Li Shandong

Exchange coupled Nd2Fe14B/α-Fe nanocomposite magnets with fine α-Fe grains obtained by low wheel speed spinning

Abstract Exchange coupled Nd2Fe14B/α-Fe nanocomposite magnets have been synthesized by melt-spinning. The effect of wheel speed on the magnetic properties and microstructure has been studied. The results show that a uniform Nd2Fe14B/α-Fe nanocomposite structure with fine α-Fe grains can be developed at an optimum wheel speed of about 12 m/s. After a low temperature annealing, the magnetic properties can reach maximum values: iHc=432.2 kA/m, Jr=1.08 T, (BH)max=115 kJ/m3. With further increasing the wheel speed, the volume fraction of an amorphous phase increases. The microstructure of the ribbons becomes nonuniform after a subsequent high temperature crystallization, and several α-Fe grains abnormally grow to about 100 nm, deteriorating the magnetic properties of the ribbons.

Thermal behaviour and magnetic properties of B-rich NdFeB nanocomposite hard magnetic alloys with partial substitution of Dy for Nd

B-rich NdFeB base alloys with partial substitutions of Dy for Nd ((Nd1-xDyx)4.5Fe77B18.5 (x = 0-0.4)) were chosen to study the influence of Dy on thermal behaviour and magnetic properties. It was found that partial substitution of Dy for Nd results in a dramatic enhancement of coercivity jHc from 183 to 261 kA m-1 and of maximum magnetic product (BH)max from 110.6 to 133.7 kJ m-3. However, the saturation magnetization Ms only decreases about 22.8 kA m-1 for every 0.1 increment of Dy content x. Differential thermal analysis traces show that the crystallization process of the initial amorphous ribbons is a three-peak process. X-ray diffraction and differential thermal analysis indicate that the three peaks correspond to the formation of crystalline Fe3B, Nd2Fe14B, and α-Fe, respectively. With increase of Dy concentration, jHc increases to a maximum value at about x = 0.1, and then reduces gradually. The enhancement of coercivity arises from the increase of the anisotropy field for the hard magnetic phase. The reduction of intrinsic coercivity jHc is attributed to the emergence of α-Fe after annealing at the third peak and the reduction of the fraction of hard magnetic phase due to the Dy addition. In order to obtain the optimum magnetic properties, a special annealing method was used.

Magnetic Properties and Intergranular Action in Bonded Hybrid Magnets

Abstract Magnetic properties and intergranular action in bonded hybrid magnets, based on NdFeB and strontium ferrite powders were investigated. The long-range magnetostatic interaction and short-range exchange coupling interaction existed simultaneously in bonded hybrid magnets, and neither of them could be neglected. Some magnetic property parameters of hybrid magnets could be approximately obtained by adding the hysteresis loops of two magnets pro rata.

Effect of amorphous grain boundaries on the magnetic properties of B-rich nanocomposite permanent magnets

Abstract A typical B-rich Nd–Fe–B permanent magnetic alloy with composition Nd 4.5 Fe 77 B 18.5 has been chosen to study the effect of an amorphous grain boundary on the exchange coupling interaction. Amorphous ribbons have been prepared by melt spinning. After optimized annealing, there is a residual amorphous grain boundary phase in the specimen. This sample exhibits better magnetic properties than the completely crystallized specimen. The remanence, remanence ratio, intrinsic coercivity and energy product of the optimum sample with a residual amorphous phase are 1.32 T, 0.86, 212 kA/m, and 130.3 kJ/m 3 , respectively. X-ray diffraction, Mossbauer spectra and high-resolution TEM observation suggest that the enhancement of the remanence ratio in the magnet with a residual amorphous intergranular layer results from the amorphous grain boundary which is favorable for enhancing the exchange coupling interaction between neighboring grains. The role of the amorphous grain boundary in the exchange coupled magnet has been discussed.

Enhancement of ferromagnetic resonance in Al2O3-doped Co2FeAl Heusler alloy film prepared by oblique sputtering

Large and variable in-plane uniaxial magnetic anisotropy in a nanocrystalline (Co2FeAl)97.8(Al2O3)2.2 soft magnetic thin film is obtained by an oblique sputtering method without being induced by magnetic field or post annealing. The in-plane uniaxial magnetic anisotropy varies from 50 Oe to 180 Oe (1 Oe = 79.5775 Am−1) by adjusting the samples position. As a result, the ferromagnetic resonance frequency of the film increases from 1.9 GHz to 3.75 GHz.

Microwave ferromagnetic properties of as-deposited Co2FeSi Heusler alloy films prepared by oblique sputtering

The Co2FeSi films are deposited on Si (100) substrates by an oblique sputtering method at ambient temperature. It is revealed that the microwave ferromagnetic properties of Co2FeSi films are sensitive to sample position and sputtering power. It is exciting that the as-deposited films without any magnetic annealing exhibit high in-plane uniaxial anisotropy fields in a range of 200 Oe–330 Oe (1 Oe = 79.5775 Am−1), and low coercivities in a range of 5 Oe–28 Oe. As a result, high self-biased ferromagnetic resonance frequency up to 4.75 GHz is achieved in as-deposited oblique sputtered films. These results indicate that Co2FeSi Heusler alloy films are promising in practical applications of RF/microwave devices.

Surface Spins Pinning Effect on the Magnetic Properties in Co3O4Nanocrystallites Covered with Polymer Decomposition Residues

We prepare two kinds of Co3O4 antiferromagnetic nanocrystallite systems with different surface structures: one grain surface is covered by polymer decomposition residues (PDRs) (referred to as CS) and the other is naked (NS). It has been found that the magnetic properties of the CS sample deviate greatly from those of the NS sample. For example, the CS sample exhibits an open loop up to 8 T at 4.2 K, while the two branches of the hysteresis loop for the NS sample superpose together when the field is in excess of 3 T. The average permanent magnetic moments per Co3O4 cell for the CS sample are about three times larger than that of the NS sample. The coercivity and loop shift for the CS sample are enhanced remarkably in comparison with the NS sample, i.e., from 73.8 and 11.0 kA/m for the NS sample to 116.5 and 25.5 kA/m for the CS sample, respectively. The anomalous magnetic properties and their enhancements for the CS sample are attributed to the surface spin pinning effect by PDRs.

Ultrahigh frequency tunability of aperture-coupled microstrip antenna via electric-field tunable BST*

A composite ceramic with nominal composition of 45.0 wt%(Ba0.5Sr0.5)TiO3–55.0 wt%MgO (acronym is BST–MgO) is sintered for fabricating a frequency reconfigurable aperture-coupled microstrip antenna. The calcined BST–MgO composite ceramic exhibits good microwave dielectric properties at X-band with appropriate dielectric constant ɛr around 85, lower dielectric loss tan δ about 0.01, and higher permittivity tunability 14.8% at 8.33 kV/cm. An ultrahigh E-field tunability of working frequency up to 11.0% (i.e., from 9.1 GHz to 10.1 GHz with a large frequency shift of 1000 MHz) at a DC bias field from 0 to 8.33 kV/cm and a considerably large center gain over 7.5 dB are obtained in the designed frequency reconfigurable microstrip antenna. These results demonstrate that BST materials are promising for the frequency reconfigurable antenna.