L. Si

Structure and magnetic characterization of amorphous and crystalline Nd–Fe–Al alloys

Abstract Glass formation has been studied in Nd 60 Fe 30 Al 10 alloy produced by melt-spinning, water quenching and copper mold chill casting. Partially amorphous alloys were obtained by melt-spinning at low wheel speeds of 5 to 15 m/s and by water quenching of a 1-mm diameter rod, while fully amorphous alloys were obtained by melt-spinning at higher wheel speeds of 20 and 30 m/s and chill casting of a 1-mm diameter rod. A high coercivity was observed in the partially amorphous ribbon melt-spun at 5 m/s and water quenched rod, and in the fully amorphous chill cast rod, while low values of coercivity were obtained in fully amorphous ribbons melt-spun at high speeds of 20 and 30 m/s. Crystallization of water quenched and chill cast samples after heat treatment at high temperature resulted in a substantial reduction of the high coercivity. Results of X-ray diffraction indicate that formation of Nd and a ternary Fe–Nd–Al phase with an unknown crystal structure were present after crystallization. TEM results and a magnetic study of the heat treated samples indicate that as long as there is an amorphous phase produced by low cooling rate, the high coercivity remains. The high coercivity of bulk amorphous samples is discussed. The unknown ternary Fe–Nd–Al phase is antiferromagnetic with a Neel temperature at about 260 K.

Mechanism of mechanical crystallization of amorphous Fe–Mo–Si–B alloy

Crystallization processes of an amorphous (Fe0.99, Mo0.01)78Si9B13 alloy induced by mechanical milling and annealing at pressures from 0 to 7.0 GPa were studied. It is found that the milling time needed for the crystallization of the amorphous alloy and its crystallization products are related to the milling intensity. The crystallization products are an α-Fe(Mo, Si) disordered solid solution at a lower milling intensity, while at a higher milling intensity they are α-Fe(Mo, Si), Fe–Si–B, and Fe2B phases. By comparing the mechanical crystallization of the amorphous alloy with its high pressure crystallization, it is suggested that crystallization of the amorphous alloy driven by ball milling results from the simultaneous action of local pressure (4–6 GPa) and local temperature (600–700 K), which are produced by the collision of steel balls. The local pressure decreases the thermodynamic potential barrier of nucleation and increases the diffusion activation energy in the process of mechanical crystallizati...

Copper complex with a magnetic ordering temperature above 400 K

A monohydrated CuII dimer [Cu2(Sala)2(H2O)]n [H2Sala=N-(2-hydroxybenzyl)-L-alanine] exists as a coordination-helical polymer in the solid state. Thermal dehydration at 340–373 K converts this helical polymer to three-dimensional chiral network polymer, [Cu2(Sala)2]n. Both the hydrated and the anhydrous samples exhibit canted antiferromagnetism at room temperature. A compensation temperature was observed at around 20 K. The hydrated sample has a magnetic ordering temperature of 300 K, while the anhydrous structure possessed a magnetic ordering temperature as high as 435 K.

Magnetoresistivity and metamagnetism of the Nd33Fe50Al17 alloy

A ternary phase has been identified in the rare-earth transition metal Nd–Fe–Al system. This phase has a composition close to Nd5(Fe3Al)12 and is antiferromagnetic with a Neel temperature of approximately 260 K. A clear step appears in magnetization curves of the isotropic ribbon at temperatures below 140 K, indicating metamagnetism. Magnetoresistivity (MR) has been observed in this compound. MR increases with decreasing temperature and is estimated to be 7.2% at 4.2 K. This compound exhibits MR of 1% in the paramagnetic state at room temperature.

