Magnetic properties of single-crystalline RNiC2 compounds (R = Ce, Pr, Nd and Sm)

~ ELSEVIER

Journal of Magnetism and Magnetic Materials 182 (1998) 161-171

iownalof am~etlsm ,JR magnetic materials

Magnetic properties of single-crystalline RNiC 2 compounds (R = Ce, Pr, Nd and Sin) Hideya Onodera*, Yutaka Koshikawa 1, Masashi Kosaka, Masayoshi Ohashi, Hiroshi Yamauchi, Yasuo Yamaguchi Institute for Materials Research, Tohoku University. Sendal 980-77, Japan

Received 4 June 1997; received in revised form 4 August 1997

Abstract Magnetic properties of CeNiC2, PrNiC2, NdNiC2 and SmNiC2 compounds have been investigated by means of magnetization measurement on the single-crystalline samples. CeNiC/ is a antiferromagnet of T~ = 19.8 K with a moment direction parallel to the a-axis. Two order-order transitions appear at 2.2 and 10.0 K. In a magnetization curve at 1.5 K of a Van Vleck paramagnet PrNiC2, there appear two anomalous increases at 17.5 and 140 kOe. NdNiCz is also a antiferromagnet of TN = 17.2 K with a moment of 2.45 p.~ parallel to the a-axis. There appears an order-order transition at 4.0 K. The magnetic structure is transformed directly into ferromagnetic one by a field of 38 kOe at 4.2 K. SmNiC2 is a novel ferromagnet of Tc = 17.5 K with a moment of 0.32 ~B parallel to the a-axis. Besides the ferromagnetic transition is of first order. There appears three anomalous changes in the magnetizations at T,1 = 4.3 K, T~2 = 13.0 K and T,3 = 25.0 K. The susceptibilities around 300 K presumably stand for a valence fluctuation of Sm ions. ~ 1998 Elsevier Science B.V. All rights reserved. PACS: 75.25; 75.30C Keywords: Rare-earth compounds; Magnetic anisotropy; Antiferromagnet; Ferromagnet; Valence fluctuation

1. Introduction D u r i n g the last decade, R N i C 2 c o m p o u n d s (R: a rare earth) with the o r t h o r h o m b i c C e N i C 2 - t y p e

* Corresponding author. Tel.: 81-22-215-2037; fax: 81-22-2152036; e-mail: [email protected]. 1Present address. Olympus Optical Co., Ltd., Kuboyamachou 2-3, Hachioji, Tokyo 192, Japan.

structure have been i n v e s t i g a t e d by m a g n e t o m e t r i c , n e u t r o n diffraction a n d MiSssbauer e x p e r i m e n t s on the p o w d e r a n d single-crystalline s a m p l e s [1--11]. T h e Ni a t o m s c a r r y no m a g n e t i c m o m e n t , since YNiC2 a n d L a N i C 2 c o m p o u n d s show very w e a k t e m p e r a t u r e - d e p e n d e n t m a g n e t i c susceptibilities of 10 . 7 e m u / g [1, 10]. T h e R m o m e n t s o r d e r antif e r r o m a g n e t i e a l l y in R N i C 2 (R = Nd, G d , Tb, Dy, Ho, Er a n d Tm) c o m p o u n d s at t r a n s i t i o n t e m p e r a t u r e s below 30 K. Recently, o u r p r e c e d i n g w o r k

0304-8853/98/$19.00 (C' 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 0 4 - 8 8 5 3 ( 9 7 ) 0 1 0 1 1-1

162

H. Onodera et al. / Journal of Magnetism and Magnetic Materials 182 (1998) 161-171

Table 1 Lattice parameters and magnetic properties of CeNiC2, PrNiC2, NdNiC2 and SmNiC2 Compound

CeNiC2

PrNiC2

NdNiC2

SmNiC2

Lattice parameter a (A) b (,~) c (,~)

3.869(5) 4.542(5) 6.154(5)

3.828(5) 4.546(5) 6.147(5)

3.787(5) 4.537(5) 6.126(5)

3.703(5) 4.529(5) 6.098(5)

Van Vleck para. -

Antiferro. 17.2 4.3

Tt2 (K)

