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Magnetic properties of single crystalline NpAs-NpSe mixed compounds
Journal of Magnetism and Magnetic Materials 104-107 (1992) 43-44 North-Holland
AI4/
Magnetic properties of single crystalline NpAs-NpSe mixed compounds K. Mattenberger a, O. Vogt a, j. Rebizant b, J.C. Spirlet b, F. Bourdarot c, P. Burlet c, j. Rossat-Mignod c, M.N. Bouillet c, A. Blaise c and J.P. Sanchez c a Laboratorium fiir Festkdrperphysik ETHZ, CH-8093 Ziirich, Switzerland b CEC, IRC, European Institute for Transuranium Elements, W-7500 Karlsruhe, Germany c CEN Grenoble, DRFMC/SPSMSMDN 85X, F-38041 Grenoble Cedex, France The antiferromagnetic 3k-structure of NpAs transformes to a ferrimagnetic one in magnetic fields < 60 kOe. The magnetic moment of the Np ion as determined by neutrons is 2.5~B. NpSe becomes antiferromagnetic below 38 K with a not yet resolved magnetic structure. The moment is only 1.35/xB. In samples of NpAs mixed with 5, 10 and 15% NpSe we observed ferrimagnetic structures. The structure of the 5% sample was determined using neutrons and is described in detail. Admixture of NpSe increases the ferromagnetic exchange forces and reduces the ionic magnetic moment. The moment reduction is clearly evidenced by M6ssbauer experiments. The exchange forces are anisotropic. Antiferromagnetic exchange favours the (100) direction, ferromagnetic exchange aligns the spins along (111 ). NpAs at 4.2 K is a 3k antiferromagnet. By applying magnetic fields higher than 60 kOe the antiferromagnetic spin structure is broken up and a ferrimagnetic structure with a net moment of 1.6/, B per Np ion is induced. This net moment is definitely lower than the free-ion value g J = 2.57. For the ordered moment a value of 2.5/x~ is found using neutrons. The complete magnetic phase diagram M ( T , H ) is today well established [1]. The N6el temperature is 175 K and a paramagnetic Curie temperature of 184 K was derived from the C u r i e - W e i s s plot [2]. NpSe is a type II antiferromagnet with the Fourier component perpendicular to k below 38 k. A multi-k ordering seems probable [3]. The ionic moment, measured by M6ssbauer experiments, is about 1.35p~B, much lower than gJ = 2.57, indicating a strong influence of the electronic environment on the trivalent Np ion. It is evident that a combination of magnetically so different compounds should lead to interesting magnetic properties. Therefore, we started an investigation of mixed crystals of NpAs and NpSe. We have grown single crystals with weights of a few mg of NpAs mixed with 5, 10 and 15% NpSe. The magnetization of the X-ray oriented crystals was measured as a function of applied field and temperature. The C u r i e - W e i s s law is obeyed for all compositions. The resulting effective number of magnetons is close to the free-ion value g ( J ( J + 1)) t/2 = 2.87. The paramagnetic Curie temperature 610 decreases monotonously from 185 K to reach 172 K for the 15% sample. The paramagnetic susceptibility was found to be isotropic. At 4.2 K we observed hysteresis loops in all our samples. The axis of maximum magnetization (easy axis) was found to be the (110) direction. Yet in all cases the obtained value (2/z B for the 5% NpSe sam-
pie, 1.8p. B for the 15% NpSe sample) is well below the expected 2.57/x B. Thus, it seems reasonable to argue that in the three N p A s - N p S e compositions investigated the magnetic order at 4.2 K is ferri- and not ferromagnetic. The broadness of the hysteresis loops increases with increasing NpSe concentration while the anisotropy and magnetic moments decrease slightly. Fortunately, one crystal, namely the 5% NpSe sample, was large enough to allow for neutron diffraction studies which are summarized in fig. 1. Indeed, we note a ferrimagnetic structure at low temperature which is followed by ferromagnetism between 131 and 147 K and an incommensurate structure above 147 K up to the N6el temperature. The ferrimagnetic structure at low temperature is the resultant of ferromagnetically ordered moments along (111) and antiferromagnetically ordered moments along (100). Fig. 2 visualizes this non-collinear ferrimagnetic structure [4].
