J. Z. Han

Preparation and magnetic properties of MnBi

MnBi with low temperature phase was fabricated by melt-spinning and subsequently annealing. The influence of quenching speeds, compositions and annealing conditions on the formation of low temperature phase MnBi was systematically investigated. It was found the amorphous MnBi ribbons could transform into low temperature phase by heat treatment in a temperature range of 533–593 K. The coercivity of MnBi was greatly improved by porphyrization, and exhibited a positive temperature coefficient. The maximum energy product BHmax of the anisotropic bonded magnet is 7.1 MGOe (56 kJ/m3) and 4.0 MGOe (32 kJ/m3) at room temperature and 400 K.

Structure and exchange bias of Ni50Mn37Sn13 ribbons

Ni50Mn37Sn13 ribbons were produced by the melt-spinning method. Structure and magnetic measurements on the Ni50Mn37Sn13 ribbons indicated that it is ferromagnetic below 340 K and undergoes an austenitic-to-martensitic phase transition just below room temperature. The austenitic phase has the cubic L21 structure, with the excess manganese atoms occupying the 4(b) sites. The martensitic phase has an orthorhombic structure. With increasing applied magnetic field, the martensite start temperature Ms and martensite finish temperature Mf shift to lower temperatures because of the field-induced phase transition. The exchange-bias effect is observed and strongly varied with different cooling fields at low temperature. At 5 K, the exchange-bias field reaches a maximum of 290 Oe when the cooling field increases to 200 Oe, and then reduces sluggishly with further increase of the cooling field because of weakening of exchange coupling between antiferromagnetic and ferromagnetic interfaces.

High frequency electromagnetic properties of interstitial-atom-modified Ce2Fe17NX and its composites

The magnetic and microwave absorption properties of the interstitial atom modified intermetallic compound Ce2Fe17NX have been investigated. The Ce2Fe17NX compound shows a planar anisotropy with saturation magnetization of 1088 kA/m at room temperature. The Ce2Fe17NX paraffin composite with a mass ratio of 1:1 exhibits a permeability of μ′ = 2.7 at low frequency, together with a reflection loss of −26 dB at 6.9 GHz with a thickness of 1.5 mm and −60 dB at 2.2 GHz with a thickness of 4.0 mm. It was found that this composite increases the Snoek limit and exhibits both high working frequency and permeability due to its high saturation magnetization and high ratio of the c-axis anisotropy field to the basal plane anisotropy field. Hence, it is possible that this composite can be used as a high-performance thin layer microwave absorber.

Magnetic properties of the anisotropic MnBi/Sm2Fe17Nx hybrid magnet

In order to improve the magnetic properties of MnBi compound, anisotropic MnBi/Sm2Fe17Nx hybrid magnet was prepared by grinding of high purity MnBi ribbons and Sm2Fe17Nx particles together. The smooth hysteresis loops of the hybrid magnets indicated that the mixture of the hard/hard phase magnetic components was well exchange coupled. As compared to the single MnBi phase magnet, the remanent magnetization and maximum energy product (BH)max of the composited magnets were improved. As an optimized result, the exchange coupled magnet of MnBi/Sm2Fe17Nx = 3/7 yielded both high remanence and coercivity from 250 K to 380 K. A maximum energy product (BH)max of 18 MGOe was achieved at 300 K, and remained 10 MGOe at 380 K, implying the MnBi/Sm2Fe17Nx magnets can be specially utilized in the high temperature environment.

Exchange bias of CoO1−δ/(NiFe,Fe) system with blocking temperature beyond Néel temperature of bulk CoO

The exchange bias effects in pulsed laser deposited CoO1−δ/(NiFe,Fe) bilayers were investigated. An anomalously high blocking temperature (TB) of 325 K was obtained for the as-deposited CoO1−δ/ferromagnet bilayers, which is 33 K higher than the Neel temperature of the bulk CoO (TN = 292 K). The amount of oxygen vacancies δ in the CoO1−δ film can be controlled by a post-annealing treatment in the O2 atmosphere. The TB of the CoO1−δ/ferromagnet bilayers increases with an increasing δ from δ = 0 to δ = 0.16. Our results indicate that the high TB of CoO1−δ/ferromagnet bilayers originates from the enhancement of the magnetic interaction in the CoO1−δ layer owing to O2− vacancies.

