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Evidence of very high coercive fields in orthoferrite phases of PLD grown thin films
Journal of Magnetism and Magnetic Materials 195 (1999) 291}298
Evidence of very high coercive "elds in orthoferrite phases of PLD grown thin "lms D.S. Schmool*, N. Keller, M. Guyot, R. Krishnan, M. Tessier Laboratoire de Magne& tisme et d'Optique, CNRS } Universite& de Versailles, 45 Avenue des Etats-Unis, 78035 Versailles, France Received 19 October 1998; received in revised form 18 December 1998
Abstract Thin magnetic layers have been prepared by pulsed laser deposition (PLD) from targets of the orthoferrites: DyFeO , GdFeO SmFeO and YFeO . All layers were deposited onto quartz substrates at 4503C under oxygen partial pressures ranging from 0 to 80 mTorr. The resulting layers display evidence of various magnetic phases, including garnet and orthoferrite phases, as evidenced by X-ray di!raction. The magnetic properties of the thin "lms have been analysed using SQUID and vibrating sample magnetometries. A simple model is proposed to aid the analysis of hysteresis loops for samples of more than one magnetic phase. The orthoferrite phases display very high coercive "elds, in the range 1.5}1.8 T. 1999 Elsevier Science B.V. All rights reserved. Keywords: Orthoferrites; Thin "lms; Pulsed laser deposition; SQUID magnetometry
1. Introduction Orthoferrites have a crystalline structure which is close to that of the perovskites [1,2]. In general, the orthoferrites are antiferromagnetic due to the antiparallel alignment of the magnetic moments of the Fe sublattices. In the case of rare-earth and yttrium orthoferrites, however, a weak ferromagnetism is present due to a small canting in the alignment of the antiferromagnetically coupled lattices. This small canting, which is of the order of 10\ rad [3], is enough to produce a very small net
* Corresponding author. Tel.: #33-1-3929-4658; fax: #331-3925-4652.
magnetic moment in the range 0.04 } 0.05 l molecule\ [4]. The rare-earth orthoferrites show a strong uniaxial anisotropy, and hence attracted much attention in the 1960s as a candidate for bubble domain materials. It was, in fact, in these materials that the "rst bubble domains were observed [5]. However, due to the di$culties in producing thin "lms of these materials they were eventually abandoned in favour of the well-known magnetic garnets. Recently we have succeeded in preparing thin "lms of yttrium orthoferrites, YFeO , by pulsed laser deposition. These displayed coercive "elds of up to almost 1 T [6]. (This value is now believed to be substantially underestimated since the samples in that study were not completely magnetically
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saturated.) These samples were deposited at room temperature and post annealed at temperatures of about 9003C, in air, in order to obtain the desired orthoferrite phase. It is well known that depositing on to heated substrates results in a better formation of ferrites, therefore we undertook a series of experiments to optimise the growth conditions in order to obtain the orthoferrites at a relatively low temperature. In this series of experiments we maintained a "xed laser power and using a constant substrate temperature, we varied the oxygen partial pressure. We have investigated the system RFeO , where R"Dy, Gd, Sm and Y. We report here some of the results of magnetic measurements, where we have concentrated on the samples prepared in high vacuum, without the introduction of oxygen, since, as will be shown, these showed the strongest magnetic moments in the unannealed state. These have then been further analysed by SQUID magnetometry. These samples are seen to be multiphased. We also present a simple model for the extraction of magnetic moments and coercive "elds from hysteresis loops from such multiphase samples.
2. Experimental The thin magnetic layers were prepared by pulsed laser deposition (PLD) in a vacuum chamber, with a base pressure of typically 5;10\ mbar. All layers were produced by ablation using a pulsed Nd : YAG laser, which is scanned in a square raster over the surface of the target material, placed directly below a heated quartz substrate. The laser was operated at a wavelength of 355 nm using a pulse width of 6 ns and a repetition rate of 10 Hz. Under these conditions the laser #uence is of the order of 7 J cm\. The substrate temperature during deposition was typically 4503C, with oxygen partial pressures maintained in the range of 0 to 80 mTorr. Thin "lms were prepared from targets (produced by sintering the high-purity raw materials (with a purity of '99.999%)) of the orthoferrites: DyFeO , GdFeO , SmFeO and YFeO , whose structure has been veri"ed using X-ray powder di!raction. Magnetic properties have been assessed using a Quantum Design SQUID magnetometer using
"elds up to 3 T in the temperature range 5}290 K. Room-temperature VSM measurements have also been performed. X-ray di!raction (XRD) experiments, using a Cu K radiation, were performed on a the as-grown samples for characterisation purposes.
