N. Akdoğan

Dose dependence of ferromagnetism in Co-implanted ZnO

We have studied the structural, magnetic, and electronic properties of Co-implanted ZnO(0001) films grown on Al2O3 (112¯0) substrates for different implantation doses and over a wide temperature range. Strong room temperature ferromagnetism is observed with magnetic parameters depending on the cobalt implantation dose. A detailed analysis of the structural and magnetic properties indicates that there are two magnetic phases in Co-implanted ZnO films. One is a ferromagnetic phase due to the formation of long range ferromagnetic ordering between implanted magnetic cobalt ions in the ZnO layer and the second one is a superparamagnetic phase, which occurs due to the formation of metallic cobalt clusters in the Al2O3 substrate. Using x-ray resonant magnetic scattering, the element specific magnetization of cobalt, oxygen, and Zn was investigated. Magnetic dichroism was observed at the Co L2,3 edges as well as at the O K edge. In addition, the anomalous Hall effect is also observed, supporting the intrinsic nat...

Intrinsic room temperature ferromagnetism in Co-implanted ZnO

We report on the structural and magnetic properties of a cobalt-implanted ZnO film grown on a sapphire substrate. X-ray diffraction and transmission electron microscopy reveal the presence of a( 1 0 ¯ 1 0)-oriented hexagonal Co phase in the Al2O3 sapphire substrate, but not in the ZnO film. Co clusters, with a diameter of about 5–6 nm, form a Co rich layer in the substrate close to the ZnO/Al2O3 interface. Magnetization measurements indicate that there exist two different magnetic phases in the implanted region. One originates from the Co clusters in Al2O3, the other one belongs to a homogeneous ferromagnetic phase with a ferromagnetic Curie temperature far above room temperature. In the latter case, the ferromagnetism can be attributed to Co substitution on Zn sites in the ZnO layer. We have observed magnetic dichroism at the Co L2,3 and O K edges at room temperature as well as the multiplet structure in x-ray absorption spectra around the Co L3 edge, supporting the intrinsic nature of the observed ferromagnetism in a Co-implanted ZnO film. The magnetic moment per substituted cobalt is found to be about 2.81 µB, which is very close to the theoretical expected value of 3 µB/Co for Co 2+ in its high spin state.

High-temperature ferromagnetism in Co-implanted TiO2 rutile

We report on structural, magnetic and electronic properties of Co-implanted TiO2(1 0 0) rutile single crystals for different implantation doses. Strong ferromagnetism at room temperature and above is observed in TiO2 rutile plates after cobalt ion implantation, with magnetic parameters depending on the cobalt implantation dose. While the structural data indicate the presence of metallic cobalt clusters, the multiplet structure of the Co L3 edge in the XAS spectra provides evidence that a sizeable portion of the dopants occupy substitutional Co 2+ sites. The detailed analysis of the structural and magnetic properties indicates that there are two magnetic phases in Co-implanted TiO2 plates. One is a ferromagnetic phase due to the formation of long range ferromagnetic ordering between implanted magnetic cobalt ions in the rutile phase, and the second one is a superparamagnetic phase which originates from the formation of metallic cobalt clusters in the implanted region. Using x-ray resonant magnetic scattering, the element specific magnetizations of cobalt, oxygen and titanium in Co-implanted TiO2 single crystals are investigated. Magnetic dichroism was observed at the Co L2,3 edges as well as at the O K edge. Anomalous Hall effect measurement indicates n-type carriers in Co-implanted TiO2 rutile. The interaction mechanism, which leads to ferromagnetic ordering of substituted cobalt ions in the host matrix, is also discussed.

Anisotropy of ferromagnetism in Co-implanted rutile

Magnetic anisotropy of cobalt implanted single-crystalline rutile has been studied by means of magneto-optical Kerr effect (MOKE) and superconducting quantum interference device (SQUID) techniques. We observed for the first time strong angular dependence of the remanent magnetization and coercive field in the plane of the implanted surface: twofold anisotropy for the (100)-substrate and fourfold anisotropy for the (001)-substrate samples. The observation opens up new possibilities to tailor magnetic anisotropies of the material. Possible origins of ferromagnetism and anisotropies in dielectric and diamagnetic single-crystalline TiO2 samples after Co-ion implantation are discussed.

Spin polarization of oxygen atoms in ferromagnetic Co-doped rutile TiO2

Of central interest in the research of dilute magnetic semiconductors is the coupling mechanism leading to a ferromagnetic ground state. Using x-ray resonant magnetic scattering, we have analyzed the element specific magnetic hysteresis curves of Co, Ti, and oxygen in Co-doped TiO2 synthesized by ion implantation. Magnetic dichroism was observed at the Co L2,3 edges, as well as at the O K edge, indicative of a spin polarization of oxygen atoms in the TiO2 host matrix. The hysteretic shapes and the coercive field values measured at the Co L3 and O K edges are identical (1.9kOe at 30K).

