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Sn2+ transferred hyperfine fields in the Europium monochalcogenides doped with tin monochalcogenides
Solid State Communications, Vol. 18, Pp. 143—148, 1976.
Pergamon Press.
Printed in Great Britain
Sn2~TRANSFERRED HYPERFINE FIELDS IN THE EUROPIUM MONOCHALCOGENIDES DOPED WITH TIN MONOCHALCOGENIDES* N. Bykovetz Department of Physics and Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, PA 19174, U.S.A. (Received 17 July 1975 byJ. Tauc)
EuS, EuSe, and EuTe (but not EuO) are found to form solid solutions with the corresponding tin monochalcogenides. Transferred hyperfine fields (‘-~50 kOe) observed at the tin nuclei in dilute solid solutions using Mossbauer spectroscopy are shown to be supertransferred from the 2nd Eu neighbors. Experiments are indicated in which the Sn2~fields can be utilized for testing zero-point spin deviation predictions and the theory of the Heisenberg non-magnetic impurity problem.
WE REPORT that the europium (mono)chalcogenides EuS, EuSe, and EuTe, form pseudobinary solid solutions with the corresponding divalent tin chalcogenides for Sn2~concentrations up to at least 10 mole %•1 .2 Attempts to prepare the corresponding oxide systems (Eui_~Sn~O) by analogous means were unsuccessful. Other attempts at preparation provide evidence that a synthesis of the oxide systems at elevated temperatures is precluded altogether.3
are of interest in themselves, in that they provide data for the testing of thf-field theories in crystallographi. cally simple systems; they are also of value in that they immediately suggest a host of other, related, chemical systems in which thf-fields, suitable for the same purpose, should be observable. In addition, however, the thf-field measurements in low Sn2~concentration solutions provide a new means3 for probing the various magnetic structureswhich exist in EuSe5 and in the solid solutions EuS 3 and EuSe1_~Te~. Moreover, it appears likely that1_~Se~ low-concentration Sn2~-dopedsolutions may prove suitable for tests of zero-point spin deviation predictions of spin-wave theory, and for tests of the theoretical predictions for the non-magnetic impurity problem in a Heisenberg ferromagnet or antiferromagnet.
1% Sn2~-dopedsamples of EuS, EuSe, and EuTe, show sizable and well-defined transferred hyperfme fields at the tin nuclei at temperatures well below inception of magnetic ordering. Transferred hyperfme (thf-) fields have been observed previously in tin within magnetic alloys and in tetravalent tin within certain magnetic insulators, but ours appears to be the first observation of thf-fields in divalent tin. Largely as a result of the multiple magnetic structureswhich exist in EuSe,45 we were able to identify the origin of the Sn2~hyperfine fields. The Sn2~hyperfine field results *
The Sn2~transferred hyperfine fields were determined by means of Mossbauer spectroscopy in 1% Sn24-doped samples prepared with 90% “9Sn-enriched SnCh (where Ch represents on of the chalcogens; in our case 5, Se, or Te). Figure 1(c) gives an ifiustration of a Sn2~Mossbauer spectrum for the case of doped EuS at 4.2 K showing a near-maximum
Work supported in part by the Office of Naval Research (0014-67-A-0216-003), and through the Laboratory for Research on the Structure ofMatter, in part by the National Science Foundation MRL Program, and by the Advanced Research Projects Agency of the Department of Defense,
field sphttmg. In both EuS (a ferromagnet), and EuTe (a Type II, MnO-like antiferromagnet), a unique field was observed at the nuclei of the Sn2~’impurities at 143
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FIG. 1. Mossbauer spectra obtained via the 24 keY transition of ll9mSfl using a Ba~~9mSnO source 3 at R.T. (a) SnS absorber at 4.2 K showing the quadrupole splitting due to the non-cubic structure of SnS. (b) A 0.9% (enriched) SnS in EuS solid solution (at 20K) showing the 2~ disappearance ions occupy sites of the ofquadrupole cubic symmetry. increased splittingThe as the Sn isomer shift (~I.S.)shows that divalent tin is more ionic in the EuS rather than the SnS matrix (likewise in the case of the selenide and the telluride). (c) A 1.4% (enriched) SnS in EuS solid solution showing the magnetic splitting in Sn2~ measured at 4.2 K in zero external field. temperatures below magnetic ordering (Tc = 16.5 K for EuS, and TN = 9.6 K for EuTe). However in EuSe, which is know to exhibit a complex behavior4’5 below its first-order magnetic phase transition of 4.6 K, a unique field6 was seen only above 3 K while below this temperature two distinct fields6 were observed, indicating the existence of magnetically inequivalent sites. Figure 2 shows a plot of the measured Sn2~fields H~extrapolated to T = as a function of the lattice constant for EuS (52.6 ±0.5 kOe) and for EuTe (43.5 ±0.5 kOe), but includes only the larger of the two fields observed in EuSe (50.0 ± 1.0 kOe), i.e., the field seen only below 3 K. The saturation value of the smaller EuSe field (the one omitted from Fig. 2) was found to be 5 ±1 kOe. For EuS the sign of the Sn2~field was determined to be
FIG. 2. Magnitudes of the hyperfme fields in Sn2~ impurities within the europium chalcogenides as a function of lattice constant. The solid line connects values of the measured field Heff. The dashed line connects values of the transferred hyperfme field HtI~t(= IHeu H(huhi); in the case of EuS, it is assumed that the demagnetization field is zero ~olydomain crystallites) so that the dipolar field, H is equal to the Lorentz field (~itM).The dotted line —
~,
