phys. stat. sol. (a) 196, No. 1, 121 – 124 (2003) / DOI 10.1002/pssa.200306368

Nuclear magnetic resonanceof 57Fe in bismuth–yttrium iron garnets
H. Štěpánková*, 1, T. Okuda2, E. Gamaliy1, A. Snezhko1, N. Adachi2, and T. Kaneuji2
1

Charles University, Faculty of Mathematics and Physics, V Holešovičkách 2, 180 00 Prague 8,Czech Republic
2
Nagoya Institute of Technology, Gokiso-cho. Showa-ku, Nagoya 466-8555, Japan
Received 1 July 2002, accepted 15 October 2002
Published online 19 March 2003
PACS 75.50.Gg, 76.60.–kHyperfine magnetic interaction at iron nuclei in a bismuth – yttrium iron garnet system BixY3–xFe5O12,0 ≤ x ≤ 3, was studied using nuclear magnetic resonance. 57Fe NMR spectra were measured by spin echo
technique at 4.2 K in zero external magnetic field. The spectrum of bismuth iron garnet film Bi3Fe5O12
prepared by ion beam sputtering corresponds to the iron garnet structure having magnetization along[111] direction: two a-lines at 74.00 MHz and 75.05 MHz with the intensity ratio 1 : 3 assigned to octahedral iron sites and a single d-line at 66.46 MHz of tetrahedral iron sites. The resonant frequencies areslightly shifted in comparison with yttrium iron garnet Y3Fe5O12. Satellite lines or line broadening appear
in spectra of partially substituted samples due to a distribution of Y3+ and Bi3+ cations in dodecahedralsites.

1. Introduction Nuclear magnetic resonance (NMR) is a method sensitive to the local properties ofmagnetic materials. It has been applied to YIG containing various substituents or intrinsic substituting
defects on iron and yttrium cation sites [1–6]. NMR spectrum of 57Fe nuclei for an ideal YIG structure
contains a single d-line assigned to iron ions in tetrahedral crystallographic sites and two lines a1 and a2
having integral intensity ratio 1 : 3 which correspond to octahedral sites. Spectrum of a-sites splits into
two lines because of the magnetic nonequivalency of a-sites with respect to the [111] direction of magnetization. Resonant frequencies are proportional to the local magnetic field at iron nuclei by gyromagnetic ratio 1.377 MHz/T.
Cationic substitution modifies hyperfine magnetic field at nuclei in its vicinity. Low concentration ofsubstitution gives rise to the characteristic satellite lines which appear in the spectrum besides the mainlines (corresponding to the nuclei with no substituent in their vicinity) supposing that the induced changeof frequency exceeds the linewidth. In case of a complete replacement of Y3+ in dodecahedral (c) cationsites the pattern of the spectrum is expected to be analogous to that of YIG (provided magnetization
direction remains parallel to [111]), with possible shifts of the resonant frequencies.
In the present paper we report on 57Fe NMR spectra dependence on the replacement of Y3+ indodecahedral sites by Bi3+. The Bi3+ substitution is well known to modify iron garnets magnetic properties and especially to enhance magnetooptical effects. Because of the considerable difference in ionicradii of Y3+ (0.1019 nm) and Bi3+ (0.117 nm) [7] a bulk sample of bismuth iron garnet (BIG) cannot be*

Corresponding author: e-mail: helena.stepankova@mff.cuni.cz, Phone: +420 2 2191 2773, Fax: +420 2 2191 2567

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122

synthetized. Nevertheless, techniques of film preparation by direct epitaxial growth from vapour phaseproved to be successful [8].
2. Experimental BIG single crystal film and bismuth–yttrium iron garnet single crystal film of high Bi
concentration (x = 2.75) were prepared by ion beam sputtering (IBS). Characteristics of these samples
are given in Table 1. Three comparative samples with low x were used: pure single crystal YIG filmgrown onto gadolinium galium garnet substrate by liquid phase epitaxy (LPE), bulk single crystal(x = 0.02) grown from flux and polycrystalline sample (x = 0.15) prepared by ceramic technique.
NMR spectra were recorded at 4.2 K in zero external magnetic field. Spin echo pulse sequence withcoherent data summation and Fourier transformation was employed.
3. Results and discussion Spectra of BixY3–xFe5O12 (x = 0, 0.02, 0.15, 2.75 and 3) are plotted in Fig.1.
The spectrum of BIG consists of a single line centered at 66.46 MHz which corresponds to iron nuclei ind-sites and of two lines at 74.00 MHz and 75.06 MHz (with relative integral intensities ~1 : 3) assigned
to iron in a-sites. This scheme of a spectrum is characteristic for an iron garnet having magnetization
direction along ~ [111]. Comparison of resonant frequencies of BIG and YIG films is given in Table 2.
The spectral lines of BIG sample exhibit more than an order higher inhomogeneous broadening in comparison with YIG epitaxial films and the lines shape is asymmetric with the slower decrease towards
lower frequencies. The most probable reason for the line broadening is a higher amount of intrinsic defects, nevertheless a distribution of magnetization directions close to [111] or distribution of demagnetizing fields cannot be excluded.
Keeping in mind that NMR yields more precise values of the hyperfine fields, a good agreement isfound between the hyperfine field on iron nuclei in BIG as well as in YIG obtained from our NMR spectra at 4.2 K and by Mössbauer conversion electron spectroscopy at 6 K [9].
Resonant frequency of a given iron crystallographic site can be described as a sum of two contributions: a dominating isotropic part I and a small anisotropic part, i.e. a contribution dependent on themagnetization direction with respect to the local coordinate system of the site [3, 4]. For magnetization
parallel to [111] the resonant frequency of d-spectrum equals directly to the isotropic part and a parameter characterizing the anisotropy cannot be determined from this spectrum. In case of the a-spectrum
the isotropic part is a weighted mean of the a1 and a2 resonant frequencies and the anisotropy can be
expressed by means of the splitting δa between a1 and a2 lines. Comparison of BIG and YIG given inTable 2 shows that in BIG the isotropic part is higher for d-sites, while it is smaller for a-sites. The splitting of a-lines does not change considerably.
X-ray diffraction performed for yttrium-bismuth [10] and bismuth iron garnets [11] indicates that
replacement of Y3+ by Bi3+ leads to a shortening of Fe–O2– distances in the tetrahedron while in theoctahedron they lengthen. In this respect the comparison of iron hyperfine fields in BIG and YIG is interesting since the higher covalency related to the shorter interatomic distances is believed to lower
resonant frequencies. The found spectral shifts of BIG in respect to the YIG reveal, however, just an
opposite tendency, i.e., decrease in resonant frequency with increasing size of the octahedron, and inTable 1 Characterisation of iron garnet films prepared by IBS.

