SYNTHESIS AND CHARACTERIZATION OF NANOCRYSTALLINE FePtP FILMS
T. M. SELVAKUMARI*, P. MUTHUKUMAR, S. GANESANa
Department of Physics, Angel College of Engineering & Technology, Tirupur- 641665, Tamilnadu, India
aDepartment of Physics, Government College of Technology, Coimbatore-641001, Tamilnadu, India
In this work we have synthesized FePtP films using sodium hypophosphite as source material for phosphorous. The surface morphology, crystalline structure, grain size and magnetic properties of the plated films prepared at various current densities, bath temperature and concentration of phosphorous source material (NaH2PO2) have been compared. The results shows that the structural & magnetic properties of this thin film depends on the phosphorus content in the film. When the bath temperature increases the phosphorus content decreases & hence coercivity increases. Also coercivity increases after annealing. Reasons for variation in magnetic properties and structural characteristics were discussed.
(Received July 19, 2010; accepted October 21, 2010)
Keywords: FePtP, Electrodeposition, Hard mangetic films, Magnetic properties
1. Introduction
In recent years, the well-known hard magnetic material FePt has received a lot of attention due to its high magneto-crystalline anisotropy. Potential application areas of FePt include high density magnetic storage and their use as micromagnets in micro-electromechanical systems (MEMS) [1-5]. The possibilities of these electroplated materials, retaining hard and soft magnetic properties up to several microns thickness, gives researches opportunity to explore them for micro fabrication of MEMS devices. Recently, much effort is being made to electrodeposits also materials of the group of L10 ordered alloys, like FePt[8,9] and CoPt[10,11], because they exhibit a significantly higher uniaxial magneto crystalline anisotropy. As the formation of the L10 phase is kinetically hindered at room temperature, post annealing of the films is necessary.
Electrodeposited and post annealed FePt and CoPt films can reach coercivities exceeding 1T[11].
Various FePt and CoPt based ternary alloys have been considered for meeting the challenges of improved corrosion resistance and lower stress with superior hard magnetic properties. In the present study we investigated in detail the effects phosphorus on electrodeposited magnetic FePtP films at various bath temperature and current density. Also we discussed their structural and magnetic characterization.
2. Experimental details
FePtP films were electrodeposited on polycrystalline Cu substrate from a single bath containing: H2PtCl6 :0.02M, (NH4)2 SO4 : 0.1 M , FeSO4 : 0.2M, NaH2PO2 : 0.2M and 0.4M The solution pH was adjusted to 3 by adding a small amount hydrochloric acid. Electro-deposition was carried out with varying current densities bath temperature and concentration of NaH2PO2.
The thickness of the deposits was tested using digital micrometer (Mitutoyo, Japan).
Magnetic properties of deposited films were studied using vibrating sample magnetometry. X ray diffractometry (XRD) and scanning electron microscopy (SEM) were used to study the structure and morphology of these magnetic films respectively. From XRD data crystallite size of the deposited FePtP film and its stress were calculated. The integral film composition was measured by energy dispersive X-ray spectroscopy(EDAX). Hardness of the deposit was obtained using Vicker’s hardness tester using diamond intender method. Adhesion of the film was tested by bend test (bending the film with substrate to 1800) and by scratch test (draw equal lines by pin and paste an adhesive tape over the scratch and pull it. If the film comes with tape then adhesion is poor).
These tests are widely used in the field of electroplating [Murphy and Frost (2006)].
3. Results and discussion 3.1 Magnetic properties
Magnetic properties of electrodeposited films were studied using Vibration Sample Magnetometer. Magnetic properties are strongly dependent on bath temperature. The coercivity of electrodeposited FePtP films increased with increase in bath temperature. The bath used for deposition was solution with NaH2PO2: 0.2 M. From the results it was observed that on increasing the current density and bath temperature coercivity and remanent values are increasing, whereas magnetic saturation values were decreasing. Also coercivity increases after annealing.
Electrodeposition were carried out from an electrolyte with 0.4 M NaH2PO2. The deposited samples were subjected to thickness and magnetic studies. The results are presented in Table 1. deposits had low remanent and coercive values, which were very low compared to the values obtained for 0.2 M NaH2PO2. This is because of the higher phosphorus content in the deposit, which affects the magnetic properties of the deposit. The above phenomenon confirms that the FePtP deposits are of hard magnetic nature. The coercivity increases after annealing the samples at 1550C in vacuum for 30 minutes and is shown in Table 2.