renresents a linear extension of the antiferromagnetic Ht~~ up to EuS. The EuS fields are negative; the EuSe and EuTe fields must,in consequence, be positive (the sign convention is described in Table 1). negative 12 kOe. by the application of an external field of Mossbauer [Fig. 1(a) and (b)] and X-ray crystallographic3 evidence shows that in the case of all three of the chacogenides the Sn2~ions enter the Eu2~sites substitutionally. This fact provides the basis for the analysis of the Sn2~fields presented in Table 1. On the basis of this analysis we are led to conclude that the Sn2~thf-fields shown in Fig. 2 are for the case of EuSe and EuTe, entirely, and for EuS, either entirely or nearly entirely, supertransferred from the 2nd Eu neighbors and have a direction opposite to the magnetization of the 2nd neighbors. Moreover, the data support (though not conclusively) the conclusion that the transferred contribution of the 1st Eu neighbors in EuSe (and by inference in EuTe) is negligibly small. The results obtained in EuSe are central to the analysis. There are three zero-applied-field magnetic structures in EuSe (NNSS, NNS, and NSNS4’5), and therefore in principle four magnetically inequivalent sites (see Table 1). Between 2.8 and 4.6K, only the NNSS structure is present.4’5 Thus we can
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Table 1. Hyperfine field contributions in various zero-external-field magnetic structures at a diva!ent diamagnetic cation impurity located substitutionally in the EuCh(NaCl) lattice Allgmnent of Structurea
Eu neighbors’~
Heff
=
Hthlf + ~
HthP(EuSe)dI
1st
2nd
(kOe)
NNN (Ferromagnetic)
+ 12
+6
NNSS (Antiferromagnetic) NNS (Ferrimagnetic) (2/3 of sites) NNS(Ferrimagnetic) (1/3 of sites) NSNS (Type II Antiferromagnetic)
+6 +6
0 0
1 + 6H2 + 24H3 + 12H4 + HthP 6H1 + 0 +0 +0 +H~ 6II~+ 0 + 6H3 + 6H4 + Hd~
0
—6
0
—6H2
0
—6
0
—
12ff
—
12H3 +0
6H2 + 0
4.58 + 4.27 + 4.27 +
+Hth~) + 3.95
+ 12114 + H~p + 395
a The magnetic structures of the europium chalcogenides can in all cases be described as a stacking of ferromag-
netically ordered (111) planes. NSNS, for example, designates a structure in which successive (111) planes have oppositely directed spins (N = North, S = South) [see, e.g., reference 4]. EuO and EuS order only ferromagnetically. EuSe and EuTe also order in the MnO-like NSNS structure. The other two structures(NNSS, and NNS) pertain only to EuSe. b Expressed relative to the direction of the magnetic moment (or spin) of the Eu ion which the diamagnetic impurity has replaced. C The expression of the measured field Heff at the nucleus of the diamagnetic ion expressed in terms of the transferred h~perfinefield (HtM) contributions of the first four Euthpneighbor given inshells, footnote and (d). of the dipolar field of the lattice (H I’). The sign convention for Hthf is the same as for H d For the antiferromagnetic structures HdIP was obtained by a summation of (point) dipole contributions = 7i.~B)from all Eu ions within a sphere of diameter = 40 lattice constants. In the ferromagnetic and fernmagnetic structuresit is assumed that the demagnetization field is zero (polydomai 1i crystallites), so that HthP = Lorentz field (~irM)+ field contributions from within the Lorentz sphere. The numerical values pertain to T = 0, Hext = 0, a0 = 6.190 A. The following sign convention is used. A + sign means Hd1P has the same direction as the magnetic moment of the Eu ion which the diamagnetic impurity has replaced.
2~fields immediately associate of the Sn observed in EuSe withthe thissmaller magnetic structure. Moreover, the magnitude of the smaller field (H~ 0= 5of±the 1 kOe) can be accounted entirely by the value dipolar field calculatedforfor the NNSS structure (H 7t~= + 4.3 kOe) if H~ is assumed to be positive (evidence against the alternative assumption will be mentioned in further discussion). If then the field acting on the tin nuclei in the NNSS structure is in fact essentially of dipolar origin, we can see from Table 1 that the 1st Eu neighbor thf-contributions in EuSe must be vanishingly small (i.e. 6H1 ~ 0). Below K EuSe is magnetically multiphase the2.8 antiferromagnetic NSNS structure in consisting admixture of with the ferrimagnetic NNS structure (and possibly also with part of the original NNSS structure).5 Consideration of the expressions for Heff in Table 1 pertinent to the four possible inequivalent sites of the three
EuSe structures (NNSS, NNS(2/3), NNS(1/3), and NSNS) shows that the only assignment of the Sn2~ fields consistent with our observation of two (rather 2” fields bethan or four) substantially Sn field is seen low 3three K is one in which the largerdistinct (50 kOe) to come from the NSNS structure and also from the “corresponding” sites (see lower curly bracket in the first column of Table 1) of the NNS structure, whereas the smaller (5 kOe) field comes from the other NNS sites and possibly also from residual NNSS material (see upper curly bracket in Table 1). The ferrimagnetic NNS structure is believed to5’8 exist thewithin temperaTheonly factinthat ture range 1.8 to 2.8 K. limits of discrimination we observed only two Sn2~ fields in this region of temperature (and down to 1.5 K) implies that third and fourth Eu neighbor contributions (H 3 and H4) sum, we andneed probably separ9 Asasaaresult to consider ately, are negligible. ‘-‘
Sn2” IN THE EUROPIUM MONOCHALCOGENIDES
