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NMR evidence for the existence of domain walls in dilute PdCo and PdFe alloys
1059
Journal of Magnetism and Magnetic Materials 54-57 (1986) 1059-1060 NMR EVIDENCE PdFe ALLOYS F. H I P P E R T ,
FOR THE EXISTENCE
P. D E L L O U V E
OF DOMAIN
WALLS IN DILUTE
PdCo AND
a n d H. A L L O U L
Laboratoire de Physique des Solides, Bhtiment 510, Universit~ de Paris Sud, 91405 Orsay, France
Domain walls are evidenced by NMR experiments in PdCo and PdFe alloys. The PdFe behave as soft ferromagnets. In contrast a monodomain state is only achieved in fields H --- 8 kG in the PdCo case. The domain structure of the PdCo alloys is found to depend on the metallurgical treatment.
Dilute P d l _ x C o ~ and P d l _ x F e x alloys have been known for long to exhibit giant moments ( = 10/x B per impurity atom) and ferromagnetic order below a critical temperature T~---40 K / % impurity [1]. However, important differences exist between the two systems. Co moments have an orbital character in dilute Pd a_xCo x alloys as demonstrated by the positive sign of the hyperfine field ( H E = 220 kG) [2] and by magnetoresistance [3] and magnetostriction [4] data. The orbital character of the Fe moments, if any, is negligible. P d l _ x F e x alloys behave as standard soft ferromagnets, the ac susceptibility being limited by demagnetization effects at all temperature below Tc. In contrast an anomalous magnetoresistance is observed in Pda_xCo x alloys at low temperature [5] and ac susceptibility measurements on needle shaped samples only show a maximum at T~, smaller then the demagnetization factor [6]. All these observations, as well as recent small angle neutron scattering studies [6] suggested the existence of local spin disalignments and of an anomalous ferromagnetic order in the dilute Pd a _xCOx alloys. We have therefore carried out comparative N M R studies of dilute Pd l_xFe~ and Pda_xCo ~ alloys at low temperature in order to get information on the domain structure of these materials. This seemed particularly of interest as previous N M R studies failed to detect any domain wall in Pd 1 xCo~ [2]. It is well known since the pionering work of Portis and Gossard [7] that N M R is a useful tool to distinguish between the magnetization processes linked with domain Wall motions and those linked with domain rotations. In a ferromagnet the effective rf field seen by the nuclei h elf is due to the coherent rotation of the electronic spins induced by the applied rf field h~: heft= ~hl, where ~ is an enhancement factor. In a m o n o d o m a i n sample ~7 is produced by the rotation of the domain magnetization: ~ = H L / H t , where H E is the hyperfine field and H t the total static field acting on the electronic spins. In a polydomain sample, domain wall motions, which are responsible for the high susceptibility, also yield a huge enhancement factor for the nuclei located within the domain walls [7,8]. The field evolu0304-8853/86/$03.50
tion of ~ brings therefore information on the domain structure. We studied the 59Co resonance (centered about 220 MHz) and the 57Fe resonance (centered about 42 MHz) at 1.2 K, in zero field and in presence of an applied field H < 13 kG, using spin echo techniques and frequency variable spectrometers. The samples, Pd0.g7 Co0.03 and Pd0.99Fe0.01, consist of thin foils, 20 ~ m thick, obtained by cold rolling the bulk alloys. The P d F e foils were annealed at 800°C for a couple of hours, the PdCo foils at 1050°C for 48 h. In a pulsed N M R experiment the optimum echo signal is obtained for yn~lhlt = "~/2, where ~'n is the gyromagnetic factor of the nuclei and t is the width of the first rf pulse. The measurement of t allows therefore the determination of 7/. The intensity I of the echo is always proportional to 7- In the case of broad lines, when yn~hl is smaller than the linewidth, the number of detected spins increases as ~/h I and I is then proportional to r/2hl . The field evolution of the quantity I~l-Zhl 1 brings therefore information on the number of spins which contribute to the resonance. In the presence of an applied field, once a monodomain has been achieved, one expects ~ cc Ht-a. In the Pd0.99Fe0.01 alloy this regime is achieved for H >__300 G but in the case of Pd0.97C00.03 an applied field H = 7 k G is required: fig. 1 shows the field dependence of ~/-a for H > 7 k G for the Pd0.97Co0.03 sample. The signals detected in zero applied field in both systems are greatly enhanced. By comparing the widths of the rf pulses in zero field an in the monodomain regime one deduces ~/(H = 0) ~ 70000 for the Pd0.99Fe0.0a sample and ~ ( H = 0) -~ 3000 for the Pd0.97Co0.03 sample. These ~7 values are only estimates as ~/(H = 0) depends on hi, decreasing by a factor of five as h a increases from 0.05 to 1 Oe. Similarly enhanced signals are detected in moderate applied fields, H _< 100 G for the P d F e case and H _< 4 k G for the PdCo case, although the spin echo intensity I decreases as H increases (see fig. 2b). These high enhancement factors cannot be associated with domain rotations. They can only be ascribed to nuclei located within domain walls. The difference between the values
© E l s e v i e r S c i e n c e P u b l i s h e r s B.V.
