Tunneling Magnetoresistance Devices Research Articles

Epitaxial growth of electron doped double perovskites [formula omitted] with [formula omitted] and Ca

Ferrimagnetic double perovskites are interesting for tunnel magnetoresistance (TMR) devices due to their high Curie-temperature and their half-metallicity. Because of the preferential formation of LaCrO 3, the synthesis of bulk samples of electron-doped La x Sr 2 - x CrWO 6 is difficult. However, due to the non-equilibrium growth conditions during pulsed laser deposition (PLD), the material can be grown as high-quality epitaxial film. We report the growth parameters of the compound La x Sr 2 - x CrWO 6 on SrTiO 3 substrates. Using reflection high-energy electron diffraction (RHEED) a layer-by-layer growth over the whole film thickness has been observed. Ca 2CrWO 6 grown on NdGaO 3 has worse crystalline quality and shows paramagnetic behavior.

Low-current spin-transfer switching and its thermal durability in a low-saturation-magnetization nanomagnet

A spin-transfer magnetization switching technique is a promising candidate as a writing mechanism for a high-density magnetic random access memory because of its scalability. The required switching current Ic, however, is still too large for this technique to be applied to MRAM using tunneling magnetoresistive devices. Here, it is demonstrated that reducing the saturation magnetization Ms of magnet cells is an effective way to decrease Ic. Use of a CoFeB film with μ0Ms of 0.75T as a magnet cell reduced Ic measured with a continuous current by an order of magnitude. We changed the duration of a writing current pulse from 1μs to 5s to investigate thermal effects on the switching process, and predicted that CoFeB magnet cells with low Ic can be compatible with the thermal durability required for MRAM applications.

Magnetically soft, high moment grain-refined Fe films: Application to magnetic tunnel junctions

The effect of N-doping on the microstructure and magnetic properties of thin Fe layers has been employed to construct all Fe-electrode magnetic tunnel junctions that displayed the tunneling magnetoresistance (TMR) effect. Using low nitrogen doses, a reduction in coercivity was achieved due to grain refinement, without a concurrent decrease in the saturation magnetization of the Fe films caused by the formation of crystalline iron nitride phases. It was demonstrated that this N-induced grain refinement can be applied beneficially to control the switching field of the “free” layer in magnetic trilayer structures. In general the ability to control magnetic softness without reducing saturation magnetization will prove important for incorporating high spin-polarized materials into spin valves and TMR devices.

Microstructure of pulsed laser deposited ceramic–metal and polymer–metal nanocomposite thin films

Ceramic–metal (MgO combined with Fe, Ti and Ni80Nb20) and polymer–metal (polycarbonate combined with Ag and Pd) nanocomposite multilayers were deposited at room temperature by laser ablation (at 248 nm). The multilayers were characterized by X-ray reflectometry, infrared spectroscopy and transmission electron microscopy. In the case of MgO/metal multilayers, well-layered structures are produced down to layer periodicities of 1.2 nm, necessary for tunneling magnetoresistance devices and X-ray mirrors in the ‘water window’. The interface roughness in the case of polymer/metal multilayers is found to be a strong function of the metal layer thickness and also the nature of the metal.

Analytical investigations of tunnel magnetoresistance layers.

The basic elements of tunnel magnetoresists are two magnetic layers separated by an insulating barrier layer. The uniformity of this only 1-2 nm thick barrier layer up to dot edges and the chemical composition of the layers are properties important for the efficiency of tunnel magnetoresistance devices. These key-properties have been investigated by analytical TEM methods like high resolution TEM imaging and energy-filtered imaging. With regard to the chemical composition the TEM results have been confirmed by XPS investigations. The subsequent oxidation of the barrier is one of the most critical steps of the deposition procedure of the layer stacks. An undersized oxygen dose leads to an incomplete oxidation of the barrier layer with uncontrollable tunnel behaviour. An overdose of oxygen leads to oxygen diffusion in the layers beneath the barrier and uncontrollable magnetic hardness of the lower magnetic electrode.

Ballistic magnetoresistance of electroplated nickel devices

Electroplated nickel nanojunctions (NJs) showing ballistic magnetoresistance up to 100 000% could be the potential workhorse to drive the next generation of magnetic storage beyond one trillion bits per square inch. Understanding the mechanism of this kind of device is extremely important for both academia and industry. In this paper, the detailed oxidation process and the structures of junctions after deposition are analyzed. Depending on the details of structures within the junction, a device of this kind can work as a multibarrier tunneling magnetoresistance (MB-TMR) device, Coulomb blockade enhanced MB-TMR, or even a spin-dependent atomiclike quantum dot. In terms of device fabrication, better control of the oxidation after plating is crucial.

