- Email: [email protected]
Magnetic behavior of YFexAl12−x
Physica B 326 (2003) 460–464
Magnetic behavior of YFexAl12x G.M. Kalviusa,*, F.E. Wagnera, D.R. Noakesb, E. Schreiera, R. W.applingc, U. Zimmermannd, W. Sch.afere, W. Kockelmanne, I. Halevyf, J. Galf a
Physics Department, TU Munich, James-Franck-strasse, 85747 Garching, Germany b Physics Department, Virginia State U., Petersburg, VA 23806, USA c Physics Department, Uppsala U., 75121 Uppsala, Sweden d Paul Scherrer Institute, 5232 Villigen PSI, Switzerland e Mineralogical-Pertrological Institute, U. Bonn, 53115 Bonn, Germany f Nucl. Engineering Department, Ben Gurion U., 84105 Beer-Sheva, Israel
Abstract Previous mSR studies on the RFe6 Al6 (R ¼ Tb; Ho, Er) ferrimagnets ðTN E340 KÞ showed effects of frustration due to competing exchange between the R and Fe sublattices. This puts the compounds on the borderline between spin glass . and long-range order. In continuation of this work, mSR and 57 Fe Mossbauer spectroscopy were carried out on YFe6 Al6 and YFe7 Al5 : In these compounds, no moments exist on the normally strongly magnetic R sublattice. Neutron . diffraction was unable to detect magnetic Bragg peaks, but the mSR and Mossbauer spectra clearly reveal a ferrimagnetic transition near 340 K in both materials. Not all of the Fe ions order at this temperature. The Fe ions on the Fe sublattice (8f) order only below B70 K: Since competing exchange is absent, the Fe sublattices possess inherent frustration reflected in distributions of moment size and orientation, short correlation lengths and strong spin fluctuations. r 2002 Elsevier Science B.V. All rights reserved. . Keywords: Magnetic frustration; Ferrimagnetism; Mossbauer spectroscopy
The RFex Al12x intermetallics crystallize in the ThMn12 structure (I4/mmm), which contains four different lattice sites. In an ideal material all R ions are located on site 2a. Site 8i is solely occupied by Al ions and site 8f exclusively by Fe ions, while site 8j is randomly occupied by Fe and Al ions according to the value of x: We previously studied [1] the x ¼ 6 ferrimagnetic ðTN B340 KÞ compounds with R ¼ Tb; Ho, Er by mSR: The RFex Al12x materials are prone to site exchange, *Corresponding author. Tel.: +4989-28912501; fax: +49893206780. E-mail address: [email protected] (G.M. Kalvius).
but the samples used in the mSR work were the same as those used in neutron diffraction studies. It was shown there that site exchange is absent within the accuracy of the data [2]. The main findings of the mSR study were that frustration brings the intermetallics near the border between spin-glass-like behavior and long-range order (LRO), the latter barely dominating, and that the frustration causes longitudinal spin fluctuations to persist down to 50 K: These fluctuations are the origin of the slow non-Brillouin-like rise of magnetic Bragg peak intensity on cooling seen in neutron diffraction [2]. Frustration in the RFex Al12x intermetallics is thought to originate
0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 6 6 6 - 6
G.M. Kalvius et al. / Physica B 326 (2003) 460–464
-1
Relaxation rate (µs )
signal) of 23 relative intensity, plus a monotonically depolarizing part of 13 strength (longitudinal signal). The transverse relaxation rate ltrans ; which is the damping rate of the oscillatory pattern, is mainly determined by the width of the static distribution of Bm : The longitudinal rate llong is given by the fluctuation rate of Bm ; which reflects the dynamics of the magnetic spins. An extremely large magnitude of ltrans in YFe6 Al6 was found, causing the transverse signal to be damped so strongly that it was completely lost in the initial dead time of the spectrometer, but llong was (as usual) small enough to render the longitudinal signal detectable. A loss of B23 of signal strength then indicates here the onset of LRO. Fig. 1 shows the temperature dependences of signal intensity and relaxation rate, indicating magnetic transitions at TN E340 K and T2 E 70 K: The signal between TN and T2 is too strong
4
YFe6Al6
3 2 1 0 0.20
Signal Amplitude
