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A μSR study of the magnetic properties of CeAgSb2
Physica B 289}290 (2000) 38}42
A lSR study of the magnetic properties of CeAgSb J.A. Dann, A.D. Hillier, J.G.M. Armitage, R. Cywinski* School of Physics and Astronomy, University of St Andrews, Fife KY16 9SS, Scotland, UK
Abstract Zero and longitudinal "eld muon spin relaxation measurements have been performed on the Kondo lattice system CeAgSb in the magnetically ordered (¹(10 K) and paramagnetic (¹'10 K) states. The resulting spectra are consistent with a single muon site which senses a unique coherent internal "eld of 53 mT at low temperatures and dynamic atomic "elds, arising from extremely rapid RKKY-dominated paramagnetic #uctuations of the Ce> spins, at higher temperatures. Small stepwise increases in the temperature dependence of the muon relaxation rate at 60 and 17 K appear to be coincident with the Kondo temperature and the onset of coherence, respectively. 2000 Elsevier Science B.V. All rights reserved. Keywords: Kondo lattice; Heavy fermion; Magnetic order
Cerium-based intermetallic compounds exhibit a remarkably rich and diverse range of magnetic phenomena. Many show characteristic heavy fermion behaviour, and for these compounds the Kondo lattice is often regarded as an appropriate descriptive model. In Kondo lattice systems the indirect, inter-site, RKKY interaction responsible for magnetic order competes with the demagnetising e!ects of the Kondo interaction arising from on-site interactions. The crucial parameter in this competition is the coupling constant for exchange between the localised atomic spin and the conduction electron spins, g"N J, where J is the mag$ netic exchange constant and N the density of $ states at the Fermi energy. Whilst ¹ Jg, the 0))7 Kondo temperature ¹HJexp(!1/g). Doniach [1] has derived a generalised phase diagram for a one* Corresponding author. Tel.: #44-0-1334-463108; fax: #44-0-1334-463104. E-mail address: [email protected] (R. Cywinski).
dimensional Kondo lattice, which predicts a critical value for g below which long-range local moment magnetic order obtains and above which a non-magnetic heavy ground state, due to Kondo reduction of the e!ective moment, k , is found.
Systematic studies of Ce-based intermetallics suggest that the Doniach phase diagram may provide a broadly qualitative description of the behaviour of three-dimensional Kondo lattice systems. For example, the application of pressure, either chemical or external, e!ectively increases the parameter g and leads to a collapse of magnetic order and the local Ce moment. For most Ce-based heavy fermion compounds the generally reduced Ce moments are found to couple antiferromagnetically (e.g. CeAl and CeB ), although the oscillatory RKKY interaction can also lead to a ferromagnetic ground state (e.g. CeRu Ge ). Whichever the case, any new addition to the Kondo lattice family is always greeted with some interest and in this respect the recently reported CeAgSb is no exception.
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 0 ) 0 0 2 3 8 - 6
J.A. Dann et al. / Physica B 289}290 (2000) 38}42
Fig. 1. Crystallographic structure of CeAgSb , shown as a single unit cell in the c-direction and as 1.5 unit cells in the aand b-directions. The proposed muon sites at (, , 0.6) and symmetry related positions are also shown.
