Remanence Characteristic of Nanostructure of Hard／Soft Magnetic Multilayered Systems

The relation between microscopic properties (e.g., layer thickness, easy axis orientation) and the macroscopic magnetic properties such as remanent magnetization of the ferromagnetic multilayer system is investigated based on a simple micromagnet approach. We concentrate on a multilayer design with periodic boundary condition, where alternating soft/hard layers build a nanostructured multilayer. For any easy axis direction in the soft and hard layers a simple explicit expression of remanence of the system has been derived analytically. We find that the remanence clearly depends on the thickness of the soft magnetic layer and is nearly independent of the thickness of hard magnetic layer. On the other hand, the remanence increases upon reducing the angle enclosed by the saturation magnetization and the easy axis directions of soft magnetic layer. However, it is unsensitive to the easy axis direction of hard magnetic layer, but there exists a maximum remanence for a certain easy axis direction of hard magnetic layer.

Enhanced hydrogen evolution reaction in FePt film with remanence due to decrease in domain walls

As an effective strategy to improve the properties of electrocatalysts,magnetic field-assisted electrocatalytic water splitting has attracted increasing attention recently.However,the corresponding enhancements mostly depend on the exertion of an external magnetic field during electrochemical reactions,which results in a high cost of industrial production,and makes the magnetic field manipulation of electrocatalysis become a challenging task.In this work,instead of the external magnetic field,a bias magnetic field is self-supplied by the remanence state of a ferromagnetic electrocatalyst of FePt.Owing to the assistance of this bias magnetic field,the FePt film in the remanence state shows the overpotential of 229 mV during hydrogen evolution reaction,which is much lower than that in its demagnetization state(283 mV).Our findings demonstrate that the remanence in ferromagnetic electrocatalysts can improve the catalytic performance,which is attributed to the decrease in domain walls.

Simultaneously enhancing coercivity and remanence of hot-deformed Nd-Fe-B magnets by flake copper powder assisted DyF_(3) interflake addition

It is still a challenge to simultaneously enhance coercivity(H_(cj))and remanence(J_(r))of hot-deformed Nd-Fe-B magnet due to the coercivity-remanence trade-off dilemma.Here,we achieved this balance between H_(cj)and Jr by flake Cu powder assisted DyF_(3)interflake addition.The Hcj increases from 1218 to 1496 kA/m and Jr increases from 1.32 to 1.34 T compared with the original magnet.Results show that the width of coarse grain layers reduces because of the introduction of flake Cu,which increases the contact areas of the adjacent grains at ribbon interfaces and suppresses the excessive growth of grains.The stronger degree of texture and higher density compared with the original magnet should take the responsibility for the increase of J_(r).Additionally,the aggregation regions of rare earth rich(RE-rich)phase reduce and the betterment of the microstructure is another reason for the enhancement of Jr in the flake Cu aided DyF_(3)hot-deformed magnet.This strategy of using flake powder additives provides a promising method for optimizing microstructure and enhancing magnetic properties of hot-deformed Nd-Fe-B magnets.

Strategy of preparing SmCo based films with high coercivity and remanence ratio achieved by temperature and chemical optimization

SmCo based films with excellent intrinsic magnetic properties have promising applications in micro-electro-mechanical system(MEMS).However,due to the complexity of phase composition and uncontrollable crystallization degree of SmCo hard magnetic phase in the film,both the coercivity(Hc)and remanence(Mr)of films are difficult to enhance simultaneously.In this paper,SmCo based films were deposited with a Cr underlayer and capping layer on single crystal Si substrates via magnetron sputtering process.The effects of annealing parameters and Sm/Co atomic ratio on the phase structure and coercivity of films are discussed.By adjusting the Sm/Co atomic ratio from 1:5 to 1:4,Co soft magnetic phase disappears and the single phase SmCo5 is obtained,leading to the increase of coercivity of the films from 30 to 34 kOe.The influence of deposition temperature and Cu doping on magnetic properties of SmCo based films was investigated.When the deposition temperature increases from room temperature to 250℃,the coercivity will further increase from 34 to 51 kOe.However,a severe kink is observed in the demagnetization curves due to the poor exchanged coupling.An analysis of transmission electron microscopy(TEM)confirms that the average size of non-hard magnetic amorphous phase exceeds the effective exchanged coupling length of SmCo5,which contributes to the decoupling and low remanence ratio.Therefore,doping Cu and applying a post-annealing process can significantly improve the crystallization degree of the films.Both the coercivity and the remanence ratio of the demagnetization curves are greatly enhanced.We propose a plausible strategy to prepare the SmCo based films with high coercivity and remanence ratio by temperature and chemical optimization,which can be utilized in high performed MEMS devices.

