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Although the magnetization reversal process in the permanent magnets has long been studied, it has not been fully revealed. The recent progress of the computational science realizes the atomistic calculations for the thermally activated magnetization reversal process in permanent magnets. In contrast, the experimental study on the magnetization reversal has remain to be the macroscopic evaluation. In this study, Nd-Fe-B hot-deformed magnets are micropatterned, and a staircase-like magnetization curve corresponding to the elemental domain-wall depinning event at each grain boundary is successfully observed. Each elemental domain-wall depinning event fluctuates with respect to thermal activation, and the stochastic analysis based on the Neel-Arrhenius model gives the energy barrier parameters H0 and E0 of each depinning event, which are the intrinsic domain-wall depinning field and energy barrier height, respectively. Three types of Nd-Fe-B hot-deformed magnets with different coercivities are adopted for this stochastic analysis. As a result, E0 exhibits very little dependence on H0, and its slope becomes steeper for the lower-coercivity magnet. The stochastic micromagnetics simulation based on the Landau-Lifshitz-Gilbert equation for the two-grain model with various inter-grain exchange coupling reproduces the experimentally observed relationship between E0 and H0. Moreover, the behavior for the lower-coercivity magnet can be explained by assuming the presence of a low magnetic anisotropy layer on the grain surface.

We report a precise measurement of the sensor behavior of the field effect transistor (FET) formed with the MoS2 channel when the channel part is exposed to Cl2 gas. The gas exposure and the electrical measurement of the MoS2 FET were executed with in situ ultrahigh-vacuum (UHV) conditions in which the surface analysis techniques were equipped. This makes it possible to detect how much sensitivity the MoS2 FET can provide and understand the surface properties. With the Cl2 gas exposure to the channel, the plot of the drain current versus the gate voltage (Id–Vg curve) shifts monotonically toward the positive direction of Vg, suggesting that the adsorbate acts as an electron acceptor. The Id–Vg shifts are numerically estimated by measuring the onset of Id (threshold voltage, Vth) and the mobility as a function of the dosing amounts of the Cl2 gas. The behaviors of both the Vth shift and the mobility with the Cl2 dosing amount can be fitted with the Langmuir adsorption kinetics, which is typically seen in the uptake curve of molecule adsorption onto well-defined surfaces. This can be accounted for by a model where an impinging molecule occupies an empty site with a certain probability, and each adsorbate receives a certain amount of negative charge from the MoS2 surface up to the monolayer coverage. The charge transfer makes the Vth shifts. In addition, the mobility is reduced by the enhancement of the Coulomb scattering for the electron flow in the MoS2 channel by the accumulated charge. From the thermal desorption spectroscopy (TDS) measurement and density functional theory (DFT) calculations, we concluded that the adsorbate that is responsible for the change of the FET property is the Cl atom that is dissociated from the Cl2 molecule. The monotonic shift of Vth with the coverage suggests that the MoS2 device sensor has a good sensitivity to detect 10–3 monolayers (ML) of adsorption corresponding to the ppb level sensor with an activation time of 1 s.

A Ga-doped Nd-Fe-B sintered magnet has attracted significant attention as a heavy-rare-earth-free high-performance magnet. We have studied the temperature dependent magnetization reversal process of a Ga-doped Nd-Fe-B sintered magnet based on the first-order reversal curve (FORC) analysis. The FORC diagram pattern of the Ga-doped Nd-Fe-B sintered magnet changes from single spot in the high field region at room temperature to double spots in the low and high field regions at 200 °C, indicating that the dominant magnetization reversal process changes from single domain type to multidomain type. The single domain magnetization reversal at room temperature is well confirmed by using the soft X-ray magnetic circular dichroism microscopy observation. This change in the magnetization reversal process is well discussed by the temperature dependent local demagnetization field and the saturation field of multidomain state. Moreover, we have demonstrated the quantitative analysis of the FORC diagram pattern, which makes a deeper understanding of the magnetization reversal process of the Ga-doped Nd-Fe-B sintered magnet.

