Journal of Magnetism and Magnetic Materials 262 (2003) 368–373 Magnetic study of nanocrystalline iron particles in alumina matrix A Fnidikia,*, C Doriena, F Richommea, J Teilleta, D Lemarchanda, N.H Ducb, J Ben Youssef c, H Le Gallc a Groupe de Physique des Materiaux UMR CNRS 6634, Universit!e de Rouen, Site Universitaire du Madrillet, B.P 12, 76801 Saint-Etienne-Du-Rouvray Cedex, France b Cryogenic Laboratory, Faculty of Physics, Vietnam National University, Hanoi, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam c Laboratoire de Magn!etisme de Bretagne, CNRS, B.P 809, 29285 Brest Cedex, France Abstract X-ray diffraction, Mossbauer effect and magnetisation investigations have been performed on sputtered Fe40(Al2O3)60 thin films Ultrafine Fe particles of nanometer size in an amorphous Al2O3 matrix have been formed by annealing in the temperature range from 100 C to 500 C Their particle sizes, however, show a rather wide distribution Mossbauer spectra are constituted of both paramagnetic and ferromagnetic contributions The paramagnetic contribution is associated with small grains of Fe, whereas the magnetic component is contributed by large iron grains This assumption is supported by the ZFC- and FC-measurements, in which the ‘‘blocking’’ temperatures of 60 and 90 K were evidenced for as-deposited and 200 C-annealed films, respectively r 2003 Elsevier Science B.V All rights reserved PACS: 75.50.Tt, 75.75.+a Keywords: Nanomagnetism; Mossbauer spectra; Superparamagnetism Introduction Metal/insulator granular films consisting of nanometer sized ferromagnetic metals immersed in an insulating medium have been intensively studied in the last decade because of their interesting giant magnetoresistance (GMR) ef*Corresponding author Group de Physique des MateriauxUMR CNRS 6634, Universite de Rouen, Mont-Saint-Aignam Cedex 76821, France Tel.: +33-35-14-67-65; fax: +33-35-1466-52 E-mail address: abdeslem.fnidiki@univ-rouen.fr (A Fnidiki) fect—called tunnelling magnetoresistance (TMR) [1,2] These materials exhibit also other novel phenomena such as superparamagnetism [3], giant magnetic coercivity [3,4], etc., making them good candidates for future technological applications Giant magnetic coercivity has been reported for the ferromagnetic transition metal (T=Fe, Co)based granular films by several authors ([4,5] and references therein) The coercivity of these granular systems is as large as about 60 mT at room temperature as well as about 250 mT at T ¼ K It is sensitive to grain size [3] A surface contribution to giant magnetic coercivity was reported for 0304-8853/03/$ - see front matter r 2003 Elsevier Science B.V All rights reserved doi:10.1016/S0304-8853(03)00064-7 A Fnidiki et al / Journal of Magnetism and Magnetic Materials 262 (2003) 368–373 granular systems having uniformly nanometer sized particles [6] Recent magnetic studies of Fe/ Al2O3 granular films [7], however, have shown that the presence of single domain regions is responsible for the high value of the coercivity, where magnetisation reversal takes places only by rotation of saturation magnetisation vectors In Ref [7], a so-called blocking temperature, TB ; at which the metastable hysteretic response is lost, was found and superparamagnetic behaviour of nanocrystalline iron particles in alumina matrix was reported In this paper, we focus our attention on the Mossbauer spectroscopic evaluation of nanocrystalline iron crystallites evolved by annealing in the sputtered Fe40(Al2O3)60 granular films The results are discussed in connection with structural and magnetic data Experimental The Fe40(Al2O3)60 thin films were deposited on a glass substrate at 300 K using a triode RFsputtering system The film thickness is 560 nm The composition was analysed using energy dispersive