Biblio

The relationship between magnetic domain structures and magnetic properties of Nd-doped Sm(Fe, Cu, Zr, Co)7.5 was investigated. In the preparation process, slow cooling between sintering and solution treatment was employed to promote homogenization of microstructures. The developed magnet achieved a maximum energy product, [BH]m, of 33.8 MGOe and coercivity, Hcb, of 11.2 kOe at 25 °C, respectively. Moreover, B-H line at 150 °C was linear, which means that irreversible demagnetization does not occur even at 150 °C. Temperature coefficients of remanent magnetic flux density, Br, and intrinsic coercivity, Hcj, were 0.035%/K and 0.24%/K, respectively, as usual the conventional Sm-Co magnet. Magnetic domain structures were observed with a Kerr effect microscope with a magnetic field applied from 0 to -20 kOe, and then reverse magnetic domains were generated evenly from grain boundaries. Microstructures referred to as “cell structures” were observed with a scanning transmission electron microscope. Fe and Cu were separated to 2-17 and 1-5 phases, respectively. Moreover, without producing impurity phases, Nd showed the same composition behavior with Sm in a cell structure.

We report on the significant enhancement of perpendicular magnetic anisotropy of Ni/NiO multilayers after mild annealing up to 90 min at 250 °C. Transmission electron microscopy shows that after annealing, a partial crystallization of the initially amorphous NiO layers occurs. This turns out to be the source of the anisotropy enhancement. Magnetic measurements reveal that even multilayers with Ni layers as thick as 7 nm, which in the as-deposited state showed inplane anisotropy with square hysteresis loops, show reduced in-plane remanence after thermal treatment. Hysteresis loops recorded with the field in the normal-to-film-plane direction provide evidence for perpendicular magnetic anisotropy with up and down magnetic domains at remanence. A plot of effective uniaxial magnetic anisotropy constant times individual Ni layer thickness as a function of individual Ni layer thickness shows a large change in the slope of the data attributed to a drastic change of volume anisotropy. Surface anisotropy showed a small decrease because of some layer roughening introduced by annealing.

The relationship between integrated media noise power and linear density in heat-assisted magnetic recording (HAMR) is discussed. A noise plateau for intermediate recording density has been observed in HAMR, similar to that found in perpendicular magnetic recording (PMR). Here, we show, by changing the temperature profile of the heat spot in HAMR, that we can tune the noise plateau regions to different recording densities. The heat spot with sharp temperature gradient favors a plateau at high recording density, while the heat spot with gradual temperature gradient favors a plateau at low recording density. This effect is argued to be a consequence of the competition between transition noise and remanence noise in HAMR.

In this paper, we present a study about the remanence state and coercivity in 1-D chain of cobalt hollow nanospheres, by using micromagnetic simulation. The high coercivity values (Hc is determined in the range of 600-1800 Oe) and the monotonic decrease of remanence are attributed to the shape anisotropy effect due to an increase in the aspect ratio value. The configuration of magnetization in remanence showed the onion state for hollow spheres (HSs) with Re = 15 nm, whereas for Re = 30 nm, appear the curling-vortex (CV) state. Finally for a cluster of chains, constituted by cobalt HSs, with random orientations the CV state is preserved.

Macroscopic hysteresis loops and microscopic magnetic moment distributions have been determined by 3-D model for Co nanowire with various easy axis deviations from applied field. It is found that both the coercivity and the remanence decrease monotonously with the increase of easy axis deviation as well as the maximum magnetic product, indicating the large impact of the easy axis orientation on the magnetic performance. Moreover, the calculated angular distributions and the evolution of magnetic moments have been shown to explain the magnetic reversal process. It is demonstrated that the large demagnetization field in the two ends of the nanowire makes the occurrence of reversal domain nucleation easier, hence the magnetic reversal. In addition, the magnetic reversal was illustrated in terms of the analysis of the energy evolution.

High-temperature magnetic properties of the anisotropic bulk SmCo7/Fe(Co) nanocomposite magnets prepared by multistep deformation have been investigated and compared with the corresponding isotropic nanocomposites. The anisotropic SmCo7/Fe(Co) nanocomposites with a Fe(Co) fraction of 28% exhibit much higher energy products than the corresponding isotropic nanocomposites at both room and high temperatures. These magnets show a small remanence (α = -0.022%/K) and a coercivity (β = -0.25%/K) temperature coefficient which can be comparable to those of the conventional SmCo5 and Sm2Co17 high-temperature magnets. The magnetic properties of these nanocomposites at high temperatures are sensitive to the weight fractions of the Fe(Co) phase. This paper demonstrates that the anisotropic bulk SmCo7/Fe(Co) nanocomposites have better high-temperature magnetic properties than the corresponding isotropic ones.

