Saturation magnetization in ferromagnetic thin films

The present status of the theory of saturation magnetization of ferromagnetic thin films is reviewed. The extensions of the Heisenberg trace method and the spin-wave method to the case of ferromagnetic thin films is fully discussed together with a comparison of the theoretical results with experimental data. The effect of anisotropic magnetic fields on magnetization and Curie temperature is presented, and the dependence of the magnetization and Curie temperature on film thickness is studied. Very recent progress and new results of the theory are stressed. The application of the Green function method to ferromagnetic thin films is also examined.     

The effect of mechanical stress on the magnetization curves of Ni and FeSi-single crystals at strong fields

The spatial distribution of the total energy is used to determine the magnetic behavior of ferromagnetic single crystals. For a single ferromagnetic domain there exists only a homogeneous magnetization the direction of which must correspond to a minimum energy direction. Magnetization curves are calculated by observing the stable positions of the magnetization under the application of strong magnetic fields. These magnetization curves show significant variations under different states of mechanical stress for FeSi and Ni crystals     

Spin-orbit-driven ferromagnetic resonance

Ferromagnetic resonance is the most widely used technique for characterizing ferromagnetic materials. However, its use is generally restricted to wafer-scale samples or specific micro-magnetic devices, such as spin valves, which have a spatially varying magnetization profile and where ferromagnetic resonance can be induced by an alternating current owing to angular momentum transfer. Here we introduce a form of ferromagnetic resonance in which an electric current oscillating at microwave frequencies is used to create an effective magnetic field in the magnetic material being probed, which makes it possible to characterize individual nanoscale samples with uniform magnetization profiles. The technique takes advantage of the microscopic non-collinearity of individual electron spins arising from spin-orbit coupling and bulk or structural inversion asymmetry in the band structure of the sample. We characterize lithographically patterned (Ga,Mn)As and (Ga,Mn)(As,P) nanoscale bars, including broadband measurements of resonant damping as a function of frequency, and measurements of anisotropy as a function of bar width and strain. In addition, vector magnetometry on the driving fields reveals contributions with the symmetry of both the Dresselhaus and Rashba spin-orbit interactions.     

Positive and negative exchange bias in IrMn/NiFe bilayers

We present the observation of double shifted hysteresis loops in IrMn/NiFe bilayer structures. The bilayer structures were fabricated using high vacuum DC magnetron sputtering system. The hysteresis loops of the as deposited samples show the double shifted loops at NiFe layer thicknesses 5 nm and 6 nm, whereas the IrMn layer thickness was kept constant at 15 nm. The results were interpreted as the contribution of both positive and negative exchange bias fields. We suppose that this phenomenon is occurring due to the ferromagnetic (FM) layer exchange coupled with the antiferromagnetic (AFM) layer in two different magnetization directions. The ferromagnetic coupling of the interface spins in some regions of the film generates the hysteresis loop shift toward negative fields and antiferromagnetic coupling toward positive fields in the other regions. The double shifted hysteresis loops disappeared after magnetic field annealing of the samples above Neel temperature of the AFM layer. The X-ray diffraction patterns of the sample show the IrMn (111) crystalline growth necessary for the development of exchange bias field in this system. The correlation between the Magnetic Force Microscopy (MFM) domain structures of the as deposited sample and the magnetization reversal process of the double shifted hysteresis loops were discussed. The results suggest that the larger multidomain formation in the AFM layer with different magnetization directions was responsible for the positive and negative exchange bias fields in IrMn/NiFe bilayer samples.     

Magnetic coupling phenomena in EuS/Pb thin film double layers

The magnetic properties of EuS/Pb double layers (a ferromagnetic insulator on a superconducting metal) were investigated by means of 50 Hz hysteresis. The magnetic coupling between the two layers does not set in spontaneously. Two procedures for achieving magnetic coupling are described: first, by means of a d.c. magnetic field perpendicular to the film plane; secondly, with an a.c. field in the film plane. The coupled states are characterized by a slowing of the 50 Hz magnetization reversals. The degree of slow-down depends on the strength of the magnetic coupling.     