x–3

–2.75

Target composition
Substrate ( (111) plane)
Substrate lattice constant (nm)Thickness (µm)
Lattice constant (nm)Faraday rotation (°/µm)
at 635 nm, room temperature–Bi3Fe5O12–Nd3Ga5O12–1.2509–2.75–1.2629–(Y0.25Bi2.75)2.44/3Fe4.56O12–(GdCa)3(GaMgZr)5O12–1.2499–1.75–1.2638

–5.35

–5.1

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Table 2 Comparison of 57Fe NMR spectral parameters of BIG and YIG films

BIGYIG

fd(MHz)∆d(MHz)fa1(MHz)∆a1(MHz)fa2(MHz)∆a2(MHz)

Ia(MHz)δa(MHz)

66.46
64.96

0.57
0.025

74.00
75.06

~0.40
0.023

75.05
76.06

0.50
0.041

74.7975.81

1.05
1.00

( fd, fa1, fa2 ... resonant frequencies of d, a1 and a2 lines; ∆d, ∆a1, ∆a2 ... linewidths;
Ia = (fa1 + 3 fa2)/4, δa = fa2 – fa1)

creasing frequency in case of the decreasing tetrahedron. This indicates a substantial impact of the bismuth electron structure on electron transfer in Fe–O-cation triads.
In samples with x = 0.02 and 0.15 we have detected resolved satellites in spectra induced by the presence of a Bi substituent in the nearest c-neighbourhood of the resonating iron. Two satellite lines arepronounced in the d-spectrum (shifted by 0.54 and 0.81 MHz from the main d-line). a-spectrum contains
one satellite line close to a1-main line (shifted by –0.215 MHz ), and three satellite lines near a2, two ofthem overlap (shifted by –0.16 MHz and, for the double satellite, by –0.31 MHz).
For high values of x (close to 3) a mirror structure of spectra is expected. It means that the spectra
contain satellites corresponding to the resonating irons with a single Y3+ cation in their nearest
c-neighbourhood regularly occupied (in BIG) by Bi3+. These satellite lines should be shifted from the BIG
main lines in an opposite direction than satellites in YIG with small concentration of Bi. The inhomoge-

x = 0, film (LPE)da2a1phys.stat.sol.

x = 0.02, single crystal

25 x

925 x

Intensity (arb. u.)

Bi induced satellites

x = 0.15, polycrystalline sample

x = 2.75, film (IBS)x = 3, film (IBS)

Fig. 1 57Fe NMR spectra ofirongarnets
BixY3–xFe5O12
measured at 4.2 K in zeroexternal magnetic field.

a2da1

64

65

66

67

73

Frequency (MHz)

7475

76

H. Štěpánková et al.: Nuclear magnetic resonance of 57Fe in bismuth – yttrium iron garnets

124

neous line broadening in the spectrum of a film with x ~ 2.75 disables detailed analysis, yet still the mirror position of satellite lines is evident at least in the spectrum of d-sites.
Assuming that the additivity principle is valid for a contribution of individual Bi → Y substitutions in
the neighbor c-site of the resonating iron nucleus to its resonant frequency, the NMR frequencies for BIGcan be predicted on the basis of the YIG main lines frequencies and the shifts of Bi3+ induced satellites
observed in spectra of the yttrium-bismuth iron garnets with low Bi content. Considering numbers of thenearest c-neighbours of the d and a iron sites and neglecting small differences in dipolar and Lorentz
magnetic fields we obtained for BIG (in MHz): fd ≡ Id = 66.31, fa1 = 73.76, fa2 = 74.79, Ia = 74.53,
δa = 1.03. These values differ somewhat from those obtained from the experiment (see Table 2) predominately in the isotropic parts for which the estimated values are systematically lower.
4. Conclusions The 57Fe NMR spectrum measured in a single crystal film of BIG prepared by IBS
reflects the garnet crystal structure having magnetization along ~[111] direction. The modification of a
hyperfine interaction induced by replacement of Y3+ with Bi3+ was found. It evidences a considerable
difference in the electronic structure of these cations.
Acknowledgement
University.

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