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Cu multilayer nanowires
Accepted Manuscript First-Order-Reversal-Curve (FORC) diagrams of alternative chain of soft/ hard magnetic CoFe/Cu multilayer nanowires E. Jafari-Khamse, M. Almasi Kashi, A. Ramazani PII:
S1567-1739(15)30127-9
DOI:
10.1016/j.cap.2015.12.001
Reference:
CAP 4125
To appear in:
Current Applied Physics
Received Date: 10 June 2015 Revised Date:
20 November 2015
Accepted Date: 2 December 2015
Please cite this article as: E. Jafari-Khamse, M.A. Kashi, A. Ramazani, First-Order-Reversal-Curve (FORC) diagrams of alternative chain of soft/ hard magnetic CoFe/Cu multilayer nanowires, Current Applied Physics (2016), doi: 10.1016/j.cap.2015.12.001. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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First-Order-Reversal-Curve (FORC) diagrams of alternative chain of soft/ hard magnetic CoFe/Cu multilayer nanowires E. Jafari-Khamse1, M. Almasi Kashi1,2, *, A. Ramazani1,2
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Department of physics, University of Kashan, Kashan, Iran
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Institute of nanoscience and nanotechnology, University of Kashan, Kashan, Iran *
[email protected]
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Abstract
The effect of interactions on the soft and hard phases and interference region that commonly
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appears in the First Order Reversal Curve (FORC) of interacting two-phase magnetic systems was investigated. To obtain an interacting two-phase system, a new method was introduced for the first time to electrodeposit a two-phase magnetic nanowire (NW) composed of hard and soft phases with high magnetization into nanopores of the anodized aluminum oxide template using
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the single-bath ac-pulse electrodeposition technique. Two-phase behavior was obtained by multilayer and grainy configurations of the CoFe and Cu layers as two type layers with controllable thickness through the related pulse numbers. It was found that interphase interaction
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can be observed in FORC diagrams with three factors; (i) the shift in center of the soft phase feature along the interaction field axis without the change in coercivity, (ii) shift in center of the
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hard phase feature along the coercivity axis and (iii) appearance of an additional interference region. However, order of the shifts directly correlates with the order of demagnetizing intraphase interaction through the hard phase and magnetic moment contribution of the soft phase. The interference region contribution was found strongly correlates with irreversible magnetic moment contribution of the soft and hard phases. Keywords: Two-phase magnetic nanowires; FORC; Interference.
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1. Introduction Following the results coming from the previous studies on the first-order-reversal-curve
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(FORC) diagrams as a new experimental tool for characterizing the interaction and coercive field distributions, two magnetic phase materials have recently attracted special attention. Information obtained from decoding the interactions in the FORC diagrams make it very attractive in
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characterizing the hard/soft two-phase magnets with interphase interaction [1-5]. Despite variety of the multi-phase materials (as magnetic soft/hard nanocomposites and multilayer
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nanowires/thin films with sequence of the magnetic/nonmagnetic or magnetic/magnetic segments), they share some of the features corresponding to the switching/interaction fields of the constituent phases [6, 7]. However, the interphase interactions produce two elongated FORC features with opposite FORC distribution signs depending on strength of the coupling between the constituents. In the recent years a great deal of attention has been focused on developing the
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new nanostructures for use in the future applications as giant Magnetoresistance (GMR) [8], drug delivery [9], adjusting the magnetic domain wall movement [10] and data storage [11]. Among different types of the nanostructures, magnetic/nonmagnetic multilayer nanowires (MNW) offer
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an additional degree of freedom due to high ordering degree of the elements with high aspect
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ratio along a nanowire (NW).
Different techniques containing ac- or dc-pulse electrodeposition from a single-or dual-bath have been introduced to prepare these nanostructures with controlled geometry [12-16]. However, growth in a single bath has a benefit of avoiding oxidation during the exchange between baths. Employing the ac-pulse electrodeposition with off-time between pulses also provides an opportunity to precise control of the purity and composition in each segment of the multilayer configuration [17, 18]. 2
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FORC diagrams were usually used to obtain detailed and compact information about magnetic behavior of the samples. To obtain a FORC, the sample was saturated and then the H was dropped to a reversal field (Hr). This process was repeated for several times while in each
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step, saturation was achieved in a lower Hr. The FORC distribution was generally evaluated as second derivative of the magnetization (M) respect to H and Hr as follows [19]:
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r
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1 2 ρ = − ∂ M ( H , H r ) ∂H ∂H
(1)
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and then plotted as a counter plot in a rotated coordinate system; Hc= (H+Hr)/2 and Hu= (H-Hr)/2 as coercive and interaction field axes, respectively. The coercivity is defined as the magnetic intensity needed to reduce to zero the magnetic flux density of a fully magnetized magnetic specimen or to demagnetize a magnet. However, the reversal field is the required magnetic
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intensity to align the magnetic moments along the applied field.
As reported in several references [20-22] considering mean-field approach through a magnetic system with wide distribution of coercivity (as two-phase magnetic systems), interaction between
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the phases can be decoded from FORC diagrams through the shift direction in location of the soft and hard phase features and appearance of an additional interference region. However,
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considering nearest neighbor interactions can be interpreted segmentation in the FORC distribution along the coercive field axis [22]. The reported results clearly show FORC approach as a powerful technique for qualitative and quantitative decoding the interactions in the magnetic systems. In this regard, interaction between the magnetic segments along a NW and also NW arrays predominantly determine magnetic behavior of the whole system. The synthesis of MNWs with alternative soft/hard magnetic segments separated in nanoscale distances opens an area to study the interphase interaction where appears in the FORC diagrams of two-phase magnetic 3
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systems as a feature connected to the feature related to the soft phase and extended away from the coercivity axis depending on the type and coupling strength [23]. Stancu et al. [24] reported that extension of the interference region strongly depends on FORC distribution ratio of the soft
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and hard phases. However, the effect of interactions on the constituent phases and interference region leaves an open new research area to FORC distribution of these interacting systems.
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Therefore, in this paper we experimentally focus on this possibility, investigation on the effect of interactions on the soft and hard phases. Alternation of two type layers with different magnetic
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behaviors along a NW produces two-phase behavior. Furthermore, the proposed method allows us to tune the magnetic properties of the NWs formed in the porous alumina template. We also observed that the existence of the coupled reversal process at each phase can be understood by analyzing the FORC diagrams.
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2. Experimental
Common two-step anodization in 0.3 M oxalic acid was used to prepare the highly ordered anodized aluminum oxide (AAO) template. Details of the experiment have been reported
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elsewhere [25]. First and second anodization times were 5 and 3 h, respectively. The obtained template has 30 and 100 nm pore diameter and interpore distances, respectively. Based on the
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second anodization time, the average length of the pores was evaluated to be about 20 µm [26]. A new method was introduced for the first time to prepare a NW with two magnetic phases along the length composed of a hard phase and a soft phase with high magnetization. A single-bath containing 0.178 M FeSO4.7H2O, 0.178 M CoSO4.7H2O, 0.0075 M CuSO4.7H2O, 45 g.l-1 boric acid and 1 g.l-1 ascorbic acid was used as an electrolyte with 2.5 constant pH value. Nanowires were obtained using the ac-pulse electrodeposition technique. In this method, during the pulse
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time, metal species were reduced on the cathode surface and no deposition process occurred during the off-time between pulses [27]. The pulse-and off-times were chosen in a manner that they were sufficient for reduction and recovery of the metal ions at the surface of cathode,
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respectively. However, the magnetic elements were dissolved and replaced by Cu during the offtime [28]. Therefore, to obtain multilayer configuration, two different reduction/oxidation voltages and off-times were used to obtain the alternative pure CoFe and Cu segments,
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respectively [29].
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CoFe / Cu multilayer NWs were then ac-pulse electrodeposited into nanopores with different CoFe and Cu pulses to control thickness of the each segment. Impurity of each one was controlled through the maximum deposition current during the reduction in the constant 10 and 32 mA values to deposit the Cu and CoFe segments, respectively. MNWs were ac-pulse electrodeposited into the templates using 14 V as oxidation sinusoidal pulse with 0 and 96 ms
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off-time between pulses for CoFe and Cu segments, respectively. Asymmetric pulses were selected to reduce reoxidation possibility of the metals. Highly pure segments were obtained by the proposed method as reported in our previous works [15, 16 and 27]. The soft and hard
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magnetic phase behaviors were obtained by various thickness ratios of the similar CoFe/Cu
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layers in two type layers.
