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First order reversal curve investigation of the hard and soft magnetic phases of annealed CoFeCu nanowire arrays
Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Contents lists available at SciVerse ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
First order reversal curve investigation of the hard and soft magnetic phases of annealed CoFeCu nanowire arrays Q1
M. Almasi-Kashi a,b,n, A. Ramazani a,b, E. Golafshan b, M. Arefpour b, E. Jafari-Khamse a a b
Department of Physics, University of Kashan, Kashan, Iran Institute of Nanoscience and Nanotechnology, University of Kashan, Kashan, Iran
art ic l e i nf o
a b s t r a c t
Article history: Received 11 April 2013 Received in revised form 20 June 2013 Accepted 21 June 2013
(CoFe)1 xCux (x ¼ 0.12–0.84) nanowire arrays were ac-pulse electrodeposited into anodic aluminum oxide templates. The electrodeposition was performed in a constant electrolyte while Cu content was controlled by off-time between pulses. Nanowires with 30 nm diameter and the certain lengths with the both bcc-CoFe and fcc-Cu phases were obtained. Magnetization and coercivity of the nanowires decreased with increasing the Cu content. Annealing improved the coercivity and a remarkable increase in magnetization of nanowires with high Cu contents was observed. A two-phase treatment was seen for annealed samples with high Cu contents. First order reversal curves showed a hard magnetic phase with almost constant magnetic properties and coercivity of 2500 Oe. The results showed that main source of the various magnetic behaviors of the samples may be attributed to increase in soft magnetic phase. A single domain treatment with a narrow interacting field and coercive field distributions was also observed for the annealed samples with high Cu content. & 2013 Published by Elsevier B.V.
Keywords: Annealing Alloy nanowire FORC Magnetic nanowire Coercivity Two-phase behavior
1. Introduction Recently, 1D magnetic nanostructures in particular magnetic nanowires have received considerable attention due to their different magnetic properties when compared with the bulk magnetic materials. High aspect ratio and high ordering of the magnetic nanowires are the main sources to GMR effect [1–4]. The Q2 deposition based on alumina template has been found to be a simple and versatile approach to fabricate the nanowires as perpendicular magnetic media. It has been proved that the acpulse electrodeposition technique improves pore filling, crystalline structure and growth rate of the deposited nanowires [5–7]. Since the coercivity and magnetization of magnetic nanowires remarkably depend on their dimension and composition, various alloy magnetic nanowires with different aspect ratios are prepared [8]. Because of relatively low crystalline energy, high saturation and transition temperatures, CoFe nanowires have recently received more attention [9]. Adding non-magnetic elements (such as Zn [10], P [11], B [12], and Pb [13]) to magnetic nanowires is a common method to control their magnetic properties. Appearance of the various magnetic phases due to presence of the non-magnetic impurity and/or annealing treatment is practically important and offers interesting approaches for researchers [14,15].
n
Corresponding author at: University of Kashan, Department of Physics, Ghotb,
Q5 P.O. Box 87147.71147, Kashan 87317_51167, Iran. Tel./fax: +98 3615552935. E-mail address: [email protected] (M. Almasi-Kashi).
Hysteresis loop as a common way to study the magnetic properties does not present a perfect picture of the magnetic phases and interacting field distribution. A more precise method to investigate the magnetic properties is the first order reversal curve (FORC) which was introduced by Mayergoyz et al. [16] and is known as finger print of the magnetic systems [17]. This method enables studying the coercive field and magnetostatic interaction distributions [18–20]. A single curve of FORC diagram consists of the magnetization as a function of applied (H) and reversal (Hr) fields. The FORC distribution is calculated as follows: ρðH r ; HÞ ¼ ð∂2 MðH r ; HÞÞ=ð∂H r ∂HÞ
ð1Þ
and presented in a contour plot form with 2 orthogonal axes; critical field Hc (Hc ¼ (H Hr)/2) and local interaction field Hu (Hu ¼(H+Hr)/2) [15,16]. This method can be used to study the magnetization behavior of a sample as a contribution of singledomain (SD), multi-domain (MD), pseudo-single domain (PSD), superparamagnetic (SP) particles, coercive field distribution and magnetic interactions between the magnetic domains [18,19]. The effect of various parameters such as particle size [20], structure and temperature [21] on the magnetic properties can be precisely studied by FORC diagrams [22]. In the present work FeCoCu alloy nanowires with various Cu contents were ac-pulse electrodeposited into the porous aluminum oxide (AAO) templates and their magnetic properties and microstructure were examined using the FORC diagrams.
0921-4526/$ - see front matter & 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.physb.2013.06.033
Please cite this article as: M. Almasi-Kashi, et al., Physica B (2013), http://dx.doi.org/10.1016/j.physb.2013.06.033i
M. Almasi-Kashi et al. / Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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3
2
1 Fig. 1. (a) Typical top view AFM image of the nanopore arrays prepared by the two-step anodization method, (b) cross-section SEM macrograph of embedded nanowires in the template and (c) typical cross section SEM micrograph of as-prepared nanowire arrays in which alumina template in regions 1 and 3 was removed by NaOH solution.
