DSpace at VNU: Nanostructure and magnetic properties of Fe0.56Cu0.44 films

Journal of Magnetism and Magnetic Materials 262 (2003) 420–426 Nanostructure and magnetic properties of Fe0.56Cu0.44 films N.H Duca,*, D.T Huong Gianga, A Fnidikib, J Teilletb a Cryogenic Laboratory, Faculty of Physics, Vietnam National University, Nguyen Trai, Thanh Xuan, Hanoi 334, Viet Nam b GPM-UMR 6634, Site Universitaire du Madrillet, B.P 12, 76801 Saint-Etienne-Du-Rouvray Cedex, France Abstract X-ray diffraction, high-resolution transmission electron microscopy, Mossbauer effect and magnetisation investigations have been performed on sputtered Fe0.56Cu0.44 thin films, in which the Fe concentration is near the percolation threshold Segregation into FCC-Cu rich and BCC-Fe rich phases takes place in the as-deposited film This state is described in terms of the interfacial transient concentration and ferromagnetism, which originate from the ferromagnetic BCC-Fe in the centre of the individual grains, the iron-rich crystalline Fe(Cu) alloy lying near the interface and the paramagnetic copper-rich FCC-Cu(Fe) matrix Annealing effects cause not only the evolution of the grain size of the BCC-nanocrystalites, but also the enrichment of Fe in this phase, leading to an increase of the interfacial sharpness In addition, the magnetic coercive field is found to be enhanced The coercivity shows a value as large as 24.2 mT for the sample annealed at 400 C A contribution of the positive surface magnetic anisotropy (KS ¼ þ0:1 mJ/m2) to the coercivity is deduced r 2003 Elsevier Science B.V All rights reserved PACS: 75.50.Bb; 75.50.Tt; 75.50.Vv; 75.70.Rf Keywords: Granular film; Mossbauer spectra; Magnetic coercivity; Surface magnetic anisotropy Introduction Granular solids, composed of nanometer-sized magnetic metal particles in a non-magnetic matrix, currently provide a wealth of scientific interest as well as of potential applications for magnetic recording, optical devices and sensors [1–2] When the size of the magnetic particles is reduced to a few tens of nanometers, they exhibit a number of outstanding physical properties Its origins may be related, on the one hand, to the particle size, and on the other hand to the high density of *Corresponding author E-mail address: duc@netnam.org.vn (N.H Duc) topological defects arising at the grain surfaces The surface atoms create a new phase and any characteristic property of this phase may become a dominant one for the whole system Indeed, surface effects on the giant magnetoresistance were reported for Fe–Ag granular films [3] Enhancement of the magnetic coercivity (up to a value as large as about 60 mT at room temperature as well as about 250 mT at T ¼ K) has been reported for Fe- and Co-based granular films by several authors [1,4–11] In these cases, Chen et al [7,10] have assumed that the surface anisotropy dominates the magnetic properties Their so-called relaxation model is rather suitable to explain peculiarities in magnetic properties of granular 0304-8853/03/$ - see front matter r 2003 Elsevier Science B.V All rights reserved doi:10.1016/S0304-8853(03)00061-1 N.H Duc et al / Journal of Magnetism and Magnetic Materials 262 (2003) 420–426 systems However, the approach seems to be imperfect in, for instance, discussing the sign of the surface anisotropy Recently, a contribution of the surface anisotropy to the magnetic coercivity was reported for granular Cu0.8Fe0.2 films below the percolation threshold [12] In these films, isolated BCC-Fe nanoparticles (size from to 35 nm) were considered to be well embedded in a non-magnetic FCC-Cu(Fe) matrix In Ref [12], both the strength and the sign of the surface magnetic