Sr-ferrite Magnets Research Articles

Preparation and magnetic properties of lanthanum- and cobalt-substituted M-type Sr base sintered magnets

In this work, the influence of the time points of the simultaneous or separate partial substitution of La3+ for Sr2+ and of Co2+ for Fe3+ in Sr ferrite on the magnetic properties of anisotropic M-type Sr ferrite magnets was investigated. It was found that the optimum values of the remanence B r and of the coercivity H ci of Sr ferrite magnet were obtained by La–Co simultaneous substitution before calcination. On the other hand, H ci is increased causing a minor decrease in B r by La separate substitution before calcination and Co separate substitution after calcination. Whereas, La–Co simultaneous substitution after calcination has a deteriorating effect on the B r of M-type Sr ferrite. Optimum magnetic properties and conditions in the experiment will eventually be used as high-end permanent magnets for the high-efficiency motor application in automobiles with B r > 4500 ± 50 Gauss and H ci > 4500 ± 50 Oe.

Investigation on the La Replacement and Little Additive Modification of High-Performance Permanent Magnetic Strontium-Ferrite

In this work, an experiment was carried out to investigate the preparation condition of anisotropic, Fe-deficient, M-type Sr ferrite with optimum magnetic and physical properties by changing experimental parameters, such as the La substitution amount and little additive modification during fine milling process. The compositions of the calcined ferrites were chosen according to the stoichiometry LaxSr1-xFe12-2xO19, where M-type single-phase calcined powder was synthesized with a composition of x = 0.30. The effect of CaCO3, SiO2, and Co3O4 inter-additives on the Sr ferrite was also discussed in order to obtain low-temperature sintered magnets. The magnetic properties of Br = 4608 Gauss, bHc = 3650 Oe, iHc = 3765 Oe, and (BH)max = 5.23 MGOe were obtained for Sr ferrite hard magnets with low cobalt content at 1.7 wt%, which will eventually be used as high-end permanent magnets for the high-efficiency motor application in automobiles with Br > 4600 ± 50 G and iHc > 3600 ± 50 Oe.

Magnetic domain imaging of a very rough fractured surface of Sr ferrite magnet without topographic crosstalk by alternating magnetic force microscopy with a sensitive FeCo-GdOx superparamagnetic tip

We imaged the magnetic domain of an extremely rough surface (with a roughness of ∼1 μm) of the anisotropic Sr ferrite sintered magnet without any topographic crosstalk by alternating magnetic force microscopy (A-MFM) using a sensitive FeCo-GdOx superparamagnetic tip. The magnetic moment of the FeCo-GdOx superparamagnetic tip is driven by an external AC magnetic field applied out of the plane direction to the magnetic sample. The static magnetic field is from the rough fractured ferrite sample parallel to the direction of the external AC magnetic field and is imaged by modulating the magnetic moment of the superparamagnetic tip. By using the frequency demodulation phenomena, A-MFM can extract the magnetic signal without any topography crosstalk versus the conventional MFM method. The intensity and the polarity of the static magnetic field originate from highly rough fractured hard magnetic Sr ferrite samples, and these were successfully detected and identified. This technique with the as-fabricated FeCo-GdOx superparamagnetic tips gives information about the intensity as well as polarity of magnetic fields from the magnetic domain structure of very rough fractured magnetic materials without any topographic crosstalk. This is crucial for the development of high performance hard magnets and magnetic devices.

Active magnetic force microscopy of Sr-ferrite magnet by stimulating magnetization under an AC magnetic field: Direct observation of reversible and irreversible magnetization processes

Understanding the dynamic magnetization process of magnetic materials is crucial to improving their fundamental properties and technological applications. Here, we propose active magnetic force microscopy for observing reversible and irreversible magnetization processes by stimulating magnetization with an AC magnetic field based on alternating magnetic force microscopy with a sensitive superparamagnetic tip. This approach simultaneously measures sample's DC and AC magnetic fields. We used this microscopy approach to an anisotropic Sr-ferrite (SrF) sintered magnet. This is a single domain type magnet where magnetization mainly changes via magnetic rotation. The proposed method can directly observe the reversible and irreversible magnetization processes of SrF and clearly reveal magnetic domain evolution of SrF (without stimulating magnetization—stimulating reversible magnetization—stimulating irreversible magnetization switching) by slowly increasing the amplitude of the external AC magnetic field. This microscopy approach can evaluate magnetic inhomogeneity and explain the local magnetic process within the permanent magnet.