2. Experiments
3. Results and discussion
The AAO templates have been prepared using the 2-step anodization process [21]. 0.3 mm thick highpurity aluminum foils (99.9999%) were electropolished in a 1:4 volume mixture of perchloric acid and ethanol. The foils were then anodized in a 0.3 M oxalic acid solution at 17 1C for 10 h at 40 V. The anodized foils were then immersed in a mixture of 0.2 M chromic and 0.5 M phosphoric acid solutions at 60 1C for 10 h to remove the anodized layer. The foils were then re-anodized as the first step for 2 h. Following the second anodization step, voltage was systematically reduced to 12 V to promote thinning of the barrier layer. The ac-pulse electrodeposition technique was employed to deposit CoFeCu nanowires into the nanopores. The off-time between pulses was 10, 150, 250, 400 ms and the reduction and oxidation times were 5 ms. A sine waveform with 18/18 V reduction/oxidation voltage was employed during the electrodeposition process. The electrolyte was a solution composed of 52.971 g/l CoSO4 7H2O, 32.551 gr/l FeSO4. 7H2O, 2.500 g/l CuSO4 5H2O, 40.00 g/l H3BO3 and 1.00 g/l C6H8O6. The pH value of electrolyte solution was adjusted to be 2.5 and the effective electrodeposition time (the total electrodeposition time minus entire off-time between pulses) was 120 s. Scanning probe microscopy (SPM) and scanning electron microscopy (SEM) were employed to investigate the morphology of the samples. The structure and composition of the CoFeCu nanowire arrays were studied by X-ray diffraction (XRD) pattern and electron dispersive spectroscopy (EDS). An Alternating gradient force magnetometer (AGFM) was employed to measure the magnetic properties of the as-deposited and annealed samples at 580 1C in a mixture of 15% hydrogen and 85% argon gases for 30 min.
Fig. 1(a) shows typical top view AFM image of the nanopore arrays prepared by the two-step anodization method. Hexagonally arranged nanopores with almost 30 nm diameter and 100 nm interpore distances can be seen. For clarification, a typical cross section SEM micrograph of the nanowires embedded in nanopore arrays is shown in Fig. 1(b) using back-scattered electrons to maximize atomic number contrast. The cross sectional view presents the well-developed nanopores in profile, indicating non-intercrossing and parallel holes with high aspect ratio. The structure of AAO membranes and CoFeCu nanowires could be seen in this cross section. The Cu content of the nanowires with off-time between pulses was investigated by EDS. It is noticeable that alumina template was removed to enhance the precision of composition investigation of the nanowires. Fig. 1(c) shows a typical cross section SEM micrograph of the dissolved CoFeCu nanowire arrays in NaOH solution. As seen in regions 1 and 3 in which the template was properly removed the released nanowires behave as strong magnets and attracted each other, while the remaining alumina in region 2 prevented the nanowires to approach together (nanowires in this region are remarkably parallel). Of course this region, located far from the activation volume of X-ray then has no significant effect on the EDS results. The EDS microanalyses of prepared samples at 10 and 400 ms off-times are typically shown in Fig. 2 and the obtained EDS results of all the samples are tabulated in Table 1. It is found that deposition rate of the copper and cobalt-iron is not the same during the process. The Cu content of nanowires initially enhances from 12 at% at 10 ms off-time up to 82 at% at 250 ms off-time and then the increment rate reduces.
Please cite this article as: M. Almasi-Kashi, et al., Physica B (2013), http://dx.doi.org/10.1016/j.physb.2013.06.033i
3
C Co
Fe
Co
C
Co
C
Fe
Energy (keV)
Intensity (a. u.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
Intensity (a. u.)
M. Almasi-Kashi et al. / Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎
C Fe
Co
Fig. 3. X-ray diffraction pattern of as-prepared CoFeCu nanowires fabricated at 10 ms and 250 ms off times.
Fe
Energy (keV) Fig. 2. EDS patterns of the CoFeCu nanowire arrays deposited at (a) 10 and (b) Q3 400 ms off-times.