3. Results and discussion In Fig. 1, we show the XRD spectrum for the SmFeO sample, grown without oxygen, i.e. in high vacuum. While we observe evidence for the presence of the desired orthoferrite phase, there also appear peaks which correspond to the garnet-like phase, an oxide phase and a magnetite (Fe O ) phase, these are indicated in the "gure. The presence of these phases are discussed with respect to the results of magnetic measurements. Magnetic measurements on the as-grown samples as a function of the partial oxygen pressure during deposition show the great sensitivity of the samples to the growth environment. In Fig. 2, we show the results of the room-temperature VSM measurements, giving the magnetic moment as a function of the oxygen partial pressure (P ) during sample preparation, for the SmFeO sample. In the low oxygen pressure regime we observe a relatively strong magnetic signal which decreases sharply with increasing O pressure, practically vanishing at an oxygen pressure above 20 mTorr and reappearing at 80 mTorr. A similar variation is observed for the GdFeO sample, also indicating a minimum around P "40 mTorr. Changes in the net magnetic moment of the samples are due to the type and quantity of the magnetic phases that form in the as-deposited samples. This can be revealed from the X-ray di!raction and from careful analysis of the magnetometric data. Since the low oxygen pressure samples showed such an enhanced magnetic moment we decided to further investigate these samples using SQUID magnetometry. For the YFeO sample, we veri"ed that the or thoferrite phase could be obtained after annealing to high temperatures (the annealing was performed at 8603C for 1 h in air). This con"rmed the results of our previous study [6] giving the same magnetooptic properties. From the di!erences before and
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Fig. 1. X-ray di!raction pattern for the SmFeO sample, with 0 mTorr oxygen, in the as-grown state. The "gure shows the presence of the orthoferrite, garnet and oxide phases for the Sm and magnetite. See text for details.
Fig. 2. Magnetic moment for the as-grown SmFeO samples as a function of the oxygen partial pressure during sample deposition. The inset shows an expanded view of the samples grown with an oxygen atmosphere.
after annealing, where the orthoferrite phase is almost fully recovered (we estimate that the annealed sample contains 91}92% orthoferrite), we see that the cationic ratio between the Y and Fe components is retained. This implies the existence of other phases before annealing. In addition to the observed garnet phase, it therefore seems possible that
phases of magnetite (Fe O ) and the pure oxide (Y O ) should also be present in the as-grown samples. The similarity of the results for the rareearth samples implies a similar situation in these samples. Magneto-optic measurements on the asgrown and annealed samples con"rm the di!erences before and after annealing and the recuperation of the orthoferrite phase in the post-annealed state [7]. Regarding the changes of the magnetic moment as a function of the oxygen partial pressure during sample growth (see Fig. 2), the very sharp reduction of the magnetic moment with the introduction of O could arise from a substantially reduced mag netite content. A de"ciency of oxygen leads to a partial reduction of Fe> to Fe> which would favour the formation of Fe O at low O pressures. This would seem quite plausible given the signi"cantly stronger moment of magnetite as compared to those of the orthoferrite and garnet materials. Magnetite has the largest magnetic moment among the iron oxide materials. It is also the case that the iron garnets exhibit a more elevated magnetic moment than the rare-earth and yttrium orthoferrites [3,5]. The curve of magnetic moment versus oxygen deposition pressure (Fig. 2) displays a minimum in the 20}60 mTorr region, this may be
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because the Fe O phase is almost completely sup pressed in this pressure regime and that the garnet phase content is quite low or absent. In the region above 60 mTorr, we observe an increase in the magnetic moment, albeit quite small, which could be attributed to a better formation of the orthoferrite phase and possibly to an increase in the relative quantity of the garnet phase. In this way it is possible to account for the changes of magnetic moment in the as-grown samples as arising from the formation of varying quantities of the phases: Fe O , R Fe O , RFeO and R O , as a function of the oxygen partial pressure during sample deposition. In Fig. 3 we show a representative hysteresis loop, in this case for the as-grown SmFeO sample, with 0 mTorr O , obtained from SQUID mag netometry. In this "gure we see evidence that the sample is not a single magnetic phase as shown by the di!erence between the ascending and descending branches of the hysteresis cylce. This is best demonstrated with the aid of Fig. 4. In Fig. 4(a) we illustrate, schematically, the hysteresis loops for two magnetic phases (A and B) of saturation magnetisations, M and M , respectively, and coercive "elds, H and H , respectively. For a sample ! ! with a mixture of phases A and B we expect a hysteresis loop which will be an addition of those shown in Fig. 4(a), this is illustrated in Fig. 4(b), where we assume an equal content of the two
Fig. 3. Hysteresis loop for the Sm sample taken at 250 K. This sample was prepared under high-vacuum conditions, i.e. 0 mTorr O .