High magnetization Co-GdOx superparamagnetic granular films as magnetic coating materials for high-sensitivity alternating magnetic force microscopy tip

The Coy(GdOx)1−y, CoyAg1−y, (Fe70Co30)y(AlOx)1−y superparamagnetic (SP) granular films were fabricated by magnetron co-sputtering technique and compared for their magnetic properties. The effect of the different matrix (GdOx, Ag, AlOx) is investigated for refining and separating magnetic particles in non-magnetic matrix by calculating the magnetic volume fraction (y), and the results indicate that the GdOx matrix can more effectively refine and separate magnetic particles. The Co0.44(GdOx)0.56 SP film with the maximum Co volume fraction (y = 0.44) approaches the highest magnetization of 608 emu/cm3 at 20 kOe, and initial susceptibility (1.74 × 10−5 H/m) as compared with Co-Ag, Fe70Co30-AlOx films and some systems of other researchers at room temperature (RT). The magnetic particle size and size distribution in these SP films is calculated by fitting with Langevin function and the results of calculation are consistent with the high-resolution transmission electron microscopy (HRTEM) results. To further confirm the SP behavior, the blocking temperature of the Co0.44(GdOx)0.56 film with the maximum Co volume fraction is measured as ∼145 K, which is below RT. Finally, the Co0.33(GdOx)0.67 thin film having a linear magnetization curve is used for the fabrication of the SP tip and further used for the A-MFM measurement on the bulk Sr-Ferrite (SrF) magnet. The fabricated Co0.33(GdOx)0.67 SP tip along with A-MFM shows an excellent result as compared to the state of art FePt hard magnetic tip using conventional MFM, and it can directly detect the intensity and identify the polarity of the stray field originating from a bulk Sr-Ferrite (SrF) permanent magnet without topography crosstalk in ambient atmosphere.

Magnetic property enhancement of cobalt-free M-type strontium hexagonal ferrites by CaCO3 and SiO2 addition

An experiment was performed to investigate whether CaCO3 and SiO2 addition enhances the magnetic properties of cobalt-free M-type Sr ferrites, in which experimental parameters such as additive composition and the CaCO3/SiO2 ratio were changed. The specimens were prepared using conventional ceramic techniques. CaCO3 addition promoted densification and uniform grain growth, resulting in high remanence; however, with the simultaneous addition of CaCO3 and SiO2, coercivity decreased to 3046 Oe in Sr ferrite hard magnets without Co3O4. SiO2 addition suppressed grain growth. The addition of appropriate concentrations of CaCO3 and SiO2 as additives after calcination was shown to be very beneficial for tailoring a dense microstructure with relatively small grains. A CaCO3/SiO2 ratio of approximately 1.8 was found to be optimum, and the magnetic properties of Br = 4.3 kG, iHc = 3.7 kOe, and (BH)max = 4.5 MGOe were obtained for Sr ferrite hard magnets without Co3O4, which are acceptable for motor applications under output power <1 kW.

High Technology Applications of Barium and Strontium Ferrite Magnets

Ceramic magnets as barium ferrite or strontium ferrite have many applications in high technology. One of the reasons is the low cost when compared to competitor materials, as Alnico, MnBi, MnAl or NdFeB. In this study, the advantages and disadvantages of Ba and Sr ferrite magnets are discussed. One clear advantage is that ferrites are already oxides, and do not present the corrosion problems typical of NdFeB and other metallic alloys. As ferrites are oxides, the processing is much easier and cheaper. For example sintering can be done at air, and milling under wet condition. One of the main conclusions is the excellent ratio cost/benefit of ferrites, giving advantage in many applications. Special attention is given for application of ferrites in high efficiency motors.

Magnetostrictions and Magnetic Properties of Nd-Fe-B and SrFe12O19

The magnetostrictions of polycrystalline Nd-Fe-B and Sr-ferrite at different temperatures are reinvestigated using a strain gauge rotating-sample method. It is found that the magnetostriction λs of Nd-Fe-B is +52×10−6, and that of Sr-ferrite is −25 × 10−6 under a magnetic field of 8 T at room temperature. The maximum energy product (BH)max of the Nd-Fe-B magnet is improved when the powders are magnetically aligned perpendicular to the pressing direction, whereas that of the Sr-ferrite magnet is better when the powders are aligned parallel to the pressing direction. These experimental results suggest that the magnetostriction can generate compressive strain anisotropy resulting from the inverse effect of the magnetostriction. Thus, the magnetization of materials with a negative coefficient of magnetostriction are easier to be aligned normal to the stress direction, while for the materials with a positive coefficient of magnetostriction, the magnetization is easier to be aligned along the stress direction. Therefore, the magnetostriction anisotropy can be used to improve the alignment of the magnetic powders as well as the performance of the magnets.

밀 스케일을 사용한 Sr-페라이트의 특성에 미치는 산화제의 영향

We have been studied the effects of oxidant on the properties of Sr-ferrite magnets using mill scale for motor. The small-added (0.5 wt%) NaNO3 oxidant improved significantly the degree of oxidation and the grindability of mill scale, and then highly enhanced the magnetic properties of anisotropic Sr-ferrite sintered magnets; such as the remanent flux density from 3.55 to 3.80 kG, the intrinsic coercivity from 2.75 to 3.22 kOe, and the maximum energy product from 2.90 to 3.45 MGOe.