from the competition between the antiferromagnetic (AFM) coupling between the R and Fe sublattices and the ferromagnetic (FM) exchange within those sublattices [3]. The crystallographic disorder in the 8j sublattice should also contribute. The situation is fundamentally different in YFex Al12x : The 2a site now contains a nonmagnetic ion (Y). Neutron scattering in YFe6 Al6 could not detect magnetic Bragg peaks, suggesting that LRO is absent (and line shape analysis gave no indication of short-range order). In YFe7 Al5 ; LRO was not found by neutrons as well [4], but the analysis of a diffuse satellite peak (at 12 K) to the 200/101 (lattice) reflection pointed towards non-statistical distribution of Fe and Al ions in the mixed 8j site, with ferromagnetic short-range order ( Fe clusters. In (SRO) for small (less than 20 A) . contrast, published Mossbauer data showed Zeeman splitting at 4:2 K for both Y-containing compounds (as well as for the corresponding Rcontaining alloys), meaning that LRO is present [5]. Temperature dependences, however, were not reported. mSR spectroscopy on the same powder samples of YFe6 Al6 (4–400 K) and YFe7 Al5 (4–295 K) used in neutron diffraction was carried out in order to gain more insight into this . confusing situation. 57 Fe Mossbauer spectra (4–350 K) were also taken. The more extensive data exist for YFe6 Al6 : They will be presented first. The existence of LRO at room temperature and below became immediately clear from low field transverse field (TF) measurement. These showed that the applied field did not penetrate into the sample, which implies ferro- or ferrimagnetic order. The spontaneous magnetization of those magnets coupled to hysteresis prevents an external field from entering the bulk of the sample as long as the field does not saturate the domain structure (a nice demonstration of this behavior can be found in Ref. [6]). In contrast, the staggered magnetization of an antiferromagnet allows even weak fields to enter the bulk of the material. In short, the TF results support a ferrimagnetic structure as present in the corresponding materials containing heavy rare earth ions. The zero field (ZF) mSR signal of a magnetically LRO polycrystalline compound consists of two terms, an oscillating part (transverse
461
0.15 0.10 0.05
T2
0.00 0
TN
50 100 150 200 250 300 350 400
Temperature (K) Fig. 1. Temperature dependence of the mSR signal amplitude (bottom) and relaxation rate (top) in YFe6 Al6 : Circles indicate data from a cryostat in the GPS facility, triangles data from an oven in the GPD facility. Since the spectrometer performances are somewhat different, the GPD signal at 280 K was adjusted to the GPS amplitude at 275 K:
G.M. Kalvius et al. / Physica B 326 (2003) 460–464
to be only the 13 longitudinal signal, with the transverse signal lost around 340 K: Thus the compound only partially enters LRO below TN until finally, at T2 ; most of the signal is lost. The remaining part is then compatible with the longitudinal signal when all Fe ions participate in LRO. The mSR spectra in the intermediate region show exponential relaxation, yet l is about an order of magnitude larger than expected for a free paramagnetic state. The non-ordered portion is not truly paramagnetic, but under the influence of strong dynamic short-range spin correlations. Even for T > TN the relaxation rate is high, meaning that the short-range correlations persist and a truly free paramagnetic state is not yet reached. Also seen just above T2 is the typical rise in l on approaching a magnetic transition from the high temperature side due to critical slowing down of spin fluctuations. Below T2 the relaxation rate ðllong Þ shows again that the static limit in LRO is approached slowly as was the case in the RFe6 Al6 materials. 57 . Fe Mossbauer spectra provide direct confirmation of the scenario of partial ordering at TN and full ordering at T2 : In the paramagnetic state ðT > TN ¼ 345 KÞ one observes (as expected) a quadrupolar doublet pattern (see top panel of Fig. 2). Its apparent asymmetry results from paramagnetic spin fluctuations which are too slow . within the 57 Fe Mossbauer time window to be fully motional narrowed (which would be the case in a normal paramagnet). The cause is the presence of dynamic short-range correlations as already . discussed. 