Antiferro. 19.8 2,2 10.0

Tt3 (K) Moment direction

Ferro. 17.5 4.3 13.0 25.0

Ila

[ja

Ila

Paramagnetic Curie temp. 0~, (K) 0pb (K) 0~, (K) Xo(emu/g)

4.1 - 61 - 38 1.2 × 10-5

12.7 - 20.4 6.3 6.9 × 10 - 6

24.6 - 17.8 - 6.0 0

(2.54)" (2,54)" -

(3.58)a (3.58)" (3.58)" -

3.45 3.62 3.60 2.45

- 30 340

40 210

- 100 - 770

Magnetism TN, Tc (K) Ttl (K)

Moment (ga/f.u.) /2aff Peeeb

/~eCff #order CEF parameter (K/a~) A° A22

(2.54) a

-0.32

aThe theoretical value of R3+ is assumed.

have revealed that the magnetic properties of RNiC2 depends strongly on competition between magnetic interactions and magnetic anisotropy as follows [10]. In the heavy rare earth RNiC2 compounds, crystalline electric field (CEF) parameters, A ° and A22, drastically change its magnitude and sign between H o N i C 2 and ErNiC2, i.e. the C E F parameters become large by one order in ErNiC2 c o m p a r e d with those in HoNiC2. This drastic C E F change is not a c c o m p a n i e d with any structural change. The R replacement brings a b o u t a n o r m a l lanthanide contraction of lattice parameters, where the decrease of lattice parameter a is slightly large c o m p a r e d with those of b and c. Consequently, the R m o m e n t s in ErNiC2 and T m N i C 2 with the positive second-order Stevens factor as align along the a-axis whose direction is the same as that in

N d N i C 2 with the negative as. There m a y occur enhancement of magnetic transition temperatures by the strong C E F interactions in ErNiC2 and TmNiC2, and hence the Nbel temperature of H o N i C 2 is the lowest a m o n g the heavy rare earth c o m p o u n d s , i.e. the magnetic transition temperature in this series deviates remarkably from the de Gennes rule. Furthermore, it surely seems that the antiferromagnetic structure is dependent on the strength of CEF. In GdNiC2, TbNiC2, D y N i C 2 and H o N i C z whose C E F parameters are relatively small, there a p p e a r noncollinear c o m m e n s u r a t e a n d / o r i n c o m m e n s u r a t e structures [5-9]. The magnetic interactions in RNiC2 m a y be advantageous to the noncollinear arrangement of R moments. O n the other hand, the R m o m e n t s in ErNiC2 and T m N i C 2 are forced to align collinearly by the

H. Onodera et al. / Journal of Magnetism and Magnetic Materials 182 (1998) 161-171

strong uniaxial anisotropy, and hence a simple antiferromagnetic structure with a propagation vector of [0 0 1] is realized [3, 4]. Less informations on the magnetic properties of the light R compounds have been reported than those of the heavy ones. NdNiC2 is an antiferromagnet with TN = 7 K and the propagation vector of [~~ ~ 0] where the Nd moments align along the a-axis [1,2]. PrNiC2 shows no magnetic transition down to 4.2 K [1]. Very recently, Motoya et al. have revealed that CeNiC2 is an antiferromagnet of TN = 20 K with two order order transitions at 2.2 and 10 K. The transition at 10 K is a commensurate-incommensurate one [11]. There is no study on SmNiC2 and EuNiC2. In the present work, the magnetic properties were investigated on the CeNiC2, PrNiC2, NdNiC2 and SmNiC2 compounds, whose single crystals were successfully prepared (see Table 1). The purpose of this work is to obtain more precise information on their magnetic properties.

2. Experimental Single-crystalline RNiC2 compounds were synthesized by the procedure described in the previous reports [-6, 10]. X-ray diffraction measurements on the powdered samples were carried out using Fe-K~ radiation. The diffraction patterns exhibit only the lines characteristic of the CeNiCz-type structure. Fig. 1 shows the dependence of the lattice parameters on R a + ionic radius, where the present results are shown together with our previous results [6-10]. These values are similar to those reported previously within experimental errors [1, 3, 12, 13]. The single-crystalline samples for the present measurements were shaped into spheres with diameter of 1-2 mm. The magnetization was measured as functions of temperature and magnetic field using magnetometers with sample-extraction, S Q U I D and vibrating-sample methods. The temperature dependence of magnetization was measured in the range 2.0-300 K. The magnetic fields below 10 kOe were used to determine the magnetic transitions and the paramagnetic susceptibility, as well as those up to 150 kOe to measure magnetization

