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Öğe Comparing Different Approaches to Form Cobalt Oxide Layer on CoPt Nanoparticles(Adiyaman University, 2020) Kaya, Dogan; Adanur, Idris; Akyol, Mustafa; Karadag, Faruk; Ekicibil, AhmetWe have studied the effect of preparation methods, under argon gas and in the air environment, on the cobalt oxide formation of CoPt nanoparticles by the polyol process. The formation of cobalt oxide for both samples was investigated by the x-ray diffraction (XRD) method and cobalt oxide peaks are observed in the air prepared sample. Rietveld refinement analyses revealed that all samples exhibit a chemically distorted cubic crystal structure. The average particle size was determined <8 nm by scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDS) was revealed the chemical compositions with possible oxygen formation in the structure. The blocking temperature is reduced to 28 K in the air prepared sample due to cobalt oxide formation. The hysteresis gap disappeared above the blocking temperature and no saturation is observed up to ±5 T external field due to the system switching from ferromagnetic state to paramagnetic state. Similarly, the coercive field was decreased from 1021 Oe to zero with increasing the temperature from 5 K to 300 K. The formations of the cobalt oxide layer did not interact with CoPt nanoparticles, therefore, the maximum exchange bias was observed about 93 Oe at 5 K. © 2020, Adiyaman University. All rights reserved.Öğe Detailed investigation of structural and magnetic properties of multiphase binary Pd-Co alloys prepared by modified polyol process(Elsevier Science Sa, 2021) Kaya, Dogan; Adanur, Idris; Akyol, Mustafa; Karadag, Faruk; Ekicibil, AhmetPd-Co based magnetic alloy nanoparticles were prepared by the modified polyol process and stabilized by polyvinylpyrrolidone (PVP) and 3-aminopropyl-trimethoxysilane (APES) capping agents and further reduction of metal salts with Sodium borohydride (NaBH4) at high temperature to form desired NPs. We have a detailed investigation of the effect of Pd concentration in the Pd-Co alloy NPs on structural and magnetic properties. XRD and Rietveld refinement analyses were confirmed that the multiphase structures of fcc-PdCo, fcc-Co, and hcp-Co phases coexist at low Pd loading samples. Over %50 of Pd loading resulted in a single fcc-PdCo phase with reduced lattice parameter to 4.0079 angstrom and d((111))-space to 2.31 angstrom. TEM and SEM images reveal well dispersed and uniformly distributed NPs with an average particle sizes of below 7 nm. The elemental compositions and the characteristic OH, CH, CO stretching peaks of capping agents were confirmed by EDS and FT-IR spectrums, respectively. M(T) and M(H) curves revealed that there are multi magnetic phase transitions in the Pd-Co structure as a function of Pd loading from superparamagnetic to ferromagnetic phase or back to superparamagnetic phase by reducing the temperature from 300 K to 5 K. We observed that the blocking temperature (T-B) could not be detected due to 5% Pd loading below 300 K, while it reduced up to 55 K at high Pd loading of 62%. The coercive field (H-c) was increased to similar to 1900 Oe for Pd0.62Co0.38 sample due to the smallest particle size as 5.26 nm. The highest amount of Co resulted in maximum saturation magnetization (M-s) up to 65.5 emu/g for 5% Pd concentration. We measured the M-r/M-s ratios were less than 0.5, which is due to the internal stress that results in the uniaxial magnetic anisotropy in the structure. The maximum K-eff and mu(f.u). values were found to be over 12.9 x 10(6) erg/cm(3) and 0.72 mu(B) at 5 K for 5% Pd concentration, respectively. (C) 2021 Elsevier B.V. All rights reserved.Öğe Effect of Ag-doping process into the yttrium iron garnet (Y3Fe5O12) thin films on the structural, magnetic and optical properties(Springer, 2024) Demiryurek, Nazan; Akyol, Mustafa; Karadag, Faruk; Ekicibil, AhmetIn this work, the effect of Ag doping process (directly and as a nanoparticle) into the Y3Fe5O12 (YIG) thin films on the structural, morphological, optical and magnetic properties was studied in detail. Ag-doped YIG thin films were grown on thermally oxidized Si substrates by following sol-gel and spin-coating methods. All films were crystallized without cracks by two-stages heat treatment process. The XRD patterns confirm the formation of YIG peaks, and metallic Ag peaks which settled into the structure without bonding