1060
F. Hippert et aL / Domain walls in dilute PdCo and PdFe alloys
" 0'075f
~. o.os[-
! i
Pd97 [03
°~zsL~~ 0
I
I "
5
t5t
_ ~
~-
(Q)
Pd97 [03
,
10
15
I
HIkG) Fig. 1. Inverse of the enhancement factor as a function of the applied field in Pd0.97Co0.03 at 1.2 K for u =194 MHz ( H L = 192 kG) in the monodomain regime. The ~/ value is deduced from the width of the rf pulse. The absolute values are in agreement with v/= H L / H . The accuracy of the data is not sufficient to estimate the anisotropy field which should not exceed 900 G [9].
]0302 t ~b}
,
illI~
IT
I
0
10
H (kG)
of ~ ( H = 0 ) measured in the P d C o a n d PdFe alloys indicates a weaker mobility of the d o m a i n walls in the P d C o case, in agreement with ac susceptibility data. I T l - Z h l I is m u c h smaller in zero field t h a n in the m o n o d o m a i n regime when all the sample contributes to the N M R signal (fig. 2b). This could of course be expected as only a small fraction of the nuclear spins are located in the d o m a i n walls. However, any more accurate estimation of this fraction is h i n d e r e d b y the fact that in the p o l y d o m a i n regime 71 a n d I ~ - 2 h ~ 1 are f o u n d to d e p e n d o n h 1. In conclusion we evidenced the existence of d o m a i n walls in P d C o as well as in P d F e alloys. The applied field induces a s m o o t h transition towards a m o n o d o m a i n state. However, the field required to achieve this m o n o d o m a i n state is m u c h higher in the P d C o case. In agreement with the N M R results, m a g n e t i z a t i o n data o n the same Pd0.97Co0.03 sample reveal that the m a g n e t i z a t i o n is difficult to saturate (see fig. 2a). Surprisingly d o m a i n walls are still detected in the N M R experiments in fields for which the magnetization is not far from saturation. W e have no definite explanation for this fact at present. T h e present results which evidence d o m a i n walls in P d C o alloys are in contradiction with those of ref. [2] o b t a i n e d o n p o w d e r e d samples. In order to elucidate this c o n t r a d i c t i o n we studied a n o t h e r Pd0.97Co0.03 sample which consists of cold rolled n o n - a n n e a l e d foils. We f o u n d that the m o n o d o m a i n regime is only reached for H>__ 12 k G a n d ~ / ( H = 0 ) = 170. Therefore the d o m a i n structure of the P d C o alloys depends strongly o n the metallurgical state. T h e results of ref. [2], where only weakly e n h a n c e d signals could be detected, can then be u n d e r s t o o d as the powders used in that experiment certainly c o n t a i n e d a strong density of defects. It must be emphasized that the linewidth of the 59Co resonance, which is quite large ( A z , / z , = 0 . 3 for Pd0.97Co0.03) is f o u n d to be identical in powders [2] a n d in well annealed samples a n d is therefore i n d e p e n d e n t of the metallurgical state. In contrast the linewidth of the 57Fe
Fig. 2. (a) Magnetization versus field at 4.2 K measured on the NMR Pd0.97Co0.o3 sample; (b) 17 2hl 1 as a function of the applied field at T = 1.2 K for u = 194 MHz for the Pd0.97Co0.03 alloy. (The spin echo intensities I have been corrected for the decay of the transverse magnetization.) 1~1 2h~1 is found roughly constant for H >~ 7 kG as expected in the monodomain regime. No data points have been drawn for H = 5 kG as a wide distribution of ~/ values is then detected.
