NdFeB thin films, showing a notable out-of-plane c-axis texture, were prepared by radiofrequency plasma-assisted pulsed laser deposition technique. Their optical, morphological and magnetic properties were investigated. Thermal analysis was performed in order to evaluate the thermal behaviour and stability, in air, and in nitrogen dynamic atmospheres. The effects of deposition time, nitrogen and argon plasma use, and substrate temperature, are discussed.
NdFeB thin films have been widely investigated for their applications in micromagnetic, magneto-electronic, and/or microelectromechanical devices, and even magnetic recording media. Various methods, e.g. magnetron sputtering, molecular beam epitaxy, vacuum arc deposition method, pulsed laser deposition (PLD), etc, have been employed to synthesize NdFeB based thin films, as such, or with special additives. During thin film deposition, a number of parameters, e.g. target composition, substrate composition and/or buffer layer material, temperature, laser wavelength, laser fluence, deposition rate, the presence of a radiofrequency (RF) plasma (in reactive or inert gas) during PLD, can affect the structure and magnetic properties of the films]. In literature, most of the work is focused upon the effects of the substrate temperature with respect to the structure and magnetic properties of PLD grown NdFeB thin films; few studies are discussing the influence of deposition rate, laser wavelength, or ambient atmosphere (gas or vacuum), on the structure and magnetic properties of the films. In order to better understand the physical and chemical processes, which are related to heat absorption in a material, thermal analysis of materials is (usually) needed before and/or after functionalizing them; few studies have been published on thermal characterization, and thermal stability enhancement, for rare-earth based alloys in bulk or as thin films. We report here on the thermal behaviour, in air and in nitrogen dynamic atmospheres, of the bulk (target) material, and the effect of RF-PLD in reactive (nitrogen) or inert (argon) gas, during thin film growth, on the microstructure and optical properties.
The alloy that was used for the experiments presented in this paper was produced at ICPE-CA, starting from FeNd, FeB and pure Fe pellets, with additional FeDy, that were melted together into an induction furnace (Leybold-Heraeus). Before the material was ready to be poured into a thick copper disc-shaped crucible for quick solidification, some aluminium powder was added to the composition (instead of gallium, to improve corrosion resistance of * Corresponding author. Tel.: þ40 740 044 770; fax: þ40 318 115 383. E-mail address: firstname.lastname@example.org (C. Constantinescu). Contents lists available at ScienceDirect Current Applied Physics journal homepage: www.elsevier.com/locate/cap 1567-1739/$ e see front matter 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cap.2013.09.002 Current Applied Physics 13 (2013) 2019e2025 the composition). The structure, in mass percents, is Nd30Fe63.6Dy5Al0.4B; the Nd2Fe14B hard magnetic phase is forming only after thermal annealing. The target used for PLD and RF-PLD experiments was a 2 2 cm, 7 mm thick, square piece of alloy. A complete description of the metallurgical procedure and target preparation is presented elsewhere. Simultaneous thermal analysis (sTA) of the NdFeB compound, meaning thermogravimetric analysis (TG), differential thermogravimetric analysis (DTG), differential scanning calorimetry (DSC), and differential thermal analysis (DTA), were carried out in dynamic air and nitrogen atmospheres (150 cm3 min1 ), under nonisothermal linear regimes, using a horizontal Diamond TG/DSC Analyzer from PerkinElmer Instruments. The samples were heated in the temperature range of RTe1000 C, in alumina crucibles. Each time, the employed heating rate was of 10 K min1 . PLD and RF-PLD are techniques that exploit high-power laser pulses, e.g., from a Nd:YAG, an excimer, or any another similar laser, in order to ablate a small amount of matter from a solid target with each pulse. Under proper process parameters (e.g. background gas pressure, substrate temperature, and/or laser fluence), the film has an epitaxial growth and the stoichiometry of the film is a replica of that of the target. We used a Nd:YAG laser (4u/266 nm) in our experimental work, irradiating the targets with 20 000e40 000 pulses. Several series of samples were obtained on platinumcovered silicon substrates. The substrates were firstly cleaned in an ultrasonic bath for 15 min, using acetone and isopropanol as cleaning mediums, rinsed with water, then dried under pressured