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Pressure dependence of magnetic properties of Nb1−yFe2+y
Journal of Magnetism and Magnetic Materials 104-107 (1992) 1317-1318 North-Holland
Pressure dependence of magnetic properties of Nbl_yFe2+y Y. Yamada t, J.G.M. Armitage, R.G. Graham and P.C. Riedi J.F. Allen Research Laboratories, Department of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9SS, Scotland, UK The pressure dependence of magnetic properties was investigated in antiferromagnetic (stoichiometric) and ferromagnetic (off-stoichiometric) NbFe 2 with the C14 Laves phase structure. The C14 Laves phase intermetallic compound NbFe 2 was originally considered to be a Pauli paramagnet. Recently, the properties of stoichiometric and off-stoichiometric Nb 1 yFez+y have been investigated in detail, using magnetisation [1,2] and N M R [2-4] measurements and it was found that stoichiometric NbFe 2 is a weak antiferromagnet with a N6el temperature of about 10 K. The antiferromagnetic state in NbFez is destroyed by an external magnetic field > 6 kOe. The magnetic susceptibility and nuclear spinlattice relaxation of this compound in the paramagnetic state above 30 K show the existence of ferromagnetic spin fluctuations. The antiferromagnetism is suppressed by a very small deviation to the Nb excess side of stoichiometry and at around y = -0.004 the compound becomes paramagnetic. However, further deviation from stoichiometry leads to ferromagnetic order in spite of the decrease in the Fe concentration. Ferromagnetism also appears on the Fe excess side, as is often seen in Fe- or Co-rich AFe 2 or ACo 2 Laves phase compounds. Thus, it is considered that ferroand antiferromagnetic spin fluctuations coexist in this system, and that the antiferromagnetism at stoichiometry is very delicately balanced. The purpose of the present study is to investigate the volume dependence of the magnetic properties of Nb~_yFez+y by NMR measurements at high pressure. The sample was placed in a tuned N M R coil inside a liquid filled B e - C u pressure cell which could be pressurised to 15 kbar at room temperature. The cell was then cooled down to 4.2 K and the 93Nb N M R spectrum recorded using a phase coherent spin echo spectrometer which measured the integral of the spin echo as a function of magnetic field at a fixed frequency. The pressure was measured by means of a semiconductor transducer inside the cell. The sample of stoichiometric NbFe z was the same as used in ref. [2] and the samples with y = +0.01, - 0 . 0 2 and - 0 . 0 4 were prepared by the same method.
Figure la shows the N M R spectrum of antiferromagnetic NbFe 2 at 3 MHz and 4.2 K at atmospheric pressure. The lineshape has already been explained by an antiferromagnetic powder pattern with a considerable distribution of the antiferromagnetic hyperfine field [4]. The line width corresponds to the magnitude of antiferromagnetic moment in the sample. Figure lb shows the N MR spectrum under a pressure of 12.2. kbar. Other experimental conditions are the same as in figure la. The line width at 12.2 kbar is much smaller than that at atmospheric pressure ( ~ 2 / 3 ) indicating that the antiferromagnetism is suppressed by pressure. Figure 2 shows the pressure dependence of the full width at 51 and ~1 the maximum height. Both the widths show a similar dependence. They are seen to increase slightly up to about 2 kbar, then to decrease with further increase in pressure. This result may show that the anti ferromagnetic moments initially increase slightly with decreasing lattice constant, but then decrease markedly. The compressibility of NbFe 2 is not known. If we use the value for the Laves phase YFe2, ~ 1.0 M b a r - 1, instead [5], the decrease in volume is only 1.2% even at the maximum pressures achieved in the present study. In such a small region the change of a physical quantity
~.
,~
{a)
K=O
,
NbFe2
4.2K 3 MHz
,
....
.~. (b) Y1
' " v | l r" ~ " ~ ' r r
0
a Permanent address: Department of Electrical Engineering, Himeji Institute of Technology, Shosha, Himeji 671-22, Japan.
1
2 3 4 5 External field (kOe)
6
Fig. 1.93Nb NMR spectra in NbFe 2 at 3 MHz at 4.2 K under (a) p = 0 (atmospheric) and (b) p = 12.2 kbar.
