Comparison between disaccommodation and ferromagnetic resonance measurements in polycrystalline nickel ferrites

FerroMagnetic Resonance (FMR) measurements of polycrystalline nickel ferrites at 11 GHz from 77 to 400 K have been carried out. The different contributions to the FMR linewidth have been studied by a computational technique based on the parameters previously obtained for single-crystal nickel ferrite. A new qualitative comparison between the FMR and disac-commodation measurements by means of the induced anisotropy formalism is presented.     

High-density all-optical magnetic recording using a high-NA lens illuminated by circularly polarized pulse lights

We propose a method for high-density all-optical magnetic recording. Our analyses, based on the vector diffraction theory, show that owing to the inverse Faraday effect, circularly polarized laser pulses focused by a high numerical aperture (NA) lens can induce a small magnetization domain. For an example, the FWHM of the effective magnetization domain is 0.4646λ when NA=0.85. The magnetization direction is basically perpendicular to the surface of the optic-magneto film within the effective magnetization domain and the switching direction of magnetization can be controlled by the helicity of the incident circularly polarized light. These characteristics are useful to next-generation high-density all-optical magnetic storage.     

Domain morphology in ultrathin ferromagnetic films with perpendicular magnetization

We determine the minimal domain structure for the equilibrium thickness of stripes as well as for the minimal energy of the domain configuration in ultrathin films of ferromagnetically coupled spins, where the easy direction of magnetization is perpendicular to the film. It is found that the equilibrium thickness of stripes and walls depend on the exchange energy. The normalized anisotropy, f, depends on interplay between the magnetic and anisotropy energies and is almost independent of the exchange energy inside the wall. The results are compared with the experimental data for thin Ag/Fe/Ag (0 0 1) films and a good coincidence is obtained between both results.     

Investigations of the first order magnetic phase transition in dysprosium

The first order magnetic phase transition in polycrystalline sample of dysprosium (Dy) has been investigated in detail by dc-resistance vs. temperature (R-T) measurements. Distinct signature of coexistence of two magnetically ordered phases (i.e. ferromagnetic and antiferromagnetic), originating as a result of the metastable behavior associated with the first order phase transition (FOPT) is found near 90 K. By successfully recording the minor hysteresis loops within the hysteretic R-T regime of Dy, a varying degree of phase coexistence in the FOPT is demonstrated.     

Magnetoresistance measurement of permalloy thin film rings with triangular fins

Magnetization reversals in permalloy rings controlled by nucleation sites using triangular fins at the same side and diagonal with respect to the field direction are demonstrated by magnetoresistance measurement and micromagnetic simulation. In the ring with triangular fins at the same side, there exists two-step reversal from onion to flux-closure state (or vortex state) and then from flux-closure (or vortex state) to reverse onion state; in the ring with diagonal triangular fins, one-step reversal occurs directly from onion to reverse onion state. The reversal processes are repeatable and controllable in contrast to an ideal ring without triangular fins where one-step and two-step reversals occur randomly in sweep-up and sweep-down processes.     

Dependence of Interlayer AF Coupling on Ferromagnetic Layer Thickness in [Pt/Co]5/Ru/[Co/Pt]5 Multilayers

We study magnetization reversal in the interlayer coupled [Pt/Co]5/Ru/[Co/Pt]5 multilayers （MLs） by means of the measurement of extraordinary Hall effect （EHE）. Fitting experimental data to a simple model, we determine the interlayer coupling strength for various thicknesses of the ferromagnetic layers at a fixed Ru spacer thickness. It is found that the dependence of interlayer coupling strength on the Pt layer thickness is much stronger than the previous report in the ferromagnetic/nonmagnetic/ferromagnetic multilayers.     

Simulations of magnetic microstructure in thin film elements used for programmable motion of magnetic particles

The results of two-dimensional micromagnetic modeling of magnetization patterns in Permalloy ellipses under the influence of rotating constant-amplitude magnetic fields are discussed. Ellipses of two different lateral sizes have been studied, 0.5 μm×1.5 μm and 1 μm×3 μm. The amplitude of the rotating magnetic field was varied between simulations with the condition that it must be large enough to saturate or nearly saturate the ellipse with the field applied along the long axis of the ellipse. For the smaller ellipse size it is found that the magnetization pattern forms an S state and the direction of the net magnetization lags behind the direction of the applied field. At a critical angle of the rotating magnetic field the direction of the magnetization switches by a large angle to a new S state. Both the critical angle and the angle interval of the switch depend on field amplitude. For this new state, it is instead the applied field direction that lags behind the magnetization direction. The transient magnetization patterns correspond to multi-domain patterns including two vortices, but this state never exists for the equilibrated magnetization patterns. The behavior of the larger ellipse in rotating field is different. With the field applied along the long-axis of the ellipse, the magnetization of the ellipse is nearly saturated with a vortex close to each apex of the ellipse. As the field is rotated, this magnetization pattern remains and the net-magnetization direction lags behind the direction of the field until for a certain angle of the applied field an equilibrium multi-domain state is created. Comparisons are made with corresponding experimental results obtained by performing in-field magnetic force microscopy on Permalloy ellipses.     