57 Fe Mossbauer spectra below TN are also presented in Fig. 2. One first observes the sum of a quadrupolar and a six-line Zeeman pattern. The remaining quadrupole subspectrum undergoes Zeeman splitting at T2 E70 K: A surprising result is the relative intensity (1:2) of the subspectra, the more intense one ordering at T2 : It means that LRO in the pure Fe sublattice (8f) is depressed as an effect of frustration. One might have expected that the magnetic dilution in the Fe–Al sublattice (8j) suppresses LRO. The temperature dependence of the hyperfine field for the two Fe sublattices is shown in Fig. 3. The T-0 hyperfine field is not much different for the two sites, but only about half ðB16 TÞ of that of Fe metal, corresponding to
100
YFe7Al5 95
295K 100
Rlative Transmissione
462
95 90
100K
100
95
60K 90 100 99 98 97
4K -6
-4
-2
0
2
4
6
Velocity (mm/s) . Fig. 2. 57 Fe Mossbauer spectra of YFe6 Al6 at selected temperatures. The top spectrum is taken for T > TN :
Fe moments on the order of 1mB : The resonance lines of the 8f magnetic subspectrum are considerably broadened. This broadening can have two sources: a statistic distribution of hyperfine fields due to local spin disorder, or a relaxation rate still . within the 57 Fe Mossbauer time window, meaning that the ordered magnet has not reached the full quasi-static limit (quasi-static, since very fast spin wave excitations will always be present, but are . sensed by the Mossbauer resonance only as the temperature dependence of the effective hyperfine field). It is close to impossible to separate the effects of the two line broadening mechanism. The least-squares fits shown in Fig. 2 and also in Fig. 4 assume a dominance of the static broadening
G.M. Kalvius et al. / Physica B 326 (2003) 460–464
20
100
18
YFe6 Al6
95
16
90
YFe6Al6
14 85
347K
12 10 8 6 4 2 0 0
50
100
150
200
250
300
350
Temperature (K) . Fig. 3. Temperature dependence of the Mossbauer hyperfine field for the two Fe sites in YFe6 Al6 :
mechanism, in accordance with the large transverse relaxation rate in the mSR spectra. The results then imply a distribution of hyperfine field with a relative width of DBhf =Bhf E0:25 in YFe6 Al6 ; the value for YFe7 Al5 being somewhat smaller. A distribution of Bhf arises only from a corresponding variation of local moment, in contrast to mSR; where spin-directional disorder also contributes to DBm : The results for YFe7 Al5 are much the same, except for the somewhat reduced line broadenings which are particularly apparent in the spectra around 60 K: Its origin is likely a difference in spin dynamics but direct evidence cannot be given. From 295 K (our highest mSR temperature in this sample) down, we again see a somewhat reduced mSR signal amplitude, followed by the sudden change of signal intensity around 70 K: The 57 Fe . Mossbauer spectra (Fig. 4) confirm the same scenario of partial ordering in YFe7 Al5 : TN is again around 330 K and the paramagnetic spectra are essentially indistinguishable from the spectrum shown in the top panel of Fig. 2. The saturation hyperfine fields for both Fe sites are also rather similar and not much different from those of YFe6 Al6 : The main difference is the relative intensity of the fraction ordering at T2 : YFe7 Al5 contains additional Fe ions which are all located in
Relative Transmission
Hyperfine Field (T)
463
100 98 96 94 92 90 88
100K
100 98 96
60K
94 100 98 96
4K
94
-6
-4
-2
0
2
4
6
Velocity (mm/s) . Fig. 4. Fe Mossbauer spectra of YFe7 Al5 at selected temperatures below TN : 57
