[en] The interplay between activation volumes and microstructure is investigated in nanocrystalline Fe_73.5Cu₁Nb₃Si_13.5B₉ (Finemet) alloys. Experiments are performed beyond the Curie point of the amorphous matrix, where relaxation effects are relevant. Measurements are analyzed within a theoretical framework where hysteresis and relaxation phenomena are jointly described. In highly crystallized samples magnetization processes are characterized by a unique length scale. In poorly crystallized samples the system behavior is controlled by a distribution of characteristic volumes related to structural disorder. [copyright] 2001 American Institute of Physics

[en] Hysteresis and relaxation properties are studied in nanocrystalline Finemet-type materials, prepared with different crystalline volume fractions, beyond the Curie point of the amorphous matrix. The dependence on temperature and field rate of the measured coercive fields is discussed. The connection between activation volumes and structural aspects is analyzed

[en] The influence of tensile and compressive stresses on the magnetic losses in non-oriented Fe-(3 wt%)Si laminations has been investigated. It is shown that, while moderately affected by tensile stresses, the energy losses strongly increase upon compression. This is assumed to derive from a corresponding evolution of the domain distribution among easy axes, leading to an increase of both coercive field and excess losses. A rationale for the correlated dependence of the DC and AC losses on stress can be found in the framework of the statistical theory of losses

[en] Hysteresis and relaxation properties are studied in nanocrystalline Finemet-type materials beyond the Curie point of the amorphous matrix. Measurements of hysteresis loops and return branches at a temperature near the maximum hardening are used to investigate the microstructure of the material. Data are discussed by a unified model of hysteresis and thermal activation

[en] An overview is given of the present understanding of hysteresis phenomena in magnetic materials. The problem is addressed from three approximate viewpoints: the connection between rate-independent hysteresis and micromagnetics; the modifications brought into this picture by thermal relaxation effects; the role of rate-dependent magnetization mechanisms, like eddy-current-damped domain wall motion

[en] Ferrites have always been a subject of great interest from point of view of magnetic application, since the fist compass to present date. In contrast, the scientific interest for iron based magnetic oxides decreased after Oersted discovery as they where replaced by coil as magnetizing sources. Neel discovery of ferrimagnetism boosted again interest and leads to strong developments during two decades before being of less interest. Recently, the evolution of power electronics toward higher frequency, the down sizing of ceramics microstructure to nanometer scale, the increasing price of rare-earth elements and the development of magnetocaloric materials put light again on ferrites. A review on three ferrite families is given herein: harder nanostructured Ba₂₊Fe₁₂O₁₉ magnet processed by spark plasma sintering, magnetocaloric effect associated to the spin transition reorientation of W-ferrite and low temperature spark plasma sintered Ni-Zn-Cu ferrites for high frequency power applications.

[en] Structural and magnetic characterization of isotropic Mn–Al–C bulk samples obtained by spark plasma sintering (SPS) is reported. This technique, to the best of our knowledge, has not been used for preparation of Mn–Al-based permanent magnets previously. Transformation from the parent ϵ-phase to the ferromagnetic τ-phase occurred on heating in the process of sintering. The phase constitution of the melt-spun precursors and consolidated samples was determined by x-ray diffraction. Magnetic hysteresis loops were recorded using a vibrating sample magnetometer. The compositional dependence of the coercivity, magnetization and density of the sintered materials is analysed. To combine good magnetic properties with proper densification, further optimization of the production parameters is necessary. (paper)

[en] Hard-soft composite (BaFe₁₂O₁₉:Mg₀.₁Ni₀.₃Zn₀.₆Fe₂O₄ (2:1) films, were deposited by ‘Pulsed Laser Deposition’ (PLD) technique on Si (100) substrate using different deposition time - 30, 60, 90 and 120 minutes. Influence of deposition time on structural and magnetic properties were studied via X-ray diffraction (XRD) and vibrating sample magnetometer (VSM). XRD confirms the presence of soft and hard phases in all the prepared thin films. Small amount of secondary phase - Fe₂O₃ is also detected in all the thin films except for the deposition time - 90 mins. With deposition time average grain diameter of both hard (BaFe₁₂O₁₉) and soft (Mg₀.₁Ni₀.₃Zn₀.₆Fe₂O₄) phase increases. Increase in the distance between the magnetic ions (Ni²⁺ and Fe³⁺) at tetrahedral (A) and octahedral [B] site leads to increase in the hopping length at A and B site except for the the deposition time of 60 minutes. Magnetic measurements shows that the coercivity and magnetization of the prepared thin films respectively ranges between 112.07 - 213.03 Oe and 1.4 x 10^-7 - 3.15 x 10^-7 Am². (paper)

[en] In this paper we study adiabatic transformations in a model of first-order phase transformations made of superposition of bistable units. A differential equation with hysteresis operators is derived and how to compute numerically the T(H) trajectories is discussed. The numerical method is applied to an idealized phase transformation displaying an instability. We discuss the differences of the temperature change between the controlled magnetic field case and the controlled phase transformation case

[en] Soft magnetic Mg_0.1Ni_0.3Zn_0.6Fe₂O₄ and hard magnetic BaFe₁₂O₁₉ bulk nanocrystalline ferrites were synthesized using the sol-gel auto-combustion method, and were used as targets to deposit soft-hard thin films by the pulsed laser deposition (PLD) method. Various soft-hard thin films with different preparation conditions were deposited on Si (100) substrate, which can be effectively utilized to get better magnetic properties. The prepared films were characterized by the X-ray diffraction (XRD), atomic force microscopy (AFM) and magnetic measurements. XRD confirms the presence of soft and hard phases in the thin films. Coercivity of the prepared films ranges from 1.67 to 2.66 kA/m. AFM images show clustering of grains at the film surface with a characteristic columnar growth.