磁特性。

2 结果与讨论

2.1 CaSi氧化物对Sr_1- _xLa _xFe_11.6- _xCo _xO₁₉预烧料微结构的影响

图1是主配方为Sr_1- _xLa _xFe_11.6- _xCo _xO₁₉( x= 0~0.3 mol)样品的XRD图谱。由图1可知, x=0~0.2均可得到单一的M相, x=0.3时出现少量CoFe₂O₄和α-Fe₂O₃杂相, 这是由于掺杂量较大时, LaCo的等量掺杂导致Co元素出现富余, 从而形成杂相^{[ 10, 11]}。

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	图1 Sr_1- _xLa _xFe_11.6- _xCo _xO₁₉( x=0~0.3 mol)预烧料的XRD图谱Fig. 1 X-ray diffraction patterns of Sr_1- _xLa _xFe_11.6- _xCo _xO₁₉samples ( x=0-0.3 mol)

由图2可知, 当一次预烧前没有添加CaSi氧化物时, 随着掺杂量的增加, 晶粒生长受到抑制, 特别是图(d1)的抑制现象非常明显, 说明LaCo掺杂使晶粒的生长受到抑制, 根据Kools^{[ 7]}的RIGGI作用机理, LaCo掺杂时, 掺杂元素溶入晶格内部导致自由能的降低, 且比晶粒生长降低的自由能大, 从而抑制晶粒的生长。这说明镧钴掺杂不仅改变材料的内禀属性, 也影响晶粒的生长。加入0.15wt% CaCO₃和0.2wt% SiO₂的氧化物后, 在 x=0时, CaSi氧化物加入后对晶粒生长抑制非常明显(图2(a1), (a2)), 主要原因是硅钙组合可以形成液相, 液相均匀分布在晶粒的表面阻碍了晶粒的生长。这说明在抑制晶粒生长方面, 加入0.15wt% CaCO₃和0.2wt% SiO₂的氧化物与 x=0.3的LaCo掺杂都非常明显。

	Figure Option View Download New Window
	图2 Sr_1- _xLa _xFe_11.6- _xCo _xO₁₉预烧样品的形貌Fig. 2 Morphology of Sr_1- _xLa _xFe_11.6- _xCo _xO₁₉calcined samples

但LaCo和CaSi同时存在时, 晶粒生长被抑制的情况发生改变。图2(b2, c2, d2)与图2(a2)和图2 (d1)相比, 晶粒的平均粒度变大, 存在晶粒生长被抑制的现象减弱。根据RIGGI理论, 晶粒生长的情况与自由能有关系^{[ 7]}, 图2(b2, c2, d2)中, 当掺杂元素LaCo和CaSi同时存在时, 杂质在晶格中的溶入过程、液相形成过程同时存在。在两个过程共同作用下, 总的自由能决定了晶粒的生长情况。且镧钴含量不同, 晶粒生长被抑制的现象在硅钙加入后受到减弱的程度亦不同, x=0.1时抑制效果减弱的现象最明显, 晶粒的尺寸最大。

因此, 可以通过分别加入CaSi氧化物、LaCo

(a1) x=0; (a2) x =0, 0.15wt% CaCO₃ +0.2wt% SiO₂;

(b1) x =0.1; (b2) x =0.1, 0.15wt% CaCO₃ +0.2wt% SiO₂;

(c1) x =0.2; (c2) x =0.2, 0.15wt% CaCO₃ +0.2wt% SiO₂;

(d1) x =0.3; (d2) x =0.3, 0.15wt% CaCO₃ +0.2wt% SiO₂;

掺杂以及两者的组合去调控晶粒的生长, 得到理想的微观形貌。预烧晶粒的均匀细化对降低第二次球磨时间和减小粉料的粒度分布是有利的, 同时会减少球磨过程中杂质的掺入量。

