Structural and magnetic properties of Al-doped yttrium iron garnet ceramics: 57Fe internal field NMR and Mössbauer spectroscopy study

Accepted Manuscript Structural and magnetic properties of Al-doped yttrium iron aluminum garnet 57 ceramics: Fe internal field NMR and Mössbauer spectroscopy study Laxmi Narayan Mahour, M. Manjunatha, Harish Kumar Choudhary, Rajeev Kumar, A.V. Anupama, R. Damle, K.P. Ramesh, Balaram Sahoo PII:

S0925-8388(18)33453-4

DOI:

10.1016/j.jallcom.2018.09.213

Reference:

JALCOM 47632

To appear in:

Journal of Alloys and Compounds

Received Date: 25 June 2018 Revised Date:

21 August 2018

Accepted Date: 17 September 2018

Please cite this article as: L.N. Mahour, M. Manjunatha, H.K. Choudhary, R. Kumar, A.V. Anupama, R. Damle, K.P. Ramesh, B. Sahoo, Structural and magnetic properties of Al-doped yttrium iron aluminum 57 garnet ceramics: Fe internal field NMR and Mössbauer spectroscopy study, Journal of Alloys and Compounds (2018), doi: https://doi.org/10.1016/j.jallcom.2018.09.213. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Structural and magnetic properties of Al-doped yttrium iron aluminum garnet ceramics: 57Fe internal field NMR and Mössbauer spectroscopy study Laxmi Narayan Mahoura,†, Manjunatha Mb,†, Harish Kumar Choudharya,†, Rajeev Kumara,†,

a

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Anupama A.V.a,†, R. Damleb, K.P. Rameshc, Balaram Sahooa, *

Materials Research Centre, Indian Institute of Science, Bengaluru, India-560012 b

Department of Physics, Indian Institute of Science, Bengaluru, India-560012

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Department of Physics, Bangalore University, Bengaluru, India-560056

†

Authors contributed equally.

*

Corresponding author:

E-mail address: [email protected] (B. Sahoo) Phone: +91-80-22932943

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Abstract The structural and magnetic properties of Al substituted yttrium-iron garnet (Y3AlxFe5xO12,

x = 0, 0.1, 0.2, 0.3, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6 and 1.8) ceramic powders synthesized

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using solution combustion method were investigated. Post combustion, the samples were calcination at 1045 °C for 6 h and subsequently at 1200 °C for 6 h to obtain phase pure garnets.  d structure. The X-ray diffraction (XRD) results confirm the formation of garnets with Ia3 occupancy of Y3+ ions in the dodecahedral site and the distribution of Al3+ and Fe3+ ions in the tetrahedral and octahedral sites in the bcc structure of the garnet were confirmed by Rietveld

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refinement of XRD patterns, Mössbauer spectroscopy and 57Fe internal field NMR spectroscopy. For low Al content, Al3+ ions have preference to occupy tetrahedral (Td) sites than the octahedral

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(Oh) sites. At higher Al content the distribution of Al tends towards a ratio of 3:2 at the tetrahedral:octahedral site. Increase in Al doping results in the decrease in the lattice parameter due to smaller size of Al3+ as compare to Fe3+ ion. All the studied samples show coral-networklike surface morphology. The saturation magnetization (MS) values decrease from ~26.94 emu/g to ~ 0.17 emu/g with increase in Al content from 0.0 to 1.8. Further addition of Al makes the sample paramagnetic at RT. Substitution of non-magnetic Al3+ reduces the saturation

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magnetization rapidly due to the decrease in the superexchange interaction in the crystal.

Keywords: Yttrium Iron Garnet; Solution combustion; Reitveld refinement; Mössbauer spectroscopy; 57Fe Internal Field NMR.

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1. Introduction Yttrium iron garnet (YIG) is a ferrimagnetic material having wide range of applications in the field of microwave, acoustics, optics and magneto-optics etc [1, 2]. They are considered as materials

for

high

frequency

microwave filters,

or

acoustic

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ideal

transmitters and transducers. The Al doped YIG ferrites can be considered as suitable for electromagnetic wave absorption, switching and sensing applications. YIG based materials also finds applications in solid-state lasers, Faraday rotators [3], data storage and various non-linear optics applications [4].

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YIG belong to the cubic crystal structure with Ia3d space group. YIG has specific formula Y3Fe5O12, but the unit cell of YIG consists of eight formula units (160 atoms) having 24

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Y3+, 40 Fe3+ and 96 O2- ions. YIG has three crystallographic lattice sites. Among these lattice sites, the 24 Fe3+, 16 Fe3+ and 24 Y3+ ions occupy the tetrahedral, octahedral and dodecahedral sites, respectively [5, 6].The ion distribution in YIG is generally represented by [7](Fe3)[Fe2]O12, where [7], () and [] represent dodecahedral, tetrahedral and octahedral sites, respectively [8]. Many different methods such as solid state reaction [9], crystallization from glass[10],

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co-precipitation [11-13], mechanical alloying [14], sol-gel auto-combustion [11, 15, 16], solution combustion synthesis [17], spark plasma sintering [18] , solvothermal /hydrothermal [19-21], polymeric precursor route [7], glycol route

