Azimuthally Controlled Magnetic and Dielectric Properties of

Article pubs.acs.org/JPCC

Azimuthally Controlled Magnetic and Dielectric Properties of Multiferroic Nanocrystalline Composite by Magnetic Coupling and Charge Hopping Yu Tang, Yi Zhang, Guodong Ma, Ning Ma, and Piyi Du* State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: Multiferroic composite is an environmentally friendly material with the extraordinary multisusceptible and high storage properties. It is significant to find an effective way for designing and fabricating high-quality materials. In this context, the BTO/NZFO nanocrystalline multiferroic composite thin films with and without (100) oriented spinel ferrite were prepared by RF magnetron sputtering. The saturation magnetization, coercivity, and initial susceptibility of composite thin films with (100) oriented spinel NZFO phase are 1.05, 1.75 to 2.08, and 1.15 times as high as those of the composites without orientation, respectively. The dc conductivity and dielectric loss of the composite thin film with (100) oriented NZFO are 18 and 33.3% lower than those of composite without orientation, respectively. It is exhibited that the magnetic and dielectric properties of the BTO/NZFO multiferroic composite thin films are azimuthally controlled by distribution of the Zn/Fe−O−Ni/Fe superexchange coupling and charge hopping between Fe2+−Fe3+ pair, respectively. Obviously, controlling the arrangement of magnetic couplings and charge hopping is an effective route to fabricate the multiferroic composite with the optimal or specific magnetic and dielectric properties.

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good dielectric and magnetic properties.12−14,23,24 Single-crystal silicon is a chemically stable substrate without interacting with the targeted thin films even as high temperature and adjusts expectedly the growth orientation of magnetic phase in the composite thin film.25,26 RF Magnetron sputtering is a physical vapor deposition (PVD) method used extensively in commercial process.27,28 The consequence is thus universal and meaningful broadly for improving performance of the multiferroic composite. In this work, nanocrystalline BTO/NZFO composite thin films with and without (100) orientation of spinel NZFO phase were prepared. The intrinsic characteristics of the composites dependent on the orientation of ferromagnetic phase were investigated in detail. The azimuthal control of the magnetic and dielectric properties by Zn/Fe−O−Ni/Fe superexchange coupling and Fe2+−Fe3+ pair were revealed. An effective and efficient way of controlling the arrangement of magnetic coupling and charge hopping to fabricate the multiferroic with outstanding and controllable magnetic and dielectric properties was provided.

INTRODUCTION Multiferroic is an excellent environmentally friendly multifunctional material due to its potential applications in novel electronic devices, such as sensors, transducers, and memories with high storage capacity and low cost.1−5 Recently, to further improve the performance of these devices and relieve the everincreasing pressure on energy and environment, more and more research on improvement of dielectric and magnetic properties of the material has been provoked.6−18 It is known that the dielectric properties of the dielectrics, such as permittivity and tunability, of the dielectrics can be promoted to be optimal by the azimuthal control of dipoles.19−21 Other than the common way that improves the dielectric properties through doping and controlling size,22 the dipole azimuthal control motivates the intrinsic nature to the greatest extent and contributes the high dielectric properties to dielectric materials. Likewise, the magnetic moment of superexchange coupling and the direction of charge hopping in multiferroic also possesses the switchable characteristics. Therefore, it is significant to reveal that azimuthal control mechanism of the magnetic and dielectric properties by magnetic coupling and charge hopping and provide an effective way with distinct physical pictures to make the performance of multiferroic to be optimal. Herein, the multiferroic composite system of BaTiO3 (BTO) − Ni0.5Zn0.5Fe2O4 (NZFO) thin film deposited on singlecrystal silicon substrate with (100) and (111) orientation by RF magnetron sputtering is proposed. It is worth noting that BTO/NZFO is a typical multiferroic composite system with © 2015 American Chemical Society

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EXPERIMENTAL SECTION

BTO and NZFO precursor powders were prepared by sol−gel process and citric acid combustion method, respectively.24 The two precursor powders were mixed with different ratios and Received: July 4, 2015 Published: July 10, 2015 17995

DOI: 10.1021/acs.jpcc.5b06429 J. Phys. Chem. C 2015, 119, 17995−18005

Article

The Journal of Physical Chemistry C

Figure 1. XRD patterns of (1−x)BTO/xNZFO composite thin films deposited on single-crystal silicon substrate with (a) (100) and (b) (111) orientation.