Fig. 4. (a) Schematic diagram of hysteresis loops for two magnetic phases, A and B, where the saturation magnetisations and coercive "elds are indicated. (b) Combined hysteresis loop for a system of equal quantities of phases A and B. (c) Di!erence curve for the double magnetic phase indicated in (b), where dM is the di!erence between the ascending and descending portions of the hysteresis cycle in (b). Values of the coercive "elds for the two phases are shown along with the saturation magnetisation of the hard magnetic phase (B) and the remnance for the softer magnetic phase (A), M . 0#+
D.S. Schmool et al. / Journal of Magnetism and Magnetic Materials 195 (1999) 291}298
phases. This complex loop contains all the information, i.e. magnetisations and coercive "elds, of the individual loops, and can be more easily analysed by subtracting the ascending and descending portions of the hysteresis cycle. Such a &di!erence' curve is shown in Fig. 4(c). The central peak will, in general, give the value of 2M (where M de0#+ 0#+ notes the remnance magnetisation) for the magnetically softer material, A, and the value of H can ! also be evaluated, as indicated. On either side of the central peak a plateau region is observed before reaching the magnetically saturated region, where d "0. The height of the plateau region above + d "0 will give twice the saturation magnetisation + of the harder magnetic phase, 2M , see Fig. 4(c). From the transition of dM"2M to dM"0 it is possible to evaluate the coercive "eld, H , of ! phase B, as illustrated. It will be noted that the total saturation magnetisation M "M #M , is 1 2-2 obtained from the combined hysteresis loop, Fig. 4(b). Thus we see that we can obtain the saturation magnetisations and coercive "elds of both magnetic phases uniquely, provided that the magnetic phases have su$ciently di!ering coercive "elds. Using the data from the SmFeO sample, the hysteresis loop of which is shown in Fig. 3, we obtain the di!erence curve (dM versus H), given in Fig. 5. From this curve it is evident that there are, at least, two clearly distinct magnetic phases. The
Fig. 5. Di!erence curve for the Sm sample, where the data is taken from Fig. 3. Signals for the soft magnetic phase (see the explanation in the text) and orthoferrite phase are clearly distinguishable.
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central peak is derived from the signal of a relatively soft magnetic phase, which has a relatively small coercive "eld (in the region of 0.06 T) and a fairly strong magnetic moment. The plateau region observed on either side of the central peak is due to a very much harder magnetic phase, which is associated with the samarium orthoferrite. The latter shows a much weaker saturation magnetic moment, about 7.5;10\ emu and a very large coercive "eld, in the region of 1.3 T. This curve is representative for the other samples studied (Y, Dy and Gd), measured in the temperature range 5}290 K. Using the di!erence curve and the hysteresis loops in conjunction, it is possible to extract the magnetic moments and the coercive "elds of the two magnetic phases uniquely, as a function of temperature. In Fig. 6 we show the temperature dependence of the magnetic moment of these two phases for the Dy sample grown in high-vacuum conditions, i.e. 0 mTorr O , where we indicate the magnetic moment of the orthoferrite phase, M , and the magnetic moment of the remainder of the sample, M . (Note the di!erent scales for the two phases.) We see that both phases have a similar temperature dependence. We clearly see a strong enhancement of the magnetic moments for both phases towards lower temperatures. This behaviour is observed, to varying degrees, in the samples for Y and Gd. Neither the Sm garnet nor orthoferrite display, in their bulk form, such an increase at low temperature, and this is not observed in the Sm
Fig. 6. Temperature dependence of the magnetic moments for the orthoferrite and softer magnetic (residual) phases for the DyFeO sample. This sample was prepared in high vacuum.
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sample studied here. A magnetically observable signature of many of the rare-earth iron garnets is the compensation point, Sm Fe O is an exception. It is noted that such compensation points should be present for both the Dy and Gd iron garnets (226 and 286 K, respectively [3]). Curiously, however, we observe no evidence of these. Allowing for the presence of an Fe O phase in these samples, we would expect that the softer magnetic phase will be comprised of both the magnetite and the garnet phases. However, since the magnetic moment of the magnetite is much larger than that of the rare-earth garnets at higher temperatures, we may expect this to dominate the net magnetic moment of the softer phase and thus mask the compensation point. The magnetite and garnet magnetic phases cannot be clearly distiguished, from the hysteresis loops themselves, in the manner apparent for the orthoferrite phase. The latter having a much greater value of coercive "eld as to be easily separable. In Table 1 we give the relative quantities of the di!erent phases present in the samples, where we account for the following phases: RFeO , R Fe O , Fe O and R O (this oxide phase is not necessarily formed completely). These have been evaluated in the following manner. For an N-phase magnetic system we can write the net magnetic moment as m " < m , (1) 2-2 , , , where < represents the relative volume of phase , N and is dimensionless when the following condition applies: < "1. (2) , , In general, our systems, as described above, present three magnetic phases. We can, however, simplify matters since our analysis allows us to separate one of the magnetic phases (i.e. the hard orthoferrite phase). The residual is therefore made up of the two-component magnetite}garnet softer phase. In the cases where the garnets show a strong enhancement of magnetic moment at low temperatures [3,5], we can further separate the magnetic moments for these two phases, since the magnetite shows no such behaviour. Since the Dy and Gd
Table 1 Percentages of the various phases in the samples and coercivity values taken from SQUID data
% Orthoferrite % Magnetite % Garnet % Oxide or nonmagnetic residual H (¹) ! H (¹) !
