Magnetization behaviour in ultrathin Co/Cu/Ni/Cu(001) trilayers

The magnetization of ultrathin multilayer systems depends in a very sensitive way on their composition. We consider the temperature dependence of magnetization and its orientation in three types of trilayers: 2ML Co/2ML Cu/4ML Ni/Cu(001), 1ML Co/2ML Cu/4ML Ni/Cu(001) and 1ML Co/2ML Cu/3ML Ni/Cu(001). The composition of each system leads to different interdependence of ordering temperatures characterizing the ferromagnetic films. It is shown that in all cases the sublayer magnetizations change gradually their directions as a function of temperature. The use of Néel sublattices concept in Valenta approach allows us to present all dependences in a layer resolved mode which leads to the conclusion that the spin reorientation process runs through non collinear magnetic superstructures.     

Effects of High Magnetic Field and Post-Annealing on the Evaporated Ni/Si (100) Thin Films

The effects of high magnetic field and post-annealing on the structural, electrical and magnetic properties of the evaporated Ni films were investigated and compared. The in-situ application of a 6 T magnetic field during evaporation or post-annealing at 200°C did not change the crystal structures of the films. However, the magnetic field makes the films exhibit the smallest grain size and the lowest surface roughness. Crystallinity was improved for both the 6 T films and the annealed films. This leads to the enhancement of saturation magnetization (Ms). The value of Ms for the 0 T films was 588 emu/cm3, while those for the 6 T films and the post-annealing films without magnetic field were 704 and 647 emu/cm3, respectively. In addition, the 6 T films also exhibited the lowest resistivity. These results indicate that the in-situ application of high magnetic field was a much more efficient method than the post-annealing treatment in the increase of film quality and properties.     

Magnetic Correlations in Some FeNi/Cu(100) Multilayers

We present a first-principles electronic structure based study of magnetic correlations in the paramagnetic phase of some Fe and Ni multilayers on Cu(100) substrates. We have used the screened Korringa–Kohn–Rostoker method to calculate the electronic structure within the spin-density functional theory and used the disordered local moments model to describe the paramagnetic phase above the Curie temperature. We find that in the Ni/Cu(100) films, there are no local moments formation for any thickness. For the multilayers of Fe and Ni, we find that there is some, although very small, local moment formation in the interior of the Ni film if the Ni layer is more than 6 monolayers thick. However, the Ni layers facilitate the magnetic correlation between the Fe layers. The Curie temperatures of the multilayers also depend upon the Ni layer thickness.     

Superconductivity in Ni/Pb modulated films

The measurements of critical temperature and critical magnetic fields were carried out in superconducting Ni/Pb modulated films. These films consisted of thick Pb component layers (d_pb = 400 Å) and thin layers of Ni with thickness d_Ni ranging from 8 Å to 30 Å. The dependence of critical temperature on the thickness of Ni component layers was compared with theoretical model. Our measurements of parallel critical field as a temperature function did not give any evidence of dimensional crossover of superconductivity induced by temperature.     

Broadband microwave absorption in [NiFe/FeMn]n exchange-coupled multilayer films

[NiFe/FeMn]_n exchange-coupled multilayer films have been fabricated on the silicon substrate by magnetron sputtering deposition. The static and dynamic magnetic properties of multilayer films have been investigated with varying numbers of layers. The results show that the linewidth and permeability of imaginary resonance peak are increased with increasing numbers of layers. For the NiFe/FeMn/NiFe sample, the resonance frequency shows a different shift with applying external magnetic field along the direction of easy and hard magnetization axis of the sample, respectively, indicating a different magnetic reversal process in two ferromagnetic layers. It proved that the increase of linewidth was originated from the different interface exchange coupling.     

Magnetic Properties of Ferromagnetic and Antiferromagnetic Spins (1/2,1/2,1/2) Ising Model: a Monte Carlo Simulation

The magnetic properties of ferromagnetic and antiferromagnetic spins (1/2,1/2,1/2) Ising model are investigated by Monte Carlo simulation. The critical temperature is obtained for different values of exchange interactions. The total magnetization has been determinated for different values of exchange interactions and for different values of external magnetic field. The magnetic hysteresis cycle is obtained for different values of temperature and for different values of exchange interactions.     

