Magnetic properties of stage-1 NiCl2-GUC2

Magnetic properties of stage-1 NiCl2-GUC2

Svnthetic Metals, 34 (1989) 519 524 5]9 MAGNETIC PROPERTIES OF STAGE-I NiCI:-GICs.* J, T. NICHOLLS and G. DRESSELHAUS Massachusetts Institute of Te...

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Svnthetic Metals, 34 (1989) 519

524

5]9

MAGNETIC PROPERTIES OF STAGE-I NiCI:-GICs.* J, T. NICHOLLS and G. DRESSELHAUS Massachusetts Institute of Technology, Cambridge, MA 02139 (U.S.A.)

Abstract The magnetic properties of stage-2 NiC12-GICs are very similar to those of stage-2 CoC12GICs, including behavior which has been ascribed to the 2D XY model. The main differences between these systems are in the magnitude of the spin and in the anisotropy energy. We report here a similarity in the magnetic behavior between the corresponding stage-1 compounds, based on ac susceptibility measurements.

Introduction Pristine NiC12 and CoC12 have been studied as possible XY magnets because the anisotropy favors the spins lying down in tile x y plane. In these metal chlorides, the in-plane exchange iateraction J is strong and ferromagnetic, whereas the interplanar exchange interaction 3' is weak and antiferromagnetic. Upon intercalation, the strong in-plane exchange constants of the pristine NiCl2 and COC12 are almost unaffected, but a dramatic decrease in J ' is expected. Below the 3D ordering temperature, the spins align ferromagnetieally oil the intercalate layers, and these sheets are ordered antiferromagnetically along the c-axis. The ground state of the free Ni 2+ ion is 3F4 and has a 3d s configuration. The L=3 states, common to both COC12 and NiC12, are split by a cubic crystal field into three sets of levels (T:g, Tlg, A2g). In NiC12, the crystal field parameter that determines the order of these three levels is of the opposite sign to that in CoC12 and the 3A2g level is lowest in energy. The orbital momentum is quenched in the 3A29 level and the spin is unchanged at S = 1. In contrast, in CoC12 there is further splitting of the 4Tlg level by the spin-orbit interaction changing the real spin S = 3/2 to an effective S = 1/2 state. Based on the crystal field analysis it is of no surprise that the leading contributions to the magnetic Hamiltonian 7-i for the NiC12 compounds[I] are isotropic Heisenberg exchange terms: n = -2JE#,. i>j

#j -

+ DE(s? i>k

- sL - sL)

(1)

i

where the index j (k) is over nearest neighbor intraplanar (interplanar) spins. The values J = 21.7 K, and J ' = - 0 . 8 K have been measured for pristine NiC12 by magnetization[2,3] and neutron scattering[I] experiments. The last term in the Hamiltonian is the small anisotropy that favors the spins to lie in the xy plane rather than along the c-a×is, and its magnitude D=0.4 K can be explained by the dipole-dipole interaction.Ill A six-fold in-plane anisotropy field h6 is probably present in pristine NiC12; however the neutron scattering experiment could not determine the orientation of the spins within the x y plane.[1] tSupported by NSF Grant #DMR 88-19896. 037%6779/89/$3.50

© Elsevier Sequoia/Printed in The Netherlands

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T

(002)

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NiCI2-GIC

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Stage -1

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~3 (005)

,,4-,-

0

t 0

1oo61 I

0

20

(007~ (008)

40 60 28 (degrees)

80

100

Figure 1: (00/) x - r a y diffraction scan of H O P G - b a s e d stage-1 NiC12-GIC. No resolvable peaks for any higher stage or for pristine graphite are observed.

The value of the anisotropy of the g-value, g ~ - gcc ~- 2.156 - 2.096 -- 0.06 obtained in the stage-2 NiC12 compound[4] is an indication that the intercalation compound also shows XY behavior. This was confirmed with neutron scattering.[5] It is expected that a similar XY anisotropy will be found in the intermediate stage-1 compounds, and it is further assumed that the above Hamiltonian will also describe stage-1 NiC12-GICs. The weak anisotropy in the stage-2 NiCl~-GIC appears not to have a significant effect on the transition temperature, which is at T ~ J S 2 / k s , as are those of CoC12-GICs which have much stronger spin anisotropy. In this paper we report new results on the magnetic properties of stage-1 NiC12-GICs and show these properties to be consistent with XY spin ordering. This first stage compound, on which we have reported earlier[6], provides an important link between pristine NiCl~ and stage-2 NiC12-GICs; the latter is considered a good candidate for observing two-dimensional planar magnetic (2D XY) behavior. Samples The reaction kinetics and sample preparation conditions for stage-2 NiC12-GICs have been extensively investigated.t7] The intercalation reaction of metal chlorides requires the presence of a C12 atmosphere; in general, the higher the chlorine pressure Pea2, the lower the stage obtained. We have made stage-1 samples using high temperatures (660°C) and C12 pressures of about 4 atmospheres at room temperature. The greater difficulty in synthesizing stage-1 NiC12-GICs compared to stage-1 CoCI~-GICs is due to the higher melting point of the former (Tmp(NiC12)--1001°C, Tmp(CoC12) ~-- 724°C), which causes slower kinetics at a given reaction temperature (e.g., 560°C).

