Ab initio calculations of magnetic structure and lattice dynamics of Fe/Pt multilayers

The magnetization distribution, its energetic characterization by the interlayer coupling constants and lattice dynamics of (001)-oriented Fe/Pt multilayers are investigated using density functional theory combined with the direct method to determine phonon frequencies. It is found that ferromagnetic order between consecutive Fe layers is favoured, with the enhanced magnetic moments at the interface. The bilinear and biquadratic coupling coefficients between Fe layers are shown to saturate fast with increasing thickness of nonmagnetic Pt layers which separate them. The phonon calculations demonstrate a rather strong dependence of partial iron phonon densities of states on the actual position of Fe monolayer in the multilayer structure.     

Ferromagnetic resonance study of sputtered Co|Ni multilayers

We report on room temperature ferromagnetic resonance (FMR) studies of [ t Co|2t Ni]  × N sputtered films, where 0.1 ≤ t ≤ 0.6 nm. Two series of films were investigated: films with the same number of Co|Ni bilayer repeats (N = 12), and samples in which the overall magnetic layer thickness is kept constant at 3.6 nm (N = 1.2/t). The FMR measurements were conducted with a high frequency broadband coplanar waveguide up to 50 GHz using a flip-chip method. The resonance field and the full width at half maximum were measured as a function of frequency for the field in-plane and field normal to the plane, and as a function of angle to the plane for several frequencies. For both sets of films, we find evidence for the presence of first and second order anisotropy constants, K₁ and K₂. The anisotropy constants are strongly dependent on the thickness t, and to a lesser extent on the total thickness of the magnetic multilayer. The Landé g-factor increases with decreasing t and is practically independent of the multilayer thickness. The magnetic damping parameter α, estimated from the linear dependence of the linewidth ΔH, on frequency, in the field in-plane geometry, increases with decreasing t. This behaviour is attributed to an enhancement of spin-orbit interactions with decreasing Co layer thickness and in thinner films, to a spin-pumping contribution to the damping.     

Effect on alloying at the Fe/Ni(001) interfaces on the interlayer exchange coupling

We investigate the stability of various ordered FeNi alloys at the interfaces of Fe/Ni superlattices by using ab initio density functional calculation. We consider an Fe_0.5Ni_0.5 ordered alloy of one or two monolayers thick at different positions beyond the interface and the possibility of an interdiffusion of a complete monolayer of Ni(Fe) in Fe(Ni) slab. An interfacial atomic layer of Fe_0.5Ni_0.5 exchanged with its adjacent Ni monolayers, leading to a buffer zone of Ni₃Fe composition is found to be the most stable structural configuration. For this atomic arrangement we investigate the magnetic profile and the resulting interlayer exchange coupling between the Ni slabs for Fe spacer thickness of 0 to 4 monolayers.     

Micromagnetic structure in multilayer films with moderate perpendicular anisotropy

Micromagnetic simulations have been performed in order to obtain deeper insight into the domain structures within multilayer films, as they are expected to differ from those of single films. These 2D calculations have been done in the case of multilayers exhibiting a moderate perpendicular anisotropy, with no indirect exchange coupling between the magnetic layers, where a “weak stripe" domain structure develops. First, these results are compared quantitatively to the very detailed experimental data available in the literature on the (Co/Au)_N system. More generally, the nucleation of a stripe pattern in multilayers is discussed as a function of the magnetic parameters and the number of magnetic layers in the stack. Compared to a single film, two main differences appear in the equilibrium domain period and the magnetization profiles. The physical origin of these effects is discussed. Received 12 January 2001 and Received in final form 15 May 2001     

