Strain-induced switching between noncollinear and collinear spin configuration in magnetic Mn5Ge3 films

Abstract

We report the temperature-dependent magnetic and structural properties of epitaxial Mn5Ge3 thin films grown on Ge substrates. Utilizing density-functional theory (DFT) calculations and various experimental methods, we reveal mechanisms governing the switching between collinear and noncollinear spin configuration in Mn5Ge3. The Mn atoms in Mn5Ge3 occupy two distinct Wyckoff positions with fourfold (Mn1) and sixfold (Mn2) multiplicity. The DFT calculations reveal that below a critical distance of approximately 3.002 Å the coupling between Mn2 atoms is antiferromagnetic (AFM) while ferromagnetic (FM) above that critical distance. The FM coupling between Mn1 atoms is weakly affected by the strain. The observed noncollinear spin configuration is due to the coexistence of AFM and FM coupling at low temperatures. The findings give insight in developing strain controlled spintronic devices. The author Y.X. (File No. 201706340054) acknowledges the financial support by China Scholarship Council. M.B. is funded by the National Science Centre Grant No. UMO-2016/23/D/ST3/03446. Access to computing facilities of PL-Grid Polish Infrastructure for Supporting Computational Science in the European Research Space and of the Interdisciplinary Center of Modeling (ICM), University of Warsaw is gratefully acknowledged. M.B. made use of computing facilities of TU Dresden ZIH within the project "TransPheMat". The low temperature X-ray diffraction was performed in MGML (http://mgml.eu/), which was supported within the program of Czech Research Infrastructures (Project No. LM2018096). Author contribution Author Y.X. performed the basic magnetic and transport measurements. Y.Y. and C.Z. performed angular dependent magnetoresistance measurements. P.D. and D.K. performed the temperature dependent XRD measurements. M.B. performed the DFT calculations and related analyses. J.G. contributed to the discussion about structural analyses. D.K. and X.Z. contributed to the discussion about transport analyses. M.H., S.Z., L.C., Y.-J.Z. and M.W. contributed to the revision of the manuscript. S.P. designed and supervised the whole project.

Critical behavior in the Mn$_{5}$Ge$_{3}$ ferromagnet

“…In addition to the magnetic phase transition at Curie temperature, there is a cusp near T Mn ∼ 70 K, which is clearly shown in the dρ / dT curve. Such a cusp is commonly observed in Mn 5 Ge 3 thin films and nanostructures but never observed in bulk crystal before [20,22,23]. For the first time, we observed it in the single crystals.…”

“…Mn atom occupies two different Wyckoff positions: 4d (MnI) and 6g (MnII) with the magnetic moments of 1.96 µ B and 3.23 µ B , respectively [19]. The magnetic moment direction of MnI and MnII atoms has been reported to be parallel to the c axis of the hexago-nal structure from T c to around 70 K [20]. The difference among various Mn-Mn interactions leads to an anisotropic exchange and complex magnetic ordering in different temperature regimes.…”

“…Neighboring MnII atoms are ferromagnetically coupled in the fully relaxed unit cell. Nevertheless, by applying compressive strain, the MnII-MnII atomic distance decreases and the corresponding FM coupling is suppressed, then transformed into AFM coupling [20,27]. In addition, it is believed that the coexistence of AFM and FM coupling observed at low temperature may lead to the low-temperature noncollinear spin configuration which is commonly observed in Mn 5 Ge 3 thin film or nanowire [20,22,23,27].…”

“…Nevertheless, by applying compressive strain, the MnII-MnII atomic distance decreases and the corresponding FM coupling is suppressed, then transformed into AFM coupling [20,27]. In addition, it is believed that the coexistence of AFM and FM coupling observed at low temperature may lead to the low-temperature noncollinear spin configuration which is commonly observed in Mn 5 Ge 3 thin film or nanowire [20,22,23,27]. Mn 5 Ge 3 has been widely studied for its magnetocaloric properties due to the large effect near room temperature, which makes it a potential candidate for magnetic refrigeration with the advantage of being environment friendly [28][29][30][31].…”

Tuning of the Magnetocaloric Properties of Mn5Ge3 Compound by Chemical Modification

“…This is because the value of MCE strongly depends on the value of the magnetic moment, which is particularly high for Mn and thus also for Mnbased materials. One of the most interesting compounds based on Mn is Mn 5 Ge 3 , which has attracted attention in many research fields for more than 50 years, e.g., in magnetic domain structure studies [29] or in spintronics as a possible efficient spin-injector due to the high spin-polarization of Mn 5 Ge 3 and compatibility with Ge-based technology [30][31][32][33][34][35]. Mn 5 Ge 3 is a promising magnetocaloric material because it is a soft ferromagnet showing second-order phase transition with a Curie temperature (T C ) near room temperature, a large magnetic entropy change of ∆S m = 7-9 J kg −1 K −1 for a magnetic field change of 5 T [36][37][38], and an adiabatic temperature change of ∆T ad = 1 K for a magnetic field change of 1 T [36].…”