Two MNWs with different pulse ratios of the CoFe and Cu segments were prepared; 5/20 and 800/100 pulses of CoFe/Cu as samples 1 and 2, respectively. The pulse ratio of the magnetic/nonmagnetic segments was selected in a manner that MNWs with high magnetization and different magnetic behaviors as soft and hard magnetic wires were obtained. Regarding to the pulse ratio, rod-like configuration of the magnetic segments in second type layer was expected. However in the other type, formation of the magnetic particles distributed in a Cu 5
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matrix is not far from expectation. In order to obtain an interacting two-phase magnetic system, two mentioned layers were alternatively deposited along a NW with four different pulse numbers of the first to second type layers; 400 to 1 as sample 3, 400 to 3 as sample 4, 200 to 1 as sample 5
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and 800 to 1 as sample 6. The overall structure was repeated 10 times to increase the magnetic moment of the whole systems. Regarding different thicknesses of the two type segments, different total thicknesses of the nanowires ranging from ~ 2 µm to ~ 6 µm was expected.
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Typical time dependence of the deposited charge for the each type layer in each repetition of sample 4 is shown in Fig. 1a. It clearly shows multi-segment deposition of the charge into the
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nanopores which can be clearly observed in the second type layer segment. However, that of the first type layer has not exactly showed a perfect periodic ordering due to low pulse numbers of the magnetic element and limitation of the recording system. To clarify this issue, Fig. 1b shows magnified deposited charge related to one set of the layers. The inset figure schematically shows
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expected nanowire configuration along the length.
The microstructure and chemical composition of the samples were studied by X-ray diffraction (XRD) and electron dispersive spectroscopy (EDS), respectively. The layer thickness
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of the obtained MNWs was estimated by transmission electron microscope (TEM). To prepare
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the NWs for TEM measurement, the template were etched in 0.3 M NaOH solution for about 2h and then ultrasonically dispersed in distilled-water before dropping on a carbon coated copper grid. The magnetic measurements including hysteresis loops and FORC diagrams were performed by vibrating sample magnetometer (VSM) at room temperature. Maximum applied field (H) of 6000 Oe and field steps of 200 Oe were used to obtain the detailed FORC. However, smoothing factor value of 2 was selected to draw the detailed diagrams with the minimum noise
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contribution. It is worthy to note that the NW arrays embedded into the AAO template were used to magnetic measurements.
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3. Results and discussion 3.1. Single-phase magnetic nanowires
Figure 2 shows XRD patterns of the samples 1 and 2. Separate peaks related to magnetic
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(CoFe) and non-magnetic (Cu) elements can be observed in both the patterns (See Fig. 2 (a) and
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(b)) implying independent formation of two CoFe and Cu phases. As can be seen, the intensity ratio of CoFe to Cu peaks follows the same trend of the related pulse ratios; 0.12 and 10 for the samples 1 and 2, respectively. It can be related to increase in thickness of the sequential CoFe and Cu layers with increasing the related pulse numbers. Moreover, in the case of sample 1, difference between the peak intensity of two elements and estimated crystallite size of CoFe (~
from the pulse ratio.
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13 nm) guide us to conclude that it maybe resulted from grainy structure of the layer as expected
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Elemental analysis of the samples was performed by EDS. The obtained results for the samples 1 and 2 are presented in Fig. 3. As shown, the atomic percentages of the sample 1
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synthesized at 5/20 pulse numbers of the magnetic/nonmagnetic elements were 28.94 at.% and 71.06 at.%, respectively while that of the sample 2 synthesized at 800/ 100 pulse numbers of the magnetic/nonmagnetic elements were 94.47at.% and 5.53at.%, respectively. It clearly indicates significant reduction in Cu to CoFe content ratio with increasing the CoFe pulse numbers from 5 to 800. The observed difference between the pulse numbers and EDS results may be related to Cu substitution of the magnetic elements during the electrodeposition process. The EDS analysis
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also confirmed same weight percentages of Co and Fe in the magnetic segments as expected from the same electrodeposition rate of Fe2+ and Co2+ ions in the electrolyte.
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Hysteresis loops of the samples 1 and 2 when H applied parallel (Out of plane, OOP) and perpendicular (In plane, IP) to the wires easy axis are shown in Fig. 4. Magnetic moment of the samples has been normalized respect to that of the sample 2. Regarding large difference between
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the remanent magnetic moment and coercivity of the sample 2 in two OOP and IP measurement directions, dominant shape anisotropy along the wires axis is clearly observed. However, low
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difference between the coercivity and remanent magnetic moment of the sample 1 in two directions clearly confirmed grainy configuration of the CoFe distributed in Cu medium. Lower saturation field along the wires axis has been resulted from the magnetized interaction between the magnetic grains.
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To obtain detailed information of interaction field, FORC diagrams were measured in OOP state at room temperature. FORC diagram of the sample 1 (Fig. 5 (a)) shows a single feature weakly distributed along the both Hc and Hu axes indicating a soft magnetic material as expected
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from Fig. 4a. However, sample 2 (Fig. 5 (b)) shows a single domain treatment with high coercivity as a hard phase which highly distributed along the Hu axis. In the case of sample 2,
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FORC diagram with a “wishbone” shape can clearly observe illustrating presence of the demagnetizing mean-field interactions [28]. As reported by Bêron et al. [30], non-uniform length distribution may be a possible source for the observed tail along the Hc axis. Low distribution of the tail in higher coercivities also demonstrates uniformity in thickness of the magnetic segments deposited along a NW.
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Deconvolution of FORC data to separate reversible and irreversible components can be obtained from the proposed method in ref [31] only near the descending branch of the major loop in H → Hr limit. The intrinsic shape anisotropy of nanowires along the wires axis plays an
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important role in identifying the reversible and irreversible contributions [32]. In addition, magnetostatic interaction and size of the magnetic grains affect the reversible contribution in magnetization reversal process. Each part of major loop was reconstructed from integration of
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magnetic moment changes along the descending field branch of FORC plot and shown in Fig. 6.Comparing the results revealed that the reversible contribution reduces from 72% for sample 1
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to 14% for sample 2 confirming grainy and rod-like structure of the samples 1 and 2, respectively. The fifty percentage of reversible contribution in magnetization reversal process for non-interacting single domain particles is reported to be dominated by the shape anisotropy. Therefore, the layers configuration mainly determines hysteric behavior of the whole system.
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3.2. Two-phase magnetic nanowires
XRD patterns of the samples synthesized under different pulse numbers of two type layers,
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samples 1 and 2, along a NW are depicted in sequence in Fig. 7. Separate CoFe and Cu peaks are observed which their intensity varies depending on pulse number of the each type layer.