터널링 자기저항 소자의 정전기 방전 시뮬레이션

본 연구에서는 인체모델(humman body model; HBM)을 터널링 자기저항(tunneling magneto resistance; TMR)소자에 연결하여 정전기에 대한 방전특성을 연구하였다. 이를 위해 제조된 TMR 시편을 전기적 등가회로 바꿔 HBM에 연결하여 PSPICE를 이용해 시뮬레이션 하였다. 이러한 등가회로에서 접합부분의 모델링 요소들의 값을 변화시켜 방전특성을 관찰할 수 있었다. 그 결과 시편의 저항과 정전용량 성분의 값이 다른 요소들에 비해 수배에서 수백 배까지 커서 정전기 방전(electrostatic discharge; ESD) 민감도를 좌우하는 주요한 요소임을 알 수 있었다. 여기에서 ESD현상에 대한 내구성을 향상시키기 위해서는 정전용량을 증가시키는 것 보다 접합면과 도선의 저항값을 줄이는 것이 유리하다. 그리고 직류 전압에 대해 절연층의 전위 장벽이 낮아져 많은 전류가 흐르게 되는 항복(breakdown)전압과 셀의 물리적 구조 및 성질이 변형되어 회복되지 못하는 파괴(failure)전압을 측정하여 DC 상태에서의 내구성을 연구하였다. 이 결과를 HBM 전압에 대한 파괴 전압으로 간주하여 TMR 소자가 견딜 수 있는 HBM 전압을 예측할 수 있었다. Electrostatic discharge characteristics were studied by connecting human body model (HBM) with tunneling magnetoresistance (TMR) device in this research. TMR samples were converted into electrical equivalent circuit with HBM and it was simulated utilizing PSPICE. Discharge characteristics were observed by changing the component values of the junction model in this equivalent circuit. The results show that resistance and capacitance of the TMR junction were determinative components that dominate the sensitivity of the electrostatic discharge(ESD). Reducing the resistance oi the junction area and lead line is more profitable to increase the recording density rather than increasing the capacitance to improve the endurance for ESD events. Endurance at DC state was performed by checking breakdown and failure voltages for applied DC voltage. HBM voltage that a TMR device could endure was estimated when the DC failure voltage was regarded as the HBM failure voltage.

절연막층의 플라즈마 산화시간에 따른 CoFe/AlO/CoFe/NiFe 구조의 터널자기저항 효과 연구

We investigated the evolution of magnetoresistance and magnetic property of tunneling magnetoresistive(TMR) device with microstructure and plasma oxidation time. TMR devices have potential applications for non volatile MRAM and high density HDD reading head. We prepared the tunnel magnetoresistance(TMR) devices of Ta(<TEX>$50{\AA}$</TEX>)/NiFe(<TEX>$50{\AA}$</TEX>)/IrMn(<TEX>$150{\AA}$</TEX>)/CoFe(<TEX>$50{\AA}$</TEX>)/Al(<TEX>$13{\AA}$</TEX>)-O/CoFe(<TEX>$40{\AA}$</TEX>)/FiFe(<TEX>$400{\AA}$</TEX>)/Ta((<TEX>$50{\AA}$</TEX>) structure which have <TEX>$100{\times}100\mu\textrm{m}^2$</TEX> junction area on <TEX>$2.5{\times}2.5\textrm{cm}^2$</TEX> Si/<TEX>$SiO_2$</TEX>((<TEX>$1000{\AA}$</TEX>) substrates by an inductively coupled plasma(ICP) magnetron sputter. We fabricated the insulating layer using an ICP plasma oxidation method by with various oxidation time from 30 sec to 360 sec, and measured resistances and magnetoresistance(MR) ratios of TMR devices. We found that the oxidized sample for oxidation time of 80 sec showed the highest MR radio of 30.31 %, while the calculated value regarding inhomogeneous current effect indicated 25.18 %. We used transmission electron microscope(TEM) to investigate microstructural evolution of insulating layer. Comparing the cross-sectional TEM images at oxidation time of 150 sec and 360 sec, we found that the thickness and thickness variation of 360 sec-oxidized insulating layer became 30% and 40% larger than those of 150 sec-oxidized layer, repectively. Therefore, our results imply that increase of thickness variation with oxidation time may be one of the major treasons of the MR decrease.

