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NMR and magnetic susceptibility of CeCu5−xNix intermetallic compounds
0022-3697/82/030159-05$03.M1/0 Pewmoo Press Ltd.
J. pkyr Ckm S/ids Vol. 43. No. 3. PP. W-203.1982 PhtCdillGMtBitlIh.
NMR AND MAGNETIC SUSCEPTIBILITY OF CeCus-,Ni, INTERMETALLIC COMPOUNDS IULIUPOP, RODICAPOP and MARINCOLDEA Cluj-NapocaUniversity,Physics Department,3400Cluj-Napoca,Romania (Receiued26 August 1980;acceptedin revisedform 8 July 1981) Ah&ad-The 63CuKnight shift and magneticsusceptibilityof the CeCu5_,Ni,intermetallicsystem have been investigatedas a function of temperature.The compoundsstudiedwere all paramagnetsin the temperaturerange 9&7OOKwith 8.
1. R?IR~DU~~N
where
In a previous paper[l] we have pointed out in the paramagnetic state of the compound CeCuSby NMR investigationsa s-f reversal coupling (JSr> 0) between conduction electrons and localized f electrons. This problem is interesting both from the physical and practical point of view. It is therefore important to see if reversal couplingis commonfor the structure type RCu5, particularly in the presence of a ferromagnetic component from the iron series, For this purpose we have studied the magnetic characteristics of the CeCuS_,Ni, (x = 0.3, 0.5, 0.75 and 1.2) intermetallic system through magnetic susceptibilityand Knight shift measurements.
x0 =
f,$ + xdia
•t ,&
+ xi,
+ xt.
(2)
In expression (2) the parameters have the following meaning. ,$ is the Pauli paramagnetismof the free conduction electrons, f being the enhancementfactor of the Pauli susceptibility,Xdiais the diamagnetismof the ion core, ,&, is the orbital paramagnetismof the d conduction electrons, xf, is the Van-Vleck paramagnetismof the localizedf electrons on Ce atoms and xr. is the Landau diamagnetismof the conduction electrons. The calculation of these contributions to x0 has been described in previous papers[2,3], and their numerical 2.ExPaMlmAL values are listed in Table 1. The compoundsinvestigatedwere prepared in an arcThis table contains also the calculated densities, the melting furnace under a pure argon atmosphere. The paramagneticCurie temperatures,the effective magnetic purity of the starting elements was 99.999%for Cu, moment values per unit formula &,) and the magnetic 99.8%for Ce and 99.9%for Ni. X-ray and metallographic moments per nickel atom (PNi)evaluated in two ways, investigationsconfirmed the existence of a single phase namely from the relation in all compounds. The magnetic susceptibility was measured between PC-~ = v/cfC&& + fNicLL) (3) 90-700K at 94500e with a Faraday-type magnetic balance, havinga sensitivityof 1Om8 e.m.u./g. where fc. and fNi are the molar fractions, and from the The Knight shift of the 63Cunuclei was measured nickel contributions to the bulk magnetic susceptibility. between 180-42OKwith a JNM-3 spectrometer at The nickel contributions to the magnetic susceptibility 9.212MHz, with reference to the 63Cu resonance in were determined by subtracting the CeCus magnetic susceptibilityfrom that of CeCu,_,Ni,. The linear temCuC12powder perature dependence of the reciprocal magnetic suscep3. RESULTS ANDDISCUSSION tibilityof the nickel contributionsgiven in Fig. 2, allowed an estimate of the magneticmomentsof the 3d localized The magnetic susceptibilitymeasurementsin the temperature range 9&7OOKshowed paramagneticbehaviour electrons on the Ni atom to be made. The values of all compounds investigated with negative paramag- obtained by the two methods are in very good netic Curie temperatures 19,
*=&+%I
199
I.PoP,R.PoP andhf. COLDEA
200
1I
7 si-.lo-c L 0. emu -1 mol f---
\ i
A
CeCutj
l
Ce Cuq NiO,3
o Ce CU@I'O,~ 3!
x Cc Cu4,25N;0,75 CC CU3,BN;j2
l
Fig. 1.
