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Optical and magneto-optical properties of Ce:YAG
Journal of Alloys and Compounds 451 (2008) 146–148
Optical and magneto-optical properties of Ce:YAG M. Kuˇcera ∗ , P. Hasa, J. Hakenov´a Charles University, Faculty of Mathematics & Physics, Ke Karlovu 5, 12116 Prague, Czech Republic Available online 19 April 2007
Abstract Extensive studies of optical absorption and magneto-optical effects (Faraday rotation and MCD) of Ce3+ -doped yttrium aluminum garnet (Ce:YAG) were performed in the range of 4f–5d transitions at temperatures from 4 to 300 K and in magnetic fields up to 7 T. The two spectral bands at 460 and 340 nm originating from the Ce ions have basically different temperature dependence of the magneto-optical effects indicating different ground state properties of involved optical transitions. The results are discussed using the moment analysis of the magneto-optical spectra. © 2007 Elsevier B.V. All rights reserved. PACS: 75.50.Gg; 78.20.Ls Keywords: Ce:YAG; Cerium; Magneto-optics; MCD; Faraday rotation; YAG
1. Introduction
2. Experimental
Novel high performance scintillators are often based on Ce3+ doped complex oxide compounds due to their fast scintillation response, high efficiency and favorable mechanical and chemical properties [1]. The Ce-doped yttrium aluminum garnet, Y3 Al5 O12 , exhibits intense broad-band green emission from crystal-field split 5d states to the ground 4f state. The ground state yields two multiplets, 2 F5/2 and 2 F7/2 , due to the spin–orbit interaction, which are separated by some 0.25 eV in YAG. These multiplets are split in the crystal field of D2 symmetry of dodecahedral Ce sites into Kramers doublets. The excited 5d1 configuration is split by the crystal field into five components [2]. The emission occurs from the lowest 5d crystal-field component to the 2 F5/2,7/2 levels of the ground state. The degeneracy of ground Kramers doublets may be removed by magnetic field. The magneto-optical effects thus represent a sensitive tool to study the details of the structure of involved energy levels. The aim of this work is to shed more light on the ground and excited state properties of Ce3+ ions using the magnetooptical (MO) experiments. In these studies the Faraday rotation (FR), magnetic circular dichroism (MCD), along with the optical absorption and luminescence of Ce:YAG were measured in the range of allowed 4f–5d transitions.
High quality single-crystal samples were grown by the Czochralski method. The samples contained 0.5 wt.% of cerium. Any trace impurities were detected neither in optical absorption nor in luminescence emission experiments. Measurements of luminescence kinetics show on single exponential decay with a time constant of 62 ns. The optical absorption, Faraday rotation and MCD were measured at temperatures between 4 and 300 K and in the spectral range from 250 to 800 nm. The magneto-optical effects were measured in magnetic fields up to 7 T and with angle resolution better than 0.002◦ using the rotating polarizer modulation.
∗
Corresponding author. Tel.: +420 221911 329; fax: +420 224922 797. E-mail address: [email protected] (M. Kuˇcera).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.04.144
3. Results and discussion The spectra of the optical absorption measured at selected temperatures are displayed in Fig. 1. Two broad spectral bands observed near 2.7 eV (460 nm) and 3.65 eV (340 nm) are typical for Ce:YAG. As we can see in Fig. 1 their temperature dependence differs: the oscillator strengths (proportional to band αdω) of the low energy band at 460 nm increase slightly, in about 15%, when temperature is decreased from 300 to 77 K. On the contrary, the oscillator strengths for the high-energy band at 340 nm decrease about 3.5× at this temperature range. However, below 77 K the oscillator strengths of both spectral bands are almost temperature independent. This behavior was discussed in detail by Robbins [3]. He proposed interpretation in terms of transitions between the crystal field split of 4f1 ground and 5d1 excited states of Ce3+ ions in YAG and explained the thermal effects due to changes in the relative populations on the ground state components.
M. Kuˇcera et al. / Journal of Alloys and Compounds 451 (2008) 146–148
Fig. 1. Spectra of the optical absorption coefficient of Ce:YAG (0.5 wt.% of Ce) single crystal measured at various temperatures.
