Rotational and domain wall motion aftereffect in a patterned array of small particles

Aftereffect for magnetization processes by rotation and by domain wall motion was investigated on the same, single domain, two-state system of a square 2D (two-dimensional) array of garnet particles. Aftereffect measurements were performed magnetooptically. The particles are thermally stable, the particle energy is 10(-6) erg compared to the thermal energy of 10(-12) erg. No aftereffect of rotation switching of the system of "up" and "down" magnetized particles could be observed at room temperature. At increased temperatures thermally activated switching, very weakly depending on magnetic field, is observed. Each individual particle can be demagnetized into a metastable stripe domain structure. The barrier for DW (domain wall) motion is much lower than the barrier for rotation, and a significant aftereffect was measured on the same particles, demagnetized into a domain structure. The observed time dependence for DW aftereffect is exponential, M(t)/Ms=d* exp(-et), where d=0.04, and e=1/tau follows the increase of the magnetization with field, de/dH=0.021 /s/Oe

Hysteretic properties of a two dimensional array of small magnetic particles: a test-bed for the Preisach model

The magnetization process of a regular two-dimensional array of small, strongly uniaxial single domain magnetic garnet particles, groups of particles, and major loop properties of a "macroscopic" sample, has been investigated experimentally and simulated numerically. These particles correspond to the assumptions of a simple Preisach model. The switching mode is by rotation. Each particle has a square hysteresis loop, with no reversible or apparent reversible component. Requirements of wiping-out and congruency properties are satisfied. From measurements of the up- and down switching fields on individual particles, the major loop can be reconstructed, and it is shown to be in in excellent agreement with the measured one. The transition from individual to collective behavior is smooth and the properties of a system, consisting of 100 particles, correspond to the major loop behavior. The numerically simulated major hysteresis loops agree very well with the measured loops, the switching sequence and the magnetization curve for particle assembly was derived from the calculated interaction fields and found to be in a very good agreement with the measured values, demonstrating the reliability of numerical modeling. A new property, not included into the existing models, is the magnetization dependence of the standard deviation of the interaction field

Domain-wall coercivity in ferromagnetic systems with nonuniform local magnetic field

Domain-wall (DW) coercive field, H-CW, which characterizes pinning of DW's in soft magnetic materials, decreases strongly with increasing value of gradient, G, of the effective local DW-position-restoring magnetic field. Particular shapes of the dependence, H-CW(G), can be calculated from the mean energy dissipation of the DW moving over the particular profile of the DW pinning field, H-p. In this paper, H-CW(G) is calculated from a wall-pinning field, H-p, which is expressed as a stochastic function of the DW coordinate, x(DW). The wall-pinning field, H-p, is described as a Wiener-Levy stochastic process modified by two correlation lengths in such a way that H-p is stationary for large DW displacements and dH(p) /dx(DW) is well defined for small DW displacements. The computed H-CW(G) is close to a hyperbolic decrease, but it approaches finite values if G-->O and it decreases in a much steeper way than alpha 1/G for high values of G, which agrees with the experimental observations. Experimentally, the dependence H-CW(G) was measured on close-packed arrays of cylindrical bubble domains in two thin films of magnetic garnets, where the local field gradient, G, was controlled within the range 10(9)-10(10) A/m(2) by changing distances between neighboring DW's. The DW coercive field, H-CW, extrapolated from the measured values for G-->O was close to 80 A/m for both samples, while H-CW(G approximate to 10(10) A/m(2)) was several times smaller. Fitting the calculated H-CW(G) dependence to the experimental data, we obtained values of the Wiener-Levy correlation lengths well comparable to the DW width parameters

Correlation between ultrasonic velocity and magnetic adaptive testing in flake graphite cast iron

A recently developed nondestructive method, called Magnetic Adaptive Testing was applied for investigation of flake graphite cast iron samples having various metallic matrices and graphite structures. MAT is typical by its low required magnetization of samples, because it is based on measurement of families of minor magnetic hysteresis loops. The flat samples were magnetized by an attached yoke and sensitive descriptors of their magnetic/structural state were obtained from evaluation of the measured data. Ultrasonic velocity measurements were performed and results of the non-destructive magnetic tests were compared with these data. A very good correlation was found between the magnetic descriptors and ultrasonic velocity.Web of Science667s17717

Nondestructive detection of local material thinning in ferromagnetic materials by Magnetic Adaptive Testing

A recently developed nondestructive method, called Magnetic Adaptive Testing, which is based on systematic measurement and evaluation of minor magnetic hysteresis loops was applied for detection of local material thinning in a thick, L-shaped ferromagnetic carbon steel, which simulates T-tubes with reinforcing plates. Artificially made slots were reliably detected with a good signal/noise ratio from the other side of the specimen, even through a covering ferromagnetic plate