Magnetic Field Switching Research Articles

Failure and reliability analysis of STT-MRAM

Spin Transfer Torque Magnetic RAM (STT-MRAM) promises low power, great miniaturization prospective (e.g. 22nm) and easy integration with CMOS process. It becomes actually a strong non-volatile memory candidate for both embedded and standalone applications. However STT-MRAM suffers from important failure and reliability issues compared with the conventional solutions based on magnetic field switching. For example, a read current could write erroneously the stored data, the variability of ultra-thin oxide barrier drives high resistance variation and the injected current in the nanopillar induces lower lifetime etc. This paper classifies firstly all the possible failures of STT-MRAM into “soft errors” and “hard errors”, and analyzes their impact on the memory reliability. Based on this work, we can find some efficient design solutions to address respectively these two types of errors and improve the reliability of STT-MRAM.

Spin-transfer-torque switching in spin valve structures with perpendicular, canted, and in-plane magnetic anisotropies

We exploit canted anisotropies as possible means to enhance spin-transfer-torque (STT) and reduce switching currents. The STTs in spin-valve structures with perpendicular, canted, and, as a reference, in-plane magnetic anisotropies were studied. For perpendicular magnetic anisotropy and canted spin valves the thicknesses and number of Co and Pt layers were varied to obtain different angles of the magnetic anisotropy with respect to the sample plane. Point contact measurements were used to measure the change in the switching-field of the magnetization with the change in the bias current applied to the point contact. A larger STT effect, as evidenced by a larger change in the switching magnetic field for the unit change in the dc bias current, was observed for the sample with 45∘ tilt in magnetization compared to a sample with 12∘ tilt. Tilted magnetization of the reference layer causes precessional switching, decreasing the switching energy and time. Micromagnetic simulations were performed to explain the experimental observations.

Prospective self‐gated nonenhanced magnetic resonance angiography of the peripheral arteries

Most nonenhanced MRA techniques for evaluating peripheral artery disease (PAD) require cardiac synchronization through physiological gating. Electrocardiographic gating is the most popular method for cardiac synchronization; however, it is subject to interference from switching magnetic field gradients and radiofrequency pulses. A method is described for self-gated nonenhanced MRA that does not require the use of electrocardiographic gating. Imaging was prospectively triggered by detecting the acceleration of blood flow during systole with a reference-less phase contrast navigator. The technique was implemented for nonsubtractive nonenhanced MRA using quiescent-interval single-shot MRA. The lower extremity peripheral arteries of eight healthy subjects were imaged using electrocardiographic-, pulse-, and self-gated quiescent-interval single-shot. Self-gated quiescent-interval single-shot triggered with 99% accuracy. There were no significant differences in relative contrast, contrast-to-noise ratio, or image quality between self-gated and electrocardiographic-gated quiescent-interval single-shot MRA (P > 0.05). Image quality with pulse gating was inferior.

Analysis and Correction of Count Rate Reduction During Simultaneous MR-PET Measurements With the BrainPET Scanner

In hybrid magnetic resonance-positron emission tomography (MR-PET) studies with the Siemens 3T MR-BrainPET scanner an instantaneous reduction of the PET sensitivity was observed during execution of certain MR sequences. This interference was investigated in detail with custom-made as well as standard clinical MR sequences. The radio-frequency pulses, the switched gradient fields and the constant magnetic field were examined as the relevant parameters of the magnetic resonance imaging (MRI) system as well as the air temperature within the PET detectors. Our investigation comprised the analysis of the analog PET signals, the total count rates, the geometric distribution of the count rate reduction within the BrainPET detector as well as reconstructed images. The fast switching magnetic field gradients were identified to distort the analog PET detector signals. The measured count rate reduction was found to be less than 3%, but only up to 2% in the case of echo planar imaging sequences, as applied in functional MRI. For clinical sequences routinely used in hybrid MR-BrainPET measurements, a correction method has been designed, implemented, and evaluated .

Magnetization states and switching in narrow-gapped ferromagnetic nanorings

We study permalloy nanorings that are lithographically fabricated with narrow gaps that break the rotational symmetry of the ring while retaining the vortex ground state, using both micromagnetic simulations and magnetic force microscopy (MFM). The vortex chirality in these structures can be readily set with an in-plane magnetic field and easily probed by MFM due to the field associated with the gap, suggesting such rings for possible applications in storage technologies. We find that the gapped ring edge characteristics (i.e., edge profile and gap shape) are critical in determining the magnetization switching field, thus elucidating an essential parameter in the controls of devices that might incorporate such structures.