In order to shed more light on the ground and excited state properties of Ce3+ ions we performed the measurements of the magneto-optical Faraday rotation and MCD. Both the FR and MCD are linear in magnetic field up to 7 T. The MO spectra measured at various temperatures are displayed in Fig. 2. The paramagnetic Ce3+ ions induce very high rotations in Ce:YAG samples, especially at low temperatures, which are proportional to the magnetic field. It is worth mentioning, that Ce3+ ions
Fig. 2. Spectra of the specific Faraday rotation (a) and magnetic circular dichroism (b) in Ce:YAG (0.5 wt.% of Ce) single crystal at selected temperatures.
147
induce huge MO effects in number of ferromagnetic and paramagnetic compounds [4,5]. The MO effects were measured with higher spectral resolution compared to the absorption spectra and a fine structure with zero-phonon line at 2.535 eV (489.2 nm) was observed at low temperatures. The complex Faraday effect is defined as ωl φF ≡ θ + iψ = (N+ − N− ) (1) 2c where the real, θ, and imaginary, ψ, parts represent, respectively, the FR and MCD, N± are complex refractive indices for rightand left-hand polarized light, and l is the sample thickness. The FR and MCD are connected through the Kramers–Kronig relations and it is seen in Fig. 2 that the FR has s-shape with zero crossing point at resonance while the MCD has maximum at resonance. The most remarkable feature of the MO spectra is quite different temperature dependence of the two spectral bands: contrary to the absorption, the low energy MO band at 460 nm is strongly temperature dependent. Its intensity increases about two orders of magnitude with temperature decrease towards liquid helium and it has obviously 1/T dependence as will be discussed hereafter. On the other hand, the intensity of the highenergy band at 340 nm decreases with temperature in a similar way as a corresponding absorption band. This behavior suggests on different origin of these two spectral bands in spite of the fact, that both MO bands exhibit the same, so called paramagnetic, spectral shape. Namely, the paramagnetic MO effects originate from the population difference of the ground Kramers doublet split by the magnetic field due to the Boltzmann distribution on involved states. In such a case the intensity of the MO effects is expected to be proportional to the magnetic field and to inverse temperature. As concerned the 340 nm band, its decreasing intensity at low temperatures shows on depopulation of the ground state Kramers doublet for this optical transition. On the other hand, the temperature independent diamagnetic contribution to the MO effects, originating from the split excited states, is expected to be small in view of small Zeeman splitting of these states (of the order of 10−4 eV in applied fields of 1 T). We can infer more specific information on individual contributions to the MO spectra using the moment analysis of the experimental data in Fig. 2. Provided that the Zeeman splitting is small compared to kB T and to the linewidth, the complex Faraday effect can be described by a formula [6]: ∂f (ω) C Ne e2 L2 φF = −A + f (ω) B + Bz (2) me cnε0 ∂ω kT where Ne is concentration of active Ce3+ ions, L2 = (n2 + 2)2 /9 is the correction for a local field, n is index of refraction, Bz is magnetic field. The complex function f(ω) describes the frequency dependencies of the FR and MCD and in a semiclassical approach f (ω) = ω(ω − iΓ0 )/(ω02 − ω2 + Γ02 + i2ωΓ0 ), ω0 is the resonance frequency and Γ 0 the linewidth. The parameters A, B, and C represent, respectively, the diamagnetic, mixing, and paramagnetic contributions to the MO effects and they are proportional to the matrix elements of involved optical transitions [6]. The quantitative estimation of individual A, B, or C terms can be obtained from the integral relations based on the moment
148
M. Kuˇcera et al. / Journal of Alloys and Compounds 451 (2008) 146–148
Fig. 3. Dependence of the zero-th moment, M0 , of the low energy MCD band at 2.7 eV (460 nm) on inverse temperature.