Deposition of [Ni–Fe/Al–O/Co–Fe] films with tunneling magnetoresistance effect using the interfacial modulation technique

[Ni–Fe/Al–O/Co–Fe] films with tunneling magnetoresistance (TMR) were deposited by the dual ion beam sputtering method. An interfacial modulation technique using Kr ion bombardment on the initial growth region of each layer in TMR devices changed the crystallographic structures and interfacial conditions of the films. Kr was used as the sputtering gas in order to reduce the damage to growing films by recoiled particles. The effect of Kr ion bombardment on the initial growth region seemed to be effective in decreasing the film resistivity of Ni–Fe and Co–Fe films, especially at the optimized bombarding energy of 100 eV. In addition, stoichiometric Al2O3 films could be attained by reactive ion beam sputtering with O2 concurrently flow to the substrate. These optimized films were applicable as the ferromagnetic and insulation layers in TMR devices.

Micromagnetic simulation of tunneling magnetoresistance junctions with parallel hard bias

Tunneling magnetoresistance (TMR) films and devices were simulated to understand the response of the free layer with a parallel hard bias. In order to determine the effect of the granular hard bias material micromagnetic simulation was used to model both the hard bias and TMR material. Minimizing hysteresis and Barkhausen jumps in the response of the device involves an optimization of the spacing between the free layer and the hard bias coupled with the shape of the device edges.

Near-room-temperature tunneling magnetoresistance in a trilayer La0.67Sr0.33MnO3/La0.85Sr0.15MnO3/La0.67Sr0.33MnO3 device

A trilayer La0.67Sr0.33MnO3/La0.85Sr0.15MnO3/La0.67Sr0.33MnO3 tunneling magnetoresistance (TMR) device has been epitaxially produced on a SrTiO3 substrate. The current–voltage curves show that tunneling process dominates the carrier transfer across the junction near room temperature. However, the tunneling effect becomes weak with decreasing temperature. Magnetoresistance of about 4%–6% has been observed below room temperature. The temperature dependence of TMR appears correlated to both the tunneling effect cross the junction and the spin polarization in La0.67Sr0.33MnO3 layers. The temperature dependence of the TMR effect in this device is different from that of other devices based on colossal magnetoresistance materials.

Tunnel magnetoresistance devices processed by oxidation in air and UV assisted oxidation in oxygen

Tunnel magnetoresistance (TMR) devices were processed by sputter deposition of Co, Al and NiFe on oxidized Si wafers. After the Al deposition, an ex-situ oxidation in air at room temperature or an in-situ oxidation enhanced by ultraviolet (UV) irradiation in high purity oxygen at 100 mbar follows. The electrical and magnetic properties of the junctions are measured and discussed concerning specific junction resistance, magnetoresistance ratio, long time stability of the junctions, and failure rate of the processes. Some microscopic experiments provided consistent information of the tunnel barrier. MR ratios between 15% and 20% were measured for the different oxidation processes.

Prerequisites for high-quality magnetic tunnel junctions: XPS and NMR study of Co/Al bilayers

Aluminum films with thicknesses ranging from 1 nm to 12 nm have been sputtered on 20 nm thick Co layers. The properties of the Co/Al bilayers were studied by X-ray photoemission spectroscopy (XPS) and spin-echo nuclear magnetic resonance (NMR). Both methods show independently that a 1 nm Al film covers the Co surface completely. XPS and NMR also showed that layers thicker than 1 nm Al are not oxidized completely in ambient air. Similarities to and deviations from niobium with Al overlayers (Nb/Al) are described. Prerequisites for the fabrication of tunneling magnetoresistance devices based on Co or NiFe ferromagnets and an aluminum oxide barrier are discussed.

Electrostatic discharge testing of tunneling magnetoresistive (TMR) devices

We report on an experimental and SPICE simulation study of electrostatic discharge (ESD) damage of tunneling magnetoresistive (TMR) junctions. The human body model (HBM) ESD failure voltage was found to be 6 V/sub HBM/, higher than the 2 V DC breakdown voltage. This difference is explained in terms of the dependence of the HBM failure voltage on the resistance and capacitance of the TMR device. A simple SPICE model for a TMR device is used to study the response of a TMR device during an ESD event.