Hard magnetic properties and magnetocaloric effect in amorphous NdFeAl ribbons

Abstract Structure and magnetic properties of amorphous melt-spun NdFeAl and subsequently crystallized ribbons were investigated. An amorphous phase was formed after quenching by melt spinning with a copper wheel surface speed of 30 m/s. This amorphous phase exhibited hard magnetic behavior at low temperatures with a Curie temperature of 110 K and a coercivity of 1526 kA/m at 4.2 K. A hexagonal phase with the lattice parameters a=5.5111 A and c=8.7448 A was formed in the NdFeAl ribbon after annealing above the crystallization temperature. The magnetic entropy change was calculated directly from isothermal magnetic measurements. The results showed that the amorphous sample had a relatively high magnetocaloric effect, indicating that the amorphous alloy can be considered as a candidate for magnetic refrigeration applications.

Co dependence of Curie temperature in amorphous Fe-Co-Zr-B-Nb alloys with high glass-forming ability

Effects of Co substitution for Fe on the Curie temperature (Tc), glass-forming ability (GFA) and thermal stability of amorphous Fe61−xCoxZr5B30Nb4 (FCZBN) alloys were studied for Co content ranging from 0 to 15 at. %. The Tc shows a sinusoid-like behaviour with increasing Co content, revealing two maxima at 3 and 12.5 at. % Co and a minimum at 7.5 at. % Co. Co content dependences of glass transition (Tg), crystallization (Tx) and reduced glass transition temperatures (Trg) of the amorphous alloys are almost completely opposite to that of the Tc. The Tc decreases with increasing Tg and Trg, but increases with increasing Co content. The Co content dependence of the Tc is suggested to relate to both Co content and high GFA of the amorphous alloys.

Dependence of pulsed-laser deposition parameters on the microstructure and magnetic property of Nd–Fe–B thin films grown at high substrate temperature

We systematically studied the dependencies of substrate temperature, laser fluence, deposition time, ambient gas pressure and laser frequency on the structural and magnetic properties of Nd–Fe–B thin films synthesized by pulsed-laser deposition at high substrate temperature Ts. A coercive force of 2.0–2.4 kOe was obtained on samples with a thickness of 70–90 nm and an average grain size of 50–70 nm grown at Ts of 620–650 °C. We showed a clear trend through statistical analysis that the coercivity decreases with increasing film thickness up to 800 nm due to an increase in the crystalline grain size. Oxidation layer and defects on the grain surface may also have greatly reduced the nucleation field and thus resulted in a low coercivity and low saturation magnetization of the samples. Comparison of the films made by PLD and sputtering under similar conditions has been discussed.

Growth of multi-walled carbon nanotubes on mechanical alloying-derived Al2O3-Ni nanocomposite powder

Mechanical alloying was employed to produce the nanocomposite Al2O3–Ni. It was found that the mechanical alloying of a mixture of NiO and α-Al2O3 generated a highly disordered structure. Reduction under a hydrogen atmosphere led to formation of nano-sized Ni crystallites in the Al2O3 matrix. Relatively high BET specific surface areas indicate that the sub-micron nanocomposite particles have a porous structure. In comparison with the co-precipitated powder, the mechanical alloying-derived powder shows smaller particle/agglomerate size and much higher Ni reducibility. Multi-walled carbon nanotubes with a high production yield were successfully synthesized by using the mechanical alloying-derived nanocomposite as the catalyst.

A structural, magnetic and Mössbauer investigation on melt-spun Nd0.33(Fe0.75Al0.25)0.67 ribbons

A tetragonal phase with a = 9.778 A and c = 11.516 A is formed in the Nd0.33(Fe0.75Al0.25)0.67 alloy after melt spinning and short period annealing at 873 K. The tetragonal phase is probably metastable and transforms slowly into the stable -Nd3Fe7-xAlx phase during heat treatment at 873 K. This phase is antiferromagnetic with a Neel temperature of 260±5 K. Metamagnetism is observed at a temperature of 140 K or below. The magnetic properties have been characterized using a vibrating sample magnetometer and Mossbauer spectroscopy. Magneto-resistivity of up to 7.2% is accompanied by metamagnetism. At room temperature, 1% of the magneto-resistivity is measured in the paramagnetic state.