CeAgSb crystallises with the tetragonal ZrCuSi structure (space group P4/nmm) shown in Fig. 1, with a"b"4.364 As and c"10.722 As , and Ce at (, , 0.238), Ag at (, , ) and Sb at (, , 0) and (, , 0.674) [2]. Magnetic and transport measure ments provide strong evidence that CeAgSb is indeed a condensed Kondo lattice system [3, 4]. The magnetic component of the electrical resistivity shows a characteristic Kondo-like !ln(¹) dependence above 50 K with a maximum in the resistivity at 24 K signifying the onset of coherence. A further, sharp, drop in the resistivity at ¹ "10 K co incides with the onset of magnetic order. Estimates of the Kondo temperature from thermopower and neutron scattering measurements suggest a relatively high value of ¹H (&60}80 K) placing CeAgSb "rmly on the high-g side of the Doniach phase diagram, a view recently con"rmed by measurement of the pressure dependence of the magnetic transition temperature [5]. The precise nature of the magnetic ground state below ¹ of CeAgSb remains controversial and
unresolved. Magnetisation measurements [3, 4] in-
39
dicate that in the paramagnetic state the Ce moment has a value of 2.58l , close to the free ion moment of Ce>. However, below ¹ a saturation
moment of only 1.4l per Ce atom has led to suggestions that the ordered state of CeAgSb is either a complex antiferromagnet with a small resultant ferromagnetic moment [3] or a ferrimagnet [4], or perhaps that the reduced Ce moment may be due to crystal "eld e!ects [5]. AC susceptibility shows an extremely sharp peak at 10 K, attributed to the onset of antiferromagnetism at ¹ "¹ , ,
accompanied by a broad shoulder peaking close to 17 K and extending over the temperature regime in which resistivity indicates the onset of coherence [5]. The linear thermal expansion provides no evidence of the magnetic transition at 10 K but is dominated by a pronounced maximum at 17 K, coinciding with the shoulder in the AC susceptibility and extending over a correspondingly broad temperature range. This feature has been associated with the stabilisation of coherence between the Ce sites [5]. Our own DC magnetisation measurements on single crystal CeAgSb suggest that the magnetic ground state is highly anisotropic with the c-axis as the easy direction and a readily saturated c-axis component of the Ce moment of 0.5l . However, the magnetisation of our polycrystalline samples remains unsaturated at "elds as high as 10 T, at which the Ce moment is approximately 1.5l . Our preliminary attempts to solve the low-temperature magnetic structure of CeAgSb using conventional neutron powder di!raction methods have been unsuccessful. We observe only very small additional magnetic contributions to nuclear Bragg peaks, indicative of a ferromagnetic component associated with Ce moments of &0.75$0.25l aligned along the c-axis. Within experimental error no additional magnetic Bragg peaks arising from additional antiferromagnetic components are observed. Recognising the important role played by zero and longitudinal "eld muon spin relaxation (lSR) in extending our understanding of heavy fermion phenomenology [6], we have turned to the lSR technique in the hope of resolving the rather contradictory bulk magnetic properties of CeAgSb . Our measurements were carried out on a polycrystalline sample using the MuSR spectrometer at the
40
J.A. Dann et al. / Physica B 289}290 (2000) 38}42
ISIS pulsed muon source. After appropriate corrections for variations in e$ciency of the backward and forward detectors and for self-attenuation of the decay positrons by the sample and sample holder, the zero-"eld relaxation spectra in the paramagnetic state of CeAgSb (¹'10 K) were consis tent with a relaxation function of the form A G (t)"a (#(1!pt)e\NR ) ) e\HR#a , (1) where a (&0.03) represents a small time- and temperature-independent background contribution arising from those muons stopping in the high-purity silver sample holder. A "t of this composite function to the data gives a temperatureindependent value of p"0.064 ls\ for the relaxation rate associated with a static Gaussian dipole "eld distribution arising principally from the Sb nuclear moments. This value for p, when compared with calculations of the second moment of the internal nuclear dipole "eld distribution across the CeAgSb unit cell, is consistent only with the muon localising at the (, , 0.6) and symmetry related sites, as shown in Fig. 1: all other potential sites have a calculated p which is at least three times larger. The Kubo}Toyabe component of Eq. (1) could be fully decoupled in a longitudinal "eld of only a few mT, con"rming that this component is indeed of nuclear origin. The remaining exponential component, associated with the #uctuating atomic "eld distributions arising from the Ce moments, was una!ected by a longitudinal "eld. The temperature dependence of this component was therefore, for simplicity, measured in a longitudinal "eld of 30 mT (inset: Fig. 2). The resulting values of j, shown in Fig. 2, are extremely small and relatively temperature independent. There is no evidence of either a critical divergence of j as ¹ is ap proached, nor of an extended critical region. This behaviour is expected for spin #uctuations dominated by the RKKY interaction between the Ce f-spins. The frequency of such #uctuations is expected to be temperature independent and also proportional to g/N [6]. Small j thus implies $ a large g/N , which is fully consistent with the $ proposed assignment of CeAgSb to the high-g side of the Doniach phase diagram.
