J. G. Booth

Magnetic properties of the FeAl1−xCux system

The substitution of a 3d transition metal (T) for Al in the equiatomic intermetallic compound FeAl results in the onset of ferromagnetism beyond a certain critical concentration of (T). The system appears magnetically and structurally similar to the equivalent Co(GaT) alloys for which the critical concentration appeared to be related to an average electron concentration parameter. The present paper discusses Cu substitution in FeAl, i.e., the series FeAl1−xCux. It was found using x‐ray and neutron diffraction techniques that the alloys remain single phase up to x=0.35 and that the crystallographic order remains B2 throughout. The site occupancy has also been determined and compared with the predictions of theoretical models. Ferromagnetism is observed to occur in the alloys with x>0.2. For these ferromagnetic alloys the inverse susceptibility against temperature curves (for T>TC) all have the same slope independent of composition suggesting an interpretation in terms of an itinerant model. This interpreta...

Spin fluctuations in both the ordered and paramagnetic phases of MnSi! MnSi a heavy Fermi liquid?

Polarised neutrons and polarisation analysis have been used to determine the diffuse magnetic scattering from MnSi as a function of temperature. Measurements at 50 K (1.72Tc) and 100 K (3.45Tc) in the paramagnetic phase confirmed the presence of strong spatial correlations previously reported at 300 K (10Tc) and 580 K (20Tc). The spatial correlations between regions of spin density separated by ∼ 12 Å are of a ferromagnetic nature and determine the static susceptibility. Integration of the paramagnetic response over an inverse atomic volume yields a moment per manganese atom of ∼1.2μB, a value considerably larger than the 0.4 μB observed in Bragg scattering and magnetisation measurements at 4.2 K. Measurements made at 11 K in the ordered magnetic phase reveal strong diffuse scattering corresponding to ∼0.8 μB per Mn atom. The increase in the diffuse scattering observed in the paramagnetic phase corresponds to the passage of the Bragg component into the background.The presence of significant scattering in the ordered phase and the very unusual wave vector dependence of the diffuse scattering observed at 11 K has been interpreted assuming MnSi to be a heavy Fermi liquid. Furthermore we believe these measurements give the first direct observation of the exchange hole.

Magnetic properties and microstructures of mischmetal-FeB-(Al, Ti and Al-Co) permanent magnets

The magnetic characteristics of anisotropic MM-FeB- (Al, Ti and Al-Co) permanent magnets have been investigated by using hot-pressing and die-upsetting process. The best magnetic properties obtained in these studies were HC = 5.1 kOe, Br = 5.4 kG with (BH)max = 5.1 MGOe for hot-pressed MM-FeB-Al-Co magnets and HC = 3.6 kOe, Br = 6.7 kG, (BH)max = 6.8 MGOe for die-upset MM-FeB-Al-Co magnets. Higher squareness of demagnetization curve was obtained in anisotropic die-upset MM-FeB- (Al, Al-Co) magnets. X-ray diffraction and STEM investigations revealed that the higher magnetic properties in die-upset magnets were resulted from alignment of the c-axis along the die-upsetting direction. The magnetic anisotropy of the die-upset magnets and the densification of the hot-pressed magnets were increased by partial substitution of Al and Al-Co for Fe.

Magnetic field induced transitions in Mn5Si3

Abstract The magnetic properties of the intermetallic compound Mn5Si3 with D88-type crystallographic ordering have been investigated at high fields and low temperatures. Magnetisation measurements in fields up to 12 T reveal that it is possible to induce transitions from the low temperature non-collinear antiferromagnetic phase by the application of a critical field which reduces in an approximately quadratic fashion with increasing temperature. The transition takes place over a field range of about 4 T wide and is accompanied by a large increase in susceptibility and hysteresis. At higher fields the behaviour seems to be unsaturated and similar to that observed for the collinear antiferromagnetic phase which is stable in the range 70–95 K. In addition, for both phases another transition which appears to be of second order occurs at a much smaller field involving a decrease in susceptibility for the non-collinear phase and an increase in susceptibility for the collinear phase.