A Stoner-Wohlfarth-type model is used to demonstrate the effect of the buildup of magnetic charges near the grain boundaries of low anisotropy polycrystalline materials, revealed by measuring the magnetization during positive-field warming after negative-field cooling. The remnant magnetization after negative-field cooling has two different contributions. The temperature-dependent component is modeled as an assembly of particles with thermal relaxation. The temperature-independent component is modeled as an assembly of particles overcoming variable phenomenological energy barriers corresponding to the change in susceptibility when the anisotropy constant changes its sign. The model is applicable to soft-magnetic materials where the buildup of the magnetic charges near the grain boundaries creates demagnetizing fields opposing, and comparable in magnitude to, the anisotropy field. The results of the model are in qualitative agreement with published data revealing the magneto-thermal characteristics of polycrystalline gadolinium.

One method to increase bit density in magnetic memory devices is to use multi-state structures, such as a ferromagnetic nanoring with multiple domain walls (DWs), to encode information. However, there is a competition between decreasing the ring size in order to more densely pack bits and increasing it to make multiple DWs stable. This paper examines the effects of ring geometry, specifically inner and outer diameters (ODs), on the formation of 360° DWs. By sequentially increasing the strength of an applied circular magnetic field, we examine how DWs form under the applied field and whether they remain when the field is returned to zero. We examine the relationships between field strength, number of walls initially formed, and the stability of these walls at zero field for different ring geometries. We demonstrate that there is a lower limit of 200 nm to the ring diameter for the formation of any 360° DWs under an applied field, and that a high number of 360° DWs are stable at remanence only for narrow rings with large ODs.

We demonstrate how a unique nanomodulation within a continuous ferromagnetic film can induce magnetic dipoles at predefined, submicrometer scale locations, which can tune the global magnetic properties of the film due to dipole-dipole interactions. Arrays of tunable magnetic dipoles are generated with in-plane and out-of-plane directions, which can be rotated in-plane within the three-dimensional (3-D) modulated structure of a continuous film. In-plane magnetic dipole rotation enables a methodology to control overall magnetic properties of a ferromagnetic thin film. Formation of magnetic dipoles and their tunability were studied in detail by magnetic force microscopy, high-resolution magnetic measurements, and micromagnetic simulation of a nanomodulated Ni45Fe55 alloy film. A pattern larger than a single magnetic domain would normally form a vortex in the remanent state. However, here the unique 3-D nanostructure prevents vortex formation due to the competition between in-plane and out-of-plane dipole-dipole interaction giving rise to a metastable state. Experimentally, at zero remanence, the magnetization goes through a transformation from a metastable to a stable state, where the dipole-dipole interaction depends on their geometrical arrangement. Thus, the magnetic properties of the continuous film can be varied by the proposed pattern geometry. A detail analytical study of the dipolar energy for the system agrees well with the experimental and simulated results.

Having significant role in the storing, delivering and conversion of the energy, the permanent magnets are key elements in the actual technology. In many applications, the gap between ferrites and rare earths (RE) based sintered permanent magnets is nowadays filled by RE bonded magnets, used in more applications, below their magnetic performances. Therewith, the recent trends in the RE market concerning their scarcity, impose EU to consider alternative magnets (without RE) to fill such gap. The paper presents the chemical synthesis of the exchange coupled SrFe12O19/CoFe2O4 nanocomposites, based on nanoferrites. The appropriate annealing leads to the increasing of the main magnetic characteristics, saturation magnetization MS and intrinsic coercivity Hc, in the range of 49 - 53 emu/g, respectively 126.5 - 306 kA/m. The value reached for the ratio between remanent magnetization and saturation magnetization is higher than 0.5, fact that proved that between the two magnetic phases occurred exchange interaction.

Due to its low cost and availability, magnetic sensors nowadays are often incorporated into security systems to detect or localize threats. This paper, with the help of a correlated pre-published work, describes preliminary steps to ensure reliable results that could help in reducing inaccuracies/ errors in case of considering a security system that detects Magnetic IEDs employing AMR-based magnetic field sensors.

Arrays of nanosized hollow spheres of Ni were studied using micromagnetic simulation by the Object Oriented Micromagnetic Framework. Before all the results, we will present an analysis of the properties for an individual hollow sphere in order to separate the real effects due to the array. The results in this paper are divided into three parts in order to analyze the magnetic behaviors in the static and dynamic regimes. The first part presents calculations for the magnetic field applied parallel to the plane of the array; specifically, we present the magnetization for equilibrium configurations. The obtained magnetization curves show that decreasing the thickness of the shell decreases the coercive field and it is difficult to obtain magnetic saturation. The values of the coercive field obtained in our work are of the same order as reported in experimental studies in the literature. The magnetic response in our study is dominated by the shape effects and we obtained high values for the reduced remanence, Mr/MS = 0.8. In the second part of this paper, we have changed the orientation of the magnetic field and calculated hysteresis curves to study the angular dependence of the coercive field and remanence. In thin shells, we have observed how the moments are oriented tangentially to the spherical surface. For the inversion of the magnetic moments we have observed the formation of vortex and onion modes. In the third part of this paper, we present an analysis for the process of magnetization reversal in the dynamic regime. The analysis showed that inversion occurs in the nonhomogeneous configuration. We could see that self-demagnetizing effects are predominant in the magnetic properties of the array. We could also observe that there are two contributions: one due to the shell as an independent object and the other due to the effects of the array.