"1-
V
5
.O
m
NpAso.95Seoo5
>, 6
~_
]-
Ferromagnetic -
4
~
-
-
H = 0
. . . . . . .
[111 X//4 ~
To= 131K TIc= 147 K z~ [k,2,0] o [2,k,0] v [2,O,k]
3 ¢~
~0 ¢) --
Type I ~
0
, Ferri
,
i 50
i17o
[011].
[
100
MFerr° = 215~B MAF
T -
0 78 PB
2 I"
'['(K)
Inc
Fig. 1. Neutron diffraction on NpAso.95Seo.05.
0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
44
K. Mattenberger et al. / Magnetic properties of NpAs-NpSe
Fig. 2. Non-collinear ferrimagnetic structure of NpAso.~sSeo.~5.
Since large enough crystals were not available for the other concentrations, we have measured the temperature dependence of the magnetization at high fields. The data for the samples with 5 and 10% NpSe show that the (111 ) magnetization deviates from normal ferromagnetic behaviour already above 150 K another indication that at low temperatures we are encountering a ferri- rather than a ferromagnetic structure. In the 15% sample a phase transition occurs at a much lower temperature, namely about 75 K. Our experiments have shown that this transition does not depend critically on the strength of the applied external field (fig. 3). The conclusion that this sample becomes ferromagnetic above 75 K seems legitimate.
].611 S- ....
~,%, NpAs'85:,~:,Se'15q
,o
o < 100>
References
+ <110> ~' ~,
' * 2,. "),T
08 I
H = 20 kOe
*<111>
' J ",
'~
O.4 ~ Q
I
(}
"
100
2(X)
The 4.2. K M6ssbauer spectra of the As-rich samples are well represented with a single Np site experiencing combined magnetic and quadrupolar interactions. For pure NpSe the lines are broad, thus indicating that the Np ions experience different hyperfine fields. The isomer shift and the negative signs of the induced quadrupole interaction clearly indicate the occurrence of Np 3~ ions in the whole concentration range. The hyperfine field exhibits a maximum at a concentration of 5% NpSe. The field increase ( ~ 300 kG) observed when substituting with 5% Se is attributed to a magnetic structure change. The smooth decrease of the hyperfine field when the Se content is further increased is to be correlated with a decrease of the ordered moments. They remain close to the Np 3+ free-ion value (2.5/x B) up to 15% NpSe while for pure NpSe the moment is only 1.35#B. By combining all the experimental results we conclude: addition of NpSe to NpAs induces an initial increase of the ferromagnetic exchange forces and a decrease of the antiferromagnetic exchange. At the same time the ionic magnetic moment decreases. Exchange forces are anisotropic - antiferromagnetic exchange prefers the (100) direction whereas ferromagnetic exchange favours the (111) direction. Thus inducing by mixing a transition from antiferromagnetism to ferromagnetism, the system gets into a frustrated state. Depending on the temperature, pure ferromagnetism along (111) or a superposition of antiferromagnetism along (100) and ferromagnetism leading to a ferrimagnetic structure is observed. The fact that exchange forces are anisotropic and depend critically on the electronic environment is a strong indication that the magnetic behaviour of the investigated compounds is determined by hybridization.
~00
Temperature [ K I
Fig. 3. Magnetization versus temperature of NpAso.85Seo.is,
[1] P. Burlet, J.M. Fournier, E. Pleska, S. Qudzel, J. RossatMignod, J.C. Spirlet, J. Rebizant and O. Vogt, J. de Phys. 49 (1988) C8-469. [2] J. Rebizant, J.C. Spirlet, K. Mattenberger and O. Vogt, 14~mes Journe6s des Actinides, Davos, Switzerland (1984). [3] P. Burlet, A. Blaise, M.N. Bouillet, F. Bourdarot, J. Rossat-Mignod, J.P. Sanchez, J. Rebizant, J.C. Spirlet and O. Vogt, 21~mes Journ~es des Actinides, Montechoro, Portugal (1991), [4] E. Pleska, Phil. Thesis Univ. Joseph Fourier, Grenoble, France (1990) unpublished.
