ELSEVIER Journal of Magnetism and Magnetic Materials 162 (1996) 183-188

~ Journal of magnetism

magnetic materials

Magnetic state of thin DyFe amorphous layers

P. Perera a M.J. O ' S h e a a,* H.H. H a m d e h ~

Cardwell Hall, Department of Physics, Kansas State Universio', Manhattan, KS 66506-2601, USA b Department of Physics, Wichita State Unitersi~', Wichita, KS 67260, USA

Received 27 October 1995: revised 19 February 1996

Abstract

We have prepared isolated DysoFeso layers in the form of multilayers by sputtering, and have studied their magnetic state as a function of the DysoFes 0 layer thickness, t. These layers are amorphous and there is a strong random magnetic anisotropy in them. We find a decrease in the magnetic transition temperature with decreasing t due to finite size effects and an increase in the anisotropy with decreasing t due to the influence of the interface. M/Sssbauer spectroscopy is used to identify the presences of two phases which we associate with 57Fe at the interface and 57Fe in the bulk. Mtissbauer measurements show the presence of magnetic order in the bulk phase at low temperatures and there is evidence for partial magnetic ordering in the interracial region at low temperatures.

Kew'ords: Amorphous systems - multilayers; Anisotropy - random; MiSssbauer spectroscopy ; Magnetic ordering

1. Introduction

The properties of magnetic materials structured on the scale of nanometers are modified significantly by finite size effects and the presence of a large inter- face area and present new and interesting physics. Such systems can be in the form of isolated particles [1-4], granular systems [5,6], or multilayers [7-9]. In the case of magnetic systems a new contribution to the magnetic anisotropy appears at an interface or surface and modifies the magnetic behavior, includ- ing hysteresis and the overall magnetic easy direc- tion. This contribution can be important in both multilayers and nanoscale particles [8-13], signifi- cantly increasing the total value of the magnetic

* Corresponding author. Email: mjoshea@ksuvm.ksu.edu.

anisotropy K and modifying the magnetic structure. Finite size effects lead to a reduction in the magnetic transition temperature T c at smaller layer thicknesses for thin magnetic layers [14-16] and affect T c in more complex ways for transition-metal-based parti- cles [17,18].

The purpose of this work is to determine how finite size effects manifest themselves in systems with random magnetic anisotropy and to determine if it is possible to separate out the magnetic behaviors of interfacial and bulk material. We have chosen DysoF%0 amorphous layers for these studies since their magnetic structure in bulk form is known. A strong random magnetic anisotropy associated with Dy exists in this system [19,20], and the magnetic coupling between the Fe and Dy is antiferromag- netic. This leads to a magnetic order in which the Dy moments form a fan about a direction opposite to the local Fe moment direction and is referred to as

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1 8 4 P. Perera et al. / Journal of Magnetism and Magnetic Materials 162 (1996) 183-188

speromagnetic order. In multilayers with anisotropic rare-earths the single-ion anisotropy, along with the break in symmetry at the interface, lead to a strong interface anisotropy [9,12]. We prepared the DyFe layers in the form of DYs0 Fes0/Mo multilayers and probed their magnetic properties using magnetization measurements and MSssbauer spectroscopy.

MiSssbauer spectroscopy is a powerful technique for probing the microscopic nature of magnetism in magnetic materials, and in many cases can separate out the magnetic properties at different sites. A particularly important application of this is to sepa- rate out the Mtissbauer signals from Fe sites at the interface in a nanophase material (multilayer or par- ticulate) from those in a bulk phase of a material and we describe such an application below. Bulk and surface M~ssbauer spectra have been separated out in other nanoscale particle systems which contain Fe, including Fe particles [21] and Fe -O particles [22,23].

2. Sample preparation and structure

Multilayers were prepared by Ar ion sputtering using a DysoF%0 target and an Mo target. The substrate (Ta) was switched between positions over each gun to build up multilayers of DysoFes0(t nm)/Mo(18 nm). The Mo layer being 18 nm is thick enough that no magnetic interactions occur between layers, so that the layers are essentially isolated. Each sample is capped with a 30 nm layer of Mo to protect it from oxidation. Four samples with t = 522,

22, 4.5 and 2.5 nm were prepared. The first one was a single layer, while the other three each have ten bilayers of DYsoFes0/Mo. These thicknesses were chosen to span the range from t > 50 nm, where the layers are essentially bulk, to t < 10 nm, where finite size effects begin to become important. Fig. 1 shows an X-ray diffractogram of the 4.5 nm multilayer. A broad maximum is present at about 29 ° and no peaks associated with crystalline Fe, Dy or alloys of these elements are present. The peaks at 38 ° and 41 ° 20 are associated with the Ta substrate and Mo layer, respectively. The structural correlation length b is determined from

b = 0.9A/B cos 0 B , ( 1 )

where A is the wavelength of Cu K s radiation (0.15406 nm), B is the full width at half maximum

c-

[ I I

2O 4O 2e (cteg.)