[22]etc. was reported in the literature for the

synthesis of garnet. It was observed that the phase purity and the magnetic properties [23] of the

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garnets were greatly influenced by the synthesis method. The conventional synthesis methods, like high energy ball-milling, are time consuming and require high temperature annealing. In case of sol-gel combustion synthesis and co-precipitation methods, control of pH [24] and choice

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of precipitant are crucial for the phase purity of garnets [14]. On the other hand, the combustion synthesis is simple, quick, needs low cost precursors and produces fine nanocrystalline powder with high yield. The enthalpy of the reaction is governed by the type of fuel and the ratio of fuel to metal nitrates, which governs the physical properties like crystallite size and surface area of the particles [25]. In addition, doping of foreign cations such as Al, Cr, Si, Co, Nd at the tetrahedral and octahedral sites can lead to tailored magnetic and electrical properties [8, 26-33]. There are only few reports on the Al doped YIG [31-33]. The structure (occupancy of ions) and change in magnetization of the Al substituted YIG (Y3Fe5-xAlxO12, x= 0, 1, 2, 3, 4 and 3

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5) (YIAG) samples prepared by solution combustion method were reported earlier [5]. The Y3+ ions occupies the dodecahedral (Dh) sites whereas Al3+ is occupied in both tetrahedral (Td) and octahedral (Oh) sites with the preference to occupy the Td sites at low concentrations [31-33]. The undoped Y3Fe5-xAlxO12 sample (x = 0), YIG, is soft ferrimagnetic, where the saturation

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magnetization decreased rapidly with increase in Al content due to decrease in superexchange interactions and became paramagnetic for x ≥ 2 [6]. The magnetic order in YIAG samples disappears at a lower concentration (< 40%) of Al substituted at Fe sites. Similarly, the substitution of non-magnetic Al in YIG reduces the internal field at the iron sites. Herein, we

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have systematically substituted Al at the Fe sites of the YIG structure, with a very small concentration step size of x = 0.1. This complements our previous report where the concentration was varied with a step of x=1.0.The variation of the internal field can be measured using the

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local techniques such as 57Fe internal field nuclear magnetic resonance (IFNMR) and Mössbauer spectroscopy. According to our knowledge, this is the first report on the estimation of the hyperfine field of 57

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Fe nuclei at the tetrahedral and octahedral sites in YIG structure using the

Fe IFNMR spectroscopy.

The hyperfine field values obtained by IFNMR spectroscopy were verified using 57

Fe

Fe Mössbauer and IFNMR spectroscopy provide

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Mössbauer spectroscopy at RT. Although

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same information about the Fe-ions present at different environments in terms of their magnetic hyperfine field, IFNMR technique provides the hyperfine field in the frequency domain, unlike that of energy domain values of Mössbauer spectroscopy. As a small variation in magnetic

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hyperfine field (i.e., in the range of Tesla, as in case of our YIG sample) can lead to a large range of frequencies, the frequency domain

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Fe-IFNMR spectroscopy provides better resolution and

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more accurate results. Our study using the Rietveld refinement of the XRD patterns, Mossbauer spectroscopy, VSM and

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Fe IFNMR spectroscopy, on the super-exchange interaction in Al

doped YIG system provides insight into the understanding of the magnetic behavior. The YAIG ceramics are potentially suitable for microwave absorption [34], switching,

sensing and catalytic applications (due to high surface area of their coral network type of shape). Hence, in this work we synthesized (Y3AlxFe5-xO12, x = 0, 0.1, 0.2, 0.3, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6 and 1.8) samples and studied the magnetic properties as a function of Al substitution using XRD, 57Fe IFNMR and Mössbauer spectroscopy. 4

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2. Materials and methods For solution combustion synthesis, oxidizers (usually nitrates) and fuel were reacted for combustion. The heat of the exothermic reaction helps in the formation of phase pure sample in a single step [35]. For the synthesis of Al doped YIG samples, 2.8726 g of Y(NO3)3•6H2O was

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dissolved in 10 ml de-ionized (D.I.) water. Stoichiometric amounts of oxidizers, (Fe(NO3)3•9H2O and Al(NO3)3•9H2O), were dissolved in 10 mL of DI water and added to the above solution. Glycine (NH2-CH2-COOH) was added as fuel to the above solution. Here the ratio of fuel to oxidizer was calculated equating the total oxidizing valancy of fuel to the oxidizers. The resultant

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solution was stirred for 10 minutes to make the solution homogeneous. The mixture was then transferred to a muffle furnace, preheated to ~ 500 ± 25 °C. After ~ 10 min, the combustion

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process was initiated and the auto ignition took place. On completion of reaction, brown-fluffy product was obtained. The product was scraped form the beaker and ground in an agate mortar and pestle. The obtained powder was calcined at 1045 °C for 6 h. As it will be discussed later in XRD results that the calcination temperature was not high enough and the samples with x = 0, 0.1, 0.2 and 0.3, were not phase pure but contains 3-4 % of perovskite (YFeO3) impurity phase. To obtain phase purity, the samples were again calcined at 1200 °C for 6 h. The code of the

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samples and their compositions are enlisted in Table 1.

