Showing papers on "Single domain published in 1980"

Magnetic blocking temperatures of single‐domain grains during slow cooling

Abstract: Acquisition of TRM by a cooling assemblage of identical monodomain grains involves a transition from superparamagnetic to blocked behavior over a narrow but indefinite range of temperature. Blocking temperature may, nevertheless, be precisely defined by comparing the blocked magnetization, after cooling, with the curve of equilibrium magnetization against temperature. At the blocking temperature TB the relaxation time τ of the magnetization is of the same order as the cooling time constant θ, which is the time required for τ to increase by a factor e. An approximate analytic solution of the differential equation for magnetization in a cooling system gives for TB:e (TB)/kT = 1n(1.78Cθ) where e is the activation energy for rotation of magnetic moments, k is Boltzmann's constant, and C is the frequency factor (∼1010 Hz). Because e varies with temperature, the cooling time constant θ is given by 1/θ = (1/kTB)(de/dT - e/TB)dT/dt. The 95% blocking interval is about 7.2θ. For steady cooling it is reasonable to assume θ to be constant through this interval; small deviations from constancy have a negligible effect upon TB. In small external fields the variable grain geometry to be found within a real assemblage can be incorporated in a parameter e0, which is the activation energy at O°C. Curves of e0 against TB for magnetite and haematite, at cooling rates from 6°C per minute to 3°C per million years, show that (1) a change in e0 by a factor of 2 corresponds to 12 orders of magnitude change in cooling rate and (2) for a broad distribution of grain sizes a large proportion of the size range will acquire their magnetization within 100°C–200°C of the Curie point. Using these curves, blocking temperature in cooling assemblages can be related to laboratory demagnetization temperatures; the differences are greater at slower cooling rates and lower temperatures and tend to zero as the Curie temperature is approached. Intensity of magnetization should increase by a few percent for each order of magnitude decrease in cooling rate. Magnetization acquired at temperatures near the Curie point should dominate in a chemically homogenous monodomain assemblage with a broad spread of grain sizes. Possible tests of the theory and its application to the determination of cooling history are discussed.

Time–temperature relations in the magnetization of assemblies of single domain grains

Abstract: A theory for important magnetization processes such as magnetic viscosity and the production of a moment on cooling in a magnetic field is presented here. The results for viscous phenomena differ from those obtained by others in that the time dependence of the viscosity depends on the size distribution of the magnetic grains. The moment produced on cooling in a magnetic field is calculated for an arbitrary distribution of grains for the first time. The effect of the cooling rate on the moment produced is found not only to be independent of that distribution but also of the temperature dependence of the saturation magnetization.

Studies of the perpendicular magnetization mode in Co-Cr sputtered films

Abstract: The extremely high resolution properties in a perpendicular magnetic recording system were studied with a recently developed Co-Cr film medium. Bitter technique was used to observe magnetization distribution in the medium, and the transition of magnetization was directly observed on the top and the bottom surfaces of the film. It was proved that an ideal perpendicular magnetization mode, having a very narrow transition width, takes place even in a high recording density over 40 kBPI. From the angular variation of H c and the micro-structure of the Co-Cr film, it was concluded that the high recording resolution of the film results from the fine columnar particles which show the rotational mechanism of magnetization reversal accompanied by a perpendicular magnetocrystalline anisotropy. The Co-Cr film was also found to be superior in perpendicular anisotropy to any other Co-M films studied here in.

Magnetic domain observations of nucleation processes in fine particles of intermediate titanomagnetite

Abstract: The stable carriers of the palaeomagnetic record in rocks are commonly fine grains of magnetite or titanomagnetite (Fe3–xTixO4) between 1 and 15 µm in diameter. The magnetic properties of these particles are therefore of interest to palaeomagnetists who must rely on them to preserve the Earth's magnetic field record over geological time. We report here domain observations of titanomagnetites in samples from Leg 49 of the Deep Sea Drilling Project. The results indicate that a major contributor to ‘pseudosingle-domain’ (PSD) saturation remanence is single domain remanence of particles larger than the classical single-domain threshold size which fail to nucleate domain walls in this magnetization state.