the 8j sublattice. The weaker non-ordered fraction in the mixed magnetic state thus supports in full the assignment of site response given above. The failure to detect magnetic LRO by neutrons probably rests in finite correlation lengths in combination with the relatively small Fe moments, but is not understood in detail. From the mSR relaxation rate in gadolinium metal just above the Curie temperature, Karlsson [7] deduced a correla( for dynamic paramagnetic tion length of B10 A clusters. This clearly did not produce the mSR response for a LRO magnet. In neutron scattering work on YFe7 Al5 ; the analysis of a diffuse satellite peak led to a correlation length of the order of ( [4]. Magnetic Bragg peaks were not seen. One 20 A may roughly estimate for the present situation a
464
G.M. Kalvius et al. / Physica B 326 (2003) 460–464
lower bound for the ferrimagnetic correlation ( and an upper bound around length around 10 A ( . 100 A: The Mossbauer data are of no help in this context because of the unresolved mixture of static and dynamic line broadening. The strong local spin disorder, persistent spin fluctuations and partial LRO ordering found in the present study indicate the presence of frustration even in the Y-containing alloys without magnetic rare earth ions. This must be an inherent property of the Fe sublattices since competing exchange with the R sublattice is absent. If frustration is predominantly due to nearest-neighbor exchange it seems possible that it is reduced by magnetic dilution in the 8j sublattice. Still, the observation of weaker frustration in the 8j
sublattice is astonishing and not really understood. It cannot be explained in the framework of naive frustration models, which would have predicted the opposite effect [3].
References [1] G.M. Kalvius, et al., Physica B 289–290 (2000) 225. [2] W. Sch.afer, et al., J. Magn. Magn. Mater. 177–181 (1998) 808. [3] I. Felner, et al., J. Phys. Chem. Solids 42 (1981) 369. [4] W. Sch.afer, et al., Alloys Compounds 225 (1995) 440. [5] I. Felner, I. Nowik, J. Magn. Magn. Mater. 58 (1986) 169. [6] A.B. Denison, et al., Helv. Phys. Acta 52 (1979) 460. [7] E.B. Karlsson, Hyperfine Interactions 64 (1990) 351.
Magnetic behavior of YFexAl12x G.M. Kalviusa,*, F.E. Wagnera, D.R. Noakesb, E. Schreiera, R. W.applingc, U. Zimmermannd, W. Sch.afere, W. Kockelmanne, I. Halevyf, J. Galf a
Physics Department, TU Munich, James-Franck-strasse, 85747 Garching, Germany b Physics Department, Virginia State U., Petersburg, VA 23806, USA c Physics Department, Uppsala U., 75121 Uppsala, Sweden d Paul Scherrer Institute, 5232 Villigen PSI, Switzerland e Mineralogical-Pertrological Institute, U. Bonn, 53115 Bonn, Germany f Nucl. Engineering Department, Ben Gurion U., 84105 Beer-Sheva, Israel
Abstract Previous mSR studies on the RFe6 Al6 (R ¼ Tb; Ho, Er) ferrimagnets ðTN E340 KÞ showed effects of frustration due to competing exchange between the R and Fe sublattices. This puts the compounds on the borderline between spin glass . and long-range order. In continuation of this work, mSR and 57 Fe Mossbauer spectroscopy were carried out on YFe6 Al6 and YFe7 Al5 : In these compounds, no moments exist on the normally strongly magnetic R sublattice. Neutron . diffraction was unable to detect magnetic Bragg peaks, but the mSR and Mossbauer spectra clearly reveal a ferrimagnetic transition near 340 K in both materials. Not all of the Fe ions order at this temperature. The Fe ions on the Fe sublattice (8f) order only below B70 K: Since competing exchange is absent, the Fe sublattices possess inherent frustration reflected in distributions of moment size and orientation, short correlation lengths and strong spin fluctuations. r 2002 Elsevier Science B.V. All rights reserved. . Keywords: Magnetic frustration; Ferrimagnetism; Mossbauer spectroscopy
The RFex Al12x intermetallics crystallize in the ThMn12 structure (I4/mmm), which contains four different lattice sites. In an ideal material all R ions are located on site 2a. Site 8i is solely occupied by Al ions and site 8f exclusively by Fe ions, while site 8j is randomly occupied by Fe and Al ions according to the value of x: We previously studied [1] the x ¼ 6 ferrimagnetic ðTN B340 KÞ compounds with R ¼ Tb; Ho, Er by mSR: The RFex Al12x materials are prone to site exchange, *Corresponding author. Tel.: +4989-28912501; fax: +49893206780. E-mail address: [email protected] (G.M. Kalvius).