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processes. The high magnetic fields were produced using a water-cooled Bitter-type magnet at the High Field Laboratory for Superconducting Materials (HFLSM) in T o h o k u University. When we measure the magnetization of powdered sample in the high field for comparison, the sample powdered from the single crystal is sealed in a polytetrafluoroethylene (Teflon) capsule with helium gas and BN powders, and then the sample powder particles are free to rotate in the magnetic field.

3. Results and discussion 3.1. CeNiC 2

CeNiC2 is an antiferromagnet of TN = 20 K, and its magnetic structure at 4.5 K is described by a propagation vector of [½ ½ 0] [11]. There occurs a commensurate-incommensurate transition at 10 K. The Ce moment direction could not be determined precisely because of the limited number of observed magnetic reflection lines. Fig. 2 shows the temperature dependence of the magnetizations M and its differentials d M / d T of CeNiC2 which are measured under a field applied along the three crystallographic axes. Small antiferromagnetic cusps are observed at TN = 19.8 K in the curves along the a- and b-axis, and there

164

H. Onodera et al. / Journal o f Magnetism and Magnetic Materials 182 (1998) 161-171 i

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appears no anomaly at TN along the c-axis. It seems likely that there occurs an order-order transition around T~I = 2.2 K where anomalous changes of magnetization appear similarly in all the M - T curves. This transition well corresponds to an anomaly at 2.2 K observed by the specific heat measurement [11]. The other order-order transition occurs at Tt2 = 10.0 K. As reported by neutron diffraction experiment [11], the transition a t Tt2 is a commensurate-incommensurate one where the propagation vector changes from [:.~ 1 ~1 03 to [0.465 0.465 0]. The commensurate-incommensurate transitions have been also found in DyNiC2 and HoNiC2, where the R moments align noncollinearly [5, 8]. The susceptibility and the magnitude of antiferromagnetic cusp at TN is rather larger in the M - T curve along the a-axis than those along the b- and c-axis. This suggests that the Ce moments align along the a-axis. It is noteworthy that in the RNiC2 system the magnitude of antifer-

romagnetic cusp as well as that of susceptibility corresponds well to the magnitude of moment component along the applied field. In ErNiCz and TmNiC2 which have the R moment parallel to the a-axis, there appears the antiferromagnetic cusp only in the M - T curve along the a-axis and no anomaly is observed around TN along the b- and c-axis [10]. In TbNiCz, DyNiC2 and HoNiC2 which have noncollinear antiferromagnetic arrangements of the R moments, there appear the cusps along the three crystallographic axes by the magnitudes corresponding to the moment components parallel to the axes [6-8, 10]. Fig. 3 shows the temperature dependence of the reciprocal susceptibilities of CeNiC2 along the three axes. The reciprocal susceptibilities do not satisfy the Curie-Weiss law in any axis. The fittings were done satisfactorily by assuming temperatureindependent constant susceptibilities Xo and the effective paramagnetic moment #,ff to be 2.54 laB/ f.u. which is the theoretical value for Ce 3 +. The paramagnetic Curie temperature is deduced to be 0p = 4.1 K, 0b-- - 6 1 K and 0v = - 3 8 K along the a-, b- and c-axis, respectively. By the procedure described before [9, 10, 14], the CEF parameters, A2° and A 2 are obtained to be - 30 and 340 K/a~, respectively. The A ° values are smaller than the A 2 values, i.e. the A ° value is not a leading term in the CEF interaction. In order to easily predict the Ce moment direction, the coordinate describing the CEF can be transformed among the three crystallographic axes so as to give the maximum Mz°t value. a

c

H. Onodera et al. / Journal of Magnetism and Magnetic Materials 182 (1998) 161 17l

When the z-direction is assumed to be the a-axis, the maximum IA°L values are obtained to be 180 K / a 2. By the CEF theory in the lowest-order approximation, the magnetocrystalline anisotropy energy E a and the anisotropy constant K x can be expressed in terms of the second-order CEF parameter A ° as follows:

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where 0 is the angle between the easy magnetization direction and the z-axis, ~j the second-order Stevens factor and ( r 2 ) the average value of r 2 for 4f electrons in a given J state. When the sign of IA°I is known, one can predict an easy magnetization direction of the R moment. The R moment is parallel to the z-axis when ~j and ]A°l have different signs, and perpendicular to the z-axis when as and [A°L have the same sign. The positive A ° value implies that the Ce moment direction is parallel to the a-axis (z-axis), since the negative second-order Stevens factor :~j for Ce 3+ requires the moment direction to be parallel to the z-axis. The values of constant term of susceptibility are deduced to be )~ = 1.14x 10 -5 emu/g, Zb = 1.11 x 10 -s emu/g and Z; = 1.23 x 10-5 emu/g, respectively, and the average is 1.2 x 10-s emu/g. Although these values are considerably large compared with ~ 10- ~ emu/g in LaNiC2 and YNiC2 [1, 10], the reason is not clear in the present. Fig. 4 shows the magnetization M - H curves of CeNiC2 at 4.2 K measured in the high magnetic fields up to 150 kOe. It is clear that CeNiC2 is magnetically anisotropic. Although the magnetization along the a-axis increases more rapidly than those along the other axes, there occurs no fieldinduced transition up to 150 kOe. Unfortunately, the magnitude of ordered moment was not determined under the present field. No information on the field-induced transition and/or the saturation magnetization was obtained in the M - H curves at various temperatures up to 22 K. The magnetization process of free powder is also shown in the figure, where it is nearly same to that along the a-axis. This fact makes sure the interpretation of the M T curves described above, i.e. the Ce moments align parallel to the a-axis.

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H (kOe) Fig, 4. Magnetization curves along the three crystallographic axes of CeNiC2 at 4.2 K under the applied field up to 150 kOe. The curve of a powder sample whose particles are free to rotate in the applied field is also shown here for comparison.

3.2. PrNiC2

It has been reported that PrNiC2 has no magnetic order down to 4.2 K [1]. This is confirmed by the present measurements as shown in Fig. 5 which shows the temperature dependence of the magnetizations and the reciprocal susceptibilities down to 2 K along the three crystallographic axes of PrNiC2. The magnetic transition temperatures are 19.8 K in CeNiC2 and 17.2 K in NdNiC2 as mentioned later. It is, therefore, hard to consider that the magnetic interaction become weak specially in PrNiC2 compared with those in CeNiC2 and NdNiC2. Consequently, it is considered that PrNiC2 is a Van Vleck paramagnet with a singlet ground state. As shown in Fig. 5b, the reciprocal susceptibilities do not satisfy the Curie-Weiss law in any axes as same as the case of CeNiC2. The fittings were done satisfactorily by assuming temperature-independent (constant) susceptibilities Zo and the effective paramagnetic moment Par to be 3.58 g,/f.u, which is the theoretical value for Pr 3 +. The paramagnetic Curie temperature is obtained to be 0~ = 12.7 K, 0 b = - 2 0 . 4 K and 0~, = - 6 . 3 K along the a-, b- and c-axis, respectively. The CEF parameters, A ° and A 2 are obtained to be 40 and 210 K / a g , respectively. The values of constant susceptibility are deduced to be X~ = 7.63x 10 -~', Xbo= 6.47 x 10 - 6 and Z~ = 6.49 x 10 - 6 emu/g, respectively, and the average is 6.9 x 10-6 emu/g.

166

H. Onodera et al. / Journal of Magnetism and Magnetic Materials 182 (1998) 161-171 0.6

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Fig. 6. Magnetization curves along the three crystallographic axes of PrNiC2 at 1.5 K.