Figure 2. TEM images of 0.6BTO/0.4NZFO composite thin films deposited on silicon substrate with (a) (100) and (b) (111) orientation and images of single phased NZFO thin films deposited on silicon substrate with (c) (100) and (d) (111) orientation.

then ball-milled in ethanol for 12 h. Afterward, the mixture was added to 5% PVA and pressed into disks under uniaxial pressure of 100 MPa. The (1−x)BTO/xNZFO disks were sintered at 1200 °C for 10 h in atmospheric ambience. The sintered ceramic disks were polished with 600-mesh waterproof abrasive paper and 600-mesh crystalline abrasive paper, forming the sputtering targets with 100 mm in diameter and 20 mm in thickness. Single-crystal silicon with (100) and (111) orientation used as the substrates were ultrasonically cleaned in hydrofluoric acid and then dried before being loaded into the sputtering chamber of JPG-560C12 (SKY Institute of Micro-

electronics of Chinese Academy of Sciences, Beijing, China) RF magnetron sputtering system. During the sputtering process at an applied power of 200 W, the oxygen partial pressure and working pressure of the mixture gas of Ar and O2 was kept at 0.4 and 1.1 × 10−2 mbar, respectively. The products were sintered at 800 °C for 4 h in oxygen atmosphere after sputtering. The constituent phases of composite thin films were identified by a RIGAKUD/MAXC type X-ray diffractometer (Tokyo, Japan) using Cu Kα target. The wavelength of the Xray source is 0.1540562 nm, and the step width is 0.02° with a 17996


Article


Figure 3. Schematic diagram of lattice structure. (a) Cubic cell of AB2O4 spinel and (b) cations at B sites in AB2O4 spinel lattice.

Figure 4. Magnetic properties of (1−x)BTO/xNZFO composite thin films with and without (100) oriented NZFO under an out-of-plane magnetic field. (a) Magnetic hysteresis loops; the inset shows the enlargement of the loops near zero magnetic field. (b−d) Variation of saturation magnetization (Ms), coercivity (Hc), and initial susceptibility (μi) as a function of x, respectively, in which the insets show the ratios of Ms, Hc, and μi of composite thin films with and without (100) oriented NZFO, respectively.

system (PPMS-9T) produced by Quantum Design Cooperation (USA).

scanning speed of 4 per minute between 10 and 90°. The morphology and microstructure were observed by transmission electron microscope (Philips CM200). Au top electrodes of 40 nm thick and 0.2 mm in diameter were sputtered as the top electrodes to measure the dielectric properties by the impedance analyzer (Agilent 4294A) with an oscillating voltage of 10 mV over the frequency region of 40 Hz to 110 MHz. The magnetic properties were measured by the vibrating sample magnetometer (VSM) and physical property measurement

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RESULTS AND DISCUSSION Figure 1 illustrates the X-ray diffraction (XRD) patterns of asprepared xBTO/(1-x)NZFO composite thin film deposited on a) (100) and b) (111) oriented single crystal silicon substrates, in which the molar fraction of ferrite, x, varies from 0.2 to 1.0. Only the diffraction peaks corresponding to the perovskite BTO and the spinel NZFO phases are observed, without any 17997


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Figure 5. (a) Sketch map of A−O−B couplings and external field in spinel NZFO lattice shown in spherical coordinate system X−O−Y. (b) Three A−O−B couplings with different polar and azimuthal angles in the first quadrant. (c) Resultant A−O−B couplings during magnetization process.

other impurity phase. The NZFO in all composite thin films with different x on (111) Si substrate grow along with (100) orientation but no specific orientation on (100) Si substrate. The peak intensities of the NZFO phases in the composite thin films increase notably with increasing fraction of NZFO. By contrast, there was no specific orientation for the tetragonal perovskite BTO grown in the composite thin films on both (111) and (100) Si substrates. Figure 2 shows the TEM micrographs of the 0.6BTO/ 0.4NZFO composite thin films (Figure 2a,b) and the singlephased NZFO thin films (Figure 2c,d) on (100) and (111) Si substrates, respectively. The selected area diffraction (SAD) pattern of the single-phase NZFO on (111) Si substrate is shown in the inset of Figure 2d. The well-grown grains of BTO and NZFO with size fc), the properties of percolative composite are mainly controlled by those of conducting phase. Therefore, for the 0.2BTO/0.8NZFO composite thin film, whose percolation threshold is ∼0.2 under out-of-plane field and 0.5 under in-plane field, the permittivity of the composite is dominantly controlled by the NZFO phase. Considering the dipolar model,35,37−40 the real and imaginary parts of permittivity of NZFO phase can be expressed as ε′NZFO = ε″NZFO =

ε(0)NZFO − ε∞ , NZFO 2 1 + ω2τNZFO

Figure 11. Dielectric susceptibility of spinel NZFO ferrite as functions of polar and azimuthal angles of the external electric field direction. The top, front, and right views are shown in Figure S6 (Supporting Information).

