Journal of Alloys and Compounds 315 (2001) 28–35 L www.elsevier.com / locate / jallcom Magnetic properties of GdT Al and GdT Al compounds (T5Cr, Mn, Cu) ¨ N.P Duong, J.C.P Klaasse, E Bruck, F.R de Boer, K.H.J Buschow* Van der Waals-Zeeman Institute, University of Amsterdam, Valckenierstraat 65 – 67, 1018 XE Amsterdam, The Netherlands Received 26 September 2000; accepted 28 September 2000 Abstract We have studied the magnetic properties of GdT Al and GdT Al compounds with T5Cr, Mn, Cu by means of standard magnetization and susceptibility measurements, magnetization measurements in high fields up to 35 T, and measurements of the specific heat In these GdT Al compounds, the T element does not carry a magnetic moment In all these cases, the coupling between the Gd moments is fairly weak and leads to antiferromagnetic ordering at rather low temperatures We have analyzed our data in terms of a simple mean-field two-sublattice model Reliable values of the intersublattice coupling strength were derived from high-field measurements made on free powder particles In the GdT Al compounds, more than one crystallographic site is occupied by the T elements, leading to relatively strong changes in the magnetic ordering temperatures A comparison is made with the magnetic properties of GdFe Al and GdFe Al compounds in which the Fe moments strongly determine the magnetic properties High-field measurements are presented also for the latter two compounds 2001 Elsevier Science B.V All rights reserved Keywords: Rare earth compounds; Transition metal compounds; Megnetically ordered materials; Magnetic measurements; Heat capacity Introduction Rare-earth compounds of the type RT Al form for almost all rare-earth elements with T5Cr, Mn, Fe and Cu These compounds adopt the relatively simple ThMn 12 structure in which there is only one single rare-earth site [1,2] Neutron-diffraction investigations have shown that the T atoms occupy almost exclusively the 8f position in the latter structure type [3–5] From results of magnetic measurements and neutron diffraction made on compounds in which T is non-magnetic (T5Cr, Mn, Cu), it can be derived that the R–R interactions are antiferromagnetic, leading to fairly low magnetic-ordering temperatures [6– 8] In the RFe Al compounds, however, also the Fe atoms carry a magnetic moment and the Fe–Fe interaction leads to comparatively high magnetic-ordering temperatures, often in excess of 150 K [5,7–9] Although primarily of an antiferromagnetic nature, the Fe–Fe interactions and the concomitant magnetic structures are complex [5,9,10] *Corresponding author Tel.: 131-20-5255-714; fax: 131-20-5255788 E-mail address: buschow@science.uva.nl (K.H.J Buschow) This has consequences for the R–Fe interactions and the resultant molecular field experienced by the R moments Because of the weakness of the R–R interaction it leads to an equally complex ordering of the R moments not easily accessible from experiments In order to obtain additional experimental information on the magnetic ordering processes in this class of magnetic materials, we have performed additional measurements on several of the compounds, supplemented with results on the corresponding GdT Al compounds in which also the 8j site is partly occupied by T atoms Experimental The GdT Al and GdT Al compounds with T5Cr, Mn, Fe and Cu were prepared in polycrystalline form by melting stoichiometric amounts of the elements (of at least 99.9% purity) in an arc furnace The ingots of GdT Al were subsequently vacuum annealed at 8008C for several weeks All samples were characterized by X-ray diffraction and shown to be almost single phase with Bragg peaks consistent with the ThMn 12 structure The amount of impurity phases was estimated to be less than 3% The 0925-8388 / 01 / $ – see front matter 2001 Elsevier Science B.V All rights reserved PII: S0925-8388( 00 )01288-3 N.P Duong et al / Journal of Alloys and Compounds 315 (2001) 28 – 35 29 specific heat was measured for some of these compounds in zero field in the temperature range from 0.4 to 200 K Polycrystalline disks of about g were spark-cut for specific-heat measurements The experimental setup used for our data collection comprises the possibility for measurements by means of the standard adiabatic method The temperature dependence of the magnetisation of the samples was studied in a SQUID magnetometer in the temperature range 4.2–300 K The field dependence of the magnetisation at 4.2 K of all compounds was measured at the High-field Installation at the University of Amsterdam Samples for these measurements