Switching processes in thin ferromagnetic film memory elements

The switching properties of small, rectangular areas of thin ferromagnetic films, relevant to their utilization as memory elements in digital computers, are discussed. An attempt is made to derive the switching properties theoretically by superposition of the calculated demagnetizing field effects upon the known intrinsic film properties. Experiments performed using homogeneous quasi-static applied fields show good agreement with the theory. In the case of high easy direction applied fields the complexity of the magnetization distributions necessitates a number of simplifying assumptions in the theoretical treatment, and here the agreement is poorer. The treatment is sufficiently accurate to yield relationships between the intrinsic film properties, the dimension of the element, and the available output flux. In practical configurations the element is switched by inhomogeneous fields produced by fast current pulses in strip lines, and its state is ascertained by the observation of an EMF induced by the rotating magnetization. The differences between the configuration observed here and the practical one are discussed.     

Magnetic and screening properties of amorphous ferromagnetic ribbons

We have studied the magnetic and screening properties of cylindrical shields made of amorphous ferromagnetic ribbons. It is established that the relative magnetic permeability of this material can exceed 106. The action of an alternating decaying magnetic field (demagnetization) converts amorphous ferromagnetic ribbons into the state of anhysteretic magnetization, which is characterized by extremely high (above 2 × 10⁷) values of the relative magnetic permeability in magnetic fields at a level of 10 nT. The results of measurements were used to estimate the coefficient of attenuation of the Earth’s magnetic field inside cylindrical shields with open ends, depending on the diameter and the number of layers of an amorphous ferromagnetic ribbon.     

Ferromagnetic Liquid Metal Putty-Like Material with Transformed Shape and Reconfigurable Polarity

It is remarkably desirable and challenging to design reconfigurable ferromagnetic materials with high electrical conductivity. This has attracted great attention due to promising applications in many fields such as emerging flexible electronics and soft robotics. However, the shape and magnetic polarity of existing ferromagnetic materials with low conductivity are both hard to be reconfigured, and the magnetization of insulative ferrofluids is easily lost once the external magnetic field is removed. A novel reconfigurable ferromagnetic liquid metal (LM) putty-like material (FM-LMP) with high electrical conductivity and transformed shape, which is prepared through homogenously mixing neodymium–iron–boron microparticles into the gallium-based LM matrix, and turning this liquid-like suspension into the solid-like putty-like material by magnetization, is reported to achieve this. The induction magnetic field of FM-LMP is mainly attributed to the magnetic alignment of the dispersed ferromagnetic microparticles, which can be conveniently demagnetized by mechanical disordering and reversibly reconfigured through microparticle realignment by applying a weak magnetic field. FM-LMP with a low fraction of microparticles can be used as printable conductive ink for paper electronics, which are further exploited for applications including magnetic switching, flexible erasable magnetic recording paper, and self-sensing paper-based soft robotics using magnetic actuation.     

A coupling phenomenon between the magnetization of two ferromagnetic thin films separated by a thin metallic film--Application to magnetic memories

A new type of magnetic interaction between two ferromagnetic thin films separated by a third metallic nonmagnetic thin film is studied. This coupling tends to bring the magnetization of the ferromagnetic materials into parallel alignment. The coupling is shown to be dependent upon the intermediate material and its thickness, and on the nature of the ferromagnetic media present. These interactions are observed for intermediate metal thicknesses up to 300 Å in the case of Palladium. The measured surface energy of this coupling is of the order of 0.1 erg/cm². A study of the variation of the coupling with temperature is given.     

Resistive switching via the converse magnetoelectric effect in ferromagnetic multilayers on ferroelectric substrates

A voltage-controlled resistive switching is predicted for ferromagnetic multilayers and spin valves mechanically coupled to a ferroelectric substrate. The switching between low- and high-resistance states results from the strain-driven magnetization reorientations by about 90°, which are shown to occur in ferromagnetic layers with a high magnetostriction and weak cubic magnetocrystalline anisotropy. Such reorientations, not requiring external magnetic fields, can be realized experimentally by applying moderate electric field to a thick substrate (bulk or membrane type) made of a relaxor ferroelectric having ultrahigh piezoelectric coefficients. The proposed multiferroic hybrids exhibiting giant magnetoresistance may be employed as electric-write nonvolatile magnetic memory cells with nondestructive readout.     