Table 1 Composition of the CoFeCu nanowire arrays as a function of off-time between pulses. Off-time (ms)
10
150
250
400
Fe content (at%) Co content (at%) Cu content (at%)
51 37 12
41 39 30
8 10 82
7 9 84
Since same electrolyte was used to deposit all the samples, increasing of Cu content with off-time may be related to effective deposition competition between Cu and Co–Fe atoms. It seems that replacement of magnetic Co and Fe atoms with non-magnetic Cu atoms has occurred during the off-time between pulses through electroless process. However, the rate of this replacement reduces during the electrodeposition. Based on this point we can control Cu content of CoFeCu alloy nanowires by controlling the off-time between pulses. As a complementary investigation, the X-ray diffraction pattern of the samples confirms the EDS results (see Fig. 3). X-ray diffraction pattern of the samples shows that with increasing the off-time between pulses from 10 to 250 ms, the intensity of fcc-Cu (1 1 1) and (2 0 0) peaks dominantly increases while that of bcc-CoFe (1 1 0) peak reduces which indicates replacement of Co and Fe atoms with Cu and or amorphous phase formation of CoFe due to Cu addition. Hysteresis loops of alloy nanowires were obtained with external magnetic field applied parallel to the wires' axis (out-of-plane) at room temperature. The hysteresis loops of the both as-prepared and annealed samples are displayed in Fig. 4. As shown coercivity, magnetization and squareness of the as-prepared samples decrease with increasing of the off-time. Reduction of magnetization with off-time also confirms EDS result. Magnetization of the nanowires prepared at 10 and 250 ms off-times is seen to improve up to 10% and 60%, respectively after annealing. Coercivity also
shows the same treatment with higher increment rate. After annealing, it increases from 2100 Oe to 2800 Oe for the sample synthesized at 10 ms off-time while it increases from 1200 Oe to 2400 Oe for the sample synthesized at 400 ms off-time. In nanowires with higher Cu content, the magnetic atoms were individually distributed between non-magnetic Cu atoms and could not show their real contribution in the magnetization. Formation of magnetic CoFe clusters during the annealing enhances the saturation magnetization and coercivity of these samples. In contrast, due to formation of relatively large magnetic grains, annealing has weakly affected the obtained magnetic clusters of the samples with low Cu content which result in small increase in the magnetization. To investigate the effect of annealing on the microstructure of nanowires, X-ray diffraction pattern of the annealed nanowires prepared at 10 and 250 ms off-times are displayed in Fig. 5. Comparing with as-prepared samples (see Fig. 3), the annealing magnifies the peak intensity of bcc-FeCo (1 1 0) especially in the case of sample prepared at 250 ms offtime. Two-phase behavior of the annealed samples prepared at higher off-times is notable for their magnetic treatment. Hysteresis loop of the annealed samples (Fig. 4b) shows an increase in the soft phase at 150, 250 and 400 ms off-times, while that of the sample prepared at 10 ms off-time shows only a single hard magnetic phase. It therefore may be said that appearance of the soft phase in annealed samples is a direct result of Cu enhancement with off-time. For detailed investigation, FORC diagrams were employed. It should be noted that the nanowire arrays embedded into the AAO template were used for FORC analysis. Since the hysteresis loop of the as-prepared samples shows the same shape with a single magnetic phase, typically FORC diagram of the sample prepared at 150 ms off-time is shown in Fig. 6(a). To plot this diagram the maximum applied field was 3000 Oe with 125 Oe intervals and smoothing factor was 2. To clarify, spreading of distribution function along the Hc axis is shown in Fig. 6(b). The irreversible component of FORC diagrams consists of contours close to the central peak which relatively distributed in the Hu direction indicating single domain (SD) systems with relatively low interacting behavior [22,23]. The experimental results show a peak at 1170 Oe along the Hc axis (HcFORC) in coincidence with the global array coercivity (HcArray) obtained through hysteresis loop [20]. The coincidence between two coercivities shows symmetric coercivity distribution along the Hc direction with full width at half maximum (FWHM) of 1000 Oe (see Fig. 6(b)). It also shows a narrow distribution along the Hu direction which correlated with
Please cite this article as: M. Almasi-Kashi, et al., Physica B (2013), http://dx.doi.org/10.1016/j.physb.2013.06.033i
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M. Almasi-Kashi et al. / Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 4. The out-of-plan hysteresis loops of (a) as-prepared and (b) annealed CoFeCu nanowire arrays prepared at different off-times.
Fig. 5. X-ray diffraction pattern of annealed CoFeCu nanowires fabricated at 10 ms and 250 ms off times.
magnetostatic interaction between the nanowires [24]. Since magnetostatic interaction between the nanowires is directly proportional with magnetization [25] then (CoFe)30Cu70 nanowires with low magnetization does not expect to show an interacting treatment. On the other hand wide distribution along the Hc axis could be related to existence of large amount of Cu which causes to form the nanowires with different coercivities. Fig. 7 shows the FORC diagram of the annealed samples prepared at 10–400 ms off-times in which the applied field is parallel with nanowires axis. A single domain phase which remarkably distributed along the Hu axis is seen for the (CoFe)82Cu18 nanowires prepared at 10 ms off-time (see Fig. 7(a)). The cross sectional view of FORC distribution along the Hu axis is presented in Fig. 8. As can be seen diagram width reduces with increasing the off-time. Spreading of diagram of the (CoFe)82Cu18 nanowires in Hu direction with 2100 Oe FWHM indicates high magnetostatic interaction between the
Fig. 6. (a) Out-of-plan FORC diagrams and (b) cross section FORC distribution along the Hc axis of CoFeCu nanowire arrays synthesized at 150 ms off-time.
nanowires which is expected for samples with higher magnetization. This treatment is almost the same as what was reported elsewhere for the only FeCo nanowire arrays [26]. The FORC diagram of the annealed (CoFe)30Cu70 nanowires shows notable reduction (to 400 Oe) in
Please cite this article as: M. Almasi-Kashi, et al., Physica B (2013), http://dx.doi.org/10.1016/j.physb.2013.06.033i
M. Almasi-Kashi et al. / Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 7. Out-of-plan FORC diagrams of annealed CoFeCu samples fabricated at (a) 10, (b) 150, (c) 250 and (d) 400 ms off-times.
