Trapped magnetic field and levitation force properties of multi-seeded YBCO superconductors with different seed distance

Journal of Alloys and Compounds 829 (2020) 154400

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Trapped magnetic field and levitation force properties of multi-seeded YBCO superconductors with different seed distance Burcu Savaskan a, *, Sait Barıs Guner b, Akiyasu Yamamoto c, Kemal Ozturk d a

Energy Systems Engineering, Faculty of Technology, Karadeniz Technical University, 61830, Trabzon, Turkey Department of Physics, Faculty of Arts and Sciences, Recep Tayyip Erdogan University, 53100, Rize, Turkey c Department of Applied Physics, Tokyo University of Agriculture and Technology (TUAT), Tokyo, Japan d Department of Physics, Faculty of Science, Karadeniz Technical University, 61080, Trabzon, Turkey b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 October 2019 Received in revised form 20 January 2020 Accepted 16 February 2020 Available online 25 February 2020

The performance of large grain (RE)BCO superconducting bulks has the significant potential for magnetic-force based applications such as trapped field magnets and magnetic levitation devices. These materials can trap high magnetic fields of several tesla at temperature below 77 K and also provide stable levitation and high load capacity in magnetic levitation systems. Top seeded melt growth process and multi-seeding technique have been used to enlarge bulk (RE)BCO samples, however many factors affect the growth of multi-seeded bulk (RE)BCO. These include the geometry of sample, the distance between seed crystal, the seed combination and alignment. In this work, we investigated the effects of the distance (d) between two Nd1.8Ba2.4Cu3.4O7 (Nd-123) seed crystals on the characteristics of the (100)//(100) grain junction of top-seeded melt growth (TSMG) processed YBCO bulk superconductors. Four cylindrical shaped YeBaeCueO samples of dimensions 25 mm in diameter and 7 mm in height with two seed crystals were prepared for this investigation. The fabrication process of these samples is described and their trapped magnetic field and vertical levitation force were measured at 77 K and lower temperatures under Zero-field-cooled (ZFC) and Field-cooled (FC) regimes. We found that both the trapped magnetic field and the vertical levitation force decreased with increasing d value. The trapped field and the vertical levitation force values of the samples with the decreasing the distance between two seed crystals from 16 mm to 1 mm, increased from 0,62 T to 0,78 T and from 13.08 N to 19.41 N, respectively. A peak trapped field value of 4.20 T and vertical levitation force of 19. 41 N has been achieved in this study for YBCO with the distance between two seed crystals of 1 mm. The obtained results can be contributing the properties on improvement of the levitation force and trapped magnetic field and these properties induce widespread usage of multi-seeded YBCO bulks in technological applications. © 2020 Elsevier B.V. All rights reserved.

Keywords: Crystal growth Multi-seeding YBCO Trapped magnetic field Magnetic levitation force

1. Introduction Single grain (RE)BCO (RE is a rare-earth element, here RE ¼ Y) bulk superconductors, cuprate family of high-temperature superconductors (HTS), most commonly YBCO, have two main advantages are (i) ability to trap higher magnetic fields compared to produced by conventional permanent magnets (PMs) (ii) ability to have passive components compared to active magnetic bearing of a magnetic levitation system [1e3]. But even the maximum field produced by the conventional iron-based magnets, which are limited generally to less than 1.5 T, whereas YBCO in bulk form can

* Corresponding author. E-mail address: [email protected] (B. Savaskan). https://doi.org/10.1016/j.jallcom.2020.154400 0925-8388/© 2020 Elsevier B.V. All rights reserved.

trap over ten times higher than those generated by permanent magnets [4]. Because of the higher trapped field properties, the compact superconducting bulk magnets are generally called as “Trapped field magnets (TFMs) or Supermagnets”. Large single grain (RE)BCO bulk superconducting materials have significant potential to trap high magnetic fields of over 17 T below 30 K and up to 3 T at 77 K, using the field-cooling magnetization (FCM). Recently, Patel et al. have reported the highest trapped field in a bulk superconductor to date of 17.7 T in between two stack HTS tape at 8 K [5]. Furthermore, in an arrangement of two bulk YBCO samples, 26.5 mm in diameter and 15 mm in thickness, which was reinforced with an epoxy resin and carbon fibre, reported a trapped field of 17.24 T at 29 K, for two bulk records [6]. Nariki et al. reported a trapped field of 3 T in Gd-Ba-Cu-O sample of 65 mm in diameter