but the samples used in the mSR work were the same as those used in neutron diffraction studies. It was shown there that site exchange is absent within the accuracy of the data [2]. The main findings of the mSR study were that frustration brings the intermetallics near the border between spin-glass-like behavior and long-range order (LRO), the latter barely dominating, and that the frustration causes longitudinal spin fluctuations to persist down to 50 K: These fluctuations are the origin of the slow non-Brillouin-like rise of magnetic Bragg peak intensity on cooling seen in neutron diffraction [2]. Frustration in the RFex Al12x intermetallics is thought to originate
0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 6 6 6 - 6
G.M. Kalvius et al. / Physica B 326 (2003) 460–464
-1
Relaxation rate (µs )
signal) of 23 relative intensity, plus a monotonically depolarizing part of 13 strength (longitudinal signal). The transverse relaxation rate ltrans ; which is the damping rate of the oscillatory pattern, is mainly determined by the width of the static distribution of Bm : The longitudinal rate llong is given by the fluctuation rate of Bm ; which reflects the dynamics of the magnetic spins. An extremely large magnitude of ltrans in YFe6 Al6 was found, causing the transverse signal to be damped so strongly that it was completely lost in the initial dead time of the spectrometer, but llong was (as usual) small enough to render the longitudinal signal detectable. A loss of B23 of signal strength then indicates here the onset of LRO. Fig. 1 shows the temperature dependences of signal intensity and relaxation rate, indicating magnetic transitions at TN E340 K and T2 E 70 K: The signal between TN and T2 is too strong
4
YFe6Al6
3 2 1 0 0.20
Signal Amplitude
from the competition between the antiferromagnetic (AFM) coupling between the R and Fe sublattices and the ferromagnetic (FM) exchange within those sublattices [3]. The crystallographic disorder in the 8j sublattice should also contribute. The situation is fundamentally different in YFex Al12x : The 2a site now contains a nonmagnetic ion (Y). Neutron scattering in YFe6 Al6 could not detect magnetic Bragg peaks, suggesting that LRO is absent (and line shape analysis gave no indication of short-range order). In YFe7 Al5 ; LRO was not found by neutrons as well [4], but the analysis of a diffuse satellite peak (at 12 K) to the 200/101 (lattice) reflection pointed towards non-statistical distribution of Fe and Al ions in the mixed 8j site, with ferromagnetic short-range order ( Fe clusters. In (SRO) for small (less than 20 A) . contrast, published Mossbauer data showed Zeeman splitting at 4:2 K for both Y-containing compounds (as well as for the corresponding Rcontaining alloys), meaning that LRO is present [5]. Temperature dependences, however, were not reported. mSR spectroscopy on the same powder samples of YFe6 Al6 (4–400 K) and YFe7 Al5 (4–295 K) used in neutron diffraction was carried out in order to gain more insight into this . confusing situation. 57 Fe Mossbauer spectra (4–350 K) were also taken. The more extensive data exist for YFe6 Al6 : They will be presented first. The existence of LRO at room temperature and below became immediately clear from low field transverse field (TF) measurement. These showed that the applied field did not penetrate into the sample, which implies ferro- or ferrimagnetic order. The spontaneous magnetization of those magnets coupled to hysteresis prevents an external field from entering the bulk of the sample as long as the field does not saturate the domain structure (a nice demonstration of this behavior can be found in Ref. [6]). In contrast, the staggered magnetization of an antiferromagnet allows even weak fields to enter the bulk of the material. In short, the TF results support a ferrimagnetic structure as present in the corresponding materials containing heavy rare earth ions. The zero field (ZF) mSR signal of a magnetically LRO polycrystalline compound consists of two terms, an oscillating part (transverse
