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A non-destructive analysis of the influence of sputter etching on the magnetic surface properties of manganese zinc ferrite
Surface Science 135 (1983)3344340 North-Holland Publishing Company
334
A NON-DESTRUCTIVE ANALYSIS OF THE INFLUENCE ETCHING ON THE MAGNETIC SURFACE PROPERTIES MANGANESE ZINC FERRITE J.W.D.
MARTENS,
W.L. PEETERS
Philips Research Laboratones, Received
3 February
and P.Q.J. NEDERPEL
5600 JA Emdhooen,
1983; accepted
OF SPUTTER OF
for publication
The Netherlunds 15 August
1983
Spectroscopic ellipsometry and the polar magneto-optical Kerr effect (0.6 < hu < 4.5 eV) have been used to study non-destructively the influence of RF sputter etching on the magnetic surface properties of the (100) face of manganese zinc ferrite single crystals. The effect of the sputtering can be described by the formation of a non-magnetic surface layer, which is presumably amorphous.
1. Introduction Single crystalline manganese zinc ferrites in various compositions are widely used in recording heads for video applications. In the production of such recorder heads, RF argon sputter etching is used to clean the ferrite surface prior to the deposition of a thin film (e.g. a glass) which is used to define the non-magnetic gap in the ring type video head. As ion bombardment is known [l] to disturb the atomic order, sputter etching is likely to influence the magnetic properties of the ferrite surface. We therefore studied the influence of RF sputter etching on the magnetic surface properties of manganese zinc ferrite single crystals by means of ellipsometry and the polar magneto-optical Kerr effect.
2. The polar magneto-optical
Kerr effect 121
In the polar Kerr geometry, an external magnetic field is applied normal to the sample surface. Normal incident linearly polarized light is upon reflection converted to elliptically polarized light with the main axis of the ellipse rotated with respect to the polarization of the incident light beam. The complex Kerr effect is defined by the rotation 8, and the ellipticity qk of the reflected light. To first approximation the complex Kerr effect is linear in the magnetization 0039-6028/83/0000-0000/$03.00
0 1983 North-Holland
J. W. D. Martens et al. / Manganese zinc ferrite
335
M and the external field merely serves to orient M perpendicular to the surface. When the sample is cubic, the normal modes for propagation of electromagnetic waves are left and right handed circularly polarized light (LCP and RCP). The Kerr rotation equals half the difference in phase shift between LCP and RCP upon reflection, whereas the Kerr ellipticity equals the ratio of the difference and summation of the amplitude reflection coefficients for LCP and RCP light. The effects are usually very small: the differences in complex refractive index fi = n - ik for LCP and RCP are two orders of magnitude smaller then the average refractive index defined as fi = (fi++z_)/2,
(1)
where the subscripts + and - denote RCP and LCP light respectively. The values of A, were calculated from the complex dielectric constant <, determined by spectroscopic ellipsometry and the Kerr rotation OK and ellipticity 77x using the equation (~,)2=~+(~K+if3K)(~-l)(~)1’2.
(2)
3. Experimental Single crystals of Mn,,,Zn,,, Fe&Fe,O, were grown by the Bridgman technique [3]. Samples of 8 x 16 x 1.5 mm3 were cut with the (100) direction perpendicular to the largest surface. After mechanically polishing, the samples received a final Syton * polishing treatment. Compositional analysis has been performed by electron microprobe and wet chemical analysis. The complex dielectric constant of the Syton polished surface has been determined in the photon energy range 0.6 < hv < 5.4 eV using two ellipsometers with rotating polarizers described in ref. [4]. The polar magneto-optical Kerr rotation and ellipticity have been determined at 0.001’ accuracy in the photon energy range 0.65 < hv -c 4.5 eV using a set-up with a piezobirefringent modulator described elsewhere [5]. A 1200 kA/m magnetic field was used to saturate the samples magnetically. The optical and magneto-optical measurements were performed at room temperature with the samples held in air. Sputter etching has been performed in a Perkin Elmer 2400-8L 8” sputter coater at 200 W RF power and 20 mTorr argon pressure. Two selected samples, having identical Kerr spectra after Syton polishing, were sputtered during 5, 10, 15 and 60 min. * Syton is a brand name of the Monsanto Company.
