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Magnetic and dielectric characterization of xBiFeO3:(1-x)SrFe12O19 multiferroic composites
Journal of Alloys and Compounds 808 (2019) 151700
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Magnetic and dielectric characterization of xBiFeO3:(1-x)SrFe12O19 multiferroic composites rez a, A.M. Bolarín-Miro a, F. Pedro-García a, C.A. Corte s-Escobedo b, J.P. Martínez-Pe nchez-De Jesús a, * n c, F. Sa A. Barba-Pingarro noma del Estado de Hidalgo Mineral de la Reforma, 42184, Hidalgo, Mexico Area Acad emica de Ciencias de la Tierra y Materiales, Universidad Auto n e Innovacio n Tecnolo gica, 02250, Ciudad de M Instituto Polit ecnico Nacional, Centro de Investigacio exico, Mexico c Centro de Ingeniería de Superficies y Acabados (CENISA), Facultad de Ingeniería. UNAM, Circuito Exterior, Ciudad Universitaria, 04510, Ciudad de M exico, Mexico a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 April 2019 Received in revised form 30 July 2019 Accepted 4 August 2019 Available online 5 August 2019
Multiferroic composites xBiFeO3:(1-x) SrFe12O19 (0.5 x 1, Dx ¼ 0.1) were produced by mixing powders of BiFeO3 and SrFe12O19 obtained by high energy ball milling assisted with heat treatment. To study their multiferroic properties, the ferromagnetic, dielectric and magnetodielectric behavior was evaluated for each composite. The composites were produced using powders of BiFeO3 and SrFe12O19 that were mixed, pressed at 800 MPa, and sintered at 700 C for 4 h. XRD analysis confirms the presence of both ferroic phases, BiFeO3 and SrFe12O19, and small amounts of the secondary phase, Bi2Fe4O9 (mullite). The quantity of this secondary phase increases with the concentration of strontium hexaferrite. The remanent magnetization values are 17.9 emu/g and 2.54 emu/g for x ¼ 0.5 and x ¼ 0.9, respectively. The coercive field does not change with the composition; it exhibits a nearly constant value of 5.5 kOe for all the samples containing strontium hexaferrite. The addition of strontium hexaferrite produces diminution of the relative permittivity (xr) and dielectric losses (tan d). At 5 MHz, the composite with x ¼ 0.9 shows the highest relative permittivity (16.78), and a diminution of dielectric loses of 43.88% due to the higher resistivity of the strontium hexaferrite. The magnetodielectric measurements showed an increase in the relative permittivity of the composites due to a reduction of the resistivity in agreement with the Maxwell-Wagner behavior when a magnetic field was applied. This study show evidence for magnetoresistive behavior by pure bismuth ferrite at room temperature, which has not been previously reported. © 2019 Elsevier B.V. All rights reserved.
Keywords: Bismuth ferrite Strontium hexaferrite Magnetoresistance Dielectric Multiferroic Composite
1. Introduction Bismuth ferrite (BiFeO3, BFO) has a perovskite crystal structure distorted in the [111] direction and crystallizes in the rhombohedral space group R3c. Moreover, to the best of our knowledge, is the only single phase material which shows multiferroic behavior above room temperature, specifically, presenting ferroelectricity and antiferromagnetism [1e3]. However, the antiferromagnetic order limits their potential applications [4]. To overcome this problem, doping with magnetic cations such as Ni or Co has been proposed [5,6]. Nevertheless, high doping concentrations destabilize its rhombohedral crystal structure and, consequently, reduce its
* Corresponding author. E-mail address: [email protected] (F. S anchez-De Jesús). https://doi.org/10.1016/j.jallcom.2019.151700 0925-8388/© 2019 Elsevier B.V. All rights reserved.
dielectric properties and limit its performance as a multiferroic material. In this sense, the interest in producing multiferroic composites, combining ferroelectric and ferromagnetic materials, has grown. These composites are usually high energy density materials that can be configured to store and release energy (electrical, magnetic and mechanical) in a well-regulated manner, making them highly useful in sensors, actuators and signal processing devices [7,8]. It is worth mentioning that the strontium hexaferrite (SrFe12O19, HFS) phase is a known hard magnetic material with high values of saturation magnetization (MS z 64 emu/g), coercive field (Hc z 5 kOe) and remanent magnetization (Mr z 45 emu/g) [9]. In addition, it has a high resistivity (2 108 Ucm) [10] and high relative permittivity (εr > 100). Recently, hexaferrites have been reported as multiferroics with coexisting spontaneous electric and magnetic polarizations characterized by a large magnetoelectric
2
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coupling [11e13]. A mixture of BFO as the ferroelectric material with HFS as the ferromagnetic material is of significant research interest in order to combine the high electrical polarization of the BFO [14] together with the high magnetization presented by the well-known strontium hexaferrite [15]. In these composites, the magnetoelectric effect can arise from the mechanical deformation of the ferroelectric and ferromagnetic phases according to Van Suchtelen [16], or there could be a magnetocapacitive effect arising from the magnetoresistance of the constitutive phases (BFO) [17,18]. Some studies of this composite show a material with high chemical and thermal stability, signal inversion of Temperature Coefficient of Capacitance (TCC) and interesting results in the radio frequency range, together with the ferromagnetic order [19]. Despite the combination of ferroelectric and ferromagnetic properties, other issues determine the performance of the composites. Some of these issues are: i) the formation of secondary phases due to the chemical interaction of the composite constituents during sintering, ii) the higher conductivity of the ferromagnetic phase, which disrupts the electric polarizability and dielectric properties of the ferroelectric phase and, finally, iii) the sintering process playing an important role in the mechanical interaction between the phases, such that a porous microstructure will result in a poorer magneto-electric coupling [20]. The chemical instability issue in these composites is complex and has a variety of sources, beginning with the difficulty in obtaining pure BFO; this is due to the stochiometric nature of the BiFeO3 and the high volatility of Bi3þ [21]. The former problems can be solved either by chemical stabilization of the BFO with an addition of dopant ions, such as Sr2þ or Ca2þ, or by the reduction of the annealing temperature when mechanical activation is used to synthesize this phase [22]. Another source of chemical instability is the interaction between both phases during the sintering process; this problem is produced by the chemical composition of SrFe12O19 (HFS), which leads to a high iron concentration medium that promotes the formation of BFO secondary phases, such as mullite (Bi2Fe4O9). In this binary composite, the resistivity of the ferromagnetic phase HFS (z0.89$108 Ucm) is greater than the resistivity of the ferroelectric one BFO (z1$106 Ucm). Therefore, a reduction of the dielectric losses due to the low resistivity of the bulk BFO can be expected after the addition of the ferromagnetic phase. The low resistivity values in ferrites are due to a conduction mechanism caused by electron hopping between Fe2þ and Fe3þ ions; this conduction mechanism is temperature and frequency dependent, in agreement with the Maxwell-Wagner theory. The conduction related issues can be attenuated with Zn and Cu substitutions in the HFS phase [23] as well as by a careful control of the morphology of the composites to produce a well-dispersed ferromagnetic phase in a ferroelectric matrix such that the conducting ferroelectric particles will be separated by a high insulating ferromagnetic phase [24]. In fact, the dielectric properties of various ferroelectricferromagnetic composites have been reported in the literature [25e27], such as PbTiO3eSrFe12O19 [28], ZnFe2O4eNa0.5Bi0.5TiO3 [29], xBaTiO3:(1-x)SrxFe12O19 [30] and, recently, the material studied in this work, xBiFeO3:(1-x)SrFe12O19 [31]. In all the reported composites, the coexistence between the electrical and magnetic properties has attracted special attention with respect to their multifunctional properties. For example, new multiferroic composites that are capable of maintaining up to four polarization states, two ferroelectric (-PS,þPs), and two ferromagnetic (-Ms,þMs) states can be used to manufacture four logic state memories, increasing the density of information storage [32]. Another application of multiferroic composites is the development of electric field-controlled magnetoresistance memories [33] and