nitrogen gas. The samples were heated up to 700 C on the substrate holder and kept at a fixed chosen temperature during thin film growth. For the RF-PLD procedure, nitrogen or argon gas was introduced through a MKS mass flow controller setup into a RF plasma torch, with the pressure stabilized at 6 102 mbar (flow rate similar to the one used during TA); for the vacuum deposited samples, the pressure during thin film growth was stabilized at 7 106 mbar. RF power was 75 W, using a CESAR 1310 RF power supply (13.56 MHz, maximum power 1000 W); plasma is generated perpendicular to the ablation plume. A complete description of the PLD/RF-PLD setup is presented elsewhere, in Refs.. Thin film morphology and roughness was analysed by atomic force microscopy (AFM) on a “Nomad” setup produced by “Quesant Instrument Corporation”. These investigations were made in noncontact mode using a silicon carbide tip (10 nm radius of curvature). Scanning electron microscopy (SEM) measurements were performed with an Inspect F FEG-SEM. The electron acceleration voltage can be set between 200 V and 30 kV; the lateral resolution is approximately 2 nm. Optical measurements were performed by using a Woolam Vertical Variable Angle Spectroscopic Ellipsometer (V-VASE), equipped with a high-pressure Xe discharge lamp incorporated in an HS-190 monochromator. Spectroscopicellipsometry (SE) measurements were performed between 350 and 1800 nm spectral range, at energies between 1 and 5 eV (step of 0.01 eV), and fixed angle of incidence (75). Formation of specific crystalline phases was determined by X-ray diffraction (XRD), on a “PAN’alytical X’Pert PRO MRD” setup. Vibrating sample magnetometry (VSM) was performed on a “LakeShore” setup for magnetic characterizations.
3. Results and discussion
3.1. Thermal analysis
The thermal behaviour in air of NdFeB may be observed in the TG, DTG, DSC, and DTA curves, at a heatflow of 10 K min1 from RT and up to 1000 C (Fig. 1). Fig. 1 reveals a 0.5% mass loss from room temperature up to 250 C. This mass loss process consists of two stages: at first, it is slow and continuous (up to 220 C), after which the process is more rapidly due to the destruction of stronger bonds, mainly gases from chemosorption: CO2 and H2O, determined by Fourier-transform infrared spectroscopy (data not shown here); the process is not completely understood at the moment. This decomposition process is complete by 270 C. An
endothermic phase transformation (DH ¼ 0.92 J g1 ) takes place at 300 C, related to the Curie temperature of the material. No chemical reaction takes place between 270 and 380 C. After 400 C, the material progressively gains weight through oxidation: between 540 and 783 C there is 2.67% increase in comparison to initial mass. For the same temperature domain it undergoes an exothermic phase transition, with the enthalpy variation DH ¼ 56.9 J g1 . The enthalpy calculation was performed using the Pyris software of the PerkinElmer TG/DSC equipment. In the range 783e1000 C the nitriding of the material is produced; this phenomenon was confirmed by X-ray diffraction on the thin films. The nitriding process is not accompanied by a thermal effect, while the increase in mass is of 3.147%. Increasing the temperature further, the rate of the mass gain is constant up to the end of the experiment (1000 C). From 400 to 1000 C the material has a (slightly) variable thermal capacity, due to alteration of chemical structure by oxidation and nitriding; the nitriding process does not seem to end at 1000 C, but this was our experimental limit in temperature. The gain is 5.823% for this temperature interval, and the oxidation process seems to generate a compound that is not stoichiometrical (lack of oxygen in the structure). This aspect is related to the granular microstructure of the NdFeB alloy, leading only to superficial oxidation of grains. From 540 C to 783 C the effect is exothermic, after which the process has no thermal effect; there is still a change in the thermal capacity due to chemical changes in the structure, as previously mentioned (interstitial bonding of oxygen and/or among the edge of the grains). The same thermo-analytical investigations of NdFeB, but in nitrogen flow, with a 10 K min1 heating rate, up to 1000 C, are presented in Fig. 2. From RT up to 243 C, a minor mass increase of 0.12% takes place, while up to 341 C the sample is stable, both from thermal and gravimetric point of view. From 341 to 1000 C, the mass gain of 3.243% may be exclusively attributed to the nitriding of the material. For this thermal domain, no thermal effects were observed, but only an increase in the caloric capacity of the material.