3.0x10-7
Table 2 Population of both weak and strong regions of the annealed nanowires with variation of Cu content.
Off-time 10 ms 150 250 400
2.0x10-7
Off-time (ms)
ρ
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
5
10 150 250 400
1.0x10-7
Population (%) Strong region
Weak region
100 89.06 86.54 65.97
– 10.94 13.46 34.03
0.0 -2000
-1000
0
1000
2000
Hu (Oe) Fig. 8. The change of ρ (FORC distribution) along the Hu axis in Hcarray for the annealed samples.
FWHM of distribution function in the Hu direction (Figs. 7(b) and 8). Reduction in FWHM of distribution function along the Hu direction of (CoFe)18Cu82 and (CoFe)16Cu84 nanowires is also seen in Figs. 7(c), (d) and 8. Due to increase in Cu content with off-time, reduction in the width is not far from expectation. It is also notable that single magnetic phase of Fig. 7(a) has changed to 2-phase behavior for the annealed samples prepared at 150, 250 and 400 ms off-times (Fig. 7 (b), (c) and (d)). FORC diagrams represent increasing the off-time causes to form a weak soft phase in turn spreading the FORC
distribution along the Hc direction. Coercivity of the annealed samples is distributed in weak and strong regions. Population of the regions was estimated and tabulated in Table 2. As can be seen, majority of population is related to the hard phase (strong region). Increasing the off-time between pulses; increasing the Cu content (as non-magnetic element) of nanowires, leads to reduce the population of the hard phase and increase the coercive field distribution of the weak region. For more detailed investigation of coercivity distribution variation, HcFORC and Hcarray are compared and tabulated in Table 3. As shown, the Hcarray decreases monotonously with increasing the Cu content, but HcFORC does not significantly vary. Increasing the Cu content (due to increase in off-time) increases the Hcarray from 1450 to 2800 Oe ( 100% increment), while HcFORC varies around 2600 Oe. Since a weak soft phase with low population is present in all the samples (see Table 2), then HcFORC is calculated as coercivity of the strong hard
Please cite this article as: M. Almasi-Kashi, et al., Physica B (2013), http://dx.doi.org/10.1016/j.physb.2013.06.033i
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
Table 3 Variation in Hcarray and HcFORC of the CoFeCu nanowires with Cu content of nanowires. Cu content (at%)
Hcarray (Oe)
HcFORC (Oe)
12 30 82 84
2820 2400 2310 1450
2300 2800 2600 2500
6- The hard phase of CoFeCu alloy nanowires with high Cu contents showed non-interacting single domain behavior.
Acknowledgment Authors are grateful to the University of Kashan for supporting this work by Grant no. (159023/3).
phase. A relatively weak soft phase is observed around the Hc ¼0 in Fig. 7(b) which magnifies in Fig. 7(c) and (d). Therefore, it may be said that variation in Hcarray (average coercivity of all nanowire arrays) of the annealed samples is correlated with appearance of a soft phase especially at higher off-times. After annealing, since the strong hard phase remains almost unchanged with increasing of Cu content (from 12% to 84%), then change in hysteresis loop of the annealed samples may be related to increase in the soft phase. The magnetic treatment of the annealed nanowires can be summarized as following: in the low Cu contents, existence of CoFe phase in CoFeCu solid solution increases coercivity up to a maximum of 2500 Oe. A soft phase appears in annealed Cu-rich CoFeCu alloy nanowires due to formation of small clusters of Co and Fe with low coercivity during the annealing. 4. Conclusions (CoFe)1 xCux nanowires with different Cu contents were ac-pulse electrodeposited through variation of off-times between pulses. Microstructures and magnetic properties of the prepared nanowires were examined and led to the following results: 1- Increasing the off-time between pulses increased the Cu content of nanowires from 12% to 84% by substitution of Co and Fe atoms with Cu during the electrodeposition. 2- X-ray analysis revealed formation of CoFeCu solid solution with separate Cu and CoFe phases. 3- Increasing the Cu content reduced coercivity and magnetization of the CoFeCu nanowires. 4- Annealing led to form two separate soft and hard phases especially in the samples with high Cu content. 5- FORC analysis confirmed existence of a hard phase in all the annealed CoFeCu nanowires and a soft one in the annealed samples with high Cu contents.
