AMORPHOUS Gd57Al43 - A NEW "FERROGLASS" ALLOY

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AMORPHOUS Gd57Al43 - A NEW ”FERROGLASS”

ALLOY

V. Skumryev, H. Gamari-Seale, D.-X. Chen, V. Petkov, K. Rao, A. Apostolov

To cite this version:

JOURNAL DE PHYSIQUE

Colloque C8, Suppl6ment au no 12, Tome 49, d6cembre 1988

AMORPHOUS Gd57A143-

A NEW

"FERROGLASS" ALLOY

V. Skumryev H. Gamari-Seale (2), D-X Chen (I), V. Petkov (=), K. V. Rao (I) and

A. Apostolov (3)

( I ) Royal Institute of Technology, Stockholm, Sweden

(2) National Res. Center "Democritos'~

,

Athens, Greece

(3) Faculty of Physics, Sofia University, Sofia, Bulgaria

Abstract.

-

The magnetic properties of amorphous Gd57A143 ribbon are investigated by ac and dc susceptibility measure- ments 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 ferrc- to 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) al- loys show a wide range of magnetic properties ranging from ferromagnetism to spin-glass (SG) state due to the existance of both exchange fluctuations and ran- dom 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 6 0 - t o 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 sus- 4 0 - 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 aver- age number of Gd-Gd nearest neighbors. These ob- servation 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 tempera- tures

irf.

The maximum is also observed at the low field dc susceptibility given in figure 1. The maximum dis- appears 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 warm- imaginary X U components of the ac susceptibility measured

ing runs from 4.2 K (Fig. 2). A strong frequence de- at ac field 3 Oe.

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 satura- tion magnetization at 4.2 K - 6.5 p B is smaller than the theoretical one (7.9 pB) and higher than those re- ported 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 a t 4.2 K) and relaxation phenomena are ob- served below

Tf.

The temperature dependence of in- verse susceptibility in the paramagnetic region contin- uously deviates from liniarity (Fig. 3) and it is impos- iible to determine precisely the moment and temperature. Despite of this we can say that the Gd "paramagnetic moment" is too large and the para- magnetic 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 para- to 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 in- terpretations for the occurence of re-entrant transition from ferro- t o SG-like state in a

-

Gd~~A143. First, the phase diagram constructed by Sherri~ngton and Kirk- patrick predicts such a re-entrant behaviour with de- treasing temperatures. Despite somta limitations this model can be useful to explain the properties of sys- tems as a

-

GdS7Ald3 in which fluctuations in the ex- change are important. The positive paramagnetic tem- perature indicates that ferromagnetic interactions are predominant. In RE alloys the exchange is attributed to RKKY interactions, its sign and magnitude depen- dent 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].

Acknowledgements

The authors wish to thank Ms. J. Warchulska and Dr. T. Mydlarz (ILHMFLT-Wroclaw, Poland) for help with magnetic measurements.

Fig. 3. -Initial magnetization isotherms; insert- the inverse dc susceptibility.

It is difficult to estimate the temperature of mag- netic ordering from the u = f (T) dependence (Fig. 1) because the transition is found to be too smeared and field dependent. Such a behaviour is presumed to be due to short-range order effects. The critical temper- atures Tc=90 K and Tf=42 K were determined from the inflections on the X" (T) component of ac suscep tibility [6].

The above mentioned field-cooling effects, the max- imum in ac susceptibility, the strong temperature de- pendence of Hc and relaxation phenomena have been observed in a-Gd-A1 system [l-51. They are also ob-

[I] Buschow, K. H., Solid State Commun. 27 (1978) 275.

[2] Coey, J. M. D., von Molnar, S. and R. Gambino, Solid State Commun. 24 (1977) 167.

[3] McGuire,

T.,

Mizoguchi, T., Gambino, R. and Kirkpatrick, S., J. Appl. Phys. 4 9 (1978) 1869. [4] Malozemoff, A., Imry, Y. and Barbara, B., J.

Appl. Phys. 53 (1982) 7672.

[5] Shirakawa, Aoki, Aoki, K. and Masumoto, T., J. non-Cryst. Solids 61 (1984) 1371.

[6] Goldfard, R., Rao, K. V. and Chen, H. S., J. Magn. Magn. Mater. 54-57 (1906) 111.

[7] Fert, A. and Levy, P., Phys. Rev. Lett. 45 (1980) 1583.