3.2. Atomic force microscopy
The AFM revealed smoother surfaces for samples deposited by RF plasma-assisted PLD (regardless of gas nature: nitrogen or argon), compared to vacuum deposited samples. Images presented in Fig. 3 are for samples deposited at 650 C, 2 J/cm2 laser fluence, 40 000 pulses, with and without RF assistance during PLD. No significant difference in roughness appears to be related to the number of pulses, i.e. lower thickness vs. higher thickness of the thin films (images not shown here). SEM images revealed similar characteristics of the films (images not shown here); in cross-section measurements, the thickness was found to be w95 nm.
In ellipsometry, the change of the polarization state of linearly polarized light is measured upon reflection at the surfaces. The complex reflection coefficient r is defined by the equation:
where Rp and Rs are the reflection coefficients for the parallel and perpendicular polarizations, respectively. The quantity r is expressed in the two angles J and D. Although the actual ellipsometry measurement is relatively simple, the analysis of the results is more complicated. An accurate model is required for the system under consideration, which enables simulation or fitting of the results. The optical model we used to measure the samples is made of several layers, as described in Refs.. The values for the optical constants of NdFeB thin films determined by SE were fitted using different procedures; the best fit was obtained with a Lorentz oscillator, having the complex dielectric function written as:
εn_Lorentz = AnBrnEn /(E2n -E2 -iBrnE)
where An is an oscillation amplitude, dimensionless; Br-broadening (expressed in eV), and En is the position of oscillation (eV). The total dielectric function is given by:
ε = ε1+ iε2 = e1 offset +þ εnLorentz
where e1 offset is a purely real constant, equivalent to εinf. The Lorentz parameters and the thickness corresponding to the fit of our experimental data are specified in Table 1. The value determined for roughness by SE is generally smaller than the value determined by AFM, due to the fact that the area scanned by the spectroscopic-ellipsometer is larger . The thickness of a thin film deposited at 40 000 laser pulses, 266 nm laser wavelength, and 2 J/cm2 laser fluence, was found to range between 90 and 95 nm. The experimental and theoretical (fitted) data for J and D are presented in Fig. 4. The refractive indices (n), and extinction coefficients (inset) of the thin film are shown in Fig. 5; with values of 2.43 between 300 and 600 nm, and a maximum of 2.46 at 360 nm.
Thermal analysis of NdFeB in bulk and thin films was performed in order to establish the chemical and physical transformations, and thermal stability of the alloy. Atomic force microscopy and scanning electron microscopy images reveal continuous and smooth surface of the samples, with few droplets; a lower roughness was observed in samples grown by RF-PLD (in argon or nitrogen). Spectroscopicellipsometry investigations reveal a thickness of 90e95 nm for the films, and a refraction index n of 2.43 between 300 and 600 nm, with a maximum of 2.46 at 360 nm. X-ray diffraction evidenced hard magnetic phases only in samples deposited at temperatures between 600 and 800 C. Further studies, on the nitrogen inclusion processes and stoichiometry issues related to the corrosion resistance of NdFeB-based thin films, are due for samples grown by laser- and plasma-assisted techniques.