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Please cite this article as: M. Almasi-Kashi, et al., Physica B (2013), http://dx.doi.org/10.1016/j.physb.2013.06.033i
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Contents lists available at SciVerse ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
First order reversal curve investigation of the hard and soft magnetic phases of annealed CoFeCu nanowire arrays Q1
M. Almasi-Kashi a,b,n, A. Ramazani a,b, E. Golafshan b, M. Arefpour b, E. Jafari-Khamse a a b
Department of Physics, University of Kashan, Kashan, Iran Institute of Nanoscience and Nanotechnology, University of Kashan, Kashan, Iran
art ic l e i nf o
a b s t r a c t
Article history: Received 11 April 2013 Received in revised form 20 June 2013 Accepted 21 June 2013
(CoFe)1 xCux (x ¼ 0.12–0.84) nanowire arrays were ac-pulse electrodeposited into anodic aluminum oxide templates. The electrodeposition was performed in a constant electrolyte while Cu content was controlled by off-time between pulses. Nanowires with 30 nm diameter and the certain lengths with the both bcc-CoFe and fcc-Cu phases were obtained. Magnetization and coercivity of the nanowires decreased with increasing the Cu content. Annealing improved the coercivity and a remarkable increase in magnetization of nanowires with high Cu contents was observed. A two-phase treatment was seen for annealed samples with high Cu contents. First order reversal curves showed a hard magnetic phase with almost constant magnetic properties and coercivity of 2500 Oe. The results showed that main source of the various magnetic behaviors of the samples may be attributed to increase in soft magnetic phase. A single domain treatment with a narrow interacting field and coercive field distributions was also observed for the annealed samples with high Cu content. & 2013 Published by Elsevier B.V.
Keywords: Annealing Alloy nanowire FORC Magnetic nanowire Coercivity Two-phase behavior
1. Introduction Recently, 1D magnetic nanostructures in particular magnetic nanowires have received considerable attention due to their different magnetic properties when compared with the bulk magnetic materials. High aspect ratio and high ordering of the magnetic nanowires are the main sources to GMR effect [1–4]. The Q2 deposition based on alumina template has been found to be a simple and versatile approach to fabricate the nanowires as perpendicular magnetic media. It has been proved that the acpulse electrodeposition technique improves pore filling, crystalline structure and growth rate of the deposited nanowires [5–7]. Since the coercivity and magnetization of magnetic nanowires remarkably depend on their dimension and composition, various alloy magnetic nanowires with different aspect ratios are prepared [8]. Because of relatively low crystalline energy, high saturation and transition temperatures, CoFe nanowires have recently received more attention [9]. Adding non-magnetic elements (such as Zn [10], P [11], B [12], and Pb [13]) to magnetic nanowires is a common method to control their magnetic properties. Appearance of the various magnetic phases due to presence of the non-magnetic impurity and/or annealing treatment is practically important and offers interesting approaches for researchers [14,15].
n
Corresponding author at: University of Kashan, Department of Physics, Ghotb,
Q5 P.O. Box 87147.71147, Kashan 87317_51167, Iran. Tel./fax: +98 3615552935. E-mail address: [email protected] (M. Almasi-Kashi).
Hysteresis loop as a common way to study the magnetic properties does not present a perfect picture of the magnetic phases and interacting field distribution. A more precise method to investigate the magnetic properties is the first order reversal curve (FORC) which was introduced by Mayergoyz et al. [16] and is known as finger print of the magnetic systems [17]. This method enables studying the coercive field and magnetostatic interaction distributions [18–20]. A single curve of FORC diagram consists of the magnetization as a function of applied (H) and reversal (Hr) fields. The FORC distribution is calculated as follows: ρðH r ; HÞ ¼ ð∂2 MðH r ; HÞÞ=ð∂H r ∂HÞ
ð1Þ
and presented in a contour plot form with 2 orthogonal axes; critical field Hc (Hc ¼ (H Hr)/2) and local interaction field Hu (Hu ¼(H+Hr)/2) [15,16]. This method can be used to study the magnetization behavior of a sample as a contribution of singledomain (SD), multi-domain (MD), pseudo-single domain (PSD), superparamagnetic (SP) particles, coercive field distribution and magnetic interactions between the magnetic domains [18,19]. The effect of various parameters such as particle size [20], structure and temperature [21] on the magnetic properties can be precisely studied by FORC diagrams [22]. In the present work FeCoCu alloy nanowires with various Cu contents were ac-pulse electrodeposited into the porous aluminum oxide (AAO) templates and their magnetic properties and microstructure were examined using the FORC diagrams.
0921-4526/$ - see front matter & 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.physb.2013.06.033
Please cite this article as: M. Almasi-Kashi, et al., Physica B (2013), http://dx.doi.org/10.1016/j.physb.2013.06.033i
M. Almasi-Kashi et al. / Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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1 Fig. 1. (a) Typical top view AFM image of the nanopore arrays prepared by the two-step anodization method, (b) cross-section SEM macrograph of embedded nanowires in the template and (c) typical cross section SEM micrograph of as-prepared nanowire arrays in which alumina template in regions 1 and 3 was removed by NaOH solution.
