[Show abstract][Hide abstract] ABSTRACT: The operation of racetrack memories is based on the motion of domain walls in atomically thin, perpendicularly magnetized nanowires, which are interfaced with adjacent metal layers with high spin-orbit coupling. Such domain walls have a chiral Néel structure and can be moved efficiently by electrical currents. High-capacity racetrack memory requires closely packed domain walls, but their density is limited by dipolar coupling from their fringing magnetic fields. These fields can be eliminated using a synthetic antiferromagnetic structure composed of two magnetic sub-layers, exchange-coupled via an ultrathin antiferromagnetic-coupling spacer layer. Here, we show that nanosecond-long current pulses can move domain walls in synthetic antiferromagnetic racetracks that have almost zero net magnetization. The domain walls can be moved even more efficiently and at much higher speeds (up to ∼750 m s(-1)) compared with similar racetracks in which the sub-layers are coupled ferromagnetically. This is due to a stabilization of the Néel domain wall structure, and an exchange coupling torque that is directly proportional to the strength of the antiferromagnetic exchange coupling between the two sub-layers. Moreover, the dependence of the wall velocity on the magnetic field applied along the nanowire is distinct from that of the single-layer racetrack due to the exchange coupling torque. The high domain wall velocities in racetracks that have no net magnetization allow for densely packed yet highly efficient domain-wall-based spintronics.
Full-text · Article · Feb 2015 · Nature Nanotechnology
[Show abstract][Hide abstract] ABSTRACT: We demonstrate a highly efficient and simple scheme for injecting domain walls into magnetic nano-wires. The spin transfer torque from nanosecond long, uni-polar, current pulses that cross a 90° magnetization boundary together with the fringing magnetic fields inherently prevalent at the boundary, allow for the injection of single or a continual stream of domain walls. Remarkably, the currents needed for this "in-line" domain-wall injection scheme are at least one hundred times smaller than conventional methods.
[Show abstract][Hide abstract] ABSTRACT: Domain walls can be driven by current at very high speeds in nanowires formed from ultra-thin, perpendicularly magnetized cobalt layers and cobalt/nickel multilayers deposited on platinum underlayers due to a chiral spin torque. An important feature of this torque is a magnetic chiral exchange field that each domain wall senses and that can be measured by the applied magnetic field amplitude along the nanowire where the domain walls stop moving irrespective of the magnitude of the current. Here we show that this torque is manifested when the magnetic layer is interfaced with metals that display a large proximity-induced magnetization, including iridium, palladium and platinum but not gold. A correlation between the strength of the chiral spin torque and the proximity-induced magnetic moment is demonstrated by interface engineering using atomically thin dusting layers. High domain velocities are found where there are large proximity-induced magnetizations in the interfaced metal layers.
Full-text · Article · May 2014 · Nature Communications
[Show abstract][Hide abstract] ABSTRACT: Spin-polarized currents provide a powerful means of manipulating the magnetization of nanodevices, and give rise to spin transfer torques that can drive magnetic domain walls along nanowires. In ultrathin magnetic wires, domain walls are found to move in the opposite direction to that expected from bulk spin transfer torques, and also at much higher speeds. Here we show that this is due to two intertwined phenomena, both derived from spin-orbit interactions. By measuring the influence of magnetic fields on current-driven domain-wall motion in perpendicularly magnetized Co/Ni/Co trilayers, we find an internal effective magnetic field acting on each domain wall, the direction of which alternates between successive domain walls. This chiral effective field arises from a Dzyaloshinskii-Moriya interaction at the Co/Pt interfaces and, in concert with spin Hall currents, drives the domain walls in lock-step along the nanowire. Elucidating the mechanism for the manipulation of domain walls in ultrathin magnetic films will enable the development of new families of spintronic devices.
Full-text · Article · Jun 2013 · Nature Nanotechnology
[Show abstract][Hide abstract] ABSTRACT: Unconventional current-driven domain wall motion (CDDWM) faster than 300
m/s has been reported in perpendicularly magnetized atomically thin
Co/Ni bilayers deposited on Pt underlayers , making these materials
extremely promising for DW-based memory and logic devices. The driving
force of the motion is believed to originate from Pt/Co interface. To
investigate the interfacial effect we carried out AMR and AHE experiment
on thin Pt/Co/Ni and Au/Co/Ni perpendicularly magnetized layers since
CDDWM in Au/Co/Ni is induced by conventional spin transfer torque. We
find that AMR ratios on Pt/Co/Ni as functions of magnitude and
orientation of applied magnetic fields are significantly different from
those on Au/Co/Ni. Also it is observed that the AHE values for Pt/Co/Ni
are greater than those for Au/Co/Ni by more than one order of magnitude.
Recently anisotropic interface magnetoresistance  was reported in
Pt/Co/Pt sandwiches, but its underlying mechanism is not yet to be
understood. Here we discuss the origin of the unconventional AHE and AMR
behaviors from Pt/Co/Ni.  Kwang-su Ryu, Luc Thomas, See-Hun Yang,
S.S.P. Parkin, Applied Physics Express 5, 093006 (2012).  A. Kobs et
al, PRL 106, 217207 (2011).
