The physical and mechanical properties of any material are usually changed when it is bombarded with neutrons, ions, electrons or gamma rays. These property changes are
1.5 Ions implantation
due to the atomic rearrangements, called radiation damage, brought about by the ra-diation, and the kind of rearrangement depends on the kind of rara-diation, i.e., neutrons, being uncharged particles, are highly penetrating.
When incident on a solid, a particle can travel relatively large distances, in the solid, before a collisions with atoms. Each collision, however, causes displacement of one or more atoms into interstitial positions, leaving vacancies behind. Interstitials and va-cancies are collectively called point defects to distinguish them from a line imperfection like a dislocation. Similarly, ion implantation and focused ion-beam processing, which are widely used in the semiconductor industry, are steadily gaining ground in magnetic thin film materials processing. A useful aspect of irradiation is that can make some materials magnetically softer [25] or harder [26].
1.5.1 Physics of ion irradiation
Radiation damage arises from the interactions of energetic particles with a target ma-terial as they are slowed down and transfer their energy by a series of collisions. At the beginning, the ion kinetic energy is reduced mainly by the electronic stopping pro-cesses, and ion trajectory is relatively unchanged. When the ion has slowed down sufficiently, the collisions with nuclei (the nuclear stopping) become more probable and finally dominant. When atoms of the solid receive significant energies by the ion, they will be removed from their lattice positions, and produce a cascade of further collisions in the material. This series of nuclear collisions results in atomic displacements and generates defects that alter microstructure and mechanical properties.
Nuclear stopping results from the elastic collisions between the ion and atoms in the sample and increases when the mass of the ion increases, dominating the stopping pro-cess at low energy. The stopping power is a function of the repulsive potential V(r) between two atoms. For very light ions slowing down in heavy materials, the nuclear stopping is weaker than the electronic stopping at all energies. Electronic stopping refers to a process by which the ion is slowed and its energy is transferred to the target material through inelastic collisions between bound electrons in the medium and the ion moving through it. The collisions may result both in excitations of bound electrons of the medium and in excitations of the electron cloud of the ion. In most materials, these electronic excitations have little e↵ect on damage production, and energy dissipa-tion e↵ects are neglected in most cases, such as in ion implantadissipa-tion of metallic systems.
1. ELEMENTS OF MAGNETISM
However, electronic stopping may play an important role in the displacement process.
Electronic stopping increases linearly with the square root of the ion energy, while at lower ion energies (<10 1 MeV) the nuclear stopping becomes more relevant. In par-ticular, the nuclear collisions create a sequence of atomic displacements (cascades) that produce a localized high concentration of defects. The atomic displacement sequence creates a primary knock-on atom (PKA), which is the atom struck by a bombarding irradiation particle. One part of the losses energy is given to the neighboring atoms, producing secondary displacement events. Then the distance between successive colli-sions becomes progressively shorter. When the energies decrease below a few hundred eV, the mean free path between collisions is of the order of a couple of atomic distances.
Thus, a large amount of energy is deposited in a small volume, generating a high con-centration of defects in a localized region.
An important parameter, useful for theoretical and experimental model, is the dis-placement energy. This is the energy needed to overcome the lattice force inside the material and to move the atoms one atomic spacing away from its original site. If the atom comes back into its original site, the energy will be given up into phonons. For metal displacement energy is 25 eV and for semiconductor is 15 eV [27].
Since implantation doses are common higher than 1012 ions/cm2, ion trajectories can be predicted employing statistical means (i.e., Monte Carlo simulation). The average depth of the implanted ions is called the projected range Rp, and the distribution of the implanted ions respect to the depth can be approximated as Gaussian with a standard deviation p. The ion concentration at depth x, can be written as:
n(x) = n0exp{ (x Rp)2
2 p2 } (1.49)
where n0 ⇡ 0.4p is the peak concentration ( is the implantation dose), Rp is the projected range, and p is the standard deviation.
The ion and dislodged target atoms can cause further damage spreading their energy over many moving particles during loss energy process. Hence, after many ions have been implanted, the crystalline target will have changed toward a highly disordered state. If the target temperature is sufficiently high, the competing process of self-annealing can occur to repair some or all of the damage as it is generated. There is an
1.5 Ions implantation
Figure 1.18: - Profile of Boron ions implantation in Si. Shallow implantation are present below surface.
implantation critical dose, defined as a minimum necessary to amorphize the target, at which surface target is eroded or damaged. Damage can a↵ect the results of subsequent processing steps. For example, point defects are known to influence di↵usion in silicon.
Damaged oxide layer etch faster than continuous oxide one because some of the bonds are already broken. Considering the sputtering yield (Y ), it is possible to understand if these phenomenas are negligible or not. The thickness decreasing due to damage is given by:
t = Y A
Na⇢ (1.50)
where A is molecular weight, Na is Avogrado’s number and ⇢ is the density, implan-tation dose.
1.5.2 E↵ect on magnetic material
The consequences of ion irradiation on magnetic material have been found mainly on magnetic properties. The displace of atoms generated by ion collisions involves two di↵erent aspects: the purely collisional atom mixing on one hand, and the chemical heat of mixing on the other. At low mixing rate, the mixing rate is linear (for He irradiation) or almost quadratic (for Ga irradiation) with the dose. The modification of the profile concentration at the interfaces due to collision may be characterized by an interdi↵usion length ldif. [27]. At high ions dose, ldif. reaches several monolayers so that the alloying of atoms is strong.
l2dif f.= (Ndis /Ns)a2 (1.51)
1. ELEMENTS OF MAGNETISM
where Ndis is the average number of displacement generated by each ion, Ns is the atomic surface density of the material, is the ion dose and a lattice parameter. For example, Ga ion implantation, by focused ion beam, leads to the reduction of exchange coupling between magnetic grains and the formation of extended domain wall pinning defects in Co film. These e↵ects lead to a square hysteresis loops and a dramatic in-crease in coercivity (as shown in [28]). On the other hand, a Ga+ dose of 1014 ions / cm2 induces a complete transition from the ordered L10 to the disordered A1 phase in FePt thin film, leading to a drastic decrease of the magnetic anisotropy and coercivity, and to a spin reorientation transition from out-of-plane to in-plane [25].
Ion irradiation is revealed to be a very flexible tool to tailor the main magnetic prop-erties (e.g., anisotropy, coercive field, exchange interaction, Curie temperature) of thin films and multilayers, giving rise to several e↵ects, depending on the chosen parameters, like intermixing, modification of morphology, interface quality and strain, modification of crystallinity and chemical composition, production of vacancies and pinning centers [29, 30, 31, 32].