Temperature dependence of magnetic stripe domain period in ultrathin films

In ultrathin films, due to the thermal activation and temperature dependencies of the magnetic parameters, magnetization reversal processes are strongly affected by thermal effects. We analyze changes of domain periods of ultrathin cobalt and L₁₀$\mathrm{L}{1}_0$ films in a wide temperature range. With regard to the temperature dependencies of the film magnetic parameters we calculate the equilibrium stripe period as a function of temperature. It is shown that on film heating the equilibrium domain structure (DS) period decreases and at the reorientation phase transition (RPT) approaches its minimal value corresponding to the temperature independent period of the sinusoidal domain structure. Just below the RPT temperature (or thickness) the stripe domain period was found to exponentially decrease with temperature. Irreversible temperature changes of the domain period affected by coercivity are also discussed.     

Designing magnetics of capped perpendicular media with minor-loop analysis

A novel method of analyzing the switching field distribution (SFD) and magnetic correlation length of perpendicular magnetic recording media that uses major and minor magnetization loops is proposed. By applying the analysis to a series of capped perpendicular media, we found that a thick capping layer with a low saturation magnetization effectively reduced SFD without rapidly increasing the magnetic correlation length. Transmission electron microscope observation suggests that the SFD is narrowed by the increased uniformity of intergranular exchange coupling via the thick capping layer. Evaluations of recording characteristics demonstrated a close correlation between narrower SFDs and improved recording performance. Reducing exchange coupling dispersion is a clear solution for improving the performance of recording media.     

Structural and magnetoresistive properties of half metallic Co2Mn1−xSi thin films

Polycrystalline Co₂Mn_1−xSi (CMS) thin films with Mn-deficiency can grow on different types of substrates such as MgO (1 0 0) single crystal, α-sapphire (0 0 0 1) and Si coated with SiO₂ either by using V or Ta/Cu as the seed layer. The magnetic property, especially the coercivity of the CMS thin films strongly depends on the crystalline structure and microstructure of the CMS thin film, hence it is affected by the substrate and also the seed layer. Very soft CMS thin film with coercivity of about 20 Oe has been obtained when MgO (1 0 0) is used as the substrate. Magnetic tunnel junctions (with MR ratio of about 9%–18%) by utilizing the CMS as one of ferromagnetic electrodes have been successfully fabricated. The degradation of the magnetoresistive effect of the MTJ after magnetic annealing is attributed to the diffusion of the Mn-atoms into the tunnel barrier during the annealing process.     

Experimental analysis of the fast magnetization dynamics in high perpendicular anisotropy Co/Pt nanostructures

We present preliminary results for magnetization dynamics in high perpendicular anisotropy Co/Pt cross-shaped nanostructures submitted to a single/train out-of-plane field pulses. For this, a fabrication process has been developed to obtain high-quality microwave test structures. Using extraordinary Hall effect, we measure quasistatic hysteresis loops while applying either a single or a train of field pulses of given amplitude (32 mT at maximum), variable duration (0.2–10 ns) and repetition frequency (1–100 kHz). We get that field pulses affect only one side of the hysteresis loops without any clear change in the squareness of the loop. Such behavior can be most probably attributed to pulse-induced nucleation of reverse domains in the cross area. The measured shift in coercive field decreases linearly with the logarithm of the pulse duration and two switching regimes that get connected for pulse duration around 1 ns have been observed. Switching probability with single pulse showed that field pulse of 10 ns and 32 mT of magnitude has the same effect of 2 mT static fields.     

Transmission X-ray microscopy using X-ray magnetic circular dichroism

X-ray magnetic circular dichroism (X-MCD) was used as a large, element-specific and quantitative magnetic contrast mechanism in the soft X-ray microscopes at BESSY I (Berlin) and the ALS (Berkeley). The present state and potential of magnetic transmission X-ray microscopy (MTXM) is outlined. The possibility to record images in varying magnetic fields and the high spatial resolution down to 25 nm were used to image out-of-plane magnetized (4 ?Fe / 4 ?Gd)×75 systems. Magnetic domains could be studied in arrays of circular and square dots with lateral dimensions down to 180 nm. Hysteresis loops of individual dots were deduced using the direct proportionality of the X-MCD contrast to the sample magnetization. Images of a 3 nmCr / 50 nmFe / 6 nmCr film demonstrate for the first time that MTXM is also able to observe in-plane magnetized domains. In the future the possible applications of MTXM will be extended with regard to the strength of the external field, the available energy range and the sample conditions by building a dedicated transmission X-ray microscope for magnetic imaging at BESSY II. Received: 22 May 2001 / Accepted: 4 July 2001 / Published online: 5 October 2001     

