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2009 MAGNETIC SYSTEMSEnhancement magnetic moment in the single phase nanostructure Gd 3 Fe 5 O 12Rare earth iron garnets (REIG) are promising candidatesfor use in high performance microwave and electrochemicaldevices owing to its high resistivity, high Curie temperature,and high chemical stability and possess uniquemagnetic, optical, thermophysical and mechanical properties.Gd 3 Fe 5 O 12 (R 3+3 c[Fe 3+2 ] a (Fe 3+3 ) dO 2−12h) particles weresynthesized in polycrystalline form by the solid-state reactiontechnique from a mixture of α−Fe 2 O 3 and Gd 2 O 3 innominal compositions of 5 : 3. The X-ray diffractogramsshow that all the milled Gd 3 Fe 5 O 12 garnet particles retaintheir single phase structure (Ia3d space group). The averagegrain size decreases with milling and reaches 29 nm forthe 40 hours milled sample. The 57 Fe Mössbauer spectrawere recorded at 300 K and 77 K for the different samples.There is no evidence for the presence of Fe 2+ chargestate. On increasing the milling time, one observes the progressiveoccurrence of a central quadrupolar feature at both300 K and 77 K, in addition to three magnetic sextets withdecreasing intensity but increasing line width compared tothat of the bulk. The quadrupolar feature has to be decomposedat both temperatures into two broad line quadrupolardoublets: the presence of ferric and ferrous species isthen identified. The Fe 2+ content linearly increases withthe milling time (figure 118).The measured magnetization (M) for the as-prepared GdIGis found equal to 15.9 µBmol −1 at 4.2 K. When the grainsize is reduced below 100 nm, the M is strongly appliedfield dependent and no saturation is observed even underthe highest applied field of 320 kOe (figure 119). In the170 − 320 field range the magnetization curves of the differentmilled samples are very well (within 1%) describedby the approach law M = M sat (1 − b/H 2 ). According toNéel the b coefficient is determined by the magnetocrystallineanisotropy and given by b = 8K 2 /105(M sat ) 2 .18.For the 35h milled sample M sat is only equal to 14 µBmol −1and remains much smaller than the bulk value. The socalculatedvariation of K is reported in figure 120 versusthe Fe2+ content where a large increase of K is observedwhen the milling time increases. The modified formulaeof our GdIG particle is given by Gd 3+3(Fe 3 3(1−r) + Fe2+ 3r )[Fe 3 2(1−s) + Fe2+ 2s ] O2− 12−2.5ptaking into account the presenceof oxygen vacancies related to the Fe 2+ content (p). Assumingthat only the Fe 3+ contribute to the ferrite magnetizationr and s values were calculated using the p and M satvalues deduced from the Mössbauer spectra and high fieldbehavior analysis respectively. The particular behavior ofthe 35h milled samples is then explained by the fact thanthe octahedral site contains the largest part of Fe 2+ ion. Wenote that (i) the T comp values of the nanogarnets are few degreeshigher than that of the bulk; (ii) However, the Curietemperature (T C ) of the various sized nanocrystalline GdIGsamples are found to be significantly higher than that of thebulk.Figure 118:Figure 119:Figure 120:Fe 2+ content versus the milling time.Isothermal magnetization curves at 4.2K.Anisotropy constant versus the Fe 2+ content.M. GuillotC. N. Chinnasamy, B. Latha, T. Sakai, S. D. Yoon, C. Vittoria, V. G. Harris, (Center for Microwave Magnetic Materialsand Integrated Circuits, Northeastern University, Boston). J. M. Greneche (LPEC, Universit du Maine, Le Mans).87