ca*4 .7Nio.3
456.377.967 -20 2.57 5.45 -8.0 4.15 4.51 -1.81 4.4 15.5 16.653.01 0.421 0.460
C&u4 .5Nio.5
455.407.950 -28 2.60 5.44 -8.0 4.52 4.51 -1.81 4.96 12.8.13.582.49 0.650 0.643)
cecu4.25N10.75454.197.929 -32 2.66 5.44 -8.0 4.94 4.51 -1.81 5.89 18.9 19.263.54 0.371 0.832 451.987.890 -35 2.75 5.40 -8.0 5.03 4.51 -1.807.15 11.4 11.67 2.16 0.926 0.933 C~CU~_~N~~ .2
NMRand magneticsusceptibilityof CeCur_,Ni,intermetallic compounds
201
. CeCq,+.4i0,~ O @C”qN’o,s ’
CcCuL,25Ni0,75
0
CeCu38Nil
2
>
1
80
TIK) 0
Fig. 2.
investigationwill represent the subject of a forthcoming paper[l2]. No field dependence of the magneticsusceptibility was observed, which clearly shows that in the samples investigated there was no free Ni. This conclusion is supported also by the NMR line widths. The existence of any ferromagnetic phase or free Ni would be revealed by broadening of the NMR line in such a manner that it would not be possible to record the spectra. The presence of Ni atoms in the lattice gives rise to a pronounced increase in the magnetic susceptibility(see Figs. 1 and 2), which reflect an increase of the magnetic interactions between magnetic ions. This will also be strongly reflected in the NMR linewidths. In fact, the linewidthfor the CeCu9.8Ni,.T compoundis so large that it cannot be recorded. The existence of two kinds of paramagneticions in these intermetalliccompoundsresults in a characteristic shape of the temperaturedependenceof the Knightshift. The thermal variation of the Knight shift (Fig. 3) presents a large minimum,which shifts towards higher temperature as the Ni concentration increases. The bulk Knight shift results from a superpositionof the Knight shifts determined by the polarizations of conduction electrons by the 4f and 3d spins localizedon Ce and Ni atoms. The increasingof the Knight shift at high temperature is caused by the 4f localized electron spins on the Ce atoms, while the decreasingof the Knightshift in the low temperature region is determined by 3d localized electronspins on the Ni atoms. For comparison,in Fig. 3 is also given the thermal variation of the Knight shift for the CeCuscompound,whichhas dKld T > 0. This picture shows clearly that s-f coupling in the investigated CeCu5_,Ni, compounds has the same sign as in the CeCuscompound,i.e. the reversal couplingis specificfor this structure type. Furthermore the values of the Knight shift are much greater than those of CeCu5.
In order to estimate the coupling constants JS-, and Is-,,, in Figs. 4 and 5 are plotted the Knight shifts vs the magneticsusceptibilityof CeCus(whichis caused mainly by the magnetic moments of the Ce atoms), and of the nickel contributionin these compounds,respectively. As one can see from Fig. 4, the dependence K(x~& has two linear portions with negativeand positive slopes. The linear dependence with negative slope is caused by the polarization of the conduction electrons by the 4f spins, which gives rise to a reversal s-f coupling with JSr> 0. A linear dependence with positive slope is not specific for CeCu5,the Knight shift in this temperature region beingcaused mainlyby s-d coupling.Thus in Fig. 5 is given even this part of the Knight shift vs the nickel susceptibilitycontribution.The fact that the dependence K(X& is linear showsthat in this temperatureregion s-d polarizationprevails. From the slope of these linear dependenceshave been determined the numerical values for the coupling constants JSr and Jsd, which are listed in Table 2. These values were calculatedassumingan uniform polarization of the conduction electrons by the localized f and d spins using the well known relations[4,5]. K
=
K
0
l +
[
J&e - ‘2xt &k&J3
I
for the high temperature region and
for the low temperature region, K. and KA being the extrapolated Knight shift values for ~cccUs + 0 and XNr+ 0, respectively, and go= and gNi are the corresponding Land6 factors. From Table 2 one can observe that with increasingnickel content the values of Jsf increase and those of Jsd decrease, all being positive and of the same
202
I. POP, R. POP and hf. COLDEA
K
x
(% I
CC Cy25N’0,?5
O cc %,5N’0,5 l
Ce C\rb,7Ni0,3
300 Fig. 3.
A
CcCu5
.
C@C,7Ni0,3
o
CcCub
5N;~ 5
>
Fig. 4.
,
LOO
NMR and magnetic susceptibility of CeCus_,Ni, intermetallic compounds
K(% 1
I
l
203
CeCU~,,N~~,~
o Cc Cu4,5Ni0,5
0,240 -
Fig. 5.
