Mapping motion of antiferromagnetic interfacial uncompensated magnetic moment in exchange-biased bilayers

In this work, disordered-IrMn3/insulating-Y3Fe5O12 exchange-biased bilayers are studied. The behavior of the net magnetic moment ΔmAFM in the antiferromagnet is directly probed by anomalous and planar Hall effects, and anisotropic magnetoresistance. The ΔmAFM is proved to come from the interfacial uncompensated magnetic moment. We demonstrate that the exchange bias and rotational hysteresis loss are induced by partial rotation and irreversible switching of the ΔmAFM. In the athermal training effect, the state of the ΔmAFM cannot be recovered after one cycle of hysteresis loop. This work highlights the fundamental role of the ΔmAFM in the exchange bias and facilitates the manipulation of antiferromagnetic spintronic devices.


A. Fabrication and measurement details
A series of IrMn 3 (=IrMn)/Y 3 Fe 5 O 12 (=YIG) (20 nm) bilayers were fabricated by pulsed laser deposition (PLD) and DC magnetron sputtering on (111)-oriented, single crystalline Gd 3 Ga 5 O 12 (GGG) substrates. The base pressures of the PLD and sputtering systems were 1.0 × 10 −6 Pa. The YIG layer was epitaxially grown via PLD from a stoichiometric polycrystalline target using a KrF excimer laser with the pule energy of 285 mJ. The substrate temperature was 625 • C during the deposition of the YIG layer. Then, the sample was annealed at the same temperature in an O 2 pressure of 1 × 10 4 Pa for 4 hours.
After the sample was cooled to the ambient temperature, it was transferred without the air exposure from the PLD chamber to the sputtering chamber through a load-lock chamber.
Afterwards, the IrMn layer was deposited at ambient temperature from an IrMn alloy target by magnetron sputtering, in order to avoid interfacial diffusion. The Ar pressure was 0.3 Pa during deposition of the IrMn layer. The deposition rate of IrMn was about 0.1 nm/s. Structural properties and film thickness were characterized by X-ray diffraction (XRD) and X-ray reflectivity (XRR) using a D8 Discover X-ray diffractometer with Cu Kα radiation (wavelength of about 1.54Å). The epitaxial growth of the YIG film was proved by pole figures with Φ and Ψ scan at 2θ fixed for the (008) reflection of the GGG substrate and YIG film. Transmission electronic microscopy (TEM) experiments were carried out in FEI/Philips CM-20 TEM with a LaB 6 filament ope rated at 200 kV at the Laboratory for Electron and X-ray Instrumentation, University of California Irvine. Cross-sectional TEM specimens were prepared in a FEI Quanta 3D FEG dual-beam system with focused ion beam (FIB). A typical FIB procedure recommended by FEI Company was used to prepare the specimens. The thin film was well protected by electron beam deposited Pt-layer before the film was exposed under the Ga-ion beam for further ion beam Pt deposition. The final thinning step using a low energy (2 kV) ion beam is crucial to minimize an amorphous layer, a damaged layer caused by Ga-ion beam, on both sides of the TEM specimens.
Magnetization hysteresis loops of the samples were measured using physics properties of measurement system. The magnetization (134 emu/cm 3 ) of the YIG film is close to 2 the theoretical value (131 emu/cm 3 ) and the coercivity is very small, 6 Oe. The films were patterned into normal Hall bar, and the transverse Hall resistivity (ρ xy ) and the longitudinal resistivity (ρ xx ) were measured by physical property measurement system (PPMS).  Figure S1 shows typical high resolution TEM images. The 20 nm thick YIG is grown epitaxially on the GGG (111) substrate ( Fig. S1(a)) and the IrMn layer is polycrystalline ( Fig. S1(b)). In Fig. S1(a), the fine Pt particles on top of the IrMn thin film form the protection layer which was deposited during TEM specimen preparation. The white area at the IrMn/YIG interface may be produced during the preparation of the TEM sample because the milling rate at the interface is slightly higher than that in YIG. The overlapping of YIG and IrMn was observed at the interface due to the YIG surface roughness, where the root mean square surface roughness of the YIG layer is 0.35 nm.

C. Anomalous Hall conductivity of IrMn single layer films
In comparison, a 5 nm thick IrMn single layer film was deposited at ambient temperature on the GGG substrate. Figure S2 shows that the Hall resistivity versus the external magnetic field H at 10 K. The Hall resistivity is proportional to the magnitude of the H, and the anomalous resistivity and thus the AHC equal zero. Therefore, the AHC in IrMn/YIG bilayers in Fig.2 in the text arises from the interaction between the IrMn and YIG layers and the noncollinear spin structure on the kagome lattice can be excluded in the explanations of the present galvanomagnetic results 1 .

D. Exchange bias training effect
In experiments, magnetic moments of the IrMn/YIG/GGG sample and the GGG substrate were first measured and the magnetic moment of the IrMn/YIG bilayer can then be obtained by subtracting the contribution of the substrate from the magnetic moment of the IrMn/YIG/GGG sample. Since the magnetic moment of the GGG substrate at low T is much larger than that of the IrMn/YIG bilayer, the magnetic noise is very large in the magnetization hysteresis loops. Since the noise becomes worse at low T , we can only show the magnetization hysteresis loops of the EB training effect at high T . Figure S3 shows that the magnetization loops with consecutive cycles. The exchange bias training effects measured by the planar Hall effect and magnetization loops are similar to each other and the reversal mechanism of the FM magnetization can be reflected by the ∆M AF M in planar Hall loops in Fig.3 in the text.