The effect of the deposition parameters on the magnetic and magnetotransport properties of laser ablated Cu–Co thin films

Sensors and Actuators A 106 (2003) 203–207

The effect of the deposition parameters on the magnetic and magnetotransport properties of laser ablated Cu–Co thin films R. López Antón, M.L. Fernández-Gubieda Ruiz∗ Departamento de Electricidad y Electrónica, Universidad del Pa´ıs Vasco, Apartado 644, 48080 Bilbao, Spain

Abstract Cu90 Co10 samples have been prepared by laser ablation under different deposition parameters and have been subjected to thermal treatment. The microstructure of the samples has been studied through the fitting of the hysteresis loops and magnetoresistance (MR) curves. The as-deposited samples consist of superparamagnetic (SPM) clusters and diluted Co atoms. Upon annealing, greater SPM clusters and a ferromagnetic phase appear. The presence of the ferromagnetic phase is also confirmed by X-ray diffraction (XRD). We have found that the deposition parameters greatly influence the microstructure, affecting the shape of the Co nanoclusters and the percentage of Co atoms contributing to the SPM phase, therefore affecting the magnetotransport properties. © 2003 Elsevier B.V. All rights reserved. Keywords: Laser ablation; Granular systems; Magnetoresistance; Magnetic properties

1. Introduction

2. Experimental

In recent years, the phenomenon of “giant magnetoresistance” (GMR), i.e. a great variation of the electrical resistance when a magnetic field is applied, has attracted great scientific and technological attention, given its promising capabilities for information storage systems and sensor technology. Granular Cu–Co alloys are among the systems that present giant magnetoresistance [1,2]. At room temperature, the solubility of Co in Cu is practically negligible. However, some special preparation methods make it possible to obtain a metastable solid solution of a small quantity of Co in Cu. In this situation, the energetically more favourable segregation of small precipitates of Co takes place whenever possible (after annealing or sometimes at preparation), giving rise to the so-called granular alloys. In particular, laser ablation technique, given its simplicity of use and its ability to obtain thin samples [3–5], is very convenient for the preparation of sensor devices based on GMR materials. In this work, several Cu90 Co10 thin films have been successfully obtained by laser ablation and subjected to different thermal treatments. X-ray diffraction (XRD), magnetic and magnetoresistance measurements have been performed to study the changes in the microstructure induced by the deposition parameters and the annealing temperature, and the influence of these changes on magnetotransport properties.

The samples were deposited using a LAMBDA PHYSIK Compex 102 laser (248 nm, 300 mJ by pulse). The used target comprised a disc with cobalt and copper sectors in proportions appropriate to the desired film composition (about 10 at.% Co). The target was rotated by a step-by-step drive. The samples were obtained varying the laser pulse frequency ν (Hz), and the target angular speed ω (rpm). In this way, three different samples were obtained (called in the following 19/150, 20/30, 15/15) nominated as a function of the deposition parameter, ν (Hz)/ω (rpm). The thin films were deposited onto Si substrates at room temperature. The pressure in the deposition chamber was about 10−5 mbar. The thickness of the thin films, measured by profilometry, was about 250 nm for the 19/150 sample and about 700 nm for the other two samples. After growth, small pieces of the samples were annealed for 30 min at 450, 500 and 550◦ C under a constant Ar flux in order to avoid oxidation. XRD spectra of several of the samples were obtained at room temperature using image plate technique (with λ = 0.68885 Å) in line 9.1 of the Daresbury Laboratory. This technique is not well suited for determining the lattice parameter given the indetermination in the zero, but it is very sensible to the existence of different phases. Magnetic measurements were performed at room temperature in a SQUID magnetometer under an applied magnetic field up to 40 kOe. The MR measurements, defined as MR = 100 × [R(H) − R(H = 0)]/R(H = 0), were performed at room temperature

∗ Corresponding author. Tel.: +34-94-6012552; fax: +34-94-6013071. E-mail address: [email protected] (M.L. Fern´andez-Gubieda Ruiz).

0924-4247/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0924-4247(03)00167-5

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using the well-known four-probe technique, with magnetic field applied perpendicular and parallel to the plane of the sample. The MR of the samples annealed at 450 ◦ C were also measured at low temperature (10 K) and high magnetic field (70 kOe). 3. Results and discussion The XRD pattern evidence the existence of a unique fcc-Cu-rich phase in the as-deposited samples and the appearance of a fcc-Co-rich phase as we anneal the sample (Fig. 1). The estimated size of the Co grains is between 10 and 22 nm. With this cluster size, they are expected to present a ferromagnetic response [6]. The hysteresis loops of the as-deposited samples show a superparamagnetic behaviour, with neither coercivity nor remanence and not saturating even under an applied field of 10 kOe (see Fig. 2). The annealed samples present remanence magnetisation, coercivities between 70 and 420 Oe, and an unsaturating behaviour.

