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Linearly polarized light induced changes of domain structure in cobalt doped YIG films
~ Journal of ELSEVIER
Journal of Magnetism and Magnetic Materials 196-197 (1999) 828-829
mnalnetlsm magnetic materials
Linearly polarized light induced changes of domain structure in cobalt doped YIG films I. Davidenko, A. Maziewski*, A. Stupakiewicz Institute of Physics, UniversiO, of Bialystok, 41, Lipowa, 15-424 Bialystok, Poland
Abstract Experimental and theoretical results of the investigation of photoinduced changes of magnetic domain structures (DSt inside the irradiated region of an epitaxial YIG : Co film are given. Domain reconstruction was studied under argon laser light pulses. Velocity of photoinduced magnetic anisotropy (PMA) growth was much faster than PMA decay velocity. (C' 1999 Elsevier Science B.V. All rights reserved. Keywords." Photomagnetic effects; Photoinduced anisotropy; Garnets
The essence of photoinduced magnetic effects (PME) in yttrium iron garnets with different doping is connected with changes of magnetic anisotropy in the optically irradiated region (review of papers in Ref. [1]). In the present work polarization-sensitive changes of magnetization in YIG : Co films were observed experimentally and described theoretically. Experimental studies were performed on about 10 gm thick YzCalFe3.9Coo.lGelO12 samples grown by liquid-phase epitaxy on (0 0 1) plane of G G G substrate. Magnetic properties of these samples at room temperature are reported in Ref. I-2] : 4tOMs = 90 G, constants of cubic and uniaxial magnetic anisotropy KI = 104 erg/cm 3 and KI: = -- 2.5 x 103 erg/cm 3, respectively. Magnetization in domains was found slightly inclined from the (1 1 1)-type directions. Under decreasing temperature magnetization orientation in a domain goes towards [1 1 0] or [1 i 0] axes. Domain structure images were observed using a lowpower halogen lamp, registrated by CCD camera and digitized by a frame grabber connected to an IBM PC. PME were induced by a linearly polarized argon laser beam (2 = 488 nm, 5 mW) in the stationary option [3] and pulse regime using a Pockels cell. Sample excitation -
-
* Corresponding author. Fax: + 4885-745-72-23;e-mail:[email protected].
was performed by focusing laser beam on to the R = 50 ~un spot on the film surface in the spatial region containing a domain wall (DW) between two "black" and "'white" domains with magnetization in-plane components along [1 1 0] and [1 1 0] directions, respectively (see Fig. lat. The sample was placed in an optical cryostat. The reported measurements were carried out at T = 160 K. Polarization characteristics were investigated in the stationary regime. Magnetization changes occurred through the DW depend shift and the change of its shape was caused by the PMA presence. Amplitude and direction of bending of the DW depend on the state of light polarization (see Fig. lb, Fig. lc and Fig. ldl and illumination time (see Fig. lc and Fig. ld). In tile pulse regime the sample was illuminated with a sequence of polarized (EIJ[1 1 0])light pulses with different durations of light pulse tl and darkness interval t2 (see Fig. 2a). After influence of the pulse series it was observed that the system reaches a stationary state with constant DW placement corresponding to the chosen t~ and t2 values and light intensity. The obtained experimental results were explained within the scope of the PMA model. The physical nature of PMA appearance is related to optical recharge of magnetic anisotropic impurity centers in the crystal lattice. Highly anisotropic ions Co 2+ localized in octahedral sites of garnet lattice play the role of such centers in YIG : Co. Under light excitation electron
0304-8853/99/$ - see front matter t'~ 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 9 5 7 - 3
I. Davidenko et al. /Journal of Magnetism and Magnetic Materials 196-197 (1999) 828-829
lib
-'-
,
829
t2 :!
:
,. t
i
t
Fig. 2. Numerical simulations of PMA field changes under the influence of a sequence of light pulses. The solid line in (b) shows PMA dynamics under pulses defined in (a). Fig. 1. The domain structures images. Initial state (a). After illuminationwith polarization and irradiation time t: Ell [1 i 0], t about 40 s (DS final state) (b). EII [1 1 0], t = 5 s (c). Elk [-1 1 0], t about 40 s (DS final state) (d).