anisotropy was deduced and discussed Magnetic properties of granular systems, however, depend not only on the particle size, but also on their distribution and density In this paper, we focus attention on the microstructure and magnetic properties of sputtered Fe0.56Cu0.44 films, in which the Fe concentration is near the percolation threshold 421 ter The source was a 57Co in rhodium matrix The films were set perpendicular to the incident gbeam The spectra were fitted with a least-squares technique using a histogram method relative to discrete distributions, constraining the line width of each elementary spectrum to be the same Isomer shifts are given relative to a-Fe at 300 K The average ‘‘cone-angle’’ b between the incident g-ray direction (i.e the film-normal direction) and that of the hyperfine field Bhf (or the Fe-magnetic moment direction) is estimated from the lineintensity ratios : x : : : x : of the six Mossbauer lines, where x is related to b by sin2 b ẳ 2x=4 ỵ xị: Experimental results and discussion 3.1 Microstructure Experimental Fe0.56Cu0.44 thin films were deposited on a glass substrate at room temperature by using a triode rf-sputtering system To avoid corrosion and oxidation, the film stacks were covered with a 10 nm-thick Nb layer on top The film thickness was 380 nm The composition was analysed using energy dispersive X-ray (EDX) spectroscopy After depositing, the samples were annealed in a vacuum of  10À5 Torr, in the temperature range from 100 C to 400 C 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 and confirmed by high-resolution transition electron microscopy (HRTEM) The magnetisation was measured with a vibrating sample magnetometer (VSM) in magnetic fields up to 1.4 T applied in the film-plane and film-normal directions Conversion electron Mossbauer spectra (CEMS) at room temperature were recorded using a conventional spectrometer equipped with a home-made helium–methane proportional coun- The X-ray diffraction patterns are presented in Figs 1(a)–(f) for the as-deposited film and films annealed at 100 C, 200 C, 300 C, 350 C and 400 C, respectively For the as-deposited film, one observes a broad Bragg peak, which seems to be formed by an overlap of (1 0)-BCC and (1 1)FCC reflections (see the principal peak around y ¼ 26 ) This indicates a coexistence of fine Ferich BCC and Cu-rich FCC nanograins No appreciable change is observed after annealing between TA ¼ 100 C and 200 C After annealing at TA X300 C, however, a clear splitting of the principal peak is observed The lower-angle peak is located at yFCC-Cu ¼ 25:5 (i.e (1 1) reflections of FCC-Cu) and it almost remains at the same position with increasing TA ; while the higher diffraction angle yBCC-Fe (i.e (1 0) reflections of BCC-Fe) slightly increases This reflects not only the formation of FCC-Cu and BCC-Fe grains, but also the Fe enrichment in the BCC-phase The separation of the BCC-Fe (1 0) and BCC-Cu (1 1) peaks is more visible with further increasing TA : Finally, at TA ¼ 400 C, the XRD result exhibits the presence of eight rather sharp peaks corresponding to five theoretical peaks of FCC-Cu and three theoretical peaks of BCC-Fe This indicates a total decomposition of Fe and Cu in the sample This is consistent with results reported 422 N.H Duc et al / Journal of Magnetism and Magnetic Materials 262 (2003) 420–426 Cu (111) Fe (110) (200) (200) (f) (220) (200) (311) (222) Intensity (arb units) (e) (d) (c) (b) (a) 15 20 25 30 35 40 45 50 55 60 Theta (degrees) Fig X-ray diffraction patterns of the Fe0.56Cu0.44 thin films: (a) the as-deposited film; after annealing at (b) 100 C; (c) 200 C; (d) 300 C; (e) 350 C and (f) 400 C previously by