Magnetic detection of small fractions of ferromagnetic martensite within the paramagnetic austenite matrix of TWIP steel

In order to accurately determine a small fraction of a ferromagnetic phase in a paramagnetic matrix, the difference in ferromagnetic and paramagnetic responses to an applied magnetic field has been analyzed. The fraction was examined by field-dependent magnetization measurement at different temperatures as well as by thermo-magnetic measurements at constant magnetic fields. The method was successfully applied for the determination of small martensite fractions in a cold-rolled twinning-induced plasticity austenitic steel at three different thickness reductions.     

Magnetic properties of sputtered multilayers of Mo/Ni

We have studied the d.c. magnetization of Mo/Ni artificial superlattices in the temperature range 5–300 K and in magnetic fields of up to 10⁷/4πAm^-1 (10kG). The saturation magnetization and the Curie temperature behavior are consistent with expectations based on thin film effects. However, there are indications of ferromagnetic coupling across the normal metal and the coercive field shows an unexpected peak at a layer thickness of 25 Å.     

Epitaxial Orientation and Planar Hall Effect for MnAs Films Grown on GaAs(001)

We study the planar Hall effect in high quality thin ferromagnetic films of MnAs grown on GaAs(001) exhibiting hysteresis due to the hindered rotation of the magnetic moment in the plane. The saturation magnetic field H _s, which is necessary to align the magnetization along the hard axis, depends sensitively on the epitaxial orientation of the film. By using out-of-plane magnetic fields directions, we show that H _s is strongest along the direction of the c-axis of the MnAs crystal, thus demonstrating the importance of the crystal field anisotropy for the planar Hall effect.     

Anomalous magnetic source defect modelling

Defect-induced field perturbations occurring during electromagnetic testing of ferromagnetic material can be modelled as combinations of anomalous eddy current and missing magnetization sources. At low frequencies, such as are used in Remote Field Eddy Current (RFEC) inspection of steel tubes, the anomalous magnetization defect sources dominate and give strong responses to circumferential slits orthogonal to the axial magnetic field. Here we present the results of finite element calculations and computer animations of the time-varying fields from a missing magnetization model of a slit defect. These are compared with the defect-induced anomalous field patterns obtained by phasor vector subtraction of calculations for slit defect and defect-free problems. It is noted that the ferromagnetic material in which the defect is located must be included in the missing magnetization model.Research supported by Natural Sciences and Engineering Research Council of Canada, Pipetronix Ltd., Province of Ontario, Gas Research Institute and Infolytica Corp.     

High-resolution optical imaging of magnetic-domain structures

High-resolution optical techniques for imaging magnetic domains in ferromagnetic materials such as confocal microscopy and scanning near-field optical microscopy (SNOM) are reviewed. The imaging capabilities of different techniques and image formation are discussed in the case of in-plane as well as out-of-plane magnetic anisotropy in different illumination configurations. It is shown that the magnetooptical resolution of near-field measurements depends on the film thickness and is limited by the diffraction on magnetic domains throughout the film. For thin magnetic films, subwavelength resolution can be attained. In addition to well-known near-field magnetooptical effects (out-of plane magnetization sensitivity of linear near-field microscopy and in-plane magnetization sensitivity of nonlinear near-field measurements), linear SNOM imaging of in-plane magnetization in thin magnetic films as well as nonlinear imaging of out-of-plane domains has been demonstrated. Thus, linear and second-harmonic near-field imaging of both in-plane and out-of-plane oriented magnetic domains can be achieved in transparent and opaque specimens. This enables applications of SNOM for studies of all kinds of magnetic materials. High-resolution optical imaging is required for characterization of the micro-magnetic and magnetooptical properties of novel magnetic structures in order to adopt a bottom-up approach in the search for new materials with improved characteristics.     