Dy
Gd
Sm
Y
43.8 10.7 14.7 30.8
40.9 1.7 22.3 35.1
* * * *
91.6 * 4.2 4.2
1.52 0.020
1.52 0.026
1.65 0.064
1.8 0.044
Sample annealed to 8603C for 1 h.
garnets have compensation points in the temperature range studied, at these points the magnetic signal (for the softer magnetic phase) should be entirely due to the magnetite. Extrapolating back to 5 K, from bulk magnetic behaviour of the Fe O , we can evaluate the di!erence between the two phases and thus ascertain their respective magnetic moments. Now, for a two-phase system we have, from Eq. (2): < "1!< . (3) Taking the ratio of the two magnetic signals, we can write < m < m . a" " (4) < m (1!< )m Comparing with the ratio of the bulk values for these materials, we have m M b" " . m M
(5)
Combining Eqs. (4) and (5) we can obtain the relative volume of phase A as a < " . a#b
(6)
< is then simply obtained from Eq. (3). Using a similar approach we can obtain the relative volume of the orthoferrite phase. Since the target is comprised of almost exclusively the orthoferrite phase, we must consider the following chemical equations, which occur during the deposition process, to account for the presence of the other
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magnetic phases (i.e: Fe O and R Fe O ) in the samples: 5RFeO PR Fe O #R O ,
(7)
12RFeO P4Fe O #6R O #O .
(8)
(This implies the presence of a non-magnetic oxide phase.) This then allows us to estimate the relative quantities of all the phases in the samples. Since the R O phase does not contribute to the magnetic signal, we cannot assume that this phase is completely formed, hence this refers to the quantity of the oxide or remaining material. In the case of the Sm sample, this was not possible since the Sm garnet phase exhibits neither a strong low-temperature enhancement of magnetic moment nor a compensation point. In the case of the annealed YFeO sample, it is reasonable to assume that there is no magnetite phase present in the sample. The calculations of phase quantities have therefore been made on this assumption. Also indicated in Table 1 are the average values of the coercive "elds for both the orthoferrite and the residual softer magnetic phase. The former display very large values, from 1.5 to 1.8 T, where it should be noted that the latter value refers to the yttrium sample which was annealed at 8603C for 1 h. In Fig. 7 we give the temperature dependence of the coercive "eld for the orthoferrite phase (here for
Fig. 7. Temperature dependence of the coercive "eld for the orthoferrite phase of the DyFeO sample.
297
the DyFeO sample, which is representative of the other samples studied). We see that the value of H remains roughly constant with increase of tem! perature and has a value of about 1.5 T region.
4. Conclusions The as-deposited layers grown at 4503C, in the absence of oxygen, from orthoferrite targets show evidence of several phases, among which the orthoferrite and garnet phases could be identi"ed. The indications are that phases of magnetite and the rare-earth oxides are also present in the as-grown samples, which would be expected from stoichiometric considerations. The magnetic phases of the orthoferrite and the magnetite and garnet phases together are clearly distinguished from SQUID magnetometry. This can be more clearly demonstrated by taking the di!erence of the ascending and descending branches of the hysteresis loops, from which values of magnetic moment and coercive "eld can be obtained for both phases. While the garnet phases (R Fe O ; R"Y, Dy, Gd, Sm) together with the magnetite show relatively strong magnetic moments and small coercive "elds, the orthoferrite phases (RFeO ) have weak saturation magnetic moments and very high coercive "elds, which range from about 1.5 T in DyFeO and GdFeO to 1.65 T in SmFeO and 1.8 T in YFeO . This allows the evaluation of the relative volumes of all the phases present. The variation of the magnetic moment, in the as-grown samples, as a function of increasing O pressure during deposition can be understood as a suppression of the magnetite phase in an oxidising atmosphere. Further changes of the magnetic moment will then be due to the varying quantites of the garnet and orthoferrite phases. By a careful analysis of the magnetometric data and a consideration of the chemical transformations, we have shown that we can estimate the relative quantities of the various phases that form in the as-grown samples. The orthoferrite phase can be almost completely recuperated from the mixed phase as-grown samples by annealing at 8603C for 1 h. We have estimated that we recover over 91% of the orthoferrite phase by such a process in the case
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of the yttrium sample. Similar results are expected for the rare-earth samples.
Acknowledgements One of the authors (DSS) would like to thank the EC for "nancial support under the TMR programme, grant number: ERB4001GT971610. We would like to thank M.-C. Brianso for performing the X-ray di!raction experiments.
References [1] S. Geller, E.A. Wood, Acta Crystallogr. 9 (1956) 563. [2] S. Geller, V.B. Bala, Acta Crystallogr. 9 (1956) 1019. [3] R.S. Tebble, D.J. Craik, Magnetic Materials, Wiley-Interscience, New York, 1969. [4] D. Treves, J. Appl. Phys. 36 (1965) 1033. [5] D.J. Craik (Ed.), Magnetic Oxides, Wiley, New York, 1975. [6] R. Krishnan, A. Lis", M. Guyot, V. Cagan, J. Magn. Magn. Mater. 147 (1995) L221. [7] D.S. Schmool, N. Keller, M. Guyot, R. Krishnan, M. Tessier, unpublished.
