Gábor Vértesy

Interaction effects in switching of a two dimensional array of small particles

The magnetostatic interaction between elements of a regular two-dimensional array of rectangular garnet pixels has been investigated. The up- and down switching fields of an individual pixels, have been measured magnetooptically. The switching characteristics of a given pixel depends on the state of the neighbors. In this work the effect of the magnetization state of the first 5 coordination shells on a central, test pixel was investigated experimentally and numerically. The interaction field, i.e. the demagnetizing/magnetizing effects, is calculated exactly and in the dipole approximation for 24 pixels surrounding the central pixel. A good agreement between experimental and numerical results has been obtained.

Improving the sensitivity of Fluxset magnetometer by processing of the sensor core

The processing of the sensor core material was investigated on the operation of the Fluxset magnetic field sensor. Static and dynamic magnetic properties were measured, and the signal of the pick-up coil was investigated as a function of processing. The sensitivity and signal/noise ratio of the sensor can be improved significantly by polishing of the ribbon and heat annealing.

Magnetic patterning perpendicular anisotropy FePd alloy films by masked ion irradiation

The nanopatterning of magnetic films by ion implantation is reported. Highly L10-ordered Fe47Pd53 epitaxial alloy films on a MgO(001) substrate were covered by a monolayer of silica spheres in a Langmuir film balance. Using this sphere layer as an implantation mask, the samples were irradiated by Ne+ or Fe+ ions with energies of 35 keV and 100 keV, respectively. After the silica mask was removed, the samples were characterized via conversion electron Mossbauer spectroscopy, longitudinal and polar magneto-optical Kerr effect, and atomic force and magnetic force microscopy. We find that the magnetic stripe domains observed in the nonirradiated sample were converted into a regular 2D magnetic pattern of hcp character upon 1 × 1015/cm2 35 keV neon or 1 × 1014/cm2 100 keV iron irradiation, with the direction of magnetization remaining out of plane in the nodes of the hcp lattice and relaxed into the film plane in the inter-node region, resulting in an overall in-plane magnetic softening of the film.

Switching mechanism of single domain particles in a two-dimensional array

The mechanism of switching of uniaxial, single domain, single crystalline epitaxial garnet particles on a 2D square array was investigated. The anisotropy field of the sample is 2 kOe, while the Gaussian switching field distribution is 280/spl plusmn/85 Oe. The dominant contribution to the incoherent switching is from inhomogeneous internal fields and canted moments at the corners and edges, where the switching starts. The interaction field with the surrounding particles, reduced nucleation barriers at crystalline defects, thermal activation, and dynamical effects in FMR also contribute to the observed switching field and its standard deviation.

Measurement of Flat Samples With Rough Surfaces by Magnetic Adaptive Testing

Magnetic nondestructive tests of structural degradation of ferromagnetic construction materials must be often carried out on flat samples whose magnetic circuit is artificially closed by an attached soft magnetic yoke. Quality of magnetic contact between the sample surface and faces of the yoke has a strong influence on the measured signal, and especially with unpolished surfaces, it is difficult to be re- produced. The presented experiment suggests a partial solution of the problem via application of nonmagnetic spacers inserted between the sample and the yoke. The spacers distort and decrease the signal, but they also decrease fluctuation of the magnetic contact quality and smooth scatter of the measured data. As the goal of the nondestructive test is not absolute measurement of magnetic parameters of the material, but relative comparison of some convenient magnetic features, which relate to degradation of microstructure of the sample, application of the spacers proves to be not only acceptable, but it is very helpful.