Low remanence temperature coefficient Sm1-xErx(Co,Fe,Cu,Zr)z magnets operating up to 400℃

Er-doped Sm1-xErx(CobalFe0.15Cu0.08Zr0.03)7.8(x=0,0.1,0.2,0.3)magnets with a low remanence temperature coefficient were prepared by powder metallurgy method.The influence of Er content on the remanence and microstructure was investigated.X-ray diffractometer(XRD)analysis showed that the magnets with different Er contents consist of 2:17 R phase and 1:5 H phase.Scanning electron microscopy(SEM)analysis showed that the composition of the matrix is consistent with stoichiometric composition and no obvious precipitated phase appears.With the increase in doped Er amount,the temperature stability of Sm1-xErx(CobalFe0.15Cu0.08Zr0.03)7.8(x=0,0.1,0.2,0.3)is getting better.When x is up to 0.3,the magnets with a low remanence temperature coefficient are obtained and the remanence descends tardily from 0.86 to 0.80 T as the temperature rises from room temperature to 400℃.These results indicate that Er substitution for Sm in SmCobased permanent magnets together with optimal composition and proper heat treatment could achieve a desired magnetic performance combined with high thermal stability.

Security strategy of powered-off SRAM for resisting physical attack to data remanence

This paper presents a security strategy for resisting a physical attack utilizing data remanence in powered- off static random access memory （SRAM）. Based on the mechanism of physical attack to data remanence, the strategy intends to erase data remanence in memory cells once the power supply is removed, which disturbs attackers trying to steal the right information. Novel on-chip secure circuits including secure power supply and erase transistor are integrated into conventional SRAM to realize erase operation. Implemented in 0.25μm Huahong-NEC CMOS technology, an SRAM exploiting the proposed security strategy shows the erase operation is accomplished within 0.2 μs and data remanence is successfully eliminated. Compared with conventional SRAM, the retentive time of data remanence is reduced by 82% while the operation power consumption only increases by 7%.

Remanence acquisition and its alteration on sediments

Previous palaeomagnetic work shows that the mechanism of remanence acquisition on sediments is very complicated. Different magnetizing processes under different conditions may produce some anomalous magnetic signals which have nothing to do with the changes of the geomagnetic field and are difficult to explain.;The depositional remanent magnetization (DRM) may produce an inclination error and a bedding error, while postdepositional remanent magnetization (PDRM) may result in a time delay between the magnetic and sedimentary ages. When there are some different magnetic phases with different lock-in depths in the sediments, it is difficult to judge the depth of the reversal or excursion, or even the artificial magnetic signal may be produced. Even if there is only one magnetic phase in the sediment, the recorded magnetic signals may also be different if the lock-in depth is changed comparative to the lasting period of one reversal or excursion. Demagnetization may also change the primary magnetic records and the changes of water content may result in the displacement of magnetic boundary. The so-called "unstable core", representing subtle changes in mineralogy into and out of a self-reversal region or a viscous magnetization region, gives a complicated palaeomagnetic record with many apparent changes in the palaeomagnetic direction. Moreover, some physical disturbances, such as biodisturbance, deformation, drilling processes and drying effects, may result in the remanence alteration. Some palaeomagnetic investigations demonstrate that good magnetic signals are invariably achieved on homogenous mud or clay sediments, but the worse signals on inhomogeneous profiles, especially on sandy layers.

Unraveling the correlation between the remanence ratio and the dipolar field in magnetic nanoparticles by tuning concentration, moment, and anisotropy

Well-dispersed, uniform cobalt ferrite (CoFe2O4) nanoparticles (NPs) with diameters of 9, 11, 14, and 30 nm were synthesized by thermal decomposition of a metal–organic salt. Multiple variables, including the interparticle distance, moment, and anisotropy, were altered by dilution in a silica matrix and reduction in hydrogen to reveal the intrinsic correlation between the ratio of remanence to saturation magnetization (Mr/Ms) and interparticle dipolar interactions, the strength of which was estimated by the maximum dipolar field Hdip. To date, this correlation has not been systematically investigated experimentally. To prevent the particles from agglomerating, the reduction was performed after dilution. The results revealed that the correlation between Mr/Msand Hdiproughly followed Mr/Ms∝ 1/lgHdipindependent of the size, distance, moment, and anisotropy of the magnetic nanoparticles. In particular, the correlation was closer for the nanoparticle systems that had higher concentrations or moments, that is, stronger dipolar interactions. For the single-phase CoFe2O4nanoparticles, deviation from Mr/Ms∝ 1/lgHdipcan be attributed to the effects of surface spin, and for the slightly reduced nanoparticles, this deviation can be attributed to the pinning effect of CoFe2O4on CoFe2. [Figure not available: see fulltext.] © 2016, Tsinghua University Press and Springer-Verlag Berlin Heidelberg.