the electric field control of a ferromagnetic material coupled to a ferroelectric-antiferromagnetic multiferroic, such as BFO [34]. The aim of this work is to obtain multiferroic composites with magnetoelectric coupling from ferroelectric and ferromagnetic phases, xBFO:(1-x)HFS, varying x from 0.5 to 1, by using mechanically assisted synthesis. We expect the coexistence of low dielectric constant and high dielectric constant regions due to a MaxwellWagner (M W) polarization in the composite. The influence of the composition on the dielectric, magnetic and electric properties will be analyzed. Moreover, the magnetodielectric coupling in the studied composites is discussed. 2. Experimental details Fe2O3 (Sigma Aldrich, 99% purity), Bi2O3 (Sigma Aldrich, 99% purity) and SrO (Sigma Aldrich, 99% purity) were used as precursors to obtain ferroelectric BiFeO3 and ferromagnetic SrFe12O19 phases. For each case, a stoichiometric mixture of 5 g of powder oxides were loaded in a steel vial together with steel balls (ball to powder weight ratio 8:1), according to the following reactions:
Bi2 O3 þ Fe2 O3 /2BiFeO3
(eq. 1)
SrO þ 6Fe2 O3 /SrFe12 O19
(eq. 2)
Each mixture was milled using a shaker mixer mill (SPEX 8000D) for 5 h in intervals of 90 min with a rest time of 30 min between each interval to avoid overheating the system. After the mechanochemical process, the mixtures were thermally treated for 2 h at 650 C for the BFO, and 2 h at 850 C to obtain HFS, according to the experimental conditions established in previous studies [15,22]. X-ray diffraction analysis was used to confirm the presence of the desired phases. The binary composites xBFO:(1-x)HFS, (0.5 x 1, Dx ¼ 0.1) were produced by means of high-energy ball milling, using a ball to powder weight ratio of 60:1 for 2 min and adding 5 wt % of ethylene bis stearamide (C38H76N2O2) powder as binder. After that, the powders were uniaxially pressed at 800 MPa, obtaining cylindrical test samples of 10 mm diameter, which were sintered at 700 C for 4 h in air. Phase identification was carried out by X-ray diffraction using a Siemens D500 diffractometer with CuKa1 (l ¼ 1.541874 Å) with increments of 0.02 (2q) in an interval 2q of 10e80 . The Rietveld refinement method was used to calculate the amount of the different phases. Magnetic characterization was carried out with a MicroSense EV7 vibrating sample magnetometer (VSM) with a maximum field of 18 kOe. The dielectric properties of the composites were measured with a Hioki 3532-50 LCR meter in the frequency range of 50 Hz- 5 MHz. Additionally, with the aim of observing the morphology and phases distribution of the composites, scanning electron micrographs were taken using a JEOL-100-CX II system. Energy-dispersive X-ray spectroscopy (EDS) microanalysis was used to determine the elemental composition in SEM micrographs. The magnetodielectric studies were performed with the MicroSense EV7 VSM and the Hioki 3532-50 LCR meter using a custom designed sample holder. 3. Results and discussion Fig. 1 shows the X-ray diffraction (XRD) patterns for cylindrical test samples of (x)BFO:(1-x)HFS, with x varying from 0.5 to 1.0 (for reference, the x ¼ 0 sample is included). In the XRD pattern corresponding to the sample x ¼ 0, the main presence of strontium hexaferrite (SrFe12O19, ICSD 16158, P63mmc) can be observed, accompanied with small amounts of hematite (Fe2O3, ICSD 22505, R-3c). This is attributed to an incomplete reaction between the precursor oxides, revealed through the Rietveld refinement. The
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Fig. 1. DRX patterns of cylindrical test samples of xBFO:(1-x)HFS, 0.5 x 1.0 composites sintered at 700 C for 4 h.
XRD pattern corresponding to x ¼ 0.5 shows that the higher relative intensity peaks of diffraction belong to bismuth ferrite (BiFeO3, ICSD #75324, R3c), accompanied by a lower portion of mullite (Bi2Fe4O9, ICSD #26808, Pbam) as the secondary phase. Additionally, the highest relative intensity peak of SrFe12O19 is detected approximately 32.5 in 2q, just after the BiFeO3 “twin peaks”, which are the bismuth ferrite representative diffraction peaks. The largest relative intensities belonging to strontium hexaferrite are small according to the weight ratio, 50 wt % and 50 wt % for BiFeO3 and SrFe12O19, respectively. This result can be explained by two reasons. i) One reason is the intensity of scattered radiation from the atom IA, given by the equation:
IA ¼ Z Ie
(eq. 3)
where Z is the number of electrons in an atom and Ie is the intensity of scattered radiation from a single electron; hence, the BFO diffraction peak intensity is larger due to the higher number of electrons present in the Bi atoms relative to the electrons in Fe and Sr atoms. ii) The second reason is the intensity formula: the intensity formula takes into account more factors to determine the total intensity of the scattered radiation, such as, the number of unit cell per unit volume (N), the integrated intensity for the hkl set of planes (I), volume of the crystal (V), electron charge), speed of light (c), mass of an electron (m), temperature factor, Lorentz and polarization factors (Lp), absorption factor (A) and the structure factor (Fhkl).
Ihkl ¼
! 3 N 2 e4 l V 2 TLp j jF hkl A 2m2 c4
3
phase BiFeO3 is characterized by the presence of the “twin peaks” located approximately 32 of 2q and mullite as a minority phase. Fig. 2 shows an amplified zone of the XRD patterns from 30 to 37 of xBFO:(1-x)HFS samples and the representative peaks of the constitutive phases. The stoichiometric nature of the BiFeO3 and the high iron concentration of the strontium hexaferrite produce a chemical interaction due to the diffusion of iron and strontium into BiFeO3, resulting in the formation of Bi2Fe4O9 as secondary phase and the transition of BiFeO3 structure from rhombohedral to orthorhombic, visible as a change in the relative intensities of the bismuth ferrite twin peaks as shown by Pedro [35] and Bhushan [36]. The inhibition of the Bi2Fe4O9 phase is difficult through mechanical-assisted synthesis. The X-ray diffraction analysis confirms the existence of a chemical interaction between the strontium hexaferrite and the bismuth ferrite. To quantify the phase weight percent, Rietveld refinements were carried out, and the results are shown in Table 1. For sample x ¼ 0 (pure HF), the main presence of strontium hexaferrite can be quantified (99 wt %), accompanied with a small amount of hematite (1 wt %). For x ¼ 0.9, 0.8, 0.7, 0.6 and 0.5, samples present a mixture of SrFe12O19, BiFeO3 and Bi2Fe4O9 for which the phase concentrations are close to the proposed compositions. Bismuth ferrite is characterized as an intermetallic compound with a narrow region in the Bi2O3eFe2O3 phase diagram, in which a minor change in stoichiometry tends to form another phases, such as Bi25FeO40 or Bi2Fe4O9. In this case, the Fe2O3 excess comes from the SrFe12O19. For x ¼ 1.0 (pure BFO), it is possible to quantify the presence of the BiFeO3 and mullite, approximately 92 and 8 wt %, respectively. The refinement parameters show a good adjustment, as shown in Table 1. The magnetic hysteresis loops at room temperature of different composites of xBFO:(1-x)HFS, 0.5 x 1.0, are depicted in Fig. 3. For x ¼ 1, which corresponds to the BFO sample, a linear behavior is observed showing a small magnetic susceptibility of 6.64 104 associated with the antiferromagnetic and paramagnetic behavior of BiFeO3 and Bi2Fe4O9, respectively, at room temperature [22,37]. The composition of x ¼ 0.0, which corresponds to pure HFS, shows the magnetic hysteresis loop of a hard ferromagnetic material [15],
(eq. 4)
From this point of view, it is important to observe that N indicates the number of unit cells per unit volume. When comparing the lattice parameters of the BFO (a ¼ 5.58 Å, c ¼ 13.90 Å) to the lattice parameters of the HFS (a ¼ 5.884 Å, c ¼ 23.05 Å), it is evident that N will be greater for BFO than for HFS. It is attributed to the small size of the BFO unit cell, due to a larger amount of unit cells per volume unit of the BFO relative to the unit cells per volume of the HFS. This explains the observed relative intensities in the XRD patterns. In addition, mullite increases with increased bismuth ferrite. According to the Bi2O3eFe2O3 phase diagram, an excess in Fe2O3 stoichiometry promotes the mullite phase, and, in this case, excessive hematite could be provided by strontium hexaferrite. Fig. 1 also shows that, for x ¼ 1.0 (pure BFO), the perovskite
Fig. 2. Zoom in of the diffraction pattern (30e37 of 2-theta) from DRX patterns for xBFO:(1-x)HFS, 0.5 x 1.0, sintered at 700 C for 4 h.
