Effect of fringing fields in storage rings

Electron beams in storage rings of third-generation synchrotron radiation sources feature a low emittance and extended lifetime. The provision of such characteristics requires a detailed study of higher order effects related to magnetic fields and cooperative effects associated with beam density. Fringing fields, being an unavoidable attribute of magnets of any type, may significantly affect the beam dynamics, since they appear in equations of particle motion of first and higher orders. A simple technique for evaluating the effect of fringing fields on the beam dynamics is suggested. Numerical results obtained with this technique for the quadrupoles of the CANDLE storage ring [1] are reported.     

Possibility of observing chiral-symmetry effects in ferromagnetic nanoparticles

The statistics of left-and right-handed vortical magnetization states in a system of submicron-sized ferromagnetic disks is studied experimentally. Lattices of elliptic cobalt particles with a planar size of 750 × 450 nm and a thickness of 25 nm were formed using high-resolution electronic lithography. To visualize the magnetization distributions in the disks, magnetic force microscopy was used. The experimental results indicate the conservation of magnetic chiral symmetry in cobalt ferromagnetic particles of the above size and shape.     

Influence of the surface anisotropy on the ground state of thin ferromagnetic films

A study is made of the ground state of a ferromagnetic film with a surface anisotropy which varies within wide limits from the easy-plane to the easy-axis type (– < 4) and with a bulk anisotropy of the easy-axis type ( 4). A rigorous analytic solution of the problem is given. The formulas obtained make it possible to calculate the distribution of the magnetization across the thickness of the film as a function of the parameters of the system. It is shown that there are two phase transitions from an inhomogeneous to a homogeneous state.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 71–76, July, 1980.The authors are grateful to B. A. Ivanov for his valuable comments and discussion of the results obtained.     

Interplay between domain-wall resistance and surface scattering in ferromagnetic thin films

A positive domain-wall resistance at low temperatures has been reported in the literature for cobalt films with a perpendicular magnetic easy axis, in contrast to the behavior observed in microstructures made from cobalt, iron, and permalloy films with an in-plane easy axis and to that seen in perpendicular iron–palladium thin-film compounds. This phenomenon is unexpected if only domain-wall or Fermi-surface properties are considered. It can, however, be understood if an existing domain-wall resistance is considered which is compensated by a magnetoresistance effect arising from surface scattering in a thin film with closure domains. A theory that properly accounts for this interplay between domain-wall resistance and surface scattering is presented and employed to analyze existing experimental data. PACS 75.60.Ch; 75.70.Ak; 75.70.Pa     

Effect of surface orientation and thickness on the magnetization of anisotropic FCC ferromagnetic films

The dependence on temperature of the layer magnetization of a Heisenberg ferromagnetic ultrathin film in presence of magnetocrystalline single-ion anisotropy was theoretically investigated in the framework of a Green's function approach using the random phase approximation (RPA). The effect of surface orientation and of film thickness N on the Curie temperature T_C was carefully investigated in the case of face centered cubic (FCC) films: the steepest increase of T_C(N) was found in the case of the FCC(1 1 1) orientation and the smoothest in the FCC(1 1 0) one. Our results for T_C(N) were successfully fitted by a finite-size scaling relation [T_C(∞)−T_C(N)]/T_C(N)=(N/N₀)^−λ, giving a shift exponent λ≃1.5, irrespectively of the surface orientation. Finally, the temperature evolution of the magnetization profile was analyzed, as well as its limiting shape at T_C.     