Evidence of very high coercive "elds in orthoferrite phases of PLD grown thin "lms D.S. Schmool*, N. Keller, M. Guyot, R. Krishnan, M. Tessier Laboratoire de Magne& tisme et d'Optique, CNRS } Universite& de Versailles, 45 Avenue des Etats-Unis, 78035 Versailles, France Received 19 October 1998; received in revised form 18 December 1998
Abstract Thin magnetic layers have been prepared by pulsed laser deposition (PLD) from targets of the orthoferrites: DyFeO , GdFeO SmFeO and YFeO . All layers were deposited onto quartz substrates at 4503C under oxygen partial pressures ranging from 0 to 80 mTorr. The resulting layers display evidence of various magnetic phases, including garnet and orthoferrite phases, as evidenced by X-ray di!raction. The magnetic properties of the thin "lms have been analysed using SQUID and vibrating sample magnetometries. A simple model is proposed to aid the analysis of hysteresis loops for samples of more than one magnetic phase. The orthoferrite phases display very high coercive "elds, in the range 1.5}1.8 T. 1999 Elsevier Science B.V. All rights reserved. Keywords: Orthoferrites; Thin "lms; Pulsed laser deposition; SQUID magnetometry
1. Introduction Orthoferrites have a crystalline structure which is close to that of the perovskites [1,2]. In general, the orthoferrites are antiferromagnetic due to the antiparallel alignment of the magnetic moments of the Fe sublattices. In the case of rare-earth and yttrium orthoferrites, however, a weak ferromagnetism is present due to a small canting in the alignment of the antiferromagnetically coupled lattices. This small canting, which is of the order of 10\ rad [3], is enough to produce a very small net
* Corresponding author. Tel.: #33-1-3929-4658; fax: #331-3925-4652.
magnetic moment in the range 0.04 } 0.05 l molecule\ [4]. The rare-earth orthoferrites show a strong uniaxial anisotropy, and hence attracted much attention in the 1960s as a candidate for bubble domain materials. It was, in fact, in these materials that the "rst bubble domains were observed [5]. However, due to the di$culties in producing thin "lms of these materials they were eventually abandoned in favour of the well-known magnetic garnets. Recently we have succeeded in preparing thin "lms of yttrium orthoferrites, YFeO , by pulsed laser deposition. These displayed coercive "elds of up to almost 1 T [6]. (This value is now believed to be substantially underestimated since the samples in that study were not completely magnetically
0304-8853/99/$ } see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 9 ) 0 0 1 0 2 - X
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saturated.) These samples were deposited at room temperature and post annealed at temperatures of about 9003C, in air, in order to obtain the desired orthoferrite phase. It is well known that depositing on to heated substrates results in a better formation of ferrites, therefore we undertook a series of experiments to optimise the growth conditions in order to obtain the orthoferrites at a relatively low temperature. In this series of experiments we maintained a "xed laser power and using a constant substrate temperature, we varied the oxygen partial pressure. We have investigated the system RFeO , where R"Dy, Gd, Sm and Y. We report here some of the results of magnetic measurements, where we have concentrated on the samples prepared in high vacuum, without the introduction of oxygen, since, as will be shown, these showed the strongest magnetic moments in the unannealed state. These have then been further analysed by SQUID magnetometry. These samples are seen to be multiphased. We also present a simple model for the extraction of magnetic moments and coercive "elds from hysteresis loops from such multiphase samples.
2. Experimental The thin magnetic layers were prepared by pulsed laser deposition (PLD) in a vacuum chamber, with a base pressure of typically 5;10\ mbar. All layers were produced by ablation using a pulsed Nd : YAG laser, which is scanned in a square raster over the surface of the target material, placed directly below a heated quartz substrate. The laser was operated at a wavelength of 355 nm using a pulse width of 6 ns and a repetition rate of 10 Hz. Under these conditions the laser #uence is of the order of 7 J cm\. The substrate temperature during deposition was typically 4503C, with oxygen partial pressures maintained in the range of 0 to 80 mTorr. Thin "lms were prepared from targets (produced by sintering the high-purity raw materials (with a purity of '99.999%)) of the orthoferrites: DyFeO , GdFeO , SmFeO and YFeO , whose structure has been veri"ed using X-ray powder di!raction. Magnetic properties have been assessed using a Quantum Design SQUID magnetometer using
"elds up to 3 T in the temperature range 5}290 K. Room-temperature VSM measurements have also been performed. X-ray di!raction (XRD) experiments, using a Cu K radiation, were performed on a the as-grown samples for characterisation purposes.