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Table 1 Rietveld analysis results of cylindrical test samples xBFO:(1-x)HFS composites, with x varying from 0.5 to 1.0, Dx ¼ 0.1. Composition wt.%BFO:wt.%HFS X BFO 90BFO:10HFS
80BFO:20HFS
70BFO:30HFS
60BFO:40HFS
50BFO:50HFS
HFS
Phase, Space group Phase (wt. %)
1.0 BiFeO3, R3c Bi2Fe4O9, Pbam 0.9 SrFe12O19, P63mmc BiFeO3, R3c Bi2Fe4O9, Pbam 0.8 SrFe12O19, P63mmc BiFeO3,R3c Bi2Fe4O9, Pbam 0.7 SrFe12O19, P63mmc BiFeO3, R3c Bi2Fe4O9, Pbam 0.6 SrFe12O19, P63mmc BiFeO3,R3c Bi2Fe4O9, Pbam 0.5 SrFe12O19, P63mmc BiFeO3, R3c Bi2Fe4O9, Pbam 0.0 SrFe12O19, P63mmc Fe2O3, R3c
92.42 7.57 10.59 81.32 8.07 18.33 67.71 13.94 28.47 57.13 13.88 36.21 52.13 12.23 44.06 45.26 10.67 98.973 1.02
c2 1.31 1.71
1.56
1.43
1.67
1.34
1.47
with high specific magnetization (Ms of 63 emu/g at 18 kOe), remanent magnetization (Mr of 33 emu/g), and high coercivity (Hc ~5.5 kOe), in good agreement with the characteristic values reported for strontium hexaferrites [15]. Samples with concentrations of x ¼ : 0.5, 0.6, 0.7, 0.8 and 0.9, show a similar behavior, a ferrimagnetic order with a high coercive field, approximately 5 kOe, which is attributed to the presence of the hard ferrimagnetic phase, SrFe12O19. The coercivity is an intrinsic property and does not depend on weight content; hence, its values remain constant regardless of if the samples have low or high concentration of strontium hexaferrite, as shown in Fig. 3. Conversely, the specific magnetization of the composites depends on the weight percent of each phase that forms it. The Ms values are not related to any structural transformation; they are attributed to the presence of different amounts of antiferromagnetic and ferrimagnetic phases [38]. If we assume a simple additive effect of magnetization, the specific magnetization is directly dependent of the HFS content according to the equation:
Fig. 3. Magnetic hysteresis loops at room temperature of xBFO:(1-x)HFS, 0.5 x 1.0 cylindrical test samples, sintered at 700 C for 4 h.
Ms ¼ ðwt:%ÞHFS ,MHFS þ ðwt:%ÞBFO ,MBFO
(eq. 5)
where ðwt:%ÞHFS is the weight percent of HFS in the composite, MHFS is the specific magnetization of HFS, ðwt:%ÞBFO is the amount of BFO in the composite, and MBFO is the specific magnetization of BFO. As BFO and mullite present antiferromagnetic order, the HFS concentration largely determines the composite's specific magnetization due to a difference of three orders of magnitude between the specific magnetization values of the phases. The loops do not show different magnetic behaviors as is shown by Singh [28] and de Morais [19], thus confirming a higher chemical homogeneity of the obtained composites. Table 2 shows magnetic properties obtained from the magnetic hysteresis loops at room temperature. To validate the homogeneity and the relationship between specific magnetization and the amount of HFS (wt. %) a fitting of the experimental data of the variation of Ms with the composition of HFS was done, as shown in Fig. 4. The fitting confirms a linear dependence of the specific magnetization with the strontium ferrite concentration, as described by the additive effect mentioned above. The dielectric characterization of the composites was performed using dielectric spectroscopy; the relative permittivity and dielectric losses (tan d) in the frequency range of 100 Hz to 5 MHz are shown in Figs. 5 and 6, respectively. As Fig. 5 shows, the relative permittivity for all the studied compositions decreases as the frequency increases. This is in good agreement with the Maxwell-Wagner behavior, in which the high relative permittivity at low frequencies is due to an accumulation of charges at the grain boundaries which have a larger resistivity than the grains of the samples [39]. In addition, in Fig. 5a, the bismuth ferrite phase (x ¼ 1) shows the highest relative permittivity at low frequencies. It is attributed to a higher conductivity of bismuth ferrite compared to strontium hexaferrite, which enhances the mobility of the charge carriers in the grains and its concentration at the boundaries. At low frequencies (and even at the 1 MHz range when the conductivity is large), the polarization is due to the Maxwell-Wagner effect. According to this effect, in a composite with two regions, which have different conductivities, such as grain/grain boundaries, the charge carries are trapped in the interface due to the difference in conductivity; therefore, the material with a larger conductivity leads to a higher charge carrier concentration at the interface, resulting in a higher electric polarization. In addition, it is also attributed to the displacement of the Bi3þ ions and their two unpaired 2s electrons, which should be present at low and high frequencies [40]. The presence of strontium hexaferrite in the composites tends to reduce its polarizability; however, this tendency is not uniform, nor linear, as the change in the magnetization. This can be explained by the complex arrangement of grains and grain boundaries of up to three different phases with different dielectric properties in the composites. Fig. 6 shows the dielectric losses (tan d) of the composites xBFO:(1-x)HFS, 0.5 x 1.0, at different frequencies. Fig. 6 has been separated in two figures, 6a and 6b, depending of the composition, in order to better visualize the behavior. This measurement allows identifying if there are some relaxation phenomena. As Fig. 6 shows, there are two relaxation processes in the studied frequency range. The first one occurs at a frequency of approximately 1 kHz, and the second is detected between 100 and 200 kHz, attributed to the BFOeHFS intergrain Maxwell-Wagner relaxation. The process at 1 kHz is slightly observable in the pure strontium hexaferrite phase (x ¼ 0). However, both relaxation processes are clearly visible in some compositions (x ¼ 0.5 and 0.6) due to the higher conductivity of the bismuth ferrite phase. In addition, Fig. 6a and b demonstrate a reduction of the
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5
Table 2 Coercivity field (Hc), specific magnetization (Ms) and remnant magnetization (Mr) of xBFO:(1-x)HF composites, with x varying from 1.0 to 0.5, Dx ¼ 0.1. Composition wt.%BFO:wt.%HFS
x
Hc (kOe)
MS at 18 kOe (emu/g)
Mr (emu/g)
100BFO:0 HFS 90BFO:10HFS 80BFO:20HFS 70BFO:30HFS 60BFO:40HFS 50BFO:50HFS 0BFO 100HFS
1 0.9 0.8 0.7 0.6 0.5 0
e 5.5 5.5 5.5 5.5 5.5 5.5
20$103 4.26 11.12 18.36 25.56 32.53 62.83
e 2.54 6.09 11.03 13.96 17.90 35.38
Fig. 4. Specific magnetization (MS) at 18 kOe versus BFO concentration (x). The red line is a linear fit, and the dots correspond to the experimental values. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
dielectric losses with the increase of the strontium hexaferrite content, and pure bismuth ferrite (x ¼ 0) shows the highest values of dielectric losses, except in the relaxation peak of the strontium hexaferrite at 1 kHz. The higher resistivity of this phase produces a reduction of the loss current; this is the current that flows through the sample, which is the loss component of the tan d, following the equation:
tan d ¼
Il Ic
(eq. 6)
where Il is the loss current and Ic the capacitive current. According to the results previously presented,
the
0.6BFO:0.4HFS composite (x ¼ 0.6) shows a good combination of remanent magnetization (25.56 emu/g) and relative permittivity at 5 MHz (11.88), despite its high ferromagnetic phase content. For this reason, representative SEM micrographs of this composition were taken in order to qualitatively analyze the homogeneity, porosity and phase distribution of the composite. As Fig. 7a shows by presenting a micrograph of secondary electrons (SE), a smooth surface produced by small and uniform grains and low porosity of the compact material can be observed. In Fig. 7b, the same specimen is shown in back scattering mode. Three materials can now be differentiated based on contrast; they can be distinguished the presence of three different phases: one dark gray, one light gray, and another white, which are attributed to SrFe12O19, BiFeO3 and Bi2Fe4O9, respectively. According to Lloyd [41], the compounds that contain atoms with greater atomic number appears brighter in a back scattered electron image. In our study, the compound with lower atomic number is SrFe12O19, thus, it appears darkener in the analysis. However, an unexpected behavior between BiFeO3 and Bi2Fe4O9 is observed, because the Bi2Fe4O9 appears as the brightest phase, and it has lower atomic number than BiFeO3. The brighter phase corresponding to Bi2Fe4O9 is attributed to the presence of Au (with the highest atomic number) in Bi2Fe4O9. This is produced during the sputtering process, due to a deflection of the sputtered Au in the HFS and BFO phases; It was confirmed by means of EDS analysis, included in Fig. 7. Fig. 8 shows the magnetodielectric behavior of the pure bismuth ferrite (x ¼ 1). It was evaluated through the analysis of the variation of relative permittivity and dielectric losses with the frequency, under different magnetic applied field, expressed indirectly as the real part (ε0 r) of the relative permittivity (Fig. 8a), and the imaginary part (ε00 r) of the relative permittivity (Fig. 8b), respectively. An interesting variation of the dielectric properties can be appreciated when high fields are applied. It is attributed to an increase in the resistivity of the phase probably due to an interfacial magnetoresistance caused by spin dependent tunneling or spin dependent
Fig. 5. Relative permittivity versus frequency of xBFO:(1-x)HF composites, with x varying from (a) 0.7 to 1, and (b) 0.5 to 0.6 and x ¼ 0.