The exchange bias and coercivity of FM/AFM films: Monte Carlo simulation

The exchange bias (H_E) and coercivity (H_C) of the ferromagnet/antiferromagnet (FM/AFM) films have been simulated with Monte Carlo method. The simulated results indicate that, the value of H_E decreases with increasing temperature, and the values of H_E and the blocking temperature T_b at which H_E=0 reduce evidently with decreasing absolute value of interlayer exchange coupling JI. It also is found that for the large absolute values of JI, the maximum in H_C occurs very close to T_b. At the same time, it is observed that the diluted ratio of FM at FM/AFM interface influences clearly the value of H_E. The simulated results are consistent with the experimental facts. The maximum behaviour in the H_C−T curves has been explained by the interplay of the softening of some fraction of the spins in the AFM layer near T_N′ and the disorder of the spins in FM layer near Curie temperature T_C.     

Effects of size and surface anisotropy on thermal magnetization and hysteresis in the magnetic clusters

Based on Monte Carlo simulation, the spin configurations, thermal magnetization and hysteresis loops of the clusters coated by the surface shell with radial anisotropy are studied. Interestingly, a new multidomain containing a few of subdomains whose easy directions are along those of the configurational anisotropy, a magnetization curve in steps and a first order phase transition from the single domain to the multidomain in the thermal and field magnetization processes, are found, which is as a result of the interplay of the configurational anisotropy, the size effect, the surface anisotropy, the applied field and the thermal fluctuation. In this first order transition, we find a critical temperature, a critical surface anisotropy and a critical size. The simulated temperature dependence of the coercivity of the cluster with the surface anisotropy can be fitted by H_c (T)=H_c (0)(1-C_αT^α) with low value of α, which explains well the experimental results of the nanoparticles. Moreover, it is found that the hysteresis loops and coercivity are strongly affected by the cluster size and the thickness of the surface layer.     

Tunable magnetic anisotropy of antiferromagnetic superlattice and resultant exchange bias of ferromagnetic layer on it

Exchange biasing of ferromagnetic layer deposited on the antiferromagnetic superlattice was investigated in (Co₇₀Fe₃₀/Ru)_29.5/Ru/Co₉₀Fe₁₀ multilayers. Uniaxial magnetic anisotropy (K_AF) was induced and tuned in the antiferromagentic superlattice by uniaxial substrate bending method through the inverse effect of magnetostriction. The exchange bias increased and tended to be saturated with increasing the K_AF, while it was not observed at K_AF=0.     

Effect of the surface anisotropy and the shape on the magnetization behavior in an egg-shaped nanoparticle

Using the classical Heisenberg model and Monte Carlo simulation, we compute the magnetization behavior of a ferromagnetic nanoparticle with an egg-shape in an external magnetic field along the symmetry axis. The particle is in a single-domain state with a surface anisotropy axis perpendicular to the surface of the particle. At low temperature, it is found that exchange bias appears in the hysteresis loop, which does not exist in the spherical and ellipsoidal nanoparticles. The bias field produced by the frozen spins on the surface of the egg-shaped nanoparticle may be the reason to produce the phenomenon of the exchange bias.     

Seedlayer interface enhanced magnetic anisotropy in CoPt (0 0 0 2)-textured films

The effects of interface roughness of Ta seedlayer on the structural and magnetic properties of Co₇₂Pt₂₈(20 nm)/Ru(30 nm)/Pt(2 nm)/Ta(5 nm)/glass were investigated. Uniaxial perpendicular magnetic anisotropy (8.6×10⁶ ergs/cc), coercivity (5.5 kOe) and nucleation field (−2.8 kOe) in the Co₇₂Pt₂₈ thin film sputter-deposited on relatively smooth surface of Ta seedlayer were achieved. The results showed that relatively smoother interface roughness of Ta seedlayer improved the CoPt/Ru (0 0 0 2) texture and magnetic properties.     

Influence of current amplitude on the nonlinear asymmetric ac volt-ampere characteristics in amorphous ribbons with GMI effect

Asymmetric giant magnetoimpedance (GMI) effect in amorphous ribbons is very promising due to its possible application in developing the highly sensitive linear magnetic-field sensors. In this paper, the model of the nonlinear asymmetric volt-ampere characteristics in field-biased Co-based amorphous ribbon has been established through the time derivative of the longitudinal magnetization component in the ribbon. Its harmonics are also developed by Fourier analysis. The influence of the current amplitude on the nonlinear asymmetric effect has been studied. It is found that when the current amplitude is 8.54 mA, the sensitivity of the first harmonic voltage on the external field is equal to that of the second harmonic voltage. The results obtained are useful for developing the high-sensitive magnetic-field sensors.     

