- Email: [email protected]
Magnetic anisotropy and magneto-optical properties of ErLiF4 single crystals
Journal of Magnetism and Magnetic Materials 140-144 (1995) 1193-1194
,.,.
journal of magnetism
~
and
magnetic materials
ELSEVIER
Magnetic anisotropy and magneto-optical properties of ErLiF4 single crystals M. Escorne
a
j. O s t o r 6 r o
a,*
j. G o u z e r h b j . y . G e s l a n d c
a Laboratoire de Chimie M6tallurgique et Spectroscopie des Terres Rares, CNRS, 92195 Meudon Cedex, France b Laboratoire de Magn~tisme et Mat~riaux Magn~tiques, CNRS, 92195 Meudon Cedex, France c Universit£ du Maine Cristallog6n~se, UniversitO du Maine, 72017 Le Mans Cedex, France
Abstract Magnetic susceptibility X, Faraday rotation and optical measurements have been performed on oriented ErLiF4 single crystals. The magnetic properties remain anisotropic in the whole temperature range (1.5 to 1000 K) with the largest X observed for H parallel to the a axis. Wavelength and temperature dependence of the refractive indexes n o and n~ have been investigated. The temperature dependence of the Verdet constants for [001] crystal at the two wavelengths 632.8 and 1152 nm are compared to the corresponding magnetic susceptibility up to 550 K.
YI-x Lnx LiF4 (Ln = rare earth) compounds are interesting materials both for fundamental studies as well as applications such as laser hosts. Most works have been performed on doped materials. We present here the results concerning the magnetic and magneto-optical properties of pure ErLiF4 single crystals. ErLiF4 is a uniaxial compound o having the° tetragonal scheelite structure: a = 5.162 A, c = 10.70 A, space group I 4 1 / a (C46h), Er 3+ ions being located in crystallographic sites of S 4 symmetry. The single crystals of ErLiF4 investigated in this paper were pulled from the melt by the Czochralski technique [1]. Magnetization and magnetic susceptibility measurements were performed on oriented samples cut from the boule, using DSM10 susceptometer, as a function of temperature from 1.5 to 1000 K in a magnetic field up to 20 kOe applied parallel to the a and c crystallographic axis. The temperature dependences of X - I ( T , H = 11 kOe) for H IIa and H [[ c axis are presented in Fig. 1. X is field independent. The experimental accuracy is estimated to be _+ 1-2%. In the 1.5-300 K temperature range, the experimental x ( T ) values are in good agreement with previous results [2]. The magnetic properties remain anisotropic in the whole temperature range with the largest susceptibility observed when H is applied along the a axis. The temperature variation of X~-1 is linear in the 50 to 1000 K range. A least squares fit using the Curie-Weiss law Xa 1= CJ(T-0,,) gives C a = 12.14 mol K emu -1 and 0a = (6.3 ± 0 . 5 ) K. It is to be noted that, below 20 K, 0a
obtained using Eq. (1) is ~ 0 K. The calculated paramagnetic moment is 9.85/XB, which is close to the theoretical value of 9.57 /x B for the free Er 3+ ion. In contrast to X~- l(T), the high temperature (T > 50 K) variation of Xc is slightly nonlinear. The anisotropic character of the magnetic properties is more evidenced in Fig, 2a where we plotted the temperature variation of the product X T. When H is applied parallel to the a axis, xaT saturates when T > 300 K whereas xcT presents a net positive slope. The difference between the two curves is still 7% at 1000 K. According to Hansen et al. [2], the 'infinite temperature' difference between X~-I and X~71 can be calculated using 1
1
Xc
Xa
°
tE. . . . .
'~ 50 [ 0
o
1)
+ 1)gj2
,
(1)
. ,+ c axlso°* o==
.... / a axis
7
I
cx,s aaxis
L
1
500
T(K)
1000
Fig. 1. Temperature dependence of X- 1 for H parallel to a and c axis. Inset: low temperature region.
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD1 0304-8853(94)01591-0
20NtxZ j ( j
100 1-~ 12 ,
0 ~
* Corresponding author. Fax: +33-1-45 07 58 44; email: os [email protected].
