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High field magnetoresistance of UFe4Al8
Physica B 211 (1995) 139-141
ELSEVIER
High field magnetoresistance of UFe4A18 G. Bonfait a'*, A.P. Gonqalves a, J.C. Spirlet b, M. Almeida b aDepartamento de Quimica, ICEN-1NETI, P-2686 Sacav~m, Portugal bEC, JRC, Institute for Transuranium Elements, Postfaeh 2340, D-76125 Karlsruhe, Germany
Abstract Magnetoresistance measurements up to 16 T on UFe4A18 single crystal are reported. The results show a strong anisotropic magnetoresistance. The normal magnetoresistance is always negative. No new phase transitioni has been detected up to 16 T.
1. Introduction
2. Experimental results and discussion
The magnetic structure of UFe4Ala has been the source of controversial interpretations mainly due to the difficulty to obtain single crystals and probably also due to a non-trivial magnetic order. UFe4AI8 crystallises in the body centred tetragonal structure of ThMna2, the U atom standing in a cage formed by four Fe atoms. Magnetisation [1, 2] and Mossbauer [3] results showed the existence of a magnetic order at 155 K involving the Fe atoms; neutron powder diffraction [4, 5] showed that, below this temperature, the magnetic lattice is the same as the crystallographic one, but led to contradictory interpretations of the magnetic configuration of the Fe and U atoms. Recently, the obtention of a single crystal allowed us to obtain more information about the magnetic behaviour of this compound: the magnetic moment at low temperature is rather low ( ~ 1.6#B/formula unit) and the c-axis is a hard axis for the magnetisation. Here, we present resistivity and magnetoresistivity (MR) measurements up to 16 T on the same single crystal that was previously used for magnetic measurements [2].
During this experimental run, the sample'h°lder was rotated twice, allowing measurements in two Configurations:
* Corresponding author.
Hlta, ILH(Ib]b), called Pr, - HIIb, llIH(lllb), called PlL-
Due to the crystallographic equivalence b~tween the a and b axes, these two configurations are th¢ standard transverse and parallel configurations for MR]withHllb. The resistivity at zero magnetic field (Fig. 1) shows three different regimes. For T > 155 K, the I resistivity increases when T decreases and this behav]our is attributed to scattering of the conduction electrbns by the spin fluctuations of the magnetic atoms (Feior U). At T¢ = 155 K, a peak in the resistivity clearly ~enotes the magnetic transition. For 100 K < T < 150 K~ the resistivity has a small temperature dependence shgwing that a scattering channel disappeared at T¢. Below1100 K, the resistivity begins to decrease steeply d~wn to a T2-regime below 3 0 K (p ~ Po + 0.016 T2 P-Ocm/K2) which can be interpreted as the effect o! the electron magnon interaction in FM materials. HOwever, the domain walls have an important effect on the resistivity
0921-4526/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 4 ) 0 0 9 6 6 - X
G. Bonfait et al./Physica B 211 (1995) 139-141
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T (K) Fig. 1. Resistivityof UFecAI8versus temperature for 0 and 16 T (Hill). The inset shows Ap/p = (p(16 T) - p(O))/p(O). [6]; therefore, further measurements in the single-domain state must be done to confirm this temperature behaviour. A 16 T field lowers the resistivity on the whole temperature range. The three different regimes persist with different characteristics and it can be deduced that the magnetic transition temperature does not significantly change. The T2-regime still exists reinforcing our interpretation of electron magnon scattering and the smaller coefficient (0.010 laf~cm/K2) reflects the higher energy of the magnetic excitations. Ap/p = (p(16 T) p(O))/p(O) reaches a maximum (in absolute value) at 70 K. It is often proposed in this type of compounds that the actinide atoms gradually order at low temperature, a phenomenon that could explain this maximum of magnetoresistance. At low temperature and low fields, the M(H) curves show steps [2], which are very clearly seen as a nonmonotone behaviour in the p(H) curves. This phenomenon, interpreted as an effect of the domain walls, will be described in more detail in an other publication [6]. In this paper, we will focus on the results obtained by a sweep of the field from high value to zero, representing the effect of the magnetic field on the resistivity in the single-domain state. Our results show a strong difference between the parallel (PlI) and transverse (PT) configuration (Fig. 2). This effect anisotropic magnetoresistance (AMR) - can have two different origins [7, 8]. The cross-section for the electron scattering is affected by the spin-orbit coupling and, therefore, becomes dependent of the angle between the current and the magnetisation. In this case, PT is
17.
~\'~kq x ~\~x 230
......