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Fig. 6. Dielectric losses versus frequency of xBFO:(1-x)HF composites, with x varying from: (a) 0.7 to 1, and (b) 0.5 to 0.6 and x ¼ 0.
Fig. 7. SEM micrographs of 0.6BFO:0.4 H F composite obtained by: (a) secondary electrons (SE) and, (b) back scattered electrons (BSE).
Fig. 8. (a) Real relative permittivity (ε0 r), and (b) imaginary relative permittivity (ε'‘r) of pure BFO (x ¼ 1) under different applied magnetic fields of 0, 4, 10 and 15 kOe.
scattering [42,43]. These phenomena have been reported in the multiferroic perovskites lanthanum-strontium manganites [43,44] at low temperatures and close to the magnetic Curie temperature at approximately 100 K. The magnetoresistance arises from the progressive domain rotation, which causes a spin polarization of the charge carriers and increases the resistivity at the interfaces with different domain directions. However, under a magnetic field of
15 kOe, the positive magnetoresistance changes to a negative magnetoresistance. This change in behavior is due to almost all the domains having the same orientation at high magnetic fields; thus, the conductivity is increased. One of the effects of an increased resistivity is the change of the relaxation process present at approximately 10 kHz to lower frequencies, as shown in Fig. 8. The relative permittivity values at 0, 4, 10 and 12 kOe converge at
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50 kHz; this is due to the frequency suppression of the MaxwellWagner polarization mechanism. When the magnetoresistance is negative, there is an increment of the charge carriers, which has the effect of extending the M W polarization region beyond 5 MHz. The dielectric losses (Fig. 8b) show high values at low frequencies and a reduction when the frequency is increased, in agreement to the Maxwell-Wagner behavior. The dielectric losses at 4, 10 and 12 kOe reach their minimum at low frequencies and have lower losses respect to the 0 (nonfield increased magnetoresistance), and 15 kOe (negative magnetoresistance) applied fields due to the positive magnetoresistive effect. The change in the relative permittivity values due to the change in resistivity is attributed to the modification in mobility of the charge carriers, which diminishes at high resistance values and increases the relaxation time of the space charge polarization; hence, the relaxation frequency is reduced, as shown in Fig. 8. When the BFO sample is under the influence of a magnetic field larger than 14 kOe, the magnetoresistivity becomes negative. This is related to the spin polarization of the charge carriers and the progressive alignment of the antiferromagnetic domains, which at low fields, start to rearrange, and the resistivity increases. However, at high fields, the realignment is complete, and the resistivity goes down. These changes in resistivity produce a variation of the capacitance values due to the changes in mobility of the charge carriers. Fig. 9 shows the magnetocapacitance at 5 MHz under the effect of different magnetic applied fields. For the sample x ¼ 1.0 (pure BFO), there is no change in the capacitance until a field of 14 KOe or larger, which is consistent with the magnetoresistance mechanism proposed before. For x ¼ 0.0 (pure HFS), the magnetoresistance behavior is similar to that of MnZn Ferrite as shown by Hu et al. [45], where the resistivity is reduced when a magnetic field of 1 kOe or larger is applied, increasing the capacitance values. The composites BFOeHFS show the same behavior when a field is applied. The increase in the capacitance values when a field is applied and the effect of the BFO over the magnetocapacitance is ambiguous and need further studies. The samples x ¼ 0.6 and 0.7 show an increase relative to the sample x ¼ 0, and the samples with x ¼ 0.5 and 0.9 exhibit a decrease in the magnetocapacitance values; however, the sample with x ¼ 0.8 shows a negative magnetocapacitance value, which can be attributed to the skin effect producing an increase of resistivity at high frequencies. Samples with x ¼ 0.0, 0.5, 0.6, 0.8 showed a small change in magnetocapacitance when the field changed from 5 to 6 kOe, which could be related to
7
the high coercive field of the HFS. However, in order to explain the behavior, further studies must be carried out to determine the precise mechanisms involved in magnetic field dependent resistance of the samples. The performed studies in the composites did not show magnetoelectric coupling between BFO and HFS phases since no perturbation of the composites magnetic field was observed when an A.C. electric field was applied. 4. Conclusions Multiferroic composites (x)BFO:(1-x)HFS were successfully synthesized by high energy ball milling assisted with heat treatment. The composites were obtained by mixing and sintering both materials. There was a chemical interaction between the ferroic phases, producing the transition of BFO from orthorhombic to rhombohedral and the growth of the Bi2Fe4O9 phase. All the compositions exhibit multiferroic character, since they present ferroelectricity and ferromagnetism at room temperature. All the studied composites show ferromagnetic order accompanied by diminution of the dielectric losses due to the high resistivity of the HFS phase. In addition, the relative permittivities of the composites measured at high frequency (5 MHz) show high values when bismuth ferrite is present. A maximum value of relative permittivity εr of 16.78 was obtained for the composite with x ¼ 0.9, which showed a remanent magnetization Mr of 2.54 emu/g. The data confirm the ferroelectric and ferromagnetic behavior of the composites when compared to the null remanent magnetization of the BFO ferroelectric-antiferromagnetic phase and the lower relative permittivity εr ¼ 5.95 at 5 MHz for the pure strontium hexaferrite. Evidence of magnetoelectric behavior was not found in the composites, but the results confirmed magnetodielectric coupling due to the room temperature spin-dependent magnetoresistance of the BFO phase. The BFO magnetoresistive behavior offers the possibility of developing sensors or magnetically tuned filters. The observed magnetoresistance shown by the BFO need further detailed studies to confirm the precise mechanism that produces it, with the aim to control it for its possible applications. Acknowledgments The authors thank the Center of Nanosciences and Micro and Nanotechnology, as well as CIITEC of the National Polytechnic rrez for Institute; in particular, we thank Dr. Hugo Martinez Gutie the morphological characterization and Mariana Alvarez Torres for the XRD characterization. References
Fig. 9. Magnetocapacitance of xBFO:(1-x)HF composites, with x varying from 0 to 1, Dx ¼ 0.1, at 5 MHz.