[Show abstract][Hide abstract] ABSTRACT: Spin-orbit-induced anisotropic transport in magnetic materials, studied
for more than a century, has recently experienced a renewed interest
thanks to the formulation of anisotropic spin scattering in terms of
Berry's curvature. Anisotropic magnetoresistance (AMR) is related to the
scattering of the transport electrons on the orbitals of localized
electrons, depending on the magnetization direction. The contributions
of the interfaces on AMR has been scarcely studied. We consider a
trilayer composed of one ferromagnetic layer sandwiched between two
normal metals. The normal metals display spin Hall effect (SHE), whereas
the ferromagnetic layer polarize the flowing current. We propose that
SHE present in the top and bottom layers might contribute to the AMR.
The charge and spin currents are analyzed by drift-diffusion equations
including the role of inverse SHE as well as anomalous Hall effect.
Longitudinal and transverse spin accumulations at the interfaces are
captured through spin dependent conductance and the mixing conductance.
It is shown that the presence of a spin accumulation in the normal metal
close to the interface is transformed into a charge current through
inverse SHE hence altering the conductivity of the normal metal. The
obtained total resistivity calculation indicates its own spin
accumulation profile dependance.
[Show abstract][Hide abstract] ABSTRACT: We report an asymmetry of magnetic disorder in exchange-biased IrMn(tIrMn=5–20 nm)/CoFe(50 nm) films observed by means of a Kerr microscope, capable of direct domain observation. From the correlation between the magnetization half-reversal time and applied magnetic field, we find that the magnetization switching in all the films occurs via a thermally activated reversal mechanism for both branches of hysteresis loops. Surprisingly, in the forward branch reversal where the applied magnetic field is antiparallel to the direction of exchange-bias field, degree of magnetic disorder decreases as exchange-bias field increases, which is definitely contrasted with the case of backward branch reversal. This result is likely ascribed to the fact that the local values of exchange-bias field and coercive field are oppositely fluctuating with each other in the film.
Full-text · Article · Jan 2013 · Journal of Magnetism and Magnetic Materials
[Show abstract][Hide abstract] ABSTRACT: Kerr microscopy is used to investigate domain wall motion in response to
nanosecond-long current pulses in perpendicularly magnetized
micron-sized Co/Ni/Co racetracks. Domain wall velocities greater than
300 m/s are observed. The velocity is independent of the pulse length
for a wide range of current densities. However, the domain wall dynamics
depends on the pulse length just above the threshold current for motion,
where slow creep motion occurs, and at very high current densities,
where domain nucleation takes place. We also observe a tilting of the
domain wall that cannot be accounted for by the Oersted field from the
[Show abstract][Hide abstract] ABSTRACT: In this study, we investigated the stochastic nature of domain reversal dynamics in exchange-biased IrMn/CoFe film using a time-resolved Kerr microscope. Interestingly, the statistical distributions of the magnetization half-reversal times for both forward and backward applied fields show that the magnetization reversal dynamics is much more stochastic for the backward branch, where an applied field is parallel to the exchange-bias field direction. The enhanced stochasticity is ascribed to the large degree of magnetic disorder during backward reversal, which induces discrete and random Barkhausen jumps, whereas the forward branch reversal is dominated by a thermally activated depinning process caused by a single potential barrier. This result can be explained by the asymmetry of the magnetic disorder between both branches of a hysteresis loop.
Full-text · Article · Mar 2012 · Journal of Applied Physics
[Show abstract][Hide abstract] ABSTRACT: One of the key problems for realization of domain wall motion devices is
the reliable and energy efficient injection of domain walls (DWs) into
magnetic nanowires. In this work, we explore the injection of domain
walls in perpendicular magnetic anisotropy nanowires (Co/Ni multilayers)
which are locally softened by ion irradiation. We observe a minimum in
the domain wall injection critical current, which occurs where the
anisotropy of the irradiated region transitions from out of plane to in
plane anisotropy. Furthermore, we find that the irradiated site acts as
a pinning site for the DWs. At the irradiation site, we are able to
create localized nanosecond long pulsed magnetic fields used to inject
the DWs. By performing DC resistance measurements after each injection
event, we are able to probe for the existence of the domain wall, and
also find the strength of the ion irradiated pinning site. Using the
above technique, we have demonstrated a five fold reduction in the
domain wall injection current.
[Show abstract][Hide abstract] ABSTRACT: We have demonstrated the key components of the Racetrack Memory. In particular, we have shown that a series of DWs can be moved at high speed along magnetic nanowires by using nanosecond long current pulses for both in-plane and perpendicularly magnetized racetracks. The spacing between DWs has been reduced significantly by using a perpendicularly magnetized RT. Higher storage densities can be achieved by optimizing the materials of the RT (Fig.11) and by scaling the racetrack width.