Fig. 1. The real and imaginary part of the complex polished (100) Mno,Zn, 3Fej;7Fc,0, surface.
Mn
Fig. 2. The Kerr rotation
Ot
Zn
03
Fe’ Fe 0 03 2 L
and ellipticity
dielectric
constant
C= t’-if”
For a Syton
[Xl01
of the Syton polished (100) Mn,,Zn
0.3 Fe”0.3Fe,O,
surface.
J. W. D. Martens
et al. / Manganese zinc
ferrite
337
4. Results The real and the imaginary part of the complex dielectric constant for the Syton polished surface are given in fig. 1. Both the real part C’ as well as the imaginary part E” are relatively smooth curves. The decrease in C” with decreasing photon energy indicates a decrease in optical absorption. Typical values for the optical absorption coefficient cx are 4.8 X lo5 cm-’ at 5 eV and 5.2 x lo4 at 0.6 eV. The absorption thus changes by one order of magnitude when decreasing the photon energy from 5 to 0.6 eV. The Kerr rotation and ellipticity in degrees are given in fig. 2. The results are displayed in single rotations and ellipticities, i.e. OK = [B,(M) OK( - M)]/2, where M is the magnetization. Fig. 3 then shows the Kerr ellipticity for the sputtered surfaces. The sputter time in minutes is indicated. Sputtering changes the Kerr ellipticity only at the high photon energy side of the spectrum. It appears that after some lo-15 min a stationary state is reached, whereafter the Kerr ellipticity changes only slightly.
Mno LZn03Fe;3Fe20L
(100)
Fig. 3. The Kerr ellipticity spectrum for the Syton polished (100) Mn,,4Zn,,,Fe&Fe,0, surface and after 5, 10, 15 and 60 min sputter etching at 200 W RF power and 20 mTorr argon pressure.
J. W. D. Martens
338
5.
et al. / Manganesezinc
/errite
Discussion
The structure observed in the Kerr spectra of fig. 2 are much more pronounced then the optical spectra of fig. 1. From the combination much information can be obtained concerning the atomic origin of these effects. At present however we will use the Kerr effect in a purely phenomenological way. Furthermore, it has been noted that the measurements at high photon energies will be much more surface sensitive when compared to low energy measurements due to the differences in optical absorption. A stratified media calculation has therefore been used to fit the experimental results. in this calculation the effective rotation and ellipticity have been determined from the differences in phase and amplitude for RCP and LCP as indicated above. In the most simple model presented here we assumed one layer on top of an unaltered manganese zinc ferrite. The surface layer has been assumed to be non-magnetic (i.e. to have zero Kerr ellipticity and rotation) thus, using eqs. (1) and (2) fi+= H_= ti. The value of the complex refractive index of the layer has been assumed to be identical to that of the crystalline ferrite as calculated from the dielectric constant given in fig. 1.
0.1
surface assuming Fig. 4. The calculated Kerr ellipticity spectra of the (100) Mn,, Zn,,Fe&Fe,O, a non-magnetic surface layer of 0, 5, 10, 15 and 20 nm thickness with optical properties of crystalline ferrite.
J. W. D. Martens
et al. / Manganese zinc ferrite
339
For this one layer model we calculated the amplitudes for RCP and LCP light while changing the layer thickness between 0 and 20 nm. The resulting amplitudes were used to calculate the Kerr ellipticity as indicated above. The results are given in fig. 4. One observes a change in qk at energies over - 2 eV and at the high energy side an approach to zero which is particularly clear for the 20 nm layer thickness. Physically this indicates that the light gradually probes less of the magnetic ferrite. Thus the effect becomes with increasing thickness more and more determined by the non-magnetic surface layer, resulting in a vanishing of the magneto-optical effects. The 5 and 7.5 nm layer calculations fit the experimental stationary condition best, suggesting that the effect of sputter etching may be described by the formation of a non-magnetic surface layer of 5-7.5 nm thickness with optical properties similar to crystalline manganese zinc ferrite. This however does not necessarily indicate that the surface layer has remained crystalline. For cobalt ferrite, a material with optical properties comparable to manganese zinc ferrite [5], our experiments indicate great similarity in optical properties for the amorphous and crystalline state, justifying the approximation of the optical constants of the surface layer by those of the crystalline bulk material. Physically the layer has been heavily distorted by the sputter etching, destroying both the crystalline and magnetic order over a depth of 5-7.5 nm. This was confirmed by normal incidence Rutherford backscattering experiments of the 60 min sputtered sample. From the amount of displaced Mn and Fe atoms a distorted layer thickness of 6-8 nm was calculated.
