Single and biphase magnetic microwires: Microwave behavior and temperature dependence
Author
El Kammouni, RhimouAdvisor
Vázquez Villalabeitia, ManuelEntity
UAM. Departamento de Física de la Materia Condensada; CSIC. Instituto de Ciencia de Materiales de Madrid (ICMM)Date
2015-04-08Subjects
Materiales magnéticos - Tesis doctorales; FísicaNote
Tesis doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física de la Materia Condensada. Fecha de lectura: 08-04-2015Esta obra está bajo una licencia de Creative Commons Reconocimiento-NoComercial-SinObraDerivada 4.0 Internacional.
Abstract
This work is has been devoted to the study of single and biphase magnetic microwires, from fabrication to their magnetic characterization at low but particularly at microwave frequencies.
As single phase microwires we consider glass-coated amorphous microwires with very soft magnetic behavior (both, positive magnetostriction Fe-based and vanishing magnetostriction CoFe-based samples). In biphase microwires, we grow an external layer of polycrystalline character with either soft (FeNi) or medium-hard (CoNi) magnetic character so that, we consider soft/soft and soft/hard magnetic biphase systems.
Microwires have been fabricated by combined glass-coating rapid-solidification technique (the amorphous core) and sputtering/electrodeposition technique (the external shell). After their fabrication, the microwires were characterized from a geometry, composition and structure points of view. Quite a number of different samples were produced as a function of core and external shell magnetic phases as well as for different thickness of intermediate insulating Pyrex thickness and external layer. In addition, selected samples were selected for studies on the influence of thermal annealing at temperatures up to reaching first steps of crystallization at 600ºC.
A first magnetic characterization was performed at low-frequency magnetic fields in a Vibrating Sample Magnetometer, VSM, to determine the magnetization processes and main magnetic magnitudes as coercivity and remanence of each magnetic phase. This has been performed for all the series of samples mentioned above.
Studies have been extended to different measuring temperature particularly in the range up to 900ºC; where after partial crystallization significant deterioration of properties is confirmed. The study about the coercive field of individual and bimagnetic phase systems is analyzed in view of the relevance for technological applications of bimagnetic microwires. Also, the magnetic phase transitions (ferro to paramagnetic) of some individual phases are first identified.
The main part of the study has been devoted to the magnetic properties at high frequency, in the microwave regime. The ferromagnetic resonance (FMR) spectra have been determined for single and biphase samples with different geometry and compositional characteristics as indicated above. FMR experiments in wires can be
basically divided into two categories. The former is done by means of network analyzer up to around 15 GHz and as a function of applied magnetic field. The latter has been performed in a coaxial or microstrip microwave circuit making use of classical FMR spectrometers and waveguide microwave techniques.
A first study was focused on the influence of glass Pyrex on single and biphase microwires. For single-phase microwires, the increase of the Pyrex thickness results in a continuous strengthening of the circular magnetoelastic anisotropy of the CoFe-based core as deduced from FMR and confirmed by low-frequency measurements. For biphase microwires three absorption peaks are observed: two of them can be ascribed to each magnetic phase since FMR frequencies obey the Kittel condition for a thin film. A third absorption peak is observed at lower frequencies that does not follow such an equation and can be ascribed to a pure geometrical effect of these biphase microwires.
Then, the effect of annealing treatment at temperatures up to 700ºC on high frequency behavior was investigated. FMR behavior was analyzed by fitting to Kittel equation for annealed samples. The observed different absorption peaks are correlated with the presence of the soft core and the peaks presented at low frequency do not obey such Kittel condition. In this regards, it has been confirmed that the peaks observed at lower frequency corresponded to a capacitance effect.
Additional studies have been presented investigating the microwave absorption phenomena of single and biphase magnetic microwires with soft magnetic behavior using two alternative techniques: (i) absorption measurements in the temperature range of -271_25ºC using a spectrometer operating at X-band frequency, at 9.5GHz, and (ii) room temperature, RT, ferromagnetic resonance measurements in a network analyzer up to 15 GHz. For single phase CoFeSiB microwire, a single absorption is observed, whose DC field dependence of resonance frequency at RT fits to a Kittel law behavior for in-plane magnetized thin film. The temperature dependence behavior shows a monotonic increase in the resonance field, Hr, with temperature. For biphase,CoFeSiB/FeNi, microwires, the absorption phenomena at RT also follow the Kittel condition. The observed opposite evolution with temperature of resonance field, Hr, in two different thick FeNi samples is interpreted considering the opposite sign of magnetostriction of the respective FeNi layers.
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