Bi-YIG system: The effect of cation
relocation on structural, hyper fi ne, and magnetic properties
Mohammad Niyaifar*,1, Hory Mohammadpour**,1, Azadeh Aezami1, and Jamshid Amighian2
1Department of Physics, Ahvaz Branch, Islamic Azad University, Ahvaz, Iran
2Department of Physics, Najafabad Branch, Islamic Azad University, Esfahan, Iran Received 29 August 2015, revised 4 October 2015, accepted 30 October 2015 Published online 25 November 2015
KeywordsBi-YIG, cation relocation, Fourier transform infrared spectroscopy, magnetic properties, M€ossbauer spectroscopy
*Corresponding author: e-mail[email protected], Phone:þ0989351055293, Fax:þ986134435288
**e-mail[email protected], Phone:þ989396082209, Fax:þ986134435288
BixY3–xFe5O12 (x¼0.0, 0.9, 1, 1.1, 1.2, and 1.3) were synthesized by the sol–gel method to investigate the effect of Bi substitution on the structural and magnetic properties of YIG. The XRD results indicate that all the diffraction peaks belong to the pure phase. The M€ossbauer analysis shows an increase in the Fe3þð Þd=Fe3þ½ a ratio and also a line broadening at tetrahedral sites, which suggests a relocation of yttrium ions at
dodecahedral sites with iron ions at octahedral sites. The saturation magnetization fluctuates with increasing bismuth content, which has been explained based on M€ossbauer results.
The change in vibrational band broadening and the observation of a new band in the far-FTIR spectra confirm the proposed cation relocation.
ß2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction Bismuth-substituted yttrium iron gar- net (Bi-YIG) has provoked great interest since its discovery [1]. It is one of the best materials for magneto- optical devices due to high transparency in the near-IR spectral range [2, 3]. These properties make it possible to fabricate a remarkable magneto-optical microscope with high-magnetic sensitivity and high-time resolution [4].
According to structural analysis, YIG belongs to the space group Ia3d(O10h ) and cations are located at the center of corresponding oxygen polyhedral, i.e., yttrium ions at dodecahedral {24c} site (a disturbed cube) and iron ions at octahedral [16a] and tetrahedral (24d) sites [5, 6].
The previous works show that bismuth has a complicated role in the saturation magnetization of the Bi-YIG system. In fact, despite the additive effect of Bi3þon magnetization (due to hybridization of bismuth 6p orbitals in the iron 3d bands) [7–9], the Bi substitution causes a structural distortion that leads to a decrease in saturation magnetization [10–12].
It is expected that at high levels of Bi substitution the saturation magnetization reaches a constant variation, even though in some works saturation magnetization is reported tofluctuate at high level of bismuth substitution [13, 14].
This ambiguity of saturation magnetization behavior motivated us to investigate the variation of saturation magnetization at high levels of Bi substitution. The variations of Ms behavior are explained by M€ossbauer spectroscopy and a vibration sample magnetometer. The results show the possibility of relocation of iron ions at octahedral sites with yttrium ions at dodecahedral sites, which leads to changes of Ms at high level of substitution.
2 Experimental The samples were synthesized by the sol–gel method. A gel was obtained from an aqueous solution of citric acid, Fe(NO3)39H2O (Aldrich, 99.95%), Y(NO3)36H2O (Aldrich, 99.8%), and Bi(NO3)35H2O (Aldrich, 99.99%). The citrate to nitrates ratio was set at 1:3.
Then, NH3H2O was added to adjust pH¼2, the resulting solution was heated at 808C for 2 h to prepare the gel. The gel was then dried at 1008C for 48 h. Further annealing treatments were performed in air at 9008C for 3 h to obtain single-phase Y3–xBixFe5O12 samples.
Crystalline phase identification was studied by an X-ray diffractometer (Seifert ID3003 model) using Cu Ka
Radiation (l¼1.5406 Å), with a step size of 0.048 (2u)
(AGFM, Meghnatis Daghigh Kavir Co., Kashan, Iran) with a maximumfield of6 kOe at 300 K. M€ossbauer spectra of the samples were recorded using a M€ossbauer spectrometer (Model CM 1101Russia) with velocity in the range of10 to 10 mm s1under constant acceleration at room tempera- ture. Theg-rays were provided by57Co source in Rh matrix.
The M€ossbauer spectra were analyzed by Mossfit software.