X-ray spectroscopy (EDX) After depositing, samples were annealed for h in a vacuum of  10À5 Torr in the temperature range from 100 C to 500 C The microstructure of the sample was investigated using a JEOL 200 FX electron microscope operating at 200 kV The structure of the samples was investigated by X-ray diffraction using a cobalt anticathode (lCo-Ka ¼ 0:1790 nm) The grain size was calculated from the full-width at half-maximum (FWHM) of the principal diffraction peaks using the Scherrer relation Various magnetic properties, such as magnetic hysteresis loops, zero-field cooled (ZFC) and field cooled (FC) magnetisation, were measured with a SQUID magnetometer in magnetic fields up to T, in the temperature range from 4.2 to 300 K The conversion electron Mossbauer spectra (CEMS) at room temperature were recorded using a conventional spectrometer equipped with a home-made helium–methane proportional coun- 369 ter The source was a 57Co in rhodium matrix The films were set perpendicular to the incident g-beam The spectra were fitted with a least-squares technique using a histogram method relative to discrete distributions, constraining the linewidth of each elementary spectrum to be the same Isomer shifts are given relative to BCC-Fe at 300 K Experimental results and discussion For the Fe40(Al2O3)60 films under investigation, the experimental XRD patterns show three characteristic peaks of (1 0), (2 1) and (2 0) BCCFe reflections No Al2O3 reflections were detected indicating the amorphous structure of the alumina matrix In Figs 1(a)–(e) however, only the patterns ranging from 2y ¼ 48 to 56 , corresponding to the (1 0) reflections, is displayed For the asdeposited film, the Bragg peak centred at 2y ¼ 52 is rather broad The average grain size of BCC-Fe particles (dFe ) is estimated by applying the Scherrer formula for the width of the (1 0) BCC-Fe diffraction peak It turns out that dFe equals about nm for the as-deposited sample The microstructure of the sputtered film was investigated by TEM using a 60 nm thick Fe40(Al2O3)60 film deposited on a carbon thin substrate The dark field electron micrograph in Fig reveals that the mean size of the BCC Fe grains is nm The annealing at TA ¼ 100 C and 200 C makes no appreciable change in the XRD patterns After annealing at TA X300 C; the (1 0) diffraction peak becomes narrower and its intensity increases with the annealing temperature, see e.g Fig 2e for the sample annealed at 500 C In this case, dFe increases up to about nm Finally, it is worthwhile to mention that, the (1 0) BCC-reflection peaks shift to higher 2y angles with increasing TA : This indicates that the annealing causes not only the evolution of crystallites, but also the increase of the Fe-concentration The Mossbauer spectra recorded at room temperature are shown in Fig The obtained hyperfine parameters are listed in Table For the as-deposited film, the CEM spectrum consists of a paramagnetic asymmetric doublet (with fraction A Fnidiki et al / Journal of Magnetism and Magnetic Materials 262 (2003) 368–373 370 (a) Intensity (a.u.) (b) (c) (d) (110) Fe (e) 48 50 52 54 56 2θ (degrees) Fig X-ray diffraction patterns of the Fe40(Al2O3)60 thin films: (a) as-deposited film, (b) after annealing at 100 C, (c) À200 C, (d) À300 C and (e) À500 C (a) (b) 20 nm Fig Dark field electron micrograph (a) and corresponding electron diffraction pattern of the as-deposited thin film Apar ¼ 51%) and a magnetic broadened sextet (with fraction Aferro ¼ 49%:) (see Table 1) The asymmetric doublet can be associated with the contribution from iron grains with small sizes, and also probably from Fe atoms diluted in the alumina matrix The broadened magnetic sextet is associated with the contribution of iron crystallites with large grain sizes It was fitted with