Childress et al [11] The average size of the BCC-Fe particles (dFe ) is estimated from FWHM using the Scherrer relation The obtained results are listed in Table Note that, for Fe0.56Cu0.44 films, dFe increases from nm for the as-deposited film to 50 nm for the film annealed at 400 C For Fe0.20Cu0.80 films however, dFe is less than 30 nm [9] The cross-section TEM 2-D color mapping images of the 100 C- and 400 Cannealed Fe0.56Cu0.44 films are presented in Figs 2(a) and (b), respectively The corresponding electron diffraction patterns are shown in the insets These results confirm not only the particle size of the BCC-Fe crystallites, but also their interconnections In particular, it can also be seen that, while the segregation was almost completed in the 400 C-annealed Fe0.56Cu0.44 film, a clear transient concentration is still observed in the 100 Cfilm This result is rather useful to discuss the information on the hyperfine parameters deduced from M.ossbauer spectrometry studies (see below) 3.2 Mossbauer spectra In Fig 3, the CEM spectra and their corresponding hyperfine field distributions PðBhf Þ are shown for the investigated Fe0.56Cu0.44 granular films For the as-deposited film, the CEM spectrum consists of both magnetic and paramagnetic Table The grain size (dFe ), magnetic coercivity (m0 HC ), saturation magnetisation (m0 MS ) and effective anisotropy constant (Keff ) for Fe0.56Cu0.44 granular films Sample dFe (nm) m0 HC (mT) m0 MS (T) Keff (kJ/m3) As-deposited TA ¼ 100 C TA ¼ 200 C TA ¼ 300 C TA ¼ 350 C TA ¼ 400 C 12 13 18 26 50 3.0 3.4 3.4 7.4 17.4 24.2 0.705 0.695 0.710 0.717 0.712 0.718 À5.12 À5.63 À5.66 À12.65 À29.74 À41.40 contributions (see Fig 3(a)) This sample is already magnetic with the relative magnetic and paramagnetic Mossbauer fractions of 85% and 15%, respectively An average hyperfine field of the magnetic phase /Bhf S ¼ 29:4 T and an average ‘‘cone-angle’’ /bS ¼ 68 can be reported However, it is worthwhile to mention that the hyperfine field is distributed in a rather broad range (from 20 to 34 T) These findings imply that the Cu-rich FCC matrix is paramagnetic The BCC Fe-rich nanograins are strongly coupled, but they exhibit an interfacial transient concentration and ferromagnetism, which originates from the ferromagnetic BCC-Fe in the centre of the individual grains, the iron-rich crystalline Fe(Cu) alloy lying near the interface and the paramagnetic N.H Duc et al / Journal of Magnetism and Magnetic Materials 262 (2003) 420–426 (a) 423 (b) Fe Cu 30 nm 30 nm Fig TEM 2-D color mapping images of the Fe0.56Cu0.44 thin films annealed at 100 C (a) and 400 C (b) TEM diffraction patterns of the corresponding phases are shown in the insets copper-rich FCC-Cu(Fe) matrix This is in good agreement with the TEM color mapping results After annealing at TA ¼ 100 C and 200 C, the CEM spectra are almost identical (see Figs 3(b)– (c)) A weak increase in /Bhf S (up to 32.0 T) and a decrease in /bS (down to 56 ) started to occur in the sample annealed at 300 C (see Table 2) In accordance with the XRD results, this increase of /Bhf S is associated to the evolution of the grain size as well as the enrichment of Fe in the BCCphase The decrease of the /bS value reflects the enhancement of an out of plane magnetisation component For the sample annealed at TA ¼ 350 C, the hyperfine field distribution is narrowing, /Bhf S ¼ 32:4 T and the paramagnetic fraction remains 6% only Finally, after annealing at 400 C, the CEM spectrum consists of one sextuplet of BCC-Fe (isomer shift d ¼ and /Bhf S ¼ 33 T, see Table 2) This confirms the complete decomposition of Fe and Cu metals In this case, a sharp BCC-Fe/FCC-Cu interface is thought to be