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Changing the pulse numbers of each type layer has not affected the crystallography growth directions of the CoFe and Cu. As expected, each peak intensifies with increasing the number of the related pulses suggesting increase in the deposited material and thickness. TEM images of the samples 3 and 5 are depicted in Fig. 8. Based on difference in atomic number of the magnetic and non-magnetic elements, multilayer structure with dark and light contrast is seen in sequence. Uniform diameter along the NW length (in both the segments) is
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clearly seen which it follows pore diameter of the template (~ 30 nm). Absence of multilayer structure in second type layer as soft phase can be clearly observed due to (i) low pulse numbers of the first type layer which led to a grainy structure as confirmed by magnetic measurements
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and (ii) Cu substitution during the electrodeposition process as mentioned in EDS results. Thickness of the each layer for the all samples based on the ones depicted in TEM images is evaluated and tabulated in Table 1. As can be seen, thickness of the each layer increases with
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increasing the related pulse numbers. It should be noted that thicknesses of two phases are
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consistent with the expectation of repetition of the pulse number when the samples are grown. OOP and IP hysteresis loops of the all two-phase samples are shown in Fig. 9. Magnetic moment of the samples has been normalized to that of related to the sample 4. Difference in magnetic moment of the all samples can be explained by the different magnetic elements in the samples. Comparing of the saturation magnetization of the single- and multi-phase samples
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confirmed that magnitude of the saturation magnetization is proportional to the composition of the magnetic elements. Moreover, the origin of the kink observed in the M-H hysteresis (Fig 9 (b) and (c)) can be explained by difference between remanent magnetic moment and coercivity
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of two phases in the sample. Dominant shape anisotropy along the wires axis is clearly observed
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confirming rod shape of the magnetic segments in hard phase layer which led to favorable magnetic easy axis direction was kept along the wires axis with the change in thickness of the each type layer. Remanent to saturation magnetic moment ratio as squareness increases from 0.45 to 0.69 from sample 3 to 4. However, it decreases to 0.57 and 0.23 with reducing the hard phase contribution indicating reducing its contribution in the magnetization process. FORC diagrams of the samples with various thicknesses of the soft and hard phases which measured in OOP state are depicted in Fig. 10. It shows that thickness ratio of two phases 10
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significantly affects magnetic behavior of the whole system. As shown, the contour plots consist of three different contours ones close to the central peak related to soft phase and two others along the Hc axis in high coercivities related to hard phase and the last in a 150o angle relative to
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the applied field axis related to interphase interaction. As can be seen, color of the each contour varies with the change in the thickness ratio of the two phases. Hc of the irreversible peak at higher portion along the Hc axis, hard phase, as HcFORC, the average coercivity obtained from
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hysteresis loop as HcArray, difference between two coercivities as ∆, magnetic moment contribution ( = ∬ , ) of each phase and interference region and the shift in
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center of the soft phase along the Hu axis were tabulated in Tables 2 and 3. In order to estimate the M contribution of the each feature, 3-dimensional Gaussian functions were fitted to the features along the Hc axis related to the soft and hard phases and interference region, then volume under the each surface was considered as their total M contribution based on
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deconvolution of the overlapped surfaces. Calculation of magnetic moment contribution of the soft and hard phases in the magnetization process regarding the reversible contribution of the soft phase in Table 3 is also suggests proportionality between the composition and saturation
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magnetization of the elements. Comparing two coercivities obtained from hysteresis loops and FORC diagrams indicates strong dependence on the M contribution of the soft phase. The results
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in Table 2 show that ∆ value grows up with increasing the soft phase contribution in agreement with our previous expectation. However, small weight percentages of the soft phase in samples 4 and 5 led to reduce contribution of this phase in the coercivity distribution. Cross-sectional view of the FORC diagrams parallel to Hc axis crossing from maximum distribution of the feature related to the soft phase clearly shows two features weakly overlapped along the axis (Fig. 11 (a)). However, cross-sectional view of the FORC diagrams parallel to Hu 11
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axis crossing from maximum distribution of the feature located in high coercivity related to hard phase is shown in Fig. 11(b). It clearly shows that of sample 2highly distributed along the Hu axis (FWHM ~ 1720Oe) indicating high demagnetizing magnetostatic interaction between the
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magnetic segments separated by non-magnetic parts. FORC diagrams of the samples combining the hard and soft phases indicate two separate features along the Hc axis related to the soft and hard phases and an additional interference region connected to the feature located in low
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coercivity indicating demagnetizing magnetostatic interaction between two phases [32, 33]. This region is accompanied with a negative distribution as a positive/negative coupled distribution
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which their distribution ratio seems depending on the overlap strength of two phases. Interaction between the soft and hard phases in these NWs in OOP measurement state can be generally classified to two main groups; magnetizing interaction between the segments along a NW length and demagnetizing one between the NW arrays. However, magnetostatic interaction
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between the NWs coupled through the soft magnetic segments can be obtained as total interaction field of soft and hard phases considering total M contribution of each phase presented
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in Table 3 [34].
Magnetostatic interaction can be calculated using the relation as follows [35]:
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Hint = Ms r2 (a (d) l + b (d) tanh (l/ld)) where Ms is the saturation magnetization of the ferromagnetic material, r is the nanowire radius, d is average interwire spacing and ld the dipolar length. a and b are phenomenological functions of d. Regarding to the known length and magnetization contribution of the soft and hard magnetic segments of the two-phase nanowires (Tables 1 and 2), magnetostatic interaction between two phases can be calculated.
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Increasing thickness of the hard phase segment increases the demagnetizing interaction between the hard segments of the NW arrays; FWHM of the distribution along the Hu axis as a qualitative estimation of the interaction increases from 710 Oe for sample 3to 1390Oe for sample 4.
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However, FORC diagram of the sample 5 shows weaker soft phase due to reduction in total length of the related segments and an interacting hard phase regarding high FWHM of 1310 Oe in Hu direction. Increasing the thickness of the soft phase segment (Sample 6) decreases the
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coupling between the hard segments thereby decreases the magnetostatic interaction between the NW arrays. However, demagnetizing interaction between the coupled hard segments reduces; the
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FWHM decreases to 587Oe. Considering constant diameter of the NWs and increasing the magnetic moment of sample 6 comparing with sample 3, reducing the interaction between the coupled hard segments of NW arrays(reducing the FWHM to ~ 587Oe) is due to decrease in the length of the coupled hard phases through the each wire. Two-step magnetic hysteresis loops and
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FORC diagrams in Figs. 9 and 10 clearly show almost independent reversal magnetizations in the soft and hard phase segments. Fig. 12 (a) shows FWHM of the distribution along the Hu axis qualitatively follows the same trend of the interaction field regarding M contribution and length
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of the each phase. Fig. 12 (b) also depicts that average coercivity of the array nanowires measured along the wires axis as magnetic easy axis exactly resulted from average magnetostatic
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interaction obtained from the coupled magnetic segments along the NW. Interphase interaction can also be evaluated from FORC diagrams by three different factors including shift of the soft phase feature along the Hu axis in a constant coercivity, shift of hard phase feature along the Hc axis and M contribution in the interference region. As can be seen, the magnetostatic interphase interaction in two-phase samples led to shift the coercivity distribution off from Hc axis. Increasing thickness of the hard phase segment in sample 4 led to more shift the 13
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center of the hard phase towards the Hu = 0from 1912 to 1673Oe along the Hc axis while that of the soft phase moves towards +Hu from 178 to 700 Oe without change in coercivity. However, the features related to the hard and soft phases move in opposite directions with the change in the
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soft phase thickness indicating increase in the magnetizing interphase interaction while the other is until dominant in agreement with the above mentioned relation (See Table 3).
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In an axial magnetized NW arrays, coupled hard segments along the length of a long nanowire act in a manner that the opposite magnetic sources localized in two ends of the wire thereby
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produces a demagnetizing field in neighboring nanowires. This field can be observed as a shift in coercivity distribution respect to Hc axis [20]. Shift of the soft phase feature along the Hu axis is dependent on the interaction between the hard segments presented as FWHM of the hard phase feature and also M contribution of the soft phase. Comparing the interference region, region 3 in Fig. 10 (a), in samples with different ratios of M contribution of soft to hard phases also revealed
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that of the interference region is proportional with irreversible portion of the soft phase appeared in the FORC diagrams. The obtained results are in agreement with the ones obtained from the experiment proposed by Stancu et al. [21]confirming that the interphase interaction illustrated as
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interference region in FORC diagrams is directly depended on the ratio between M contribution
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of two phases. However, comparing the interference region contribution and soft and hard contributions revealed that the interference region contribution simultaneously is dependent on that of the soft phase and interaction between hard and soft phases. It is not far from expectation that interaction between two phases depends on magnetic moment contribution of the one phase and the field proportional to the magnetization of the other phase. Each reconstructed part of major loop of two-phase nanowires is also shown in Fig. 13. The change in thickness of the hard and soft layers from sample 3 to 5 (according to Table 1) 14
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indicates dominant contribution of the shape anisotropy along the wires axis led to almost the similar reversibility contribution of theses samples (29-31%). However, increasing the soft phase contribution increases the role of the grainy structure and maybe the number of the small
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particles against the dominant shape anisotropy which increases the reversibility to 68%. Therefore, comparing the soft phase and reversibility contributions confirms the impact effect of
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the irreversible part of the soft phase on the interference region. 4. Conclusions
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MNWs were ac pulse electrodeposited into the nanopores of AAO template with two different types of layers as soft and hard magnetic phases deposited along the length of a NW. Considering different magnetic behavior of the two type layers, two magnetic phase behavior was obtained. Coupling strength between soft and hard phases as interference region
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corresponding with interphase interaction was controlled by thickness of each phase related to number of the each layer along a NW. M contribution of each FORC feature was estimated from deconvolution of the Gaussian peaks centered in each feature. The results showed that the
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interphase interaction was strongly depended on the M contribution ratio of the soft and hard phases. However, magnetostatic interaction between the hard phases of the NW arrays is
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dependent on the hard segments coupled through the soft segments. Interphase interaction can be observed in FORC diagrams with the shift in center of the soft phase feature along the interaction field axis without the change in coercivity, shift in center of the hard phase feature along the coercivity axis and appearance of an additional interference region. However, order of the shifts directly correlates with the order of demagnetizing intraphase interaction through the hard phase and magnetic moment contribution of the soft phase. The interference region contribution was
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also found strongly correlates with irreversible magnetic moment contribution of the soft and hard phases.
