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Radicals as EPR probes of magnetization of gadolinium stearate Langmuir–Blodgett film
Materials Science and Engineering C 22 (2002) 201 – 207 www.elsevier.com/locate/msec
Radicals as EPR probes of magnetization of gadolinium stearate Langmuir–Blodgett film Yu.A. Koksharov a,*, I.V. Bykov b, A.P. Malakho c, S.N. Polyakov d, G.B. Khomutov a, J. Bohr e a
Faculty of Physics, Moscow State University, 119992 Moscow, Russia Vernadski Institute of Geochemistry and Analytical Chemistry RAS, Moscow, Russia c Department of Material Science, Moscow State University, 119992 Moscow, Russia d Institute of Nuclear Physics, Moscow State University, Moscow, Russia e Department of Physics, Technical University of Denmark, Lyngby, Denmark
b
Abstract In the present work we have applied the method of the EPR spin probes which allows performing simultaneously EPR and magnetization measurements to the investigation of magnetism of the Gd stearate Langmuir – Blodgett (LB) films. For this purpose we have prepared and studied by the EPR technique the Gd and Y stearate LB films. Placing the small BDPA crystal on the film surface we have found that for the Gd LB sample the effective g-value of the radical’s resonance depends on the film orientation in respect to the external magnetic field direction. The relative shift of the EPR signal corresponded to the magnetization of the film along the field direction. Such effect has not been observed for the Y stearate LB film. The data obtained give an experimental proof for the room temperature magnetic ordering in the gadolinium stearate LB film. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Rare-earth metal; Langmuir – Blodgett films; Thin magnetic films; EPR; Spin probes; Magnetization; 2-D magnetism
1. Introduction Langmuir– Blodgett (LB) technique can give ultra thin mono- and multilayer molecular films with monatomic layers of magnetic atoms and interesting magnetic properties [1 –4]. The high sensitivity of the electron paramagnetic resonance (EPR) spectroscopy makes this method rather effective for study magnetism in the thin films [5]. Because of very small amount of magnetic atoms present in LB film special efforts are required to detect the EPR signal. Usually, thin (about 100 Am) polymer substrates, like mylar (polyethylene terephtalate) sheets [2,3], are used for EPR measurements in order to have the possibility to place several pieces of the LB film into the active resonance cavity. However, in magnetically ordered films the linewidth of the EPR line could be so large that the signal peak-to-peak height becomes negligible. In that case the conventional EPR measurements are non-informative. In order to break the EPR ‘‘silence’’ the LB film could be doped or decorated by spin probes. Now the method of spin probes has become * Corresponding author. Tel.: +7-95-9392973; fax: +7-95-9391489. E-mail address: [email protected] (Yu.A. Koksharov).
customary for researchers working in different areas from the solid-state physics to the biophysics [6 –9]. In the present work we have applied the method of the EPR spin probes to the investigation of magnetism of Gd stearate LB films. Though Gd3 + is the very convenient EPR center, which is often used as the EPR probe in different magnetic matrices [10], its EPR signal may be rather broad in magnetically ordered state due to the large fluctuations of the local magnetic fields [5]. Surely, other reasons can cause the significant linewidth, for example, unresolved EPR fine structure. In Gd stearate LB films the EPR signal is practically negligible at room temperature [11]. Only high-temperature experiments make the EPR signal detectable [11], however, the strong enough heating can destroy the LB film planar structure. It is desirable to get experimental evidences for magnetic ordering in the LB films at ambient temperatures. For this purpose we have prepared and studied by the EPR technique the Gd and Y stearate multilayer LB films. Placing the small crystal of the complex with benzene of a,g-bis-diphenylene-h-phenyl allyl (BDPA) on the film surface we have found that for the Gd stearate LB film sample the effective g-value of the radical’s resonance depends on the film orientation in respect to the external
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magnetic field direction. The relative shift of the EPR signal has corresponded to the magnetization of the film along the field direction. Such effect has not been observed for the yttrium stearate LB film.