Measurement of remanent magnetic moment using a torsion pendulum with single frequency modulation method

In Tian Qin spaceborne gravitational-wave detectors, the stringent requirements on the magnetic cleanliness of the test masses demand the high resolution ground-based characterization measurement of their magnetic properties. Here we present a single frequency modulation method based on a torsion pendulum to measure the remanent magnetic moment mr of 1.1 kg dummy copper test mass, and the measurement result is(6.45 ± 0.04(stat) ± 0.07(syst)) × 10^(-8)A · m^(2). The measurement precision of the mr is about 0.9 n A · m^(2), well below the present measurement requirement of Tian Qin. The method is particularly useful for measuring extremely low magnetic properties of the materials for use in the construction of space-borne gravitational wave detection and other precision scientific apparatus.

High repetition granular Co/Pt multilayers with improved perpendicular remanent magnetization for high-density magnetic recording

Thanks to the strong perpendicular magnetic anisotropy(PMA), excellent processing compatibility as well as novel spintronic phenomenon, Co/Pt multilayers have been attracting massive attention and widely used in magnetic storage.However, reversed magnetic domains come into being with the increasing layer repetition ‘N’ to reduce magneto-static energy, resulting in the remarkable diminishment of the remanent magnetization(Mr). As a result, the product of Mrand thickness(i.e., the remanent moment-thickness product, Mrt), a key parameter in magnetic recording for reliable data storing and reading, also decreases dramatically. To overcome this issue, we deposit an ultra-thick granular [Co/Pt]80multilayer with a total thickness of 68 nm on granular SiNxbuffer layer. The Mrt value, Mrto saturation magnetization(Ms) ratio as well as out of plane(OOP) coercivity(Hcoop) are high up to 2.97 memu/cm^(2), 67%, and 1940 Oe(1 Oe = 79.5775 A·m^(-1)),respectively, which is remarkably improved compared with that of continuous [Co/Pt]80multilayers. That is because large amounts of grain boundaries in the granular multilayers can efficiently impede the propagation and expansion of reversed magnetic domains, which is verified by experimental investigations and micromagnetic simulation results. The simulation results also indicate that the value of Mrt, Mr/Msratio, and Hcoopcan be further improved through optimizing the granule size, which can be experimentally realized by manipulating the process parameter of SiNxbuffer layer. This work provides an alternative solution for achieving high Mrt value in ultra-thick Co/Pt multilayers, which is of unneglectable potential in applications of high-density magnetic recording.

Anisotropy of Stable Single Domain Ferrimagnetic Particles in a Rock Sample from Gyroremanent Magnetization and Comparison with Other Anisotropy Methods

The orientation of stable single domain (SSD) ferrimagnetic particles in an igneous rock sample was determined by a sensitive technique utilizing gyroremanent magnetization (GRM). Components of GRM were measured in the sample upon exposure to an alternating field (AF) at various orientations in 3 orthogonal planes. The major components of GRM exhibited a sin(2θ) dependence on AF orientation in the respective perpendicular planes. This was in accordance with theory [1] and contrary to some previously reported experimental results on magnetic recording tape, which produced a distorted sin(2θ) dependence of the GRM [1]. The explanation is likely due to the SSD ferrimagnetic particles in the rock sample being more dispersed (less interacting) compared to the highly interacting SSD particles in the magnetic tape sample of the previous study. The GRM results were consistent with another remanence anisotropy method, anisotropy of isothermal remanent magnetization (AIRM). This method again measures the anisotropy of the remanence carrying ferrimagnetic particles, but the IRM is also acquired by larger multidomain (MD) particles as well as by the SSD particles. The results were also consistent with the visible rock anisotropy (petrofabric), the anisotropy of magnetic susceptibility (AMS), and the shear wave velocity anisotropy. A comparison of all the methods demonstrated that the fine SSD particles, which make up only a small proportion of the rock, were aligned in quite a similar orientation to that of the main rock forming minerals that constituted the bulk of the sample.

