Structural and magnetic properties of polycrystalline ...

Structural and magnetic properties of polycrystalline Fe3O4 thin film Muhammed Shameem P. V., Laxman Mekala, Dushyant Singh, and M. Senthil Kumar Citation: AIP Conference Proceedings 1728, 020333 (2016); doi: 10.1063/1.4946384 View online: http://dx.doi.org/10.1063/1.4946384 View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/1728?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Influence of oxygen vacancies on the electronic structure and magnetic properties of NiFe2O4 thin films J. Appl. Phys. 111, 093906 (2012); 10.1063/1.4704690 Effect of silver addition on structural, electrical and magnetic properties of Fe3O4 thin films prepared by pulsed laser deposition J. Appl. Phys. 111, 073907 (2012); 10.1063/1.3702463 Structural, electrical, and magnetic properties of polycrystalline Fe3−x Pt x O4 (0 ≤ x ≤ 0.10) films J. Appl. Phys. 109, 073905 (2011); 10.1063/1.3563080 Enhancement of electrical properties in polycrystalline Bi Fe O 3 thin films Appl. Phys. Lett. 89, 192902 (2006); 10.1063/1.2385859 Magnetic properties of reactively sputtered Fe1−x O and Fe3O4 thin films J. Appl. Phys. 75, 431 (1994); 10.1063/1.355869

Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions IP: 103.21.125.76 On: Tue, 10 May 2016 15:34:29

Structural and Magnetic Properties of Polycrystalline Fe3O4 Thin Film MuhammedShameem P.V1,b), LaxmanMekala1, DushyantSingh1and M. SenthilKumar1,a) 1

Department of Physics, Indian Institute of Technology Bombay, Mumbai 400 076 a) Corresponding author: [email protected] b) [email protected]

Abstract.The Fe3O4 thin films of 2000Å thickness have been grown at room temperature by dc magnetron sputtering technique and their structural and magnetic properties are studied. The structural study shows that the film is random polycrystalline with no preferred orientation.The isothermal in-plane magnetization measurement performed at various temperatures shows a consistent increase in the coercivity (HC) and exchange bias field(HEB)as the temperature lowered, with a significant increase below 100K.The occurrence of small exchange bias indicates the existence of a non-negligible exchange coupling between the core spins and surface spins of the Fe3O4 grains. The ZFC-FC measurements show that samples are having a wide particle size distribution, along with the existence of stronger interaction between the nearby Fe3O4 grains. Keywords:Polycrystalline magnetic thin film, Half-metallic ferrimagnet,Magnetization and Exchange bias.

INTRODUCTION Fe3O4 (magnetite) is a half-metallic ferrimagnetic materialhaving large spin polarization at room temperature with Curie temperature ofTC≈ 860K, making it as one of the potential material for future spin based devices such as spin injector in spin field effect transistor(spin-FET), magnetic electrode in spin valves based on giant magnetoresistance(GMR) and tunneling magnetoresistance(TMR) [1, 2].Fe3O4 is having cubic inverse spinel crystal structure, the room temperature band structure calculation shows that only minority electrons(spin down) are present at the Fermi level with a gap in the majority spin electrons(spin up) band leading to 100% spin polarization[3]. Hence recently received much attention for the fabrication and studying the structural, microstructural, magnetic and magnetotransport properties of the Fe3O4 thin films. Here we studied the structural properties of Fe3O4 thin filmsas well as the magnetic properties at different temperatures.

EXPERIMENTAL Fe3O4thin filmshaving thickness, t ≈ 2000Åare deposited over Si(111) and glass substrates at room temperature by dc reactive magnetron sputtering using a Fe target.The Ar gas pressure PAr = 4×10-3 mbar, oxygen partial pressure percentage Po2= 2.5%, target-substrate distance, sputtering power etc.are pre-calibrated to get the best quality films with well-defined stoichiometry, and an adequate pre-deposition time is given to clean the target.The thickness of the film is measured using BrukerDektakXTprofilometer. The structural studies areperformed by Rigaku high resolution X-ray diffractometer (HRXRD) with Cu-Kα radiationand by high resolution transmission electron microscope(HRTEM).The in-plane magnetization measurements were carried out, in the magnetic field range of -5T

International Conference on Condensed Matter and Applied Physics (ICC 2015) AIP Conf. Proc. 1728, 020333-1–020333-4; doi: 10.1063/1.4946384 Published by AIP Publishing. 978-0-7354-1375-7/$30.00

020333-1 Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions IP: 103.21.125.76 On: Tue, 10 May 2016 15:34:29

to +5T and the temperature range of 2-380K by using vibrating sample magnetometer(VSM) of Quantum designs physical properties measurement system(PPMS).

RESULTS AND DISCUSSION The HRXRD profile of a Fe3O4(2000Å) thin film is shown in Fig.1. The peaks positions and their relative intensities are completely consistent with the powder XRD data of Fe3O4, indicating that the film is random polycrystalline with no preferred orientation. The average grain size calculated by Scherrer formula [4] is found to be ≈ 200 Å.

20

40

60

2θ (degree)

Fe 3 O 4 (533)

Fe 3 O 4 (422) Fe 3 O 4 (511) Fe 3 O 4 (440)

Fe 3 O 4 (400)

Fe 3 O 4 (222)

Fe 3 O 4 (220)

Fe 3 O 4 (111)

Intensity (arb. unit)

Fe 3 O 4 (311)

80

FIGURE 1.HRXRD data of a Fe3O4 thin film

Fig.2 shows the HRTEM selected area electron diffraction (SAED) pattern of Fe3O4 thin film deposited over TEM grid.As consistent with the XRD data, the continuous rings and their corresponding intensity shows that the film is polycrystalline with no preferred orientation.