2.1.Material Characterizations

All the synthesized Al-substituted yttrium-iron garnet (Y3AlxFe5-xO12, x = 0, 0.1, 0.2, 0.3,

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0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6 and 1.8) samples were characterized by X-ray diffraction (using PANalytical X-Ray diffractometer; Cu Kα radiation). The Rietveld refinement [36] of the relevant

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structural parameters were obtained from the XRD patterns using “FullProf” software. Scanning electron microscopy (SEM, FEI Inspect F50) was used to obtain the morphology of the samples. The structural and magnetic properties of all the synthesized samples were characterized at room temperature (RT) by

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Fe Mössbauer spectroscopy in transmission mode. The spectra were

recorded using a gas filled proportional counter. The velocity of Doppler drive was calibrated using α-Fe foil at RT. “NORMOS” computer program [37] was used for least squares fitting of the Mössbauer spectra. The magnetic hysteresis loops of the samples were obtained using “Quantum design 9T” PPMS.

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Table 1. Sample codes and the formula of the synthesized Y3AlxFe5-xO12 samples. Sample code Al concentration Fe concentration Formula (x) (5-x) GAl-0.0 0.0 5.0 YFe5O12 0.1

4.9

YAl0.1Fe4.9O12

GAl-0.2

0.2

4.8

YAl0.2Fe4.8O12

GAl-0.3

0.3

4.7

YAl0.3Fe4.7O12

GAl-0.4

0.4

4.6

YAl0.4Fe4.6O12

GAl-0.6

0.6

4.4

YAl0.6Fe4.4O12

GAl-0.8

0.8

4.2

YAl0.8Fe4.2O12

GAl-1.0

1.0

4.0

GAl-1.2

1.2

3.8

GAl-1.4

1.4

GAl-1.6

1.6

GAl-1.8

1.8

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GAl-0.1

YAl1.0Fe4.0O12

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YAl1.2Fe3.8O12

3.6

YAl1.4Fe3.6O12

3.4

YAl1.6Fe3.4O12

3.2

YAl1.8Fe3.2O12

A home-built pulsed IFNMR spectrometer was used to characterize the magnetic

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properties of trivalent iron (Fe3+) ions present at the tetrahedral and octahedral sites in the YIAG samples at RT and at 77 K. The working principle of the IFNMR instrument is provided elsewhere [38]. For the measurements, a specially designed coaxial probe was excited with a pulse sequence of ‘π/2 – τ – π/2’ (two equal pulses sequence). The reason for using two pulse

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sequences (π/2 – τ – π/2) instead of spin echo pulse sequence (π/2 – τ – π) [39, 40] is justified elsewhere [41]. An optimized pulse width of 1 µs, for which the echo amplitude was maximum,

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was chosen as the π/2 pulse. The delay time τ of 30 µs was maintained before the arrival of the second π/2 pulse (1 µs). The echo amplitude (NMR signal) was recorded as a function of frequency varying from 50 MHz to 80 MHz in steps of 0.1 MHz. This frequency range was estimated based on the hyperfine field obtained from the Mössbauer spectroscopy using the formula  =



(where γ is the gyromagnetic ratio of iron (1.382×106 rad/T) and Bhf is the

internal field at the Fe-sites). The IFNMR spectra for all the samples were measured at RT and at 77 K, respectively.

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3. Results and discussion 3.1 X-ray diffraction Fig. 1(a) and Fig. 1(b) show the XRD patterns of all the samples calcined at 1045 °C and

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1200 °C, respectively. Fig. 1(a) shows that the samples with x = 0, 0.1, 0.2 and 0.3, contain 3-4 % of yttrium ferrite perovskite (YFeO3) phase with orthorhombic structure with space group Pnma, as secondary phase and the rest of the samples are possessing only cubic crystal structure (garnet phase) belonging to Ia3d space group. Fig. 1(b) shows that calcining the samples at 1200

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°C produces phase pure garnet. The Reitveld refined XRD patterns of the samples are shown in Fig. 2. The sharp XRD peaks indicate high crystallinity of the samples. As the Al content increases it is easier for Al to occupy the Fe sites because the smaller size of Al3+ (than Fe3+)

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facilitates this occupation. Hence, YIG sample needs highest annealing temperature for the formation of phase pure structure and the samples with high Al content required lower temperature. Furthermore, we have calculated the average crystallite size and lattice strain using the Williamson-Hall equation:

 

+ 4 sin 

(1)

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 cos  =

where D is the mean size of the ordered crystalline domains (crystallite size), k is the shape factor with a value close to unity for spherical crystallites (k = 0.9), λ is the wavelength of the Xray used (Cu Kα, λ = 1.54 Å), β is the full width at half maximum (FWHM), θ is the diffraction

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angle (Bragg angle) and ε is the lattice strain.