Anomalous magnetization processes and non-symmetrical domain wall displacements in L10 FePt particulate films

Anomalous magnetization processes and non-symmetrical domain wall displacements in the minor loop of L1₀ FePt particulate films were investigated by magnetization measurements and in situ magnetic force microscopy. Magnetization (M) decreases dramatically on increasing the magnetic field to ∼3 kOe after which M becomes small and constant in the range of 5–20 kOe as observed in the successive measurement of minor loops. The domain wall displacement is non-symmetrical with respect to the field direction. The anomalous magnetization behavior was attributed to the non-symmetrical domain wall displacement and large magnetic field required for domain wall nucleation. Energy calculations from modeling suggest that non-symmetrical domain wall displacement is caused by the existence of metastable domains in which the domain edges are stuck to the particle boundaries.     

A new method for M–H and λ–H determination using the magnetostrictive delay line technique

In this paper, a new technique for the determination of M–H loop and λ–H loop is proposed, based on the magnetostrictive delay line (MDL) technique and used for long magnetostrictive ribbons, wires and rods of uniform cross-section. The principle of the M–H loop determination is based on the biasing field effect at the MDL search coil, while the principle of the λ–H loop is based on the biasing and pulsed field effects at the MDL excitation point.     

Modelling magnetic polarisation J50 by different methods

Two different methods for modelling the angular behaviour of magnetic polarisation at 5000 A/m (J₅₀) of electrical steels were evaluated and compared. Both methods are based upon crystallographic texture data. The texture of non-oriented electrical steels with silicon content ranging from 0.11 to 3%Si was determined by X-ray diffraction. In the first method, J₅₀ was correlated to the calculated value of the average anisotropy energy in each direction, using texture data. In the second method, the first three coefficients of the spherical harmonic series of the ODF and two experimental points were used to estimate the angular variation of J₅₀. The first method allows the estimation of J₅₀ for samples with different textures and Si contents using only the texture data, with no need of magnetic measurement, and this is advantageous, because texture data can be acquired with less than 2 g of material. The second method may give better adjust in some situations but besides the texture data, it requests magnetic measurements in at least two directions, for example, rolling and transverse directions.     

Biased magnetization reversal in bi-phase multilayer microwires

We report on the magnetic behaviour of a novel family of two-magnetic-phase multilayer microwires consisting of: (i) a bistable FeSiB glass-coated amorphous microwire as soft nucleus, and (ii) a polycrystalline CoNi outer microtube as harder layer. Such bi-phase microwires are prepared by combined quenching and drawing plus sputtering and electroplating techniques. The stray field produced by the harder outer layer after premagnetizing it to saturation is used to bias the magnetization reversal process of the soft nucleus via dipolar magnetostatic coupling. A detailed analysis of the asymmetric low-field magnetization reversal process of the soft nucleus is presented together with the study of the fluctuating switching field and its asymmetric behaviour. The study of the domain wall characteristics under the presence of a nucleation coil at one end of the microwire allows us to draw conclusions on the role of the bias field generated by the premagnetized hard outer layer.     

Novel Bilayer Structures for Short Wavelength High Density Magneto-Optical Data Storage

We report a novel bi-layer thin film structure for high density magneto-optical (MO) data storage, which combines the advantages of blue wavelength and magnetically induced superresolution (MSR) recording. A double-layer system of exchange-coupled light rare-earth (LRE) element doped NdGdFeCo and traditional TbFeCo is used as the recording medium. The experimental results demonstrate that this NdGdFeCo/TbFeCo double layer has large Kerr rotation under blue wavelength. Centre aperture detection (CAD) MSR effect with temperature rising is also observed. Theoretical calculation is also carried out to verify the experimental results. These results collectively suggest that the new bilayer structure is very promising in next generation high density MO data storage.     