MAJOR LOOP RECONSTRUCTION FROM SWITCHING OF INDIVIDUAL PARTICLES

Major hysteresis loops of groups of isolated 60 μm square garnet particles of a regular two-dimensional array, have been measured magnetooptically. Individual loops for each particle were measured, and the statistics of the distribution of coercivities and interaction fields was determined. It is shown that from the measured coercivity distribution and calculated magnetostatic interaction fields the major hysteresis loop can be reconstructed. The switching sequence, and the major loop of an assembly of 5×5 particles were calculated numerically for two cases: first, when calculating the magnetostatic interaction, the 25 particles were assumed to be isolated; second, the major loop of the same 25 particles, embedded into a 9×9 square, was reconstructed taking into account the interactions among all 81 particles. The numerically simulated major hysteresis loops agree very well with the measured loops, demonstrating the reliability of numerical modeling.

Stability of domain patterns and domain wall coercivity

Abstract The stability of demagnetized stripe domain patterns of epitaxial magnetic garnets with respect to applied field pulses has been observed to be strongly dependent on the magnetic history of the sample. The highest stability with regard to irreversible changes of the wall positions has been measured after the demagnetization by a slowly decreasing ac field normal to the specimen which is commonly considered to yield the lowest total energy domain structure with the zero field stripe period. In such a case the stability field has been found just equal to the samples coercivity. The measurement is believed to make it possible to decide experimentally which of the alternative domain structures of a certain type is the lowest energy one.

Growth of Bi and Ga substituted YIG and LuIG layers by LPE method

Bi substituted Y- and Lu- iron garnets are very good materials for different kinds of magneto-optical applications. In this work we report the preparation of Bi,Ga: YIG and Bi,Ga: LuIG layers on Gd-Ga garnet (GGG) substrates grown by conventional, isotherm dipping LPE technique using PbO-Bi2O3-B2O3 system as a solvent. Electron microprobe analyses of the garnet films were performed by means of energy-dispersive spectrometer and the refractive index of the layers was determined by spectroscopic ellipsometry. The layer growth rate (v), saturation magnetisation (4 piM(S)) and the Bi content of the layers were determined as a function of the supercooling of the melt (AT) and of the substrate rotation rate (omega) at a given melt composition. From these data the Deltav(L)/DeltaT, Delta4 pi Ms-S/DeltaT, Delta4 piM(S)/Delta(omega)(1/2), Delta Bi/DeltaT and Delta Bi/Delta(omega)(1/2) growth coefficients were calculated. The physical parameters of the films (thickness, 4 piM(S), refractive index, Faraday rotation and the figure of merit) can be very sensitively changed by these growth coefficients. The Faraday rotation (Phi (p)) of our Y2.5Bi0.5Fe3.8Ga1.2O12 and Lu2Bi1Fe4.1Ga0.9O12 garnet layers is similar to 4.8x10(3) and similar to8.3x10(3)deg/cm at 633 nm wavelength, respectively.

Magnetization process in a classical system of Preisach-type particles

Abstract The magnetization process of a regular two-dimensional array of small individual, strongly uniaxial magnetic garnet particles, groups of particles and major loop properties of a “macroscopic” sample has been investigated experimentally in an optical magnetometer. These particles correspond to the assumptions of a simple Preisach model. The switching mode is coherent 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 field on individual particles, the major loop can be reconstructed, and it is shown to be in nearly exact correspondence 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 interaction field for an assembly of 25 particles was calculated numerically. The switching sequence and the magnetization curve for the 25 particles assembly was derived from the calculated interaction fields and found to be in a very good agreement with the measured values.

Temperature dependence of domain‐wall coercive field in magnetic garnet films

The coercive properties of magnetically uniaxial liquid‐phase epitaxy garnet films were investigated between 10 K and the Neel temperature (TN≤500 K). Two independent methods, the results of which are nearly identical (magnetical response of oscillating domain walls and the method of coercive loops measured in a vibrating sample magnetometer), were used. Besides the usual domain‐wall coercive field, Hdw, the critical coercive pressure, pdw, was also introduced as it describes in a direct way the interactions of the domain walls with the wall‐pinning traps. Both Hdw and pdw were found to increase exponentially with decreasing temperature. Three different types of wall‐pinning traps were identified in the sample and their strength, their rate of change with temperature, and their temperature range of activity were determined.