Papers by Keyword: Magnetic Anisotropy

Abstract: New types of stress sensitive and magnetic field tunable microwave composite materials are discussed where embedded short ferromagnetic microwire inclusions are used as controllable radiative elements. The dc external magnetic field is applied to the whole composite structure. And, the local stress is transferred to the individual microwires through the accommodating composite matrix. The spatial and angular distributions of microwires can be random, partly ordered, or completely ordered. For a wide frequency range, the free-space microwave response of a wire-filled composite can be characterized by a complex effective permittivity with resonance frequency dispersion. The latter depends on the conductive and magnetic properties of the microwire inclusions that contribute to the ac microwire magnetoimpedance (MI). In the vicinity of the so-called antenna resonance frequency, which is defined by the length of microwires and matrix dielectric constant, any variations in the MI of the microwires will result in large changes of the effective permittivity, and hence the reflection and transmission coefficients for an incident microwave. The field or stress dependence of the effective permittivity arises from the corresponding field or stress sensitivity of the MI in the ferromagnetic microwires with induced circumferential or helical magnetic anisotropy, respectively. The strong field tunable effect in the proposed composite materials can be utilized to introduce reconfigurable microwave properties in coatings, absorbers, and randomizers, and also in new media such as microwave metamaterials and bandgap wire structures. A maximum field tunability of 30 dB was achieved for free-space transmission measurements when the external magnetic field changed from zero to ~40 Oe. The stress sensitivity of reflection and transmission coefficients opens up new possibilities for the distant non-destructive testing and evaluation of composite materials both in the laboratory environment and large scale applications. The stress tunability of transmission coefficient may reach up to 5-8 dB within the elastic limit. The reflection coefficient usually demonstrates less tunability in both cases (field and stress dependent) and may require a multilayer structure to achieve better results, but it is always strong enough for the stress sensing applications.

Abstract: Effects of shape and distribution of the hard phases (Fe, Cr)7C3 and Cr7C3 on wear resistance of Fe5 deposited metal obtained by plasma arc welding with electromagnetic stirring were investigated. The deposited layers were subsequently characterized by SEM observation, wear tests and hardness measurements. The hardness of the deposited layers was increased and then decreased with increasing the applied current. With the current of 3 A and the electromagnetic frequency of 10 Hz, the hardness of deposited metal reached maximum value of about HRC 68, which was increased about 19% compared with that of the deposited metal without electromagnetic stirring. The wear weight loss of the deposited metal with 3 A and 10 Hz is greatly decreased. It is confirmed that the shape and the distribution of hard phase significantly affect wear resistance of the deposited metal. The slag M7C3 is transformed into hexagon during electromagnetic stirring with 3 A and 10 Hz. And the regular distribution of hexagon M7C3 in the deposited metal resulted in the excellent wear resistance.

Abstract: The effect of capping with Cu, Au and Pt of an array of Co nanoparticles is revised. The magnetic surface anisotropy KS was found to be the dominant contribution to the effective anisotropy Keff of the particles. Recent X-ray Magnetic Circular Dichroism (XMCD) measurements show that there is hybridization between the 3d Co electrons and the d and 4p electrons of the capping metal. By comparison to the mechanisms which give rise to the surface anisotropy in thin films, it is argued that this hybridization governs the modification of KS, and hence, of Keff.

Abstract: Ferromagnetic L10 ordered alloys are extensively studied nowadays as good candidates for high density magnetic storage media due to their high magnetic anisotropy, related to their chemical order anisotropy. Epitaxial thin bilayers NiPt/FePt/MgO(001) have been grown at 700 K and annealed at 800 K and 900 K. At 800 K, the L10 long-range order increases without measurable interdiffusion. At 900 K, the interdiffusion takes place without destroying the L10 long-range order. This surprising observation can be explained by different diffusion mechanisms that are energetically compared using molecular dynamics simulations in CoPt in the second moment tight binding approximation. In addition, the frequencies of the normal modes of vibration have been measured in FePd, CoPt and FePt single crystals using inelastic neutron scattering. The measurements were performed in the L10 ordered structure at 300 K. From a Born-von Karman fit, we have calculated the phonon densities of states. The migration energies in the 3 systems have been estimated using the model developed by Schober et al. (1981). The phonon densities of states have also been used to calculate several thermodynamic quantities as the vibration entropy and the Debye temperature.

Abstract: The magnetic anisotropy of Co/Ti-doped Ba ferrite particles for application in recording media is studied. A method which deduces the anisotropy from the measurement of the reduced perpendicular magnetization and allows to find also the distribution of anisotropy fields is employed. The effect of the doping ions content, of the alignment degree of the particles and of the variation of the temperature on the anisotropy is analysed and discussed, taking account of the contemporary presence of magnetocrystalline and shape anisotropy, with different easy magnetization axes, in the Ba ferrite particles.

Abstract: We have measured the magnetization as a function of temperature and magnetic field in layered perovskite manganites of La2-2xSr1+2xMn2O7 single crystals (x=0.313, 0.315, 0.320, 0.350) in order to know their magnetic structures. All the present manganites exhibit magnetic transitions from ferromagnetic to paramagnetic at 76K, 107K, 120K and 125K for x=0.313, 0.315, 0.320 and 0.350, respectively. For x=0.350 and 0.320, the magnetic structure is a planar ferromagnetism whose easy axis is in the ab-plane at all temperatures below the Curie temperature. On the other hand, for x=0.315 and 0.313, the magnetic structure is an uniaxial ferromagnetism whose easy axis is along the c-axis below 85K and 66K, respectively, and a planar ferromagnetism above the temperature. From the results described above, we made the detailed magnetic phase diagram of layered perovskite manganite La2-2xSr1+2xMn2O7 (0.313≤x≤0.350).

Abstract: The magnetic parameters of steel depend on the mutual orientation of the applied magnetic field, the (applied) stress and the crystalline structure. The magnetic anisotropy can be modeled in terms of the magneto-elastic and magneto-crystalline energy distributions. By investigation of the magnetic anisotropic behaviour of steels with respect to stress, a rapid, nondestructive and possibly non-contact measurement of the residual stress can be devised that can find application in manufacturing, machining, forming and life-time assessment of steel and steel components. Reliability, robustness and versatility will constitute the main challenges to further develop these techniques for common industrial applications.