analysis of the MCD spectra. We take advantage of the fact that the MCD spectrum was obtained through the whole spectral band. Then, in view of symmetry properties of the spectral function f(ω), the zero-th and the first moment satisfy relations: ψ C M0 ≡ dω = γ B + (3) kB T band ω ψ γ M1 ≡ (ω − ω0 ) dω = A (4) ω h ¯ band where γ is a constant. These relations enable us to calculate the A, B, and C terms by the numerical integration of particular MCD spectral bands. The calculated zero-th moment M0 of the low energy band at 460 nm as a function of inverse temperature is shown in Fig. 3. The linear fit provides values Bγ = 3.5(3) m−1 T−1 and Cγ/kB = 2420(50) m−1 T−1 . The calculated the first moment M1 is very small for all temperatures, Aγ = 1(1) eV m−1 T−1 . Consequently the paramagnetic term dominates at 460 nm, the values of diamagnetic A and mixing B terms are in several orders of magnitude smaller and are
rather within the accuracy of the experiment. In particular, these findings correlate with the fact that the Zeeman splitting of the excited state is very small and thus the diamagnetic contribution is negligible. Fairly different results were obtained for higher energy band at 340 nm. There is not any obvious dependence of the M0 moment on temperature, likewise the A term cannot be determined since the calculated values of M1 vary with temperature. This shows that the parameter γ in Eqs. (3) and (4) is not constant due to the concentration changes, Ne , with temperature. This supports the suggestion that the decreasing intensity of 340 nm band towards low temperatures is due to the depopulation of the corresponding ground state doublet for this transition in accord with absorption measurements. In conclusion, the moment analysis of the MCD spectra shows on different ground state properties and different selection rules associated with the two particular spectral bands at 2.7 eV (460 nm) and 3.65 eV (340 nm). The crystal field of tetragonal symmetry splits the 2 F5/2 multiplet into three Kramers doublets, the magnitude of the splitting was estimated by Robbins [3] to be ∼0.037 eV. This value is close to the kT ∼0.025 eV for 300 K and hence considerable changes in the relative population of particular states are expected. The low energy band is dominated by the paramagnetic C term in the MO spectra and its intensity is proportional to the ground state magnetic moment. Acknowledgements This work was supported by the Grant agency of the Czech Republic, grant No. 202/05/2471. The authors wish to thank to J. Kvapil for providing the samples. References [1] [2] [3] [4] [5] [6]
M. Nikl, Measur. Sci. Tehnol. 17 (2006) R37. G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer, Berlin, 1994. D.J. Robbins, J. Electrochem. Soc. 126 (1979) 1550. M. Kuˇcera, J. Bok, K. Nitsch, Solid State Commun. 69 (1989) 1117. J. Schoenes, R. Pittini, J. Appl. Phys. 81 (1997) 4853. P.J. Stephens, J. Chem. Phys. 52 (1970) 3489.
Optical and magneto-optical properties of Ce:YAG M. Kuˇcera ∗ , P. Hasa, J. Hakenov´a Charles University, Faculty of Mathematics & Physics, Ke Karlovu 5, 12116 Prague, Czech Republic Available online 19 April 2007
Abstract Extensive studies of optical absorption and magneto-optical effects (Faraday rotation and MCD) of Ce3+ -doped yttrium aluminum garnet (Ce:YAG) were performed in the range of 4f–5d transitions at temperatures from 4 to 300 K and in magnetic fields up to 7 T. The two spectral bands at 460 and 340 nm originating from the Ce ions have basically different temperature dependence of the magneto-optical effects indicating different ground state properties of involved optical transitions. The results are discussed using the moment analysis of the magneto-optical spectra. © 2007 Elsevier B.V. All rights reserved. PACS: 75.50.Gg; 78.20.Ls Keywords: Ce:YAG; Cerium; Magneto-optics; MCD; Faraday rotation; YAG
1. Introduction
2. Experimental
Novel high performance scintillators are often based on Ce3+ doped complex oxide compounds due to their fast scintillation response, high efficiency and favorable mechanical and chemical properties [1]. The Ce-doped yttrium aluminum garnet, Y3 Al5 O12 , exhibits intense broad-band green emission from crystal-field split 5d states to the ground 4f state. The ground state yields two multiplets, 2 F5/2 and 2 F7/2 , due to the spin–orbit interaction, which are separated by some 0.25 eV in YAG. These multiplets are split in the crystal field of D2 symmetry of dodecahedral Ce sites into Kramers doublets. The excited 5d1 configuration is split by the crystal field into five components [2]. The emission occurs from the lowest 5d crystal-field component to the 2 F5/2,7/2 levels of the ground state. The degeneracy of ground Kramers doublets may be removed by magnetic field. The magneto-optical effects thus represent a sensitive tool to study the details of the structure of involved energy levels. The aim of this work is to shed more light on the ground and excited state properties of Ce3+ ions using the magnetooptical (MO) experiments. In these studies the Faraday rotation (FR), magnetic circular dichroism (MCD), along with the optical absorption and luminescence of Ce:YAG were measured in the range of allowed 4f–5d transitions.