461
0.15 0.10 0.05
T2
0.00 0
TN
50 100 150 200 250 300 350 400
Temperature (K) Fig. 1. Temperature dependence of the mSR signal amplitude (bottom) and relaxation rate (top) in YFe6 Al6 : Circles indicate data from a cryostat in the GPS facility, triangles data from an oven in the GPD facility. Since the spectrometer performances are somewhat different, the GPD signal at 280 K was adjusted to the GPS amplitude at 275 K:
G.M. Kalvius et al. / Physica B 326 (2003) 460–464
to be only the 13 longitudinal signal, with the transverse signal lost around 340 K: Thus the compound only partially enters LRO below TN until finally, at T2 ; most of the signal is lost. The remaining part is then compatible with the longitudinal signal when all Fe ions participate in LRO. The mSR spectra in the intermediate region show exponential relaxation, yet l is about an order of magnitude larger than expected for a free paramagnetic state. The non-ordered portion is not truly paramagnetic, but under the influence of strong dynamic short-range spin correlations. Even for T > TN the relaxation rate is high, meaning that the short-range correlations persist and a truly free paramagnetic state is not yet reached. Also seen just above T2 is the typical rise in l on approaching a magnetic transition from the high temperature side due to critical slowing down of spin fluctuations. Below T2 the relaxation rate ðllong Þ shows again that the static limit in LRO is approached slowly as was the case in the RFe6 Al6 materials. 57 . Fe Mossbauer spectra provide direct confirmation of the scenario of partial ordering at TN and full ordering at T2 : In the paramagnetic state ðT > TN ¼ 345 KÞ one observes (as expected) a quadrupolar doublet pattern (see top panel of Fig. 2). Its apparent asymmetry results from paramagnetic spin fluctuations which are too slow . within the 57 Fe Mossbauer time window to be fully motional narrowed (which would be the case in a normal paramagnet). The cause is the presence of dynamic short-range correlations as already . discussed. 57 Fe Mossbauer spectra below TN are also presented in Fig. 2. One first observes the sum of a quadrupolar and a six-line Zeeman pattern. The remaining quadrupole subspectrum undergoes Zeeman splitting at T2 E70 K: A surprising result is the relative intensity (1:2) of the subspectra, the more intense one ordering at T2 : It means that LRO in the pure Fe sublattice (8f) is depressed as an effect of frustration. One might have expected that the magnetic dilution in the Fe–Al sublattice (8j) suppresses LRO. The temperature dependence of the hyperfine field for the two Fe sublattices is shown in Fig. 3. The T-0 hyperfine field is not much different for the two sites, but only about half ðB16 TÞ of that of Fe metal, corresponding to
100
YFe7Al5 95
295K 100
Rlative Transmissione
462
95 90
100K
100
95
60K 90 100 99 98 97
4K -6
-4
-2
0
2
4
6
Velocity (mm/s) . Fig. 2. 57 Fe Mossbauer spectra of YFe6 Al6 at selected temperatures. The top spectrum is taken for T > TN :
Fe moments on the order of 1mB : The resonance lines of the 8f magnetic subspectrum are considerably broadened. This broadening can have two sources: a statistic distribution of hyperfine fields due to local spin disorder, or a relaxation rate still . within the 57 Fe Mossbauer time window, meaning that the ordered magnet has not reached the full quasi-static limit (quasi-static, since very fast spin wave excitations will always be present, but are . sensed by the Mossbauer resonance only as the temperature dependence of the effective hyperfine field). It is close to impossible to separate the effects of the two line broadening mechanism. The least-squares fits shown in Fig. 2 and also in Fig. 4 assume a dominance of the static broadening
G.M. Kalvius et al. / Physica B 326 (2003) 460–464
20
