Micromagnetic Configuration of Variable Nanostructured Cobalt

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Surfaces, Interfaces, and Applications

Micromagnetic Configuration of Variable Nanostructured Cobalt Ferrite: Modulating and Simulations towards Memory Devices Junli Zhang, Shimeng Zhu, Weixing Xia, Jun Ming, Fa-Shen Li, and Jiecai Fu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b07502 • Publication Date (Web): 16 Jul 2019 Downloaded from pubs.acs.org on July 16, 2019

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ACS Applied Materials & Interfaces

Micromagnetic Configuration of Variable Nanostructured Cobalt Ferrite: Modulating and Simulations towards Memory Devices Junli Zhanga#*, Shimeng Zhua#, Weixing Xiab, Jun Mingc, Fashen Lia and Jiecai Fua* aKey

Laboratory of Magnetism and Magnetic Materials of the Ministry of Education, School of

Physical Science and Technology, Lanzhou University, Lanzhou 730000, P. R. China bKey

Laboratory of Magnetic Materials and Devices, Ningbo Institute of Material Technology and

Engineering, Chinese Academy of Sciences, Ningbo 315201, P. R. China cState

Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied

Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China. *Corresponding authors E-mail: [email protected] (Junli Zhang); [email protected] (Jiecai Fu) #These

authors contributed equally to this work.

ABSTRACT. Magnetic nanostructures with flux-closure state or single domain state have a widespread application in diverse memory devices. However, an insight into the modulation of these variable states within one specific magnetic material is rarely reported but still needed. Herein, these micromagnetic configurations within prototypical cobalt ferrite (CoFe2O4) nanostructures in different size and dimension were studied by modulating the assembly of CoFe2O4 building blocks. We find that the CoFe2O4 nanowire (NW) has a multi-domain structure when the diameter is about 90 nm, in which the domain walls (DWs) locate preferentially at the grain boundary and can convert to singledomain state when the diameter is reduced. Alternatively, a flux closure domain state is obtained when the CoFe2O4 nanostructure changing from NW to nanosheet (NS), where the DWs location depends on the overall shape of NS. In addition, we further confirm that the magnetic anisotropy and magnetostatic energy are two main factors affecting the micromagnetic configuration in CoFe2O4 1

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nanostructures by crystallographic analysis and micromagnetic simulations. Our experimental and simulation results demonstrate that the modulation of morphology and dimension are efficient to tailor the micromagnetic configuration in magnetic nanostructures. KEYWORDS. micromagnetic configuration, CoFe2O4 nanostructures, single domain state, domain wall, flux-closure state, electron holography.

INTRODUCTION Magnetic materials with variable nanostructures have widespread applications in the fields of memory devices1,

2, 3,

spintronic devices4, 5, magnetic fluids6, biosensor7 and biomedicine8 owing to the

uniqueness of their micromagnetic configuration. For example, one-dimensional (1D) nanostructures can suppress the Walker limit because of the transverse domain walls (DWs)9, 10, which is particularly important for the magnetic recording media and logical device11. The 2D nanosheets/films can be widely developed for magnetoelectric random access memory applications due to the curling or vortex magnetic configuration12, 13. Besides, the magnetic multilayers/single crystal are promising for the new spintronics devices benefiting from the unique Skyrmion structure appeared14,

15.

However, the

modulation of micromagnetic configurations is not clear, and it has not been fully studied when the nanostructures were changed systematically in sizes, morphologies and dimensions. It is well-known that the micromagnetic configuration relates to their intrinsic magnetic properties, morphology, size, crystallographic environment and defect etc., 16, 17 18. To date, the exchange stiffness and saturation magnetization in magnetic films were found to be the main factors to the shape and size of its vortex core19,

20,

while the magnetic configuration and domain wall location in Fe3O4

nanoparticle arrays (film) were governed by the magnetostatics energy21. The exchange, Dzyaloshinskii-Moriya interaction and magnetic anisotropy can be tuned by adjusting the thickness of magnetic multilayers, leading to the formation of Skyrmion22, 23. And the magnetocrystalline and 2


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shape anisotropy dominating the micromagnetic configuration in low dimensional ferrite nanostructure were also demonstrated24. However, a systematic understanding of the relationship between the micromagnetic configuration and the factors of dimension, size and crystalline structure is still lack, particularly for the same kind of magnetic material. This is significant in nanomagnetism for fundamental research and device applications, because we can tune the micromagnetic configuration through modulating these geometrics (e.g., size, morphology and dimensions), thereby discovering more intriguing properties and potential applications. Herein, the relations between the magnetic configuration and geometric parameters in the CoFe2O4 nanostructures are studied, in which the energy potential can be modulated by controlling the diameter and crystalline orientation of CoFe2O4 nanoparticles. The uniqueness of the CoFe2O4 nanostructures in different sizes and dimensions are achieved by the electrospinning technique, and then the micromagnetic configuration is feasible to be investigated in detail by electron holography (EH). Through local reconstruction of their micromagnetic configuration, we find that the domain structure can transit from multi-domain state to single domain state when the diameter of CoFe2O4 NW was reduced, and DWs of the thicker NW preferentially lie on the particle boundary. Moreover, the micromagnetic configuration can be tuned from single domain state to flux-closure domain state when the nanostructure was changed from 1D NW to 2D nanosheet (NS). Our findings suggest the potential of modulated dimension and size of nanostructures for the development of diverse magnetic memory devices.

RESULTS AND DISCUSSIONS Features of Synthetic Strategy. One great feature of our presented electrospinning technique is the discovered effect of humidity (i.e., amount of water vapor in the electrospinning chamber), which could be a new strategy to prepare the different nanostructured CoFe2O4, as shown in Figure 1. We 3




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Determining factors for micromagnetic configuration. Generally, the magnetic moment should be aligned in the axial direction if shape anisotropy dominates the micromagnetic configuration in an individual NW. But we observe the different magnetic moment distribution in Figure 2c. Thus, magnetocrystalline anisotropy as another possible factor affecting the micromagnetic configuration needs to be fully considered, which directly relates to the crystalline structure and orientation. The crystallographic orientation of particles 1-4 (Figure 2e-h) are confirmed to be [201], [001], [001] and [101] respectively, which is consistent well with the ideal CoFe2O4 crystal in Figure S3. Afterwards, we can confirm the easy axis of cubic system from the value of first (K1) and second order (K2) magnetocrystalline anisotropy constant based on the magnetocrystalline anisotropy energy as below: =

(

1

2 2

+

2 2

+

2 2

)+

2

2 2 2

(2)

Therefore, the easy axis of cubic magnetic materials is mainly determined by the values of K1 and K2. Three situations can be classified as shown in Figure 3a-c: i) the easy axis of cubic system is along the [111] direction when K1 < 0 and direction when >

1

. 9 2

4 9 2

Micromagnetic Configuration of Variable Nanostructured Cobalt

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