EISEVIER Applied S&ace Science 96-98 (1996) 802-806

applied surface science

Ferrimagnetic thin films prepared by pulsed laser deposition

M. Guyot *, A. Lisfi, R. Krishnan, M. Porte, P. Rougier, V. Cagan Laboracoire de Magn&isme et d’oprique de Versailles, CNRS, F-92195 Meudon, France

Received 22 May 1995

Abstract

Thin films, with thickness ranging from 25 nm up to 1.5 pm, of well-known ferrimagnetic materials such as NiFe,O,, CoFe,O,, Nio,sZnosFe,O,, and BaPe,,O,, have been prepared by PLD. Films made at low substrate temperature (T< 500°C) are amorphous, but can crystallize by post-annealing in air in the temperature range 506800°C. Films deposited at temperature between 500 and 800°C are polycrystalline, the grain size (from 50 nm to 1 pm) and surface roughness (l-100 nm) depending upon deposition parameters. The polycrystalline films are ferrimagnetic with a saturation magnetization close to the bulk value (J, = 0.3 T for Ni-ferrites). Spine1 films are isotropic as deduced from torque balance measurements. Coercivities are rather high for the spine1 films (up to 500 Oe for Ni-ferrite) and even higher for the hexafetrites (3 kOe).

1. Introduction

In the case of insulating ferrimagnetic materials, thin film preparation up to now was mostly restricted to garnet films prepared by LPE, due to the intensive researches on bubble memories and later on VBL memories. More recently attempts have been made to prepare hexaferrite films mostly by RF sputtering [Il.

Pulsed laser deposition (PLD) is now successfully used to prepare high T, superconducting films [2], which implies that other oxide-based materials could also be prepared by using this technique [3-61. We report on the results of the preparation of some ferrite and hexaferrite thin films by PLD.

* Corresponding author.

2. PLD deposition set up

Fig. 1 shows the schematic of our PLD system. In practice, there are two dichroic mirrors (presently 355 nm designed), which allow the removal of the fundamental and the green first harmonic from the beam, the energy of which being absorbed by an absorber installed behind the first mirror. Prior to entering the vacuum deposition chamber, the 9 mm diameter beam is concentrated through a 400 mm lens to the desired energy density onto the target; playing with the laser power supply and the lens position allows us to adjust the energy density from 0.2 up to 5 J/cm*. The beam incidence is 5 45” with respect to the normal of the target surface. The target of the appropriate composition is fixed onto a support which can rotate at a few turns per minute around its vertical axis. The substrate faces the target

0169-4332/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved. SSDI 0169-4332(95)00586-Z

h4. Guyor et al./Applied Surface Science 96-98 (1996) 802-806 803

y Minor

Fig. I. Scheme of the pulsed laser deposition set-up

at a distance adjustable from 30 to 70 mm. The substrate holder includes a heater, which allows us to maintain the substrate up to N 900°C during deposi- tion; the temperature is measured with a thermocou- ple in contact with the back of the substrate; the temperature precision is rather poor (&5O”C). Fi- nally the atmosphere inside the deposition chamber is controlled: after installation of the target and of the substrate, the vacuum chamber is pumped down to 10m6 Torr, and during deposition the oxygen pressure can be adjusted between 1 and 100 mTorr.

3. Samples and measurements

3. I. Film preparation

The goal of this study was to test the possibility of film deposition of well-known ferrimagnetic ma- terials. The rectangular shaped (30 X 40 X 5 mm3) chosen targets were stoichiometric spinels (NiFe,O,, CoFe,O,, Ni,,Zn,,,Fe,O,) and an hexaferrite (BaFe,,O,,). We have observed that the onset of the ablation process occurs if the energy density exceeds 0.3 J/cm’; the use of energy density higher than 3 J/cm* leads to bad quality films, so we have gener- ally used - 1.5 J/cm’. Another important deposi- tion parameter is the target-substrate distance. In our system we adopted 40 mm as the standard distance, which led to a typical deposition rate of 0.3 rim/s.. From the combination of the above parameters and of the deposition time, typically 20-60 min, we

obtained films with thickness ranging from 25 nm up to N 1.5 pm.

3.2. Structural and chemical measurements

We used different substrates (glass, quartz, MgO), 7-15 mm diameter and 0.5-l mm thick. Films deposited onto a substrate at room temperature or at temperature below - 450°C are amorphous as seen from X-ray diffraction patterns. A post-annealing in air (or oxygen), in the temperature range 500-800°C depending on the film composition, leads to crys- talline films. On the other hand, films deposited on heated substrates in the same temperature range, are polycrystalline. Fig. 2 shows two examples of X-ray diffraction patterns, taken on two CoFe,O, films: on the top the substrate temperature was 450°C and on the bottom it was 670°C. For this latter the rays characteristic of a random polycrystalline CoFe,O, are well visible, while for the former, only the amorphous substrate is seen.