9MwB°Ctj(2J+3)(ZJ+
M. Escorne et al. /Journal of Magnetism and Magnetic Materials 140-144 (1995) 1193-I 194
1194 15
0
(a)
'--~
200
a axis
......T < I
00 q 500
500 T ( K ) I000 r T(K) 1000
c m - 1 ( _ _ - - __).
where M w / N is the mass per formula unit, B ° the crystal field parameter at the rare earth site in ErLiF4, a j the Stevens multiplicative factor [3], and gj the Land6 factor. We plotted in Fig. 2b, the experimental A x ( T ) results and the calculated values using Eq. (1) with the values of the B ° parameter, (430___ 50) cm -1, deduced from crystal field theory [2,4-6]. In the high temperature limit, the experimental A X is more in agreement with the lowest value B ° = 380 c m - 1 . The determination of the ordinary refractive index n o was performed on [001] platelet as a function of the wavelength in the 4 0 0 - 2 4 0 0 nm range using the absorption method which is influenced by the different transitions of Er 3÷ (Fig. 3a); n0(A) is given by the envelope (broken curve). The room temperature wavelength dependence of the difference n o - n, perpendicular to the optic axis, and its temperature variation, deduced from the analysis of the elliptic vibration, at the two constant wavelengths of 632.8 and 1152 nm are given in Figs. 3b and 3c. The Faraday rotation measurements of ErLiF4 were performed at two wavelengths: 632.8 and 1152 nm on [001] platelet as a function of temperature (6 K < T < 550 K) and magnetic field up to 20 kOe applied along the c 1.8 0,029
•
600
632.8 nm
Fig. 2. (a) Temperature dependence of xT for the a and c orientations. (b) Temperature variation of Xc 1_ / ~ - 1; the broken lines are calculated Ax (Eq. 1) for B° = 380 ( . . . . . . ) and 500
;{ . "
T ( K )
o
_0
~ -25
0
400
o
] 0,029
F/b)
~ m
T(K)
~
~ -3o-4b/
600~
ErLiF4
"~
\
c axis
1152nm
_60 Fig. 4. (a) Temperature dependence of the reciprocal Verdet constant V-~ at 632.8 and 1152 nm for H parallel to the c axis. (b) Temperature variation of V / X for experimental Vexp (filled symbols) and paramagnetic Vp (empty symbols) Verdet constants. axis. For 632.8 nm, this wavelength is situated on the edge of an absorption peak due to 4115/2 ---->4F9/2 transitions of Er 3+ [4]. At both wavelengths, the experimental FR isotherms q~(H) are linear functions of the magnetic field within experimental accuracy ( + 2 % ) in the whole temperature range. The Verdet constant V(T, A) = dCh/dH is deduced by a least squares fit of the different isotherms. We have plotted in Fig. 4a, the temperature variation of V - 1 for the two wavelengths. The V I(T) curves are not linear like the corresponding magnetic susceptibility X,-1(T) is. In contrast to magnetic susceptibility X values, the diamagnetic contribution to the Verdet constant of a paramagnetic compound is more important. In a first approximation, it can be estimated here by the diamagnetic Verdet constant of LaF 3 [7]. In order to compare V and X, we plotted in Fig. 4b the temperature variation of V/Xc for Vexp and the deduced paramagnetic contribution Vp (assuming Vdia to be temperature independent): Vp/X,. is more temperature independent than V~xp/X,. particularly at 1152 nm, whereas the 632.8 nm data may be influenced by the proximity of the absorption band. Acknowledgement: We thank J.M. Frigerio for his assistance in optical measurement.
1
References \ t" " "0.026 ~'{\ %"# • 400
0.014
k ( nm ) 800
\
16
\~
(a)
0
T(
K ) 300
" •
T=3OOK 1.4 400
~
~
~
~
I 1400
~
i I ~' ( n m )
B 2400
Fig. 3. (a) Room temperature wavelength dependence of the ordinary re#active index; n0(A) is given by the broken line. (b) Wavelength variation of n o - n ~ at 300 K. (c) Temperature dependence of n o - n, at 632.8 and 1152 nm.
[1] K. Rotreau, J.Y. Gesland, P. Daniel and A. Bulou, Mater. Res. Bull. 28 (1993) 813. [2] P.E. Hansen, T. Johansson and R. Nevald, Phys. Rev. B 12 (1975) 5315. [3} M.T. Hutchings, Solid State Phys. 16 (1964) 227. [4] M.R. Brown, K.G. Roots and W.A. Shand, J. Phys. C 2 (1969) 593. [5] N. Karayianis, J. Phys. Chem. Solids 32 (1971) 2385. [6] C.K. Jayasankar, M.F. Reid and F.S. Richardson, Phys. Stat. Sol. (b) 155 (1989) 559. [7] M.J. Weber, Faraday rotator materials, Lawrence Livermore Laboratory Report, June 1982.