2.2 CaSi氧化物对Sr_1- _xLa _xFe_11.6- _xCo _xO₁₉样品磁特性的影响

为了分析这种明显的晶粒形貌变化对磁特性的影响, 图3给出了Sr_1- _xLa _xFe_11.6- _xCo _xO₁₉预烧料样品的VSM测试结果。掺杂量 x<0.2, 相同预烧温度下, 样品的 σ_s上升, 但当 x=0.3时 σ_s下降, 由XRD图谱的分析结果可知, 主要是由于CoFe₂O₄和α-Fe₂O₃杂相的出现。与未加CaSi氧化物的样品相比, 在Sr_1- _xLa _xFe_11.6- _xCo _xO₁₉配方中加入CaSi氧化物能得到高的 σ_s。对于 H_cJ而言, 在 x=0~0.3的掺杂范围内, 随着掺杂量 x的增加, 各向异性场会增加^{[ 1]}, 根据Haneda等^{[ 11]}所提出的公式 ( J_s表示饱和磁极化强度、 K₁表示各向异性常数、 H_d表示退磁场、 H_in表示晶粒之间的相互作用场), H_cJ应该呈上升趋势。如图3所示, 当未加CaSi氧化物时, H_cJ随着 x的变化满足公式。但当加入CaSi氧化物后, H_cJ表现为非线性变化, 在 x=0.1和0.3时 H_cJ出现明显下降, 认为引起该现象的原因主要是CaSi氧化物对预烧料晶粒形貌的调控作用导致的, 由样品的微观形貌分析可知, 图2(a2)均匀细小的晶粒是高 H_cJ的主要原因, 尽管此时没有掺杂镧钴。随着LaCo掺杂量的增加, 当 x≤0.2时, 有CaSi氧化物同时存在的情况下, σ_s和 H_cJ均有所提高, 在 x=0.2处达到最大, H_cJ和 σ_s比未加入CaSi氧化物分别提高4.76%和6.2%。这说明一次预烧前加入CaSi氧化物能在LaCo掺杂的基础上进一步提高样品的磁性能。

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	图3 Sr_1- _xLa _xFe_11.6- _xCo _xO₁₉在有/无硅钙氧化物时的磁特性对比Fig. 3 Comparison of magnetic properties for Sr_1- _xLa _xFe_11.6- _xCo _xO₁₉samples prepared with or without additive

图4是烧结样的 M_r /M_s的对比, M_r /M_s是反映样品微观形貌是否理想的重要参数, M_r /M_s越高, 说明样品抗退磁的能力越强, 越能得到较高的磁能积。如图4所示, 随着 x的增加, x≤0.2时, M_r /M_s是降低的, 这是由于掺杂后粒度分布加宽, 不容易得到均匀一致的晶粒所致^{[ 1]}。加入硅钙氧化物后, x=0时, M_r /M_s提高, 但由于此时并没有LaCo元素, M_r /M_s的提高不显著, 在掺杂LaCo后, M_r /M_s得到明显提高, 当 x=0.2时, M_r /M_s达到最大。当 x=0.3时, 由于出现杂相, M_r /M_s下降。说明在相同的预烧温度下, 可以通过同时添加一定量的LaCo和CaSi氧化物以调控预烧料的晶粒生长, 使反应得到加强, 预烧料能够得到高的 σ_s、 H_cJ和 M_r /M_s, 实现综合磁性能的提高。

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	图4 Sr_1- _xLa _xFe_11.6- _xCo _xO₁₉在有/无硅钙氧化物时的 M_r /M_S对比Fig. 4 Comparison of M_r /M_sfor Sr_1- _xLa _xFe_11.6- _xCo _xO₁₉samples prepared with or without additive

图5给出了在有/无CaSi氧化物时烧结样品 B_r和 H_cJ的指标对比。未添加硅钙氧化物时, 随着掺杂量的增加, B_r和 H_cJ总体上升, 这主要是由于掺杂LaCo的作用。在 x=0时, 加入CaSi氧化物后能得到高的 H_cJ, 主要由于晶粒生长被抑制, 但 B_r较低。随着 x含量的增加, 加入硅钙氧化物后 B_r上升趋势更明显, 但只有 x=0.2时, B_r和 H_cJ均达到最大值。说明当氧化物的量为0.15wt% CaCO₃和0.2wt% SiO₂时, 能够得到最佳性能。

	Figure Option View Download New Window
	图5 Sr_1- _xLa _xFe_11.6- _xCo _xO₁₉在有/无硅钙氧化物时 B_r和 H_cJ对比Fig. 5 Comparison of B_r and H_cJ for Sr_1- _xLa _xFe_11.6- _xCo _xO₁₉samples prepared with or without additive

3 结论

1) 配方Sr_1- _xLa _xFe_11.6- _xCo _xO₁₉中加入0.15wt% CaCO₃和0.2wt% SiO₂的添加剂能有效调控预烧料的晶粒形貌, 并有利于反应过程的完成, 能够得到比不加CaSi氧化物时更高的 σ_s和 H_cJ, 在 x=0.2时分别提高4.76%和6.2%。

2) LaCo掺杂样品的 M_r /M_s通常会下降, 在配方中加入CaSi氧化物能有效抑制 M_r /M_s的下降, 说明由掺杂LaCo引起的晶粒不均匀生长或者不完全反应的现象可被改善或者消除, 这对所研制的铁氧体的实际应用有利。

The authors have declared that no competing interests exist.