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only 1st and 2nd neighbor effects (see column 2 of Table 1). Since in the NSNS structure (and at the corresponding NNS sites) the first neighbor contributions necessarily cancel, it follows that the 50 kOe field (as shown in Fig. 2) consists solely of 2nd neighbor contributions and a small dipolar field (ignored in previous literature on EuCh hyperfme fields; see last column of Table 1). In EuTe (NSNS structure) the measured field at the Sn2” nuclei must, likewise, consist only of transferred 2nd neighbor contributions (6H 2) and the dipolar field (assuming that here too H3 and H4 are negligible). Thus, as in the case of EuSe, the HtM field value for EuTe shown in Fig. 2 consists entirely of 2nd neighbor contributions. If 1st neighbor transferred field contributions in EuTe do exist, they would show up only in the ferromagnetic state (12H 2” field 1). Ifinour interpretation of the Sn observed EuSe is valid, so that forsmaller EuSe 6H 1 = 0, then in EuTe the 1st neighbor contributions are probably also negligible (one expects the 1St neighbor contributions to decrease with an increase in lattice constant). It is possible that the 1st neighbor contributions in = EuSe such that theequal measured field, Heff 6H are actually thp, is fortuitously in 1+H magnitude to HthP, but has the opposite sign, so that our data (H~= 151 kOe) should in fact be interpreted as implying that 6H 1 = —9 ±1 kOe. If this were thein 2” thf-field case,to however, one projected would expect the Sn of the 2nd EuS exceed the contribution neighbors (dotted line in Fig. 2) by at least (—)1 8 kOe (1 2H 1), whereas we see an excess of only kOe. More conclusive (and more direct) on8the existence or non-existence of Sn2” 1stevidence neighbor effects in EuSe and EuTe must await Sn21’ field measurements in the ferromagnetic state (obtainable via applied magnetic fields,1°and for EuSe, possibly also via chemistry (EuSe or 1 pressure”). 1_~S~),’ In EuS it is not possible to separate out any 1st neighbor contributions directly. Our data (Fig. 2) is suggestive of a possible 1st neighbor contribution 1 2H~ 8 kOe (i.e., < 15% of the 2nd neighbor thffield), although this added amount could merely be due to a nonlinear variation ofH 2 with 12 If any direct (as opposed tolattice 90°-superconstant. contribution from 1st neighbors exists in transferred) EuS one would expect to observe a rapid change in the Sn24’ field in going toward Sn2”-doped EuO. —
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As Sn2”’-doped EuO cannot be prepared at elevated temperatures,3 and low-temperature synthesis is Unlikely, it may prove worthwile for this reason to attempt a Eui_~Sn~O preparation by the more difficult approach of Sn2”-ion implantation. Since the Sn2”’ thf-fields shown in Fig. 2 represent the transferred contribution of the 2nd neighbors alone (with the possible exception of the EuS field, as noted above), we conclude that the slow variation of Htht with lattice constant is indicative of a supertransfer mechanism, i.e., a (180°)spin-density polarization via the intervening ligands. Moreover, since there is a large difference in ionic radius 4” (0.71 A) and Sn2~(1.12 A) one canbetween expect that Sn the two outer 5s electrons of Sn2”’ provide the largest contribution to the spin-density unpairing at the Sn2~”impurity nucleus. It may thus be possible with fairly reasonable theoretical effort13 to correlate the Sn2~ supertransferred fields we have observed with the transferred fields which have been observed at the EuCh ligand nuclei in other studies (e.g. ref. 20). We2”note that ingiven our above, analysisit of theassumed magnitudes thf-fields was that of the Sn zero-point spin deviations (ZPSD) can be magnetic disregarded. It has been suggested14 on the basis of spin-wave theory estimates that such deviations in the NSNS EuSe could be as large as 9% at T = OKstructure (thoughofthis seems highly unlikely);the ZPSD in EuTe, on the other hand, are expected to be smaller. Thus EuSe could prove to be a good case study with regard5toThe testing the influence of ZPSD problem of extracting such aon hyperfine ZPSD fromfields.’ the previously measured5 Eu2”’ hyperfme fields is beset with various difficulties;3 moreover, a possible determination of the effect via the ligand (77Se) NMR is precluded (at least in undoped EuSe samples) by the fact that the ligand thf-field contributions in the NSNS structure suffer complete cancellation. As a result, Sn2”-doped EuSe samples appear to provide the best means of getting at the problem. A determination of the difference between the saturation values of the Sn21’ thf-fields in the ferromagnetic state and the NSNS antiferromagnetic state would (under certain conditions) a direct measure of the ZPSD difference.16 Theprovide existence of measurable ZPSD in the magnetic structures of EuSe would, of course, modify somewhat our previous analysis of the Sn2” fields (the magnitude of the EuSe ffthf in Fig. 2
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would be increased a few percent, bringing the Hthf curve closer to linear), We point out also that the Sn2”’-doped EuCh provide the first opportunity to test the theory of the non-magnetic impurity problem in a 3-dimensional Heisenberg ferromagnet.’7 Our preparation and study of the Sn2”-doped systems was in fact motivated entirely by this problem.3”7 The relevant measurements for Sn2’~-dopedEuS have been carried out, and are discussed elsewhere.3 The effects are small;however, and for this reason ferromagnetically drdered Sii21’doped EuSe is expected to be a more suitable system.3 Sn21’-doped EuTe appears to be the best candidate for the corresponding antiferromagnetic problem.18 We note in co>flclusion that in view of the existence of the Sn2”’ thf-fields which we have observed 50 kOe) and the existence of the Eu2”’ thf-fields in EuCh 30 kOe, if assumed likewise supertransferred19 ) previously reported in the literature,5 10 14 ~ it is quite probable that thf-iIelds of this order of magnitude should be observable in the EuCh series (‘-‘j
(“j
‘
‘
‘
‘
147
with other substitutional impurities, particularly diamagnetic ones. EuCh systems doped with the alkaline earths are known to exist (in the case of Ca2”, Sr2”, and Ba2”’) and would be especially useful with regard to the question of the behavior of Hti~between EuS and EuO. With regard to rare earth dopants, the diamagnetic Yb2” as well as Sm21’ (a Van Yleck paramagnet with a large ferromagnetic Eu2”-Sm2”’ exchange interaction), appear to offer the most interesting prospects.