AI4/
Magnetic properties of single crystalline NpAs-NpSe mixed compounds K. Mattenberger a, O. Vogt a, j. Rebizant b, J.C. Spirlet b, F. Bourdarot c, P. Burlet c, j. Rossat-Mignod c, M.N. Bouillet c, A. Blaise c and J.P. Sanchez c a Laboratorium fiir Festkdrperphysik ETHZ, CH-8093 Ziirich, Switzerland b CEC, IRC, European Institute for Transuranium Elements, W-7500 Karlsruhe, Germany c CEN Grenoble, DRFMC/SPSMSMDN 85X, F-38041 Grenoble Cedex, France The antiferromagnetic 3k-structure of NpAs transformes to a ferrimagnetic one in magnetic fields < 60 kOe. The magnetic moment of the Np ion as determined by neutrons is 2.5~B. NpSe becomes antiferromagnetic below 38 K with a not yet resolved magnetic structure. The moment is only 1.35/xB. In samples of NpAs mixed with 5, 10 and 15% NpSe we observed ferrimagnetic structures. The structure of the 5% sample was determined using neutrons and is described in detail. Admixture of NpSe increases the ferromagnetic exchange forces and reduces the ionic magnetic moment. The moment reduction is clearly evidenced by M6ssbauer experiments. The exchange forces are anisotropic. Antiferromagnetic exchange favours the (100) direction, ferromagnetic exchange aligns the spins along (111 ). NpAs at 4.2 K is a 3k antiferromagnet. By applying magnetic fields higher than 60 kOe the antiferromagnetic spin structure is broken up and a ferrimagnetic structure with a net moment of 1.6/, B per Np ion is induced. This net moment is definitely lower than the free-ion value g J = 2.57. For the ordered moment a value of 2.5/x~ is found using neutrons. The complete magnetic phase diagram M ( T , H ) is today well established [1]. The N6el temperature is 175 K and a paramagnetic Curie temperature of 184 K was derived from the C u r i e - W e i s s plot [2]. NpSe is a type II antiferromagnet with the Fourier component perpendicular to k below 38 k. A multi-k ordering seems probable [3]. The ionic moment, measured by M6ssbauer experiments, is about 1.35p~B, much lower than gJ = 2.57, indicating a strong influence of the electronic environment on the trivalent Np ion. It is evident that a combination of magnetically so different compounds should lead to interesting magnetic properties. Therefore, we started an investigation of mixed crystals of NpAs and NpSe. We have grown single crystals with weights of a few mg of NpAs mixed with 5, 10 and 15% NpSe. The magnetization of the X-ray oriented crystals was measured as a function of applied field and temperature. The C u r i e - W e i s s law is obeyed for all compositions. The resulting effective number of magnetons is close to the free-ion value g ( J ( J + 1)) t/2 = 2.87. The paramagnetic Curie temperature 610 decreases monotonously from 185 K to reach 172 K for the 15% sample. The paramagnetic susceptibility was found to be isotropic. At 4.2 K we observed hysteresis loops in all our samples. The axis of maximum magnetization (easy axis) was found to be the (110) direction. Yet in all cases the obtained value (2/z B for the 5% NpSe sam-
pie, 1.8p. B for the 15% NpSe sample) is well below the expected 2.57/x B. Thus, it seems reasonable to argue that in the three N p A s - N p S e compositions investigated the magnetic order at 4.2 K is ferri- and not ferromagnetic. The broadness of the hysteresis loops increases with increasing NpSe concentration while the anisotropy and magnetic moments decrease slightly. Fortunately, one crystal, namely the 5% NpSe sample, was large enough to allow for neutron diffraction studies which are summarized in fig. 1. Indeed, we note a ferrimagnetic structure at low temperature which is followed by ferromagnetism between 131 and 147 K and an incommensurate structure above 147 K up to the N6el temperature. The ferrimagnetic structure at low temperature is the resultant of ferromagnetically ordered moments along (111) and antiferromagnetically ordered moments along (100). Fig. 2 visualizes this non-collinear ferrimagnetic structure [4].