参考文献

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2002

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J. Magn. Magn. Mater.. 2002, 242(2):1270-1276

LaCo-substituted ferrite magnets, a new class of high-grade ceramic magnets; intrinsic and microstructural aspects

Kools F , Morel A , Grössinger R

The science and technology of conventional ferrite magnets is reviewed, including their historical evolution and models to explain their properties. Next, a survey is given of the new LaCo-type ferrite magnets, representing a breakthrough in magnetic performance. The increased performance is explained with the classical models. Increased market share of M-ferrite magnets as well as re-intensification of background research is expected.

... Grössinger等^[1,2,3]的研究表明, La、La-Co、Ca-La-Co等稀土元素的掺杂取代在不降低饱和磁化强度的情况下能大幅度提高六角铁氧体的磁晶各向异性, 因而能获得综合磁性能高的永磁铁氧体 ...

... 根据Kools等^[1]的报道, 随着LaCo掺杂量的增加, 烧结样品会出现杂相和晶粒粒度分布加宽的双峰现象, 引起磁性能的降低 ...

... 3的掺杂范围内, 随着掺杂量x的增加, 各向异性场会增加^[1], 根据Haneda等^[11]所提出的公式 (J_s表示饱和磁极化强度、K₁表示各向异性常数、H_d表示退磁场、H_in表示晶粒之间的相互作用场), H_cJ应该呈上升趋势 ...

... 2时, M_r/M_s是降低的, 这是由于掺杂后粒度分布加宽, 不容易得到均匀一致的晶粒所致^[1] ...

2004

2.289

0.0

J. Alloys Compd.. 2004, 370(1/2):331-334

Recent improvement of hard ferrite permanent magnets based on La-Co substitution

Tenaud P , Morel A , Kools F

Recent improvements performed on hard ferrite magnets with the Sr_1−xLa_xFe_12−yCo_yO₁₉ composition are presented. The influence of the composition on both structural and magnetic properties of samples withy=x andy/x=0.75 is emphasized. Our investigations have allowed to optimize the permanent magnet properties of La–Co substituted hard ferrite magnets. The optimum composition contains more lanthanum than cobalt at the benefit of both lower irreversible losses and lower raw material costs.

2003

4.661

0.0

Phys. B. 2003, 327(2/3/4):202-207

Magnetic properties of a new family of rare-earth substituted ferrites

Grössinger R , Tellez Blanco C , Kupferling M

The magnetic properties of substituted hard-magnetic M-type ferrites are surveyed. A combined La–Co substitution leads to magnets of compositions Ba_1−xLa_xFe_12−xCo_xO₁₉ and Sr_1−xLa_xFe_12−xCo_xO₁₉ (x= 0, 0.1, 0.2, 0.3, 0.4). Due to this substitution, the anisotropy as well as the coercivity increases. The effect of the ion size was studied on the system (Sr_1−yCa_y)_1−xLa_xFe_12−x Co_0.75xFe²⁺_0.25xO₁₉ (x= 0.2, 0.4;y= 0, 0.33, 0.66, 1). Additionally, Sm substitution which leads to a strong increase of the coercivity was studied, but the effect on the anisotropy is small (1%).

1985

2.646

0.0

Solid State Ionics. 1985, 16(6):251-259

Reaction-induced grain growth impediment during sintering of strontium hexaferrite with silica addition

Kools R

The present paper, referrinf to our previous studies on the action of silica during sintering of SrO excess strontium hexaferrite (SrFe₁₂O₁₉) is focused on the mechanism of the grain growth impediment. It is concluded that none of the three “classical” mechanisms - pore drag, precipitate drag and impurity drag - can explain satisfactorily the observed grain growth behaviour. Suggested by the observed correlation between dissolving of silica in hexaferrite and stopped grain growth, a new mechanism is put forward :“Reaction-induced grain growth impediment”.

In this mechanism, a chemical force (F_c) is introduced, which counteracts the driving force for grain growth (F_s). This chemical force results from the fact that the concentration gradients of the solute at both sides of the curved grain boundary are different during the (dissolving) reaction of a reactive secondary phase. The ratio F_c/F_s determines what happens : impeded grain growth, stopped grain growth or even growth of the smaller grains resulting in a decreased spread in grain size.