&lt;I&gt;γ&lt;/I&gt;-Ray Irradiation on Microsized Nd-Fe-B and Sr-Ferrite Magnets at Low Temperature

The gamma-ray irradiation on microsized Nd-Fe-B and Sr-Fe permanent magnet at low temperature and room temperature was investigated. The change of shape and magnetic properties of two kinds of magnet powder before and after irradiation at low temperature was measured. The crystal structure of each permanent magnet powders was analyzed using X-ray powder diffraction (XRD) and the size and shape were characterized by scanning electron microscope (SEM). The changed magnetic properties of magnet such as saturation magnetization (M(s)) and coercivity (H(c)) were measured by VSM.

The Influence of Low Temperature on γ-Ray Irradiated Permanent Magnets

The temperature effect on the magnetic property of gamma-ray irradiated Nd-Fe-B and Sr-Ferrite magnets has been investigated. When the permanent magnets are exposed to gamma-ray, it's magnetic and other related properties are declined with degree of dose. The decreased magnetic property by gamma-ray irradiation at low temperature is similar with the result of magnet at high temperature. The temperature effect on the gamma-ray irradiation at exposed moment is also regarded as one of the important parameters for the reduced magnetic properties. The gamma-irradiation at low temperature was carried out at 195 K, and the changed properties of two kinds of magnets before and after gamma-irradiation were comparatively studied. The increased demagnetization of the magnets were studied by Hall probe. And changed Curie temperature and micro-crystal structure of each permanent magnet by gamma-ray irradiation has been also studied. Moreover the strong and broad single line shape of ESR signal in the resonance magnetic field is attributed to unpaired electron of Fe2+ in the sample by the effect of gamma-ray irradiation.

&lt;I&gt;In Situ&lt;/I&gt; Lorentz Microscopy and Electron Holography of Magnetization Process in Ferrite Magnets

The magnetic domain structures and magnetization processes of anisotropic Ba and Sr ferrite magnets are investigated by Lorentz microscopy and electron holography. By utilizing a sharp magnetic needle made of a sintered Nd-Fe-B magnet in a transmission electron microscope, the magnetization process in ferrite magnets is visualized for the first time by in situ Lorentz microscopy. In the anisotropic Ba ferrite specimen, magnetization reversal occurs suddenly along the grain boundaries over a large volume, which includes the volumes of some grains located around the specimen edges. On the other hand, in the anisotropic Sr ferrite specimen, magnetization reversal is not observed due to the large coercivity of the specimen. During the magnetization process from the demagnetized state in the Sr ferrite specimen, the domain wall inside a grain moves gradually along the grain boundary and then stops at the grain boundary.

Ｌａ‐Ｃｏ置換Ｍ形Ｓｒフェライト磁石の保磁力の温度係数に関する検討

The relationship between the temperature coefficient of coercivity {α(HcJ)} and the ratio Sv to HcJ was investigated for M-type Sr-ferrite substituted by La and Co (Sr1-xLaxFe12-xCoxO19) in the temperature range from 173to 423 K, where Sv and HcJ were the magnetic viscosity constant, which essentially determined the thermal stability, and the coercivity, respectively. The value of x was varied between 0, 0.3, and 0.6. The value of α(HcJ) for x = 0 with Sv/HcJ = 0.038 was 0.35%/K, whereas that for x = 0.3 with Sv/HcJ = 0.034 was 0.1%/K at 298 K. Furthermore, α(HcJ) for x = 0.6 with Sv/HcJ = 0.026 was almost zero at 298 K. The following relation of α(HcJ) to Sv/HcJ was also found to hold in these M-type Sr-ferrite magnets substituted by La and Co, as in the case of Nd-Dy-Fe-Al-B-Sn sintered magnets. α(HcJ) was expressed as {28.5k − (δE/δT)H}/kT ·(Sv/HcJ), where, k, E, T, and H were the Boltzmann constant, the activation energy, the absolute temperature, and the magnetic field, respectively. The equation shows that smallerSv/HcJ values are necessary for ensuring small α(HcJ) values, which are very important for permanent magnets.

Magnetic properties of Sr-ferrites synthesized in molten [formula omitted] flux

The Sr-ferrite powders, SrFe 12 O 19 , were synthesized by the molten salt method using ( NaCl + KCl ) mixture. Particle morphology was homogeneous and hexagonal platelet like. Both particle size and thickness increased as the reaction temperature and time increased. The sintering density of Sr-ferrite magnet prepared with powders by the molten salt method showed the maximum value at the sintering temperature of 1200 ∘ C . The magnetic properties of the Sr-ferrite magnet were investigated with various sintering temperatures. The maximum values of remanent magnetization ( σ r , 45 emu/g) and coercivity field ( H c j , 298 kA/m) occurred at the sintering temperatures of 1150– 1200 ∘ C . The Sr-ferrite magnet by a molten salt method showed higher remanent magnetization and coercivity field than those of the Sr-ferrite magnet prepared with the same starting materials by a conventional ceramic process.

