Strength Of Intergranular Exchange Coupling Research Articles

Microstructure and magnetic properties of multi-main-phase Ce-Fe-B spark plasma sintered magnets by dual alloy method

Abstract Ce-Fe-B spark plasma sintered (SPSed) magnets prepared with different ratio of alloys of Ce-Fe-B and Pr-Nd-Fe-B by dual alloy method are investigated in this paper. As expected, the remanent magnetization Jr, coercivity Hci and maximum energy product (BH)max of SPSed magnets increase obviously with increasing weight percentage of Pr-Nd-Fe-B alloy. The magnetic properties of Jr = 0.71 T, Hci = 915 kA/m, (BH)max = 72 kJ/m3 are obtained for the SPSed magnets with 80 wt% Pr-Nd-Fe-B alloy. There are three Curie temperatures in this type magnet, which implies the coexistence of three hard magnetic phases. The second Curie temperature depends on the Pr-Nd-Fe-B content. With increasing Pr-Nd-Fe-B alloy content, the volume fraction and width of coarse grain zone decrease. It is shown by the microstructure analysis that the rare earth elements diffuse during sintering process resulting in the formation of Pr-Nd-Ce-Fe-B hard magnetic phase. The intergranular exchange coupling strength is enhanced with increasing Pr-Nd-Fe-B content. The present research may be a potential reference for further research and development of this type Ce-containing nanocrystalline permanent magnetic materials.

Special microstructure evolution and enhanced magnetic properties of Ce-Fe-B-based spark plasma sintered magnets with core-shell structure by NdCu addition

Ce-Fe-B-based rare earth permanent magnets are prepared by spark plasma sintering (SPS) technique, and low melting NdCu alloy is added to improve its magnetic properties and thermal stabilities. The X-ray diffraction (XRD) result indicates that the lattice constants of the unit cell of 2:14:1 phase increase with NdCu addition. The remanent magnetization, coercivity and maximum energy product are enhanced remarkably from 0.37 T, 227 kA/m, 16 kJ/m3 for the sintered magnets without NdCu addition to 0.47 T, 476 kA/m, 30 kJ/m3 for those with 20 wt% NdCu addition. The temperature coefficients of remanence (α) and coercivity (β) of the spark plasma sintered magnets (denoted as SPSed magnets thereafter) are improved from −0.46%/K, −0.60%/K to −0.34%/K, −0.55%/K in the range of 300–400 K. There are two Curie temperatures in the Ce-based magnets with NdCu addition, which implies the coexistence of two hard magnetic phases. The second Curie temperature depends on the NdCu content. It is shown by the microstructure analysis that a typical core-shell structure is formed with Ce-rich core and Ce-lean shell by NdCu addition. Furthermore, it is displayed that Nd atoms prefer to diffuse into the Ce-lean flakes. The Nd-Ce exchange mechanism and immigration behavior of elements are finally investigated. The intergranular exchange coupling strength is enhanced in NdCu-added magnet. It is found out that the core-shell structure plays an important role in the special microstructure evolution and the enhancement of the magnetic properties for the typical Ce-Fe-B-based SPSed magnets by NdCu addition.

Magnetic properties of granular CoCrPt:SiO 2 thin films deposited on GaSb nanocones

We report on the effect of microstructure and geometrically induced modifications of the magnetic properties of granular CoCrPt:SiO2 films with weakly interacting magnetic grains deposited on pre-structured GaSb nanocone templates fabricated by an ion erosion technique. By tuning the irradiation conditions, nanocone patterns of different cone sizes were prepared (from 28 to 120 nm in diameter and 32 to 330 nm high, respectively). The influence of the intergranular exchange coupling was also investigated by varying the SiO2 content from 8 to 12 at.%. Deposition of CoCrPt:SiO2 on samples with small nanocones leads to a close magnetic grain packing, which results in the formation of extended magnetic domains larger than the average distance between the GaSb cones. In contrast, on larger nanocones, the magnetic coating grows on the side-walls, with a large separation between neighboring cones, leading to magnetic single-domain regions, which are correlated to the underlying structure. Magnetometry indicates that both remanence and coercivity decrease with increasing cone size and/or SiO2 content due to a combined effect of the angular distribution of the magnetic easy axis of the grains and the intergranular exchange coupling strength.

