Lubna Shah

Effect of Zn interstitials on the magnetic and transport properties of bulk Co-doped ZnO

We have demonstrated that the bound magnetic polaron model is responsible for ferromagnetism in Co?ZnO semiconductors, where the carriers are provided by the interstitial zinc (Zni). Our experiment is unique since by changing the temperature, we are able to cross the carrier concentration threshold above which a long-range ferromagnetic order is established. Consequently, the ferromagnetic order is observed at room temperature but is weakened at temperatures below 100?K. To support our conclusion we have performed a systematic investigation on the structural, magnetic and transport properties which all give consistent results in the context of our proposed two-region model, i.e. (a) a Zni layer where carriers are sufficient to couple Co ions ferromagnetically and (b) a region with little carriers that remain in a paramagnetic state.

In-situ characterization of rapid crystallization of amorphous CoFeB electrodes in CoFeB/MgO/CoFeB junctions during thermal annealing

We report the crystallization study of CoFeB/MgO/CoFeB magnetic tunnel junctions using in-situ, time-resolved synchrotron-based x-ray diffraction and transmission electron microscopy. It was found that the crystallization of amorphous CoFeB electrodes occurs on a time scale of seconds during the postgrowth high temperature annealing. The crystallization can be well fit by the Johnson–Mehl–Avrami model and the effective activation energy of the process was determined to be 150 kJ/mol. The solid-state epitaxy mode of CoFeB was found to involve separate crystallization at different locations followed by subsequent merging of small grains, instead of layer-by-layer growth of CoFeB film along the MgO template.

Evolution of barrier-resistance noise in CoFeB/MgO/CoFeB tunnel junctions during annealing

The low-frequency resistance noise in sputtered-deposited magnetic tunnel junctions with MgO barriers has been measured as a function of annealing time at different annealing temperatures. The noise has a 1/f spectrum and it is quantified by a Hooge-like parameter α given in units of μm2. Unannealed devices have the highest noise levels and their α parameters exhibit a pronounced dependence on the voltage bias across the junction. A significant increase in tunneling magnetoresistance (TMR) is observed for short annealing times (on the order of minutes) at high temperatures and it is correlated with a large reduction in noise and in its bias dependence. The maximum TMR and minimum noise levels are reached at a later time that depends on temperature, being shorter at higher annealing temperatures. Devices annealed at 380 and at 430 °C exhibit the same minimum noise levels, α≈2×10−10 μm2. The origin of the resistance noise, its annealing time evolution, and its bias dependence are discussed and they are attr...

Magnetic noise evolution in CoFeB/MgO/CoFeB tunnel junctions during annealing

We report on the evolution of equilibrium magnetoresistive (MR) 1/f noise due to the exchange-biased magnetic layer in MgO-based magnetic tunnel junctions as a function of annealing time at 380 and 430 °C. The resistance susceptibility and MR noise are observed to increase rapidly with annealing time at a fixed temperature. The magnetic losses responsible for MR noise are not significantly affected by the structural crystallization at the CoFeB/MgO interface during short annealing times. After prolonged annealing, the decrease in magnetic losses is attributed to reduced disorder in the magnetic layers that result in thermally driven fluctuations in local micromagnetic structure.

Role of dopant, defect, and host oxide in the observed room temperature ferromagnetism : Co-ZnO versus Co-CeO2

A comparative study on the room temperature ferromagnetism of Co doped ZnO and CeO2 bulk samples is presented. Co–ZnO system has been observed to switch between ferromagnetic and paramagnetic by controlling the donor defects, interstitial zinc: Zni. On the other hand, Co–CeO2 is always ferromagnetic. However, ferromagnetism increases/decreases with increase/decrease in donor defects, oxygen vacancies: VO. Systemic structural, magnetic, and transport analyses reveal that the nature of donor defects and host oxide plays a vital role in establishing ferromagnetism. This study provides an insight into the underlying mechanisms that are responsible for the ferromagnetism in Co–ZnO and Co–CeO2. Moreover, the proposed mechanisms are supported by the electronic structure of magnetic impurity ions and defects.