with the YIG components. In both Ag doping processes, the coercive field (H-c) and saturation magnetization (M-s) values of the samples containing 3% Ag are significantly lower than the other samples. The M-s values of 5% Ag doped samples were found the highest in both series. The M-s values of the films between in-plane and out-of-plane measurement increased with the Ag concentration. The optical measurements indicate an absorption peak in the range of 0-4 eV in both sample series and the optical band gap of the films decreased with the Ag doping due to its metallic characteristic. The ferromagnetic resonance measurements indicate that the lowest FMR linewidth as 92 Oe is seen in the 1% Ag-doped YIG sample. The relatively cheap and easy production of the used method and additive material may enable the widespread the usage of Ag-doped YIG thin films in magneto-optical devices.Öğe Effect of B site partial Ru substitution on structural magnetic and magnetocaloric properties in La0.7Pb0.3Mn1-xRuxO3 (x=0.0, 0.1 and 0.2) perovskite system(Elsevier, 2020) Ayas, Ali Osman; Cetin, Selda Kılıç; Akyol, Mustafa; Akca, Gonul; Ekicibil, AhmetThe La0.7Pb0.3Mn1-xRuxO3 (x = 0.0, 0.1 and 0.2) perovskite manganite compounds have been fabricated by solid-state reaction method to investigate structural, magnetic and magnetocaloric properties. All samples are indexed in the Rhombohedral structure with R (3) over tildec space group by using the Rietveld refinement method. The Curie temperature decreases from 336 K to 313 K when Ru (x = 0.1) is added to the structure and it becomes 335 K for x = 0.2. Maximum magnetic entropy change and relative cooling power values were calculated as 3.17, 3.15, 3.06 J kg(-1) K-1 for 5 T magnetic field change and 214.40, 160.20, 128.38 J kg(-1) under 4 T magnetic field change for x = 0.0, 0.1 and 0.2 in La0.7Pb0.3Mn1-xRuxO3, respectively. From the H/M vs M-2 plots obtained from isothermal magnetization curves, it is found that all samples exhibit a second-order magnetic phase transition, which shows reversible magnetocaloric effect. Under the light of all observed results, La0.7Pb0.3Mn1-xRuxO3 (x = 0.0, 0.1 and 0.2) perovskite manganite compounds can be considered as candidate materials above room temperature range magnetic cooling systems. (C) 2019 Elsevier B.V. All rights reserved.Öğe Effect of boron content on structure and magnetic properties in CoFe2O4 spinel nanocrystals(Elsevier Science Sa, 2018) Akyol, Mustafa; Adanur, Idris; Ayas, Ali Osman; Karadag, Faruk; Ekicibil, AhmetWe study the effect of boron content on the structural and magnetic properties of CoFe2O4 spinel nanocrystallines synthesized by sol-gel method. The crystal structure and phase identification of samples are studied by using X-ray diffraction experiment and Rietveld analysis. Rietveld refinement results reveal that all samples have cubic symmetry with space group Fd3m. The cationic distributions are obtained from Rietveld refinement that boron ions are settled into both tetrahedral and octahedral sites in spinel lattice. The crystallite sizes of samples are found in a range of 47-67 nm that is in the limit of single domain in such structure. All samples show ferromagnetic nature and magnetic transition was not seen in the temperature range of 5-400 K. The magnetic domains are pinned with adding boron ions into the CoFe2O4 spinel structure at low temperatures. Thus, an increment in the propagation field (H-p) and temperature (T-p) by boron content in CoFe2O4 structure is observed. In addition, the saturation magnetization (M-s) normalized by crystal size increases with increasing boron concentration. The temperature dependence of magnetic properties of the samples taken by experimental data are confirmed with the Neel-Arhenius model by adding thermal dependence of magnetocrystalline anisotropy term. The results indicate that boron-doping into the spinel structure enhances ferromagnetic coupling and suppresses super-exchange interaction between tetrahedral (X) and octahedral (Y) sites. (C) 2018 Elsevier B.V. All rights reserved.