Study of grain interactions in perpendicular magnetic recording media using first order reversal curve diagrams

It is demonstrated that, for perpendicular magnetic recording (PMR) media, first order reversal curve analysis can independently measure the grain coercive field distribution (or switching field distribution) and the strength of grain interactions, i.e., the demagnetization (mean-field) factor and the dispersion of grain interaction fields around the mean-field. The coercive field distribution is used to determine the intrinsic anisotropy field distribution of PMR media. The temperature dependence of the demagnetization factor shows that the strength of inter-granular exchange coupling is increasing with increasing medium thickness and it is decreasing with increasing temperature.

Grain geometry induced reversal behaviour alteration

According to the experimental investigations, it is reported that the magnetic switching field is varied by the stacking orders in the recording media composed of stacked structure. The lattice distortion due to the lattice mismatch between two layers has been suggested as an origin of the difference; however, it has not been clearly explained up to now. In this paper we studied the possibility of the contribution of the convex surfaces in recording media. The convex surfaces vary the ratio between local magnetic anisotropy and intergranular exchange coupling strength that influences the magnetic behaviours of the stacked granular structure. An analytic model including the local geometric inequivalency is presented. From the analytic equations, it is found that the uniaxial anisotropy of the concave bottom layer can be overcome more easily than other parts, with the assistance of the intergranular exchange. Consequently, the harder bottom layer can be reversed earlier than the softer top layer, under some appropriate conditions. The analytic results are proved by finite element micromagnetic simulations.

Magnetic cluster size and cluster size distribution study on perpendicular media

For granular media, the cluster size is closely related to the grain size and, at the same time, strongly affected by the intergranular exchange coupling strength. We develop the minimum-width detection scheme to analyze the magnetic cluster size and cluster size distribution for perpendicular media. We observed that the median magnetic cluster size strongly depends on the intergranular exchange interaction strength. For the exchange decoupled sample, the magnetic cluster size is close to the grain size of 7.6 nm. With increasing magnitude of the exchange interaction, the cluster size increases to about 19 nm for Hex=10 kOe. Similar to the median cluster size, the standard deviation of the magnetic cluster size also increases with increasing exchange interaction strength. However, the fractional standard deviation of the cluster size, defined as the ratio between the distribution and the median cluster size, shows little dependency on the intergranular exchange interaction strength.

Ｃｏ／Ｐｔ人工格子垂直磁気記録媒体へのＢ添加効果とＮｉＡｌシード層効果

The effect of boron doping on the magnetic and recording performance was examined in [Co (0.4 nm)/Pt (1 nm)]9 superlattice media, and ways of reducing media noise while maintaining a large perpendicular anisotropy Ku were discussed. The boron was doped only in the Co layer. The Ku was found to decrease as the boron content increased beyond 15 vol.%, while maintaining a large value of 5.7×106 erg/cm3 even at 30 vol.% in the Co layer. Moreover, the addition of boron was found to reduce the strength of intergranular exchange coupling, probably owing to the reduction of the exchange stiffness, resulting in an improvement of the signal-to-medium-noise ratio, SNmR. It was also shown that the use of an NiAl seed layer for Co/Pt superlattice media enhances grain segregation, resulting in an improvement of SNmR. However, no significant improvement in SNmR was achieved by the boron doping for [Co (0.4 nm)/Pt (1 nm)]9 media with an NiAl seed layer.

Magnetization reversal and activation volume of perpendicular recording media

Magnetization reversal and thermal agitation of magnetization are quantitatively evaluated using a pulse magnetometer for several kinds of perpendicular magnetic recording media. In media with well-segregated grains, magnetization reversal is confirmed to proceed by coherent switching of magnetization, however, intergranular exchange coupling reduces the remanence coercivity, especially in superlattice media and amorphous/granular composite media. The activation volume in CoCr-based alloy media is experimentally indicated to be nearly the same as that of physical grain volume. It is successfully shown that media noise has a clear relation to both activation diameter and the strength of intergranular exchange coupling. It is likely that the values of activation diameter and the strength of intergranular exchange coupling are main factors to determine the cluster size of magnetization, and they must be reduced to improve the media noise performance.