5
10
15
( #o H) [T] Fig. 2. Transverse and parallel magneto-resistivity versus magnetic field at 2 and 4 K (left scale) and 20 K (right scale). The offset between the two scales is to emphasize the very small temperature dependence of the magneto-resistivity between 2 and 20 K.
usually lower than Pll but PT > Pll has already been found in some rare-earth compounds [9]. The second origin is the effect of the strong internal field existing in FM metals which, at low temperature and in pure samples, curves the trajectory of the electrons. This leads to a positive MR when the magnetisation is perpendicular to the electrical current, then in this case PT > PlI" Due to the fact that the AMR subsists up to high temperature in our compound, the first reason seems to be more appropriate to explain our results. These need to be decomposed in two different contributions: one, almost independent of the magnetic field, as indicated by the parallelism between PT and PlI, and the other - always negative - much more dependent of the magnetic field. The T 2-regime for p(T) suggests that the electron-magnon is the source of this negative MR. In this case, Yamada and Takada [10] showed that the resistivity scales as #H/kB T. This is not the case in our results where the curves at 4.2 and 2 K are identical and a field higher than 5 T is needed to obtain a difference between the curve pT(H) at 2 K and 20 K (Fig. 2). Such a temperature-independent behaviour is easier to explain in a simple band magnetism model: the effect of the magnetic field is simply to fill the spin-up band, the decrease of the MR indicating that the Fermi level moves to a region where the density of states is higher. Such an interpretation would explain the increase of ]dPll/dHI at low temperatures for fields higher than 10 T, by a Fermi level reaching an enhanced density of states region. This band description explains the low
G. Bonfait et al./Physica B 211 (1995) 139-141 .
~
.
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.
I
~
i
i
i
i
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i
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141
must be decomposed in two parts. One, almost field independent at low temperature, causes a high AMR; the other, much more field dependent, indicates either the e l e c t r o n - m a g n o n interaction or a band-filling ieffect. The large and unusual (PT > PlI) A M R needs a careful analysis in order to try to attribute this effect to one of the U or Fe atom. Such an analysis would help tO separate the influence of each of these atoms on the ~ransport properties.
Acknowledgements Work partially supported by a N A T O collaborative Research G r a n t 920996.
260 5
10
15
( oH) [T] Fig. 3. Transverse (full lines) and parallel (dashed lines) magneto-resistivity versus magnetic field. magnetic moment found in the magnetisation measurements at low temperature. A temperature dependence of the p(H) curves appears only for T > 40 K (Fig. 3), temperature reminiscent of the m a x i m u m of the MR at 16 T (inset of Fig. 1) and of the beginning of the decrease of the spontaneous magnetisation [6]. This new regime can be attributed to the beginning of the disorder of the U atoms.
3. Conclusion The comparison between magnetoresistance and magnetisation results shows a strong coupling between the conduction electrons and the magnetic atoms. The MR
References [1] A. Baran, W. Suski and T. Mydlarz, J. Less-Common Met. 96 (1984) 269. [2] M. Godinho, G. Bonfait, A.P. Gonqalves, M. AJlmeidaand J.C. Spirlet, J, Magn. Magn. Mater. Proc. ICIW94. [3] J. Gal, I. Yaar, D. Regev, S. Fredo, G. Shani, E. Arbaboff, W. Potzel, K. Aggarwal, J.A, Pereda, G.M. Kalvius, F.J. Litterst, W. Sch~iffer and G. Will, Phys. Rev. B 42 (1990) 8507. [4] W. Sch~iffer,G. Will, J. Gal and W. Suski, J. LeSs-Common Mater. 149 (1989) 237. [5] B. Ptasiewicz-Bak, A. Baran, W. Suski and J. Leciejewicz, J. Magn. Magn. Mater. 76-77 (1988) 439. [6] G. Bonfait et al., Phys. Rev. B, to be submitted. [7] I.A. Campbell and A. Fert, in: Ferromagnetic Materials, Vol. 3, ed. E.P. Wohlfarth (North-Holland, Amsterdam, 1982). [8] L. Berger, J. Appl. Phys. 49 (1978) 2156. [9] A. Fert, R. Asomoza, D.H. Sanchez, D. Sp~njaard and A. Friederich, Phys. Rev. B 16 (1977) 5040. [10] H. Yamada and S. Takada, J. Phys, Soc. Japan 48 (1972) 1828.
ELSEVIER
High field magnetoresistance of UFe4A18 G. Bonfait a'*, A.P. Gonqalves a, J.C. Spirlet b, M. Almeida b aDepartamento de Quimica, ICEN-1NETI, P-2686 Sacav~m, Portugal bEC, JRC, Institute for Transuranium Elements, Postfaeh 2340, D-76125 Karlsruhe, Germany
Abstract Magnetoresistance measurements up to 16 T on UFe4A18 single crystal are reported. The results show a strong anisotropic magnetoresistance. The normal magnetoresistance is always negative. No new phase transitioni has been detected up to 16 T.