formed 3.3 Magnetisation and magnetic coercivity Figs 4(a)–(c) illustrates the magnetic hysteresis loops measured in magnetic fields applied in the film-plane and along the film-normal directions for the as-deposited film and the films annealed at TA ¼ 200 C and 400 C, respectively The magnetic data are summarised in Table Note that the as-deposited sample is a soft magnetic material with saturation magnetisation MS ¼ 0:705 T and coercivity m0 HC ¼ mT: With increasing annealing temperature, the saturation magnetisation slightly increases The coercivity m0 HC ; however, remains almost constant for TA p200 C, but strongly increases for TA X300 C A m0 HC value as large as about 24.2 mT is achieved at room temperature for film annealed at TA ¼ 400 C This coercivity is quite large as compared with that of pure BCC-Fe (m0 HC B1 mT) In addition, we also observed an indication of the existence of a (spontaneous) perpendicular magnetisation component (see, e.g., magnetic hysteresis loops measured in magnetic fields applied along the film-normal direction in Figs 4(a) and (b)) This was already mentioned in the discussion of the average Mossbauer ‘cone-angle’ /bS in Section 3.2 Below, we will connect it to a contribution of the surface magnetisation According to the conventional theory, the effective magnetic anisotropy constant Keff can be estimated from the coercivity m0 HC and the saturation magnetisation MS by using the relation: Keff ¼ 12am0 HC MS ; ð1Þ N.H Duc et al / Journal of Magnetism and Magnetic Materials 262 (2003) 420–426 424 Velocity (mm/s) -10 +10 30 1.02 (a) 20 10 30 1.00 1.01 (b) 20 10 30 1.00 1.01 (c) 20 P (Bhf ) Emission 10 1.00 1.02 30 (d) 20 10 80 (e) 60 40 20 1.00 1.02 100 1.00 1.03 (f) 50 0 1.00 10 20 30 Bhf (T) Fig Mossbauer spectra and hyperfine-field distributions of the Fe0.56Cu0.44 thin films: (a) the as-deposited film; after annealing at (b) 100 C; (c) 200 C; (d) 300 C; (e) 350 C and (f) 400 C Table Hyperfine parameters for Fe0.56Cu0.44 granular films: isomer shift (d), hyperfine field (/Bhf S), Mossbauer cone-angle (/bS) and paramagnetic fraction (Apara) Sample d (mm/s) /Bhf S (T) /bS (deg.) Apara (%) As-deposited TA ¼ 100 C TA ¼ 200 C TA ¼ 300 C TA ¼ 350 C TA ¼ 400 C 0.052 0.040 0.040 0.010 0.006 29.4 29.4 30.5 32.0 32.4 33.0 68 68 68 56 55 55 15 15 15 14 where a is a proportionality constant, which depends on the magnetisation reversal mechanism According to the Stoner–Wohlfarth model, i.e for randomly distributed single domain particles, one has aE1 ([7] and references therein) The obtained Keff values are listed in Table Assuming an assembly of fine, spherical Fe particles with average diameter dFe ; Keff can be expressed in terms of the volume (KV ) and surface (KS ) anisotropy constants, as [12] Keff ẳ KV ỵ 6KS =dFe 2ị N.H Duc et al / Journal of Magnetism and Magnetic Materials 262 (2003) 420–426 24 1.0 TA = 30 °C ⊥ // dFe.Keff (mJ/m2) M (10-3 kA m2) 18 12 425 -6 -12 -18 0.0 -1.0 -2.0 -24 (a) -1.2 -0.8 -0.4 0.0 0.4 0.8 -3.0 1.2 16 TA = 200 °C M (10-3 kAm2) 12 80 -4 -8 -12 -16 (b) -1.2 20 -0.8 -0.4 0.0 0.4 0.8 1.2 0.8 1.2 TA = 400 °C 16 M (10-3 kAm2) 40 60 dFe (nm) Fig The plot of dFe Keff as a function of dFe for the Fe0.56Cu0.44 thin films ⊥ // 20 12 ⊥ // -4 -8 -12 -16 -20 -1.2 -0.8 -0.4 (c) 0.0 0.4 µ° H(T) Fig Magnetic hysteresis loops for the Fe0.56Cu0.44 thin films: (a) the as-deposited films; after annealing at (b) 200 C and (c) 400 C or dFe Keff ẳ dFe KV ỵ 6KS : 3ị In Fig 5, dFe Keff is plotted as a function of dFe As can be seen from this figure, a large deviation from linearity is observed for small grain sizes A similar result was observed in Fe0.2Cu0.8 films [12] In Ref [12], this behaviour