0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
1318
Y Yamada et al. / Pressure dependence of magnetic properties o f N b 1 ~.Fe2 + y
(b) |
° 2 0 I- •
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(a) |
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°6 I 0.5[-
,
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I
[
J
i
4 6 Pressure
l
J
l
l
8 (kbar)
l
10
12
Fig. 2. Pressure dependence of the full width at xl and I maximum of the 93Nb NMR spectrum of NbFe 2 measured at 3 MHz at 4.2 K.
is usually small a n d linear in pressure. Thus, the present result reflects the a n o m a l o u s a n d delicately bala n c e d a n t i f e r r o m a g n e t i c state in N b F e 2. Figure 3 shows the N M R spectra of weakly ferrom a g n e t i c Nb]_~,Fe2+y with y = - 0 . 0 2 ( T c = 6 K) at
N bl_yFe2°y
14=0 t~
y =-0-02 10 MHz
i c
a
z
_J 9
10
11 12 13 14 External field (kOe)
Fig. 3. 93Nb N M R spectra o f Nh, yFe2+y with y = - 0 . 0 2 at 10 M H z at 4.2 K under p = 0, 7.8 and 10.5 kbar.
zero, 7.8 a n d 10.5 kbar. T h e spectra show a largc negative shift a n d a b r o a d width a t t r i b u t a b l e to the i n h o m o g e n e o u s f e r r o m a g n e t i c magnetisation. It is clear that the pressure suppresses the s h o u l d e r on the highfield side a n d decreases the overall shift of the spectrum, indicating that pressure suppresses ferromagnetism in this system. T h e d e c r e a s e in the shift of the centre of gravity of the s p e c t r u m from K = 0 induced by the pressure is a b o u t 6 % at 10.5 kbar, which suggests t h a t the d e c r e a s e in the f e r r o m a g n e t i c magnetisation at 4.2 K in this c o m p o u n d is such that a In c r / a P ~ - 6 M b a r 1. For the sample with y = - 0 . 0 4 ( T c = 24 K) the value is - 9 M b a r i. T h e d e c r e a s e of the f e r r o m a g n e t i c shift in the Fe excess sample with y = +0.01 ( T c = 24 K) is less sensitive to pressure, only 2.5% at 10.5 kbar. However, it should be n o t e d t h a t an these pressure effects are very m u c h larger than those f o u n d in o t h e r Laves p h a s e c o m p o u n d s , such as Y F e : or Z r F e 2 w h e r e 0 In o-laP ~ - 0 . 8 M b a r 1 [6]. T h e difference in the effect of pressure on the Nb spectra on the Nb a n d Fe excess sides of stoichiometry can be a t t r i b u t e d to the difference of the m e c h a n i s m responsible for the a p p e a r a n c e of f e r r o m a g n e t i s m in the two cases. O n the Nb excess side, the increase in distance b e t w e e n the Fe atoms caused by the occupation of Fe sites by larger Nb atoms may have an i m p o r t a n t role in the a p p e a r a n c e of ferromagnetism. O n the o t h e r h a n d , in the Fe-rich systems, the ferrom a g n e t i s m is believed to arise from Fc atoms occupying Nb sites a n d forming f e r r o m a g n e t i c clusters with n e i g h b o u r i n g Fe atoms. In this case, the ferromagnetism associated with the Fe clusters is expected to d e p e n d only weakly on the lattice constant.
References [l] M. Shiga and Y. Nakamura, J. Phys. Soc. Jpn. 56 (1987) 4040. [2] Y. Yamada and A. Sakata, J. Phys. Soc. Jpn. 57 (1988) 46. [3] Y. Yamada, H. Nakamura, Y. Kitaokoa, K. Asayama, K. Koga, A. Sakata and T. Murakami, J. Magn. Magn. Mater. 90 & 91 (1990) 712. [4] Y. Yamada, H. Nakamura, Y. Kitaoka, K. Asayama, K. Koga, A. Sakata and T. Murakami, J. Phys. Soc. 59 (1990) 2976. [5] H. Klimber, M. Rosen, M.P. Dariel and U. Atzmony, Phys. Rev. B 10 (1974) 2968. [6] J.G.M. Armitage, T. Dumelow, R.H. Mitchell, P.C. Riedi, J.S. Abell, P. Mohn and K. Schwarz, J. Phys. F 16 (1986) L141.
