Annealing Effect on the Structural and Magnetic Properties of Granular Cu80Fe10Ni10 Ribbons

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DOI 10.1007/s10948-015-3018-5

ORIGINAL PAPER

Annealing Effect on the Structural and Magnetic Properties of Granular Cu ₈₀ Fe ₁₀ Ni ₁₀ Ribbons

R. Moubah·A. Fnidiki·N. Omari·M. Abid· E. K. Hlil·H. Lassri

Received: 7 January 2015 / Accepted: 4 February 2015

Abstract We report on the effect of annealing process on the structural and magnetic properties of granular Cu80Fe10Ni10ribbons prepared by melt spinning technique.

Energy-filtered transmission electron microscopy measurements show that the precipitates (Fe10Ni10) are localized at the grain boundaries of the Cu matrix for the non- annealed samples. After an annealing treatment at 600^◦C, a strong diffusion of the precipitates within the Cu matrix is observed. The temperature dependence of magnetization has been analyzed using the Bloch law. Spin wave stiffness constantD and the exchange constant A were calculated from the experimental results. Furthermore, we show that the random magnetic anisotropy (RMA) model is applica- ble to describe the experimental results. Other fundamental parameters have also been calculated using the approach to saturation magnetization. Ab initio calculations based on Korringa-Kohn-Rostocker (KKR) method are carried out in order to characterize both electronic and magnetic properties.

R. Moubah ()·H. Lassri

LPMMAT, Faculté des Sciences, Université Hassan II - Casablanca, 5366 Maârif, Morocco e-mail: [email protected]

A. Fnidiki

Faculté des Sciences de Rouen, Groupe de Physique des Matériaux, UMR CNRS 6634, Site Universitaire du Madrillet, Avenue de l’Université, BP 12, 76801 Saint-Etienne du Rouvray Cedex, France

N. Omari·M. Abid

LPTA, Faculté des Sciences, Université Hassan II - Casablanca, 5366 Maârif, Morocco N. Omari·E. K. Hlil

Institut N´eel, CNRS et Universit´e Joseph Fourier, BP 166, 38042 Grenoble Cedex, France

Keywords Granular ribbon·Superparamagnetic· Magnetization·Spin waves·Random magnetic anisotropy·Electronic structure calculations

1 Introduction

Device miniaturization has made the study of nanometric magnetic materials a goal of great interest. By reducing the dimensions of a magnetic material, many of its fundamental properties can be changed, such as the ordering temperature, type of ordering, or magnetic anisotropy. In this context, granular magnetic systems consisting of nanometric magnetic precipitates embedded in a non-magnetic (metallic or insulated) matrix can be viewed as a model system to investigate. These systems can be prepared using different techniques such as melt spinning, vapor deposi- tion, mechanical alloying, or electrodeposition [1,2]. More- over, using annealing treatments, their microstructure can be tuned [3]. From a fundamental point of view, such systems are interesting due to their particular properties, such as superparamagnetism [4–6], kinetics and phase transfor- mation [7,8], or spin glass behavior [9,10]. Furthermore, the discovery of giant magnetoresistance (GMR) in metallic granular systems such as CuFeNi [11–17], CuCo [17–23], CuCoNi [24], AuFe [15,25], or CoAg [26,27] has increased their potential for applications. However, the relationship between the magnetic and transport properties and the microstructure is still not clear, as a result of their complex microstructure [27–35].

In this paper, we investigate experimentally and theoret- ically the structural and magnetic properties of the granular Cu80Fe10Ni10 ribbons annealed at different temperatures.

