Capturing dual behavior of the parallel coercivity in FeNi/Cu nanowire arrays by fine-tuning of segment thicknesses
Graphical abstract
Introduction
Understanding fundamental nanoscale properties of magnetic nanowires (NWs) is a necessary task to develop sophisticated nanodevices utilized in applied research fields of magnetic storage media [1,2], logic circuits [3], spintronics [4,5] and so forth. In particular, researchers have been able to show promising results of giant magnetoresistance (GMR) [6], tunneling magnetoresistance (TMR) [7] and magnetothermopower (MTP) [8,9] effects in magnetic multilayered NW (MNW) arrays, proposing them as suitable NW systems for careful investigations into magnetic properties including magnetization reversal processes [10,11] and magnetostatic couplings [[12], [13], [14]]. As a hallmark, the magnetic switching and couplings involve the effective role of parallel (axial) coercive field in such a way that one would be capable of changing magnetization direction of specific parts while also unchanging the magnetization of the remaining ones [15]. Thereby, successful applications of the nanomagnets inside NW storage device can be provided by tunable properties induced by the magnetic coercivity [16].
One of the straightforward approaches to tuning the parallel magnetic properties is through the NW length. In other words, the anisotropy field (induced mainly by shape and magnetocrystalline anisotropies)[17,18] and magnetostatic interactions [19] (comprising inter- and intra-wire couplings) can be influenced along the length of NWs, thus changing magnetization reversal processes [[20], [21], [22]]. Such kinds of tuning can be made by porous aluminum oxide (PAO) template-based electrochemical deposition techniques [23,24], resulting in large and dense NW arrays with different aspect ratios. In the particular case of MNW system, the number of segments along the NW length and thickness of each ferromagnetic/non-magnetic segment can be finely adjusted using the pulsed electrochemical deposition method [25], changing the total NW’s and each segment’s aspect ratio, respectively. Therefore, the parallel magnetic properties of MNWs would depend considerably on the specific characteristics of individual segments, and respective inter- and intra-wire couplings between segments along the NW length and neighboring NWs [26,27].
Although intense study has been carried out on single (integrated) NWs and MNWs with conventional (e.g., hysteresis curve) and advanced [e.g., first-order reversal curve (FORC)] analyses, there is yet much to understand about threshold conditions in which the magnetic properties of MNWs start to distinguish from their single NW counterparts, likely leading to the emergence of novel effects such as GMR, TMR and MTP [7,8,28]. This aim may be realized by fine-tuning of both ferromagnetic and non-magnetic segment thicknesses along the MNW length.
Basically, it is well-known that single NW arrays saturated parallel to their long axis show reduced magnetic properties when increasing NW length, which has been attributed to an enhancement in magnetostatic interactions between neighboring NWs and a decrease in the magnetization irreversibility [29]. On the other hand, increasing (decreasing) ferromagnetic (non-magnetic) segment thickness of MNWs has improved corresponding parallel magnetic properties due to reduced (increased) shape demagnetizing factor (intra-wire coupling) from disk-shaped to wire-like segments, according to the literature [[30], [31], [32]]. For instance, increasing length of ca. 50 nm diameter FeNi NWs [33] and Ni NWs [34] from 2.5 to 10 μm and 4–12 μm decreased the corresponding parallel hysteresis curve coercivity (HcHyst) from 800 to 430 Oe and 750 to 560 Oe, respectively. Moreover, increasing total length and ferromagnetic segment thickness of ca. 45 nm diameter Ni/Cu MNWs [35] from 25 nm to 2.3 μm and 35–110 nm increased the corresponding HcHyst from about 300 to 600 Oe and 250 to 820 Oe, respectively. For the case of non-magnetic segment adjustment, decreasing Cu spacer thickness of 45 nm diameter Fe/Cu [36] and FeNi/Cu [37] MNWs from 120 to 15 nm and 25 to 2.5 nm enhanced the corresponding HcHyst from 430 to 650 Oe and 395 to 570 Oe.
In this paper, we comprehensively study parallel magnetic properties of FeNi/Cu MNW array systems fabricated in 50 nm diameter PAO templates using an alternating current (AC) pulsed electrochemical deposition technique. To this end, we fine-tune both ferromagnetic (FeNi) and non-magnetic spacer (Cu) thicknesses within the range of disk-shaped to rod-like segment morphology. The magnetic properties are thoroughly investigated by hysteresis curve and FORC analyses. In this way, the roles of subtle morphological and magnetic characteristics such as the spacer thickness and magnetization reversibility (MR) in parallel HcHyst are revealed, initiating different behavior of single NWs and MNWs.
Section snippets
Fabrication of 50 nm diameter PAO templates
High purity aluminum foil (99.99%; purchased from Chempure) was cleaned in acetone by sonication, followed by electropolishing it under a voltage of 20 V at 3 °C for 5 min using a mixture of perchloric acid and ethanol solutions (1:4 in volume) [38]. To form PAO templates with highly ordered nanopores, a two-step anodization process was used as described elsewhere [39]. In brief, the first anodization step was carried out using 0.3 M oxalic acid under 40 V at 15 °C for 5 h [40]. The oxide layer
Morphology, crystalline structure and composition of MNWs
FeNi/Cu MNWs with varied segment thicknesses were fabricated in PAO templates by changing FeNi and Cu deposition pulse numbers from 300 to 1500 and 100 to 1500, respectively, as described in the experimental details. TEM images of MNWs with respective FeNi and Cu deposition pulse numbers of 300 and 800, 480 and 100, 570 and 300, 750 and 500, 1140 and 1500, and 1500 and 150 are shown in Fig. 1(a)–(f). As can be seen, ca. 50 nm diameter FeNi/Cu MNWs with TFeNi/TCu values of 25/12, 40/2, 60/6,
Conclusions
Using a pulsed AC electrochemical technique in a single bath, FeNi/Cu MNW arrays with fine-tuned FeNi (25–260 nm) and Cu (2–23 nm) segment thicknesses were fabricated in 50 nm diameter PAO templates. The XRD analysis showed that the crystalline properties of the MNWs oriented along the [111] direction were not considerably influenced by thickness tuning of the segments. By magnetically saturating the MNWs in the parallel direction, HcHyst, HcIrrev, ΔHu and MR as a function of TFeNi for varied T
CRediT authorship contribution statement
M.H. Abbas: Investigation, Visualization, Writing - original draft. A. Ramazani: Supervision, Conceptualization, Methodology, Software, Writing - original draft. A.H. Montazer: Writing - original draft, Writing - review & editing. M. Almasi Kashi: Resources.
Acknowledgements
The authors acknowledge University of Kashan for providing financial support for this research by Grant No. (159023/57).
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