RESUMEN
The intrinsic magnetic low-frequency noise (LFN) is of fundamental scientific interest to the study of magnetic tunnel junctions (MTJs). To gain insight into its mechanism, the fluctuation-dissipation theorem, which describes the linear relation between magnetic LFN and magnetic sensitivity product, has been utilized. However, deviation from the linear correlation has been reported in some studies. To understand and effectively control the magnetic LFN, a more elaborate analytical description and further experimental validation are required. In this work, the magnetic LFN contributed from the magnetization fluctuation in the pinned layer of MTJs with various shape anisotropies was investigated. The MTJs with different shape anisotropies, achieved by altering their aspect ratios, possessed distinct demagnetizing factors. Large magnetic noise was correlated with the increase of magnetic phase loss of ferromagnetic layers during magnetization reversal at which magnetization fluctuation was enhanced. Upon increasing the shape anisotropy, a notable reduction of the magnetic phase loss in the antiparallel (AP) state was observed while it exhibited a slight decrease in the parallel (P) state, revealing that the increase of the shape anisotropy caused a more pronounced suppression of the equilibrium magnetization fluctuation in the AP state. These phenomena were computationally validated by constructing a macrospin model to describe the thermally-induced magnetization fluctuation in the pinned layer. This work reveals the physical relation between MTJ shape anisotropy and magnetic LFN. The effect of the shape anisotropy on the magnetic LFN can be extended to other types of in-plane uniaxial anisotropies.
RESUMEN
We report here on the reproducibility of measurements on a second-order gradiometer superconducting quantum interference device magnetometer of two different yttrium iron garnet spheres, both having a diameter of 1 mm: 1) the National Institute of Standards and Technology magnetic moment standard reference material (SRM) and 2) a commercial sample. It has been suggested that rotating the sample rod around its axis can move the sample center toward the center of the second-order gradiometer coil. The observed value of the magnetic moment will be theoretically a minimum when the radial offset is 0, and this value will increase in a "quadratic" manner with the radial offset. When the magnetic moment of the SRM was repeatedly measured as a function of rotation angle φ from 0° to 360°, we observed a sinusoidal variation in the measured values. The radial offset dependence of the observed magnetic moment was experimentally confirmed by the measurements of the commercial sphere placed in a hole in several cylindrical containers, wherein the distance between the center of the hole and the center of the container was r. The r-dependence of the minimum from each φ-dependent measurement series is qualitatively consistent with the theoretical curve. When the φ-dependent measurements for the SRM in a capsule were repeated 12× over 21 months, the relative standard deviation of the minimums improved up to 0.1%. Knowledge of these facts will be necessary for the accurate measurement of the magnetic moment of other sample forms (e.g., powders).
RESUMEN
The magnetoelastic behavior of multiferroic heterostructures-coupling of magnetic anisotropy or domain dynamics to structural deformations-has been intensively studied for developing materials for energy-efficient, spin-based applications. Here, we report on a large, interface-dominated magnetostriction in (Co/Ni)4/Pb(Mg1/3Nb2/3)O3-PbTiO3 multiferroic heterostructures. Ferromagnetic resonance spectroscopy under voltage-induced strains enabled estimation of the saturation magnetostriction as a function of Ni thickness. The volume and the interface components to the saturation magnetostriction are (6.6 ± 0.9) × 10-6 and (-2.2 ± 0.2) × 10-14 m, respectively. Similar to perpendicular magnetic anisotropy in Co/Ni, the large, negative magnetostriction originates from the Co/Ni interfaces. This interfacial functionality delivers an effect over 300% larger than the bulk contribution and can enable low-energy, nanoelectronic devices that combine the tunable magnetic and magnetostrictive properties of Co/Ni multilayers with the ferroelectric properties of Pb(Mg1/3Nb2/3)O3-PbTiO3.
RESUMEN
When a polarized light beam is incident upon the surface of a magnetic material, the reflected light undergoes a polarization rotation1. This magneto-optical Kerr effect (MOKE) has been intensively studied in a variety of ferro- and ferrimagnetic materials because it provides a powerful probe for electronic and magnetic properties2, 3 as well as for various applications including magneto-optical recording4. Recently, there has been a surge of interest in antiferromagnets (AFMs) as prospective spintronic materials for high-density and ultrafast memory devices, owing to their vanishingly small stray field and orders of magnitude faster spin dynamics compared to their ferromagnetic counterparts5-9. In fact, the MOKE has proven useful for the study and application of the antiferromagnetic (AF) state. Although limited to insulators, certain types of AFMs are known to exhibit a large MOKE, as they are weak ferromagnets due to canting of the otherwise collinear spin structure10-14. Here we report the first observation of a large MOKE signal in an AF metal at room temperature. In particular, we find that despite a vanishingly small magnetization of M ~0.002 µB/Mn, the non-collinear AF metal Mn3Sn15 exhibits a large zero-field MOKE with a polar Kerr rotation angle of 20 milli-degrees, comparable to ferromagnetic metals. Our first-principles calculations have clarified that ferroic ordering of magnetic octupoles in the non-collinear Néel state16 may cause a large MOKE even in its fully compensated AF state without spin magnetization. This large MOKE further allows imaging of the magnetic octupole domains and their reversal induced by magnetic field. The observation of a large MOKE in an AF metal should open new avenues for the study of domain dynamics as well as spintronics using AFMs.