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1

0 0

500

1000

1500

Mognetic field (Oe) Figure 2: In-plane a.c. susceptibility X~ vs. H of a HOPG-based stage-1 NiC12-GIC samples at different temperatures: (a) 4.2 K (b) 17.4 K (c) 19.0 K (d) 21.8 K and (e) 23.0 K. The dominant peak is at H¢~.

In this study highly oriented pyrolytic graphite (HOPG) pieces, with a mass of 1 mg or less, have been intercalated with NiC12. The intercalation reaction was carried out in a quartz ampoule and took 1 2 months, after which the samples were blue, typical of stage-1 acceptor compounds. From the peak positions of the (00/) x-ray diffraction scan, shown in Fig. 1, the repeat distance Ic=9.36+0.05 /~ was obtained. This is consistent with the staging formula Ic = (n - 1)co + d, where Co = 3.35 /~. Note that in Fig. 1 there is no indication of any minority phase as in earlier samples[6] which were based on single crystal flakes. It is unusual to have greater success in synthesizing HOPG-based GIC samples; normally HOPG is regarded as one of the more difficult graphite hosts to intercalate.

Experimental In-plane a.c. susceptibility measurements, X~a, were performed on a HOPG-based stage 1 NiC12-GIC sample. The modulating field is parallel to the in-plane applied field H. Figure 2 shows five different traces of X ~ ( H ) vs H, the data being taken at the indicated temperatures T. All the traces in Fig. 2 are shown on the same scale and have not been offset; the amplitude of the He2 peak goes through a maximum at ~ 19 K. At the lowest temperature T=4.2 K (plot(a)) the field value of the susceptibility maximum, interpreted as a field-induced phase transition, is He2 = 1100 Oe. As the temperature increases, the transition field moves to lower field values, as shown in plots (b)--*(e). This peak structure persists up to T = 21.8 K (plot(d)), but is not observed at T = 23.0 K (plot(e)). Slightly different susceptibility results were obtained with the earlier samples[6] based on natural graphite flakes. Namely, two anomalies of approximately equal strength at Hcl and H ~ were observed. The HOPG-based samples reported here show a clear and dominant peak at He2, whereas the peak at He1 is barely observable. Due to the prominence of the

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I I I I I 10 15 20 25 Temperature(K)

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Figure 3: The H - T phase diagram derived from Xa~(H) data, showing data points for the H~2 transition. The solid line is a guide to the eye.

He2 peak we are able to track its temperature dependence down to the lowest temperatures. The values of He2 as a function of temperature T are plotted as a phase diagram in H - T space, as shown in Fig. 3.

Discussion The N~el temperature, the temperature at which the layers begin to couple antiferromagnetically, is interpreted as being close to the temperature at which the peak structure in X~(H) disappears. We estimate TN=22.0rkO.5 K, in agreement with our previously reported resistivity measurements.[6] Our value of TN is higher than any transition temperature (T~t or Tcu) quoted for a stage-2 NiClz compound.[8] A similar hierarchy of transition temperatures is observed in the stage-1 and stage-2 CoC12-GICs.[9] In the 3D ordered phase of this system, we would expect a number of transitions when a magnetic field is applied in the xy-plane. In zero field, the compound is an antiferromagnet (AF); at a certain field the antiferromagnetic phase is expected to flop into the spin-flop (SF) phase, where the spins on alternate layers are aligned at angles of =1=7r/3with respect to the magnetic field applied along an easy direction.[10] At even higher fields, there will be an eventual alignment of the spins along the applied field, leading to the spin aligned paramagnetic (P) phase. Experimentally, the ratio Hc~/Hcl ~ 2 in stage-1 NiC12-GICs (see traces (b) and (c) in Fig. 2) is similar to that observed in stage-1 CoCI~-GICs (300/160) (Ref. [10]) and in stage-2 NiC12-GICs (25/15) (Ref. [8]) but cannot be explained by the mean field theory proposed by Szeto et a/.[10] where the ratio H(SF --~ P ) / H ( A F ---* SF) is 3.0 at T = 0 K. The peak structure at H~I in X~ vs. H for stage-1 CoC12-GICs is not observed in all samples; more recent works on better characterized samples generally find the peak at Hc~ to be more dominant. The strong sample dependence of the Hd structure in both stage-1 NiClz and CoCI~-GICs is not well understood, but may be due to the range of angles of the easy axes on the various magnetic layers with respect to the applied magnetic field.

