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Journal of Magnetism and Magnetic Materials 146 (1995) 187-190

Influence of irradiation on amorphous films " L.V. Nemtsevich *, T.A. Tochitskii, M.U. Sheleg Institute of Solid State Physics & Semiconductors of Belorussian Academy of Science, P. Brovki 17, 220726 Minsk, Belarus

Abstract

Experimental studies are made on the effect of ion, electron and ",/-irradiationson thermal stability and microstructure of amorphous electrodeposited CoP alloy films. It is found that irradiation results in a deterioration of the thermal stability of the amorphous state and under definite dose of boron or hydrogen ions the effect of ion-stimulated atoms regulation is observed. It is shown that ions irradiation destroy the granular microstructure. It leads to the destruction of the perpendicular magnetic anisotropy in these films and greatly improves their magnetic characteristics.

Amorphous ferromagnetic films find wide employment in microelectronic and computer science of late. However, the metastable character of the amorphous state makes the physical properties of metallic glasses susceptible to different external influences such as temperature, pressure, irradiations, etc. Therefore investigations of the sensitivity of metallic glasses to radiations are very important in view of the practical use. In the present work the effect of boron or hydrogen ions, electrons, and ",/-irradiations on thermal stability, microstructure, magnetic properties and microhardness of amorphous electrodeposited CoP alloy films has been investigated. The Cox00_xPx alloy films, where x = 12 to 20 at% were electrodeposited from sulphate solutions [1] onto polished copper substrates.

* Although originally accepted for publication in the ICM'94 Proceedings, this paper has not been published therein because it was not presented at the Conference. * Corresponding author: Fax: + 7-0172-324 694.

The as-deposited films were irradiated with either a 3 MeV electron flow of 2 × 1017 e - / c m 2, 1.3 MeV ",/-irradiation in the dose range up to 1 X 1018 ~ / c m 2, 100 keV boron ions with ion current density ( j ) of 1.2 p.A/cm e in the dose range of 5 X 1015 to 7 X 1016 B + / c m 2, or 2 keV hydrogen ions at j = 50 ixA/cm 2 in the dose range of 3 x 1015 to 1 x 1019 H + / c m 2. Both the as-deposited and irradiated films were subjected to the isothermal annealing in a vacuum of 3 x 10 -5 Torr for 2 h at temperatures varying in the range from 373 to 573 K every 20 K. The structure of the films was investigated with X-ray diffraction using CoKa-radiation and transmission electron microscopy (TEM) on aMB100JIM. The magnetic properties of the films (coercive field H c, saturation field H s, rectangular hysteresis loops Br/B s) were determined using an inductive loop tracer. The films microhardness ( H V) was measured with a Vickers microhardness tester. The temperature Tx of incipient crystallization of an amorphous films was determined by X-ray and electron diffraction and using the change in coercive field under annealling [2].

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the diffraction halo maxima is noted on the X-ray and electron diffraction patterns (Fig. lb). However, the thermal stability of amorphous CoP alloy films is

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Fig. 1. X-ray diffraction patterns of (a) the as-deposited CoP alloy films containing 16 at% P; (c) after annealing at T = 513 K; (b) after hydrogen ion irradiation with doses of 3 × 1015 to 6 × 1017 H+/cm2; (d) 1 × 10 TM H+/cm2; (e) 6 × 10 TM H+/cm2; (f) 9 × 10 TM H + / c m 2

In X-ray and electron diffraction studies the asdeposited films with the above-mentioned compositions exhibit the well-known halo maxima (Figs. la, 2). The TEM image of the as-deposited CoP alloy film shows an inhomogeneous granular microstructure due to fluctuations of the mass density and composition [3] with granules on a scale of 80 to 100 nm. Themselves granules are inhomogeneous, there are individual small (2-5 nm) fragments within them. Depending on composition and thickness of the films, the amorphous CoP alloy films have following characteristics: H c = 0.2 to 1.5 Oe, B r / B s ---0.3 to 0.4, H S = 30 to 40 Oe, H v = 700 to 760 k g / m m 2. The B H loops of films with thickness of about 0.5 to 1 p.m indicate perpendicular magnetic anisotropy (PMA). After exposure to hydrogen ion irradiation with doses of 3 × 1015 to 6 X 10 iv H + / c m 2 or boron ion implantation with doses of (5-8) x 10 a5 B + / c m 2, to an electron flow of 2 X 1017 e - / c m ~ or "y-irradiation up to a dose of 1 × 10 ls "y/cm 2, no splitting of

