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MÖSSBAUER EFFECT AND MAGNETIC
PROPERTIES OF AN AMORPHOUS Fe2O3
A. van Diepen, Th. Popma
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 12, Tome 37, De'cembre 1976, page C6-755
MOSSBAUER
EFFECT AND MAGNETIC PROPERTIES
OF
AN AMORPHOUS
Pe203
A. M. VAN DIEPEN and Th. J. A. POPMA Philips Research Laboratories, Eindhoven, The Netherlands
RbumB. - Un oxyde amorphe de fer 011) a et6 prkpark par dkcomposition thermique d'un akrosol sur un substrat chauffk. Les mesures de la susceptibilitk magnktique montrent une temp&- rature de N k l de 80 K. En dessous de 50 K on observe un moment thermo-remanent. Le moment paramagnetique est de 2,5 p~ par ion de fer, indiquant la prksence d'aggrkgats antiferromagnk- tiques. Les spectres Mijssbauer montrent un doublet paramagnetique au-dessus de 80 K avec un dkplacement isomerique de
+
0,15 mmls. En dessous de 80 K une separation hyperfine est obser- vke avec, ?L 5 K, Hhr = 470 kOe et AEQ = 0,11 mmls. 11 en est d6duit que les ions Fe3+ sont entourks par un octakdre dkforrne d'ions W-. Leurs axes de symktrie sont orient& arbitrairement dans la maille apkriodique, laissant une large dispersion des angles v entre Hhn et Vzz.Abstract.
-
An amorphous iron (111) oxide was prepared by thermal decomposition of an aerosol on a heated substrate. Magnetic susceptibility measurements show a Neel temperature of 80 K. Below 50 K a thermoremanent moment is observed. The paramagnetic moment amounts to 2.5 PB per iron ion, indicating antiferromagnetic clustering. Mijssbauer spectra show a para- magnetic doublet above 80 K with an isomer shift of+
0.15 mm/s and a quadrupole splitting of 1.01 mm/s ; below 80 K a hyperfine split spectrum is found with, at 5 K, Hhf = 470 kOe andAEo = 0.11 mmls. It is derived that the Fe3+ ions are coordinated by a distorted octahedron of 0 2 - ions. Their symmetry axes are randomly oriented in the aperiodic lattice, leaving a wide
distribution of angles W between Hhf and Vzz.
1. Introduction. - In previous studies [l, 21 we reported on the preparation and magnetic properties of an amorphous oxide of yttrium and iron with garnet composition. A complication of those materials was the fact that they were obtained as small particles which led to a superparamagnetic behaviour superim- posed on more intrinsic properties. We now have been able to prepare an amorphous iron oxide as a continuous thin film with a thickness of a few microns. This provides the opportunity to study the magnetic properties of amorphous iron-oxide materials more
directly.
2. Experimental.
-
An amorphous iron (111) oxide was prepared by thermal decomposition of an aerosol upon a heated substrate in air. The aerosol was generated by pneumatically atomizing a solution of iron (111) chloride in butyl acetate. As a substrate a thin plate of fused quartz was used. The temperature and the character of the substrate determines the phase of Fe203 obtained.The material formed on fused quartz below 250 OC is amorphous, between 250 and 300 OC the metastable P-Fe20, with a bixbyite structure is obtained and above 400 OC the stable form of a-Fe20, is formed.
The amorphous character of the material studied was checked by X-ray and electron diffraction. The composition was shown to be Fe203 by the fact that heating at 400 OC resulted in a transition to a-Fe@, without a change in weight.
Magnetic susceptibility measurements were taken between 4 and 300 K with a PAR vibrating sample magnetometer. The amorphous Fe203 had to be scraped off from the substrate in order to avoid the contribution of the diamagnetic susceptibility of the substrate and to reduce the lateral dimensions of the film to a level acceptable for measurements. With paraffine the parts of the film were fixed in the sample holder.
The magnetization (M) as a function of the magnetic field (H) was measured at several temperatures between 4 and 300 K. At low fields all curves show a small non-linear dependence of M on H while the dependence is linear for higher fields. A typical example of such a behaviour is given in the insert of figure 1. In the region 80-300 K the high field suscep- tibility can be described by a Curie-Weiss law (Fig. 1). The magnetic moment per iron ion as calculated from the slope of the reciprocal susceptibility versus tempe- rature amounts 2.5 p,. An asymptotic Curie tempera-
A. M. VAN DIEPEN AND Th. J. A. POPMA
FIG. 1.
