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    Effect of Ho substitution on structural, magnetic and magnetocaloric properties of Co(CrFe)O4
    (Springer, 2021) Kivrak, Burak; Gulkesen, Semiramis; Ayas, Ali Osman; Akyol, Mustafa; Ekicibil, Ahmet
    In this study, structural, magnetic and magnetocaloric properties of Ho-substituted Co(CrFe)O-4 spinel compounds produced by the sol-gel method were investigated. The Ho concentration in Co(CrFe)O-4 spinel structure was changed from 0 to 20% by 5% steps. It is observed that the Ho3+ ions create a new phase HoCrO3/HoFeO3 in the host spinel lattice. SEM images of samples indicate that the average particle size increases with Ho content. The paramagnetic to ferro-/ferrimagnetic phase transition temperature increases from 312 to 344 K with Ho content. While the fluctuation in saturation magnetization of samples was detected at low temperature, it almost becomes a linear change with the Ho content. The maximum magnetic entropy change under 7 T magnetic field was found as - 1.39, - 1.50, - 1.20, - 0.98 and - 0.91 J/kgK for Ho-free, 5, 10, 15 and 20 wt% Ho content in CoCrFeO4 spinel structure, respectively.
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    Effect of Ta insertion between Pt and CoFeB on interfacial magnetic anisotropy in Pt/CoFeB/MgO multilayer thin-film stack
    (Springer, 2020) Akyol, Mustafa; Kivrak, Burak; Tumen, Kutluhan Utku; Ekicibil, Ahmet
    The effect of a thin Ta layer inserted between Pt and CoFeB layers on perpendicular magnetic anisotropy (PMA) with MgO barrier layer has been studied. The crystallinity was studied by performing high-resolution x-ray diffraction (HR-XRD) technique. While the crystal peak of Pt is observed in all sample stack that is oriented as (111) face-centered cubic crystal structure, a very weak and broad alpha-Ta (110) peak is observed when Ta layer thickness is above 0.8 nm. Magneto-optical Kerr effect (MOKE) measurements show that the PMA could be enhanced by inserting Ta layer between Pt and CoFeB layer. The magnetically dead layer thickness (t(dead)) and the interfacial anisotropy energy density (K-i) of the various Ta-inserted layer thickness were found as 0.135 nm; 0.75 erg/cm(2), 0.141 nm; 1.02 erg/cm(2), 0.187 nm; 1.15 erg/cm(2), for t(Ta) = 0.0, 0.5, and 1.0 nm, respectively. Both t(dead) and K-i increase with Ta-inserted layer thickness in Pt/Ta(t)/CoFeB/MgO multilayer film stack. The sources of interfacial magnetic anisotropy can be hybridization, crystallinity properties, and/or B/Ta diffusion effect at the interfaces in Pt/Ta(t)/CoFeB/MgO multilayer film stack.
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    Investigation of structural and magnetic characteristic of pure and M-doped (M: Fe and Cu) MoS2 thin films
    (Springer, 2022) Kivrak, Burak; Akyol, Mustafa; Ekicibil, Ahmet
    In this study, structural and magnetic properties of pure and M-doped MoS2 (M: Fe and Cu) thin film were investigated. Two-step film growing process were used to create a thin film which are (i) spin coating and (ii) thermal decomposition. Raman spectroscopy results indicate that two characteristic vibration modes of E-2(g)1 (in-plane) and A(18) (out-of-plane) 2H MoS2 were observed in all samples. The surface morphology of samples studied by scanning electron microscopy (SEM) images shows that MoS2 crystals formed a back-to-back triangle shape. However, the triangle-shaped grains transform to nanorods when M is doped into the MoS2. Pure and Fe-doped samples show magnetic transition temperatures at similar to 247 and 95.5 K, respectively. In constrast, the Cu-doped sample shows strong diamagnetic characteristics in measurements. In addition, the magnetization values increase with the introduction of the Fe atoms into the main structure. According to the magnetic hysteresis curves of samples, pure and Fe-doped samples show ferromagnetic characteristics, while the Cu-doped sample shows weak magnetic characteristics in both 5 and 300 K.