Rayleigh-Benard convection in a micropolar ferromagnetic fluid

The problem of Rayleigh-Benard convection in a micropolar ferromagnetic fluid layer permeated by a uniform, vertical magnetic field is investigated analytically with free-free, isothermal, spin-vanishing, magnetic boundaries. The influence of the various micropolar and magnetization parameters on the onset of stationary convection has been analysed. It is observed that the micropolar ferromagnetic fluid layer heated from below is more stable as compared with the classical Newtonian ferromagnetic fluid. The nature of influence of the magnetization parameters on convection in the micropolar ferromagnetic fluid is similar to that in the case of Newtonian ferromagnetic fluids. The influence of the micropolar parameters on convection in the ferromagnetic case is akin to its role in the non-magnetic fluid case. The critical wave number is found to be insensitive to the changes in the micropolar fluid parameters, but sensitive to the magnetization parameters.     

Properties of Electrodeposited CoFeNi/Cu Superlattices: The Effect of CoFeNi and Cu Layers Thicknesses

CoFeNi/Cu superlattices were grown on Ti substrate by electrodeposition as a function of the ferromagnetic and non-magnetic layer thicknesses. In order to examine the effect of the Cu layer thickness on the film properties, the Cu layer thickness was changed from 0.5 to 6 nm, while the CoFeNi layer thickness was kept constant at 4 nm. Also, for the CoFeNi layer effect, the CoFeNi layer thickness was changed from 2 to 15 nm, while the Cu layer thickness was fixed at 4 nm. The structural analysis studied by X-ray diffraction indicated that the superlattices have face-centered-cubic structure. Magnetic characteristics were investigated by vibrating sample magnetometer. From the hysteresis curves, the coercivity and the saturation magnetization were determined. It was found that the easy-axis of the films is parallel to the film plane. Magnetoresistance measurements were made by the Van der Pauw method at the room temperature with magnetic fields up to ±12 kOe. All superlattices exhibited giant magnetoresistance (GMR). As the ferromagnetic layer thickness increased up to 4 nm, the GMR value increases up to 22 % and then decreases. The superlattices saturated at the lower magnetic layers with increasing ferromagnetic layer thickness. The maximum GMR value was obtained to be 22 % for a superlattice with 375[CoFeNi(4 nm)/Cu(4 nm)].     

Band model approach for the magnetic stiffness within ferromagnetic thin films

The magnetic stiffness within ferromagnetic thin films, considered as a tendency of the film to be homogeneous with respect to the distribution of magnetization directions, is defined on the basis of the domain structure as well as of the spin wave theory. The transversal stiffness parameter in thin films is calculated by means of the band model approach within a Hartree-Fock type of approximation. This parameter depends on the film thickness and, what is particularly interesting, on the surface effects.     

Pure magnetooptic diffraction and Kerr microscopy of periodic domain structures

The fabrication of a periodic domain structure in a ferromagnetic thin film is reported. This periodic domain structure is formed in a thin continuous magnetic film by coupling it to a periodic array of magnetic elements grown on top. When the array and the continuous film are exchange decoupled, magnetostatic interactions produce in the continuous layer a domain structure replica of the topographic pattern at selected field values. The present work reports a direct confirmation of this periodic domain structure in the flat continuous film by Kerr microscopy, which is responsible for the pure magnetooptic diffraction. The effect on the magnetization processes of oneand two-dimensional structures with different periodicities and dimensions is studied in detail and compared with micromagnetic simulations, for Co and Fe films.     

Analysis of the Anisotropy Field and the Saturation Magnetization for Ultrathin Ferromagnetic Films

We study the magnetic in-plane anisotropic fields for a two-dimensional film by including magnetic field in the basal plane for an easy axis film. We present the balance between the applied field and in-plane anisotropic field at equilibrium. We have also investigated the approach to saturation of magnetization and numerically solving the nonlinear equation for equilibrium, and results are discussed in connection with experimental data reported for Co films.