Electrical properties of ferromagnetic Gd3−xS4 near its Curie temperature

Mat. Res. B u l l . , Vol. 15, pp. 995-1000, 1980. Printed in the USA. 0025-5408/80/070995-06502.00/0 Copyright (c) 1980 Pergamon P r e s s Ltd.

ELECTRICAL

PROPERTIES OF FERROMAGNETIC ITS CURIE TEMPERATURE

Gd3_xS 4 NEAR

M. Sato, K. Niki, G. Adachi , and J. Shiokawa Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamadakami, Suita, Osaka 565, Japan

(Received May 12, 1980; Communicated b y J. B. Goodenough)

ABSTRACT Nonstoichiometric Gd3_xS 4 ( 0 < x < 1 / 3 ) , which exhibits a metallic behavior, was obtained by heating an insulating Gd2s 3 at various temperatures under a vacuum. Electrical and magnetic properties of the samples obtained have been investigated from 4.2 K to 300 K. A maximum in resistivity was observed in the curve of resistivity vs. temperature. The temperature T , at which the maximum emerged, was very close to t~e Curie temperature T c for the sample. An increase in resistivity at Tp, ~p, is proportional to ~ exp(E/kBTp) , where pp is the resistivity obtained the extrapolation from the linear portion to Tp in the p vs. T curve. Using the relationship obtained, a model based on the formation of magnetic polaron was proposed for this system.

Introduction Gadolinium sulfides Gd3_xS 4 (0~ x ~ 1 / 3 ) crystallize in a bcc Th3P 4 structure (i). These compounds possess some interesting physical properties (2-4) because of their unique crystal structure. The data of magnetic susceptibility measurements show that gadolinium contained is in a 3+ ionic state in the Th3P 4 structure. Since a sulfur atom cannot accept more than two electrons to become a sulfide anion, there are extra electrons for compositions with x < 1 / 3 , and these electrons must be in a conduction band. As a result, the structure can be characterized in more detail by the formula (Gd3+)3-xVx(S2-)4(e-)l_3x where

V indicates

vacancies

The conduction *Author

in the cation positions.

electrons

introduced

to w h o m all correspondence 995

into the samples with

should be addressed.

996

M. SAT O , et al.

Vol. 15, No. 7

x < 1/3 have a significant influence on the magnetic properties of Gd3_xS 4 system (5). Since the dominant magnetic interaction comes from 4f localized electrons of Gd 3+ ions, the magnetic ordering in Gd3_xS 4 is usually considered to be based on the f-f indirect exchange via the conduction electrons. The aim of this paper is to investigate the electrical behavior for Gd3_xS 4 system on the basis of their magnetic properties.

Experimental Preparation. N o n s t o i c h i o m e t r i c Gd3_xS 4 prepared by the following two-step procedures. First, stoichiometric Gd2S 3 was obtained by the method described in Ref.(6). For the preparation of Gd3_xS4, Gd2S 3 which was pressed into pellet at 200 kg/cm 2 was heated on a molybdenum plate in an induction furnace at various temperatures (1400 - 1600 °C) for 3 - 6 hr in vaccum ( ~ i 0 -4 mmHg). Analysis. The gadolinium content in the n o n s t o i c h i o m e t r i c samples obtained was ascertained with an EDTA titration method in a neutral or weak hydrochloric medium using a xylenol orange indicator. Measurements of some physical properties. The electrical resistivity was determined from 4.2 K to 300 K. The data were obtained by a four-probe technique using a direct current. The magnetic measurements were carried out with a Shimazu magnetic balance MB-II in the temperature range 4.2 - 300 K. X-ray powder diffraction patterns were obtained on all products using a Rigaku Denki "Rota-flex" diffractometer with a scintillation detector and a CuK~ radiation. An internal standard of a high-purity silicon (99.999 %) was used, and the lattice parameter was refined by a least-square method for unambigously indexed reflections.

Results

and Discussion

Six samples were obtained for this study. X-ray powder diffraction patterns of these samples were all identical with that of the Th3P 4 structure. In Table 1 are given the analytical values and the lattice constants for Gd3_xS 4 obtained. Temperature dependences of the electrical resistivity for a series of Gd3_xS 4 are shown in Fig. i. At high temperature all samples indicate a common metallic behavior, that is, the resistivity linearly increases with the rise of temperature. A maximum in resistivity is, however, observed in the curve of resistivity vs. temperature for every cases at low temperature. The maximum increases in height and shifts to low temperature

Vol. 15, No. 7

FERROMAGNETIC Gd3_xS 4

]

TABLE Compositions

and

lattice

S a m p l e No.