Anisotropic magnetoresistance of epitaxial Pr0.5Sr0.5MnO3 film

The magnetic field and temperature dependent anisotropic magnetoresistance (AMR) of the epitaxial grown Pr0.5Sr0.5MnO3 thin films was investigated. It was found that the magnetoresistance exhibited the characteristics of magnetic polaron hopping. A two-fold symmetric AMR occurred in the ferromagnetic region (∼220 K < T < ∼150 K), while a four-fold symmetric AMR appeared under a high magnetic field in the antiferromagnetic orbital ordered region (T < ∼150 K). The angular dependence of the resistance showed a hysteresis effect under magnetic field at low temperature. It is believed that these phenomena are attributed to the spin canting effect, which originates from the melting of orbital ordering under the external magnetic field in the antiferromagnetic region.

The manipulation of magnetic properties by resistive switching effect in CeO2/La0.7(Sr0.1Ca0.9)0.3MnO3 system

The bipolar resistance switching behavior was observed in the epitaxially grown CeO2/La0.7(Sr0.1Ca0.9)0.3MnO3 (CeO2/LSCMO) heterojunctions on SrTiO3 substrate using pulsed laser deposition technology. It was found that the magnetization of CeO2/LSCMO heterojunction varies with the resistance state of the device when the external triggered voltage is higher than the set and reset voltages. The magnetization could be reversibly changed by exerting external set and reset voltages on the junction. The electron tunneling accompanied by a trapping/detrapping process at the interface is likely responsible for the modulation of the magnetization in this insulator/manganite device.

A separation of antiferromagnetic spin motion modes in the training effect of exchange biased Co/CoO film with in-plane anisotropy

The motion of antiferromagnetic interfacial spins is investigated through the temperature evolution of training effect in a Co/CoO film with in-plane biaxial anisotropy. Significant differences in the training effect and its temperature dependence are observed in the magnetic easy axis and hard axis (HA) and ascribed to the different motion modes of antiferromagnetic interfacial spins, the collective spin cluster rotation (CSR) and the single spin reversal (SSR), caused by different magnetization reversal modes of ferromagnetic layer. These motion modes of antiferromagnetic spins are successfully separated using a combination of an exponential function and a classic n−1/2 function. A larger CSR to SSR ratio and a shorter lifetime of CSR found in the HA indicates that the domain rotation in the ferromagnetic layer tends to activate and accelerate a CSR mode in the antiferromagnetic spins.

Thickness induced uniaxial anisotropy and unexpected four-fold symmetry in Co/SiO2/Si films

Co films with thickness ranging from 20 to 160 nm have been fabricated on SiO2/Si substrates by pulsed laser deposition method (PLD). It was found that that the Co crystal tends to have a structure of bulk hcp Co with the increase of the Co film thickness, and the coercivity of the Co film decreases with increasing film thickness due to the change of the grain size. A uniaxial anisotropy was found in Co films with thickness less than 120 nm, while the Co films with thickness more than 120 nm show an unexpected four-fold anisotropy which is ascribed to the existence of two types (directions) of strongly exchange-coupled hcp Co grains.

Magnetic properties of Nd(Fe1-xCox)10.5M1.5 (M=Mo and V) and their nitrides

In this work, alloys of Nd(Fe1-xCox)10.5M1.5 (M=Mo and V) were prepared via arc melting and heat treatment. The nitrides of these alloys were synthesized using a gas-solid state reaction method. The influence of Co substitution for Fe in NdFe10.5Mo1.5 and NdFe10.5V1.5 alloys and their nitrides were investigated. It was found that the lattice parameters a, c, and unit cell volume V decrease with increasing Co content x for Nd(Fe1-xCox)10.5Mo1.5. As compared to their parent alloys, the lattice parameters and unit cells volume increase after nitrogenation, which gives rise to higher Curie temperature, magnetization and magnetocrystalline anisotropy field for nitrides. A small amount of Co substitution for Fe (x≤0.3) can enhance the magnetic properties including Curie temperature, saturation magnetization and magnetocrystalline anisotropy field of the alloys and their nitrides, while higher concentration of Co (x>0.3) will deteriorates these magnetic properties, especially for the nitrides, due to the modific...