2. Experimental details The Gd and Y stearate LB films have been deposited on the silicon substrate by the standard vertical dipping method as described elsewhere [12]. The LB films with 21 and 53 double-layers were formed. To obtain free stable radicals inside the films we have used crystals of BDPA (Aldrich Chemicals). Stearic acid (SA), GdCl3, YCl3, and acetic acid were obtained from Serva. Milli-Q water purification system was used to produce water with an average resistivity of 18 MV cm for all experiments. A small (about 3%) amount of BDPA dissolved in chloroform has been added to the spreading chloroform solution of stearic acid (10 4 M). The EPR spectra were recorded with a computerized Varian E-4 X-band EPR spectrometer at room temperature. The reference sample MnO in MgO has been used to check the absolute value of the magnetic field. Thin (about 1 Am in depth) plate-like rhombic crystals of BDPA have been placed onto the macroscopic film surfaces in order to study the space distribution of magnetic field. Commercially available 1.4-MB floppy disk has been used as an anisotropic magnetic media for model experiments. Dimensions of the samples have been measured using a light microscope.
3. Results and discussion Surface pressure-monolayer area isotherms of SA monolayer are shown in Fig. 1. For the rare earth (RE) containing subphase (RE cation concentration 10 4 M, 10 3 M acetate, pH 5.3) the isotherms demonstrate an absence of monolayer liquid phase, which indicates a complete salt formation. Fig. 2 demonstrates the X-ray diffraction (XRD) spectra for the Gd and Y stearate LB films. A number of equidistant narrow Bragg reflections in the range 2h = 10 – 100 point to the very good periodicity of the films structure. Results of Fourier-transform infrared (FTIR) measurements are shown in Fig. 3. The absence of the band at 1703 cm 1, which is due to the CjO stretching mode of COOH groups, and the presence of the band near 1550 cm 1, which corresponds to the symmetric stretching vibrations in carboxylate groups, indicate that the practically all stearate molecules are bound with RE ions in the LB films formed. Before describing our EPR results, we should note that the EPR signal of Gd3 + ions in the formed LB films has not been detected in our experiments. Fig. 4 shows EPR spectrum of the BDPA complex inside Gd stearate LB film (21 bilayers). The EPR signal has been rather weak, so about 160 scanning procedures have been required to get the acceptable signal-to-noise ratio. The signal linewidth can be estimated approximately as 0.3 mT. The single crystals of BDPA in our experiments showed a more narrow EPR line (the minimal value of DHpp is about 0.04 mT). We have made sure that the EPR signal of BDPA in Y stearate LB films also has a linewidth of about 0.3 mT and, hence, the
Fig. 1. Typical surface pressure-monolayer area isotherms for SA monolayers with only stearic acid (a), with BDPA (b) and with Gd3 + ions in the aqueous subphase (c). The isotherms for SA monolayers on the subphase with Gd3 + and Y3 + cations were practically identical.
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Fig. 2. X-ray diffraction patterns for the yttrium (i) and gadolinium (ii) stearate LB films with 53 layers. For convenience, the pairs of the identical spectra with the different magnification are shown. The spectra for the films with 53 and 21 layers are practically the same, except for a slight broadening of the individual peaks due to the reduced number of the layers.
Fig. 3. FTIR spectra for Gd stearate LB film (21 layers) and Y stearate LB film (53 layers). As a control the data are shown for the LB film from only stearic acid.
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Fig. 4. (i) EPR spectrum of BDPA inside Gd stearate LB film (after 160 scanning operations); (ii) residual EPR spectra of the resonator in the absence of the sample. The crosses are experimental points, the solid lines are the guides for the eye.