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Magnetic and dielectric characterization of xBiFeO3:(1-x)SrFe12O19 multiferroic composites rez a, A.M. Bolarín-Miro a, F. Pedro-García a, C.A. Corte s-Escobedo b, J.P. Martínez-Pe nchez-De Jesús a, * n c, F. Sa A. Barba-Pingarro noma del Estado de Hidalgo Mineral de la Reforma, 42184, Hidalgo, Mexico Area Acad emica de Ciencias de la Tierra y Materiales, Universidad Auto n e Innovacio n Tecnolo gica, 02250, Ciudad de M Instituto Polit ecnico Nacional, Centro de Investigacio exico, Mexico c Centro de Ingeniería de Superficies y Acabados (CENISA), Facultad de Ingeniería. UNAM, Circuito Exterior, Ciudad Universitaria, 04510, Ciudad de M exico, Mexico a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 April 2019 Received in revised form 30 July 2019 Accepted 4 August 2019 Available online 5 August 2019
Multiferroic composites xBiFeO3:(1-x) SrFe12O19 (0.5 x 1, Dx ¼ 0.1) were produced by mixing powders of BiFeO3 and SrFe12O19 obtained by high energy ball milling assisted with heat treatment. To study their multiferroic properties, the ferromagnetic, dielectric and magnetodielectric behavior was evaluated for each composite. The composites were produced using powders of BiFeO3 and SrFe12O19 that were mixed, pressed at 800 MPa, and sintered at 700 C for 4 h. XRD analysis confirms the presence of both ferroic phases, BiFeO3 and SrFe12O19, and small amounts of the secondary phase, Bi2Fe4O9 (mullite). The quantity of this secondary phase increases with the concentration of strontium hexaferrite. The remanent magnetization values are 17.9 emu/g and 2.54 emu/g for x ¼ 0.5 and x ¼ 0.9, respectively. The coercive field does not change with the composition; it exhibits a nearly constant value of 5.5 kOe for all the samples containing strontium hexaferrite. The addition of strontium hexaferrite produces diminution of the relative permittivity (xr) and dielectric losses (tan d). At 5 MHz, the composite with x ¼ 0.9 shows the highest relative permittivity (16.78), and a diminution of dielectric loses of 43.88% due to the higher resistivity of the strontium hexaferrite. The magnetodielectric measurements showed an increase in the relative permittivity of the composites due to a reduction of the resistivity in agreement with the Maxwell-Wagner behavior when a magnetic field was applied. This study show evidence for magnetoresistive behavior by pure bismuth ferrite at room temperature, which has not been previously reported. © 2019 Elsevier B.V. All rights reserved.
Keywords: Bismuth ferrite Strontium hexaferrite Magnetoresistance Dielectric Multiferroic Composite
1. Introduction Bismuth ferrite (BiFeO3, BFO) has a perovskite crystal structure distorted in the [111] direction and crystallizes in the rhombohedral space group R3c. Moreover, to the best of our knowledge, is the only single phase material which shows multiferroic behavior above room temperature, specifically, presenting ferroelectricity and antiferromagnetism [1e3]. However, the antiferromagnetic order limits their potential applications [4]. To overcome this problem, doping with magnetic cations such as Ni or Co has been proposed [5,6]. Nevertheless, high doping concentrations destabilize its rhombohedral crystal structure and, consequently, reduce its
* Corresponding author. E-mail address: [email protected] (F. S anchez-De Jesús). https://doi.org/10.1016/j.jallcom.2019.151700 0925-8388/© 2019 Elsevier B.V. All rights reserved.
dielectric properties and limit its performance as a multiferroic material. In this sense, the interest in producing multiferroic composites, combining ferroelectric and ferromagnetic materials, has grown. These composites are usually high energy density materials that can be configured to store and release energy (electrical, magnetic and mechanical) in a well-regulated manner, making them highly useful in sensors, actuators and signal processing devices [7,8]. It is worth mentioning that the strontium hexaferrite (SrFe12O19, HFS) phase is a known hard magnetic material with high values of saturation magnetization (MS z 64 emu/g), coercive field (Hc z 5 kOe) and remanent magnetization (Mr z 45 emu/g) [9]. In addition, it has a high resistivity (2 108 Ucm) [10] and high relative permittivity (εr > 100). Recently, hexaferrites have been reported as multiferroics with coexisting spontaneous electric and magnetic polarizations characterized by a large magnetoelectric
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coupling [11e13]. A mixture of BFO as the ferroelectric material with HFS as the ferromagnetic material is of significant research interest in order to combine the high electrical polarization of the BFO [14] together with the high magnetization presented by the well-known strontium hexaferrite [15]. In these composites, the magnetoelectric effect can arise from the mechanical deformation of the ferroelectric and ferromagnetic phases according to Van Suchtelen [16], or there could be a magnetocapacitive effect arising from the magnetoresistance of the constitutive phases (BFO) [17,18]. Some studies of this composite show a material with high chemical and thermal stability, signal inversion of Temperature Coefficient of Capacitance (TCC) and interesting results in the radio frequency range, together with the ferromagnetic order [19]. Despite the combination of ferroelectric and ferromagnetic properties, other issues determine the performance of the composites. Some of these issues are: i) the formation of secondary phases due to the chemical interaction of the composite constituents during sintering, ii) the higher conductivity of the ferromagnetic phase, which disrupts the electric polarizability and dielectric properties of the ferroelectric phase and, finally, iii) the sintering process playing an important role in the mechanical interaction between the phases, such that a porous microstructure will result in a poorer magneto-electric coupling [20]. The chemical instability issue in these composites is complex and has a variety of sources, beginning with the difficulty in obtaining pure BFO; this is due to the stochiometric nature of the BiFeO3 and the high volatility of Bi3þ [21]. The former problems can be solved either by chemical stabilization of the BFO with an addition of dopant ions, such as Sr2þ or Ca2þ, or by the reduction of the annealing temperature when mechanical activation is used to synthesize this phase [22]. Another source of chemical instability is the interaction between both phases during the sintering process; this problem is produced by the chemical composition of SrFe12O19 (HFS), which leads to a high iron concentration medium that promotes the formation of BFO secondary phases, such as mullite (Bi2Fe4O9). In this binary composite, the resistivity of the ferromagnetic phase HFS (z0.89$108 Ucm) is greater than the resistivity of the ferroelectric one BFO (z1$106 Ucm). Therefore, a reduction of the dielectric losses due to the low resistivity of the bulk BFO can be expected after the addition of the ferromagnetic phase. The low resistivity values in ferrites are due to a conduction mechanism caused by electron hopping between Fe2þ and Fe3þ ions; this conduction mechanism is temperature and frequency dependent, in agreement with the Maxwell-Wagner theory. The conduction related issues can be attenuated with Zn and Cu substitutions in the HFS phase [23] as well as by a careful control of the morphology of the composites to produce a well-dispersed ferromagnetic phase in a ferroelectric matrix such that the conducting ferroelectric particles will be separated by a high insulating ferromagnetic phase [24]. In fact, the dielectric properties of various ferroelectricferromagnetic composites have been reported in the literature [25e27], such as PbTiO3eSrFe12O19 [28], ZnFe2O4eNa0.5Bi0.5TiO3 [29], xBaTiO3:(1-x)SrxFe12O19 [30] and, recently, the material studied in this work, xBiFeO3:(1-x)SrFe12O19 [31]. In all the reported composites, the coexistence between the electrical and magnetic properties has attracted special attention with respect to their multifunctional properties. For example, new multiferroic composites that are capable of maintaining up to four polarization states, two ferroelectric (-PS,þPs), and two ferromagnetic (-Ms,þMs) states can be used to manufacture four logic state memories, increasing the density of information storage [32]. Another application of multiferroic composites is the development of electric field-controlled magnetoresistance memories [33] and
the electric field control of a ferromagnetic material coupled to a ferroelectric-antiferromagnetic multiferroic, such as BFO [34]. The aim of this work is to obtain multiferroic composites with magnetoelectric coupling from ferroelectric and ferromagnetic phases, xBFO:(1-x)HFS, varying x from 0.5 to 1, by using mechanically assisted synthesis. We expect the coexistence of low dielectric constant and high dielectric constant regions due to a MaxwellWagner (M W) polarization in the composite. The influence of the composition on the dielectric, magnetic and electric properties will be analyzed. Moreover, the magnetodielectric coupling in the studied composites is discussed. 2. Experimental details Fe2O3 (Sigma Aldrich, 99% purity), Bi2O3 (Sigma Aldrich, 99% purity) and SrO (Sigma Aldrich, 99% purity) were used as precursors to obtain ferroelectric BiFeO3 and ferromagnetic SrFe12O19 phases. For each case, a stoichiometric mixture of 5 g of powder oxides were loaded in a steel vial together with steel balls (ball to powder weight ratio 8:1), according to the following reactions:
Bi2 O3 þ Fe2 O3 /2BiFeO3
(eq. 1)
SrO þ 6Fe2 O3 /SrFe12 O19
(eq. 2)
Each mixture was milled using a shaker mixer mill (SPEX 8000D) for 5 h in intervals of 90 min with a rest time of 30 min between each interval to avoid overheating the system. After the mechanochemical process, the mixtures were thermally treated for 2 h at 650 C for the BFO, and 2 h at 850 C to obtain HFS, according to the experimental conditions established in previous studies [15,22]. X-ray diffraction analysis was used to confirm the presence of the desired phases. The binary composites xBFO:(1-x)HFS, (0.5 x 1, Dx ¼ 0.1) were produced by means of high-energy ball milling, using a ball to powder weight ratio of 60:1 for 2 min and adding 5 wt % of ethylene bis stearamide (C38H76N2O2) powder as binder. After that, the powders were uniaxially pressed at 800 MPa, obtaining cylindrical test samples of 10 mm diameter, which were sintered at 700 C for 4 h in air. Phase identification was carried out by X-ray diffraction using a Siemens D500 diffractometer with CuKa1 (l ¼ 1.541874 Å) with increments of 0.02 (2q) in an interval 2q of 10e80 . The Rietveld refinement method was used to calculate the amount of the different phases. Magnetic characterization was carried out with a MicroSense EV7 vibrating sample magnetometer (VSM) with a maximum field of 18 kOe. The dielectric properties of the composites were measured with a Hioki 3532-50 LCR meter in the frequency range of 50 Hz- 5 MHz. Additionally, with the aim of observing the morphology and phases distribution of the composites, scanning electron micrographs were taken using a JEOL-100-CX II system. Energy-dispersive X-ray spectroscopy (EDS) microanalysis was used to determine the elemental composition in SEM micrographs. The magnetodielectric studies were performed with the MicroSense EV7 VSM and the Hioki 3532-50 LCR meter using a custom designed sample holder. 3. Results and discussion Fig. 1 shows the X-ray diffraction (XRD) patterns for cylindrical test samples of (x)BFO:(1-x)HFS, with x varying from 0.5 to 1.0 (for reference, the x ¼ 0 sample is included). In the XRD pattern corresponding to the sample x ¼ 0, the main presence of strontium hexaferrite (SrFe12O19, ICSD 16158, P63mmc) can be observed, accompanied with small amounts of hematite (Fe2O3, ICSD 22505, R-3c). This is attributed to an incomplete reaction between the precursor oxides, revealed through the Rietveld refinement. The
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Fig. 1. DRX patterns of cylindrical test samples of xBFO:(1-x)HFS, 0.5 x 1.0 composites sintered at 700 C for 4 h.