First-order reversal curve (FORC) diagram, which visualizes the variation of magnetic susceptibility on a field plane, has been applied to a Nd-Fe-B sintered magnet. The FORC diagram exhibits the characteristic behavior of two remarkable spots in low-field and high-field regions. The high-field FORC spot corresponds to the irreversible magnetization reversal at a coercive field, whereas the low-field FORC spot indicates the appearance of a large magnetic susceptibility state during the demagnetization process. Moreover, this low-field FORC spot becomes dominant at high temperature, accompanied by a significant reduction in coercivity. These results suggest that the low-field FORC spot has a strong correlation with the degradation of magnetic properties of a Nd-Fe-B sintered magnet. To clarify the actual magnetization reversal processes corresponding to these two FORC spots, soft X-ray magnetic circular dichroism (XMCD) microscopy observation was employed with similar field sequences of the FORC measurements. Consequently, the low-field FORC spot is mainly attributed to the domain wall motion in multi-domain grains, whereas the high-field FORC spot corresponds to the magnetization reversal of single-domain grains. These indicate that a FORC diagram is a powerful evaluation method for the magnetization reversal processes of permanent magnets.

There is a growing interest in the development of rare-earth element-free hard magnets. The L10 FeNi phase found in Fe-based meteorites is promising based on its high-magnetocrystalline anisotropy and saturation magnetization. Not only the production, but also the characterization of L10 FeNi phase is challenging due to similar X-ray scattering factors of Fe and Ni. Here, we report on the confirmation of L10 FeNi phase precipitated in a multi-phase FeNiSiBPCu alloy by anomalous X-ray diffraction (AXRD). This is a powerful technique, which can differentiate between ordered and disordered phases along with the elements present in a phase. We measured integrated X-ray intensity with energy near the Fe (~7–7.2 keV) and Ni (8.25–8.4 keV) absorption edges for superlattice and fundamental reflections of L10 FeNi. A drastic change in integrated intensity with energy of X-ray was observed. Increase in intensity around superlattice reflection at Ni-absorption edge clearly confirms the presence of a chemically ordered L10 FeNi phase, but the results obtained at Fe edge are surprising. The analysis of AXRD results suggested that (110) diffraction peak of Fe2B phase overlaps with (001) of L10 FeNi. Understanding gained from AXRD results also allows us to estimate the long-range order parameter (S) from X-ray diffraction measurements.

[Co (0.29 nm)/Ni (0.71 nm)]10 multilayers were deposited on Ta (30 nm) and Pt (30 nm) buffer layers, and the variation of the magnetization dynamics by the adsorption of Fe3O4 nanoparticles was studied by vector network analyzer—ferromagnetic resonance method. The surface roughness of the Co/Ni multilayer was 0.52 nm, and significantly increased after the adsorption of the Fe3O4 nanoparticles which was captured by large field gradient on the demagnetized Co/Ni multilayer. The anisotropy field $H_{k}$ , damping constant $\alpha $ , and anisotropy distribution $\Delta H_{k}$ of the Co/Ni on Ta buffer layer were 2.6 kOe, 0.012, and 0.32 kOe, respectively. By adsorbing of the Fe3O4 nanoparticles, the stray field from the nanoparticles $H_{{\text {np}}}$ was applied to the Co/Ni, and $H_{k} + {H}_{{\text {np}}}$ was estimated to increase to 3.7 kOe. Moreover, $\alpha $ and $\Delta H_{k}$ also increased to 0.020 and 0.42 kOe by the adsorption, respectively. Similarly, $H_{k} + H_{\mathrm{np}}$ , $\alpha $ , $\Delta H_{k}$ of Pt buffered Co/Ni also increased from 2.4 kOe, 0.033, 0.43 kOe to 4.2 kOe, 0.055, 0.53 kOe, respectively. These results suggest that the adsorption of Fe3O4 nanoparticles significantly modifies the magnetization dynamics of Co/Ni multilayer.

First-order reversal curve (FORC) diagram has been previously adopted for the analyses of magnetization reversal process and/or quantitative evaluation of coercivity and interaction field dispersions in various magnetic samples. Although these kinds of information are valuable for permanent magnets, previously reported FORC diagrams of sintered Nd-Fe-B magnets exhibit very complicated patterns. In this paper, we have studied the FORC diagrams of hot-deformed Nd-Fe-B magnets under various conditions. Contrary to the previous reports on sintered Nd-Fe-B magnets, the FORC diagram of the hot-deformed Nd-Fe-B magnet exhibits a very simple pattern consisting of a strong spot and a weak line. From this FORC diagram pattern, it is revealed that the coercivity dispersion of the hot-deformed Nd-Fe-B magnets is surprisingly small. Moreover, this feature of the FORC diagram pattern is very robust and unaffected by changes in various conditions such as grain boundary diffusion process, temperature, and field direction, whereas these conditions significantly change the coercivity and the shape of magnetization curve. This fact indicates that the magnetization reversal process of the hot-deformed Nd-Fe-B magnets is almost unchanged against these conditions.