Table 1: Magnetic properties of FePtP films NaH2PO2 (M) Temperature
0c
Magnetic saturation
(emu)
Remanent (emu)
Coercivity (Oe)
0.2
30 50 70
.75 .71 .65
0.08 0.12 0.19
700 950 1550
0.4 30
50 70
0.82 0.77 0.72
0.06 0.09 0.15
600 780 1050
Table 2. Magnetic properties of FePtP films after annealing at 1550C NaH2PO2
(M)
Temper ature
0c
Magnetic saturation (emu) Remanent (emu)
Coercivity (Oe)
0.2
30 50 70
0.71 0.68 0.62
0.14 0.17 0.21
1200 1650 2400 0.4 30
50 70
0.72 0.69 0.65
0.13 0.16 0.19
1100 1520 2010
3.2 Surface analysis
3.2.1. Morphological observation
Microstructures of electrodeposited FePtP films obtained from various bath temperatures like 300C, 500C and 700C and various concentrations of NaH2PO2 were examined using scanning electron microscope. The backscattered electron image revealed a finely granular structure of FePtP electrodeposits as shown in Fig. 1. Examination of the micrographs indicated that initially, the deposit obtained from 0.2 M NaH2PO2 with high bath temperature crystallites are smaller and granular. The uniform grains through out the entire region have been noticed with a perfect crystal structure. But the deposit obtained from 0.4 M NaH2PO2 crystallites are bigger and low granular.
(a) (b)
Fig. 1. Microstructures of FePtP films at (a) NaH2PO2 : 0.2 M and (b) NaH2PO2 0.4 M
3.2.2 Structural analysis
Electrodeposited FePtP films were subjected to XRD studies. The X-ray wavelength used was 1.5406 A0 of Cu Kα radiation. Films obtained from various bath temperatures like 300C, 500C and 700C and various concentrations of NaH2PO2 were studied for their structural characteristics as shown in Fig 3. FePtP films had face centered tetragonal structure and exhibited (111) plane predominantly. The (111) plane peak is shifted in all XRD patterns because of phosphorous . In the presence of phosphorous few low intensity peaks like (002) and (221) were also observed because of the formation intermetallic FePtP compound(fct) during electrodeposition.Stress of the films were calculated from XRD pattern using the formula:
Youngs modulas= stress/strain and presented in Table 3. FePtP film prepared from low
concentration of NaH2PO2 and high bath temperature acts as a grain refiner and stress reliver. But on increasing attentiveness of NaH2PO2 stress is also increased.
Fig. 2. XRD pattern for FePtP films of (a) NaH2PO2: 0.2 Mand (b) NaH2PO2:0.4 M
Crystallite sizes of the deposited films were calculated from XRD pattern using the formula i.e., crystalline size= 0.9λ/βcosθ .These values clearly show that the crystallite size of the FePtP deposit by electro-deposition process are in nano scale. The crystallite size of the deposits are given in Table 3. This analysis reveals the effect of phosphorous on the crystallite size of the deposit. The structural properties of FePtP films after annealing the samples at 1550C in vacuum for 30 minutes and is shown in Table 4.
Table 3. Structural and mechanical of FePtP films
NaH2PO2 (mol/L) Bath temperature
0C
Crystalline Size(nm) Stress (MPa)
Vickers Hardness (VHN)
30 34 150 333
50 29 137 355
0.2
70 24 124 380
30 39 160 321
50 32 149 345
0.4
70 29 140 360
Table 4. Structural and mechanical properties of FePtP films after annealing at 1550C.
NaH2PO2 (M) Bath temperature Crystalline Size(nm) Stress (MPa)
Vickers Hardness (VHN)
30 39 168 320
50 35 152 341
0.2
70 29 140 369
30 43 179 312
50 38 163 335
0.4
70 33 151 348
3.3 Mechanical properties
Adhesion of the film with the substrate is tested by bend test and scratch test. It showed that the film is having good adhesion with the substrate. Hardness of these films was examined using a Vicker’s hardness tester by the diamond intender method. The results are reported in Table 3 and 4. The results shows that hardness increases with increasing bath temperature. This may be due to lower stress associated with FePtP film. Hardness of the film decreases when concentration of increases because of high stress.
3.4 Elemental analysis
Elements present in the film were analyzed by energy dispersive X-ray spectroscopy (EDX) and the results showed that the films obtained from high temperatures have low phosphorus content and their magnetic properties are high.
4. Conclusions
We have presented the first report of continuous electrodeposited FePtP films. FePtP film having good hard magnetic properties are electrodeposited from a bath containing the following composition: H2PtCl6 :0.02M, (NH4)2 SO4 : 0.1 M , FeSO4 : 0.2M, NaH2PO2 : 0.2M at the current density 6 mA cm-2 . Increase in the concentration of phosphorous will decrease the hard magnetic properties. This method using a novel bath when compared to vapor deposition methods, opens an alternative route for the production of FePtP films that may be useful for MEMS applications
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