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    Investigation of structural, magnetic and microwave absorption properties of NixCo1-xFe2O4/Ni:ZnO (x:0.0, 0.5, and 1.0) embedded epoxy composites
    (Springer Heidelberg, 2022) Kivrak, Burak; Tumen, Kutluhan Utku; Karaaslan, Muharrem; Akyol, Mustafa
    In this study, NixCo1-xFe2O4/Ni:ZnO (x:0.0, 0.5, and 1.0) epoxy-based composite structures were manufactured by sol-gel method. The structural, morphological and magnetic characteristics were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM). Electromagnetic wave (EMW) absorption measurements were carried out using a vector network analyzer (VNA) between 8 and 12 GHz (X band). Ni substitution with Co decreases the crystallite size, average particle size, magnetic saturation (M-s) and the coercive field (H-c) values of cobalt-ferrite spinel structure. EMW characterization results showed that minimum reflection loss (RL) - 30.91 dB (99.9% absorption) at 10.76 GHz, and the maximum bandwidth of 1.81 GHz were observed for the RAM1 composite. In addition, the substitution of Ni2+ into the cobalt-ferrite structure resulted in the shifting absorption peaks to the higher frequency region (towards K-u band).
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    PANI-coated YIG/CFO hybrid composites as advanced electromagnetic wave absorber through X-band
    (Elsevier Science Sa, 2023) Kivrak, Burak; Sergun, Atalay; Karaaslan, Muharrem; Akyol, Mustafa
    Polyaniline (PANI)-coated Y3Fe5O12(YIG)/CoFe2O4(CFO) hybrid composites were synthesized via sol-gel and oxidative polymerization methods to achieve advanced microwave absorption properties. The structural, morphological, magnetic, and microwave absorption characteristics of the resulting samples were analyzed by performing X-ray diffraction (XRD), scanning electron microscopy (SEM), vibration sample magnet-ometer (VSM), and vector network analyzer (VNA). XRD results revealed that CFO and YIG have cubic crystal structures with Fd3m and Ia3d space symmetry, respectively, with an average crystallite size of 35.3 nm and 42.9 nm. The Ferro/ferrimagnetic signals were detected from the ferrite samples' room temperature magnetic hysteresis loops. The incorporation of PANI and YIG into the composites resulted in a decrease in their magnetic saturation (Ms) and coercive field (Hc) values. VNA measurements demonstrated that nearly all samples achieved 90% absorption (& LE;-10 dB) of incident electromagnetic waves throughout the X-band frequency range. The minimum reflection loss (RLmin) value reaches - 47.34 dB at 8.14 GHz, with an effective bandwidth of 4.00 GHz, covering the entire X-band and beyond. These impressive absorption characteristics suggest that the manufactured composites possess potential as viable candidates for defense applications conducted at X-band frequency.& COPY; 2023 Elsevier B.V. All rights reserved.
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    Phase (1T/2H) dependent electromagnetic wave absorbing performance of flower-like MoS2 nanosheets
    (Elsevier, 2023) Aka, Cemal; Kivrak, Burak; Teksen, Fikret Alpay; Akyildiz, Hasan; Akgol, Oguzhan; Karaaslan, Muharrem; Akyol, Mustafa
    A study was carried out to determine the phase-dependent electromagnetic wave (EMW) absorption performance of MoS2 particles via systematically diversified hydrothermal synthesis conditions such as time, temperature, precursor molarity, and oxalic acid concentration. The formation of mixed-phase structure was verified depending on the presence of the characteristic diffraction peaks belong to 1T (trigonal, P3?m1 space group) and 2H (hexagonal, P63/mmc space group) phases in the X-ray diffraction (XRD) patterns. Phase-pure 2H-MoS2 was obtained at 200 degrees C after 12 h of reaction and using 5 mmol oxalic acid. Microscopic examinations indicate that MoS2 particles formed by assembling several nanosheets to create a flower-like morphology. The size, shape, and the distance between the nanosheets were observed to change with increasing temperature, time, and acid concentration. The band gap was found to be modified depending on the phase distribution and decreased from 1.54 eV (2H) to a minimum of 0.64 eV (1T/2H) with the formation of the 1T (metallic) phase. Finally, the minimum reflection loss was measured as -67.73 dB (99.9999 % of absorption at 9.05 GHz) with an effective absorption bandwidth (EAB) of 3.52 GHz for the sample composed of 1T/2H mixed-phased structure.