XRD pattern corresponding to x ¼ 0.5 shows that the higher relative intensity peaks of diffraction belong to bismuth ferrite (BiFeO3, ICSD #75324, R3c), accompanied by a lower portion of mullite (Bi2Fe4O9, ICSD #26808, Pbam) as the secondary phase. Additionally, the highest relative intensity peak of SrFe12O19 is detected approximately 32.5 in 2q, just after the BiFeO3 “twin peaks”, which are the bismuth ferrite representative diffraction peaks. The largest relative intensities belonging to strontium hexaferrite are small according to the weight ratio, 50 wt % and 50 wt % for BiFeO3 and SrFe12O19, respectively. This result can be explained by two reasons. i) One reason is the intensity of scattered radiation from the atom IA, given by the equation:
IA ¼ Z Ie
(eq. 3)
where Z is the number of electrons in an atom and Ie is the intensity of scattered radiation from a single electron; hence, the BFO diffraction peak intensity is larger due to the higher number of electrons present in the Bi atoms relative to the electrons in Fe and Sr atoms. ii) The second reason is the intensity formula: the intensity formula takes into account more factors to determine the total intensity of the scattered radiation, such as, the number of unit cell per unit volume (N), the integrated intensity for the hkl set of planes (I), volume of the crystal (V), electron charge), speed of light (c), mass of an electron (m), temperature factor, Lorentz and polarization factors (Lp), absorption factor (A) and the structure factor (Fhkl).
Ihkl ¼
! 3 N 2 e4 l V 2 TLp j jF hkl A 2m2 c4
3
phase BiFeO3 is characterized by the presence of the “twin peaks” located approximately 32 of 2q and mullite as a minority phase. Fig. 2 shows an amplified zone of the XRD patterns from 30 to 37 of xBFO:(1-x)HFS samples and the representative peaks of the constitutive phases. The stoichiometric nature of the BiFeO3 and the high iron concentration of the strontium hexaferrite produce a chemical interaction due to the diffusion of iron and strontium into BiFeO3, resulting in the formation of Bi2Fe4O9 as secondary phase and the transition of BiFeO3 structure from rhombohedral to orthorhombic, visible as a change in the relative intensities of the bismuth ferrite twin peaks as shown by Pedro [35] and Bhushan [36]. The inhibition of the Bi2Fe4O9 phase is difficult through mechanical-assisted synthesis. The X-ray diffraction analysis confirms the existence of a chemical interaction between the strontium hexaferrite and the bismuth ferrite. To quantify the phase weight percent, Rietveld refinements were carried out, and the results are shown in Table 1. For sample x ¼ 0 (pure HF), the main presence of strontium hexaferrite can be quantified (99 wt %), accompanied with a small amount of hematite (1 wt %). For x ¼ 0.9, 0.8, 0.7, 0.6 and 0.5, samples present a mixture of SrFe12O19, BiFeO3 and Bi2Fe4O9 for which the phase concentrations are close to the proposed compositions. Bismuth ferrite is characterized as an intermetallic compound with a narrow region in the Bi2O3eFe2O3 phase diagram, in which a minor change in stoichiometry tends to form another phases, such as Bi25FeO40 or Bi2Fe4O9. In this case, the Fe2O3 excess comes from the SrFe12O19. For x ¼ 1.0 (pure BFO), it is possible to quantify the presence of the BiFeO3 and mullite, approximately 92 and 8 wt %, respectively. The refinement parameters show a good adjustment, as shown in Table 1. The magnetic hysteresis loops at room temperature of different composites of xBFO:(1-x)HFS, 0.5 x 1.0, are depicted in Fig. 3. For x ¼ 1, which corresponds to the BFO sample, a linear behavior is observed showing a small magnetic susceptibility of 6.64 104 associated with the antiferromagnetic and paramagnetic behavior of BiFeO3 and Bi2Fe4O9, respectively, at room temperature [22,37]. The composition of x ¼ 0.0, which corresponds to pure HFS, shows the magnetic hysteresis loop of a hard ferromagnetic material [15],
(eq. 4)
From this point of view, it is important to observe that N indicates the number of unit cells per unit volume. When comparing the lattice parameters of the BFO (a ¼ 5.58 Å, c ¼ 13.90 Å) to the lattice parameters of the HFS (a ¼ 5.884 Å, c ¼ 23.05 Å), it is evident that N will be greater for BFO than for HFS. It is attributed to the small size of the BFO unit cell, due to a larger amount of unit cells per volume unit of the BFO relative to the unit cells per volume of the HFS. This explains the observed relative intensities in the XRD patterns. In addition, mullite increases with increased bismuth ferrite. According to the Bi2O3eFe2O3 phase diagram, an excess in Fe2O3 stoichiometry promotes the mullite phase, and, in this case, excessive hematite could be provided by strontium hexaferrite. Fig. 1 also shows that, for x ¼ 1.0 (pure BFO), the perovskite
Fig. 2. Zoom in of the diffraction pattern (30e37 of 2-theta) from DRX patterns for xBFO:(1-x)HFS, 0.5 x 1.0, sintered at 700 C for 4 h.
