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The perovskite compounds of LaMn 1 - x Co x O 3 ( 0.2 ⩽ x ⩽ 0.5 ) have been studied by means of NMR spectroscopy in powder samples prepared by the solid state reaction and sol–gel methods. The NMR signals were observed in the frequency range 250–650 MHz at 4.2 K in zero external magnetic field. A relatively narrow spectral line assigned to 55 Mn in Mn 4 + is situated around 270–320 MHz in all spectra. Additional spectral lines are observed at higher frequencies. Line intensities and forms are dependent on the cobalt concentration and preparation techniques.

57Fe NMR spectra of lutetium iron garnet (LuIG) and yttrium iron garnet (YIG) samples prepared by liquid mix process were measured at 4.2 and 77 K. We detected and identified patterns of satellite lines induced by lutetium antisite defects Lu(a) in LuIG and yttrium antisite defects Y(a) in YIG. Frequency shifts of satellite lines induced by Lu(a) were smaller compared to shifts of corresponding satellite lines induced by Y(a). Concentration of the defects determined from intensities of spectral lines was significantly higher for Lu(a) in LuIG than for Y(a) in YIG.

The known macroscopic data for the temperature dependence of the normalized spontaneous magnetization, ms(T), of iron, cobalt and nickel are reconsidered and compared with our normalized zero-field hyperfine field data, hhf(T), obtained by Mossbauer spectroscopy or nuclear magnetic resonance. It is observed that for all three metals hhf(T) is not proportional to ms(T) and systematically smaller than ms(T). This is attributed to smaller magnetic interactions in the domain walls compared to the volume of the domains. The temperature dependence of ms(T) and hhf(T) can be described in the whole range 0 S=3/2 and 2 .

Further investigations on magnets with pure spin moments are presented showing that the temperature dependence of the order parameter can accurately be described over a large temperature range by a...

NMR spectra on 57 Fe nuclei in yttrium iron garnet (YIG) containing Y 3+ (a) defects and Al 3+ (d) substitutions were measured in the temperature range 4.2–360 K in zero external magnetic field. The temperature dependencies of satellite lines caused by Y 3+ (a), Al 3+ (d) impurities were determined. Theoretical description of the temperature behavior of the main and satellite lines has been carried out in the frame of model based on the Bethe–Peierls–Weiss approximation.

The magnetism in three polycrystalline samples of the electron doped manganites ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Pr}}_{x}{\mathrm{MnO}}_{3}$ $(x=0.025,$ 0.05, 0.1) was investigated by NMR on ${}^{55}\mathrm{Mn}$ and ${}^{141}\mathrm{Pr}$ nuclei, neutron diffraction, and magnetic measurements. In all systems the coexistence of majority antiferromagnetic and minority ferromagnetic phases was found. At 4.2 K distinct NMR signals from ferromagnetic domains and antiferromagnetic matrix as well as from the domain walls were detected. Magnetic moment at 4.2 K increases sharply with increasing x from $0.06{\ensuremath{\mu}}_{B}$ pfu $(x=0.025)$ to $0.9{\ensuremath{\mu}}_{B}$ pfu $(x=0.1)$ accompanied by an equally sharp increase of the ferromagnetic NMR signal. For the $x=0.1$ sample the temperature dependence of the ferromagnetic moment determined by neutrons and NMR agrees well with the magnetic measurements. For smaller x the NMR gives essentially the same dependence, while the bulk magnetization decreases with increasing temperature more rapidly, indicating reduction of the ferromagnetic volume when temperature is raised. For a more complete characterization of the system studied the temperature dependencies of the electrical resistivity and thermopower were also measured.