Fourier transform infrared (FTIR) spectra were recorded using a Perkin–Elmer spectrometer, model 2000, between 150 and 700 cm1. The specimens were made of KBr powder by pressing into a pellet form.
3 Results and discussion The XRD patterns of the Y3–xBixFe5O12samples are shown in Fig. 1. The diffraction patterns belong to the garnet structure for the samples with x1:3. The average crystallite size and the lattice constants of the samples were calculated. The average crystallite sizes of different samples were in the range of 48–62 nm and the calculation shows that the lattice constant increases from 12.21 to 12.38 Å as bismuth content increases (Fig. 2). This is attributed to the larger Bi3þ ionic radius (1.170 Å) compared to the Y3þionic radius (1.019 Å) [15].
The M€ossbauer spectra of the samples at room tempera- ture are shown in Fig. 3, where the dots present the experimental spectrum and the solid lines are related to the least squares fitting of the experimental data. Table 1 summarizes the M€ossbauer refined parameters. The M€ossbauer spectrum of the sample with x¼0 shows the presence of two sets of sextets overlapping with each other, corresponding to high-spin Fe3þat the tetrahedral (24d) and octahedral (16a) sites. The magnetic hyperfinefield (Hhf) of the octahedral sites (48 T) is larger than that of tetrahedral sites (39 T). The large difference between the magneticfieldsHhf
on the a and d sublattices is connected to the different
covalences of the Fe–O bond at a and d sites of the garnets [16–18]. The isomer shift (IS) of iron ions atasite is larger than that fordsites. This is due to the difference in degree of covalency of Fe–O bonds ataanddsites. It is well known that, in ionic–covalent bands, Fe3þion has the 3d54sd electronic configuration rather than 3d5. The more covalency at the tetrahedral site increases the 4sdelectron density at the
Figure 1 X-ray diffraction patterns of the samples BixY3–xFe5O12 (x¼0.0, 0.9, 1, 1.1, 1.2, and 1.3).
Figure 2 Variation of the lattice constant of BixY3–xFe5O12as a function of Bi content.
Figure 3 The M€ossbauer spectra of BixY3–xFe5O12(x¼0.0, 0.9, 1, 1.1, 1.2, and 1.3).
nucleus and hence the isomer shift (IS) ofasites is larger than that ofdsites, wheredcan be regarded as a measure of the ionicity of the compound [19, 20]. For the sample withx¼0.9, the M€ossbauer spectra for both a and d sites, gradually broaden with the increase of bismuth content. This line broadening is ascribed to the distortion ofaanddsites which is due to the larger ionic radius of Bi3þ, which leads to the changes in length and angle of the Fe–O–Fe bond.
The M€ossbauer spectra for the samples with x1 clearly exhibit an unexpected sextet (in addition to those attributed to iron ions ataanddsites) that has largerHhf
compared to theHhfofaanddsites. Based on the previous reports and by considering the hyperfine parameters of the unknown sextet, it is possible to assign iron ions to dodecahedral sites (Fe3þfcg) by relocation of Fe3þ½a with Y3þfcg. Indeed, there are few reports that show the possibility of Fe ions at dodecahedral site [21, 22]. The large magnitude of Hhffor Fe3þ½a may be attributed to the lower covalency atc site compared to those of ataand dsites. Therefore, the cation distribution of the samples with x1 may be rewritten as below:
Y3þx3Bi3þx
cFe3þ2
aFe3þ3
dO212
! Y3þ3xyFe3þy Bi3þx n o
c Fe3þ2yY3þy h i
aFe3þ3
dO212: Hence, the M€ossbauer spectra of the samples arefitted by three sextets assigned to iron ions at tetrahedral, octahedral, and dodecahedral sites andyis estimated by considering the ratio of Fe3þð Þd=Fe3þ½ a to be 1.5. Also from Fig. 3, it can be seen that the absorption intensity of sextet that is assigned to Fe3þfcg increases for the samples with 1x1:3. This shows more relocation of Fe3þ½a with Y3þfcg ions. The magnetic hyperfine
interactions ata,c, anddsites show a slight decrease as the xvalue increases. It is well known thatJadJddJaaJac, Jdc[23], so by considering the correlation between super- exchange interactions and the magnetic hyperfine interac- tions [16], the reduction inHhfcan be expected.
The isomer shift of all the samples shows a decrease at theaanddsites. It should be noted that the iron ions inaand dsites are trivalent and no changes in shielding of s electrons is expected. Therefore, the variation of IS may be attributed to the increase of 4sd electrons.