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Fig. 2. TEM photographs of (a) the as-deposited amorphous CoP alloy film containing 16 at% P; (b) after boron ion irradiation with a dose of 1×1016 B + / c m 2 ; (c) 7×1016 B + / c m 2. (Magnification = 30000 × .)

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found to depend on the dose and type of irradiation. While the temperature Tx of incipient crystallization of the as-deposited nonirradiated films equals 493 K (Fig. 3, curve 1), TX for films irradiated by "y or electron flows varies from 473 to 483 K (Fig. 3, curves 2,3) and Tx of ion implanted films versus from 453 to 463 K (Fig. 3, curve 4). On achieving a definite hydrogen or boron ion irradiation dose the effect of irradiation-stimulated crystallization of CoP films is observed. After hydrogen ion irradiation with doses of ( 1 - 3 ) × 1018 H ÷ / c m 2 the diffraction peak of the hcp structure of Co appears on the X-ray diffraction patterns (Fig. ld). After boron ion irradiation with doses of (1-3) × 1016 B + / c m 2 in addition to the amorphous phase the reflexes of the fcc phase and cobalt phosphide (Co2P) monocrystalline phases appear on the electron diffraction patterns (Fig. 2b). Further increase of hydrogen irradiation dose leads to a further splitting of the halo diffraction maxima. In the higher dose region of (3-6) × 1018 H + / c m 2 the intensity of the hcp phase increases and a different diffraction peak corresponding to the crystalline phase of Co2P appears on the X-ray diffraction pattern (Fig. le). At the maximum dose of 9 × 1018 H + / c m 2, in addition to the hcp and COEP phases there appears some weak diffraction peak with (200) spacing due to the equilibrium phase based on the fcc phase (Fig. lf). It should be noted that the irradiation-induced phase transformation corresponds to the amorphous-crystalline transition of CoP alloy films during isothermal annealing. I5

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Fig. 3. The coercive field versus the annealing temperature for (1) as-deposited amorphous CoP alloy film containing 16 at% P and (2) film irradiated by electrons of 2 × 1017 e - / c m 2, (3) fluences of 1×10 TM y / c m 2, (4) by boron ions with dose of 7>(1016 B+/cm 2,

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Fig. 4. The coercive field of the amorphous CoP alloy films containing 16 at% P as a function of the hydrogen (a) or boron (b) ions irradiation.

Contrary to hydrogen irradiation after boron irradiation with doses of ( 5 - 7 ) × 1016 B + / c m 2, the X-ray and electron diffraction patterns evidence the amorphous state of irradiated films (Fig. 2c). However, the diameter of the first diffraction ring is smaller compared to that of nonirradiated amorphous films. This fact indicates that the obtained amorphous alloy is characterized by greater interatomic distances than as-deposited amorphous alloy. According to Fig. 2 with increasing boron ions dose the deformation and destruction of the granules are noted; the narrowing and full disappearance of the quasi grain boundaries are observed. The micropores and through pores disappear too. Ions bombardment has influence on magnetic and mechanical characteristics of the CoP alloy films. Fig. 4a shows that H c rises by two orders of magnitude with hydrogen ion doses increasing above 3 × 1017 H + / c m 2. H v of the films increases by 1.5-2 times too. After boron ion irradiation with doses of 8 × 1015 to 3 × 1016 c m -2 according to Fig. 4b maximum values in coercive field are observed. In