Magnetic Memory Effect of Nanocomposites

AbstractThe magnetic memory effect (MME) is the ability of magneto‐sensitive materials to remember the magnetic field strength (Hdef), at which they were deformed recently. They respond close to Hdef either by recovering their initial shape at a switching magnetic field strength Hsw under stress‐free conditions or by building up stress with a peak maximum at Hσ,max under constant strain conditions. This paper explores whether such a MME can be created for polymer‐based nanocomposites. The concept is based on temperature‐memory polymers (TMP) as matrix, in which silica coated iron(III)oxide nanoparticles (mNP) are dispersed. The MME was explored in a cyclic magneto‐mechanical test, in which the nanocomposite sample was elongated to ϵm while being exposed to an alternating magnetic field at Hdef. The magnetic memory was read out by determining Hσ,max or Hsw. A linear correlation between Hσ,max (or Hsw) and Hdef in a range from 15 to 23 kA m−1 at a fixed frequency of f = 258 kHz is observed and demonstrates the excellent magnetic memory properties of the investigated nanocomposites containing either crystallizable or amorphous, vitrifiable domains as controlling units. The deformation ϵm at Hdef can be fixed with an accuracy of more than 72% and the initial shape can be recovered almost completely by more than 86%. The MME allows the design of magnetically programmable devices such as switches or mechanical manipulators.

Switching Fields of Individual Co Nanoislands

We explore the magnetization reversal process of individual Co nanoislands grown on Cu(111) by low temperature spin-polarized scanning tunneling microscopy (spin-STM) and spectroscopy (spin-STS). We measure hysteresis loops of the differential conductance of single Co islands in magnetic fields of up to 4 T. From such hysteresis loops we extract the magnetic switching field of single Co islands as a function of island size. Tentatively we analyze the size dependence of the switching field using the venerable model of thermally assisted coherent magnetization reversal. We present evidence for the failure of that model to explain our experimental results. We propose that the magnetization reversal process within individual Co nanoislands on Cu(111) is a non-coherent process.

Damping dependence in microwave assisted magnetization reversal

This letter reports a demonstration of microwave assisted magnetization reversal (MAMR) in a CoFeB film and the damping dependence in MAMR through the measurement of ferromagnetic resonance (FMR). Spin-pumping in non-ferromagnetic/ferromagnetic films provides a large range variation of Gilbert damping constants in magnetic samples when changing the thickness of non-ferromagnetic layers without changing the ferromagnetic film. An evident dependence of switching fields on the damping constant is observed in the presence of microwaves. The trend of the experimental data is well reproduced by a numerical simulation based on the Landau–Lifshitz–Gilbert equation. The result indicates that the large damping decreases the efficiency of microwaves in reducing the magnetization switching field.

Origin of magnetic switching field distribution in bit patterned media based on pre-patterned substrates

Using a combination of synchrotron radiation based magnetic imaging and high-resolution transmission electron microscopy we reveal systematic correlations between the magnetic switching field and the internal nanoscale structure of individual islands in bit patterned media fabricated by Co/Pd-multilayer deposition onto pre-patterned substrates. We find that misaligned grains at the island periphery are a common feature independent of the island switching field, while irregular island shapes and misaligned grains specifically extending into the center of an island are systematically correlated with a reduced island reversal field.

Influence of Speed Variation of a Transverse Magnetic Field on a Magnetization of HTS Cylinder

In this paper, we numerically study the influence of the speed variation of a magnetic source on the distribution of current density, magnetization, and dissipated energy of a high-temperature superconducting cylinder described by a Jn power law. The results presented come from the resolution of a nonlinear diffusion problem of electric field by a mixed finite-element finite-volume discretization method. This method is robust, stable, and converges for large values of n. The calculations carried out for n, varying from 1 to 200, show that when the external magnetic field quickly varies from 0 to its maximal value, the maximum values of penetration, the magnetization, and the energy dissipation are obtained when the switching of magnetic field occurs. For a periodic magnetic field, we note that any change of the period results in variation of the magnetization and the dissipated energy.