Exchange bias properties of [Co/CoO]n multilayers

In this study, the exchange bias properties of four polycrystalline multilayer stack samples of antiferromagnetic (AF) CoO and ferromagnetic (FM) Co in the form of [CoO/Co]n with n = 1, 2, 3, and 5 are reported. The samples were grown on top of Si (001) substrates by using magnetron sputtering method. X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) were used to determine the structural properties of the samples. XPS measurements of cobalt oxide layer revealed the coexistence of different phases in cobalt oxide as CoO and Co3O4, the latter of which lowers the blocking temperature. The blocking temperature is also affected by the finite size scaling effects observed in AF layers. In-plane ferromagnetic resonance (FMR) measurements revealed uniaxial in-plane magnetic anisotropy for the samples. Low temperature vibrating sample magnetometer measurements provided exchange bias with a stepwise character. Observed steps are believed to be due to magnetization reversals of indivi...

Six-fold in-plane magnetic anisotropy in Co-implanted ZnO (0001)

Magnetic anisotropies of Co-implanted ZnO (0001) films grown on single-crystalline Al2O3 (112¯0) substrates have been studied by ferromagnetic resonance (FMR) technique for different cobalt implantation doses. The FMR data show that the easy and hard axes have a periodicity of 60° in the film plane, in agreement with the hexagonal structure of the ZnO films. This six-fold in-plane magnetic anisotropy, which is observed for the first time in ZnO-based diluted magnetic semiconductors, is attributed to the substitution of cobalt on Zn sites in the ZnO structure, and a clear indication for long range ferromagnetic ordering between substitutional cobalt ions in the single-crystalline ZnO films.

Interface induced manipulation of perpendicular exchange bias in Pt/Co/(Pt,Cr)/CoO thin films

Perpendicular exchange bias has been manipulated by changing ferromagnetic film thickness and spacer layer in Pt/Co/(Pt,Cr)/CoO thin films. The exchange bias characteristics, blocking temperature, and magnetization of thin films strongly depend on the spacer layer (Pt,Cr) between ferromagnetic and antiferromagnetic layers. While Pt/Co/Pt/CoO thin films show perpendicular exchange bias, Pt/Co/Cr/CoO has exchange bias with easy magnetization axis in the film plane. We have also observed very small hysteretic behavior from the hard axis magnetization curve of Pt/Co/Cr/CoO film. This can be attributed to misalignment of the sample or small perpendicular contribution from Pt/Co bottom interface. We have also investigated the temperature and spacer layer dependent exchange bias properties of the samples. We observed higher HEB and HC for the thicker Co layer in the Pt/Co/Pt/CoO sample. In addition, onset of exchange bias effect starts at much lower temperatures for Pt/Co/Cr/CoO thin film. This clearly shows that Cr spacer layer not only removes the perpendicular exchange bias, but also reduces the exchange interaction between Co and CoO and thus lowers the TB.

The effect of annealing on the junction profile of CoFeB/MgO tunnel junctions

The tunnelling magnetoresistance of CoFeB/MgO tunnel junctions is exceptionally high, although the electrodes and the barrier are grown at room temperature in the amorphous state. For their functionality annealing steps up to high temperatures are required. We have analyzed in detail the changes in the chemical and magnetization profile upon annealing up to 360°. The multilayers used for this study are similar to those which are used in magnetic tunnel junctions, however with five repeats. In particular, we have used hard non-resonant and soft resonant magnetic x-ray scattering in order to unravel any changes upon annealing. The multilayers exhibit superior structural quality, which hardly changes with annealing. Surprisingly, only little recrystallization of the CoFeB and the MgO layers can be discerned by x-ray diffraction.

Coexistence of perpendicular and in-plane exchange bias using a single ferromagnetic layer in Pt/Co/Cr/CoO thin film

We studied the temperature dependence of magnetization and exchange bias in a Pt/Co/Cr/CoO multilayer thin film. These magnetic multilayers are of particular interest since the easy axis of ultra-thin Co is strongly affected by the interfacial anisotropies of neighbouring Pt and Cr layers. The room temperature measurements show that the sample has a magnetic easy axis only in the film plane. However, upon cooling the sample, the easy axis of the magnetization departs from its initial orientation and typical easy-axis hysteresis loops are obtained for both in-plane and perpendicular directions. In accordance with this change in the magnetization direction at lower temperatures, the sample shows an unexpected coexistence of perpendicular and in-plane exchange bias below the antiferromagnetic transition of CoO. The temperature dependence of the exchange bias field for both directions is also significantly different. Along the film plane, the exchange bias field monotonically decreases and disappears at 220 K with increasing temperature. For the perpendicular direction, however, the exchange bias field increases and reaches a maximum value at 80 K. Then it decreases and disappears at 150 K with further increasing temperature. The mechanisms behind this anomalous temperature dependence of the exchange bias as well as the step-like behaviour in the hysteresis curves are discussed.