Acknowledgements The author would like to express gratitude to the following: R.H. Hartford ( a most defmite crypto-coauthor) for his close collaboration and generous help in vanous forms throughout the entire course of this study; to G.R. Davidson for participation in the initial stages of experimental work, many useful discussions, and the use of his GRD3 computer program for fitting Mossbauer spectra; F. Holtzberg and M.W. Shafer (both of IBM Corp.) for their(EuSe generous donation hostrespectively); material single crystals and EuTe, andof EuO and to Profs. H. Callen and M.E. Caspari for reading the manuscript and making very helpful suggestions. —
REFERENCES 1.
The author would like to thank HARTFORD R.H. for bringing to his attention the possible existence of these compounds (Hartford prepared and studied Eu 0,9Sn0,1Se; unpublished result). The method of preparation and the crystallographic aspects will be reported elsewhere (see also reference 3).
2.
The preparation of Eu1_~Sn~Te for x > 0.90 was reported by MATHUR M.P., DIES D.W., JONES C.K., PATTERSON A. & CARR W.J., Jr.,App!. Phys. 42, 1693 (1971). Since SnTe (alone of the SnCh) is isomorphic with EuTe, it is quite certain that the solid solutions of the tellunides exist for all ratios of composition. BYKOVETS N., PhD. dissertation, University of Pennsylvania (to be published).
3. 4. 5. 6.
FISCHER P., HALG W., VON WARTBURG W., SCHWOB P. & VOGT 0., Phys. Kondens, Mater. 9,249 (1969). KOMARUT.,HIHARAT.&KOIY.,J.Phys. Soc. Japan 31,1391(1971). 9Sn Mossbauer spectrum above 3°Klooks like Determined a somewhat within broadened limits ofFig. resolution 1(b) dip, of and the below Mossbauer 3°Klike data.a The superposition U of Fig. 1(b) and (c). Our observations on Sn21’ agree with the NMR results for Eu21’ in EuSe where a single magnetically inequivalent site is observed above 2.8°Kand two sites below 2.8°K(reference 5).
7.
Extrapolation to T = 0 was made by plotting H vs T2, as in referenceS.
8.
GRIESSEN R., LANDOLT M. & OTT H.R., Solid State Commun. 9, 2219 (1971).
9.
From the results obtained in reference 5 for the Eu21’ fields, we can conclude that for the Eu21 thf-fields, also H 3 +H4 0. SAUER CH., KOBLER U., ZINN W. & KALVIUS G.M.,J. Phys. 35, C6-269 (1974).
10. 11. 12.
SCHWOB P.,Phys. Kondens. Mater. 10, 186 (1969). One might expect a linear variation of H2 by anology with the (approximately) linear variation in the 2ndneighbor exchange interaction, J2, generally assumed in previous literature (see KASUYA T., IBMJ. Res. &
148
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Dev. 14,214 (1970) and references therein), We point out in this connection that in previous literature it has
always been assumed that J 2 increases in magnitude from EuS to EuTe, whereas our result for the variation of H2 (Hthfcurve in Fig. 2) suggests the precise opposite.
13. 14. 15,
16, 17. 18. 19. 20.
See, e.g., SAWATZKY G.A. & VAN DER WOUDE F,, J, Phys. 35, C6-47 (1974). HARTFORD R.H. & CASPARI M.E., Mater. Res. Bull, 6, 989 (1971). More detailed ZPSD predictions can be found in SWENDSEN R.H.,J. Phys. C: Solid State Phys. 6,3763 (1973). Predicted ZPSD are usually very small and have proved difficult to detect. See discussion and references in GESCHWIND S. in Hyperfine Interactions (Edited by FREEMAN A,J. & FRANKEL RB.), p.258, Academic Press, NY (1967), See reference 3 for more detailed discussion, SWENDSEN R.H.,Phys. Rev. B6, 1903 (1972). 2tdoped MnF This problem has been previously studied in Zn 2. See WALKER L,R., CHAMBERS B.C., HONE D. & CALLEN H., Phys. Rev. BS, so 1144 2”’ it has not been possible, far,(1972). to separate the 1st and 2nd neighbor thf-contributioris. Measurements ForEuSe Eu and EuTe show that 12H in 2”’), we 1 + l2H2 (seeand reference thatSn2’~fields, H1 = 0 (as for Sn see that the 2nd neighbor contributions 6112 6OkOe 30 kOe, have the10). sameAssuming sign as the BUDNICK J.I., RAJ K., BURCH T.J. & HOLTZBERG F.,J. Phys. 32, Cl-763 (1971).
Pergamon Press.
