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EuH2: A new ferromagnetic semiconductor
Journal of Magnetism and Magnetic Materials 31-34 (1983) 255-256
E u H z: A N E W F E R R O M A G N E T I C R. B I S C H O F , E. K A L D I S
255
SEMICONDUCTOR
a n d P. W A C H T E R
Laboratorium flit FestkSrperphysik, E T H Zfirich, 8093 Ziirich, Switzerland
EuH 2 w a s synthesized from the elements at 500°C. It was found to be ferromagnetic on the basis of susceptibility measurements (0 = 23.13 K, /~ft = 7.94), magnetization (M s = 7.1#a ) and initial susceptibility (T~ = 18.3 K). An optical absorption edge was found at i.85 eV which exhibits a red shift of 0.073 eV upon cooling below the magnetic ordering temperature.
The discovery of the first ferromagnetic semiconductor CrBr a in 1960 by Tsubokawa [1] has opened a new field in magnetism which has developed into one of the most fruitful areas in physics. Since then two main branches of ferromagnetic semiconductors have evolved, those containing divalent Eu, noticibly the Eu chalcogenides and the Cr compounds with mainly the Cd and H g - C r chalcogenides. (For a review of the Eu compounds see refs. [2] and [3].) On the other hand the rare earth hydrides are a field of current interest because these materials are technically important for the storage of hydrogen. E u H 2 was first described by Zanowick and Wallace [4] and was found to be ferromagnetic. However, no information as to its metallic or semiconducting behavior has been
~
6
I
17
~20
~.
18
19 20 Temperat
21
~
"
, , J Slote=.12661~t
given.
We have synthesized polycrystaUine samples of E u H 2 by direct reaction of Eu metal (purity 4N) with (Pd : Ag) diffusion purified hydrogen at 500°C. The reaction took place in a H V tight Cahn balance until the desired stoichiometry was reached. The samples were slowly cooled and the lattice constant and -symmetry were obtained from a D e b y e - S c h e r r e r diagram. The structure is primitive orthorhombic (pnma) with a = 6.247 A, b = 3.805 ,~ and c = 7.196 ,~. In fig. 1 we show the reciprocal magnetic susceptibility which is linear above about 50 K with a slope corresponding to a #eft = 7-948#n, which is to be compared with a theoretical /Lett 7.937 for a J = 7, g = 2 system. The paramagnetic Curie temperature was found to be 0 = 23.13 K. The insert of fig. 1 shows the initial susceptibility measured in an ac field of 5 0 e . Due to the demagnetizing factor the initial susceptibility exhibits a kink at Tc for a ferromagnet with a temperature independent value below T~. Thus T~ is observed to be 18.3 K. The magnetization was measured at 4.2 K. The maxim u m value M s was 7.1/~ a at 90 kOe and the initial slope of the curve corresponds to the demagnetizing factor for a spherical sample. Thus all magnetic measurements point to a classical ferromagnet, where experimental values are in excellent agreement with theory, assuming a spin only divalent Eu ion. =
0304-8853/83/0000-0000/$03.00
0
400
200
300
Temperature (K)
Fig. 1. Inverse and initial (insert) susceptibility of EuH 2.
Regarding the electronic structure an optical re-emission technique was applied using a sample thickness such that macroscopically no light was transmitted through the sample. Thus absorption power equals 1 re-emission power. In fig. 2 the optical absorption is plotted versus the wavelength and an absorption drop in the red p a r t of the spectrum can be observed. The position of the absorption edge is given by the steepest slope [5] of the curve which is at 666 nm or 1.85 eV at 300 K. This measurement establishes for the first time the semiconducting behavior of EuH 2 which can thus be compared with the Eu chalcogenides [2,3]. In 1964 [6] a red shift of the absorption edge of a ferromagnetic semiconductor upon cooling below the magnetic ordering temperature was discovered [3] which follows a spin correlation function. In fig. 3 this red shift is shown with and without an external magnetic field. The spin correlation function has maximum slope at T~ which is also verified experimentally for EuH 2. A magnetic field only enhances the spin alignment near To. The magnitude of the red shift in EuH 2 amounts to 0.073 eV between 0 and 35 K.
© 1983 N o r t h - H o l l a n d
256
R. Bischof et al. / EuH2: a new ferromagnetic semiconductor
Ioo
1.85
g b.I
Absorption Edge 6 6 6 n m = 1.85 eV
~aa
""~
1.90 6C
40
=12 kOe
H=O ~
400
i
1.9~
q200
'
20'00 ' Wovekmgth (nm)
I I
0
~0
I 2o J rc
3'0 Temperoture (K)
Fig. 2. Absorption power of EuH 2 at 300 K.
Fig. 3. Red shift of EuH 2 in zero field and an applied field of 12 kOe.