Fig. 2. Hysteresis loops (left) and MR curves (right) of the 19/150 sample with and without annealing. The experimental data (䉫), the total fit (—), that of the FM phase (– – –) and that of the SPM phase (- - -), are represented. In the detail are shown the goodness of the fits at low fields.

The evolution of the microstructure has been analysed through the simultaneous fit of the hysteresis loops and MR curves, both of them obtained with the field parallel to the film plane. First, we considered that the hysteresis in the annealed samples was due to dipolar interactions between the superparamagnetic clusters, as suggested by Allia et al. [7], but the quality of the fit was not good. Finally, we have fitted the hysteresis loops considering both ferromagnetic (FM) and superparamagnetic (SPM) contributions which gives cluster size distribution f(D) for the SPM phase as a function of cluster diameter D according to the following functions [8] as M(H) = MFM (H) + MSPM (H) (1a)  
S R 2MFM πMFM H + HC (1b) tan arctan MFM (H) = S π HC 2MFM    ∞ µH S f(D)L dD (1c) MSPM (H) = MSPM KB T 0

Fig. 1. X-ray spectra of the 19/150 sample with the different thermal treatments.

S beThe first term is the FM contribution, MFM (H), MFM R ing the saturation magnetisation, MFM the magnetic remanence and HC the coercivity field. The second term is the S SPM one, MSPM (H), being MSPM the superparamagnetic

R. L´opez Ant´on, M.L. Fern´andez-Gubieda Ruiz / Sensors and Actuators A 106 (2003) 203–207

saturation magnetisation, and L the well-known Langevin function, depending on cluster size through the magnetic S S moment per cluster, µ, as µ = MCofcc V = MCofcc πD3 /6 S (MCofcc = 1450.2 emu/cc is the spontaneous magnetisation of fcc Co). We have chosen as f(D) the logarithmic-normal distribution [9]:   −ln2 (D/Dm ) 1 exp (2) f(D) = √ 2ρ2 2πDρ which depends on two parameters: Dm and ρ, which are ¯ and standard deviation σ [9]. related to the mean diameter D The field dependence of the MR has been studied taking into account the contribution of each magnetic phase, and considering that both phases follow a quadratic dependence on the magnetisation [9]:  2 MSPM (H) MR(H) = ASPM S MSPM  2  2  R M M (H) FM FM  + AFM  − (3) S S MFM MFM where ASPM and AFM indicate the superparamagnetic and ferromagnetic contributions to the MR. In these fits, performed simultaneously in a feedback process, we have fixed R to the value found experimentally. Therefore, D , ρ, MFM m S S is linked toM S ) are the free paramHC and MSPM (MFM SPM eters of the magnetic loops whereas Dm , ρ, ASPM and AFM are the parameters of the fits of the MR loops. Results found are shown in Table 1, and Fig. 2 shows the fits obtained for the 19/150 sample. We have also estimated the percentage of the total Co S forming the SPM clusters taking into account that MSPM = N µ, ¯ where N is the number of clusters per volume unit and µ ¯ is the mean magnetic moment per cluster defined as µ ¯ = S ¯ 3 /6. The amount of Co that enters in the SPM MCofcc πD phase can be calculated from N V¯ (normalised by the 10% of Co in the sample) [10]. On the other hand, the amount of Co in the FM phase is S /M S obtained from MFM Cofcc (normalised by the 10% of Co). The remaining percentage (100% − % Co SPM − % Co FM) represents the amount of Co atoms that do not contribute magnetically because they are diluted in the Cu matrix. The results of these calculi are presented in Table 1. All the as-deposited samples present a small fraction of Co atoms forming the SPM particles (about 15%), being the rest of the Co atoms diluted in the Cu matrix, not contributing to the magnetism of the sample. The mean diameter of the SPM particle distribution is about 3 nm and the standard deviation is about 1.8 nm. These values are consistent with those found in the literature [8,9]. As we anneal the samples, the microstructure evolves differently depending on the deposition conditions. In particular, for the 19/150 sample, the percentage of Co in the SPM phase is always smaller than 20%, with a mean diameter

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Table 1 ¯ and standard deviation σ of the SPM particle size Mean diameter D distribution, % Co SPM (% Co FM), percentage of Co atoms in the SPM (FM) phase, for the as-deposited and annealed samples, and saturation value of the MR obtained at 10 K and 7 T for samples annealed at 450 ◦ C Tann (◦ C)

Samples 19/150

20/30

15/15

¯ (nm) D a.d. 450 500 550

2.7 3.1 3.2 4.8

(6) (4) (4) (4)

3.1 2.7 3.5 4.4

(2) (3) (2) (3)

3.2 2.2 3.1 4.7

(2) (2) (2) (3)

σ (nm) a.d. 450 500 550

2.1 2.0 1.7 2.0

(8) (7) (6) (7)

1.6 0.8 1.1 1.3

(3) (2) (2) (3)

1.7 1.2 1.5 1.4

(4) (2) (3) (3)

% Co SPM a.d. 450 500 550

12 18 16 11

% Co FM a.d. 450 500 550

– 29 (3) 40 (4) 52 (5)