Öğe Effect of Gd-doping in Ni/NiO core/shell magnetic nanoparticles (MNPs) on structural, magnetic, and hydrogen evolution reaction(Aip Publishing, 2022) Adanur, Idris; Karazehir, Tolga; Dogru Mert, Basak; Akyol, Mustafa; Ekicibil, AhmetIn this study, Gd-x-doped Ni/NiO MNPs (x: 0.0%, 2.5%, 5.0%, and 10.0%) with a protective polyvinylpyrrolidone (PVP) layer have been synthesized via a polyol reduction process. The x-ray diffraction patterns revealed that samples have a cubic structure with Fm3m space group and no change in the crystallite structure was observed with doping Gd3+ ions. The crystallite size (D-c) decreases from 2.70 to 1.27 nm when Gd is doped into Ni/NiO MNPs. Transmission electron microscopy analysis revealed that the Ni/NiO MNPs with Gd(5%) concentration are formed as spherical multicore-like shape core/shell MNPs with a protective PVP layer. The magnetic hysteresis measurements taken at 10 and 300 K show that the saturation magnetization (M-s) decreases with increasing Gd3+ ions in the structure. The highest effective magnetic moment (mu(eff)) was obtained as 10.34 mu(B) in the NG-2 sample. We ascribe that the high mu(eff) value in this sample is due to the increase in d-f exchange interaction between Ni(3d(7)) and Gd(4f(7)) and the contribution of the dipole moment of PVP molecules. The electrochemical measurements showed that the current density values were 0.294 and 0.319 mA/cm(2) at-1.3 V in the absence of Gd (NG-0) and Gd(5%) doped (NG-2) samples, respectively. beta c was 159 and 132 mV/dec for NG-0 and NG-2 samples, respectively. The diminishing of beta c and the charge resistance (Rct) proved that the Gd doped catalyst enhanced the hydrogen evolution activity and the Gd(5%) doped sample exhibited the highest catalyst performance.Öğe Effect of heavy metal layer thickness on spin-orbit torque and current-induced switching in Hf¦CoFeB¦MgO structures(Amer Inst Physics, 2016) Akyol, Mustafa; Jiang, Wanjun; Yu, Guoqiang; Fan, Yabin; Gunes, Mustafa; Ekicibil, Ahmet; Amiri, Pedram KhaliliWe study the heavy metal layer thickness dependence of the current-induced spin-orbit torque (SOT) in perpendicularly magnetized Hf broken vertical bar CoFeB broken vertical bar MgO multilayer structures. The damping-like (DL) current-induced SOT is determined by vector anomalous Hall effect measurements. A non-monotonic behavior in the DL-SOT is found as a function of the thickness of the heavy-metal layer. The sign of the DL-SOT changes with increasing the thickness of the Hf layer in the trilayer structure. As a result, in the current-driven magnetization switching, the preferred direction of switching for a given current direction changes when the Hf thickness is increased above similar to 7 nm. Although there might be a couple of reasons for this unexpected behavior in DL-SOT, such as the roughness in the interfaces and/or impurity based electric potential in the heavy metal, one can deduce a roughness dependence sign reversal in DL-SOT in our trilayer structure. Published by AIP Publishing.Öğe 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, AhmetIn 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.Öğe Effect of reducing agent and surfactant on the morphology, structural and magnetic properties of Ni magnetic nanoparticles(Taylor & Francis Ltd, 2018) Adanur, Idris; Akyol, Mustafa; Ekicibil, AhmetWe report the effect of reducing agent and surfactant on the morphology, structural and magnetic properties of Ni magnetic nanoparticles (MNPs) synthesised by the polyol process. The samples in this work were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and vibrating sample magnetometry (VSM). The XRD results indicate that samples synthesised by EG-NaOH and 1.2H-OAm/OAc have a cubic structure with space group Fm-m. The average crystallite sizes of these samples are found to be approximate to 20.7 and 16.2nm, respectively. The sample synthesised by a combination of EG-OAm/OAc grows in nanorod form, whereas that synthesised by EG-NaOH and 1.2H-OAm/OAc has a spherical shape. The temperature dependence of the magnetisation, (M-T) measurements, indicate that the EG-NaOH sample has a superparamagnetic transition, while the 1.2H-OAm/OAc sample shows ferromagnetic behaviour in the temperature range of 5-380K. The magnetic anisotropy constants of the EG-NaOH and 1.2H-OAm/OAc samples extracted from the M-T experiments, are 141.3x10(5) and 10.5x10(5)erg/cm(3), respectively. In addition, the magnetic hysteresis measurements taken at 5 and 300K show that the 1.2H-OAm/OAc sample is more stable than the sample synthesised by EG-NaOH. The overall results are important for the use of the Ni MNPs for low- and room-temperature applications in bio- and nano-technology.Öğe 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, AhmetThe 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.