X-ray rocking curve and ferromagnetic resonance investigations of ion-implanted magnetic garnet

Abstract

Detailed analyses of x-ray rocking curves and ferromagnetic resonance spectra were used to characterize properties of <111>-oriented Gd, Tm, Ga:YIG films implanted with Ne+, He+, and H ⁺ ₂. For each implanted species the range of doses begins with easily analyzed effects and ends with paramagnetism or amorphousness. Ion energies were chosen to produce implanted layer thicknesses of 3000 to 6000 Å. Profiles of normal strain, lateral strain, and damage were obtained. The normal strain increases with dose and near amorphousness is 2.5%, 3.4%, and 3.9% for Ne+, He+, and H ⁺ ₂, respectively. Lateral strain is 0 for all values of normal strain, implying absence of plastic flow. Comparison of these results with the reported decrease in lateral stress implies either a large reduction in Young's modulus or a transition to rhombohedral equilibrium unit cell. Damage is modelled by a spherically-symmetric Gaussian distribution of incoherent atomic displacements. Due to the use of (444), (888), and (880) reflections the sensitivity is greatest for the c sites occupied by Gd, Tm, and Y. The standard deviation of displacements increases linearly with strain with proportionality constant 0.25, 0.18, and 0.13 Å/% for Ne+, He+, and H ⁺ ₂, respectively. For maximum strains up to 1.3% annealing in air reduces the strain without changing the shape of the profile. The behavior of the strain with annealing is nearly independent of implanted species or doses. After annealing at 600 °C the strain is 40% of the original value. Magnetic profiles obtained before and after annealing were compared with the strain profiles. The local change in anisotropy field DeltaHk with increasing strain shows an initially linear rise for both He+ and Ne+. The slope is −4.1 kOe/%, in agreement with the magnetostriction effect estimated from the composition. For strain values between 1 and 1.5%, DeltaHk saturates reaching peak values of −3.6 kOe for He+ and −2.8 kOe for Ne+. At strain values near 2.3% for He+ and 1.8% for Ne+, DeltaHk drops to nearly 0 and the material is paramagnetic. For peak strains greater than 1.3% for He+ and 1.1% for Ne+ the relation between uniaxial anisotropy and strain is not unique. The saturation magnetization 4piM, the ratio of exchange stiffness to magnetization (A/M) and the cubic anisotropy H1 decrease with strain reaching 0 at 2.3% and 1.8% for He+ and Ne+, respectively. At these strain values the damping coefficient alpha is 50% and 80% greater than bulk value for He+ and Ne+, respectively. For higher observed strains the material remains paramagnetic. Upon annealing of samples implanted with low doses of Ne+ and He+ the anisotropy field follows uniquely the behavior with strain for unannealed material. At 600 °C the magnetization returns to bulk value but the ratio A/M remains 20% low. For H ⁺ ₂ implantation the total DeltaHk consists of a magnetostrictive contribution due to strain and of a comparable excess contribution associated with the local concentration of hydrogen. The profile of excess DeltaHk agrees with calculated LSS range. The presence of hydrogen results in a reduction of 4piM not attributable to strain or damage. For a peak strain of 0.60% and a peak total DeltaHk of −4.5 kOe, the magnetization is only 40% of bulk value. After annealing up to 350 °C the excess DeltaHk diminishes and redistributes itself to the regions neighboring the peak damage. At 400 °C the excess is nearly 0. For higher annealing temperatures the only component of DeltaHk is magnetostrictive. At 600 °C, the magnetization, the ratio A/M, and alpha return to bulk values.

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