Using NMR, anisotropy of the hyperfine field on 57Fe nuclei was measured in Y3Fe5O12 with nonmagnetic, trivalent substitutions on d (Ga3+) and a (Y3+ - yttrium antisite defect) sites which are occupied by ferric ions in a pure, stoichiometric system. The measurements were performed by the spin-echo method at liquid helium temperature. Thorough analysis of the hyperfine field anisotropy on the 57Fe nuclei in an ideal environment and in an environment where one of Fe3+ nearest cation neighbours is replaced by the nonmagnetic impurity is given. When the Fe3+ on which the NMR is measured is on the a site and the impurity is Ga3+(d), the results may be interpreted in terms of the superposition model. On the other hand, the results for Fe3+ on the d site and Y3+(a) are in clear disagreement with the predictions of this model.

Magnetite single crystals with Al, Ga and Ti substitutions were measured by means of 57Fe nuclear magnetic resonance (NMR) technique. Satellite structure of NMR spectra well above the Verwey temperature was detected and analyzed to obtain information on the distribution of substituting cations on iron sublattices. It was confirmed that Al and Ti enter iron octahedral B-sites. Ga strongly prefers tetrahedral A-sites, nevertheless a low presence of Ga cations in B-sites was detected.

Presence of defects in ferrites gives rise to a modification of the hyperfine field on nuclei of those iron ions, which are situated in the neighborhood of the defect. If the concentration of defects is small, satellite structure of the NMR spectra of 57 Fe nuclei is observed. Analysis of this satellite structure provides valuable information. In particular concentration of the defect, its distribution over various sublattices, magnetic moment and the valency may be obtained. In the present paper we describe how the analysis of the satellite structure is performed, summarize the requirements on the NMR experiment and illustrate the method using several examples of defects in the yttrium iron garnet.

The relaxation of 57Fe nuclei in yttrium iron garnet (YIG) is very sensitive to the way in which charge compensation is achieved in these magnetic insulators. Charge compensation becomes necessary either if nontrivalent impurities are incorporated in the garnet structure or in the presence of intrinsic defects. From measurements of the nuclear relaxation times T1 and T2 in epitaxial YIG films doped with small amounts of Ca, Si or Pb, we conclude that the relaxation behavior at low temperatures depends critically on the presence of certain intrinsic defects, namely Fe2+. These ions also increase the optical absorption in the near infrared.

Using 55Mn nuclear magnetic resonance (NMR), the magnetic phase separation into antiferromagnetic and ferromagnetic phases was found in Ca0.95La0.05MnO3 manganite. At 4.2 K, distinct NMR signals from ferromagnetic domains, antiferromagnetic matrix, and from the domain walls were detected. Predominant relaxation mechanism in the ferromagnetic phase is the Suhl-Nakamura interaction. This suggests that ferromagnetic domains in Ca0.95La0.05MnO3 have mesoscopic rather than nanoscopic dimensions.

The NMR spectrum of the polycrystalline sample of gadolinium iron garnet enriched by 155Gd isotope (91.6%) was detected. The interpretation of this spectrum consisting of 12 lines gives the values of electric field gradient (EFG) tensor, isotropic part and anisotropic components of hyperfine field on 155Gd nuclei. NMR measurements were accompanied by the ab initio calculations of the EFG tensor.

Temperature dependence of 57 Fe NMR was measured above the Verwey transition in three Fe 3− x Al x O 4 ( x =0, 0.005, 0.03) single crystals. The satellite lines corresponding to Fe (A) which have a single Al (B) nearest neighbor were detected in the NMR spectra of x =0.005 and 0.03 compounds. Temperature dependence of NMR linewidth for x =0.03 indicates that the electron density becomes inhomogeneous as T → T V .

The nuclear spin-lattice relaxation of59Co in amorphous Fe-Co-B alloys was studied by NMR andthe results were compared with nuclear orientation measurements. The NMR relaxation rates were evaluated taking into account the electric quadrupole interaction. Then the results of both methods are in good agreement. It was confirmed that the spin-lattice relaxation is independent of the external magnetic field in the magnetically saturated samples andit was found that it does not depend on Co concentration.

