PRESSURE EFFECT ON THE MAGNETIC PROPERTIES OF Fe-La AMORPHOUS ALLOYS

Pressure dependknce of XAC (T) for amorphous Fe,Laloo-, has been measured up to 20 kbar. With increasing pressure, the ferromagnetic phase for x = 87.5 reduces and finally the spin-glass state is realized below 110 K. Paramagnetic to spin-glass transition temperature observed for x = 92.5 becomes low by applying pressure. Spin-glass behavior has been observed in Fe-rich amorphous alloy systems with early transition metals such as Fe-Y, Fe-Zr and Fe-Hf [l-41. TMs indicates that the alloy system is a random spin system with competing ferroand antiferromagnetic interactions. Recently, we investigated the magnetic phase diagram of Fe,Laloo-, amorphous alloys with 60 5 x 5 92.5 [5]. In the concentration range 60 < x < 90, the alloy is ferromagnetic, but shows a re-entrant spin-glass behavior below a lower temperature than the Curie temperature. The Curie temperature rises to a maximum of 285 K at a: = 30 and then drops sharply as a: -+ 90. The amorphous alloy with x _> 90 has no ferromagnetic phase and exhibits a direct transition from the paramagnetic phase to the spin-glass phase at about 110 K characterized by a cusp in the temperature dependence of the AC susceptibility. In the present study, we have measured the pressure dependence of the AC susceptibility for the Fe-rich FeLa amorphous alloys up to 20 kbar to clarify the spinglass behavior of the alloy system. The bulk samples about 0.2 mm thick with nominal compositions of 87.5 and 92.5 at.% Fe were prepared by high rate DC sputtering on water-cooled Cu substrates 45 mm diameter. These samples were confirmed to be amorphous by X-ray diffraction. The AC susceptibility in the temperature range from 4.2 K to 300 K were measured at various hydrostatic pressures up to 20 kbar by a conventional mutual inductance method with an AC field of 13 Oe at 80 Hz using a Be-Cu clamp pressure cell. We measured the AC susceptibility (XAC) of two amorphous samples Fe,Laloo-, of a: = 87.5 and 92.5 with re-entrant spin-glass and spin-glass behavior, respectively [5]. Figure l a shows the temperature dependence of XAC for x = 87.5 at various pressures. At 0 kbar, XAC incremes sharply at the Curie temperature (Tc) 241 K to a maximum value and then decreases gradually as the temperature decreases, following an abrupt drop-off a t a lower temperature. This drop-off of XAC is related to a transition from the ferromagnetic to the re-entrant spin-glass state. Since the onset of Fig. 1. Temperature dependence of XAC at various pressures for amorphous Fes7.5La12.5 (a) and Feg2.sLa7.5 (br the drop-off is rather vague, the re-entrant spin-glass temperature (TRSG) is defined by a shoulder in the temperature dependence of XAC. AS the pressure increases, Tc decreases rapidly while TRSG increases, indicating a reduction of the ferromagnetic phase. At 20 kbar, XAC dose not show any sharp increase corresponding to a direct transition from the paramagnetic to the ferromagnetic state, but exhibits a cusp characterizing the spin-glass transition 110 K. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19888524 C8 1144 JOURNAL DE PHYSIQUE Figure l b shows the temperature dependence of XAC for x = 93.5 at various pressures. The temperature dependence of XAC at 0 kbar shows the cusp at 110 K, indicating the direct transition from the paramagnetic to the spin-glass state. With increasing pressure, the transition temperature (TsG) decreases gradually and the cusp of XAC collapses. The pressure dependence of the transition temperatures Tc and TRSG for x = 87.5 and TSG for x = 92.5 are plotted in the magnetic phase diagrams in figures 2a and b. The shifts of Tc and TRSG are estimated to be -4.8 K/kbar and 5.0 K/kbar below 10 kbar, respectively. The shift of Tc is as large as that of Fe-Ni invar alloy [6]. On the other hand, the shift of TSG for x = 92.5 is -1.4 K/kbar, whose absolute value is about one-fourth of that of TitsG for x = 87.5. 300 (a) a-Fe87.5La12.5 x 2 90 [5]. Matsuura et al. measured the interatomic distance between Fe-Fe atoms in the Fe-rich amorphous Fe-La system [7]. The distance becomes short very slightly with increasing Fe concentration. This suggests that the drastic change in the magnetic properties with Fe concentration is not related mainly to the change in the interatomic distance between Fe-Fe atoms, but to the change in the partial Fe-Fe coordination number. Recently, Kakehashi calculated the magnetic phase diagram of the fcc Fe-Ni alloys on the basis of the finite-temperature theory of the local environment effect [8]. According to the numerical calculations, the spin-glass state is realized in the concentration range above 65 at.% Fe. The antiferromagnetic coupling, which is the origin of the formation of the spin-glass state, is caused between Fe atoms with a large number of Fe nearest neighbors. The spin-glass state observed in the Fe-rich amorphous Fe-La system is considered to be derived from the same origin as in the fcc Fe-Ni system. On the other hand, the present high pressure experiments indicate that the reduction of Fe-Fe atomic distance suppresses the ferromagnetic ordering and facilitates the formation of the spin-glass state. These results are considered to be due to a decrease in the average exchange interaction between Fe atoms. The collapse of the cusp of XAC observed at x = 92.5 by the application of pressure suggests that the antiferromagnetic interaction prevails between Fe atoms at high pressures. P '0 5 10 15 20 25 P (kbar)