No preview · Article · Dec 2011 · Electron Devices Meeting, 1988. IEDM '88. Technical Digest., International
[Show abstract][Hide abstract] ABSTRACT: We report in this paper the decrease of the stochasticity of magnetization half-reversal time with increasing domain wall (DW) pinning in Fe films investigated by means of time-resolved magneto-optic Kerr microscopy. The domain images in the films reveal that the density of DW pinning sites increases with increasing Fe thickness. However, we found that the stochasticity of the magnetization half-reversal time significantly decreases with increasing DW pinning. The major reason for the reduced stochasticity is shown to be due to a thermally activated DW creep mechanism that becomes dominant during magnetization reversal due to increased DW pinning.
Full-text · Article · Aug 2011 · New Journal of Physics
[Show abstract][Hide abstract] ABSTRACT: We have investigated the Barkhausen critical scaling behavior of NixFe1−x (x = 0−0.5) films using a magneto-optical microscope, capable of time-resolved domain observation. Real-time direct observations of the domain evolutions in these films revealed that magnetization reversal occurred with a sequence of random Barkhausen jumps. From more than 1000 repeated experiments with each sample, it was found that the distribution of the Barkhausen jump size followed a power-law distribution. The scaling exponent of the power-law distribution was found to have the same value of 1.1, independent of the film composition, revealing a universal critical scaling behavior in these alloy films.
Preview · Article · Mar 2011 · Journal of Applied Physics
[Show abstract][Hide abstract] ABSTRACT: We report a breakdown of Barkhausen critical-scaling behavior in NiO/Fe films with increasing domain-wall pinning by means of a Kerr microscope capable of direct domain observation. The time-resolved domain images in the films revealed that the Barkhausen jump size is generally decreased with increasing NiO thickness, showing an enhanced domain-wall pinning effect. Interestingly enough, the power-law scaling behavior of the Barkhausen jump size distribution gradually disappears as pinning of domain walls in the Fe layer is increased. This is ascribed to the fact that the magnetization reversal mechanism is changed from a Barkhausen avalanche dominant mode to domain-wall creep dominant mode.
[Show abstract][Hide abstract] ABSTRACT: We report a large converse magnetoelectric (CME) effect at room temperature in a multiferroic heterostructure formed from thin layers of perpendicularly magnetized CoxPd1-x alloys deposited on a piezoelectric single-crystal of lead magnesium niobate-lead titanate PMN-PT(001). The CME results from a strain-induced reorientation of the CoPd magnetization. By varying the composition and thickness of the CoxPd1-x film, a large converse magnetoelectric coupling constant, α = 8×10−7 s/m, at room temperature was found for 10 nm Co0.25Pd0.75. This large CME effect results from combining a highly magnetostrictive CoPd alloy with highly piezoelectric PMN-PT.
Preview · Article · Dec 2010 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: Spin precessions in the stripes of α-MnAs films prepared on GaAs(001) are investigated using an all-optical pump-probe method. We find that a large-angle spin precession appears while the stripe width decreases. In addition, the large-angle precession considerably changes the resonance frequency, resulting in a significant decrease in the relaxation time. These changes in the precessional motion are mainly ascribed to the dephasing of the nonuniform spin waves existing at the large-angle precession, as experimentally confirmed by varying the precession angle via tuning pump fluence. Micromagnetic simulations using a single Gilbert damping constant well predict the experimental observations, which verifies the interpretation of the change in the precessional motion.
[Show abstract][Hide abstract] ABSTRACT: We have investigated the magnetization reversal behavior in exchange-coupled NiO/Fe films with varying the NiO thicknesses using a magneto-optical microscope magnetometer capable of direct domain observation in real time. Interestingly enough, the magnetization reversal mechanism is gradually changed from a domain wall-motion process to a nucleation process as the NiO thickness increases. This result clearly demonstrates that the exchange coupling effect between the NiO and Fe layers increases the domain wall pinning effect of the Fe layer, resulting in the nucleation reversal mode.
Full-text · Article · May 2010 · Journal of Applied Physics
[Show abstract][Hide abstract] ABSTRACT: We present a systematic change of the magnetic domain structure with temperature in epitaxial ferromagnetic MnAs film on GaAs (001), observed in a wide temperature range of 15–45°C by magnetic force microscopy. Interestingly, it is found that, as temperature increases, the domain structure within the ferromagnetic α-MnAs stripes shows a mixture of head-on and simple domains at 15°C and then, takes a complete transition to simple ones above 15°C. This change could be understood by change in the demagnetizing factor of the cross-section of the ferromagnetic stripes with temperature.
[Show abstract][Hide abstract] ABSTRACT: We report the first simultaneous measurements of the magnetoresistance (MR) and the magnetic domain images in two-dimensional permalloy thin films by utilizing a magneto-optic magnetic microscope. In this way, we could directly map the MR to the domain patterns without any ambiguity. For 30-nm-thick permalloy films with in-plane uniaxial magnetization, negative MR was observed when the domain wall was nucleated. An analysis of the MR change with the anisotropic magnetoresistance resulted in a rather large domain wall thickness of 1.7 μm compared to the well-known theoretical value of 0.3 μm. We discuss possible origins of this discrepancy in terms of wide domain tails, wavy domain boundaries, and intrinsic properties associated with permalloy. This new method can be utilized to investigate the magnetic properties of newly emergent materials.
No preview · Article · Aug 2008 · Journal- Korean Physical Society