From the Reitveld refinement results, we observed that yttrium undoubtedly occupies the

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dodecahedral site, Al goes to both the tetrahedral and octahedral sites, but it preferentially occupy the tetrahedral site at low concentration of Al doping (x ≤ 1.8). The site occupancy of Al3+ and Fe3+ ions in Td and Oh sites obtained from the Reitveld refinement results are listed in Table 2. Around 82-85 % of Al goes to tetrahedral site and rest of it goes in the octahedral site. This confirms our earlier observation [5, 6]. As we know that in YIG crystal the spins of Fe3+ ions are antiparallely arranged in Td and Oh sites, in 3:2 ratio. The net magnetization is the difference between the magnetic moment of tetrahedral and octahedral sites. Therefore, substituting the magnetic Fe3+ ions with non magnetic Al3+ ion in the Td and Oh sites can alter its magnetic properties. As we observed from the Rietveld refined XRD results that Al prefers to 7

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occupy Td site than Oh site. This results in deficiency of net Fe3+ ions in the Td site. So, we expect a tendency of change in magnetic property from ferrimagnetic into paramagnetic nature of the samples (as the tetrahedral-octahedral superexchange interaction between the Fe+3 ions will be reduced due to presence of more non magnetic Al ions at the tetrahedral sites). This

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change in occupancy of Fe3+ ions will alter its magnetic properties which we will discuss in later

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section.

Figure. 1 The powder XRD patterns of Y3AlxFe5-xO12 samples calcinated at (a) 1045 °C and (b) 1200 °C. The (121) peak of the YFeO3 impurity is encircled in (a).

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Figure 2. The Rietveld refined XRD patterns of Y3AlxFe5-xO12 samples. The red dots in the figure are the experimental data points and the red colored lines are the fits according to the Reitveld refinement. The Bragg’s reflection positions are indicated as olive colored vertical lines and the difference between obtained XRD data and Reitveld refined pattern is shown as blue colored line at the bottom of each figure. 9

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Table 2. Site occupancy of Al and Fe ions in Tetrahedral (Td) and Octahedral (Oh) sites obtained from the Reitveld refinement results and the garnet unit cell formula calculated based on the Al and Fe occupancy. Al-occupancy

Fe-occupancy

(%)

(%)

code

Unit cell formula

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Sample

Oh

Td

Oh

GAl-0.0

0.0

0.0

60.0

40.0

[7](Fe3)[Fe2]O12

GAl-0.1

82.0

18.0

60.0

40.0

[7](Al0.082Fe2.934)[Al0.018Fe1.966]O12

GAl-0.2

82.0

18.0

60.0

40.0

[7](Al0.164Fe2.840)[Al0.036Fe1.960]O12

GAl-0.3

83.3

16.7

58.6

GAl-0.4

81.3

18.7

58.5

GAl-0.6

84.2

15.8

57.2

GAl-0.8

85.1

14.9

55.2

GAl-1.0

85.0

15.0

GAl-1.2

83.3

16.7

GAl-1.4

83.4

16.6

GAl-1.6

83.4

GAl-1.8

83.4

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Td

[7](Al0.250Fe2.754)[Al0.050Fe1.946]O12

41.5

{Y3}(Al0.325Fe2.692)[Al0.075Fe1.908]O12

42.8

{Y3}(Al0.505Fe2.516)[Al0.095Fe1.884]O12

44.8

{Y3}(Al0.681Fe2.318)[Al0.119Fe1.882]O12

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41.4

46.2

{Y3}(Al0.850Fe2.150)[Al0.150Fe1.850]O12

52.6

47.4

{Y3}(Al1.000Fe2.000)[Al0.200Fe1.800]O12

51.0

49.0

{Y3}(Al1.167Fe1.838)[Al0233Fe1.762]O12

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53.8

49.0

51.0

{Y3}(Al1.334Fe1.666)[Al0.266Fe1.734]O12

16.7

46.9

53.1

{Y3}(Al1.500Fe1.500)[Al0.300Fe1.700]O12

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16.6

Variation of the crystallite size and lattice strain for the samples calcined at 1200 °C are

shown in Fig. 3(a). The crystallite size varies between 80-60 nm with a slight decreasing trend as Al content increases. The lattice strain is almost independent of Al content with a value of ~ 3×10-3 (Fig. 3(a)). This indicates that after annealing the samples at 1200 °C, the stress in the lattice is relaxed.

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Figure 3. (a) Variation of crystallite size and (b) lattice strain, and (c) tolerance factor with Al content in the samples calcined at 1200 °C.

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The lattice parameters obtained from the Rietveld refinement results are shown in Fig. 3(b). For YIG sample, we have obtained lattice parameter value of 12.3804 Å. The lattice parameter values decrease linearly with increase in the Al concentration. This decrease in lattice parameter can be attributed to the smaller size of Al3+ ion than Fe3+ ions. A similar behavior was

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reported for Al and Cr co-substituted YIG (Y3AlxCrxFe5-2xO12, x = 0-1.0) [42]. For x = 1.8 sample, the lattice parameter is found to be 12.2798 Å. This trend also suggests that smaller sizes Al3+ is substituted in the lattice. The shrinkage of the unit cell because of Al3+ introduction may be attributed to the smaller size of Al3+ (0.56Å) as compared to Fe3+ (0.67Å) .