Instantons and solitons in a monolayer film of ferromagnetic grains with biaxial anisotropy

We investigate domain wall and domain structure in a monolyer film consisting of ferromagnetic grains with biaxial anisotropy, which are seen to be the static versions of instanton and soliton, respectively. The equation of motion of the magnetization vector is reduced to the (1 + 2)-dimensional sine-Gordon field equation in strong anisotropy limit and the instanton and soliton configurations are obtained analytically. Various new domain structures in the ferromagnetic film are found.     

Monte Carlo simulation of the reorientation transition in Heisenberg model with dipole interactions

The change of magnetic states in ultrathin films with temperature have been simulated by Monte Carlo method. A Heisenberg model with long-range dipole interactions was adopted in our calculations. The results were qualitatively in good agreement with the experimental phenomena. That is at low temperatures the magnetization is perpendicular to the plane, and at higher temperatures but below the Curie point, the magnetization is mostly within the plane. In between these two regions, the magnetization seems to be suppressed. The simulations show that the loss of magnetization is a consequence of the special magnetic states in which the local domains orientations are reverse with the neighbor ones.     

Out-of-Plane Torque Influence on Magnetization Switching and Susceptibility in Magnetic Multilayers

Based on both the spin diffusion equation and the Landau-LlTshitz-Gilbert （LLG） equation, we demonstrate the influence of out-of-plane spin torque on magnetization switching and susceptibility in a magnetic multilayer system. The variation of spin accumulation and local magnetization with respect to time are studied in the magnetization reversal induced by spin torque. We also research the susceptibility subject to a microwave magnetic field, which is compared with the results obtained without out-of-plane torque.     

Dynamic hysteresis of magnetic aggregates with non-integer dimension

We investigate the dynamic hysteresis of nanoscale magnetic aggregates by employing Monte Carlo simulation, based on Ising model in non-integer dimensional space. The diffusion-limited aggregation (DLA) model with adjustable sticking probability is used to generate magnetic aggregates with different fractal dimension D. It is revealed that the exponential scaling law A(H₀, ω)∼H₀^α·ω^β, where A is the hysteresis area, H₀ and ω the amplitude and frequency of external magnetic field, applies to both the low-ω and high-ω regimes, while exponents α and β decrease with increasing D in the low-ω regime and keep invariant in the high-ω regime. A mean-field approach is developed to explain the simulated results.     

Micromagnetic studies on magnetic spectra of submicron ferromagnetic particles with different aspect ratio

Two resonance modes occur when the magnetic spectra of rectangular ferromagnetic particles with different aspect ratio are studied using micromagnetic simulation. The intensity of the higher mode is stronger when the particles have the aspect ratio larger than 1.6. When the aspect ratio reduces to 1.6, only one strong resonance peak remains with some tiny satellite peaks appeared. These behaviors are attributed to the competition between the exchange interaction and dipolar coupling, which has different result in the edge and middle part, respectively.     

Dynamics of two interacting dipoles

We study the deterministic spin dynamic of two interacting magnetic moments with anisotropy and dipolar interaction under the presence of an applied magnetic field, by using the Landau–Lifshitz equation with and without a damping term. Due to different kinds of interactions, different time scales appear: a long time scale associated with the dipolar interaction and a short time scale associated with the Zeeman interaction. We found that the total magnetization is not conserved; furthermore, for the non-dissipative case it is a fluctuating function of time, with a strong dependence on the strength of the dipolar term. In the dissipative case there is a transient time before the total magnetization reaches its constant value. We examine this critical time as a function of the distance between the magnetic moments and the phenomenological damping coefficient, and found that it strongly depends on these control parameters.     

Magnetization behavior of hard/soft-magnetic composite pillar

Hard/soft-magnetic composite pillar array medium is proposed for ultra-high-density recording media. Magnetization reversal process for a single hard/soft-magnetic composite pillar in the medium is calculated using the Landau–Lifshitz–Gilbert equation. Magnetization reversal of the soft-magnetic unit helps the magnetization reversal for the hard-magnetic unit, and the effective coercivity for the hard-magnetic unit is greatly reduced. Thereby saturation recording to the high-K_u-hard-magnetic material used for perpendicular magnetic recording will be realizable.     

New approach to spin dynamics simulation in magnetic system

New approach to spin dynamics simulation in magnetic system is presented. In the approach, we substitute new algorithm for Landau-Lifshitz-Gilbert dynamics, which enable us to achieve the final stable state of magnetic system much faster than traditional spin dynamics simulation. A square-shaped sample with 32×32×4 size under different conditions is calculated. Our results show the new approach can largely reduce the computational time.