Robert Street

Structure and magnetic properties of mechanically alloyed SmxCo1−x

Abstract A study of the phase structure and magnetic properties of mechanically alloyed and heat-treated Sm x Co 1− x with x = 0.13 to 0.24 has been carried out. Remanence enhancement above 0.5 M s ( M s = saturation) was found in samples with x = 0.12 to 0.17 after annealing at 700 °C. A maximum energy product of 16.6 M G Oe was measured for Sm 0.13 Co 0.87 . Coercive forces above 50 kOe were obtained for x = 0.17–0.20 after heat treatment at approximately 800 °C. The highest value of 57 kOe was measured for Sm 0.19 Co 0.81 .

Measurement of magnetic viscosity in a Stoner-Wohlfarth material

Abstract A Stoner-Wohlfarth model has been used to simulate conventional magnetic viscosity experiments. The magnetic viscosity parameter Λ was found to vary depending on the method used for measurement of the irreversible magnetisation. The difference in Λ is found to be related to a parameter η which describes the dependence of reversible magnetisation on irreversible magnetisation. The correct value of Λ at any field has been related to the population of particles which reverse at that field and it is revealed that distributions of volume and orientation can obscure the interpretation of the measured Λ. Thus the measurement of Λ as an indicator of reversal mechanism is questioned.

Magnetic hardening of mechanically alloyed Sm2Fe17Nx

Abstract Samples of Sm2Fe17Nx having the Th2Zn17 structure were prepared using mechanical alloying and subsequent heat treatment. The magnetic behaviour is found to depend significantly on heat treatment temperature. Coercive forces of up to 31.3 kOe at 300 K have been measured from hysteresis loops with maximum applied fields of 50 kOe. At 423 K the coercive force was 17 kOe. The variations of coercive force, magnetization and remanence as functions of maximum applied field are consistent with the view that the magnetization of this material is pinning controlled.

Structure and Magnetic Properties of Ultrafine Fe Powders by Mechanochemical Processing

Ultrafine Fe powders have been synthesized by mechanochemical processing. The kinetics of the formation of metallic Fe by the reduction of FeCl2 during mechanical milling has been studied. The influence of milling conditions and reductant on structure and magnetic properties was investigated. It is shown that the production of uniform nano-sized powders by mechanochemical processing requires a high particle-to-by-product ratio and avoidance of combustion by the use of low energy milling conditions or by the addition of diluents.

Structure and magnetic properties of mechanically alloyed SmxFe100−x nitride

The influence of samarium concentration and nitriding kinetics on the structure of mechanically alloyed SmxFe100−x nitride was studied. The study of the nitriding kinetics confirmed that the nitrogen concentration in these materials cannot be varied continuously. Chemical analysis showed that a stoichiometric nitride Sm2Fe17Nx with x = 2.66 was formed after nitriding. A maximum coercive force of 36.1 kOe was measured in Sm14Fe86 nitride. An energy product of 12.8 MGOe with a coercive force of 29.5 kOe was obtained for Sm13Fe87 nitride. This work shows that further significant improvement in energy product can be achieved only by the development of anisotropic samples.

Mechanically Alloyed Hard Magnetic Materials

Abstract Recent developments in the synthesis of hard magnetic materials by mechanical alloying are reviewed. In rare earth-transition metal compounds the nanocrystalline structure developed by mechanical alloying and heat treatment results in ultra-high coercivity and isotropic behaviour associated with random grain alignment. Mechanically alloyed two phase exchange-coupled materials exhibit remanence enhancement, and the reversible and irreversible magnetisation behaviour of this novel class of permanent magnet is also discussed.

A study of melt-spun SmCo5 ribbons

Abstract Anisotropic SmCo5 ribbons have been prepared by melt-spinning using low wheel velocity. Dendritic structure was observed, with needle-like crystallites having their long axis preferentially aligned in the ribbon plane. Ribbon prepared with a wheel velocity of 6 m s−1 exhibited a remanence of 80 emu g−1 (about 80% of the saturation magnetisation) and a coercive force of 9.6 kOe.

The interpretation of magnetic viscosity

The consideration of a simple model, based on the Stoner and Wohlfarth model of magnetic reversal including time-dependence, allows the measurement of the key viscosity parameter to be related to the micromagnetic changes occurring in the material. An aligned system with a log-normal distribution of volumes has been studied and the measured value of at each field related to the mean volume of the particles which are reversing at that field. It is also shown that, when the system is not aligned, there is no longer a simple relationship between and the physical processes occurring. This is because in this instance is a complex average over the ensemble.

Variation of the magnetic domain structure with reversal field (invited)

A rate dependence of the magnetic domain structure has been observed in a Pt/Co multilayer. The form of the domain structure as a function of the rate of change of magnetization was studied using nanosecond pulsed magnetic fields. At low fields the magnetization pattern consists of a dendritic like growth of the domains. As the magnitude of the pulse field is increased the patterns change to a more circular structure with smooth boundaries. The change in structure can be quantified and is discussed in terms of a field dependence where different behaviors can be clearly identified. The structure observed results from a narrowing of the distribution of energy barriers to reversal as the field is increased.

Evolution of ferromagnetism in LaMnO 3+d

An extensive study has been made of the magnetic properties of LaMnO3+δ with finely tuned hole concentrations, x (=2δ), ranging between 0·08 and 0·15. As x increases in the range studied, the spontaneous magnetisation at low temperature increases from only about 20% to almost 100% of the value for full polarisation of Mn spins, and the coercivity Hc decreases by more than an order of magnitude, e.g. from 4·2 kOe to 120 Oe at 5 K. The magnetic ordering temperature Tc takes a minimum at x = xc ~ 0·12. For x < xc, the inverse susceptibility shows a marked decrease as Tc is approached from above. Measurements of minor hysteresis loops for the sample with x = 0·08 show that magnetisation is reversible with respect to field changes except near Hc. These observations are discussed in terms of a two-phase exchange coupling (TPEC) model in which it is assumed that at low temperature the system separates into two phases, a hole-free antiferromagnetic phase and an optimally doped ferromagnetic phase, and the two phases are coupled through superexchange interactions at the interface.