Self-demagnetizing Field Research Articles

Modelling of Magnetic Stray Fields in Multilayer Magnetic Films with In-Plane or Perpendicular Anisotropy

The magnetic stray field is an unavoidable consequence of magnetic multilayers, which may have a significant influence on the performance of spintronic devices. Based on Maxwell’s magnetostatics theory, here we numerically calculated the distributions of magnetic stray fields and self-demagnetizing fields in a series of patterned multilayer thin-film structures with either an in-plane or a perpendicularly magnetized ferromagnetic layer. The stray field above the ferromagnetic layer is inhomogeneous, showing the dramatic changes near the sample edge, but the uniformity in the center region could be improved with the increasing sample size. The stray field strength tends to zero for large samples, increases with the increase in the hard-layer thickness, and decreases with the increase in the distance D away from the ferromagnetic layer. In the multilayer samples, the separately simulated stray field and self-demagnetizing field within the soft layer agree well with the classic magnetostatic relationship of B=μ0(Hd+M). For the in-plane magnetized trilayer sample, the magnetic-flux density within the soft ferromagnetic layer slightly decreases in the antiparallel magnetization alignment and increases in the parallel alignment state with the increase in the intermediate non-magnetic-layer thickness. In contrast, for the sample with the perpendicular magnetization, the magnetic-flux density decreases as the non-magnetic layer is thickened for both the antiparallel and parallel state. This study may provide a theoretical basis for the design of thin-film spintronic devices.

Irreversible demagnetization mechanism of permanent magnets during electromagnetic buffering

The permanent magnets will be irreversibly demagnetized under high temperature and high velocity during the electromagnetic buffering. In this study, the magnetic field induced by eddy currents and the self-demagnetizing field of permanent magnet are taken into consideration together for demagnetization analyse. The magnetic Reynolds number is used to express the eddy currents demagnetization. The correction coefficient being expressed as the index of the air-gap width, the inner cylinder thickness, iron pole axial length and the permanent magnet demagnetization coefficient is introduced by magnetic path analysis to represent the self-demagnetization effect and the demagnetization extent. The electromagnetic buffer (EMB) prototype is tested under intensive impact loads of different strengths at room temperature. The accuracy of the nonlinear irreversible demagnetization finite element model is verified by demagnetization on damping force, velocity and displacement. Finally, high-velocity demagnetization and high-temperature demagnetization are analysed in order to obtain the distribution law of irreversible demagnetization.

Photographic observation of magnetic domain structure with three-dimensional local magnetization direction

The direction of magnetization of a magnetic material is possibly oriented three-dimensionally because of the presence of magnetic anisotropy field, self-demagnetizing field, and stray field. Therefore, the three-dimensional detection of the direction of magnetization is required. The method of magnetic domain observation by photographic imaging utilizing the Kerr effect is widely used. If the perpendicular magnetization components exist, there is a problem that obliquely incident light has superimposed longitudinal Kerr and polar Kerr effects. To perform the three-dimensional detection of magnetization direction, it is necessary to eliminate the influence of the polar Kerr effect from the Kerr effect of obliquely incident light. We report the photographic observation of the magnetic domain structure and the detection of the three-dimensional local magnetization direction using the Kerr effect, applying only an in-plane saturation magnetic field.

Magnetic recording in rocks

Charting the movement of tectonic plates or the evolution of a dynamo ultimately relies on the behavior of often imperfect mineral grains in Earth’s magnetic field.

A Comparative Study on the Prediction of the Saddle Point Magnetic Configuration in Nanostructured Cells of Single Magnetic Thin Films and Synthetic Antiferromagnets

Methods are described in this study to predict the magnetic configuration at the saddle point in nanostructured cells of single magnetic thin films and synthetic antiferromagnets. The prediction is done indirectly by calculating the self-demagnetizing field at the saddle point, which is then compared with the value calculated under the ideal single domain state. The saddle point magnetic configuration in the single magnetic thin films deviates more from the ideal single domain state than in the synthetic antiferromagnets. As the cell geometry varies from rectangle to ellipse, the deviation decreases monotonically in the single magnetic thin films, while it remains nearly constant in the synthetic antiferromagnets.

Revisiting magnetization processes in granular hard magnetic materials

We show that the generally accepted demagnetizing field corrections are not applicable to the broad class of modern granular magnetic materials, where magnetization reversal occurs via discrete switching of the moments of individual grains, and thus the self-demagnetizing field of a given grain is proportional to the spontaneous magnetization Ms, not to the sample magnetization M. This leads to the fact that the self-demagnetizing field must be considered as a contribution to the coercive field and that the slope of the magnetization variation is higher than the classical demagnetizing field slope. This description of demagnetization processes is confirmed by numerical modeling.