60 ¸
4 0
.o
exchange between ions C o 2 + and Co 3+ via iron ions occurs, due to great separation between cobalt ions for small concentrations. This process could be described by an analogy to photoconductivity, taking into consideration photogeneration, transport and trap of charge cartiers resulting in redistribution of anisotropic impurity centers [4]. The pulse technique [4] makes it possible to determine a time scale for description of growth and decay of photoinduced magnetic anisotropy which could be described by an effective field HL [3]. It is natural to assume, that in the general case, different times zl and z2 characterize the field growth and decay, respectively. It is connected with different physical mechanisms responsible for these processes. After many pulses with pulse duration tl and darkness interval t2 (see Fig. 2a) the light photoinduced anisotropy field approaches the following amplitude: 1 - exp[ - tl/zl] H m~x HL(tl,t2) = 1 -- exp[ -- tx/zxJexp[ -- tz/z2] L
(1)
within the scope of this simplified model, P M A field growth is accomplished according to low H L ~ ( 1 - e x p [ - q / q ] ) and its decay according to HL ~ exp[ - t2/~2]. Numerical simulations are plotted in Fig. 2b. D W begins to move from an equilibrium state when condition HL > Hs is satisfied, Hs is the field of D W start connected with film coercivity [3]. Experimental dependencies of D W shift amplitude b on tt for different t2 are present in Fig. 3. We have extrapolated these dependencies to zero aimed at determination of the start fields HL of D W displacement. HL(t],tz) = Hs, so start times t] of D W displacement could be achieved for different t2. t] were extrapolated from b(tl) dependencies measured for different t2. As
20 ¸
~
0 0
q
I 2
o.2
I 3
I 4
t2 [ms] -t~ [ m s
Fig. 3. Dependencies of DW shift amplitude on tl for different t2 (points-experiment, solid lines-extrapolation). Inset: dependency of t] on t2 for the condition b = 0.
shown in the inset, t](t2) points were theoretically fitted using Eq. (1) and the t] definition. Two orders of magnitude of difference between characteristic times were found: ~2/~1 ~ 200. Experiments were conducted at different temperatures [4]. The technique using light pulses allows to measure directly the P M A field dynamics with separation of the processes of P M A growth and decay. Analysis of temperature dependence of P M A dynamics will be useful for the explanation of the microscopic mechanism of polarization-sensitive PME. The work was partially supported by the Polish grant No. 2P03B 061 14. The authors are grateful to Mr.A.Wasilewicz for his contribution in the construction of our light modulator.
References
[1] E.L Nagaev, Phys. Stat Sol (B) 145 (1988) 11 and references therein. [2] A. Maziewski, J.Magn.Magn.Mater. 88 (1990) 325. [3] A.B. Chizhik, I.I. Davidenko, A. Maziewski, A. Stupakiewicz, Phys. Rev. B 57 (1998) 14366. [4] The work is under preparation for publication.
Journal of Magnetism and Magnetic Materials 196-197 (1999) 828-829
mnalnetlsm magnetic materials
Linearly polarized light induced changes of domain structure in cobalt doped YIG films I. Davidenko, A. Maziewski*, A. Stupakiewicz Institute of Physics, UniversiO, of Bialystok, 41, Lipowa, 15-424 Bialystok, Poland
Abstract Experimental and theoretical results of the investigation of photoinduced changes of magnetic domain structures (DSt inside the irradiated region of an epitaxial YIG : Co film are given. Domain reconstruction was studied under argon laser light pulses. Velocity of photoinduced magnetic anisotropy (PMA) growth was much faster than PMA decay velocity. (C' 1999 Elsevier Science B.V. All rights reserved. Keywords." Photomagnetic effects; Photoinduced anisotropy; Garnets
The essence of photoinduced magnetic effects (PME) in yttrium iron garnets with different doping is connected with changes of magnetic anisotropy in the optically irradiated region (review of papers in Ref. [1]). In the present work polarization-sensitive changes of magnetization in YIG : Co films were observed experimentally and described theoretically. Experimental studies were performed on about 10 gm thick YzCalFe3.9Coo.lGelO12 samples grown by liquid-phase epitaxy on (0 0 1) plane of G G G substrate. Magnetic properties of these samples at room temperature are reported in Ref. I-2] : 4tOMs = 90 G, constants of cubic and uniaxial magnetic anisotropy KI = 104 erg/cm 3 and KI: = -- 2.5 x 103 erg/cm 3, respectively. Magnetization in domains was found slightly inclined from the (1 1 1)-type directions. Under decreasing temperature magnetization orientation in a domain goes towards [1 1 0] or [1 i 0] axes. Domain structure images were observed using a lowpower halogen lamp, registrated by CCD camera and digitized by a frame grabber connected to an IBM PC. PME were induced by a linearly polarized argon laser beam (2 = 488 nm, 5 mW) in the stationary option [3] and pulse regime using a Pockels cell. Sample excitation -
-
* Corresponding author. Fax: + 4885-745-72-23;e-mail:[email protected].