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Table 1 Rietveld analysis results of cylindrical test samples xBFO:(1-x)HFS composites, with x varying from 0.5 to 1.0, Dx ¼ 0.1. Composition wt.%BFO:wt.%HFS X BFO 90BFO:10HFS
80BFO:20HFS
70BFO:30HFS
60BFO:40HFS
50BFO:50HFS
HFS
Phase, Space group Phase (wt. %)
1.0 BiFeO3, R3c Bi2Fe4O9, Pbam 0.9 SrFe12O19, P63mmc BiFeO3, R3c Bi2Fe4O9, Pbam 0.8 SrFe12O19, P63mmc BiFeO3,R3c Bi2Fe4O9, Pbam 0.7 SrFe12O19, P63mmc BiFeO3, R3c Bi2Fe4O9, Pbam 0.6 SrFe12O19, P63mmc BiFeO3,R3c Bi2Fe4O9, Pbam 0.5 SrFe12O19, P63mmc BiFeO3, R3c Bi2Fe4O9, Pbam 0.0 SrFe12O19, P63mmc Fe2O3, R3c
92.42 7.57 10.59 81.32 8.07 18.33 67.71 13.94 28.47 57.13 13.88 36.21 52.13 12.23 44.06 45.26 10.67 98.973 1.02
c2 1.31 1.71
1.56
1.43
1.67
1.34
1.47
with high specific magnetization (Ms of 63 emu/g at 18 kOe), remanent magnetization (Mr of 33 emu/g), and high coercivity (Hc ~5.5 kOe), in good agreement with the characteristic values reported for strontium hexaferrites [15]. Samples with concentrations of x ¼ : 0.5, 0.6, 0.7, 0.8 and 0.9, show a similar behavior, a ferrimagnetic order with a high coercive field, approximately 5 kOe, which is attributed to the presence of the hard ferrimagnetic phase, SrFe12O19. The coercivity is an intrinsic property and does not depend on weight content; hence, its values remain constant regardless of if the samples have low or high concentration of strontium hexaferrite, as shown in Fig. 3. Conversely, the specific magnetization of the composites depends on the weight percent of each phase that forms it. The Ms values are not related to any structural transformation; they are attributed to the presence of different amounts of antiferromagnetic and ferrimagnetic phases [38]. If we assume a simple additive effect of magnetization, the specific magnetization is directly dependent of the HFS content according to the equation:
Fig. 3. Magnetic hysteresis loops at room temperature of xBFO:(1-x)HFS, 0.5 x 1.0 cylindrical test samples, sintered at 700 C for 4 h.
Ms ¼ ðwt:%ÞHFS ,MHFS þ ðwt:%ÞBFO ,MBFO
(eq. 5)
where ðwt:%ÞHFS is the weight percent of HFS in the composite, MHFS is the specific magnetization of HFS, ðwt:%ÞBFO is the amount of BFO in the composite, and MBFO is the specific magnetization of BFO. As BFO and mullite present antiferromagnetic order, the HFS concentration largely determines the composite's specific magnetization due to a difference of three orders of magnitude between the specific magnetization values of the phases. The loops do not show different magnetic behaviors as is shown by Singh [28] and de Morais [19], thus confirming a higher chemical homogeneity of the obtained composites. Table 2 shows magnetic properties obtained from the magnetic hysteresis loops at room temperature. To validate the homogeneity and the relationship between specific magnetization and the amount of HFS (wt. %) a fitting of the experimental data of the variation of Ms with the composition of HFS was done, as shown in Fig. 4. The fitting confirms a linear dependence of the specific magnetization with the strontium ferrite concentration, as described by the additive effect mentioned above. The dielectric characterization of the composites was performed using dielectric spectroscopy; the relative permittivity and dielectric losses (tan d) in the frequency range of 100 Hz to 5 MHz are shown in Figs. 5 and 6, respectively. As Fig. 5 shows, the relative permittivity for all the studied compositions decreases as the frequency increases. This is in good agreement with the Maxwell-Wagner behavior, in which the high relative permittivity at low frequencies is due to an accumulation of charges at the grain boundaries which have a larger resistivity than the grains of the samples [39]. In addition, in Fig. 5a, the bismuth ferrite phase (x ¼ 1) shows the highest relative permittivity at low frequencies. It is attributed to a higher conductivity of bismuth ferrite compared to strontium hexaferrite, which enhances the mobility of the charge carriers in the grains and its concentration at the boundaries. At low frequencies (and even at the 1 MHz range when the conductivity is large), the polarization is due to the Maxwell-Wagner effect. According to this effect, in a composite with two regions, which have different conductivities, such as grain/grain boundaries, the charge carries are trapped in the interface due to the difference in conductivity; therefore, the material with a larger conductivity leads to a higher charge carrier concentration at the interface, resulting in a higher electric polarization. In addition, it is also attributed to the displacement of the Bi3þ ions and their two unpaired 2s electrons, which should be present at low and high frequencies [40]. The presence of strontium hexaferrite in the composites tends to reduce its polarizability; however, this tendency is not uniform, nor linear, as the change in the magnetization. This can be explained by the complex arrangement of grains and grain boundaries of up to three different phases with different dielectric properties in the composites. Fig. 6 shows the dielectric losses (tan d) of the composites xBFO:(1-x)HFS, 0.5 x 1.0, at different frequencies. Fig. 6 has been separated in two figures, 6a and 6b, depending of the composition, in order to better visualize the behavior. This measurement allows identifying if there are some relaxation phenomena. As Fig. 6 shows, there are two relaxation processes in the studied frequency range. The first one occurs at a frequency of approximately 1 kHz, and the second is detected between 100 and 200 kHz, attributed to the BFOeHFS intergrain Maxwell-Wagner relaxation. The process at 1 kHz is slightly observable in the pure strontium hexaferrite phase (x ¼ 0). However, both relaxation processes are clearly visible in some compositions (x ¼ 0.5 and 0.6) due to the higher conductivity of the bismuth ferrite phase. In addition, Fig. 6a and b demonstrate a reduction of the
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Table 2 Coercivity field (Hc), specific magnetization (Ms) and remnant magnetization (Mr) of xBFO:(1-x)HF composites, with x varying from 1.0 to 0.5, Dx ¼ 0.1. Composition wt.%BFO:wt.%HFS
x
Hc (kOe)
MS at 18 kOe (emu/g)
Mr (emu/g)
100BFO:0 HFS 90BFO:10HFS 80BFO:20HFS 70BFO:30HFS 60BFO:40HFS 50BFO:50HFS 0BFO 100HFS
1 0.9 0.8 0.7 0.6 0.5 0
e 5.5 5.5 5.5 5.5 5.5 5.5
20$103 4.26 11.12 18.36 25.56 32.53 62.83
e 2.54 6.09 11.03 13.96 17.90 35.38
Fig. 4. Specific magnetization (MS) at 18 kOe versus BFO concentration (x). The red line is a linear fit, and the dots correspond to the experimental values. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
dielectric losses with the increase of the strontium hexaferrite content, and pure bismuth ferrite (x ¼ 0) shows the highest values of dielectric losses, except in the relaxation peak of the strontium hexaferrite at 1 kHz. The higher resistivity of this phase produces a reduction of the loss current; this is the current that flows through the sample, which is the loss component of the tan d, following the equation:
tan d ¼
Il Ic
(eq. 6)
where Il is the loss current and Ic the capacitive current. According to the results previously presented,
the
0.6BFO:0.4HFS composite (x ¼ 0.6) shows a good combination of remanent magnetization (25.56 emu/g) and relative permittivity at 5 MHz (11.88), despite its high ferromagnetic phase content. For this reason, representative SEM micrographs of this composition were taken in order to qualitatively analyze the homogeneity, porosity and phase distribution of the composite. As Fig. 7a shows by presenting a micrograph of secondary electrons (SE), a smooth surface produced by small and uniform grains and low porosity of the compact material can be observed. In Fig. 7b, the same specimen is shown in back scattering mode. Three materials can now be differentiated based on contrast; they can be distinguished the presence of three different phases: one dark gray, one light gray, and another white, which are attributed to SrFe12O19, BiFeO3 and Bi2Fe4O9, respectively. According to Lloyd [41], the compounds that contain atoms with greater atomic number appears brighter in a back scattered electron image. In our study, the compound with lower atomic number is SrFe12O19, thus, it appears darkener in the analysis. However, an unexpected behavior between BiFeO3 and Bi2Fe4O9 is observed, because the Bi2Fe4O9 appears as the brightest phase, and it has lower atomic number than BiFeO3. The brighter phase corresponding to Bi2Fe4O9 is attributed to the presence of Au (with the highest atomic number) in Bi2Fe4O9. This is produced during the sputtering process, due to a deflection of the sputtered Au in the HFS and BFO phases; It was confirmed by means of EDS analysis, included in Fig. 7. Fig. 8 shows the magnetodielectric behavior of the pure bismuth ferrite (x ¼ 1). It was evaluated through the analysis of the variation of relative permittivity and dielectric losses with the frequency, under different magnetic applied field, expressed indirectly as the real part (ε0 r) of the relative permittivity (Fig. 8a), and the imaginary part (ε00 r) of the relative permittivity (Fig. 8b), respectively. An interesting variation of the dielectric properties can be appreciated when high fields are applied. It is attributed to an increase in the resistivity of the phase probably due to an interfacial magnetoresistance caused by spin dependent tunneling or spin dependent
Fig. 5. Relative permittivity versus frequency of xBFO:(1-x)HF composites, with x varying from (a) 0.7 to 1, and (b) 0.5 to 0.6 and x ¼ 0.