were crushed to fine particles that were able to rotate freely in the sample holder towards their equilibrium directions Results and discussion 3.1 RT4 Al8 compounds Results of magnetic measurement on GdFe Al are displayed in Fig These results are in accordance with ă those obtained previously [8] From results of Mossbauer spectroscopy it can be derived that the Fe sublattice orders magnetically around 165 K [8], whereas no indication of this ordering becomes apparent in the temperature dependence of the magnetisation It can be seen in Fig that the only indication of magnetic ordering is at low tempera- Fig Temperature dependence of the magnetization of a powdered material of GdFe Al , fixed with epoxy, measured in T with increasing temperature from to 350 K (left scale), and temperature dependence of the reciprocal susceptibility (right scale) Fig Temperature dependence of c /T for GdFe Al tures, where M(T ) rises strongly well below 50 K The exact value of the ordering temperature, T C 526 K, has been obtained from specific heat measurements shown in Fig Although the specfic heat data are not very accurate above about 100 K, they not give any indication of ordering of the Fe sublattice at about 165 K Quite a different behaviour is found for the GdT Al in which the T component (T5Cu, Mn, Cr) is nonmagnetic A plot of the reciprocal susceptibility vs temperature for GdCu Al is shown in Fig A similar behaviour is found for GdCr Al The values of the asymptotic Curie temperature uP and effective moment m´ff derived from these curves are listed in Table The values listed for T N in the same table have been derived from the shallow maximum in the corresponding M(T ) curves measured in low fields at low temperatures The data listed for these two compounds are in satisfactory agreement with those of Felner and Nowik [7] Results for GdMn Al have been reported previously [8], Curie Weiss behaviour being observed from room temperature to 4.2 K The values found for T N , uP and m´ff have been included in Table Surprising is the absence of any indication of magnetic ordering of the Gd sublattice This absence is clearly apparent also in the temperature dependence of the ac-susceptibility shown in Fig From the temperature dependence of the specific heat we have obtained evidence that long-range magnetic ordering is absent above K Details of this investigation have been reported in a separate paper [11] In a simple molecular field approach, assuming two (equal) magnetic Gd sublattices that are antiferromagnetically coupled, we can express the the intrasublattice coupling parameter n 11 5n 22 and the intersublattice cou- N.P Duong et al / Journal of Alloys and Compounds 315 (2001) 28 – 35 30 Fig Temperature dependence of the real part of the ac susceptibility for GdMn Al The inset shows the imaginary part Fig Temperature dependence of the reciprocal susceptibility for GdCu Al measured in T on powdered material fixed with epoxy pling parameter n 12 in terms of the experimental parameters T N and uP by means of the relations n 11 (uP T N ) /C (1) n 12 (uP T N ) /C (2) The values obtained by means these equations for the intrasublattice n 11 and intersublattice n 12 exchange coupling constant are listed in Table Neutron diffraction results obtained for DyCu Al and HoCu Al showed that the magnetic structure is simply antiferromagnetic with ferromagnetically ordered (001) planes stacked antiferromagnetically along the c direction [12] The R moments at the corners of the unit cell are antiparallel with those in the center of the cell Hence we can identify the intrasublattice coefficient with n 11 with the ferromagnetic coupling within the (001) planes and the intersublattice coefficient n 12 with the antiferromagnetic coupling between adjacent planes In Fig 5a and b, the results of the high-field measurements on free-powder samples of GdCr Al and GdCu Al at 4.2 K are shown The magnetisation curve of all compounds starts from the origin which confirms the antiferromagnetic nature of the magnetic ordering in these compounds In the bending process, the magnetisation increases more or less linearly with increasing magnetic field The values of n 12 obtained by means of the expression M5 m0 H / un 12 u for all compounds are given in Table In fields up to 35 T, the bending process is seen to reach completion for both compounds The values of the Gd moment in the forced parallel configuration, mGd , are also listed in Table Finalizing this section, we come to the following conclusion The Gd–Gd interaction in RT Al compounds with