Magnetoresistance behaviour in CoFe/Cu multilayers: thin Cu layer effect

The magnetoresistance properties of the CoFe/Cu multilayers have been investigated as a function of thin non-magnetic Cu layer thickness (from 2.5 to 0.3 nm). CoFe/Cu multilayers were electrodeposited on Ti substrates from a single electrolyte containing their metal ions under potentiostatic control. The structural analysis of the films was made using X-ray diffraction. The peaks appeared at 2θ ≈ 44°, 51°, 74° and 90° are the main Bragg peaks of the multilayers, arising from the (111), (200), (220) and (311) planes of the face-centered cubic structure, respectively. The magnetic characterization was performed by using vibration sample magnetometer in magnetic fields up to ±1600 kA/m. At 0.6, 1.2 and 2.0 nm Cu layer thicknesses, the high saturation magnetization values were observed due to antiferromagnetic coupling of adjacent magnetic layers. Magnetoresistance measurements were carried out using the Van der Pauw method in magnetic fields up to ±1000 kA/m at room temperature. All multilayers exhibited giant magnetoresistance (GMR), and the similar trend in GMR values and GMR field sensitivity was observed depending on the Cu layer thickness.     

Positive exchange bias in epitaxial permalloy/MgO integrated with Si (1 0 0)

In magnetic random access memory (MRAM) devices, soft magnetic thin film elements such as permalloy (Py) are used as unit cells of information. The epitaxial integration of these elements with the technologically important substrate Si (1 0 0) and a thorough understanding of their magnetic properties are critical for CMOS-based magnetic devices. We report on the epitaxial growth of Ni_82.5Fe_17.5 (permalloy, Py) on Si (1 0 0) using a TiN/MgO buffer layer. Initial stages of growth are characterized by the formation of discrete islands that gradually merge into a continuous film as deposition times are extended. Interestingly, we find that the magnetic features of Py films in early stages of island coalescence are distinctly different from the films formed initially (discrete islands) and after extended deposition times (narrow distribution of equiaxed granular films). Isothermal in-plane and out-of-plane magnetic measurements performed on these transitional films show highly anisotropic magnetic behavior with an easy magnetization axis lying in the plane of the film. Importantly, when this sample is zero-field cooled, a positive exchange bias and vertical loop shift are observed, unusual for a soft ferromagnet like Py. Repeated field cycling and hysteresis loops up to the fields of 7T produced reproducible hysteresis loops indicating the existence of strongly pinned spin configurations. Classical interface related exchange bias models cannot explain the observed magnetic features of the transitional Py films. We believe that the anomalous magnetic behavior of such Py films may be explained by considering the highly irregular morphology that develops at intermediate growth times that are possibly also undergoing a transition from Bloch to Neel domain wall structures as a function of Py island size. This study broadens the current understanding of magnetic properties of Py thin layers for technological applications in magneto-electronic devices, integrated with Si (1 0 0).     

Enhancement of saturation magnetization in epitaxial (111) BiFeO3 films by magnetic annealing

We report the enhancement of the saturation magnetization in BiFeO₃ films achieved by magnetic annealing. The saturation magnetization of the film annealed in a magnetic field is found to be 80 emu/cc, which is an increase nearly by a factor of 10 compared to the as-grown one. From the investigation of optical second harmonic generation (SHG), we observe the presence of s_out-polarization under s_in-polarization in the film annealed in a magnetic field, which implies that the spin state is homogeneous antiferromagnetic one rather than cycloidal one. We interpret the observed large enhancement in the saturation magnetization to be due to the magnetic phase transition from cycloidal to homogenous antiferromagnetic one.     