Single-layer and multilayered films for perpendicular recording based on a Co-Cr alloy

Results of materials research into the dependence of the magnetic properties of Co-Cr recording media on the structural properties are discussed in relation with results from recording experiments. The magnetic properties depend strongly on the preparation conditions. An increase in substrate temperature results in an increase of the coercivity, but also gives rise to inhomogeneities of the Co-Cr film in the growth direction and to lateral inhomogeneities due to Cr segregation. The inhomogeneities in the growth direction are suppressed by the application of a non-magnetic, amorphous Ge layer between the substrate and the Co-Cr layer, resulting in a magnetic layer with a large perpendicular anisotropy. From the slope of the hysteresis loops we obtain, using a model proposed by Kooy and Enz, an estimate for the mean size of the stripe domains, which is found to increase with increasing layer thickness and decrease with increasing substrate temperature. The change in the size of the stripe domains is consistent with the trends calculated from noise spectra obtained in recording experiments. Multilayers of thin Co-Cr layers alternated with non-magnetic Ge layers are also discussed.     

Magnetostriction relative to pretransition in Ni50.5Mn24.5Ga25 single crystal

The strain behaviors as well as the structural and magnetic changes relative to the pretransition in the Ni_50.5Mn_24.5Ga₂₅ single crystals have been characterized by various methods, such as pretransition strain, magnetostriction, magnetization measurements, and TEM observations. A large magnetostriction up to 505 ppm measured in the [001] direction of the sample is obtained at the pretransition temperature with only a low magnetic field of about 1 kOe applied along the [010] direction. We found that not only the pretransition strain pronounces a more large change, but also the magnetostriction at a certain temperature exhibits a more large magnitude for field applied along the [010] direction than with field along the [001] direction. It is concluded that the magnetoelastic interaction is responsible for the premartensitic transition, and the magnetoelastic interaction in the [010] direction is stronger than that in the [001] direction.     

Magnetic domain wall pinning and coercivity in FePt/MgO and FePt/Pt thin films

FePt thin layers have been epitaxied either on Pt(0 0 1) or on MgO(0 0 1) substrates, and magnetically characterized using extraordinary Hall effect magnetometry and magnetic force microscopy. The coercivity originates in both cases from the pinning of domain walls on structural defects. Whereas the coercivity increases with the FePt layer thickness in FePt/Pt samples, it decreases in FePt/MgO samples. This discrepancy is explained on the basis of structural observations, and of atomistic simulations of magnetic domain wall pinning.     

Growth temperature dependence of the hysteretic behavior of Ni0.5Zn0.5Fe2O4 thin films

Herein, a discussion of the effect of deposition temperature on the magnetic behavior of Ni_0.5Zn_0.5Fe₂O₄ thin films. The thin films were grown by r.f. sputtering technique on (1 0 0) MgO single-crystal substrates at deposition temperatures ranging between 400 and 800 °C. The grain boundary microstructure was analyzed via atomic force microscopy (AFM). AFM images show that grain size (φ∼70-112 nm) increases with increasing deposition temperature, according to a diffusion growth model. From magneto-optical Kerr effect (MOKE) measurements at room temperature, coercive fields, H_c, between 37and 131 Oe were measured. The coercive field, H_c, as a function of grain size, reaches a maximum value of 131 Oe for φ ∼93 nm, while the relative saturation magnetization exhibits a minimum value at this grain size. The behaviors observed were interpreted as the existence of a critical size for the transition from single- to multi-domain regime. The saturation magnetization (21 emu/g<M_s<60 emu/g) was employed to quantify the critical magnetic intergranular correlation length (L_c≈166 nm), where a single-grain to coupled-grain behavior transition occurs. Experimental hysteresis loops were fitted by the Jiles-Atherton model (JAM). The value of the k-parameter of the JAM fitted by means of this model (k/μ_o∼50 A m²) was correlated to the domain size from the behavior of k, we observed a maximum in the density of defects for the sample with φ∼93 nm.     

Spin-spiral and domain wall structures in monolayer Fe/W(1 1 0)

The stability of spin-spiral and domain wall structures in an Fe monolayer on a W(1 1 0) substrate is theoretically investigated. By analyzing the exchange parameters obtained from first principles total energy calculations, we find that a competition between the nearest-neighbor ferromagnetic and long-distant antiferromagnetic exchange interactions leads to a stabilization of the spin-spiral structures. When the strong magnetocrystalline anisotropy (MCA) arising from the Fe/W(1 1 0) interface is introduced, however, the formation of the spin-spiral structures is suppressed and the ground state appears to be the ferromagnetic state—as observed in experiments. In addition, the strong MCA is found to play a key role in determining the domain wall structures.