Staging influence on the magnetic properties of NiCl2 intercalation compounds

Synthetic Metals, 40 (1991) 325-332

325

Staging influence on the magnetic properties of NiC12 intercalation compounds M. E1 H a l i d i Universit~ Hassan H, Facultd des Sciences If, Casablanca (Morocco)

G. C h o u t e a u Service National des Champs Intenses, Centre National de la Recherche Scientifique, B.P. 166X, 38042 Grenoble Cddex (France) (Laboratoire associd d l'Universitd Joseph Fourier, Grenoble)

R. Y a z a m i Laboratoire d'Ionique et d'Electrochimie du Solide, UA CNRS No. 1213, B.P. 75, 38402 Grenoble Cddex (France)

(Received August 10, 1990; accepted January 28, 1991)

Abstract A first-stage NiC12 graphite intercalation compound (GIC) was prepared using very fine natural graphite powder under a high chlorine pressure and at high temperature. A second-stage NiC12-GIC was also prepared on highly oriented pyrolytic graphite (HOPG). The structure of the compounds consists of NiCle layers inserted between the graphene ones. It is shown that the magnetic behaviour is three dimensional for both compounds. The ordering temperature decreases with the stage due to the increase of the interlayer distance.

R6sum6 Un compos6 de premier stade intercalaire du graphite a 6t6 pr~par~ par insertion de la mol6cule NiC12. Le mat~riau de d6part est une poudre de graphite tr~s fine. La r6action d'intercalation se produit ~ haute temperature, sous haute pression de chlore. On a 6galement pr6par6 un compos6 de deuxi~me stade sur du graphite pyrolytique tr~s bien orient6 (HOPG). La structure des deux compos6s est faite de plans de NiC12 ins~r6s entre les plans de graphite. Le comportement magn6tique est tridimensionnel. La temp6rature d'ordre d~croit avec le stade ~tcause de l'augmentation de la distance interplanaire.

1. I n t r o d u c t i o n Graphite intercalation compounds (GICs) have a two-dimensional structure. Many molecules, for instance chlorides or fluorides, may be inserted between the carbon layers (graphene or graphitic layers). The resulting c o m p o u n d is c h a r a c t e r i z e d b y i t s s t a g e , s, w h i c h d e s c r i b e s t h e s e q u e n c e o f t h e s t a c k i n g . T h e d i s t a n c e b e t w e e n t w o i n t e r c a l a t e d l a y e r s Ic is g i v e n b y t h e relation Ic = (S -- 1)de + d l

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326 where dl is the interlayer distance for the first-stage compound and-d~ the distance between the graphene layers in the pure graphite (d~ = 0.335 rim). In the case of chlorides, M~CI~ with M = C o or Ni, three parameters influence the magnetic properties: J, the ferromagnetic exchange interaction between the M ions in the plane; J ' , the antiferromagnetic exchange interplane interaction (J/J' >> 1); and D, an anisotropy constant which keeps the magnetic moments in the planes. For pure NiC12 J/J' = 10 and the single ion anisotropy D = - 0 . 4 K [1]. The structure is of the CdC12 type [2]. It is quite difficult to obtain a first-stage compound and it was thought for a long time that only the second-stage NiC12-GIC could be synthesized. Recently, Yazami and Touzain succeeded in preparing a first-stage NiCle-GIC

[3]. The aim of this paper is to compare the magnetic properties of a firststage and a second-stage compound.

2. Experimental 2.1. S a m p l e p r e p a r a t i o n The preparation of the first-stage compound has been described in detail elsewhere [3, 4]. We emphasize the reaction conditions: high chlorine pressure (10 atm) and high temperature of reaction (700 °C). The specific surface area of the powder is also an important parameter. The structure of the resulting compound was determined by X-ray diffraction and electron microscopy. The repeat distance along the c-axis, Ic, is equal to 0.937 nm. The chemical analysis leads to the average composition Cs.gNiC12.a~, implying a filling factor of 70%, close to that of the stage-2 compound [5]. The second-stage compound was prepared under the same conditions of chlorine pressure and temperature using a highly oriented pyrolytic graphite (HOPG). The average formula is C12.5NiC12.1 and the interlayer distance I c = 1 . 2 7 8 _ 0 . 0 0 6 nm is in very good agreement with the general formula given above for s = 2. For the given conditions (temperature and Pc~) of the intercalation reaction, the final stage depends only on the graphite specific surface area. 2.2. Measuremcmts techniques The magnetization was measured as a function of the field and temperature between 4.2 and 300 K up to 11 T using a superconducting coil. The sensitivity is about 5 × 10 -3 e.m.u. The temperature is determined in zero field with a carbon resistor ( T < 30 K) and a platinum resistor (T> 30 K) thermometer. The accuracy is better than 0.05 K in the whole range. The temperature regulation is achieved using a capacitance bridge in order to be insensitive to the field. A.c. susceptibility measurements have also been performed in the same temperature range at frequencies ranging from 1 to 10 kHz. The a.c. field was 0.03 mT. Both in-phase and out-of-phase components were recorded.