Understanding of magnetization behavior in magnetic nanostructures is a key issue for future magnetic/spintronic devices. Time-resolved X-ray magnetic circular dichroism (XMCD) is a powerful measurement technique with potential of nanometer spatial resolution, picosecond time resolution, and element selectivity. We have carried out time and spatially resolved XMCD microscopy measurement on a Co/Pt multilayer dot by detecting XMCD at the Pt $L_{3}$ edge at BL39XU of SPring-8 using hard X-ray. Transient magnetic response of Co/Pt multilayer dots with diameter of $2.6~\mu \text{m}$ was investigated against pulsed RF field with frequency around 2.5 GHz and the maximum amplitude of about 400 Oe. By synchronizing excitation RF field pulse with X-ray pulses, the growth of magnetization precession of the induced Pt magnetic moment in the dot was clearly observed with sub-100 ps time resolution and submicrometer spatial resolution. The developed measurement setup can be applicable for a wide range of studies for time-resolved hard X-ray experiments.

Magnetocapacitance (MC) effect, observed in a wide range of materials and devices, such as multiferroic materials and spintronic devices, has received considerable attention due to its interesting physical properties and practical applications. A normal MC effect exhibits a higher capacitance when spins in the electrodes are parallel to each other and a lower capacitance when spins are antiparallel. Here we report an inverse tunnel magnetocapacitance (TMC) effect for the first time in Fe/AlOx/Fe3O4 magnetic tunnel junctions (MTJs). The inverse TMC reaches up to 11.4% at room temperature and the robustness of spin polarization is revealed in the bias dependence of the inverse TMC. Excellent agreement between theory and experiment is achieved for the entire applied frequency range and the wide bipolar bias regions using Debye-Fröhlich model (combined with the Zhang formula and parabolic barrier approximation) and spin-dependent drift-diffusion model. Furthermore, our theoretical calculations predict that the inverse TMC effect could potentially reach 150% in MTJs with a positive and negative spin polarization of 65% and −42%, respectively. These theoretical and experimental findings provide a new insight into both static and dynamic spin-dependent transports. They will open up broader opportunities for device applications, such as magnetic logic circuits and multi-valued memory devices.

Microwave-assisted magnetization switching MAS has been expected as one of the prominent future ultra-high-density magnetic recording technologies. In this paper, we have studied the radio frequency RF field dependence of the switching behavior of Co/Pt nanodot arrays with various dot diameters. While the critical frequency increases and the minimum switching field decreases with increasing the RF field, MAS behavior critically depends on the dot size. For small dots less than 130i¾źnm in diameter, the critical frequency and the minimum switching field exhibit linear variation with the RF field. On the other hand, for the larger dots dot diameter i¾ź 230i¾źnm, they exhibit not only profound MAS effect but also obvious change in the behavior at a certain RF field. These changes of MAS behaviors are explained as a consequence of the change of magnetization precession mode. On the other hand, the dispersions of MAS effect exhibit no remarkable variation on the RF field.

The tunnel magnetocapacitance (TMC) effect in two-dimensional single nanogranular layered Fe / MgF 2 films is investigated both experimentally and theoretically. We measured the frequency dependence of TMC ratios in a frequency range of 20 Hz–1 MHz and discovered that TMC ratios strongly depend on the frequency, reaching a peak value at a specific frequency. We observe that the largest TMC ratios occur at lower frequencies and that TMC values steadily reduce with increasing frequency. Notably, we achieved a maximum TMC ratio of 1.45%, which is the largest low-field TMC ever reported for granular films. A combination of the Debye–Frohlich (DF) model and the Julliere formula is used to fit the experimental data to theory, and an excellent agreement between the calculated values and the experimental data is obtained. To perfectly fit the experimental data, the conventional DF model is extended to a composite model in which three capacitors (with three different relaxation times) are introduced. Our findings will give further insights into the exact mechanism of the TMC effect in nanogranular films and will open broader opportunities for device applications, such as magnetic sensors and impedance tunable devices.

