Magnetization reversal in perpendicular exchange-biased multilayers

Co/Pt multilayers with perpendicular magnetic anisotropy exhibit an exchange bias when covered with an IrMn layer. The exchange bias field, which is about 7 mT for 3 Co/Pt bilayer repetitions and a Co layer thickness of 5 Å, can be increased up to 16.5 mT by the insertion of a thin Pt layer at the Co/IrMn interface. The interfacial magnetic anisotropy of the Co/IrMn interface (KSCo/IrMn =-0.09 mJ/m2) favours in-plane magnetization and tends to tilt the Co spins away from the film normal. Dynamical measurements of the magnetization reversal process reveal that both thermally activated spin reversal in the IrMn layer and domain wall nucleation in the Co/Pt multilayer influence the interfacial spin structure and therefore the strength of the perpendicular exchange bias field.     

Influence of Pt thickness on magnetization reversal processes in (Pt/Co)3 multilayers with perpendicular anisotropy

We present a detailed study of the magnetization reversal in perpendicularly magnetized (Pt/Co)₃ multilayers with different values of the platinum interlayer thickness t_Pt. To study the magnetization reversal in our samples we combined measurements of relaxation curves with the direct visualization of domain structures. Magnetization reversal was dominated by domain wall propagation for t_Pt=1 nm and by domain nucleation for t_Pt=0.2 nm, while a mixed process was observed for t_Pt=0.8 nm. We interpret our results within the framework of a model of thermally activated reversal where a distribution of activation energy barriers is taken into account. The reversal process was correlated with the energy barrier distribution.     

Magnetization behavior of nanomagnets for patterned media application

Bit patterned media (BPM) which utilize each magnetic nanostructured dot as one recorded bit has attracted much interest as a promising candidate for future high-density magnetic recording. In this study, the magnetization reversal behaviors of nanostructured L1₀-FePt, Co/Pt multilayer (ML), and CoPt/Ru dots are investigated. For Co/Pt and CoPt/Ru nanodots, the bi-stable state is maintained in a very wide size range up to several hundred nm, and the magnetization reversal is dominated by the nucleation of a small reversed nucleus with the dimension of domain wall width. On the other hand, the critical size for the bi-stability of L1₀-FePt is about 60 nm, and its magnetization reversal proceeds via domain wall displacement even for such a small dot size. These reversal behaviors, depending on the magnetic materials, might be attributed to the difference in structural inhomogeneity, such as defects. In addition to the magnetic properties, the structural uniformity of the material could be crucial for the BPM application.     

Magnetization reversal dynamics in Au/Co/Au(111) ultrathin films: Effect of roughness of the buffer layer

We present a study of the magnetization reversal dynamics in ultrathin Au/Co/Au films with perpendicular magnetic anisotropy, for a Co thickness of 0.5, 0.7 and 1 nm. In these films, the magnetization reversal is dominated by domain nucleation for t_Co=0.5, 0.7 nm and by domain wall propagation for t_Co=1 nm. The prevalence of domain nucleation for the thickness range 0.5-0.7 nm is different from results reported in the literature, for the same system and for the same thickness range, where the magnetization reversal took place mainly by domain wall motion. We attribute this difference to the effect of roughness of the Au buffer layer on the morphology of the magnetic layer.     

Magnetization reversal in [Ni/Pt]6/Pt(x)/[Co/Pt]6 multilayers

The magnetization reversal is studied in magnetron sputtered artificial superstructures of the form [Ni/Pt]₆/Pt(x)/[Co/Pt]₆ with perpendicular anisotropy, in which the [Co/Pt]₆ stacks have higher coercivity than the [Ni/Pt]₆. For x≥2 nm the two stacks reverse separately and exhibit characteristic stepped loops with a “plateau” in the region between the two switching fields. First-Order Reversal Curves (FORCs) reveal that the maximum coupling is obtained for x=1.5 nm. While each of the Ni/Pt and Co/Pt stacks by itself is thin enough to reverse in large domains when they are coupled, formation of maze like domains is observed. In this case some reversibility of the demagnetization curves associated with interfacial domain wall pinning appears while in the rest of the cases the reversal mechanism is based on lateral domain wall pinning with low reversibility. In the loops monitored by Extraordinary Hall Effect (EHE) measurements this “plateau” appears as a hump due to the different sign of the EHE coefficient between the [Ni/Pt]₆ and [Co/Pt]₆.     

Influence of magnetocrystalline anisotropy on the magnetization reversal mechanism in exchange bias Co/CoO bilayers

We investigated the reversal mechanism in a Co/CoO exchange bias bilayer with a pronounced magnetocrystalline anisotropy in the ferromagnet. The anisotropy, which is induced by the growth of a highly textured Co layer, imposes a distinct reversal mechanism along the magnetically easy and hard direction. It is shown that exchange bias can be induced along both directions, despite the magnetocrystalline anisotropy. The interplay between the magnetocrystalline anisotropy and exchange bias induces a different reversal mechanism for the subsequent reversals in the two crystallographic directions. Along the hard axis, the magnetization reverses according to the reversal mechanism observed before in polycrystalline exchange bias bilayers, i.e. domain wall nucleation and motion for the first reversal and coherent rotation for the subsequent ones. Along the easy axis, domain wall motion remains the dominant reversal mechanism and magnetization rotation has only a minor contribution.     