Pressure dependence of magnetic properties of Nbl_yFe2+y Y. Yamada t, J.G.M. Armitage, R.G. Graham and P.C. Riedi J.F. Allen Research Laboratories, Department of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9SS, Scotland, UK The pressure dependence of magnetic properties was investigated in antiferromagnetic (stoichiometric) and ferromagnetic (off-stoichiometric) NbFe 2 with the C14 Laves phase structure. The C14 Laves phase intermetallic compound NbFe 2 was originally considered to be a Pauli paramagnet. Recently, the properties of stoichiometric and off-stoichiometric Nb 1 yFez+y have been investigated in detail, using magnetisation [1,2] and N M R [2-4] measurements and it was found that stoichiometric NbFe 2 is a weak antiferromagnet with a N6el temperature of about 10 K. The antiferromagnetic state in NbFez is destroyed by an external magnetic field > 6 kOe. The magnetic susceptibility and nuclear spinlattice relaxation of this compound in the paramagnetic state above 30 K show the existence of ferromagnetic spin fluctuations. The antiferromagnetism is suppressed by a very small deviation to the Nb excess side of stoichiometry and at around y = -0.004 the compound becomes paramagnetic. However, further deviation from stoichiometry leads to ferromagnetic order in spite of the decrease in the Fe concentration. Ferromagnetism also appears on the Fe excess side, as is often seen in Fe- or Co-rich AFe 2 or ACo 2 Laves phase compounds. Thus, it is considered that ferroand antiferromagnetic spin fluctuations coexist in this system, and that the antiferromagnetism at stoichiometry is very delicately balanced. The purpose of the present study is to investigate the volume dependence of the magnetic properties of Nb~_yFez+y by NMR measurements at high pressure. The sample was placed in a tuned N M R coil inside a liquid filled B e - C u pressure cell which could be pressurised to 15 kbar at room temperature. The cell was then cooled down to 4.2 K and the 93Nb N M R spectrum recorded using a phase coherent spin echo spectrometer which measured the integral of the spin echo as a function of magnetic field at a fixed frequency. The pressure was measured by means of a semiconductor transducer inside the cell. The sample of stoichiometric NbFe z was the same as used in ref. [2] and the samples with y = +0.01, - 0 . 0 2 and - 0 . 0 4 were prepared by the same method.
Figure la shows the N M R spectrum of antiferromagnetic NbFe 2 at 3 MHz and 4.2 K at atmospheric pressure. The lineshape has already been explained by an antiferromagnetic powder pattern with a considerable distribution of the antiferromagnetic hyperfine field [4]. The line width corresponds to the magnitude of antiferromagnetic moment in the sample. Figure lb shows the N MR spectrum under a pressure of 12.2. kbar. Other experimental conditions are the same as in figure la. The line width at 12.2 kbar is much smaller than that at atmospheric pressure ( ~ 2 / 3 ) indicating that the antiferromagnetism is suppressed by pressure. Figure 2 shows the pressure dependence of the full width at 51 and ~1 the maximum height. Both the widths show a similar dependence. They are seen to increase slightly up to about 2 kbar, then to decrease with further increase in pressure. This result may show that the anti ferromagnetic moments initially increase slightly with decreasing lattice constant, but then decrease markedly. The compressibility of NbFe 2 is not known. If we use the value for the Laves phase YFe2, ~ 1.0 M b a r - 1, instead [5], the decrease in volume is only 1.2% even at the maximum pressures achieved in the present study. In such a small region the change of a physical quantity
~.
,~
{a)
K=O
,
NbFe2
4.2K 3 MHz
,
....
.~. (b) Y1
' " v | l r" ~ " ~ ' r r
0
a Permanent address: Department of Electrical Engineering, Himeji Institute of Technology, Shosha, Himeji 671-22, Japan.
1
2 3 4 5 External field (kOe)
6
Fig. 1.93Nb NMR spectra in NbFe 2 at 3 MHz at 4.2 K under (a) p = 0 (atmospheric) and (b) p = 12.2 kbar.
0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
1318
Y Yamada et al. / Pressure dependence of magnetic properties o f N b 1 ~.Fe2 + y
(b) |
° 2 0 I- •
o•
!