⎛ nπ ⎞ + σdc,NZFO tan⎜ ⎟Bωn − 1 + ε∞ ,NZFO ⎝2 ⎠

[ε(0)NZFO − ε∞ ,NZFO]ωτNZFO 2 1 + ω2τNZFO

+

σdc,NZFO ω

while the minimum of 0.75N′p0,NZFOχ′0,NZFO appears at ⟨110⟩ directions. From the effective dielectric susceptibility distributed along the different direction, it is seen that when the electric field is applied vertically in the composite with (100) oriented NZFO, the static permittivity ε(0)NZFO|(100)⊥ can be obtained as 1 + 0.805N′p0,NZFOχ′0,NZFO. When the electric field is applied horizontally in the composite with (100) oriented NZFO, the static permittivity ε(0)NZFO|(100)// is the average of all possible electric field paralleled to Y′−O′−Z′ plane and given as 1 + 0.786N′p0,NZFOχ′0,NZFO. For randomly oriented thin films, all of the dipoles are uniformly distributed in the 3D space. The static permittivity ε(0)NZFO|random is the average of all possible electric field and calculated as 1 + 0.785N′p0,NZFOχ′0,NZFO by vector sphere model. It is obvious that when the external electric field is applied along the ⟨100⟩ direction of NZFO lattice in the thin film with (100) oriented NZFO, a maximum angle appears between the resultant dipolar and external field directions, resulting in the maximum response rate of the resultant Fe2+− Fe3+ pairs and thus maximum static permittivity of NZFO. When the external electric field is applied for NZFO randomly oriented, the angle between the resultant dipolar and external field directions is lower than that of the (100) oriented one, resulting in the lower response rate of the Fe2+−Fe3+ pair dipole and the static permittivity of NZFO. However, the real permittivity of the NZFO is the cocontribution of the static permittivity and the permittivity contributed by space charge polarization, which is directly related to the dc conductivity shown as eq 10. The real and imaginary parts of permittivity of BTO phase ε′BTO and ε″BTO are expressed as

(1 + Bωn)

(10)

where ε(0)NZFO, ε∞,NZFO, and τNZFO are the static permittivity, high-frequency permittivity, and relaxation time of NZFO, respectively. ω is the angular frequency. B and n are the prepower factor and power factor, which describe the frequency dependence of complex permittivity. The value of B ≈ 1 and 0 < n < 1. The first and second terms of eq 10 are the contributions from dipole response and the space-charge polarization, which is directly related to the dc conductivity, respectively.36,40 It is known that the charge hopping between Fe2+−Fe3+ pairs can be approximately considered as the electric dipole of ferrite.33,37 Like the previous discussion for dc conductivity, the electron -pair configuration can also be used in analyzing permittivity in NZFO phase. Hence, the effective dielectric susceptibility χ′|[hkl] contributed by one NZFO dipole with [hkl] direction, which is the response rate of dipole under the changeable external electric field, can be expressed as21 χ ′NZFO |[hkl] = sin ζ |[hkl]p0,NZFO χ ′0,NZFO

(11)

where ξ|[hkl] is the deviation angle of external electric field from the dipole along the [hkl] direction. p0,NZFO and χ′0,NZFO are the dielectric moment and static dielectric susceptibility of NZFO, respectively. The total dielectric susceptibility contributed by NZFO lattice is thus the sum of the contributions from all possible Fe2+−Fe3+ pairs. As previously mentioned, 12 possible Fe2+−Fe3+ pairs of NZFO uniformly distribute along with the ⟨110⟩ directions of lattice. The number of each Fe2+−Fe3+ pairs along the [hkl] direction is 1/12 of total Fe2+−Fe3+ pairs (N′). Therefore, the dielectric susceptibility of NZFO is expressed as χ ′NZFO =

ε′BTO =

1 N ′p0,NZFO χ ′0,NZFO (sin ζ |[110] + 12 ··· + sin ζ |[011])

ε″BTO =

(12)

ε(0)BTO − ε∞ ,BTO 2 1 + ω 2τBTO + ε∞ ,BTO

⎛ nπ ⎞ + σdc,BTO tan⎜ ⎟Bωn − 1 ⎝ 2 ⎠

[ε(0)BTO − ε∞ ,BTO]ωτBTO 1+

2 ω 2τBTO

+

σdc,BTO ω

(1 + Bωn) (13)

The 3D plot of the dielectric susceptibility of NZFO as a function of external field direction of β′ ∈ [0, 90°] and α′ ∈ [0, 90°] can be schematized as the “rainbow surface” in Figure 11, in which the maximum dielectric susceptibility of NZFO of 0.805N′p0,NZFOχ′0,NZFO is obtained at the ⟨100⟩ directions,

The permittivity of composite thin film is approximately compatible with the Kirkpatrick model24,36 with which the corresponding parameters of BTO and NZFO based on eqs 10 and 13 are adopted. (See the detail in the Supporting 18003


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effective route to design the multiferroic with the excellent and required magnetic and dielectric properties.

