Properties of non-crystalline EulG and DylG obtained from Mossbauer and magnetization data

The well-known dependence on preparation conditions of the microstructural polymorphism in nc networks is often also reflected in their magnetic properties. This is particularly evident for non-metallic nc materials where superexchange coupling dominates. The latter is very sensitive to slight variations in bond lengths and angles. For example. this counterplay of local structural and magnetic properties has successfully been investigated by Mossbauer spectroscopy on nc ferrous halides l-3. Here we report about the characterization of nc EuIG and DyIG with the help of Mossbauer and macroscopic magnetization measurements.


INTRODUCTION
The well-known dependence on preparation conditions of the microstructural polymorphism in nc networks is often also reflected in their magnetic properties.This is particularly evident for non-metallic nc materials where superexchange coupling dominates.The latter is very sensitive to slight variations in bond lengths and angles.For example.this counterplay of local structural and magnetic properties has successfully been investigated by Mossbauer spectroscopy on nc ferrous halides l -3 .
Here we report about the characterization of nc EuIG and DyIG with the help of Mossbauer and macroscopic magnetization measurements.

EXPERIMENTAL
Nc EuIG and DyIG have been prepared by slow dc-sputtering (8kVAr+) from crystalline garnet targets.The argon pressure was lo-5Torr.Both. the target and the mylar subs t rat ewe r e ke pt at•"" 8 0 K. Sam p 1 e s 0 f t h i c knesses between some 103 up to l05~ were obtained with deposition times ranging from 10 to 100 hours.Mossbauer absorotion studies on 57Fe and 151Eu were carried out in the temperature range between 4.2 and 300 K.Additional measurements with 161Dy are presently limited to the range above 70 K.Macroscopic magnetization data between 4.2 and 200 K were obtained with a vibrating sample magnetometer.X-ray characterization of the samples was done at room temperature.

. RESULTS
The sputtered garnet samples are X-ray amorphous.Crystallization can be achieved by heating in air to 800-1000 K. 57Fe Mossbauer spectra of nc EuIG and OyIG taken at 300 K in the paramagnetic regime are shown in Fig. l.In Fig. 2 we show spectra in the magnetically ordered state at 4.2 K.The high temperature spectra can be interpreted by a superposition of three quadrupole patterns reflecting diffe~ I'"i g. 1 :  I).
For an accurate determination of the magnetic ordering temperature Tm we performed thermal scanning 7 for the resonances of 57Fe, 151Eu resp.1610y.We found Tm=62.±0.5 K for nc EuIG and 70.:0.5K for nc OyIG.The 57Fe magnetic hf patterns below Tm were fitted by a superposition of three sets for 6-line effective field patterns.Their isomer shifts and the relative intensities were fixed to the  I).No variation in spectral shape was found by changing the angle between the direction of the t-rays and the plane of the absorber substrate.This indicates the absence of a preferred orientation of the Fe moments.The distribution of the magnetic hf fields can be satisfactorily described by a sum of three Lorentzians.The effective hf fields Bhf for the three Fe sites and the relative widths of the distributions at 4.2 K are also summarized in Table I.In Fig. 3 the variation of Shf with temperature is plotted for nc OyIG.The shape of the distribution fUnction of Bhf (as shown in the inserts in Fig. 2 for 4.2 K) is qualitatively maintained up to Tm.The 'Eu 3 + ions experience only an induced hf magnetic field.A value of 8T was found at 4.2 K which is about 1/8 of the value for crystalline EuIG8.The quadrupole interaction above Tm corresponds to e2qQgd~130MHz for 151Eu 3 + in nc Eu1G and ~1600MHz for 161Dy3+ in nc OyIG.
From macroscopic magnetization measurements we find roughly a curie-Weiss behaviour above 90 K.The extrapolated Curie-Weiss temperatures are -220 K and -120 K for nc EuIG and OyIG, resp.The magnetic moments are (24",2)IJS per ficitive formula unit EU3Fe5012 and (45",2)IJS for OY3Fe5012.The increase of moment below 90 K indicates ferrimagnetic order.No remanence is found down to 30 K.Even at 4.2 K a large amount of spins is unblocked.The magnetization in a field of 5T is about half the value estimated for any collinear ferrimagnetic arrangement of the Fe 3 +, Fe2+ and Eu 3 + resp.Oy3+ moments.The compensation temperature of nc OyIG is about (35",2)K but is strongly masked by the unblocked contribution to the magnetization.

01 SCUSSION
The appearance of different valence states in sputter-deposited materials is well-known 9 .
Here we can trace it to a preferential oxygen sputtering from the target: surface probes also contain Fe 2 +.During evaporation total molecular units may be conserved thus allowing a close structural relationship between target and sputtered sample.We assume that the nc structure and its stoichiometry are stabilized by an oxygen deficiency J. Per formula unit RE3Fe5012-J we estimated /"'0.4 for nc Eu1G and OyIG.These values compare with!"'-~0.5 reported for melt quenched nc GdIG10.
The large efg for octahedral Fe 3 + together with the increased isomer shift can be connected to a widening of the lattice with a corresponding distortion of the 0 2 -octahedra 6 .The isomer shift of tetrahedral Fe 3 + is enhanced in a similar way.The major part of the reduction of the saturation hf magnetic fields for Fe 3 + can be explained by a distribution of the molecular field due to the dis-

CONCLUSION
The microchemical composition and structure of sputtered nc EuIG and DyIG can be satisfactorily explained by a small oxygen deficiency due to preferential oxygen sputteri~.Macroscopic magnetization suggests ferrimagnetic order possibly of sperimagnetic type.The sharp magnetic transition and the distribution of hf fields preclude a wide distribution of magnetic clusters with varying order parameters.Although a part of the deviations of the magnetic hf parameters from the values for the corresponding crystalline materials can be explained by a distribution in the molecular field, the strong reduction of Tm must be attributed to a decrease of the average molecular field due to the distorted superexchange bonds.The present Mossbauer experiments will be completed by further 1610y studies below Tm on nc DyIG in order to determine the sUblattice magnetization of the rare-earth sites.
of nc EuIG and OyIG at 300 K. (source 30mCi 57CoRh.esf7f.absorber thi ckness 0.10 and 0-:T5mg Fe/cm 2 ) ent Fe sites.The values of their spectral parameters are given in Tabl e I.One can tentatively identify tetrahedral and octahedral Fe 3 + sites.The hf parameters of the third site are close to those known for tetrahedral Fe 2 + (e.~ in silica garnets 6 ).The relative intensities of the three subspectra were corrected for different Lamb-Mossbauer factors (as estimat~ from their temperature dependence between 80 and 300 K) in order to obtain the occupancy of the three sites (Table

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Fig.2: 57 Fe absorption spectra of nc EuIG and OyIG at 4.2 K, the inserts give the probability distribution of Shf values valid above Tm (TableI).No variation in spectral shape was found by changing the angle between the direction of the t-rays and the plane of the absorber substrate.This indicates the absence of a preferred orientation of the Fe moments.The distribution of the magnetic hf fields can be satisfactorily described by a sum of three Lorentzians.The effective hf fields Bhf for the three Fe sites and the relative widths of the distributions at 4.2 K are also summarized in TableI.In Fig.3the variation of Shf with temperature is plotted for nc OyIG.The shape of the distribution fUnction of Bhf (as shown in the inserts in Fig.2for 4.2 K) is qualitatively maintained up to Tm.The 'Eu 3 + ions experience only an induced hf magnetic field.A value of 8T was found at 4.2 K which is about 1/8 of the value for crystalline EuIG8.The quadrupole interaction above Tm corresponds to e2qQgd~130MHz for