resonance in Pd0.99Fe0.01 is smaller by an order of m a g n i t u d e (,Sp/t., = 0.017). In conclusion, although d o m a i n walls were evidenced in the PdCo alloys as well as in the PdFe alloys, the d o m a i n properties of b o t h alloys are different. PdFe alloys behave as soft ferromagnets. In contrast achieving a m o n o d o m a i n state in a PdCo sample requires high applied fields a n d the d o m a i n structure depends on the metallurgical treatment. A n a n o m a l o u s small angle neutron scattering, d e p e n d e n t o n the metallurgical state, is also observed in the P d C o alloys [6] revealing disalignm e n t s of the spins in zero a n d m o d e r a t e applied fields. It is t e m p t i n g to ascribe all these a n o m a l o u s properties of the PdCo alloys to the existence of an orbital mom e n t on the Co site, as such a m o m e n t can couple easily by magnetostrictive coupling to the local strains. [1] J. Crangle and W.R. Scott, J. Appl. Phys. 36 (1965) 921. [2] N. Katayama, K. Kumagai, T. Kohara, K. Asayama, I.A. Campbell, N. Sana, S. Kobayachi and J. Itoh, J. Phys. Soc. Japan 40 (1976) 429. [3] S. Senoussi, I.A. Campbell and A. Fert, Solid State Commun. 21 (1977) 269. [4] G. Creuzet, A. Hamzid and I.A. Campbell, Solid State Commun. 39 (1981) 451. [5] A. Hamzid, S. Senoussi and I.A. Campbell, J. Phys. F 10 (1980) L165. [6] I. Mirebeau, M. Hennion, F. Hippert and I.A. Campbell, J. Magn. Magn. Mat. 54-57 (1986). [7] A.M. Portis and A.C. Gossard, J. Appl. Phys. 31 (1960) 2055. [8] S.P. Retnikov and V.B. Ustinov, Soviet Phys. Solid State 11 (1969) 834. [9] D.M.S. Bagguley and J.A. Robertson, J. Phys. F4 (1974) 2282.
Journal of Magnetism and Magnetic Materials 54-57 (1986) 1059-1060 NMR EVIDENCE PdFe ALLOYS F. H I P P E R T ,
FOR THE EXISTENCE
P. D E L L O U V E
OF DOMAIN
WALLS IN DILUTE
PdCo AND
a n d H. A L L O U L
Laboratoire de Physique des Solides, Bhtiment 510, Universit~ de Paris Sud, 91405 Orsay, France
Domain walls are evidenced by NMR experiments in PdCo and PdFe alloys. The PdFe behave as soft ferromagnets. In contrast a monodomain state is only achieved in fields H --- 8 kG in the PdCo case. The domain structure of the PdCo alloys is found to depend on the metallurgical treatment.
Dilute P d l _ x C o ~ and P d l _ x F e x alloys have been known for long to exhibit giant moments ( = 10/x B per impurity atom) and ferromagnetic order below a critical temperature T~---40 K / % impurity [1]. However, important differences exist between the two systems. Co moments have an orbital character in dilute Pd a_xCo x alloys as demonstrated by the positive sign of the hyperfine field ( H E = 220 kG) [2] and by magnetoresistance [3] and magnetostriction [4] data. The orbital character of the Fe moments, if any, is negligible. P d l _ x F e x alloys behave as standard soft ferromagnets, the ac susceptibility being limited by demagnetization effects at all temperature below Tc. In contrast an anomalous magnetoresistance is observed in Pda_xCo x alloys at low temperature [5] and ac susceptibility measurements on needle shaped samples only show a maximum at T~, smaller then the demagnetization factor [6]. All these observations, as well as recent small angle neutron scattering studies [6] suggested the existence of local spin disalignments and of an anomalous ferromagnetic order in the dilute Pd a _xCOx alloys. We have therefore carried out comparative N M R studies of dilute Pd l_xFe~ and Pda_xCo ~ alloys at low temperature in order to get information on the domain structure of these materials. This seemed particularly of interest as previous N M R studies failed to detect any domain wall in Pd 