Fig. 2. Temperature dependence of j. The arrows indicate the magnetic transition temperature, ¹ , the temperature of the
onset of coherence between Ce sites, ¹ , and the Kondo temper! ature, ¹H, from [5]. Errors are the size of the data points unless shown. The solid line is a guide to the eye. Inset: lSR spectra at 11 K in zero "eld and a longitudinal "eld of 30 mT. The solid lines represent "ts of Eq. (1) to the data.
j is not entirely temperature independent. A small stepwise increase to j"0.02 ls\ is observed at approximately 60 K, together with a further marked increase to 0.05 ls\ below 17 K as the ordering temperature is approached. It may be of some signi"cance that the former corresponds roughly with the Kondo temperature, ¹*, [5], whilst the latter coincides with the developing coherence between the Ce sites [5], (although the occurrence of short-range order as a precursor to the onset of magnetic order cannot be excluded). Similar stepwise changes in an otherwise temperature-independent j close to ¹* and ¹ , albeit on , an order of magnitude larger scale, have also been observed for the controversial heavy fermion CePt Sn , [7]. Below ¹ "10 K the onset of magnetic order in
CeAgSb is signalled by the appearance of coherent oscillations in the zero-"eld spectra. As shown in Fig. 3 the muon spectra below 10 K are well described by the expression for a multi-domain polycrystalline ordered magnet. i.e. (2) A G (t)"a cos(ut)e\NR#a e\HR#a , where the "rst term represents the Gaussian-damped coherent oscillations arising from the two
J.A. Dann et al. / Physica B 289}290 (2000) 38}42
Fig. 3. Zero "eld lSR spectra of CeAgSb at (a) 1.5 K and (b) 8 K. The solid lines represent "ts of Eq. (2) to the data. Inset: Temperature dependence of the reduced square root intensity +I (¹)/I (¹"0), of the magnetic component of the (1, 0, 1)
neutron Bragg re#ection (open symbols), together with that of the reduced internal "eld at the muon site, B (¹)/B (¹"0) (full I I symbols). The solid line represents a Brillouin function.
transverse components of the internal "eld at the muon site and the lightly damped second term arises from dynamic #uctuations of the longitudinal "eld component. Whilst a is close to the expected value of a /3, a falls increasingly short of the expected 2a /3 as the frequency, u"B /c, I of the "eld-induced oscillations increase towards low temperatures. This is simply a consequence of the instrumental convolution of the oscillatory signal with the "nite (&70 ns) muon pulse width at ISIS. The temperature dependence of u, and hence of B , closely follows the same Brillouin I function as the ferromagnetic component of the magnetisation determined from the square root intensity of the (1, 0, 1) neutron Bragg re#ection as shown in Fig. 3. Extrapolation of u to zero temperature indicates an internal "eld of 53 mT at the muon site. From these results we believe that a simple ferrimagnetic model [4] for the magnetic structure of
41
CeAgSb can be rejected. Ferrimagnetism requires that at least two magnetically inequivalent Ce sites with di!ering Ce moments must evolve below ¹ from the two crystallographically equivalent Ce
sites within the unit cell. This in turn would lead to the appearance of at least two crystallographically equivalent but magnetically distinct muon sites, each with a di!erent B . However, the spectra colI lected in the ordered and paramagnetic states of CeAgSb indicate a single crystallographic and magnetic muon site. Discrimination between a simple ferromagnetic structure, suggested by our preliminary neutron di!raction data, and a complex antiferromagnetic structure with a resultant ferromagnetic component suggested by bulk magnetic measurements [3], is