The electron structure and site preferences of Zn and Fe cations in Y hexaferrite system were calculated. The hyper ne magnetic elds on Fe nuclei were determined using WIEN2k and corrections for hyper ne contact interaction. The calculated elds were compared to Fe nuclear magnetic resonance (NMR) experiment in Ba2Zn2Fe12O22 single crystal with an aim of interpretation of experimental NMR spectrum.

Presence of defects in ferrites gives rise to a modification of the hyperfine field on nuclei of those iron ions, which are situated in the neighborhood of the defect. If the concentration of defects is small, satellite structure of the NMR spectra of 57Fe nuclei is observed. Analysis of this satellite structure provides valuable information. In particular concentration of the defect, its distribution over various sublattices, magnetic moment and the valency may be obtained. In the present paper we describe how the analysis of the satellite structure is performed, summarize the requirements on the NMR experiment and illustrate the method using several examples of defects in the yttrium iron garnet.

An impurity in the vicinity of the 57 Fe nucleus induces a specific structure of satellite lines in the NMR spectrum, the intensities of which are proportional to the impurity concentration. Recently achieved sensitivity makes it possible to detect as little as 0.02% impurity. Spectra of nominally pure YIG epitaxial films, containing Pb, Ga, Al, Si, Pt, Ca impurities and ‘antisite’ defects are presented to illustrate the approach.

57 Fe NMR spectra of epitaxial yttrium iron garnet film containing small amount of gallium were measured at 4.2 K in an external magnetic field applied along several directions in the plane of the film. The isotropic part and the anisotropy tensor of the hyperfine interaction of iron on a -sites having Ga 3+ as the nearest d -neighbours were deduced from the experimental data. The validity of the ‘independent bond’ model for this case is discussed.

We review the results of the NMR on57Fe nuclei in a number of ferrimagnetic garnets Y3−x RxFe5O12 (R =rare earth or Bi) and Y3Fe5−xMxO12 (M=Al, Ga, In). For suitable concentrations of substituted ions (x∼0.1–0.4) satellite lines in the NMR spectra of both tetrahedral and octahedral Fe ions are observed. These satellites correspond to Fe ions in the vicinity of which single substitution is situated. The splitting between the satellite and the parent line has its origin in the change of the dipolar field and in the change of crystal field effects (including the transfer of electrons). While the dipolar field is easily calculated, once the magnetic moments and the geometry is known, the origin of the change of the crystal field effects is much less clear. Experimental results show that it is anisotropic and that in some cases the anisotropy may be substantial. This anisotropy is discussed within the framework of the semiempirical “independent bond” method.

An analysis of the59Co NMR spectra of RECo2 compounds in zero and non-zero external magnetic field is presented. The proposed model is based on the assumption that the hyperfine interaction can be described by the symmetric second rank tensor. The local symmetry of the Co site enables to reduce number of the tensor components so that they can be determined from NMR data. This approach is used for the interpretation of the59Co NMR spectra and their external field dependence reported by Hirosawa and Nakamura in /1/. An agreement between the predicted and measured external field dependence of Bef was achieved.

The NMR spectra of57Fe in Gd: YTG polyeryslallaline samples were measured between liquid-helium and room temperatures. In spectra of57Fe on tetrahedral sites, beside the main line, weaker and broader peak at higher frequencies appears. It can be decomposed in to the three satellites corresponding to Fe3+ ion with one Gd3+ ion in the nearest (two satellites) and next nearest dodecahedral site. The temperature dependence of the stellite resonance frequency indicates, that the part of the transferred hyperfine field on57Fe with the Gd3+ in the nearest c-site depends on the magnetic moment of Gd3+ ion. The resonance frequency of the satellite corresponding to the57Fe with Gd3+ in the next nearest c-site follows the temperature dependence of the dipolar field, the change of the transferred field is small.