Table 2. Compound
ca%
0.2419 0.008
-,
-
C&u4 .7wio.3
0.327
cecu4 p.
0.3185 0.053 0.1840 0.123
.5
cscu4'25Ni0 . .75 -_.__ -
0.042 0.1800 0.153
0.3610 0.076 0.1867 0.118
order of magnitude as for the other rare-earth intermetalliccompoundsK-1 11. 4. CONCLUSIONS NMR and magnetic susceptibilityinvestigationshave revealed the existence of reversal s-j coupling in the intermetallic system CeCu,_,Ni,, which has the RCu5 type structure. The presence of nickel atoms in these compoundsdoes not changethe signof s-f coupling,the only effect being one of enhancement, which is confirmedby the values of the .LI couplingconstants in these compoundscomparedto that in CeCu5. The positive values of the J,, and Jsd coupling constants indicate a parallel coupling between the s-f and s-d spins. These results may be interpreted as a startingpoint for the investigationof reversal couplingin the ordered state of this class of compounds.Such investigationsare now in progress.
NEFERENCES
1. POD I.. Rus E.. Coldea M. and POD . 0... /. Phvs. . Chem. Solids.
40:683 (1979): 2. Coldea M. and Pop I., Phil. Msg. 28,881 (1973). 3. Pop I., Rus E., Coldea M. and Pop O., Phys. Status Solidi
(a), 54,365 (1979). 4. Jaccarino V., Mathias B. T., Peter M., Suhl M. and Wemick M. J., Phys. Rev. Letters 5, 251 (1960). 5. Jaccarino~V.,J. Appl Phys. 32, ilES (l%l). 6. Coldea M. and Pop I., Ada. Phys. Polo&a A48,SlS (1975). 7. Pop I., Coldea M. and Wallace W. E., J. Solid Sfate Chen. 24, 115(1978). 8. de Wijn H. W., Buschow K. H. J. and van Diepen A. M., Phys. Status Solidi 30.759 (1968). 9. Buschow K. H. J., van Diepen A. M. and de Wijn H. W., J. Appl. Phys. 41,4609 (1970). 10. Barnes R. Cl., Borsa F. and Peterson D., 1. Appl. Ays. 36, 940 (1%5). 11. Rao V. U. S. and Vijayaraghavan, Phys. Letf. 19, 168(1966). 12. Pop I. and Pop R. (to be published).
J. pkyr Ckm S/ids Vol. 43. No. 3. PP. W-203.1982 PhtCdillGMtBitlIh.
NMR AND MAGNETIC SUSCEPTIBILITY OF CeCus-,Ni, INTERMETALLIC COMPOUNDS IULIUPOP, RODICAPOP and MARINCOLDEA Cluj-NapocaUniversity,Physics Department,3400Cluj-Napoca,Romania (Receiued26 August 1980;acceptedin revisedform 8 July 1981) Ah&ad-The 63CuKnight shift and magneticsusceptibilityof the CeCu5_,Ni,intermetallicsystem have been investigatedas a function of temperature.The compoundsstudiedwere all paramagnetsin the temperaturerange 9&7OOKwith 8.
1. R?IR~DU~~N
where
In a previous paper[l] we have pointed out in the paramagnetic state of the compound CeCuSby NMR investigationsa s-f reversal coupling (JSr> 0) between conduction electrons and localized f electrons. This problem is interesting both from the physical and practical point of view. It is therefore important to see if reversal couplingis commonfor the structure type RCu5, particularly in the presence of a ferromagnetic component from the iron series, For this purpose we have studied the magnetic characteristics of the CeCuS_,Ni, (x = 0.3, 0.5, 0.75 and 1.2) intermetallic system through magnetic susceptibilityand Knight shift measurements.
x0 =
f,$ + xdia
•t ,&
+ xi,
+ xt.