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Acknowledgment Authors are grateful to the University of Kashan for supporting this work by Grant No
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(159023/31).
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[17]H. Chiriac, N. Lupu, L. Stoleriu, P. Postolache, A. Stancu, “Experimental and micromagnetic first-order reversal curves analysis in NdFeB-based bulk ‘‘exchange spring’’type permanent magnets”, J. Magn. Magn. Mat., 316 (2007) 177–180. [18] S.Alikhanzadeh-Arani, M. Almasi-Kashi, A. Ramazani, “Magnetic characterization of FeCo nanowire arrays by first-order reversal curves”, Curr. Appl. Phys. 13 (2013) 664-669.
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[19]S. Samanifar, M. Almasi Kashi, A. Ramazani, M. Alikhani, “Reversal modes in FeCoNi nanowire arrays: Correlation between magnetostatic interactions and nanowires length”, J. Magn. Magn.Mater. 378 (2015) 73–83. [20]D. A. Gilbert, G. T. Zimanyi, R. K. Dumas, M.Winklhofer, A. Gomez, N.Eibagi, J. L.Vicent, K. Liu, “Quantitative Decoding of Interactions inTunable Nanomagnet Arrays Using FirstOrder Reversal Curves”, Sci. Report. DOI: 10.1038/srep04204.
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[21]C. R. Pike, A. P. Roberts, K. L.Verosub, “Characterizing interactions in finemagnetic particle systems using first order reversal curves”, J. Appl. Phys. 85 (1999) 6660–6667. [22]C.-I. Dobrot, A. Stancu, “What does a first-order reversal curve diagram really mean? A study case: Array of ferromagnetic nanowires”, J. Appl. Phys., 113, (2013) 043928-043938.
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[23]F. Béron, L.-P.Carignan, D. Ménard, A. Yelon, “Extracting Individual Properties from Global Behaviour: First-order Reversal Curve Method Applied to Magnetic Nanowire Arrays”, Source: Electrodeposited Nanowires and Their Applications, Book edited by: NicoletaLupu, ISBN 978-953-7619-88-6, pp. 228, February 2010, INTECH, Croatia. [24]L. Clime, F. Béron, P. Ciureanu, M. Ciureanu, R.W. Cochrane, A. Yelon, “Characterization of individual ferromagnetic nanowires by in-plane magnetic measurements of arrays”, J. Magn. Magn. Mat., 299 (2006) 487–491. [25] I. Bodale, L. Stoleriu, A. Stancu, “Reversible and Irreversible Components Evaluation in Hysteretic Processes Using First and Second-Order Magnetization Curves”, IEEE Trans. Magn., 47 (2011) 192-197. [26] E. Jafari-Khamse, M. Almasi Kashi, A. Ramazani, “Angular dependence of interactions in polycrystalline Co nanowire arrays”, Mat. Chem. Phys., 159 (2015) 128-138. 18
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[27]Fanny Béron,L. Clime, M. Ciureanu, D. Ménard, R. W. Cochrane, A. Yelon, “Reversible and quasireversible information in first-order reversal curve diagrams”, J. Appl. Phys., 101 (2007) 09J107-09J109.
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[28] D. A. Gilbert, G. T. Zimanyi, R. K. Dumas, M. Winklhofer, A. Gomez, N. Eibagi, J. L. Vicent, K. Liu, “Quantitative Decoding of Interactions in Tunable Nanomagnet Arrays Using First Order Reversal Curves”, Sci. Rep. [29]A. Ramazani, M. Almasi Kashi, F. Eghbal and E. Jafari-Khamse, “The effect of deposition parameters on themagneticbehavior of CoFe/Cu multilayer nanowires”, Eur. Phys. J. Plus130 (2015) 2-9.
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[30]F.Béron, D. Ménard, A.Yelon, “First-order reversal curve diagrams of magnetic entities with meaninteraction field: A physical analysis perspective”, J. Appl. Phys. 103 (2008) 07D90807D910. [31]M.Winklhofer,R. K. Dumas, K. Liu, “Identifying reversible and irreversible magnetization changes in prototypepatterned media using first- and second-order reversal curves”, J. Appl. Phys. 103 (2008) 07C518-07C520. [32]M.Kumari, M.Widdrat,E. Tompa, R. Uebe, D. Schuler,M.Posfai, D.Faivre, A. M.Hirt, “Distinguishing magnetic particle size of iron oxide nanoparticleswith first-order reversal curves”, J. Appl. Phys. 116 (2014) 124304-124309.
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[33]D.R. Cornejo, R.D. Noce, T.R.F. Peixoto, N. Barelli, P.T. A. Sumodjo, A.V. Benedetti, “First order reversal curve analysis of nanocrystalline Pd80Co20 alloy films”, J. Alloy. Compd. 479 (2009) 43–48.
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[34] L. Clime, S. Y. Zhao, P. Chen, F. Normandin, H. Roberge, T. Veres, “The interaction field in arrays of ferromagnetic barcode nanowires”, Nanotechnol., 18 (2007) 435709-435715.
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[35] L. Clime, F. Beron, P. Ciureanu, M. Ciureanu, R. W. Cochrane, A. Yelon, “Characterization of individual ferromagnetic nanowires by in-plane magnetic measurements of arrays”, Journal of Magnetism and Magnetic Materials 299 (2006) 487–491.
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Figure captions Fig. 1. (a) Typical time dependence of the deposited charge and (b) magnified deposited charge
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of a set. Fig.2. XRD pattern of sample (a) 1 and (b) 2. Fig. 3. EDS spectrum of sample (a) 1 and (b) 2.
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Fig. 4.OOP and IP hysteresis loops of the sample (a) 1 and (b) 2 normalized to magnetic moment
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of the sample 2.
Fig. 5.FORC diagram of the sample (a) 1 and (b) 2 measured in OOP state. Fig. 6.(a, c) Magnetic moment changes along descending-field branch of the FORC plot decomposed into reversible and irreversible components and (b, d) reconstructed reversible and
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irreversible parts of the major loop of (a, b) sample1 and (c, d) sample 2. Fig. 7.XRD patterns of the samples 3, 4 and 5.
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Fig. 8.TEM images of the samples numbered as (a) 3 and (b) 5. Fig. 9. OOP and IP hysteresis loops of the samples 3-6. Magnetic moments were normalized to
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that of the sample 4.
Fig. 10.FORC diagrams of the samples (a) 3, (b) 4, (c) 5 and (d) 6 measured in OOP state. The features numbered as 1, 2 and 3 correspond to soft, hard and interference regions, respectively. Fig.11. Cross-sectional view of FORC diagrams along the (a) Hcand (b) Huaxes crossing from maximum distribution.
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Fig. 12. Variation of (a) interaction field (Hint) between the hard phase segments and half width of hard phase distribution along the Hu axis (FWHM/2) and (b) total Hint and average coercivity
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(Hc) of samples numbered as 1-6. Fig. 13.Reconstructed reversible and irreversible parts of the major loop of (a, b) sample 3, (c, d) sample 4, (e, f) sample 5 and (g, h) sample 6. Magnetic moment changes along descending-field
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branch of the FORC plot decomposed into reversible and irreversible components presented in
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Table.1. Thickness of the both type layers obtained from TEM images.
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Hc (Oe) HcFORC HcArray ∆ = HcFORC-HcArray
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Table. 2. Comparison between HcArray and HcFORCand M contribution of the reversible portion in the all samples. Sample 4 1670 1500 170
Sample 5 2200 1880 320
Sample 6 2080 620 1460
Soft phase shift along the +Hu axis (Oe)
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Table 3.M contribution of each FORC feature, soft phase shift along the +Hu axis.