broadening of the EPR signal is due to the chemical environment of the BDPA complex in the films. It is not surprising, since, for example, in the chloroform solution BDPA shows an EPR linewidth of about 0.7 mT ( c 0.15 mT after desiccation). We should note also that the EPR linewidth in BDPA is anisotropic and can vary from 0.04 up to 0.07 mT. As a result, the BDPA resonance signal is ‘‘sample-dependent’’ to some extent. Since any possible magnetic contribution to the linewidth of the EPR signal from the spin probes inside the films has been masked by the broadening, which is likely due to the fast spin relaxation, we have used a variant of the ‘‘EPR decoration’’ method. We have attached a BDPA crystal (about 100 Am in the largest dimension) to the film surface in order to measure the separation between the EPR signals for different locations of the BDPA crystal on the film and different positions of the film in respect to the external magnetic field. Our calculations of the magnetic field space distribution near the uniformly magnetized thin plate have shown that, in the ideal case, the largest effect can be observed if the BDPA crystal is located at the corner of the film. However, there are some smeared regions on the LB film borders, and the most preferable place for the BDPA crystal attachment is the middle of the edge of the silicon substrate, which coincides with the film edge. Fig. 5 demonstrates the EPR spectra of the BDPA crystal attached to the center of the Gd stearate LB film edge. During experiments the normal to the film has been oriented either parallel (N-position) or perpendicular (?-position) to the external magnetic field direction. Since resonance conditions are sensitive to the sample orientation, we have
measured the reference EPR signal (MnO in MgO) for each sample position. If the film is magnetized according to the applied magnetic field, we have anticipated the following condition for the resonance fields HRES: HRES,N < HRES,?. The resonance field can be estimated as HRES=(HL + HR)/2, where HL and HR are the left and right peaks of the resonance line, respectively. We have found that both HL and HR have been really moved to the higher fields for the ?-position in comparison to the N-position of the film (Fig. 5). The separation between the EPR lines for the two film orientations is equal approximately to 0.014 F 0.04 mT. For the Y LB film no noticeable shift of HRES has been detected (Fig. 6). To estimate quantitatively the Gd LB film magnetization we have performed similar experiments on a 1.4-MB floppy disk (Fig. 7a,b). Fig. 7a shows the EPR spectra for two disk orientations and different distances of the spin probe from the magnetic surface. The crystal of BDPA has been attached to a rectangular piece of the disk as shown in the inset of Fig. 7b. We have changed the distance yZ between the crystal and the disk surface (along the perpendicular direction to the surface) with the help of microscope cover glasses, each of 200 Am in depth. Fig. 7b shows the experimentally found separation yH between the BDPA EPR lines for N- and ?-disk positions as a function of yZ, and the results of calculations within a simple magnetostatic model. This model allows calculating the magnetic field produced by the rectangular uniformly magnetized magnetic film. Details of the calculations will be published elsewhere. Parameters of the model are as follows: M is the film volume saturation magnetization (in general, M is a vector), d is the thickness of the magnetic layer. For a
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Fig. 5. (a) EPR spectra of the BDPA crystal placed on the Gd stearate LB film (21 layers) surface. The low field signal is the Mn2 + reference resonance. (b) The Mn2 + reference resonance on a large scale; (c) BDPA crystal EPR spectra on a large scale. The external magnetic field Hext either parallel [curves (i) in (a) and circles in (b) and (c)] or perpendicular [curves (ii) in (a) and solid lines in (b) and (c)] to the film surface. The separations between peaks for two film orientations are as follows: yH1 = 0.017 mT, yH2 = 0.010 mT.
Fig. 6. (a) EPR spectra of the BDPA crystal placed on the Y stearate LB film (53 layers) surface. The low field signal is the Mn2 + reference resonance. (b) The Mn2 + reference resonance on a large scale; (c) BDPA crystal EPR spectra on a large scale. The external magnetic field Hext either parallel [circles in (b) and (c)] or perpendicular [solid lines in (b) and (c)] to the film surface.