The mean grain size and surface roughness have been measured by using an atomic force microscope (AFM); Fig. 3 shows an oblique view of - 1 X 1 pm2 taken on the surface a Ni,,Zn,,,Fe,O, film, the vertical scale being 150 nm per division. For this particular film obtained after 40 min of deposition onto a quartz substrate heated at 590°C the thickness is - 1.2 pm, the mean grain size is 470 nm and the surface roughness is - 50 nm.

For some of me films, we have determined the composition by microprobe analysis. For two NiFe,O, films, A and B, prepared from the same target, under very different conditions, (substrate temperature 260°C for A and 650°C for B), the chemical formula obtained were Ni,.,,Fe, .9804 and NiO,,,Fe,,,,O, respectively, which are very close to each other.

3.3. Magnetic measurements

Magnetic measurements can also be used to cross- check the composition of the film. Fig. 4 shows the hysteresis loop of a NiFe,O, film, recorded by using an alternating field gradient magnetometer (AFGM) the magnetizing field being applied perpendicular to the film surface. In such conditions, owing to the low anisotropy of this material, the perpendicular

804 M. Guyot et al. /Applied Surface Science Sk-98 (1996) 802406

loop is controlled by the shape demagnetizing effect (H = NM = 4nM). Consequently the saturation field H, is a measurement of 47rM,. In the present case H, = 3100 Oe which is in good agreement with the admitted value of the bulk magnetization of NiFe,O, at room temperature, 49rM, = 3400 G. We have observed such a good correlation on other film com- positions also.

On the other hand, from the same figure we also deduced the saturation magnetization, from the mea-

sured moment, taking into account the sample di- mensions. This leads to 47rM, = 1850 G, which is - 50% less than the admitted value and the value deduced from the saturation field. This large differ- ence is attributed, on the one hand, to large errors in the determination of the film volume (mainly thick- ness), and on the other hand, to the AFGM measure- ment conditions, not adapted for large diameter sam- ples.

The coercive fields depends on both the composi-

Fig. 2. Typical X-ray diffraction patterns obtained on CoFe,O, films prepared with different substrate temperature: (top ) 450 and (bottom) 670°C.

M. Guyot et al. /Applied Surface Science 96-98 (lY96) 802-806

Fig. 3. Oblique view, obtained with AFM, of 1 x I @rn* of a well crystallized NioSZn,,FezO, film; note the verti, :al scale is 150 nm per division

tion and the preparation conditions, and are rather high for the spine1 films (from 50 up to 500 Oe>, and even higher for the Ba hexaferrites (- 3 kOe). As a comparison, Fig. 5 shows the loop of another NiFe,O, film (128 nm thick and 160 nm mean grain size). The coercive field is - 5 times higher than the

Fig. 4. Perpendicular B - H loop of a NiFe,O, film (thickness 1.5 pm, mean grain size 300 nm). the arrow indicates the extrapolated saturation field, H, = 3100 Oe, which is close to the admitted saturation magnetization of hulk NiFe,O, (4vA4, = 3400 G).

one shown on Fig. 4. The values of the coercive fields have been crosschecked by Faraday rotation loops. In Fig. 6, one can see the Faraday loop recorded for the same Ni-ferrite film as the one shows in Fig. 5; after correction of the substrate contribution (diamagnetic) the coercive field ob-

H IO4 - I I

+2E+4

Fig. 5. Same as Fig. 4 for an other NiFe,O, film 128 nm thick, 160 nm mean grain size: H, = 3000 G. Note the much higher coercivity H, = 423 Oe.

806 M. Guyot et al. /Applied Surface Science 96-98 (19%) 802-806

P Ni-Ferdm

Fig. 6. Faraday rotation loop measured on the same sample as on Fig. 5. The N&ferrite film contribution (b) is obtained after correction of the substrate contribution on the rough curve (a). Note the film coercivity H, = 400 Oe, in good agreement with Fig. 5.

tained (N 400 Oe) agrees with the one obtained from M-H loops with the AFM (423 Oe).

4. Conclusions

We have shown that ferrimagnetic films can suc- cessfully be prepared by pulsed laser deposition (PLD). The substrates have to be heated in order to

obtain crystalline films. The films have the same compositions as the target, but the magnetic proper- ties are somewhat different from the bulk. The mag- netic moments seem slightly smaller than expected, and the coercive fields even for ferrites like Ni,,,Zn,,Fe,O, are rather high, which could be attributed to size effects (film thickness and/or grain size).

Acknowledgements

We would like to thank Dr. Y. Dumond for EPMA analyses.

References

[l] A. Morisako et al., IEEE Trans. Mag. MAG-24 (1988) 3024. [2] B. Roas et al., Phys. Lett. 53 (1988) 1557. [3] H.J. Masterson et al., J. Appl. Phys. 73 (1993) 3917. [4] I. Cheung and J. Horwitz, MRS Bull. (1992) p. 30. [5] CM. Williams, D.B. Chrisey, P. Lubitz, K.S. Grabowski and

C.M. Cottel, J. Appl. Phys. 75 (1994) 1676. [6] R. Krishnan, A. Lisfi, M. Guyot and V. Cagan, J. Magn.

Magn. Mater., submitted.