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Localized and itinerant f electrons in concentrated spin glass systems: Mössbauer, magnetization and neutron diffraction studies
PHYSICA i/Z
Physica B 180 & 181 (1992) 73-75 North-Holland
Localized and itinerant f electrons in concentrated spin glass systems: Miissbauer, magnetization and neutron diffraction studies W. Schafer”,
G. Will”, W. Potzelb,
G.M.
Kalviusb,
I. Yaar’, E. Arbaboff’
and J. Gal’
“Mineralogisches Institut der Universitiit Bonn, Poppelsdorfer SchloJ, D-5300 Bonn 1, Germany hPhysik-Department El5, Technische Universitiit Miinchen, D-8046 Garching bei Miinchen, Germany ‘Ben-Gurion University and Nuclear Research Center of the Negev. 84105 Beer Sheva, Israel
Localized and itinerant behaviour of the f electrons strongly influence the thermodynamic properties of concentrated spin glass systems. This is demonstrated for some AnFe,Al, (An = actinides) and REFe,AI, (RE = rare earths) intermetallic systems using combined neutron diffraction, Miissbauer and AC-DC studies.
1. Introduction The electronic structure and magnetic properties of the light actinides (An) are often compared to the 3d electron transition group and to the localized 4f rare earth (RE) metallic systems. The reason for this comparison lies in the interplay between localized and itinerant (band like) behaviour of the 5f electrons, especially in the intermetallic compounds. Recently we have reported that some An and RE intermetallics show spin glas behaviour [l, 21. In the present contribution our Miissbauer, AC-DC susceptibility and neutron diffraction studies, with and without applied external magnetic fields show that the itinerant or localized f-electron character strongly influences the spin dynamic properties especially in spin glass systems.
switching off
5h after
VI0
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H
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:.
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.
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.
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.
,r l .-. . l -..*
*,
.
*
200
.:
. . .I .
‘G
..
*-
,““““““““‘n’,,“,,“,
600
before switching
2. Results
on H
E
400
.F
2.1. HoFe, Al, - a localized
spin glass
We have reported in several previous publications on the spin glass (SG) behaviour of the REFe,Al, intermetallic system. HoFe,Al, shows unusual SG behaviour. Irreversibilities in the magnetic Bragg scattering with and without applied external magnetic fields derived from neutron diffraction reveal time dependent isothermal and thermoremanent effects, as shown for example in fig. 1. However, the AC susceptibility shows no cusp in the vicinity of the SG temperature T,, , which is unusual for spin glass systems (fig. 2). Further evidence for a SG state in the present system comes from the DC isothermal and thermoremanent magnetization and the 57Me MGssbauer studies. We relate this unusual behaviour to the establishment of a localized SG system. In addition, a new thermodynamical transition from aligned to random 0921.4526/92/$05.00
.
.
. .
0
1992 - Elsevier
Science
Publishers
.:,
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-
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-0
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,
:
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.
:*
.*.*
-. *.*..#. z*
. 8.
*:
..
.
4,: ..
~-n---rrrll-l~~
200
IO
IS
SCATTERING
ANGLE
20 (deg)
Fig. 1. Front part of the HoFe,Al, neutron diffraction pattern under variable experimental conditions: under external magnetic field of 6T (above), 5 h after switching off the external field (middle), and before switching on the external field (below). For information on crystal structure and magnetic reflections, see ref. 3.
spin glass state T,,(H) = 40-100 K was found localized 4f HoFeAl, [l]. The relation To,(H+ holds
B.V. All rights
=)+
T,,(O)
for this localized reserved
9
T,,(H-+O)+O
concentrated
SG system.
in the
W. Schiifer et al. I Localized and itinerant f electrons in concentrated spin glass systems
74
(4
NpFeqAlg
HoFeqAIs
-\_
2OQO
(220) (211)
1000
80
150
II.5
185
200
TEMPERATURE
255
0
1
0
(K)
50
I
I
150
100 TEMPERATURE
Fig. 2. Temperature dependence of AC susceptbility curves of NpFe,Al, (above), UFe,AI, (middle), and HoFe,Al, (below). Contrary to the An compounds, no cusp is observed in HoFe,Al,.
2.2.