extremely di$cult as a unique internal "eld at all crystallographically equivalent muon sites can be obtained within either model. Moreover, it should be noted that the magnetic "eld at the proposed muon site at (, , 0.6) in the CeAgSb unit cell is largely dominated by the atomic moment of the nearest-neighbour Ce atom at a distance of 1.75 As . Even extreme variations in the relative orientations of the Ce moments in more distant atomic shells (the closest of which is at 4.7 As ) are unable to reduce this magnetic "eld signi"cantly below a magnitude of 150}400 mT, estimated assuming a collinear arrangement of Ce moments consistent with the various bulk magnetic measurements. We can therefore conclude only that CeAgSb is certainly magnetically ordered be low ¹ "10 K, with Kondo-reduced Ce moments
aligned such as to produce a net ferromagnetic moment parallel to the c-axis. The anomalously low internal "eld of 53 mT at the muon site may thus be a consequence of electronic screening of the muon. The authors gratefully acknowledge "nancial support from the UK Engineering and Physical Sciences Research Council, and experimental support from the sta! at ISIS.
References [1] S. Doniach, Physica B 91 (1977) 231.
42
J.A. Dann et al. / Physica B 289}290 (2000) 38}42
[2] O. Sologub et al., J. Sol. Stat. Chem 115 (1995) 441. [3] M. Houshiar et al., J. Magn. Magn. Mater. 140}144 (1995) 1231. [4] Y. Muro et al., J. Alloys Comp. 257 (1997) 23.
[5] M. Thornton et al., J. Phys.: Condens. Matter 10 (1998) 9485. [6] A. Amato, Rev. Mod. Phys. 69 (1997) 1119. [7] G.M. Luke et al., Physica B 206&207 (1995) 222.
A lSR study of the magnetic properties of CeAgSb J.A. Dann, A.D. Hillier, J.G.M. Armitage, R. Cywinski* School of Physics and Astronomy, University of St Andrews, Fife KY16 9SS, Scotland, UK
Abstract Zero and longitudinal "eld muon spin relaxation measurements have been performed on the Kondo lattice system CeAgSb in the magnetically ordered (¹(10 K) and paramagnetic (¹'10 K) states. The resulting spectra are consistent with a single muon site which senses a unique coherent internal "eld of 53 mT at low temperatures and dynamic atomic "elds, arising from extremely rapid RKKY-dominated paramagnetic #uctuations of the Ce> spins, at higher temperatures. Small stepwise increases in the temperature dependence of the muon relaxation rate at 60 and 17 K appear to be coincident with the Kondo temperature and the onset of coherence, respectively. 2000 Elsevier Science B.V. All rights reserved. Keywords: Kondo lattice; Heavy fermion; Magnetic order
Cerium-based intermetallic compounds exhibit a remarkably rich and diverse range of magnetic phenomena. Many show characteristic heavy fermion behaviour, and for these compounds the Kondo lattice is often regarded as an appropriate descriptive model. In Kondo lattice systems the indirect, inter-site, RKKY interaction responsible for magnetic order competes with the demagnetising e!ects of the Kondo interaction arising from on-site interactions. The crucial parameter in this competition is the coupling constant for exchange between the localised atomic spin and the conduction electron spins, g"N J, where J is the mag$ netic exchange constant and N the density of $ states at the Fermi energy. Whilst ¹ Jg, the 0))7 Kondo temperature ¹HJexp(!1/g). Doniach [1] has derived a generalised phase diagram for a one* Corresponding author. Tel.: #44-0-1334-463108; fax: #44-0-1334-463104. E-mail address: [email protected] (R. Cywinski).
dimensional Kondo lattice, which predicts a critical value for g below which long-range local moment magnetic order obtains and above which a non-magnetic heavy ground state, due to Kondo reduction of the e!ective moment, k , is found.