3. Results and discussion In Fig. 1, we show the XRD spectrum for the SmFeO sample, grown without oxygen, i.e. in high vacuum. While we observe evidence for the presence of the desired orthoferrite phase, there also appear peaks which correspond to the garnet-like phase, an oxide phase and a magnetite (Fe O ) phase, these are indicated in the "gure. The presence of these phases are discussed with respect to the results of magnetic measurements. Magnetic measurements on the as-grown samples as a function of the partial oxygen pressure during deposition show the great sensitivity of the samples to the growth environment. In Fig. 2, we show the results of the room-temperature VSM measurements, giving the magnetic moment as a function of the oxygen partial pressure (P ) during sample preparation, for the SmFeO sample. In the low oxygen pressure regime we observe a relatively strong magnetic signal which decreases sharply with increasing O pressure, practically vanishing at an oxygen pressure above 20 mTorr and reappearing at 80 mTorr. A similar variation is observed for the GdFeO sample, also indicating a minimum around P "40 mTorr. Changes in the net magnetic moment of the samples are due to the type and quantity of the magnetic phases that form in the as-deposited samples. This can be revealed from the X-ray di!raction and from careful analysis of the magnetometric data. Since the low oxygen pressure samples showed such an enhanced magnetic moment we decided to further investigate these samples using SQUID magnetometry. For the YFeO sample, we veri"ed that the or thoferrite phase could be obtained after annealing to high temperatures (the annealing was performed at 8603C for 1 h in air). This con"rmed the results of our previous study [6] giving the same magnetooptic properties. From the di!erences before and
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Fig. 1. X-ray di!raction pattern for the SmFeO sample, with 0 mTorr oxygen, in the as-grown state. The "gure shows the presence of the orthoferrite, garnet and oxide phases for the Sm and magnetite. See text for details.
Fig. 2. Magnetic moment for the as-grown SmFeO samples as a function of the oxygen partial pressure during sample deposition. The inset shows an expanded view of the samples grown with an oxygen atmosphere.
after annealing, where the orthoferrite phase is almost fully recovered (we estimate that the annealed sample contains 91}92% orthoferrite), we see that the cationic ratio between the Y and Fe components is retained. This implies the existence of other phases before annealing. In addition to the observed garnet phase, it therefore seems possible that
phases of magnetite (Fe O ) and the pure oxide (Y O ) should also be present in the as-grown samples. The similarity of the results for the rareearth samples implies a similar situation in these samples. Magneto-optic measurements on the asgrown and annealed samples con"rm the di!erences before and after annealing and the recuperation of the orthoferrite phase in the post-annealed state [7]. Regarding the changes of the magnetic moment as a function of the oxygen partial pressure during sample growth (see Fig. 2), the very sharp reduction of the magnetic moment with the introduction of O could arise from a substantially reduced mag netite content. A de"ciency of oxygen leads to a partial reduction of Fe> to Fe> which would favour the formation of Fe O at low O pressures. This would seem quite plausible given the signi"cantly stronger moment of magnetite as compared to those of the orthoferrite and garnet materials. Magnetite has the largest magnetic moment among the iron oxide materials. It is also the case that the iron garnets exhibit a more elevated magnetic moment than the rare-earth and yttrium orthoferrites [3,5]. The curve of magnetic moment versus oxygen deposition pressure (Fig. 2) displays a minimum in the 20}60 mTorr region, this may be
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because the Fe O phase is almost completely sup pressed in this pressure regime and that the garnet phase content is quite low or absent. In the region above 60 mTorr, we observe an increase in the magnetic moment, albeit quite small, which could be attributed to a better formation of the orthoferrite phase and possibly to an increase in the relative quantity of the garnet phase. In this way it is possible to account for the changes of magnetic moment in the as-grown samples as arising from the formation of varying quantities of the phases: Fe O , R Fe O , RFeO and R O , as a function of the oxygen partial pressure during sample deposition. In Fig. 3 we show a representative hysteresis loop, in this case for the as-grown SmFeO sample, with 0 mTorr O , obtained from SQUID mag netometry. In this "gure we see evidence that the sample is not a single magnetic phase as shown by the di!erence between the ascending and descending branches of the hysteresis cylce. This is best demonstrated with the aid of Fig. 4. In Fig. 4(a) we illustrate, schematically, the hysteresis loops for two magnetic phases (A and B) of saturation magnetisations, M and M , respectively, and coercive "elds, H and H , respectively. For a sample ! ! with a mixture of phases A and B we expect a hysteresis loop which will be an addition of those shown in Fig. 4(a), this is illustrated in Fig. 4(b), where we assume an equal content of the two
Fig. 3. Hysteresis loop for the Sm sample taken at 250 K. This sample was prepared under high-vacuum conditions, i.e. 0 mTorr O .
