Three-Dimensional Magnetic Page Memory

The increasing need to store large amounts of information with an ultra-dense, reliable, low power and low cost memory device is driving aggressive efforts to improve upon current perpendicular magnetic recording technology. However, the difficulties in fabricating small grain recording media while maintaining thermal stability and a high signal-to-noise ratio motivate development of alternative methods, such as the patterning of magnetic nano-islands and utilizing energy-assist for future applications. In addition, both from sensor and memory perspective three-dimensional spintronic devices are highly desirable to overcome the restrictions on the functionality in the planar structures. Here we demonstrate a three-dimensional magnetic-memory (magnetic page memory) based on thermally assisted and stray-field induced transfer of domains in a vertical stack of magnetic nanowires with perpendicular anisotropy. Using spin-torque induced domain shifting in such a device with periodic pinning sites provides additional degrees of freedom by allowing lateral information flow to realize truly three-dimensional integration

Thermally induced magnetic switching in bit-patterned media

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Journal of Applied Physics 122, 043907 (2017) and may be found at https://doi.org/10.1063/1.4992808.We have studied the thermal variation of the switching field of magnetic islands at room temperature. A model bit-pattern media composed of an assembly of islands with 80 nm width was fabricated by sputter deposition onto a pre-patterned substrate. Using direct magnetic-contrast imaging of the islands under applied field, we extract the switching probabilities of individual islands. Based on an analytical model for the thermally activated switching of the islands, we are able to determine the intrinsic magnetic anisotropy of each island and, consequentially, a distribution of anisotropies for the island ensemble investigated. In the distribution, we identify a separated group of islands with a particularly small anisotropy. We attribute this group to islands containing misaligned grains triggering the magnetic reversal. At room temperature and slow field sweep rates, the observed thermal broadening of the switching-field distribution is small compared to the intrinsic broadening. However, we illustrate that thermal fluctuations play a crucial role at high sweep rates by extrapolating our results to technological relevant regimes

The Paramagnetic Meissner Effect (PME) in Metallic Superconductors

The experimental data in the literature concerning the Paramagnetic Meissner Effect (PME) or also called Wohlleben effect are reviewed with the emphasis on the PME exhibited by metallic, s-wave superconductors. The PME was observed in field-cool cooling (FC-C) and fieldcool warming (FC-W) m(T)-measurements on Al, Nb, Pb, Ta, in compounds such as, e.g., NbSe2, In-Sn, ZrB12, and others, and also in MgB2, the metallic superconductor with the highest transition temperature. Furthermore, samples with different shapes such as crystals, polycrystals, thin films, biand multilayers, nanocomposites, nanowires, mesoscopic objects, and porous materials exhibited the PME. The characteristic features of the PME, found mainly in Nb disks, such as the characteristic temperatures T1 and Tp and the apparative details of the various magnetic measurement techniques applied to observe the PME, are discussed. We also show that PME can be observed with the magnetic field applied parallel and perpendicular to the sample surface, that PME can be removed by abrading the sample surface, and that PME can be introduced or enhanced by irradiation processes. The PME can be observed as well in magnetization loops (MHLs, m(H)) in a narrow temperature window Tp < Tc, which enables the construction of a phase diagram for a superconducting sample exhibiting the PME. We found that the Nb disks still exhibit the PME after more than 20 years, and we present the efforts of magnetic imaging techniques (scanning SQUID microscopy, magneto-optics, diamond nitrogen-vacancy (NV)-center magnetometry, and low-energy muon spin spectroscopy, (LE-µSR)). Various attempts to explain PME behavior are discussed in detail. In particular, magnetic measurements of mesoscopic Al disks brought out important details employing the models of a giant vortex state and flux compression. Thus, we consider these approaches and demagnetization effects as the base to understand the formation of the paramagnetic signals in most of the materials investigated. New developments and novel directions for further experimental and theoretical analysis are also outlined

Magnetoresistance and Structural Characterization of Electrospun La1−xSrxMnO3 Nanowire Networks

Nanowire network fabrics of La1−xSrxMnO3 (LSMO) with different doping levels x = 0.2, 0.3, and 0.4 were fabricated by means of electrospinning. The resulting nanowires are up to 100 μm long with a mean diameter of about 230 nm. The nanowires form a nonwoven fabric-like arrangement, allowing to attach electric contacts for magnetoresistance (MR) measurements. The resistance in applied magnetic fields and the MR effect were measured in the temperature range 2 K < T < 300 K in magnetic fields up to 10 T applied perpendicular to the sample surface. An MR ratio of about 70% is obtained for x = 0.2 at 10 T applied field and T = 20 Kr. The highest low-field MR of 5.2% (0.1 T) is obtained for the sample with x = 0.2. Magnetization measurements reveal the soft magnetic character of the samples. A thorough analysis of the microstructure of these nanowire networks is performed including scanning electron microscopy (SEM) and transmission electron microscopy (TEM)