Thus it appears that EuH 2 is a normal ferromagnetic semiconductor with all its typical features and it should interpolate somewhere in the Eu chalcogenide series. However, this is not the case; for a gap E c of 1.85 eV the Curie temperature is much too high, in fact it is higher than in EuS (16.5 K) which has a gap of only 1.65 eV [2]. Since in the Eu chalcogenides a c a t i o n cation super-exchange via neighboring 5d states is the d o m i n a n t exchange mechanism (see e.g. ref. [2] and references therein) a large E o, i.e. a large 4 f - 5 d separation, yields a low Jl and thus a low T~. The crystal structure of EuH 2 and the arrangement of nearest Eu neighbors and of the anion configuration is very complex. There are 12 nearest Eu neighbors in different and non-symmetric directions around a central Eu with distances ranging between 3.722 ~, and 4.0793 .A, having an average of 3.94 A. These separations are less than 4.4 tk, the critical overlap for b a n d formation of 5d electrons [7] thus a 5d conduction band will exist. The low symmetry of the crystal structure does not permit a simple crystal field analysis into t2g and % functions. Nevertheless the t 2s functions can be arranged to produce a direct overlap between only 10 of the nearest Eu neighbors. As a consequence the 5d b a n d is expected to be narrower than in the Eu chalcogenides in spite of the fact that the average Eu separation is less than in EuS (4.21/k). This is also born out by a formula [2] stating that the red shift AEoc tz/Jta with t the
transfer integral, being proportional to the 5d band width and Jfa the inner atomar exchange. The experimental value of AE for EuH 2 leads to a band width like in EuSe, with E o also being comparble to EuSe. However, since an experimental relation also exists between AE and Ji, the nearest neighbor exchange parameter, [2] A E tx JI( E G ) 2 / J 2, we estimate on this basis J l / k B tx 0.066 K, too small to yield such a high Curie temperature. However, if in molecular field theory J2 is also positive (as in EuO) the observed Curie temperature can be explained. Although this is speculation at the moment, more experimental parameters are needed to understand this new material. References
[1] I. Tsubokawa, J. Phys. Soc. Japan 15 (1960) 1664. [2] P. Wachter, in: Handbook on the Physics and Chemistry of Rare Earths, Vol. 2, eds. K.A. Gschneidner and L. Eyring (North-Holland, Amsterdam, 1979) p. 507. [3] B. Batlogg, E. Kaldis, A. Schlegel and P. Wachter, Phys. Rev. BI2 (1975) 3940. [4] R.L. Zanowick and W.E. Wallace, Phys. Rev. 126 (1962) 537. [5] G. Busch and P. Wachter, Phys. Kondens, Mat. 5 (1966) 232. [6] G. Busch, P. Junod and P. Wachter, Phys. Lett. 12 (1964) 11. [7] J.B. Goodenough, in: Magnetism and the Chemical Bond (J. Wiley, New York, 1963).
E u H z: A N E W F E R R O M A G N E T I C R. B I S C H O F , E. K A L D I S
255
SEMICONDUCTOR
a n d P. W A C H T E R
Laboratorium flit FestkSrperphysik, E T H Zfirich, 8093 Ziirich, Switzerland
EuH 2 w a s synthesized from the elements at 500°C. It was found to be ferromagnetic on the basis of susceptibility measurements (0 = 23.13 K, /~ft = 7.94), magnetization (M s = 7.1#a ) and initial susceptibility (T~ = 18.3 K). An optical absorption edge was found at i.85 eV which exhibits a red shift of 0.073 eV upon cooling below the magnetic ordering temperature.
The discovery of the first ferromagnetic semiconductor CrBr a in 1960 by Tsubokawa [1] has opened a new field in magnetism which has developed into one of the most fruitful areas in physics. Since then two main branches of ferromagnetic semiconductors have evolved, those containing divalent Eu, noticibly the Eu chalcogenides and the Cr compounds with mainly the Cd and H g - C r chalcogenides. (For a review of the Eu compounds see refs. [2] and [3].) On the other hand the rare earth hydrides are a field of current interest because these materials are technically important for the storage of hydrogen. E u H 2 was first described by Zanowick and Wallace [4] and was found to be ferromagnetic. However, no information as to its metallic or semiconducting behavior has been
~
6
I
17
~20
~.
18
19 20 Temperat
21
~
"
, , J Slote=.12661~t
given.
We have synthesized polycrystaUine samples of E u H 2 by direct reaction of Eu metal (purity 4N) with (Pd : Ag) diffusion purified hydrogen at 500°C. The reaction took place in a H V tight Cahn balance until the desired stoichiometry was reached. The samples were slowly cooled and the lattice constant and -symmetry were obtained from a D e b y e - S c h e r r e r diagram. The structure is primitive orthorhombic (pnma) with a = 6.247 A, b = 3.805 ,~ and c = 7.196 ,~. In fig. 1 we show the reciprocal magnetic susceptibility which is linear above about 50 K with a slope corresponding to a #eft = 7-948#n, which is to be compared with a theoretical /Lett 7.937 for a J = 7, g = 2 system. The paramagnetic Curie temperature was found to be 0 = 23.13 K. The insert of fig. 1 shows the initial susceptibility measured in an ac field of 5 0 e . Due to the demagnetizing factor the initial susceptibility exhibits a kink at Tc for a ferromagnet with a temperature independent value below T~. Thus T~ is observed to be 18.3 K. The magnetization was measured at 4.2 K. The maxim u m value M s was 7.1/~ a at 90 kOe and the initial slope of the curve corresponds to the demagnetizing factor for a spherical sample. Thus all magnetic measurements point to a classical ferromagnet, where experimental values are in excellent agreement with theory, assuming a spin only divalent Eu ion. =
0304-8853/83/0000-0000/$03.00
0
400
200
300
Temperature (K)
Fig. 1. Inverse and initial (insert) susceptibility of EuH 2.
