Crystal structure and magnetic properties of Cr doped barium hexaferrite Sunil Kumar, Sweety Supriya, Rabichandra Pandey, Lagen Kumar Pradhan, and Manoranjan Kar
Citation: AIP Conference Proceedings 1942, 130040 (2018); doi: 10.1063/1.5029110 View online: https://doi.org/10.1063/1.5029110 View Table of Contents: http://aip.scitation.org/toc/apc/1942/1 Published by the American Institute of Physics
Crystal Structure and Magnetic Properties of Cr Doped Barium Hexaferrite Sunil Kumara), Sweety Supriya, Rabichandra Pandey, Lagen Kumar Pradhan and Manoranjan Kar Department of Physics, Indian Institute of Technology, Patna, Bihta-801103, India a) Corresponding author:
[email protected] Abstract. The Cr3+ substituted BaFe12O19 has been synthesized by modified sol-gel method to tailor the magnetic anisotropy and coercivity for technological applications. Some basic studies have revealed that this substitution leads to unusual interactions among the magnetic sublattices of the M-type hexaferrite. In order to investigate these interactions, BaFe12-xCrxO19 (x=0.0, 0.5, 1.0, 2.0, and 4.0) M-type hexaferrites were characterized by employing XRD (X-ray Diffractometer). It is confirmed that, all the samples are in nanocrystalline and single phase, no impurity has been detected within the XRD limit. The magnetic hysteresis (m-H) loops revealed the ferromagnetic nature of nanoparticOHV 13V 7KHFRHUFLYH¿HOGZHUHincreasing with the increasing Cr3+ content, but after the percolation limit it decreases. The magnetocrystalline anisotropy is increasing with the Cr3+ concentration in samples and high values of magnetocrystalline anisotropy revealed that all samples are hard magnetic materials. Magnetic hysteresis loops were analyzed using the Law of Approach to Saturation method.
INTRODUCTION Magnetic materials have drawn great attention in intensive research and development due to their promising electronic and magnetic properties for the use in technological applications. Among the magnetic materials, ferromagnetic materials having very high saturation magnetization, very high coercivity, magnetic anisotropy field, and excellent chemical stability [1]. The M-type hexaferrites are important ferromagnetic oxides. Among them, the barium hexaferrite (BaFe12O19) is magnetoplumbite structures and, have been rigorously investigated and used as permanent magnets due to its high anisotropic magnetic property [1]. The M-type hexaferrite materials crystallizes in a hexagonal crystal structure, which belongs to P63/mmc space group. The crystal structure is comprised of 64 ions per unit cell on 11 distinct crystallographic sites. The 24 Fe3+ ions are distributed in five different crystallographic sites, such as three octahedral sites (12k, 2a, and 4f2), one tetrahedral site (4f1) and one trigonal bipyramidal site (2b) [2]. The ferromagnetic structure of M-type hexaferrite is given by the Gorter model shows three parallel (12k, 2a, and 2b) and two anti-parallel (4f1 and 4f2) sites, which are coupled by superexchange interactions through the Oí ions [3]. For the enhancement of fundamental electrical and magnetic properties of hexaferrites, a number of studies were carried out on the synthesis methods, and cationic substitutions of divalent or multivalent ions and of their mixture [2]. In order to improve the magnetic properties of the barium hexaferrites, many studies have been concerned with the cationic substitutions. From the above literature, It is suggested that a dopants can be used to control the magnetic properties as well as electrical properties of barium hexaferrites [4-5]. The magnetocrystalline anisotropy need to be tuned along the saturation magnetization. Cr is a very good magnetic element and, ionic radius is different to that of Fe. Hence, the magnetic properties can be tuned. The magnetic properties of Cr-substituted barium hexaferrites, BaFeí[CrxO19 (0.0-4.0) are investigated and discussed. It is observed that, although saturation magnetization decreases with the increase in Cr concentration, but the coercivity increases up to a certain percentage of Cr.
DAE Solid State Physics Symposium 2017 AIP Conf. Proc. 1942, 130040-1–130040-4; https://doi.org/10.1063/1.5029110 Published by AIP Publishing. 978-0-7354-1634-5/$30.00
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EXPERIMENTS The chromium substituted barium hexaferrite BaFe12-xCrxO19 with x =0.0, 0.5, 1.0, 2.0, 3.0 and 4.0 were synthesized using the sol gel method [6]. The starting chemical such as: barium nitrate, aluminum nitrate, iron nitrate and citric acid are procured from either Merck or Alfa Aesar with >99% purity. The ratio of citric acid and metal ion is maintained 1:3 with PH of the solution is ~ 7. The synthesized powder was ground and sintered at 500 OC and 1200 O C for 2 hours at each temperature with 4 degree centigrade per minute ramping rate. The prepared samples were characterized by employing the XRD (Rigaku X-ray Diffractometer Model TTRX III) technique to understand the phase purity and crystal structure of the samples. The magnetic hysteresis loops (m-H) were recorded at room temperature by employing the vibrating sample magnetometer (VSM) Lakeshore 7500, USA equipped with a magnet of 20 kOe.