– 14 (2) 60 (3) 74 (4)

5.1

8.4

MR (%) (at 10 K) 450

(4) (3) (3) (4)

17 19 19 16

(2) (2) (3) (4)

17 37 20 14

(2) (2) (2) (2)

– 34 (3) 67 (3) 72 (4) 13

that increases from 2.7 to 4.8 nm with the thermal treatment. After annealing at 450 ◦ C, an FM phase appears, which increases from 30 to 50% as the annealing temperature increases. For the 20/30 sample, the percentage of Co in SPM phase is again smaller than 20%, with negligible changes with Tann . The mean diameter of the SPM clusters evolves from 3 to 4.4 nm. Meanwhile, the percentage of Co atoms in the FM phase increases from 14 to 74% for Tann of 450 and 550 ◦ C, respectively. Finally, for the 15/15 sample, the annealing at 450 ◦ C gives place to an important increase of the SPM phase (up to roughly 40%) and an important diminution of the mean diameter of the SPM particles. Increasing the annealing temperature enlarges the size of the SPM particles and decreases the percentage of this phase. The FM phase increases its contribution with the annealing temperature, reaching 70% for the sample annealed at 550 ◦ C. The percentage of Co not contributing to the magnetism of the samples (as high as 80% for the as-deposited samples) is important even for the samples annealed at 550 ◦ C (about 12% for the 20/30 and 15/15, and 37% for the 19/150 one). In all the samples the coercive field of the FM phase increases with the annealing temperature, from about 100 to 400 Oe. This behaviour, together with the size of the Co clusters determined by XRD, suggests that the FM phase is formed by monodomain FM Co clusters.

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for the 15/15 sample, the one with the highest percentage of Co in SPM phase, we found better values.

4. Conclusions We have studied the influence of the deposition parameters on the magnetic and magnetotransport properties of Cu10 Co90 thin films obtained by laser ablation. The microstructure of these films and its evolution with the thermal treatment has been determined by fitting the hysteresis loops and MR curves considering both an SPM and an FM contribution. The deposition parameters modify not only the shape of the clusters but also the percentage of Co in the SPM and FM phases. The sample 15/15, the one with the highest percentage of Co in SPM phase, presents the highest MR value, 13% at 10 K.

Acknowledgements Fig. 3. MR curves at room temperature with the magnetic field perpendicular (䊏) and parallel (䊊) to the plane of the film for 19/150 (left) and 15/15 (right) samples.

In order to get a better insight of the structure, we have also obtained MR curves with the field applied perpendicular to the samples in order to check the existence of anisotropy. In Fig. 3, we compare the MR curves (parallel and perpendicular) for the samples 19/150 and 15/15. As can be observed, the 19/150 sample exhibits an isotropic behaviour whereas the 15/15 one (and the 20/30 one) presents a clear anisotropy, reaching lower MR values when the field is perpendicular to the sample. This different behaviour is induced by the deposition conditions: in the deposition of the 19/150 film, the laser impacts once in the Co and then nine times in the Cu, giving place to a very homogeneous dispersion of the Co atoms, which favours the formation of spherical Co clusters and explains the high percentage of Co diluted in the Cu matrix even after annealing. Meanwhile, in the case of the 15/15 one, the laser hits six consecutive times in the Co and 54 times in the Cu, promoting the formation of frustrated multilayers, where the Co atoms and small clusters are not so homogeneously dispersed but ordered preferentially in incomplete layers. This structure of the as-deposited sample gives place, after the thermal treatments, to the formation of clusters with planar symmetry (which are responsible for the anisotropy) and to higher percentages of Co in the SPM and FM phases. Another interesting point is the maximum value reached for the MR. It is well known that the granular Co–Cu system reaches its maximum GMR value after annealing at 450 ◦ C [11,12]. Therefore we have also obtained these MR values at 10 K and under fields of 70 kOe (in order to reach the saturation value of the MR). These values, also presented in Table 1, are similar to those found in the literature [3–5] but,

This work has been partially financed by the Spanish CICYT under project MAT99-0667. We also thank the help provided in sample characterisation by M. Insausti, from the University of the Basque Country, and A. Cebollada, from the Instituto de Microelectrónica de Madrid.

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Biographies M.L. Fernandez-Gubieda Ruiz is lecturer in the University of the Basque Country from 1995. She graduated from the Faculty of Sciences in the

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University of the Basque Country in 1986 and received her PhD degree in 1991. In research, she is involved in preparation and characterisation of magnetic materials and in the study of the influence of the structure on magnetic properties. Her research activity includes laser ablation, electrodeposition, magnetoresistive thin films, metallic glasses, Mössbauer spectroscopy and EXAFS.

R. Lopez is currently a researcher at the University of the Basque Country, having finished his PhD studies very recently (Summer 2002). The subject of his thesis has been the preparation and characterisation of thin films of magnetoresistive materials. He graduated from the same university in 1996. His research activity includes laser ablation, electrodeposition, magnetoresistive thin films and metallic glasses.