non-magnetic nanoparticles films with peculiar properties produced by ultrashort pulsed laser deposition

Applied Surface Science 254 (2007) 1053–1057 www.elsevier.com/locate/apsusc

Magnetic/non-magnetic nanoparticles films with peculiar properties produced by ultrashort pulsed laser deposition V. Iannotti a,*, S. Amoruso b, G. Ausanio a, A.C. Barone a, C. Campana c, C. Hison a, X. Wang b a

CNR-INFM Coherentia, Dip.to di Scienze Fisiche, Universita` degli Studi di Napoli ‘‘Federico II’’, P.le V. Tecchio 80, I-80125 Napoli, Italy b CNR-INFM Coherentia, Dip.to di Scienze Fisiche, Universita` degli Studi di Napoli ‘‘Federico II’’, Complesso Universitario di Monte S. Angelo, Via Cintia, I-80126 Napoli, Italy c Dip.to di Ingegneria dei Materiali e della Produzione, Universita` degli Studi di Napoli ‘‘Federico II’’, P.le V. Tecchio 80, I-80125 Napoli, Italy Received 30 May 2007; received in revised form 25 July 2007; accepted 8 August 2007 Available online 15 August 2007

Abstract Films of magnetic nanoparticles uniformly mixed with non-magnetic nanoparticles have been produced by ultrashort pulsed laser deposition. These films present innovative characteristics with respect to their counterparts produced by standard techniques, as for example nanosecond laser ablation or sputtering, due to the peculiar shape and preferential distribution of their constituent nanoparticles. In the present investigation, the difficult coalescence among the deposited nanoparticles, specific characteristic of the ultrashort pulsed laser deposition, is particularly stressed for what concerns its effect on the collective magnetic behaviour. In particular, we observed that, even for a significant fraction of magnetic particles, the films exhibit an unusual high remanent magnetization, together with relatively low values of saturation and coercive fields, showing a strong squareness of the hysteresis loops. In perspective, these nanogranular films appear very promising for potential application as permanent magnets and in magnetic recording. # 2007 Elsevier B.V. All rights reserved. Keywords: Ultrashort pulsed laser deposition; Nanogranular magnetic films; High remanence ratio

1. Introduction The synthesis and analysis of nanoparticles (NPs) of various elements and compounds are receiving increasing attention due to their particular significance for both fundamental research and technological applications [1,2]. The high interest in NPs and films made of NPs aggregates is based on their peculiar physical properties, which cannot be found in corresponding bulk materials, associated with the particles small size and large surface-to-volume ratio [3,4]. Different techniques have been used for the NPs production, such as arc discharge, vapour and electrochemical deposition, sputtering and nanosecond pulsed laser deposition (ns PLD). Recently, ultrashort pulsed laser deposition (uPLD) in vacuum has been demonstrated to be a powerful and versatile tool for

* Corresponding author. Tel.: +39 081 7682612; fax: +39 081 2391821. E-mail address: [email protected] (V. Iannotti). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.08.027

the production of metal and semiconductor NPs films [5–8]. What characterizes the uPLD from standard (nanosecond, ns) PLD is that the NPs are ejected directly from the target, by its thermo-mechanical fractioning, as consequence of the extreme temperature and pressure conditions induced by femtosecond (fs) laser pulse excitation, via different mechanisms, such as phase explosion and fragmentation [9,10]; the ejected nanodrops expand in vacuum at high velocities (0.1–1 km/ s), being subsequently collected on the deposition substrate, where they solidify as independent nanograins [11]. In the classical ns PLD, the target material is atomized in a plume and the nanoparticles are mainly formed through condensation into a background gas [12]. The uPLD assures the deposition of NPs reproducing the stoichiometry of the target material, strongly reducing the inclusion of impurities and avoiding all the complications introduced by the presence of a background gas, as the plume expansion may develop in vacuum. Recent results on the peculiarities of the NPs films synthesized via uPLD show that the constituent nanograins

