Endoh, Yasuo

Hosoito, Nobuyoshi

Kawaguchi, Kenji

Shinjo, Teruya

Kyoto University Research Information Repository

TitleMagnetic Properties of 'Sub' Monolayered Superlattice ofFe/Mg (Commemoration Issue Dedicated to Professor ToshioTAKADA On the Occasion of His Retirement)Author(s)Endoh, Yasuo; Kawaguchi, Kenji; Hosoito, Nobuyoshi; Shinjo,TeruyaCitationBulletin of the Institute for Chemical Research, KyotoUniversity (1986), 64(4): 243-248Issue Date1986-12-06URL http://hdl.handle.net/2433/77156RightType Departmental Bulletin PaperTextversionpublisherKyoto UniversityBull. Inst. Chem. Res., Kyoto Univ., Vol. 64, No. 4, 1986         Magnetic Properties of  `Sub' Monolayered                Superlattice of Fe/Mg       Yasuo ENDOH*, Kenji KAWAGUCHI**, * * *, Nobuyoshi HOSOITO* *                          and Teruya SHINJO**                              Received June 10, 1986    Magnetization measurements have been carried out on a `sub' monolayered artificial super-structure film of [Fe1Mg16147o evaporated on a mylar substrate. An appreciable anisotropy was observed, which shows the easy direction lying out of the film plane at 4.2 K. Although ysteresis appears at this temperature, the Arrott plot obtained from the data at the higher magnetic fields more than 1000 Oe suggests that the magnetic properties of this sample is considerably different from usual ferromagnets.      KEY WORDS: Magnetism/ Fe/Mg superlattice/ Submonolayer/ Ar-                  rott plot/ I. INTRODUCTION     Recent development in the fabrication techniques can provide metallic super-lattices of tailored modulation not only for the periodicity but also for the composi-tion or the combination of two elements. Among various classes of novel materials Fe/Mg artificial superstructure films (hereafter abbreviated as ASFs) have attracted a special attention due to the fact that Fe and Mg are mutually insoluble in thermal equilibrium state. (1) According to the Mossbauer measurements, a large hyper-fine field on Fe nuclei similar to the value in bulk Fe metal was observed in the ASF which consists of monolayered Fe interfaced with thick Mg layers. (2) This fact is just extraordinary because the surface layer of Fe interfaced with many non-mag-netic elements more or less loses its magnetic moment. Eventually the monolayer-ed Fe ASF with thick non -magnetic metal layers becomes non-magnetic such as Fe/V ASF. Thus the monolayered Fe/Mg ASF may possess 2-dimensional ferro-magnetic features.    Very recently a new result was observed that the hyperfine field on Fe nuclei in an [Fe1Mg16] ASF is still 70% of the bulk value at 4.2 K. (3) Note that [Fe,Mg16]470 represents that this ASF consists of 470 bilayers combined with 1 A Fe and 16 A Mg. It means that each Fe lattice in this particular ASF should not be extended in 'Fe' layers, but nearly a half of the lattice in each layer is occupied by Fe atoms, since the atomic radius of Fe atoms should be nearly 2 A. Thus we define this sample as a `sub' monolayered superstructure film. Then we raise a fundamental * g J A : Department of Physics, Faculty of Science, Tohoku University, Sendai 980  ** JIIQ =1, ifiEnf : Laboratory of Solid State Chemistry, Institute for Chemical     Research, Kyoto University, Uji, Kyoto 611. *** Present address: National Chemical Laboratory for Industry, Tsukuba, Ibaraki 305. ( 243 )                     Y. ENDOH, K.  KAWAGUCHI, N. HOSOITO and T. SHINJO question on the magnetic state with Fe lattices of the finite size. In this paper we mainly report ferromagnetic properties of [Fe1Mg16]470 ASF at low temperatures.     In the next section the magnetization measurements and the results will be described. The third section will be devoted to discussion and conclusion.                   