IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 1, JANUARY 2014
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Surface Roughness Effects on Magnetization Reversal of Magnetic Ring Elements Tzong-Rong Ger , Hao-Ting Huang , Chen-Yu Huang , and Mei-Feng Lai Department of Power Mechanical Engineering, National Tsing Hua University, Hsinchu, Taiwan Institute of Nanoengineering and Microsystems, National Tsing Hua University, Hsinchu City 30013, Taiwan The effect of surface roughness on magnetoresistance of permalloy ring structure is investigated. Microstructured permalloy rings with surface roughness varying from 4.2 to 21 nm were fabricated using electron beam lithography and a chemical etching process. It is found experimentally that the first and second nucleation fields decrease obviously with the surface roughness increasing, and the field range for the flux-closure state is not associated with surface roughness. The results indicate that surface roughness in permalloy ring structure is an important factor that influences the magnetic behaviors of the ring. This result can provide important information for future designs of ring-shaped magnetic devices. Index Terms—Magnetization reversal, magnetoresistance, permalloy films.
I. INTRODUCTION
magnetoresistance and morphological observation of ring elements with different surface roughnesses are provided.
S
INCE the year 2000, ferromagnetic micro- and nano-structures have drawn more and more attention [1]–[3], especially the circular shape, as it easily forms stable flux-closure states that can significantly reduce field leakage and avoid the influence of edge domains. Ferromagnetic rings not only have potential applications on data storage systems [4], but are also a good physical system for investigating the domain wall motion, domain wall nucleation and annihilation, and magnetic reversal process due to their symmetric shape. In most of the previous works [5]–[9], it is found that in a magnetization reversal process an onion state transforms into flux-closure state and finally to the reverse onion state. Generally speaking, the magnetic configurations are determined by geometric parameters, such as diameter, width, and thickness [10]. It was also proposed that surface roughness of magnetic thin films influences magnetic properties, such as magnetic anisotropy, coercivity, magnetoresistance (MR), and magnetic domain structure [11]. Although there have been extensive studies on the relationship between surface roughness and magnetic properties in various magnetic materials such as Co [12], permalloy [13], Fe/Co [14] in continuous thin films, little work has been devoted to the influence of surface roughness on magnetic properties of ferromagnetic patterned geometries, which are promising for the applications of anisotropic magnetoresistance (AMR) and giant magnetoresistance (GMR) sensors. In this study, we investigate the effect of surface roughness on magnetoresistance of a micro-scale permalloy ring element. The surface roughness is controlled by chemistry etching time in this experiment. In addition to the surface roughness, the film thickness that also determines the magnetic properties of the films [15] is discussed in this paper. Experimental measurements of
Manuscript received May 06, 2013; accepted July 08, 2013. Date of current version December 23, 2013. Corresponding author: M.-F. Lai (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2013.2273875
II. EXPERIMENT The permalloy rings were fabricated by a two-step standard electron beam lithography (JEOL, JSM-6390, Japan) and electron-beam evaporation (FULINTEC, FU-PEB-500, Taiwan) through liftoff procedures. First, ring patterns were exposed in the resist, polymethyl methacrylate (PMMA) on the SiO -coated silicon substrate using an electron beam. A commercial scanning electron microscope was modified for direct writing. Later, the samples were developed in a 3:1 mixture of 2-propanol and methyl isobutyl ketone, and then the ring-shaped trenches were formed in PMMA. We used the electron-beam evaporation system for the deposition of the Permalloy material, and the thickness was monitored by a quartz crystal. After the liftoff process in acetone, all resist was removed and the Permalloy rings were obtained on the SiO -coated silicon substrate. One more step of electron beam lithography was needed here to make the current/voltage electrodes of nonmagnetic material. Normally 10 nm of Cr followed by 120 nm of Au are used. Subsequently, surface roughness was obtained by chemical etching process using buffered oxide etchant. By controlling etching time, ring elements with various surface roughnesses could be obtained. In our study, the outer diameter, inner diameter, line width, and thickness of each ring m, m, and nm, respectively. were m, The surface morphologies of the ring samples were analyzed using atomic force microscope [(AFM) Bruker, Innova, Santa Barbara, CA, USA]. The analysis of the surface roughness based on AFM images was performed by the image analysis software. We measured the magnetoresistance at room temperature using A dc sensing current and apply in-plane magnetic fields in the direction perpendicular to the axis connecting the two electrodes [16]. Fig. 1 shows the scanning electron microscopy , (SEM) image of the experimental setup. The four Au leads , , and , and the direction of the external field are indicated on the figure. The MR loops of the ring were measured Oe applied field with 5 Oe field step. between
0018-9464 © 2013 IEEE
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IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 1, JANUARY 2014
Fig. 1. SEM picture of the experimental setup. Outer diameter, inner diameter, m, m, and nm, respectively. Four Au and thickness of the ring are , , , and , and the direction of the external field are indicated leads, on the figure.