High quality single-crystal samples were grown by the Czochralski method. The samples contained 0.5 wt.% of cerium. Any trace impurities were detected neither in optical absorption nor in luminescence emission experiments. Measurements of luminescence kinetics show on single exponential decay with a time constant of 62 ns. The optical absorption, Faraday rotation and MCD were measured at temperatures between 4 and 300 K and in the spectral range from 250 to 800 nm. The magneto-optical effects were measured in magnetic fields up to 7 T and with angle resolution better than 0.002◦ using the rotating polarizer modulation.
∗
Corresponding author. Tel.: +420 221911 329; fax: +420 224922 797. E-mail address: [email protected] (M. Kuˇcera).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.04.144
3. Results and discussion The spectra of the optical absorption measured at selected temperatures are displayed in Fig. 1. Two broad spectral bands observed near 2.7 eV (460 nm) and 3.65 eV (340 nm) are typical for Ce:YAG. As we can see in Fig. 1 their temperature dependence differs: the oscillator strengths (proportional to band αdω) of the low energy band at 460 nm increase slightly, in about 15%, when temperature is decreased from 300 to 77 K. On the contrary, the oscillator strengths for the high-energy band at 340 nm decrease about 3.5× at this temperature range. However, below 77 K the oscillator strengths of both spectral bands are almost temperature independent. This behavior was discussed in detail by Robbins [3]. He proposed interpretation in terms of transitions between the crystal field split of 4f1 ground and 5d1 excited states of Ce3+ ions in YAG and explained the thermal effects due to changes in the relative populations on the ground state components.
M. Kuˇcera et al. / Journal of Alloys and Compounds 451 (2008) 146–148
Fig. 1. Spectra of the optical absorption coefficient of Ce:YAG (0.5 wt.% of Ce) single crystal measured at various temperatures.
In order to shed more light on the ground and excited state properties of Ce3+ ions we performed the measurements of the magneto-optical Faraday rotation and MCD. Both the FR and MCD are linear in magnetic field up to 7 T. The MO spectra measured at various temperatures are displayed in Fig. 2. The paramagnetic Ce3+ ions induce very high rotations in Ce:YAG samples, especially at low temperatures, which are proportional to the magnetic field. It is worth mentioning, that Ce3+ ions
Fig. 2. Spectra of the specific Faraday rotation (a) and magnetic circular dichroism (b) in Ce:YAG (0.5 wt.% of Ce) single crystal at selected temperatures.
147
induce huge MO effects in number of ferromagnetic and paramagnetic compounds [4,5]. The MO effects were measured with higher spectral resolution compared to the absorption spectra and a fine structure with zero-phonon line at 2.535 eV (489.2 nm) was observed at low temperatures. The complex Faraday effect is defined as ωl φF ≡ θ + iψ = (N+ − N− ) (1) 2c where the real, θ, and imaginary, ψ, parts represent, respectively, the FR and MCD, N± are complex refractive indices for rightand left-hand polarized light, and l is the sample thickness. The FR and MCD are connected through the Kramers–Kronig relations and it is seen in Fig. 2 that the FR has s-shape with zero crossing point at resonance while the MCD has maximum at resonance. The most remarkable feature of the MO spectra is quite different temperature dependence of the two spectral bands: contrary to the absorption, the low energy MO band at 460 nm is strongly temperature dependent. Its intensity increases about two orders of magnitude with temperature decrease towards liquid helium and it has obviously 1/T dependence as will be discussed hereafter. On the other hand, the intensity of the highenergy band at 340 nm decreases with temperature in a similar way as a corresponding absorption band. This behavior suggests on different origin of these two spectral bands in spite of the fact, that both MO bands exhibit the same, so called paramagnetic, spectral shape. Namely, the paramagnetic MO effects originate from the population difference of the ground Kramers doublet split by the magnetic field due to the Boltzmann distribution on involved states. In such a case the intensity of the MO effects is expected to be proportional to the magnetic field and to inverse temperature. As concerned the 340 nm band, its decreasing intensity at low temperatures shows on depopulation of the ground state