AMORPHOUS Gd57Al43 - A NEW ”FERROGLASS” ALLOY

The magnetic properties of amorphous Gd57A143 ribbon are investigated by ac and dc susceptibility measurements together with low and high field (140 kOe) magnetization study. The alloy is found to be "ferroglass" (re-entrant spin glass) with Curie temperature T,=90 K and ferrcto spin glass transition at Tf=42 K. The Gd moment is found to be 6.5 p s . The radial distribution function is obtained from X-ray experiments. Rare earth (RE) intermetallic amorphous (a) alloys show a wide range of magnetic properties ranging from ferromagnetism to spin-glass (SG) state due to the existance of both exchange fluctuations and random anisotropy fields acting on the non-S-state ions. In order to study the role of exchange fluctuations it is useful to investigate alloys with S-state ions. For Gd, an S-state ion, the single ion anisotropy is expected 60 to be negligible. The magnetic properties of a-Gd-A1 alloys and films have been investigated in [I-51. In this brief communication the results from ac and dc sus40 ceptibility measurements together with magnetization study in low and high steady fields (up to 140 kOe) on a Gd57A143 alloy are presented. The alloy is obtained as amorphous ribbon by melt spinning. The amorphism is confirmed from DSC and o so l o o iso T, K X-ray diffraction measurements. The first peak of the Radial distribution function (RDF) obtained in Fig. 1. Temperature dependence of the magnetization the later experiments is well resolved and centered at u and low field dc susceptibility xdc: (-) measurements 3.5 A, a distance slightly shorter than the ~ ~ l d ~ ~ h ~ i d t after zero-field cooling; (-) data for field-cooled sample. diameter of Gd (3.6 A). The area under the first peak, which is over 12, is approximately equal to the average number of Gd-Gd nearest neighbors. These observation suggests that the local short range order in a Gd57A143 has a "closed packed" co-ordination. s k The temperature dependence of magnetization in different magnetic fields is given in figure 1. After the 10 alloy had been cooled from paramagnetic state in the absence of an external magnetic field (Hco,l= 0) and subsequently measured at increasing T in a constant external field HqP1 of less than 5 kOe, a maximum in 5 the u = f (T) dependence is observed at the temperatures irf. The maximum is also observed at the low field dc susceptibility given in figure 1. The maximum disappears when the sample is cooled in a magnetic field. The Tf shifts to a lower temperatures with increase in 0 Happl. 0 20 40 60 80 T,K The and imagnary components Fig. 2. Temperature dependence of the real X' and the ac susceptibility have been measured during the warmimaginary X U components of the ac susceptibility measured ing runs from 4.2 K (Fig. 2). A strong frequence deat ac field 3 Oe. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19888624 C8 1364 JOURNAL DE PHYSIQUE pendence has been observed in both X' (T) and X" (T) between 20 K and 60 K. The isotherms of initial magnetization indicating conventional soft ferromagnetic behaviour are given in figure 3. The moment of Gd derived from the saturation magnetization at 4.2 K 6.5 pB is smaller than the theoretical one (7.9 pB) and higher than those reported for a Gd78A122 in [I]. It should be noted that a strong paraprocess exists above the technical saturation indicating that the spin structure remains non-collinear. A sharp increase in the coercivity (H,= 300 Oe at 4.2 K) and relaxation phenomena are observed below Tf. The temperature dependence of inverse susceptibility in the paramagnetic region continuously deviates from liniarity (Fig. 3) and it is imposiible to determine precisely the moment and temperature. Despite of this we can say that the Gd "paramagnetic moment" is too large and the paramagnetic temperature is positive. served in many cluster glasses as well as in amorphous systems containing non-S state RE. We conclude that with decreasing temperature the a Gd~7A143 passes from parato ferromagnetic and then to spin-glass like state with transition temperatures 1:=90 K and Tf= 42 K respectively. In the case of amorphous alloys containing non-S state RE the SG-like structure is prolduced mainly by strong anisotropy fields. There are two plausible interpretations for the occurence of re-entrant transition from ferroto SG-like state in a Gd~~A143. First, the phase diagram constructed by Sherri~ngton and Kirkpatrick predicts such a re-entrant behaviour with detreasing temperatures. Despite somta limitations this model can be useful to explain the properties of systems as a GdS7Ald3 in which fluctuations in the exchange are important. The positive paramagnetic temperature indicates that ferromagnetic interactions are predominant. In RE alloys the exchange is attributed to RKKY interactions, its sign and magnitude dependent on various Gd-Gd distances and the conduction electron density. However, for the alloy investigated here the Gd-Gd distances are very large (3.5 A) and as proposed in [l] the role of the indirect exchange via 5d electrons can be important. Another interpretation can be the anisotropy effects. The source of anisotropy in Gd alloys is not well understood but it can be the exchange anisotropy [7] or splitting of ~ d ~ + ground state as a result of other L, S states into the ground state [8].