1. Introduction
2. Experimental results and discussion
The magnetic structure of UFe4Ala has been the source of controversial interpretations mainly due to the difficulty to obtain single crystals and probably also due to a non-trivial magnetic order. UFe4AI8 crystallises in the body centred tetragonal structure of ThMna2, the U atom standing in a cage formed by four Fe atoms. Magnetisation [1, 2] and Mossbauer [3] results showed the existence of a magnetic order at 155 K involving the Fe atoms; neutron powder diffraction [4, 5] showed that, below this temperature, the magnetic lattice is the same as the crystallographic one, but led to contradictory interpretations of the magnetic configuration of the Fe and U atoms. Recently, the obtention of a single crystal allowed us to obtain more information about the magnetic behaviour of this compound: the magnetic moment at low temperature is rather low ( ~ 1.6#B/formula unit) and the c-axis is a hard axis for the magnetisation. Here, we present resistivity and magnetoresistivity (MR) measurements up to 16 T on the same single crystal that was previously used for magnetic measurements [2].
During this experimental run, the sample'h°lder was rotated twice, allowing measurements in two Configurations:
* Corresponding author.
Hlta, ILH(Ib]b), called Pr, - HIIb, llIH(lllb), called PlL-
Due to the crystallographic equivalence b~tween the a and b axes, these two configurations are th¢ standard transverse and parallel configurations for MR]withHllb. The resistivity at zero magnetic field (Fig. 1) shows three different regimes. For T > 155 K, the I resistivity increases when T decreases and this behav]our is attributed to scattering of the conduction electrbns by the spin fluctuations of the magnetic atoms (Feior U). At T¢ = 155 K, a peak in the resistivity clearly ~enotes the magnetic transition. For 100 K < T < 150 K~ the resistivity has a small temperature dependence shgwing that a scattering channel disappeared at T¢. Below1100 K, the resistivity begins to decrease steeply d~wn to a T2-regime below 3 0 K (p ~ Po + 0.016 T2 P-Ocm/K2) which can be interpreted as the effect o! the electron magnon interaction in FM materials. HOwever, the domain walls have an important effect on the resistivity
0921-4526/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 4 ) 0 0 9 6 6 - X
G. Bonfait et al./Physica B 211 (1995) 139-141
140
280 k '
'
'
,
~
. . . .
~1:
:~
.,, 0ot ~
,.-.,, 3 0 0
.
,
,
I
,
~ o
~'~
+ 4.2K
270 O
~L~
.-I
E
.
/
/
Y
.
.
.
.
.
.
.
II bO O
I
250 7~
-o.os~ r ( K )
270
Q.
/
~_
/
.
.
.
I
0 .
.
L j Lq 2K . 4.2K
-0.10 .
240
.
Hal
330
::L
,
.
.
.
.
.
.
.
I
.
.
.
.
.
.
.
.
.
.
.
100 .
.
100
I
200
.
.
....
!
200 .
.
.
.
.
.
0
.
300
T (K) Fig. 1. Resistivityof UFecAI8versus temperature for 0 and 16 T (Hill). The inset shows Ap/p = (p(16 T) - p(O))/p(O). [6]; therefore, further measurements in the single-domain state must be done to confirm this temperature behaviour. A 16 T field lowers the resistivity on the whole temperature range. The three different regimes persist with different characteristics and it can be deduced that the magnetic transition temperature does not significantly change. The T2-regime still exists reinforcing our interpretation of electron magnon scattering and the smaller coefficient (0.010 laf~cm/K2) reflects the higher energy of the magnetic excitations. Ap/p = (p(16 T) p(O))/p(O) reaches a maximum (in absolute value) at 70 K. It is often proposed in this type of compounds that the actinide atoms gradually order at low temperature, a phenomenon that could explain this maximum of magnetoresistance. At low temperature and low fields, the M(H) curves show steps [2], which are very clearly seen as a nonmonotone behaviour in the p(H) curves. This phenomenon, interpreted as an effect of the domain walls, will be described in more detail in an other publication [6]. In this paper, we will focus on the results obtained by a sweep of the field from high value to zero, representing the effect of the magnetic field on the resistivity in the single-domain state. Our results show a strong difference between the parallel (PlI) and transverse (PT) configuration (Fig. 2). This effect anisotropic magnetoresistance (AMR) - can have two different origins [7, 8]. The cross-section for the electron scattering is affected by the spin-orbit coupling and, therefore, becomes dependent of the angle between the current and the magnetisation. In this case, PT is
17.