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A new method was introduced for the first time to electrodeposit a two-phase magnetic nanowire.
•
Two-phase behavior was obtained by multilayer and grainy configurationsof CoFe
The shifts order correlates with order of demagnetizing intraphase interaction through the hard phase.
The interference region contribution was found correlates with irreversible
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magnetization of the soft and hard phases.
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and Cu layers.
S1567-1739(15)30127-9
DOI:
10.1016/j.cap.2015.12.001
Reference:
CAP 4125
To appear in:
Current Applied Physics
Received Date: 10 June 2015 Revised Date:
20 November 2015
Accepted Date: 2 December 2015
Please cite this article as: E. Jafari-Khamse, M.A. Kashi, A. Ramazani, First-Order-Reversal-Curve (FORC) diagrams of alternative chain of soft/ hard magnetic CoFe/Cu multilayer nanowires, Current Applied Physics (2016), doi: 10.1016/j.cap.2015.12.001. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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First-Order-Reversal-Curve (FORC) diagrams of alternative chain of soft/ hard magnetic CoFe/Cu multilayer nanowires E. Jafari-Khamse1, M. Almasi Kashi1,2, *, A. Ramazani1,2
2
Department of physics, University of Kashan, Kashan, Iran
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1
Institute of nanoscience and nanotechnology, University of Kashan, Kashan, Iran *
[email protected]
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Abstract
The effect of interactions on the soft and hard phases and interference region that commonly
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appears in the First Order Reversal Curve (FORC) of interacting two-phase magnetic systems was investigated. To obtain an interacting two-phase system, a new method was introduced for the first time to electrodeposit a two-phase magnetic nanowire (NW) composed of hard and soft phases with high magnetization into nanopores of the anodized aluminum oxide template using
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the single-bath ac-pulse electrodeposition technique. Two-phase behavior was obtained by multilayer and grainy configurations of the CoFe and Cu layers as two type layers with controllable thickness through the related pulse numbers. It was found that interphase interaction
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can be observed in FORC diagrams with three factors; (i) the shift in center of the soft phase feature along the interaction field axis without the change in coercivity, (ii) shift in center of the
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hard phase feature along the coercivity axis and (iii) appearance of an additional interference region. However, order of the shifts directly correlates with the order of demagnetizing intraphase interaction through the hard phase and magnetic moment contribution of the soft phase. The interference region contribution was found strongly correlates with irreversible magnetic moment contribution of the soft and hard phases. Keywords: Two-phase magnetic nanowires; FORC; Interference.
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1. Introduction Following the results coming from the previous studies on the first-order-reversal-curve
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(FORC) diagrams as a new experimental tool for characterizing the interaction and coercive field distributions, two magnetic phase materials have recently attracted special attention. Information obtained from decoding the interactions in the FORC diagrams make it very attractive in
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characterizing the hard/soft two-phase magnets with interphase interaction [1-5]. Despite variety of the multi-phase materials (as magnetic soft/hard nanocomposites and multilayer
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nanowires/thin films with sequence of the magnetic/nonmagnetic or magnetic/magnetic segments), they share some of the features corresponding to the switching/interaction fields of the constituent phases [6, 7]. However, the interphase interactions produce two elongated FORC features with opposite FORC distribution signs depending on strength of the coupling between the constituents. In the recent years a great deal of attention has been focused on developing the
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new nanostructures for use in the future applications as giant Magnetoresistance (GMR) [8], drug delivery [9], adjusting the magnetic domain wall movement [10] and data storage [11]. Among different types of the nanostructures, magnetic/nonmagnetic multilayer nanowires (MNW) offer
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an additional degree of freedom due to high ordering degree of the elements with high aspect
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ratio along a nanowire (NW).
Different techniques containing ac- or dc-pulse electrodeposition from a single-or dual-bath have been introduced to prepare these nanostructures with controlled geometry [12-16]. However, growth in a single bath has a benefit of avoiding oxidation during the exchange between baths. Employing the ac-pulse electrodeposition with off-time between pulses also provides an opportunity to precise control of the purity and composition in each segment of the multilayer configuration [17, 18]. 2
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FORC diagrams were usually used to obtain detailed and compact information about magnetic behavior of the samples. To obtain a FORC, the sample was saturated and then the H was dropped to a reversal field (Hr). This process was repeated for several times while in each
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step, saturation was achieved in a lower Hr. The FORC distribution was generally evaluated as second derivative of the magnetization (M) respect to H and Hr as follows [19]:
2
r
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1 2 ρ = − ∂ M ( H , H r ) ∂H ∂H
(1)
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and then plotted as a counter plot in a rotated coordinate system; Hc= (H+Hr)/2 and Hu= (H-Hr)/2 as coercive and interaction field axes, respectively. The coercivity is defined as the magnetic intensity needed to reduce to zero the magnetic flux density of a fully magnetized magnetic specimen or to demagnetize a magnet. However, the reversal field is the required magnetic
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intensity to align the magnetic moments along the applied field.
As reported in several references [20-22] considering mean-field approach through a magnetic system with wide distribution of coercivity (as two-phase magnetic systems), interaction between
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the phases can be decoded from FORC diagrams through the shift direction in location of the soft and hard phase features and appearance of an additional interference region. However,
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considering nearest neighbor interactions can be interpreted segmentation in the FORC distribution along the coercive field axis [22]. The reported results clearly show FORC approach as a powerful technique for qualitative and quantitative decoding the interactions in the magnetic systems. In this regard, interaction between the magnetic segments along a NW and also NW arrays predominantly determine magnetic behavior of the whole system. The synthesis of MNWs with alternative soft/hard magnetic segments separated in nanoscale distances opens an area to study the interphase interaction where appears in the FORC diagrams of two-phase magnetic 3
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systems as a feature connected to the feature related to the soft phase and extended away from the coercivity axis depending on the type and coupling strength [23]. Stancu et al. [24] reported that extension of the interference region strongly depends on FORC distribution ratio of the soft
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and hard phases. However, the effect of interactions on the constituent phases and interference region leaves an open new research area to FORC distribution of these interacting systems.
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Therefore, in this paper we experimentally focus on this possibility, investigation on the effect of interactions on the soft and hard phases. Alternation of two type layers with different magnetic
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behaviors along a NW produces two-phase behavior. Furthermore, the proposed method allows us to tune the magnetic properties of the NWs formed in the porous alumina template. We also observed that the existence of the coupled reversal process at each phase can be understood by analyzing the FORC diagrams.
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2. Experimental
Common two-step anodization in 0.3 M oxalic acid was used to prepare the highly ordered anodized aluminum oxide (AAO) template. Details of the experiment have been reported
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elsewhere [25]. First and second anodization times were 5 and 3 h, respectively. The obtained template has 30 and 100 nm pore diameter and interpore distances, respectively. Based on the
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second anodization time, the average length of the pores was evaluated to be about 20 µm [26]. A new method was introduced for the first time to prepare a NW with two magnetic phases along the length composed of a hard phase and a soft phase with high magnetization. A single-bath containing 0.178 M FeSO4.7H2O, 0.178 M CoSO4.7H2O, 0.0075 M CuSO4.7H2O, 45 g.l-1 boric acid and 1 g.l-1 ascorbic acid was used as an electrolyte with 2.5 constant pH value. Nanowires were obtained using the ac-pulse electrodeposition technique. In this method, during the pulse
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time, metal species were reduced on the cathode surface and no deposition process occurred during the off-time between pulses [27]. The pulse-and off-times were chosen in a manner that they were sufficient for reduction and recovery of the metal ions at the surface of cathode,
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respectively. However, the magnetic elements were dissolved and replaced by Cu during the offtime [28]. Therefore, to obtain multilayer configuration, two different reduction/oxidation voltages and off-times were used to obtain the alternative pure CoFe and Cu segments,
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respectively [29].
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CoFe / Cu multilayer NWs were then ac-pulse electrodeposited into nanopores with different CoFe and Cu pulses to control thickness of the each segment. Impurity of each one was controlled through the maximum deposition current during the reduction in the constant 10 and 32 mA values to deposit the Cu and CoFe segments, respectively. MNWs were ac-pulse electrodeposited into the templates using 14 V as oxidation sinusoidal pulse with 0 and 96 ms
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off-time between pulses for CoFe and Cu segments, respectively. Asymmetric pulses were selected to reduce reoxidation possibility of the metals. Highly pure segments were obtained by the proposed method as reported in our previous works [15, 16 and 27]. The soft and hard
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magnetic phase behaviors were obtained by various thickness ratios of the similar CoFe/Cu
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layers in two type layers.
