H. Komatsu

The structure of short- and medium-range order of Fe-rich amorphous Fe-La alloys

The ferromagnetic state of iron-rich amorphous FexLa1-x alloys (a-FexLa1-x) is known to become unstable with increasing iron concentration and at x=0.9 a cusp characteristic of a spin glass appears in the AC susceptibility. In order to elucidate the correlations between the magnetic properties and the structure, the authors measured both large-angle and small-angle X-ray scatterings for a-FexLa1-x(0.65<or=x<or=0.9) alloys. From large-angle scattering measurements, interatomic distances and partial coordination numbers between Fe-Fe, Fe-La and La-La atoms were determined. The Fe-Fe interatomic distances for these alloys are all very close to the critical value (=2.54 AA) at which an antiferromagnetic exchange competes with a ferromagnetic one. The Fe-Fe coordination number of 8.6 for a-Fe0.9La0.1was found to be close to 8.8 for crystalline (Fe0.86Al0.14)13La alloy where the ferromagnetic ordering collapses and the antiferromagnetic one dominates. The small-angle scattering for a-Fe0.9La0.1 shows pronounced high intensity in contrast to those for others (x=0.7 and 0.8). This fact suggests that inhomogeneous regions exist in a-Fe0.9La0.1. From the correlation function gamma (r) and Q-invariant 2 pi 2 gamma (0), the authors conjecture that the inhomogenous regions consist of iron clusters whose size is widely distributed and their volume fraction is estimated to be about 2%. The correlations of iron clusters with the spin glass state of a-Fe0.9La0.1 are discussed.

Magnetic properties and density of Fe-Ce amorphous alloys

Abstract Several kinds of FeCe amorphous alloys were prepared by high-rate dc sputtering in order to investigate their magnetic properties. The coercive force at 4.2 K is very small as that of FeZr amorphous alloys, suggesting that Ce atoms do not have large magnetic moment due to the valence fluctuation, that is, a tetravelent character. The magnetic cooling curves and the temperature dependence curves of ac susceptibility reveal that the FeCe amorphous alloy system shows a spin glass behavior in a wide concentration range in a similar manner as the FeLu amorphous alloy systems. The Curie temperature is much lower than that of FeLa and FeZr amorphous alloy systems. The room temperature densities in the crystalline and amorphous phases at the Laves phase composition are much larger than the value expected from Vegards law due to the valence fluctuation of Ce.

Differential magnetic susceptibility of amorphous Fe-Y alloys

The spin-glass behaviour of amorphous FexY100-x alloys (70<or=x<or=90) has been investigated by means of differential magnetic susceptibility dM/dH. In low magnetic fields, the dM/dH versus T curves show the peaks, indicating that the spin-glass state occurs through two steps. Two freezing temperatures are defined from the two peaks in the curve, i.e. the higher temperature is Tg and the lower temperature is Tf. From the results of the magnetization measurement as a function of temperature, i.e. the M versus T curve, it is indicated that the magnetization starts to decrease at Tg and a thermal irreversibility appears below Tf with decreasing temperature. With increase in the applied magnetic field, a third peak appears at around 150 K in the dM/dH versus T curve for amorphous Fe84Y16 alloy. The position of this peak corresponds to the upper inflection point in the M versus T curve, being defined as the Curie temperature TC. Therefore, re-entrant behaviour appears in this system on application of a magnetic field. The concentration dependences of TC and Tf for a field of 500 Oe are very similar to those obtained in zero field for other amorphous Fe-RE alloys (RE: rare-earth metal) such as Fe-Lu and Fe-Ce systems.

Itinerant electron metamagnetic transition in exchange enhanced paramagnetic compounds Lu(Co1-xSnx)2

Magnetization measurements of C-15 type Laves phase compounds Lu(Co1-xSnx)2 have been carried out up to 100 T in order to investigate the itinerant electron metamagnetic transition. By partial replacement of Co by Sn, the lattice constant increases and the critical magnetic field decreases linearly with increasing x. The susceptibility increases until x=0.04 and then decreases above x=0.06. A clear itinerant electron metamagnetic transition has been observed in the whole concentration range. It has been demonstrated that this transition occurs even in a pure compound LuCo2 and its critical magnetic-transition field is determined to be 74 T, not so different from the value predicted by the band calculation. On the other hand, the observed magnetization jump of LuCo2 is about 0.5 mu B/Co, which is much larger than the theoretical value.