We investigated the magnetization dynamics of Co/Pt multilayer dots with diameters in the micron scale by X-ray magnetic circular dichroism (XMCD) with a focused beam with submicron dimensions. XMCD measurement with soft and hard X-rays revealed that the magnetic moments of Co and Pt strongly couple and precess with the same cone angle in ferromagnetic resonance conditions. The transient magnetization behavior of Co/Pt multilayer dots was investigated by pump–probe XMCD measurement with a scanning focused X-ray beam. A non-uniform magnetization distribution with concentric geometry including the phase of magnetization precession was visualized with excitation frequencies up to 3.05 GHz.

One of the most important achievements in the field of spintronics is the development of magnetic tunnel junctions (MTJs). MTJs exhibit a large tunneling magnetoresistance (TMR). However, TMR is strongly dependent on biasing voltage, generally, decreasing with applying bias. The rapid decay of TMR was a major deficiency of MTJs. Here we report a new phenomenon at room temperature, in which the tunneling magnetocapacitance (TMC) increases with biasing voltage in an MTJ system based on Co40Fe40B20/MgO/Co40Fe40B20. We have observed a maximum TMC value of 102% under appropriate biasing, which is the largest voltage-induced TMC effect ever reported for MTJs. We have found excellent agreement between theory and experiment for the bipolar biasing regions using Debye-Fröhlich model combined with quartic barrier approximation and spin-dependent drift-diffusion model. Based on our calculation, we predict that the voltage-induced TMC ratio could reach 1100% in MTJs with a corresponding TMR value of 604%. Our work has provided a new understanding on the voltage-induced AC spin-dependent transport in MTJs. The results reported here may open a novel pathway for spintronics applications, e.g., non-volatile memories and spin logic circuits.

We investigate the microwave spectroscopies of a micrometer-sized single permalloy (Py) chiral structure on coplanar waveguides (CPWs). Under an external dc magnetic field applied in a direction perpendicular to the microwave propagation, the Py chiral structure loaded on the center of the CPW signal line shows Kittel-mode ferromagnetic resonance. Contrastingly, the structure on the signal-line edge highlights two additional resonances: spin-wave resonance at a higher frequency, and unique resonance at a lower frequency of approximately 7.8 GHz. The resonance signal at 7.8 GHz originates from magnetically induced, geometry-driven resonance, although the resonance frequency does not depend on the external magnetic field. Moreover, the displacement of the Py structures on the signal line results in nonreciprocal microwave transmission, which is traced back to the edge-guide mode.

In this paper, we report that W/Fe/W (001) epitaxial trilayer films exhibit very large negative uniaxial magnetic anisotropy $K_{u}$ . The magnitude of $K_{u}$ increases with decreasing the Fe layer thickness and also exhibits significant temperature dependence varying from $-2.1 \times 10^{7}$ erg/cc at 300 K to $-3.5 \times 10^{7}$ erg/cc at 10 K for the Fe layer thickness of 0.5 nm. The large negative $K_{u}$ is attributable to interface magnetic anisotropy in nature at the W/Fe interface and can be quantitatively reproduced by the firstprinciples calculation. The X-ray magnetic circular dichroism measurement at room temperature has revealed that the interfacial W atom is polarized in antiparallel with the magnetic moment of Fe. The induced magnetic moment of the interfacial W is almost the half of the values obtained by the first-principles calculation and the previous experiment at low temperature. This small induced W moment may be ascribed to the significant temperature dependence of the negative $K_{u}$ .

We examined microwave-assisted switching properties of an antiferromagnetically coupled (AFC) granular perpendicular medium consisting of CoCrPt–TiO2(5)/Co(0.7)/Ru(0.9)/Co(0.7)/CoCrPt–TiO2(3) (in nanometers). In a saturation remanent state, the magnetization of the lower CoCrPt–TiO2 + Co layer switched its direction to form AFC with that of the upper layer because of a thin-film thickness. We assessed the microwave-assistance effect on magnetization switching from the remanent state to the saturation state of the thick upper layer using an anomalous Hall effect. The reduction of a switching field was most enhanced by the application of microwaves of 10 GHz frequency. The switching field reduction by microwave assistance for the AFC medium, obtained by compensating a thermal agitation effect, was estimated as 26%.

In this paper, we review the theory and experiment on microwave-assisted magnetization switching in perpendicular magnetic nanodots. Within the framework of the single-macrospin model, the switching condition under radio frequency fields can be analytically derived from the steady-state solutions of the Landau-Lifshitz-Gilbert equation. When the Gilbert damping is not so large, this analysis perfectly coincides with the Stoner-Wohlfarth model under the rotating frame. This calculation also predicts the critical frequency above which no microwave assistance effect is operative. While the above analysis almost explains the experimental results on nanodots of a Co/Pt multilayer, the nanodots exhibits unusual size-dependent switching behaviors. For the dot as small as 50 nm, the switching behaviors agree with the analytical calculations based on the single-macrospin model. In contrast, for the dot diameter , the significantly enhanced assistance effect is observed. This enhancement for larger dot can be explained by excitation of spatially nonuniform precessional motion in the dots.