Finding stabilized coercivity in a study of thickness and temperature dependent Co/Ag/Ge(001) ultrathin films

Epitaxial magneto-optical Co/Ag/Ge(001) ultrathin films were investigated at both room temperature and low temperature. Interesting variations in coercivity as a function of thickness and temperature suggest the existance of interfacial exchange interactions. The operative Ag layers required to inhibit intermixing of Co-Ge alloys from the oscillation of coercivity with Ag film thickness were determined. When the Ag film thickness was reduced to the submonolayer range, perpendicular anisotropy was unexpectedly found. Thus unusual phenomenon can be qualitatively discussed in terms of effective magnetic anisotropy by the contribution of reduced Co layers and by surface magnetic anisotropy.     

Magnetic anisotropy of the ultrathin Fe layers in Fe/NiFe/Fe/Cu multilayers and their effect to the magnetoresistance

The effects of the deposition of ultrathin ⁵⁷Fe layers on both sides of the NiFe layers in NiFe/Cu multilayers were investigated by focusing on their structural, magnetic and magnetoresistance properties. Conversion electron Mössbauer spectroscopy measurements showed an out-of-plane magnetic anisotropy of the Fe layers. The magnetoresistance curves showed an unusual shape, where up to three peaks were observed. Eight variables computer simulations, based on a phenomenological model that considers bilinear and biquadratic couplings between layers with cubic and in-plane uniaxial anisotropies, were used in order to calculate the best-fitting magnetization curves for the NiFe/Cu and Fe/NiFe/Fe/Cu multilayers. Both model and Mössbauer spectroscopy results showed that it is the rotation of the Fe magnetic moment from out-of-plane to in-plane orientation that provokes the unusual magnetoresistance curve shape. The observed reduction of the magnetoresistance amplitude with the addition of one monolayer of Fe in the NiFe/Cu multilayer was attributed to a less-effective spin-dependent scattering that occurs at Fe/Cu and Fe/NiFe interfaces than at the NiFe/Cu interfaces.     

Field dependence of the magnetization of adsorbed3He films at ultra low temperatures

We present measurements of the magnetization of pure³He films adsorbed on graphite at a density of p = 0.235 atoms/Ų, which corresponds to the 2D Heisenberg ferromagnetic regime. Different NMR frequencies (461.3kHz and 1.004 MHz) were used to study the magnetic field dependence of the nuclear magnetization. Measurements were performed on a Papyex sample to investigate the influence of the platelet size. The results are discussed in the context of theoretical models presented recently to describe these systems.     

Thermomagnetic data writing and erasing in DyCo/NiFe (TbFe/NiFe) film structures

A new principle of data writing and erasing is proposed based on the exchange coupling between magnetic layers with mutually orthogonal orientations of the effective magnetization. The new method provides for a significant decrease in energy consumption. The possibility of using bilayer magnetic films of the (rare earth metal-transition metal)/NiFe type possessing unidirectional anisotropy as the media for thermomagnetic writing and erasing of magnetooptical data is studied.     

The magnetic properties of ultrafine particles of Ni(OH)2

Ultrafine particles of Ni(OH)₂ were prepared by precipitation from aqueous solution. The susceptibility of the ultrafine particles shows a distinctive maximum at temperature which is rather lower than T_N of the large particles. Using the c-axis oriented ultrafine particles prepared by the sedimentation method, the magnetic susceptibility and the magnetization curve perpendicular and parallel to the c-axis of the ultrafine particles were measured. The magnetization curve showed a small hysteresis in low field and a remanent magnetization which increased with decrease of the particle size. Two-step jump was observed in the magnetization curve of the ultrafine particles when the external field was applied along the c-axis. The magnetic properties characteristic of the Ni(OH)₂ ultrafine particles are discussed.     