Torque magnetometry of perpendicular anisotropy exchange-spring heterostructures

International audienceThe field-induced magnetic configurations in a [Co/Pd]15 /TbFeCo exchange-spring system with perpendicular magnetic anisotropy are studied using torque magnetometry. The experimental results are compared to a 1D micromagnetic simulation. The good agreement between experiments and simulations allows us to deduce the evolution of the in-depth magnetic configuration as a function of the applied field orientation and amplitude. The chirality transition of the interfacial domain wall developing in the structure can also be determined with this technique

Influence of ion irradiation on switching field and switching field distribution in arrays of Co/Pd-based bit pattern media

International audienceWe have used ion irradiation to tune switching field and switching field distribution ͑SFD͒ in polycrystalline Co/Pd multilayer-based bit pattern media. Light He + ion irradiation strongly decreases perpendicular magnetic anisotropy amplitude due to Co/Pd interface intermixing, while the granular structure, i.e., the crystalline anisotropy, remains unchanged. In dot arrays, the anisotropy reduction leads to a decrease in coercivity ͑H C ͒ but also to a strong broadening of the normalized SFD/ H C ͑in percentage͒, since the relative impact of misaligned grains is enhanced. Our experiment thus confirms the major role of misorientated grains in SFD of nanodevice arrays. Today a major research effort in magnetism is targeted toward achieving ultrahigh density data storage with nano-scale magnets. Spin-transfer magnetic random access memory ͑spin-RAM͒ and bit patterned media ͑BPM͒ technologies are currently part of the most promising media. The implementation of both of these technologies relies on achieving in-detail physical understanding and control of the magnetization reversal mechanism in each nanoscopic individual bit to ensure reproducibility of the bit properties in order to avoid write errors. Perpendicular magnetic anisotropy ͑PMA͒ materials, such as polycrystalline Co/Pd, Co/Pt, and Co/Ni multilayers, are believed to be promising materials for both spin-RAM and BPM applications. 1–4 Indeed, they have a well defined high amplitude uniaxial anisotropy that provides good thermal stability while offering low critical current in spin-transfer devices 2 and tunable switching fields in BPM.

Reproducible formation of single magnetic bubbles in an array of patterned dots

International audienceThe formation conditions of single magnetic bubbles through in-plane field demagnetizationare investigated in an array of Co/Ni circular dots by magnetic force microscopy andcompared to micromagnetic calculations. We demonstrate high success rates in nucleatingstable bubbles. The efficiency of single bubble formation is found to depend not only on thedot size, material thickness and intrinsic material parameters but also on the bubble nucleationpath. Experimental phase diagrams and micromagnetic calculations highlight the influenceof the starting in-plane field amplitude and dipolar interactions in stabilizing the bubble.The identification of a systematic procedure for controlling nucleation of single bubbles,multidomain states or a uniform state is important from a technological point of view, openinga path toward the realization of reprogrammable magnonic crystals for the control of spinwavepropagation

Development of a preclinical model of donation after circulatory determination of death for translational application

BACKGROUND: Extracorporeal membranous oxygenation is proposed for abdominal organ procurement from donation after circulatory determination of death (DCD). In France, the national Agency of Biomedicine supervises the procurement of kidneys from DCD, specifying the durations of tolerated warm and cold ischemia. However, no study has determined the optimal conditions of this technique. The aim of this work was to develop a preclinical model of DCD using abdominal normothermic oxygenated recirculation (ANOR). In short, our objectives are to characterize the mechanisms involved during ANOR and its impact on abdominal organs. METHODS: We used Large White pigs weighing between 45 and 55 kg. After 30 minutes of potassium-induced cardiac arrest, the descending thoracic aorta was clamped and ANOR set up between the inferior vena cava and the abdominal aorta for 4 hours. Hemodynamic, respiratory and biochemical parameters were collected. Blood gasometry and biochemistry analysis were performed during the ANOR procedure. RESULTS: Six ANOR procedures were performed. The surgical procedure is described and intraoperative parameters and biological data are presented. Pump flow rates were between 2.5 and 3 l/min. Hemodynamic, respiratory, and biochemical objectives were achieved under reproducible conditions. Interestingly, animals remained hemodynamically stable following the targeted protocol. Arterial pH was controlled, and natremia and renal function remained stable 4 hours after the procedure was started. Decreased hemoglobin and serum proteins levels, concomitant with increased lactate dehydrogenase activity, were observed as a consequence of the surgery. The serum potassium level was increased, owing to the extracorporeal circulation circuit. CONCLUSIONS: Our ANOR model is the closest to clinical conditions reported in the literature and will allow the study of the systemic and abdominal organ impact of this technique. The translational relevance of the pig will permit the determination of new biomarkers and protocols to improve DCD donor management