RESULTS AND DISCUSSIONS
15 20 25 30 35 40 45 50 55 60 65 70
2T (Degree)
x=0.5 x=0.0
(2014)
(220)
(207) (2011)
(303)
x=0.0 (209)
(110) (112) (107) (108) (114) (203) (116) (206) (1011)
(006)
(101) (102)
x=0.5
x=1.0
(114)
x=1.0
x=4.0 x=2.0
(107)
x=2.0
Intensity (A.U.)
BaFe12-xCrxO19
x=4.0
Intensity (A.U.)
BaFe12-xCrxO19
32.0
32.5
33.0
33.5
34.0
2T (Degree)
34.5
35.0
FIGURE 1. (a) XRD patterns of the samples BaFe12-xcrxO19 for x= 0.0, 0.50, 1.0, 2.0, and 4.0. (b) The enlarge view of (107) and (114) peaks.
X-ray diffractions (XRD) patterns of BaFe12-xCrxO19 (x=0.0,0.5,1.0, 2.0 and 4.0) are shown in figure1 (a). All the observed peaks were indexed and compared to the standard XRD patterns obtained from M-type barium hexaferrite JCPDS file no: 27–1029 [7] with P63/mmc space group. Analysis of all the XRD patterns confirms that no impurity phases are present in the samples. The crystallite size calculated by employing the Willimson Hall method (W-H method) [8] and values are enlisted in table I. The enlarge view of (107) and (114) peaks are shown in figure 1(b) for clarity. It is observed that the XRD peaks are shifting towards the higher ș value (right side) with increasing Cr3+ content in the BHF. It could be due to the smaller size of Cr3+ LRQ ǖ WKDQWKHKRVW)H3+ LRQ ǖ Also, the substitution of Cr3+, suggest that the development of lattice strain due to ionic radius mismatched of host Fe3+ and guest Cr3+ atom. It is interesting to note that the crystallite size of all the samples are almost same. TABLE 1. The structural and magnetic parameters of BaFe12-xCrxO19 for x= 0.0, 0.50, 1.0, 2.0, and 4.0 (Ms, Mr, Hc and K1). Samples BaFe12-xCrxO19
Magnetic parameters
Crystallite size W-H method
Ms (emu/g)
Mr (emu/g)
Hc (kOe)
K1 (106 erg/cm3)
x=0.0
72
53.14
26.65
4.45
3.56
x=0.5
73
34.83
17.86
5.62
1.95
x=1.0
76
28.64
15.39
4.21
1.45
x=2.0
73
16.39
9.88
3.67
0.61
x=4.0
68
11.81
7.69
2.58
0.37
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BaFe Cr O 11.5 0.5 19
2.3 2.2
5.0
BaFe12-xCrxO19
4.5
2.1
4.0
2.0
3.5
1.9
3.0
1.8
0.8
1.0
1.2
4SinT
1.4
1.6
Coercivity (kOe)
6.0 Latice Stain 5.5 Coercivity
2.4
Experimental Data points Linear Fit
Lattice Strain (10-3)
Ehkl cosT
0.054 0.051 0.048 0.045 0.042 0.039 0.036 0.033 0.030 0.027 0.024 0.021 0.6
2.5
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
1.8
(x) Cr content
FIGURE 2. (a) Typical W-H plot for (BaFe11.5Cr0.5O19). (b) strain and coercivity plot with Chromium concentration in the BHF.