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at 77 K and this result is very important in the practical applications because of using the boiling point of liquid nitrogen temperature [7]. Fuchs et al. also reported a trapped field of 16 T at 24 K for Zrdoped and Ag-impregnated YBCO sample of 25 mm in diameter [8]. It is important to note that the trapped field ability of multi-seeded samples has been measured rarely comparing with the single seeded samples. For instance, a comparatively trapped field study, by Shi et al. [9] performed on a single grain, a multi-seeded and a close packed YBCO bulk samples, indicated that a multi-seeded sample has the lowest trapped field value although it has maximum magnetic levitation force value. The fabrication method of bulk (RE)BCO is an important way to improve the trapped field and levitation properties. However, the presence of weak links between the grain boundaries in bulk (RE) BCO reduces the flow of supercurrent and results in a significant reduction in the critical current density, Jc, and therefore trapped field, BT, (according to the Bean model, trapped field depend on Jc as BT f Jc d). The top-seeded melt growth (TSMG) process has been used commonly to fabricate high performance and large single grain form YBCO bulk superconductors. In the TSMG process; the bulk crystal grows from the Nd/SmBCO seed crystal, the size and crystallographic orientation of YBCO grain can be controlled by placing seed arrangement on the top surfaces of sample [10]. However, there are significant challenges in this processing technique. The growth of bulk in the form of large sinle grain YBCO sample typically requires rather complex and long processing time due to very low growth rate of Y123 phase [10e12]. Additionally, it is obvious that the fabrication of large-size bulks is essential for engineering applications but it takes a very long time. Due to the size limitation of bulk single seeded superconductors, in the recent years multi-seeded samples have been fabricated to obtain high trapped field and levitation force performance especially for engineering applications. TSMG process with multi-seeding technique, in which uses two or more separate seeds to control the nucleation, was developed initially by Schatzle et al. [13]. Comparing with the a single grain grown from a single seed, multi-seeding process provides some advantages such as boundary control, faster sample growth, the fabrication of larger size and time saving [10,12,14e16]. Multiseeded samples, reported recently, have been used in magnetic levitation systems such as Maglev and superconducting bearings. Deng et al. [17] observed that a multi-seeded sample has higher the maximum levitation force than that of a single seeded sample. Shi et al. [9] also reported that large single grains of bulk (RE)BCO at 77 K with high trapped field may not always be required for levitation applications. In a comparison study of the levitation force between the single-seeded with multi-seeded GdBCO-Ag samples, Shi et al. [18] reported that higher levitation force of multi-seeded sample is attractive for based on levitation applications. In addition, Kim et al. [10] reported the magnetic and levitation properties were decreased as the number of seeds and seed distance increased while the processing time was reduced by providing many growth sites for Y123 grains. Such behaviour has also been reported by Choi et al. [19], in a comparative study of the levitation forces obtained from single grain and multi-grain YBCO superconductor using 1e5 seeds. This suggests that the levitation force decreased with increasing the number of seeds because of weakly connected grain boundaries of junctions. Alternatively, several groups [20e23] have described a new method for successful growth of (RE)BCO bulk superconductors using buffered seed with different buffer layer compositions in order to ensure uniform the seed into the growing bulk sample. Additionally, Tang et al. [24] have applied the buffer process in multi-seeded YBCO sample with Y211 buffer and reported a trapped field of 0.45 T at 77 K.

In order to improve using potential of YBCO bulk superconductors in technological applications, the trapped magnetic field and levitation force properties of the multi-seeded YBCO bulk superconductors were investigated in detail. In this paper, we fabricated YBCO superconductors with two-seeded by a top-seeded melt growth process using seeds with (100)//(100) grain boundary junctions. We first evaluated the trapped field properties of multiseeded samples from liquid nitrogen temperature (77 K) to 20 K by field-cooling magnetization (FCM). Secondly, the magnetic levitation force was measured between the fabricated cylindrical YBCO sample and a 19 mm diameter rare-earth permanent magnet (PM) under zero field-cooling and field-cooling conditions at the measurement temperatures of 77 and 80 K. The effects of the distance (d) between two seeds on the trapped magnetic field and levitation force properties are reported and results discussed.