327 3. R e s u l t s

3.1. High f i e l d m a g n e t i z a t i o n Figure 1 shows the magnetization curves obtained at 1.8 K for the stage1 and stage-2 compounds. For stage-2 c o m p o u n d we com pare the magnetization meas u red when the field is applied along the c-axis with the magnetization meas ur ed when the field is perpendicular to the c-axis. The anisotropy is weak. We find the saturation m o m e n t Ms = 1.67 tLB/ Ni ato m for field H perpendicular to the c-axis and Ms= 1.49 t~B/Ni atom for H parallel to it. In the case of CoC12 the anisotropy is m uch bigger: Ms = 2.7 t~B/Co atom for H perpendicular to the c-axis and Ms = 1.95 tLB/Co atom for H parallel to it [6]. F o r the stage-1 NiC12, Ms = 1.53 t~B/Ni atom. It is difficult to com pare this latter value with those of the second stage since the stage-1 c o m p o u n d is a p o wd er sample. However, it is of the same order of magnitude, showing that in both c o m p o u n d s nickel is in the same valency state. In all cases, large high field s upe r i m pos ed susceptibilities are observed. They may corr e s p o n d to van Vleck contributions or to anisotropy. Table 1 summarizes the data. In addition, as shown in Fig. 1, the magnetization saturates in fields of the o r d er of 0.3 T. Such a low field is indicative of the presence of strong ferromagnetic interactions. 3.2. A.c. susceptibility Figure 2 shows the real part, X' (in arbitrary units), of the a.c. susceptibility of the stage-2 c o m p o u n d for both directions: H perpendicular (curve a) and parallel (curve b) to the c-axis. The applied field is of the order of 0.3 Oe and the frequency is 1270 Hz. The sample is a rectangular platelet of 0.3 m m height along the c-axis and 2.6)< 1.5 mme cross section. When H is applied parallel to the c-axis the demagnetizing field coefficient can be evaluated as 0.8. Thus, the internal field is equal to 0.2 )
I

I

+ ÷ / ÷ + + + + + +

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I

÷ ÷ + ~ ÷ + + + + + ~- + +

40 + _ _ x X x N × x N "~30

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N o

x

x o

N o

o

o

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I

J

I

I

I

2

4

6

8

10

H (T)

Fig. 1. Magnetization (in e.m.u./g) of a stage-1 compound (+) and a stage-2 compound (×, H perpendicular to the c-axis; O, H parallel to the c-axis) at 4.2 K up to 11 T.

328 TABLE 1

Saturation magnetization and high field susceptibility for NiCl2 stage-1 and stage-2 GICs. Data for CoC12 stage-1 compound are given for comparison Compound

Saturation moment (Bohr magneton/Ni atom)

High field susceptibility (106 Bohr magneton/2 + ion)

NiCI2 stage-1

1.51 1.90 (ref. 7)

1.64

NiC12 stage-2

1.49 (par.) 1.67 (perp.)

1.314 0.985

CoC12 stage-1

1.95 (par.) 2.70 (perp.)

2.83 2.09

i

r

a

1200

i Stage-2

g 1270Hz

~ 800

x~ 400

0

b 1.

, :.:.::.-__,_q...... l0

..