non-magnetic T components is antiferromagnetic and leads to fairly low magnetic ordering temperatures 3.2 RT6 Al6 compounds It has been mentioned already that neutron diffraction results have shown an almost ideal preferential occupation of the T atoms on the 8f sites Nevertheless, a small deviation from the ideal site occupation cannot be excluded In order to obtain an impression of the effect of the Table Magnetic properties of several GdT Al compounds (T5Cr, Mn, Fe, Cu)a HFFP ) Compound TN (K) uP (K) Gd n 11 (T.f.u /mB ) Gd n 12 (T.f.u /mB ) Gd( n 12 (T.f.u /mB ) Gd m eff ( mB / at.) mS at 4.2 K ( mB / f.u.) GdCr Al GdMn Al GdCu Al 2, 35 26.5 28.5 [8] 218.5 0.1 20.38 1.06 20.89 20.8 23.46 21.14 – 23.86 8.22 8.06 [8] 8.31 7.0 7.0 7.0 a The values obtained for the intersublattice constant n 12 by means of high-field measurements on free powders are indicated by the superscript HFFP N.P Duong et al / Journal of Alloys and Compounds 315 (2001) 28 – 35 31 Fig (a) Field dependence of the magnetisation of GdCu Al measured on free powder at 4.2 K (b) Field dependence of the magnetisation of GdCr Al measured on free powder at 4.2 K Fig Temperature dependence of the reciprocal susceptibility for GdCu Al measured in T on a piece of bulk material fixed with epoxy Fig Temperature dependence of the reciprocal susceptibility for GdMn Al measured in and T on a piece of bulk material fixed with epoxy 32 N.P Duong et al / Journal of Alloys and Compounds 315 (2001) 28 – 35 Fig Temperature dependence of the real part of the ac susceptibility for GdMn Al The inset shows the imaginary part occupation of the other two sites (8j and 8i) by T atoms we have used an excess of T atoms forcing these atoms to occupy also these other two sites Results of neutron diffraction have shown that the 8i site is very reluctant to accept T atoms, so that it is predominantly the 8j site where the excess T atoms is accommodated [13] Fig Temperature dependence of the GdMn Al measured in various applied fields magnetization for Fig 10 Temperature dependence of the magnetization of a powdered material of GdCr Al , fixed with epoxy, measured in T with increasing temperature from to 300 K (left scale), and temperature dependence of the reciprocal susceptibility (right scale) N.P Duong et al / Journal of Alloys and Compounds 315 (2001) 28 – 35 Table Magnetic properties of GdCu Al 33 a Compound TN (K) uP (K) Gd n 11 (T.f.u /mB ) n Gd 12 (T.f.u /mB ) ( HFFP ) n Gd 12 (T.f.u /mB ) Gd m eff ( mB / at.) mS at 4.2 K ( mB / f.u.) GdCu Al 10 243.5 22.2 23.55 23.65 8.2 6.8 a The value obtained for the intersublattice constant n 12 by means of high-field measurements on free powder is indicated by the superscript HFFP From the results shown in Fig for GdCu Al we derive that the influence of an excess Cu atoms is to shift T N to lower temperatures The data obtained for T N and uP in GdCu Al have been used to calculate the intersublattice and intrasublattice molecular field constants The corresponding values have been listed in Table If we compare these data with those listed in Table 1, we see that the excess Cu atoms has led to a sign reversal of the intrasublattice coefficient n 11 The absolute value of intersublattice coefficient with n 12 has remained nearly the same Results for GdMn Al are shown in Figs 7–9 There is no longer any Curie–Weiss behaviour for this compound in the temperature range considered here This is most likely due to the occurrence of a magnetic moment for the excess Mn atoms on the 8j sites, as suggested by Coldea et al [14] Also the magnetic ordering temperature has increased strongly and shifted from about K in GdMn Al to 36 K in GdMn Al Fig shows that the antiferromagnetic ordering can be broken in relatively low magnetic fields The excess T atoms has an even more dramatic effect in the Cr compounds It can be seen in Fig 10 that ferromagnetic behaviour is found in GdCr Al (T C 5175 K), in agreement with results obtained previously by Felner et al [15] Results obtained with high field strengths for the GdT Al compounds are shown in Figs 11–13 These data confirm the antiferromagnetic nature of the magnetic state in GdCu Al and GdMn Al In GdCu Al , the bending of the two sublattice moments leads to an initial linear behaviour From the slope, the intrasublattice molecular field constant has been calculated As seen in Table 2, the corresponding value of n 12 is in good agreement with that obtained from T N and uP It is seen, however, that the slope of the isotherm of GdCu Al becomes less steep above about 10 T and that the bending process seems