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Fig. 7. (a) EPR spectra of the BDPA crystal for different distances yZ between the crystal and the floppy disk surface: (i) yZ = l (the BDPA crystal on the quartz holder, no magnetic film); (ii) yZ = 1 – 20 Am; (iii) yZ = 200 Am; solid and dotted lines denote the spectra for N- and ?-disk positions, respectively; (b) the separation yH between the BDPA EPR lines for N- and ?-disk positions as a function of yZ; the dotted lines denote results of calculations within the model of uniformly magnetized magnetic film (parameters of the model see in the text). Inset shows the place of attachment of the BDPA crystal to the rectangular (1.77 2.07 mm2) piece of the floppy disk.
sufficiently small film the value of the magnetic field near its surface is proportional to the product ms = M d (other film dimensions being constant). That is, the magnetic field near the film depends only on the surface density of the magnetic dipole moment ms. We have found that the experimental and calculated values of yH(yZ) are in good accordance with the following characteristics of the floppy disk: msf,N c 1.1 10 8 T m, msf,? c 5.4 10 8 T m. It corresponds to the volume magnetization Mf,N = 0.11 T, assuming d = 100 nm. For additional testing of the validity of these parameters, we have measured and calculated the shift of the EPR signal from the reference position (the spectrum (i) in Fig. 7a) when the BDPA crystal is placed in the center of the piece of floppy film in the N-position. The experimental (0.049 mT) and calculated (0.042 mT) values are rather closed. Our results agree also with the data about the magnetic field distribution near the surface of the floppy disk, which has been obtained by the scanning Hall probe microscope [13]. Now we are able to estimate the magnetic characteristics of the Gd LB film. Taking into account the data presented in Figs. 5 and 7, we can get the value of the surface magnetization for the Gd LB film by the relation: mN,Gd c (0.1/ 7.5) ms,N = 1.47 10 10 T m. According to our previous finding [11,12], the average distance between Gd ions in the LB monolayer is equal to about 0.4 nm. Therefore each Gd ion in the film has a magnetic moment (in projection on the perpendicular to the film surface) of not less than l = 1 . 47 16 10 1 0 10 2 0 / 21 = 0. 11 1 0 2 9 T m3 c 1.2 lB. This value is sufficiently less than the mag-
netic moment for the free Gd3 + ion (7.5 lB), but it is not negligible and can indicate weak ferromagnetism. That result is in good agreement with earlier obtained evidences for the high temperature magnetic ordering in Gd stearate LB films [11,12,14,15].
4. Conclusions We present an experimental study of qualitative characteristics of room temperature magnetic ordering in gadolinium stearate LB film. We have used the spin probe method, which allows performing simultaneously EPR and magnetization measurements and can be very effective in the investigations of extremely thin magnetic films. Acknowledgements This work was supported by the Russian Foundation for Basic Researches (grants 02-03-33158 and 00-04-48330) and NATO linkage Grant (PST.CLG.976539). Yu.A.K also thanks for partial support INTAS (grants 99-1086, 01-483) and ISTC (no. 1838). References [1] M. Pomerantz, F.H. Dacol, A. Segmuller, Phys. Rev. Lett. 23 (1978) 246.
Yu.A. Koksharov et al. / Materials Science and Engineering C 22 (2002) 201–207 [2] Y. Ando, T. Hiroike, T. Miasshita, T. Miyazaki, Thin Solid Films 278 (1996) 144. [3] L. Pasimeni, U. Segre, A. Toffoletti, L. Valli, A. Marigo, Thin Solid Films 284 – 285 (1996) 656. [4] A. Brugger, Ch. Schoppmann, M. Schurr, M. Seidl, G. Sipos, C.Y. Hahn, J. Hassmann, O. Waldmann, H. Voit, Thin Solid Films 338 (1999) 231. [5] M. Farle, K. Baberschke, Phys. Rev. Lett. 58 (1987) 511. [6] M. Pozˇek, H.U. Habermeier, A. Maier, M. Mehring, Physica, C 269 (1996) 61. [7] Yu.N. Shvachko, H.-H. Wang, J.M. Williams, Proceedings of V Russian Conference on Physics and Chemistry of Elementary Chemical Processes, Russia, Chernogolovka, 1997. [8] A.M. Suvarna, C.S. Sunandana, Physica, C 276 (1997) 65. [9] G.B. Khomutov, I.V. Fry, M.E. Huflejt, L. Packer, Arch. Biochem. Biophys. 277 (1990) 263.