Magnetization reversal in a permanent magnet takes place through multiple and simultaneous events of nucleation and domain wall depinning. Thus, detection and analysis of elemental magnetization reversal events are essentially important to understand the coercivity mechanism of a permanent magnet. In this study, we have fabricated a micro-patterned Nd-Fe-B hot-deformed magnet using mechanical polishing and focused ion beam, and anomalous Hall effect (AHE) detection has been adopted to measure the magnetization reversal of the sample. During the micro-patterning process, the degradation of magnetic property is carefully evaluated. Consequently, the micro-patterned Nd-Fe-B hot-deformed magnet with the thick of 5 μm and the width of 13 μm is fabricated, and subsequently, the discrete steps on the AHE curve are successfully detected. The magnetization reversal unit size estimated from the step height is ∼1 μm2, which is almost the same as observed in the magneto-optical Kerr microscopy. We have clearly demonstrated that this technique has significant potential to study the physical nature of elemental magnetization reversal events in permanent magnets.

We have fabricated a device for microwave-assisted switching (MAS) experiments with perpendicularly magnetized CoCrPt-SiO2 granular film with anisotropy field Hk = 18.8 kOe. The device was carefully designed to be able to apply a microwave field of 0.95 kOe in amplitude continuously, allowing direct evaluation of microwave-assisted switching properties of the medium without any assumptions about time-dependent switching behavior. The coercivity of CoCrPt-SiO2 medium linearly decreases with increasing microwave frequency, and the maximum coercivity reduction ratio reaches 80% for hrf = 0.95 kOe at frf = 25 GHz after subtracting the effect of temperature rise due to microwave field application. The linear slope of coercivity against the frequency becomes steeper with increasing microwave field amplitude, which is a unique frequency and amplitude dependencies in granular media. Correlation length obtained by X-ray magnetic circular dichroism microscopy measurement decreases by applying a microwave field with higher frequency, suggesting that the magnetic clustering dimensions of the medium possibly depend on microwave frequency during microwave-assisted switching.

According to previous reports, Co–W alloy films with the hexagonal close-packed (hcp) exhibit high magnetic anisotropy and magnetically induced phase separation due to the miscibility gap formed along the magnetic phase transition, both of which are favorable for high density magnetic recording media. In the present study, the crystal structure, magnetic anisotropy energy (MAE), and their correlation are systematically investigated for Co100−xWx films (x=5, 10, 15) epitaxially grown on MgO (111) single crystals covered with Ru buffer layers by DC magnetron sputtering. The MAE strongly depends on the films thickness as well as the W content x and exceeds that of pure hcp-Co. Comparing with the elaborately analyzed crystal structures, such as lattice parameters (a, c), crystal axis ratios c/a, crystal axis dispersion, volume ratio of the fcc phase to the hcp phase, and deformation/growth stacking fault densities, the MAE is found to be closely correlated with the crystal axis ratio c/a, being qualitatively consistent with the classic single-ion anisotropy model and the recent first principles calculations.

Fe–B–P submicron particle chains and (Ni0.6Zn0.4)Fe2O4 nanoparticles were synthesized by chemical methods. (Ni0.6Zn0.4)Fe2O4 nanoparticles were used to fill in the space among Fe–B–P chains under a magnetic field. The synthetic composite of Fe–B–P chains filled with 20 vol.% (Ni0.6Zn0.4)Fe2O4 nanoparticles exhibits the highest intrinsic permeability(142). It is due to the enhanced performance of Fe–B–P particle chains and the effect of (Ni0.6Zn0.4)Fe2O4 nanoparticles, which both decrease the demagnetization field. The composite film also shows a better power loss feature on coplanar transmission line than pure Fe–B–P chains, and the resonance frequencies of the composite films on coplanar line shift to about 20–30 GHz, which is caused by a large demagnetizing field due to the narrow line width of coplanar line.

