E. Brück

Effect of exchange coupling in nanocrystalline CoPt bulk magnets

The effect of the atomic disorder–order transformation on remanence enhancement and coercivity in isotropic CoPt magnets has been studied by isothermal annealing at 675 °C, well below the transformation temperature, for 10–80 min after quenching from 900 °C. The results of x-ray diffraction, alternating-current susceptibility, and magnetization measurements indicate that the strong remanence enhancement effect originates from exchange coupling between the face-centered-tetragonal ordered hard-magnetic precipitated phase and the face-centered-cubic disordered soft-magnetic matrix phase during the initial ordering transformation. The nonuniform distribution of the magnetic anisotropy results in a lower reversibility, unlike the so-called exchange spring magnet.

Remanence enhancement in nanocrystalline CoPt bulk magnets

Abstract The effect of the atomic disorder–order transformation on remanence enhancement and coercivity in isotropic CoPt magnets has been studied by isothermal annealing at 675°C, well below the transformation temperature, for 10–80 min after quenching in ice–water from 900°C. The results of X-ray diffraction, transmission electron microscopy, alternating-current (ac) susceptibility and magnetization measurements indicate that the strong remanence enhancement effect originates from exchange coupling between the face-centered-tetragonal (fct) ordered hard-magnetic precipitated phase and the face-centered-cubic (fcc) disordered soft-magnetic matrix phase during the initial ordering transformation. The non-uniform distribution of the magnetic anisotropy results in a lower reversibility, unlike the so-called exchange spring magnet.

Remanence enhancement and coercivity in two-phase CoPt bulk magnets

We have studied the effect of the atomic disorder–order transformation on remanence enhancement and coercivity in CoPt ingots by isothermal annealing at various temperatures well below the transformation point. The relative amount of the low-temperature hard-magnetic face-centered-tetragonal (fct) phase precipitated in the high-temperature magnetically soft face-centered-cubic (fcc) phase was determined by means of x-ray diffraction. As a function of annealing temperature, the mean grain size and the relative amount of the fct phase increase at the cost of the fcc phase. These changes were followed by means of magnetic measurements. We observed a continuous increase in coercivity with increasing annealing temperature, eventually reaching a maximum. The Kneller–Hawig model was used to explain the occurrence of remanence enhancement and the continuously changing degree of exchange coupling between the magnetically soft and hard phases.

Magnetic properties and microstructure of Fe-Pt based alloys

Summary form only given. The Fe-Pt alloys have a complete solid solubility range at high temperature with a disordered face-centred cubic (fcc) AuCu type structure in which the Fe and Pt atoms are statistically distributed over the crystallographic sites. For alloys around the equiatomic composition, ageing at lower temperatures leads to a transformation to an ordered L1/sub 0/ type face-centred tetragonal (fct) structure with alternating layers of Fe and Pt. This tetragonal phase gives rise to a strong increase of the magnetic hardness of the alloys, which makes them suitable for permanent magnets to be used, for instance, in dentures. We have studied the effect of isothermal annealing on the hard magnetic properties and the microstructure of Fe-Pt based alloys containing small amounts of Nb and Al.

Magnetovolume effects of Y–Fe–Co–Ti intermetallics

Thermal expansion and magnetic properties of the YFe11−xCoxTi compounds have been investigated by various experimental methods. A large magnetovolume effect has been clearly observed below TC for the Fe-rich compounds. When the transition metal concentration changes from Fe to Co rich, the invar-like effect becomes less pronounced and almost disappears close to the pure-Co compound. The dependence of the spontaneous volume magnetostriction on temperature and Co concentration has been estimated from the thermal expansion measurements. It has been found that, for the YFe11−xCoxTi compounds, the largest spontaneous volume magnetostriction (about 2.1%) occurs at x=3, close to the concentration in which a maximum in saturation magnetization was found. The anomalous thermal expansion can be attributed to the volume dependence of the magnetic energy, which is very sensitive to the distance between first-neighbors transition-metal atoms for Fe-rich compounds.

