In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

https://www.andrew.cmu.edu/user/dl0p/laughlin/pdf/PMS.pdf

Amorphous and nanocrystalline materials for applications as soft magnets

Magnetocaloric effect: From materials research to refrigeration devices

In the past 20 years, there has been a surge in research on the magnetocaloric response of materials, due mainly to the possibility of applying this effect for magnetic refrigeration close to room temperature. This review is devoted to the main families of materials suitable for this application and to the procedures proposed to predict their response. Apart from the possible technological applications, we also discuss the use of magnetocaloric characterization to gain fundamental insight into the nature of the underlying phase transition.

The Magnetocaloric Effect and Magnetic Refrigeration Near Room Temperature: Materials and Models

ICTAC Kinetics Committee recommendations for collecting experimental thermal analysis data for kinetic computations

We determined the magnetic entropy change and refrigerant capacity of melt spun Ni503Mn355Sn144 ribbons around both the structural and the magnetic transitions for a field of 20kOe The maximum entropy changes at the structural and magnetic transitions were of 41 and −11Jkg−1K−1 Ribbons studied show a larger refrigerant capacity around the magnetic transition (46Jkg−1) than around the structural transition (26Jkg−1), suggesting that the temperature range at the magnetic transition is more adequate for a refrigerant cycle than that at the structural transition

Magnetocaloric effect in melt spun Ni50.3Mn35.5Sn14.4 ribbons

We demonstrate composite media with ferromagnetic wires that exhibit a frequency region at the microwave regime with scattering spectra strongly dependent on an external magnetic field or stress. These tunable composite materials have recently been proposed theoretically; however, no direct experimental verification has been reported. We used composite materials with predominantly oriented CoFeCrSiB glass-coated amorphous wires having large magnetoimpedance at GHz frequencies. The free space measurements of reflection and transmission coefficients were conducted in the frequency range 1-8 GHz in the presence of an external static magnetic field or stress applied to the whole sample. In general, the transmission spectra show greater changes in the range of 10dB for a relatively small magnetic field of few Oe or stress of 0.1 MPa. The obtained results are quantitatively consistent with the analytical expressions predicted by the effective medium arguments. The incident electromagnetic wave induces an electrical dipole moment in each wire, the aggregate of which forms the effective dipole response of the whole composite structure in the radiative near or far field region. The field and stress dependences of the effective response arise from a field or tensile stress sensitivity of the ac surface impedance of a ferromagnetic wire. In the vicinity of the antenna resonance the variations in the magneto-impedance of the wire inclusions result in large changes of the total effective response. A number of applications of proposed materials is discussed including the field tunable microwave surfaces and the self-sensing media for the remote non-destructive evaluation of structural materials.

http://arxiv.org/pdf/cond-mat/0602644.pdf

Ferromagnetic Wires Composite Media with Tunable Scattering Spectra at Microwaves

We report a method of imaging of the magnetization reversal process using analysis of real-time images of magnetic domain structures in cylindrically shaped microwires. This method uses wide-field polarizing optical microscopy and is based on the magneto-optical Kerr effect (MOKE). The aperture diaphragm in MOKE microscope was used to control the incident angles of the light rays that reached the non-planar surface of the microwire and also determined the MOKE geometries. The movement of the non-central position of the hole in this diaphragm leads to a change in the orientation of the plane of incidence of the light along the perpendicular or the parallel direction to the axial direction of the wire. The visualization of the surface magnetic domain structures is obtained using polar and longitudinal MOKE geometries. The hysteresis loops were obtained by plotting the averaged image contrast as a function of the external magnetic field. The separation of the all-magnetization components is performed using different MOKE geometries in a microscope. We demonstrate the use of vector magnetometry to analyze the orientation of the magnetization in a cylindrically shaped microwire under the influence of an external magnetic field.

Direct imaging of the magnetization reversal in microwires using all-MOKE microscopy.

We report the results of a systematic study on stress tunable absorption of glass-coated amorphous Co68.7Fe4Ni1B13Si11Mo2.3 microwires and their composites. The magnetic microwires possess good stress-impedance properties and yield a stress dependence of absorption at gigahertz frequencies. The stress compensates the reverse effect of magnetic field on absorption. There exist strong stress dependences of the effective permittivity and transmission parameters. Composite failure due to the wire damage results in a dramatic change of the sign and magnitude of effective permittivity. The double peak is identified in the stress dependence of field tunability, in contrast to the single peak for the magnetic field tunability. All these results indicate that the present composites are very promising for detecting the ambient stress levels and interrogating the structural integrity.

/pdf/stress-tunable-properties-of-ferromagnetic-microwires-and-sltgggo8em.pdf

Stress tunable properties of ferromagnetic microwires and their multifunctional composites

Magnetic and mechanical properties of Co-rich amorphous microwires of nominal compositions (Co1–xMnx)75Si10 B15 (0.08 < x < 0.11) and Co56.5Fe6.5Ni10B16Si11 with nearly zero magnetostriction constant have been studied. Drastic changes of the hysteresis loop with the decrease of x from rectangular to flat shape were observed at x < 0.1, while the Co56.5Fe6.5Ni10B16Si11 microwire showed rectangular hysteresis loops. Compositions with x ≥ 0.1 have small but positive magnetostriction constant while negative λs values have been observed at x ≤ 4 0.09, indicating rectangular character of the hysteresis loop in samples with nearly zero and even for small negative λs. External stress dependence of switching field, H*, showed an increase of H*; with s for samples with x = 0.11 and a decrease of H* with σ for x = 0.1. Besides, microwires of the same composition but with different dimensions showed different H*(σ) dependencies.

J. Gonzalez

Papers

Magnetocaloric effect in melt spun Ni50.3Mn35.5Sn14.4 ribbons

Ferromagnetic Wires Composite Media with Tunable Scattering Spectra at Microwaves

Direct imaging of the magnetization reversal in microwires using all-MOKE microscopy.

Stress tunable properties of ferromagnetic microwires and their multifunctional composites

Physical properties of nearly zero magnetostriction Co-rich glass-coated amorphous microwires