(2)
In expression (2) the parameters have the following meaning. ,$ is the Pauli paramagnetismof the free conduction electrons, f being the enhancementfactor of the Pauli susceptibility,Xdiais the diamagnetismof the ion core, ,&, is the orbital paramagnetismof the d conduction electrons, xf, is the Van-Vleck paramagnetismof the localizedf electrons on Ce atoms and xr. is the Landau diamagnetismof the conduction electrons. The calculation of these contributions to x0 has been described in previous papers[2,3], and their numerical 2.ExPaMlmAL values are listed in Table 1. The compoundsinvestigatedwere prepared in an arcThis table contains also the calculated densities, the melting furnace under a pure argon atmosphere. The paramagneticCurie temperatures,the effective magnetic purity of the starting elements was 99.999%for Cu, moment values per unit formula &,) and the magnetic 99.8%for Ce and 99.9%for Ni. X-ray and metallographic moments per nickel atom (PNi)evaluated in two ways, investigationsconfirmed the existence of a single phase namely from the relation in all compounds. The magnetic susceptibility was measured between PC-~ = v/cfC&& + fNicLL) (3) 90-700K at 94500e with a Faraday-type magnetic balance, havinga sensitivityof 1Om8 e.m.u./g. where fc. and fNi are the molar fractions, and from the The Knight shift of the 63Cunuclei was measured nickel contributions to the bulk magnetic susceptibility. between 180-42OKwith a JNM-3 spectrometer at The nickel contributions to the magnetic susceptibility 9.212MHz, with reference to the 63Cu resonance in were determined by subtracting the CeCus magnetic susceptibilityfrom that of CeCu,_,Ni,. The linear temCuC12powder perature dependence of the reciprocal magnetic suscep3. RESULTS ANDDISCUSSION tibilityof the nickel contributionsgiven in Fig. 2, allowed an estimate of the magneticmomentsof the 3d localized The magnetic susceptibilitymeasurementsin the temperature range 9&7OOKshowed paramagneticbehaviour electrons on the Ni atom to be made. The values of all compounds investigated with negative paramag- obtained by the two methods are in very good netic Curie temperatures 19,
*=&+%I
199
I.PoP,R.PoP andhf. COLDEA
200
1I
7 si-.lo-c L 0. emu -1 mol f---
\ i
A
CeCutj
l
Ce Cuq NiO,3
o Ce CU@I'O,~ 3!
x Cc Cu4,25N;0,75 CC CU3,BN;j2
l
Fig. 1.
ca*4 .7Nio.3
456.377.967 -20 2.57 5.45 -8.0 4.15 4.51 -1.81 4.4 15.5 16.653.01 0.421 0.460
C&u4 .5Nio.5
455.407.950 -28 2.60 5.44 -8.0 4.52 4.51 -1.81 4.96 12.8.13.582.49 0.650 0.643)
cecu4.25N10.75454.197.929 -32 2.66 5.44 -8.0 4.94 4.51 -1.81 5.89 18.9 19.263.54 0.371 0.832 451.987.890 -35 2.75 5.40 -8.0 5.03 4.51 -1.807.15 11.4 11.67 2.16 0.926 0.933 C~CU~_~N~~ .2
NMRand magneticsusceptibilityof CeCur_,Ni,intermetallic compounds
201
. CeCq,+.4i0,~ O @C”qN’o,s ’
CcCuL,25Ni0,75
0
CeCu38Nil
2
>
1
80
TIK) 0
Fig. 2.
investigationwill represent the subject of a forthcoming paper[l2]. No field dependence of the magneticsusceptibility was observed, which clearly shows that in the samples investigated there was no free Ni. This conclusion is supported also by the NMR line widths. The existence of any ferromagnetic phase or free Ni would be revealed by broadening of the NMR line in such a manner that it would not be possible to record the spectra. The presence of Ni atoms in the lattice gives rise to a pronounced increase in the magnetic susceptibility(see Figs. 1 and 2), which reflect an increase of the magnetic interactions between magnetic ions. This will also be strongly reflected in the NMR linewidths. In fact, the linewidthfor the CeCu9.8Ni,.T compoundis so large that it cannot be recorded. The existence of two kinds of paramagneticions in these intermetalliccompoundsresults in a characteristic shape of the temperaturedependenceof the Knightshift. The thermal variation of the Knight shift (Fig. 3) presents a large minimum,which shifts towards higher temperature as the Ni concentration increases. The bulk Knight shift results from a superpositionof the Knight shifts determined by the polarizations of conduction electrons by the 4f and 3d spins localizedon Ce and Ni atoms. The increasingof the Knight shift at high temperature is caused by the 4f localized electron spins on the Ce atoms, while the decreasingof the Knightshift in the low temperature region is determined by 3d localized electronspins on the Ni atoms. For comparison,in Fig. 3 is also given the thermal variation of the Knight shift for the CeCuscompound,whichhas dKld T > 0. This picture shows clearly that s-f coupling in the investigated CeCu5_,Ni, compounds has the same sign as in the CeCuscompound,i.e. the reversal couplingis specificfor this structure type. Furthermore the values of the Knight shift are much greater than those of CeCu5.
