pssapplications and materials science

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phys. stat. sol. (a) 205, No. 8, 1828 – 1830 (2008) / DOI 10.1002/pssa.200723632

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applications and materials science

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Perpendicular anisotropy in Fe/Ag multilayers

J. Balogh^{*, 1}, Cs. Fetzer², D. Kaptás¹, L. F. Kiss¹, I. S. Szűcs², I. Dézsi², and I. Vincze¹

1 Research Institute for Solid State Physics and Optics, Budapest, Hungary

2 KFKI Research Institute for Particle and Nuclear Physics, Budapest, Hungary Received 13 December 2007, revised 10 April 2008, accepted 25 April 2008 Published online 10 June 2008

PACS 75.20.–g, 75.70.Cn, 75.75.+a, 76.80.+y

* Corresponding author: e-mail baloghj@szfki.hu

[Ag(2.6 nm)/Fe(x nm)]₁₀ multilayers (0.2 ≤x ≤ 1) have been prepared on Si(111) substrate by vacuum evaporation of ⁵⁷Fe enriched iron from a heated tungsten crucible in a base pressure of 10^–7 Pa. Ag was deposited by electron beam from a cold copper crucible. For ex-situ transmission Mössbauer spectroscopy measurements the samples were capped with a thick capping layer (55 nm Ag and 100 nm B). The deposited layers were removed from the substrate by using scotch tape. Field cooled (FC) and zero field cooled (ZFC) magnetization curves were measured by a superconducting quantum interference device (SQUID) magnetometer.

The samples show superparamagnetic behavior in the whole Fe thickness range. This is indicated by the devia- tion of the FC and ZFC magnetization curves, as shown in Fig. 1 for a few representative samples. The blocking temperature, as defined by the temperature of the maximum in the ZFC magnetization, is 34 K for x = 0.2 (well reproduc- ing the value of Ref. [2]) and rapidly increases with increasing x until x = 0.5. The broadening of the distribution of the size of the superparamagnetic grains with increasing x can also be inferred from the ZFC curves. Samples with x ≥ 0.6 are also superparamagnetic, as it is indicated by in- field Mössbauer measurements not shown here, but for the majority of the Fe atoms the blocking temperature (TB) is

well above room temperature. The low temperature anom- aly of the FC-ZFC curves of these samples (for x = 0.7 see Fig. 1) can be attributed to an about 10 – 20% of the Fe atoms forming smaller grains with TB below room temperature.

The direction of the spontaneous magnetization changes from in plane to out of plane at around x = 0.6.

This finding is qualitatively in agreement with results on samples prepared under different experimental conditions [1 – 3]. The magnetic anisotropy of multilayers can be affected by many different contributions [4]. For example the density and the orientation of the Ag steps of the growing superlattice were shown [5] to influence the direction of the magnetization. Our preliminary results on granular alloys prepared by co-evaporation of the elements indicate an in plane magnetization in a Fe concentration range equivalent to the present multilayer samples. In case of our multilayers the observed epitaxial growth [6] can explain the appearance of perpendicular magnetic anisotropy, since the coherent growth of the layers involves the presence of strains even in case of a small lattice mismatch. The transi- tion from coherent to incoherent growth modes with increasing Fe layer thickness decreases the strain and can be accompanied by a change of the magnetic anisotropy direction [4].

The direction of the spontaneous magnetization changes from out of plane to in plane at around x = 0.6 in [Ag(2.6 nm)/

Fe(x nm)]₁₀ multilayers (0.2 ≤x≤ 1) prepared on Si(111) substrate by vacuum evaporation. Transmission Mössbauer spectroscopy measurements of removed samples with a thick capping layer are compared to conversion electron Mössbauer spectroscopy measurements of samples on the Si substrate

with a thin capping layer. The stress arising because of the application of a thick capping layer and the removal of the samples from the substrate is shown to have negligible effect on the spontaneous magnetization. The results support that the appearance of the perpendicular anisotropy below x = 0.6 is an intrinsic property of Fe/Ag multilayers.

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phys. stat. sol. (a) 205, No. 8 (2008) 1829

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0 50 100 150 200 250 300 350

160 180 200 220

M(emu/g)

T (K)

0 2 4 6 8 10 12 14

Figure 1 Temperature dependence of the magnetization of the x = 0.4 (diamonds), 0.5 (triangles), and 0.7 (squares) samples measured in 10 Oe magnetic field after cooling the sample in zero (full symbols) and 10 Oe (open symbols) applied field. Note the smaller magnetization scale for x = 0.4 and 0.5.

Since the magnetic anisotropy might also be influenced by extrinsic stress, we examined the dependence of the direction of the spontaneous magnetization on the temperature, on the thickness of the capping layer and on the sample removal. For the latter two purposes in the case of x = 0.4 and 0.7 two identical multilayer samples have been

-8 -6 -4 -2 0 2 4 6 8

-4 -2 0 2 4

Intensity(arbitraryunits)

Velocity (mm/s) 80K

[Ag(2.6 nm)/⁵⁷Fe(0.4 nm)]₁₀ 300K

Figure 2 CEMS and transmission Mössbauer spectra (up and down) of the x = 0.4 multilayer at 80 K and 300 K. The sample measured by CEMS was capped with 5 nm Ag after deposition on Si(111) substrate. The transmission sample was capped with 55 nm Ag + 100 nm B and removed from the Si substrate.