The M€ossbauer analysis shows that the sextet that is assigned to the tetrahedral site increasingly broaden with increasing bismuth content for the samples withx1. This is probably due to the presence of diamagnetic Y3þions at octahedral sites, which causes a spin canting at tetrahedral sites. The absorption studies shows that for the samples with x<1 the ratio of Fe3þð Þd=Fe3þ½ a remains at 1.53, however, for the samples withx1 this ratio increases up to 1.8. Hence, the increase in line broadening atdsites together with the increase in Fe3þð Þd=Fe3þ½ a confirms the cation relocation betweenaandcsites.
Figure 4 shows the hysteresis loops of the samples Y3–xBixFe5O12 (x¼0.0, 0.9, 1, 1.1, 1.2, and 1.3) at room temperature. The variation of saturation magnetization (Ms) as a function ofxis shown in Fig. 5. TheMsvalue exhibits a decreases for the sample withx¼0.9, then it shows a sudden rise atx¼1.1, which is the highest value forMs. Finally, a fall is observed for 1:1<x1:3. These variations ofMs
can be explained in terms of two magnetic effects;“spin canting of iron ions ataanddsites due to the size effect of Bi3þions and the relocation of Fe3þions at octahedral sites with Y3þat dodecahedral sites”. It is known that in a YIG system, the nonmagnetic Y3þions occupy dodecahedral {c}
Table 1 The M€ossbauer hyperfinefitting parameters and the cation distribution of BixY3–xFe5O12(x¼0.0, 0.9, 1, 1.1, 1.2, and 1.3).
composition and cation distribution of Y3–xBixFe5O12 site IS (mm s1)
QS (mm s1)
Hhf
(T)
W (mm s1)
area (%)
Y3Fe5O12 a 0.374 0.02 48.24 0.39 39.4
d 0.145 0.09 39.35 0.52 60.6
Y2.2Bi0.9Fe5O12 a 0.375 0.01 48.23 0.41 39.3
d 0.143 0.02 39.34 0.54 60.8
Y2Bi1Fe5O12Y3þ1:88Fe3þ0:12Bi3þ1
cFe3þ1:88Y3þ0:12
aFe3þ3
dO212 a 0.476 0.01 49.45 0.41 36.8
d 0.223 0.08 40.53 0.59 58.6
c 0.376 0.10 51.96 0.32 4.6
Y1.9Bi1.1Fe5O12Y3þ1:75Fe3þ0:15Bi3þ1:1
cFe3þ1:85Y3þ0:15
aFe3þ3
dO212 a 0.447 0.03 48.13 0.42 36.1
d 0.214 0.01 39.50 0.66 58.57
c 0.367 0.07 50.76 0.31 5.34
Y1.8Bi1.2Fe5O12Y3þ1:75Fe3þ0:28Bi3þ1:2
cFe3þ1:72Y3þ0:28
aFe3þ3
dO212 a 0.430 0.01 48.04 0.42 33.57
d 0.206 0.03 39.35 0.71 58.57
c 0.354 0.11 50.52 0.35 7.86
Y1.7Bi1.3Fe5O12Y3þ1:36Fe3þ0:34Bi3þ1:3
cFe3þ1:66Y3þ0:34
aFe3þ3
dO212 a 0.412 0.05 47.98 0.53 32.47
d 0.208 0.11 39.12 0.93 58.5
c 0.346 0.03 50.5 0.53 9.03
sites, while octahedral [a] and tetrahedral (d) sites are occupied by magnetic Fe3þions. Hence, according to Neel’s theory, the magnetic moments of two iron ions ina sites ðFe3þ½aÞare aligned antiparallel to those of three iron ions ind sites ðFe3þðdÞÞ. Therefore, the net magnetic moment can be written as [24]
Mnet¼Mð Þd M½ aMf gc ; Mf gc ¼0
ð1Þ whereM½a,MðdÞ, andMfcgare the magnetization ata,d, and csites. The observed decrease inMs for the samples with x¼0:9 can be attributed to the reduction in Ms due to division of the ideal magnetic direction of sublattices [10–12]. In fact, the previous works show that the presence of Bi in YIG structure seriously increases the exchange interaction [7–9], however, for the sample withx¼0.9 this additive effect on magnetization might be covered by the effect of distortion on magnetic ordering.
