RAPID COMMUNICATIONS PHYSICAL REVIEW B 83, 100406(R) (2011) Spin-polarized positron annihilation measurements of polycrystalline Fe, Co, Ni, and Gd based on Doppler broadening of annihilation radiation Atsuo Kawasuso, * Masaki Maekawa, Yuki Fukaya, Atsushi Yabuuchi, and Izumi Mochizuki Advanced Science ResearchCenter, Japan AtomicEnergy Agency, 1233 Watanuki, Takasaki, Gunma 370-1292, Japan (Received 6 January 2011; revised manuscript received 18 February 2011; published 14 March 2011) The Doppler broadening of annihilation radiation (DBAR) spectra of Fe, Co, Ni, and Gd polycrystals measured using spin-polarized positrons from a 68 Ge- 68 Ga source in magnetic fields exhibited clear asymmetry upon field reversal. The differential DBAR spectra between field-up and field-down conditions were qualitatively reproduced in calculations considering polarization of positrons and electrons. The magnitudes of the field-reversal asymmetry for the Fe, Co, and Ni samples was approximately proportional to the effective magnetization. The magnetic field dependence of the DBAR spectrum for the Fe sample showed hysteresis that is similar to a magnetization curve. These results demonstrate that spin-polarized positron annihilation spectroscopy will be useful in studying magnetic substances. DOI: 10.1103/PhysRevB.83.100406 PACS number(s): 75.50.−y, 78.70.Bj, 71.60.+z The electron momentum distribution of a magnetic sub- stance observed using spin-polarized positrons exhibits so- called field-reversal asymmetry due to the time-reversal symmetry breaking arising from excess electron spins. 1 This spectroscopic feature is analogous to that of the magnetic Compton scattering performed with circularly polarized x rays. One advantage of spin-polarized positron annihilation spectroscopy (SP-PAS) may be the depth selectivity by employing monochromatic positron beams. 2–4 Some impor- tant magnetic effects, such as giant magnetoresistance and tunneling magnetoresistance, occur near the interface between magnetic and nonmagnetic layers. Spin-injection electrodes, which will be used in spin devices, are normally thin films. Spin phenomena such as the spin Hall effect 5 and the giant Rashba effect 6 occur near surfaces. These are potential applications of SP-PAS. A pioneering research on surface magnetism using spin-polarized positron beam was performed by the Michigan group. 7 Taking advantage of the fact that PAS is a powerful tool to detect vacancy defects, SP-PAS might be used in studying vacancy-induced magnetism. In the 1960s, extensive studies were performed on magnetic substances using the angular correlation of annihilation radiation (ACAR) method with spin-polarized positrons. 8–18 However, thereafter, only a limited number of works have been carried out. 19–24 The detection limit of the field-reversal asymmetry of the electron momentum distribution in the SP-PAS experiment depends on positron polarization. To perform better SP- PAS experiments, highly spin-polarized positrons are needed. Positrons emitted from radioisotopes are longitudinally spin polarized due to the parity nonconservation in the weak interaction. 25,26 The longitudinal spin polarization of a positron is given as its helicity, v/c, where v and c are positron and light speeds, respectively. This means that highly spin-polarized positrons can be obtained from radioisotopes with high Q values. The average helicities of positrons from 22 Na and 68 Ge- 68 Ga are 0.7 and 0.94, respectively, and hence the latter radioisotope may be a better choice. Having positrons emitted into a cone angle θ , the average longitudinal spin polarization is decreased by a factor of (1 + cos θ )/2. Selection of faster positrons and restriction of the cone angle are options for enhancing spin polarization. 27 In this study, for future applications of SP-PAS to spin-electronics materials, we produced a 68 Ge- 68 Ga source and conducted Doppler broadening of annihilation radiation (DBAR) measurements for simple ferromagnetic substances (Fe, Co, Ni, and Gd). We considered the observed field- reversal asymmetry of DBAR spectra based on first-principles calculations. Samples used in this study were polycrystalline Fe(4N), Co(5N), Ni(5N), and Gd(3N) with the dimension of 15 × 15 × 2 mm 3 . The samples were mechanically and electrochemically polished and subjected to heat treatment at 1150 ◦ C for 2 h in vacuum. Through a nuclear reaction of 69 Ga(p,2n) 68 Ge induced by 20 MeV proton irradiation of a GaN substrate (8 mm φ), a positron source ( 68 Ge- 68 Ga, 20 MBq) was produced (total fluence: 9 × 10 17 protons). In the present experimental condition, the longitudinal spin polarization of positrons emitted from the source was determined to be 0.7 through the magnetic field dependence of the S parameter related to the self-annihilation of spin-singlet positronium in α-SiO 2 . 28 The samples and the source were placed in the center of the gap of an electromagnet keeping a distance of 7 mm at room temperature. To detect annihilation radiation only from the samples, the source was shielded by lead blocks. The DBAR spectra were measured using a high-purity Ge detector with an energy resolution of 1.4 keV at 511 keV. Here, a photon energy of E γ = 1 keV corresponds to an electron momentum of p = 3.92 × 10 −3 m 0 c. By changing field polarity, the DBAR spectra [N ↑ (p) and N ↓ (p)] were obtained for each sample. The subscript ↑ or ↓ denotes that the positron polarization and the magnetic field directions were parallel (field-up) or antiparallel (field-down). In each spectrum, more than 5 × 10 6 events were accumulated. All the spectrum area intensities were normalized to unity. Figure 1 shows the DBAR spectra of the Fe sample obtained in the field-up and field-down conditions. It is seen that the spectrum in the field-up condition is broader than that in the field-down condition. Similar features were observed for the other samples. Figure 2 shows the differential DBAR spec- tra [N ↑ (p) − N ↓ (p)] for these samples. The finite differential intensities mean that there exists field-reversal asymmetry. Roughly speaking, the field-reversal asymmetry appears due 100406-1 1098-0121/2011/83(10)/100406(4) ©2011 American Physical Society