III. RESULTS AND DISCUSSION A. Surface Morphology Fig. 2(a)–(d) shows AFM surface images of the permalloy ring elements after chemical etching. Ring surface becomes rougher with increasing etching time. The different degrees of surface roughness were obtained with different etching times. The average roughness (Ra) was obtained through the same five regions of ring structure with various degrees of roughness as indicated by the arrows in the Fig. 2(a)–(d). Fig. 2(a) shows the initial ring surface morphology without etching process, and the Ra is 4.2 nm. Fig. 2(b) shows the ring surface morphology with Ra nm after the ring was etched for 10 s. Fig. 2(c) shows the ring surface morphology with Ra nm after the ring was etched for 20 s. Fig. 2(d) shows the ring surface morphology with Ra nm after the ring was etched for 30 s. Fig. 2(e) indicates the profile uniformity of permalloy ring after etching process. It can be observed that the standard deviation of surface roughness increases with the etching time. There was no effect between the etching treatment and the composition of permalloy ring element confirmed by energy dispersive X-ray spectroscopy (EDS) (data not shown). B. Magnetic Properties Fig. 3 shows the experimental results of MR curves of four different surface roughnesses of rings with the same element. The onion state, flux-closure state (vortex state), and the reverse onion state, which correspond to the external field regions I, II, and III as the field sweeps down, respectively, are shown schematically in the lower parts of the figures. These spin configurations are similar to the ones observed and identified in [7] and [8]. In the transition from onion state (state I) to flux-closure state (state II) the spins form a closed flux loop configuration, and the magnetoresistance increases abruptly because in flux-closure state the magnetization and the current have the same orientation. In the last transition, from state flux-closure state (state II) to reverse onion state (state III), the magnetoresistance decreases considerably due to the nucleation of the two transverse domain walls in the reverse onion state.
Fig. 2. AFM images of etched rings with different surface roughness: (a) Ra nm, (b) Ra nm, (c) Ra nm, and (d) Ra nm. (e) Surface roughness of permalloy ring after various etching time.
In the sweep-up and down process each of the MR loops measurements exhibit two pronounced steps corresponding to nucleation fields (the first nucleation field, that is, from state I to state II) and (the second nucleation field, that is, from state II to state III). Between the two transition fields there is a high plateau, and MR curves of ferromagnetic rings with such plateaus have been observed and discussed in literature [17], [18]. From Fig. 3(a), it can be observed that is 25.24 Oe (average of the absolute values of and ), is 101 Oe (average of the absolute values of and ) and (field width of the plateau corresponding to the flux-closure state) is 23.98 Oe with Ra nm. From Fig. 3(b), it can be observed that is 24.2 Oe, is 70.6 Oe and is 23.04 Oe with Ra nm. From Fig. 3(c), it be can observed that is 23.22 Oe, is 69 Oe and is 22.06 Oe with Ra nm. From Fig. 3(d), it can be observed that is 11.7 Oe, is 67.4 Oe and is 24.8 Oe with Ra nm.