Two MNWs with different pulse ratios of the CoFe and Cu segments were prepared; 5/20 and 800/100 pulses of CoFe/Cu as samples 1 and 2, respectively. The pulse ratio of the magnetic/nonmagnetic segments was selected in a manner that MNWs with high magnetization and different magnetic behaviors as soft and hard magnetic wires were obtained. Regarding to the pulse ratio, rod-like configuration of the magnetic segments in second type layer was expected. However in the other type, formation of the magnetic particles distributed in a Cu 5
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matrix is not far from expectation. In order to obtain an interacting two-phase magnetic system, two mentioned layers were alternatively deposited along a NW with four different pulse numbers of the first to second type layers; 400 to 1 as sample 3, 400 to 3 as sample 4, 200 to 1 as sample 5
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and 800 to 1 as sample 6. The overall structure was repeated 10 times to increase the magnetic moment of the whole systems. Regarding different thicknesses of the two type segments, different total thicknesses of the nanowires ranging from ~ 2 µm to ~ 6 µm was expected.
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Typical time dependence of the deposited charge for the each type layer in each repetition of sample 4 is shown in Fig. 1a. It clearly shows multi-segment deposition of the charge into the
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nanopores which can be clearly observed in the second type layer segment. However, that of the first type layer has not exactly showed a perfect periodic ordering due to low pulse numbers of the magnetic element and limitation of the recording system. To clarify this issue, Fig. 1b shows magnified deposited charge related to one set of the layers. The inset figure schematically shows
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expected nanowire configuration along the length.
The microstructure and chemical composition of the samples were studied by X-ray diffraction (XRD) and electron dispersive spectroscopy (EDS), respectively. The layer thickness
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of the obtained MNWs was estimated by transmission electron microscope (TEM). To prepare
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the NWs for TEM measurement, the template were etched in 0.3 M NaOH solution for about 2h and then ultrasonically dispersed in distilled-water before dropping on a carbon coated copper grid. The magnetic measurements including hysteresis loops and FORC diagrams were performed by vibrating sample magnetometer (VSM) at room temperature. Maximum applied field (H) of 6000 Oe and field steps of 200 Oe were used to obtain the detailed FORC. However, smoothing factor value of 2 was selected to draw the detailed diagrams with the minimum noise
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contribution. It is worthy to note that the NW arrays embedded into the AAO template were used to magnetic measurements.
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3. Results and discussion 3.1. Single-phase magnetic nanowires
Figure 2 shows XRD patterns of the samples 1 and 2. Separate peaks related to magnetic
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(CoFe) and non-magnetic (Cu) elements can be observed in both the patterns (See Fig. 2 (a) and
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(b)) implying independent formation of two CoFe and Cu phases. As can be seen, the intensity ratio of CoFe to Cu peaks follows the same trend of the related pulse ratios; 0.12 and 10 for the samples 1 and 2, respectively. It can be related to increase in thickness of the sequential CoFe and Cu layers with increasing the related pulse numbers. Moreover, in the case of sample 1, difference between the peak intensity of two elements and estimated crystallite size of CoFe (~
from the pulse ratio.
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13 nm) guide us to conclude that it maybe resulted from grainy structure of the layer as expected
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Elemental analysis of the samples was performed by EDS. The obtained results for the samples 1 and 2 are presented in Fig. 3. As shown, the atomic percentages of the sample 1
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synthesized at 5/20 pulse numbers of the magnetic/nonmagnetic elements were 28.94 at.% and 71.06 at.%, respectively while that of the sample 2 synthesized at 800/ 100 pulse numbers of the magnetic/nonmagnetic elements were 94.47at.% and 5.53at.%, respectively. It clearly indicates significant reduction in Cu to CoFe content ratio with increasing the CoFe pulse numbers from 5 to 800. The observed difference between the pulse numbers and EDS results may be related to Cu substitution of the magnetic elements during the electrodeposition process. The EDS analysis
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also confirmed same weight percentages of Co and Fe in the magnetic segments as expected from the same electrodeposition rate of Fe2+ and Co2+ ions in the electrolyte.
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Hysteresis loops of the samples 1 and 2 when H applied parallel (Out of plane, OOP) and perpendicular (In plane, IP) to the wires easy axis are shown in Fig. 4. Magnetic moment of the samples has been normalized respect to that of the sample 2. Regarding large difference between
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the remanent magnetic moment and coercivity of the sample 2 in two OOP and IP measurement directions, dominant shape anisotropy along the wires axis is clearly observed. However, low
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difference between the coercivity and remanent magnetic moment of the sample 1 in two directions clearly confirmed grainy configuration of the CoFe distributed in Cu medium. Lower saturation field along the wires axis has been resulted from the magnetized interaction between the magnetic grains.
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To obtain detailed information of interaction field, FORC diagrams were measured in OOP state at room temperature. FORC diagram of the sample 1 (Fig. 5 (a)) shows a single feature weakly distributed along the both Hc and Hu axes indicating a soft magnetic material as expected
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from Fig. 4a. However, sample 2 (Fig. 5 (b)) shows a single domain treatment with high coercivity as a hard phase which highly distributed along the Hu axis. In the case of sample 2,
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FORC diagram with a “wishbone” shape can clearly observe illustrating presence of the demagnetizing mean-field interactions [28]. As reported by Bêron et al. [30], non-uniform length distribution may be a possible source for the observed tail along the Hc axis. Low distribution of the tail in higher coercivities also demonstrates uniformity in thickness of the magnetic segments deposited along a NW.
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Deconvolution of FORC data to separate reversible and irreversible components can be obtained from the proposed method in ref [31] only near the descending branch of the major loop in H → Hr limit. The intrinsic shape anisotropy of nanowires along the wires axis plays an
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important role in identifying the reversible and irreversible contributions [32]. In addition, magnetostatic interaction and size of the magnetic grains affect the reversible contribution in magnetization reversal process. Each part of major loop was reconstructed from integration of
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magnetic moment changes along the descending field branch of FORC plot and shown in Fig. 6.Comparing the results revealed that the reversible contribution reduces from 72% for sample 1
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to 14% for sample 2 confirming grainy and rod-like structure of the samples 1 and 2, respectively. The fifty percentage of reversible contribution in magnetization reversal process for non-interacting single domain particles is reported to be dominated by the shape anisotropy. Therefore, the layers configuration mainly determines hysteric behavior of the whole system.
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3.2. Two-phase magnetic nanowires
XRD patterns of the samples synthesized under different pulse numbers of two type layers,
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samples 1 and 2, along a NW are depicted in sequence in Fig. 7. Separate CoFe and Cu peaks are observed which their intensity varies depending on pulse number of the each type layer.