Spin glass behaviour
of AnFe,Al,
(An = U, Np)
200
(K)
loo0 r-iiizb)
H = 7T
intermetallics
Combined neutron diffraction, AC and DC susceptibility, and Mossbauer studies of UFe,AI, and NpFe,Al, argue for the establishment of a SG state below T&H+ 0) = 130 K and 120 K, respectively. The SG temperature T,,;(H) decreases with application of an external magnetic field H. The AC susceptibility (x,e) shows a sharp cusp at T,,;(H = 0) (fig. 2). The isothermal and thermoremanent magnetization differ markedly and show temperature dependent irreversibilities below T,,(H). The Mossbauer spectra reveal onset of hyperfine splitting, indicating frozen spins below T,,(O). The neutron diffraction studies of UFe,Al, and NpFe,Al, show that after switching off an external magnetic field greater than 3 T the U and Np moments (2a site in space group 14/mmm) are frozen parallel to the direction of the external field (fig. 3). We label the freezing temperature under high external fields T,,, and show that it serves as the upper limit of T,,(H) with T,,(H+O)+ T,,,;. For concentrated SG systems, T,,;(O) is a reproducible value, independent of the observation time window, which indicates a true thermodynamic phase transition at T,,; and T,,,;. The
0 0
100
200
TEMPERATURE
301D
(K)
Fig. 3. Temperature dependence of integrated neutron diffraction intensities of UFe,AI, without external field (a) and under an applied field of 7T (b). The enhancements of the (1 I 0), (3 1 0), and (3 3 0) reflections are due to an antiferromagnetic order of the Fe site; (2 0 0, 10 I), (2 2 0,2 1 I), and (2 0 2.4 0 0,3 2 1) superimposed reflections belong to the ferromagnetic U site (see ref. [3]). correlation Ts,(H-+O)+
To,;.
holds for an itinerant glasses.
T,,(H+m)-+O
SG system similar to iron 3d spin
75
W. Schtifer et al. I Localized and itinerant f electrons in concentrated spin glass systems 3.
Conclusions
Neutron diffraction with and without applied magnetic field combined with AC and DC and Mossbauer techniques, turns out to be important tools for investigating localized as well as itinerant properties of concentrated spin glass systems.
Acknowledgement This work has been supported by the German Federal Minister for Research and Technology (BMFT)
under contract 03GA2BEE.
no.
03W12BON,
03KA2TUM
and
References [l] J. Gal, I. Yaar, E. Arbaboff, H. Etedgi, F.J. Litterst, K. Aggarwal, J.A. Pereda, G.M. Kalvius, G. Will and W. Schafer, Phys. Rev. B 40 (1989) 745. [2] 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. Schafer and G. Will, Phys. Rev. B 42 (1990) 8507. [3] W. Schafer, G. Will and J. Gal, Eur. J. Solid State Inorg. Chem. 28 (1991) 563.
Physica B 180 & 181 (1992) 73-75 North-Holland
Localized and itinerant f electrons in concentrated spin glass systems: Miissbauer, magnetization and neutron diffraction studies W. Schafer”,
G. Will”, W. Potzelb,
G.M.
Kalviusb,
I. Yaar’, E. Arbaboff’
and J. Gal’
“Mineralogisches Institut der Universitiit Bonn, Poppelsdorfer SchloJ, D-5300 Bonn 1, Germany hPhysik-Department El5, Technische Universitiit Miinchen, D-8046 Garching bei Miinchen, Germany ‘Ben-Gurion University and Nuclear Research Center of the Negev. 84105 Beer Sheva, Israel
Localized and itinerant behaviour of the f electrons strongly influence the thermodynamic properties of concentrated spin glass systems. This is demonstrated for some AnFe,Al, (An = actinides) and REFe,AI, (RE = rare earths) intermetallic systems using combined neutron diffraction, Miissbauer and AC-DC studies.
1. Introduction The electronic structure and magnetic properties of the light actinides (An) are often compared to the 3d electron transition group and to the localized 4f rare earth (RE) metallic systems. The reason for this comparison lies in the interplay between localized and itinerant (band like) behaviour of the 5f electrons, especially in the intermetallic compounds. Recently we have reported that some An and RE intermetallics show spin glas behaviour [l, 21. In the present contribution our Miissbauer, AC-DC susceptibility and neutron diffraction studies, with and without applied external magnetic fields show that the itinerant or localized f-electron character strongly influences the spin dynamic properties especially in spin glass systems.
switching off
5h after
VI0
:
H
14 . . . . . 121 .: .
:.
.’
.
(._
y.:,
.
\
.
,r l .-. . l -..*
*,
.
*
200
.:
. . .I .
‘G
..
*-
,““““““““‘n’,,“,,“,
600
before switching
2. Results
on H
E
400
.F
2.1. HoFe, Al, - a localized
spin glass
We have reported in several previous publications on the spin glass (SG) behaviour of the REFe,Al, intermetallic system. HoFe,Al, shows unusual SG behaviour. Irreversibilities in the magnetic Bragg scattering with and without applied external magnetic fields derived from neutron diffraction reveal time dependent isothermal and thermoremanent effects, as shown for example in fig. 1. However, the AC susceptibility shows no cusp in the vicinity of the SG temperature T,, , which is unusual for spin glass systems (fig. 2). Further evidence for a SG state in the present system comes from the DC isothermal and thermoremanent magnetization and the 57Me MGssbauer studies. We relate this unusual behaviour to the establishment of a localized SG system. In addition, a new thermodynamical transition from aligned to random 0921.4526/92/$05.00
.