2. Experiments
3. Results and discussion
The AAO templates have been prepared using the 2-step anodization process [21]. 0.3 mm thick highpurity aluminum foils (99.9999%) were electropolished in a 1:4 volume mixture of perchloric acid and ethanol. The foils were then anodized in a 0.3 M oxalic acid solution at 17 1C for 10 h at 40 V. The anodized foils were then immersed in a mixture of 0.2 M chromic and 0.5 M phosphoric acid solutions at 60 1C for 10 h to remove the anodized layer. The foils were then re-anodized as the first step for 2 h. Following the second anodization step, voltage was systematically reduced to 12 V to promote thinning of the barrier layer. The ac-pulse electrodeposition technique was employed to deposit CoFeCu nanowires into the nanopores. The off-time between pulses was 10, 150, 250, 400 ms and the reduction and oxidation times were 5 ms. A sine waveform with 18/18 V reduction/oxidation voltage was employed during the electrodeposition process. The electrolyte was a solution composed of 52.971 g/l CoSO4 7H2O, 32.551 gr/l FeSO4. 7H2O, 2.500 g/l CuSO4 5H2O, 40.00 g/l H3BO3 and 1.00 g/l C6H8O6. The pH value of electrolyte solution was adjusted to be 2.5 and the effective electrodeposition time (the total electrodeposition time minus entire off-time between pulses) was 120 s. Scanning probe microscopy (SPM) and scanning electron microscopy (SEM) were employed to investigate the morphology of the samples. The structure and composition of the CoFeCu nanowire arrays were studied by X-ray diffraction (XRD) pattern and electron dispersive spectroscopy (EDS). An Alternating gradient force magnetometer (AGFM) was employed to measure the magnetic properties of the as-deposited and annealed samples at 580 1C in a mixture of 15% hydrogen and 85% argon gases for 30 min.
Fig. 1(a) shows typical top view AFM image of the nanopore arrays prepared by the two-step anodization method. Hexagonally arranged nanopores with almost 30 nm diameter and 100 nm interpore distances can be seen. For clarification, a typical cross section SEM micrograph of the nanowires embedded in nanopore arrays is shown in Fig. 1(b) using back-scattered electrons to maximize atomic number contrast. The cross sectional view presents the well-developed nanopores in profile, indicating non-intercrossing and parallel holes with high aspect ratio. The structure of AAO membranes and CoFeCu nanowires could be seen in this cross section. The Cu content of the nanowires with off-time between pulses was investigated by EDS. It is noticeable that alumina template was removed to enhance the precision of composition investigation of the nanowires. Fig. 1(c) shows a typical cross section SEM micrograph of the dissolved CoFeCu nanowire arrays in NaOH solution. As seen in regions 1 and 3 in which the template was properly removed the released nanowires behave as strong magnets and attracted each other, while the remaining alumina in region 2 prevented the nanowires to approach together (nanowires in this region are remarkably parallel). Of course this region, located far from the activation volume of X-ray then has no significant effect on the EDS results. The EDS microanalyses of prepared samples at 10 and 400 ms off-times are typically shown in Fig. 2 and the obtained EDS results of all the samples are tabulated in Table 1. It is found that deposition rate of the copper and cobalt-iron is not the same during the process. The Cu content of nanowires initially enhances from 12 at% at 10 ms off-time up to 82 at% at 250 ms off-time and then the increment rate reduces.
Please cite this article as: M. Almasi-Kashi, et al., Physica B (2013), http://dx.doi.org/10.1016/j.physb.2013.06.033i
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C Co
Fe
Co
C
Co
C
Fe
Energy (keV)
Intensity (a. u.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
Intensity (a. u.)
M. Almasi-Kashi et al. / Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎
C Fe
Co
Fig. 3. X-ray diffraction pattern of as-prepared CoFeCu nanowires fabricated at 10 ms and 250 ms off times.
Fe
Energy (keV) Fig. 2. EDS patterns of the CoFeCu nanowire arrays deposited at (a) 10 and (b) Q3 400 ms off-times.