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Changing the pulse numbers of each type layer has not affected the crystallography growth directions of the CoFe and Cu. As expected, each peak intensifies with increasing the number of the related pulses suggesting increase in the deposited material and thickness. TEM images of the samples 3 and 5 are depicted in Fig. 8. Based on difference in atomic number of the magnetic and non-magnetic elements, multilayer structure with dark and light contrast is seen in sequence. Uniform diameter along the NW length (in both the segments) is
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clearly seen which it follows pore diameter of the template (~ 30 nm). Absence of multilayer structure in second type layer as soft phase can be clearly observed due to (i) low pulse numbers of the first type layer which led to a grainy structure as confirmed by magnetic measurements
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and (ii) Cu substitution during the electrodeposition process as mentioned in EDS results. Thickness of the each layer for the all samples based on the ones depicted in TEM images is evaluated and tabulated in Table 1. As can be seen, thickness of the each layer increases with
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increasing the related pulse numbers. It should be noted that thicknesses of two phases are
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consistent with the expectation of repetition of the pulse number when the samples are grown. OOP and IP hysteresis loops of the all two-phase samples are shown in Fig. 9. Magnetic moment of the samples has been normalized to that of related to the sample 4. Difference in magnetic moment of the all samples can be explained by the different magnetic elements in the samples. Comparing of the saturation magnetization of the single- and multi-phase samples
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confirmed that magnitude of the saturation magnetization is proportional to the composition of the magnetic elements. Moreover, the origin of the kink observed in the M-H hysteresis (Fig 9 (b) and (c)) can be explained by difference between remanent magnetic moment and coercivity
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of two phases in the sample. Dominant shape anisotropy along the wires axis is clearly observed
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confirming rod shape of the magnetic segments in hard phase layer which led to favorable magnetic easy axis direction was kept along the wires axis with the change in thickness of the each type layer. Remanent to saturation magnetic moment ratio as squareness increases from 0.45 to 0.69 from sample 3 to 4. However, it decreases to 0.57 and 0.23 with reducing the hard phase contribution indicating reducing its contribution in the magnetization process. FORC diagrams of the samples with various thicknesses of the soft and hard phases which measured in OOP state are depicted in Fig. 10. It shows that thickness ratio of two phases 10
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significantly affects magnetic behavior of the whole system. As shown, the contour plots consist of three different contours ones close to the central peak related to soft phase and two others along the Hc axis in high coercivities related to hard phase and the last in a 150o angle relative to
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the applied field axis related to interphase interaction. As can be seen, color of the each contour varies with the change in the thickness ratio of the two phases. Hc of the irreversible peak at higher portion along the Hc axis, hard phase, as HcFORC, the average coercivity obtained from
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hysteresis loop as HcArray, difference between two coercivities as ∆, magnetic moment contribution ( = ∬ , ) of each phase and interference region and the shift in
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center of the soft phase along the Hu axis were tabulated in Tables 2 and 3. In order to estimate the M contribution of the each feature, 3-dimensional Gaussian functions were fitted to the features along the Hc axis related to the soft and hard phases and interference region, then volume under the each surface was considered as their total M contribution based on
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deconvolution of the overlapped surfaces. Calculation of magnetic moment contribution of the soft and hard phases in the magnetization process regarding the reversible contribution of the soft phase in Table 3 is also suggests proportionality between the composition and saturation
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magnetization of the elements. Comparing two coercivities obtained from hysteresis loops and FORC diagrams indicates strong dependence on the M contribution of the soft phase. The results
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in Table 2 show that ∆ value grows up with increasing the soft phase contribution in agreement with our previous expectation. However, small weight percentages of the soft phase in samples 4 and 5 led to reduce contribution of this phase in the coercivity distribution. Cross-sectional view of the FORC diagrams parallel to Hc axis crossing from maximum distribution of the feature related to the soft phase clearly shows two features weakly overlapped along the axis (Fig. 11 (a)). However, cross-sectional view of the FORC diagrams parallel to Hu 11
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axis crossing from maximum distribution of the feature located in high coercivity related to hard phase is shown in Fig. 11(b). It clearly shows that of sample 2highly distributed along the Hu axis (FWHM ~ 1720Oe) indicating high demagnetizing magnetostatic interaction between the
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magnetic segments separated by non-magnetic parts. FORC diagrams of the samples combining the hard and soft phases indicate two separate features along the Hc axis related to the soft and hard phases and an additional interference region connected to the feature located in low
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coercivity indicating demagnetizing magnetostatic interaction between two phases [32, 33]. This region is accompanied with a negative distribution as a positive/negative coupled distribution
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which their distribution ratio seems depending on the overlap strength of two phases. Interaction between the soft and hard phases in these NWs in OOP measurement state can be generally classified to two main groups; magnetizing interaction between the segments along a NW length and demagnetizing one between the NW arrays. However, magnetostatic interaction
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between the NWs coupled through the soft magnetic segments can be obtained as total interaction field of soft and hard phases considering total M contribution of each phase presented
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in Table 3 [34].
Magnetostatic interaction can be calculated using the relation as follows [35]:
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Hint = Ms r2 (a (d) l + b (d) tanh (l/ld)) where Ms is the saturation magnetization of the ferromagnetic material, r is the nanowire radius, d is average interwire spacing and ld the dipolar length. a and b are phenomenological functions of d. Regarding to the known length and magnetization contribution of the soft and hard magnetic segments of the two-phase nanowires (Tables 1 and 2), magnetostatic interaction between two phases can be calculated.
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Increasing thickness of the hard phase segment increases the demagnetizing interaction between the hard segments of the NW arrays; FWHM of the distribution along the Hu axis as a qualitative estimation of the interaction increases from 710 Oe for sample 3to 1390Oe for sample 4.
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However, FORC diagram of the sample 5 shows weaker soft phase due to reduction in total length of the related segments and an interacting hard phase regarding high FWHM of 1310 Oe in Hu direction. Increasing the thickness of the soft phase segment (Sample 6) decreases the
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coupling between the hard segments thereby decreases the magnetostatic interaction between the NW arrays. However, demagnetizing interaction between the coupled hard segments reduces; the
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FWHM decreases to 587Oe. Considering constant diameter of the NWs and increasing the magnetic moment of sample 6 comparing with sample 3, reducing the interaction between the coupled hard segments of NW arrays(reducing the FWHM to ~ 587Oe) is due to decrease in the length of the coupled hard phases through the each wire. Two-step magnetic hysteresis loops and
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FORC diagrams in Figs. 9 and 10 clearly show almost independent reversal magnetizations in the soft and hard phase segments. Fig. 12 (a) shows FWHM of the distribution along the Hu axis qualitatively follows the same trend of the interaction field regarding M contribution and length
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of the each phase. Fig. 12 (b) also depicts that average coercivity of the array nanowires measured along the wires axis as magnetic easy axis exactly resulted from average magnetostatic
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interaction obtained from the coupled magnetic segments along the NW. Interphase interaction can also be evaluated from FORC diagrams by three different factors including shift of the soft phase feature along the Hu axis in a constant coercivity, shift of hard phase feature along the Hc axis and M contribution in the interference region. As can be seen, the magnetostatic interphase interaction in two-phase samples led to shift the coercivity distribution off from Hc axis. Increasing thickness of the hard phase segment in sample 4 led to more shift the 13
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center of the hard phase towards the Hu = 0from 1912 to 1673Oe along the Hc axis while that of the soft phase moves towards +Hu from 178 to 700 Oe without change in coercivity. However, the features related to the hard and soft phases move in opposite directions with the change in the
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soft phase thickness indicating increase in the magnetizing interphase interaction while the other is until dominant in agreement with the above mentioned relation (See Table 3).
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In an axial magnetized NW arrays, coupled hard segments along the length of a long nanowire act in a manner that the opposite magnetic sources localized in two ends of the wire thereby
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produces a demagnetizing field in neighboring nanowires. This field can be observed as a shift in coercivity distribution respect to Hc axis [20]. Shift of the soft phase feature along the Hu axis is dependent on the interaction between the hard segments presented as FWHM of the hard phase feature and also M contribution of the soft phase. Comparing the interference region, region 3 in Fig. 10 (a), in samples with different ratios of M contribution of soft to hard phases also revealed
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that of the interference region is proportional with irreversible portion of the soft phase appeared in the FORC diagrams. The obtained results are in agreement with the ones obtained from the experiment proposed by Stancu et al. [21]confirming that the interphase interaction illustrated as
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interference region in FORC diagrams is directly depended on the ratio between M contribution
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of two phases. However, comparing the interference region contribution and soft and hard contributions revealed that the interference region contribution simultaneously is dependent on that of the soft phase and interaction between hard and soft phases. It is not far from expectation that interaction between two phases depends on magnetic moment contribution of the one phase and the field proportional to the magnetization of the other phase. Each reconstructed part of major loop of two-phase nanowires is also shown in Fig. 13. The change in thickness of the hard and soft layers from sample 3 to 5 (according to Table 1) 14
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indicates dominant contribution of the shape anisotropy along the wires axis led to almost the similar reversibility contribution of theses samples (29-31%). However, increasing the soft phase contribution increases the role of the grainy structure and maybe the number of the small
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particles against the dominant shape anisotropy which increases the reversibility to 68%. Therefore, comparing the soft phase and reversibility contributions confirms the impact effect of
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the irreversible part of the soft phase on the interference region. 4. Conclusions
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MNWs were ac pulse electrodeposited into the nanopores of AAO template with two different types of layers as soft and hard magnetic phases deposited along the length of a NW. Considering different magnetic behavior of the two type layers, two magnetic phase behavior was obtained. Coupling strength between soft and hard phases as interference region
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corresponding with interphase interaction was controlled by thickness of each phase related to number of the each layer along a NW. M contribution of each FORC feature was estimated from deconvolution of the Gaussian peaks centered in each feature. The results showed that the
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interphase interaction was strongly depended on the M contribution ratio of the soft and hard phases. However, magnetostatic interaction between the hard phases of the NW arrays is
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dependent on the hard segments coupled through the soft segments. Interphase interaction can be observed in FORC diagrams with the shift in center of the soft phase feature along the interaction field axis without the change in coercivity, shift in center of the hard phase feature along the coercivity axis and appearance of an additional interference region. However, order of the shifts directly correlates with the order of demagnetizing intraphase interaction through the hard phase and magnetic moment contribution of the soft phase. The interference region contribution was
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also found strongly correlates with irreversible magnetic moment contribution of the soft and hard phases.