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20 Fig. 6 shows the magnetization M - H curves of PrNiC2 at 1.5 K measured in the high magnetic fields up to 150 kOe. It is clear that PrNiC2 has relatively weak magnetic anisotropy. It is remarkable that the anomalous increases along the a-axis are observed around 17.5 and 140 kOe. The M - H curves along the a-axis at various temperatures are shown in Fig. 7. Although the anomalous increase around 17.5kOe disappears at 4.2K, the one around 140 kOe is clear at 4.2 K and dims down at 10 K. The 4f CEF levels in PrNiCz can be calculated using the CEF parameters A ° and A~ deduced from the paramagnetic susceptibilities described above. The ground state is a singlet of -0.1131 - 4 ) + 0.4821- 2 ) - 0.71410) + 0.48212) - 0.11314), and the first excited level is also a singlet of 0.2811 - 3) - 0 . 6 4 9 I - 1 ) + 0.64911)-0.28113). These levels are separated by 0.01 K. This separation is very sensitive to the values of CEF parameters. It is

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remarkable that the transition probability between the ground and first-excited singlets equals zero. This explains well that PrNiC2 is a Van Vleck paramagnet and there occurs no Van Vleck transition down to 2 K, since there is no exchangeinduced moment by the level transition between the two singlets. The second excited state is also a singlet of -0.3271- 4) + 0.6271- 2) + 0f0) - 0.62712) + 0.32714), which is separated by 26.9 K from the

H. Onodera et al. / Journal of Magnetism and Magnetic Materials 182 (1998) 161 171

ground state. However, the 4f CEF levels derived using only A ° and A 2 do not reproduce the magnetization processes shown in Figs. 6 and 7. In order to interpret the present results on the basis of the 4f CEF levels, therefore, it is necessary to obtain the precise information on the higher-order CEF parameters.

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3.3. NdNiC2 NdNiC2 is an antiferromagnet with TN = 7 K [1], and its magnetic structure at 4 K is described by a propagation vector of [½½ 0] with the Nd moments aligned along the a-axis [3]. The present work, however, provides a different result from the previous reports. Fig. 8 shows the temperature dependence of the magnetizations and the reciprocal susceptibilities along the three axes of NdNiC2. The N6el temperature is determined to be 17.2 K which is rather higher than the reported value of 7 K. The large antiferromagnetic cusp appears in the M - T curve along the a-axis, suggesting that the Nd moment aligns along the a-axis. This present result is consistent with the magnetic structure reported by Yakinthos et al. [3]. Furthermore, there occurs an order- order transition at T, = 4.0 K where the anomalous change of magnetization appears also in all the M T curves. The differences in the reciprocal susceptibilities along three axes are small as shown in Fig. 8b. Being different from the cases of CeNiC2 and PrNiC2, three curves satisfy the Curie-Weiss law above 100 K without assuming the temperature-independent constant susceptibility Zo. The paramagnetic Curie temperature is deduced to be 0 ~ = 2 4 . 6 K , 0 b = - - 1 7 . S K and 0~ = - 6.0 K along the a-, b- and c-axis, respectively. The CEF parameters, A ° and A 2, are obtained to be - 100 and - 7 7 0 K/ag, respectively. The coordinate transformation among the three crystallographic axes gives the maximum IA°I value of 450 K/a2o when the z-direction is assumed to be the a-axis. The positive A ° value explains well the Nd moment direction parallel to the a-axis, since the second-order Stevens factor :¢s is negative for Nd s +. The values of paramagnetic moment of ~fr = 3.45 laB/f.u., ~eff b 3.62 laR/f.u, and #err¢= 3.60 ~ts/f.u. are close to the theoretical value of 3.62 ~tB for Nd 3 + ion. =

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100

200

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T (K) Fig. 8. Temperature dependence of (a) the magnetizations and (b) the reciprocal susceptibilities along the three crystallographic axes of NdNiC2.

Fig. 9 shows the magnetization M H curves of NdNiC 2 at 4.2 K measured in the high magnetic fields up to 150 kOe. It is clear that NdNiC2 has a strong magnetic anisotropy. The magnetization along the a-axis increases rapidly around 37.1 kOe, and reaches the value 2.45 laB/f.u, which is smaller than the full moment 3.27 lab of Nd 3+ ion. The magnetization process of free powder made of the single crystal is also shown in the figure, where the magnetizations are smaller than those along the a-axis. This confirms also that the Nd moments align parallel to the a-axis. It is remarkable that the magnetic structure is transformed directly into the ferromagnetic one at He. TbNiC2 which has the same propagation vector with that of NdNiC2 is transformed into the intermediate phase at Hc 1 [6]. The intermediate structure gives the half of the full magnetization at the ferromagnetic arrangement. This intermediate structure also appears in DyNiC2 with the propagation vector of [~: 1 1 0] [83