Magnetooptical study of the near-surface micromagnetic structure and local magnetic properties of NiFe/Cu nanocomposite microwires

The local near-surface magnetic properties and micromagnetic structure of NiFe/Cu nanocomposite microwires have been studied using the scanning Kerr microscopy technique. The local magnetization curves and the profiles of magnetization components (both parallel and perpendicular to the magnetic field H applied parallel to the microwire sample length) were measured using a 1-μm-diameter probing light beam that was scanned along the sample. The magnetization component parallel to H in all samples has the same sign, while the component perpendicular to the field changes sign. The observed magnetization distributions are indicative of the presence of circular near-surface domains with a ±70°–80° orientation of magnetization in the neighboring domains relative to the sample axis. It is shown that the dominating mechanism of magnetization reversal in the near-surface regions of nanocomposite microwires is via the rotation of magnetization in the circular domains.     

Magnetic Studies of Vapor-Deposited Co/Si(100) and Co/Glass Thin Films

Magnetic force microscopy and alternating gradient field magnetometry techniques were used to investigate the static magnetic properties of vapor-deposited cobalt films with different thicknesses ranging from 50 to 195 nm. Brillouin light scattering (BLS) and ferromagnetic resonance (FMR) techniques were performed to study the dynamic properties of these films. Despite thicknesses well above the theoretical critical thickness that allows the presence of stripe domains, only the thicker films exhibit a magnetic stripe domain structure. The magnetocrystalline anisotropy factors, deduced and computed from BLS and FMR measurements, were found to decrease with thickness. Values of these computed effective anistotropy factors, of up to 7×10⁶ erg?cm^?3, have been found. All these results will be discussed and correlated.     

Field induced vortex dynamics in magnetic Ni nanotriangles

The magnetization states in Ni triangular dots under an applied magnetic field have been studied using variable-field magnetic force microscopy (VF-MFM) imaging. In order to understand their dynamics we performed micromagnetic simulations which are in remarkable agreement with the experimental MFM results. The nanostructures present magnetic vortices as ground states which move under an external magnetic field. The combination of micromagnetic simulations and MFM imaging allows us to identify correctly the vortex chiralities and polarizations. The triangular geometry produces an improved contrast of the vortex core. Additionally, the vortices of different chiralities present clearly different MFM images under an?applied field.     

A micromagnetic model of dual-layer magnetic-recording thin films

The micromagnetic model, which is capable of simulating magnetic layers having different magnetic and geometric properties, is applied to the study of the magnetic properties of dual-layer media characterized by a three-dimensional isotropic distribution of anisotropy axes in both layers, using parameters typical of cobalt-alloy films. In the absence of exchange interactions between the layers, a correlation is found between squareness ratios, average magnetostatic energy densities, and structural dimensions of the media. An in-phase magnetization reversal of the layers occurs with increasing interlayer exchange coupling. A complex relationship is found between coercivity and media parameters     

Vortex Self-Organization in the Presence of Magnetic Pinning Arrays

Lateral nanostructuring is an efficient tool to control vortex confinement in superconductors. This will be illustrated by studying pinning phenomena in type-II superconducting Pb films with a lattice of submicron magnetic dots. We consider rectangular Co dots with in-plane magnetization and circular Co/Pt dots with out-of-plane magnetization. The domain structure of the Co dots can be changed from multi- to single-domain, resulting in an enhancement of their stray field. After covering this Co dot array with a Pb film, we demonstrate the influence of the local magnetic stray field of the dots on their flux pinning efficiency. The Co/Pt dots have a single-domain structure with their magnetic moment out of plane. Depending on the magnetic history, the magnetic moment of all dots can be aligned in positive or negative direction, or a random distribution of positive and negative magnetic moments of the dots can be achieved. For a Pb film covering this Co/Pt dot array, we observe an asymmetric magnetization loop due to the magnetic interactions between the vortices and the magnetic dots.     

Eigenschaften von elektrolytisch im magnetischen feld abgelagerten NiFe schichten

The paper presents the results of research on the magnetic properties of non-magnetostrictive NiFe films formed electrolytically in magnetic fields of various intensities.The magnetic properties change drastically for films obtained in a magnetic field greater than the anisotropy field. These changes are assigned to the magnetic coupling between imperfections of the net or the iron atoms and magnetization during the process of deposition.