2. Material and methods 2.1. Sample preparation Multi-seeded YBCO superconductor bulk samples were fabricated via the Top-Seeded Melt-Growth (TSMG) process. The powders of Y123 (YBa2Cu3O7) and Y211 (Y2BaCuO6) were prepared using Y2O3 (purity 99.99%), BaCO3 (purity 99.8%), CuO (purity 99.7%) and calcinated at 900  C for 20 h and then at 920  C for 15 h, by solid-state reaction method. The compositions of the precursor powders are the ratio of 75 wt% Y-123 þ 25 wt% Y-211 þ 0.5 wt% of CeO2. The mixed precursor powders were pressed as uniaxial into pellets of diameter 30 mm and height 9 mm Nd1.8Ba2.4Cu3.4O7 (The size of Nd-123 seed crystal is 3 mm  2 mm x 2 mm) single crystal seeds were placed on the top surface of pressed pellet for melt processing in air using a conventional TSMG heating profile, as shown in Fig. 1. The successfully grown samples were subsequently annealed in pure oxygen at a temperature at 450 ᵒC for 7 days following TSMG process. Two seeds are arranged with a style of (100)//(100) grain junction; i.e., the growth fronts two adjacent grains encountered at the a or b axis directions of the bulk and formed a grain boundary (GB) at that position, as the arrows shown schematically in Fig. 2. Fig. 3 shows photographs of the top surfaces of the YBCO samples fabricated for this study with the trapped field measured at the centres of the sample and measured levitation force in the ZFC regime at 77 K. MS-0 is a YBCO single grain seeded using a single

Fig. 1. Heating profiles used to top-seed melt processing YBCO samples [25].

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Fig. 2. The schematic view of the top surface of the TSMG processed YBCO grains fabricated using two seed crystal with (100)//(100) grain junction.

Nd-123 seed crystal, while MS-1, MS-8 and MS-16 are multi-seeded YBCO samples, in diameter 25 mm and in height 7 mm after the heat process. In this study, the distance (d) between the two seeds centres are determined as d ¼ 1, 8 and 16 mm and they are named as MS-1, MS-8 and MS-16 samples, respectively. 2.2. Characterization The phase composition of the samples were carried out in a a Rigaku D/Max-IIIC X-ray diffractometer (XRD) with CuKa radiation over the range 20 e55 with a scan speed of 0.2 min1 at room temperature. The X-ray diffraction (XRD) analysis was performed on a small specimen cut from the top surface of the MS-0 and MS-8 samples and the results are shown in Fig. 4. As can be seen in the XRD plot, all the intensified peaks correspond to the high (0 0 l) diffraction peaks of Y123 phase. The result indicates the well c-axis orientation of the YBCO grain and also grain boundaries close to the (100) plane are included in the surfaces. Comparing the XRD diffraction peaks of MS-0 with MS-8 sample, no obvious change is discovered, namely the samples have the same crystallographic orientation, as observed in previously study [10]. Also, in our previous study [25], the critical current densities in the self-field, Jc (0), were determined to be 4.35  108 A/m2 and 5.6  108A/m2 at 77 K, taken a small pieces cut from under the seed and the edge of bulk, for single-seeded YBCO bulk sample which fabricated the same conditions. 2.3. Trapped field measurements All field-cooling magnetization (FCM) experiments were conducted with a 5 T low-temperature superconducting solenoid magnet system with a room-temperature core of 60 mm in diameter at Department of Applied Physics, Tokyo University of Agriculture and Technology (TUAT). The trapped field was measured by using two transversal Hall sensors (Lakeshore, HGT-2101-10) mounted on the centre and 6 mm away from the centre of the

Fig. 4. X-ray diffraction patterns for the top surfaces of the MS-0 and MS-8 samples.

bulk surface using varnish. The top surfaces of samples were polished for trapped field measurements. At the first step of the experiments, liquid nitrogen (LN2) was used as coolant to cool the bulk. Each sample was cooled to 77 K using LN2 under 1 T applied field perpendicular to its top surface. The field-cooling magnetization procedures were as follows:  The sample was placed at the centre of the bore of superconducting magnet system.  The applied field was set to 1 T, in which the bulk was progressively cooled down into a superconducting state. The fieldsweep rate was 0.02 A/s during the field adding and removing processes.  After a sufficient cooling time of 25 min, the applied field was removed.  The trapped field on the top surface of each sample was measured using two Lakeshore Hall sensors (HGT-2101-10). The two Hall sensors, represented as H1 and H2, were placed at the centre of each top surface at positions of 0 and 6 mm in radial direction from the centre of the surface.