We report the NMR spectra of gallium substituted magnetite measured at temperatures 4.2, 77 and 273 K. A single crystal of Fe 3− x Ga x O 4 , x = 0.05 has been prepared by a floating zone technique. The Ga-lines found in the NMR spectrum above Verwey temperature T V ∼ 117 K indicate a preferency of Ga substitution into the tetrahedral A-sites. The well-resolved structure of Ga-lines has been identified and assigned to both Ga isotopes below T V . A weak quadrupolar interaction contributes only to the line broadening.

/sup 57/Fe NMR spectra and relaxation rates in lead hexaferrite single crystals PbFe/sub 12/O/sub 19/, PbFe/sub 12-x/Ga/sub x/O/sub 19/, (x=0.42, 1.2) and PbFe/sub 12-x/Al/sub x/O/sub 19/, (x=0.21) have been measured at 4.2 K using spin echo technique. Besides the main lines corresponding to the nonequivalent positions of Fe/sup 3+/ ions in the magnetoplumbite structure, the satellite lines caused by substitution in the vicinity of resonating nuclei have been observed. The dependence of the NMR spectrum on the external magnetic field has been used to separate the overlapped satellites from different iron sites. The most distant satellites have been assigned to the resonance of iron nuclei at 4f/sub 1/ site with substitution at 12 k and 2a sites in their vicinity. A strong preference of 2a site for Al substitution has been found. >

The nuclear quadrupole resonance (NQR) on Ba nuclei was studied in four samples of ${\mathrm{BaBiO}}_{3}$ prepared in different ways and, in addition, in ${\mathrm{BaPbO}}_{3}.$ The spectrum of ${}^{137}\mathrm{Ba}$ at $T=4.2 \mathrm{K}$ consists of a relatively broad line centered near 18 MHz for all ${\mathrm{BaBiO}}_{3}$ samples and near 13 MHz for ${\mathrm{BaPbO}}_{3}.$ The integrated intensity of the ${}^{137}\mathrm{Ba}$ resonance in the ceramic sample of ${\mathrm{BaBiO}}_{3}$ synthesized at $800\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ is approximately twice as large as that in ceramic samples and a single crystal prepared at $930--1080\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ The decrease of the NQR signal can be attributed to the partial disordering of charge-disproportionated Bi ions on the two inequivalent sites. The broadening of the resonance indicates that local distortions of the crystal structure exist in both compounds. The point-charge model was used to analyze the electric-field gradient on the Ba sites.

NMR of 57 Fe nuclei is used to study the Y antisite defects (yttrium ions on the sites nominally occupied by iron) in yttrium-iron garnet (YIG). In spectra of both tetrahedral and octahedral Fe 3+ ions, satellite lines appear, corresponding to Fe 3+ ions with this defect in their vicinity. NMR was measured on a number of nominally pure YIG samples - single crystals, thin films as well as polycrystalline samples. The content X Y per formula unit of the Y antisite defect (Y a ) was determined from the amplitude of the satellite lines and it lies in the interval 0.001< x y < 0.03. In most cases the satellite lines caused by the Y a defect are obscured by the satellites originating from other impurities. In two high purity single crystals the satellites caused by Y a dominate, however. In the NMR spectra of these single crystals the satellite lines which correspond to the Felt ions in seven different crystallographically inequivalent positions with respect to the defect are resolved at low temperatures. Further unresolved satellites emerge from the main lines as the temperature is increased.

The dependence of 57 Fe NMR spectra in YIG on the external magnetic field direction was measured at 4.2K. The high purity single crystal of a spherical form was used. The external magnetic field of 0.75T was applied in several directions lying in (110) plane. The satellite structure of the spectra caused by an antisite defects Y(a) (Y 3+ ion on the a site, nominally occupied by Fe 3+ ion) enabled us to study the effect of a nonmagnetic impurity on the 57 Fe hyperfine field anisotropy. The tensors of hyperfine interaction for 57 Fe in d- sites with the nearest and next nearest Y(a) defect were obtained from NMR data. The results were confronted with the predictions of the independent bond model.

