Magnetic domain texture and the Dzyaloshinskii-Moriya interaction in Pt/Co/IrMn and Pt/Co/FeMn thin films with perpendicular exchange bias

Risalat A. Khan, Hans T. Nembach, Mannan Ali, Justin M. Shaw, Christopher H. Marrows, and Thomas A. Moore

Phys. Rev. B 98, 064413 – Published 15 August 2018

Abstract

Antiferromagnetic materials present us with rich and exciting physics, which we can exploit to open new avenues in spintronic device applications. We explore perpendicularly magnetized exchange biased systems of Pt/Co/IrMn and Pt/Co/FeMn, where the crossover from paramagnetic to antiferromagnetic behavior in the IrMn and FeMn layers is accessed by varying the thickness. We demonstrate, through magneto-optical imaging, that the magnetic domain morphology of the ferromagnetic Co layer is influenced by the Néel order of the antiferromagnet (AFM) layers. We relate these variations to the anisotropy energy of the AFM layer and the ferromagnet-antiferromagnet (FM-AFM) interlayer exchange coupling. We also quantify the interfacial Dzyaloshinskii-Moriya interaction (DMI) in these systems by Brillouin light scattering spectroscopy. The DMI remains unchanged, within experimental uncertainty, for different phases of the AFM layers, which allows us to conclude that the DMI is largely insensitive to both AFM layer spin order and exchange bias. Understanding such fundamental mechanisms is crucial for the development of future devices employing chiral spin textures, such as Néel domain walls and skyrmions, in FM-AFM heterostructures.

Received 22 May 2018

DOI:https://doi.org/10.1103/PhysRevB.98.064413

Physics Subject Headings (PhySH)

Dzyaloshinskii-Moriya interaction Exchange bias Magnetic domains Magnetic phase transitions Spintronics

Antiferromagnets Thin films

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Risalat A. Khan^1,*, Hans T. Nembach², Mannan Ali¹, Justin M. Shaw², Christopher H. Marrows¹, and Thomas A. Moore¹

¹School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
²Quantum Electromagnetics Division, National Institute of Standards and Technology, Boulder, Colorado 80305, USA

^*R.A.Khan@leeds.ac.uk

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Vol. 98, Iss. 6 — 1 August 2018

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Images

Figure 1
Coercivity and exchange bias for the systems of Pt(2 nm)/Co( $t_{Co}$ )/IrMn(5 nm) and Pt(2 nm)/Co( $t_{Co}$ )/FeMn(4 nm): (a) Coercivity $μ_{0} H_{c}$ as a function of Co layer thickness $t_{Co}$ from which the optimum thickness is chosen to be 1 nm for the IrMn, and 0.6 nm for the FeMn system. The solid lines are guides to the eye. (b) Polar MOKE hysteresis loops of Pt(2 nm)/Co(1 nm)/IrMn(5 nm) and Pt(2 nm)/Co(0.6 nm)/FeMn(4 nm) measured at room temperature. (c) Exchange bias field $μ_{0} H_{ex}$ as a function of Co layer thickness.
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Figure 2
IrMn layer thickness dependence of the exchange bias field $μ_{0} H_{ex}$ (blue down triangles) and coercive field $μ_{0} H_{c}$ (red up triangles) for Pt(2 nm)/Co(1 nm)/IrMn( $t_{IrMn}$ ). The solid lines are guides to the eye. Onset of exchange bias occurs at $\approx 2.3 nm$ of IrMn, at which point the coercivity peaks.
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Figure 3
Kerr microscope difference images showing the propagation of magnetic domains after $μ_{0} H_{z}$ field pulses, which were applied for a few seconds, and ranged from 5 to 100 mT depending on the coercivity of the sample. The bright/dark regions (relative to grey) represent the areas swept out by DWs during the field pulse. The domain texture changes significantly as a function of IrMn layer thickness. At low IrMn thicknesses (paramagnet phase), the domains are large (a)–(b). At 1.7 nm thickness, the AFM phase sets in and bubble domains are nucleated from isolated pinning sites (c). An increase in nucleation density occurs (d) at a slightly thicker IrMn layer. Further increasing the thickness the DWs become rough (e)–(f) due to the onset of EB. Eventually, the DWs become rougher when the EB stabilizes (g)–(h).
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Figure 4
Temperature dependence of the coercivity $μ_{0} H_{c}$ (red up triangles) and exchange bias field $μ_{0} H_{ex}$ (blue down triangles) for selected IrMn thicknesses. The solid lines are guides to the eye. Samples were initially cooled to 5 K in a 200 mT field. The blocking ( $T_{B}$ ) and Néel temperatures ( $T_{N}$ ) of the three samples are indicated by vertical dashed lines. At 2 nm IrMn (a) $T_{B} = 200$ K, while $T_{N}$ lies just above room temperature. $T_{B}$ and $T_{N}$ move to lower temperatures as the IrMn thickness is decreased to 1.4 nm (b) and then to 1.1 nm (c).
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Figure 5
(a) FeMn layer thickness dependence of the exchange field $H_{ex}$ (blue down triangles) and coercive field $H_{c}$ (red up triangles). The solid lines are guides to the eye. (b)–(e) Kerr microscope difference images showing the changes in domain structure as a function of FeMn layer thickness.
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Figure 6
BLS spectra of the Damon-Eshbach spin-wave modes with a wave vector of $k = 16.7 μ m^{- 1}$ for a sample of Pt(2 nm)/ Co(1 nm)/IrMn(2.4 nm) with perpendicular magnetic anisotropy. The solid lines are fits of the data using the transmission function of the tandem multipass interferometer in the BLS spectrometer, ${({(f - f_{0})}^{2} - Δ f^{2})}^{- 6}$ , where $f$ is the frequency, $f_{0}$ is the resonance frequency, and $Δ f$ is the linewidth. The data are normalized using the fit.
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