100
18
YFe6 Al6
95
16
90
YFe6Al6
14 85
347K
12 10 8 6 4 2 0 0
50
100
150
200
250
300
350
Temperature (K) . Fig. 3. Temperature dependence of the Mossbauer hyperfine field for the two Fe sites in YFe6 Al6 :
mechanism, in accordance with the large transverse relaxation rate in the mSR spectra. The results then imply a distribution of hyperfine field with a relative width of DBhf =Bhf E0:25 in YFe6 Al6 ; the value for YFe7 Al5 being somewhat smaller. A distribution of Bhf arises only from a corresponding variation of local moment, in contrast to mSR; where spin-directional disorder also contributes to DBm : The results for YFe7 Al5 are much the same, except for the somewhat reduced line broadenings which are particularly apparent in the spectra around 60 K: Its origin is likely a difference in spin dynamics but direct evidence cannot be given. From 295 K (our highest mSR temperature in this sample) down, we again see a somewhat reduced mSR signal amplitude, followed by the sudden change of signal intensity around 70 K: The 57 Fe . Mossbauer spectra (Fig. 4) confirm the same scenario of partial ordering in YFe7 Al5 : TN is again around 330 K and the paramagnetic spectra are essentially indistinguishable from the spectrum shown in the top panel of Fig. 2. The saturation hyperfine fields for both Fe sites are also rather similar and not much different from those of YFe6 Al6 : The main difference is the relative intensity of the fraction ordering at T2 : YFe7 Al5 contains additional Fe ions which are all located in
Relative Transmission
Hyperfine Field (T)
463
100 98 96 94 92 90 88
100K
100 98 96
60K
94 100 98 96
4K
94
-6
-4
-2
0
2
4
6
Velocity (mm/s) . Fig. 4. Fe Mossbauer spectra of YFe7 Al5 at selected temperatures below TN : 57
the 8j sublattice. The weaker non-ordered fraction in the mixed magnetic state thus supports in full the assignment of site response given above. The failure to detect magnetic LRO by neutrons probably rests in finite correlation lengths in combination with the relatively small Fe moments, but is not understood in detail. From the mSR relaxation rate in gadolinium metal just above the Curie temperature, Karlsson [7] deduced a correla( for dynamic paramagnetic tion length of B10 A clusters. This clearly did not produce the mSR response for a LRO magnet. In neutron scattering work on YFe7 Al5 ; the analysis of a diffuse satellite peak led to a correlation length of the order of ( [4]. Magnetic Bragg peaks were not seen. One 20 A may roughly estimate for the present situation a
464
G.M. Kalvius et al. / Physica B 326 (2003) 460–464
lower bound for the ferrimagnetic correlation ( and an upper bound around length around 10 A ( . 100 A: The Mossbauer data are of no help in this context because of the unresolved mixture of static and dynamic line broadening. The strong local spin disorder, persistent spin fluctuations and partial LRO ordering found in the present study indicate the presence of frustration even in the Y-containing alloys without magnetic rare earth ions. This must be an inherent property of the Fe sublattices since competing exchange with the R sublattice is absent. If frustration is predominantly due to nearest-neighbor exchange it seems possible that it is reduced by magnetic dilution in the 8j sublattice. Still, the observation of weaker frustration in the 8j
sublattice is astonishing and not really understood. It cannot be explained in the framework of naive frustration models, which would have predicted the opposite effect [3].
References [1] G.M. Kalvius, et al., Physica B 289–290 (2000) 225. [2] W. Sch.afer, et al., J. Magn. Magn. Mater. 177–181 (1998) 808. [3] I. Felner, et al., J. Phys. Chem. Solids 42 (1981) 369. [4] W. Sch.afer, et al., Alloys Compounds 225 (1995) 440. [5] I. Felner, I. Nowik, J. Magn. Magn. Mater. 58 (1986) 169. [6] A.B. Denison, et al., Helv. Phys. Acta 52 (1979) 460. [7] E.B. Karlsson, Hyperfine Interactions 64 (1990) 351.