We investigate the ferromagnetic resonance (FMR) of a single chiral structure of a ferromagnetic metal—the magnetochiral (MCh) metamolecule. Using a strain-driven self-coiling technique, micrometer-sized MCh metamolecules of metallic permalloy (Py) are fabricated without any residual Py films. The magnetization curves of ten Py MCh metamolecules obtained by an alternating gradient magnetometer show soft magnetic behavior. In cavity FMR with a magnetic-field sweep and coplanar-waveguide (CPW) FMR with a frequency sweep, the Kittel-mode FMR of the single Py metamolecule is observed. The CPW-FMR results, which are consistent with the cavity-FMR results, bring about the effective g factor, effective magnetization, and Gilbert damping of the single metamolecule. Together with calculations using these parameters, the angle-resolved cavity FMR reveals that the magnetization in the Py MCh metamolecule is most likely to be the hollow-bar type of configuration when the external magnetic field is applied parallel to the chiral axis, although the expected magnetization state at remanence is the corkscrew type of configuration.

Microwave assisted magnetic recording (MAMR) is one of the promising candidates for further high density magnetic recording system. In MAMR, precessional motion of magnetization is resonantly excited by rf fields with GHz frequency to switch magnetization in reduced writing fields.

Amorphous CoxFe1-x-B particles with magnetic softness were fabricated by chemical reduction of metal ions. Fabrication conditions to obtain particles with good magnetic softness are described. CoxFe 1-x-B particles with x= 0.90-0.95 exhibit superb softness because of their nearly zero-magnetostrictive property. The soft properties of the particles are thermally stable compared to other compositions such as Fe-B-P or Co50 Fe50-B. Chains of CoxFe1-x-B particles with x=0.95 produced in a magnetic field were studied in detail. The high permeability feature associated with the high aspect ratio of the chain shape exhibits only a slight change after annealing up to 400 degrees. The frequency dispersion of initial permeability exhibits two resonance peaks that are ascribed to the shape anisotropy of the chains.

Gilbert damping constants α of Co/Pd multilayers with various layered structures were estimated by time resolved magneto-optical Kerr effect. The [Co(tCo)/Pd(tPd)]6 multilayers exhibiting a perpendicular magnetic anisotropy (PMA) were prepared by dc magnetron sputtering on a Pd (1 nm)/Ta (30 nm)/oxidized Si substrate. The PMA of the multilayer having tCo/tPd=0.55 increased with decreasing the bilayer period, tCo+tPd, while all the multilayers showed almost the same α of 0.09. The same trend was seen for Co/Ni multilayers with a constant tCo/tNi, although the values of α are different, i.e., 0.035 for Co/Ni . From the estimation of the damping α of the multilayers with various tCo/tPd, the α was confirmed to be enhanced when the ratio tCo/tPd becomes lower than 0.8. Moreover, the linear relationship between the α and tPd/tCo was found. These results suggest that the damping α of both Co/Pd and Co/Ni multilayers are not closely correlated with their PMA, and mainly determined by the composition of the multilayers.

SUMMARY A voltage pulse generator for magnetic field application was built using a coaxial cable as a capacitor. The pulse duration was fixed to 10 ns and the pulse field rise time was controlled from 70 ps to 4 ns using low-pass filters. Magnetization switching experiments were carried out on a Co/Pt multilayer dot with diameter of 300 nm with a perpendicular pulse field. The required pulse field amplitude for magnetization switching is independent of the pulse rise time, suggesting that the thermally activated process is dominant in the perpendicular field geometry. © 2013 Wiley Periodicals, Inc. Electron Comm Jpn, 96(12): 9–14, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ecj.11569

Ni80Fe20 permalloy nanoparticles with narrow size distribution and homogeneous composition have been prepared by the polyol processing at 180 degrees C for 2 h and their particle sizes can be tunable in the size range of 20-440 nm by proper addition of K2PtCI4 agent. X-ray diffraction results show that the NiFe nanoparticles are of face centered cubic structure. The addition of K2PtCl4 does not affect the composition of NiFe NPs but decreases the particle size remarkably. Both saturation magnetization and coercivity of the as-prepared NiFe nanoparticles decrease with decreasing particle size. Annealed at 280 degrees C, however, the saturation magnetization of various sized NiFe nanoparticles increases drastically and approaches to the bulk for the -440 nm NiFe particles, and a maximum coercivity (-270 Oe) happens at a critical size of -50 nm. The magnetic property dependency of these NiFe nanoparticles on annealing has been discussed by considering the surface chemistry.