Magnetic properties and domain structures in narrow NiFe stripes

Abstract: The coercivity of narrow stripes of 81 percent Ni-19 percent Fe films has been found to increase rapidly, by up to an order of magnitude, as the stripe width decreases and approaches 1 μm. Furthermore, the coercivity increases with decreasing film thickness as it does in sheet films, but in the stripes the changes in coercivity with film thickness are much larger than in sheet films. For example, in stripes of 2 μm × 100 μm, H c increases from 10 Oe to 35 Oe as the thickness decreases from 1800 A to 300 A. The increase in coercivity with decreasing stripe width may be explained by a buckling of the magnetization perpendicular to the length of the stripe. This buckling process was made visible by decoration of domain walls with Ferrofluid, and is shown to lead to the formation of walls perpendicular to the stripe. These walls do not move, but block reverse domains from propagating down the stripe. In the narrowest stripes (2 μm) fields larger than 100 Oe are required to collapse the 360° wall segments which eventually form. A theoretical model for this buckling process is given which shows that a minimum in energy occurs when the stripe buckles with a buckling wavelength approximately equal to the width of the stripe. These findings suggest that, as structures of NiFe like those in magnetic bubble devices and in magnetic recording heads are made smaller, coercivity and dispersion will rise significantly, leading in many cases to undesirable magnetic behavior.

Magnetization measurements on single crystals of uranium chalcogenides and pnictides

Abstract: 1. (a) US, USe and UTe are ferromagnets as well as a great variety of mixed crystals. The magnetic moments are strictly confined to the 〈111〉 easy axis. Moments measured along any other axis are merely the projections of the moment induced along the 〈111〉 axis. The shape of the hysteresis loop depends critically on temperature even far below the Curie temperature, due to thermal activation of monoatomic Bloch walls. Even if the maximum magnetization is observed along the 〈111〉 direction, it is in some cases easier to reach saturation by applying a magnetic field along another axis. The observed anisotropy cannot be of a simple uniaxial nature. 2. (b) UN, UP, UAs, USb, and UBi are antiferromagnets together with many mixed crystals. Magnetic moments in the ordered state and spin structures were determined by neutron diffraction experiments. We can observed magnetic transitions in experimentally achievable fields (200 kOe for our experiments). Such measurements permit us to establish a magnetic phase diagram. In many antiferromagnets we encounter predominant domain effects. Up to 70 kOe of applied field are necessary to obtain single domain crystals.

Rotational hysteresis and magnetization curling of elongated single-domain particles in Alnico permanent magnets

Abstract: The rotational hysteresis energy has been measured as a function of the applied field for Alnico IX with a preferred orientation and a single crystal of Ticonal 900. The results have been compared with the theoretical values obtained from the Stoner-Wohlfarth and Shtrikman-Treves theories, The results are in good general agreement with the Shtrikman-Treves theory, and from measurements of the rotational hysteresis integrals and the angular dependence of the coercivities it has been concluded that changes in the magnetization occur principally by the curling mechanism.

Characterization of effects of plane-front solidification and heat treatment on magnetic properties of Bi/MnBi composites

Abstract: Eutectic Bi/MnBi composites (3.2 volume percent MnBi) have been plane-front directionally solidified using the Bridgman-Stockbarger method. For the growth velocity range investigated, the morphology of the composites consists of an aligned ensemble of MnBi rods with mean rod diameters lying below the theoretical size for single domain behavior. Magnetization as a function of temperature, applied field and angle with respect to the solidification direction was used to characterize the magnetic properties. These measurements have been correlated with particle size distributions and volume fraction data. Previous studies have shown that solidification processing conditions and post-solidification heat treatment significantly affect the magnetic properties due to the presence of a nonequilibrium magnetic phase. The nonequilibrium phase has been isolated by varying the processing parameters and the temperature dependence of its magnetization determined. In addition, the transformation kinetics of the nonequilibrium to the equilibrium MnBi phase was investigated by monitoring the magnetization change during isothermal annealing. The intrinsic coercivity of the equilibrium component was found to depend on whether a given percent of equilibrium phase is produced by directional solidification or post-solidification heat treatment anneal.

Magnetic fluid as a continuum with internal degrees of freedom

Abstract: A closed system of equations governing thermomechanics of magnetic fluids is formulated with magnetization relaxation included. The material parameters are estimated. Approximate formulations of the dynamic megnetization equations are presented and the ranges of their validity are discussed. The dimensionless groups, which determine the magnetic fluid mechanics are introduced and their ranges are found. Particular effects due to the magnetization relaxation are considered.

