Secondary ion emission of Fe–Ni alloys in the temperature range including the Curie point
Introduction
It is known that the magnetic phase transitions influence on various processes in magnetic materials, including thermal expansion [1], [2], oxidation [3], [4], sublimation [5], chemical reaction [6] and structure rearrangement [7], [8], [9]. The first result concerning the ion-stimulated emission of atomic particles near Curie temperature Tc was reported for the process of sputtering (see Ref. [10]). A step-like increase (by 10–20%) of sputtering yield Y for nickel single crystal (Tc=365°C) under transition from ferro- to paramagnetic state (f–p transition) and a high narrow peak at the Curie point was observed [10], [11], [12], [13]. Similar results were obtained then for gadolinium (Tc=17°C) [14], [15]. The details of temperature dependence of Ni sputtering in Refs. [16], [17], [18], [19], [20] were reported.
The decrease in the sputtering yield of Ni and Gd in ferromagnetic state is connected with an increase of the potential energy of atomic interaction. The theory developed in Refs. [21], [22] (see also [12], [20], [23]), shows that a variation of the interaction potential for f-state change the collision cascades, and increases the binding energy of surface atoms, in such a way that the coefficient of sputtering Y decreases by ∼15%. The fast rise of sputtering at the Curie point was explained in Ref. [24] by a substantial increase of sublimation during ion bombardment of nickel target at that temperature.
The magnetic phase transition influence also on the secondary ion emission from nickel [25], [26], [27], [20], [23] and gadolinium [28], [20], [23]. For both elements the emission of secondary ions decrease under f–p transition and somewhat increase at the Curie point. A decrease of secondary ion emission in paramagnetic state of target causes a rise of neutralization probability of ions near a surface. For nickel this occurs due to increase of electron states density near Fermi level at the temperature higher than Tc (see Refs. [20], [23]).
It is of interest to continue investigating the temperature dependence of secondary-particle emission for materials with fairly similar elemental composition and different Curie points. The following iron–nickel alloys: 35% Ni 65% Fe (Tc=240°C), 40% Ni 60% Fe (Tc=360°C), 50% Ni 50% Fe (Tc=530°C), and also the alloy 79% Ni 16% Fe 5% Mo (Tc=345°C) were chosen for this study. These ferromagnetic alloys are distinguished by their unique properties and are widely used. Invar N35 (35% Ni) has an uniquely low coefficient of thermal expansion, the permalloys N40 and N50 (40% Ni and 50% Ni) have a high conductivity and permeability, and molybdenum permalloy MN79 (79% Ni 17% Fe 4% Mo) is characterized by high values of permeability and resistivity [29].
In this work the temperature dependence of the total mass spectra and energy spectra of secondary ions of nickel, iron, and impurities in the case of irradiation of Fe–Ni alloy single crystals by 10 keV argon ions has been studied. In addition, the modification in surface composition of the alloys arising as a result of sputtering has been investigated.
Section snippets
Experimental
The measurements were performed using an automatically-controlled set-up (see Ref. [30]) comprising a mobile 180° spherical energy analyzer (made at Physics Faculty of MSU workshop) connected to a quadrupole mass spectrometer MS-7303 (NTO RAN, Chernogolovka, Russia). The energy resolution was 0.5 eV for a transmission energy of 20 eV. The unit, comprising a set of lenses and the 90° deflector, provided a constant transmission coefficient when scanning over the energy spectrum and for the
Spatial distribution
For the Ni+ and Fe+ ions emitted from the (1 1 1) face of invar 35% Ni 65% Fe single crystal the azimuthal angular distribution of emission (for a 45° polar angle of observation) was measured. The result is shown in Fig. 1. The emission maximum was found near the 〈1 1 0〉 close-packed directions for both Ni+ and Fe+ ions. The degree of anisotropy (the ratio Imax/Imin of ion yields at the maximum and minimum of the distribution) was somewhat higher for Ni+ than for Fe+ ions. The emission composition
Conclusions
1. The secondary ion emission is investigated for ferromagnetic Fe–Ni alloy single crystals with fairly similar elemental composition and different Curie points. These were invar with 35% Ni (Tc=240°C), permalloys with 40% Ni (Tc=360°C) and 50% Ni (Tc=530°C), and also an alloy with 79% Ni 16% Fe 5% Mo (Tc=345°C).
2. The spatial distribution of Ni+ and Fe+ secondary ions emitted from the (1 1 1) face of invar and permalloy single crystals was shown to be anisotropic with pronounced maxima of ion
Acknowledgments
We thank the Russian Foundation for Basic Research (Project No. 02-02-17918) and the UK Royal Society (Grant No. 15 294) for supporting this work.
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