Fig. 3. The photographs of the top (seeded) surfaces of the YBCO single and multi-grain bulk superconductors processed by the TSMG process and using the Nd123 seed crystal. (a) MS-0 a YBCO single grain sample with max. Bz ¼ 0.70 T, Fzmax ¼ 19.41 N at 77 K. (b) MS-1 a multi-seeded YBCO sample with max. Bz ¼ 0.78 T, Fzmax ¼ 19.24 N at 77 K. (c) MS-8 a multi-seeded YBCO sample with max. Bz ¼ 0.64 T, Fzmax ¼ 17.12 N at 77 K. (d) MS-16 a multi-seeded YBCO sample with max. Bz ¼ 0.62 T, Fzmax ¼ 13.08 N at 77 K.

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 The trapped field profiles to give magnetic field mapping of the magnetised samples, 1.5 mm above top surfaces at FCM in a magnetic field of 1 T, were measured using a rotating array of Hall probes.

by a vertical traverse of z ¼ 50 mm. In FC regime; the Fz(z) measurements were performed by cooling the sample at z ¼ 1.5 mm above the PM. Then, the measurements were carried out during the PM was vertically moving from z ¼ 1.5 mm to a maximum vertical distance of z ¼ 50 mm, followed by a traverse to z ¼ 1.5 mm.

2.4. Levitation force measurements

3. Results and discussion

Magnetic levitation force (Fz, vertical force) measurements in zero field cooled (ZFC) and field cooled (FC) regimes were performed at 77 K and 80 K by using Magnetic Levitation Force Measurement System. Cylindrical Nd-Fe-B permanent magnet, 19 mm diameter and 10 mm thickness, (PM) with a magnetization of m0M ¼ 0.48 T was used as a magnetic field source in this system. The detailed information of this system can be found in Ref. [26]. In ZFC regime, firstly, the sample was fixed approximately on the central axis of the PM. Then the system was vacuumed and the cooling distance between sample and PM was chosen as z ¼ 50 mm. When the sample was cooled at z ¼ 50 mm above PM, the Fz(z) measurements were carried out during the vertical traverse of the PM from z ¼ 50 mm to a minimum distance of z ¼ 1.5 mm, followed

3.1. Trapped field at the top surface of the YBCO samples Fig. 5 shows the trapped field of a single seeded and multiseeded bulk YBCO samples with various distances (d) between two seed crystals (d ¼ 1, 8 and 16 mm) measured at 77 K, after FC magnetization. The maximum trapped field value at 77 K is obtained for MS-1 sample as 0.78 T comparing the MS-0, MS-8 and MS-16 samples (max. trapped field values are 0.70, 0.64 and 0.62 T, respectively), which is the best values observed. This means that MS-1 sample, by this measurement, of the highest quality among the four measured samples. This result clearly suggests, and is in good agreement with previously studies, that the trapped field of multi-seeded samples decrease when the distance between the

Fig. 5. The field measured during magnetization at the top surface of YBCO bulk samples with (100)//(100) grain junction (a) MS-0 (b) MS-1 (c) MS-8 and (d) MS-16 samples by two Hall probes, represented as H1 (red line) and H2 (black line), were placed at the centre of the sample at positions 0; 6 mm in radial direction from the centre of the surface, respectively. Insets show the 3-D trapped field distribution of the MS-0, MS-1 and MS-8 samples by a static 1 T FCM excitation at a gap of 1.5 mm in LN2. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

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seed crystal increases [10,14,27,28]. As reported by Cardwell and Lo [27], Y123 grains growth from the seed crystal with rectangular shape, then two 123 grains grow further and then occur a thin melt channel between the grains. Especially when the distance between the two seeds crystals is large, the melt channel tends to be well formed. Kim et al. reported that the residual melt-forming phases (CuO and BaCuO2) were observed around the grain boundaries because they formed separation of the peritectic melt Ba3Cu5O8 into eutectic melt during cooling and results in the decrease of the magnetic properties at grain junctions [16,28]. Indeed, in very recent work by Shi et al. [19], it was reported that the value of trapped field decreases when the distance between two seed crystal increases due to the increased depth of the grain boundary and so connectivity of the grain junctions becomes poorer as d increase.