Magnetic tunneling junction based magnetic field sensors: Role of shape anisotropy versus free layer thickness

Al2O3- and MgO-based magnetic tunnel junction (MTJ) sensors were designed and fabricated using microfabrication techniques. This study revealed that in the case of Al2O3-based sensors, the shape anisotropy in the free NiFe electrode resulted in a linear and hysteresis-free tunneling magnetoresistance (TMR) curve. These sensors exhibited TMR values between 27% and 30% and sensitivity up to 0.4%/Oe over a magnetic field range of − 40 to 40 Oe. In the case of CoFeB/MgO/CoFeB MTJ sensors, shape anisotropy alone was not sufficient to achieve a linear and hysteresis-free MR response. A superparamagnetic free layer was used to achieve the desired sensor response. MgO-based sensors had about 90% TMR and 1.1%/Oe sensitivity over the same field range as Al2O3-based MTJs.

Effect of rapid thermal annealing on microstructural, magnetic, and microwave properties of FeGaB alloy films

We have performed rapid thermal annealing (RTA) experiments on crystalline and amorphous (Fe70Ga30)1−xBx films in order to understand and reduce the linewidth of ferromagnetic resonance (FMR). In the absence of cubic anisotropy in amorphous films, the effective anisotropy is small, resulting in a narrow FMR linewidth. The FMR linewidth in both crystalline and amorphous films can be further reduced by RTA. In crystalline films, a minimum FMR linewidth can be achieved by annealing the samples at a relatively high temperature for a short time. The effect is attributed to the reduction in the uniaxial anisotropy. In amorphous films, the reduction in FMR linewidth is achieved at low temperature and short annealing time. Elevated temperatures and prolong time may crystallize the amorphous structure and introduce a much large cubic anisotropy, resulting in a large FMR linewidth.

Enhancement of ferromagnetism in Mn-doped Si via B codoping

Room temperature ferromagnetic Mn0.026BXSi0.974−X bulk samples with X=0.001, 0.005, and 0.01 were fabricated by arc melting and followed by thermal annealing at 1000 °C. The effects of carrier density on the ferromagnetism were studied. Due to the high carrier densities, all samples showed metallic behavior. Kondo effect was observed when the temperature was below 10 K. Weakly localized carriers monotonously increased with increasing boron concentration, resulting in the enhancement of exchange coupling among Mn ions. The study indicated that the ferromagnetism originates from the hole mediated Ruderman–Kittel–Kasuya–Yosida mechanism.

Properties of (Zn,Cr)Te semiconductor deposited at room temperature by magnetron sputtering

We report the fabrication of (Zn,Cr)Te films at room temperature by magnetron sputtering. Various structural and elemental characterizations revealed there was only a zinc blende phase from the ZnTe host and Cr atoms were distributed uniformly in these films. The magnetization measurement by superconducting quantum interference device magnetometer clearly showed that the samples were ferromagnetic at low temperatures with Curie temperature around 150K. The magnetic circular dichroism measurements confirmed that the observed ferromagnetism was originated from the interaction of substitutional Cr ions and ZnTe host. Transport measurement revealed typical semiconductor behaviors with the large negative magnetoresistance observed.

Ferromagnetism in sputtered Co doped CeO2 thin film on Al2O3 (0001) substrate

Diluted Co doped CeO2 films with the stoichiometry of Ce0.97Co0.03O2-d have been prepared via long range magnetron sputtering under varied O2 flux. Films were thoroughly characterized using X-ray diffraction (XRD), atomic force microscopy (AFM), optical transmission spectroscopy, X-ray Photoelectron Spectroscopy (XPS) and magnetic measurements. The results indicate all film are ferromagnetism (FM) at room temperature, of which the saturated magnetization moments (MS) are changed according to the O2 pressure adopted in deposition. This phenomenon is well consistent with the oxygen vacancy (VO, or so called F-center) mediated FM mechanism which was recently proposed to interpret FM found in insulating diluted magnetic oxides (DMOs) system. For further exploration of the oxygen dependent of the MS, oxygen non-stoichiometry was characterized by XPS. It shows that film deposited at lower O2 pressure contains less oxygen weight or more oxygen vacancies, lead to an enhanced MS and vice versa.