was performed by focusing laser beam on to the R = 50 ~un spot on the film surface in the spatial region containing a domain wall (DW) between two "black" and "'white" domains with magnetization in-plane components along [1 1 0] and [1 1 0] directions, respectively (see Fig. lat. The sample was placed in an optical cryostat. The reported measurements were carried out at T = 160 K. Polarization characteristics were investigated in the stationary regime. Magnetization changes occurred through the DW depend shift and the change of its shape was caused by the PMA presence. Amplitude and direction of bending of the DW depend on the state of light polarization (see Fig. lb, Fig. lc and Fig. ldl and illumination time (see Fig. lc and Fig. ld). In tile pulse regime the sample was illuminated with a sequence of polarized (EIJ[1 1 0])light pulses with different durations of light pulse tl and darkness interval t2 (see Fig. 2a). After influence of the pulse series it was observed that the system reaches a stationary state with constant DW placement corresponding to the chosen t~ and t2 values and light intensity. The obtained experimental results were explained within the scope of the PMA model. The physical nature of PMA appearance is related to optical recharge of magnetic anisotropic impurity centers in the crystal lattice. Highly anisotropic ions Co 2+ localized in octahedral sites of garnet lattice play the role of such centers in YIG : Co. Under light excitation electron
0304-8853/99/$ - see front matter t'~ 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 9 5 7 - 3
I. Davidenko et al. /Journal of Magnetism and Magnetic Materials 196-197 (1999) 828-829
lib
-'-
,
829
t2 :!
:
,. t
i
t
Fig. 2. Numerical simulations of PMA field changes under the influence of a sequence of light pulses. The solid line in (b) shows PMA dynamics under pulses defined in (a). Fig. 1. The domain structures images. Initial state (a). After illuminationwith polarization and irradiation time t: Ell [1 i 0], t about 40 s (DS final state) (b). EII [1 1 0], t = 5 s (c). Elk [-1 1 0], t about 40 s (DS final state) (d).
60 ¸
4 0
.o
exchange between ions C o 2 + and Co 3+ via iron ions occurs, due to great separation between cobalt ions for small concentrations. This process could be described by an analogy to photoconductivity, taking into consideration photogeneration, transport and trap of charge cartiers resulting in redistribution of anisotropic impurity centers [4]. The pulse technique [4] makes it possible to determine a time scale for description of growth and decay of photoinduced magnetic anisotropy which could be described by an effective field HL [3]. It is natural to assume, that in the general case, different times zl and z2 characterize the field growth and decay, respectively. It is connected with different physical mechanisms responsible for these processes. After many pulses with pulse duration tl and darkness interval t2 (see Fig. 2a) the light photoinduced anisotropy field approaches the following amplitude: 1 - exp[ - tl/zl] H m~x HL(tl,t2) = 1 -- exp[ -- tx/zxJexp[ -- tz/z2] L
(1)
within the scope of this simplified model, P M A field growth is accomplished according to low H L ~ ( 1 - e x p [ - q / q ] ) and its decay according to HL ~ exp[ - t2/~2]. Numerical simulations are plotted in Fig. 2b. D W begins to move from an equilibrium state when condition HL > Hs is satisfied, Hs is the field of D W start connected with film coercivity [3]. Experimental dependencies of D W shift amplitude b on tt for different t2 are present in Fig. 3. We have extrapolated these dependencies to zero aimed at determination of the start fields HL of D W displacement. HL(t],tz) = Hs, so start times t] of D W displacement could be achieved for different t2. t] were extrapolated from b(tl) dependencies measured for different t2. As
20 ¸
~
0 0
q
I 2
o.2
I 3
I 4
t2 [ms] -t~ [ m s
Fig. 3. Dependencies of DW shift amplitude on tl for different t2 (points-experiment, solid lines-extrapolation). Inset: dependency of t] on t2 for the condition b = 0.
shown in the inset, t](t2) points were theoretically fitted using Eq. (1) and the t] definition. Two orders of magnitude of difference between characteristic times were found: ~2/~1 ~ 200. Experiments were conducted at different temperatures [4]. The technique using light pulses allows to measure directly the P M A field dynamics with separation of the processes of P M A growth and decay. Analysis of temperature dependence of P M A dynamics will be useful for the explanation of the microscopic mechanism of polarization-sensitive PME. The work was partially supported by the Polish grant No. 2P03B 061 14. The authors are grateful to Mr.A.Wasilewicz for his contribution in the construction of our light modulator.
References
[1] E.L Nagaev, Phys. Stat Sol (B) 145 (1988) 11 and references therein. [2] A. Maziewski, J.Magn.Magn.Mater. 88 (1990) 325. [3] A.B. Chizhik, I.I. Davidenko, A. Maziewski, A. Stupakiewicz, Phys. Rev. B 57 (1998) 14366. [4] The work is under preparation for publication.