Fig. 4. (a) Schematic diagram of hysteresis loops for two magnetic phases, A and B, where the saturation magnetisations and coercive "elds are indicated. (b) Combined hysteresis loop for a system of equal quantities of phases A and B. (c) Di!erence curve for the double magnetic phase indicated in (b), where dM is the di!erence between the ascending and descending portions of the hysteresis cycle in (b). Values of the coercive "elds for the two phases are shown along with the saturation magnetisation of the hard magnetic phase (B) and the remnance for the softer magnetic phase (A), M . 0#+
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phases. This complex loop contains all the information, i.e. magnetisations and coercive "elds, of the individual loops, and can be more easily analysed by subtracting the ascending and descending portions of the hysteresis cycle. Such a &di!erence' curve is shown in Fig. 4(c). The central peak will, in general, give the value of 2M (where M de0#+ 0#+ notes the remnance magnetisation) for the magnetically softer material, A, and the value of H can ! also be evaluated, as indicated. On either side of the central peak a plateau region is observed before reaching the magnetically saturated region, where d "0. The height of the plateau region above + d "0 will give twice the saturation magnetisation + of the harder magnetic phase, 2M , see Fig. 4(c). From the transition of dM"2M to dM"0 it is possible to evaluate the coercive "eld, H , of ! phase B, as illustrated. It will be noted that the total saturation magnetisation M "M #M , is 1 2-2 obtained from the combined hysteresis loop, Fig. 4(b). Thus we see that we can obtain the saturation magnetisations and coercive "elds of both magnetic phases uniquely, provided that the magnetic phases have su$ciently di!ering coercive "elds. Using the data from the SmFeO sample, the hysteresis loop of which is shown in Fig. 3, we obtain the di!erence curve (dM versus H), given in Fig. 5. From this curve it is evident that there are, at least, two clearly distinct magnetic phases. The
Fig. 5. Di!erence curve for the Sm sample, where the data is taken from Fig. 3. Signals for the soft magnetic phase (see the explanation in the text) and orthoferrite phase are clearly distinguishable.
295
central peak is derived from the signal of a relatively soft magnetic phase, which has a relatively small coercive "eld (in the region of 0.06 T) and a fairly strong magnetic moment. The plateau region observed on either side of the central peak is due to a very much harder magnetic phase, which is associated with the samarium orthoferrite. The latter shows a much weaker saturation magnetic moment, about 7.5;10\ emu and a very large coercive "eld, in the region of 1.3 T. This curve is representative for the other samples studied (Y, Dy and Gd), measured in the temperature range 5}290 K. Using the di!erence curve and the hysteresis loops in conjunction, it is possible to extract the magnetic moments and the coercive "elds of the two magnetic phases uniquely, as a function of temperature. In Fig. 6 we show the temperature dependence of the magnetic moment of these two phases for the Dy sample grown in high-vacuum conditions, i.e. 0 mTorr O , where we indicate the magnetic moment of the orthoferrite phase, M , and the magnetic moment of the remainder of the sample, M . (Note the di!erent scales for the two phases.) We see that both phases have a similar temperature dependence. We clearly see a strong enhancement of the magnetic moments for both phases towards lower temperatures. This behaviour is observed, to varying degrees, in the samples for Y and Gd. Neither the Sm garnet nor orthoferrite display, in their bulk form, such an increase at low temperature, and this is not observed in the Sm
Fig. 6. Temperature dependence of the magnetic moments for the orthoferrite and softer magnetic (residual) phases for the DyFeO sample. This sample was prepared in high vacuum.
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sample studied here. A magnetically observable signature of many of the rare-earth iron garnets is the compensation point, Sm Fe O is an exception. It is noted that such compensation points should be present for both the Dy and Gd iron garnets (226 and 286 K, respectively [3]). Curiously, however, we observe no evidence of these. Allowing for the presence of an Fe O phase in these samples, we would expect that the softer magnetic phase will be comprised of both the magnetite and the garnet phases. However, since the magnetic moment of the magnetite is much larger than that of the rare-earth garnets at higher temperatures, we may expect this to dominate the net magnetic moment of the softer phase and thus mask the compensation point. The magnetite and garnet magnetic phases cannot be clearly distiguished, from the hysteresis loops themselves, in the manner apparent for the orthoferrite phase. The latter having a much greater value of coercive "eld as to be easily separable. In Table 1 we give the relative quantities of the di!erent phases present in the samples, where we account for the following phases: RFeO , R Fe O , Fe O and R O (this oxide phase is not necessarily formed completely). These have been evaluated in the following manner. For an N-phase magnetic system we can write the net magnetic moment as m " < m , (1) 2-2 , , , where < represents the relative volume of phase , N and is dimensionless when the following condition applies: < "1. (2) , , In general, our systems, as described above, present three magnetic phases. We can, however, simplify matters since our analysis allows us to separate one of the magnetic phases (i.e. the hard orthoferrite phase). The residual is therefore made up of the two-component magnetite}garnet softer phase. In the cases where the garnets show a strong enhancement of magnetic moment at low temperatures [3,5], we can further separate the magnetic moments for these two phases, since the magnetite shows no such behaviour. Since the Dy and Gd
Table 1 Percentages of the various phases in the samples and coercivity values taken from SQUID data
% Orthoferrite % Magnetite % Garnet % Oxide or nonmagnetic residual H (¹) ! H (¹) !
