Nguyen N. Phuoc

Ultrawideband microwave noise filter: Hybrid antiferromagnet/ferromagnet exchange-coupled multilayers

Hybrid multilayers of [FeNi–FeMn]N/[MnIr–FeCo]N deposited onto flexible substrates oriented for wideband noise filters were fabricated using rf sputtering deposition. By combining the two types of multilayers with various thicknesses, where FeNi–FeMn is designed for the low frequency range and MnIr–FeCo for the high frequency range, one can realize a hybrid thin film for wideband noise filters. Its broad range from 1.5 to 7 GHz (the absorption width where the reflection loss is less than −10 dB) makes this hybrid thin film promising for future applications.

Tuning magnetization dynamic properties of Fe–SiO2 multilayers by oblique deposition

The static and dynamic magnetic properties of Fe–SiO2 multilayers fabricated onto Si(100) substrates by a radio frequency sputter-deposition system are investigated as functions of deposition angle and Fe thickness. By changing the oblique deposition angle, one can effectively tune the ferromagnetic resonance frequency from 1.7 to 3.5 GHz. In addition, the frequency linewidth is significantly changed with the oblique deposition angle when the Fe layer is thick, but it is almost constant in the case of small Fe thickness.

Anomalous Temperature Dependence of Magnetic Anisotropy in Gradient-Composition Sputterred Thin Films

An abnormal behavior of increasing magnetic anisotropy with temperature in magnetic thin films fabricated by a gradient-composition sputtering technique is observed and consistently confirmed by two independent measurements of static magnetic hysteresis loops and dynamic magnetic permeability spectra. This peculiar behavior is suggested to be due to the physical origin of stress-induced magnetic anisotropy.

Exchange coupling driven omnidirectional rotatable anisotropy in ferrite doped CoFe thin film.

Isotropic magnetic materials with high resonant frequencies are useful for applications in microwave devices. Undoped CoFe thin films, as common soft magnetic materials with high saturation magnetization, show isotropic characteristics but no high frequency response. Here, we use ferrite doped CoFe thin film to realize a resonant frequency higher than 4.5 GHz at all orientations. The exchange coupling between ferrimagnet and ferromagnet is assumed to play a key role on the omnidirectional rotatable anisotropy.

Magnetization dynamics in permalloy films with stripe domains

Through a systematic investigation of the field-dependent dynamic magnetization of a series of NiFe films with and without stripe domains in conjunction with the static magnetization process, we demonstrate that the experimental rotatable anisotropy field is not a fixed value but strongly varied with the external in-plane magnetic field, being qualitatively associated with the emergence of stripe domains. Moreover, the frequency linewidth spectra of the films with stripe domains show an abnormal behavior with three distinct regimes which are strongly correlated with both the static magnetization process and the competition between external magnetic field and dynamic anisotropy field. The results are discussed in terms of the effect of inhomogeneous magnetization associated with the formation of stripe domains and the field-dependent dynamic anisotropy that cause the broadening of frequency linewidth.

Enhancement of exchange bias and ferromagnetic resonance frequency by using multilayer antidot arrays

A systematic investigation of the dependences of the exchange bias and the ferromagnetic resonance frequency on the pore size of the antidot arrays fabricated by depositing Permalloy-FeMn multilayer thin films onto self-organized porous anodic aluminum oxide membranes was carried out. The magnetic and microwave properties of the antidot arrays with different pore sizes ranging from 30 to 80 nm are characterized and compared with that of the continuous thin films. It was found that the exchange bias field and the ferromagnetic resonance frequency are increased with the increase of the pore size, which may tentatively be interpreted in the framework of the random field model. It was also found that by using the antidot arrays in the best condition (the FeNi thickness is 20 nm and the pore size is 80 nm), one can enhance the exchange bias field from 65 to 135 Oe and the ferromagnetic resonance frequency from 3.1 to 4.1 GHz accordingly.

Magnetization behaviors for FeRh single crystal thin films

The structural and magnetic phase transition in FeRh thin films are investigated. The (001) oriented single crystal FeRh thin films fabricated onto MgO (100) substrate possess lattice parameter a smaller than c, because of the compressive stress along the a axis from MgO substrate. With increasing annealing temperature, paramagnetic FeRh thin films transform into ferromagnetic and then antiferromagnetic-ferromagnetic (AF-FM) stages. The transition temperature of AF-FM increases with annealing temperature, while the thermal hysteresis width decreases. The M‐H loops of the film annealed at 700°C show an opening at high magnetic fields during the transition state. Note that the opening in M‐H loops disappears when AF phase has transformed into FM phase. The origin of the opening is not known but possibly due to the combination of hysteresis loops of FM and AF phases, where FM phase is soft magnetic and the AF phase FeRh shows a hysteresis behavior with coercivity due to the reduction of magnetic anisotropy a...

Tuning of the magnetization dynamics in as-sputtered FeCoSiN thin films by various sputtering gas pressures

In this work, we investigate the influence of various sputtering gas pressures on the high-frequency magnetization dynamics in as-sputtered FeCoSiN granular thin films. The permeability spectra are measured with the shorted microstrip transmission-line perturbation method and analyzed with the Landau–Lifshitz–Gilbert equation. The dependence of the effective damping coefficient on the external fields is fitted with a power law. The measurement and fitting results show that both the effective and the intrinsic damping coefficients in the magnetization dynamics can be conveniently and effectively tuned by changing the sputtering gas pressure. The physical origin of the influences is suggested to be related to the stress in the films.

Influence of Si concentration on the magnetization dynamics in as-sputtered FeCoSiN thin films at high frequencies

In this work, the high-frequency magnetic permeability spectra of as-sputtered FeCoSiN films with various Si concentrations were investigated. The soft magnetic properties with an induced in-plane uniaxial anisotropy can only be obtained within some composition ranges because of the formation of different granular microstructures. The permeability spectra measured without any external fields (He) were well fitted based on the phenomenological Landau–Lifshitz–Gilbert equation. Results show that with the increase in Si concentration, the saturated magnetization 4πMs, the resonance frequency fr, the permeability μ, and the qualify factor Q values decrease, while the damping coefficient α and resonant frequency linewidth Δf increase. The increase in Gilbert damping coefficient α or G is ascribed to the increase in mosaicity or magnetic ripples with higher volume proportion of Si-rich matrix. The investigations on Δf-He relations indicate the extrinsic damping contribution from the two-magnon scattering in FeC...

Influence of Cu underlayer on the high-frequency magnetic characteristics of as-sputtered FeCoSiN granular thin films

In this work, we investigate the influence of Cu underlayers on the magnetic properties of as-sputtered FeCoSiN thin films. Different from their traditional improvement in the static magnetic properties of sputtered films, Cu underlayers are found to have a significant influence on the dynamic magnetic properties of as-sputtered FeCoSiN thin film at high frequencies. The permeability spectra and the analyses based on the Landau?Lifshitz?Gilbert equation show that the damping coefficient in the magnetization dynamics significantly increases with the underlayers thickness, and thus the resonant frequency linewidth increases. The extent of increase shows that using the Cu underlayer is an effective method for tuning the damping in the magnetization dynamics of granular thin films. The increase in the damping coefficient is suggested to be related to the release of the films stress by the underlayer.