Synthesis of the new RGaMnO 5 (R = Ho, Er, Tm) oxides: Structural and magnetic features
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Teresa Ferna´ndez-Dı´az Institute Laue-Langevin 156X, F-38042 Grenoble Cedex 9, France (Received 17 October 2008; accepted 28 April 2009)
Polycrystalline HoGaMnO5, ErGaMnO5, and TmGaMnO5 oxides have been first prepared by soft chemistry procedures followed by high oxygen pressure treatments, to stabilize Mn4+ cations. Their crystal structures and magnetic behavior have been studied at room temperature and 5 K by neutron powder diffraction (NPD) data in complement with magnetization measurements. RGaMnO5 are orthorhombic, Pbam space group, and their crystal structures contain infinite chains of edge-sharing Mn4+O6 octahedra, interconnected by pyramidal Ga3+O5 and RO8 units. For R = Ho, a = 7.2810(4), b = ˚ ; for R = Er, a = 7.2575(3), b = 8.4357(3), and c = 5.6613 8.4526(4), and c = 5.6668(3) A ˚ ; and for R = Tm, a = 7.2438(3), b = 8.4124(3), and c = 5.6509(2) A ˚ , at room (2) A temperature. Above 300 K the reciprocal magnetic susceptibility follows a Curie-Weiss law. In the paramagnetic region, a positive Weiss constant suggests the presence of ferromagnetic interactions, which have been investigated by low-temperature NPD for R = Er, Tm. The 5 K patterns show a detectable long-range magnetic ordering over the Mn and R positions, ferromagnetically aligned along the x-direction.
I. INTRODUCTION
The family of RMn2O5 (R = rare earths element or Y) compounds has long been studied.1 More recently, these oxides have attracted great attention since the report of magnetoelectric effects in TbMn2O5.2 The characteristics of multiferroism include a spontaneous magnetization that can be switched by an applied electric field, a spontaneous electrical polarization that can be reoriented by an applied magnetic field, and a strong coupling that exists between these two properties.3,4 All of the RMn2O5 (R = Y, La. . .Lu) materials are isostructural (space group Pbam, Z = 4) and contain infinite chains of Mn4+O6 octahedra sharing edges, linked together by Mn3+O5 and RO8 units.5 The orthorhombic crystal structure contains two crystallographic sites for Mn atoms, with different oxygen coordination. Mn4+ ions are located at the 4f sites, octahedrally coordinated of oxygens, whereas Mn3+ ions occupy the 4h sites and they are bonded by five oxygens, forming a distorted tetragonal pyramid. In these compounds, the direct and indirect interactions between the two subsystems of magnetic d ions Mn3+ and Mn4+, being of the ferro- or a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0361 J. Mater. Res., Vol. 24, No. 9, Sep 2009
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antiferromagnetic type, are subtly dependent on the features of the environment. In fact, it has been shown that small variations of the exchange integrals with temperature lead to the complex phase diagram for this multiferroic family.6 For instance, RMn2O5 (R = Dy, Tb, Ho) display incommensurate antiferromagnetic ordering below 40 K, becoming commensurate on further cooling.7 For R = Tb, Ho, a commensurate-inc
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