S.F. da Cunha

Magnetic properties of Ce(Fe1-xAlx)2 for x⩽0.20

Abstract Magnetization measurements of the system Ce(Fe 1- x Al x ) 2 for x ⩽0.20 show that the substitution of Fe by Al in CeFe 2 has not a simple dilution effect. Together with an accentuated decrease in the iron mean magnetic moment and in Curie temperatures was observed, above a certain concentration, a transition from ferromagnetism to a canted spin phase upon decreasing temperature. For the higher concentration range the system exhibits magnetic freezing effects. A preliminary magnetic phase diagram was proposed.

Changeover in the order of the magnetic phase transition in the intermetallic compounds (Er1−xTbx)Co2

Abstract Electrical resistivity, magnetization and AC susceptibility measurements have been carried out to investigate the order of the magnetic phase transition in (Er 1− x Tb x )Co 2 compounds. In this system a changeover from first- to second-order transition at about x c =0.60 was observed. This result is discussed in the framework of the generalized Inoue-Shimizu phenomenological model. Moreover, a microscope explanation about the order of the phase transition in terms of spin fluctuation and metamagnetism of the Co-3d subsystem is offered.

Magnetic properties of the pseudo-binary intermetallic compounds: Hf(Fe1−xAlx)2

Abstract From magnetization measurements in polycrystalline Hf(Fe 1− x Al x ) 2 samples, it was observed that the magnetization is not saturated even at high fields (70 kOe). It is suggested that in Hf(Fe 1− x Al x ) 2 , a high anisotropy energy and consequently narrow domain walls exist. Magnetization vs. temperature curves present a different behaviour if the sample is cooled with field (FC) or without (ZFC) field. This difference is applied field dependent and is a maximum in the range 0.45 x T C vs. x ), two different slopes were found and T C vanishes at x =0.63, approximately. To compare the Hf(Fe 1− x Al x ) 2 and Zr(Fe 1− x Al x ) 2 results, we performed the same measurements on a Zr (Fe 0.68 Al 0.32 ) 2 sample and found a similar irreversibility of ZFC magnetization which has not been detected in the literature.

Coercive field and domain wall properties in (DyχY1-χ) Fe2 and (ErχY1-χFe2 intermetallic compounds

Isothermal d.c. magnetization measurements were performed for (RχY1-χ)Fe2 (RDy, Er, 0<χ<1) at several temperatures. The observed pronounced decay of the coercive field Hc with temperature was fitted with an exponential curve at low temperatures for concentrations near 30%. The coercive field Hc and the propagation field Hp showed maximum values in the concentration range 30%–40% for Dy or Er. These results can be explained by intrinsic domain wall pinning. Time relaxation effects of the remanent magnetization were detected and attributed to domain wall surface deformations.

89Y nuclear-magnetic-resonance measurements in (DyxY1-x)Fe2 compounds

In a previous work, 89Y measurements at 4.2 K in (RxY1−x)Fe2 compounds, with R=Tb and Ho and 0.05≤x≤0.20, have shown broad nuclear‐magnetic‐resonance (NMR) spectra, due to transferred hyperfine interactions. 89Y NMR signals could not be observed with R=Dy in the same composition range. In the present work, compounds were prepared with Dy in the concentration range 0.005≤x≤0.02 to investigate the origin of this anomaly. Compared to the pure YFe2 compound, the observed NMR spectra are broadened and show a shift towards higher frequencies. With increasing Dy concentration, the NMR signals fall very rapidly. Magnetization measurements at 4.2 K were performed in samples with Tb, Ho, and Dy; the results indicate that the behavior of the Dy compounds is qualitatively different in the low‐magnetic‐field region of the curves, requiring a minimum applied field to start increasing the magnetization. The NMR data and the magnetization results of the Dy compounds can both be explained as arising from a much lower mobi...

Magnetic properties of Ce(Fe1−xAlx)2 (Al-RICH)

Magnetic properties of the Laves phase system Ce(Fe1−xAlx)2 were investigated by means of magnetization and electrical resistivity measurements for 0.60 ≤x< 0.90. X-ray powder diffractograms reveal that at least up to 40% of iron in CeAl2 (x = 0.60) the samples are single-phase. A magnetic phase diagram is proposed where, in this range of concentration, the system changes from a ferromagnetic to a quasi-ferrimagnetic phase around x = 0.80. At low temperatures the mixed phase character was indicated by spin-glass behaviour, walls pinning of ferromagnetic domains and frustration.

Electrical resistivity of the pseudo-binary system Ce(Fe1-xAlx)2

Abstract The Laves phase pseudo-binary system Ce(Fe 1- x Al x ) 2 for x ≤0.20 has been studied by means of electrical resistivity measurements from 1.5 to 300 K. It is shown that with Al addition, the long range magnetic order of CeFe 2 is destroyed and that a spin glass phase brings in a minimum in the total resistivity with T min proportional to x . The freezing temperatures T f are always smaller than T min and there appears a negative coefficient of the T 3/2 dependence below T f . The minimum in dϱ/d T is well correlated with T f .

Intrinsic domain wall pinning in Hf(Fe0.5Al0.5)2

Abstract The magnetization versus temperature curve for Hf(Fe 0.5 Al 0.5 ) 2 in low applied field presents a maximum and also an irreversibility up to near the Curie temperature. For temperatures lower than approximately 7 K, a critical field (propagation field) exists in the virgin magnetization curves. Even in high fields (70 kOe) the magnetization values are not saturated, suggesting high anisotropy energy and consequently narrow domain walls. An exponential temperature decrease of the coercive field is observed. These phenomena can be explained by intrinsic domain wall pinning. The time effect in the demagnetization region of the hysteresis loop was measured and it is related to the wall deformation (kink creation). A brief comparison with other systems is made.

RECOVERY OF ERCO2 FERMI LEVEL BY SUBSTITUTION OF CO BY NI AND FE

Magnetic and electrical resistivity measurements performed in Er(Co1−xNix)2 have shown that the critical concentration for the first-order magnetic phase transition is near to xc=0.10. For higher Ni concentration the results suggest that Co is no longer magnetic. Experimental results presented here for Er(Co0.8Ni0.2xFe0.2(1−x))2 and Er(Co0.9Ni0.1xFe0.1(1−x))2 systems, show that when Co is replaced by Fe, the 3d band is filled up, whereas for Ni substitution that band is depleted. The saturation moment obtained from isothermal magnetization measurements indicates that Co recovers its moment and couples parallel to the Fe moment and antiparallel to the Er moment. Characteristics of spin fluctuation are also observed. These results show that with a suitable combination of Co, Ni, and Fe we can recover the original density of the states of the ErCo2 compound.

Electrical resistivity of the pseudo-binary compound Ce(Fe0.8Al0.2)2

Abstract Electrical resistivity measurements on the alloy Ce(Fe 0.8 Al 0.2 ) 2 as a function of temperature in the range of 1.5 to 300 K shows that the long range magnetic order of the CeFe 2 is destroyed and that the spin glass phase appears with a negative coefficient of T 3/2 . The high residual resistivity is also discussed.