On the microstructure and magnetization process of γ-Fe 2 O 3

Abstract: Experimental evidence is reported for γ-Fe 2 O 3 single particles being made up of several microcrystals chained together and their boundaries being sources of demagnetizing fields. This leads to a very defined picture of the magnetization reversal in partially alligned γ-Fe 2 O 3 particles: rotation is incoherent but the magnetization vectors are scattered in such a way that H ci vs. angle Ψ (between the orientation direction and the applied field) does not follow the known magnetization reversal modes. The importance of the reduction temperature in the process: α-FeOOH \underrightarrow{red.} Fe 3 O 4 \underrightarrow{ox.} γ-Fe 2 O 3 becomes extremely important if considered as the parameter which rules the pore closure within the particles, i.e. the size and number of microcrystals chained in a single particle.

New method for routine measurement of coercivity in magnetic bubble films

Abstract: We describe a new technique for measuring coercivity in magnetic bubble films which consists of placing the film in a weak field gradient (∼1 Oe./μm) in order to obtain a set of finger-like domains. The unconstrained ends of these domains are caused to move back and forth in response to an oscillatory field, and the coercivity is obtained from an extrapolation of the linear portion of the response vs. drive field curve. We present a comparison between coercivity values in materials with 3μm and 1.7μm stripe-widths obtained using the new technique and bubble translation. Good correlation is observed for both types of material, the values obtained with the new technique being somewhat higher than the bubble translation values. The difference is ascribed to material non-uniformities.

On the mechanism of particle interaction in magnetic recording media

Abstract: The mechanism of particle interaction in particulate magnetic recording media has been investigated using a simple model for the particle switching process. The model allowed simulation of about 1000 interacting particles. It was found that the exact definition of the particle collective is decisive. Furthermore, the average interaction field appears to consist of two independent parts, one related to the well-known demagnetizing field and the other which only depends on the structure of the particle arrangement.

The angular distribution of the magnetization in magnetic recording tapes

Abstract: The angular distribution of the magnetization in a recording tape in, for example, the remanent state is obtained by fitting realistic models of the distribution to experimental data. The results on a wide range of tapes show that the distribution (assumed to take a double Gaussian form) tends to be cylindrically symmetrical about the tape axis. Values of the angular standard deviation usually lie between 0.4 and 0.8 radians.

The effect of the magnetization distribution within anisotropic alnico magnets upon field calculation

Abstract: Magnetization characteristics of an anisotropic alnico permanent magnet have been measured at 10° increments from the preferred to the transverse axes. These are shown to correspond to results obtained in a magnetometer. Although the magnetization varies in magnitude and direction throughout the volume of alnico magnets, the rotation of magnetization may therefore be modelled using J vs. H characteristics. In fact, it is found that only curves for mutually perpendicular directions should be used, and these are incorporated in a rapid method for modelling these magnets. This new technique allows a non-uniform magnetization distribution to be determined from self-demagnetizing fields that have been calculated using a uniform magnetization distribution, thus eliminating a lengthy iterative procedure.

Direct observation of domain nucleation in GdCo5 single crystals

Abstract: Magnetization reversal in GdCo5 single crystals was studied by domain observations and magnetization measurements. Dendritelike domains appear after reversal at the nucleation field. These patterns suggest that the first reversed domain nucleates at one point and spreads out through the entire sample. The lifetime of moving walls of the dendritelike domain increases with the increasing diameter of the spheric or spheroidal samples along the easy axis of magnetization. The velocity of wall tips is estimated to be approximately 5 m/sec at room temperature except for extremely small samples.

Domain wall pinning in the hard permanent magnet Sm 2 Co 10 Cu 1.48 Fe 3.16 Zr 0.194

Abstract: We have considered the ferromagnetic domain wall pinning caused by a planar type defect characterized by a change in the magnitude of the exchange constant and the uniaxial magnetic anisotropy and in the angular deviation of the magnetic anisotropy axis. Our model is appropriate for bulk material when the magnitude of the radius of curvature of the defect is not large compared to the width of the domain wall. Expressions are obtained and graphed for the coercive force due to such pinning. It is shown that the angular mismatch between the defect and the host material breaks the degeneracy between clockwise and counterclockwise rotating Bloch walls. Further, the results are applied to the Japanese commercial TDK material, Sm 2 Co 10 Cu 1.48 Fe 3.16 Zr 0.194 , which shows a fine cellular structure of approximately 500 A scale.