Dipolar coupling effect in magnetic bilayer system

Equilibrium micromagnetic structures in a bilayer system composed of two thin cobalt films separated by a non magnetic spacer are systematically analysed. These 2D magnetization distributions are obtained by numerical computations according to different set of magnetic and geometric parameters. The coupling effect due to the dipolar long range interaction (or stray-field effect) between the two Co layers is studied through the evolution of the magnetic pattern in the stack with or without an applied field and compared to a continuous film of same thickness. Special attention is paid to the hysteresis process in a bilayer. Even though the general aspect of the magnetization distribution looks like a Landau-Lifshitz structure, the absence of any core in the vortex of the magnetic structure is analysed in relation to a possible disappearence of (topological) hysteresis. Received 12 December 2001     

Interfacial polarisation effect on the interlayer couplings in Co/Rh sandwiches

The structural, magnetic and transport properties of Co/Rh sandwiches grown by ultra high vacuum evaporation and sputtering have been studied. High-energy electron diffraction observations during the growth reveal that both Co and Rh layers have been stabilised in the (111) fcc structure for the evaporated sandwiches. X-ray measurements performed on sputtered samples show a predominant fcc polycrystalline structure of the stacks with a preferential (111) texture. Magnetisation and magnetoresistance measurements show a very strong antiferromagnetic exchange coupling for thin Rh layers, reaching for 4.8? Rh, the strongest ever observed in exchange coupled systems. This value is in good agreement with the value of obtained by ab initio calculations for Co/Rh (hcp) superlattices. This is explained by the magnetic nature of the Co/Rh interfaces. Indeed, the variation of the measured saturation magnetisation as a function of the Co layer thickness shows no evidence of Co moment reduction for the Co atoms located at the interfaces, even for the very thin layers. The value of the preserved magnetic moments of the cobalt atoms at the interfaces is confirmed by ab initio calculations for Co/Rh superlattices taking the intermixing into account. Received: 18 February 1998 / Received in final form: 30 April 1998 / Accepted: 29 May 1998     

Effect of the structural quality of the buffer on the magnetoresistance and the exchange coupling in sputtered Co/Cu sandwiches

The effect of the structural quality of the buffer stack on the structural properties, giant magnetoresistance (GMR) and the quality of the antiferromagnetic coupling has been investigated for Co/Cu/Co sandwiches prepared by DC-magnetron sputtering. Three kinds of buffers were employed: type A: Cr(6 nm)/Co(0.8 nm)/Cu(10 nm), type B: Fe(6 nm)/Co(0.8 nm)/Cu(10 nm) and type C: Cr(4 nm)/Fe(3 nm)/Co(0.8 nm)/Cu(10 nm). For B and C type buffers, the antiferromagnetic alignment is very interesting at zero field with a coupling strength larger than 0.4 erg/cm² and a GMR signal reaching 5% at room temperature. However, for the A type buffer the antiferromagnetic coupling completely disappears, while the GMR drops to about 0.8%. X-ray diffraction, atomic force microscopy and transmission electron microscopy have been performed in order to understand the origin of the observed difference in the magnetic properties. The results show a strong difference in the average surface roughness, 1.15 nm and 0.35 nm, respectively for the A and C types buffers, and demonstrate that the quality of the surface of the buffer is the key to optimize both the GMR and the indirect exchange coupling. Received 11 July 2000     

Use of the chemical potential for a limited number of fermions with a degenerate groundstate

A full analysis of domain structure using a micromagnetic model is developed in order to get a clear insight into the behaviour of transverse initial susceptibility as a function of dc applied field for thin films and bilayers exhibiting both in-plane and perpendicular anisotropy. The numerical simulations are in good agreement with available experimental results in case of single layers with the so-called stripe domain pattern while some predictions are done for bilayers. As the main result, it is shown that in low field, the transverse initial susceptible cannot be interpreted without the knowledge of the static domain pattern while, above saturation, it is only affected by the in-plane anisotropy. Received 10 October 2002 Published online 4 February 2003 RID="a" ID="a"e-mail: ml@lpmtm.univ-paris13.fr RID="b" ID="b"UMR CNRS 8634     