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[10] B. Elshner, A. Loidl, in: K.A. Gschneidner Jr., L. Eyring (Eds.), Handbook on Physics and Chemistry of Rare Earth, vol. 24, NorthHolland, Amsterdam, 1997, p. 221, Chap. 162. [11] A.M. Tishin, Yu.A. Koksharov, J. Bohr, G.B. Khomutov, Phys. Rev. B 55 (1997) 11064. [12] G.B. Khomutov, Yu.A. Koksharov, I.L. Radchenko, E.S. Soldatov, A.S. Trifonov, A.M. Tishin, J. Bohr, Mater. Sci. Eng., C 8 – 9 (1999) 299. [13] A. Sandhu, H. Masuda, A. Oral, S.J. Bending, J. Cryst. Growth 227 – 228 (2001) 899. [14] A.M. Tishin, O.V. Snigirev, G.B. Khomutov, S.A. Gudoshnikov, J. Bohr, J. Magn. Magn. Mater. 234 (2001) 499. [15] T.V. Murzina, A.A. Fedyanin, T.V. Misuryaev, G.B. Khomutov, O.A. Aktsipetrov, Appl. Phys. B 68 (1999) 537.
Radicals as EPR probes of magnetization of gadolinium stearate Langmuir–Blodgett film Yu.A. Koksharov a,*, I.V. Bykov b, A.P. Malakho c, S.N. Polyakov d, G.B. Khomutov a, J. Bohr e a
Faculty of Physics, Moscow State University, 119992 Moscow, Russia Vernadski Institute of Geochemistry and Analytical Chemistry RAS, Moscow, Russia c Department of Material Science, Moscow State University, 119992 Moscow, Russia d Institute of Nuclear Physics, Moscow State University, Moscow, Russia e Department of Physics, Technical University of Denmark, Lyngby, Denmark
b
Abstract In the present work we have applied the method of the EPR spin probes which allows performing simultaneously EPR and magnetization measurements to the investigation of magnetism of the Gd stearate Langmuir – Blodgett (LB) films. For this purpose we have prepared and studied by the EPR technique the Gd and Y stearate LB films. Placing the small BDPA crystal on the film surface we have found that for the Gd LB sample the effective g-value of the radical’s resonance depends on the film orientation in respect to the external magnetic field direction. The relative shift of the EPR signal corresponded to the magnetization of the film along the field direction. Such effect has not been observed for the Y stearate LB film. The data obtained give an experimental proof for the room temperature magnetic ordering in the gadolinium stearate LB film. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Rare-earth metal; Langmuir – Blodgett films; Thin magnetic films; EPR; Spin probes; Magnetization; 2-D magnetism
1. Introduction Langmuir– Blodgett (LB) technique can give ultra thin mono- and multilayer molecular films with monatomic layers of magnetic atoms and interesting magnetic properties [1 –4]. The high sensitivity of the electron paramagnetic resonance (EPR) spectroscopy makes this method rather effective for study magnetism in the thin films [5]. Because of very small amount of magnetic atoms present in LB film special efforts are required to detect the EPR signal. Usually, thin (about 100 Am) polymer substrates, like mylar (polyethylene terephtalate) sheets [2,3], are used for EPR measurements in order to have the possibility to place several pieces of the LB film into the active resonance cavity. However, in magnetically ordered films the linewidth of the EPR line could be so large that the signal peak-to-peak height becomes negligible. In that case the conventional EPR measurements are non-informative. In order to break the EPR ‘‘silence’’ the LB film could be doped or decorated by spin probes. Now the method of spin probes has become * Corresponding author. Tel.: +7-95-9392973; fax: +7-95-9391489. E-mail address: [email protected] (Yu.A. Koksharov).