(a) |
-~
•
i
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!
°6 I 0.5[-
,
I
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I
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i
4 6 Pressure
l
J
l
l
8 (kbar)
l
10
12
Fig. 2. Pressure dependence of the full width at xl and I maximum of the 93Nb NMR spectrum of NbFe 2 measured at 3 MHz at 4.2 K.
is usually small a n d linear in pressure. Thus, the present result reflects the a n o m a l o u s a n d delicately bala n c e d a n t i f e r r o m a g n e t i c state in N b F e 2. Figure 3 shows the N M R spectra of weakly ferrom a g n e t i c Nb]_~,Fe2+y with y = - 0 . 0 2 ( T c = 6 K) at
N bl_yFe2°y
14=0 t~
y =-0-02 10 MHz
i c
a
z
_J 9
10
11 12 13 14 External field (kOe)
Fig. 3. 93Nb N M R spectra o f Nh, yFe2+y with y = - 0 . 0 2 at 10 M H z at 4.2 K under p = 0, 7.8 and 10.5 kbar.
zero, 7.8 a n d 10.5 kbar. T h e spectra show a largc negative shift a n d a b r o a d width a t t r i b u t a b l e to the i n h o m o g e n e o u s f e r r o m a g n e t i c magnetisation. It is clear that the pressure suppresses the s h o u l d e r on the highfield side a n d decreases the overall shift of the spectrum, indicating that pressure suppresses ferromagnetism in this system. T h e d e c r e a s e in the shift of the centre of gravity of the s p e c t r u m from K = 0 induced by the pressure is a b o u t 6 % at 10.5 kbar, which suggests t h a t the d e c r e a s e in the f e r r o m a g n e t i c magnetisation at 4.2 K in this c o m p o u n d is such that a In c r / a P ~ - 6 M b a r 1. For the sample with y = - 0 . 0 4 ( T c = 24 K) the value is - 9 M b a r i. T h e d e c r e a s e of the f e r r o m a g n e t i c shift in the Fe excess sample with y = +0.01 ( T c = 24 K) is less sensitive to pressure, only 2.5% at 10.5 kbar. However, it should be n o t e d t h a t an these pressure effects are very m u c h larger than those f o u n d in o t h e r Laves p h a s e c o m p o u n d s , such as Y F e : or Z r F e 2 w h e r e 0 In o-laP ~ - 0 . 8 M b a r 1 [6]. T h e difference in the effect of pressure on the Nb spectra on the Nb a n d Fe excess sides of stoichiometry can be a t t r i b u t e d to the difference of the m e c h a n i s m responsible for the a p p e a r a n c e of f e r r o m a g n e t i s m in the two cases. O n the Nb excess side, the increase in distance b e t w e e n the Fe atoms caused by the occupation of Fe sites by larger Nb atoms may have an i m p o r t a n t role in the a p p e a r a n c e of ferromagnetism. O n the o t h e r h a n d , in the Fe-rich systems, the ferrom a g n e t i s m is believed to arise from Fc atoms occupying Nb sites a n d forming f e r r o m a g n e t i c clusters with n e i g h b o u r i n g Fe atoms. In this case, the ferromagnetism associated with the Fe clusters is expected to d e p e n d only weakly on the lattice constant.
References [l] M. Shiga and Y. Nakamura, J. Phys. Soc. Jpn. 56 (1987) 4040. [2] Y. Yamada and A. Sakata, J. Phys. Soc. Jpn. 57 (1988) 46. [3] Y. Yamada, H. Nakamura, Y. Kitaokoa, K. Asayama, K. Koga, A. Sakata and T. Murakami, J. Magn. Magn. Mater. 90 & 91 (1990) 712. [4] Y. Yamada, H. Nakamura, Y. Kitaoka, K. Asayama, K. Koga, A. Sakata and T. Murakami, J. Phys. Soc. 59 (1990) 2976. [5] H. Klimber, M. Rosen, M.P. Dariel and U. Atzmony, Phys. Rev. B 10 (1974) 2968. [6] J.G.M. Armitage, T. Dumelow, R.H. Mitchell, P.C. Riedi, J.S. Abell, P. Mohn and K. Schwarz, J. Phys. F 16 (1986) L141.