Phase coexistence and transitions between antiferromagnetic and ferromagnetic states in a synthetic antiferromagnet

In synthetic antiferromagnets (SAFs), antiferromagnetic (AFM) order and synthesis using conventional sputtering techniques is combined to produce systems that are advantageous for spintronics applications. Here we present the preparation and study of SAF multilayers possessing both perpendicular magnetic anisotropy and the Dzyaloshinskii-Moriya interaction. The multilayers have an antiferromagnetically aligned ground state but can be forced into a full ferromagnetic (FM) alignment by applying an out-of-plane field $\sim 100$ mT. We study the spin textures in these multilayers in their ground state as well as around the transition point between the AFM and FM states at fields $\sim$ 40 mT by imaging the spin textures using complementary methods: photoemission electron, magnetic force, and Lorentz transmission electron microscopies. The transformation into a FM state by field proceeds by a nucleation and growth process, where skyrmionic nuclei form and then broaden into regions containing a ferromagnetically aligned labyrinth pattern that eventually occupies the whole film. Remarkably, this process occurs without any significant change in the net magnetic moment of the multilayer. The mix of antiferromagnetically and ferromagnetically aligned regions on the micron scale in the middle of this transition is reminiscent of a first-order phase transition that exhibits phase coexistence. These results are important for guiding the design of spintronic devices whose operation is based on spin textures in perpendicularly magnetized SAFs.

Figure 1

SAF multilayer structure. (a) SAF multilayer stack, with numbers in brackets denoting layer thicknesses in angstroms. (b). TEM image of a cross section showing a SAF multilayer deposited on top of an Si/${\mathrm{SiO}}_x$ substrate. (c). EELS chemical map from the box region in (b) displaying the elemental distribution of tantalum, platinum, cobalt, and iron.

Figure 2

SQUID VSM $M\left(H\right)$ loops at room temperature of the two samples: (a) data for S1 and (b) data for S2. Standard errors from the measurement of magnetization are presented; in addition, there is a systematic error from the volume normalization that is estimated to be 5%. (a) also includes a pictorial description of the magnetic state in each pair of SAF coupled layers at each step of the loop. Dashed boxes show the approximate regions of interest for the three microscopy techniques in Figs. 3–6. Here S1 and S2 are two samples grown in separate runs using the same nominal growth recipe as described in the text.

Figure 3

PEEM imaging of remanent SAF states. (a) Magnified view of the low-field region of the $M\left(H\right)$ loop in Fig. 2, showing the transition between the two remanent SAF states. Arrows show the fields applied prior to acquiring images at remanence. photoemission electron microscope images taken at zero field after (b) an alternating demagnetization cycle, (c) an applied field of $+12$ mT, and (d) an applied field of $+15$ mT.

Figure 4

MFM imaging of AFM-FM phase coexistence. (a) Magnified view of the transition region of the $M\left(H\right)$ loop in Fig. 2, showing the transition from the SAF state of the sample into the FM state, and the fields applied during MFM imaging. Magnetic force microscope images acquired at applied out-of-plane fields of (b) 36, (c) 39, (d) 41, (e) 44, and (f) 46 mT. There is likely a small offset in the true field experienced by the sample area due to the stray field of the tip that is expected to be $\sim 10$ mT. The associated schematics show the nature of the coupling at each image. Initially, a section of one layer flips into FM alignment, then grows and starts to encompass domain structure, which goes on to expand over the whole image area.

Figure 5

Fresnel LTEM images of S2 around the transition from AFM to FM alignment, showing the same sample area in each image with the domains growing to encapsulate more complex textures. The bubblelike features are structural defects in the imaging area. Fields the images were measured in were (a) $-67.2$, (b) $-69.1$, and (c) $-69.7$ mT, applied out of plane with respect to the sample. The dashed line and arrow in (a) represent the axis and direction of tilt of imaging.

Figure 6

Quantitative imaging of the mixed-phase state using DPC Lorentz TEM. (a) DPC image of S2 at an applied field of $-69.1$ mT. The dashed line in the top right corner shows the tilt axis, and the arrow shows the direction of the IP field. (b). Line traces of the integrated inductance through the two regions shaded in (a). Horizontal dashed lines are guides to the eye at $\pm 4$ and 0.5 T nm.