Temperature dependence of 57 Fe NMR spectra was measured below and around the Verwey transition in Fe 3− x Al x O 4 ( x =0, 0.005, 0.01, 0.02, 0.03) single crystals. All lines of substituted samples were broadened with increasing Al concentration, but we have found no change in their positions. The Verwey temperature T V decreases with increasing Al concentration in agreement with electric conductivity measurements. The satellite structure found in the vicinity of the A-line above T V disappears below T V , on the other hand a broad spectrum in the spectral region 35–45 MHz at temperature 4.2 K was detected.

NMR on a gadolinium iron garnet was measured in zero and nonzero external magnetic field at 4.2 K. 24 well separated lines of 155 Gd and 157 Gd nuclei were found in the zero field spectrum. From the spectral line positions we determined the EFG tensor and both isotropic and anisotropic components of hyperfine magnetic interaction on gadolinium nuclei. The experimental hyperfine interaction parameters were compared with the ‘ab initio’ calculations of the EFG tensor.

Effect of Ca 2+ substitution on the 57 Fe NMR spectra and relaxations was measured in YIG epitaxial films. A satellite line caused by the presence of Ca 2+ was identified in the spectra of d-sites of YIG:Ca, Ca–Ge and Ca–Si films. In a YIG:Ca series, after an initial constant region a decrease of a spin–lattice relaxation rate was observed in a dependence on the increasing Ca 2+ content. The onset of the relaxation rate decrease is supposed to correspond to the induced decrease of an amount of Fe 2+ ions, while charge compensation is reached by other mechanisms for lower calcium content.

The NMR spectrum of ${}^{57}\mathrm{Fe}$ nuclei in high purity ${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ garnet (YIG) is reported. The satellite structure of this spectrum is caused by an antisite defect ( ${\mathrm{Y}}^{3+}$ ion replacing an ${\mathrm{Fe}}^{3+}$ ion). Resolved satellite lines, which correspond to the ${\mathrm{Fe}}^{3+}$ ion in six different cation coordination spheres around the defect, are observed. In particular, satellite lines arising from the ${\mathrm{Fe}}^{3+}$ ions' distance of which from the defect is as large as 1.07 nm (11th coordination sphere) are identified unambiguously. The results yield information on the range of magnetic interactions in YIG.

We report the NQR on Ba nuclei in four samples of BaBiO 3 prepared in different ways. The spectra of 137 Ba consist of broad line centered near 18 MHz at 4.2 K. The broadening of NQR suggests a local distortions of the crystal structure. The area under the line for ceramic sample BaBiO 3 synthesized at 800°C is approximately two times larger comparing to ceramic samples and single crystal prepared at 930–1080°C. The decrease of the NQR signal is attributed to the partial disordering of charge disproportionating Bi ions on the two inequivalent sites.

NMR spectra on 57Fe nuclei in YIG containing Ga3+ (d) and Y3+ (a) impurities were measured in the wide temperature region 4.2–370 K in zero external magnetic field. The temperature dependence of satellite lines caused by Y3+ (a) and Ga3+ (d) impurities was determined. Experimental results are compared with the theoretical calculation, based on the molecular field approximation.

The 57 Fe NMR spectra of pure yttrium iron garnet, measured at 4.2 K, exhibit a well-resolved satellite structure caused by antisite defects (yttrium ions replacing iron on a sites). The analysis of the a site spectra, obtained in a magnetic field of 0.75 T applied in several directions in the (1 1 0) plane, yielded the values of the isotropic and anisotropic terms of the hyperfine field for four resolved satellite sets.

Nuclear Magnetic Resonance/Near Quadupole Resonace specra of Lu175 in lutetium iron garnet (LuIG) were measured in the range of 10–500MHz in zero external magnetic field at a temperature of 4.2K. Experimentally observed spectra had complicated structure and a theory was needed to interpret them. To this end we calculated the electronic structure of LuIG and from it the values of magnetic hyperfine fields and the components of electric-field-gradient tensor at the lutetium nuclei were determined. These parameters were used to simulate the theoretical spectra of Lu175 in LuIG. Simulated spectral lines of Lu175 at dodecahedral sublattice correspond reasonably well with the system of measured lines in the range of 10–200MHz. Several spectral lines in the range of 300–500MHz can be interpreted as the resonance of Lu175 at the octahedral sites that are nominally occupied by the ferric cations.