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The trapped magnetic field of MS-16 sample is almost zero at the 6 mm away from the center of the sample because of the connectivity of the grain junctions becomes poorer when the distance of between seeds increase, is shown in Fig. 5 d. The variation of trapped field for MS-16 sample is in agreement with previous observations [15,16,27,28] that the trapped fields of muti-seeded samples decrease when the distance between the seeds increases. Inset in Fig. 5a and b and c exhibit the trapped magnetic field profiles for the 1.5 mm above top surfaces of the MS-0, MS-1 and MS-8 samples. From insets, we can see that MS-0 and MS-1 samples have a single peak, whereas MS-8 sample exhibits nearly two peak points. As known, the profile of trapped field in single grain bulk superconductors is represented typically by shape with a cone and an obviously peak proves a well-grown single grain [17]. The trapped field profile of MS-1 sample with d ¼ 1 mm has a

Fig. 6. The field measured at the top surface of MS-1 sample by two Hall probes were placed at the centre of the sample at positions 0; 6 mm (a) 77 K, 1 T (b) 60 K, 4 T and (c) 40 K, 7 T (d) 20 K, 5 T.

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single peak, which indicates the sample has little residual impurity phases at grain boundaries. It can be seen that the trapped field profile for the MS-8 sample, shown in Fig. 5 c, changes to slightly two peak points. This result is consistent with that reported by Shi et al. [14], where three seeds were used to seed a bar-shaped sample, and the authors concluded that “the depth of the grain boundaries increases with increasing distance between seeds, this situation induced the lower trapped field”. The trapped field profile measurement was not taken for MS-16 sample, shown in Fig. 5 d, because the tapped field value is almost zero taken 6 mm away from the center and it has the lowest value at the center comparing the others. Fig. 6 shows the temperature dependence of the maximum trapped field at the bulk surface when the MS-1 sample was fieldcooled in 1 T at 77 K; in 4 T at 60 K, in 7 T at 40 K, in 5 T 20 K. The maximum trapped-field was 0.78 T at 77 K, 1.73 T at 60 K, 4.12 T at 40 K and 4.20 T at 20 K for MS-1 sample. The maximum trappedfield increased with decreasing the temperature. The measurement temperature decreasing from 77 K to 20 K the trapped field performance, obtained taking the ratio of “trapped field/applied field”, of the multi-seeded YBCO bulk has been improved from % 78 to % 84, as shown in Fig. 6 a and d respectively. The obtained lower performance of % 43 at 60 K in Fig. 6 b points out that the applied magnetic field is excess high for the measurement temperature because of the flux flow in the sample. 3.1.1. Levitation force properties of the YBCO samples Similarly to the evaluation of trapped magnetic field, vertical magnetic levitation force in the ZFC and FC regimes is also used to evaluate the performance of the multi-seeded bulk YBCO samples for levitation applications. Figs. 7 and 8 show the levitation forcevertical distance hysteresis curves of the single seeded YBCO