1 xCo~ [2]. It is well known since the pionering work of Portis and Gossard [7] that N M R is a useful tool to distinguish between the magnetization processes linked with domain Wall motions and those linked with domain rotations. In a ferromagnet the effective rf field seen by the nuclei h elf is due to the coherent rotation of the electronic spins induced by the applied rf field h~: heft= ~hl, where ~ is an enhancement factor. In a m o n o d o m a i n sample ~7 is produced by the rotation of the domain magnetization: ~ = H L / H t , where H E is the hyperfine field and H t the total static field acting on the electronic spins. In a polydomain sample, domain wall motions, which are responsible for the high susceptibility, also yield a huge enhancement factor for the nuclei located within the domain walls [7,8]. The field evolu0304-8853/86/$03.50
tion of ~ brings therefore information on the domain structure. We studied the 59Co resonance (centered about 220 MHz) and the 57Fe resonance (centered about 42 MHz) at 1.2 K, in zero field and in presence of an applied field H < 13 kG, using spin echo techniques and frequency variable spectrometers. The samples, Pd0.g7 Co0.03 and Pd0.99Fe0.01, consist of thin foils, 20 ~ m thick, obtained by cold rolling the bulk alloys. The P d F e foils were annealed at 800°C for a couple of hours, the PdCo foils at 1050°C for 48 h. In a pulsed N M R experiment the optimum echo signal is obtained for yn~lhlt = "~/2, where ~'n is the gyromagnetic factor of the nuclei and t is the width of the first rf pulse. The measurement of t allows therefore the determination of 7/. The intensity I of the echo is always proportional to 7- In the case of broad lines, when yn~hl is smaller than the linewidth, the number of detected spins increases as ~/h I and I is then proportional to r/2hl . The field evolution of the quantity I~l-Zhl 1 brings therefore information on the number of spins which contribute to the resonance. In the presence of an applied field, once a monodomain has been achieved, one expects ~ cc Ht-a. In the Pd0.99Fe0.01 alloy this regime is achieved for H >__300 G but in the case of Pd0.97C00.03 an applied field H = 7 k G is required: fig. 1 shows the field dependence of ~/-a for H > 7 k G for the Pd0.97Co0.03 sample. The signals detected in zero applied field in both systems are greatly enhanced. By comparing the widths of the rf pulses in zero field an in the monodomain regime one deduces ~/(H = 0) ~ 70000 for the Pd0.99Fe0.0a sample and ~ ( H = 0) -~ 3000 for the Pd0.97Co0.03 sample. These ~7 values are only estimates as ~/(H = 0) depends on hi, decreasing by a factor of five as h a increases from 0.05 to 1 Oe. Similarly enhanced signals are detected in moderate applied fields, H _< 100 G for the P d F e case and H _< 4 k G for the PdCo case, although the spin echo intensity I decreases as H increases (see fig. 2b). These high enhancement factors cannot be associated with domain rotations. They can only be ascribed to nuclei located within domain walls. The difference between the values
© E l s e v i e r S c i e n c e P u b l i s h e r s B.V.
1060
F. Hippert et aL / Domain walls in dilute PdCo and PdFe alloys
" 0'075f
~. o.os[-
! i
Pd97 [03
°~zsL~~ 0
I
I "
5
t5t
_ ~
~-
(Q)
Pd97 [03
,
10
15
I
HIkG) Fig. 1. Inverse of the enhancement factor as a function of the applied field in Pd0.97Co0.03 at 1.2 K for u =194 MHz ( H L = 192 kG) in the monodomain regime. The ~/ value is deduced from the width of the rf pulse. The absolute values are in agreement with v/= H L / H . The accuracy of the data is not sufficient to estimate the anisotropy field which should not exceed 900 G [9].