Temperature stability and microstructure of ultra-high intrinsic coercivity Nd-Fe-B magnets

The variations of intrinsic coercivity and remanence of sintered Nd-Fe-B magnets with ultra-high intrinsic coercivity were investigated. The results showed that the intrinsic coercivity and remanence declined simultaneously with increasing temperature, but the squareness of the magnets has hardly been changed. The temperature coefficients of remanence （α） and coercivity （β） for the magnets were calculated by two different methods, and the variations of the temperature coefficients and the microstructure of sintered Nd-Fe-B magnets were analyzed. The temperature coefficients of remanence （α） and coercivity （β） for the sintered magnets are very small, and the existence of fine microstructure is necessary to obtain sintered Nd-Fe-B magnets with ultra-high intrinsic coercivity.

Phase Transformation and Magnetic Properties of Nd_xFe_(60.5-x)Pt_(39.5)(x=0, 0.5, 1.0, 1.5) Alloys

The effect of Nd addition on the structure, phase transformation and magnetic properties of FePt based alloys was investigated. The results indicated that the transition temperature from ordered FCT to disordered FCC phase decreased with increasing Nd concentration, but for alloys quenched rapidly from the γ phase region into ice-water, it increased with increasing Nd. The Nd element not only effectively reduced the grain size of the ordered phase but also decreased the degree of the ordered phase and refined the grains of the FCC matrix phase. The remanence ratio and coereivity of the FePt based alloy as a function of the Nd content had maximum values, respectively.

Phase Formation and Magnetic Properties of Nanocomposite Nd-Fe-B Adjusted by Small Amount of Dy and Co

In order to improve and stabilize the magnetic properties of nanocomposite Nd2Fe14B/α-Fe magnetic alloys by a compositional adjustment, small amount of Dy and/or Co was added to Nd9Fe84B7 alloys. DTA analysis on the amorphous of the alloys took place as the soft magnetic phases were crystallized, and then the hard magnetic Nd2Fe14B was precipitated from them. While α-Fe and a metastable 1:7 (TbCu7-type) phase were formed simultaneously in Dy and Co-free alloys, they were crystallized separately at different temperatures after Dy or Co was added. This phase separation occurred more clearly in the Dy-treated alloys and the other soft magnetic phase Fe3B was also stabilized by Dy and/or Co. The 1: 7 phase that was stabilized by Dy and/or Co was not eliminated at 700 ℃, decreasing magnetic properties of the alloys. It was eventually disappeared above 725 ℃, but Fe3B was not eliminated even at 750 ℃ when Dy was added more than 0.5 at% or Co was added more than 2.0 at%. Amount of Nd2Fe14B in the alloys tended to increase as Dy addition increased,whereas Co addition did not lead to any appreciable change in the ratio of α-Fe and Nd2Fe14B. Moreover, Dy addition apparently increased coercivity of an alloy while Co addition had a beneficial effect on remanence. The grains in the Dytreated alloys were usually finer than those in the Co-treated alloys. The grain size of both α-Fe and Nd2Fe14B in the alloys exhibiting mr ≥ 0.72 was in the range of 20 ～ 40 nm or even larger 50 nm, which is larger than the theoretical optimum size ( ～ 10 nm). Typical magnetic properties obtained from a Nd7.5Dy1.5Fe82.5Co1.5B7 alloy annealed for 12 min at 725 ℃were iHc=4.85 kOe, Br= 11.32 kG, (BH)max = 15.73 MGOe, and mr=0.73.

Computer Simulation of Effect of Intergrain Exchange Interaction on Magnetic Properties of Nanocomposite Magnets

Effects of the intergrain exchange interaction on magnetic properties of nanocomposite magnets were investigated by using the computer simulation based on the micromagnetic theory. The simulation was carried out under the assumptions that the strength of the intergrain exchange interaction is weaker than that of the intragrain exchange interaction, that inhomogeneous nanostructures result in the distribution of the strength of the intergrain exchange interaction, and that there exists nonmagnetic intergranular phase (NMIP) between grain boundaries. The distribution of the strength of the intergrain exchange interaction was simulated by the lognormal distribution with the standard deviation of σ.The calculations for Nd 2Fe 14B/α-Fe nanocomposite magnets reveal that a suitably weak intergrain exchange interaction and small grain size enable us to improve magnetic properties. It is also found that a Nd 2Fe 14B/α-Fe nanocomposite magnet has a potential of a (BH) max value exceeding 300 kJ·m -3. On the other hand, the calculations for Nd 2Fe 14B/Fe 3B nanocomposite magnets reveal that the distribution of the strength of the intergrain exchange interaction deteriorates magnetic properties significantly. Particularly, this tendency is remarkable, when the grain size L is larger than its optimum value, 11 nm. The existence of nonmagnetic boundary layers accelerats this tendency. At σ=0.2, the calculated demagnetization curve for the model magnet composed of Nd 2Fe 14B(36%)/Fe 3B(54%)/NMIP(10%) (Valume fraction) grains (L=15 nm) agrees with that obtained experimentally for a Nd 2Fe 14B/Fe 3B nanocomposite magnet. These results suggest importance of refinement of grain size, suppression of a nonmagnetic intergranular phase, and preparation of homogeneous nanostructure for superior magnetic properties.