Regarding the electronic structure an optical re-emission technique was applied using a sample thickness such that macroscopically no light was transmitted through the sample. Thus absorption power equals 1 re-emission power. In fig. 2 the optical absorption is plotted versus the wavelength and an absorption drop in the red p a r t of the spectrum can be observed. The position of the absorption edge is given by the steepest slope [5] of the curve which is at 666 nm or 1.85 eV at 300 K. This measurement establishes for the first time the semiconducting behavior of EuH 2 which can thus be compared with the Eu chalcogenides [2,3]. In 1964 [6] a red shift of the absorption edge of a ferromagnetic semiconductor upon cooling below the magnetic ordering temperature was discovered [3] which follows a spin correlation function. In fig. 3 this red shift is shown with and without an external magnetic field. The spin correlation function has maximum slope at T~ which is also verified experimentally for EuH 2. A magnetic field only enhances the spin alignment near To. The magnitude of the red shift in EuH 2 amounts to 0.073 eV between 0 and 35 K.
© 1983 N o r t h - H o l l a n d
256
R. Bischof et al. / EuH2: a new ferromagnetic semiconductor
Ioo
1.85
g b.I
Absorption Edge 6 6 6 n m = 1.85 eV
~aa
""~
1.90 6C
40
=12 kOe
H=O ~
400
i
1.9~
q200
'
20'00 ' Wovekmgth (nm)
I I
0
~0
I 2o J rc
3'0 Temperoture (K)
Fig. 2. Absorption power of EuH 2 at 300 K.
Fig. 3. Red shift of EuH 2 in zero field and an applied field of 12 kOe.
Thus it appears that EuH 2 is a normal ferromagnetic semiconductor with all its typical features and it should interpolate somewhere in the Eu chalcogenide series. However, this is not the case; for a gap E c of 1.85 eV the Curie temperature is much too high, in fact it is higher than in EuS (16.5 K) which has a gap of only 1.65 eV [2]. Since in the Eu chalcogenides a c a t i o n cation super-exchange via neighboring 5d states is the d o m i n a n t exchange mechanism (see e.g. ref. [2] and references therein) a large E o, i.e. a large 4 f - 5 d separation, yields a low Jl and thus a low T~. The crystal structure of EuH 2 and the arrangement of nearest Eu neighbors and of the anion configuration is very complex. There are 12 nearest Eu neighbors in different and non-symmetric directions around a central Eu with distances ranging between 3.722 ~, and 4.0793 .A, having an average of 3.94 A. These separations are less than 4.4 tk, the critical overlap for b a n d formation of 5d electrons [7] thus a 5d conduction band will exist. The low symmetry of the crystal structure does not permit a simple crystal field analysis into t2g and % functions. Nevertheless the t 2s functions can be arranged to produce a direct overlap between only 10 of the nearest Eu neighbors. As a consequence the 5d b a n d is expected to be narrower than in the Eu chalcogenides in spite of the fact that the average Eu separation is less than in EuS (4.21/k). This is also born out by a formula [2] stating that the red shift AEoc tz/Jta with t the
transfer integral, being proportional to the 5d band width and Jfa the inner atomar exchange. The experimental value of AE for EuH 2 leads to a band width like in EuSe, with E o also being comparble to EuSe. However, since an experimental relation also exists between AE and Ji, the nearest neighbor exchange parameter, [2] A E tx JI( E G ) 2 / J 2, we estimate on this basis J l / k B tx 0.066 K, too small to yield such a high Curie temperature. However, if in molecular field theory J2 is also positive (as in EuO) the observed Curie temperature can be explained. Although this is speculation at the moment, more experimental parameters are needed to understand this new material. References
[1] I. Tsubokawa, J. Phys. Soc. Japan 15 (1960) 1664. [2] P. Wachter, in: Handbook on the Physics and Chemistry of Rare Earths, Vol. 2, eds. K.A. Gschneidner and L. Eyring (North-Holland, Amsterdam, 1979) p. 507. [3] B. Batlogg, E. Kaldis, A. Schlegel and P. Wachter, Phys. Rev. BI2 (1975) 3940. [4] R.L. Zanowick and W.E. Wallace, Phys. Rev. 126 (1962) 537. [5] G. Busch and P. Wachter, Phys. Kondens, Mat. 5 (1966) 232. [6] G. Busch, P. Junod and P. Wachter, Phys. Lett. 12 (1964) 11. [7] J.B. Goodenough, in: Magnetism and the Chemical Bond (J. Wiley, New York, 1963).