APPLIED PHYSICS LETTERS 92, 202506 共2008兲
Probing the magnetic states in mesoscopic rings by synchronous transport measurements in ring-wire hybrid configuration S. Jain and A. O. Adeyeyea兲 Information Storage Materials Laboratory, Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117576, Singapore
共Received 3 April 2008; accepted 30 April 2008; published online 22 May 2008兲 The magnetization reversal of individual rings has been mapped using synchronous magnetotransport measurement of ring-wire hybrid structures. This method of characterization eliminates the effect of both short circuiting introduced by placing the electrical contact probes directly on the ring and the dependence of the magnetotransport response on the contact configuration used. We have characterized the switching behavior of circular and elliptical ring elements and observed clearly distinguishable spin states for different ring shapes. © 2008 American Institute of Physics. 关DOI: 10.1063/1.2936089兴 Magnetization reversal process in ring geometries has been extensively investigated in recent years.1–3 Of particular interest has been the ring geometry, which undergoes reversal from bidomain or onion state to flux closure vortex state and then to reverse-onion state. The control of the vortex chirality, also using asymmetric ring geometries,4–6 is the key to a successful implementation of any magnetoelectronic device incorporating magnetic rings. A previously published work on ferromagnetic rings involves the characterization of magnetization reversal process using magnetometry techniques,7 magneto-optical kerr effect8 共MOKE兲, diffracted MOKE,9 and photoemission electron microscopy.10 Recently, magnetotransport measurements have also been widely used to investigate the reversal mechanism and map the magnetization states in individual nanorings.11–13 This technique, however, was implemented by fabricating the contact probes directly on the ring. Consequently, the magnetoresistance 共MR兲 response is strongly dependent on the contact configuration used13 and also results in a detrimental shunting effect.14 Any probing technique that maps the spin states of the entire ring structure without any dependence on the electrical contact configuration is desirable. In this work, we describe a simple method for probing the giant MR 共GMR兲 response in elliptical and circular ring geometries using synchronous transport measurements in a ring-wire hybrid configuration. This method does not involve the fabrication of contact probes on the ring, but instead utilizes nanowires attached to each side of the ring, thus, eliminating any dependence on the contact geometry of the probes. Polycrystalline pseudospin valve ring-wire structures of configuration SiO2 / Co共10 nm兲 / Cu共8 nm兲 / Ni80Fe20共10 nm兲 / Cu共2 nm兲 were fabricated using multilevel lithography techniques followed by deposition and lift-off process. Elliptical ring elements of 5 m / 3 m major/minor diameter and circular ring elements of outer diameter 3 m were fabricated using electron-beam 共e-beam兲 lithography and e-beam deposition. The width of the rings was kept constant at 300 nm. The width of the nanowires was designed to be 150 nm, to allow different switching fields between the ring and the nanowires. Nonmagnetic contact probes were a兲
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fabricated on top of the nanowires using both optical and e-beam lithography. This was followed by thermal evaporation of Cr共10 nm兲 / Au共150 nm兲 material and lift-off processes. MR measurements were carried out using the standard four-probe technique. Figure 1共a兲 shows the scanning electron micrograph 共SEM兲 image of an elliptical ring in ring-wire hybrid configuration. A constant dc current was passed through the outer two probes, labeled as I+ and I−. Synchronous transport measurements were carried out by simultaneously probing the voltage from the two inner contact probes 共labeled as V1兲 and the two outer contact probes 共labeled as V2兲, respectively. Figure 1共b兲 shows the SEM micrograph of the circular ring in a similar configuration. Shown in Fig. 1共c兲 is an SEM image of a circular ring with a nanowire bisecting it at the center. When the electrical current is injected into the ring, the current divides itself into two parts. The corresponding resistance measured for section V1 is
FIG. 1. SEM micrographs of 共a兲 an elliptical ring with 5 m / 3 m major/ minor diameter and ring width of 300 nm, 共b兲 circular ring of 3 m diameter and width 300 nm, and 共c兲 another circular ring of same dimensions but with a nanowire bisecting it in the center of width 150 nm, in ring-wire hybrid configuration, respectively.
