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Register a new userHigh domain wall velocity at zero magnetic field induced by low current densities in spin-valve nanostripes
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Stefania Pizzini
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Current-induced magnetic domain wall motion at zero magnetic field is observed in the permalloy layer of a spin-valve-based nanostripe using photoemission electron microscopy. The domain wall movement is hampered by pinning sites, but in between them high domain wall velocities (exceeding 150 m/s) are obtained for current densities well below $10^{12} unit{A/m^2}$, suggesting that these trilayer systems are promising for applications in domain wall devices in case of well controlled pinning positions. Vertical spin currents in these structures provide a potential explanation for the increase in domain wall velocity at low current densities.
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We present experimental results on the displacement of a domain wall by injection of a dc current through the wall. The samples are 1 micron wide long stripes of a CoO/Co/Cu/NiFe classical spin valve structure. The stripes have been patterned by electron beam lithography. A neck has been defined at 1/3 of the total length of the stripe and is a pinning center for the domain walls, as shown by the steps of the giant magnetoresistance curves at intermediate levels (1/3 or 2/3) between the resistances corresponding to the parallel and antiparallel configurations. We show by electric transport measurements that, once a wall is trapped, it can be moved by injecting a dc current higher than a threshold current of the order of magnitude of 10^7 A/cm^2. We discuss the different possible origins of this effect, i.e. local magnetic field created by the current and/or spin transfer from spin polarized current.
Domain wall motion induced by nanosecond current pulses in nanostripes with perpendicular magnetic anisotropy (Pt/Co/AlO$_x$) is shown to exhibit negligible inertia. Time-resolved magnetic microscopy during current pulses reveals that the domain walls start moving, with a constant speed, as soon as the current reaches a constant amplitude, and no or little motion takes place after the end of the pulse. The very low mass of these domain walls is attributed to the combination of their narrow width and high damping parameter $alpha$. Such a small inertia should allow accurate control of domain wall motion, by tuning the duration and amplitude of the current pulses.
Domain-wall magnetoresistance and low-frequency noise have been studied in epitaxial antiferromagnetically-coupled [Fe/Cr(001)]_10 multilayers and ferromagnetic Co line structures as a function of DC current intensity. In [Fe/Cr(001)]_10 multilayers a transition from excess to suppressed domain-wall induced 1/f noise above current densities of j_c ~ 2*10^5 A/cm^2 has been observed. In ferromagnetic Co line structures the domain wall related noise remains qualitatively unchanged up to current densities exceeding 10^6A/cm^2. Theoretical estimates of the critical current density for a synthetic Fe/Cr antiferromagnet suggest that this effect may be attributed to current-induced domain-wall motion that occurs via spin transfer torques.
Due to the difficulty in detecting and manipulating magnetic states of antiferromagnetic materials, studying their switching dynamics using electrical methods remains a challenging task. In this work, by employing heavy metal/rare earth-transition metal alloy bilayers, we experimentally studied current-induced domain wall dynamics in an antiferromagnetically coupled system. We show that the current-induced domain wall mobility reaches a maximum close to the angular momentum compensation. With experiment and modelling, we further reveal the internal structures of domain walls and the underlying mechanisms for their fast motion. We show that the chirality of the ferrimagnetic domain walls remains the same across the compensation points, suggesting that spin orientations of specific sublattices rather than net magnetization determine Dzyaloshinskii-Moriya interaction in heavy metal/ferrimagnet bilayers. The high current-induced domain wall mobility and the robust domain wall chirality in compensated ferrimagnetic material opens new opportunities for high-speed spintronic devices.
Active manipulation of spin waves is essential for the development of magnon-based technologies. Here, we demonstrate programmable spin-wave filtering by resetting the spin structure of a pinned 90$^circ$ N{e}el domain wall in a continuous CoFeB film with abrupt rotations of uniaxial magnetic anisotropy. Using phase-resolved micro-focused Brillouin light scattering and micromagnetic simulations, we show that broad 90$^circ$ head-to-head or tail-to-tail magnetic domain walls are transparent to spin waves over a broad frequency range. In contrast, magnetic switching to a 90$^circ$ head-to-tail configuration produces much narrower domain walls at the same pinning locations. Spin waves are strongly reflected by a resonance mode in these magnetic domain walls. Based on these results, we propose a magnetic spin-wave valve with two parallel domain walls. Switching the spin-wave valve from an open to a close state changes the transmission of spin waves from nearly 100% to 0% at the resonance frequency. This active control over spin-wave transport could be utilized in magnonic logic devices or non-volatile memory elements.
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