-8 -6 -4 -2 0 2 4 6 8

Intensity(arbitraryunits)

Velocity (mm/s) 80 K

[Ag(2.6 nm)/⁵⁷Fe(0.7 nm)]₁₀

300K

Figure 3 CEMS and transmission Mössbauer spectra (up and down) of the x = 0.7 multilayer samples at 80 K and 300 K. The sample measured by CEMS was capped with 5 nm Ag after deposition on Si(111) substrate. The transmission sample was capped with 55 nm Ag + 100 nm B and removed from the Si substrate.

prepared in the same run. One of them was covered with a thick capping layer (55 nm Ag and 100 nm B) and the other one with a thin capping layer (5 nm Ag). The samples with thin capping layers were examined by conversion electron Mössbauer spectroscopy (CEMS) at 80 K and 300 K.

Mössbauer spectra of the as received samples with thin capping layer (these samples were not removed from their substrates) and of the removed samples with thick capping layer are shown in Figs. 2 and 3 for x = 0.4 and x = 0.7, respectively, measured at two different temperatures. The superparamagnetic blocking temperature is around 150 K for x = 0.4 and above room temperature for x = 0.7. For this reason the Mössbauer spectrum of the x = 0.7 sample exhibits magnetic splitting at 80 K and 300 K, while the sample of the x = 0.4 consists of paramagnetic lines at 300 K. The CEMS and the transmission spectra of samples differing in the capping layer thickness agree very well and show a similar distribution of the hyperfine parameters.

Small differences of the 80 K spectra around the zero velocity arise from Fe impurities in the beryllium window of the cryostat used for the transmission measurements. The average hyperfine splitting, the width of the distribution and the average isomer shifts values were calculated by the NORMOS program and are deployed in Table 1. In measurements at elevated temperatures the distribution of the

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1830 J. Balogh et al.: Perpendicular anisotropy in Fe/Ag multilayers

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Table 1 Average values of the parameters of the hyperfine field distributions fitted to the CEMS and transmission spectra of the different samples at 80 K and 300 K. The following values are indicated: ISav, the average isomer shift (relative to α-Fe at room temperature), QSav, the average quadrupole splitting, Hav, the average hyperfine field of the magnetic components, σ, standard width of the hyperfine field distribution, I25, intensity of the 2nd and 5th lines of the sextets.

x T (K)

mode ISav

(mm/s) QSav

(mm/s) Hav

(Tesla) σ (Tesla)

I25

0.4 300 CEMS 0.14 (1) 0.44 (2) TMS 0.12 (1) 0.45 (2)

80 CEMS 0.28 (1) 24.5(4) 2.6(1) 0.85(20) TMS 0.29 (1) 23.7(3) 2.5(1) 0.85(20) 0.7 300 CEMS 0.10 (1) 23.2(3) 3.0(1) 4.00(10) TMS 0.11 (1) 23.3(3) 3.0(1) 4.00(10) 80 CEMS 0.21 (1) 31.3(3) 1.6(1) 3.80(10) TMS 0.20 (1) 30.7(3) 1.5(1) 3.80(10)

hyperfine fields reflects both the distribution of the local Fe neighborhoods and the relaxation of the magnetic moments of the superparamagnetic grains. The similarity of the hyperfine field distributions observed in the CEMS and in the transmission measurements indicates that the blocking temperature is not affected by the capping layer and by the different removal procedure of the samples.

The intensity of the 2nd and 5th lines (I25) of a magnetic sextet with respect to that of the 3rd and 5th lines is determined by the angle (θ) between the direction of the local magnetization and of the γ-ray, I25 = 4 sin²θ/ (1 + cos²θ). The γ-ray directions are perpendicular to the sample planes both for the CEMS and the transmission measurements. In the case of a hyperfine field distribution an average value of I25 can be given by supposing a uni-

0 50 100 150 200 250 300

0 1 2 3 4

I

25

T [K]

Figure 4 Average intensity of the 2nd and 5th lines of the mag- netic components as a function of temperature for the x = 0.4 (diamonds), 0.5 (triangles), 0.6 (circles), and 0.7 (squares) samples.

form direction of all the magnetic moments. I25 = 0.9 and 3.8 at 80 K (see Table 1) indicates that the average magnetization direction is close to perpendicular and in plane (θ = 36° and 80°) for x = 0.4 and 0.7, respectively, for the CEMS and the transmission spectra as well. Stresses due to the applied capping layer and the sample removal procedure do not affect the direction of the spontaneous magnetization.

The I25 amplitudes determined by transmission measurements in a broad temperature range below TB for several samples are shown in Fig. 4. It should be noted that for each sample the spectra were evaluated as a sum of magnetic and non-magnetic components, the ratio of latter gradually decreasing with decreasing temperature. For samples of x = 0.4, 0.5, and 0.7, a slight decrease of the average I25 of the magnetic components can be observed with decreasing temperature. It hardly exceeds the uncertainty arising from the correlation of line width and amplitude values and might be due to stress induced by heat dilata- tions of the different layers. Nevertheless, the observed temperature dependence cannot explain the rather sharp change in the anisotropy direction as a function of layer thickness. The largest temperature variation of I25 is observed below 150 K for x = 0.6, which is probably close to the width where the crossover between the anisotropy directions occurs and thus the most sensitive to thickness fluctuations of the layers.

In conclusion, these studies undoubtedly show that the appearance of the perpendicular anisotropy at around x = 0.6 is an intrinsic property of our Fe/Ag multilayers.

Preliminary results on granular alloys of similar Fe concentration and blocking temperature prepared by co- evaporation of the elements indicate that the out of plane spontaneous magnetization is related to the layered struc- ture. The observed epitaxial growth of the layers [6] is suggested to explain the phenomena.

AcknowledgementsThis work was supported by the Hun- garian Scientific Research Fund (OTKA) Nos. T 48965, T 46795, and T 32096.

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