Systematic studies of Ce-based intermetallics suggest that the Doniach phase diagram may provide a broadly qualitative description of the behaviour of three-dimensional Kondo lattice systems. For example, the application of pressure, either chemical or external, e!ectively increases the parameter g and leads to a collapse of magnetic order and the local Ce moment. For most Ce-based heavy fermion compounds the generally reduced Ce moments are found to couple antiferromagnetically (e.g. CeAl and CeB ), although the oscillatory RKKY interaction can also lead to a ferromagnetic ground state (e.g. CeRu Ge ). Whichever the case, any new addition to the Kondo lattice family is always greeted with some interest and in this respect the recently reported CeAgSb is no exception.
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 0 ) 0 0 2 3 8 - 6
J.A. Dann et al. / Physica B 289}290 (2000) 38}42
Fig. 1. Crystallographic structure of CeAgSb , shown as a single unit cell in the c-direction and as 1.5 unit cells in the aand b-directions. The proposed muon sites at (, , 0.6) and symmetry related positions are also shown.
CeAgSb crystallises with the tetragonal ZrCuSi structure (space group P4/nmm) shown in Fig. 1, with a"b"4.364 As and c"10.722 As , and Ce at (, , 0.238), Ag at (, , ) and Sb at (, , 0) and (, , 0.674) [2]. Magnetic and transport measure ments provide strong evidence that CeAgSb is indeed a condensed Kondo lattice system [3, 4]. The magnetic component of the electrical resistivity shows a characteristic Kondo-like !ln(¹) dependence above 50 K with a maximum in the resistivity at 24 K signifying the onset of coherence. A further, sharp, drop in the resistivity at ¹ "10 K co incides with the onset of magnetic order. Estimates of the Kondo temperature from thermopower and neutron scattering measurements suggest a relatively high value of ¹H (&60}80 K) placing CeAgSb "rmly on the high-g side of the Doniach phase diagram, a view recently con"rmed by measurement of the pressure dependence of the magnetic transition temperature [5]. The precise nature of the magnetic ground state below ¹ of CeAgSb remains controversial and
unresolved. Magnetisation measurements [3, 4] in-
39
dicate that in the paramagnetic state the Ce moment has a value of 2.58l , close to the free ion moment of Ce>. However, below ¹ a saturation
moment of only 1.4l per Ce atom has led to suggestions that the ordered state of CeAgSb is either a complex antiferromagnet with a small resultant ferromagnetic moment [3] or a ferrimagnet [4], or perhaps that the reduced Ce moment may be due to crystal "eld e!ects [5]. AC susceptibility shows an extremely sharp peak at 10 K, attributed to the onset of antiferromagnetism at ¹ "¹ , ,
accompanied by a broad shoulder peaking close to 17 K and extending over the temperature regime in which resistivity indicates the onset of coherence [5]. The linear thermal expansion provides no evidence of the magnetic transition at 10 K but is dominated by a pronounced maximum at 17 K, coinciding with the shoulder in the AC susceptibility and extending over a correspondingly broad temperature range. This feature has been associated with the stabilisation of coherence between the Ce sites [5]. Our own DC magnetisation measurements on single crystal CeAgSb suggest that the magnetic ground state is highly anisotropic with the c-axis as the easy direction and a readily saturated c-axis component of the Ce moment of 0.5l . However, the magnetisation of our polycrystalline samples remains unsaturated at "elds as high as 10 T, at which the Ce moment is approximately 1.5l . Our preliminary attempts to solve the low-temperature magnetic structure of CeAgSb using conventional neutron powder di!raction methods have been unsuccessful. We observe only very small additional magnetic contributions to nuclear Bragg peaks, indicative of a ferromagnetic component associated with Ce moments of &0.75$0.25l aligned along the c-axis. Within experimental error no additional magnetic Bragg peaks arising from additional antiferromagnetic components are observed. Recognising the important role played by zero and longitudinal "eld muon spin relaxation (lSR) in extending our understanding of heavy fermion phenomenology [6], we have turned to the lSR technique in the hope of resolving the rather contradictory bulk magnetic properties of CeAgSb . Our measurements were carried out on a polycrystalline sample using the MuSR spectrometer at the