2. MAGNETIZATION MEASUREMENTS    Magnetization measurements have been carried out with the vibration mag-netometer at Tohoku. This magnetometer was recently upgraded. Now several scans varying either temperature or field are fully controlled by a dedicated micro-computer PC-9801. The output voltage is picked up by coils attached on the pole piece surfaces of the magnet. The hole current, the thermopower of the Au(Fe)-Chromel coupled wire and so forth, are taken into the digital meters. These digital signals are transferred to the data acquisition memories in the microcomputer. Eventually these data are processed by several softwares.    The magnetic field is applied horizontally and both the magnet and the sample can be rotated around vertical axis. In the present set up we use a sample, stacked 100 pieces of [Fe1Mg16] ASF (about 1 cm') evaporated on the mylar substrate wrap-ped by a thin aluminum foil. It was placed at the end of the sample holder rod such that the film plane stands vertically. Temperature was controlled within 0.2±0.01 K. Other details of the magnetometer is described in a separate paper.    Figure 1 depicts typical results of the field scan at 4.2 K, in which the field is applied either perpendicular or parallel to the film plane. It is noted that an ap-preciable hysteresis was observed in both scans as shown in Fig. 1(b). The fact that easy direction is along perpendicular direction to the film plane is consistent with the Mossbauer measurement for [Fe1Mg16] ASF.(3)    Figure 2 shows the results of thermal scan at two different fields applied in both direction parallel and perpendicular to the film plane. At the low field of 50 Oe, a broad peak appears around 20 K besides that spontaneous magnetization quickly diminishes below 10 K.    On the other hand, the magnetization at 1000 Oe behaves differently: it satu-rates below 20 K and gradually decreases upon heating. Appreciable effect of the anisotropy continuously appears up to 100 K and finally it disappears above then. The magnetization curve always has a convex curvature with respect to the field up to the room temperature.    Arrott plot was obtained from the magnetization data at various temperatures. As is shown in Fig. 3, the curves below 20 K have sharp bends at low HIM values. Except the unusual portion at low fields and low temperatures, the Arott plot re-presents the similar behavior to that of the paramagnetic state above the Curie temperature for normal ferromagnet. It is interesting that the extrapolating point from the data at the higher magnetic field at 4.2 K crosses at a positive value on the abscissa or a negative value on the ordinate, which conflicts to the fact of the spontaneous magnetization observed at this temperature.    The magnetization curves can be fitted to the Langevin function of coth (244)   Magnetic Properties of  Submonolayered Superlattice of Fe/Mg [Fe1 Mg16] 4.2K .-------------------------------------------- 1.4 -- 1.2 - > 1.0 -O .8 -/-z i T 6             i O .4 -/~r J .2 - i-        // 0 ------------------------------------------------------------- 0 5000 10000 15000 20000                   Hex [0e] •                    (a) [Fe1 Mg16] 4.2K      1.0        .8 .6."% > 4 M 0 .2 Azi z o CD—.2: O'~ J          —..4 —.6       —.8      —1.0       —4000 —2000 0 2000 4000                  Hex [Oe]                  (b) Fig. 1. (a) Magnetization curves for [Fe1Mgls]470 ASF at 4.2K. Solid        and dashed lines respectively represent measurements under       applied field in perpendicular direction and parallel to the film       plane. Fig. 1 (b) Enlarged portion near low magnetic field in       the vicinity of zero field. (245)       Y.  ENDOH, K. KAWAGUCHI, N. HOSOITO and T. SHINJO [Fe1 Mg16] 50 Oe 2.0------------------------------------------------------ 1.5 - a z T , U ', of .5 - 00 20 40 60 80 100                  T [K]                   (a) [Fe1 Mg16] 1000 Oe 6.0------------------------------------------- I4.0-                       ~~                       ~   2.0- .~ 0 ------------------------------------------------------ 0 50 100 150200                 T [K]                  (b) Fig. 2. (a) Thermal scans under the constant magnetic field of 50 Oe.       Solid and dashed lines mean the same as described in Fig. 1.       (b) Scans at 1000 Oe. ( 246 )              Magnetic Properties of Submonolayered Superlattice of Fe/Mg  [Fe1 Mg16]  M2  /1/                               4                              27 30                                          40                                               77                             H/M             Fig. 3. Arrott plot for [Fe1Mg16]47o ASF. The numbers in the figure                    represent temperatures at each field scan. Field was applied                    in perpendicular direction to the film plane. but the fitted values of a (HIT) vary with the temperature. It means that magnetic properties may not be solely interpreted by a simple superparamagnetism.                    3. DISCUSSION AND CONCLUTIONS    We have confirmed that a `sub' monolayered Fe in Fe/Mg ASF possesses fer-romagnetic properties and the easy direction is out of plane. The features of mag-netization curves are devided by two portions as typically shown by the Arrott plot; the ferromagnetic region which disappears around 10 K and paramagnetic regions. This fact indicates that the Fe lattices are distributed in their size. The dramatic decrease of magnetization seems to indicate the characteristic features of the low dimensionality, but further analysis for the magnetization curves is not easy. Thus the detailed discussion on the 2-dimensional behavior in this material will be left in future studies.    The superparamagnetic behavior is also peculiar. Fitting these magnetization curves to the Langevin function without introducing the distribution of the size does not yield the average size throughout all temperatures measured in the present experiments. Therefore we draw a schematic picture of Fe lattices in the `sub' monolayered Fe/Mg ASF as follows. The size of Fe lattices in which all Fe mag-netic moments are ferromagnetically aligned are widely distributed. The thermal fluctuation starts at around 10 K and the residual magnetization begins to decrease. Between ferromagnetic clusters, at least partially, weak anti-ferromagnetic couplings exist (up to around 20 K) which might be an interlayer interaction through Mg layers. They may break around at 20 K. At elevated temperatures above these ( 247 )                     Y. ENDOH, K. KAWAGUCHI, N.  HosolTO and T. SHINJ0 critical temperatures, the Fe lattices behave all paramagnetic. Mossbauer spectro-scopic measurements showed that the critical temperature for this sample is about 35 K; Namely the fluctuation rate of magnetization at above this temperature is faster than the Mossbauer observation time4) In order to clarify these circum-stances more accurately, we must elucidate the magnetization processes for other specimens with different compositions in the future studies.                       ACKNOWLEDGEMENTS    One of authors (Y.E.) is indebted to Professor Emeritus T. Takada for col-laboration in the work granted as the Special Distinguished Research in 1982-1984. Authors thank Mr. M. Suzuki and N. Terao for their technical assistance during the present measurements at Tohoku. This work was supported by the grant in Aid for Scientific Research from Ministry of Education, Science and Culture.                            REFERENCES (1) T. Shinjo, K. Kawaguchi, R. Yamamoto, N. Hosoito and T. Takada, Chem. Lett., 59 (1984). (2) T. Shinjo, K. Kawaguchi, R. Yamamoto, N. Hosoito and T. Takada, Solid State Commun., 52,     257 (1984). (3) T. Shinjo, N. Hosoito, K. Kawaguchi, N. Nakayama, T. Takada and Y. Endoh, J. Magn. & Magn. Mater., 54 57, 737 (1986). (4) K. Kawaguchi, R. Yamamoto, N. Hosoito, T. Shinjo and T. Takada, J. Phys. Soc. Jpn., 55,     2379 (1986). ( 248 )

Magnetic Properties of 'Sub' Monolayered Superlattice of Fe/Mg (Commemoration Issue Dedicated to Professor Toshio TAKADA On the Occasion of His Retirement)

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Magnetic Properties of 'Sub' Monolayered Superlattice of Fe/Mg (Commemoration Issue Dedicated to Professor Toshio TAKADA On the Occasion of His Retirement)

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