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Acknowledgment Authors are grateful to the University of Kashan for supporting this work by Grant No
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(159023/31).
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[10]A. Pereira, J. L. Palma, M. Vázquez, J. C. Denardina, J. Escrig, “A soft/hard magnetic nanostructure based onmultisegmentedCoNi nanowires”, Phys. Chem. Chem. Phys. (2013) 1-3. [11]M. Almasi-Kashi, A. Ramazani, F. Kheyri, E. Jafari-Khamse, “The effect of magnetic layer thickness on magnetic properties of Fe/Cu multilayer nanowires”, Volume 144, Issue 3, 15 April 2014, Pages 230–234. [12]A. Ramazani, M. Ghaffari, M. Almasi Kashi, F. Kheiry, F. Eghbal, “A new approach to fabricate magnetic multilayer nanowires by modifying the ac-pulse electrodeposition in a single bath”, J. Phys. D: Appl. Phys. [13] G. Binaseh, P. Grunberg, F. Z. Saurenbach. Phys. Rev. B 39 (1989) 4828-4832. 17
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[28] D. A. Gilbert, G. T. Zimanyi, R. K. Dumas, M. Winklhofer, A. Gomez, N. Eibagi, J. L. Vicent, K. Liu, “Quantitative Decoding of Interactions in Tunable Nanomagnet Arrays Using First Order Reversal Curves”, Sci. Rep. [29]A. Ramazani, M. Almasi Kashi, F. Eghbal and E. Jafari-Khamse, “The effect of deposition parameters on themagneticbehavior of CoFe/Cu multilayer nanowires”, Eur. Phys. J. Plus130 (2015) 2-9.
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[30]F.Béron, D. Ménard, A.Yelon, “First-order reversal curve diagrams of magnetic entities with meaninteraction field: A physical analysis perspective”, J. Appl. Phys. 103 (2008) 07D90807D910. [31]M.Winklhofer,R. K. Dumas, K. Liu, “Identifying reversible and irreversible magnetization changes in prototypepatterned media using first- and second-order reversal curves”, J. Appl. Phys. 103 (2008) 07C518-07C520. [32]M.Kumari, M.Widdrat,E. Tompa, R. Uebe, D. Schuler,M.Posfai, D.Faivre, A. M.Hirt, “Distinguishing magnetic particle size of iron oxide nanoparticleswith first-order reversal curves”, J. Appl. Phys. 116 (2014) 124304-124309.
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[33]D.R. Cornejo, R.D. Noce, T.R.F. Peixoto, N. Barelli, P.T. A. Sumodjo, A.V. Benedetti, “First order reversal curve analysis of nanocrystalline Pd80Co20 alloy films”, J. Alloy. Compd. 479 (2009) 43–48.
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[34] L. Clime, S. Y. Zhao, P. Chen, F. Normandin, H. Roberge, T. Veres, “The interaction field in arrays of ferromagnetic barcode nanowires”, Nanotechnol., 18 (2007) 435709-435715.
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[35] L. Clime, F. Beron, P. Ciureanu, M. Ciureanu, R. W. Cochrane, A. Yelon, “Characterization of individual ferromagnetic nanowires by in-plane magnetic measurements of arrays”, Journal of Magnetism and Magnetic Materials 299 (2006) 487–491.
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Figure captions Fig. 1. (a) Typical time dependence of the deposited charge and (b) magnified deposited charge
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of a set. Fig.2. XRD pattern of sample (a) 1 and (b) 2. Fig. 3. EDS spectrum of sample (a) 1 and (b) 2.
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Fig. 4.OOP and IP hysteresis loops of the sample (a) 1 and (b) 2 normalized to magnetic moment
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of the sample 2.
Fig. 5.FORC diagram of the sample (a) 1 and (b) 2 measured in OOP state. Fig. 6.(a, c) Magnetic moment changes along descending-field branch of the FORC plot decomposed into reversible and irreversible components and (b, d) reconstructed reversible and
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irreversible parts of the major loop of (a, b) sample1 and (c, d) sample 2. Fig. 7.XRD patterns of the samples 3, 4 and 5.
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Fig. 8.TEM images of the samples numbered as (a) 3 and (b) 5. Fig. 9. OOP and IP hysteresis loops of the samples 3-6. Magnetic moments were normalized to
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that of the sample 4.
Fig. 10.FORC diagrams of the samples (a) 3, (b) 4, (c) 5 and (d) 6 measured in OOP state. The features numbered as 1, 2 and 3 correspond to soft, hard and interference regions, respectively. Fig.11. Cross-sectional view of FORC diagrams along the (a) Hcand (b) Huaxes crossing from maximum distribution.
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Fig. 12. Variation of (a) interaction field (Hint) between the hard phase segments and half width of hard phase distribution along the Hu axis (FWHM/2) and (b) total Hint and average coercivity
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(Hc) of samples numbered as 1-6. Fig. 13.Reconstructed reversible and irreversible parts of the major loop of (a, b) sample 3, (c, d) sample 4, (e, f) sample 5 and (g, h) sample 6. Magnetic moment changes along descending-field
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branch of the FORC plot decomposed into reversible and irreversible components presented in
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Table.1. Thickness of the both type layers obtained from TEM images.
Thickness of first type layer (nm)
Thickness of second type layer (nm)
3
240
90
4
240
300
5
150
90
6
480
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Sample 1 100 140 -40
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Sample 3 1910 1085 825
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Hc (Oe) HcFORC HcArray ∆ = HcFORC-HcArray
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Table. 2. Comparison between HcArray and HcFORCand M contribution of the reversible portion in the all samples. Sample 4 1670 1500 170
Sample 5 2200 1880 320
Sample 6 2080 620 1460
Soft phase shift along the +Hu axis (Oe)
56
23
178
710
4
4
84
12
700
1390
5
8
86
6
444
1310
6
20
67
13
125
587
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Table 3.M contribution of each FORC feature, soft phase shift along the +Hu axis.
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(d)
Sample 6
ACCEPTED MANUSCRIPT Fig. 11 (a) -7
1.6x10
-6
ρ (a. u.)
1.2x10
-7
8.0x10
-8
8.0x10
0.0
0
2000
4000
Hc (Oe)
-7
0.0 0
1000
2000
3000
-7
2.4x10
(b)
-7
-7
1.2x10
-8
EP
0.0
5000
TE D
ρ (a.u.)
1.8x10
6.0x10
4000
M AN U
Hc (Oe)
SC
4.0x10
RI PT
Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Sample 6
AC C
-2000
-1000
0 Hu (Oe)
1000
2000
ACCEPTED MANUSCRIPT Fig. 12
4500 4000
(a)
3500
RI PT
2500
Hint (Hard phase)
2000
FWHM/2 700
2
3
SC
Interaction (Oe)
3000
4
5
6
M AN U
Sample 4000
3000
(b)
Hint Hc
2000
TE D
1000
1
AC C
EP
0
2
3
4
Sample
5
6
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Fig. 13
ACCEPTED MANUSCRIPT •
A new method was introduced for the first time to electrodeposit a two-phase magnetic nanowire.
•
Two-phase behavior was obtained by multilayer and grainy configurationsof CoFe
The shifts order correlates with order of demagnetizing intraphase interaction through the hard phase.
The interference region contribution was found correlates with irreversible
EP
TE D
M AN U
magnetization of the soft and hard phases.
AC C
•
SC
•
RI PT
and Cu layers.