Design considerations and strategies for high-reliable STT-MRAM

Benefiting from Spin Transfer Torque (STT) switching approach, second generation of Magnetic RAM (MRAM) promises low power, great miniaturization prospective (<22nm) and easy integration with CMOS process. It becomes actually a strong non-volatile memory candidate for both embedded and standalone applications. However STT-MRAM suffers from important reliability issues compared with the conventional one based on magnetic field switching, for example, a read-current could write erroneously the stored data, the low Resistance Area (RA) value drives high sensing error rate. This paper presents the considerations and strategies from design point of view for the reliability enhancement. Mixed transient and statistical simulations have been performed by using a STT-MRAM compact model and CMOS 65nm design kit.

Microwave coupled electron tunneling measurement of Co nanoparticles

We study electron tunneling through Co nanoparticles in the presence of repeated microwave pulses at 4.2 K. While individual pulses are too weak to affect the magnetic switching field, repeated microwave pulses start to reduce the magnetic switching field at 10 μs spacing. We use I-V curve as a thermometer to show that the microwave pulses do not heat the sample, showing that magnetization in Co nanoparticles is directly excited by microwave pulses, and the relaxation time of the excitation energy is in the range of microsecond.

Pumping Spin and Charge Currents from Applying a Time-Variant Field and a Magnetic Field to a Quantum Dot

Photon-electron pumping effects on a quantum dot connected to two magnetic leads in the presence of both via-dot and over-dot tunneling channels have been investigated by the evolution operator approach. It is found that a time-variant field applied to the quantum dot may give rise to charge and spin pumping at zero bias voltage for asymmetric magnetic junctions. When the magnetic field switch on，the spin-up and spin-down current can be considerably separated，then the highly spin polarized current is also generated.

Incoherent magnetization reversal in Co–Pt nanodots investigated by magnetic force microscopy

The magnetization reversal behavior of a hexagonal close-packed Co–Pt nanodot (50 nm in diameter) array was studied by magnetic force microscopy (MFM). The magnetic switching field ( H SW) of each dot and the switching field distribution (SFD) for the array were determined by the MFM observation. The results show that the contrast intensity of the dots in the MFM image has a strong correlation with its H SW. The angular dependence of the average H SW on the direction of the applied field deviates from the coherent rotation model, and appears to reverse by a vortex-like incoherent rotation. It is deduced that the nanodot seems to consist of a hard inner core with high magnetic anisotropy and a soft magnetic shell layer surrounding the core. This core–shell structure results in the magnetic vortex-like incoherent rotation because of an exchange coupling interaction. Shell layers of various thicknesses should result in a broad SFD. This proposal is confirmed by micromagnetic simulation using the exchange coupling model, which is in good agreement with the experimental results.

Effects of Co addition on magneto-transport properties of magnetic tunnel junction consisting of CoFeB or CoFeSiB free layer

Amorphous ferromagnetic CoFeB and CoFeSiB layers with various Co concentrations were employed as free layers of magnetic tunnel junctions (MTJs), and their magnetization switching performances were compared. Both analytical measurements and micromagnetic modeling efforts were carried out to understand the dependence of magnetization switching field (Hsw) on Co concentration. In overall, the CoFeSiB free layered MTJs showed a lower Hsw compared to that of the CoFeB ones. This is due to the fact that CoFeSiB possesses a lower saturation magnetization than CoFeB and, moreover, its magnetization switching process shows coherent switching.

Phase coexistence and magnetic anisotropy in polycrystalline and nanocrystalline LaMnO3+δ

We report on the phase coexistence and magnetic anisotropy in polycrystalline (bulk) and nanocrystalline (∼15 nm) LaMnO3+δ materials, which were prepared by solid state reaction and sol-gel methods, respectively. In addition to standard magnetization measurements, radio-frequency transverse susceptibility (TS) based on a very sensitive, self-resonant tunnel diode oscillator method was used to probe magnetic anisotropy and switching fields in the samples. The results revealed a coexistence of the ferromagnetic (FM) and antiferromagnetic (AFM) phases in both samples. For the bulk sample, the AFM phase significantly changed in volume fraction at ∼30 K and completely vanished around 120 K. Size reduction to the nanometer scale (∼15 nm) significantly suppressed the AFM phase while inducing surface spin disorder in the material. The large magnetic anisotropies were probed by TS experiments in both samples. Our studies showed that the magnetic properties of bulk LaMnO3+δ were strongly modified by size reduction.

