30 research outputs found
Respective influence of in-plane and out-of-plane spin-transfer torques in magnetization switching of perpendicular magnetic tunnel junctions
The relative contributions of in-plane (damping-like) and out-of-plane
(field-like) spin-transfer-torques in the magnetization switching of
out-of-plane magnetized magnetic tunnel junctions (pMTJ) has been theoretically
analyzed using the transformed Landau-Lifshitz (LL) equation with the STT
terms. It is demonstrated that in a pMTJ structure obeying macrospin dynamics,
the out-of-plane torque influences the precession frequency but it does not
contribute significantly to the STT switching process (in particular to the
switching time and switching current density), which is mostly determined by
the in-plane STT contribution. This conclusion is confirmed by finite
temperature and finite writing pulse macrospin simulations of the current-field
switching diagrams. It contrasts with the case of STT-switching in in-plane
magnetized MTJ in which the field-like term also influences the switching
critical current. This theoretical analysis was successfully applied to the
interpretation of voltage-field STT switching diagrams experimentally measured
on perpendicular MTJ pillars 36 nm in diameter, which exhibit macrospin-like
behavior. The physical nonequivalence of Landau and Gilbert dissipation terms
in presence of STT-induced dynamics is also discussed
The influence of intergranular interaction on the magnetization of the ensemble of oriented Stoner-Wohlfarth nanoparticles
We consider the influence of interparticle interaction on the magnetization
reversal in the oriented Stoner-Wohlfarth nanoparticles ensemble. To do so, we
solve a kinetic equation for the relaxation of the overall ensemble
magnetization to its equilibrium value in some effective mean field. Latter
field consists of external magnetic field and interaction mean field
proportional to the instantaneous value of above magnetization. We show that
the interparticle interaction influences the temperature dependence of a
coercive field. This influence manifests itself in the noticeable coercivity at
( is so-called blocking temperature). The above interaction
can also lead to a formation of the "superferromagnetic" state with correlated
directions of particle magnetic moments at . This state possesses
coercivity if the overall magnetization has a component directed along the easy
axis of each particle. We have shown that the coercive field in the
"superferromagnetic" state does not depend on measuring time. This time
influences both and the temperature dependence of coercive field at
. We corroborate our theoretical results by measurements on
nanogranular films (CoFeB)-(SiO) with concentration of
ferromagnetic particles close, but below percolation threshold
Ion irradiation-induced easy-cone anisotropy in double-MgO free layers for perpendicular magnetic tunnel junctions
We have used the ferromagnetic resonance in the X-band (9.37 GHz) to investigate the effect of 400 keV Ar+ irradiation on the perpendicular magnetic anisotropy (PMA) and Gilbert damping parameter, α, of double-MgO free layers designed for application in perpendicular magnetic tunnel junctions. The samples comprised a MgO / Fe72Co8B20 / X(0.2 nm) / Fe72Co8B20 / MgO layer stack, where X stands for an ultrathin Ta or W spacer. Samples with two different total FeCoB layer thicknesses, tFCB = 3.0 nm and tFCB = 2.6 nm, were irradiated with ion fluences ranging from 1012 cm-2 to 1016 cm-2. The effective first-order PMA field, BK1, decreased nearly linearly with the logarithm of the fluence for both FeCoB thicknesses and spacer elements. The decrease in BK1, which is likely caused by an ion-induced intermixing at the FeCoB/MgO interfaces, resulted in a reorientation of the magnetization of the free layers with tFCB = 2.6 nm, initially exhibiting a perpendicular easy-axis anisotropy. For intermediate fluences, 1013 cm-2 and 1014 cm-2, easy-cone states with different cone angles could be induced in the free layer with a W spacer. Importantly, no corresponding increase in the Gilbert damping was observed. This study shows that ion irradiation can be used to tune the easy-cone anisotropy in perpendicular magnetic tunnel junctions, which is interesting for spintronic applications such as spin-torque magnetic memories, oscillators and sensors.publishe