Amorphous Fe-P-B particles with submicron sizes were synthesized by chemical precipitation in aqueous solutions. The particles having a spherical shape exhibit good magnetic softness and high saturation magnetization. When they are precipitated in a magnetic field, they are connected to each other to form a chain shape. The high aspect ratio of the chains reduces appreciably the demagnetization field associated with isolated particles, resulting in significant improvement of high permeability performance. In this paper, investigations on the external and internal structures of the chains, uniaxial anisotropy caused by aligning the chains in an external magnetic field, frequency dispersion of permeability, and thermal change of the high permeability feature are presented. Finally, comparison of permeability for particles with various sizes is made with the present chain samples. In addition to the superior high permeability feature compared to other magnetically soft particles, the chains exhibit low loss characteristics at high frequencies up to a few GHz. It is also demonstrated that permeability is improved by annealing at 150°C in vacuum. These results suggest that the particle chains have a high potential for high-frequency applications such as inductor cores and electromagnetic noise absorbers.

Co/Ni multilayers with a stack of Ta (2 nm)/[Co (tCo)/Ni (tNi)] N/Ta (30 nm) were prepared by dc magnetron sputtering, and their magnetization dynamics were measured by time-resolved magneto-optical Kerr effect (TRMOKE). The total thickness of the multilayer and perpendicular anisotropy were varied by changing the bilayer period d = tCo+ tNi and number of repeats N while tNi/tCo was kept at a constant of 2.5. The TRMOKE measurements show clear damped oscillation of the magnetization of Co/Ni multilayers after the pump pulse illumination, and the damping constant α of the Co/Ni multilayers was estimated from the TRMOKE waveform. The estimated α was found to be independent both on total thickness and anisotropy field of the multilayer and was estimated to be ~ 0.035 for all the multilayers. This means that the use of Ta capping and buffer layers is effective to evaluate intrinsic damping constant of the Co/Ni multilayer, and that independent control of α and perpendicular anisotropy are possible for the magnetic multilayers.

The microwave-assisted switching properties of a CoCrPt–TiO2 granular perpendicular medium are investigated by detecting the anomalous Hall effect. We improved the device structure to be able to generate a linearly polarized microwave field of 950 Oe without a significant increase in temperature. Coercivity decreases linearly with increasing microwave frequency and the microwave assistance effect is most enhanced by the application of microwaves of 18 GHz frequency with a coercivity reduction ratio of 55%.

A technique based on torque magnetometry is proposed for precise determination of the uniaxial magnetic anisotropy constant, K u, of perpendicularly magnetized thin films, in particular for K u larger than 106 J/m3, by electrical detection of the anomalous Hall effect and exact determination of the magnetic field direction. This technique can measure the K u value of several-nanometer-thick polycrystalline films. Further, the K u value of FePt–C granular films, which are expected to be useful for high-density magnetic recording media, is evaluated. The intrinsic K u value of 4-nm-thick FePt–C thin films fabricated at 450 °C is approximately 3 MJ/m3 for carbon contents up to 34 vol % C.

Magnetically soft amorphous particles were successfully synthesized in aqueous solutions. The advantages of the particles are their high saturation magnetization and submicron size. In addition to the original synthesizing process, particle precipitation in a magnetic field is studied. In the process of precipitation in the magnetic field the particles are bonded into long chains leading to an appreciable reduction of the demagnetizing field that has been an obstacle to the high-permeability performance of soft magnetic particles. The microstructures, magnetic softness of the chains, and significant enhancement of the permeability in the frequency range 0.05∼10 GHz are demonstrated.

Ni80Fe20 permalloy nanoparticles (NPs) have been prepared by the polyol processing at 180 °C for 2 h and their particle sizes can be precisely controlled in the size range of 20–440 nm by proper addition of K2PtCl4 agent. X-ray diffraction results show that the Ni–Fe NPs are of FCC structure, and a homogeneous composition and a narrow size distribution of these NPs have been confirmed by scanning electron microscopy assisted with energy dispersion spectroscopy of X-ray (SEM–EDX). The saturation magnetization of ~440nm NPs is 80.8 emu/g that is comparable to that of bulk Ni80Fe20 alloys, but it decreases to 28.7 emu/g for ~20 nm NPs. The coercive force decreases from 90 to 3 Oe with decreasing NP size. The wide range of particle size is exploited to seek for high permeability composite particles. The planar type samples composed of the NiFe NPs exhibit low initial permeability due to the deteriorated magnetic softness and low packing density. However, when they are mixed with Fe micron particles, the initial permeability significantly increases depending on the mixing ratio and the NiFe NP size. A maximum initial permeability is achieved to be ~9.1 at 1 GHz for the Fe-10 vol%NiFe (~20 nmΦ), which is about three times that of pure Fe micron particles. The effects of Ni–Fe particle size, volume percentage and solvent on the static and dynamic permeability are discussed.