customary for researchers working in different areas from the solid-state physics to the biophysics [6 –9]. In the present work we have applied the method of the EPR spin probes to the investigation of magnetism of Gd stearate LB films. Though Gd3 + is the very convenient EPR center, which is often used as the EPR probe in different magnetic matrices [10], its EPR signal may be rather broad in magnetically ordered state due to the large fluctuations of the local magnetic fields [5]. Surely, other reasons can cause the significant linewidth, for example, unresolved EPR fine structure. In Gd stearate LB films the EPR signal is practically negligible at room temperature [11]. Only high-temperature experiments make the EPR signal detectable [11], however, the strong enough heating can destroy the LB film planar structure. It is desirable to get experimental evidences for magnetic ordering in the LB films at ambient temperatures. For this purpose we have prepared and studied by the EPR technique the Gd and Y stearate multilayer LB films. Placing the small crystal of the complex with benzene of a,g-bis-diphenylene-h-phenyl allyl (BDPA) on the film surface we have found that for the Gd stearate LB film sample the effective g-value of the radical’s resonance depends on the film orientation in respect to the external
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magnetic field direction. The relative shift of the EPR signal has corresponded to the magnetization of the film along the field direction. Such effect has not been observed for the yttrium stearate LB film.
2. Experimental details The Gd and Y stearate LB films have been deposited on the silicon substrate by the standard vertical dipping method as described elsewhere [12]. The LB films with 21 and 53 double-layers were formed. To obtain free stable radicals inside the films we have used crystals of BDPA (Aldrich Chemicals). Stearic acid (SA), GdCl3, YCl3, and acetic acid were obtained from Serva. Milli-Q water purification system was used to produce water with an average resistivity of 18 MV cm for all experiments. A small (about 3%) amount of BDPA dissolved in chloroform has been added to the spreading chloroform solution of stearic acid (10 4 M). The EPR spectra were recorded with a computerized Varian E-4 X-band EPR spectrometer at room temperature. The reference sample MnO in MgO has been used to check the absolute value of the magnetic field. Thin (about 1 Am in depth) plate-like rhombic crystals of BDPA have been placed onto the macroscopic film surfaces in order to study the space distribution of magnetic field. Commercially available 1.4-MB floppy disk has been used as an anisotropic magnetic media for model experiments. Dimensions of the samples have been measured using a light microscope.
3. Results and discussion Surface pressure-monolayer area isotherms of SA monolayer are shown in Fig. 1. For the rare earth (RE) containing subphase (RE cation concentration 10 4 M, 10 3 M acetate, pH 5.3) the isotherms demonstrate an absence of monolayer liquid phase, which indicates a complete salt formation. Fig. 2 demonstrates the X-ray diffraction (XRD) spectra for the Gd and Y stearate LB films. A number of equidistant narrow Bragg reflections in the range 2h = 10 – 100 point to the very good periodicity of the films structure. Results of Fourier-transform infrared (FTIR) measurements are shown in Fig. 3. The absence of the band at 1703 cm 1, which is due to the CjO stretching mode of COOH groups, and the presence of the band near 1550 cm 1, which corresponds to the symmetric stretching vibrations in carboxylate groups, indicate that the practically all stearate molecules are bound with RE ions in the LB films formed. Before describing our EPR results, we should note that the EPR signal of Gd3 + ions in the formed LB films has not been detected in our experiments. Fig. 4 shows EPR spectrum of the BDPA complex inside Gd stearate LB film (21 bilayers). The EPR signal has been rather weak, so about 160 scanning procedures have been required to get the acceptable signal-to-noise ratio. The signal linewidth can be estimated approximately as 0.3 mT. The single crystals of BDPA in our experiments showed a more narrow EPR line (the minimal value of DHpp is about 0.04 mT). We have made sure that the EPR signal of BDPA in Y stearate LB films also has a linewidth of about 0.3 mT and, hence, the
Fig. 1. Typical surface pressure-monolayer area isotherms for SA monolayers with only stearic acid (a), with BDPA (b) and with Gd3 + ions in the aqueous subphase (c). The isotherms for SA monolayers on the subphase with Gd3 + and Y3 + cations were practically identical.