6. Conclusions
The combination of spectroscopic ellipsometry and the polar magneto-optical Kerr effect proved to be very useful in a non-destructive in-depth analysis of the magnetic properties of manganese zinc ferrite specifically and magnetic materials in general.
Acknowledgements
We thank J.B. Theeten and M. Erman for the use of their ellipsometers, J.P.M. Damen for supplying the crystals, L. van Oorschot for the sputter etching and Y. Tamminga for the RBS experiments.
340
J. W. D. Martens et al. / Manganese zinc Jerrite
References [l] G. Carter and J.S. Colligon, Ion Bombardment of Solids (Heinemann, London. 1968). [2] For a detailed description of magneto-optical effects see, e.g., L.D. Landau and E.M. Lifshitz, Electrodynamics of Continuous Media (Addison-Wesley, Reading, MA, 1960); P.S. Pershan, J. Appl. Phys. 38 (1967) 1482; M.J. Freiser, IEEE Trans. Magnetics MAC-4 (1968) 152; J.C. Suits, IEEE Trans. Magnetics MAC-8 (1972) 95. [3] Th.J. Berben, D.J. Perduyn and J.P.M. Damen, in: Proc. Intern. Conf. on Ferrites. Japan. 1980. [4] J.B. Theeten. F. Simondet, M. Erman and J. Pernas. Vide. Couches Minces 201 (1980) 1071; M. Erman, Thesis, Orsay University, Paris (1982). [5] J.W.D. Martens, W.L. Peeters, P.Q.J. Nederpel and M. Erman, J. Appl. Phys., submitted.
334
A NON-DESTRUCTIVE ANALYSIS OF THE INFLUENCE ETCHING ON THE MAGNETIC SURFACE PROPERTIES MANGANESE ZINC FERRITE J.W.D.
MARTENS,
W.L. PEETERS
Philips Research Laboratones, Received
3 February
and P.Q.J. NEDERPEL
5600 JA Emdhooen,
1983; accepted
OF SPUTTER OF
for publication
The Netherlunds 15 August
1983
Spectroscopic ellipsometry and the polar magneto-optical Kerr effect (0.6 < hu < 4.5 eV) have been used to study non-destructively the influence of RF sputter etching on the magnetic surface properties of the (100) face of manganese zinc ferrite single crystals. The effect of the sputtering can be described by the formation of a non-magnetic surface layer, which is presumably amorphous.
1. Introduction Single crystalline manganese zinc ferrites in various compositions are widely used in recording heads for video applications. In the production of such recorder heads, RF argon sputter etching is used to clean the ferrite surface prior to the deposition of a thin film (e.g. a glass) which is used to define the non-magnetic gap in the ring type video head. As ion bombardment is known [l] to disturb the atomic order, sputter etching is likely to influence the magnetic properties of the ferrite surface. We therefore studied the influence of RF sputter etching on the magnetic surface properties of manganese zinc ferrite single crystals by means of ellipsometry and the polar magneto-optical Kerr effect.