Öğe Effects of Ca substitution on magnetic and magnetocaloric properties in PrBa1-xCaxMn2O6 system(Academic Press Inc Elsevier Science, 2023) Kandemir, Arda; Akca, Gonul; Cetin, Selda Kılıç; Ayas, Ali Osman; Akyol, Mustafa; Ekicibil, AhmetIn this study, we have examined the effect of Ba substitution by Ca on the magnetic, and magnetocaloric properties of PrBa1-xCaxMn2O6 (x = 0, 0.2 and 0.3) double perovskite manganites produced using sol-gel method. The samples were indexed in orthorhombic structure (with space group of Pbnm). The temperature-dependent magnetization, M(T), measurements indicate that all samples have a magnetic phase transition from ferromagnetic (FM) to paramagnetic (PM) state with increasing temperature. Curie temperature (T-C) decreases with increasing Ca content in the main lattice. From the magnetic field dependence of magnetization measurements, M(H), maximum magnetic entropy change (-Delta S-M(max)) values were calculated as 4.07, 4.12, and 5.04 Jkg(-1)K(-1) under 5 T for PrBaCaMn2O6, PrBa0.8Ca0.2Mn2O6, and PrBa0.7Ca0.3Mn2O6 samples, respectively. Relative cooling power (RCP) values below 5 T were determined as 214.16, 227.14 and 279.62 Jkg(-1) based on the increasing Ca content. The -Delta S-M(max) and RCP values enhance with increasing Ca content, respectively. The nature of the phase transition, checked by Banerjee criterion and Franco's universal master curves, was found to be second order magnetic phase transition (SOMT).Öğe Electronic and magnetic properties of boron substituted CuFeO2 delafossite oxide(Taylor & Francis Ltd, 2024) Ezircan, Ali; Aslan, M. Selim; Miyazaki, Hidetoshi; Akyol, Mustafa; Ozkendir, O. Murat; Ekicibil, Ahmet; Ozturk, HakanSynchrotron x-ray diffraction (SR-XRD) and X-ray absorption fine structure spectroscopy (XAFS) were used to investigate the crystal and electronic properties of boron-substituted CuFeO2 material at room temperature. Without boron substitution, the polycrystalline structures of the trigonal (rhombohedral) Rm' over bar m' CuFeO2 (87.7%) and hexagonal 'P63/mmc' (12.3%), which were also present in each sample but in different proportions, were utilised to identify the base material. XRD patterns revealed that, beyond 10% boron substitution, the metal-oxygen bonds (Fe-O and Cu-O) weakened, resulting in the formation of new tetragonal 'I41/amd' CuFe2O4 crystals. Although the CuFeO2 structure was preserved, it is conceivable that the presence of other crystal structures could lead to the formation of new features. This state arose as a result of CuFe2O4 crystallization and the impact of boron activity on the surrounding oxygen structures. By measuring magnetisation at both swept temperatures (10-300 K) and applied magnetic fields (+/- 30 kOe), the magnetic properties of the samples were investigated. In the 10-300 K temperature range, the polycrystalline samples exhibit a ferromagnetic property without a magnetic phase transition. This suggests that replacing B with Fe in CuFe(1-x)BxO(2 )does not influence the primary magnetic property of CuFeO2. The samples' saturation magnetisation (Ms) values gradually fall as the B substitution content increases with Fe. This is because there's a chance that the non-transition metal B in CuFe1-xBxO(2) will boost antiferromagnetic superexchange Cu-O interactions while lowering the p-d exchange interaction.Öğe Evaluation of nanoparticle formation and magnetic properties by boron doping in Ni/NiO? nanoparticles(Springer, 2020) Adanur, Idris; Akyol, Mustafa; Tezcan, Fatih; Kardas, Gulfeza; Ekicibil, AhmetIn this work, Boron-doped B-x:Ni/NiO delta [x(%) = 0.0, 5.0, 10.0 and 15.0] core/shell magnetic nanoparticles were synthesized by the polyol reduction process. The XRD spectra of the samples indicate that the B addition does not cause any change in the cubic structure of Ni. The TEM photographs present that nanoparticle formation and accumulation orientation occurs in various shapes as spherical, octahedral-like and octopus-like accumulation. The average particle sizes of B-x:Ni/NiO(delta)MNPs forx(%) = 0.0, 5.0, 10.0, 15.0 were found as similar to 90, 12, 46, 5 nm, respectively, from the TEM images. It is observed from the temperature and magnetic field dependence magnetization measurements that magnetization of Ni/NiO delta core/shell MNPs increases with increasing Boron concentration from 5 to 15%. One can be deduced that this increment in the magnetization comes from the resulting of strengthening the Ni-B ferromagnetic interaction. The highest saturation magnetizations under 1 T magnetic field were found as similar to 37 and similar to 30 emu/g at 10 and 300 K, respectively.