low temperature properties of magnetic films

LOW T E M P E R A T U R E PROPERTIES OF MAGNETIC FILMS

(Invited Paper)

E. N. MITCHELL

Physics Department, University of North Carolina*)

The effect of surface oxides on the low temperature magnetic properties of permalloy films is reviewed. Emphasis is placed on the work of H a g e d o r n and M i t c h e l l and coworkers. It is shown that the unidirectional anisotropy in permalloy (typically exhibited at temperatures below 40 ~ can be correlated with the presence of ~-Fe203 as detected in electron diffraction studies. A possible mechanism for the phenomena is discussed which hinges on the depression of the Morin transition in r a as a consequence of a change in the d spacing of the ct-Fe203.

I propose in this paper to summarize the current understanding of the effects of surface oxides on the magnetic properties of permalloy films. I shall first try to summarize the observed magnetic phenomena and our current understanding of the causes of the observed magnetic phenomena, and then relate the magnetic behavior of the films to the observed oxide structures on the films. I shall draw heavily on the works of H a g e d o r n [1, 2] and M i t c h e l l and co-workers [3, 4]. In general I shall restrict my comments to permalloy films of near 82~ Ni and 18~ Fe composition except where results on other materials impact on this problem.

M it c h e 11 and B r i s c o e [3] first observed that permalloy films when cooled to low temperatures exhibited a marked increase in the anisotropy field (Hk) as measured with a conventional 1000 Hz looper. In particular they found that Hk increased by about a factor of two between 4"2 ~ and 1"3 ~ These films (about 2000 ~) were made in the usual oil diffusion vacuum system at 300 ~ and given no special oxida-

tion or redusXion treatment. H a g e d o r n [1, 2] showed that films which had been oxidized at 120 ~ for a period

of hours (after reduction in hydrogen) showed a similar behavior to that reported above but the value of Hk did not show an increase if the measurement was made in a rotating field magnetometer where the field rotated at frequencies of about 500 Hz. He further showed that if one examined the magnetic behavior of the film at low temperature in the easy direction the hysteresis loop was biased under certain circum- stances. This occurred if a field was applied along the easy axis during the cooling process and then removed before the magnetic measurements were made. He showed that the magnitude of both of these effects increased with increasing oxidation and that both effects were absent if the film was not oxidized. The onset of the above effects appeared to be at or near 40 ~ If the film was oxidized at a higher temperature (250 ~ then the onset of the phenomena appeared to be above 77 ~

Ba i l ey et al. [4] performed a set of experiments designed to determine the temperature of the onset of this phenomena as a function of oxidation time and tempera-

*)Chapel Hill, North Carolina, 27514, U.S.A.

384 Czech J. Phys B 21 (1971)

Low temperature properties of magnetic films

ture. These experiments involved measuring the temperature at which the offset in the easy axis hysteresis loop was stabilized against reversal by a biasing field along the easy axis (i.e. the temperature at which the effect was "frozen in"). They found that this transition temperature decreased slightly with increased oxidation time. Some evidence indicated that increasing the temperature at which the film was oxidized broadened the range of temperature over which the transition took place.

The magnetic behavior of the oxidized films was interpreted by H a g e d o r n [2] as a unidirectional anisotropy since this could explain both the offset of the easy axis loop and the apparent increase in the anisotropy field for small drive fields. He attributed this to an exchange coupling between an antiferromagnetic surface oxide exhibiting considerable anisotropy and the underlying permalloy. He speculated that the oxide might be FeO. This explanation in terms of a unidirectional anisotropy is quite successful in explaining the more general features of the magnetic observations though it is not completely successful in explaining thickness dependence of the phenomena. One must additionally postulate considerable hysteresis effects in some of the measurements to make them all internally consistent. For example, the increase in H k is much less than that which is observed if one calculates H k from information derived from the offset of the easy axis hysteresis loop. However, some hysteresis effects were observed directly in some of these films so such an interpretation does not appear to be very surprising.

Bailey et al. [4] tried to identify the oxide responsible for this unidirectional anisotropy by alternately oxidizing and reducing permalloy films and correlating the presence or absence of various oxides on the surface with the presence or absence of the unidirectional anisotropy. In these experiments the oxides were examined by transmission electron diffraction on specimens which were treated identically with those on which magnetic measurements were made. It was necessary to use different specimens since the permalloy had to be removed from the substrate and since the permalloy itself had to be very thin for the electron microscope studies of the oxides.