Control of the magnetic domain wall propagation in Pt/Co/Pt ultra thin films using direct mechanical AFM lithography

We create mesoscopic point and line defects by scanning probe lithography to control the magnetization reversal process in Pt/Co/Pt ultra thin film devices. The domain wall propagation near the defects is studied by the Kerr microscopy and extraordinary Hall effect measurements. The observed domain wall pinning is used to block and channel the domain expansion and to create artificial domain patterns.     

On the possibility of detecting asymmetric magnetization reversal processes in exchange bias systems by low temperature nuclear orientation

Low temperature nuclear orientation has been used for the first time to investigate the magnetization reversal processes in an exchange bias system (Co/Au/CoO), with the advantage of observing both the Co and Au layers at the same time. By monitoring the counting-rate ratio of two γ-ray detectors, the measurements may be used to distinguish between reversal processes dominated by domain wall motion as opposed to rotation of the magnetization.     

Magnetic field dependence of asymmetry in the magnetization reversal of ultrathin Co films and Co/Pt multilayers with perpendicular anisotropy

We studied the magnetization reversal in ultrathin [Co/Pt]_n films (n=1, 2, and 4) using magneto-optical Kerr microscopy. These materials demonstrate unusual asymmetries in the activity of nucleation centers and domain wall motion. It was found that application of very high holding magnetic field prior to magnetization reversal, exceeding some critical value much larger than the apparent saturation field, suppresses the subsequent ‘asymmetric’ nucleation centers, activity. We revealed that the ‘asymmetric’ nucleation centers become active again after subsequent reversal cycles coming from a smaller holding field and studied how the asymmetry returns with the decrease of applied holding field. It was found that in low-coercivity ultrathin Co films, the asymmetry in domain wall velocity decreased sharply with the applied field increase and disappeared when the reversal field is greater than μ₀H=1.5 mT.     

Field sweep rate dynamics in magnetic tunnel junctions

The dynamical behavior of magnetic tunnel junctions (MTJs) was investigated by varying the magnetic field sweep rate from 0.01 mT/s to 10 T/s in a magneto optical Kerr effect set-up. The bias fields of the pinned and free ferromagnetic electrodes were found to drastically decrease above a field sweep rate of 1 T/s. This decrease in the bias fields coincides with a change in the magnetization reversal process from domain wall motion at low-field sweep rates to domain nucleation at high-field sweep rates. The nucleation of inverse domains in the ferromagnetic layer changes the interfacial spin structure of the antiferromagnetic layer and therefore the magnitude of the exchange bias effect. Furthermore, the nucleation of domains induces a discontinuous magnetic charge density at the tunnel barrier interfaces and this reduces the interlayer coupling between the two ferromagnetic electrodes of the MTJ.     

Low-temperature study of the magnetization reversal and magnetic anisotropy of Fe,Ni, and Co nanowires

We present an extensive study of the magnetic reversal mechanism of Fe and Ni nanowires with diameters down to 6 nm, i.e. smaller than the domain wall width. The coercive field at 5 K is a factor of 3 lower than the prediction for rotation in unison. We also observe that the activation energy associated with the reversal process is proportional to the cross-section of the wires and nearly independent of the wire length. From the temperature dependence of the coercive field and the magnetic viscosity we can conclude that magnetization reversal takes place via a nucleation of a small magnetic domain, probably at the end of the wire, followed by the movement of the domain wall. For Co wires, we observe a different behavior that is dominated by the competition between the shape anisotropy and the temperature-dependent magnetocrystalline anisotropy.     

Direct observation of dendritic domain growth in perpendicular magnetic anisotropy CoFe/Pt multilayers

We present the experimental results on thermally activated magnetization reversal for [Co_0.9Fe_0.1(5.0 Å)/Pt(20 Å)]₄ multilayer. Direct domain observations show that magnetization reversal is initiated with rare nucleation and followed by dendritic growth of domain walls. Based on macroscopic magnetic parameters from experimental data, the dendritic domain growth mode is qualitatively interpreted by Monte Carlo simulations in terms of a simple uniaxial magnetic anisotropy model. Moreover, both time evolution of domain growth observation and magnetic relaxation measurements reveal that CoFe/Pt multilayer has a relatively large activation volume compared with Co/Pt multilayers.     

Detection of domain wall propagation in a mesoscopic wire

Extraordinary Hall effect was used to detect the propagation of a domain wall in magnetic devices patterned in sputter grown Pt/Co(1 nm)/Pt sandwiches with perpendicular easy magnetization axis. In such films, domain walls propagate as coherent 1D nano-object in a 2D medium with weak fluctuation energy density. In a patterned device, the competition between global wall energy and Zeeman energy strongly influences the wall propagation.     