.
. .
0
1992 - Elsevier
Science
Publishers
.:,
...
-..*
-
.;*’
‘.
-0
.x*‘.” *. . .
,
:
*
.
:*
.*.*
-. *.*..#. z*
. 8.
*:
..
.
4,: ..
~-n---rrrll-l~~
200
IO
IS
SCATTERING
ANGLE
20 (deg)
Fig. 1. Front part of the HoFe,Al, neutron diffraction pattern under variable experimental conditions: under external magnetic field of 6T (above), 5 h after switching off the external field (middle), and before switching on the external field (below). For information on crystal structure and magnetic reflections, see ref. 3.
spin glass state T,,(H) = 40-100 K was found localized 4f HoFeAl, [l]. The relation To,(H+ holds
B.V. All rights
=)+
T,,(O)
for this localized reserved
9
T,,(H-+O)+O
concentrated
SG system.
in the
W. Schiifer et al. I Localized and itinerant f electrons in concentrated spin glass systems
74
(4
NpFeqAlg
HoFeqAIs
-\_
2OQO
(220) (211)
1000
80
150
II.5
185
200
TEMPERATURE
255
0
1
0
(K)
50
I
I
150
100 TEMPERATURE
Fig. 2. Temperature dependence of AC susceptbility curves of NpFe,Al, (above), UFe,AI, (middle), and HoFe,Al, (below). Contrary to the An compounds, no cusp is observed in HoFe,Al,.
2.2.
Spin glass behaviour
of AnFe,Al,
(An = U, Np)
200
(K)
loo0 r-iiizb)
H = 7T
intermetallics
Combined neutron diffraction, AC and DC susceptibility, and Mossbauer studies of UFe,AI, and NpFe,Al, argue for the establishment of a SG state below T&H+ 0) = 130 K and 120 K, respectively. The SG temperature T,,;(H) decreases with application of an external magnetic field H. The AC susceptibility (x,e) shows a sharp cusp at T,,;(H = 0) (fig. 2). The isothermal and thermoremanent magnetization differ markedly and show temperature dependent irreversibilities below T,,(H). The Mossbauer spectra reveal onset of hyperfine splitting, indicating frozen spins below T,,(O). The neutron diffraction studies of UFe,Al, and NpFe,Al, show that after switching off an external magnetic field greater than 3 T the U and Np moments (2a site in space group 14/mmm) are frozen parallel to the direction of the external field (fig. 3). We label the freezing temperature under high external fields T,,, and show that it serves as the upper limit of T,,(H) with T,,(H+O)+ T,,,;. For concentrated SG systems, T,,;(O) is a reproducible value, independent of the observation time window, which indicates a true thermodynamic phase transition at T,,; and T,,,;. The
0 0
100
200
TEMPERATURE
301D
(K)
Fig. 3. Temperature dependence of integrated neutron diffraction intensities of UFe,AI, without external field (a) and under an applied field of 7T (b). The enhancements of the (1 I 0), (3 1 0), and (3 3 0) reflections are due to an antiferromagnetic order of the Fe site; (2 0 0, 10 I), (2 2 0,2 1 I), and (2 0 2.4 0 0,3 2 1) superimposed reflections belong to the ferromagnetic U site (see ref. [3]). correlation Ts,(H-+O)+
To,;.
holds for an itinerant glasses.
T,,(H+m)-+O
SG system similar to iron 3d spin
75
W. Schtifer et al. I Localized and itinerant f electrons in concentrated spin glass systems 3.
Conclusions
Neutron diffraction with and without applied magnetic field combined with AC and DC and Mossbauer techniques, turns out to be important tools for investigating localized as well as itinerant properties of concentrated spin glass systems.
Acknowledgement This work has been supported by the German Federal Minister for Research and Technology (BMFT)
under contract 03GA2BEE.
no.
03W12BON,
03KA2TUM
and
References [l] J. Gal, I. Yaar, E. Arbaboff, H. Etedgi, F.J. Litterst, K. Aggarwal, J.A. Pereda, G.M. Kalvius, G. Will and W. Schafer, Phys. Rev. B 40 (1989) 745. [2] 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. Schafer and G. Will, Phys. Rev. B 42 (1990) 8507. [3] W. Schafer, G. Will and J. Gal, Eur. J. Solid State Inorg. Chem. 28 (1991) 563.