Effects of TbFeCo Underlayers on Magnetic Properties of CoFe_2O_4 Thin Films

Cobalt ferrite thin films were deposited on TbFeCo(10 nm)/Si(100) and Si(100) substrates at a substrate temperature of 350 ℃ by RF magnetron sputtering. The heat treated films were analyzed by Vibrating Sample Magnetometer (VSM) and X-Ray Photoelectron Spectroscopy (XPS). Results showed that all films had high coercivity and perpendicular anisotropy especially for the films deposited on TbFeCo underlayer. TbFeCo underlayers increase the coercivity, magnetization and remanence ratio of CoFe2O4 films, films on TbFeCo underlayer had coercivity and magnetization as high as 832×103 A·m-1 and 450×103 A·m-1, and its romance ratio reaches 0.9, which was related to the Tb3+ diffusion from the underlayer into the film.

Influence of modulated structure on magnetic properties of NdFeB/Co multilayer thin films

The Nd Fe B/Co multilayer films were prepared by magnetron sputtering. After that, the samples were annealed at 600 ℃ for 20 min. The surface morphology, phase structures and magnetic properties of Mo（50 nm）/[Nd Fe B（100 nm）/Co（y）]×10/Mo（50 nm） thin films were researched by AFM, XRD and VSM, respectively. The results show that the films show stronger perpendicular magnetic anisotropy. When the thickness of Co layers is 10 nm, the coercivity Hc⊥ is the maximum, 295 k A/m. However, for y=10-20, the reduced remanence M/Ms of films has increased. When the thickness of Co layers is 20-30 nm, the Nd Fe B/Co multilayer films obtained more superior magnetic properties with M/Ms =0.95.

DEPENDENCE OF MAGNETISM OF Nd-Fe-B SINTERMAGNETS ON ORIENTATION MAGNETIC FIELD

The effect of the orientation magnetic field on the permanent magnetism of Nd-Fe-B sintermagnets is investigated. The results show that the variation law of magnetism with orientation field obviously changes at the critical orientation field and the intensity of the orientation field should he determined by the dimension ratio of the magnet for effectively utilizing its magnetic energy product.

Nanocrystalline exchange-coupled Pr-Fe-B permanent magnets

Nanocomposite Pr2Fe14B/α-Fe permanent magnets were prepared by melt spinning and subsequent crystallizahon of Pr8Fe86B6, amorphous Precursnors. The microstructure is a two-phase nanocomposite of Pr2 Fe14 B and softmagnetic α-Fe with an average size of 30nm. X-ray diffration, Thermomagnetic analysis and TEM analy0sis indicatetha amorphous Pr8Fe86B6, alloy crystallizes through the process of Am→Am→Am'+α-Fe→Pr2Fe23B3+α-Fe-Pr2Fe14B+α-FeThe highest value of remanence (Br), cocreivity (Hci) and maximum energy Product ((BH)max) of the nanocrystallinealloys are 1.10T, 340 kA/m and 110 kJ/m3 respechvely, exhibihng remarkable remanence enhancement. The effect ofannaling temperature and time on the microstructure and magnetic properties was also studied. The resultS show thatappropriate annealing temperature and time are important for obtaining the optimal microstructure and the bestmagnetic properties.

Investigation of Surface-inset Machines with Mixed Grade Magnets Considering Magnet Thickness

This paper presents a mixed grade magnet model for surface-inset machines considering the magnet thickness. In the polar coordinates, on the basis of the Laplace/quasi-Poisson equations and boundary conditions, the constructed matrix equations are solved and the air gap magnetic field in the machine is derived. Taking an 8-pole/12-slot surface-inset motor as an example, through the presented optimization process, the air gap field is optimized considering the magnet thickness, remanence and magnetization angle. In addition, the back-EMF and electromagnetic torque are analytically obtained. The optimized results show that the proposed mixed grade magnet model has larger electromagnetic torque and smaller torque ripple than the conventional one. Finally, the analytical predictions are evaluated by finite element analysis(FEA).