Table 1 Composition of the CoFeCu nanowire arrays as a function of off-time between pulses. Off-time (ms)
10
150
250
400
Fe content (at%) Co content (at%) Cu content (at%)
51 37 12
41 39 30
8 10 82
7 9 84
Since same electrolyte was used to deposit all the samples, increasing of Cu content with off-time may be related to effective deposition competition between Cu and Co–Fe atoms. It seems that replacement of magnetic Co and Fe atoms with non-magnetic Cu atoms has occurred during the off-time between pulses through electroless process. However, the rate of this replacement reduces during the electrodeposition. Based on this point we can control Cu content of CoFeCu alloy nanowires by controlling the off-time between pulses. As a complementary investigation, the X-ray diffraction pattern of the samples confirms the EDS results (see Fig. 3). X-ray diffraction pattern of the samples shows that with increasing the off-time between pulses from 10 to 250 ms, the intensity of fcc-Cu (1 1 1) and (2 0 0) peaks dominantly increases while that of bcc-CoFe (1 1 0) peak reduces which indicates replacement of Co and Fe atoms with Cu and or amorphous phase formation of CoFe due to Cu addition. Hysteresis loops of alloy nanowires were obtained with external magnetic field applied parallel to the wires' axis (out-of-plane) at room temperature. The hysteresis loops of the both as-prepared and annealed samples are displayed in Fig. 4. As shown coercivity, magnetization and squareness of the as-prepared samples decrease with increasing of the off-time. Reduction of magnetization with off-time also confirms EDS result. Magnetization of the nanowires prepared at 10 and 250 ms off-times is seen to improve up to 10% and 60%, respectively after annealing. Coercivity also
shows the same treatment with higher increment rate. After annealing, it increases from 2100 Oe to 2800 Oe for the sample synthesized at 10 ms off-time while it increases from 1200 Oe to 2400 Oe for the sample synthesized at 400 ms off-time. In nanowires with higher Cu content, the magnetic atoms were individually distributed between non-magnetic Cu atoms and could not show their real contribution in the magnetization. Formation of magnetic CoFe clusters during the annealing enhances the saturation magnetization and coercivity of these samples. In contrast, due to formation of relatively large magnetic grains, annealing has weakly affected the obtained magnetic clusters of the samples with low Cu content which result in small increase in the magnetization. To investigate the effect of annealing on the microstructure of nanowires, X-ray diffraction pattern of the annealed nanowires prepared at 10 and 250 ms off-times are displayed in Fig. 5. Comparing with as-prepared samples (see Fig. 3), the annealing magnifies the peak intensity of bcc-FeCo (1 1 0) especially in the case of sample prepared at 250 ms offtime. Two-phase behavior of the annealed samples prepared at higher off-times is notable for their magnetic treatment. Hysteresis loop of the annealed samples (Fig. 4b) shows an increase in the soft phase at 150, 250 and 400 ms off-times, while that of the sample prepared at 10 ms off-time shows only a single hard magnetic phase. It therefore may be said that appearance of the soft phase in annealed samples is a direct result of Cu enhancement with off-time. For detailed investigation, FORC diagrams were employed. It should be noted that the nanowire arrays embedded into the AAO template were used for FORC analysis. Since the hysteresis loop of the as-prepared samples shows the same shape with a single magnetic phase, typically FORC diagram of the sample prepared at 150 ms off-time is shown in Fig. 6(a). To plot this diagram the maximum applied field was 3000 Oe with 125 Oe intervals and smoothing factor was 2. To clarify, spreading of distribution function along the Hc axis is shown in Fig. 6(b). The irreversible component of FORC diagrams consists of contours close to the central peak which relatively distributed in the Hu direction indicating single domain (SD) systems with relatively low interacting behavior [22,23]. The experimental results show a peak at 1170 Oe along the Hc axis (HcFORC) in coincidence with the global array coercivity (HcArray) obtained through hysteresis loop [20]. The coincidence between two coercivities shows symmetric coercivity distribution along the Hc direction with full width at half maximum (FWHM) of 1000 Oe (see Fig. 6(b)). It also shows a narrow distribution along the Hu direction which correlated with
Please cite this article as: M. Almasi-Kashi, et al., Physica B (2013), http://dx.doi.org/10.1016/j.physb.2013.06.033i
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M. Almasi-Kashi et al. / Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 4. The out-of-plan hysteresis loops of (a) as-prepared and (b) annealed CoFeCu nanowire arrays prepared at different off-times.
Fig. 5. X-ray diffraction pattern of annealed CoFeCu nanowires fabricated at 10 ms and 250 ms off times.
magnetostatic interaction between the nanowires [24]. Since magnetostatic interaction between the nanowires is directly proportional with magnetization [25] then (CoFe)30Cu70 nanowires with low magnetization does not expect to show an interacting treatment. On the other hand wide distribution along the Hc axis could be related to existence of large amount of Cu which causes to form the nanowires with different coercivities. Fig. 7 shows the FORC diagram of the annealed samples prepared at 10–400 ms off-times in which the applied field is parallel with nanowires axis. A single domain phase which remarkably distributed along the Hu axis is seen for the (CoFe)82Cu18 nanowires prepared at 10 ms off-time (see Fig. 7(a)). The cross sectional view of FORC distribution along the Hu axis is presented in Fig. 8. As can be seen diagram width reduces with increasing the off-time. Spreading of diagram of the (CoFe)82Cu18 nanowires in Hu direction with 2100 Oe FWHM indicates high magnetostatic interaction between the
Fig. 6. (a) Out-of-plan FORC diagrams and (b) cross section FORC distribution along the Hc axis of CoFeCu nanowire arrays synthesized at 150 ms off-time.
nanowires which is expected for samples with higher magnetization. This treatment is almost the same as what was reported elsewhere for the only FeCo nanowire arrays [26]. The FORC diagram of the annealed (CoFe)30Cu70 nanowires shows notable reduction (to 400 Oe) in
Please cite this article as: M. Almasi-Kashi, et al., Physica B (2013), http://dx.doi.org/10.1016/j.physb.2013.06.033i
M. Almasi-Kashi et al. / Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Fig. 7. Out-of-plan FORC diagrams of annealed CoFeCu samples fabricated at (a) 10, (b) 150, (c) 250 and (d) 400 ms off-times.
