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Naunton J. Oxford University Press, - 82 pages. Clockwise is an intensive refresher course in general English for adults, with an emphasis on improving. Naunton J. Oxford University Press, - pages. Clockwise is an intensive refresher course in general English for adults, with an emphasis on improving. Clockwise Upper-Intermediate Classbook book. Read reviews from world's largest community for readers. This is an intensive refresher course.
Inset to panels a and d show schematically the angular configuration of two Types of NWs. Schematic pictures of each NW is shown in upper section of the corresponding panel. Legends of panels a to e are same only shown in panel c. Full size image According to the angular dependence plots, the coercivity or switching field increases as a function of magnetization angle. Therefore, we find out that the magnetization reversal is controlled by the propagation of a vortex domain wall known also as the Bloch-point domain wall in magnetic nanotubes and nanowires 31 , 33 , For all the modulated diameter nanowires, the fitting procedure was repeated for different D values including DMax, DMin, and DMean and the results are plotted in Fig.
As evidenced from the plots, the experimental data are well fitted to the maximum diameter or wider segment for Type II NWs, particularly for small angles. This is a general behavior of diameter modulated magnetic nanowires with single giant Barkhausen jump where no abrupt small jump occurs and it follows the magnetization switching at the wider diameter segment, Dmax, as has been also shown also by Palmero et al. In contrast, that is not the case for Type I NWs and it is very likely that an imaginary mean value between wide and narrow diameters called mean diameter, Dmean rather closely fits to the experimental data, though some discrepancy exists.
The discrepancy between the theoretical curves and experimental data of the coercivity for Type I NWs is observed very likely due to the small switching field in magnetization loops.
The variation of the terminating switching field or the coercivity assumed here is far beyond the theoretical curve driven for nanowires with a straight diameter of Dmax and is close to the curve plotted for Dmean. This means that the magnetization takes places by a complex contribution from either wide or narrow segments.
The increasing trend of the switching field or the coercivity is evident as a similar behavior for all types of nanowires studied, see Fig. To understand the magnetization behavior of Type I nanowires, we have conducted a careful analysis on the angular dependence to interpret the existence of the abrupt jumps and clarify i how the magnetization process occurs, ii which segment, narrower or wider in diameter, controls the process and iii the switching sequence at reversal.
The switching fields are marked by S1, S2, S3 and S4, where both S2 and S3 denote the small or initiating switching fields and both S1 and S4 are terminating final magnetization states i.
These switching fields were collected from the experimental hysteresis loops of Type I-3 NWs measured at different angles in Fig. We compare the two observed switching fields with the theoretical calculation according to Eq.
Full size image With increasing the angle, the gap between the small and large switching fields becomes wider indicating the presence of a metastable state for Type I Ni NWs. There is a good agreement between the measured switching fields and the values calculated according to Eq.
The initial and final switching fields correlate respectively to the calculated values for the wider Dmax and narrower Dmin diameter.
This indicates that the wider segment with a diameter Dmax acts as the nucleation of domain wall as is always the case. Furthermore, an intermediate metastable state is created when a domain wall reaches the transition from the wider to the narrower segment until the applied field becomes high enough to overcome that barrier and it gets inside the narrower segment. Indeed, a similar phenomenon has been previously reported by C. Faulkner et al.
The domain wall nucleates at the ends of the nanowire where segments are wider and propagates in the small switching field until it reaches the trap and it leaves the trap at the large switching field.
In the case of Type II nanowires, where the central segment has the wider diameter, we observe a single Barkhausen jump. Therefore, we can assume that the domain wall nucleates at the ends of the nanowire and once it is de-pinned at the coercivity or switching field it propagates without any barrier at the diameter transition to the segment of wider diameter. More detailed information on the influence of diameter modulating junctions at a submicron scale can be found in the Supplementary Information.
We have definitely observed significant differences between two types of nanowires based on the local loops recorded at different locations. Here, we only show data for two representative samples of the two different types of NWs.
On one hand, nanowires Type II-6 exhibit the general behavior of magnetization discussed in above which remains unchanged along the nanowire.
Little differences among the hysteresis loops are in the error range of the measurement techniques for instance variation of the coercivity about 10 Oe. The hysteresis loops were measured at different positions across the length of the nanowire. At the left side end, away from the segments junction, there is only one jump in one branch of the hysteresis loop. But two sharp jumps appear by approaching to the junction where they are both seen in either branch of the hysteresis loops.
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These experiments reveal the importance of the junction geometry distribution on the domain wall nucleation and propagation. MFM was employed to image the magnetic configuration particularly at the junction of segments. Dark and bright contrasts due to the presence of accumulated magnetic charges are observed at the ends of both types of NWs.
In addition, MFM image in Fig. The contrasts magnified in Fig. While some effect of the stray field emanating from the standard magnetic tip could not be completely discarded 46 , according to our micromagnetic simulations that will be discussed later we conclude that the magnetization starts with the nucleation of a domain wall from thick segments and reaches the junction from either side.