Fig. 1. X-ray diffractogram for the DysoFeso(7.5 nm)/Mo(18 nm) multilayer (ten bilayers) using Cu Ka radiation.

of the X-ray peak, and 0 B is the peak position. A value of b = 3 nm is obtained, indicating that the DyFe layer is close to being amorphous. In other amorphous metallic systems the structural correlation length is 1-3 nm [24].

3. Magnetization

The magnetization was measured using a Quan- tum Design SQUID magnetometer with a maximum field of 55 kOe, and a temperature range of 1.8-400 K. Fig. 2 shows the field-cooled and zero-field-cooled magnetization of selected samples of this work in an

60 t=22nm

~ 30

40

20

0 - = 0 100 200 300

r (K)

Fig. 2. Field-cooled (FC) and zero-field-cooled (ZFC) magnetiza- tions for selected DysoFes0(t nm)/Mo(18 nm) multilayers in an applied field of 200 Oe. The dash lines are linear extrapolations of the magnetization to the T axis and are used to estimate T c. The solid lines are guides for the eye.

P. Perera et al. / Journal of Magnetism and Magnetic Materials 162 (1996) 183-188 185

1500

. 1o00

5OO

0

• 4.4 / /

. . . . . . . . . . . It i~ I , , ,

100 200

T (K) 300

Fig. 3. Curie-Weiss plots for selected DY_s0F%o(t nm)/Mo(18 nm) multilayers in an applied field of 200 Oe. The dashed lines are linear extrapolations of the high-temperature 1 / X to the T axis and can be used to determine the Curie-Weiss temperature. The solid lines are guides for the eye.

applied field of 200 Oe. At layer thicknesses above 10 nm the curves are identical, while below 10 nm the value of T c, as determined from an extrapolation of the initial rise in magnetization to the temperature axis, decreases. Similar results are seen in the Curie-Weiss plots ( I / x versus T, where X = M/H) in Fig. 3, where the extrapolated Curie-Weiss tem- peratures are the same for the two multilayers with the largest t and decrease for the one multilayer shown with smaller t. We and others have already shown that finite size effects reduce the value of T c in ferromagnetic systems in this thickness range [14-16]. This result shows for the first time that a similar effect occurs in random magnetic anisotropy systems where the magnetic order is complex, namely speromagnetic.

The coercivity of these multilayers also changes with multilayer thickness, as can be seen from the magnetization curves in Fig. 4. These results, along with the variation in T c, are summarized in Fig. 5, and the coercivity shows a significant decrease with decreasing layer thickness.

For DyFe layer thicknesses t < 2.5 nm, complex magnetization curves were observed, and it is likely that a substantial amount of Fe in these multilayers is in the interfacial region.

The total magnetic anisotropy K is estimated using the magnetization-area method [25]

K=~ fM°HdM, (2) M~

• t = 522 n m

• 22

2 0 0 * 4.4

~ 1 0 0

0

- 3 0 -15 0 15 3 0 4 5

H (kOe)

Fig. 4. Magnetization curves for selected Dyso Feso(t nm)/Mo(18 nm) multilayers.

where M 0 is the saturation magnetization and M r is the remanent magnetization. We have shown that for rare-earth systems the magnetization-area method and a torque determination of K both give similar results [26], giving confidence in the magnetization-area method. Note that a 1/H 2 extrapolation of the high-field magnetization is used before K is calcu- lated from Eq. (1). Such a dependence of the magne- tization on H provides a good fit to the high-field data and is expected from theory [27]. We find that K changes from 1.2 × 10 7 e rg / cm 3 for the t = 522 and 22 nm multilayers, to 1.53 × 10 7 e rg /cm 3 for the 4.4 nm multilayer with an error bar of 5%.

160

140

120

3 0

Z" 20

10

i r i i i i l l I i i q l , l l l , , i , ~

Y 45 K

. . . . . . . . i . . . . . . . . . . . . .

10 100 1 0 0 0

t (nm)

Fig. 5. Dependence of magnetic transition temperature T c and coercivity H~ on magnetic layer thickness t.

186 P. Perera et al. /Journal of Magnetism and Magnetic Materials 162 (1996) 183-188

i i i i i

-3 -2 0 0 0 2

Velocity (mm/s)

Fig. 6. MSssbauer spectra at 290 K for selected DysoFeso(t nm)/Mo(18 nm) multilayers. The solid line is a fit to the data, as discussed in the text, and the separate fitted peaks are also shown.

4. M6ssbauer measurements

MiSssbauer spectra were obtained at 290 and 25 K using a conventional constant acceleration M6ssbauer spectrometer. A radioactive source of 30 mCi 57Co in a Rh matrix was used and the transmission spectra were obtained using a PA-1200 ranger scientific proportional counter. The absorbers were obtained by stacking together at least eight pieces of 1 cm 2 DyFe /Mo multilayer film.