The structure of the ribbons is studied by energy-filtered transmission electron microscopy (EFTEM). We describe

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Fig. 1 Low-resolution TEM image of granular Cu80Fe10Ni10

ribbons of a non-annealed sample (a). EFTEM images recorded at different element edges: Cu (b), Fe (c), and Ni (d). This figure highlights the existence of precipitates (NiFe) at the grain boundaries, as the arrowspoint out

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(b) (c) (d)

500 nm

200 nm 200 nm 200 nm

the change of magnetic properties when the particle size of the ultrafine metallic particles is changed by heat treatment. The results are analyzed using the random magnetic anisotropy (RMA) model (originally developed for amorphous alloys). The electronic and magnetic properties assumed as chemical disordered systems are studied by ab initio calculations based on appropriate Korringa-Kohn- Rostocker coherent-potential approximation (KKR-CPA) method.

2 Experimental Methods

All the ribbon samples were fabricated using melt spinning and rapid solidification process in an argon atmosphere using a steel wheel rotating at a surface speed of 25 m/s.

Their thickness is around 30 µm and the widths are ranging from 2 to 4 mm. Annealing treatments were made in vacuum under a pressure of 10⁻⁷mbar for 24 h at 500 and 600^◦C.

The structural properties of the films were investigated by transmission electron microscopy (TEM) technique, using a Tecnai F20 microscope operating at 200 kV. The microscope is equipped by a field emission gun (FEG) and a Gatan Imaging Filter (GIF) for EFTEM observations. TEM samples were prepared by mechanical polishing followed by Ar ion milling at 5 keV. Final polishing was done using low-energy ions at 2 keV. Magnetic measurements were performed at a range of temperatures 5–300 K, using a superconducting quantum interference device (SQUID) with an external applied field up to 50 kOe.

3 Results and Discussion

3.1 Structural Properties

First, we discuss the structural aspects of the samples through TEM and EFTEM observations. Typically,

(a) (b) (c)

50 nm 50 nm

50 nm

Fig. 2 EFTEM images taken at different element edges:aCu,bFe, andcNi for the Cu80Fe10Ni10ribbons annealed at 600^◦C for 24 h

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Annealed at 500 °C

Annealed at 600 °C

Without annealing

H (kOe)

M (emu/g)

Fig. 3 First magnetization curves of Cu80Fe10Ni10ribbons recorded at 5 K, for samples without and with annealing treatment at 500 and 600^◦C. Thesolid linesrepresent a fit of the data to (4)

Cu80Fe10Ni10 ribbons are composed of face−centered cubic (fcc) (Fe,Ni)-rich superparamagnetic nanoparticles, embedded in a Cu-rich fcc matrix. Figure1a shows a low- resolution TEM image of the non-annealed Cu80Fe10Ni10

ribbons. It can be seen that the sample has a grain size in a micrometric range. However, it is not possible with a con- ventional TEM to observe the different existing phases in the sample. This is due to the fact that the two expected phases (Cu and NiFe) have similar crystal structure (fcc) and close lattice parameters. In order to evidence the presence of different phases, we have carried out EFTEM measurements, which allow a selective chemical composition mapping of the different elements Cu, Fe, and Ni (Fig.1b, c, d). As can be observed, for all the images the contrast is changed at the grain boundaries, which can be understood by the presence of precipitates (NiFe) at the grain boundaries. These precipitates have similar crystal structure but different chemical composition. At the Fe and Ni edges (Fig.1c, d), the contrast is bright in the grain boundaries and dark within the grains, showing that the grain boundaries are much richer in Fe and Ni. To determine the effect of annealing on the structure of the Cu80Fe10Ni10 ribbons, we have performed the same EFTEM measurements at different element edges for the sample annealed at 600^◦C for 24 h (Fig.2) At the Cu edge, the contrast is bright within the matrix and dark inside the precipitates, confirming that the matrix is richer in Cu. Similar conclusions can be drawn from the images taken at the Fe and Cu edges where the precipitates have a bright contrast. However, we note that

the annealing process has led to the diffusion of precipitates within the Cu matrix (they are no more localized at the grain boundaries) Moreover, the grain size of precipitates becomes bigger, it has increased from 3 to 21 nm (aver- age values) for the non-annealed and annealed samples, respectively. These observations highlight the importance of annealing treatment on the structure of this system. It will be interesting to study the effect of annealing time on the thermodynamic diffusion of precipitates within the Cu matrix.