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the higher doses regions the reduction of the coercive field by 20 to 30% occurs compared to that of as-deposited films. With boron ion doses increasing from 5 × 1015 to 7 × 1017 B + / c m z the H v of the films increases by 30 to 40% and H~ reduces by 3 times. The shape of the hysteresis loops of both the boron or hydrogen ion irradiated films changes: B r / B ~ increases essentially and, consequently, the perpendicular magnetic anisotropy reduces. The transmission of small amount of excessive energy into the volume of an amorphous transition metal-metalloid alloy by bombarding with ions can lead to migration of the lighter and more mobile alloy components, i.e. the metalloid atoms [4]. As a result the change of local short-range order in the environment of transition metal atoms is possible. Evidently, irradiation-induced redistribution of phosphorus atoms around cobalt atoms promotes the increase of short-range order in the atomic arrangement, resulting in the activation of the crystalline phase nucleation process in an amorphous matrix. It is known [5] that during electrodeposition of transition metals such as Fe, Co, Ni and their alloys an amount of hydrogen is coprecipitated in the electrodeposits. The high energy hellium, hydrogen, etc. irradiation lead to an artificial increase of gas dissolubility in metals and alloys. As a result nonequilibrium Me-H, Me-He and other systems form [6]. Apparently, the CoP alloy films, irradiated by hydrogen ions are also supersaturated solid C o - P - H solutions, which crystallize at lower temperatures than in as-deposited nonirradiated speciments. As shown in Ref. [7], during electrodeposition of the CoP alloy onto copper substrates the Co atoms preferably take places corresponded to the epitaxial growth within range of the large copper grains. The simultaneous recovery of the phosphorus ions onto cathode prevents the construction of the equilibrium crystalline cobalt lattice, and at phosphorus concentrations above 12 at% the amorphous precipitations are formed. On achieving a definite boron ion irradiation dose the effect of 'epitaxial crystallization' is possible, resulting in the formation of monocrystalline regions on the scale of copper grains size. The formation of the fcc phase evidently is connected with the known fact [8] that the short-range order of

the amorphous CoP alloy is based on fcc lattice Co. It should be noted that the observed fcc structure Co is nonequilibrium: it turns into the supersaturated solids solution based on hcp lattice Co at annealing temperatures above 433 K or it turns into amorphous phase in the high boron doses region. The irradiation-induced diffuse of phosphorus atoms from quasi grains boundaries on the free surfaces [9] leads to decrease of mass density fluctuations in alloy volume and narrowing or full disappearence of the quasi grain boundaries. Moreover in near surface layers of the irradiated CoP alloy the formation of phosphorus segregations is possible and at critical boron dose the formation of the CoeP intermetallic combination is observed. The structural transformations due to ions irradiation are the reason of change of the magnetic characteristics of the alloy. The main cause of the increase in coercive field of CoP films is the formation of the crystalline phases in the amorphous matrix. The destruction of the granular microstructure in the irradiated films leads to the suppression of the PMA and, consequently, to the decrease in film coercivity. The lower the structure inhomogeneties, the more alloy homogenization leads to the increase of the films microhardness.

References [1] L.A. Citlenok and L.F. llueshenko, Ann. Belorus. Acad. of Sciences, Sect. Phys.-Math. Sciences 6 (1983) 57. [2] A.A. Glazer, A.P. Potapov and V.V. Serikov, Fiz. Metallov i Metallovedenie 48 (1979) 1165. [3] R. Sonnberger, H. Bestgen and G. Dietz, J. Phys. B. 56 (1984) 289. [4] T. Imura and M. Doi, Intern. Sem. (Toronto, 1986) p. 327. [5] L.F. Ilueshenko, M.U. Sheleg and A.V. Boltushkin, The Electrical Deposition of Magnetic Films (Nauka i Teknika, Minsk, 1978) p. 278. [6] L.S. Palatnik, L.A. Kuzma and M.Ya. Fucks, Dokl. Akad. Nauk SSSR 242 (1978) 333. [7] T.A. Tochitsldi, V.G. Shadrov and A.V. Boltushkin, Ann. Belorus. Acad. of Sci. Sect. Phys.-Math. Scien. 5 (1988) 62. [8] L.S. Palatnik and N.I. Famko, Dokl. Akad. Nauk SSSR 270 (1983) 1380. [9] A.M. Shalaev, The Irradiation-Stimulated Processes in Metals (Energoatomizdat, Moscow, 1988) p. 236.