Magnetic properties and magnetic structure of HoTiGe and ErTiGe

Abstract We have studied the crystal structure, magnetic structure and magnetic properties of the tetragonal compounds HoTiGe and ErTiGe by means of neutron diffraction and magnetic measurements. The neutron-diffraction measurements were made in the range from 1.7 K to temperatures above the corresponding magnetic-ordering temperatures ( T N =88 and 41 K, respectively). Both compounds have a simple antiferromagnetic structure with a propagation vector q =(0, 0, 0.5). The Ho moments have a preferred moment direction along the tetragonal c -axis at all temperatures in the magnetically ordered regime, while the direction is perpendicular to the c -axis for the Er moments. This difference in easy moment direction has been discussed in terms of a crystal-field-induced rare-earth-sublattice anisotropy. The magnetic measurements showed that both compounds give rise to metamagnetic transitions in their magnetically ordered states. This field-induced transition is particularly pronounced in HoTiGe where it is accompanied by a strong hysteresis in the lower-temperature range.

Complex conductivity of UNiGe in high magnetic fields

UNiGe exhibits two antiferromagnetic transitions at 42 and 50 K in zero field. Previous transport and magnetic measurements revealed metamagnetic transitions for magnetic fields applied along the b- (17 and 25 T) or c-axis (4 and 10 T). These metamagnetic transitions are accompanied by sharp changes in the magnetoresistance. For the present work, we have performed RF skin depth measurements of UNiGe in applied magnetic fields up to 50 T. This technique is relatively new, as applied to metallic samples but is a useful probe of magnetotransport since the skin depth can be simply related to the magnetoresistance.

Hybridization and pressure effects in UTX compounds

The UTX intermetallic compounds (T=transition metal and X=p-electron element) were found to crystallize mainly in two large groups, the orthorhombic TiNiSi and the hexagonal ZrNiAl structure. For both groups, magnetic-ordering phenomena depend sensitively on the 5f-ligand hybridization. The 5f-ligand hybridization is very sensitive to the interatomic distances, which can be effectively controlled by external pressure. Here, we have summarized existing and new pressure studies on various single-crystalline UTX compounds (T=Co, Ni, Ir;X=Al, Ga, Ge). We performed magnetoresistance studies in magnetic fields up to 18 T under external hydrostatic pressure up to 10 kbar. The observed dependencies of the ordering temperatures and the critical fields are compared with the different contributions of the 5f-ligand hybridization, which were deduced from tight-binding calculations. We find relatively weak pressure dependence in compounds, where 5f electrons are more localized, while substantially large pressure effec...

HIGH-FIELD MAGNETIZATION, LONGITUDINAL AND TRANSVERSE MAGNETORESISTANCE OF UIrGe

UIrGe crystallizes in the orthorhombic TiNiSi structure and undergoes an antiferromagnetic transition around 14.1 K. The low-temperature longitudinal magnetoresistance (I // B) exhibits a pronounced field-induced step at about 13 T (14 T) and a much weaker step at about 17 T (out of field range) for B // b axis (c axis). No transition was seen for B // a. Here, we report on the magnetization results in fields up to 38 T applied along the principle directions. In addition, we present new magnetoresistance results taken in the transverse (I ⊥ B)configuration in fields up to 18 T applied along the c axis. The data show intriguing differences in comparison to those taken in the longitudinal configuration. The results are discussed in terms of field-induced magnetic transitions and/or Fermi-surface changes.

Effect of temperature on hybridization and magnetism in UPdSn and UCuSn

We measured the temperature dependence of the structural parameters and the occurrence of magnetism in UPdSn and UCuSn using neutron diffraction. The data were taken in an effort to understand the role of hybridization effects for the development of the uranium magnetic moment and the occurrence of long-range magnetic order in these two compounds. The shortest U–U distance provides a measure of delocalization due to direct 5f-5f overlap, while the U–Pd (or U–Cu, respectively) and U–Sn distances give a measure of the effects of 5f-ligand hybridization. Using Rietveld refinement of our neutron diffraction data, we determined the shortest interatomic distances for temperatures between 15 K and room temperature. The changes in the interatomic distances cause changes in the hybridization effects, which in turn leads to the formation of a magnetic ground state for both compounds.