Printed in Great Britain
Sn2~TRANSFERRED HYPERFINE FIELDS IN THE EUROPIUM MONOCHALCOGENIDES DOPED WITH TIN MONOCHALCOGENIDES* N. Bykovetz Department of Physics and Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, PA 19174, U.S.A. (Received 17 July 1975 byJ. Tauc)
EuS, EuSe, and EuTe (but not EuO) are found to form solid solutions with the corresponding tin monochalcogenides. Transferred hyperfine fields (‘-~50 kOe) observed at the tin nuclei in dilute solid solutions using Mossbauer spectroscopy are shown to be supertransferred from the 2nd Eu neighbors. Experiments are indicated in which the Sn2~fields can be utilized for testing zero-point spin deviation predictions and the theory of the Heisenberg non-magnetic impurity problem.
WE REPORT that the europium (mono)chalcogenides EuS, EuSe, and EuTe, form pseudobinary solid solutions with the corresponding divalent tin chalcogenides for Sn2~concentrations up to at least 10 mole %•1 .2 Attempts to prepare the corresponding oxide systems (Eui_~Sn~O) by analogous means were unsuccessful. Other attempts at preparation provide evidence that a synthesis of the oxide systems at elevated temperatures is precluded altogether.3
are of interest in themselves, in that they provide data for the testing of thf-field theories in crystallographi. cally simple systems; they are also of value in that they immediately suggest a host of other, related, chemical systems in which thf-fields, suitable for the same purpose, should be observable. In addition, however, the thf-field measurements in low Sn2~concentration solutions provide a new means3 for probing the various magnetic structureswhich exist in EuSe5 and in the solid solutions EuS 3 and EuSe1_~Te~. Moreover, it appears likely that1_~Se~ low-concentration Sn2~-dopedsolutions may prove suitable for tests of zero-point spin deviation predictions of spin-wave theory, and for tests of the theoretical predictions for the non-magnetic impurity problem in a Heisenberg ferromagnet or antiferromagnet.
1% Sn2~-dopedsamples of EuS, EuSe, and EuTe, show sizable and well-defined transferred hyperfme fields at the tin nuclei at temperatures well below inception of magnetic ordering. Transferred hyperfme (thf-) fields have been observed previously in tin within magnetic alloys and in tetravalent tin within certain magnetic insulators, but ours appears to be the first observation of thf-fields in divalent tin. Largely as a result of the multiple magnetic structureswhich exist in EuSe,45 we were able to identify the origin of the Sn2~hyperfine fields. The Sn2~hyperfine field results *
The Sn2~transferred hyperfine fields were determined by means of Mossbauer spectroscopy in 1% Sn24-doped samples prepared with 90% “9Sn-enriched SnCh (where Ch represents on of the chalcogens; in our case 5, Se, or Te). Figure 1(c) gives an ifiustration of a Sn2~Mossbauer spectrum for the case of doped EuS at 4.2 K showing a near-maximum
Work supported in part by the Office of Naval Research (0014-67-A-0216-003), and through the Laboratory for Research on the Structure ofMatter, in part by the National Science Foundation MRL Program, and by the Advanced Research Projects Agency of the Department of Defense,
field sphttmg. In both EuS (a ferromagnet), and EuTe (a Type II, MnO-like antiferromagnet), a unique field was observed at the nuclei of the Sn2~’impurities at 143
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FIG. 1. Mossbauer spectra obtained via the 24 keY transition of ll9mSfl using a Ba~~9mSnO source 3 at R.T. (a) SnS absorber at 4.2 K showing the quadrupole splitting due to the non-cubic structure of SnS. (b) A 0.9% (enriched) SnS in EuS solid solution (at 20K) showing the 2~ disappearance ions occupy sites of the ofquadrupole cubic symmetry. increased splittingThe as the Sn isomer shift (~I.S.)shows that divalent tin is more ionic in the EuS rather than the SnS matrix (likewise in the case of the selenide and the telluride). (c) A 1.4% (enriched) SnS in EuS solid solution showing the magnetic splitting in Sn2~ measured at 4.2 K in zero external field. temperatures below magnetic ordering (Tc = 16.5 K for EuS, and TN = 9.6 K for EuTe). However in EuSe, which is know to exhibit a complex behavior4’5 below its first-order magnetic phase transition of 4.6 K, a unique field6 was seen only above 3 K while below this temperature two distinct fields6 were observed, indicating the existence of magnetically inequivalent sites. Figure 2 shows a plot of the measured Sn2~fields H~extrapolated to T = as a function of the lattice constant for EuS (52.6 ±0.5 kOe) and for EuTe (43.5 ±0.5 kOe), but includes only the larger of the two fields observed in EuSe (50.0 ± 1.0 kOe), i.e., the field seen only below 3 K. The saturation value of the smaller EuSe field (the one omitted from Fig. 2) was found to be 5 ±1 kOe. For EuS the sign of the Sn2~field was determined to be
FIG. 2. Magnitudes of the hyperfme fields in Sn2~ impurities within the europium chalcogenides as a function of lattice constant. The solid line connects values of the measured field Heff. The dashed line connects values of the transferred hyperfme field HtI~t(= IHeu H(huhi); in the case of EuS, it is assumed that the demagnetization field is zero ~olydomain crystallites) so that the dipolar field, H is equal to the Lorentz field (~itM).The dotted line —
~,