1.........'............................ 20

T (K) Fig. 2. A.e. susceptibility measured at 1270 Hz of the stage-2 compound. The a.e. field (0.3 Oe) was applied perpencUeu]ar (curve a) and parallel (curve b) to Lhe c-axe. No correction for demagnetizing effeets has been made. Arrows indicate the ordering temperatures.

should be multiplied by five. When H is perpendicular to the c-axis this effect is negligible. This shows that the apparent anisotropy of the a.c. susceptibility is only a demagnetizing factor effect. The two curves in Fig. 2 exhibit a maximum at 15.8 K (curve a) and 16.3 K (curve b). The difference in the temperatures of the two maxima is larger than the uncertainty in the thermometer measurements. In addition it should be noticed that curve a s h o w s a slight shoulder around 16.3 K. We think that these two temperatures have the same meaning as Tcu and Tc~ of CoC12-GICs: at Tcu an order takes place in the planes while at Tc~ the 3D antiferromagnetic order occurs. We shall discuss the problem of the ordering temperatures in these c o m p o u n d s later in this paper. In NiC12-GICs the anisotropy is much weaker than in CoC12-GICs. For this reason the difference ( T c u - T c l = 0 . 5 K) is smaller than for CoC12 ( T c u - T c l = l . 3 K). This is also the reason why the susceptibility peaks are much broader for NiC12 than for COC12.

329

Figure 3 shows the a.c. susceptibility for the stage-1 compound. The measurement was performed with the same a.c. field and at the same frequency as for the stage-2 compound. With the same definitions as above, we find Tcl= 17.8 K and T c , = 2 0 . 7 K. As for the CoC19-GICs, the difference Tcu-Tcl is a decreasing function of the stage. This can be qualitatively explained by the fact that when the compound becomes 'more 2D', Tc, strongly decreases since 2D systems can only order at T c = 0 ; on the other hand the 3D temperature Tc~ is less affected. However, the decrease of Tc, and Tc~ with the stage is indicative of the importance of the interlayer antiferromagnetic interactions and shows that, even in the second stage, the NiC12-GICs are 3D magnetic systems. It should be noted that we did not observe any change in the position of the susceptibility peaks for the stage-1 and the stage-2 compounds with frequency in the range 1 to 10 kHz. This means that no noticeable relaxation effects can be observed. In other words the measured susceptibility is the equilibrium one.

3.3. Static susceptibility Figures 4(a) and (b) show the static susceptibility M/H measured in various fields for the stage-2 compound in both directions (H parallel and perpendicular to the c-axis). The main feature is the difference between the zero field-cooled and the field-cooled susceptibilities below a temperature which is not very different from the temperature of the peaks of the a.c. susceptibilities. This difference can be interpreted as due to the appearance of a remanent magnetization the origin of which will be discussed in the next section. In addition it is worth noting that the quantity M/H is field dependent well above the temperature of the maximum, up to around 25 K, but in this range the remanent magnetization is not measurable. The stage-1 compound has the same overall behaviour. However, the static susceptibility exhibits a plateau at low temperatures rather than a pronounced maximum. This is an indication of stronger ferromagnetic in]

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Fig. 3. A.c. s u s c e p g b i l i t y m e a s u r e d at 1000 Hz of the stage-1 c o m p o u n d .

330 I

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I

0

I

I0

20

r (K)

Fig. 4. Static susceptibility at 5 0 e and 10 Oe for the stage-2 compound: (a), H perpendicular to the c-axis; Co), H parallel to the c-axis. Arrows indicate the direction of the temperature variation. F i g . 5. S t a t i c s u s c e p t i b i l i t y Oe and

for the stage-1

compound

between

4.2 and 30 K measured

at 5

10 Oe.

teractions in stage-1 compounds than in stage-2 compounds. In addition it can be seen in Fig. 5 that the ordering temperature measured at the maximum of the zero field-cooled susceptibility is higher than that of the stage-2 compound (see also Table 2). 3.4. The ordering temperatures Results in the literature s e e m to be s o m e w h a t c o n t r a d i c t o r y (see Table 2). W e shall discuss t h e m briefly here. It is clear that the v a l u e s o f the o r d e r i n g t e m p e r a t u r e s d e p e n d o n the criterion c h o s e n to d e t e r m i n e t h e m . T h e c h o i c e o f the t e m p e r a t u r e b e l o w w h i c h a r e m a n e n t m a g n e t i z a t i o n a p p e a r s is q u e s t i o n a b l e b e c a u s e it can b e due to a small a m o u n t o f a parasitic m a g n e t i c p h a s e (for instance NiC12), n o t d e t e c t a b l e b y X-rays. In addition, the r e m a n e n t m a g n e t i z a t i o n is strongly d e p e n d e n t o n the way it has b e e n obtained. In the p r e s e n c e o f a t o p o l o g i c a l disorder, s u c h as islands o f intercalant, w h i c h is a well-known f e a t u r e of GICs, the t e m p e r a t u r e o f the a p p e a r a n c e o f the r e m a n e n t m a g n e t i z a t i o n is field d e p e n d e n t a n d d o e s n o t define well the o c c u r r e n c e of m a g n e t i c order. F o r t h e s a m e r e a s o n s the c h o i c e o f the t e m p e r a t u r e b e l o w w h i c h the static susceptibility M/H b e c o m e s field d e p e n d e n t is n o t a p p r o p r i a t e b e c a u s e it m a y b e simply the signature o f the o c c u r r e n c e of the long-range i n t e r a c t i o n s b e t w e e n the islands or also due to o t h e r m i n o r i t y phases. It is w o r t h n o t i n g that the a b o v e criteria give always the s a m e value ofTcu = 2 1 - 2 2 K i n d e p e n d e n t of the stage, in c o n t r a d i c t i o n to the generally well-admitted t h r e e - d i m e n s i o n a l c h a r a c t e r o f the m a g n e t i s m in all t h e s e NiC12 intercalation c o m p o u n d s , at least u p to the s e c o n d stage. Since the a.c. susceptibility is f r e q u e n c y i n d e p e n d e n t (or v e r y weakly d e p e n d e n t ) one c a n say t h a t it is always m e a s u r e d at the t h e r m o d y n a m i c