Magnetic properties and microstructure of hot-pressed and die-upset Mischmetal-FeB-(Al)permanent magnets

Abstract The magnetic characteristics of some anisotropic Mischmetal (MM)-FeB and MM-FeB-Al permanent magnets have been investigated. The magnets were fabricated by hot pressing and die upsetting. The results for the hot-pressed MM-FeB magnets were Hc = 2.42 × 105A/m, Br = 0.46 T and (BH)max = 23 kJ/m3 compared with Hc = 1.39 × 105A/m, Br = 0.55 T and (BH)max = 32.8 kJ/m3 for die-upset magnets. On the other hand, hot-pressed MM-FeB-Al magnets gave Hc = 2.41 × 105A/m, Br = 0.44 T and (BH)max = 22.5 kJ/m3 compared with Hc = 1.85 × 105A/m, Br = 0.57 T and (BH)max = 38.5 kJ/m3 for die-upset magnets. X-ray diffraction and transmission electron microscope examinations revealed that the better magnetic properties in the die-upset magnets results from alignment of the c-axis along the die-upsetting direction. The magnetic anisotropy in die-upset magnets and the densification of the hot-pressed magnets are increased by Al substitution for Fe.

Magnetic phase diagrams of the Cr-Ga and Cr-Ge systems

Abstract The magnetic phase diagram defining the regions over which the AF 0 , AF 1 , AF 2 and paramagnetic states are stable have been determined for dilute Cr-Ga and Cr-Ge alloys, using neutron diffraction. The magnetic moment variation, the SDW-vector Q and γ, the fraction of electrons in E g orbitals have also been obtained.

Temperature dependence of magnetic fluctuations in the intermediate valence system CeSn3

Abstract We report the results of paramagnetic neutron scattering experiments on CeSn 3 using polarised neutrons and polarisation analysis. The temperature dependence between 15 K and room temperature is investigated. The magnetic scattering intensity within an energy window of ±5 THz (21 meV) does not depend on wavevector q (indicating the absence of spatial correlations) at all temperatures and decreases strongly with temperature to an insignificant value at low temperature. This result is inconsistent with the picture of a spin degenerate heavy Fermi liquid. At all temperatures considerable scattering intensity is observed at higher energy transfers.

On the spin-glass state of some CoAlT alloys

Abstract Low-field, low-temperature static susceptibility measurements are reported for some CoAl1-xTx alloys with small concentrations of T = Cr, Fe, Co, Ni or Cu. All these alloys show field cooling effects. With increasing amount of T atoms spin-glass, mictomagnetic or ferromagnetic behaviour is observed. The Cr, Co and Cu alloys are in a spin-glass regime and pass subsequently through a cluster glass and reentrant regimes at the examined concentrations whereas the Fe and Ni alloys are mictomagnetic.

Light‐scattering observation of anomalous parametric spin‐wave character in subsidiary absorption

The technique of Brillouin light scattering has been used for the characterization, in terms of wave number and direction of propagation, of parametric spin waves associated with subsidiary absorption in thin films of yttrium iron garnet. Measurements at 7.99, 8.47, and 9.90 GHz have revealed a spin‐wave character consistent with theory, with two notable exceptions. First, the data show no evidence for a predicted flip in the azimuthal propagation angle. Second, the measured magnon wave numbers at high fields are inconsistent with the theoretical values. The wave number and polar angle results imply that the change to low‐k spin waves is not a discontinuous transition as indicated by the model.

Magnetic and structural studies of Fe and Ni substituted CoAl alloys

Abstract Previous work has shown that the substitution of a 3d element (Ti, V, Cr or Mn) for Al in the equiatomic intermetallic compound CoAl results in the onset of ferromagnetic order beyond a critical concentration. The systems appear magnetically and structurally similar to the equivalent CoGa substituted compounds for which the critical concentration appears to be correlated with an average electron-atom ratio rather than woth any obvious change in the crystallographic order. The present paper completes the investigation of the CoAl series by presenting results for the systems CoAl1−xFex and CoAl1−xNix. X-ray and pulsed neutron diffraction techniques showed that the ordered B2 (CsCl) structure is retained for both these series but for the Ni series a second phase fcc structure was also observed. The neutron diffraction data was consistent with Fe atoms occupying Al sites alone and Ni atoms occupying Co sites with a consequent displacement of Co atoms to Al sites. A striking feature is that although CoAl is paramagnetic only a small amount of Fe (x≥0.03) produces ferromagnetic order with a subsequent rapid increase in Curie temperature (to ≈ 900 K for x=0.2). The disparity between the moments obtained from the high temperature paramagnetic and low temperature ferromagnetic regimes is suggestive of an itinerant magnetism and an analysis is attempted using spin fluctuation models of magnetism. Results for the entire series of CoAl substituted alloys are reviewed and compared with those for the related compounds of CoGa.