The magnetic hysteresis loops of BaCrxFe12-xO19 for x=0.0, 0.5, 1.0, 2.0, and 4.0 at the room temperature are shown in figure 3 (a), reveals that samples are hard magnetic material with very high coercivity. The coercive field (Hc) is maximum for the 4.16 % chromium substitution in BHF. However, it decreases with the further increase of Cr concentration beyond 4.16 %. But it is interesting to note that saturation magnetization and remanent magnetization (table 1) increases with the decrease in Cr concentration. It could be due to smaller free ion magnetic moment of Cr to that of Fe ions as well as weaker super exchange interaction between Cr-O-Fe to that comparable to Fe-O-Fe. The increase of coercive field (Hc) up to 4.16 % Cr in the barium hexaferrite could be due to increase of lattice strain in the sample. Which is observed from the XRD analysis and depicted in the figure 2 (b). The Cr3+ ions occupy Fe3+ ionic sites of the BaFe12O19 crystal lattice. In barium hexaferrites, 24 Fe3+ ions are distributed oQ ¿YH GLIIHUHQW crystallographic sites of three octahedral (2a, 12k, and 4f2), one tetrahedral (4f1), and one trigonal bipyramidal (2b) site. The sites 2a,12k, and 2b are parallel, whereas 4f1and 4f2 are anti parallel. 49.5
(b)
Experimental data Linear Fit
49.0 48.5
BaFe12O19
48.0 47.5 47.0 46.5 46.0 3.00E-009
4.00E-009 5.00E-009 1/H2 (Oe-2)
32.5 Magnetization (emu/g)
Magnetization (emu/g)
Magnetization (emu/g)
50 X =0.0 (a) BaFe12-xCrxO19 40 X =0.5 X =1.0 30 X =2.0 X =4.0 20 10 0 -10 -20 -30 -40 -50 -20 -15 -10 -5 0 5 10 15 20 Magnetic Field (KOe)
6.00E-009
32.0 31.5
(c)
Experimental data Linear Fit
BaFe11.5Cr0.5O19
31.0 30.5 30.0 29.5 3.00E-009 4.00E-009 5.00E-009 6.00E-009 1/H2 (Oe-2)
FIGURE 3. (a) Hysteresis loop of BaFe12-xcrxO19 x= 0.0, 0.5, 1.0, 2.0, and 4.0 at room temperature. (b) and (c) is the Law of $SSHURDFKWRVDWXUDWLRQ¿WWHGFXUYHIRU%D)H12O19 and BaFe11.5Cr0.5O19 respectively.
The M-type barium hexaferrite is a ferromagnetic (hard magnetic) material. Hence, the Law of Approach (LA) to saturation can be employed to understand the magnetocrystalline anisotropy of the material. The Law of approach is defined as [9],
ு
ுమ
ܯ = ܯ௦ ܺ ቀ1 െ െ
ቁ+ܪܺݔ
(1)
Here, Ms is the saturation magnetization, A is is the material property, it arises from inhomogeneities of samples and B is related to the magnetocrystalline anisotropy of the samples and x X H is associated with the field-induced increase in the spontaneous magnetization of the domains, it is only effective in the high temperature, in this case the measurement is carried out at room temperature [10]. Thus the second part of equation 1 is not considered. The
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experimental magnetic data are fitted with the Law of approach to saturation for all the samples, some of them are shown in figure 3 (b-c). The black circle points represent the experimental data and the red solid line represents the theoretical data to the LA to saturation equation. The magnetization data in the magnetic field between 12 kOe and 18 kOe has been plotted against 1/H2, and obtained straight lines have been fitted with linear equation. The intercepts of the straight lines gave the saturation magnetizations (Ms) and slopes were used to determine the anisotropy field from the relation, =ܤ
ுೌమ
(2)
ଵହ
The magneto anisotropy constant (K1) has been evaluated using the relation, ܪ =
ଶభ
(3)
ெೞ
The saturation magnetization (Ms) obtained from the intercept of linear fit, magnetocrysalline anisotropy is calculated from the slope of the linear fit with the help of equation 2 and 3 and coercivity determined from the hysteresis loops. All the magnetic parameters are enlisted in table 1.
CONCLUSION The Cr3+ substituted BaFe12O19 nanoparticles crystalline to the hexagonal symmetry with P63/mmc space group. Lattice strain is found to be maximum. Crystallite size for all the samples are comparable and it is in nm range. The maximum lattice strain ISobserved in BaFe11.5Cr0.5O19 sample. Also the maximum coercivity was observed for the same sample. There is an one to one relation between lattice strain and coercivity has been obtained. The saturation magnetization decreases with the increase in Cr concentration. Which is due to dilution of magnetic interaction between different metal ions. The saturation magnetization is calculated by employing the Law of Approach for saturation. The magnetocrystalline anisotropy (K1) decreases with Cr content in the barium hexaferrite.
ACKNOWLEDGMENTS Authors are thankful to UGC, India, ref. No.: 4051/ (NET-June 2013) for financial support and authors also acknowledge IIT Patna for providing the working platform.
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