In order to estimate the coupling constants JS-, and Is-,,, in Figs. 4 and 5 are plotted the Knight shifts vs the magneticsusceptibilityof CeCus(whichis caused mainly by the magnetic moments of the Ce atoms), and of the nickel contributionin these compounds,respectively. As one can see from Fig. 4, the dependence K(x~& has two linear portions with negativeand positive slopes. The linear dependence with negative slope is caused by the polarization of the conduction electrons by the 4f spins, which gives rise to a reversal s-f coupling with JSr> 0. A linear dependence with positive slope is not specific for CeCu5,the Knight shift in this temperature region beingcaused mainlyby s-d coupling.Thus in Fig. 5 is given even this part of the Knight shift vs the nickel susceptibilitycontribution.The fact that the dependence K(X& is linear showsthat in this temperatureregion s-d polarizationprevails. From the slope of these linear dependenceshave been determined the numerical values for the coupling constants JSr and Jsd, which are listed in Table 2. These values were calculatedassumingan uniform polarization of the conduction electrons by the localized f and d spins using the well known relations[4,5]. K
=
K
0
l +
[
J&e - ‘2xt &k&J3
I
for the high temperature region and
for the low temperature region, K. and KA being the extrapolated Knight shift values for ~cccUs + 0 and XNr+ 0, respectively, and go= and gNi are the corresponding Land6 factors. From Table 2 one can observe that with increasingnickel content the values of Jsf increase and those of Jsd decrease, all being positive and of the same
202
I. POP, R. POP and hf. COLDEA
K
x
(% I
CC Cy25N’0,?5
O cc %,5N’0,5 l
Ce C\rb,7Ni0,3
300 Fig. 3.
A
CcCu5
.
C@C,7Ni0,3
o
CcCub
5N;~ 5
>
Fig. 4.
,
LOO
NMR and magnetic susceptibility of CeCus_,Ni, intermetallic compounds
K(% 1
I
l
203
CeCU~,,N~~,~
o Cc Cu4,5Ni0,5
0,240 -
Fig. 5.
Table 2. Compound
ca%
0.2419 0.008
-,
-
C&u4 .7wio.3
0.327
cecu4 p.
0.3185 0.053 0.1840 0.123
.5
cscu4'25Ni0 . .75 -_.__ -
0.042 0.1800 0.153
0.3610 0.076 0.1867 0.118
order of magnitude as for the other rare-earth intermetalliccompoundsK-1 11. 4. CONCLUSIONS NMR and magnetic susceptibilityinvestigationshave revealed the existence of reversal s-j coupling in the intermetallic system CeCu,_,Ni,, which has the RCu5 type structure. The presence of nickel atoms in these compoundsdoes not changethe signof s-f coupling,the only effect being one of enhancement, which is confirmedby the values of the .LI couplingconstants in these compoundscomparedto that in CeCu5. The positive values of the J,, and Jsd coupling constants indicate a parallel coupling between the s-f and s-d spins. These results may be interpreted as a startingpoint for the investigationof reversal couplingin the ordered state of this class of compounds.Such investigationsare now in progress.
NEFERENCES
1. POD I.. Rus E.. Coldea M. and POD . 0... /. Phvs. . Chem. Solids.
40:683 (1979): 2. Coldea M. and Pop I., Phil. Msg. 28,881 (1973). 3. Pop I., Rus E., Coldea M. and Pop O., Phys. Status Solidi
(a), 54,365 (1979). 4. Jaccarino V., Mathias B. T., Peter M., Suhl M. and Wemick M. J., Phys. Rev. Letters 5, 251 (1960). 5. Jaccarino~V.,J. Appl Phys. 32, ilES (l%l). 6. Coldea M. and Pop I., Ada. Phys. Polo&a A48,SlS (1975). 7. Pop I., Coldea M. and Wallace W. E., J. Solid Sfate Chen. 24, 115(1978). 8. de Wijn H. W., Buschow K. H. J. and van Diepen A. M., Phys. Status Solidi 30.759 (1968). 9. Buschow K. H. J., van Diepen A. M. and de Wijn H. W., J. Appl. Phys. 41,4609 (1970). 10. Barnes R. Cl., Borsa F. and Peterson D., 1. Appl. Ays. 36, 940 (1%5). 11. Rao V. U. S. and Vijayaraghavan, Phys. Letf. 19, 168(1966). 12. Pop I. and Pop R. (to be published).