MgO multilayered films

Journal of Magnetism and Magnetic Materials 148 (1995) 80-82 .

~ ELSEVIER

Journal of magnetism and

ma.gnetlc ma,6dals

Preparation and magnetic properties of EuO/MgO multilayered fihns K. Kawaguchi *, M. Sohma, Y. Oosawa National Institute of Materials and Chemical Research, Higashi 1-1, Tsukuba, Ibaraki 305, Japan

Abstract Multilayered films with a combination of ferromagnetic semiconductor (EuO) and non-magnetic insulator (MgO) have been prepared by a molecular beam epitaxy (MBE) method. Although the stacking of the EuO atomic planes showed a preferred orientation, epitaxial growth has not been observed. The EuO layers as thin as 1.0 nm were ferromagnetic and their Curie temperature (Tc) was approximately the same as in the bulk. The coercive force increases significantly as the EuO layer thickness decreases.

The multilayering method is known as a powerful technique to produce novel materials, or to modify the physical properties of existiF.g materials, such as the supermodulus effect, unusual perpendicular anisotropies [1], giant magnetoresistance [2], et,e. Our target mat¢rial in this study is europium monoxide (EuO). E~:ropium monoxide is a typical semiconductive ferromagnet accompanied by a metal insulator transition (MIT) around the Curie temperature (Tc). Our purpose is to investigate the magnetic and transport properties of layered EuO using the multilatyering method. Some novel physical phenomena are also expected. The non-magnetic insulator, MgO, was considered as a spacer material to investigate the intrinsic properties of the layered EuO. Sample films were prepared using molecular beam epitaxy (MBE) techniques. The preparation chamber was first evacuated to ,-, 5 × 10 -~° Tort and then an oxygen atmosphere ranging between 10 -9 and 10 - 7 Tort was introduced. Most samples discussed here were deposited at an oxygen atmosphere of 10 -~ Torr. However, the introduced oxygen atmospheric pressure dropped below ~ 10 -9 Torr due to getter effect of the deposited Eu. We considered that the oxidation condition did not change significantly and fixed the variable leak valve position during deposition, because the gas inlet is directed and extended to nearby the sample holder. Since ferromagnetic EuO could not be obtained from a EuzO 3 source material [3], metal Eu ingot was evaporated. Tile hydrogen partial pressure derived from the Eu source initially increased to above 5 × 10 .-7 Torr and remained around 10 -7 Torr

* Corresponding author. Fax: + 81-298-54-4551/-4487; e-marl: [email protected].

during deposition. Europium hydride is also ferromagnetic and might disturb our EuO study. Actually, some hydrogen content in a simple EuO sample prepared under the same conditions was detected by secondary ion mass spectrometry. However, in a series of simple EuO films (SEFs), no hydride X-ray diffraction (XRD) peak was observed as reported in our previous work [3]. The magnetic properties of the SEFs were highly dependent on the oxygen atmospheric pressure and the SEFs composed of EuO single phase indicated that the saturation magnetization value was the same as in the bulk EuO. Thus some hydrogen contamination without hydride formation might exist in the artificial multilayered films (AMFs), however the contribution from the contaminants to the magnetic properties of the samples is negligible. Both an electron beam gun (e-gun) and a Knudsen cell (K-cell) were employed to evaporate Eu. The other preparing conditions were the same as in the previous report [3]. Three kinds of non-magnetic insulators, CaF2, MgAI204, and MgO were exploited to make multilayered films with EuO. Since these three materials are cubic and the lattice mismatching with EuO is small (1.066 for C a F z ( 1 0 0 ) / E u O ( 1 0 0 ) , 0.966 for MgAi204(220) / E u O ( l l l ) , and 1.006 for M g O ( l l 0 ) / E u O ( l l l ) ) , heteroepitaxial growth was expected. Typical XRD patterns for the four systems are displayed in Fig. 1. All four samples shaw E u O ( l l l ) or EuO(200) preferred orientation in the high angle region. Superstructure reflections at low angles imply the formation of well ordered multilayer structures, except for sample (c). Therefore, both CaF2 and MgO seem promising judged from these results. However, the sample of E u O / C a F 2 looked transparent and was non-conductive. Thus we considered MgO as the most appropriate multilayering partner. Though the lattice

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81

K. Kawaguchi et aL /Journal o f Magnetisra and Magnetic Materials 148 (1995) 80-82

low-angle

high-angle CaF2(111) buffer-layer

I

(a} Mot~I/'l~N

(b)I~O/lM~,,O

(a)

0(111)

J~,__U k. (b) EuO(200) EuO(200)

(e)

z w I.z

Fig. 2. Polar scanning plots of RBS channeling. Plot (a) is for an

, *~ , ~ . . . . .

~EuO(~00)

i

~.j

~ ~

(d) EuO(400)

epitaxial AMF of MolN(3.4 tam)/TiN(2.4 nm) as a reference [4] and (b) is for an AMF of Erie(5.0 nm)/MgO(3.0 nm). Channeling measurements of Me signal for (a) and Eu signal for (b) were performed using 1.5 MeV He2+.