a wide A Fnidiki et al / Journal of Magnetism and Magnetic Materials 262 (2003) 368–373 371 Velocity (mm/s) -10 50 +10 40 1.02 30 20 10 0 10 20 30 10 20 30 10 20 30 10 20 30 10 20 30 50 1.00 1.02 40 30 20 10 1.00 1.02 P(Bhf ) (%) Emission (%) 40 30 20 10 1.00 1.02 30 20 10 1.00 1.02 40 30 20 10 1.00 Bhf (T) Fig Mossbauer spectra and hyperfine field distributions of the Fe40(Al2O3)60 thin films: (a) as-deposited film, (b) after annealing at 100 C, (c) À200 C, (d) À300 C and (e) À500 C hyperfine field distribution PðBhf Þ in order to take into account the different environments of the iron atoms Indeed, as can be seen from Fig 3a, the fitted hyperfine field exhibits a rather wide distribution A low fraction with Bhf ¼ 32 T characterising the pure BCC-Fe is observed The main contribution, however, is centred at Bhf ¼ 28 T A similar behaviour was found for the Fe/Cu granular films [6] In Ref [6], it was described in terms of the transient composition and ferromagnetism At present, one can add to this picture an effect of the wide distribution of Fegrain sizes The CEM spectrum remains almost identical for the film annealed at TA ¼ 100 C (see Fig 3b) After annealing at 200 C, the relative BCC-Fe Mossbauer fraction starts to increase, while the corresponding paramagnetic contribution decreases This process takes place strongly in the sample annealed at TA ¼ 300 C and then is slowing down at further heat treatments At TA ¼ 500 C; more than 96% of the Fe atoms are found to be in the BCC-Fe phase As the annealing A Fnidiki et al / Journal of Magnetism and Magnetic Materials 262 (2003) 368–373 As-deposited TA ¼ 100 C 200 C 300 C 500 C Accuracy Doublet Sextet Doublet Sextet Doublet Sextuplet Doublet Sextet Doublet Sextet Bhf (T) A (%) — 18.5 — 18.3 — 20.7 — 25.3 — 28.2 0.1 51 49 51 49 24 76 91 96 temperature increases, the main peak observed at 28 T on the hyperfine field distribution for the asdeposited film is shifted toward the characteristic value (33 T) of bulk BCC-Fe This is in agreement with the increasing size of the Fe grains observed in XRD The remaining traces of the doublet in the Mossbauer spectrum indicate that the formation of BCC-Fe was not completed In accordance with the XRD results, the increase of the relative fraction of BCC-Fe phase is associated with the increase of the Fe-grain size as well as of the Feconcentration in the grains with increasing annealing temperature, indicating that the Fe atoms dissolved in the Al2O3 matrix diffuse to form the BCC-Fe phase Fig illustrates the ZFC and FC curves measured in a magnetic field (of mT) applied in the film-plane, for the as-deposited, 200 C- and 500 C-annealed films, respectively It is clearly seen that the ZFC and FC magnetisation bifurcates at different temperatures in different samples These temperatures are regarded as the experimentally measured blocking temperature TB ; at which the spin are aligned or not along the field TB equals 60 K for the as-deposited film It reaches to 90 K in the 200 C-annealed films, in accordance with the increase of the Fe grain size For uniaxial particles, TB can be taken as KV/ 25kB, where K is the magnetic-anisotropy constant, kB is the Boltzmann constant and V is the volume of the nanocrystallite [7] Substituting the values of TB ; K ¼ 50 kJ/m3 for Fe, TA = 30 o C M (arb unit) Sample TB 100 200 T (K) TB 300 TA = 200 o C M (arb unit) Table Hyperfine parameters for (Fe)40(Al2O3)60 granular films: hyperfine field (/Bhf S) and Mossbauer fractions (A) 100 200 T (K) M (arb unit) 372 300 TA = 500 o C 100 200 T (K) 300 Fig ZFC and FC curves of the (a) as-deposited film, (b) after annealing at 200 C, (c) À500 C kB ¼ 1:38  10À23 J into the above expression, we find the mean diameter of the Fe particles d ẳ 3V =4pị1=3 to be 8.5 and 10 nm for