All of the diffractions patterns could be identified as being due to three compounds. In addition to the lines of permalloy, the characteristic lines of Fe30 4 (spinel ferrite) and ~-Fe20 3 (hematite) were observed. The lines of c~-Fe203 disappeared on reduction at 250 ~ and reappeared on oxidation at 120 ~ and c~-Fe203 seems to be the most likely candidate for the cause of the magnetic effects. No evidence of an oxide of nickel was observed nor was any evidence found for the presence of FeO regardless of the length of time the oxidation was carried out at 120 ~ (up to 200 hours). The patterns attributable to Fe304 were present independent of the chemical treatment of the film. Unfortunately the characteristic patterns of Fe304 and NiFe204 are undistinguishable at the level of precision with which this experiment was performed. Thus either of these materials could have been correlated with the oxidation and reduction process and this correlation been masked by the predominance of the other compound. In the presence of so much Ni it would seem likely that at least some NiFe204 was present.

Czech. J. Phys. B 21 (1971) 3 8 5

E. N. Mitchell

F o l e y et al. [5, 6, 7] have extensively studied the oxidation of bulk iron-nickel alloys at temperatures from 600-900 ~ They have studied metals which were 30~ Ni-70~ Fe; 41~o Ni-59~ Fe; and 78~ Ni-18~ Ee-4~ Mo. The presence of Mo in the last alloy and the difference in the composition of the others relative to the alloy discussed here makes correlations somewhat tenuous. Some generalizations may be relevant, however. Both the ferrite structure and that due to ~-FezO 3 are observed in his studies, though the ferrite is the most common. FeO is not observed and NiO is only observed if the oxidation temperature approaches 900 ~ When ~-Fe203 and a ferrite are present the ferrite is next to the metal and the ~-Fe20 3 on

AL

Fig. 1. Crystal structure of ~-Fe203. Only the iron sites are shown. The solid arrows show the orientation of the spins of the iron atoms below the Morin transition and the dashed arrows the

orientation above the Morin transition,

top of that. There is also evidence that oxidation at lower temperatures is more favorable for the formation of ~-FezO3 relative to the ferrite.

All of these observations lead one to speculate that when permalloy is oxidized at modest temperatures a ferrite is first formed on the surface of the metal and that this oxide is subsequently further oxidized to form some ~-Fe203 which can then be re- duced with relative ease. It is postulated that this oxide gives rise to the observed unidirectional anisotropy. Hematite is rhombohedral [8] in structure (see figure 1). Each unit cell contains 2 molecules and the iron atoms are placed in pairs centered about (0, 0, 0) and (1/2, 1/2, 1/2) sites in the cell. In bulk form its Curie temperature is about 530 ~ below which it is weakly ferromagnetic with the spins in the (111) plane though alternately directed in alternate planes (dashed arrows in the figure). The spins in adjacent planes are not directed exactly opposite each other but are canted slightly relative to each other. The net effect is to give rise to a small net moment in the (111) plane. At 260 ~ ~-Fe203 undergoes another transition (Morin) in which the spins align along the [111] direction (solid arrows in figure 1) but with alternate planes oppositely directed so that the oxide becomes a strict antiferromagnet. Above the Morin transition the oxide exhibits little anisotiopy [8, 9]. Below the Morin transition the material exhibits marked anisotropy relative to the [111] axis.

3 8 6 Czeeh. J. Phys B 21 (197D

Low temperature properties of magnetic films

The maximum anisotropy [8] observed in ~-Fe/O 3 is considerably higher than the unidirectional anisotropy observed in the oxidized permalloy and so could be a candidate to explain the observed unidirectional anisotropy.

Unfortunately the transition under discussion in this paper does not occur near either of the above temperatures. Schrtieer and Nin inge r [10] using M6ssbauer techniques have shown that the Morin transition in microcrystals of a-Fe/O 3 (of the order of 500 ~ diameter) is depressed from 260 ~ to 166 ~ presumably due to an increase in the lattice spacing of the hematite. The situation in this case is much more complicated since one is not studying free microcrystals but one oxide imbedded in and/or attached to another. Schr~Seer and Nin inger show that a spacing change of 0.2G could explain the effect they observe. On this basis dimensional changes of at most 0.5G could explain all of the observations on oxidized permalloy under discussion here. Following this line of reasoning then one postulates that when the ferrite is oxidized to ~-Fe20 3 the thin layer of ~-Fe20 3 is in a highly distorted state (presumably extended) and this gives rise to a lowered Morin transition temperature in the cr 3. It is further postulated that the transition in the permalloy observed by H a g e d o r n and Bailey et al. is attributed to the Morin transition in the hematite.