168

H. Onodera et al. / Journal of Magnet&m and Magnetic Materials 182 (1998) 161-171

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as well as in HoNiCz, ErNiC2 and TmNiC2 whose virgin magnetic structures are described by the propagation vector of [0 0 1] [3-5, 10]. The feature of the field-induced transition in NdNiC2 is unique in the RNiCz series. It is possible that the magnetic interactions are modified by the lattice parameter change like the cases of heavy rare earth compounds [10]. Fig. 10 shows the magnetization processes along the a-axis below 100 kOe at various temperatures. The critical field He is nearly constant up to 10 K. A single metamagnetic transition below 10 K splits into two-step transitions at He1 and HoE. The transition at He2 is observed barely at 21 K which is beyond TN = 17.2 K, and smears out above 21 K. The magnetic phase H - T diagram of NdNiC2 is shown in Fig. 11 where IF means an induced ferromagnetic phase. The transition above TN is attributable to the existence of magnetic short-range order. The similar behaviors have been observed in GdNiC2 [9] and TbNiCz [6].

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The magnetic properties of SmNiC2 are reported for the first time in the present work. Fig. 12 shows the temperature dependence of the susceptibilities and the differentials d M / d T of SmNiC2 which are measured under an applied field along the three crystallographic axes. It is a surprise that SmNiC2

Fig. 11. Magnetic H - T phase diagram of NdNiC2 in fields applied along the a-axis. IF means a field-inducedferromagnetic phase. is a ferromagnet of Tc = 17.5 K, although all the other RNiC2 compounds reported hitherto are antiferromagnetic. Furthermore, it seems surely that the transition at Tc is of first order. An

H. Onodera et al. / Journal of Magnetism and Magnetic Materials 182 (1998) 161-171

[ × 10-:] - - , ~ 1

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30

40

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i

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Fig. 12. Temperature dependence of (a) the susceptibilitiesand (b) the differentials dM/dT along the three crystallographic axes of SmNiC2 below 40 K.

T (K) Fig. 13, Temperature dependence of (a) the magnetic susceptibilities and (b) the reciprocal susceptibilities along the three axes of SmNiCz.

order-order transition at T t l = 4.3 K is also of first order, although the magnetizations change by small amounts in the three M - T curves. In addition to these first-order transitions, the appearance of anomalies at Tt2 -- 13.0 K and Tt3 ~- 25.0 K is probably attributed to certain kinds of magnetic transition. However, the anomaly at Tt3 is not able to attribute to an antiferromagnetic transition, since all the dM/dT changes along the three axes do not behave like an antiferromagnetic cusp. It is also impossible to attribute a population change of the 4f C E F levels, since all the magnetizations along three axes decrease at Tt3. If anything, the anomaly at Tt2 is positively attributed to the population change of C E F levels, since the magnetizations along the b- and c-axis increases as if they would compensate the decrease of magnetization along the a-axis. Fig. 13 shows the temperature dependence of the magnetic susceptibility and the reciprocal suscepti-

bility along the three crystallographic axes. The reciprocal susceptibilities do not satisfy the Curie-Weiss law, since there occurs thermal excitations from the ground J multiplet of J = ~ to the excited multiplet of J = ~ in Sm compounds. Fig. 14 again shows the susceptibilities in the temperature range near to 300 K. As is clear from Fig. 14, the susceptibility at 3 0 0 K is about 0.22 emu/mol which is about twice larger than the value expected for Sm 3 + and rather smaller than that for Sm 2 +. Nevertheless, it is required to postulate some contribution of Sm 2 + ions in order to explain the magnitudes of susceptibility around 300K. The estimated amounts x of Sm 2+ in SmNiC2 are shown as a function of temperature in the upper part of the figure. The lattice parameters of SmNiC2 show a normal lanthanide contraction behavior of R 3+ as shown in Fig. 1. A valence fluctuating SmRuSn3 c o m p o u n d also shows the normal lanthanide contraction behavior, although

H. Onodera et al. / Journal of Magnetism and Magnetic Materials 182 (1998) 161-171

170

.