Metal-terminated graphene nanoribbons

We have investigated structure, electronic, and magnetic properties of metal-terminated zigzag graphene nanoribbons (M-ZGNRs) by first-principles calculations. Two families of metal terminations are studied: (1) 3d-transition metals (TMs) Fe, Co, and Ni and (2) noble metals (NMs) Cu, Ag, and Au. All systems have spin-polarized edge states with antiferromagnetic (AFM) ordering between two edges, except Co-ZGNRs and Ni-ZGNRs which exhibit negligibly small energy differences between AFM and ferromagnetic states with the given ribbon width. In the AFM state the TM terminations transform semiconducting ZGNRs into metallic ones while the band gap remains in ZGNR with NM terminations. Ferromagnetic states of M-ZGNRs with TM terminations show a high degree of spin polarization at the Fermi energy. We predict a large magnetoresistance in Fe-ZGNR junctions with a low, uniform magnetic switching field.

Desktop fast-field cycling nuclear magnetic resonance relaxometer

In this paper a new type of Fast Field Cycling (FFC) Nuclear Magnetic Resonance (NMR) relaxometer with low power consumption (200 W) and cycle to cycle field stability better than 10 −4 is described. The new high-permeability magnet was designed to allow for good magnetic field homogeneity and allows for the sample rotation around an axis perpendicular to magnetic field, operating with magnetic fields between 0 and 0.21 T. The power supply of the new relaxometer was specially developed in order to have steady state accurate currents and allow for magnetic field switching times less than 3 ms. Additional control circuits were developed and included to compensate the Earth magnetic field component parallel to the field axis and to compensate for parasitic currents. The main aspects of the developed circuits together with some calibrating experimental results using the liquid crystal compounds 5CB and 8CB are presented and discussed.

Non-Cartesian sampled centric scan SPRITE imaging with magnetic field gradient and B0( t) field measurements for MRI in the vicinity of metal structures

This paper proposes the possibility of spatially resolved MRI measurements undertaken inside metallic cells. MRI has been rarely usable inside conducting vessels due to the eddy currents in the walls caused by switching magnetic field gradients, which render most advanced MRI pulse sequences impossible. We propose magnetic field gradient waveform monitoring (MFGM) for MRI of samples inside metallic cells. In this work the MFGM method was extended to measure the B 0 field temporal evolution associated with gradient waveforms. MFGM was used to observe and correct eddy current effects associated with a metallic cell. High quality centric scan SPRITE images result from such corrections. MRI of samples held under pressure, most notably rock core samples, traditionally employs cells that are non-magnetic and fabricated from polymeric materials. The natural material for high-pressure MRI is however non-ferromagnetic metal given their high tensile strengths and high thermal conductivity. MRI measurement of macroscopic samples at high pressure would be generally possible if metallic pressure vessels could be employed. This study will form the basis of new MRI compatible metallic pressure vessels, which will permit MRI of macroscopic systems at high pressure.

Co/Ni]N-Based Synthetic Antiferromagnet with Perpendicular Anisotropy and Its Application in Pseudo Spin Valves

The dependence of magnetic properties on layer repetition number and Ru thicknesses have been studied for perpendicularly magnetized synthetic antiferromagnet (SAF) in a structure of [Co/Ni] <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</sub> /Ru/[Co/Ni] <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> . The optimum SAF with strong antiferromangetic coupling field and large switching field of the net magnetization have been determined and utilized as the reference layer in the pseudo spin valves. Compared with the rapid drop of GMR signal for the normal [Co/Ni]-based pseudo spin valves after annealing at low temperature (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> ) of 150°C, the spin valve with SAF reference layer exhibits much stable thermal stability due to the large switching field difference between the free and reference layers which avoids the simultaneous magnetization rotation. The GMR signal of the SAF spin valve sample is 6.0% at room temperature, it decreases very gradually with the increase of T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> . We attribute the slow GMR reduction observed in the SAF spin valve to the effects of domain formation and perpendicular anisotropy deterioration caused by high temperature anneals.