Information.) According to experimental data shown in Figure 8, the ratio of the real part of permittivity of the 0.2BTO/ 0.8NZFO composite thin film with and without (100) oriented NZFO under external electric field out-of-plane and in-plane, at the applied frequency of 1 MHz, is obtained as 1.02 and 1, respectively, which are very close to the experimental data of 1.04 and 0.98, and the ratio of the imaginary part of permittivity of them is 69.53 and 73.22%, respectively, which are also agreeable with the experimental data of 69.5 and 72.5%. (The calculation in detail is given in the Supporting Information.) It is obvious that the permittivity of the BTO/NZFO composite thin film is azimuthally controlled by the charge hopping between the Fe2+−Fe3+ pair in the conductive NZFO phase. As is known, the dielectric loss, which is characteristically represented by the lowest value of ε″/ε′, is significantly contributed by dc conductivity, which is azimuthally dependent on the charge hopping and orientation of the NZFO inside the thin film. According to discussion for conductivity, the dielectric loss of composite thin film with (100) oriented NZFO will be much smaller than that with randomly oriented ferrite. This azimuthally controllable characteristic results in the dielectric loss of composite thin film with (100) oriented NZFO under the out-of-plane (ε″|(100)⊥/ε′|(100)⊥ = 0.008) and in-plane electric field (ε″|(100)///ε′|(100)// = 0.009) are 33.3 and 25% lower than that of the composite thin films without oriented NZFO (ε″|random/ε′|random = 0.012), respectively.

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ASSOCIATED CONTENT

* Supporting Information S

Calculation for the real saturation magnetization, dc conductivity of NZFO lattice without orientation. View of 3D plot of the real saturation magnetization, initial susceptibility, dc conductivity, and dielectric susceptibility of spinel NZFO ferrite under the external field in the direction with different polar and azimuthal angles from different viewpoint. Derivation for the initial susceptibility, dc conductivity of the BTO/NZFO composite thin film. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b06429.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate the financial support from the Natural Science Foundation of China under grant nos. 51272230 and 50872120 and Zhejiang Provincial Natural Science Foundation under grant no. Z4110040.

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CONCLUSIONS The nanocrystalline BTO/NZFO composite thin films were successfully prepared by RF magnetron sputtering deposited on silicon substrate. All of the magnetic properties of BTO/NZFO composite are azimuthally controlled by Zn/Fe−O−Ni/Fe superexchange couplings in NZFO. The magnetic moments of Zn/Fe−O−Ni/Fe couplings, which are configured in NZFO phase and assigned parallel and rotated to a very small spatial angle along an external magnetic field in the BTO/NZFO composite thin film with oriented NZFO, contribute higher saturation magnetization than that of the thin film with randomly distributed NZFO. The initial susceptibility contributed by the response rate of all Zn/Fe−O−Ni/Fe couplings in the initial state reaches the highest in the (100) oriented NZFO with external field along ⟨100⟩ direction in which the largest angle between external magnetic field and resultant coupling appears. The resultant easy direction of magnetization for all coupling is different in the thin film with oriented NZFO from that of thin film without orientation of NZFO and contributes both higher magnetocrystalline anisotropy field and higher coercivity in the thin film with (100) NZFO and both lower ones in the thin film with randomly oriented NZFO. The dielectric properties of the BTO/NZFO composite are also controlled azimuthally by the charge hopping between Fe2+− Fe3+ pairs in NZFO. The resultant direction of charge hopping in the composite with (100) oriented NZFO occupies the larger angle between hoping and measuring (out-of-plane or inplane) directions and contributes smaller dc conductivity than that of thin film with randomly oriented ferrite. The azimuthally controllable characteristic of the dc conductivity and static permittivity makes the real and imaginary parts of permittivity azimuthally be controlled by the charge hopping in the conducting phase. The dielectric loss of the thin film with oriented NZFO is thus much smaller than that of the thin film without orientation. Obviously, controlling the arrangement and orientation of magnetic coupling and charge hopping is an

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Azimuthally Controlled Magnetic and Dielectric Properties of

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