C. Piqué

Double magnetic phase transition inND4Fe(DPO4)2andNH4Fe(HPO4)2

B. F. Alfonso,1 C. Pique,1 C. Trobajo,2 J. R. Garcia,2 E. Kampert,3 U. Zeitler,3 J. Rodriguez Fernandez,4 M. T. Fernandez-Diaz,5 and J. A. Blanco6 1Departamento de Fisica, Universidad de Oviedo, E-33204 Gijon, Spain 2Departamento de Quimica Organica e Inorganica, Universidad de Oviedo, E-33006 Oviedo, Spain 3Institute for Molecules and Materials, High Field Magnet Laboratory, Radboud University Nijmegen, Toernooiveld 7, 6525 ED Nijmegen, The Netherlands 4CITIMAC, Facultad de Ciencias, Universidad de Cantabria, E-39005 Santander, Spain 5Institute Laue-Langevin, BP 156X, F-38042 Grenoble, France 6Departamento de Fisica, Universidad de Oviedo, E-33007 Oviedo, Spain Received 3 June 2010; revised manuscript received 14 September 2010; published 20 October 2010

Powder neutron diffraction investigation of the crystal and magnetic structures of NH4Fe(HPO4)2 and its deuterated form

The NH4Fe(HPO4)2 and its deuterated form, ND4Fe(DPO4)2, were investigated in detail from powder neutron diffraction data allowing a precise determination of the hydrogen positions and low temperature magnetic structures. Below TC = 17.82 K they order with a ferrimagnetic structure and the magnetic moments lying in the crystallographic plane ac. As the temperature is lowered to Tt = 3.52 K the system undergoes a magnetic phase transition to an equal moment antiphase structure characterized by the propagation vector close to AF ? (1/16,0,1/16) and a magnetic moment for the Fe3+ ions of 4.8 ?B at 1.89 K. The low symmetry of this crystal structure and the complex pattern of competing superexchange pathways are responsible for the existence of this double magnetic phase transition.

Neutron powder diffraction investigation in ammonium iron(III) bis (hydrogenphosphate)

The deuterated form of ammonium iron(III) bis (hydrogenphosphate), ND4Fe(DPO4)2, was investigated in detail from neutron powder diffraction data with a wavelength λ = 4.724 A. The material undergoes two successive magnetic phase transitions which are associated with the Fe3+ magnetic moments. One at TC = 17.82 ± 0.05 K is attributed to the ferrimagnetic order with the magnetic moments, μFI = 4.19 ± 0.02 μB at 4 K, lying on the crystallographic plane ac. The other transition is found to be at Tt = 3.52 ± 0.05 K due to an antiferromagnetic arrangement, with an equal moment antiphase structure that is characterized by a long-period propagation vector close to AF ≈ (1/16,0,1/16) and a magnetic moment for the Fe3+ ions of μAF = 4.41 ± 0.03 μB at 1.5 K. The low symmetry of its triclinic crystal structure and the complex pattern of competing superexchange pathways seem to be responsible for the existence of this double magnetic phase transition.

Heat capacity of RFexMn12−x (R = Gd, Tb and Dy) compounds: wiping out a cooperative 4f–4f exchange interaction by breaking the 3d–4f magnetic symmetry

Using adiabatic calorimetry the heat capacity of a series of RFex Mn12−x (R = Gd, Tb and Dy) compounds has been measured from 3 to 350 K. The substitution of Fe for Mn in RFex Mn12−x influences both the magnetic interactions on the 3d sublattice and the magnetism of R (the Neel temperature doubles on going from x = 0 to 6 and the compounds become ferromagnetic for x = 8 with Curie temperatures of around 300 K). In pure TbMn12 the heat-capacity data shows a λ-type anomaly associated with the independent cooperative magnetic ordering of the R sublattice (∼5 K), while the anomaly related to the Mn magnetic ordering (∼100 K) is rather smooth, as observed in other itinerant magnetic systems such as YMn12. In contrast, the substitution of Fe for Mn leads, on the one hand, to a more localized magnetic behaviour of the 3d sublattice, and, on the other, to magnetic polarization effects between the 3d and 4f sublattices, together with the disappearance of the cooperative magnetic ordering of the R sublattice due to the breaking of the antiferromagnetic symmetry in the 3d sublattice. This is reflected in the heat-capacity curve through a smooth Schottky-like anomaly. In the case of Gd compounds the magnitude of the exchange molecular-field parameter has been deduced by fitting the magnetic contribution to the heat capacity within a simple mean-field model. From this analysis we found that this molecular field acting on the rare-earth site increases with the iron concentration, reaching values as large as 48 T for the concentration x = 6. A similar analysis of the heat capacity in the ordered phase on the Tb compounds also leads to an enhancement of the molecular field with increasing Fe content. These results allow checking the possible crystal-field parameters for these RFex Mn12−x compounds.

Magnetic structures of RFexMn12 − x compounds (R = Tb and Y)

Abstract The magnetic ordering of RFe x Mn 12 − x compounds (R = Tb and Y) has been studied by powder neutron diffraction. The main coupling yields a non-collinear antiferromagnetic (AF) ordering of the transition metals (3d) sublattice, with the mean FeMn magnetic moments much larger than in the AF RMn 12 compounds. In samples with higher Fe content a 3d ferromagnetic (F) contribution is also observed, coexisting with the AF one, whose moments are significantly reduced. In TbFe 6 Mn 6 a F component develops at low temperature on the Tb ions owing to the polarisation induced by the 3d sublattice.

Heat capacity in RFexMn12 − x compounds

Abstract We have measured the heat capacity of a series of RFe x Mn 12 − x compounds (R = Y, Gd, Tb, Lu; x = 2, 4, 6, 8) from 3 to 350 K, using adiabatic calorimetry. Magnetic ordering is observed at 204 K for x = 2, and 240 K for x = 4 and 6; no defined anomaly has been found for x = 8. In the low-temperature range there is a smooth Schottky-like behaviour coming from low-energy levels of the rare-earth ions. No λ-type ordering anomaly of the rare-earth sublattice has been detected. In the case of Gd compounds the magnitude of the exchange-field parameter has been deduced fitting the magnetic contribution to the heat capacity with a simple molecular-field model. From this analysis we found that the exchange molecular field on the rare earth increases with the iron concentration. On the other hand the values of the electronic coefficient γ are quite enhanced, γ = 245 mJ/mol K 2 for YFe 8 Mn 4 .