2. The polar magneto-optical
Kerr effect 121
In the polar Kerr geometry, an external magnetic field is applied normal to the sample surface. Normal incident linearly polarized light is upon reflection converted to elliptically polarized light with the main axis of the ellipse rotated with respect to the polarization of the incident light beam. The complex Kerr effect is defined by the rotation 8, and the ellipticity qk of the reflected light. To first approximation the complex Kerr effect is linear in the magnetization 0039-6028/83/0000-0000/$03.00
0 1983 North-Holland
J. W. D. Martens et al. / Manganese zinc ferrite
335
M and the external field merely serves to orient M perpendicular to the surface. When the sample is cubic, the normal modes for propagation of electromagnetic waves are left and right handed circularly polarized light (LCP and RCP). The Kerr rotation equals half the difference in phase shift between LCP and RCP upon reflection, whereas the Kerr ellipticity equals the ratio of the difference and summation of the amplitude reflection coefficients for LCP and RCP light. The effects are usually very small: the differences in complex refractive index fi = n - ik for LCP and RCP are two orders of magnitude smaller then the average refractive index defined as fi = (fi++z_)/2,
(1)
where the subscripts + and - denote RCP and LCP light respectively. The values of A, were calculated from the complex dielectric constant <, determined by spectroscopic ellipsometry and the Kerr rotation OK and ellipticity 77x using the equation (~,)2=~+(~K+if3K)(~-l)(~)1’2.
(2)
3. Experimental Single crystals of Mn,,,Zn,,, Fe&Fe,O, were grown by the Bridgman technique [3]. Samples of 8 x 16 x 1.5 mm3 were cut with the (100) direction perpendicular to the largest surface. After mechanically polishing, the samples received a final Syton * polishing treatment. Compositional analysis has been performed by electron microprobe and wet chemical analysis. The complex dielectric constant of the Syton polished surface has been determined in the photon energy range 0.6 < hv < 5.4 eV using two ellipsometers with rotating polarizers described in ref. [4]. The polar magneto-optical Kerr rotation and ellipticity have been determined at 0.001’ accuracy in the photon energy range 0.65 < hv -c 4.5 eV using a set-up with a piezobirefringent modulator described elsewhere [5]. A 1200 kA/m magnetic field was used to saturate the samples magnetically. The optical and magneto-optical measurements were performed at room temperature with the samples held in air. Sputter etching has been performed in a Perkin Elmer 2400-8L 8” sputter coater at 200 W RF power and 20 mTorr argon pressure. Two selected samples, having identical Kerr spectra after Syton polishing, were sputtered during 5, 10, 15 and 60 min. * Syton is a brand name of the Monsanto Company.
Fig. 1. The real and imaginary part of the complex polished (100) Mno,Zn, 3Fej;7Fc,0, surface.
Mn
Fig. 2. The Kerr rotation
Ot
Zn
03
Fe’ Fe 0 03 2 L
and ellipticity
dielectric
constant
C= t’-if”
For a Syton
[Xl01
of the Syton polished (100) Mn,,Zn
0.3 Fe”0.3Fe,O,
surface.
J. W. D. Martens
et al. / Manganese zinc
ferrite
337
4. Results The real and the imaginary part of the complex dielectric constant for the Syton polished surface are given in fig. 1. Both the real part C’ as well as the imaginary part E” are relatively smooth curves. The decrease in C” with decreasing photon energy indicates a decrease in optical absorption. Typical values for the optical absorption coefficient cx are 4.8 X lo5 cm-’ at 5 eV and 5.2 x lo4 at 0.6 eV. The absorption thus changes by one order of magnitude when decreasing the photon energy from 5 to 0.6 eV. The Kerr rotation and ellipticity in degrees are given in fig. 2. The results are displayed in single rotations and ellipticities, i.e. OK = [B,(M) OK( - M)]/2, where M is the magnetization. Fig. 3 then shows the Kerr ellipticity for the sputtered surfaces. The sputter time in minutes is indicated. Sputtering changes the Kerr ellipticity only at the high photon energy side of the spectrum. It appears that after some lo-15 min a stationary state is reached, whereafter the Kerr ellipticity changes only slightly.
Mno LZn03Fe;3Fe20L
(100)
Fig. 3. The Kerr ellipticity spectrum for the Syton polished (100) Mn,,4Zn,,,Fe&Fe,0, surface and after 5, 10, 15 and 60 min sputter etching at 200 W RF power and 20 mTorr argon pressure.