3.2 Exchange Constants

Magnetization curves were measured at 5 K in a magnetic field up to 50 kOe for the non-annealed and annealed samples at 500 and 600 ^◦C. As can be seen in Fig. 3, the non-annealed Cu80Fe10Ni10 ribbons exhibit a small magnetization with a high saturation field. After annealing treatment, the magnetization increases and the saturation field decreases, which can be understood by the increase of the NiFe precipitate size, which induces an enhancement of ferromagnetism upon annealing treatment.

In order to determine the exchange constants, we use the Bloch law. The spontaneous magnetization M ver- sus temperature curve follows Bloch’s T^3/2 law, which is mainly associated with thermal excitation of spin waves.

The change of magnetization as a function of temperature for ferromagnetic materials can be expressed using the spin wave theory using the following formula:

M(5K)−M(T )

M(5 K) =BT^3/2. (1)

TheBparameter expressed in (1) is linked to the spin wave stiffness constantDby this relation:

B =2.612 gμB

M(5K)

kB

4π D 3/2

, (2)

where g is the spectroscopic g factor (g = 2), kB is the Boltzmann’s constant, andμBis the Bohr magnetron. The measured data (M-T )have been adjusted using (1), and thus theBparameter is obtained. Using the deduced value ofB in (2), D was calculated for the annealed samples at 500 and at 600^◦C, respectively (Table1). KnowingD, we can now determine the exchange constantA(Table1) using this formula:

A(T )= M(5K)D 2gμB

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Table 1 Some magnetic parameters of Cu₈₀Fe₁₀Ni₁₀ ribbons from (1), at 5 K

Cu80Fe10Ni10 MS(emu/g) B(10⁻⁵K⁻^3/2) D(meV/ ˚A²) A(10⁻⁷erg/cm)

Annealed at 500^◦C 28.2 4.15 191.8 2.1

Annealed at 600^◦C 24.0 3.29 249.1 2.3

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Table 2 Calculated parameters of Cu80Fe10Ni10ribbons from (4), at 5 K

Cu80Fe10Ni10 Hr(kOe) Hex(kOe) KL(10⁵erg/cm³)

Annealed at 500^◦C 3.24 0.38 4

Annealed at 600^◦C 3.67 0.35 3.9

3.3 Magnetic Anisotropy Constant

To determine the magnetic anisotropy constants, we apply the random anisotropy model for the granular Cu80Fe10Ni10

ribbon. The magnetization curves shape approaching saturation has been analyzed using the following expression [37–40]:

M(H )=M0

1− a2

(H +Hex)²

, (4)

a2=H_r² 15 = 1

15 2KL

M0

2

, (5)

where, H is the applied magnetic field, M0 is the saturation magnetization,Hex is the exchange field,Hr is the anisotropy field, and a2 is a constant which is a function of the local anisotropyKLandM0. The magnetization curves for all samples fit nicely with (4) as can be seen in Fig.3. The parametersM0,a2, andHexobtained from the fitting at 5 K are listed in Table2. The deduced values ofM0

anda2were used to determineKLusing (5). The extracted values by this way are also displayed in Table 2. In the granular Cu80Fe10Ni10ribbons, the anisotropy constant calculated from the law of approach to saturation is about 4× 10⁵erg/cm³at 5 K. This value is smaller than that reported in pure Fe (K=5×10⁵erg/cm³at 5 K), indicating that the presence of Ni reduces theKL, which is expected.