GER et al.: SURFACE ROUGHNESS EFFECTS ON MAGNETIZATION REVERSAL OF MAGNETIC RING ELEMENTS
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Fig. 4. (a) Relationship between the nucleation field and the average surface roughness Ra. (b) Relationship between the second nucleation field and the average surface roughness Ra. (c) Relationship between the field width and the average surface roughness Ra.
Fig. 3. MR curves of etched rings with different surface roughness. Spatial and relationships between the domains and leads are shown in the inset. are the nucleation fields of the flux-closure state in the sweep-down and and are the nucleation fields of the reverse sweep-up processes. is the field width onion state in the sweep-down and sweep-up processes. of the plateau of flux-closure state. (a) MR curves of etched ring for Ra nm; (b) MR curves of etched ring for Ra nm; (c) MR curves of nm; (d) MR curves of etched ring for Ra nm. etched ring for Ra
Fig. 4(a) shows the relationship between the first nucleation field and the average surface roughness Ra. From the figure, it can be observed that the nucleation field slightly decreases with the increase of roughness. As the roughness reached a certain degree where the started to decline obviously, it indicated the state of the ring element accessible to form the flux-closure state. Fig. 4(b) shows the relationship between the second nucleation field and the average surface roughness Ra. It also can be observed that significantly
decreases to the surface roughness. It implies a great effect on the lower field of nucleation of domain walls forming the reverse onion state for rings with larger surface roughness. It is worth noticing that the MR ratios are respectively 0.092%, 0.069%, 0.044% and 0.067% as the Ra are 4.2 nm, 11.4 nm, 14.6 nm and 21 nm. There was no direct correlation between the roughness and MR ratio. The transitions at and occur when one of the 180 domain walls sweeps through one side of the ring and annihilates the other wall, giving rise to a vortex state. The first and second nucleation fields and correspond to the depinning of domain walls. Therefore, from Fig. 4(a) and (b), it can be deduced that the domain wall pinning effect is lower during the magnetic switching for larger surface roughness, making it easier for generation and propagation of the nucleation of domain walls (see Fig. 5). The results correspond to our previously simulated and experimental results with triangular fins or notches in the rings element, which were designed to be located at the original nucleation regions to induce the deviation of the initial spin structures and the subsequent magnetization process. The pinning effect caused by the sharp edges of the fins leads to the increment of the depinning field [19], [20]. We also observe that though increasing the roughness would lower the
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IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 1, JANUARY 2014
Fig. 5. Schematic diagrams show the spin configuration with (a) nature nucleation center without surface roughness, and (b) roughness induced multinucleation center.
magnetic field of forming the flux-closure state and the reverse onion state, the field range for the flux-closure state is unaffected by the surface roughness, as shown in Fig. 4(c). IV. CONCLUSION In summary, we verify the influence of surface roughness on the nucleation fields of a micrometer scale permalloy ring. It was found that the first nucleation field and second nucleation field decreased with respect to the increase of surface roughness. The field width for flux-closure state did not receive much effect from the surface roughness. By controlling the roughness we can alter the magnetic field required for each magnetization state of magnetic ring elements. Our results can provide important information for future designs of magnetic ring-shaped data storage or spintronic devices. ACKNOWLEDGMENT This work was supported in part by the ROC National Science Council under Grant NSC 99-2112-M-007-016-MY3, Grant NSC 99-2112-M-007-015-MY3, and Grant NSC 102–2112–M–007–006–MY3, and Grant NSC 102-2112-M007-012-MY3. REFERENCES [1] T. R. Ger, H. T. Huang, W. Y. Chen, and M. F. Lai, “Magneticallycontrollable zigzag structures as cell microgripper,” Lab Chip, vol. 13, pp. 2364–2369, 2013.
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