**3. Methodology and materials**

**Figure 9.** Schematic diagram of a typical SQUID magnetometer [42].

shows dependency on bead concentrations, which cannot be neglected (six different concentrations were tested for every bead type). All of this has to be considered when calculating the mag-

SQUID magnetometry is a very sensitive technique for magnetic characterization of materials. A SQUID can detect magnetic ordering by tracking temperature dependency on magnetization (MT), field-dependent magnetization (MH) and very weak magnetic moments. A SQUID works on the principle of electron-pair wave coherence and Josephson effect, which can be defined as the flow of current (called Josephson's current) across two superconductors separated by an insulated layer. A Josephson's junction comprises two superconducting coils separated by a very thin insulating barrier to enable electrons to pass through it. A SQUID magnetometer is made of a superconducting ring into which two Josephson's junctions are placed in parallel in a magnetic field. A current flows through the superconducting loop if a magnetic field is applied. The magnetic flux of a ferromagnetic sample placed between the superconductors in the presence of an applied field will change accordingly. This magnetic flux change will induce a current which changes the initial current circulating through the coil. The variation in the current helps to detect the magnetic moment of the

**Figure 9** shows the principle of SQUID magnetometry. A SQUID magnetometer was used to investigate MT and MH characteristics of metal ion-implanted GaN samples. During MH analysis, the hysteresis loops of ion-implanted and unimplanted samples were recorded at

netic moment at a small outer magnetic field.

24 Ion Implantation - Research and Application

material.

Epitaxial layers of GaN (n-type) about 2 μm thick were grown on sapphire (Al<sup>2</sup> O3 ) by metal organic chemical vapour deposition (MOCVD). The structure and crystal quality of the GaN epilayers were analysed by Rutherford backscattering and channelling spectrometry (RBS/C) prior to ion implantation. A He<sup>+</sup> ion beam, with energy 2.0 MeV, was used for RBS/C analysis. Sample mounting and detector specifications and configurations have already been explained in Section 2.2. The ˂0001˃ normal axis was chosen to investigate the quality of GaN. The channelling minimum yield was found to be around 1.3%, indicating that the quality of the epitaxial GaN material was high. Co<sup>+</sup> ions of energy 150 keV were implanted into the GaN epilayers at room temperatures at doses of 3 × 1016 and 5 × 1016 ions cm−2, making the wafer to self-heat at such high doses. This is advantageous since it causes 'dynamic annealing', that is, annealing during implantation. The concentration of cobalt was calculated to be around 3–5% from SRIM 2008 [37]. The estimated range of Co<sup>+</sup> ions in the sample was approximately 150 nm. The implanted samples were annealed at 700, 800 and 900°C for 5 min using rapid thermal annealing (RTA) in an ambient N<sup>2</sup> atmosphere to remove implantation damage and recrystallize them. Similarly, Cr<sup>+</sup> ions of energy 150 keV were implanted into the GaN epilayer at room temperature to a dose of 3 × 10<sup>16</sup> ions cm−2 at a tilted angle of 7o of the direction of incident beam to avoid implantation channelling effects. The chromium concentration was estimated to be about 3% as calculated from SRIM 2008 and also confirmed by simulation of random spectra using RUMP. The projected range of the Cr<sup>+</sup> ions in the samples was about 150 nm. The implanted samples were annealed at 800 and 900°C for 2 min using rapid thermal annealing (RTA) in ambient N<sup>2</sup> to re-crystallize the samples and to remove implantation damage.

X-ray diffraction (XRD, Philips X'Pert Data Collector) was used for structural analysis of ion-implanted samples, using Cu Kα radiations. High resolution X-ray diffraction (HR-XRD, BEPCII-2.5 GeV) was performed at Beijing synchrotron radiation facility (BSRF) and at Shanghai synchrotron radiation facility (SSRF). Alternating gradient magnetometry (AGM, PMC 2900-04C) was used for measuring magnetization at room temperature and a superconducting quantum interference device magnetometer (MPMS-XL Quantum Design) was used to investigate magnetic properties at 5 K. A magnetic field was applied parallel to the sample surface during magnetization measurements.