Effect of dispersion of orthorhombic anisotropy fields on the ferromagnetic resonance linewidth in iron garnet films

The effect of the dispersion of anisotropy fields on the width of the ferromagnetic resonance line is calculated in films with orthorhombic anisotropy. The results of the calculation, based on a model of linear variation of the anisotropy fields with thickness and on the assumption of additivity of the different contributions to the resulting linewidth, are in good agreement with experimental results. Zh. Tekh. Fiz. 67, 131–134 (August 1997)     

On the theory of ferromagnetic thin films

As is known, the second approximation in the calculation of the partition function by the traces method of ferromagnetic thin films gives wrong results for the coordination number equal to eight. In order to obtain correct results even for this case, the third order approximation of the partition function is developed and thus the magnetic properties of body-centred cubic iron thin films are studied. The dependence of the Curie temperature on the thickness, for different values of the ratio between the anisotropy constant and the exchange energy between two neighbours, is discussed. A value can be chosen for this ratio such that the thin film becomes ferromagnetic only for a thickness greater than a definite value.

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Ignition of hydrocarbon films by a pulsed discharge propagating above a water surface

We present the results of experimental investigations into the ignition and combustion of hydrocarbon films deposited on a water surface by a pulsed discharge propagating above the liquid in motionless air under atmospheric pressure.     

Two-stage magnetization reversal of ferromagnetic films

The two-stage magnetization relaxation of thin ferromagnetic films during the magnetization reversal process is considered in the context of the generalized Kolmogorov three-phasecrystallization model. The experimental data agree qualitatively with the considered model.     

On the domain structure of thin ferromagnetic films

The energy of Bloch walls is calculated for very thin ferromagnetic films with respect to the demagnetizing energy of a Bloch wall. A model of the stability of the domain structure in thin films is proposed.Part of this paper was delivered at the conference of solid state physics in Sopoty (Poland) November 5–11th, 1956.     

Electric hyperfine interactions in ferromagnetic binary alloys above the curie temperature

In the frame of an unscreened point charge model we calculated all possible electric field gradients for first neighbor atom arrangements in disordered binary fcc alloys. With these values and a distribution of isomer shifts we fit experimental spectra of nearly disordered alloys.     

Model for domain wall avalanches in ferromagnetic thin films

The Barkhausen jumps or avalanches in magnetic domain-walls motion between successive pinned configurations, due the competition among magnetic external driving force and substrum quenched disorder, appear in bulk materials and thin films. We introduce a model based in rules for the domain wall evolution of ferromagnetic media with exchange or short-range interactions, that include disorder and driving force effects. We simulate in 2-dimensions with Monte Carlo dynamics, calculate numerically distributions of sizes and durations of the jumps and find power-law critical behavior. The avalanche-size exponent is in excellent agreement with experimental results for thin films and is close to predictions of the other models, such as like random-field and random-bond disorder, or functional renormalization group. The model allows us to review current issues in the study of avalanches motion of the magnetic domain walls in thin films with ferromagnetic interactions and opens a new approach to describe these materials with dipolar or long-range interactions.     

Transient magnetic fields for highly stripped ions traversing ferromagnetic solids

Transient magnetic fields experienced by nuclei of single-electron ions in ferromagnetic solids have been analyzed in terms of the polarization of 1s electrons. Values obtained are well explained with spin-exchange scattering. In measurements for Si and Ni ions in Fe and Gd host at high and low velocity, respectively, it is demonstrated that the field strength also depends on the ion beam used for excitation of the nucleus. This new feature has direct relevance in the understanding of previous data.     

Loss separation for dynamic hysteresis in ferromagnetic thin films

We develop a theory for dynamic hysteresis in ferromagnetic thin films, on the basis of the phenomenological principle of loss separation. We observe that, remarkably, the theory of loss separation, originally derived for bulk metallic materials, is applicable to disordered magnetic systems under fairly general conditions regardless of the particular damping mechanism. We confirm our theory both by numerical simulations of a driven random-field Ising model, and by reexamining several experimental data reported in the literature on dynamic hysteresis in thin films. All the experiments examined and the simulations find a natural interpretation in terms of loss separation. The power losses' dependence on the driving field rate predicted by our theory fits satisfactorily all the data in the entire frequency range, thus reconciling the apparent lack of universality observed in different materials.