not yet to have reached completion even in 35 T The data displayed for GdMn Al in Fig 12 show that the bending Fig 11 Field dependence of the magnetisation of GdCu Al measured on free powder at 4.2 K Fig 12 Field dependence of the magnetisation of GdMn Al measured on free powder at 4.2 K 34 N.P Duong et al / Journal of Alloys and Compounds 315 (2001) 28 – 35 Fig 13 Field dependence of the magnetisation of GdCr Al measured on free powder at 4.2 K process does not lead to a linear part in the whole field range considered In 35 T the magnetic isotherm is still far from saturation The results displayed in Fig 13 for GdCr Al confirm the ferromagnetic nature of this compound When extrapolating the high-field branch of the isotherm to zero field, one obtains the value 8.2 mB / f.u, which is considerably higher than the value 7.0 mB / f.u expected if only the Gd atoms would carry a magnetic moment This suggests that also the Cr atoms carry a magnetic moment in this compound and that the latter are responsible for the comparatively high magnetic ordering temperature Leaving this compound out of consideration, we can state that the overall Gd–Gd interaction in GdT Al compounds is not notably different from those in GdT Al compounds In both cases it leads to antiferromagnetic ordering, albeit the ordering temperatures change with the T concentration Antiferromagnetic ordering has also been observed in the isostructural compounds GdZn 12 [16] and GdNi 10 Si (N.P ă Duong, J.C.P Klaasse, E Bruck, F.R de Boer and K.H.J Buschow, unpublished results), the corresonding magnetic ordering temperatures being T N 516 K and T N 55 K Results of magnetic measurements obtained for GdFe Al are shown in Fig 14 When comparing these data with the results shown in Fig 1, it can be noticed that there has been an enormous increase in magnetic ordering temperature The Curie temperature in GdFe Al is above room temperature and our value T C 5345 K is in agreement with results obtained by Felner et al [17] In view of our result that the Gd–Gd interaction does not depend much on the stoichiometry, we attribute this increase to a strong enhancement of the Fe–Fe interaction The results displayed for Gd 0.7 Y 0.3 Fe Al in the same figure lend Fig 14 Temperature dependence of the magnetization of GdFe Al and Gd 0.7 Y 0.3 Fe Al measured in T on powdered material fixed with epoxy N.P Duong et al / Journal of Alloys and Compounds 315 (2001) 28 – 35 35 relatively small moment in zero applied field Because the applied field is parallel to the Fe sublattice moment, these latter atoms potentially can show a large increase of their moment with increasing field strength This is probably the reason why both isotherms show a comparatively strong field dependence Assuming that the spontaneous moment in GdFe Al is around 1.7 mB / f.u., we derive with mGd 57 mB , a value for the average Fe moment of 1.5 mB , a value that is in reasonable agreement with the neutron diffraction data obtained on other members of the RFe Al series [18] References Fig 15 Field dependence of the magnetisation of GdFe Al and Gd 0.7 Y 0.3 Fe Al measured on free powder at 4.2 K further credence to this assumption, because the magnetic dilution of the Gd sublattice by 30% is seen to have hardly any influence of the magnetic ordering temperature Results of high-field measurements are shown in Fig 15 It can be seen that magnetic dilution of the Gd sublattice leads to a strong shift in upward direction of the magnetic isotherm Taking into consideration that neutron diffraction [18] has shown several RFe Al compounds to be collinear ferrimagnets, this shift in upward direction can be taken as evidence that the Fe sublattice moment dominates the Gd sublattice moment in GdFe Al We mentioned already that in RFe Al compounds also the 8j site becomes partly occupied by Fe atoms The partly and statistically occupation of the latter site 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GdMn Al GdCu Al 2, 35 26. 5 28. 5 [8] 2 18. 5 0.1 20. 38 1. 06 20 .89 20 .8 23. 46 21.14 – 23 . 86 8. 22 8. 06 [8] 8. 31 7.0 7.0 7.0 a The values obtained for the intersublattice constant n 12 by means of high-field... samples of GdCr Al and GdCu Al at 4.2 K are shown The magnetisation curve of all compounds starts from the origin which confirms the antiferromagnetic nature of the magnetic ordering in these compounds. .. Journal of Alloys and Compounds 315 (2001) 28 – 35 31 Fig (a) Field dependence of the magnetisation of GdCu Al measured on free powder at 4.2 K (b) Field dependence of the magnetisation of GdCr