"1-
V
5
.O
m
NpAso.95Seoo5
>, 6
~_
]-
Ferromagnetic -
4
~
-
-
H = 0
. . . . . . .
[111 X//4 ~
To= 131K TIc= 147 K z~ [k,2,0] o [2,k,0] v [2,O,k]
3 ¢~
~0 ¢) --
Type I ~
0
, Ferri
,
i 50
i17o
[011].
[
100
MFerr° = 215~B MAF
T -
0 78 PB
2 I"
'['(K)
Inc
Fig. 1. Neutron diffraction on NpAso.95Seo.05.
0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
44
K. Mattenberger et al. / Magnetic properties of NpAs-NpSe
Fig. 2. Non-collinear ferrimagnetic structure of NpAso.~sSeo.~5.
Since large enough crystals were not available for the other concentrations, we have measured the temperature dependence of the magnetization at high fields. The data for the samples with 5 and 10% NpSe show that the (111 ) magnetization deviates from normal ferromagnetic behaviour already above 150 K another indication that at low temperatures we are encountering a ferri- rather than a ferromagnetic structure. In the 15% sample a phase transition occurs at a much lower temperature, namely about 75 K. Our experiments have shown that this transition does not depend critically on the strength of the applied external field (fig. 3). The conclusion that this sample becomes ferromagnetic above 75 K seems legitimate.
].611 S- ....
~,%, NpAs'85:,~:,Se'15q
,o
o < 100>
References
+ <110> ~' ~,
' * 2,. "),T
08 I
H = 20 kOe
*<111>
' J ",
'~
O.4 ~ Q
I
(}
"
100
2(X)
The 4.2. K M6ssbauer spectra of the As-rich samples are well represented with a single Np site experiencing combined magnetic and quadrupolar interactions. For pure NpSe the lines are broad, thus indicating that the Np ions experience different hyperfine fields. The isomer shift and the negative signs of the induced quadrupole interaction clearly indicate the occurrence of Np 3~ ions in the whole concentration range. The hyperfine field exhibits a maximum at a concentration of 5% NpSe. The field increase ( ~ 300 kG) observed when substituting with 5% Se is attributed to a magnetic structure change. The smooth decrease of the hyperfine field when the Se content is further increased is to be correlated with a decrease of the ordered moments. They remain close to the Np 3+ free-ion value (2.5/x B) up to 15% NpSe while for pure NpSe the moment is only 1.35#B. By combining all the experimental results we conclude: addition of NpSe to NpAs induces an initial increase of the ferromagnetic exchange forces and a decrease of the antiferromagnetic exchange. At the same time the ionic magnetic moment decreases. Exchange forces are anisotropic - antiferromagnetic exchange prefers the (100) direction whereas ferromagnetic exchange favours the (111) direction. Thus inducing by mixing a transition from antiferromagnetism to ferromagnetism, the system gets into a frustrated state. Depending on the temperature, pure ferromagnetism along (111) or a superposition of antiferromagnetism along (100) and ferromagnetism leading to a ferrimagnetic structure is observed. The fact that exchange forces are anisotropic and depend critically on the electronic environment is a strong indication that the magnetic behaviour of the investigated compounds is determined by hybridization.
~00
Temperature [ K I
Fig. 3. Magnetization versus temperature of NpAso.85Seo.is,
[1] P. Burlet, J.M. Fournier, E. Pleska, S. Qudzel, J. RossatMignod, J.C. Spirlet, J. Rebizant and O. Vogt, J. de Phys. 49 (1988) C8-469. [2] J. Rebizant, J.C. Spirlet, K. Mattenberger and O. Vogt, 14~mes Journe6s des Actinides, Davos, Switzerland (1984). [3] P. Burlet, A. Blaise, M.N. Bouillet, F. Bourdarot, J. Rossat-Mignod, J.P. Sanchez, J. Rebizant, J.C. Spirlet and O. Vogt, 21~mes Journ~es des Actinides, Montechoro, Portugal (1991), [4] E. Pleska, Phil. Thesis Univ. Joseph Fourier, Grenoble, France (1990) unpublished.