40
J.A. Dann et al. / Physica B 289}290 (2000) 38}42
ISIS pulsed muon source. After appropriate corrections for variations in e$ciency of the backward and forward detectors and for self-attenuation of the decay positrons by the sample and sample holder, the zero-"eld relaxation spectra in the paramagnetic state of CeAgSb (¹'10 K) were consis tent with a relaxation function of the form A G (t)"a (#(1!pt)e\NR ) ) e\HR#a , (1) where a (&0.03) represents a small time- and temperature-independent background contribution arising from those muons stopping in the high-purity silver sample holder. A "t of this composite function to the data gives a temperatureindependent value of p"0.064 ls\ for the relaxation rate associated with a static Gaussian dipole "eld distribution arising principally from the Sb nuclear moments. This value for p, when compared with calculations of the second moment of the internal nuclear dipole "eld distribution across the CeAgSb unit cell, is consistent only with the muon localising at the (, , 0.6) and symmetry related sites, as shown in Fig. 1: all other potential sites have a calculated p which is at least three times larger. The Kubo}Toyabe component of Eq. (1) could be fully decoupled in a longitudinal "eld of only a few mT, con"rming that this component is indeed of nuclear origin. The remaining exponential component, associated with the #uctuating atomic "eld distributions arising from the Ce moments, was una!ected by a longitudinal "eld. The temperature dependence of this component was therefore, for simplicity, measured in a longitudinal "eld of 30 mT (inset: Fig. 2). The resulting values of j, shown in Fig. 2, are extremely small and relatively temperature independent. There is no evidence of either a critical divergence of j as ¹ is ap proached, nor of an extended critical region. This behaviour is expected for spin #uctuations dominated by the RKKY interaction between the Ce f-spins. The frequency of such #uctuations is expected to be temperature independent and also proportional to g/N [6]. Small j thus implies $ a large g/N , which is fully consistent with the $ proposed assignment of CeAgSb to the high-g side of the Doniach phase diagram.
Fig. 2. Temperature dependence of j. The arrows indicate the magnetic transition temperature, ¹ , the temperature of the
onset of coherence between Ce sites, ¹ , and the Kondo temper! ature, ¹H, from [5]. Errors are the size of the data points unless shown. The solid line is a guide to the eye. Inset: lSR spectra at 11 K in zero "eld and a longitudinal "eld of 30 mT. The solid lines represent "ts of Eq. (1) to the data.
j is not entirely temperature independent. A small stepwise increase to j"0.02 ls\ is observed at approximately 60 K, together with a further marked increase to 0.05 ls\ below 17 K as the ordering temperature is approached. It may be of some signi"cance that the former corresponds roughly with the Kondo temperature, ¹*, [5], whilst the latter coincides with the developing coherence between the Ce sites [5], (although the occurrence of short-range order as a precursor to the onset of magnetic order cannot be excluded). Similar stepwise changes in an otherwise temperature-independent j close to ¹* and ¹ , albeit on , an order of magnitude larger scale, have also been observed for the controversial heavy fermion CePt Sn , [7]. Below ¹ "10 K the onset of magnetic order in
CeAgSb is signalled by the appearance of coherent oscillations in the zero-"eld spectra. As shown in Fig. 3 the muon spectra below 10 K are well described by the expression for a multi-domain polycrystalline ordered magnet. i.e. (2) A G (t)"a cos(ut)e\NR#a e\HR#a , where the "rst term represents the Gaussian-damped coherent oscillations arising from the two
J.A. Dann et al. / Physica B 289}290 (2000) 38}42
Fig. 3. Zero "eld lSR spectra of CeAgSb at (a) 1.5 K and (b) 8 K. The solid lines represent "ts of Eq. (2) to the data. Inset: Temperature dependence of the reduced square root intensity +I (¹)/I (¹"0), of the magnetic component of the (1, 0, 1)
neutron Bragg re#ection (open symbols), together with that of the reduced internal "eld at the muon site, B (¹)/B (¹"0) (full I I symbols). The solid line represents a Brillouin function.