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exhibit an oblate ellipsoidal shape, with a aspect ratio depending on the laser intensity, and an ordinate disposition with the major cross section parallel to the deposition substrate [4,8,13]. Many experimental evidences demonstrate that the particles maintain their individuality and only a partial exchange interaction is active among the nearest ones [14,15]. When a multitarget material is used, the deposited uPLD film is not constituted by nanograins coalescing one another, as in the films obtained by standard production methods (e.g. ns PLD, sputtering, etc.), but it is an immiscible mixture of particles, each one composed by a single target element. Granular and multilayer Cu–Co and Fe–Ag thin films have been the subject of many investigations in the recent years due to their broad scientific and technological interest [16,17]. Most of these films have been obtained by techniques such as ac–dc sputtering [18], melt spinning [19], electrodeposition [20] and ns laser ablation in an appropriate background gas [21]. The above-mentioned preparation methods require post-process annealing of the samples in order to obtain heterogeneous nanogranular films and to optimize their properties. The uPLD has not been used up to the present time for the preparation of this kind of films, despite its great advantages of versatility and obviation of post-deposition annealing. In this work, we are focusing on the preparation of CoxCu100 x and FexAg100 x heterogeneous nanogranular magnetic films by uPLD in vacuum and on the investigation of their magnetic properties. Due to the uPLD peculiarities, the magnetic properties of the obtained films are developing in microstructural conditions significantly different from those in films obtained by other techniques. The study of these innovative nanogranular systems is of interest both from a fundamental point of view, as well as for future potential applications in electromagnetic devices exploiting their peculiar magnetic behaviour (e.g. permanent magnets and high density magnetic recording media).

all samples, the deposition time was 1 h and no post-deposition thermal processing was performed. The as-deposited films have a 10 mm  4 mm rectangular form. Preliminary analysis of NPs produced by ablation of individual, pure Co, Cu, Fe and Ag targets were performed in order to evaluate the elements deposition efficiency and estimate the appropriate permanence time of the laser beam on each target region, so to obtain the desired volume fractions in the films. In agreement with pre-existing literature [22,23], similar deposition rates for the Co–Cu elements were observed, while a rate about double was identified for Ag component with respect to the Fe element. The films absolute volume fraction accuracy was quantified in 3%, as inferred from the deposition rate control. The morphology of the deposited samples was analysed by atomic force microscopy (AFM), using a Digital Instruments Nanoscope IIIa in tapping mode (scan size and rate of 2 mm and 1 Hz, respectively), with a sharpened silicon tip having the apical curvature radius of less than 5 nm. After performing deconvolution on each AFM image, in order to avoid the tip size effect, the particles average size in a plane parallel and orthogonal to the substrate was evaluated. In this way, the threedimensional view of the deposits has been reconstructed. The size and size distribution of individual NPs were determined for each element by AFM analysis on less than one layer deposits onto mica substrates. The average film thickness was estimated by means of a profilometer (Tencor Alpha-Step 500 Surface Profilometer), with an overall accuracy of about 20 nm. The magnetic characterization of the deposited films was performed at 250 K and 10 K, by means of a vibrating sample magnetometer (VSM Oxford Instruments Maglab, 9 T), operating at a vibration frequency of 55 Hz and with the magnetizing field in the film plane.

2. Experimental

Representative AFM images of less than one layer surface of Co and Fe nanogranular films are presented in Figs. 1 and 2,

The experimental setup used in the present investigation has been reported earlier [5,6,8], and will be briefly described here. The CoxCu100 x and FexAg100 x nanogranular films were produced by means of ultrashort laser pulses obtained using a chirped pulse amplification Nd:Glass laser system. Laser pulses at 527 nm, with a duration of 0.3 ps were obtained by means of a pulse compression and second harmonic generation process. CoCu and FeAg films were deposited at a repetition rate of 33 Hz, by exploiting laser pulses with a fluence of 0.3 J/cm2 and 0.6 J/cm2, respectively. The target, made of a combination of magnetic (99.9% Co or Fe) and non-magnetic (99.9% Cu or Ag) plates (multi-target configuration), was mounted on a rotating (about 25 rpm) holder, to minimize the pit formation. The deposition of the CoCu and FeAg films was developed at room temperature, in a stainless-steel vacuum chamber evacuated at a residual background pressure 10 5 Pa, on Kapton polyimide and Silicon (1 0 0) substrate, respectively, placed parallel to the target at 30 mm. Prior to deposition, the target was cleaned with 1000 laser pulses, while a shutter was held over the substrate. For

3. Results and discussion

Fig. 1. Typical AFM image (2 mm  2 mm) of less than one layer of Co nanogranular film deposited onto mica substrate.