Metamagnetic transition in Y2Cu2O5

Abstract Neutron diffraction experiment has been performed to determine the magnetic structures in magnetic fields up to 6 T on a single crystal of Y 2 Cu 2 O 5 which shows strange metamagnetic transitions at H c1 = 3.0T and H c2 = 4.8T for H‖b . At H = 0, the determined magnetic structure is identical with that obtained for the powder sample, but new diffraction peaks appear at completely unexpected positions between H c1 and H c2 , which means a three-lattice period modulation along the b -axis but this structure change is mysterious from the crystallographic point of view.

Mössbauer Study on Fe-Rich Amorphous Fe–La Alloys

Mossbauer spectra have been measured for two Fe-rich amorphous alloys, a -Fe 91.5 La 8.5 with spin glass behavior and a -Fe 82.5 La 17.5 with reentrant spin glass behavior, in the temperature range between 4.2 K and room temperature. These spectra are analyzed using the Hesse and Rubertsch method to obtain the distribution of hyperfine fields. The distribution of hyperfine fields at 4.2 K consists of a main component ranging from 200 kOe to 400 kOe and a sub-component around 150 kOe. The average Fe-moment is estimated to be 1.96µ B for a -Fe 91.5 La 8.5 and 2.00µ B for a -Fe 82.5 La 17.5 . Hyperfine fields in a -Fe 91.5 La 8.5 appear progressively below 150 K, which is apparently higher than the spin glass temperature of 124 K determined by the ac susceptibility measurement. On the other hand, hyperfine fields in a -Fe 82.5 La 17.5 appear just below the ferromagnetic ordering temperature of 267.5 K, but increase anomalously below 70 K. This anomaly can be explained in terms of the freezing of transverse c...

Large magnetoelastic effects of amorphous CeFe alloys

Abstract The thermal expansion, magnetic properties and magnetoelastic effects in amorphous CeFe alloys have been investigated. These alloys exhibit a marked thermal expansion anomaly associated with a significantly large spontaneous volume magnetostriction. A large elastic anomaly has been confirmed even above the Curie temperature. The compressibility calculated from the elastic data obtained by Brillouin scattering is about three times that of crystalline b.c.c. Fe. These large magnetoelastic effects give rise to remarkable pressure effects on the Curie temperature, magnetization and spinwave stiffness constant.

Magnetic Properties and Phase Diagram of Amorphous Fe-Lu Alloys

A magnetic phase diagram of amorphous Fe 100- X Lu X was determined from AC susceptibility and remanent magnetization measurements. The alloys with 30≤ X ≤40 exhibit a spin glass behavior. The alloys with 10.5≤ X ≤25 are ferromagnetic, but show a reentrant spin glass behavior at low temperatures. The reentrant spin glass phase is divided into two phases with weak and strong irreversibility. The ferromagnetic state becomes unstable as X approaches to 8. For X =8, the ferromagnetic state disappears and a direct transition from the paramagnetic to spin glass state is observed. The high field magnetization measurements imply that the instability of ferromagnetism and the appearance of the spin glass state are caused by the development of antiferromagnetic interactions. In the reentrant spin glass state, the freezing of transverse components of spins is confirmed by the analysis of the Mossbauer spectra.

Magnetic properties of La(FexAl1-x)13 amorphous alloys

Several kinds of La(FexAl1-x)13 (0.80<or=x<or=0.95) amorphous alloys have been prepared by high-rate DC sputtering to investigate the magnetic properties. The magnetic phase diagram has been established by DC and AC magnetic field measurements. Above x=0.85, re-entrant spin-glass behaviour has been observed in the concentration range where antiferromagnetic order occurs in the crystalline state. The high-field susceptibility in this concentration range is extremely large and the magnetization curves are not easily saturated. The magnetic moment estimated from the magnetization curves measured up to 380 kOe in pulsed magnetic fields shows a maximum around x approximately 0.85, corresponding to 80 at.% Fe. The Curie temperature in the amorphous state decreases with increasing x and it is higher than the Neel and Curie temperatures in the crystalline state. The spin-wave stiffness constant in the amorphous state is extremely small, suggesting a magnetic instability.

Isomer shift and quadrupole splitting of amorphous iron — uniqueness of the state of amorphous pure iron

Abstract Isomer shift and quadrupole splitting of amorphous Febased binary alloys have been investigated. Concentration dependence of the isomer shift for amorphous RFe (R = Lu, Ce or Y) and Fe B or FeSb alloys is discussed. Extrapolation of the isomer shift and quadrupole splitting to amorphous pure Fe reveals the unique state of amorphous pure Fe, which can be related to the state of fcc Fe.