331 TABLE 2 Value and definition of the ordering temperatures given in the literature for various NiC12GICs Compound

Ordering temperature a (K)

Criterion

Pristine

52 49.6

maximum of X ~ c maximum of X ~ c

Stage-1

14.3 21 21 17.8 16.4

maximum of x= appearance of the remanence peak in Xac versus field maximum of X= kink in X~t,~ (50 Oe)

7 7 10 this work this work

Stage-2

18.1 (TcL) 20.3 (Tcu) 19.4 17.3 17.5 (TcL) 22 (Tc~) 15.8 (perp.) 16.3 (par.) 15 (perp.) 15.7 (par.) 25

maximum of X~ appearance of the remanence maximum of X~ maximum of X~ nonlinearity of X~u~ maximum of Xac maximum of Xa¢ kink in X ~ (10 Oe) kink in X ~ c (10 Oe) nonlinearity of X~u~

11 11 12 12 13 13 this this this this this

17.8 17.8

peak in Xac versus field peak in Xac versus field

14 14

Stage-3 Stage-5

Reference

8 9

work work work work work

~rhe indications perp. and par. mean H perpendicular or parallel to the c-axis, respectively. e q u i l i b r i u m . T h e r e f o r e , t h e t e m p e r a t u r e o f i t s m a x i m u m is a g o o d c r i t e r i o n for determining the ordering temperature as in the case of antiferromagnets. T h e k i n k in t h e s t a t i c s u s c e p t i b i l i t y M/H, m e a s u r e d i n s u f f i c i e n t l y l o w f i e l d s , is a l s o a g o o d c h o i c e p r o v i d e d it d o e s n o t v a r y t o o m u c h w i t h t h e field. Using these two criteria one finds, as expected, that the ordering temperature decreases by increasing the stage. T h e d e c r e a s e o f t h e o r d e r i n g t e m p e r a t u r e s Tc~ a n d Tc~ w i t h t h e s t a g e cannot be explained by a reduction of the size of the islands because the filling f a c t o r i s a t l e a s t e q u a l t o 7 0 % w h i c h m e a n s t h a t a l m o s t a l l t h e i s l a n d s are in contact.

4. C o n c l u s i o n s We have prepared for the first time a stage-1 intercalation compound w i t h NiCle a t h i g h t e m p e r a t u r e a n d c h l o r i n e p r e s s u r e o n f i n e l y d i v i d e d g r a p h i t e powder and compared its magnetic properties with those of a stage-2 compound prepared using the same method at the same reaction temperature and the same chlorine pressure on HOPG. The ordering temperature, chosen at the

332 m a x i m u m o f t h e a.c. s u s c e p t i b i l i t y o r a t t h e l o w e r k i n k o f t h e s t a t i c s u s c e p t i b i l i t y , is a d e c r e a s i n g f u n c t i o n o f t h e s t a g e s h o w i n g t h a t t h e m a g n e t i c p r o p e r t i e s o f s t a g e - 1 a n d s t a g e - 2 c o m p o u n d s a r e 3D.

Acknowledgements W e t h a n k R. T u r f o r p e r f o r m i n g t h e m a g n e t i c m e a s u r e m e n t s i n s t a t i c fields a n d Miss M. C h o u t e a u f o r b u i l d i n g p a r t o f t h e a.c. s u s c e p t i b i l i t y apparatus.

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