~o 4b 5b 6'0 ¢0 2

e

Fig. 1. Typical X-ray diffraction patterns for AMFs with various spacer materials, (a) EuO(5.0 nm)/CaF2(3.0 nm), (b) EuO(5.0 nm)/MgAl204(4.0 rim) on CaF2(lll) substrate, (c) EuO(5.0 nm)/MgAl204(4.0 nm), and (d) EuO(7.0 nm)/MgO(5.0 nm). All samples were deposited at 30D°C. And the substrates are cleaved MgO(100) single crystals except (b). The asterisks (*) at low angles denote superstructure reflections. matching relation described above suggests E u O ( l l 0 ) / MgO(100) orientation, the observed XRD pattern indicates a E u O ( 1 0 0 ) / M g O ( 1 0 0 ) orientation as shown in Fig. l(d). The possible atomic configuration o f the E u O ( 1 0 0 ) / MgO(100) interface is not well understood. Such an XRD pattern in Fig. l(d) showing fairly good preferred orientation suggests an epitaxial growth of EuO and MgO layers. It is difficult to obtain information on in-plane structures o f such thin films prepared on single crystal substrates from XRD measurements. Both reflection high energy electron diffraction (RHEED) and channeling with Rutherford backscattering spectrometry (RBS) will be effective for the purpose. Since the single crystal subslrates used in this study are cleaved MgO(100) small pieces (-,, 5 × 10 ram) placed on a polyimide film, the surface smoothness is not good and the sample is highly insulating. These conditions are not favorable to RHEED method and this latter method is more quantitative. Thus we preferred the RBS channeling method. Polar scanning results of RBS channeling for a E u O / M g O A M F and a M e N / T i N A M F [4] as a reference are exhibited in Fig. 2. Compared with the epitaxial M e N / T i N AMF (Fig. 2(a)), it is clear that the E u O / M g O A M F shows no atomic alignment in Fig. 2(b). That is, EuO(100) and MgO(100) planes stack with random orientation in the film plane. Magnetic and transport properties of a series of E u O / M g O AMFs deposited at low substrate temperature Ts = 100°C have alread~¢ been reported [5]. Most of the high Ts samples showed too high resistance at room temperature to be measured and ~to MIT. This differs from

our previous results [5]. A bound magnetic polaron model [6] is considered to be the most probable explanation for the MIT in EuO. Oxygen vacancies are essential for the MIT in this model. This suggests that the samples grown at high Ts are highly oxidized as compared to those o f low Ts in spite of no significant difference in the XRD measurements. Curie temperatures for the low Ts samples (T C = 120 K) were significantly higher than the bulk value (Tc = 70 K), while all the high Ts AMFs exhibited a Tc around 70 K. Such an increase in T¢ reported before [5] might be related to the oxygen deficiency. The coercive force ( H c ) as a function of EuO layer thickness is plotted in Fig. 3. The H c for both low and high T s A M F s increase as the EuO layer thickness decreases. The maxim u m H c value observed was 600 Oe for a low T s A M F of EuO(1.0 r i m ) / M g O ( 5 . 0 nm). The high T s A M F with EuO layers as thin as 1.4 nm was non-magnetic. Since the

Hc vs EuO thickness 60C zx

• Ts-3ooc ~ Ts=S00C

;

~A

(UPILF.X)

40{] 7 ~ 0,9,

~ TS-100C

~,

20C o

o

1'o

'

2'o

'

30

EuO layer thickness (nm) Fig. 3. Coercive force as a function of EuO thickness. Samples of AMFs were deposited at 100oC ( A ) or 300°C ( 0 , 0 ) . The substrates are cleaved MgO(100) single crystals ( A , O ) or polyimide (Upilex) filnas ( ~ ) , A l l H c 'values were measured at 4.5 K.

82

K. Kawaguehi et al. /Journal o f Magnetism and Magnetic Materials 148 (1995) 80-82

H c shows no meaningful dependence on the M g O layer thickness, interlayer magnetic coupling between EuO layers is unlikely. It is well known that lattice strain also affects H c [7]. Since the lattice strain o f deposited films generally depends on the substrates and on the T s, H c should change with those conditions. However, the H c values show no remarkable T s dependence and no meaningful difference between p o l y i m i d e (Upilex) and M g O single crystal substrates in Fig. 3. Therefore the lattice strain is not considered to be o f primary importance. Further detailed investigation to clarify the unusual H c behavior is now in progress.

References

[1] T. Shinjo, Metallic Supcrlattice8, ¢ds. T. Takada and T, Shinjo (Elsevier, Amsterdam, 1987) chs. 3 and 4. [2] M.N. Baibich ct al., Phys. Rev. Left. 61 (1988) 2472. [3] K. Kawaguehi and M. Sohma, Thin Solid Films 246 (1994) 1. [4] K. Kawaguehi and S. Shin, J. Appl. Phys. 67 (1990) 921. [5] M. Sohma, K. Kawaguchi and Y. Oosawa, submitted to J. Magn. Magn. Mater. [6] J.B. Torrance, M.W. Slmfer and T.R. McGuire, Phys. Rev. Lett. 29 (1972) 1168. [7] M. Goto, H. Tange and T. Hamatake, J. Appl. Phys. 52 (1981) 1914.