V. Iannotti et al. / Applied Surface Science 254 (2007) 1053–1057

Fig. 2. Typical AFM image (2 mm  2 mm) of less than one layer of Fe nanogranular film deposited onto mica substrate.

respectively. A large number of disperse NPs, with sizes ranging from a few to tens of nanometers, and some islands where NPs stuck together can be observed. The results on the size characterization (nanoparticles inplane diameter, D, and height, d) of the Co, Cu, Fe and Ag nanograins in less than one layer deposits are summarized in Table 1. The aspect ratio, D/d, is reported as an indicator of the oblate shape degree, indicating how much the single particle is flatted as consequence of the deposition process. The D size distribution for each Co, Cu, Fe and Ag element has an asymmetric shape which can be fitted to a lognormal distribution of particles diameters. The evaluated D mean values for Co, Cu, Fe and Ag particles are 23 nm, 29 nm, 17 nm and 16 nm, respectively. The corresponding standard deviation of particles diameter was determined to about 94% of the D mean value for Co and Cu and about 15% of the D mean value for Fe and Ag particles. The median diameter, Dm (obtained by determining the D value at which the integral of lognormal distribution is equal on either side of Dm) is taken as representative size for further analysis and its values are reported in Table 1. The d and D/d size distributions cannot be fitted to any known distribution. Therefore, there were evaluated for Co, Cu, Fe and Ag particles the maximum d and D/d values (defined as the nanoparticles size below which 90% of the particles are counted) dmax: 13 nm, 19 nm, 7 nm and 2 nm, respectively and (D/d)max: 19, 19, 18 and 85, respectively. In Table 1 are reported the median d and D/d values Table 1 Co, Cu, Fe and Ag nanoparticles morphology: in-plane diameter, Dm, height, dm, and aspect ratio (D/d)m representing the median values of the NPs sizes distributions Sample

Dm (nm)

dm (nm)

(D/d)m

Co Cu Fe Ag

17 21 17 16

3 3 2 1

6 6 11 20

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(determined by finding the nanoparticles size below and above which 50% of the particles are counted) as representative sizes for further analysis. The NPs size in the films is fairly similar to that measured by means of AFM in less than one layer deposits due to the difficult particles coalescence, as demonstrated by the correlation between the AFM morphology in less than one layer deposits and films X-ray analysis [14,15]. Similar AFM images were obtained for films of different compositions, with the thickness ranging from 300 to 900 nm, indicating a good morphological uniformity in the samples volume. Therefore, in agreement with other experimental results on Ni and Fe mono-component films [14], we can conclude that also for the present bi-component films deposited by uPLD, the basic structure is cauliflower-like, made of granular agglomerates inside which the NPs are sticking to one another, while maintaining their own individuality. The NPs exhibit a sensible difference between the in-plane diameter, D, and the height, d, and an ordered distribution, with the major cross section parallel to the substrate plane. The measured diameter of the magnetic nanograins in the investigated CoxCu100 x and FexAg100 x films (Table 1) is smaller or comparable to the single domain critical size, which in the case of Co and Fe is known to be 70 nm and 14 nm, respectively, for spherical particles, with no shape anisotropy [24]. Moreover, the particles with significant shape anisotropy, as in our case, can remain single domain up to much larger dimensions than their spherical counterparts. Therefore, the deposited Co and Fe nanograins are single domain. The samples morphological and topological features, together with the above-mentioned single domain characteristic, determine the films magnetic properties as obtained by VSM measurements and summarized in Tables 2 and 3. The coercive, Hc, and saturation, Hs, fields, and the remanence ratio, Mr/Ms, were evaluated from the hysteresis loops at 10 K and 250 K presented in Figs. 3 and 4. Similar results were obtained along both longitudinal and transverse in plane directions of the magnetizing field, for all studied samples. As can be seen from Tables 2 and 3 and Figs. 3 and 4, at low Co and Fe concentration (xv = 15%), the films exhibit high coercive field and remanence ratio (about 0.5), ascribed to the random distribution of non-interacting single domain particles,

Table 2 Magnetic characteristics (coercive, Hc, and saturation Hs, fields and remanence ratio, Mr/Ms) at 250 K and 10 K of CoxCu100 x nanoparticles films having different thicknesses, t Sample composition in volume fraction, xv

t (nm)

T (K)

m0Hc (T)

m0Hs (T)

Mr/Ms

Co15Cu85

500

10 250

0.053 0.050

0.6 0.5

0.46 0.35

Co25Cu75

300

10 250

0.045 0.023

0.5 0.4

0.68 0.62

Co50Cu50

400

10 250

0.047 0.020

0.5 0.4

0.63 0.63

The errors on the Hc, Hs and Mr/Ms values are 1 in the last significant digit.

