B. Altuncevahir

Istanbul Technical University

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G. Sadullahoğlu

Bülent Ecevit University

V. M. Chakka

The University of Texas at Arlington

A. R. Köymen

The University of Texas at Arlington

A. Okan Addemir

Istanbul Technical University

O. Addemir

Istanbul Technical University

Yufeng Tian

Shandong University

T. D. Black

The University of Texas at Arlington

Kai Sun

University of Michigan–Ann Arbor

Florian Bertram

Deutsches Elektronen-Synchrotron DESY

Jascha Bahlmann

Osnabrück University

Cooperative Institutions

The University of Texas at Arlington

Shandong University

Osnabrück University

Istanbul Technical University

State Key Laboratory of Crystal Materials

Chinese Academy of Sciences

University of Michigan–Ann Arbor

Deutsches Elektronen-Synchrotron DESY

University of California, Berkeley

Gebze Technical University

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The Effect of Milling Time and Sintering Temperature on Crystallization of BaFe₁₂O₁₉Phase and Magnetic Properties of Ba-Hexaferrite Magnet

Acta Physica Polonica A (2015)

G. Sadullahoğlu Burcu Ertuğ Hasan Gökçe B. Altuncevahir Mustafa Öztürk

Barium hexaferrite samples were prepared by mechanical alloying using the stoichiometric amounts of BaCO3 and Fe2O3 precursors followed by heat treatment applied in the temperature range 7001150 • C. It was found that the high energy ball mill with a milling rate enabled to obtain powders with the ner particles at the reduced milling time mechanical alloying of the initial powders linked to the formation of barium hexaferrite phase.The exothermic reaction peaks corresponding to the formation of BaFe12O19 phase shift from 928 • C to 793 • C for the increased milling time up to 6 h.This was resulted in improved magnetic properties that the Ms value of the as-blended sample sintered at 800 • C rised from 31.16 emu/g to 53.46 emu/g after milling for 6 h.The saturation magnetization and remanence values of the samples mechanically alloyed for 3 h and sintered at 1150 • C also increased to 63.57 emu/g and 31.26emu/g, respectively, more than for 800 • C and 900 • C. The increase in the annealing temperature favours the formation of BaFe12O19 phase in the samples.

10.12693/aphyspola.128.377

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Magnetic Properties and Antiferromagnetic Coupling in Inhomogeneous Zn _{1– x} Fe _x O Magnetic Semiconductor

Chinese Physics Letters (2009)

Deng Jiang-Xia Yan Shi-Shen MEI LIANG-MO J. P Liu B. Altuncevahir

Zn1−xFexO inhomogeneous oxide magnetic semiconductor films with high Fe concentration are prepared by sputtering, and fast annealing is carried out at different temperatures. It is found that magnetic properties are greatly modulated by controlling the composition inhomogeneity and subsequently fast annealing. Both ferromagnetic and paramagnetic components are found to coexist in the as-deposited Zn1−xFexO magnetic semiconductor. In particular, the antiferromagnetic coupling between the neighbouring local ferromagnetic regions is found in the as-deposited Zn0.23Fe0.77 O film, and the antiferromagnetic coupling strength increases with increasing temperature from 110 K to 300 K. We believe that this unusual antiferromagnetic coupling is mediated by thermally activated hopping carriers.

Magnetic semiconductor

10.1088/0256-307x/26/2/027502

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Citations (2)

Dielectric investigation of the sol-gel transition in the acrylamide/N,N? methylenebisacrylamide system

Journal of Applied Polymer Science (2000)

F. Salehli Huceste Çatalgil‐Giz Ahmet Giz O. Kamer B. Altuncevahir

The acrylamide/N,N′-methylenebisacrylamide gelation process was studied by time dependent measurement of real ϵI and imaginary ϵII parts of the complex dielectric permittivity. The measurements were carried out in the frequency interval 500 Hz to 13 MHz. Strong relaxation behavior of ϵI and ϵII was observed in all dielectric spectra during the transformation from the sol to the gel state. At all frequencies a maximum in the imaginary part of dielectric constant ϵII was observed. The gel point was found by extrapolating this maximum to zero frequency. The behavior of ϵI and ϵII related to the gelation mechanism is discussed. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 463–466, 2000

Gel point

10.1002/(sici)1097-4628(20000711)77:2<463::aid-app24>3.0.co;2-m

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Citations (6)

Cationic Ordering and Its Influence on the Magnetic Properties of Co-Rich Cobalt Ferrite Thin Films Prepared by Reactive Solid Phase Epitaxy on Nb-Doped SrTiO3(001)

Materials (2021)