... 针对烧结样品的粒度分布加宽现象, Kools^[4]用RIGGI(Reaction-induced grain growth impediment)作用机理来解释 ...

2011

1.363

0.0

... 该机理指出, 掺杂元素在进入晶粒的过程中会抑制晶粒的生长, 溶入过程结束晶粒重新开始生长, 当存在不同程度的掺杂取代时, 晶粒的尺寸存在差异, 除了各向异性场, 晶粒的尺寸及分布对H_cJ也有重要影响^[5,6,7], 不同尺寸的晶粒具有不同的H_cJ, 因而抗退磁能力不同, 最终导致M_r/M_s和综合磁性能下降 ...

2009

1.363

0.0

1999

0.771

0.0

J. Appl. Phys.. 1999, 85(8):5697-5698

Improved magnetic properties and growth anisotropy of chemically modified Sr ferrites

Lee J W , Cho Y S , Amarakoon V R W

Magnetic properties and microstructural characteristics ofSrO∙5.9Fe₂O₃ chemically modified with Si and Ca were investigated by changing experimental parameters such as additive composition, the ratio of Ca/Si, and sintering condition. A novel particulate coating method utilizing sol–gel reactions was used to uniformly incorporate the additives of Si and Ca. This method was very successful in obtaining homogeneous grain growth and fine grains. A sample containing the gel additives of 0.6 wt %SiO₂ and 0.7 wt % CaO and sintered at 1200 °C for 4 h was found to significantly suppress abnormal grain growth, resulting in submicron-sized grains and high density. A distinct grain boundary phase containing Si and Ca was observed by increasing the sintering temperature to 1250 °C. The resultant microstructural characteristics favorably affected magnetic properties. For example, the chemically modified sample exhibited a higher coercivity of 3530 Oe compared to a value of 2050 Oe obtained for the sample without the additives. On the other hand, an increase in the ratio of Ca/Si or in sintering temperature tended to induce a large anisotropy during grain growth. © 1999 American Institute of Physics.

... 由图2可知, 当一次预烧前没有添加CaSi氧化物时, 随着掺杂量的增加, 晶粒生长受到抑制, 特别是图(d1)的抑制现象非常明显, 说明LaCo掺杂使晶粒的生长受到抑制, 根据Kools^[7]的RIGGI作用机理, LaCo掺杂时, 掺杂元素溶入晶格内部导致自由能的降低, 且比晶粒生长降低的自由能大, 从而抑制晶粒的生长 ...

... 根据RIGGI理论, 晶粒生长的情况与自由能有关系^[7], 图2(b2, c2, d2)中, 当掺杂元素LaCo和CaSi同时存在时, 杂质在晶格中的溶入过程、液相形成过程同时存在 ...

2005

2.353

0.0

... Töpfer等^[8,9]学者对二次添加时CaO/SiO₂的最佳比例、烧结工艺的优化进行了研究, 二次工艺的调整能在一定程度上提高磁性能, 但无法消除一次预烧过程中由于LaCo元素不同程度的掺杂取代而导致的晶粒度尺寸分布加宽的现象 ...

2007

1.78

0.0

2000

0.0

... 3时出现少量CoFe₂O₄和α-Fe₂O₃杂相, 这是由于掺杂量较大时, LaCo的等量掺杂导致Co元素出现富余, 从而形成杂相^[10,11] ...

1973

1.058

0.0

Jpn. J. Appl. Phys.. 1973, 12(3):355-360

Intrinsic coercivity of substituted BaFe₁₂O₁₉

Haneda K , Kojima H

Intrinsic coercive force measurements were made on BaFe₁₂O₁₉ in which certain amounts of Fe were substituted by In, Cr, Al, Ga, ZnGe, ZnV, ZnNb or ZnTa. The specimens were prepared by the ordinary powder metallurgy method. X-ray diffraction of the specimens indicated only the presence of the magnetoplumbite phase within the experimental substitution range. A large increase in coercive force_iH_c, observed when Cr, Al or Ga were substituted, was mainly due to the onset or the approach of single-domain behaviour at higher concentrations even under the same milling condition. In the case of Al substitution, the increase of magnetocrystalline anisotropy fieldH_a also contribute greatly to the abrupt rise of_iH_c. In the case of the other substitutions, the value of_iH_c decreased without an appreciable change in the single-domain critical size; this decrease might be due to the decrease inH_a.

... 3时出现少量CoFe₂O₄和α-Fe₂O₃杂相, 这是由于掺杂量较大时, LaCo的等量掺杂导致Co元素出现富余, 从而形成杂相^[10,11] ...