transverse components of the internal "eld at the muon site and the lightly damped second term arises from dynamic #uctuations of the longitudinal "eld component. Whilst a is close to the expected value of a /3, a falls increasingly short of the expected 2a /3 as the frequency, u"B /c, I of the "eld-induced oscillations increase towards low temperatures. This is simply a consequence of the instrumental convolution of the oscillatory signal with the "nite (&70 ns) muon pulse width at ISIS. The temperature dependence of u, and hence of B , closely follows the same Brillouin I function as the ferromagnetic component of the magnetisation determined from the square root intensity of the (1, 0, 1) neutron Bragg re#ection as shown in Fig. 3. Extrapolation of u to zero temperature indicates an internal "eld of 53 mT at the muon site. From these results we believe that a simple ferrimagnetic model [4] for the magnetic structure of
41
CeAgSb can be rejected. Ferrimagnetism requires that at least two magnetically inequivalent Ce sites with di!ering Ce moments must evolve below ¹ from the two crystallographically equivalent Ce
sites within the unit cell. This in turn would lead to the appearance of at least two crystallographically equivalent but magnetically distinct muon sites, each with a di!erent B . However, the spectra colI lected in the ordered and paramagnetic states of CeAgSb indicate a single crystallographic and magnetic muon site. Discrimination between a simple ferromagnetic structure, suggested by our preliminary neutron di!raction data, and a complex antiferromagnetic structure with a resultant ferromagnetic component suggested by bulk magnetic measurements [3], is extremely di$cult as a unique internal "eld at all crystallographically equivalent muon sites can be obtained within either model. Moreover, it should be noted that the magnetic "eld at the proposed muon site at (, , 0.6) in the CeAgSb unit cell is largely dominated by the atomic moment of the nearest-neighbour Ce atom at a distance of 1.75 As . Even extreme variations in the relative orientations of the Ce moments in more distant atomic shells (the closest of which is at 4.7 As ) are unable to reduce this magnetic "eld signi"cantly below a magnitude of 150}400 mT, estimated assuming a collinear arrangement of Ce moments consistent with the various bulk magnetic measurements. We can therefore conclude only that CeAgSb is certainly magnetically ordered be low ¹ "10 K, with Kondo-reduced Ce moments
aligned such as to produce a net ferromagnetic moment parallel to the c-axis. The anomalously low internal "eld of 53 mT at the muon site may thus be a consequence of electronic screening of the muon. The authors gratefully acknowledge "nancial support from the UK Engineering and Physical Sciences Research Council, and experimental support from the sta! at ISIS.
References [1] S. Doniach, Physica B 91 (1977) 231.
42
J.A. Dann et al. / Physica B 289}290 (2000) 38}42
[2] O. Sologub et al., J. Sol. Stat. Chem 115 (1995) 441. [3] M. Houshiar et al., J. Magn. Magn. Mater. 140}144 (1995) 1231. [4] Y. Muro et al., J. Alloys Comp. 257 (1997) 23.
[5] M. Thornton et al., J. Phys.: Condens. Matter 10 (1998) 9485. [6] A. Amato, Rev. Mod. Phys. 69 (1997) 1119. [7] G.M. Luke et al., Physica B 206&207 (1995) 222.