Thermal stability of a nanostructured trilayer synthetic antiferromagnet

Thermal stability of a nanostructured trilayer synthetic antiferromagnet is investigated in the Worledge model. No or minimal shape anisotropy effect is predicted from the model due to the simplifying assumption that the self-demagnetizing field is equal to the dipole field. A slight relaxation of the assumption causes a large impact on the thermal stability. In the case that the dipole field is 90% of the demagnetizing field, the energy barrier to spin flop is doubled to 80kT in a typical ellipsoid cell (212×106nm2) for high density magnetic random access memory. The thermal stability of half-selected cells in the writing scheme based on the spin flop is also improved by relaxing the assumption over the applied field range relevant to magnetic random access memory applications.

Experimental comparison of FORC and remanent Preisach diagrams

First-order reversal curve (FORC) and remanence-based Preisach diagrams are alternative ways of determining the Preisach distribution of a sample, which incorporates information about the coercivity spectrum and the distribution of interactions and self-demagnetizing fields. We compare results of the two methods for well-characterized synthetic and natural samples containing single-domain (SD) and pseudo-SD (PSD) magnetite, maghemite, titanomagnetite and titanomaghemite. The greater time requirements of remanence as opposed to in-field measurements limited our Preisach diagrams to a few hundred points, compared to several thousand points for the corresponding FORC diagrams. Only minimal smoothing could be applied in order to limit the regions near the axes of the diagrams in which function values must be extrapolated. In spite of these restrictions, we find excellent agreement between the essential features of the distributions determined by the two methods. The main features, the location and spreading of the distribution peak, are very consistent. However, the low-coercivity part of the Preisach distribution is sometimes poorly resolved or not imaged at all for remanence-only measurements. Features in this region can be diagnostic of PSD and multidomain (MD) grains. The essential agreement between our FORC and Preisach diagrams in the region where they overlap justifies using the much faster FORC routine instead of traditional remanence-based Preisach methods to determine the Preisach distribution of palaeomagnetic samples without strong interactions. We propose a symmetric FORC protocol that would permit separation of the irreversible and reversible parts of the Preisach distribution. The irreversible part is what is determined by remanence-only methods and what is desired for characterization of the remanence behaviour of palaeomagnetic samples. The reversible part is most significant in detecting MD behaviour and screening out samples containing large PSD and MD grains.

Determination of activation volumes of reversal in perpendicular media

We discuss a method for the determination of activation volumes of reversal in perpendicular media. This method does not require correction for the self-demagnetizing field normally associated with these media. This is achieved by performing time dependence measurements at a constant level of magnetization. From the difference in time taken for the magnetization to decay to a fixed value at two fields separated by a small increment /spl Delta/H, the activation volume can be determined. We report data for both CoCrPt alloy films and a multilayer film, typical of those materials under consideration for use as perpendicular media. We find activation volumes that are consistent with the hysteresis curves of the materials. The activation volume scales qualitatively with the exchange coupling. The alloy films have significantly lower activation volumes, implying that they would be capable of supporting a higher data density.

Recording Characteristics of Flux-detectable Magneto-optical Recording Media

We investigated the effect of an additional readout layer in TbFeCo-based bilayer media on the magneto-optical (MO) recording characteristics, by conducting experiments and calculations based on the Huth formula. The bilayer medium with an appropriate readout layer can have equivalent recording characteristics to those of a monolayer one. The designed bilayer medium showed sufficient performance for high-density MO recording, providing a sufficiently strong stray field for magneto-resistive detection. On the other hand, a monolayer medium with large magnetization resulted in poor recording performance due to its self-demagnetizing field. By comparing the recording characteristics, the guidelines for designing the bilayer structure were clarified.

Magnetic susceptibility quantification for arbitrarily shaped objects in inhomogeneous fields.

Magnetic susceptibility measurement has wide-ranging applications in MR technical development and medical applications. A general susceptibility quantitation method for objects of arbitrary shapes in inhomogeneous magnetic fields is presented in this study. Based on the mean value properties of magnetic fields, the polarizing magnetic field at the location of interest inside an object can be exactly obtained in situ from the field values on a spherical surface enclosing the object. With numerical computation of the self-demagnetizing field and correction of contact shifts, magnetic susceptibilities were quantitatively measured for CuSO(4) phantoms based on their MR gradient echo phase maps.

Multidomain hematite: A source of planetary magnetic anomalies?