This might be a reason for the creation of such contrast spot on the MFM images. However, this is not the case for Type II NWs which only exhibits the most usual behavior of constant diameter NWs where the magnetic poles with opposite contrast are observed only at both ends of the nanowire.
That typical dipolar contrast is commonly ascribed to the axial magnetization, and the geometry effect can be clearly observed in accordance with previous research work on FeCoCu bamboo-like nanowires by E.
Berganza et al. Insets to c,f are the schematic representation of domain structures. Full size image Over all based on our experimental data presented so far, our finding reveals that Type I nanowires have a distinct effect on the magnetic configuration and domain wall structure. All NWs present MFM images that are compatible with the single domain configuration, but only type I NWs exhibits additional local spin divergence induced at the site of narrow-wide junction.
This may explain the effect of the junctions on the pinning and depinning of the domain wall as discussed in above. On the other hand, the presence of a vortex or a system of vortices 47 could be expected in this type of nanowires as predicted by our angular dependence magnetization measurements. However, MFM is hardly sensitive to the formation of pseudo-vortex in nanowires of similar diameter as reported before, to this pseudo-vortex due to the lack of stray field In order to obtain deeper insight into the microscopic reversal mechanism, we have performed Monte Carlo simulations on the magnetization reversal see Methods.
The applied reverse field is kept constant close to the coercivity value, which is different for the two types of NWs. Experimentally the applied field sweep is slow enough with respect to the DW motion. This would allow us to consider it constant in our simulations.
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In Fig. These data indicate that two vortex domain walls VDW form at opposite ends of the nanowire and propagate towards the center of the wire, where they eventually merge leading to complete magnetization reversal. Notice that our model of the tri-segmented nanowires supports vortex domain walls VDW in both the narrow and the wide regions of the nanowire, which is in accordance with our experimental observations by the angular dependence data where the diameters of all samples are above the critical value for vortex formation.
Figure 6 a Spin configuration of Type I and II nanowires when the domain wall approaches the junction. A color code is used to indicate the spin direction, namely red for spin up and blue for spin down. Red black line indicates the DW propagating from top-end bottom-end towards the center of the nanowire.
Notice the change in average velocity when a domain wall enters the junction. A sudden drop of the instant velocity to almost zero value for a short time — MCSS indicates a temporary pinning of the DW in the junction region of type-I NWs.
The effect of DW pinning at the junction region is not observed in type-II nanowires. Full size image In Fig.
What is interesting is that the velocity of the VDW drops when it enters the narrow region, as seen from the change of slope. In both types of NWs an overshoot of the instant velocity is observed, which is attributed to local softening of the material caused by the reduced coordination of moments in the junction. Indeed, when a cubic discretization is used to describe a smooth variation of diameter, inevitably the diameter variation is described numerically in discrete steps. Then some cells right in the transition region will have fewer than 6 nearest neighbors.
As soon as the DW exits the junction region we observe for the type-I NW a nearly vanishing velocity for a short period of time pinning effect and then the DW velocity is stabilized to a lower value, as expected since the DW velocity in the narrower area is smaller compared to the velocity in the wider area.
In type-II nanowires, we observe a stabilization of domain wall velocity to higher values as the domain wall enters the wide area, but no accompanying delay of the DW in the junction region is seen.
This behavior indicates that domain wall pinning occurs only in the wide-to-narrow junction. In this sense, different reverse fields are applied in different MW types. These results we show in Fig. The delay of domain wall propagation at the wide-to-narrow junction is captured by the MOKE measurements due to the small measuring time scale.
Based on our experimental and theoretical results, we exploited the magnetization reversal mechanism of Ni NWs that takes place by the propagation of a vortex domain wall with a net magnetization component appearing in the xy plane perpendicular to the wire axis.
We mention here that the geometrical restrictions when we encapsulate a narrower segment between the two wider segments would damp the magnetization reversal process at the junction site as a pinning site or domain wall trap site with different energy potential imposed to the reversal system as an energy barrier This type of geometrically engineered nanowires could be realized as novel domain wall mediated spintronic data storage devices.
In particular, one of the challenging issues for the implementation of such nanowires into application is to create high density of domain walls. One solution is to fabricate three-dimensional architectures using alumina nanoporous templates as has been realized in 50 which can be filled with our TS-DM NWs. Our nanowire arrays can be proposed to represent an ensemble of the tri-segmented nanowires inside the templates towards high density 3D domain wall mediated memory devices 51 , though they need to be studied from magnetic properties in array form whose magnetization behavior relies on the geometrically induced magnetic interaction and stray fields emanating from the modulating segments inside the array Maggie Baigent.
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As evidenced from the plots, the experimental data are well fitted to the maximum diameter or wider segment for Type II NWs, particularly for small angles.