-
High field reciprocal susceptibility X-1 versustemperature T of amorphous Fez03. In the insert the magneti- zation versus magnetic field at 275 K is given.
ture 8 of
-
200 K is derived while the behaviour of the susceptibility around 80 K suggest an ordering tempe- rature T, E 80 K.Below 40 K a remanent magnetic moment can be induced by cooling the material in a magnetic field. The moment measured at zero field that can be frozen in by cooling from 50 K to a temperature T i n a field of 20 kOe is given in figure 2. Thermoremanent magneti- zation~ are not unusual for small antiferromagnetic particles [3] and are also observed in mictomagnetic systems [4]. We attribute the phenomenon in our sample to clustering of several Fe3+ moments in
comparable distorted oxygen surroundings.
FIG. 2.
-
Remanent (0) and thermoremanent (0) magneti- zation Mr of Fe203 versus temperature after cooling in amagnetic field of 20 kOe.
Mossbauer spectra were measured with a conven- tional constant acceleration spectrometer at room temperature, liquid nitrogen temperature and liquid helium temperature. For these measurements the mate- rials were removed from the glass substrates and the powders obtained were glued to much thinner Fe-free mica. Spectra are given in figure 3. A sample that was sprayed and measured on a teflon substrate produced
L
--10 - 8 - 6 -L - 2 0 +2 +L +6 +B +l0
VELOCITY l rnm/s)
Fig. 3.
-
Mossbauer spectra of amorphous Fez03 taken at 300 K, 78 K and 5 K (top to bottom).the same room temperature Mossbauer spectrum as that given in figure 3.
At room temperature a paramagnetic doublet is found with quadrupole splitting AEQ = 1.01 mm/s and isomer shift IS =
+
0.15 mm/s (with respect to 57Co in Pd). At liquid helium temperature the spectrum consists of six lines ; a little extra intensity near u = 0 originates from the Fe in the beryllium windows of the cryostat. From this spectrum we derive a hyper- fine field Hhf = 470 kOe and a quadrupole shifteQVzz(3 cos2
V
-
1)/2 = 0.22 mmls, whereV
is the angle between the direction of H,, and the main axis Zof the electric field gradient. At 78 K a rather broad unresolved spectrum is observed, which has the appea- rance of a spectrum taken just below the magnetic ordering temperature. The hyperfine splitting is clearly reduced with respect to that at 5 K so that the ordering temperature must be a normal one unlike the blocking temperature that was observed in amorphous Y,Fe,O,, [l, 21. This is in agreement with the Ntel temperature derived from the magnetization measurements.
3. Discussion.
-
Both the magnetization and the Mossbauer results indicate a paramagnetic beha- viour above 80 K. The small departure from a linear M ( H ) curve (see Fig. 1) can easily be accounted for by a contaminating phase with a permanent moment, e. g., the presence of less than one weight percent Fe30,, an amount which is almost undetectable by means other than a magnetization measurement.The paramagnetic moment per iron ion as derived from the linear lIx(T) curve (2.5 pB) is much smaller than the 5 pB one expects for Fe3+ ions. Such devia- tions have been observed before in gels of iron oxide
[5, 61 and have been ascribed to the existence of
MOSSBAUER EFFECT AND MAGNETIC PROPERTIES OF A N AMORPHOUS Fez03 C6-757
oxides have high ordering temperatures because of a strong superexchange intefaction. In the amorphous compound such large antiferromagnetic exchange paths will still exist, unless. all Fe-0-Fe angles have become 900, or all Fe-0 distances have changed with respect to other Fe(II1) oxides, both possibilities being highly unlikely.
In conjunction with these large exchange energies the rather low NCel temperature can be understood by the aperiodicity of the lattice. In such a lattice it is difficult to arrange the Fe3' magnetic moments in such a way that all near neighbour interaction energies become lowered. A similar situation exists in the fcc antiferromagnet for first neighbour interactions, which leads to a reduction of TN. Small departures from an average coordination, however, may give rise to strong local ordering of a cluster of ions which is consistent with the low paramagnetic moment observed.
In the value of
l
8l
the total exchange energy shouId appear which is, at first sight, in contradiction with the low value ofl
8l
measured. However, in order to obtain a physically realistic 0 we have to extra-polate 1 / ~ from temperatures high relative to all exchange energies. Assuming that an Fe3+ ion in an oxygen surrounding has a spin of 5 p, we derive from
X
( T = 300 K) and the theoretical value of the Curie constant : 8 =-
1 000 K, which value of 0 compares well with those known for iron oxides.At room temperature the Mossbauer lines are relati- vely narrow for such an amorphous material, at half height
rlI2
= 0.62 mm/s, indicating a not too wide distribution of electric field gradients. Compare, e. g.,rIl2
= 0.90 mm/s for amorphous Y3Fe5OI2 at 300 K, while AEQ and IS are practically the same in Y3Fe5012 as they are here [l, 21. All room temperature spectra are slightly asymmetric. The extra intensity in the low- velocity component may be the result of a few percent Fe sites with a much smaller electric field gradient, either in an amorphous or a (micro)-crystalline state. The quadrupole splitting of about 1 mm/s points to quite a large deviation from spherical symmetry at the Fe-sites, comparable to that at the tetrahedral site in the garnet structure or to the d site in P-Fe203. The isomer shift is comparable to that of Fe3' in octahedral sites. It is interesting to note thatAEQ
and IS arepractically the same for amorphous Y3Fe5OI2 and amorphous Fe203, indicating a close similarity on a local scale between the two materials. It thus seems that the local surrounding of Fe3+ in amorphous oxide materials is more determined by Fe3+ and oxy- gen than by the presence of other metal ions.