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`Fig. 3(c) shows the variation of the calculated tolerance factor ( =

  

, where RA,

√ (  )

RB and RO are the radii of Th, Oh site cations and O-anion, respectively) with Al content. All the

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samples contain a low concentration of Al, where Al3+ have the preference to occupy Td sites. The values of the tolerance factor are almost similar, with a slightly decreasing trend with Al content; this indicates that there is no rapid variation in Al occupancy in the samples. Therefore, Al3+ is occupying almost 82-85 % of Td sites and 18-15 % of Oh site, which justifies the correctness of XRD results. As less the tolerance factor the more will be the hindrance in the

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growth of the particles, the observed decrease in the size of the coral branches with Al content (SEM images (Fig. 4), discussed later) is due to the observed small decrease in the tolerance factor (Fig. 3(c)). 3.2 SEM

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SEM micrographs of the samples are shown in Fig. 4. The particles are having coral shape surface morphology and Al substitution doesn’t change the morphological shape of the samples.

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However, the size of the branch-width is increasing as we increase the Al substitution. Since the particles are interconnected with each other and formed a network type structure therefore we have calculated the branch-width for each particles and the variation of these branch-width is shown in figure 5.The size of the coral branches width is found to decrease from ~0.9 µm to 0.4 µm as the Al content increases. As discussed above in sec. 3.1, this decrease in branch-size might be related to the small decrease in tolerance factor.

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Figure 4. SEM micrographs of Y3AlxFe5-xO12 powder samples calcination at 1200 °C. 3.3 Mössbauer spectroscopy In yttrium iron garnet the site occupancy of Fe ions in Td and Oh sites play important role in determining the magnetic properties of the samples.

As a versatile tool, we have used

Mössbauer spectroscopy to determine the site occupancy of Fe ions [43]. Fig. 6 shows the RT Mössbauer spectra of all the analyzed YIAG samples. We have observed that for the samples with x=0.0 to 0.6, the Mössbauer spectrum of each sample consists of two sextets and are fitted

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Figure 5. Variation of average branch-width (diameter) of the coral shaped particles with Al

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content, calculated from SEM images.

accordingly [5–10]. The sextet S1 with higher hyperfine field (Bhf) is assigned to the Fe+3 ion at the Oh site and sextet S2 with lower Bhf is assigned to the Td site [5, 6, 11, 12, 44]. Assuming the same Lamb- Mössbauer factor for both the Td and Oh sites, the integrated area under these two

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sextets provide the net occupancy of Fe3+ ions in the respective sites. The results of Mössbauer fitting parameters such as magnetic hyperfine field, quadruple splitting, isomer shift and spectral

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area (%) are listed in Table 3. The measured isomer shift and the quadruple splitting values donot vary significantly with changes in Al concentration of the sample, as seen in Table 3. This is because the substitution of Al at the Fe sites do not significantly alter the valency of Fe ions and the electric field gradients. Therefore, the observed isomer shift and quadruple splitting values do not vary drastically [45]. However, because Al is non-magnetic, the magnetic hyperfine field (decided by the super-exchange interaction) and the spectral areas corresponding to the amount of Fe atoms at the Th and Oh sites are different for different Al-concentrations. From the relative area of Fe+3 ions in the Td and Oh sites, which have antiparallel alignment of spins with respect to each other due to the superexchange interaction, the magnetic properties 14

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can be understood. From the Mössbauer results and the Rietveld refinement results, we can

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quantify the site occupancy of both Al3+ and Fe3+ ions in the Td and Oh sites of the samples.

Figure 6. The least square fit Mössbauer spectra of Y3AlxFe5-xO12 powder. D1 (yellow) and D2 (magenta) represent the doublets due to Fe atoms in octahedral and tetrahedral sites, respectively. Furthermore, S1 (light blue) and S2 (green) represent the sextets due to Fe atoms at octahedral and tetrahedral sites, respectively. The black colored dots represent the measured obtained Mössbauer spectra. The red line is the cumulative fit.

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Table 3. Mössbauer spectral parameters of the Y3Fe5-xAlxO12 samples. IS: Isomer shift, QS: Quadruple splitting, Bhf: Magnetic hyperfine field. Sample

Sub-spectra IS (± 0.005) QS (± 0.01) Bhf (± 0.5) Area (± 2)

code

designation

(mm/s)

(T)

GAl-0.0 S1

0.269

0.048

48.7

35.6

S2

0.034

0.074

39.5

64.4

GAl-0.1 S1

0.269

0.030

48.3

37.2

S2

0.032

0.076

39.2

62.8

GAl-0.2 S1

0.265

0.030

47.8

36.9

S2

0.039

0.044

39.0

63.1

GAl-0.3 S1

0.256

0.007

47.1

35.6

S2

0.036

0.056

38.5

64.4

GAl-0.6 S1

0.235

0.030

45.2

30.3

S2

0.036

0.032

37.2

69.7

GAl-1.0 S1

0.308

0.002

40.4

25.7

S2

0.095

0.012

33.4

74.3

GAl-1.2 S1

0.279

-0.049

37.7

26.6

0.080

0.044

31.3

73.4

0.236

0.012

23.9

38.1

0.201

0.064

18.2

42.8

0.195

1.309

----

19.1

GAl-1.8 D1

0.032

1.027

----

48.9

D2

0.252

0.398

----

51.0

GAl-1.6 S1 S2

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S2

(%)