FIGURE 2. HRTEM diffraction pattern of a Fe3O4 thin film


Fig. 3 (a) shows the enlarged scale plot of the isothermal in-plane magnetization curve(M-H curve) of Fe3O4(2000Å)thin film deposited over Si(111)substrate, performed at various temperatures.It is evident that Fe3O4is ferrimagnetic with wide hysteresis loop having room temperature saturation magnetization of M S ≈ 320 emu/cc at 5T(bulk value 470emu/cc)[5]. This decrease in MS and the less saturation tendency of M-H curve can be due to the non-magnetic amorphous region formed by the mismatch in the lattices of nearby grains, the structural growth defects in the lattice called antiphase boundaries (APBs), anddisordered moments which are individually strongly aligned at the grain boundaries, as reported by many authors [6, 7]. From Fig.3 (b) it is clear that as the temperature

100

900

t = 2000Å

(a)

(b)

800

50 0 300K 200K 150K 100K 50K 10K 2K

-50 -100 -150 -1500 -1000 -500

0

500

1000

t = 2000Å

50 40

700

30

600

20

500

10

400

0 0

1500

Magnetic field (Oe)

HEB (Oe)

150

HC (Oe)

Magnetization (emu/cc)

decreases from 300K to 2K the average coercivity ( ) increasesconsistently from 410Oe to 890Oe, as large field will be needed to rotate a strongly oriented moments. It is also observed that there is a small consistent increase in the magnitude of exchange bias field ( ) with decreasing temperature indicates the presence of a non- negligible exchange coupling between the core spins(which are easy to rotate) and surface spins(which are pinned and hard to rotate) of the Fe3O4grains. The small value of exchange bias field is due to reasonably large grain size (≈200 Å) leading to less efficient coupling[8, 9].

50

100 150 200 250 300 Temperature (K)

FIGURE 3 (a).The in-plane M-H data of a Fe3O4(2000Å) thin film at different temperatures. (b) The variation of HC and HEB with temperature

To study the intrinsic magnetic properties of Fe3O4 grains the zero field cooled (ZFC) and field cooled (FC) magnetization measurement were performed for different value of applied field as shown in Fig.4. In ZFC measurement, the sample was first cooled to 3K with zero field and then a required field is applied, the magnetic moment was measured as the temperature increases from 3K to 380K.In FC measurement the sample was cooled to 3K under an applied field and then by keeping the same constant field, the magnetic moment was measured as the temperature increases from 3K to 380K.It is clear that even if the applied field (H) varies from 100Oeto 1000Oethe ZFC and FC branchesremain separated and showing an extremely broad peak with little effect of magnetic field over the blocking temperature of T B≈365K.This effect is contradictory to the usually observed sharper transition fromblocked state to the superparamagnetic state in a diluted weakly interacting system having narrow particle size distribution. This indicates that our sample is having a wide particle size distribution, along with the existence of stronger interaction between the nearby Fe3O4 grains like dipole-dipole interaction, exchange interaction etc.[10, 11].


-5

Magnetic moment (10 emu)

1000 Oe

65 60 55 50

250 Oe

45

100 Oe

40 0

50 100 150 200 250 300 350 400

Temperature (K) FIGURE 4.The ZFC-FC magnetization data of a Fe3O4 (2000Å) thin film for different values of applied field.The open symbol represents ZFC data and the filled symbol represents FC data

CONCLUSION The structural and magnetic properties of room temperature reactive sputtered Fe3O4(2000Å) thin filmsare studied. The HRXRD and HRTEM studiesshow that the sample is random polycrystalline with no preferred orientation having average grain size of ≈ 200Å.The isothermal in-plane magnetization measurements performed at various temperatures indicate the existence of non-negligible exchange coupling between the core and surface spins of the Fe3O4 grains. The ZFC-FC measurements show that the samples are having wide particle size distribution, along with the existence of stronger interaction between the nearby Fe3O4 grains.

REFERENCES 1. 2. 3.

S. Soeya, J. Hayakawa, H. Takahashi, K. Ito, and C. Yamamoto, et al., Appl. Phys. Lett. 80, 823 (2002) L. B. Zhao, W.B. Mi, E.Y. Jiang, and H.L. Bai., Appl. Phys. Lett. 91, 052113 (2007) W. Wang, J. M. Mariot, M. C. Richter, O. Heckmann, W. Ndiaye, and P.D. Padova, et al., Phys. Rev. B 87, 085118 (2013) 4. A. L. Patterson., Phys. Rev. B 56, 978 (1939) 5. D. Reisinger, P. Majewski, M. Opel, L. Alff, and R. Gross., Appl. Phys. Lett. 85, 4980 (2004) 6. J. B. Moussy, S. Gota, A. Bataille, M. J. Guittet, and M. G. Soyer, et al., Phys. Rev. B 70, 174448 (2004) 7. H. Wang, T. Zhu, K. Zhao, W. N. Wang, C. S. Wang, Y. J. Wang, and W. S. Zhan., Phys. Rev. B 70, 092409(2004) 8. W.B. Mi, J. J. Shen, E. Y. Jiang, and H. L. Bai., ActaMaterialia. 55, 1919-1926 (2007) 9. J. Tang, K.Y. Wang, and W. Zhou., J. Appl. Phys. 89(11), 7690 (2001) 10. M. Knobel, W. C. Nunes, L. M. Socolovsky, E. D. Biasi, J. M. Vargas, and J. C. Denardin.,J. Nanosci.Nanotechnol.8, 2836 (2008) 11. D. W. Kavich, S. A. Hasan, S. V. Mahajan, J. H. Park, and J. H. Dickerson., Nanoscale Res. Lett. 5, 1540-1545 (2010)
