Journal of Magnetism and Magnetic Materials 272–276 (2004) 503–504 55 Mn NMR/NQR study in b-MnOs alloys T. Hama a, *, M. Matsumura a , H. Yamagata a , M. Miyakawa b , R. Umetsu b , K. Fukamichi b a Department of Materials Science, Faculty of Science, Kochi University, Kochi 780-8520, Japan b Department of Materials Science, Tohoku University, Aoba-yama 02, Aoba-ku, Sendai 980-8579, Japan Abstract The substitutional effects to the site I of Os atom were investigated microscopically by 55 Mn zero-field NMR and NQR in b-Mn 1x Os x alloys. Above the N! eel temperature T N ; the new set of NQR signal appears in lower frequency, suggesting the local lattice expansion around Os atom in xp0:06: Sufficiently below T N ; NMR signal associated with the site II appears in xX0:005: The internal field increases continuously with increasing x; which is consistent with the picture of itinerant antiferromagnet. r 2003 Elsevier B.V. All rights reserved. PACS: 75.20.En; 75.50.Ee; 76.60.Gv Keywords: b-MnOs alloy; Itinerant antiferromagnet; NQR; NMR The high-temperature b-phase of Mn metal, b-Mn, is one of a few examples of itinerant antiferromagnet (AF). b-Mn has two crystallographically inequivalent sites. Eight atoms occupy site I and 12 atoms occupy site II in the 20 atoms per unit cell. The electronic state of Mn atoms at the site II is expected to be more magnetic than at the site I since the atoms at the site II are more isolated than the atoms at the site I. No magnetic ordering down to low temperature in b-Mn metal is ascribed to two contrast points of view. One is a nearly AF [1]. Another is a spin liquid state due to the geometrical frustration of large Mn moments at the site II [2]. The study of the substitution effect is useful to determine which view is reasonable for the magnetism in b-Mn metal. In b-Mn 1x Al x alloys, where Al atoms preferentially occupy site II, the magnetic ground state changes to a spin glass state at the critical concentration xB0:03; as evidenced from the cusp and the hysteresis in field-cooled and zero field-cooled magnetic susceptibil- ity, and also from mSR [2,3]. The local moment formation in substitution of Al seems to lift the degeneracy of the spin liquid state. On the other hand, in b-Mn 1x Os x alloy, the preferential substitution of Os atoms to the site I was confirmed already by the powder X-ray diffraction, the x dependence of the specific heat coefficient and the N! eel temperature is interpreted by the self-consistent spin fluctuation theory for itinerant AF [4]. The paramag- netic susceptibility with no field-cooling effect behaves in analogy with an itinerant AF [4]. Here, the effects of the substitution of Os in b-Mn metal were investigated microscopically by 55 Mn zero- field NMR and NQR. b-MnOs alloys were prepared by rapid quenching of the high-temperature b-phase into iced water after annealing. The obtained ingots were crushed into powder with the particle size of less than 53 mm: Zero- field NMR and NQR measurements were performed using a coherent spin echo technique. The electric field gradient (EFG) at the site II is much larger than that at the site I. We can investigate the site II by NQR separately from the site I. Fig. 1 shows 55 Mn NQR spectra at 77 K ðT > T N Þ for the site II. The two lines around 4.5 and 9 MHz in x ¼ 0 come form the transitions for 7 3 2 27 1 2 and 7 5 2 27 3 2 ; respectively [5]. ARTICLE IN PRESS *Corresponding author. Tel.: +81-88-844-8286; fax: +81- 88-844-8359. E-mail address: thama@cc.kochi-u.ac.jp (T. Hama). 0304-8853/$-see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.389