,.,.
journal of magnetism
~
and
magnetic materials
ELSEVIER
Magnetic anisotropy and magneto-optical properties of ErLiF4 single crystals M. Escorne
a
j. O s t o r 6 r o
a,*
j. G o u z e r h b j . y . G e s l a n d c
a Laboratoire de Chimie M6tallurgique et Spectroscopie des Terres Rares, CNRS, 92195 Meudon Cedex, France b Laboratoire de Magn~tisme et Mat~riaux Magn~tiques, CNRS, 92195 Meudon Cedex, France c Universit£ du Maine Cristallog6n~se, UniversitO du Maine, 72017 Le Mans Cedex, France
Abstract Magnetic susceptibility X, Faraday rotation and optical measurements have been performed on oriented ErLiF4 single crystals. The magnetic properties remain anisotropic in the whole temperature range (1.5 to 1000 K) with the largest X observed for H parallel to the a axis. Wavelength and temperature dependence of the refractive indexes n o and n~ have been investigated. The temperature dependence of the Verdet constants for [001] crystal at the two wavelengths 632.8 and 1152 nm are compared to the corresponding magnetic susceptibility up to 550 K.
YI-x Lnx LiF4 (Ln = rare earth) compounds are interesting materials both for fundamental studies as well as applications such as laser hosts. Most works have been performed on doped materials. We present here the results concerning the magnetic and magneto-optical properties of pure ErLiF4 single crystals. ErLiF4 is a uniaxial compound o having the° tetragonal scheelite structure: a = 5.162 A, c = 10.70 A, space group I 4 1 / a (C46h), Er 3+ ions being located in crystallographic sites of S 4 symmetry. The single crystals of ErLiF4 investigated in this paper were pulled from the melt by the Czochralski technique [1]. Magnetization and magnetic susceptibility measurements were performed on oriented samples cut from the boule, using DSM10 susceptometer, as a function of temperature from 1.5 to 1000 K in a magnetic field up to 20 kOe applied parallel to the a and c crystallographic axis. The temperature dependences of X - I ( T , H = 11 kOe) for H IIa and H [[ c axis are presented in Fig. 1. X is field independent. The experimental accuracy is estimated to be _+ 1-2%. In the 1.5-300 K temperature range, the experimental x ( T ) values are in good agreement with previous results [2]. The magnetic properties remain anisotropic in the whole temperature range with the largest susceptibility observed when H is applied along the a axis. The temperature variation of X~-1 is linear in the 50 to 1000 K range. A least squares fit using the Curie-Weiss law Xa 1= CJ(T-0,,) gives C a = 12.14 mol K emu -1 and 0a = (6.3 ± 0 . 5 ) K. It is to be noted that, below 20 K, 0a
obtained using Eq. (1) is ~ 0 K. The calculated paramagnetic moment is 9.85/XB, which is close to the theoretical value of 9.57 /x B for the free Er 3+ ion. In contrast to X~- l(T), the high temperature (T > 50 K) variation of Xc is slightly nonlinear. The anisotropic character of the magnetic properties is more evidenced in Fig, 2a where we plotted the temperature variation of the product X T. When H is applied parallel to the a axis, xaT saturates when T > 300 K whereas xcT presents a net positive slope. The difference between the two curves is still 7% at 1000 K. According to Hansen et al. [2], the 'infinite temperature' difference between X~-I and X~71 can be calculated using 1
1
Xc
Xa
°
tE. . . . .
'~ 50 [ 0
o
1)
+ 1)gj2
,
(1)
. ,+ c axlso°* o==
.... / a axis
7
I
cx,s aaxis
L
1
500
T(K)
1000
Fig. 1. Temperature dependence of X- 1 for H parallel to a and c axis. Inset: low temperature region.
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD1 0304-8853(94)01591-0
20NtxZ j ( j
100 1-~ 12 ,
0 ~
* Corresponding author. Fax: +33-1-45 07 58 44; email: os [email protected].