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    Polyaniline-Functionalized Nanosized Cobalt Ferrite-Decorated MoS2 Composites for Broadband Electromagnetic Wave Absorption
    (Amer Chemical Soc, 2024) Kivrak, Burak; Akyildiz, Hasan; Akgol, Oguzhan; Karaaslan, Muharrem; Akyol, Mustafa
    This study presents the synthesis of polyaniline (PANI)-coated cobalt ferrite-decorated molybdenum disulfide (MoS2@CoFe2O4); a composite architecture engineered to combine unique morphological, magnetic, and dielectric properties for the electromagnetic wave/radar absorption applications. MoS2 and CoFe2O4 were produced separately using a one-step hydrothermal process. Then, the CoFe2O4 nanoparticles were integrated into the flower-like MoS2 nanosheets using the sonication method. Finally, PANI was synthesized through in situ polymerization of aniline on the surface of MoS2@CoFe2O4 to create the final design as well as to boost the dielectric performance. Structural analysis confirmed the existence of the 2H phase and a minor trace of the 1T phase in MoS2, while CoFe2O4 displayed a cubic structure without any detectable impurities. Scanning electron microscopy verified the successful distribution of CoFe2O4 particles within the MoS2 nanosheets, aligning with the design's intended configuration. The electromagnetic wave (EMW) characteristics of the composites were analyzed by using a vector network analyzer. The MoS2@CoFe2O4 demonstrated a broad effective absorption bandwidth, spanning from the X-band to the Ku-band with a minimum matching thickness of just 2 mm. The reflection loss minimum value (RLmin) reached -22.82 dB at 8.96 GHz, corresponding to a 99.47% absorption of the incident EMWs. The addition of PANI at 50% by weight further increased the RLmin value to approximately 99.999% absorption (-50 dB), indicating enhanced impedance matching and absorption efficiency.
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    Strong magnetic coupling in MnO? nanoparticles by doping Boron
    (Elsevier, 2021) Demiryurek, Nazan; Kivrak, Burak; Akyol, Mustafa; Ekicibil, Ahmet
    In this study, the effects of boron (B) doping into MnO delta were examined on structural and magnetic properties. The B-doped MnO delta MNPs were synthesized by polyol method at various B concentration changed from 0 to 15% by 5% steps. Majority MnO and a very weak Mn3O4 crystal diffraction were detected from x-ray diffraction experiments in undoped sample. In contrast to undoped sample, MnO peaks are disappeared and Mn3O4 becomes majority phase when B is doped into the main matrix, as well as observing new crystal orientation called B2Mn3O6. The crystallite size of Mn3O4 phase in undoped sample was calculated to be 62.4 nm. This value nonlinearly decreased to 33.3 nm by amount of B. From scanning electron microscopy images, the particle sizes of the synthesized nanoparticles were determined to be 5.01, 6.77, 6.23 and 7.51 nm for x = 0.0, 5.0, 10.0 and 15.0% in B-x:MnO delta, respectively. The magnetic hysteresis curves indicated that all samples exhibited ferromagnetic (paramagnetic) behavior at 5 K (room temperature). Interestingly, we observed a giant increment in the saturation magnetization (Ms) with boron content. The blocking temperature values obtained from M-T graphs were found to be 24.39, 42.03, 42.14 and 41.91 K for x = 0.0, 5.0, 10.0 and 15.0% in B-x:MnO delta, respectively. (c) 2020 Elsevier B.V. All rights reserved.