Magnetic properties of small acicular particles

Abstract: Using the optical techniques described previously, the mean dipole moments and anisotropy fields, as well as the switching field distributions were measured on gamma ferric oxide, cobalt-doped gamma ferric oxide and chromium dioxide particles suspended in a liquid. A new method for the determination of switching field distributions is described; its results agree with those of the older one. The particles were about 100 × 20 × 20 nm. The anisotropy, fields (for small reversible changes of the magnetization) ranged from 60 to 100 kA/m, i.e. much lower than expected for shape anisotropy. The observed mean switching fields (for large irreversible changes) were all between 63 and 75 kA/m, and appeared to be independent of particle size, shape or material.

A study of domain wall and magnetic domain changes in T bar by employing ferro-fluid and comparison of the results with magneto-optical Kerr effect measurements

Abstract: The movements of domain walls and the changes in the magnetic domain patterns of a T bar are studied by employing Ferro-fluid and the results are compared with the model proposed for a T bar. The oscillation of the walls and the changes of the magnetic domains in T bar due to an oscillating in-plane field are shown by different sketches. The net magnetization distribution that can be obtained from these observations are compared to the similar results obtained by Krinchik et al.

Rotatable magnetic anisotropy in epitaxial ferrite layers

Abstract: Magnetic domain structure and magnetic anisotropy were studied in monocrystalline epilayers of Mg 0.9 Mn 0.3 Fe 1.8 O 4 ferrite. The layers, several micrometers thick, were obtained by a CVD method on monocrystalline MgO substrates. Domain observations were performed by the Bitter's method. Magnetic anisotropy measurements were performed by torque and FMR methods. In the demagnetized state, a typical stripe structure of 2.0 to 2.8 μm period was observed. From the domains behavior in the in-plane magnetic fields it was found that in these epilayers the rotatable anisotropy was present. The existence of this anisotropy was confirmed by torque measurements in small in-plane fields. The magnetic parameters characterizing these layers are: 4ΠM = 3500 Gs, K 1 = - 2.2 × 104ergs/cc, K N = 2.3 × 105ergs/cc.

The application of piezoremanent magnetization to magnetic contact printing

Abstract: Piezoremanent magnetization (i.e., remanent magnetization obtained by the simultaneous application of a momentary pressure and a magnetic dc field) was studied for application to magnetic contact printing. A Co-γFe 2 O 3 tape was found most suitable for the slave tape, and the origin of the piezoremanent magnetization effect was attributed to the inverse magnetostrictive effect, the dispersion of Co atoms to vacant lattices, and crystalline anisotropy.

Effects of in-plane field on domain wall dynamics in amorphous GdCo magnetic films

Abstract: Previously we reported the dynamics of an isolated straight domain wall near the compensation point in amorphous GdCo and GdCoAu films prepared by rf-sputtering. Nonlinear dependence of the wall velocity on drive field is observed. Here we describe the effects of externally applied in-plane field and in-plane anisotropy of the film on domain wall dynamics in amorphous GdCo magnetic films. The wall whose plane is perpendicular to the in-plane easy axis shows higher velocity and smaller wall motion coercive force compared with the wall whose plane is parallel to the in-plane easy axis. The domain wall velocity depends on the directions of pulse magnetic field and external in-plane field. The effects of external in-plane field in low field region are different from those in high field region.

Coercivity mechanism of Sm(Co,Cu) alloys

Abstract: Critical fields for the pinning of a narrow domain wall, their angular dependences, and crystalline lattice strain effects are evaluated in materials of high magnetocrystalline anisotropy, such as Sm(Co, Cu) z compounds, with 5.0 < z < 6.0.

Comments on "An analysis of tilted magnetic transition in magnetic transitions in magnetic recording media"

Abstract: Expressions for the demagnetizing field caused by the surface magnetic charge produced by vertical magnetization in a tilted transition are presented. This field should be added to that used by Chang and Perez in their analysis of tilted transitions.