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Fig. 6. Dielectric losses versus frequency of xBFO:(1-x)HF composites, with x varying from: (a) 0.7 to 1, and (b) 0.5 to 0.6 and x ¼ 0.
Fig. 7. SEM micrographs of 0.6BFO:0.4 H F composite obtained by: (a) secondary electrons (SE) and, (b) back scattered electrons (BSE).
Fig. 8. (a) Real relative permittivity (ε0 r), and (b) imaginary relative permittivity (ε'‘r) of pure BFO (x ¼ 1) under different applied magnetic fields of 0, 4, 10 and 15 kOe.
scattering [42,43]. These phenomena have been reported in the multiferroic perovskites lanthanum-strontium manganites [43,44] at low temperatures and close to the magnetic Curie temperature at approximately 100 K. The magnetoresistance arises from the progressive domain rotation, which causes a spin polarization of the charge carriers and increases the resistivity at the interfaces with different domain directions. However, under a magnetic field of
15 kOe, the positive magnetoresistance changes to a negative magnetoresistance. This change in behavior is due to almost all the domains having the same orientation at high magnetic fields; thus, the conductivity is increased. One of the effects of an increased resistivity is the change of the relaxation process present at approximately 10 kHz to lower frequencies, as shown in Fig. 8. The relative permittivity values at 0, 4, 10 and 12 kOe converge at
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50 kHz; this is due to the frequency suppression of the MaxwellWagner polarization mechanism. When the magnetoresistance is negative, there is an increment of the charge carriers, which has the effect of extending the M W polarization region beyond 5 MHz. The dielectric losses (Fig. 8b) show high values at low frequencies and a reduction when the frequency is increased, in agreement to the Maxwell-Wagner behavior. The dielectric losses at 4, 10 and 12 kOe reach their minimum at low frequencies and have lower losses respect to the 0 (nonfield increased magnetoresistance), and 15 kOe (negative magnetoresistance) applied fields due to the positive magnetoresistive effect. The change in the relative permittivity values due to the change in resistivity is attributed to the modification in mobility of the charge carriers, which diminishes at high resistance values and increases the relaxation time of the space charge polarization; hence, the relaxation frequency is reduced, as shown in Fig. 8. When the BFO sample is under the influence of a magnetic field larger than 14 kOe, the magnetoresistivity becomes negative. This is related to the spin polarization of the charge carriers and the progressive alignment of the antiferromagnetic domains, which at low fields, start to rearrange, and the resistivity increases. However, at high fields, the realignment is complete, and the resistivity goes down. These changes in resistivity produce a variation of the capacitance values due to the changes in mobility of the charge carriers. Fig. 9 shows the magnetocapacitance at 5 MHz under the effect of different magnetic applied fields. For the sample x ¼ 1.0 (pure BFO), there is no change in the capacitance until a field of 14 KOe or larger, which is consistent with the magnetoresistance mechanism proposed before. For x ¼ 0.0 (pure HFS), the magnetoresistance behavior is similar to that of MnZn Ferrite as shown by Hu et al. [45], where the resistivity is reduced when a magnetic field of 1 kOe or larger is applied, increasing the capacitance values. The composites BFOeHFS show the same behavior when a field is applied. The increase in the capacitance values when a field is applied and the effect of the BFO over the magnetocapacitance is ambiguous and need further studies. The samples x ¼ 0.6 and 0.7 show an increase relative to the sample x ¼ 0, and the samples with x ¼ 0.5 and 0.9 exhibit a decrease in the magnetocapacitance values; however, the sample with x ¼ 0.8 shows a negative magnetocapacitance value, which can be attributed to the skin effect producing an increase of resistivity at high frequencies. Samples with x ¼ 0.0, 0.5, 0.6, 0.8 showed a small change in magnetocapacitance when the field changed from 5 to 6 kOe, which could be related to
7
the high coercive field of the HFS. However, in order to explain the behavior, further studies must be carried out to determine the precise mechanisms involved in magnetic field dependent resistance of the samples. The performed studies in the composites did not show magnetoelectric coupling between BFO and HFS phases since no perturbation of the composites magnetic field was observed when an A.C. electric field was applied. 4. Conclusions Multiferroic composites (x)BFO:(1-x)HFS were successfully synthesized by high energy ball milling assisted with heat treatment. The composites were obtained by mixing and sintering both materials. There was a chemical interaction between the ferroic phases, producing the transition of BFO from orthorhombic to rhombohedral and the growth of the Bi2Fe4O9 phase. All the compositions exhibit multiferroic character, since they present ferroelectricity and ferromagnetism at room temperature. All the studied composites show ferromagnetic order accompanied by diminution of the dielectric losses due to the high resistivity of the HFS phase. In addition, the relative permittivities of the composites measured at high frequency (5 MHz) show high values when bismuth ferrite is present. A maximum value of relative permittivity εr of 16.78 was obtained for the composite with x ¼ 0.9, which showed a remanent magnetization Mr of 2.54 emu/g. The data confirm the ferroelectric and ferromagnetic behavior of the composites when compared to the null remanent magnetization of the BFO ferroelectric-antiferromagnetic phase and the lower relative permittivity εr ¼ 5.95 at 5 MHz for the pure strontium hexaferrite. Evidence of magnetoelectric behavior was not found in the composites, but the results confirmed magnetodielectric coupling due to the room temperature spin-dependent magnetoresistance of the BFO phase. The BFO magnetoresistive behavior offers the possibility of developing sensors or magnetically tuned filters. The observed magnetoresistance shown by the BFO need further detailed studies to confirm the precise mechanism that produces it, with the aim to control it for its possible applications. Acknowledgments The authors thank the Center of Nanosciences and Micro and Nanotechnology, as well as CIITEC of the National Polytechnic rrez for Institute; in particular, we thank Dr. Hugo Martinez Gutie the morphological characterization and Mariana Alvarez Torres for the XRD characterization. References
Fig. 9. Magnetocapacitance of xBFO:(1-x)HF composites, with x varying from 0 to 1, Dx ¼ 0.1, at 5 MHz.
[1] J.R. Teague, R. Gerson, W.J. James, Dielectric hysteresis in single crystal BiFeO3, Solid State Commun. 8 (1970) 1073e1074, https://doi.org/10.1016/00381098(70)90262-0. [2] M. Skinner, Magnetically ordered ferroelectric materials, IEEE Trans. Parts, Packag. 6 (2) (1970) 68e90. [3] S.V. Kiselev, G.S. Zhdanov, R.P. Ozerov, Detection of magnetic arrangement in BiFeO3 ferroelectric by means of neutron diffraction study, Dokl. Akad. Nauk SSSR 145 (6) (1962) 1255e1258. [4] K.S. Nalwa, A. Garg, A. Upadhyaya, Effect of samarium doping on the properties of solid-state synthesized multiferroic bismuth ferrite, Mater. Lett. 62 (2008) 878e881, https://doi.org/10.1016/j.matlet.2007.07.002. , C.A. Corte s-Escobedo, [5] L.G. Betancourt-Cantera, A.M. Bolarín-Miro nchez-De Jesús, Structural transitions and multiL.E. Hern andez-Cruz, F. Sa ferroic properties of high Ni-doped BiFeO3, J. Magn. Magn. Mater. 456 (2018) 381e389, https://doi.org/10.1016/j.jmmm.2018.02.065. [6] F. Azough, R. Freer, M. Thrall, R. Cernik, F. Tuna, D. Collison, Microstructure and properties of Co-, Ni-, Zn-, Nb- and W-modified multiferroic BiFeO3ceramics, J. Eur. Ceram. Soc. 30 (2010) 727e736, https://doi.org/10.1016/ j.jeurceramsoc.2009.09.016. [7] C.W. Nan, M.I. Bichurin, S. Dong, D. Viehland, G. Srinivasan, Multiferroic magnetoelectric composites: historical perspective, status, and future directions, J. Appl. Phys. 103 (3) (2008), https://doi.org/10.1063/1.2836410,
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