9MwB°Ctj(2J+3)(ZJ+
M. Escorne et al. /Journal of Magnetism and Magnetic Materials 140-144 (1995) 1193-I 194
1194 15
0
(a)
'--~
200
a axis
......T < I
00 q 500
500 T ( K ) I000 r T(K) 1000
c m - 1 ( _ _ - - __).
where M w / N is the mass per formula unit, B ° the crystal field parameter at the rare earth site in ErLiF4, a j the Stevens multiplicative factor [3], and gj the Land6 factor. We plotted in Fig. 2b, the experimental A x ( T ) results and the calculated values using Eq. (1) with the values of the B ° parameter, (430___ 50) cm -1, deduced from crystal field theory [2,4-6]. In the high temperature limit, the experimental A X is more in agreement with the lowest value B ° = 380 c m - 1 . The determination of the ordinary refractive index n o was performed on [001] platelet as a function of the wavelength in the 4 0 0 - 2 4 0 0 nm range using the absorption method which is influenced by the different transitions of Er 3÷ (Fig. 3a); n0(A) is given by the envelope (broken curve). The room temperature wavelength dependence of the difference n o - n, perpendicular to the optic axis, and its temperature variation, deduced from the analysis of the elliptic vibration, at the two constant wavelengths of 632.8 and 1152 nm are given in Figs. 3b and 3c. The Faraday rotation measurements of ErLiF4 were performed at two wavelengths: 632.8 and 1152 nm on [001] platelet as a function of temperature (6 K < T < 550 K) and magnetic field up to 20 kOe applied along the c 1.8 0,029
•
600
632.8 nm
Fig. 2. (a) Temperature dependence of xT for the a and c orientations. (b) Temperature variation of Xc 1_ / ~ - 1; the broken lines are calculated Ax (Eq. 1) for B° = 380 ( . . . . . . ) and 500
;{ . "
T ( K )
o
_0
~ -25
0
400
o
] 0,029
F/b)
~ m
T(K)
~
~ -3o-4b/
600~
ErLiF4
"~
\
c axis
1152nm
_60 Fig. 4. (a) Temperature dependence of the reciprocal Verdet constant V-~ at 632.8 and 1152 nm for H parallel to the c axis. (b) Temperature variation of V / X for experimental Vexp (filled symbols) and paramagnetic Vp (empty symbols) Verdet constants. axis. For 632.8 nm, this wavelength is situated on the edge of an absorption peak due to 4115/2 ---->4F9/2 transitions of Er 3+ [4]. At both wavelengths, the experimental FR isotherms q~(H) are linear functions of the magnetic field within experimental accuracy ( + 2 % ) in the whole temperature range. The Verdet constant V(T, A) = dCh/dH is deduced by a least squares fit of the different isotherms. We have plotted in Fig. 4a, the temperature variation of V - 1 for the two wavelengths. The V I(T) curves are not linear like the corresponding magnetic susceptibility X,-1(T) is. In contrast to magnetic susceptibility X values, the diamagnetic contribution to the Verdet constant of a paramagnetic compound is more important. In a first approximation, it can be estimated here by the diamagnetic Verdet constant of LaF 3 [7]. In order to compare V and X, we plotted in Fig. 4b the temperature variation of V/Xc for Vexp and the deduced paramagnetic contribution Vp (assuming Vdia to be temperature independent): Vp/X,. is more temperature independent than V~xp/X,. particularly at 1152 nm, whereas the 632.8 nm data may be influenced by the proximity of the absorption band. Acknowledgement: We thank J.M. Frigerio for his assistance in optical measurement.
1
References \ t" " "0.026 ~'{\ %"# • 400
0.014
k ( nm ) 800
\
16
\~
(a)
0
T(
K ) 300
" •
T=3OOK 1.4 400
~
~
~
~
I 1400
~
i I ~' ( n m )
B 2400
Fig. 3. (a) Room temperature wavelength dependence of the ordinary re#active index; n0(A) is given by the broken line. (b) Wavelength variation of n o - n ~ at 300 K. (c) Temperature dependence of n o - n, at 632.8 and 1152 nm.
[1] K. Rotreau, J.Y. Gesland, P. Daniel and A. Bulou, Mater. Res. Bull. 28 (1993) 813. [2] P.E. Hansen, T. Johansson and R. Nevald, Phys. Rev. B 12 (1975) 5315. [3} M.T. Hutchings, Solid State Phys. 16 (1964) 227. [4] M.R. Brown, K.G. Roots and W.A. Shand, J. Phys. C 2 (1969) 593. [5] N. Karayianis, J. Phys. Chem. Solids 32 (1971) 2385. [6] C.K. Jayasankar, M.F. Reid and F.S. Richardson, Phys. Stat. Sol. (b) 155 (1989) 559. [7] M.J. Weber, Faraday rotator materials, Lawrence Livermore Laboratory Report, June 1982.