Structural and magnetic properties of Ni-doped yttrium iron garnet nanoparticles

R. Peña-Garcia1, Y. Guerra2, F. E. P. Santos3, C. J. Sabino1 and E. Padrón-Hernández1,2

1Universidade Federal de Pernambuco, Departamento de Física, Recife-PE, Brazil 2Universidade Federal de Pernambuco, Pós Graduação em Ciência de Materiais, Recife-PE, Brazil 3Universidade Federal do Piauí, Departamento de Física, Teresina-PI, Brazil e-mail: [email protected]

Abstract: We present a study about the structural and magnetic properties of Ni-doped yttrium iron garnet nanoparticles synthesized by sol-gel. X-ray diffractogram (XRD) indicate the replacement of iron by nickel, evidentiated in the increase of the lattice parameter from 12.3781 Å to 12.3961 Å with the increase in Ni concentration. Scanning Electron Microscopy (SEM) showed particles with a cylindrical format and aggregated, while the Energy Dispersive Spectroscopy (EDS) results corroborate the presence of Ni in YIG. The Fourier transform infrared and Raman spectroscopies confirmed the XRD results and indicated the substitution in the octahedral and tetrahedral sites. The value of saturation magnetization (Ms) for pure sample is 25.81 emu/g achieving the maximum of 26.37 emu/g for x = 0.01. The coercivity for pure sample is 55.72 Oe and increases to 66.45 Oe for x = 0.01. The magnetic measurements complement our results with the changes of the total moment and coercivity for different doping concentration. Keywords: Ni-doped YIG; magnetic properties; structural analysis; sol gel; Raman, FTIR.

Introduction

Yttrium iron garnet is the most important ferrimagnetic insulator for spintronic applications 3+ [1, 2]. The crystalline structure of YIG presents a cubic symmetry, with Fe in the octahedral and tetrahedral positions. Despite this, various researches have reported the possibility of find Fe2+ ions in the YIG structure [1]. Different properties could be achieved through the incorporation of novel elements, conditioned to a good microstructural control. In literature, little is known about the modifications of the physical properties of YIG doped with Ni ions. This work, presents the preparation and characterization of YIG doped with Ni using the sol gel method. Our study is focused on the substitution of Fe by Ni, in order to obtain further knowledge about the effect of Ni addition on the structural and magnetic properties of YIG.

Experimental Procedure

Compounds of composition Y3(Fe1-xNix)5O12 ,(x = 0, 0.01, 0.03 and 0.05), were prepared by a sol–gel method. Y(NO3)3, Fe(NO3)3, Ni(NO3)2 and 0.1M citric acid were dissolved in 50 ml of distilled water. The pH value was kept constant at 2 by adding NH4OH. The solution was heated at 95°C with continuous stirring in order to obtain the gel. A heat treatment at 150°C during 36 hours for drying was performed and followed by a heating at 350°C for 30 minutes. Then, a heating at 900°C for 2 hours was carried. The samples were characterized using XRD,

1554 Raman, FTIR, and SEM. Magnetization was measured using VSM at maximum applied magnetic field of 30 kOe at 300K.

Results and Discussion

Fig. 1a) shows XRD patterns of samples with different Ni contents. XRD data shown that, for x = 0.00, 0.01 and 0.03 samples, we have a single phase corresponding to the garnet structure. For x = 0.05, the sample presented mixed phases of hematite (Fe2O3) and YIG. This shows that the maximum content of Ni for obtaining a single phase is close to 3%. The lattice parameter a, was determined using the Rietveld refinement have shown an increasing tendency from 12.378 Å to 12.396 Å, as the Ni concentration increases from x = 0 to 0.05. This increasing could be attributed the difference between the ionic radii of Ni (0.60 Å) and Fe (0.58 Å). A decrease in the crystallite size up to x = 0.03 was observed, while a small increasing for x = 0.05 occurs. This effect is due to the internal stress caused by the addition of Ni in YIG.

a)

Fig. 1. a). XRD patterns of Y3(Fe1-xNix)5O12 samples. b) EDS spectra for x=5. An increase in the Ni peak intensity with the dopant concentration was observed. The inset is the morphology of the particles

Fig. 1b shows the EDS spectra and SEM image for x = 5. The most relevant result of this measurement is the increasing of Ni peaks with the doping. For all samples the particles are roughly cylindrical and aggregated. The agglomeration may appear due to a long range magnetic dipole-dipole particles interaction. Fig. 2a shows the FTIR spectra for all samples. Three absorption bands appear at 563 cm-1, 595 cm-1 and 656 cm-1, assigned to the asymmetric stretching of the tetrahedron Fe–O bond in YIG. We can see a small shift for smaller wave-numbers of the absorption bands. The shift increase as the dopant concentration takes higher values and confirms the replacement of Fe by Ni ions. Fig. 2b shows the Raman spectrum. As observed, there is a shift in the Raman active vibrational modes, increasing with the Ni concentration. This implies that the Ni ions, gives rise to the structural distortion in the YIG, which could alter the super-exchange interactions between Fe ions at octahedral and tetrahedral sites, thus affecting the magnetic properties. Fig. 2c presents the magnetization versus magnetic field for all samples. The value of Ms for the pure sample is 25.81 emu/g

1555 while, for x = 0.01, an increase for 26.37 emu/g is observed. For x = 0.03 the Ms present a value of 26.07 emu/g and for doping with x = 0.05, Ms is 24.19 emu/g. The substitution of Fe by Ni in the octahedral and tetrahedral sites diminishes the total magnetic moment of YIG, because each Fe contributes with 5 μB and each Ni with 2 μB. The coercive field values are dominated by the shape, agglomeration of nanoparticles as well as the pinning mechanisms for the magnetic moments within the material.

a) b)

Fig. 2. a). FTIR spectra. b) Raman spectra and c) Hysteresis loops for all samples. The upper left insertion shows the variation of Ms with the increase of Ni concentration. The c) insertion inferior right shows the low field data.

Conclusions

We conclude here that, it is possible to insert Ni ions in the YIG structure by using sol-gel method. The XRD data confirmed that the Ni enters in YIG structure because as the doping increases, the larger the lattice parameter. This assertion is supported by the FTIR and Raman analyzes presented. The magnetic measurements confirmed our results, given the variation in the Ms and coercivity

Acknowledgments

The authors thank the Brazilian Agencies CNPq, FINEP, CAPES and FACEPE.

References

[1] R. Peña-Garcia, A. Delgado, Y. Guerra, G. Duarte, L. A. P. Gonçalves, E. Padrón- Hernández. Mater. Res. Express. 4 (2017) 016103. [2] T. Tashiro, S. Matsuura, A. Nomura. Sci. Rep. 5 (2015) 15158.

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