the asdeposited and 200 C-annealed films, respectively In order to obtain better results for the d parameter, one must approach the real value of the magnetic anisotropy constant, possibly taking into account a surface contribution After annealing at 500 C, a weak bifurcation is still observed at 10 K Here, the observed ‘‘curvature’’ of the ZFC– FC curves differs from the Langevin ‘‘curvature’’ generally observed for systems containing noninteracting particles It indicates probably the existence of a strong dipolar interaction between iron grains in the samples Indeed, in all cases, the ratio of the remanence to the saturation magnetisation was found to be about 0.7 at T ¼ K A Fnidiki et al / Journal of Magnetism and Magnetic Materials 262 (2003) 368–373 1.2 0.8 M/Ms 0.4 0.0 -0.4 -0.8 -1.2 -0.10 -0.05 (a) 0.00 B (T) 0.05 0.10 1.2 373 value of the transition temperature of about 340 K observed on the ZFC–FC curve for the asdeposited sample (see Fig 4) This assumption is corroborated by the fact that the transition temperature increases with increasing annealing temperature (Fig 4) Fig exhibits also a large coercive field of the investigated sample The coercivity decreases with increasing annealing temperature, i.e with increasing Fe grain size It is worthwhile, however, to mention that this behaviour is observed in a rather small grain size region 0.8 M/Ms 0.4 Concluding remarks 0.0 -0.4 -0.8 -1.2 -0.10 -0.05 (b) 0.00 B (T) 0.05 0.10 1.2 0.8 M/Ms 0.4 Mossbauer spectrometry shows the existence of a broad distribution of iron grain sizes Ferromagnetism is formed in large sized BCC-Fe grains The existence of small particles of Fe leads strong surface effects and magnetic disorder, evidenced by Mossbauer and magnetisation measurements A simple simulation of the hysteresis loops and the ZFC- and FC-curves will be presented in a forthcoming publication -0.4 Acknowledgements -0.8 (c) -1.2 -0.10 -0.05 0.00 B (T) 0.05 0.10 Fig Hysteresis loops measured at K for the as-deposited Fe40(Al2O3)60 thin films, the 200 C- and the 500 C-annealed films (Fig 5) The value of the saturation magnetisation for the sample annealed at 500 C (748 kA/m) corresponds to 43% of Fe with an average moment of mB/at, in agreement with the Mossbauer result, indicating that the nearly total amount of Fe is in the BCC form The smaller values obtained for the as-deposited and the 200 C-annealed films (600 kA/m) indicate a strong surface effect, in accordance with the small value of the Fe grain size obtained in TEM and XRD These effects of the surface can explain the low This work is partly supported by the State Program for Natural Scientific Researches of Vietnam, within project 420.301 References [1] J.Q Xiao, J.S Jiang, C.L Chien, Phys Rev Lett 68 (1992) 3749 [2] J.L Gittlement, Y Golstein, S Bozowsky, Phys Rev B5 (1972) 3605 [3] C.L Chien, J Appl Phys 69 (1991) 5276 [4] C Chen, O Kitakami, Y Shimada, J Appl Phys 84 (1998) 2184 [5] C Chen, O Kitakami, Y Shimada, J Appl Phys 86 (1999) 2161 [6] N.H Duc, D.T Huong Giang, A Fnidiki, J Teillet, J Magn Magn Mater 262 (2003) 420, in this volume [7] D Kumar, J Narayan, A.V Kvit, A.K Sharma, J Sankar, J Magn Magn Mater 232 (2001) 161 ... increase of the relative fraction of BCC-Fe phase is associated with the increase of the Fe-grain size as well as of the Feconcentration in the grains with increasing annealing temperature, indicating... noninteracting particles It indicates probably the existence of a strong dipolar interaction between iron grains in the samples Indeed, in all cases, the ratio of the remanence to the saturation... blocking temperature, TB ; at which the metastable hysteretic response is lost, was found and superparamagnetic behaviour of nanocrystalline iron particles in alumina matrix was reported In this