Firing (°C)

scheduel (hr)

1 2 3 4 5 6

1400 1500 1550 1550 1600 1600

3 3 3 3 4 6

997

constants

for Gd3_xS 4 o b t a i n e d

Compositions x 0.34 0.29 0.29 0.28 0.26 0.25

+ + + ; + +

Latticeoconstant A

0.01 0.01 0.01 0.01 0.01 0.01

8.387 8.379 8.377 8.380 8.381 8.379

10-1

side as x increases. The m a g n e t i c data were o b t a i n e d in the t e m p e r a t u r e range 4.2300 K. The m a g n e t i c s u s c e p t i b i l i t i e s for all samples o b e y e d the C u r i e W e i s s law. Paramagnetic Curie t e m p e r a t u r e s ep, w e r e l i s t e d in Table 2. The sign of 8p for all samples is positive, i n d i c a t i n g a ferrom a g n e t i c behavior. The values of ep are approx i m a t e l y in a g r e e m e n t w i t h the values of Tp. For Gd2.74S4(No.5) the d e p e n d e n c e s of the m a g n e t i zation I on the m a g n e t i c field H and on the temperature n e a r 8 p w e r e i n v e s t i gated. The results o b t a i n e d are shown in Fig. 2. The r e l a t i o n s h i p of I vs. H

~ 1 0 -2

E

X=0.29(No.3) X=0.26(No.5) 10< i

50

Sample 1 2 3 4 5 6

No.

A

i

i

100

150

200

i

250

T(K)

FIG.

TABLE Magnetic

/

1

Temperature

dependences

resistivity

for a series

of e l e c t r i c a l of Gd3_xS 4

2

and e l e c t r i c a l

ep (K)

Tc (K)

Tp (K)

24 32 36 36 56 62

51.0 -

21.0 28.5 30.2 31.5 51.0 55.0

data

Pmax Pp aP (xl0-3~cm) (xl0-3~cm) (xl0 -3~cm) 81.81 5.378 4.346 4.688 1.297 1.609

7.238 2.023 1.947 2.349 1.075 0.902

74.57 3.364 2.399 2.339 0.222 0.149

998

M. SATO, et al.

100

20 K

-

Vol. 15, No. 7

~" X=0.34 (No.l)

Pma~ 30 K -

- -~~O

=Pmax-Pp

I I

I

T

Tp

/

E

@

X=0.29 (No.2)

Z50

,29 (N o.3) B(No.4)

/

50K

60K

K

rx

.2•X2=0.25(No.6)

3

H x 10-3 (Oe)

FIG.

2

FIG.

3

Magnetic field dependence of Log(~p/p D) vs. reciprocal of Tp, magnetization of Gd2.74S 4 (No. 5) insert; ~xample for temperatuge for various temperatures of p

below about 50 K is typical of ferromagnetic characteristics. The Curie temperature T c for this sample was determined, using the method proposed by Arrott et al. (7). As a result, the obtained Curie temperature, 51 K, coincides with its Tp. The maxima in resistivity as shown in Fig. 1 are similar to that observed in ferromagnetic EUl_xGdxS system (8,9), where the indirect exchange via conduction electrons occurs between localized 4f spins, leading to the formation of magnetic polarons. There have been no quantitative explanation for this phenomenon in these ear]ier studies. Here, the height and position of the resistivity maximum observed in the resistivity vs. temperature curve are quantitatively treated on the basis of the concep£ of magnetic polaron. The values of log(~p/pp) were plotted as a function of the reciprocal values of Tp in Fig. 3. The notations, ~ p and pp, mean such values as indicated in Fig. 3. Thus, ~p would-be an increase in resistivity due to the formation of magnetic polarons. A linearity is obtained in the relation of log(~p/pp) and i/Tp. This means the relationship ~p~ where k B is the Boltzman the dimension of energy.

pp exp(E/kBTp)

[i]

constant and E is also a constant with The value obtained for E is 0.012 eV.

FERROMAGNETIC Gd3_xS4

Vol. 15, No. 7

999

Assuming that the number of the charge carries in this system obeys the Boltzman-like distribution, the following model can be proposed. Two types ¢ of conduction electrons would exist as described & in Fig. 4. One type of electrons are magnetically trapped by 4f spins of Gd ions, forming magnetic I polarons. The mobility ! of these electrons is very small under this ! ! circumstance. The other i (~) , type of electrons are magnetically "free" electrons. The charge in this system would be (~) trapped electron predominantly transported 0 free electron by these electrons, assuming that the mobility I' 4f spin of Gd3+ of the trapped electron i " ~ magnetic polaron is negligibly small compared with that of the free electron. The E in FIG. 4 the equation [I] Scheme of electrons trapped by 4f corresponds to the energy spins and free electrons gap between the states of magnetically trapped and free electrons. The magnetic polaron resulting in the sample with a high content of conduction electrons would have little influence on the resistivity since the number of free electrons is overwhelmingly greater than that of trapped electrons. For temperatures below Tc, 4f spins are aligned ferromagnetically. Thus, the mobility of the trapped electrons increases since they move along the aligned spin field.

t

,,

t,_.,f

0

©

References l.

W. H.

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V. I. Marchenko, RZM, 51(1975).

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