3.0x10-7
Table 2 Population of both weak and strong regions of the annealed nanowires with variation of Cu content.
Off-time 10 ms 150 250 400
2.0x10-7
Off-time (ms)
ρ
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
5
10 150 250 400
1.0x10-7
Population (%) Strong region
Weak region
100 89.06 86.54 65.97
– 10.94 13.46 34.03
0.0 -2000
-1000
0
1000
2000
Hu (Oe) Fig. 8. The change of ρ (FORC distribution) along the Hu axis in Hcarray for the annealed samples.
FWHM of distribution function in the Hu direction (Figs. 7(b) and 8). Reduction in FWHM of distribution function along the Hu direction of (CoFe)18Cu82 and (CoFe)16Cu84 nanowires is also seen in Figs. 7(c), (d) and 8. Due to increase in Cu content with off-time, reduction in the width is not far from expectation. It is also notable that single magnetic phase of Fig. 7(a) has changed to 2-phase behavior for the annealed samples prepared at 150, 250 and 400 ms off-times (Fig. 7 (b), (c) and (d)). FORC diagrams represent increasing the off-time causes to form a weak soft phase in turn spreading the FORC
distribution along the Hc direction. Coercivity of the annealed samples is distributed in weak and strong regions. Population of the regions was estimated and tabulated in Table 2. As can be seen, majority of population is related to the hard phase (strong region). Increasing the off-time between pulses; increasing the Cu content (as non-magnetic element) of nanowires, leads to reduce the population of the hard phase and increase the coercive field distribution of the weak region. For more detailed investigation of coercivity distribution variation, HcFORC and Hcarray are compared and tabulated in Table 3. As shown, the Hcarray decreases monotonously with increasing the Cu content, but HcFORC does not significantly vary. Increasing the Cu content (due to increase in off-time) increases the Hcarray from 1450 to 2800 Oe ( 100% increment), while HcFORC varies around 2600 Oe. Since a weak soft phase with low population is present in all the samples (see Table 2), then HcFORC is calculated as coercivity of the strong hard
Please cite this article as: M. Almasi-Kashi, et al., Physica B (2013), http://dx.doi.org/10.1016/j.physb.2013.06.033i
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
Table 3 Variation in Hcarray and HcFORC of the CoFeCu nanowires with Cu content of nanowires. Cu content (at%)
Hcarray (Oe)
HcFORC (Oe)
12 30 82 84
2820 2400 2310 1450
2300 2800 2600 2500
6- The hard phase of CoFeCu alloy nanowires with high Cu contents showed non-interacting single domain behavior.
Acknowledgment Authors are grateful to the University of Kashan for supporting this work by Grant no. (159023/3).
phase. A relatively weak soft phase is observed around the Hc ¼0 in Fig. 7(b) which magnifies in Fig. 7(c) and (d). Therefore, it may be said that variation in Hcarray (average coercivity of all nanowire arrays) of the annealed samples is correlated with appearance of a soft phase especially at higher off-times. After annealing, since the strong hard phase remains almost unchanged with increasing of Cu content (from 12% to 84%), then change in hysteresis loop of the annealed samples may be related to increase in the soft phase. The magnetic treatment of the annealed nanowires can be summarized as following: in the low Cu contents, existence of CoFe phase in CoFeCu solid solution increases coercivity up to a maximum of 2500 Oe. A soft phase appears in annealed Cu-rich CoFeCu alloy nanowires due to formation of small clusters of Co and Fe with low coercivity during the annealing. 4. Conclusions (CoFe)1 xCux nanowires with different Cu contents were ac-pulse electrodeposited through variation of off-times between pulses. Microstructures and magnetic properties of the prepared nanowires were examined and led to the following results: 1- Increasing the off-time between pulses increased the Cu content of nanowires from 12% to 84% by substitution of Co and Fe atoms with Cu during the electrodeposition. 2- X-ray analysis revealed formation of CoFeCu solid solution with separate Cu and CoFe phases. 3- Increasing the Cu content reduced coercivity and magnetization of the CoFeCu nanowires. 4- Annealing led to form two separate soft and hard phases especially in the samples with high Cu content. 5- FORC analysis confirmed existence of a hard phase in all the annealed CoFeCu nanowires and a soft one in the annealed samples with high Cu contents.
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Please cite this article as: M. Almasi-Kashi, et al., Physica B (2013), http://dx.doi.org/10.1016/j.physb.2013.06.033i