Linewidths for the spectrum taken at liquid helium temperature are about 1 .l mm/s, which is almost twice as large as at room temperature. The quadrupole coupling eQV,,(3 cos2 W
-
1)/2 = 0.22 mm/s has to be compared with the room temperature value which is an order of magnitude larger :(assuming axial symmetry). This difference is also present in amorphoils ferric hydroxide [7, 81 while in
amorphous Y3Fe5012 as a result of the very broad lines we were not able to deduct a value for the quadru- pole coupling at 5 K. In crystalline materials such ,a difference is explained by the factor (3 cos2 W
-
l), which becomes zero for W JT 550, but it is unlikely that this holds for every iron ion in the amorphous material.The magnitude and direction of the hyperfine field at an Fe nucleus are determined by the exchange and dipole interactions between the electron spins and by the local crystalline field. If the latter were dominating, the angle W between V,, and H,, would be the same for all Fe nuclei, and the resulting magnetic structure of the material would be such that the electron spins are more or less randomly oriented in space, but strongly fixed to a local symmetry axis. This would produce a sharp six !ine Mossbauer spectrum with a clearly defined value for W. If, on the other hand, the aniso- tropy were weak, as is not unlikely for iron (111) ions, the exchange interactions would impose the type of magnetic ordering, irrespective of any local symmetry axis. In that case the angle W between V,, and H,, varies widely. Each angle corresponds to a different six-line Mossbauer spectrum. Tf a homogeneous distri- bution of W values is assumed it is not very difficult to calculate the corresponding distribution in line posi- tions and to fold these with Lorentz curves, very much the same as is, for instance, commonly done to des- cribe powder spectra in NMR 191. We calculate that for a L0rent.r linewidth equal to
+
eQV,, N 1 mm/sthe very characteristic (3 cos2 W
-
1) dependence of the lineshape is wiped out and a rather symmetric line, not far from the unshifted position, is found. Thus an overall Mossbauer spectrum is observed with hardly any quadrupolar shift. Within the statistics of our experiment we cannot observe the deviations from Lorentzian line shapes that are still allowed for3
eQVzz = 1 mm/s andr,,,
= 1 mm/s. This type of structure is very likely to occur in many other amor- phous materials without typical anisotropic ion. Data of amorphous iron (111) hydroxide [7, 81 and an amor- phous gel containing iron (111) ions 161, for instance, are also consistent with this structure.4. Conclusion.
-
The Mossbauer spectra and the magnetization data show that the amorphous iron oxide is paramagnetic above 80 K. The low value of the paramagnetic moment is ascribed to cluster formation.C6-758 A. M. VAN DIEPEN AND Th. J. A. POPMA
ferromagnetic behaviour. However, a spin-glass-like electron diffraction, Dr. P. R. Locher and L. de Vries structure cannot be ruled out completely. for computational help, Mrs. M. G. J. Kamminga and J. J. P. Verheijden for technical assistance, and Acknowledgements.
-
We are indebted to H. B. Dr. P. F. Bongers and R. P. van Stapele for stimulating Haanstra and Mrs. J. R. M. Gijsbers for doing the discussions.References
[l] POPMA, Th. J. A. and VAN DIEPEN, A. M., Mat. Res. Bull. [6] COEY, J. M. D. and READMAN, P. W., Nature 246 (1973)
9 (1974) 1119. 476.
12] Th. J.
*.
and VAN DIEPEN, A. Conference [7] VAN DER GIESSEN, A. A., Philips Res. Reports Suppl. No. 12Proceedings 24 (1 974) 123. (1968).
[3] GELLER, S., GRANT, R. W., CAPE, J. A. and ESPINOSA, G. P.,
J. Appl. Phys. 38 (1967) 1457. [81 OKAMOTO, S., SEKIZAWA, H. and OKAMOTO, S. I., in Reaeti-
[41 BECK, P. A.. Met. Trans. 2 (1971) 2015. vity of Solids (Chapman and Hall) 1972, p. 341. [5j SELW&RD,-P. W., ~agnetochekzistry 2nd Ed. (Interscience [9] BLOEMBERGEN, N. and ROWLAND, T. J., Acta Met. 1 (1953)