Thermoremanent magnetization (TRM) in hematite is larger than TRM in magnetite for grain sizes ≥10 µm. We show that hematite's weak spontaneous magnetization Ms causes its strong TRM, since the self‐demagnetizing field Hd opposing large domain wall displacements is proportional to Ms. In hematite, Hd is comparable to the Earth's magnetic field but in magnetite, Hd is 1000 times larger. As a result, Earth's field TRM of MD hematite (≈ 0.3 Am²/kg) outweighs TRM and induced magnetization of MD magnetite (≈0.01–0.02 Am²/kg and rivals TRM of single‐domain and PSD magnetite as a source of magnetic anomalies on Earth and perhaps on Mars.

Exact calculation of demagnetization field in micromagnetic simulation of shielded read heads

In micromagnetic simulation of shielded GMR heads, the self-demagnetizing field is usually calculated under the assumption that the two shields extend to infinity beyond the air bearing surface. Under this assumption, the demagnetization tensor for rectangular cells in free space and the method of images are used to calculate the self-demagnetizing field in the sensor. An unanswered question for this approach is how accurate it is. In this work, the Fourier series method is used to calculate the self-demagnetizing field of the sensor in the shielded environment exactly. Simulation results from this rigorous approach and the common approach are compared for both simple spin valve and synthetic antiferromagnet (SAF) biased spin valve heads. It is shown that the error of the common approach is very small for a SAF spin valve head and can be as large as 10% for a simple spin valve head.

Size effects on characteristic magnetic fields of a spin-valve multilayer by computer simulation

The change in characteristic magnetic fields of a spin-valve multilayer is investigated as a function of the size by computer simulation. The spin-valve modeled in this work is IrMn (9 nm)/CoFe (4 nm)/Cu (2.6 nm)/CoFe (2 nm)/NiFe (6 nm). The spin-valve dimensions are varied widely from 20 mm×10 mm to 0.5 μm×0.25 μm, but the aspect ratio defined by the ratio of the length to the width is fixed at 2.0. The magnetostatic interactions begin to affect the magnetic properties substantially at a spin-valve length of 5 μm, and, at a length of 1 μm, they become even more dominant. The main consequences of the magnetostatic interactions are a significant increase of the coercivity and a very large shift of the bias field in both the pinned and free layers. It is shown that these changes can be explained by two separate contributions to the total magnetostatic interactions: the coercivity change by the self-demagnetizing field and the change of the bias field by the interlayer magnetostatic interaction field.

Study on Magnetic Force Acting on the Magnetic Toner in the High Pixel Density Magnetic Printer With Longitudinal Recording

The purpose of this study is to realize much higher pixel density such as 2,000dpi than the current available magnetic printer of 600dpi. In the magnetic printer with longitudinal recording method, if the adjoining magnetic charges become near, self-demagnetizing field increases which decreases magnetization of the recording medium in turn. The higher the pixel density becomes, the weaker the magnetic force acting on the magnetic toner becomes. The magnetic forces acting on the magnetic toner are calculated with various characteristics of the recording medium and the toner. The analysis is effected from the view point of the magnetic force enough to attract the magnetic toner. The suitable combinations of the characteristics of the recording medium and the toner are discussed. The results are as follows. a) As the pixel density increases, the magnetic force decreases exponentially; b) At 2,000dpi the magnetic force becomes 1/10 of that of 400dpi. c) Even at 2,000dpi there exist some conditions which can generate the same magnetic force as that of 400dpi. d) One of the suitable condition is a combination of the recording medium with the residual magnetization of 0.8Wb/m2, the coercive force of 64kA/m and the thickness of 0.37μm and the magnetic toner with the diameter of 5μm and the relative magnetic permeability of 2.

Micromagnetic design of spin dependent tunnel junctions for optimized sensing performance

Pinned spin dependent tunneling devices have been fabricated into high sensitivity magnetic field sensors with many favorable properties including high sensitivity (∼10 μOe/Hz at 1 Hz and ∼100 nOe/Hz at >10 kHz), a linear bipolar output versus applied field, high processing yields, and high temperature stability and operability (over 200 °C). However, the performance of fabricated sensors has not yet approached the theoretical limit one calculates assuming ideal behavior of the sensors’ ferromagnetic layers’ magnetizations. Given a total magnetoresistive signal of 30%, and typical anisotropy fields and hard axis biasing conditions, there should be a region of linear nonhysteretic response at zero field with a slope of greater than 20%/Oe. Measured responses are 1%–3%/Oe, and exhibit some hysteresis. These less than desirable effects are the result of several factors including: (1) Self-demagnetizing fields of the soft (sensing) layer; (2) stray fields from the hard (pinned) layer; (3) imperfect pinning of the hard layer; and (4) interlayer magnetic coupling across the tunnel barrier. This paper describes, in detail, the extent to which these factors affect sensor performance, and specific steps to be taken in order to minimize their deleterious influence. Specifically, the simple pinned layer is replaced by an exchange coupled synthetic antiferromagnet (CoFe/Ru/CoFe), the soft layer is made to be significantly larger in the plane than the pinned layer, and the soft layer is made as thin as possible.

