Magnetic patterning: local manipulation of the intergranular exchange coupling via grain boundary engineering

Abstract

Magnetic patterning, with designed spatial profile of the desired magnetic properties, has been a rising challenge for developing magnetic devices at nanoscale. Most existing methods rely on locally modifying magnetic anisotropy energy or saturation magnetization, and thus post stringent constraints on the adaptability in diverse applications. We propose an alternative route for magnetic patterning: by manipulating the local intergranular exchange coupling to tune lateral magnetic properties. As demonstration, the grain boundary structure of Co/Pt multilayers is engineered by thermal treatment, where the stress state of the multilayers and thus the intergranular exchange coupling can be modified. With Ag passivation layers on top of the Co/Pt multilayers, we can hinder the stress relaxation and grain boundary modification. Combining the pre-patterned Ag passivation layer with thermal treatment, we can design spatial variations of the magnetic properties by tuning the intergranular exchange coupling, which diversifies the magnetic patterning process and extends its feasibility for varieties of new devices.

Figure 1. Demonstration on the proposed magnetic patterning method.

(a) A flowchart of the proposed magnetic patterning process. (b) Hysteresis loops of the magnetic patterned Co/Pt MLs acquired from different regions by FMOKE. (c) AFM (upper row) and corresponding MFM images (lower row) of magnetic patterned Co/Pt MLs. The left (right) shows the partially saturated (ac-demagnetized) state with different width ratios of Ag-capped to uncapped stripes.

Figure 2. Sheet film properties after RTA process.

(a) Typical out-of-plane hysteresis loops and its initial curves of Co/Pt MLs with different RTA temperatures (T_ann) and capping conditions. The examples for defining H_n and H_p are also shown here. (b) Evolutions of H_n and H_p with increasing T_ann. (c) Dependence between H_p and H_n. (d) Evolutions of M_S and H_K with different T_ann and capping conditions.

Figure 3. Magnetic analysis on the magnetic patterned Co/Pt MLs.

(a) ac-demagnetized MFM images of the uncapped (upper row) and the Ag-capped samples (lower row) at the as-deposited state, and after 250 ^oC, and 350 ^oC RTA. (b) ΔM curves with different T_ann and capping conditions. (c) T_ann dependence of the ac-demagnetized domain size (D_domain) and activation volume (V_act). (d) Dependence between D_domain and V_act. (e) Correlation between the reciprocal of activation volume (1/V_act) and nucleation field (H_n) of samples with different T_ann. The red and blue arrows indicate the different amount of activation volume change from the as-deposited to the 350 ^oC annealed for uncapped and Ag-capped cases, respectively.

Figure 4

Structural and microstructural analyses of the Co/Pt MLs with different capping conditions (a) θ–2θ X-ray diffraction patterns of Co/Pt MLs with different T_ann. The central (n = 0) and satellite (n = −2, −1, 1) peaks of the uncapped (Ag-capped) samples with increasing T_ann are shown in the upper panel (lower panel). (b) Dependence between the reciprocal of activation volume (1/V_act) and the change of strain (−Δε_z). (c) STEM-HAADF images and corresponding STEM-EELS mapping of Co L_2,3 edge are shown for the as-deposited and 350 ^oC annealed uncapped samples. The red dashlines on HAADF images indicate the position of grain boundary; The yellow dash-lines on the Co mapping point out the offset of Co layers between adjacent grains. (d) Co line scans of the 350^oC annealed sample extracted from the Co mapping (the same image shown in (c), but rotated by 90^o) on different positions: the green line shows the Co distribution across the whole grain, and the red line is along the grain boundary (as indicated on the Co mapping image). The blue circles on the Co line scan along the grain boundary indicate the distribution of out-diffusion Co via grain boundary creep.

Figure 5

Magnetic patterning by Joule heating (a) A sketch of the process flow for Joule heating annealing, where individual wire can be selected for magnetic patterning. (b) Hysteresis loops obtained by FMOKE at different regions on the magnetic patterned wire. The insertion of (b) shows the MFM image of magnetic patterned magnetic wire at the partially saturated states. The Ag-capped region is indicated by blue dot-squares with different width of Ag pre-pattern (4 and 2 μm, respectively).