Current induced multiple spin structures in nanoring magnetic tunnel junctions:  
H. X. Wei, F. Q. Zhu, X. F. Han, Z. C. Wen, and C. L. Chien

A nanoring magnet exhibits unique magnetic configurations unattainable in a disc magnet.  A magnetic nanoring can exhibit the onion state, the vortex state with no net magnetization, and the “twisted” state with a partial magnetization.   In a nanoring magnetic tunnel junction (MTJ), there are multiple configurations for exploitation, not just parallel and antiparallel in ordinary disk MTJs.  Even if we restrict the reference layer to be in the onion state, there are three states for the storage layer (Fig. 1e, 1f, and 1g) resulting in the MTJ three resistance values, R_P, R_AP, and R_TW.

Fig. 1: (a) an onion state with two DWs, and their motion under (b) a linear magnetic field and (c) a circulatory magnetic field. (d) Schematics of a NRMTJ with reference layer in the onion state. The storage layer can acquire (e) parallel onion state with R_P, (f) antiparallel onion state with R_AP, and (g) twisted state with R_TW.

We have recently achieved the first sub-µm nanoring MTJs with diameter as small as 100 nm and a ring width of 20 nm.  Starting from the onion state (Fig. 1a) with two poles (in fact two domain walls). Under a linear (or homogeneous) magnetic field, the two poles of the onion state move in opposite directions (Fig. 1b). However, under a spin current through the MTJ, the combined effect of spin transfer torque and the circulatory magnetic field, causes the onion state to revert to the “twisted state”.  Experimentally, we have observed these unique events as shown in Fig. 2.  For the 100-nm nanoring MTJ, the field-switching result (Fig. 2a) and the current-switching result (Fig. 2b) share the same low resistance state, but different high resistance state. Under current switching, the high resistance state (Fig. 2b) is only about 2/3 the value of the field-switching result (Fig. 2a), because the storage layer acquires the reverse onion state in Fig. 2a but the twisted state in Fig., 2b.

Fig. 2: Resistances vs. (a) linear magnetic field H and (b) current I of a 100-nm NR-MTJ.  (c) and (d) are the same measurements for a 200-nm NR-MTJ. The spin structures are shown in the insets.

H. X. Wei, F. Q. Zhu, X. F. Han, Z. C. Wen, and C. L. Chien, “Current-induced multiple spin structures in 100 nm ring magnetic tunnel junctions,” Phys. Rev. B. 77, 224432 (2008).

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