NMR spectra of 57 Fe nuclei in single crystals of BaFe 12 O 19 and Ba(Fe 2− x Co x )Fe 16 O 27 ( x = 0, 0.8) were measured at 4.2 K. The field dependence of the resonant frequency at iron site 4f VI (R) in external magnetic fields parallel and perpendicular to the hexagonal axis was used to trace the magnetization processes. The change of the easy direction with Co content was determined from the results.

The NMR spectra of 57Fe in nominally pure YIG were measured and a rich satellite structure caused by defects was found. We identified the satellites caused by antisite defects (yttrium on iron sites, and vice versa) and satellites caused by Pb2+ impurities. From amplitudes of the satellites the concentration of the antisite defects was determined.

57 Fe NMR spectra from magnetic domains of BaFe 2 2+ Fe 16 3+ O 27 hexaferrite single crystal were measured at 4.2K using the conventional spin echo technique. External magnetic fields up to 3 T parallel and perpendicular to the hexagonal axis were applied to separate the overlapped NMR lines and to find their correspondence to the seven nonequivalent lattice sites occupied by Fe 3+ ions. A pronounced satellite structure of lines from tetrahedral sites was found. These results were analyzed with respect to the distribution of Fe 2+ ions among the iron sites.

The NMR spectra of 57 Fe in YIG:Ga and YIG:In systems were measured between liquid He and room temperature. By comparing the temperature dependences of the satellite lines with those of the main lines, conclusions are made about the exchange interactions in YIG.

Domain 57Fe NMR spectra of La3+ substituted hexagonal ferrites Pb1−xLaxFe12O19 and Ba1−xLaxFe12O19 have been measured at 4.2 K using the spin echo technique. Besides the five main lines, the satellite lines caused by the substitution were observed. The assignment of the satellite lines to the Fe3+ sublattices was made using the dependence of the NMR spectrum on the external magnetic field. The experimental results are explained assuming the existence of Fe2+ ions in the 2a sublattice.

The NMR of 57 Fe was used to determine the distribution of Bi 3+ ions on the dodechedral sublattice of thin YIG: Bi films with (111) orientation. In a film with 11% Bi 3+ on Y sites the two sublattices which are non-equivalent with respect to the growth direction reveal a difference in the Bi concentration of 10%. The growth induced anisotropy constant was determined from the FMR spectra and allows an estimation of the change of the second order crystal field parameters for the Fe 3+ neighbors of a Bi 3+ ion.

Satellite structure in 57Fe NMR spectra of In3+ , Sc3+ , Ga3+ and Al3+ substituted yttrium iron garnets (YIG) was analysed taking into account results previously obtained for ‘antisit’ defects (Y3+ ions on Fe3+ sites) in nominally pure YIG. The influences of the diamagnetic substitution on the hyperfine field at the neighbouring iron nuclei were found to be remarkably dependent on the ionic radius of the substituent. In the case of two substituents in the vicinity of the resonating iron, an additivity of their effects within a 15% tolerance interval was verified.

The results of NMR/ON measurements on56'57'60Co isotopes in iron are presented. To avoid the uncertainties caused by local demagnetizing field inhomogenities the measurements on two cobalt isotopes in one sample have been carried out. The values of nuclearγ-factor ratiosγ57/γ60 = 1.805 (20),γ60/γ56 = 0.761 (20),γ56/γ57 = 0.726 (20) and hyperfine anomalies56Δ60 = -0.036 (10),60Δ57 = 0.017 (10) and57Δ56 = 0.018 (10) have been calculated from the experimental results.