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(mm/s)

Fig. 7(a) shows the variation of the area under sextet S1 and S2 with the Al composition

(x). For the YIG (x=0) sample the Td and Oh site areas (S2:S1) should be in the ratio 3:2 (60:40), but form the Mössbauer result we observed the relative area ratio of 64:36. For smallest Al content (x=0.1) in the sample, we observed a lowest relative area under the sextet S2 (~63%) (Fig. 7(a)). As the Al composition increases the relative area under the sextet S1 (Oh site) decreases and simultaneously the relative area under the sextet S2 (Td site) increases. This means that the Al occupancy at the octahedral site increases and that at the tetrahedral site decreases, as 16

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Al content in the samples increases. This supports our observation form the XRD results that at low Al compositions, Al+3 ions preferably occupy the Td site and as the Al composition

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increases the Al+3 ions tend to approach 3:2 ratio at the tetrahedral:octahedral sites.

Figure 7. (a) Variation of Area (%) of S1 and S2 subspectra with Al- content. (b) Variation of

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magnetic hyperfine field (Bhf) of S1 and S2 subspectra with Al- content. Fig 7b shows the variation in magnetic hyperfine field values of the S1 and S2 sextets in

the Mössbauer spectra of the analyzed samples. The hyperfine field value of both the Td and Oh site gradually decreases with increase in Al content. For YIG sample (x=0) the hyperfine field value of Td site and Oh site are ~ 40 and ~ 49 T, respectively. For the GAl-1.6 sample it has decreased to ~24 and ~18 T. This decrease in hyperfine field values suggests that the magnetic properties of the sample deteriorate with Al doping. At higher Al content, e.g., for GAl-1.6 sample we get the broad Mössbauer spectrum with a doublet. This doublet is attributed to the 17

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appearance of paramagnetic phase in GAl-1.6 sample. Further doping of Al (i.e., for GAl-1.8 sample) results in complete paramagnetic phase of the sample, with two doublet in the

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Mössbauer spectrum [5].

Fig. 8 VSM (a) The M-H loop of Y3AlxFe5-xO12 powder, (b) Variation of saturation magnetization (MS) with increase in Al- content.

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3.4 VSM measurements Room temperature M vs H curve of all the studied samples are shown in Fig. 8 (a). All the samples are soft magnetic in nature with very small coercivity value. The observed saturation

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magnetization (MS) values are plotted in Fig. 8(b). Clearly, the Al substitution in the garnet lattice deteriorates the magnetic property[14, 15]. For YIG sample, the MS value is 27 emu/g, which gradually decreases to ~ 0 emu/g for GAl-1.8 sample. From the Reitveld refinement and Mössbauer spectroscopy results (Table 2 and Table 3), it is clear that Al goes both to the Td site and Oh sites, but it has the preference to occupy the Td site. Hence, the net magnetization, which

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is difference between the Td site and Oh site Fe spin moments, alters by Al substitution. Furthermore, the nonmagnetic Al3+ substitution reduces the superexchange interaction between

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Fe3+ ions at the Td and Oh sites in the crystal[7]. The non-linear variation of the saturation magnetization (MS) with Al composition (Fig. 8(b)) can be due to the preference of Al to occupy Td site at lower doping than that at higher doping of Al. Along with that, the more substitution of Al at the Td site decrease the superexchange interaction and that transforms the sample to paramagnetic state relatively faster with compositions in comparison to ideal 3:2 occupation. Therefore, even only < 40% Al doping, i.e., for the GAl-1.8 sample, the saturation magnetization

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vanishes [5, 6]. At higher composition (≥ 40%) the general weakening of super-exchange interaction-strength makes the sample paramagnetic. 3.5 57Fe-IFNMR

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The IFNMR measurements on all the studied samples were carried out at RT and 77 K. The obtained results are discussed below.

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3.5.1 Measurements at RT

Fig. 9 shows the

temperature.

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Fe-IFNMR spectra of the YIAG samples measured at room

Fe-IFNMR signals were very weak and were detectable only at the central peak

frequencies for three samples, i.e., for (x = 0.0 − 0.2). For x = 0.0 sample, the IFNMR signals were observed at two different frequencies, one at ~ 54.4 MHz and the other at ~ 67.1 MHz. The internal hyperfine field (Bhf) corresponding to these NMR frequencies were calculated using the equation "#$ =

$ 

and the values were ~39.36 T and ~ 48.55 T. These values are in good

agreement with the values of magnetic hyperfine fields of Td and Oh sites obtained from the 19

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Mössbauer spectral results (Table 3). For x = 0.1 sample, the NMR frequencies for Td and Oh sites nuclei shifted to lower values, i.e., ~ 54.2 MHz and 66.8 MHz, respectively. However, the intensity of the signal decreases compared to the YIG sample (x=0.0). Further, for x=0.2, the two weak intense peaks are observed at ~ 53.8 MHz and ~ 66.1 MHz. For the higher concentration of

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Al in the samples (x ≥ 0.3), our spectrometer was not able to detect any NMR signal. However,

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we observed NMR signals for all the samples at 77 K, as discussed in the next section.

