Title: Jiří Prchal:Structural aspects of magnetic transitions in HoCo2 and TmCo2
Number: 8/18
Status: Closing date exceeded
Begin: Středa, Březen 14, 2018 at 02:30
Tutor: Vladimír Sechovský
Location: lecture room F2, first floor Ke Karlovu 5

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We have a pleasure to invite you to attend the joint seminar
of the Department of Condensed Matter Physics (DCMP)
and the Materials Growth and Measurement Laboratory (MGML)




Structural aspects of magnetic transitions in HoCo2 and TmCo2

lecture given by:

Jiří Prchal

DCMP, Faculty of Mathematics and Physics, Charles University

The seminar takes place in the lecture room F2 
of the Faculty of Mathematics and Physics, Ke Karlovu 5, Praha 2
on Wednesday, 14. 3. 2018 from 14:30 

Vladimír Sechovský
On behalf of the DCMP and MGML


The family of RCo2 compounds forming the cubic Laves phase (R represents a rare-earth element) has been studied already since more than half century ago for their interesting physical properties. Namely the interaction between the magnetic moments coming from the localized 4f-electrons of the rare-earth and the magnetism originating from the 3d-electrons of Co results in very interesting behavior of these compounds. The two magnetic sublattices (rare-earth and cobalt one) are ferromagnetically ordered at low temperatures and their respective orientation is either parallel or antiparallel, resulting either in ferromagnetic or ferrimagnetic structure. Particularity of the RCo2 compounds resides in the appearance of the cobalt magnetism on the border of its formation. Thus the system is very sensitive to external acting by magnetic field, pressure, substitution etc.
We focus on two representatives - HoCo2 and TmCo2. In these two compounds (together with ErCo2 and DyCo2) the magnetic ordering is connected with a first-order phase transition (FOMPT). In the case of HoCo2, the FOMPT appears at around Tc=80K (reports in the literature vary within some interval around this value, depending on the sample). With further cooling, another phase-to-phase transition appears at around TR=16K where the magnetic easy-axis changes its orientation from the cubic [100] to [110] direction. In TmCo2 the reported value of magnetic ordering temperature varies from 3.6K to 7K while both - first and second-order type of magnetic transition were observed. The magnetic easy-axis direction below the ordering temperature is along the original [111] direction in TmCo2. Especially the eventual presence of FOMPT in TmCo2 is disputed for long time.
The magnetic ordering is connected with the distortion of the crystal lattice in correspondence with the magnetic easy-axis direction. The type of the distortion below the respective transition is mostly known. Some discrepancies in the structure type of HoCo2 below TR and the order of magnetic transition in TmCo2 together with the (non-)presence of Co magnetism were subject of study in the near past. Our contribution is dealing with several interesting aspects of the magneto-structural changes in these two compounds. Our findings are based on characterization of HoCo2 and TmCo2 in both - poly- and monocrystalline form of the samples. Besides other results, the low-temperature X-ray diffraction data bring valuable view to the controversal points.

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