0003-6951/2008/92共20兲/202506/3/$23.00 92, 202506-1 © 2008 American Institute of Physics Downloaded 29 May 2008 to 137.132.123.74. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp
202506-2
Appl. Phys. Lett. 92, 202506 共2008兲
S. Jain and A. O. Adeyeye
FIG. 2. 共Color online兲 MR response for elliptical and circular ring structures when the two sections, V1 and V2, are synchronously probed and field is applied along the x direction. For clarity, only one half of the loops are shown.
冉
冊
V1 R CR D = RA + + RB I RC + RD
共1兲
for the configuration in Figs. 1共a兲 and 1共b兲, and
冉
冊
R CR DR E V1 + RE = RA + I R CR D + R DR E + R ER C
共2兲
for the configuration in Fig. 1共c兲. The corresponding resistances are labeled in Fig. 1共c兲. Figure 2 shows the MR curves for the different geometries when the field is applied along the x direction. For clarity, only one half of the loop 共from 50 to − 300 Oe兲 is shown. The open circles correspond to the data from the ring-wire hybrid configuration 共V1兲 showing the switching fields of both the ring and the nanowires, respectively. The solid circles correspond to the response from the section V2, which probes only the nanowire segment. As expected, due to the synchronized transport measurements, we observe a clear superimposition of the two curves. Figure 2共a兲 shows the GMR curve for the elliptical ring when the applied field is from positive saturation to negative saturation. The elliptical ring in ring-wire hybrid configuration undergoes various transitions corresponding to the different resistance levels of E1, E2, E3, and E4, before the entire configuration reaches saturation. E1 is the state of highest resistance corresponding to the antiparallel alignment of the spin states for Ni80Fe20 and Co layers in the section V1. In this state, the Ni80Fe20 layer in the nanowire has reversed along with the Ni80Fe20 layer in the ring at the same switch-
ing field of −42 Oe. We observed that the transition of Ni80Fe20 ring from reverse-onion state to forward-onion state does not take place through the intermediate vortex state. A slight decrease in resistance from the state E1 to E2 corresponds to the Co layer in the elliptical ring forming twisted domain walls due to the magnetostatic interactions with the top Ni80Fe20 layer.11 The Co layer finally undergoes a transition to a vortex state when the field is further increased, resulting in a reduction of the resistance level to E3. The sharp drop in resistance at −155 Oe corresponds to the Co layer magnetization in the nanowire switching to the reverse saturation field direction, and thus, decreasing the overall resistance level to E4. This state, however, still corresponds to the vortex configuration of the Co layer in the elliptical ring. The final transition from vortex to reverse onion takes place at a field of −225 Oe. Thus, the stability of the Co vortex state exists for a field range of ⬃97 Oe 关as shown in Fig. 2共a兲兴. These results are in agreement with the work done on elliptical rings elsewhere.12 Therefore, by using the synchronous transport measurement technique, we are able to eliminate the nanowire switching field contributions from the MR response of the ring-wire hybrid configuration. This method also reduces the need to fabricate local contact probes on the ring, which introduces current-shunting effects as observed by Chao et al.14 The corresponding GMR curve for circular ring geometry is shown in Fig. 2共b兲. For this geometry, we observed three different distinct states. State R1 corresponds to the configuration when only the Ni80Fe20 magnetization in the nanowire segments has switched. With further increase in applied field, the Ni80Fe20 magnetization in the circular ring switches directly from forward-onion state to reverse-onion state without forming an intermediate vortex state, corresponding to state R2, where the entire structure is in antiparallel alignment, leading to the highest resistance level. A drop in the resistance level is observed when the Co layer in the nanowires switch to form a parallel alignment with the Ni80Fe20 spins at a field of −153 Oe, corresponding to resistance level R3. This is the state in which the Co ring is still in the forward-onion state and Ni80Fe20 is in the reverse-onion state. Total parallel alignment is achieved when the Co ring directly undergoes transition to reverse-onion state. This is in contrast to the reversal mechanism of the elliptical ring structure shown in Fig. 2共a兲, where the Co vortex state was highly stable. The nonoccurrence of Co vortex state in circular ring elements was also seen in Ref. 12. The reversal mechanism of a circular ring element with a nanowire bisecting at the center is shown in Fig. 2共c兲. The nanowire introduces pinning of the domain walls at its end points which forces the ring to be directly coupled with the nanowire segment. The effect of coupling is clearly observed in the MR response shown in Fig. 2共c兲. There is only a single switching observed at a field of −40 Oe at which the Ni80Fe20 layer of the entire V1 section switches at the same time and increases the resistance to level S1. This configuration remains stable for a wide field range of 70 Oe, after which the Co layer in the circular ring reverses first 关in contrast to the circular ring of Fig. 1共b兲 where the Co ring switches at a higher field兴. This is followed by the reversal of Co magnetization in the nanowire segments. It is clearly evident from our results that this method of spin-state mapping is highly sensitive to the type and shape of the ring geometry.