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r

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T (K) Fig. 14. Temperature dependence of the susceptibilities of SmNiCz in the temperature range near to 300 K. The solid lines are the theoretical susceptibilities for Sm 3 ÷ and Sm 3 +, and the broken line in the upper part is the estimated ratio of Sm z ÷ ions.

&

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a quarter of Sm atoms are in the valence-fluctuation state [15]. Fig. 15a shows the magnetization M - H curves of SmNiC2 at 5 K. The spontaneous magnetization appears only along the a-axis. Fig. 15b shows the magnetization processes at various temperatures measured with the applied field up to 150kOe along the a-axis. When the temperature is T > Tc = 17.5 K, there appears a metamagnetic transition from paramagnetic to ferromagnetic state. It is noted that this metamagnetic transition is accompanied with large hysteresis as shown in the figure. Fig. 16 shows the magnetic H - T phase diagram. Unfortunately, it is not clear whether the metamagnetic transition occurs above 22 K, since the critical field goes over 150 kOe. The spontaneous magnetization is 0.32 ~tB/f.u. at 5.0 K which is about half of the full moment 0.72 ~tB of Sm 3+. Three possible origins are cited for the small Sm moment. The first one is a ferrimagnetic arrangement of Sm moments. The other RNiC2 compounds with antiferromagnetic coupling show the metamagnetic transition like NdNiCz or the large increase of magnetization like CeNiC2. However, there occurs no metamagnetic transition and no large increase of magnetization as shown in Fig. 15.

Hl[c

100

2~K i 15( 0

H (kOe) Fig. 15. Magnetization curves of SmNiC2 (a) along the three crystallographic axes under the applied field below 20 kOe at 5 K and (b) along the a-axis up to 150 kOe at various temperatures.

Consequently, it is hard to assume a ferrimagnetic arrangement. The second is the existence of Sm 2 + ion which has no magnetic moment. It is supposed that this possibility is rather high, since the magnitude of paramagnetic susceptibility is also interpreted well by postulating the contribution of Sm 2 + as discussed above. As the third one, the Sm moment often becomes a rather small value by the CEF effect which brings forth even mixing of some contribution of the higher J multiplet to the ground 4f state, i.e. it is not peculiar for Sm 3+ to have a small moment compared with the full moment. Consequently, it is supposed that the small value of magnetization originates from the CEF effect and/or the valence fluctuation. There are some incomprehensible magnetic behaviors in SmNiC2 as described above. The most interesting one is the first-order transition at Tc which seems to correlate closely to its ferromagnetism being different from all the other RNiC2

H. Onodera et al. / Journal of Magnetism and Magnetic Materials 182 (1998) 161 17l

150

I

I

SmNiC2

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0 HcuP

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temperatures, specific heat, t 54Sm M6ssbauer spectroscopy and so on.

Acknowledgements The authors would like to thank Professors K. Suzuki, K. Fukamichi and collaborators for their kind support in the magnetometric experiments. Thanks are also due to the staff of H F L S M , IMR, for operation of the high-field magnet.

T (K) Fig. 16. Magnetic H - T phase diagram of SmNiC2 in fields applied along the a-axis.

compounds. One of the plausible explanation is given by assuming that the magnetic transition occurs accompanied with a structural transformation such as cooperative Jahn-Teller effect, charge ordering and so on. As has been clarified by the previous works, the magnetic interactions in RNiC2 consist of ferromagnetic and antiferromagnetic ones simultaneously which compete with each other, and an antiferromagnetic structure is realized as a final result. If a structural transformation strongly modifies this competition, a ferromagnetic structure probably appears. The uniaxial anisotropy in SmNiC2 is so strong that the Sm moments are forced to align along the a-axis. Also, the R moments in CeNiC2 and NdNiC2 align along the a-axis. The Sm 3 + ion has a positive second-order Stevens factor as which is the opposite sign to those of Ce 3 ÷ and Nd 3 +. This fact implies that the values and/or signs of the C E F parameters in SmNiC2 differs by a considerable amount from those in CeNiC2 and NdNiC2. One can suppose that a structural transformation originates in large changes of the CEF parameters. In any case, it is necessary to investigate the properties of SmNiC2 by means of some different kinds of measurement such as X-ray diffraction at low

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