renresents a linear extension of the antiferromagnetic Ht~~ up to EuS. The EuS fields are negative; the EuSe and EuTe fields must,in consequence, be positive (the sign convention is described in Table 1). negative 12 kOe. by the application of an external field of Mossbauer [Fig. 1(a) and (b)] and X-ray crystallographic3 evidence shows that in the case of all three of the chacogenides the Sn2~ions enter the Eu2~sites substitutionally. This fact provides the basis for the analysis of the Sn2~fields presented in Table 1. On the basis of this analysis we are led to conclude that the Sn2~thf-fields shown in Fig. 2 are for the case of EuSe and EuTe, entirely, and for EuS, either entirely or nearly entirely, supertransferred from the 2nd Eu neighbors and have a direction opposite to the magnetization of the 2nd neighbors. Moreover, the data support (though not conclusively) the conclusion that the transferred contribution of the 1st Eu neighbors in EuSe (and by inference in EuTe) is negligibly small. The results obtained in EuSe are central to the analysis. There are three zero-applied-field magnetic structures in EuSe (NNSS, NNS, and NSNS4’5), and therefore in principle four magnetically inequivalent sites (see Table 1). Between 2.8 and 4.6K, only the NNSS structure is present.4’5 Thus we can
Vol. 18, No. 1
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Table 1. Hyperfine field contributions in various zero-external-field magnetic structures at a diva!ent diamagnetic cation impurity located substitutionally in the EuCh(NaCl) lattice Allgmnent of Structurea
Eu neighbors’~
Heff
=
Hthlf + ~
HthP(EuSe)dI
1st
2nd
(kOe)
NNN (Ferromagnetic)
+ 12
+6
NNSS (Antiferromagnetic) NNS (Ferrimagnetic) (2/3 of sites) NNS(Ferrimagnetic) (1/3 of sites) NSNS (Type II Antiferromagnetic)
+6 +6
0 0
1 + 6H2 + 24H3 + 12H4 + HthP 6H1 + 0 +0 +0 +H~ 6II~+ 0 + 6H3 + 6H4 + Hd~
0
—6
0
—6H2
0
—6
0
—
12ff
—
12H3 +0
6H2 + 0
4.58 + 4.27 + 4.27 +
+Hth~) + 3.95
+ 12114 + H~p + 395
a The magnetic structures of the europium chalcogenides can in all cases be described as a stacking of ferromag-
netically ordered (111) planes. NSNS, for example, designates a structure in which successive (111) planes have oppositely directed spins (N = North, S = South) [see, e.g., reference 4]. EuO and EuS order only ferromagnetically. EuSe and EuTe also order in the MnO-like NSNS structure. The other two structures(NNSS, and NNS) pertain only to EuSe. b Expressed relative to the direction of the magnetic moment (or spin) of the Eu ion which the diamagnetic impurity has replaced. C The expression of the measured field Heff at the nucleus of the diamagnetic ion expressed in terms of the transferred h~perfinefield (HtM) contributions of the first four Euthpneighbor given inshells, footnote and (d). of the dipolar field of the lattice (H I’). The sign convention for Hthf is the same as for H d For the antiferromagnetic structures HdIP was obtained by a summation of (point) dipole contributions = 7i.~B)from all Eu ions within a sphere of diameter = 40 lattice constants. In the ferromagnetic and fernmagnetic structuresit is assumed that the demagnetization field is zero (polydomai 1i crystallites), so that HthP = Lorentz field (~irM)+ field contributions from within the Lorentz sphere. The numerical values pertain to T = 0, Hext = 0, a0 = 6.190 A. The following sign convention is used. A + sign means Hd1P has the same direction as the magnetic moment of the Eu ion which the diamagnetic impurity has replaced.
2~fields immediately associate of the Sn observed in EuSe withthe thissmaller magnetic structure. Moreover, the magnitude of the smaller field (H~ 0= 5of±the 1 kOe) can be accounted entirely by the value dipolar field calculatedforfor the NNSS structure (H 7t~= + 4.3 kOe) if H~ is assumed to be positive (evidence against the alternative assumption will be mentioned in further discussion). If then the field acting on the tin nuclei in the NNSS structure is in fact essentially of dipolar origin, we can see from Table 1 that the 1st Eu neighbor thf-contributions in EuSe must be vanishingly small (i.e. 6H1 ~ 0). Below K EuSe is magnetically multiphase the2.8 antiferromagnetic NSNS structure in consisting admixture of with the ferrimagnetic NNS structure (and possibly also with part of the original NNSS structure).5 Consideration of the expressions for Heff in Table 1 pertinent to the four possible inequivalent sites of the three
EuSe structures (NNSS, NNS(2/3), NNS(1/3), and NSNS) shows that the only assignment of the Sn2~ fields consistent with our observation of two (rather 2” fields bethan or four) substantially Sn field is seen low 3three K is one in which the largerdistinct (50 kOe) to come from the NSNS structure and also from the “corresponding” sites (see lower curly bracket in the first column of Table 1) of the NNS structure, whereas the smaller (5 kOe) field comes from the other NNS sites and possibly also from residual NNSS material (see upper curly bracket in Table 1). The ferrimagnetic NNS structure is believed to5’8 exist thewithin temperaTheonly factinthat ture range 1.8 to 2.8 K. limits of discrimination we observed only two Sn2~ fields in this region of temperature (and down to 1.5 K) implies that third and fourth Eu neighbor contributions (H 3 and H4) sum, we andneed probably separ9 Asasaaresult to consider ately, are negligible. ‘-‘
Sn2” IN THE EUROPIUM MONOCHALCOGENIDES
146