was attributed to the variation of the surface anisotropy in the series due to the varying degree of the segregation, and then to a different modification of the band-structure in the surface phase At present, as already indicated by the Mossbauer data, this can also be ascribed to modifications of the interfacial transient concentration and ferromagnetism Approximately, we can deduce the values of KV and KS from the large grain size region, where a sharp interface is expected to exist It turns out from this approach that KV ẳ 51 kJ/m3 and KS ẳ ỵ0:1 mJ/m2 The observed negative sign of KV means that the Fe magnetic moments in the grain cores tend to be oriented in the film plane The positive sign of KS ; however, indicates that Fe-magnetic moments in the surface phase tend to be oriented along the surface normal [12] Finally, it is interesting to mention that the obtained KV value is rather close to the values of À50 and À40 kJ/m3, reported for the cubic anisotropy constant of the pure BCC-Fe [4] and the granular Fe0.2Cu0.8 films [12], respectively The obtained value of the surface anisotropy KS, however, is larger than that (+0.04 mJ/ m2) of the Fe0.2Cu0.8 granular films Thus, the grain interconnections, not reduce, but enhance KS : Consequently, this can be associated with the intergrain magnetic coupling effects However, one should also take into account effects of the Feconcentration dependence of the surface magnetic anisotropy 426 N.H Duc et al / Journal of Magnetism and Magnetic Materials 262 (2003) 420–426 Concluding remarks References Magnetic properties of Fe0.56Cu0.44 films can be described in terms of the initial formation of BCCFe nanograins with interfacial transient concentration and ferromagnetism With increasing annealing temperature, a complete separation is reached and the interfaces become sharper The enhancement of the magnetic coercivity observed in the granular films was associated with the surface anisotropy contribution, which is thought to be strengthened by the intergrain magnetic coupling as well as by the Fe-concentration enrichment [1] B Abeles, in: R Wolfe (Ed.), Applied Solid State Science: Advances in Materials and Device Research, Academic Press, New York, 1976, p [2] C.I Chen, J Appl Phys 69 (1991) 5267 [3] C Alof, B Stahl, M Ghafari, R Hahn, J Appl Phys 88 (2000) 4212 [4] C.L Chien, J Appl Phys 69 (1991) 5276 [5] J.Q Xiao, J.S Jiang, C.L Chien, Phys Rev Lett 68 (1992) 3749 [6] G Xiao, CL Chien, Appl Phys Lett 51 (1987) 1280 [7] C Chen, O Kitakami, Y Shimada, J Appl Phys 84 (1998) 2184 [8] T Hayashi, S Hirono, M Tomita, S Umemura, Nature (London) 381 (1996) 72 [9] T Murayama, M Miyamura, S Kondoh, J Appl Phys 76 (1994) 5361 [10] C Chen, O Kitakami, Y Shimada, J Appl Phys 86 (1999) 2161 [11] J.R Childress, C.L Chien, M Nathan, Appl Phys Lett 50 (1990) 95 [12] N.H Duc, N.A Tuan, A Fnidiki, C Dorien, J Teillet, J Ben Youssef, H Le Gall, J Phys.: Condens Matter 14 (2002) 6657 Acknowledgements This work was partly supported by the State Program for Natural Scientific Researches of Vietnam, within project 420.301  ... that the as-deposited sample is a soft magnetic material with saturation magnetisation MS ¼ 0:705 T and coercivity m0 HC ¼ mT: With increasing annealing temperature, the saturation magnetisation... Journal of Magnetism and Magnetic Materials 262 (2003) 420–426 Concluding remarks References Magnetic properties of Fe0.56Cu0.44 films can be described in terms of the initial formation of BCCFe... Keff can be estimated from the coercivity m0 HC and the saturation magnetisation MS by using the relation: Keff ẳ 12am0 HC MS ; 1ị N.H Duc et al / Journal of Magnetism and Magnetic Materials 262