Research on La 3+–Co 2+-substituted strontium ferrite magnets for high intrinsic coercive force

M-type strontium ferrites with substitution of Sr 2+ by rare-earth La 3+, and a little amount of Fe 3+ by Co 2+ according to the formula Sr 1− x La x Fe 12− x Co x O 19, are prepared by the ceramic process. Effects of the substituted amount of La 3+ and Co 2+ on structure and magnetic properties of Sr 1− x La x Fe 12− x Co x O 19 compounds have systematically been investigated by X-ray diffraction (XRD), vibrating sample magnetometer (VSM) and B–H hysteresis curve measurements. In our results, the suitable amount of La 3+–Co 2+ substitution may remarkably increase saturation magnetization. Intrinsic coercive force ( H cj) of Sr ferrite magnets is evidently increased without significant decrease in residual flux density ( B r) by La 3+–Co 2+ substitution.

염을 이용한 Sr 페라이트의 분말합성과 그의 자성특성

In this study we prepared the Sr-ferrite powders and magnet by a molten salt method using the (NaCl+KCL) salt mixture. Starting materials of Fe₂O₃ and SrCO₃ were mixed as the molar ratio of 5.70:1, and 0.08 mol% Al₂O₃, 0.10 mol% SiO₂ and 0.12 mol% CaO were added as additives. Sr-ferrite powders synthesized at the reaction temperatures of 800~1200℃ showed the typical M-type hexagonal ferrite phase, and hexagonal plate-like morphology with uniform distribution of 1~3 ㎛ particle size. The bulk density of the sintered Sr-ferrite magnet prepared with powders by the molten salt method showed the maximum density of 4.82 g/㎤ at the sintering temperature of 1200℃. The maxima of remanent flux density (Br, 45 emu/g) and coercive force ( i H c , 3.75 kOe) occurred at the sintering temperatures of 1150℃ and 1200℃.

Ｌａ‐Ｃｏ置換フェライト磁石の開発

The effect of the simultaneous partial substitution of Co2+ for Fe3+ and of La3+ for Sr2+ ion in Sr ferrite on the magnetic properties of anisotropic Sr ferrite magnets was investigated. It was found that the coercive force iHc of Sr ferrite magnets is increased without significant decrease in residual flux density Br by La-Co substitution. The temperature coefficient of coercive force was found to be also improved by La-Co substitution. The residual orbital magnetic moment of Co2+ ion is considered to play a main role to increase the coercive force of La-Co substituted Sr ferrite magnets.Based on these results, the new grade (9B) Sr ferrite magnets were developed, whose typical magnetic properties are Br=440mT (4400 G), iHc=350kA/m (4400Oe).

Room temperature scanning Hall probe microscopy of localized magnetic field fluctuations on the surfaces of magnetic recording media, permanent magnets and crystalline garnet films in external bias fields

A sub-micron room temperature scanning Hall probe microscope (RT-SHPM) was used for real-time imaging of surface magnetic domains of floppy disks, Sr ferrite magnets and Bi-substituted iron garnets placed in large external bias fields. Domain wall nucleation was observed in the garnets where bubble lattices expanded, collapsed and transformed into stripe domains in cyclic bias fields. Evolution of RT-SHPM images was compared with conventional vibrating sample magnetometer measurements.

磁性材料 放電プラズマ焼結法による六方晶フェライトの作製

Sr-ferrite sintered magnets were prepared by the spark plasma sintering method. This method brought the densification of Sr-ferrite at relatively low processing temperature and a remarkable short processing time. From the comparison of the magnetic properties of Sr-ferrite magnets produced by spark plasma sintering method with those by the usual ceramics method, a larger magnetization and a larger coercivity were easily and simultaneously obtained in the spark plasma sintering process. It was likely that such improvement in magnetic properties might be attributed to the densification with effective suppression of the ferrite grain growth by the spark plasma sintering method.

Improvements of magnetic properties of Sr ferrite magnets by substitutions of La and Co

The effect of the simultaneous partial substitution of Co/sup 2+/ for Fe/sup 3+/ and of La/sup 3+/ for Sr/sup 2+/ ion in Sr ferrite on the magnetic properties of anisotropic Sr ferrite magnets was investigated. It was found that the coercive force of Sr ferrite magnets is increased without significant decrease in residual flux density by La-Co substitution. Temperature coefficients of coercive force were found to be also improved by La-Co substitution.