MAGNETIC RELAXATION EFFECTS IN 57Fe/Ag SUPERLATTICES FROM CONVERSION ELECTRON MÖSSBAUER SPECTROSCOPY

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MAGNETIC RELAXATION EFFECTS IN 57Fe/Ag

SUPERLATTICES FROM CONVERSION ELECTRON

MÖSSBAUER SPECTROSCOPY

F. Volkening, B. Jonker, J. Krebs, G. Prinz, N. Koon

To cite this version:

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JOURNAL DE PHYSIQUE

Colloque C8, Supplement au no 12, Tome 49, decembre 1988

MAGNETIC RELAXATION EFFECTS IN 5 7 ~ e / ~ g

SUPERLATTICES FROM

CONVERSION ELECTRON MOSSBAUER SPECTROSCOPY

F.

-4. Volkening

(*),

B.

T.

Jonker (I),

J. J.

Krebs ( I ) ,

G.

-4. Prinz and

N. C.

Koon (I) ( I ) Naval Research Laboratory, Washington, DC 20375-5000, U.S.A.

(2) Sachs/Freernan Associates, Landover MD 20785, U.S.A.

Abstract. - The magnetic ordering process in single crystal ( 1 0 0 ) ~ ~ F e / ( 1 0 0 ) ~ ~ superlattice films has been studied via

Mijssbauer spectroscopy. Measurements were made over the temperature range 13 K

5T 5

300 K. The 6 7 ~ e layer thickness was 2.4 monolayers. The spectra were fit using a stochastic relaxation model which incorporates an order parameter. The phenomena are attributed to the two dimensional nature of these ultra-thin Fe films.

In previous publications [I, 21 we reported on the static magnetic properties of ultrathin Fe(lOO)/Ag(lOO) superlattices as probe1 via conversion electron Mossbauer spectroscopy. We found that for Fe layer thicknesses 131

5

2.4 monolayers (ML) the films exhibited a substantial perpendicular surface anisotropy. This, along with the well known [3] growth characteristics of Fe on the Ag(100) surface led us to conclude that the films are flat and continuous. These films should then be considered as members of a class of materials which are essentially two-dimensional in nature.

Previous magnetization measurements indicated the presence of superparamagnetic-like fluctuations similar to what one would expect for a 2-d system [4]. The purpose of the present experiment was t o determine via

Mossbauer spectroscopy the dynamics of the magnetic ordering process of 57~e(100)/~g(100) superlattices in zero applied field.

Using a stochastic model we were able to determine the temperature dependence of the fluctuation frequency and local order parameter. Final results are available for the 2.4 ML superlattice only. The justifi- cation for fitting the spectra assuming fluctuating hyperfine fields instead of a temperature dependent static distribution is the appearance of superparamagnetic- like fluctuations in the magnetization data [4].

The details of sample preparation have been described previously [I]. The 5 7 ~ e / ~ g layer thickness was 2.415.6 ML. The number of periods in the superlattice was 20. The conversion electron Mossbauer spectrometer used was of the cylindrical mirror anal- yser type [I].

Mossbauer spectroscopy probes both the local magnetic order and the spin-autocorrelation time r,,

(lo7

<

l/r,<10~~ S-I). An exact treatment for the Mossbauer lineshape based on the stochastic relaxation theory has been given by van der Woude and Dekker [5] and Blume and Tjon [6]. For a spin 112 system the dynamics, including the effects of partial ferromagnetic order, can be described by two parame-

ters: the magnetic order q such that the probabilities for s, to be +1/2 or -112 are (1

+

q) 12 and (1

-

q) 12, respectively, and the spin flip frequency 0. The presence of pktial magnetic order implies two spin flip frequencies 0 (+-) and 0 (-+) ; however, the two are related by detailed balance such that 0 (+-) (1

+

q) =

0 (-+) (1

-

q)

.

The other parameters in the model are fixed, as are the quadrupole splitting, isomer shift, and ground state hyperfine field.

Due to the leftlright asymmetry in the spectra a simple two magnetic site model [I] was used t o fit the spectra. The values for the quadrupole splittings, isomer shifts, and ground state hyperfine fields for the two sites are respectively: Q*

-

0.08

f

0.03, 0.14

f

0.02 mm/s, IS = 0.02

f

0.02, 0.56

f

0.05 mm/s, and H ~ ~ = 3 7 2

f

5, 360 f 5 kG. The linewidth was taken from the fit to the room temperature data and was 0.33 mm/s (FWHM) for both sites, the instrumental linewidth being 0.3 mm/s.