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To estimate the magnitude of the interplanar antiferromagnetic coupling J' we shall first ignore the low field structure and assume that the in-plane anisotropy field hs is small compared to J'. We can estimate the magnitude of J ' from the saturation field, which is the field that is required, at T = 0 K, to overcome the antiferromagnetic interplanar coupling, and is given by H,.t ~ zJ'S/g,Mts (2) where lts is the Bohr magneton. The number of next nearest magnetic neighbors on adjacent Ni 2+ layers, z = 12, is the same for both the stage 1 and pristine compounds. From our studies of the magnetization M(H) and resistivity p,(H) [6], we estimate that H,~t -- 3.0 kOe for stage-1 NiC12-GICs, and hence the value of IJ'/JI can be calculated. In Table 1 this ratio is estimated for stage-2 NiC12-GICs assuming that H,~t = 100 Oe.[8] The ratio of J'(pristine)/J'(stage-1) is ,--30 for both NiCl2 and CoC12-GICs, suggesting that a similar mechanism is responsible for the reduction of J ' in both stage-1 compounds. An alternative interpretation of the x(H) data is to assume that He2 is the saturation field. This results in a smaller vMue of J'/J also given in Table 1. The complication in interpretation arises because paramagnetic impurities could lead to higher H~t values in less pure samples. The results presented in Fig. 2 show a pronounced transition at He2, indicative of a homogeneous sample, and the magnitude of the susceptibility drops very quickly to a low value on passing through H¢2. The (00l) x-ray scans in Fig. 1 indicate a high degree of structural order in our latest samples. A number of factors may be responsible for the purely three dimensional magnetic behavior of NiC12 and CoC12 stage-1 compounds compared to the quasi-two dimensional behavior of their respective stage-2 compounds. Firstly J ' (stage-2) is reduced relative to J' ( s t a g e - I ) because the distance between the magnetic ion layers in the stage 2 compounds is increased due to the presence of an extra graphite layer. Secondly, these 3D properties depend on the enhancement of the interplanar antiferromagnetic interaction in the stage 1 compound relative to stage-2 due to the in-plane spin correlation length (~ through the formula J ' s ~ ( ~ / ~ ) ~ ~ kBTN,

(3)

where TN is the 3D ordering temperature. For stage-1 NiCI2-GICs and CoC12-GICs we estimate J ' from H,,t, while from Eq. 3 we get ~ values of 90/~ and 40/~, respectively. For the stage-2 CoCI2-GICs, neutron scattering experiments show that (~ saturates at 900+150A for T < To,,[ll] while the value of ~a calculated from Eq. 3 for stage-2 NiCI2GIC is of the same order, i.e., 480/~.

Conclusions X,Ve have prepared stage-1 NiC12-GIC samples of a higher quality than previously reported,[6] as is reflected in the clarity of the (00g) x - r a y scans and the magnitude of the field-induced peak structure at He2. Likewise the He2 vs. T phase diagram is also very smooth. The susceptibility results presented here, together with the magnetic transport properties, presented elsewhere,[6] suggest that stage-1 NiC12-GICs, like CoC12-GICs, have an antiferromagnetic stacking of the ferromagnetic layers. Neutron scattering measurements axe necessary to confirm the suggested spin structure.

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Table 1: Comparison between the properties of stage-1 and stage--2 NiC12-GICs and pristine NiC12. Property

Pristine NiC12 stage-1 NiC12-GIC

TN

54.5 K

Ucl(0) Ho2(0) Usat(0) IJ'/JI (0

0.037

IZ'/Jr(o)

22.04-0.5 K 380 Oe 1100 Oe 3000 Oe 1.5x10 -4 5.7x10 -4

stage-2 NiC12-GIC

Tel= 19.5 K (") 15 Oe(") 25 Oe (") 100 Oe (") 5.2×10 -5

(~) Ref.[8]. (b) calculated using Hs~t. {c) calculated using He2(0).

References [1] P. A. Lindgard, R. J. Birgeneau, J. Als-Nielsen, and H. J. Guggenheim. J. Phys. C: Solid State Phys., 8, 1059, (1975). [2] D. Billerey, C. Terrier, A. J. Pointon, and J. P. Redoules. J. Mag. Mag. Mats., 21, 187, (1980). [3] J. de Gunzbourg, S. Papassimacopoulos, A. Miedan-Gros, and Y. A11ain. J. Phys.

(Paris), a2, CI 125, (1971). [4] M. Suzuki, K. Koga, and Y. Jinzaki. J. Phys. Soc. Ypn, 53, 2745, (1984). [5] D. G. Wiesler, M. Suzuki, and H. Zabel. Phys. Rev., B36, 7051, (1987). [6] J. T. Nicholls, J. S. Speck, and G. Dresselhaus. Phys. Rev., B39, 10047, (1989). [7] S. Flandrois, J. M. Masson, J. C. Rouillon, J. Gaultier, and C. Hauw. Synthetic Metals,

3, 1, (1981). [8] M. Suzuki and H. Ikeda. J. Phys. C: Solid State Phys., 14, L923, (1981). [9] H. Ikeda, Y. Endoh, and S. Mitsuda. J. Phys. Soc. Japan, 54, 3232, (1985). [10] K. Y. Szeto, S. T. Chen, and G. Dresselhaus. Phys. Rev., B33, 3453, (1986). [11] D. G. Wiesler and H. Zabel. Phys. Rev. B, 36, 7303, (1987).