Correlation between lifetime and blocking temperature distribution in spin-valve structures

The blocking temperature distribution Tb(T) and the failure activation energy (as defined by a 10% drop in the magnetoresistance amplitude in a reverse field equivalent to the self-demagnetizing field of a micron size stripe height device) have been determined in spin-valve sheet films with FeMn, IrMn, PtMn, NiMn, and CrPdMn antiferromagnetic exchange biasing layers. We find a clear correlation between the expected lifetime and the fraction of loose (e.g., unblocked) antiferromagnetic grains, which we believe is due to pinned layer rotation being the main failure mechanism in these systems. For CrPdMn structures, a good agreement is found between the stability of sheet films and of finished sliders. From these data, only NiMn and PtMn appear to be suitable for disk-drive applications.

Blocking temperature distribution and long-term stability of spin-valve structures with Mn-based antiferromagnets

We have determined the blocking temperature distribution Tb(T) in spin-valve sheet films with FeMn, IrMn, PtMn, NiMn and CrPdMn antiferromagnetic layers (AFM). We find a clear dependence of Tb(T) on the field applied during the measurement, which we link to the reversal state of the pinned layer through the torque applied on the AFM. Using fields large enough to fully reverse the pinned layer, NiMn and PtMn show little or no components of the blocking temperature below 150 °C, whereas both IrMn and CrPdMn (the latter in a “synthetic” AFM design) exhibit important low-temperature trailing edges of the distribution. Accelerated annealing experiments in a low reversed field equivalent to the self-demagnetizing field in a micron-size head allows us to access the time to failure and the failure activation energy from which the expected lifetime can be assessed. We find a general correlation between the expected lifetime and the fraction of loose (e.g., unblocked) AFM spins at any given temperature. Accordingly, only NiMn and PtMn are found to exhibit a sufficient long-term stability for disk-drive operations.

High coercivity in ultrathin epitaxial micrometer-sized particles with in-plane magnetization: Experiment and numerical simulation

Arrays of $2.5\ifmmode\times\else\texttimes\fi{}{10}^{7}$ ultrathin epitaxial W(110)/Fe(110)/W(110) submicron particles with in-plane magnetization were fabricated using x-ray lithography and dry-etching techniques. A large coercive-field increase as compared to continuous films is observed which is ascribed to the short range of self-demagnetizing fields for ultrathin particles with in-plane magnetization. A method to extract the mean hysteresis loop of a single particle and the coercive field distribution function from measurements over the whole array is proposed. The analysis is in good agreement with the picture of single-domain particles and of nucleation volumes much smaller than particle dimensions. It is corroborated by micromagnetic calculations performed on isolated ideally square particles.

A COMBINED MAGNETO-OPTIC-MAGNETIC FORCE MICROSCOPE STUDY OF AMORPHOUS AND MULTILAYER FILMS

We have combined a Magnetic Force/Atomic Force Microscope (MFM/AFM) with a Magneto-optic (MO) microscope. This instrument combines the high spatial resolution of the MFM/AFM and its capability to correlate magnetic structure with the structure of the sample surface with the real-time imaging capability and large field of view of the MO microscope. Using this instrument, we studied nucleation and displacement of domain walls (DW) in amorphous TbFe alloy films and Co/Pd multilayer films. We found that in Co/Pd multilayer film with 20 Co/Pd layer pairs and 16 nm total thickness, nucleation of domains during sample remagnetizations occurs repeatedly in the same points, and that displacement of domain walls is unidirectional. The high topographic resolution of the AFM allowed us to show that domains nucleate at small defects (1-2 nm in depth) on the sample surface. The unidirectional displacement of the domain walls was found to correlate with the anisotropic structure of the sample surface. We observed reversible bending of domain wall segments pinned to defects and irreversible, jump-like displacement of domain wall segments in an applied magnetic field. The maximum reversible displacement of domain walls was 50-100 nm and the length of the segments which reversibly curved in the field was about 150 nm. Measurement of the change in radius of curvature of a DW segment in response to an applied field allowed estimation of the DW energy density and local self-demagnetizing field of the film acting on the DW. The irreversible displacement of a DW was not a continuous process. The 200 - 400 nm long DW segments exhibited jump-like motion over distances of 100-150 nm.