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202506-3
Appl. Phys. Lett. 92, 202506 共2008兲
S. Jain and A. O. Adeyeye
FIG. 3. 共Color online兲 Low-field MR response for the two circular ring structures when the magnetic field is applied along the x direction.
To further substantiate the results, we investigated the MR behavior at low fields for circular rings shown in Figs. 1共b兲 and 1共c兲. For the low-field MR measurements, the devices are first saturated in a field of −4 kOe and then reversed to a small positive field value of +150 Oe. The field is then reversed back to the negative saturation. Figure 3共a兲 shows the low-field GMR response when the state R3 has been reached. This state corresponds to the Co ring in its initial onion state, Ni80Fe20 ring transitioned to the reverseonion state, and the Ni80Fe20 and Co layers of the nanowires switched to the applied field direction. When the field is reversed back to the negative saturation from this state, the resistance remains constant till remenance, after which a slight increase in the field reduces the resistance level to R4. This corresponds to the Ni80Fe20 ring switching back to the initial onion state and having a parallel alignment with the Co layer magnetization. This is a position of complete parallel alignment of the magnetization in the entire ring-wire hybrid structure. Further increase in applied field switches the Ni80Fe20 spins in the nanowires to the applied field direction and state R5 is achieved. The resistance levels of both R5 and R1 are similar as they both correspond to similar magnetization states. Increasing the field switches the Co spins as well and a state of parallel alignment is achieved. For the circular ring bisected with a nanowire at the center, as shown in the inset of Fig. 3共b兲, the field is reversed when state S2 has been reached. The coupling between the Ni80Fe20 layers in both the ring and the bisected nanowire is clearly evident as they both switch at similar switching fields. The resistance level remains constant while the field is being reversed, until a value of −5 Oe is reached where the Ni80Fe20 ring switches back to the initial forward-onion state
and attains a higher resistance level due to its antiparallel alignment with the Co ring 共shown as state S3兲. At a slightly higher field, the Ni80Fe20 layer in the nanowire segments switches back to its initial configuration, bringing the resistance level to S4. However, we also observed an additional state S5 which corresponds to the Co ring undergoing a transition from reverse-onion state to a vortex state. This vortex state was not evident in the major loops shown in Fig. 2共c兲. Generally, the vortex configuration in a ring structure is highly dependent on its initial domain configuration at low fields. In this case, the reverse onion state reached at level S2 for the Co layer ring is responsible for its intermediate vortex state, although the stability range for this state is only ⬃20 Oe. In summary, we have demonstrated a highly efficient probing and sensitive technique for investigating the magnetization reversal mechanism for elliptical and circular ring geometries in ring-wire hybrid configuration. This technique of synchronous transport measurement has been used to clearly identify the distinct spin states of various ring elements without physically fabricating any contact probes on the ring itself. We have also shown that this method is extremely sensitive to the ring shape and measurement configurations. This work was supported by Ministry of Education 共MOE兲 under Grant No. R-263-000-437-112 and the Singapore MIT Alliance. One of the authors 共S. J.兲 would like to thank National University of Singapore 共NUS兲 for her research scholarship. 1
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