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Table 3 Magnetic characteristics (coercive, Hc, and saturation Hs, field and remanence ratio, Mr/Ms) at 250 K and 10 K of FexAg100 x nanoparticles films having different thicknesses, t Sample composition in volume fraction, xv

t (nm)

T (K)

m0Hc (T)

m0Hs (T)

Mr/Ms

Fe15Ag85

900

10 250

0.023 0.015

0.9 0.9

0.57 0.47

Fe25Ag75

600

10 250

0.012 0.009

0.6 0.6

0.68 0.64

Fe50Ag50

700

10 250

0.012 0.007

0.4 0.4

0.74 0.62

The errors on the Hc, Hs and Mr/Ms values are 1 in the last significant digit.

with random uniaxial magnetic anisotropy, as predicted by the Stoner–Wolfarth model. The films magnetic behaviour, when passing from 15% to 25% magnetic volume fraction, demonstrates characteristic features clearly different from the Stoner–Wohlfarth behaviour for randomly oriented, non-interacting particles: a strong decrease of the coercive field value (about 50%) and a remanence ratio of about 0.6, indicating either some crystallographic texture or strong particles interactions [25]. The results of X-ray diffractometric measurements for other magnetic systems prepared by uPLD [14] show a weak crystallographic texture in such materials. Therefore, the above-mentioned magnetic behaviour in our films is mainly determined by NPs interactions. Practically, the remanence increase can be ascribed to the enhanced exchange interactions determined by the random agglomeration of single domain ferromagnetic particles in clusters, which starts to be progressively more important for magnetic volume fractions higher than 15%. More exactly, the remanence increase can be attributed to the non-uniform magnetic states within an agglomerate of magnetic nanoparticles. The particles magnetizations within the cluster are not parallel, but point in slightly different directions. In the

Fig. 4. Normalized hysteresis loops at 10 K and 250 K of the investigated FeAg nanoparticles films.

limit of strong exchange interactions, the cluster is uniformly magnetized, with a random easy axis. Therefore, each agglomerate behaves like a particle with uniaxial anisotropy and, since the uniaxial anisotropy of the agglomerates is random, the remanence ratio is given by the Stoner– Wohlfarth model (Mr/Ms = 0.5). When the exchange interactions between the nanoparticles within a cluster are moderate, because the grains size is larger than the exchange length, as in the investigated Co25Cu75 and Fe25Ag75 films, the magnetization becomes non-uniform and an increase of the remanence ratio value is expected [25], as indeed observed. The increase of the remanence ratio with the magnetic volume fraction increment from 15% to 25%, followed by an almost constant behaviour at higher volume fractions is a peculiar characteristic of the NPs systems obtained by uPLD, as consequence of their difficult coalescence even at high magnetic concentrations. The small difference between the remanence ratio values at 10 K and 250 K observed in the films with magnetic fraction larger or equal to 25% (see Tables 2 and 3), suggests an increase of the exchange interactions among the magnetic nanoparticles. Finally, it is worth observing that the variation of the remanence ratio with the relative amount of magnetic component, pretty similar in both CoCu and FeAg systems and for different substrates (Kapton and Silicon), is a peculiar characteristic of the uPLD technique. In particular, the films present a singular structure and morphology which allow the magnetic particles to retain their individuality, avoiding coalescence effects even at large magnetic fractions. This is not possible with other deposition techniques which typically suffer the effects induced by coalescence phenomena at high concentration of the magnetic element. 4. Conclusions

Fig. 3. Normalized hysteresis loops at 10 K and 250 K of the investigated CoCu nanoparticles films.

The morphological and magnetic investigation of the deposited CoxCu100 x and FexAg100 x nanogranular films produced by uPLD show interesting magnetic properties

V. Iannotti et al. / Applied Surface Science 254 (2007) 1053–1057

with respect to both homogeneous bulk and continuous nanogranular counterpart films obtained by sputtering or standard PLD. Specifically, the obtained results evidence that (i) the deposited CoxCu100 x and FexAg100 x nanogranular films are formed by individual, mono-component NPs, randomly agglomerated in separate clusters due to the femtosecond uPLD process characteristics; (ii) the mono-component nanoparticles preserve their individuality and the coalescence between the magnetic particles is very difficult, even at high volume fractions, with respect to the nanogranular films obtained by other techniques like sputtering or classical PLD; (iii) the films present a peculiar magnetic behaviour, characterised by relatively low values of saturation and coercive fields, accompanied by high remanence ratio (>0.5); these are mandatory properties for permanent magnet applications. Acknowledgements This work has been supported by a PRIN’05 Project of the Italian Ministry of Education, University and Research (MIUR), entitled ‘‘Production, characterization and modelling of nanogranular films with innovations in the magnetic, magnetoresistive or magnetostrictive properties’’. References [1] A.S. Edelstein, R.C. Cammarata, Nanomaterials: Synthesis, Properties and Applications, Institute of Physics Publishing, Bristol, 1996. [2] C.J. Zhong, M.M. Maye, Adv. Mater. 13 (2001) 1507. [3] X. Batlle, A. Labarta, J. Phys. D: Appl. Phys. 35 (2002) R15.

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