Some representative data for the 2.4 ML thick sample along with fits calculated from the theory are shown in figure 1. Due to the substantial perpendicular surface anisotropy of these films the intensity in the Am = 0 lines is extremely weak [I]. The line intensity ratios were taken as 3:0.4:1 for all temperatures below 180

K.

At 180 K a better fit was obtained using dine intensity ratio of 3:2:1 instead of 3:0.4:1, sug- gesting that the moment directions are becoming more random. As is clear from the figure the model yields good agreement with the data over the whole temperature range measured. The absence of a central peak in the spectra along with the substantial perpendicular anisotropy strongly suggest that the fluctuations are not due t o small superparamagnetic clusters.

The values for q &d

0

obtained from the model are plotted versus temperature in figure 2. For T

<

100 K q varies linearly with temperature. At 200 K

the hyperfine splitting has vanished and

fl

appears to diverge, the spectrum becoming equivalent to that at room temperature. Preliminary data for 1.8 ML and 0.9 ML thick superlattices~show similar effects except

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JOURNAL DE PHYSIQUE

-8 -4 0 4 8

Velocity (rnrnls)

Fig. 1. - Mijssbauer spectra for the 2.4 ML superlattice. Solid lines represent the fits using the model described in

the text.

Fig. 2. - Temperature dependence of the order parameter

71 and relaxation frequency R for the 2.4 ML superlattice (1 mm/s =7.3x 10' s-I).

that the temperature scale is shifted downward as the thickness of the Fe layers decreases [I].

It should be remarked that in our previous work [I] the hyperfine fields (spins) below 50 K for the 2.4 ML film were assumed static except for spin waves, the low temperature line broadening assumed due to a distribution of hyperfine fields. If that picture is correct

then the fluctuations should freeze out near

T

= 50

K

and

R

should drop to zero. In order t o resolve this difficulty Mossbauer measurements at 4.2 K and in a field are planned.

The physical picture which we use is based on the investigations of Lines [7] and Berezinski and Blank [8]. Due to the low dimensionality of the film a con- siderable amount of "short range" order can exist. For a given region of strongly correlated spins of size r one can define a corresponding relaxation time r (order

lo-'

s). Inside of the region, spin waves with wave vec- tor larger than l/r can also exist, leading to a decrease in the average hyperfine field. The model is therefore one of "short range" correlated regions which 3uctuate slowly and which form an instantaneous background on which fast short wavelength spin waves are super- imposed. As the temperature is decreased both T and

T increase while the number of spin waves excited de-

creases accounting for both the observed slow fluctuai tions

R

and the dependence of q and R on temperature. In conclusion, interesting relaxation effects have been observed in the Mossbauer spectra of 5 7 ~ e / ~ g ( 1 0 0 ) single crystal films for Fe layers

5

2.4 ML thick. The observed effect can be attributed to the two-dimensional nature of the films. Addi- tional work is under way on thinner films in addition to planned measurements at 4.2 K and in a field.

[l] Volkening, F. A., Jonker, B. T., Krebs, J. J., Koon, N. C. and Prinz, G. A., J. Appl. Phys.

63 (1988) 3869.

[2] Koon, N. C., Jonker, B. T., Volkening, F. A., Krebs, J . J. and Prinz, G. A., Phys. Rev. Lett. 59 (1987) 2463.

[3] Jonker, B. T., Walker, K.-H., Kisker, E., Prinz, G. A. and Carbonne, C., Phys. Rev. Lett. 57

(1986) 142.

[4] Krebs, J. J., Jonker, B. T. and Prinz, G. A. (these Proc.).

[5] van der Woude, F. and Dekker, A. J., Phys. Sta- tus Sofidi 9 (1965) 775.

[6] Blume, M. and Tjon, J. A., Phys. Rev. 165 (1968) 446.

[7] Lines, M. E., Phys. Rev. B 3 (1971) 1749. [8] Berezinskii,

V.

L. and Blank,

A.

Ya., sov. Phys.-