J. W. D. Martens
338
5.
et al. / Manganesezinc
/errite
Discussion
The structure observed in the Kerr spectra of fig. 2 are much more pronounced then the optical spectra of fig. 1. From the combination much information can be obtained concerning the atomic origin of these effects. At present however we will use the Kerr effect in a purely phenomenological way. Furthermore, it has been noted that the measurements at high photon energies will be much more surface sensitive when compared to low energy measurements due to the differences in optical absorption. A stratified media calculation has therefore been used to fit the experimental results. in this calculation the effective rotation and ellipticity have been determined from the differences in phase and amplitude for RCP and LCP as indicated above. In the most simple model presented here we assumed one layer on top of an unaltered manganese zinc ferrite. The surface layer has been assumed to be non-magnetic (i.e. to have zero Kerr ellipticity and rotation) thus, using eqs. (1) and (2) fi+= H_= ti. The value of the complex refractive index of the layer has been assumed to be identical to that of the crystalline ferrite as calculated from the dielectric constant given in fig. 1.
0.1
surface assuming Fig. 4. The calculated Kerr ellipticity spectra of the (100) Mn,, Zn,,Fe&Fe,O, a non-magnetic surface layer of 0, 5, 10, 15 and 20 nm thickness with optical properties of crystalline ferrite.
J. W. D. Martens
et al. / Manganese zinc ferrite
339
For this one layer model we calculated the amplitudes for RCP and LCP light while changing the layer thickness between 0 and 20 nm. The resulting amplitudes were used to calculate the Kerr ellipticity as indicated above. The results are given in fig. 4. One observes a change in qk at energies over - 2 eV and at the high energy side an approach to zero which is particularly clear for the 20 nm layer thickness. Physically this indicates that the light gradually probes less of the magnetic ferrite. Thus the effect becomes with increasing thickness more and more determined by the non-magnetic surface layer, resulting in a vanishing of the magneto-optical effects. The 5 and 7.5 nm layer calculations fit the experimental stationary condition best, suggesting that the effect of sputter etching may be described by the formation of a non-magnetic surface layer of 5-7.5 nm thickness with optical properties similar to crystalline manganese zinc ferrite. This however does not necessarily indicate that the surface layer has remained crystalline. For cobalt ferrite, a material with optical properties comparable to manganese zinc ferrite [5], our experiments indicate great similarity in optical properties for the amorphous and crystalline state, justifying the approximation of the optical constants of the surface layer by those of the crystalline bulk material. Physically the layer has been heavily distorted by the sputter etching, destroying both the crystalline and magnetic order over a depth of 5-7.5 nm. This was confirmed by normal incidence Rutherford backscattering experiments of the 60 min sputtered sample. From the amount of displaced Mn and Fe atoms a distorted layer thickness of 6-8 nm was calculated.
6. Conclusions
The combination of spectroscopic ellipsometry and the polar magneto-optical Kerr effect proved to be very useful in a non-destructive in-depth analysis of the magnetic properties of manganese zinc ferrite specifically and magnetic materials in general.
Acknowledgements
We thank J.B. Theeten and M. Erman for the use of their ellipsometers, J.P.M. Damen for supplying the crystals, L. van Oorschot for the sputter etching and Y. Tamminga for the RBS experiments.
340
J. W. D. Martens et al. / Manganese zinc Jerrite
References [l] G. Carter and J.S. Colligon, Ion Bombardment of Solids (Heinemann, London. 1968). [2] For a detailed description of magneto-optical effects see, e.g., L.D. Landau and E.M. Lifshitz, Electrodynamics of Continuous Media (Addison-Wesley, Reading, MA, 1960); P.S. Pershan, J. Appl. Phys. 38 (1967) 1482; M.J. Freiser, IEEE Trans. Magnetics MAC-4 (1968) 152; J.C. Suits, IEEE Trans. Magnetics MAC-8 (1972) 95. [3] Th.J. Berben, D.J. Perduyn and J.P.M. Damen, in: Proc. Intern. Conf. on Ferrites. Japan. 1980. [4] J.B. Theeten. F. Simondet, M. Erman and J. Pernas. Vide. Couches Minces 201 (1980) 1071; M. Erman, Thesis, Orsay University, Paris (1982). [5] J.W.D. Martens, W.L. Peeters, P.Q.J. Nederpel and M. Erman, J. Appl. Phys., submitted.