sample and multi seeded YBCO samples with various seed distance (d) values (d ¼ 1, 8 and 16 mm) at 77 K and 80 K, respectively. Due to intrinsic hysteresis property of the bulk material, a big hysteresis loop is formed by two branches (descending and ascending) of the levitation curves. In ZFC case, the Fz curves indicate repulsive character and increase exponentially with the decrease in vertical distance at fixed temperature, then arrive at a maximum value, Fzmax at the closest distance z ¼ 1.5 mm, between the sample and permanent magnet (PM). The Fzmax values of the sample with d ¼ 0 is 19.41 N, while it decreases to 19.24 N, 17.12 N, 13.08 N as “d” increased to 16 mm at 77 K, shown in Fig. 7a. In Fig. 7b, when the sample was cooled close to permanent magnet, this is regarded as field cooling (FC) regime; in practice, it is set as 1.5 mm for FC. In FC case, generally magnetic flux is trapped inside the sample, because of the pinning capability and shielding currents on the surface of the sample and the levitation force becomes attractive force character(vertical stability) [29e31]. By comparing ZFC and FC regimes in Figs. 7 and 8, it can be seen that FC regime can get a smaller levitation (repulsive) force but it exhibits a good stability [32]. Namely, the attractive force in FC regime is stronger than in ZFC regime while the repulsive force in ZFC regime is stronger than in FC regime, consistent with the literature [29e32]. The hysteresis loops broaden under FC case during the descending/ascending cycle with increasing measurement temperature because of the weakness of trapped magnetic flux and induced currents circulating inside the sample. The levitation performance in the FC case is much practical in technological application, such as maglev and superconducting motors, because FC case can get more guidance force to achieve passive stability, whereas ZFC case can get larger levitation force. The maximum attractive force values at 77 K and 80 K for MS-1

Fig. 7. Levitation force-distance hysteresis curves of single-seeded and multi-seeded YBCO samples with various seed distance (d) values (d ¼ 1, 8 and 16 mm) at 77 K under (a) ZFC (b) FC regimes.

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Fig. 8. Levitation force-distance hysteresis curves of single-seeded and multi-seeded YBCO samples with various seed distance (d) values (d ¼ 1, 8 and 16 mm) at 77 K under (a) ZFC (b) FC regimes.

sample are obtained as 7.67 and 6.42 N, respectively. Another observation from Figs. 7 and 8 is that MS-16 sample exhibits consistently the lowest attractive force (in FC case) and repulsive force (in ZFC case). The lower repulsive and attractive force values at d > 1 values may be attributed to the flux enters more easily in the poorly connected grain boundaries and weakened the shielding currents through the bulk sample. These results suggest that the existence of the accumulation of non-superconducting phases around the grain junction is considered to be the main reason of the decrease of the levitation performance. In other words, magnetic levitation force performance improves when the two seeds are placed very closely, magnetic levitation reduces when the two seeds distance are larger. According to Kim et al. [28] and Shi et al. [14], the lower Fzmax values at larger d values may be attributed to the easier penetration of magnetic field to the poorly connected grain junctions due to an accumulation of non-superconducting phases at the grain boundaries. 4. Conclusion YBCO bulk samples, 25 mm in diameter with a thickness of 7 mm, were fabricated by TSMG process with multi-seeding technique using Nd123 two seeds. The trapped magnetic field and magnetic levitation force of multi-seeded YBCO samples with various seed crystal distance (d) were examined. The measured trapped field and the vertical levitation forces of the samples show that the superconducting properties decreased with increasing the distance between seed crystals. A peak trapped field value of 4.20 T for the distance between two seeds d ¼ 1 mm at 20 K and vertical levitation force of 19. 41 N and 19.24 N at 77 K under ZFC has been achieved in this study for MS-0 and MS-1 samples, respectively. As

a result, when the two seeds were arranged very closest without spacing as d ¼ 1, the sample behaviour like a single seeded bulk superconductor, whereas the longer distance of two seeds induce deterioration of the magnetic field profile because of the residual non-superconducting phases formation between the seeds. And so poorly connected grain boundaries are thought to be the cause of low levitation force and trapped field values of multi-seeded sample. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Burcu Savaskan: Supervision, Project administration, Conceptualization, Writing - original draft. Sait Barıs Guner: Methodology, Conceptualization. Akiyasu Yamamoto: Funding acquisition. Kemal Ozturk: Project administration. Acknowledgements The authors acknowledge the Scientific and Technological Research Council of Turkey (TUBITAK), for providing the financial support under grant number (2219 International Post-Doctoral Research Fellowship Program). This paper was partly supported by JSPS KAKENHI (JP18H01699) and RTE University Scientific Research Projects Coordination Department (Project Number: 2014.102.01.05). All the magnetic levitation force measurements

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were performed in the MLFMS designed by the project of TÜBITAK (Project Number: 110T622) and whose patent application number is 2013/13638 for the Turkish Patent Institute (TPE). All the authors would like to thank Dr. Yunhua Shi (at Cambridge University, Cambridge, UK) for her support.

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