On the steady motions of a flat domain wall in a ferromagnet

A new derivation is presented of Walker's exact solution to Gilbert equation, a solution which mimicks the travelling-wave motion of a flat domain wall at 180^°. It is shown that a process during which the working of the applied magnetic field exactly compensates dissipation (the Walker condition) exists both under the constitutive circumstances considered in the standard Gilbert equation and when either the internal free-energy or the dissipation, or both, are generalized by the introduction of higher-gradient terms; but that such a process cannot solve the generalized Gilbert equation. It is also shown that, when dry-friction dissipation is considered and a suitable magnetic field is applied, the associated Gilbert equation has a Walker-type solution mimicking a flat wall, at 90^° this time, which however does not satisfy the Walker condition. Received 16 November 2001     

In situ resistivity and magnetoresistance studies of Co/Au(111) single layers and bilayers

We report here on resistance and magnetoresistance (MR) studies of ultrathin Co/Au(111) single sandwiches and bilayers with perpendicular magnetization. Resistance of the films was measured in situ in ultrahigh vacuum, during depositions and as a function of a perpendicular applied magnetic field. A large MR variation with the thickness of Au coverage was observed and compared to calculations. The coercive field of the Co films shows a drastic variation with the Au coverage thickness, which reflects the theoretical anisotropy variation. It was measured as a function of temperature. For the first time, the effect of interlayer interaction on the resistivity of a Co bilayer during the growth of Co top layer, is evidenced and compared to calculations. Finally, hysteresis loops of strongly antiferromagnetically coupled bilayers are investigated. Received 3 November 1998 and Received in final form 18 January 1999     

Magnetic properties of (Co Pd ) superstructures on Pd(100) and Pd(111)

The magnetic properties of (Co_nPd_m)_r superstructures on Pd(100) and Pd(111) are evaluated using the fully-relativistic spin-polarized screened Korringa-Kohn-Rostoker method. It is found that only in the case of a Pd(111) substrate such superstructures exhibit perpendicular magnetism, while on a Pd(100) substrate the magnetization is oriented in-plane. Also investigated is the effect of interdiffusion in repeated superstructures. By using the inhomogeneous coherent potential approximation (CPA) for layered systems the effect of ordering into (repeated) superstructures can be described in an ab-initio-like manner. It is found that already small amounts of interdiffusion can be decisive for the actual value of the magnetic anisotropy energy. Received 3 November 1999 and Received in final form 18 January 2000     

Magnetostatic modes in coupled antiferromagnet/ferromagnet bilayers

We study the influence of interface effects on the magnetostatic modes propagating in a coupled ferromagnetic/antiferromagnetic bilayer. We assume that the magnetic layers are thick enough to be described by the bulk parameters and they are coupled through the interaction between the magnetic moments located at the interface. We use a phenomenological approach taking into account the presence of different magnetic layers in the system to calculate the modified dynamical response of each material. We use the corrected magnetic permeability of the layers to obtain a correlation between the interface characteristics and the physical behavior of the magnetic excitations propagating in the system.     

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.     

The study of spin ordering in FM/AFM bilayers with a mixed interface

Monte Carlo simulation is adopted to study the spin ordering in ferromagnet(FM)/antiferromagnet(AFM) bilayers with a mixed interface. We introduce the time-dependent autocorrelation function to describe the thermal stability of the spin ordering for each plane, especially the interface, in FM/AFM bilayers with uncompensated and compensated AFM surfaces. It is found that the thermal decay of spin ordering depends on the plane, the interface coupling and the interface roughness. For the uncompensated AFM surfaces, when the interface coupling is small, the thermal decay of spin ordering is faster at the interface than other planes, while the large interfae coupling makes the spin ordering at the interface become relatively stable. In the case of compensated AFM surfaces, the spin ordering at the interface is thermally disordered much before that at other planes, and the thermal decay becomes slow gradually as the interface coupling increases.     