Submicron-size amorphous particles were produced chemically from aqueous solutions. The particles in the as-made state exhibit a high saturation magnetization and magnetic softness almost equivalent to that of much larger amorphous particles produced by water atmizing. Platinum ions work effectively to control the particle size without losing magnetic softness. Magnetic softness observed for the as-made particles becomes degraded slightly by annealing. A calculation of eddy current within spheres predicts no eddy current loss even at 100 MHz for 0.3-mum particles, whereas initial permeability measurements exhibit slight indication of magnetic loss due to the broad spin resonance profile extending from 100 MHz to 10 GHz.

We have investigated the relationship between the crystal structure and the magnetic anisotropy constant of epitaxially grown Fe/Co multilayers (ML). The Fe/Co ML grown on a Ag (100) underlayer at ambient temperature exhibits body centered cubic (bcc) structure with (100) orientation for the stacking period λ less than 15 nm. The saturation field along the film normal is much larger than the demagnetization field 4πMs, indicating negatively large magnetic anisotropy Ku. The negative Ku takes the peak value as large as −1×107 erg/cc at λ = 0.75 nm. This behavior of Ku against λ is very similar to that reported for Fe/Co (110) polycrystalline ML [Vas'ko et al., Appl. Phys. Lett. 89, 092592 (2006)]. Moreover, slight lattice distortion is observed, but the magnetoelastic contribution is much smaller than the negative Ku. These results lead to the conclusion that the negative Ku of Fe/Co ML is attributable to the interfacial magnetic anisotropy with weak crystal orientation dependence.

Fine-particle composites consisting of high-permeability particles with a large size difference exhibit enhanced initial permeability. From an analysis of the initial permeability of a composite that includes Fe (1 micron) particles as a main component, we found that the microscopic demagnetizing field imposed on the particles is appreciably reduced, allowing enhancement of the initial permeability up to the intrinsic permeability of Fe particles. The significance of this effect is exemplified using composites that include Ni50Fe50 (6 microns) and amorphous Fe-Si-B-Cr (3 microns) particles. According to a calculation of the eddy current in spheres with high permeability, reduction of the particle size to less than 1 micron is essential for further improvement of the high-frequency performance of particle composites.

The time dependence of remanence coercivity and thermal stability were investigated for hard/soft-stacked media consisting of a magnetically hard granular layer underneath a very thin soft layer with a large saturation magnetization, M s . The values of remanence coercivity at measurement times t ′=10 3 and 10 −5 s (pulse field) were measured, and defined as H r and H r P . The remanence coercivity on the recording time scale, H r (1 ns), and the energy barrier, Δ E / kT , were evaluated by fitting H r and H r P to Sharrock's equation taking into account the power law variation of the energy barrier, n . The value of H r (1 ns) for a (Co–Pt)–SiO 2 (9 nm)/Co–SiO 2 (2 nm) stacked medium with an interfacial coupling control layer was about 9 kOe, which was less than half of that of a (Co–Pt)–SiO 2 (9 nm) conventional medium (=21.3 kOe). The value of Δ E / kT for the stacked medium was about 111 ( n =0.7), and was not significantly different from the conventional medium. Moreover, no significant difference in the rate of decrease of H r with increasing temperature was observed between media with and without interlayers. These results indicate that the use of a thin soft layer with high M s was effective at significantly reducing H r with no notable change in thermal stability.

A pulse generator with large amplitude of 160 V and fast rise time of less than 0.4 ns has been built using a coaxial cable as a capacitor. By combining with the anomalous Hall effect technique for a single magnetic nanodot, magnetization response of one Co/Pt nanodot of 400 nm in diameter to the pulse was examined. By applying a pulse field with the maximum amplitude of 740 Oe and the duration of 10 ns, magnetization switching of the dot by the pulse was successfully observed. The pulse amplitude dependence of switching field suggested that the pulse field and the dc bias field could be treated as a simple summation of the both fields in the time scale of 10 ns. All the results of magnetization switching by the pulse field could be explained qualitatively by classical thermal relaxation process.