Asymmetry in elementary events of magnetization reversal in a ferromagnetic/antiferromagnetic bilayer

Real-time magneto-optical indicator film images reveal distinct asymmetry in the motion of a single domain wall in a wedged-NiFe/uniform-FeMn bilayer due to the nucleation and behavior of an exchange spring in the antiferromagnetic layer. Magnetization reversal from the ground state begins at the thick end of the wedge where the exchange anisotropy field (HE) is minimal and the magnetostatic field (HMS) is maximal, whereas reversal into the ground state begins from the thin end where HE is maximal and HMS is minimal.     

Measurement of the nucleation and domain depinning field in a single Co/Pt multilayer dot by Anomalous Hall effect

Co/Pt multilayer dots with perpendicular anisotropy and with diameters of 250 and 350 nm were fabricated on top of a Hall cross configuration. The angular dependence of the magnetic reversal of the individual dot was investigated by Anomalous Hall effect measurements. At near in-plane angles (85° with the magnetic easy axis) the dot switches partially into a stable two-domain state. This allows for separate analysis of the angular dependence of both the field required for nucleation of a reversed domain, and the field required for depinning of the domain wall. The angular dependence of the depinning field fits accurately to a 1/cos(θ) behavior, whereas the angular dependence of the nucleation field shows a minimum close to 45°. The latter dependency can be accurately fitted to the modified Kondorsky model proposed by Schumacher [1].     

Magnetic domain wall propagation unto the percolation threshold across a pseudorectangular disordered lattice

The reversal process of thin FePt/Pt(001) layers with perpendicular magnetization was observed by magnetic imaging techniques. Reversal occurs through domain wall propagation across a strongly disordered rectangular lattice of linear anisotropy defects. Micromagnetic simulations of domain wall pinning allowed deriving an analytical model of the reversal process unto percolation threshold. Quantitative agreement is found between the calculated and experimental fractal dimension of the reversed domain.     

FePt nano-stripes fabricated on anodic aluminum oxide templates

A new Fe Pt nanostructure with stripe-like patterns has been prepared by direct current(DC) magnetron sputtering on anodic aluminum oxide(AAO) templates. AAO templates anodized under low voltages(7 V) demonstrate self-organized,maze-like patterns, different from the conventional porous structures obtained at high voltages. Fe Pt thin films deposited on such templates tend to replicate the morphology of the templates. Although there is no obvious spatial ordering, the dimensions of the Fe Pt nano-stripes are highly uniform, due to the constrained growth along the transverse direction of the AAO pattern. The magnetic properties are strongly influenced by this unique morphology. While continuous films demonstrate strong exchange coupling, the dominant interaction in Fe Pt nano-stripes with the same nominal thickness is magnetostatic. The morphology also dictates the magnetization reversal behaviors, with thin films dominated by domain nucleation; while nano-stripes incline to reverse their magnetization by spin rotation. Our work demonstrates that selforganized AAO templates can be used to control the morphology and magnetic behavior of Fe Pt materials.     

Composition-controlled exchange bias training effect in FeCr/IrMn bilayers

For FeCr/IrMn bilayers, the exchange bias training effect and the magnetization reversal mechanism are correlated to each other and depend on the composition of the ferromagnetic layer. For high Fe contents, the asymmetric magnetization reversal is observed. During the training effect, both exchange field and coercivity decrease monotonically, suggesting a type I training effect. For low Fe contents, the domain wall depinning takes place for the two hysteresis loop branches. Only exchange field diminution happens in the training effect. The coercivity almost does not change in the process, corresponding to a type II training effect. It is suggested that the motion of antiferromagnetic spins is modified by the magnetization reversal mechanism in the ferromagnetic layer.     

Rotation of the pinning direction in the exchange bias training effect in polycrystalline NiFe/FeMn bilayers

For polycrystalline NiFe/FeMn bilayers, we have observed and quantified the rotation of the pinning direction in the exchange bias training and recovery effects. During consecutive hysteresis loops, the rotation of the pinning direction strongly depends on the magnetization reversal mechanism of the ferromagnet layer. The interfacial uncompensated magnetic moment of antiferromagnetic grains may be irreversibly switched and rotated when the magnetization reversal process of the ferromagnet layer is accompanied by domain wall motion and domain rotation, respectively.     

Formation and evolution of giant dynamic domains in an ac magnetic field

Magnetization reversal in ferrite-garnet films placed in an ac magnetic field, bringing about the formation of metastable dynamic domains with sizes exceeding those of quasi-static domains by an order of magnitude or greater, was studied using a stroboscopic method. The formation of giant dynamic domains (GDDs) is due to the finite domain wall velocity and depends on the density of domain nucleation centers. It was shown that the GDD comblike boundary forms during the part of a field period near the moment of field polarity change. GDDs arise when the dynamic hysteresis loop shape changes from a triangle to an ellipse.