2002年6月大学英语四级考试试题及答案.pdfVIP

5 0 0 2 g u Magnetic Phases of Rare Earth Hexagonal Manganites A 9 a S. H. Curnoe a,∗, I. Munawara Department of Physics and Physical Oceanography, Memorial University of Newfoundland, St. John’s NL, A1B 3X7, Canada ] l e - r t Abstract s . t We describe the magnetic phases of hexagonal rare earth manganites RMnO3 using Landau theory. A minimal a model based on four one-dimensional magnetic order parameters is developed. m - d Key words: RMnO3, hexagonal manganites, multi-ferroics, magneto-electric n PACS: 75.10.-b; 75.30.Kz; 75.80.+q o c [ Hexagonal rare earth and yttrium manganites two two-dimensional representations E1,2 . These labels 1 RMnO3 (R = Ho, Er, Tm, Yb, Lu, Y or Sc) belong are also be used to denote the representations of the v to an unusual class of materials known as “multi- magnetic point group which correspond to magnetic 3 3 ferroics”, which display simultaneously electric and order parameters. Multiple copies of every order pa- 2 magnetic ordering. Most of the hexagonal mangan- rameter are possible for the spins of both Mn and rare 8 ites are ferroelectric below a very high temperature earth ions. There is an antiferromagnetic spin configu- 0 (TF E ≈ 900 K) and order magnetically at a lower ration involving Mn spins in the xy-plane for each 1D 5 temperature (TN ≈ 100 K). A complex phase diagram order parameter (see Ref. [2]). Additional spin config- 0 involving several different magnetic order parameters urations aligned with z for both Mn and R spins yield / has b