Jannis Thien Jascha Bahlmann Andreas Alexander Kevin Ruwisch Jari Rodewald

Here, we present the (element-specific) magnetic properties and cation ordering for ultrathin Co-rich cobalt ferrite films. Two Co-rich CoxFe3-xO4 films with different stoichiometry (x=1.1 and x=1.4) have been formed by reactive solid phase epitaxy due to post-deposition annealing from epitaxial CoO/Fe3O4 bilayers deposited before on Nb-doped SrTiO3(001). The electronic structure, stoichiometry and homogeneity of the cation distribution of the resulting cobalt ferrite films were verified by angle-resolved hard X-ray photoelectron spectroscopy. From X-ray magnetic circular dichroism measurements, the occupancies of the different sublattices were determined using charge-transfer multiplet calculations. For both ferrite films, a partially inverse spinel structure is found with increased amount of Co3+ cations in the low-spin state on octahedral sites for the Co1.4Fe1.6O4 film. These findings concur with the results obtained by superconducting quantum interference device measurements. Further, the latter measurements revealed the presence of an additional soft magnetic phase probably due to cobalt ferrite islands emerging from the surface, as suggested by atomic force microscope measurements.

Stoichiometry

10.3390/ma15010046

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Positive and negative exchange bias in CoNi/Gd/CoNi trilayers and CoNi/Gd bilayers

Journal of Magnetism and Magnetic Materials (2003)

B. Altuncevahir A. R. Köymen

Exchange bias

Hysteresis

10.1016/s0304-8853(02)01493-2

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Citations (27)

Effect of magnetic field on ball milled hard magnetic particles

APS March Meeting Abstracts (2004)

B. Altuncevahir N. Poudyal V. M. Chakka K. H. Chen T. D. Black

Ball (mathematics)

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Effect of Al and Cu Additions on Microstructure and Magnetic Properties of NdTb-FeCo-B Magnets

Acta Physica Polonica A (2014)

G. Sadullahoğlu B. Altuncevahir O. Addemir

With the addition of Al and/or Cu into the NdFeB based magnet, the soft free iron phase was not observed in the cast alloys while it exists in the microstructure of the undoped NdFeB ingots.For the undoped sample, free iron disappeared after hot deformation process.Grain orientation is not good and there are deviations in the growth direction of the lamellar grains which are not perpendicular to the pressing direction.On the other hand, there is a striking change that the intergranular chemistry of Al added ingot separated into Nd-rich phase and a ternary FeNdAl composition most probably formed at the pressing temperature of 750 • C. The highest remanent magnetization of 7.47 kG obtained for this sample, it might be due to the hard magnetic properties of this ternary composition.For the AlCu added sample, the high Hc and Br values of 7.71 kOe and 7.33 kG, respectively, can be attributed to the homogeneous distribution of the intergranular composition and good orientation of the magnetic grains by hot deformation process.

10.12693/aphyspola.125.1172

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Citations (15)

Transformation of electrical transport from variable range hopping to hard gap resistance in Zn1−xFexO1−v magnetic semiconductor films

Journal of Applied Physics (2006)

Yufeng Tian Shishen Yan Y. P. Zhang Hongqiang Song Gang Ji

Transformation of the electrical transport from the Efros and Shklovskii [J. Phys. C 8, L49 (1975)] variable range hopping to the “hard gap” resistance was experimentally observed in a low temperature range as the Fe compositions in Zn1−xFexO1−v ferromagnetic semiconductor films increase. A universal form of the resistance versus temperature, i.e., ρ∝exp[TH∕T+(TES∕T)1∕2], was theoretically established to describe the experimental transport phenomena by taking into account the electron-electron Coulomb interaction, spin-spin exchange interaction, and hard gap energy. The spin polarization ratio, hard gap energy, and ratio of exchange interaction to Coulomb interaction were obtained by fitting the theoretical model to the experimental results. Moreover, the experimental magnetoresistance was also explained by the electrical transport model.

Variable-range hopping

Exchange interaction

Magnetic semiconductor

Electric potential energy

10.1063/1.2386925

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Citations (32)

Investigation of Nd-Al-Fe Added Nd-Fe-B Alloy Produced by Arc Melting

World Academy of Science, Engineering and Technology, International Journal of Physical and Mathematical Sciences (2017)

G. Sadullahoğlu B. Altuncevahir

Arc melting

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Role of domain and interface spin structure on exchange bias in CoNi/Gd/CoNi trilayers

Journal of Applied Physics (2003)

B. Altuncevahir Sezen Demirtas A. R. Köymen

In this study, dc remanence measurements were performed on a sputter deposited CoNi(top)/Gd/CoNi trilayers. The remanence state of the CoNi(top)/Gd bilayer, which is the hard layer, determines the exchange bias field strength. If the sample is cooled from room temperature at the remanence state corresponding to the hard layer coercive field, the exchange bias becomes zero. Domains form in the hard layer at this remanence state. The angular dependence of the coercive field does not indicate magnetization reversal by domain wall motion. The bottom soft CoNi layer exchange couples to each domain in the hard layer at the interface. This causes the exchange bias field to become zero on the average. If the trilayer is cooled at its full remanence state, the formation of Hb anisotropy is observed in the angular dependence of coercivity.

Exchange bias

Domain wall (magnetism)

10.1063/1.1556203

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Citations (1)