Öğe Influence of Gd Substitution on Structural, Magnetic and Magnetocaloric Properties in (La0.8Gd0.2)0.85Ag0.15MnO3 Perovskite Manganite System(2018) Ekicibil, Ahmet; Ayaş, Ali Osman; Akyol, MustafaIn this paper, the effect of Gd substitution with La on structural, magnetic and magnetocaloricproperties in (La0.8Gd0.2)0.85Ag0.15MnO3 manganite sample prepared by sol-gel method has been studied. Thecrystal properties have been investigated by x-ray diffraction technique that shows the sample is inrhombohedral structure with R3?c space group. In addition to this structure, small amount of reflections belongsto GdMn2O5 phase is detected. Scanning electron microscope images show that the sample is constituted fromsquare shaped grains. Energy dispersive x-ray spectroscopy analysis shows that the sample includes all expectedelements. The sample exhibits magnetic phase transition from ferromagnetic to paramagnetic phase at around149 K temperature. Applied field dependence of magnetization under isothermal process M(H) shows that thenature of the phase transition is second order and maximum magnetic entropy change (-?SM) value calculatedfrom M(H) curves is found as 1.73 J/kgK under 50 kOe external magnetic field change.Öğe Investigation of Magnetic Anisotropy in the Ni-Ge and Ni(NiO)-Ge Thin Films(Elsevier, 2021) Kaya, Dogan; Akyol, Mustafa; Wang, Yitao; Guo, Quanmin; Karadag, Faruk; Ekicibil, AhmetWe studied the structural and magnetic properties of Ni-Ge and Ni(NiO)-Ge films deposited by physical vapor deposition technique on the Si/SiO2 substrate. The x-ray diffraction (XRD) analysis show that as-prepared samples exhibit clear peaks of Ge, Ni, and Au with a cubic symmetry belongs to space group Fm (3) over barm. Due to the thermal annealing process at 350 degrees C, NiO layer formed on Ni layer. The NiO formation has reduced the interaction of Ni-Ge interlayer which is confirmed by XRD data. Investigation of temperature-dependent magnetic moment revealed the Neel and the critical temperatures as 93 and 294 K, respectively. The hysteresis loops showed that the preferred magnetization direction for samples at 300 K is the in-plane (parallel) which reaches saturation easier than out-of-plane (perpendicular) loops. The formation of NiGe interlayer resulted in a magnetically dead-region which led to decrease ferromagnetic domain and the coercive field. (C) 2020 Elsevier B.V. All rights reserved.Öğe 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, AhmetIn 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.Öğe Investigation of the effect of sintering temperature on structural, magnetic and magnetocaloric properties in PrCaMn2O6 double perovskite manganite system(Springer, 2022) Ayas, Ali Osman; Kandemir, Arda; Cetin, Selda Kılıç; Akca, Gonul; Akyol, Mustafa; Ekicibil, AhmetWe have studied the effect of sintering temperature change on structural, magnetic and magnetocaloric properties of PrCaMn2O6 compound. The samples were produced by using solid-state-reaction process and then sintered at 1000, 1100 and 1200 degrees C temperatures. The Rietveld refinement results indicate that the crystal symmetry of the produced samples is tetragonal with P4mm space group. All samples include impurity phase, and their percentages decrease with increasing sintering temperature. Scanning electron Microscope images show that grain sizes of samples increase with increasing T-S. Energy-dispersive X-ray spectra indicate that samples include all required starting elements. Temperature-dependent magnetization measurements show that samples have the Curie temperatures as 42.6, 43.1, and 42.8 K for samples sintered at 1000, 1100 and 1200 degrees C, respectively. Magnetic entropy change values are found as 1.19, 1.59 and 1.72 J kg(-1) K-1 at 5 T magnetic field change for samples sintered at 1000, 1100 and 1200 degrees C, respectively. Relative cooling power values are calculated as 14.28, 20.54 and 17.20 J kg(-1) for samples sintered at 1000, 1100 and 1200 degrees C, respectively. The universal curve and the Arrott plots show that magnetic phase transition type of all samples is second order.