The NMR spectra of57Fe in domains of MFe12O19 (M=Ba, Sr, Pb) were measured by spin echo technique at 4.2 K. The change of the heavy ion causes frequency shifts of lines corresponding to Fe3+ ions in 2b and 4f2 sites while leaving other lines essentially unchanged; the significant role of different Fe3+−M2+ bonding was found. The dipolar broadening of lines in BaFe12O19 caused by random and static displacement of bipyramidal Fe3+ ions from the mirror plane is calculated and the results are compared with the experiment.

The results of NMR/ON measurements on56,57,60Co isotopes in iron are presented in the paper. To avoid the uncertainties caused by local demagnetizing field inhomogeneities the measurements on two cobalt isotopes in one sample have been carried out. The values of nuclearγ-factors ratiosγ57/γ60=1·805(20),γ60/γ56 = 0·761(20),γ56/γ57 = 0·726(20) and hyperfme anomalies56Δ60 = −0·036(10),60Δ57 = 0·017(10) and57Δ56 = 0·018(10) have been calculated from the experimental results.

The results of the nuclear magnetic resonance of 57Fe in Al-, Ga- and In-substituted Y3Fe5O12 garnets are reported. When diamagnetic Ga3+ or Al3+ ions are substituted for tetrahedral Fe3+ ions, four satellite lines in the NMR spectrum of octahedral Fe3+ ions appear. The substitution of In3+ ions for octahedral Fe3+ ions leads to three satellite lines in the spectrum of tetrahedral Fe3+ ions. The hyperfine fields of these satellites are explained using the independent bond model for the hyperfine interaction. Suprisingly large change of the anisotropic part of the hyperfine field relative to the isotropic one was found.

The NMR of 57Fe in Ho: YIG and Gd: YIG was measured on polycrystalline samples at 4.2 and 77K. In spectra of both tetrahedral and octahedral Fe3+ ions satellite lines appear which were ascribed to Fe3+ with a rare earth ion in their vicinity. From the position of the satellites the magnitude and direction of the Ho3+ magnetic moment were deduced. The results for Ho3+ ion are in good agreement with the crystal field calculations and with other magnetic data, but disagree with the neutron scattering results in HoIG garnet. The results on Gd: YIG were used to check the correctness of the analysis.

The NMR of $^{57}\mathrm{Fe}$ in Bi:YIG and La-YIG (where YIG is yttrium iron garnet) polycrystalline samples was measured between liquid-helium and room temperatures. In spectra of both tetrahedral and octahedral ${\mathrm{Fe}}^{3+}$ ions, satellite lines appear, corresponding to ${\mathrm{Fe}}^{3+}$ ions with ${\mathrm{Bi}}^{3+}$ or ${\mathrm{La}}^{3+}$ ions in their vicinity. The resonance frequency ${f}_{\mathrm{res}}$ of the satellites decreases with increasing temperature more slowly than ${f}_{\mathrm{res}}$ of the main lines. The different temperature dependences reflect the enhancement of the ${\mathrm{Fe}}^{3+}$-${\mathrm{Fe}}^{3+}$ superexchange interaction in the vicinity of the diamagnetic substitution. In the simplest model, the presence of ${\mathrm{Bi}}^{3+}$ or ${\mathrm{La}}^{3+}$ leads to two modified exchange integrals. Corresponding changes obtained by the molecular-field analysis are 4.5 and 1.6% for Bi:YIG, while they are 1.3 and 0.5% in the La-doped system. The smaller effect of the ${\mathrm{La}}^{3+}$ ion indicates that local deformation around the substitution plays only a minor role in the enhancement of the superexchange.

The room temperature NMR spectra of57Fe in hematite substituted by varying content of Al were measured and exchange integrals of the nearest four pairs Fe3+−Fe3+ were estimated: The values 261.9, −109.5, 146.6, 49.6 [10−22T2/J] for the first, second, third and fourth pair respectively were obtained.