Kramers doublet for this optical transition. On the other hand, the temperature independent diamagnetic contribution to the MO effects, originating from the split excited states, is expected to be small in view of small Zeeman splitting of these states (of the order of 10−4 eV in applied fields of 1 T). We can infer more specific information on individual contributions to the MO spectra using the moment analysis of the experimental data in Fig. 2. Provided that the Zeeman splitting is small compared to kB T and to the linewidth, the complex Faraday effect can be described by a formula [6]: ∂f (ω) C Ne e2 L2 φF = −A + f (ω) B + Bz (2) me cnε0 ∂ω kT where Ne is concentration of active Ce3+ ions, L2 = (n2 + 2)2 /9 is the correction for a local field, n is index of refraction, Bz is magnetic field. The complex function f(ω) describes the frequency dependencies of the FR and MCD and in a semiclassical approach f (ω) = ω(ω − iΓ0 )/(ω02 − ω2 + Γ02 + i2ωΓ0 ), ω0 is the resonance frequency and Γ 0 the linewidth. The parameters A, B, and C represent, respectively, the diamagnetic, mixing, and paramagnetic contributions to the MO effects and they are proportional to the matrix elements of involved optical transitions [6]. The quantitative estimation of individual A, B, or C terms can be obtained from the integral relations based on the moment
148
M. Kuˇcera et al. / Journal of Alloys and Compounds 451 (2008) 146–148
Fig. 3. Dependence of the zero-th moment, M0 , of the low energy MCD band at 2.7 eV (460 nm) on inverse temperature.
analysis of the MCD spectra. We take advantage of the fact that the MCD spectrum was obtained through the whole spectral band. Then, in view of symmetry properties of the spectral function f(ω), the zero-th and the first moment satisfy relations: ψ C M0 ≡ dω = γ B + (3) kB T band ω ψ γ M1 ≡ (ω − ω0 ) dω = A (4) ω h ¯ band where γ is a constant. These relations enable us to calculate the A, B, and C terms by the numerical integration of particular MCD spectral bands. The calculated zero-th moment M0 of the low energy band at 460 nm as a function of inverse temperature is shown in Fig. 3. The linear fit provides values Bγ = 3.5(3) m−1 T−1 and Cγ/kB = 2420(50) m−1 T−1 . The calculated the first moment M1 is very small for all temperatures, Aγ = 1(1) eV m−1 T−1 . Consequently the paramagnetic term dominates at 460 nm, the values of diamagnetic A and mixing B terms are in several orders of magnitude smaller and are
rather within the accuracy of the experiment. In particular, these findings correlate with the fact that the Zeeman splitting of the excited state is very small and thus the diamagnetic contribution is negligible. Fairly different results were obtained for higher energy band at 340 nm. There is not any obvious dependence of the M0 moment on temperature, likewise the A term cannot be determined since the calculated values of M1 vary with temperature. This shows that the parameter γ in Eqs. (3) and (4) is not constant due to the concentration changes, Ne , with temperature. This supports the suggestion that the decreasing intensity of 340 nm band towards low temperatures is due to the depopulation of the corresponding ground state doublet for this transition in accord with absorption measurements. In conclusion, the moment analysis of the MCD spectra shows on different ground state properties and different selection rules associated with the two particular spectral bands at 2.7 eV (460 nm) and 3.65 eV (340 nm). The crystal field of tetragonal symmetry splits the 2 F5/2 multiplet into three Kramers doublets, the magnitude of the splitting was estimated by Robbins [3] to be ∼0.037 eV. This value is close to the kT ∼0.025 eV for 300 K and hence considerable changes in the relative population of particular states are expected. The low energy band is dominated by the paramagnetic C term in the MO spectra and its intensity is proportional to the ground state magnetic moment. Acknowledgements This work was supported by the Grant agency of the Czech Republic, grant No. 202/05/2471. The authors wish to thank to J. Kvapil for providing the samples. References [1] [2] [3] [4] [5] [6]
M. Nikl, Measur. Sci. Tehnol. 17 (2006) R37. G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer, Berlin, 1994. D.J. Robbins, J. Electrochem. Soc. 126 (1979) 1550. M. Kuˇcera, J. Bok, K. Nitsch, Solid State Commun. 69 (1989) 1117. J. Schoenes, R. Pittini, J. Appl. Phys. 81 (1997) 4853. P.J. Stephens, J. Chem. Phys. 52 (1970) 3489.