Elsevier

Thin Solid Films

Volume 699, 1 April 2020, 137889
Thin Solid Films

Realization of resistive and magnetization switching in sol-gel derived yttrium iron garnet thin films

https://doi.org/10.1016/j.tsf.2020.137889Get rights and content

Highlights

  • Yttrium iron garnet thin films were synthesized by sol-gel method.

  • Resistive switching and magnetization variation of the film device were studied.

  • Carrier transport mechanism follows schottky emission at high resistance state.

  • ~46% saturation magnetization change could be realized via tuning electric field.

  • Resistive switching is related to oxygen vacancy and valence change (Fe2+/Fe3+).

Abstract

In this work, large-area ferrite thin films of ferromagnetic yttrium iron garnet (Y3Fe5O12, YIG) were synthesized on Pt/Ti/SiO2/Si (Pt) substrates by a sol-gel method to investigate the resistive and magnetization switching properties. The synthesized YIG thin films acquire a single garnet structure. The Pt/YIG/Pt stack illustrates unipolar resistive switching behavior with excellent switching uniformity, large memory window (102), stable cycle-to-cycle endurance, and good data storage retention (104 s). The ~46% saturation magnetization variation could be realized via the conversion between high and low resistance states by manipulating the electric field. Schottky emission is governed in the high-field region for the high resistance state. Temperature dependence of resistance and magnetization variation confirms that oxygen vacancies conductive filament model and valence state change (Fe2+ and Fe3+) are responsible for the resistive and magnetization switching mechanisms. These results indicate that YIG ferrite based stack is suitable to design the electro-magnetic coupling multifunctional nonvolatile memory devices.

Introduction

Resistive random access memory (RRAM), based on the simple metal/oxide/metal (MIM) sandwich stack, becomes one of the most promising candidates for transcending the physical limitations of conventional flash memory devices in nonvolatile data memory family, relying on its advantages of simple structure, ultrafast switching speed, high storage density, and good compatibility with magnetism or photo-electricity properties for multifunctional device application [1], [2], [3]. According to resistive switching (RS) effect in MIM memory cell, RS behavior should be switched repetitively between high resistance state (HRS) and low resistance state (LRS), can be classified into electrical polarity dependent bipolar RS and electrical amplitude dependent unipolar RS [4,5]. Unipolar RS devices, getting rid of the polarity limitation, usually obtain the merit of high integration density, therefore giving them great potential for nonvolatile memory application.

Up to now, a variety of metal oxides have been reported for nonvolatile RS memory application, such as binary metal oxides, ferrites, and ferroelectric materials [6,7]. Among these, magnetic ferrite materials are especially attractive owing to their potential for use in multifunctional electric- and magnetic- integrated device application. Recently, spinel ferrites, such as NiFe2O4, CoFe2O4 and ZnFe2O4, have exhibited excellent RS characteristics [3,6,8]. Garnet ferrite (i.e. Y3Fe5O12) is not only a key member of ferrite family but also a versatile and technologically important material. Especially, garnet Y3Fe5O12 (YIG) has abundant magnetic properties, e.g., magnetic spin pumping effect, magnetic proximity effect, magnon valve effect, magnetic-optical effect, and ferromagnetic resonance effect [9], [10], [11], [12], [13], [14]. These magnetic characteristics provide an opportunity for realizing electromagnetic functionalization in single YIG thin films. YIG based multifunctional electromagnetic memory devices not only generate additional degrees of freedom to store information, but also enhance the data storage capability. For this purpose, RS controlled magnetism provides a possibility of synergetic effect between electromagnetic properties and RRAM application, such as magnetoelectric memory with electrical writing and magnetic read operation [15], multilevel data storage device to develop four logic states [16], and magnetoelectric functionalization memory [17,18]. However, until very recently, the resistive switching behavior of YIG was initially reported by Chen et al. [19]. They employed pulse laser deposition (PLD) technique to prepare YIG thin films, examined ultrafast sub-nanosecond RS operation of YIG based RRAM memory device, and demonstrated the great potential for ultra-fast memory application [19]. Further investigations concerning YIG thin films for resistive switching application are needed, such as distribution of switching voltage and the related switching mechanism. Also, due to the limited research on RS behavior of YIG films, current topical issue of RS induced magnetism modulation has not yet been reported. Therefore, the above research situation motivated us to further explore the garnet YIG thin films for RS device application, including resistive switching mechanism and compatible ferrite thin film preparation. Compared with PLD technique, sol-gel method for synthesizing YIG thin films has many advantages, such as facile fabrication process, large-area preparation, low-cost equipment, and good chemical homogeneity.

Herein, in order to further clarify the underlying physical RS mechanism and the correlations between RS and magnetic properties in YIG based RRAM device, we synthesized the garnet YIG thin films by means of a facile sol-gel method to investigate the electric field manipulated resistive and magnetization switching properties. The crystal structure, surface and cross-sectional morphologies, and chemical valence states of YIG films were characterized. The unipolar RS behavior, switching voltage distribution, cycling endurance, and time-dependent resistance were further investigated. In addition, temperature-dependent resistance and magnetism variation provide evidences to explain the physical RS mechanism and associated magnetization switching.

Section snippets

Fabrication of YIG thin films and devices

YIG thin films were synthesized by sol-gel method, and the schematic diagram of YIG thin film based memory device is shown in Fig. 1(a). Y3Fe5O12 precursor solution (0.05 mol/L) was synthesized using Y(CH3COO)34H2O and Fe(NO3)39H2O as solutes adding to co-solvents of 2-methoxyethanol and acetic acid. The solutes with Y/Fe molar ratio 3:5 and the co-solvents (containing 2-methoxyethanol and acetic acid of volume ratio 2:1) were mixed to obtain the precursor solution, followed by spin-coating

Results and discussion

The XRD pattern of YIG thin films deposited on Pt/Ti/SiO2/Si substrate is shown in Fig. 1(b). It is found that YIG thin films acquire a cubic poly-crystalline structure. No impurities peaks are observed except for the Pt and Si diffraction peaks of substrates. The observed (211), (400), (420), (422), (640), and (642) planes can be assigned to the cubic garnet structure (JCPDS No. 43–0507) [13]. Fig. 1(c) exhibits the surface SEM image of YIG thin films, where small pores about 13 nm are

Conclusion

In summary, large-area polycrystalline YIG ferrite thin films were synthesized on Pt/Ti/SiO2/Si substrate by sol-gel method. The typical unipolar resistive switching characteristics were inspected and the related switching parameters were investigated, including switching voltage distribution, resistance ratio, cycle-to-cycle endurance, and data storage retention, in terms of Pt/YIG/Pt stack. The conduction mechanism was dominated by Ohmic conduction and Schottky emission at LRS and HRS,

CRediT authorship contribution statement

Chuangye Yao: Investigation, Writing - original draft. Aize Hao: Investigation. Santhosh Kumar Thatikonda: Investigation. Wenhua Huang: Investigation. Ni Qin: Investigation. Dinghua Bao: Conceptualization, Writing - review & editing, Supervision, Funding acquisition.

Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Nos. 51872335, 51372281) and Natural Science Foundation of Guangdong Province, China (No. 2015A030311019).

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