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Fig. 2. X-ray diffraction patterns for the yttrium (i) and gadolinium (ii) stearate LB films with 53 layers. For convenience, the pairs of the identical spectra with the different magnification are shown. The spectra for the films with 53 and 21 layers are practically the same, except for a slight broadening of the individual peaks due to the reduced number of the layers.
Fig. 3. FTIR spectra for Gd stearate LB film (21 layers) and Y stearate LB film (53 layers). As a control the data are shown for the LB film from only stearic acid.
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Fig. 4. (i) EPR spectrum of BDPA inside Gd stearate LB film (after 160 scanning operations); (ii) residual EPR spectra of the resonator in the absence of the sample. The crosses are experimental points, the solid lines are the guides for the eye.
broadening of the EPR signal is due to the chemical environment of the BDPA complex in the films. It is not surprising, since, for example, in the chloroform solution BDPA shows an EPR linewidth of about 0.7 mT ( c 0.15 mT after desiccation). We should note also that the EPR linewidth in BDPA is anisotropic and can vary from 0.04 up to 0.07 mT. As a result, the BDPA resonance signal is ‘‘sample-dependent’’ to some extent. Since any possible magnetic contribution to the linewidth of the EPR signal from the spin probes inside the films has been masked by the broadening, which is likely due to the fast spin relaxation, we have used a variant of the ‘‘EPR decoration’’ method. We have attached a BDPA crystal (about 100 Am in the largest dimension) to the film surface in order to measure the separation between the EPR signals for different locations of the BDPA crystal on the film and different positions of the film in respect to the external magnetic field. Our calculations of the magnetic field space distribution near the uniformly magnetized thin plate have shown that, in the ideal case, the largest effect can be observed if the BDPA crystal is located at the corner of the film. However, there are some smeared regions on the LB film borders, and the most preferable place for the BDPA crystal attachment is the middle of the edge of the silicon substrate, which coincides with the film edge. Fig. 5 demonstrates the EPR spectra of the BDPA crystal attached to the center of the Gd stearate LB film edge. During experiments the normal to the film has been oriented either parallel (N-position) or perpendicular (?-position) to the external magnetic field direction. Since resonance conditions are sensitive to the sample orientation, we have
measured the reference EPR signal (MnO in MgO) for each sample position. If the film is magnetized according to the applied magnetic field, we have anticipated the following condition for the resonance fields HRES: HRES,N < HRES,?. The resonance field can be estimated as HRES=(HL + HR)/2, where HL and HR are the left and right peaks of the resonance line, respectively. We have found that both HL and HR have been really moved to the higher fields for the ?-position in comparison to the N-position of the film (Fig. 5). The separation between the EPR lines for the two film orientations is equal approximately to 0.014 F 0.04 mT. For the Y LB film no noticeable shift of HRES has been detected (Fig. 6). To estimate quantitatively the Gd LB film magnetization we have performed similar experiments on a 1.4-MB floppy disk (Fig. 7a,b). Fig. 7a shows the EPR spectra for two disk orientations and different distances of the spin probe from the magnetic surface. The crystal of BDPA has been attached to a rectangular piece of the disk as shown in the inset of Fig. 7b. We have changed the distance yZ between the crystal and the disk surface (along the perpendicular direction to the surface) with the help of microscope cover glasses, each of 200 Am in depth. Fig. 7b shows the experimentally found separation yH between the BDPA EPR lines for N- and ?-disk positions as a function of yZ, and the results of calculations within a simple magnetostatic model. This model allows calculating the magnetic field produced by the rectangular uniformly magnetized magnetic film. Details of the calculations will be published elsewhere. Parameters of the model are as follows: M is the film volume saturation magnetization (in general, M is a vector), d is the thickness of the magnetic layer. For a
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Fig. 5. (a) EPR spectra of the BDPA crystal placed on the Gd stearate LB film (21 layers) surface. The low field signal is the Mn2 + reference resonance. (b) The Mn2 + reference resonance on a large scale; (c) BDPA crystal EPR spectra on a large scale. The external magnetic field Hext either parallel [curves (i) in (a) and circles in (b) and (c)] or perpendicular [curves (ii) in (a) and solid lines in (b) and (c)] to the film surface. The separations between peaks for two film orientations are as follows: yH1 = 0.017 mT, yH2 = 0.010 mT.