Öğe Magnetic and optical properties of ZnO/Ni/ZnO multilayer film on Si(100) and sapphire substrates(Elsevier Gmbh, 2022) Kaya, Dogan; Akyol, Mustafa; Tuzemen, Ebru Senadim; Ekicibil, AhmetThe optical absorption, band gap, and magnetic properties of ZnO/Ni(t)/ZnO multilayer film structures were investigated in various Ni layer thicknesses and substrates. We deposited 1, 4, and 5 nm of Ni film by thermal evaporation technique that sandwiched between ZnO films (similar to 15 nm) via radio frequency magnetron sputtering method on both Si(100) and sapphire substrates. Although x-ray diffraction (XRD) analysis confirmed the (002) crystalline plane of ZnO without any Ni crystal phases on Si(100) surface, the better crystalline directions of ZnO, hexagonal wurtzite structure, was observed on the sapphire substrate. In the XRD analysis, we observed the cubic structure of NiO film formation due to thermally oxidation of Ni ions with interactions ZnO layer. Atomic force microscopy images confirmed the effect of the Ni layer on the average island size of 23.9 +/- 2.9 nm and 25.6 +/- 6.2 of ZnO films on 4 and 5 nm Ni films, respectively. Energy dispersive x-ray spectroscopy data confirmed that there is no other atom or impurity in the sample structure. The optical transparency of the multilayer films was reduced with increasing Ni layer thickness and maximum transparency was obtained as 97% at 800 nm of wavelength for the film with 1 nm Ni. The direct optical band gap of ZnO/Ni(t)/ZnO films was found to be 3.25, 3.20, and 3.12 eV with the contribution of 1, 4, and 5 nm Ni film in the multilayer film stack. The maximum H-c is found to be 1000 Oe for Si substrates and this value is reduced to around 400 Oe due to the crystal formation of the NiO layer for sapphire substrate samples.Öğe Magnetic field dependence of magnetic coupling in CoCr2O4 nanoparticles(Elsevier, 2017) Akyol, Mustafa; Adanur, Idris; Ayas, Ali Osman; Ekicibil, AhmetCoCr2O4 spinel nanoparticles synthesized by sol-gel method have been worked extensively by performing structural and magnetic characterization techniques, as well as modeling the experimental results. The microstructure analysis shows that the crystallite size of CoCr2O4 nanoparticles, which are purely crystallized in cubic phase with space group Fd3m, is similar to 75 nm. The grain size distribution determined from scanning electron microscope images indicates that the particles are uniformly formed and distributed homogenously in the structure. A comprehensive magnetic study has been performed by measuring magnetic moment as a function of temperature and external magnetic field. The paramagnetic to ferromagnetic phase transition and non-collinear spiral magnetic transition have been observed in CoCr2O4 nanoparticles at 96 and 27 K, respectively. Interestingly, we observed field-condition shift in lock-in transition which is found as 16 and 8 K for FH and FC, respectively. The exchange bias effect is observed when the CoCr2O4 sample is cooled under magnetic field. The magnitude of exchange bias field decreases with increasing temperature from 5 to 50 K, and it is vanished above 50 K. In addition, we also worked on the magnetic entropy change around the paramagnetic to ferromagnetic phase transition. The magnetic entropy change is found as -0.87 J/kg K under 6 T magnetic field.Öğe Magnetic refrigeration: Current progress in magnetocaloric properties of perovskite manganite materials(Elsevier, 2023) Ayas, Ali Osman; Cetin, Selda Kılıç; Akca, Gonul; Akyol, Mustafa; Ekicibil, AhmetMagnetic cooling technology based on the magnetocaloric effect is a prominent research topic due to its great potential helps to solve the increasing energy need and environmental problems arising from existing technologies. Studies on magnetic cooling systems mainly include both engineering works and material research activities. This review principally covers the material research part of these studies and presents the current developments in them. Specifically, this review focuses on perovskite manganites due to their various advantages over other material families, such as large working temperature range, adjustable Curie temperature, easy production methods and low costs, chemical stability, negligible thermal and magnetic hysteresis properties. In accordance with these properties, important research points such as crystal structure, mismatch effect, magnetic properties, etc., related to the perovskite manganite materials are evaluated and interpreted. Additionally, progress on some prominent material families rather than perovskites is also briefly discussed.