Can one in this model explain the increase of the transition temperature of the oxidized permalloy with increased oxidation temperature? At least in the instance of Bailey et al. this is possible since those films were evaporated on borosilicate microscope slides and since the substrates are much thicker than the permalloy any dimensional changes observed in the oxide due to cooling would be determined by the contraction of the substrate. They observed an increase in the transition temperature of about 40 ~ upon increasing the oxidation temperature from 23 ~ to 250 ~ The higher oxidation temperature would cause the dimensions in the plane of the film to decrease about 0.08~ more on cooling which according to Schr6eer would increase the Morin transition by something like 40 ~ This is undoubtedly a fortuitous agree- ment but is nonetheless supportive of the model. One might qualitatively explain the broadening of the transition in the oxidized permalloy with increased oxidation temperatures since a film oxidized at 250 ~ was not oxidized at just 250 ~ but at a range of temperature to 250 ~ giving rise to oxides experiencing varying degrees of strain.

It is worth reiterating that this model is consistent with the observation reported above that the temperature of oxidation determines the temperature of the transition and that the length of time of oxidation primarily determines the magnitude of the unidirectional anisotropy in the permalloy. One still needs to explain why prolonged oxidation at a given temperature seems to reduce the transition temperature of the oxidized permalloy. This may be due to relief of strain in the permalloy itself since it was deposited on the glass at 250 ~ to 300 ~ and is under considerably tension as a consequence. Continued annealing at elevated temperatures may relieve this strain and actually reduce the amount by which the oxide is contracted on successive anneals. The same argument can be used with regard to the glass substrate itself.

The most difficult part of this discussion remains, however. What precisely is the

Czech. J. Phys. B 21[ (1971) 387

E. N. Mitchell

mechanism that makes the Morin transition of ~ - F e 2 0 3 significant in the generation of a unidirectional anisotropy in oxidized permalloy? Bailey et al. [4] reported that the ~-Fe/O 3 was oriented with a texture axis such that the [001] direction was normal to the plane of the film. The data which formed the basis for this conclusion have been reevaluated and it is believed that the above is not true. Rather it is concluded that the [ l l l ] axes of the crystals (see figure 1) are in the plane of the film but oriented in a nearly random fashion. There is some evidence that adjacent crystallites in certain areas are oriented with their axes at about 120 ~ with respect to each other in the plane of this film. Since these conclusions are based on evaluation of the ring due to diffraction from (111) planes in the electron diffraction patterns, one can further con- clude that the a-Fe/O3 contains many defects since the structure factor for a [111] diffraction direction is strictly zero for electron diffraction in a perfect single crystal ofu-Fe203. It appears that in all other respects the ~-Fe203 crystallites are essentially random in orientation.

Since the [111] axes are in the plane of the film there exists the possibility for unidirectional anisotropy due to exchange coupling between the permalloy and the ~-Fe203 (see summary by Jacobs and Bean [11]). In order to couple the antiferromagnet (~-FezO3) to the ferromagnet (permalloy) below the Morin transition the net moment of the layer of spins of the ~-Fe203 nearest the permalloy (i.e. nearest neighbors) must be non-zero. This is necessary if any net directional coupling is to be accomplished. With the [111] axis of the ~-Fe203 in the plane of the film this can be accomplished only if some of the Fe atoms are missing from the lattice (a necessary but not a sufficient condition). The diffraction data cited above indicate that this is the case.

If the vacancies are not all paired vacancies (i.e. not always A and B or C and D in figure 1) in the planes nearest the permalloy then a net moment will exist (on a micro- scale) to which the permalloy can be coupled. If this were the case and if the [111] axes were alligned preferentially along the easy axis of the permalloy then a unidirectional anisotropy could be generated in the permalloy by alligning the net moment in the nearest neighbor plane in the same direction as in the permalloy above the Morin transition and freezing this preferential orientation into the ~-Fe203 by cooling through the Morin transition.