]0302 t ~b}
,
illI~
IT
I
0
10
H (kG)
of ~ ( H = 0 ) measured in the P d C o a n d PdFe alloys indicates a weaker mobility of the d o m a i n walls in the P d C o case, in agreement with ac susceptibility data. I T l - Z h l I is m u c h smaller in zero field t h a n in the m o n o d o m a i n regime when all the sample contributes to the N M R signal (fig. 2b). This could of course be expected as only a small fraction of the nuclear spins are located in the d o m a i n walls. However, any more accurate estimation of this fraction is h i n d e r e d b y the fact that in the p o l y d o m a i n regime 71 a n d I ~ - 2 h ~ 1 are f o u n d to d e p e n d o n h 1. In conclusion we evidenced the existence of d o m a i n walls in P d C o as well as in P d F e alloys. The applied field induces a s m o o t h transition towards a m o n o d o m a i n state. However, the field required to achieve this m o n o d o m a i n state is m u c h higher in the P d C o case. In agreement with the N M R results, m a g n e t i z a t i o n data o n the same Pd0.97Co0.03 sample reveal that the m a g n e t i z a t i o n is difficult to saturate (see fig. 2a). Surprisingly d o m a i n walls are still detected in the N M R experiments in fields for which the magnetization is not far from saturation. W e have no definite explanation for this fact at present. T h e present results which evidence d o m a i n walls in P d C o alloys are in contradiction with those of ref. [2] o b t a i n e d o n p o w d e r e d samples. In order to elucidate this c o n t r a d i c t i o n we studied a n o t h e r Pd0.97Co0.03 sample which consists of cold rolled n o n - a n n e a l e d foils. We f o u n d that the m o n o d o m a i n regime is only reached for H>__ 12 k G a n d ~ / ( H = 0 ) = 170. Therefore the d o m a i n structure of the P d C o alloys depends strongly o n the metallurgical state. T h e results of ref. [2], where only weakly e n h a n c e d signals could be detected, can then be u n d e r s t o o d as the powders used in that experiment certainly c o n t a i n e d a strong density of defects. It must be emphasized that the linewidth of the 59Co resonance, which is quite large ( A z , / z , = 0 . 3 for Pd0.97Co0.03) is f o u n d to be identical in powders [2] a n d in well annealed samples a n d is therefore i n d e p e n d e n t of the metallurgical state. In contrast the linewidth of the 57Fe
Fig. 2. (a) Magnetization versus field at 4.2 K measured on the NMR Pd0.97Co0.o3 sample; (b) 17 2hl 1 as a function of the applied field at T = 1.2 K for u = 194 MHz for the Pd0.97Co0.03 alloy. (The spin echo intensities I have been corrected for the decay of the transverse magnetization.) 1~1 2h~1 is found roughly constant for H >~ 7 kG as expected in the monodomain regime. No data points have been drawn for H = 5 kG as a wide distribution of ~/ values is then detected.
resonance in Pd0.99Fe0.01 is smaller by an order of m a g n i t u d e (,Sp/t., = 0.017). In conclusion, although d o m a i n walls were evidenced in the PdCo alloys as well as in the PdFe alloys, the d o m a i n properties of b o t h alloys are different. PdFe alloys behave as soft ferromagnets. In contrast achieving a m o n o d o m a i n state in a PdCo sample requires high applied fields a n d the d o m a i n structure depends on the metallurgical treatment. A n a n o m a l o u s small angle neutron scattering, d e p e n d e n t o n the metallurgical state, is also observed in the P d C o alloys [6] revealing disalignm e n t s of the spins in zero a n d m o d e r a t e applied fields. It is t e m p t i n g to ascribe all these a n o m a l o u s properties of the PdCo alloys to the existence of an orbital mom e n t on the Co site, as such a m o m e n t can couple easily by magnetostrictive coupling to the local strains. [1] J. Crangle and W.R. Scott, J. Appl. Phys. 36 (1965) 921. [2] N. Katayama, K. Kumagai, T. Kohara, K. Asayama, I.A. Campbell, N. Sana, S. Kobayachi and J. Itoh, J. Phys. Soc. Japan 40 (1976) 429. [3] S. Senoussi, I.A. Campbell and A. Fert, Solid State Commun. 21 (1977) 269. [4] G. Creuzet, A. Hamzid and I.A. Campbell, Solid State Commun. 39 (1981) 451. [5] A. Hamzid, S. Senoussi and I.A. Campbell, J. Phys. F 10 (1980) L165. [6] I. Mirebeau, M. Hennion, F. Hippert and I.A. Campbell, J. Magn. Magn. Mat. 54-57 (1986). [7] A.M. Portis and A.C. Gossard, J. Appl. Phys. 31 (1960) 2055. [8] S.P. Retnikov and V.B. Ustinov, Soviet Phys. Solid State 11 (1969) 834. [9] D.M.S. Bagguley and J.A. Robertson, J. Phys. F4 (1974) 2282.