H. Sassik

Volume magnetostriction and structure of copper mold-cast polycrystalline Fe-Ga alloys

The effect of the volume expansion on the total linear magnetostriction of Fe85Ga15, Fe80Ga20, and Fe71Ga29 mold-cast bulk alloys was investigated by measuring the change in length with applied field longitudinal and perpendicular to the temperature gradient during solidification. In the magnetically saturated state, due to the contribution of the volume expansion, the saturation magnetostriction for all three alloys was about 66 % of the total linear magnetostriction. The magnetostriction is strongly dependent on the direction of the temperature gradient, being larger in this direction. The substitution of Fe by Ga atoms increases the lattice constant and causes a change to the A2 crystal structure, which was confirmed by Mossbauer spectroscopy. The thermal-expansion coefficient increases with Ga content at temperatures between 4.2 and 150K.

Magnetic and anisotropy studies of Nd-Fe-B based permanent magnets

Abstract Magnetocrystalline anisotropy and coercivity of sintered and rapidly solidified Nd-Fe-B magnets are compared. The coercivity for melt-spun ribbons with optimum cooling rate appears to be dominated by domain wall pinning, while for sintered Nd-Fe-B the coercivity is mainly controlled by nucleation of reverse domains.

Onset of ferromagnetism in U(CoxNi1−x)2

The hexagonal part of the U(CoxNi1−x)2 pseudobinary (0 < x < 0.12) has been studied in detail by means of susceptibility (10–300 K), magnetization (4.2–30 K and fields up to 70 kOe), and electrical resistivity (4.2–300 K) measurements. The system exhibits magnetic ordering at 4.2 K for 0.0 < x < 0.1 with Curie temperature (starting at 21 K for UNi2) and spontaneous magnetization decreasing rapidly with increasing Co content. An estimation of the anisotropy field for ordered samples is presented, analysing the Arrott plots obtained at 4.2 K. Additionally, the susceptibility measurements of the cubic compounds (0.4 < x < 1.0) indicate that no magnetic order occurs within this concentration range. An attempt to explain the obtained magnetic and resistivity data by means of relative position of 5f band at the Fermi level varying with the Co concentration is presented.

Transport phenomena in CeAl2

Abstract CeAl2 was first characterized as a “Kondo compound” on the basis of resistivity measurements which showed a minimum at about 15 K characteristic of the Kondo effect. In addition, the existence of a pronounced crystal field influence was inferred from resistivity, specific heat, and susceptibility data. Since the sign of the effective exchange interaction J is negative in this antiferromagnetic CeAl2 compound, Kondo scattering of the conduction electrons can take place. At low temperatures the 4f moments order magnetically and the internal magnetic field, thus produced, quenches the Kondo scattering. The aim of the paper is to present the temperature dependence of electrical resistivity ϱ, thermopower S, and thermal conductivity λ of CeAl2. These experimental data of CeAl2 will be discussed by comparing them with those of the isostructural nonmagnetic LaAl2 compound.

Onset of magnetism and the crystallographic distortion in U(Fe1−xMnx)2☆

Abstract We report on magnetic, Mossbauer, resistivity, thermal expansion and magnetostriction measurements of the pseudobinary Laves phase system U(Fe 1- x Mn x ) 2 . Both boundary compounds UFe 2 and UMn 2 exhibit below a characteristic temperature T ∗ a rhombohedral and a monoclinic lattice distortion respectively, which is one order of magnitude larger in UMn 2 than in UFe 2 . UFe 2 is ferromagnetic with a Curie temperature of 165 K, while UMn 2 is Pauli paramagnetic. The critical concentration for the onset of magnetism is found to occur at x F ∼0.37. Susceptibility measurements show that Mn carries a paramagnetic moment for concentrations 0⩽ x ⩽0.5. Furthermore the results indicate that in UFe 2 the rhombohedral distortion - causing a lattice contraction - is driven by the magnetostrictive energy, while the positive spontaneous magnetostriction of the undistorted lattice is attributed to the change in the polarization of the 3d band.

Magnetic characterization of soft magnetic materials—experiments and analysis

Abstract Various kinds of magnetic measurement methods suited for soft magnetic materials are critically surveyed. Emphasis was laid on AC-susceptibility methods, the hysteresis loop and the magnetostriction. The analysis of the temperature dependence of the magnetization and the magnetostriction is demonstrated on the amorphous system Fe–Co–B.

Austromag – a new multipurpose quasi-static high-field facility

The new quasi-static high-field installation at the T.U. Wien is described. With a regulated power supply of 10 MW, field pulses up to 40 T with a pulse duration of 1 s can be obtained. Polar and bipolar field pulses are possible. Three working stations were established: (i) a high-temperature magnetometer which allows magnetization measurements between room temperature and 500°C in external fields of 35°T; (ii) a low-temperature system with a maximum field of 40 T which operates between 1.5 and 300 K. Magnetization measurements between 1.5 and 800 K in high external fields are possible. (iii) An additional room temperature test system for developing new measurement methods. The construction of the high temperature set-up is described and an example of an experiment is shown. First magnetostriction measurements on an anisotropic technical barium ferrite are shown. Possibilities to improve the signal-to-noise ratio for magnetization measurements are discussed.

Enhancement of the coercive force with addition of Nb in α-FeSi as-quenched ribbons

Abstract The effect of the addition of Nb in Fe–Si alloys was investigated by measuring the magnetic properties and investigating the microstructure. Nb enhances (about 10 times) the coercivity and changes the microstructure as well as the temperature dependence of the coercivity. There exists indication that Nb enters in the lattice.

Austromag—a new high-field facility

A new quasistatic high-field system with a 10 MW 1 s power supply is described. The current is regulated with two 6-pulse thyristor bridges, which can be switched serial, parallel or antiparallel. For testing a magnet was constructed which generates a maximum field of 25 T in a bore of 30 mm for a plateau time of 200 ms.

Low-temperature hardening of coercivity of amorphous alloys

The temperature dependence of the coercivity was measured in Fe 85-x Co x B 15 and Co 77 B 23 as-cast amorphous ribbons. A fast increase of the coercivity was found for T < 100 K. The temperature dependence of the susceptibility, pinning field, saturation magnetostriction and saturation magnetisation was used to describe the temperature dependence of the coercivity. The hardening of the coercivity for low temperatures (T < 100 K) is well described by short-range pinning mechanism, whereas for high temperatures, magnetostrictive interactions between internal stresses and the domain walls lead to a pinning effect.