The transverse ferromagnet spin-1 Ising model of alternating magnetic superlattice

Within the framework of the effective field theory with a probability distribution technique that accounts for the single-site spin correlations, we examine the critical behavior of the transverse ferromagnetic spin-1 Ising model of an alternating magnetic superlattice. The critical temperature of the alternating magnetic superlattice has been studied as a function of the interlayer and intralayer exchange interactions and the strength of the transverse field and the thickness of the finite superlattice. Received 12 January 2000 and Received in final form 14 September 2000     

Shape anisotropy and magnetic domain structures in striped monolayers

We study the effect of dipolar interactions on a magnetic striped monolayer with a microscopic unit cell of square symmetry, and of size spins. Even if the aspect ratio r=N _x /N _y is very large, an in-plane shape anisotropy is always negligible, except if N_y is fairly small (N _y <40). In-plane domains are not possible, except for values of the dipolar coupling larger than the domain wall energy. Received: 11 July 1997 / Revised: 24 September 1997 / Accepted: 24 October 1997     

Magnetic properties of 57Fe/Ir(100) superlattices

Summary The synthesis of a new BCT Fe phase was performed in Fe/Ir(100) superlattices grown by MBE. Magnetic properties of⁵⁷Fe/Ir(100) superlattices with 4 ml Fe and variable Ir thickness (2–30 ?) are investigated by⁵⁷Fe conversion electron M?ssbauer spectroscopy in the 4.2–300 K temperature range. Two spectral components are evidence, related, respectively, to Fe atoms involved in the central part of the iron layers and at the interface between iron and iridium layers. The appearance of a high magnetic hyperfine splitting in the iron BCT structure above a volume threshold of 12 ?³ is evidenced. Marked differences are observed between the mean magnetic properties and the local ones suggesting strong relaxation effects. Paper presented at ICAME-95, Rimini, 10–16 September 1995.     

Phase transitions of a spin-one alternating magnetic superlattice

We examine the critical behavior of a magnetic superlattice which made up of two magnetic materials, A and B. Using the effective field theory with a probability distribution technique that accounts for the single-site spin correlation, we derive the analytical equation for the Curie temperature of the superlattice which alternates as ABAB...AB. The dependence of the Curie temperature on the interface coupling strength J_ab and the layer number of the finite superlattice was calculated. The effects of the surface modification are also studied. Received 2 March 2001     

Magneto-resistance of NiFe nanowire with zigzag shape

The anisotropic magneto-resistance (AMR) contributions of a zigzag-shaped NiFe wire were investigated. The magneto-resistance (MR) behaviors in different magnetic-field directions clearly reflect the angular relationships between the directions of the current and magnetic moment in the subdivisions. The resistance in remanence after magnetization along 0° in respect to the longer direction of zigzag was larger than that along 90°. Assumed that the difference appears due to the AMR contribution in the domain wall trapped at the corner, the MR ratio was estimated to be 1.2%, which is in good agreement with the AMR of the NiFe film. We clearly showed that the domain-wall resistance originates in the AMR.     

Transient domain wall displacement under spin-polarized current pulses

This paper investigates the non steady-state displacement of magnetic domain walls in a nanostrip submitted to a time-dependent spin-polarized current flowing along the nanostrip. First, numerical micromagnetic simulations show that a domain wall can move under application of a current pulse, and that the displacement resulting from a conversion of the domain wall structure is quantized. The numerical findings are subsequently explained in the framework of simplified analytic models, namely the 1D model and the point-core vortex model. We then introduce the concept of an angle linked to the magnetization of a general domain wall, and show that it allows understanding the transient phenomena quite generally. Simple analytic formulas are derived and compared to experiments. For this, charts are given for the key parameters of the domain wall mechanics, as obtained from numerical micromagnetic simulations. We finally discuss the limitations of this work, by looking at the influence of temperature elevation under current, presence of a non-adiabatic term, and of disorder.