RESUMEN
The ability to tune both magnetic and electric properties in magnetoelectric (ME) composite heterostructures is crucial for multiple transduction applications including energy harvesting or magnetic field sensing, or other transduction devices. While large ME coupling achieved through interfacial strain-induced rotation of magnetic anisotropy in magnetostrictive/piezoelectric multiferroic heterostructures has been demonstrated, there are presently certain restrictions for achieving a full control of magnetism in an extensive operational dynamic range, limiting practical realization of this effect. Here, we demonstrate the possibility of generating substantial reversible anisotropy changes through induced interfacial strains driven by applied electric fields in magnetostrictive thin films deposited on (0 1 1)-oriented domain-engineered ternary relaxor ferroelectric single crystals with extended temperature and voltage ranges as compared to binary relaxors. We show, through a combination of angular magnetization and magneto-optical domain imaging measurements, that a 90° in-plane rotation of the magnetic anisotropy and propagation of magnetic domains with low applied electric fields under zero electric field bias are realized. To our knowledge, the present value attained for converse magnetoelectric coupling coefficient is the highest achieved in the linear piezoelectric regime and expected to be stable for a wide temperature range, thus representing a step towards practical ME transduction devices.
RESUMEN
The influence of the magnetization configuration on Kondo effect in magnetic tunnel junction is investigated. In the parallel configuration, an additional resistance contribution (R*) below 40 K exhibits a logarithmic temperature dependence, indicating the presence of Kondo effect. However, in the anti-parallel configuration, the Kondo-effect-associated spin-flip scattering has a nontrivial contribution to the tunneling current, which compensates the reduction of the current directly caused by Kondo scattering, making R* disappear. These results indicate that suppression and restoration of Kondo effect can be experimentally achieved by altering the magnetization configuration, enhancing our understanding of the role of Kondo effect in spin-dependent transport.
RESUMEN
The magnetic properties of 10 nm diameter surfactant-coated cobalt (Co) nanoparticles in 1,2-dichlorobenzene (DCB) are investigated by a series of sequential magnetic moment (m) vs temperature (T) measurements. A rapid rise in magnetic moment around 250 K during warming and an abrupt drop at 234 K during cooling are observed when a nonsaturating external magnetic field is applied. Differential scanning calorimetry (DSC) measurements demonstrate that the rapid rise and abrupt drop in magnetization are associated with the melting and freezing of the solvent. Magnetic measurements of these Co nanoparticles in DCB are also used to probe their aging over a period of 70 days. The saturation magnetic moment of Co nanoparticles in DCB stored in air at room temperature decreases by nearly 40% over 70 days. Transmission electron microscopy (TEM) characterizations are reported to show the time evolution in the size, shape, and crystalline structures of DCB-immersed nanoparticles.
RESUMEN
The magnetocaloric effect is the change in temperature of a material as a result of the alignment of its magnetic spins that occurs on exposure to an external magnetic field. The phenomenon forms the basis for magnetic refrigeration, a concept purported to be more efficient and environmentally friendly than conventional refrigeration systems. In 1997, a 'giant' magnetocaloric effect, between 270 K and 300 K, was reported in Gd5Ge2Si2, demonstrating its potential as a near-room-temperature magnetic refrigerant. However, large hysteretic losses (which make magnetic refrigeration less efficient) occur in the same temperature range. Here we report the reduction (by more than 90 per cent) of these hysteretic losses by alloying the compound with a small amount of iron. This has the additional benefit of shifting the magnetic entropy change peak (a measure of the refrigerator's optimal operating temperature) from 275 K to 305 K, and broadening its width. Although the addition of iron does not significantly affect the refrigerant capacity of the material, a greater net capacity is obtained for the iron-containing alloy when the hysteresis losses are accounted for. The iron-containing alloy is thus a much-improved magnetic refrigerant for near-room-temperature applications.
RESUMEN
While disease management has been described as an important strategy for the care of patients with congestive heart failure (CHF) in the managed care setting, little is known about the impact of this approach on overall health-related quality of life. In this study the Short Form 36 (SF-36) was administered to all patients entering CHF disease management at the time of program entry and at 1 year following entry. Scores on the eight subscales and the two composite scales were calculated and compared before and after. Patients were enrolled from a mixed-model health maintenance organization (HMO) with 34,740 Medicare + Choice enrollees residing in 38 counties in central and northeastern Pennsylvania. Two hundred sixty-eight continuously enrolled patients in an HMO-sponsored CHF disease state management program with completed baseline and follow-up SF-36 surveys were sampled. All patients entered into disease management received primary care based, nurse-directed education about CHF self-management including instruction on etiology of CHF, the importance of medication compliance, home care services if indicated, monitoring weight gain, increased understanding of the warning signs of worsening CHF, and coaching on strategies to contact a physician in a timely manner when CHF worsens. Nurses also facilitated for CHF guidelines among primary care physicians, including the need to obtain a baseline assessment of cardiac function, prescribe angiotensin I-converting enzyme (ACE) inhibitors and beta blockers when appropriate, and initiated appropriate specialist referral. Compared with enrollees who did not complete a pair of SF-36 surveys, the 268 respondents were younger and had a significantly higher rate of cardiac imaging as well as use of ACE inhibitors and beta blocker medications. Analysis of the SF-36 data revealed that three of the eight (Role Physical, General Health Perceptions, and Role Emotional) subscales increased in a statistically significant manner, as did the Mental Health Composite Score. No statistically significant declines in SF-36 scores were observed. Despite limitations to our study, we found disease management for patients with CHF can be associated with significant improvements in quality of life as measured by the SF-36. Compared with nonrespondents, respondents had a higher prevalence of cardiac imaging, ACE inhibitor use, and beta blocker medication use. Our findings are also limited by a lack of a control group with the possibility that the improvements we observed were unrelated to the disease management intervention. However, our findings and success with the use of this tool indicate the SF-36 can be an important part of the ongoing assessment of patients in a disease management program for CHF.