only 1st and 2nd neighbor effects (see column 2 of Table 1). Since in the NSNS structure (and at the corresponding NNS sites) the first neighbor contributions necessarily cancel, it follows that the 50 kOe field (as shown in Fig. 2) consists solely of 2nd neighbor contributions and a small dipolar field (ignored in previous literature on EuCh hyperfme fields; see last column of Table 1). In EuTe (NSNS structure) the measured field at the Sn2” nuclei must, likewise, consist only of transferred 2nd neighbor contributions (6H 2) and the dipolar field (assuming that here too H3 and H4 are negligible). Thus, as in the case of EuSe, the HtM field value for EuTe shown in Fig. 2 consists entirely of 2nd neighbor contributions. If 1st neighbor transferred field contributions in EuTe do exist, they would show up only in the ferromagnetic state (12H 2” field 1). Ifinour interpretation of the Sn observed EuSe is valid, so that forsmaller EuSe 6H 1 = 0, then in EuTe the 1st neighbor contributions are probably also negligible (one expects the 1St neighbor contributions to decrease with an increase in lattice constant). It is possible that the 1st neighbor contributions in = EuSe such that theequal measured field, Heff 6H are actually thp, is fortuitously in 1+H magnitude to HthP, but has the opposite sign, so that our data (H~= 151 kOe) should in fact be interpreted as implying that 6H 1 = —9 ±1 kOe. If this were thein 2” thf-field case,to however, one projected would expect the Sn of the 2nd EuS exceed the contribution neighbors (dotted line in Fig. 2) by at least (—)1 8 kOe (1 2H 1), whereas we see an excess of only kOe. More conclusive (and more direct) on8the existence or non-existence of Sn2” 1stevidence neighbor effects in EuSe and EuTe must await Sn21’ field measurements in the ferromagnetic state (obtainable via applied magnetic fields,1°and for EuSe, possibly also via chemistry (EuSe or 1 pressure”). 1_~S~),’ In EuS it is not possible to separate out any 1st neighbor contributions directly. Our data (Fig. 2) is suggestive of a possible 1st neighbor contribution 1 2H~ 8 kOe (i.e., < 15% of the 2nd neighbor thffield), although this added amount could merely be due to a nonlinear variation ofH 2 with 12 If any direct (as opposed tolattice 90°-superconstant. contribution from 1st neighbors exists in transferred) EuS one would expect to observe a rapid change in the Sn24’ field in going toward Sn2”-doped EuO. —
Vol. 18, No. I
As Sn2”’-doped EuO cannot be prepared at elevated temperatures,3 and low-temperature synthesis is Unlikely, it may prove worthwile for this reason to attempt a Eui_~Sn~O preparation by the more difficult approach of Sn2”-ion implantation. Since the Sn2”’ thf-fields shown in Fig. 2 represent the transferred contribution of the 2nd neighbors alone (with the possible exception of the EuS field, as noted above), we conclude that the slow variation of Htht with lattice constant is indicative of a supertransfer mechanism, i.e., a (180°)spin-density polarization via the intervening ligands. Moreover, since there is a large difference in ionic radius 4” (0.71 A) and Sn2~(1.12 A) one canbetween expect that Sn the two outer 5s electrons of Sn2”’ provide the largest contribution to the spin-density unpairing at the Sn2~”impurity nucleus. It may thus be possible with fairly reasonable theoretical effort13 to correlate the Sn2~ supertransferred fields we have observed with the transferred fields which have been observed at the EuCh ligand nuclei in other studies (e.g. ref. 20). We2”note that ingiven our above, analysisit of theassumed magnitudes thf-fields was that of the Sn zero-point spin deviations (ZPSD) can be magnetic disregarded. It has been suggested14 on the basis of spin-wave theory estimates that such deviations in the NSNS EuSe could be as large as 9% at T = OKstructure (thoughofthis seems highly unlikely);the ZPSD in EuTe, on the other hand, are expected to be smaller. Thus EuSe could prove to be a good case study with regard5toThe testing the influence of ZPSD problem of extracting such aon hyperfine ZPSD fromfields.’ the previously measured5 Eu2”’ hyperfme fields is beset with various difficulties;3 moreover, a possible determination of the effect via the ligand (77Se) NMR is precluded (at least in undoped EuSe samples) by the fact that the ligand thf-field contributions in the NSNS structure suffer complete cancellation. As a result, Sn2”-doped EuSe samples appear to provide the best means of getting at the problem. A determination of the difference between the saturation values of the Sn21’ thf-fields in the ferromagnetic state and the NSNS antiferromagnetic state would (under certain conditions) a direct measure of the ZPSD difference.16 Theprovide existence of measurable ZPSD in the magnetic structures of EuSe would, of course, modify somewhat our previous analysis of the Sn2” fields (the magnitude of the EuSe ffthf in Fig. 2
Sn2” IN THE EUROPIUM MONOCHALCOGENIDES
Vol. 18, No. 1
would be increased a few percent, bringing the Hthf curve closer to linear), We point out also that the Sn2”’-doped EuCh provide the first opportunity to test the theory of the non-magnetic impurity problem in a 3-dimensional Heisenberg ferromagnet.’7 Our preparation and study of the Sn2”-doped systems was in fact motivated entirely by this problem.3”7 The relevant measurements for Sn2’~-dopedEuS have been carried out, and are discussed elsewhere.3 The effects are small;however, and for this reason ferromagnetically drdered Sii21’doped EuSe is expected to be a more suitable system.3 Sn21’-doped EuTe appears to be the best candidate for the corresponding antiferromagnetic problem.18 We note in co>flclusion that in view of the existence of the Sn2”’ thf-fields which we have observed 50 kOe) and the existence of the Eu2”’ thf-fields in EuCh 30 kOe, if assumed likewise supertransferred19 ) previously reported in the literature,5 10 14 ~ it is quite probable that thf-iIelds of this order of magnitude should be observable in the EuCh series (‘-‘j
(“j
‘
‘
‘
‘
147
with other substitutional impurities, particularly diamagnetic ones. EuCh systems doped with the alkaline earths are known to exist (in the case of Ca2”, Sr2”, and Ba2”’) and would be especially useful with regard to the question of the behavior of Hti~between EuS and EuO. With regard to rare earth dopants, the diamagnetic Yb2” as well as Sm21’ (a Van Yleck paramagnet with a large ferromagnetic Eu2”-Sm2”’ exchange interaction), appear to offer the most interesting prospects.