Fig. 6. (a) EPR spectra of the BDPA crystal placed on the Y stearate LB film (53 layers) surface. The low field signal is the Mn2 + reference resonance. (b) The Mn2 + reference resonance on a large scale; (c) BDPA crystal EPR spectra on a large scale. The external magnetic field Hext either parallel [circles in (b) and (c)] or perpendicular [solid lines in (b) and (c)] to the film surface.
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Fig. 7. (a) EPR spectra of the BDPA crystal for different distances yZ between the crystal and the floppy disk surface: (i) yZ = l (the BDPA crystal on the quartz holder, no magnetic film); (ii) yZ = 1 – 20 Am; (iii) yZ = 200 Am; solid and dotted lines denote the spectra for N- and ?-disk positions, respectively; (b) the separation yH between the BDPA EPR lines for N- and ?-disk positions as a function of yZ; the dotted lines denote results of calculations within the model of uniformly magnetized magnetic film (parameters of the model see in the text). Inset shows the place of attachment of the BDPA crystal to the rectangular (1.77 2.07 mm2) piece of the floppy disk.
sufficiently small film the value of the magnetic field near its surface is proportional to the product ms = M d (other film dimensions being constant). That is, the magnetic field near the film depends only on the surface density of the magnetic dipole moment ms. We have found that the experimental and calculated values of yH(yZ) are in good accordance with the following characteristics of the floppy disk: msf,N c 1.1 10 8 T m, msf,? c 5.4 10 8 T m. It corresponds to the volume magnetization Mf,N = 0.11 T, assuming d = 100 nm. For additional testing of the validity of these parameters, we have measured and calculated the shift of the EPR signal from the reference position (the spectrum (i) in Fig. 7a) when the BDPA crystal is placed in the center of the piece of floppy film in the N-position. The experimental (0.049 mT) and calculated (0.042 mT) values are rather closed. Our results agree also with the data about the magnetic field distribution near the surface of the floppy disk, which has been obtained by the scanning Hall probe microscope [13]. Now we are able to estimate the magnetic characteristics of the Gd LB film. Taking into account the data presented in Figs. 5 and 7, we can get the value of the surface magnetization for the Gd LB film by the relation: mN,Gd c (0.1/ 7.5) ms,N = 1.47 10 10 T m. According to our previous finding [11,12], the average distance between Gd ions in the LB monolayer is equal to about 0.4 nm. Therefore each Gd ion in the film has a magnetic moment (in projection on the perpendicular to the film surface) of not less than l = 1 . 47 16 10 1 0 10 2 0 / 21 = 0. 11 1 0 2 9 T m3 c 1.2 lB. This value is sufficiently less than the mag-
netic moment for the free Gd3 + ion (7.5 lB), but it is not negligible and can indicate weak ferromagnetism. That result is in good agreement with earlier obtained evidences for the high temperature magnetic ordering in Gd stearate LB films [11,12,14,15].
4. Conclusions We present an experimental study of qualitative characteristics of room temperature magnetic ordering in gadolinium stearate LB film. We have used the spin probe method, which allows performing simultaneously EPR and magnetization measurements and can be very effective in the investigations of extremely thin magnetic films. Acknowledgements This work was supported by the Russian Foundation for Basic Researches (grants 02-03-33158 and 00-04-48330) and NATO linkage Grant (PST.CLG.976539). Yu.A.K also thanks for partial support INTAS (grants 99-1086, 01-483) and ISTC (no. 1838). References [1] M. Pomerantz, F.H. Dacol, A. Segmuller, Phys. Rev. Lett. 23 (1978) 246.
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