Direct and converse magnetoelectric effects in Metglas/LiNbO3/Metglas trilayers
Electromechanical and magnetoelectric properties of Metglas/LiNbO3/Metglas trilayers have been studied in the frequency range from 20 Hz to 0.4 MHz. A trilayer of Metglas/PMN-PT/Metglas prepared in the same way was used as a reference. Though PMN-PT has much larger charge piezocoefficients than LiNbO3 (LNO), the direct magnetoelectric voltage coefficient is found to be comparable in both trilayers due to the much lower dielectric permittivity of LNO. The magnitude of the direct magnetoelectric effect in the LNO trilayers is about 0.4 V/cm Oe in the quasistatic regime and about 90 V/cm Oe at the electromechanical resonance. Calculations show that the magnetoelectric properties can be significantly improved (up to 500 V/cm Oe) via controlling the cut angle of LNO, choosing the appropriate thickness ratio of the ferroelectric/ferromagnetic layers and a better bonding between Metglas and LNO. Advantages of using LiNbO3-type ferroelectrics in magnetoelectric composites are discussed. (C) 2013 AIP Publishing LLC
Impact of ion irradiation-induced interface intermixing on the magnetic and electrical properties of magnetic tunnel junctions
The impact of 400 keV Ar+ irradiation on the magnetic and electrical properties of in-plane magnetized magnetic tunnel junction (MTJ) stacks was investigated by ferromagnetic resonance, vibrating sample magnetometry and current-in-plane tunneling techniques. The ion fluences ranged from 10^12 cm−2 to 5 × 10^15 cm−2. Below 10^14 cm−2, the anisotropy of the Ta-capped FeCoB free layer was weakly modulated, following a decrease in the saturation magnetization. The tunnel magnetoresistance (TMR), along with the exchange-bias and the interlayer exchange coupling providing a stable magnetic configuration to the reference layer, decreased continuously. Above 10^14 cm−2, a strong decrease in the saturation magnetization was accompanied by a loss of the magnetic coupling and of the TMR. We show there is an ion-fluence window where the modulation of magnetic anisotropy can occur while preserving a large TMR and stable magnetic configuration of the MTJ, and demonstrate that the layers surrounding the free layer play a decisive role in determining the trend of the magnetic anisotropy modulation resulting from the irradiation. Our results provide guidance for the tailoring of MTJ parameters via ion irradiation, which we propose as a potentially suitable technique for setting the magnetic easy-cone state in MTJ for attaining field-free, fast, and non-stochastic magnetization switching.publishe
First-principles quantum transport modeling of spin-transfer and spin-orbit torques in magnetic multilayers
We review a unified approach for computing: (i) spin-transfer torque in
magnetic trilayers like spin-valves and magnetic tunnel junction, where
injected charge current flows perpendicularly to interfaces; and (ii)
spin-orbit torque in magnetic bilayers of the type
ferromagnet/spin-orbit-coupled-material, where injected charge current flows
parallel to the interface. Our approach requires to construct the torque
operator for a given Hamiltonian of the device and the steady-state
nonequilibrium density matrix, where the latter is expressed in terms of the
nonequilibrium Green's functions and split into three contributions. Tracing
these contributions with the torque operator automatically yields field-like
and damping-like components of spin-transfer torque or spin-orbit torque
vector, which is particularly advantageous for spin-orbit torque where the
direction of these components depends on the unknown-in-advance orientation of
the current-driven nonequilibrium spin density in the presence of spin-orbit
coupling. We provide illustrative examples by computing spin-transfer torque in
a one-dimensional toy model of a magnetic tunnel junction and realistic
Co/Cu/Co spin-valve, both of which are described by first-principles
Hamiltonians obtained from noncollinear density functional theory calculations;
as well as spin-orbit torque in a ferromagnetic layer described by a
tight-binding Hamiltonian which includes spin-orbit proximity effect within
ferromagnetic monolayers assumed to be generated by the adjacent monolayer
transition metal dichalcogenide.Comment: 22 pages, 9 figures, PDFLaTeX; prepared for Springer Handbook of
Materials Modeling, Volume 2 Applications: Current and Emerging Material