Figure 9. IFNMR spectra of the Y3AlxFe5-xO12 (x=0.0-0.3) powder samples measured at RT.

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3.5.2 Measurements at 77 K

Fig. 10 shows the IFNMR spectra of all the samples measured at 77 K. Clearly, there are two IFNMR peaks at the 62 and 75 MHz, which were assigned to Fe nuclei at the Td and Oh

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sites, respectively, as discussed earlier. We have two important observations in the IFNMR spectra of the sample. Firstly, for YIG sample the IFNMR peaks intensity is highest and the intensity of both the peaks decreases gradually with the increase in Al content and vanishes for x=1.8. Secondly, there is a red shift in the NMR frequency of both the peaks as we increase the Al content in the sample. The variation of NMR frequency and echo amplitude with Al content are shown in Fig 11(a) and Fig 11(b), respectively. As observed, both the NMR frequency and the echo amplitude are decreasing with increase in the Al content. These observations support the results of the Reitveld refinement of XRD patterns, Mössbauer spectroscopy and VSM. From

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Figure 10. (a) The IFNMR spectra of the Y3AlxFe5-xO12 powder samples measured at 77 K. The variation of IFNMR frequency and echo amplitude of (b) low frequency (1st peak, Td site) and (c) high frequency (2nd peak, Oh site) peaks. 21

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the Mössbauer and Reitveld refinement results we have observed that for low Al content, e.g., for x=0.1, Al preferentially occupy the Td sites, therefore the echo amplitude of Td site should initially decrease much rapidly than the echo amplitude of Oh site. Hence, the NMR signal for Td site of x=0.1 sample drops drastically than that of the Oh site, as observed in Fig. 10 and further

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increase in Al content the rate of reduction of echo amplitude slows down. Hence, the Reitveld refinement, Mössbauer and IFNMR results are consistent and support each other. The observation of the red shift in the IFNMR signal is in line with the Mössbauer spectroscopy results that the decrease in the hyperfine field value of the Td and Oh sites with Al content

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decreases the IFNMR frequency. 3.5.3 Spin population factor (ξ)

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As described above, we have observed only very weak signal at RT only for three of our sample containing very small amount of Al (x = 0.0, 0.1 and 0.2). The observation of this weak signal may originate from a very small spin population factor (SPF) or low sensitivity of the NMR equipments to detect the spins at RT. To understand the reason for this weak or no NMR signal of our samples at RT, we calculated the values of SPF, ξ, for our studied samples. The

[41], is defined as:

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spin population factor for a particular site (Td or Oh) in the spinel structure, as described earlier

ξ=

Nm− × Sm × S f N m+

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where, Nm+ and Nm- are the number of nuclei in the spin state parallel and antiparallel to the magnetic field, Sm is the fraction of the spins magnetically ordered in the material and Sf is the

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fraction of spins present at a particular site (Td or Oh). The calculated SPF values at RT are given in Table 4 and Table 5 for the Td and Oh sites, respectively. We have determined the Boltzmann factor using the equation below:  γ Bhf  N m− = exp  −  N m+  kT 

where, γ is the gyromagnetic ratio of iron (1.382×106 Hz/T), Bhf is the hyperfine field of the Fe atoms and k is the Boltzmann constant (1.38×1023 J/K) and T is the absolute temperature. The 22

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hyperfine fields at RT for the Td and Oh site Fe atoms were obtained from Table 3 (the Mössbauer spectroscopy results). The value of this Boltzman factor seems to be same for all samples up to fourth decimal.

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The fractions of spins magnetically ordered in the materials, Sm, were obtained from the Mössbauer spectroscopy. These values were basically the ratio of the area under Zeeman-split sextet (the magnetically ordered Fe spins) to that of the total area of the spectrum. In our samples the value of Sm is 1, except for Gal-1.6 sample (Sm = 0.81).

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The fraction of spins present at a particular site (Td or Oh), Sf, were obtained by subtracting the fractional occupancy of Al present at a particular site per unit spin, as obtained from the Rietveld refinement and Mössbauer spectroscopy results, from the stoichiometric

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amount of Fe at that particular site in the structure of YIG (x=0). As at low Al content in the samples, the Al atoms prefer to occupy the Td sites and we have observed about 85 % of the total Al is present at the Td sites, the Sf values were calculated accordingly. For example, for x = 0.1,

Sf =

1  2 − (0.15 × 0.1)   . 2  2

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1  3 − (0.85 × 0.1)  GAl-0.1 sample, the S f =   for the tetrahedral site and for octahedral site 3 3

sample

Al conc. Internal field (Bhf) Frequency (x) (MHz) 0 0.1 0.2 0.3 0.6 1.0 1.2 1.6

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GAl-0.0 GAl-0.1 GAl-0.2 GAl-0.3 GAl-0.6 GAl-1.0 GAl-1.2 GAl-1.6

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Table 4. Spin Population factor for the tetrahedral site of YIAG samples.