Fig. 6 shows the room-temperature 57Fe MiSssbauer spectra for three of the multilayers exam- ined in this work. Each spectrum shows a quadrupole doublet and, as expected, no magnetic component since these materials are not magnetically ordered at room temperature. Since the DysoFes0 layers are amorphous there is considerable broadening of the spectrum. The spectra are best fitted to two doublets which we take to represent two phases of the DyFe layer in the multilayer. In fitting the data the quadrupole splitting, isomer shift, and line widths of the two doublets were freely adjusted. These two phases have different isomer shifts and different quadrupole splittings according to this fitting and these results are given in Table 1. Note that the isomer shifts are relative to pure Fe. These isomer shifts were calculated by subtracting the isomer shift

o ¢ . . " . . f . ° ~ .. - ' " . . : ,

-,o -; o ; lo Velocity (ram/s)

Fig. 7. MSssbauer spectra at 25 K for selected DYsoFes0(t nm)/Mo(18 rim) multilayers. The solid line is a fit to the data, as discussed in the text.

for pure Fe from the isomer shift for our sample at the same temperature. As the thickness of the DyFe layer is reduced the absorption associated with phase 2 which has the smaller quadrupole splitting in- creases as compared to phase 1. This suggests that phase 2 is associated with 57Fe at the interfacial region and phase 1 is associated with 57Fe deep in the bulk of the DyFe layer. The intensity of each doublet represents the amount of Fe in each phase.

At 25 K the M/Sssbauer spectra for the multilayers with layer thicknesses t = 522 and 22 nm exhibit magnetic and non-magnetic components (see Fig. 7). The corresponding hyperfine magnetic field distribu- tions were obtained using the method of Le Caer and Dubois [28]. In this method the spectra were fitted to one magnetic component with no quadrupole split- ting and phase 1 isomer shift, and another non-mag-

t = 2 2 r i m

o.

100 200 300 400

Hmf (kG)

Fig. 8. Calculated hyperfine field distribution for selected multi- layers from the M~Sssbauer spectra of Fig. 7.

P. Perera et al. / Journal of Magnetism and Magnetic Materials 162 (1996) 183-188 187

Table 1 M~issbauer parameters IS (isomer shift relative to pure Fe with both the sample and Fe at same temperature), AQS (quadrupole splitting), HMF (average hyperfine magnetic field). The calculated percentage of Fe contributing a Mtissbauer signal to each phase are also given

Sample Temperature Phase 1 Phase 2

(nm) (K) IS (ram/s) AQS (mm/s) HMF (kG) Fe (%) IS (mm/s) AQS (mm/s) HMF (kG) Fe (%)

522 290 -0.16 0.46 0 88 0.08 0.31 0 12 25 - 0.16 0 126 94 0.08 0.31 0 6

22 290 - 0.16 0.46 0 71 0.15 0.32 0 29 25 - 0.16 0 122 871 0.15 0.32 0 t9

4.5 290 - 0.15 0.53 0 30 0.13 0.38 0 70

netic component with the hyperfine field parameters of phase 2. The calculated hyperfine field distribu- tions of the two samples are essentially the same, as can be seen from Fig. 8.

The Mrssbauer magnetic signal is due primarily to the magnetic order in the bulk of the DyFe layer at 25 K. However, the larger number of Fe atoms in the magnetic components as compared to those of bulk DyFe at room temperature is evidence for par- tial magnetic order in the interfacial region, as indi- cated in Table 1.

5. Discussion and conclusions

All of the D y F e / M o multilayers have a strong magnetic anisotropy and show strong hysteresis at low temperatures due to this strong magnetic anisot- ropy. Size effects are seen in these systems similar to those that we have reported for elemental Dy layers [14]. The value of T c for our thickest layers (160 K)

is closed to that reported for an evaporated DysoFes0 amorphous film [29] (185 K).

We were able to separate out the MiSssbauer signals from the interfacial region and bulk of the DyFe layer by varying the DyFe layer thickness and observing the change in intensity of the two room- temperature quadrupole signals. Additional evidence that phase 2 is interfacial comes from the fact that the quadrupole splitting for phase 1 is larger than phase 2, suggesting that the former has lower sym- metry. We find that the interfacial region is broad and probably consists of a region of Fe which has diffused out of the DyFe layer during sample prepa- ration. The large difference in the isomer shifts of these two phases indicates a significant change in

electronic structure of the interfacial phase as com- pared to the bulk phase and is probably accompanied by a change in the Fe moment in the interfacial phase.

Thus we have observed finite size effects in the magnetic transition temperature and in the coercivity and we find a significant enhancement of the total anisotropy for our thinnest layers. This suggests that the interface is contributing a significant anisotropy in these multilayers. We were able to separate out the bulk and interfacial M~Sssbauer signals and find bulk magnetic ordering at low temperatures and some evidence for magnetic ordering at the interface.

Acknowledgements

We thank the National Science Foundation for Support under NSF OSR92-55223 and NSF DMR91-23831.

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