For the samples with 0:9>x1:1, more Bi3þ substitution shows an unexpected variations that is
fact, the M€ossbauer results showed that a small number of Fe3þions atasite have been relocated with Y3þions atc sites, so it is expected that the magnetization of a sites decreases and that ofcsites increases. However, it is worth noting that JadJddJaaJac, Jdc. Therefore, by comparing Jad withJacandJdc, it can be deduced that by relocation of iron ions between a sites and c sites the magnetic ordering of Fe3þ½a reduces, while the magnetic ordering of Fe3þ ions at d sites remain unchanged. The observed decrease for the samples with 1:1<x1:3 can be elucidated by the effect of spin canting on magnetization ofdsite. It is well known that the presence of a diamagnetic ion in a site causes a spin canting at the opposite site [25].
Hence, by considering the presence of diamagnetic Y3þion at a sites, the spin canting occurs at dsite and causes a decrease in the net magnetization.
Figure 6 shows far-FTIR transmittance spectra of the samples with x¼0.0, 0.9, 1.1, and 1.3 measured between 150 and 700 cm1. According to the factor group analysis, 17 vibrational modes in the spectra of the garnets are expected to be found. These 17 IR modes consist of three asymmetric stretching modes of the tetrahedron ydð Þ,y3
three asymmetric bending modes, and one symmetric bending yaðy4;y2Þ, two rotations/liberations R of the tetrahedron, two translations T of the tetrahedron, three translations Td of the dodecahedral cation, and three Figure 4 The variation of saturation magnetization (Ms) of
BixY3–xFe5O12as a function of Bi content.
Figure 5 Hysteresis loops of the samples Y3–xBixFe5O12(x¼0.0, 0.9, 1, 1.1, 1.2, and 1.3) at room temperature.
Figure 6 Far-IR spectra of BixY3–xFe5O12 as a function of Bi content (x¼0.0, 0.9, 1.1, and 1.3).
translations of the octahedral cationTo[26]. In this work, due to experimental limitations, the IR bands below 150 cm1were not recorded. The bands recorded at 658, 598, and 566 cm1 are assigned to the y3 modes in Y3Fe5O12, whereas the three IR bands of y4 appeared unclearly in the range of 384–476 cm1. The IR band at 362 cm1 can be ascribed to y2, whereas the IR band at 334 cm1can be ascribed to theTomode. The bands at 317 and 260 cm1 cannot be assigned to R modes in accordance with the attribution provided in Ref. [27]. The observed mode at 228 cm1can be assigned to one of the translationsT of the tetrahedral cation. The FTIR spectra show that the intensity of absorption for vibrational bandsyd
is higher than that ofyafor all the samples. From thefirst selection rule, the transitions between d orbitals are not allowed for a site that have a center of symmetry. By considering that the tetrahedral site has the lack of center of symmetry compared to that of octahedral site [28], the observation of a stronger absorption intensity forydbands is expected.
The IR spectra for the samples withx¼0.0, 0.9, 1.1, and 1.3 show a slight shift ofydandyaabsorption bands, while those related to the samples withx¼1.1 and 1.3 show an increase in asymmetric bending y4, in addition to broadening. To explain the effect of cation substitution in the IR absorption, it is worth noting that the substitution may cause some noticeable changes, such as: new bands, band shifts, and band splitting at the substituted site, whereas in other sites only broadening or band shifts may occur [29].
Hence, the observed shift and broadening in IR spectra are due to bismuth substitution at dodecahedral sites.
The increase iny4bands absorption for the samples with x¼1.1 and 1.3 can be attributed to the effect of relocation of Fe3þand Y3þions at octahedral sites, which increases the asymmetric bending. Another piece of information that may be derived from IR spectra, is the decrease in the depth of minima when Bi content is increased. This could be due to the fact that the degree of ionicity has been decreased with Bi substation.
4 Conclusions In the present work, the variation of magnetic properties of the samples is explained by two magnetic effects:“spin canting of iron ions ataanddsites due to size effect of Bi3þion”and“a relocation of Fe3þ½a with Y3þfcg ions”. For the high level of Bi substitution, the appearance of the extra sextet with the largest magnetic hyperfine interaction and the increase of broadening for the sextet atdsites, confirm the relocation of Fe3þ½a with Y3þfcg. In IR spectra, the observation of a new band at low frequency is assigned to the effect of the cation relocation. Also, this substitution leads to an increase in broadening foryd,ya, andycgroups of the bands. The band broadening at higher Bi contents has been explained.
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