• Araujo, Jp
• Ibarra, Mr
• Morellon, L.
• Pereira, Am
• Ritter, C.
• Sousa, Jb
• Magen, C.
• Algarabel, Pa

Abstract

The magnetic and crystal structures of the magnetocaloric Ho-5(SixGe1-x)(4) (x= 1, 0.75, 0.5 and 0) have been studied by neutron powder diffraction experiments. The room-temperature crystal phases of the compositions x= 1, 0.75, and 0.5 are preserved in the whole temperature range 2-300 K, i.e. the Gd5Si4-type Pnma O(I) structure for Ho5Si4, the Gd5Si2Ge2-type P112(1)/a M state in x= 0.75, and the Sm5Ge4-type Pnma O(II)phase in x= 0.5. Ho5Si4 undergoes a second order ferromagnetic (FM) transition at T-C= 77 K into a noncollinear FM structure with the magnetization oriented mainly along the a axis and weak antiferromagnetic (AFM) coupling along b and c (magnetic space group Pnm'a'). Ho5Si3Ge becomes FM at T-C= 50 K (magnetic space group P112(1)'/a). At T-C'similar to 16 K an incipient second incommensurate magnetic phase appears which becomes commensurate below 8 K with a propagation vector k=(0 0 1/4). The magnetic structure of the main phase at 30 and 2 K shows the dominance of a FM coupling along the a axis with an AFM canting along c. Ho5Si2Ge2 stands out for the complexity of its low-temperature magnetic structure. In addition to the Neel transition of the O(II)phase at T-N similar to 30 K (magnetic space group Pn'm'a'), two additional magnetic phases with propagation vectors k=(0 0 1/4) and k=(1/2 0 0) appear at 15 and 12 K, respectively. In Ho5Ge4 the main O(II) structure orders AFM with k=(0 0 0) at T-N similar to 30 K in the same magnetic space group Pn'm'a'. Below 25 K a complete structural transition from high temperature O(II)Pnma to P2(1)/m takes place within the AFM state. The magnetic structure of this new nuclear phase stays AFM with k=(0 0 0), but sees two out of six independent Ho sites non magnetic. A second magnetic transition takes place at about 18 K characterized by the appearance of a second propagation vector k=(0 0 1/2) which magnetically couples the formerly non magnetic Ho sites. Magnetic-field dependent neutron diffractograms demonstrate that FM sets in in Ho5Ge4. The onset of ferromagnetism is associated with the previously reported nucleation of a new high field O(II) Pnma phase. Contrary to the intensity of the magnetic coupling with the propagation vector k=(0 0 0), which disappears quickly with the onset of FM, a progressive decrease of the intensity associated with the state k=(0 0 1/2) suggests a possible relationship between the extent of the magnetic coupling k=(0 0 1/2) and the percentage of remnant P2(1)/m phase.

Topics

• impedance spectroscopy
• phase
• experiment
• atomic force microscopy
• neutron diffraction
• magnetization
• space group