~\'~kq x ~\~x 230
......
5
10
15
( #o H) [T] Fig. 2. Transverse and parallel magneto-resistivity versus magnetic field at 2 and 4 K (left scale) and 20 K (right scale). The offset between the two scales is to emphasize the very small temperature dependence of the magneto-resistivity between 2 and 20 K.
usually lower than Pll but PT > Pll has already been found in some rare-earth compounds [9]. The second origin is the effect of the strong internal field existing in FM metals which, at low temperature and in pure samples, curves the trajectory of the electrons. This leads to a positive MR when the magnetisation is perpendicular to the electrical current, then in this case PT > PlI" Due to the fact that the AMR subsists up to high temperature in our compound, the first reason seems to be more appropriate to explain our results. These need to be decomposed in two different contributions: one, almost independent of the magnetic field, as indicated by the parallelism between PT and PlI, and the other - always negative - much more dependent of the magnetic field. The T 2-regime for p(T) suggests that the electron-magnon is the source of this negative MR. In this case, Yamada and Takada [10] showed that the resistivity scales as #H/kB T. This is not the case in our results where the curves at 4.2 and 2 K are identical and a field higher than 5 T is needed to obtain a difference between the curve pT(H) at 2 K and 20 K (Fig. 2). Such a temperature-independent behaviour is easier to explain in a simple band magnetism model: the effect of the magnetic field is simply to fill the spin-up band, the decrease of the MR indicating that the Fermi level moves to a region where the density of states is higher. Such an interpretation would explain the increase of ]dPll/dHI at low temperatures for fields higher than 10 T, by a Fermi level reaching an enhanced density of states region. This band description explains the low
G. Bonfait et al./Physica B 211 (1995) 139-141 .
~
.
.
.
I
~
i
i
i
i
I
i
i
•
•
I
,
_210 K
320
E 300 o
0-
280 •
141
must be decomposed in two parts. One, almost field independent at low temperature, causes a high AMR; the other, much more field dependent, indicates either the e l e c t r o n - m a g n o n interaction or a band-filling ieffect. The large and unusual (PT > PlI) A M R needs a careful analysis in order to try to attribute this effect to one of the U or Fe atom. Such an analysis would help tO separate the influence of each of these atoms on the ~ransport properties.
Acknowledgements Work partially supported by a N A T O collaborative Research G r a n t 920996.
260 5
10
15
( oH) [T] Fig. 3. Transverse (full lines) and parallel (dashed lines) magneto-resistivity versus magnetic field. magnetic moment found in the magnetisation measurements at low temperature. A temperature dependence of the p(H) curves appears only for T > 40 K (Fig. 3), temperature reminiscent of the m a x i m u m of the MR at 16 T (inset of Fig. 1) and of the beginning of the decrease of the spontaneous magnetisation [6]. This new regime can be attributed to the beginning of the disorder of the U atoms.
3. Conclusion The comparison between magnetoresistance and magnetisation results shows a strong coupling between the conduction electrons and the magnetic atoms. The MR
References [1] A. Baran, W. Suski and T. Mydlarz, J. Less-Common Met. 96 (1984) 269. [2] M. Godinho, G. Bonfait, A.P. Gonqalves, M. AJlmeidaand J.C. Spirlet, J, Magn. Magn. Mater. Proc. ICIW94. [3] J. Gal, I. Yaar, D. Regev, S. Fredo, G. Shani, E. Arbaboff, W. Potzel, K. Aggarwal, J.A, Pereda, G.M. Kalvius, F.J. Litterst, W. Sch~iffer and G. Will, Phys. Rev. B 42 (1990) 8507. [4] W. Sch~iffer,G. Will, J. Gal and W. Suski, J. LeSs-Common Mater. 149 (1989) 237. [5] B. Ptasiewicz-Bak, A. Baran, W. Suski and J. Leciejewicz, J. Magn. Magn. Mater. 76-77 (1988) 439. [6] G. Bonfait et al., Phys. Rev. B, to be submitted. [7] I.A. Campbell and A. Fert, in: Ferromagnetic Materials, Vol. 3, ed. E.P. Wohlfarth (North-Holland, Amsterdam, 1982). [8] L. Berger, J. Appl. Phys. 49 (1978) 2156. [9] A. Fert, R. Asomoza, D.H. Sanchez, D. Sp~njaard and A. Friederich, Phys. Rev. B 16 (1977) 5040. [10] H. Yamada and S. Takada, J. Phys, Soc. Japan 48 (1972) 1828.