Dy
Gd
Sm
Y
43.8 10.7 14.7 30.8
40.9 1.7 22.3 35.1
* * * *
91.6 * 4.2 4.2
1.52 0.020
1.52 0.026
1.65 0.064
1.8 0.044
Sample annealed to 8603C for 1 h.
garnets have compensation points in the temperature range studied, at these points the magnetic signal (for the softer magnetic phase) should be entirely due to the magnetite. Extrapolating back to 5 K, from bulk magnetic behaviour of the Fe O , we can evaluate the di!erence between the two phases and thus ascertain their respective magnetic moments. Now, for a two-phase system we have, from Eq. (2): < "1!< . (3) Taking the ratio of the two magnetic signals, we can write < m < m . a" " (4) < m (1!< )m Comparing with the ratio of the bulk values for these materials, we have m M b" " . m M
(5)
Combining Eqs. (4) and (5) we can obtain the relative volume of phase A as a < " . a#b
(6)
< is then simply obtained from Eq. (3). Using a similar approach we can obtain the relative volume of the orthoferrite phase. Since the target is comprised of almost exclusively the orthoferrite phase, we must consider the following chemical equations, which occur during the deposition process, to account for the presence of the other
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magnetic phases (i.e: Fe O and R Fe O ) in the samples: 5RFeO PR Fe O #R O ,
(7)
12RFeO P4Fe O #6R O #O .
(8)
(This implies the presence of a non-magnetic oxide phase.) This then allows us to estimate the relative quantities of all the phases in the samples. Since the R O phase does not contribute to the magnetic signal, we cannot assume that this phase is completely formed, hence this refers to the quantity of the oxide or remaining material. In the case of the Sm sample, this was not possible since the Sm garnet phase exhibits neither a strong low-temperature enhancement of magnetic moment nor a compensation point. In the case of the annealed YFeO sample, it is reasonable to assume that there is no magnetite phase present in the sample. The calculations of phase quantities have therefore been made on this assumption. Also indicated in Table 1 are the average values of the coercive "elds for both the orthoferrite and the residual softer magnetic phase. The former display very large values, from 1.5 to 1.8 T, where it should be noted that the latter value refers to the yttrium sample which was annealed at 8603C for 1 h. In Fig. 7 we give the temperature dependence of the coercive "eld for the orthoferrite phase (here for
Fig. 7. Temperature dependence of the coercive "eld for the orthoferrite phase of the DyFeO sample.
297
the DyFeO sample, which is representative of the other samples studied). We see that the value of H remains roughly constant with increase of tem! perature and has a value of about 1.5 T region.
4. Conclusions The as-deposited layers grown at 4503C, in the absence of oxygen, from orthoferrite targets show evidence of several phases, among which the orthoferrite and garnet phases could be identi"ed. The indications are that phases of magnetite and the rare-earth oxides are also present in the as-grown samples, which would be expected from stoichiometric considerations. The magnetic phases of the orthoferrite and the magnetite and garnet phases together are clearly distinguished from SQUID magnetometry. This can be more clearly demonstrated by taking the di!erence of the ascending and descending branches of the hysteresis loops, from which values of magnetic moment and coercive "eld can be obtained for both phases. While the garnet phases (R Fe O ; R"Y, Dy, Gd, Sm) together with the magnetite show relatively strong magnetic moments and small coercive "elds, the orthoferrite phases (RFeO ) have weak saturation magnetic moments and very high coercive "elds, which range from about 1.5 T in DyFeO and GdFeO to 1.65 T in SmFeO and 1.8 T in YFeO . This allows the evaluation of the relative volumes of all the phases present. The variation of the magnetic moment, in the as-grown samples, as a function of increasing O pressure during deposition can be understood as a suppression of the magnetite phase in an oxidising atmosphere. Further changes of the magnetic moment will then be due to the varying quantites of the garnet and orthoferrite phases. By a careful analysis of the magnetometric data and a consideration of the chemical transformations, we have shown that we can estimate the relative quantities of the various phases that form in the as-grown samples. The orthoferrite phase can be almost completely recuperated from the mixed phase as-grown samples by annealing at 8603C for 1 h. We have estimated that we recover over 91% of the orthoferrite phase by such a process in the case
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of the yttrium sample. Similar results are expected for the rare-earth samples.
Acknowledgements One of the authors (DSS) would like to thank the EC for "nancial support under the TMR programme, grant number: ERB4001GT971610. We would like to thank M.-C. Brianso for performing the X-ray di!raction experiments.
References [1] S. Geller, E.A. Wood, Acta Crystallogr. 9 (1956) 563. [2] S. Geller, V.B. Bala, Acta Crystallogr. 9 (1956) 1019. [3] R.S. Tebble, D.J. Craik, Magnetic Materials, Wiley-Interscience, New York, 1969. [4] D. Treves, J. Appl. Phys. 36 (1965) 1033. [5] D.J. Craik (Ed.), Magnetic Oxides, Wiley, New York, 1975. [6] R. Krishnan, A. Lis", M. Guyot, V. Cagan, J. Magn. Magn. Mater. 147 (1995) L221. [7] D.S. Schmool, N. Keller, M. Guyot, R. Krishnan, M. Tessier, unpublished.