48.7 48.3 47.8 47.1 45.2 40.4 37.7 23.9

67.3 66.7 66.05 65.09 62.56 55.8 52.1 32.75

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N m− N m+

Sm

Sf

ξ

0.9999 0.9999 0.9999 0.9999 0.9999 0.9999 0.9999 0.9999

0.644 0.628 0.631 0.644 0.697 0.743 0.734 0.428

0.333 0.324 0.320 0.305 0.276 0.239 0.220 0.182

0.2146 0.2073 0.1983 0.1964 0.1925 0.1774 0.1614 0.0780

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Table 5. Spin Population factor for the octrahedral site of YIAG samples.

0 0.1 0.2 0.3 0.6 1.0 1.2 1.6

39.5 39.2 39 38.5 37.2 33.4 31.3 18.2

54.58 54.17 53.89 53.2 51.41 46.16 43.25 25.15

N m− N m+

Sm

Sf

ξ

0.9999 0.9999 0.9999 0.9999 0.9999 0.9999 0.9999 0.9999

0.356 0.372 0.369 0.356 0.303 0.257 0.266 0.428

0.5 0.496 0.492 0.489 0.478 0.462 0.455 0.440

0.1778 0.1860 0.1818 0.1761 0.1402 0.1169 0.1169 0.1851

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GAl-0.0 GAl-0.1 GAl-0.2 GAl-0.3 GAl-0.6 GAl-1.0 GAl-1.2 GAl-1.6

Al conc. Internal field (Bhf) Frequency (x) (MHz)

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sample

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As we can observe from Table 4 and Table 5 that the spin population factor at RT is continuously decreasing from a value of 0.215 to 0.078 for the tetrahedral site and from a value of 0.178 to 0.117 for the octahedral site. The discrepancy observed in Table 5 that for the last sample (Gal-1.6) the value of ξ higher than that of the previous sample can be assigned to the difficulty in fitting the Mössbauer spectra with two broad sextets (Fig. 6). In Fig. 9, we have observed that NMR signal deteriorates and is specifically observed only for central NMR

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frequencies corresponding to the particular site only for low Al containing samples (x = 0.0, 0.1 and 0.2). Comparing with our previous report on Li-Zn ferrite system [41], where we have observed that for any value less than 0.25, the NMR signal may not be observable. However, for

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the present case we have observed signals down to a ξ value of 0.18 (although very feeble). Taken together all the characterization results, it can be inferred that the spin population factor

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not only depends on the Boltzmann factor (

N m− ), fraction of spins ordered magnetically (Sm) N m+

and the fraction of spins present at a particular site (Sf), but also the distance between the magnetically interacting cations, the strength of the superexchange interaction, the presence of non-magnetic cations (such as Y in the present case). Hence, a wide variety of samples needs to be studied at different temperatures in order to obtain an accurate empirical formula for predicting or understanding the observation of IFNMR signal. Hence, our investigation provides the impetus for the development of IFNMR spectroscopy in the study of ferrimagnetic samples.

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4. Conclusion Phase pure yttrium-aluminium-iron garnet (Y3AlxFe5-xO12, x = 0, 0.1, 0.2, 0.3, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6 and 1.8) ceramic powder samples were successfully prepared using solution

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combustion method. All the samples show coral-network shaped morphology with a reduction in width (diameter) of coral branches. The lattice parameter reduces due to the smaller ionic radii of Al3+ ion than Fe3+ ion. The crystallite size varies between 80-60 nm in a decreasing trend with increase in Al- content. The Rietveld refinement of the XRD patterns, Mössbauer spectroscopy

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and IFNMR spectroscopy results conclude that for low Al doping in YIG, Al prefers to occupy tetrahedral site than octahedral site, which influences the magnetic properties of the garnet, as well. This preference of Al3+ ion for the tetrahedral site can be due to the smaller ionic radii of

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dopant Al3+ (0.56Å) compared to that of host Fe3+ (0.67Å). Further addition of Al, the hyperfine field (Bhf), saturation magnetization (MS) and NMR frequency monotonously decreases suggesting that the magnetic property of the samples deteriorate with Al doping. The nonmagnetic Al3+ substitution rapidly reduces the magnetization for smaller Al-doping and the internal field NMR intensity decreases initially rapidly followed by a slow decrease (especially for the tetrahedral site) due to preferential tetrahedral occupation of Al. This results in the

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decrease in strength of the super-exchange interactions in the crystal. Further substitution of Al (above x = 1.8) transforms the garnet to paramagnetic phase.

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Highlights 1. Yttrium-aluminium-iron garnet powders were synthesized via solution combustion

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method. 2. Al-doping reduces lattice parameter and magnetism due to the smaller ionic radii of Al3+.

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3. For small Al-concentration, Al prefers to occupy tetrahedral site than octahedral site of YIG.

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4. Preference of Al3+ to go tetrahedral site is due to the smaller ionic radius of Al3+. 5. Internal field NMR intensity decreases rapidly followed by a slow decrease as Al

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content increases.