However, the diffraction data indicate that the [111] directions of the various crystallites are distributed isotropically in the plane of the film. If a crystallite is oriented so that its [111] axis is at an angle (O) with respect to the easy axis (see figure 2) in the permalloy then assuming random orientation of the [111] axes in the plane of the film there will be a crystallite oriented with its [111] axis at angle ( - O ) relative to the easy axis in the plane of the film. The uncompensated spins of each of these in the presence of an orienting field above the Morin transition will line up with a component in the same direction as the magnetization of the permalloy. The net effect on the permalloy will be comparable to a single crystallite with its [111] axis along the easy axis of the permalloy and so from the argument above one sees that

388 Czech. j. Phys. B 21 (1971)

Low temperature properties of maonetic films

this will contribute to the observed unidirectional anisotropy when the system is cooled through the Morin transition. In this fashion one can generate a unidirectional anisotropy whose source is the magnetic anisotropy of the hematite even though the hematite exhibits no preferred orientation in the plane of the film.

Three peripheral points are worthy of comment. The exchange anisotropy described here is a nearest neighbour phenomena [11]. The dependence of the unidirectional anisotropy on oxidation time [2] (roughly thickness) is caused not only by making the oxide thicker but also by increasing the fraction of the surface covered by

~ Cl111

Fig. 2. Schematic diagram of the relation between the ~-Fe203 and the permalloy. The arrow through the circle represents the permalloy and its easy axis. The arrows through the diamonds

represent the [111] axes of the hematite.

e-Fe203. One would not therefore necessarily expect a linear dependence of the above anisotropy on oxidation time. The second point relates to hysteresis effects in this anisotropy which H a g e d o r n [2] discusses and which are probably due to a break- down in the preferential coupling cited above. This occurs as the magnetization of the permalloy is repeatedly cycled by the rotating field in a rotating field magnetometer for example [2]. Finally the role of the ferrite in this mechanism has not been discussed.

In summary then it is shown that the unidirectional anisotropy in permalloy at low temperature (and the apparent increase in uniaxial anisotropy) can be explained in terms of an antiferromagnetic exchange coupling between permalloy and ~-FezO3. The temperature at which this coupling takes place is determined by the Morin transition in ~-FezO 3 which is in turn modified by changes in the lattice spacings in ~-Fe203.

of The author would like to thank Professors L. D. R o b e r t s and C. S. S m i t h of the University Nor th Carolina for their helpful discussions during the course of this work.

Submit ted to the IVth I C M T F 24. 7. 1970.

Received 15. 12. 1970.

Czech. J. Phys. B 21 (1971) 389

E. N. Mitchell: Low temperature properties of magnetic films

References

[1] H a g e d o r n F. B.: J. Appl. Phys. 38 (I967), 1362. [2] H a g e d o r n F. B.: J. Appl. Phys. 38 (1967), 3641. [3] M i t c h e l l E. N., B r i s coe C. V.: J. Appl. Phys. 37 (I966), 1489. [4] Ba i l ey S. B., P e t e r l i n T. N., R i c h a r d R. T', M i t c h e l l E. N.: J. Appl. Phys. 41 (1970),

194.

[5] F o l e y R. T., D r u c k J. V., F r y x e l l R. E.: Journal of Electrochemical Society 102 (1955), 440.

[6] F o l e y R. T., G u a r e C. J., S c h m i d t H. R.: Journal of Electrochemical Society 104 (1957), 413.

[7] F o l e y R. T., G a u r e C. J.: Journal of Electrochemical Society 106 (1959), 936. [8] C i n a d u G., F l a n d e r s P. J., S h t r i k m a n S.: Phys. Rev. 162 (1967), 419. [9] N a g a m i y a T., Y o s i d a K., K u b o R.: Advances in Physics 4 (1955), 1.

[10] S c h r 6 e e r D., N i n i n g e r R. C. Jr.: Phys. Rev. Letters 19 (1967), 632. [11] J a c o b s I. S., Bean C. P.: in Magnetism. Ed. G. T. Rado and H. Sut-d, Academic Press,

Inc., New York 1963, Vol. III, p. 271.

390 Czech j. Phys. B 21 (1971)

low temperature properties of magnetic films

Documents