Superparamagnetic behavior in Fe ultrathin films on GaN(0001)

P. K. J. Wong, W. Zhang, X. G. Cui, I. G. Will, Y. B. Xu, Z. K. Tao, X. Li, Z. L. Xie, R. Zhang
2010 2010 8th International Vacuum Electron Sources Conference and Nanocarbon  
Here we report on the studies on the growth, structural and magnetic properties of ultrathin Fe grown on GaN(OOOl) by molecular beam epitaxy. The films and their surfaces were monitored by in-situ reflection high energy electron diffraction (RHEED) and a crystal thickness monitor. The magnetic properties of the samples were determined by a superconducting quantum interference device (SQUID) magnetometer. The 2 )lm thick GaN films used were prepared by metal-organic chemical vapor deposition
more » ... apor deposition (MOCVD) with a 20 nm low temperature (LT) buffer layer on c-Ab0 3 (0001). Prior loading into vacuum, the GaN surface was cleaned in a 1:1 HCI:H20 solution then a 1 :99 HF:H20 solution for 2 min each with DI water rinses between and after etching. This ex-situ chemical processes combined with a vacuum anneal are useful to remove oxygen and carbon contaminants on the GaN surface [1] . After annealing at 600°C for 1 h followed by cooling to room temperature (RT), streaky and sharp bulk-terminated (lx1) RHEED patterns were obtained, indicating a smooth GaN surface. Fe films with various thicknesses ranging from 2.5 to 150 ML were grown at RT at a chamber pressure of 1x10-1o mbar bye-beam evaporation at a rate of 2 A min-1 . Before samples retrieval for ex-situ characterization, the samples were capped with a 3.0 nm thick Au layer. Evolution of the Fe(llO) growth has been studied in-situ with the RHEED patterns as a function of Fe film thickness with the incident e-beam along the [11 2 0] of the GaN(OOOl). The streaky (Ix1) pattern of the clean GaN(OOOl) surface disappears upon a Fe coverage of 2.5 ML, indicating that the growth proceeds via a 3D mechanism as previously reported by He et al [2] . The faint main streaks of Fe exhibiting a bcc structure start to emerge at a deposition of 8 ML. Clear RHEED streaks of the Fe film become unambiguous at the deposition of 10 ML from where onward, less intense reflexes are visible beside the main streaks and the Fe patterns appear spotty. We further demonstrate that by reducing tFe down to a few ML, superparamagnetism (SPM) of the ultrathin Fe can be activated at the ambient temperature. Fig. l(a) illustrates the hysteresis loop of an as-deposited 5 ML Fe(llO) film on GaN(OOOl) taken at RT and reveals that the loop on one hand has an unsaturated magnetization and on the other hand possesses tiny but noticeable M r and He. These two characteristics as a whole imply a coexistence of SPM and weak FM in the ultrathin film. In order to gain further insight into this mixed magnetic state, temperature dependence of the magnetization M( T) in the form of field cooling (FC) and zero field cooling (ZFC) curves of the 5 ML sample was measured by SQUID in a T range between 5 and 300 K as illustrated in Fig. 1 (b) . The ZFC magnetization curve was performed by applying a small in-plane field of 300 Oe to the sample at 5 K and then warming it with Mbeing measured 978-1-4244-6644-3/10/$26.00 ©2010 IEEE
doi:10.1109/ivesc.2010.5644240 fatcat:3jbt4idrkndlpo23kwbpadsqiy