Making use of theHexandAmentioned above, the length over which the local anisotropy axes show a correlationRa

can be determined (for nanocrystalline materials,Rais equal

-0.6 -0.4 -0.2 0.0 0.2 -45

-30 -15 0 15 30

45 Cu

Fe Ni

Energy (Ry)

Density of State (states/Ry)

Total DOS Cu80Fe10Ni10

Fig. 4 Total DOS of disordered Cu80Fe10Ni10ribbons

-0.6 -0.4 -0.2 0.0 0.2 -30

-20 -10 0 10 20 30

Energy (Ry)

Cu s p d

Fig. 5 The l-decomposed DOS of Cu (s, p, d)-like states in Cu80Fe10Ni10

to the grain size). According to Chudnovsky’s model [37–

40],Racan be expressed as Ra=

2A

M0Hex

1/2

, (6)

For our nanocrystalline material, Ra is found to be 23 ± 2 nm. Above Ra, where the short range order takes place, the directions of anisotropy are considered to be randomly distributed. One can notice that the obtained value of 23

± 2 nm is comparable to that determined by EFTEM measurements (21 nm).

3.4 Electronic Structure Calculations

In order to investigate both the electronic and magnetic structures of Cu80Fe10Ni10 ribbons, we have carried out ab initio calculations based on the KKR-CPA approximation [41] with the parameterization of Vosko, Wilk, and Nusair (VWN) [42]. The fact that Cu80Fe10Ni10 is considered as a chemical disordered system justifies the choice of this method. The form of the crystal potential is approximated by a muffin-tin potential, and the

-0.6 -0.4 -0.2 0.0 0.2 -30

-20 -10 0 10 20 30

Energy (Ry)

Fe s p d

Fig. 6 The l-decomposed DOS of Fe (s, p, d)-like states in Cu80Fe10Ni10

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-0.6 -0.4 -0.2 0.0 0.2 -40

-20 0 20 40

Energy (Ry)

Ni s p d

Fig. 7 The l-decomposed DOS of Ni (s, p, d)-like states in Cu80Fe10Ni10

wave functions in the respective muffin-tin spheres were expanded in real harmonics up to l = 2, where l is the angular momentum quantum number defined at each site.

HigherKpoints up to 144 in the irreducible part of the first Brillouin zone are considered. Calculations are performed using MACHIKANEYAMA 2002v08 package made by Akai [43].

Figure 4 displays the total density of states (DOS) of Cu80Fe10Ni10, extracted from these calculations. The main contribution is attributed to the Cu atom due to the fact that its concentration is 4 times higher than that of both Fe and Ni. We note that the shape is not symmetrical with respect to energy axis, which is a characteristic of a magnetic system.

Thel-decomposed DOS ofs,p, ord-like states, reported in Figs.5,6, and7, provide a more detailed picture and allow concluding that both Ni and Fe contribution have mainly a dcharacter. Calculated magnetic moment values are found to be 0.0023, 2.35, and 0.042μBfor Cu, Fe, and Ni atoms, respectively; with an antiferromagnetic coupling between Cu and Fe and Ni. The calculated values present a small Fe magnetic moment enhancement and an abrupt Ni magnetic moment reduction. We have also calculated the total magnetization value which is found to be equal to 0.226μB. This value is in line with that determined from magnetization measurements using a SQUID device, which is 0.29± 0.02μB.

4 Conclusions

In summary, the structural and magnetic properties of the granular Cu80Fe10Ni10 ribbons have been investigated. We have demonstrated a strong diffusion of the NiFe precipitates within the Cu matrix, after annealing treatment at 600^◦C for 24 h. The RMA model originally developed for amorphous alloys describes well the experimental M(H )

curves for our nanocrystalline Cu80Fe10Ni10ribbons. More- over, we have also succeeded to determine some essential parameters such as exchange and random anisotropy fields, random anisotropy constant, and local correlation length of anisotropy axes. These values are in agreement with those determined experimentally, evidencing the applicability of the used models. The local anisotropy is found to be about 4

×10⁵erg/cm³5 K.Abinitio calculations demonstrate that the calculated magnetic moment is in line with the measured value. Finally, this work will be helpful for understanding the magnetism of the granular Cu80Fe10Ni10ribbons.

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Annealing Effect on the Structural and Magnetic Properties of Granular Cu80Fe10Ni10 Ribbons