Acknowledgements The author would like to express gratitude to the following: R.H. Hartford ( a most defmite crypto-coauthor) for his close collaboration and generous help in vanous forms throughout the entire course of this study; to G.R. Davidson for participation in the initial stages of experimental work, many useful discussions, and the use of his GRD3 computer program for fitting Mossbauer spectra; F. Holtzberg and M.W. Shafer (both of IBM Corp.) for their(EuSe generous donation hostrespectively); material single crystals and EuTe, andof EuO and to Profs. H. Callen and M.E. Caspari for reading the manuscript and making very helpful suggestions. —
REFERENCES 1.
The author would like to thank HARTFORD R.H. for bringing to his attention the possible existence of these compounds (Hartford prepared and studied Eu 0,9Sn0,1Se; unpublished result). The method of preparation and the crystallographic aspects will be reported elsewhere (see also reference 3).
2.
The preparation of Eu1_~Sn~Te for x > 0.90 was reported by MATHUR M.P., DIES D.W., JONES C.K., PATTERSON A. & CARR W.J., Jr.,App!. Phys. 42, 1693 (1971). Since SnTe (alone of the SnCh) is isomorphic with EuTe, it is quite certain that the solid solutions of the tellunides exist for all ratios of composition. BYKOVETS N., PhD. dissertation, University of Pennsylvania (to be published).
3. 4. 5. 6.
FISCHER P., HALG W., VON WARTBURG W., SCHWOB P. & VOGT 0., Phys. Kondens, Mater. 9,249 (1969). KOMARUT.,HIHARAT.&KOIY.,J.Phys. Soc. Japan 31,1391(1971). 9Sn Mossbauer spectrum above 3°Klooks like Determined a somewhat within broadened limits ofFig. resolution 1(b) dip, of and the below Mossbauer 3°Klike data.a The superposition U of Fig. 1(b) and (c). Our observations on Sn21’ agree with the NMR results for Eu21’ in EuSe where a single magnetically inequivalent site is observed above 2.8°Kand two sites below 2.8°K(reference 5).
7.
Extrapolation to T = 0 was made by plotting H vs T2, as in referenceS.
8.
GRIESSEN R., LANDOLT M. & OTT H.R., Solid State Commun. 9, 2219 (1971).
9.
From the results obtained in reference 5 for the Eu21’ fields, we can conclude that for the Eu21 thf-fields, also H 3 +H4 0. SAUER CH., KOBLER U., ZINN W. & KALVIUS G.M.,J. Phys. 35, C6-269 (1974).
10. 11. 12.
SCHWOB P.,Phys. Kondens. Mater. 10, 186 (1969). One might expect a linear variation of H2 by anology with the (approximately) linear variation in the 2ndneighbor exchange interaction, J2, generally assumed in previous literature (see KASUYA T., IBMJ. Res. &
148
Sn2” IN THE EUROPIUM MONOCHALCOGENIDES
Vol. 18, No, 1
Dev. 14,214 (1970) and references therein), We point out in this connection that in previous literature it has
always been assumed that J 2 increases in magnitude from EuS to EuTe, whereas our result for the variation of H2 (Hthfcurve in Fig. 2) suggests the precise opposite.
13. 14. 15,
16, 17. 18. 19. 20.
See, e.g., SAWATZKY G.A. & VAN DER WOUDE F,, J, Phys. 35, C6-47 (1974). HARTFORD R.H. & CASPARI M.E., Mater. Res. Bull, 6, 989 (1971). More detailed ZPSD predictions can be found in SWENDSEN R.H.,J. Phys. C: Solid State Phys. 6,3763 (1973). Predicted ZPSD are usually very small and have proved difficult to detect. See discussion and references in GESCHWIND S. in Hyperfine Interactions (Edited by FREEMAN A,J. & FRANKEL RB.), p.258, Academic Press, NY (1967), See reference 3 for more detailed discussion, SWENDSEN R.H.,Phys. Rev. B6, 1903 (1972). 2tdoped MnF This problem has been previously studied in Zn 2. See WALKER L,R., CHAMBERS B.C., HONE D. & CALLEN H., Phys. Rev. BS, so 1144 2”’ it has not been possible, far,(1972). to separate the 1st and 2nd neighbor thf-contributioris. Measurements ForEuSe Eu and EuTe show that 12H in 2”’), we 1 + l2H2 (seeand reference thatSn2’~fields, H1 = 0 (as for Sn see that the 2nd neighbor contributions 6112 6OkOe 30 kOe, have the10). sameAssuming sign as the BUDNICK J.I., RAJ K., BURCH T.J. & HOLTZBERG F.,J. Phys. 32, Cl-763 (1971).