Ｃｏ（‐Ｂ）／Ｐｄ人工格子垂直磁気記録媒体の磁気異方性と磁化の熱擾乱

Magnetic properties and recording performance are discussed for [Co/Pd (1 nm)]9 and [(Co-27 at% B)/Pd (1 nm)]9 superlattice perpendicular recording media as a function of the Co or Co-B layer thickness, δCo or δCoB. The perpendicular magnetic anisotropy Ku of these media shows maximum values of 5.7 x 106 erg/cm3 (Co/Pd) and 3.2 x 106 erg/cm3 (Co-B/Pd) at δCo or δCoB of around 0.2-0.3 nm. The KuV/kT (V; the activation volume, k; the Boltmann constant, T; the absolute temperature) of the Co/Pd medium has large values of more than 200 (δCo>0.3nm), while the KuV/kT in both media significantly decreases as the thickness decreases, which is not coincident with the thickness dependence of Ku. The value of Ku/2πMs2 (Ms; the saturation magnetization) in both media increase monotonically as the thickness decrease, and is ∼10 at δCo or δCoB of around 0.2-0.3 nm. The signal-to-medium-noise ratio SNmR of the Co-B/Pd media is higher than that of the Co/Pd media, and increase as δCoB decrease. TEM images and magnetic characterization revealed that these differences in SNmR are mainly due to the difference in the strength of intergranular exchange coupling.

Experimental and theoretical analysis of rotational hysteresis loss in CoCrTa perpendicular recording media

We measured the value of rotational hysteresis loss W/sub r/ for various CoCrTa perpendicular media, and discussed the dependence of W/sub r/ and the hysteresis integral R/sub h/ on the perpendicular anisotropy and the strength of intergranular exchange coupling. Simulated results using a model based on the Landau-Lifshitz-Gilbert equation were compared with the experimental results. The experimental and theoretical studies revealed that improvement of the c-axis alignment significantly reduces W/sub r/, resulting in a small R/sub h/ value of /spl sim/0.2. Increase of intergranular exchange coupling shifts the applied field at which W/sub r/ reaches a maximum, H/sub p/, to a lower field and produces a large R/sub h/. Decrease of K/sub u/ also shifts H/sub p/ to a lower field but is accompanied by a reduction of R/sub h/. These results imply that the values of R/sub h/ and H/sub p/ are strongly related to the structural and magnetic properties, and analysis of rotational hysteresis loss appears to be a useful technique to characterize perpendicular media.

Characterization of Magnetization Reversal Mechanism for Magnetic Recording Media by Rotational Hysteresis Loss Analysis

Experimental characterization of magnetic anisotropy and intergranular magnetic coupling for currently developed recording media was carried out with magnetic torque analysis. In thin film media, the magnitude of the magnetic fieled where rotational hysteresis loss vanishes showes an almost constant value in each meterial independent of coercive force Hc. It is concluded that this field nearly corresponds to magentic anisotropy field of grains. Experimental results shows that the increase of Hc in almost all thin film media is mainly caused by the decrease of the strength of intergranular exchange coupling. On the other hand, with decreasing the strength of intergranular coupling, the number of grains where Hc is determined by reversible rotation of magnetization gradually increases. Futhermore, the existence of incoherent magnetization reversal in grains is suggested in the real films especially in CoCrTa media.

Coercivity angular dependence in longitudinal thin film media

Computer simulation study of coercivity out-of-plane angular dependence in thin film longitudinal recording media combined with experimental measurements is presented. A theoretical model developed for studying micromagnetic processes in polycrystalline thin films was utilized for the calculations. Films with crystalline easy axes of the grains randomly oriented in three dimensions as well as in the film plane are investigated. The coercivity angular dependence is analyzed by varying magnetostatic and intergranular exchange interactions broad ranges. The coercivity angular dependence is characterized by two quantities: the angular position of the coercivity maximum and the normalized difference between the maximum coercivity and in-plane coercivity. The former quantity exhibits significantly different behavior with separately varying the magnetostatic interaction strength and intergranular exchange coupling. The difference may allow us to distinguish the two types of interactions and it may allow us to determine the crystalline anisotropy constant and an effective intergranular exchange coupling strength in the film simultaneously from experimental measurements. Specifically designed experiments are carried out to verify the theoretical calculations. Reasonable agreements between the calculations and experimental measurements are obtained.