History dependence of the magnetic properties of single-crystalFe1−xCoxSi

Abstract

We report the magnetization, ac susceptibility, and specific heat of optically float-zoned single crystals of Fe1−xCoxSi, 0.20 ≤ x ≤ 0.50. We determine the magnetic phase diagrams for all major crystallographic directions and cooling histories. After zero-field cooling, the phase diagrams resemble that of the archetypal stoichiometric cubic chiral magnet MnSi. Besides the helical and conical state, we observe a pocket of skyrmion lattice phase just below the helimagnetic ordering temperature. At the phase boundaries between these states evidence for slow dynamics is observed. When the sample is cooled in small magnetic fields, the phase pocket of skyrmion lattice may persist metastably down to lowest temperatures. Taken together with the large variation of the transition temperatures, transition fields, and the helix wavelength as a function of composition, this hysteresis identifies Fe1−xCoxSi as an ideal material for future experiments exploring, for instance, the topological unwinding of the skyrmion lattice.

Magnetic relaxation phenomena in the chiral magnet Fe1−xCoxSi : An ac susceptibility study

“…In the field cooled case, the helical phase covers a much smaller section of the phase diagram and is suppressed to fields B C1 < 5 mT in favor of the conical phase that covers a much larger part of the phase diagram. This suppression of the helical phase is consistent with previous neutron scattering experiments [25] and with (ac) susceptibility measurements of Fe 1−x Co x Si with different degrees of Co doping [24]. This strong temperature and history dependence of B C1 has not been observed for MnSi and Cu 2 OSeO 3 , but is similar to the doped compounds Mn 1−x Fe x Si and Mn 1−x Co x Si [37], where the helical phase also covers a wide section of the magnetic phase diagram for ZFC.…”

“…The unwinding or decay of the skyrmion lattice correlations in Fe 0.7 Co 0.3 Si is considerably slower than for MnSi, where much higher cooling rates of ∼700 K s −1 are required to freeze the skyrmion lattice correlations below the A phase [17,[24][25][26]. In other words, the slow decay of skyrmion lattice correlations is likely related to the high degree of chemical disorder in the system and can be (partially) prevented with conventional cooling rates.…”

“…However, it is known that there is a strong hysteretic behavior of Fe 1−x Co x Si [8,24,25] that also depends on the cooling rate [25]. For this reason, the ac susceptibility was also measured following the FC and Fast We start the discussion of the history dependence by comparing ZFC with FC.…”

“…Different from the archetype chiral system MnSi and other systems of the same family, Fe 1−x Co x Si appears to have a phase diagram depending on the magnetic history of the sample [8,24,25] and also on the applied cooling rates through T C [25]. Additionally, neutron scattering shows that skyrmion lattice correlations may persist down to the lowest temperatures depending on the magnetic history [8,25,26].…”

“…Several dc magnetization [18,20,24,[27][28][29] and some ac susceptibility studies have been performed so far, but only with an ac drive frequency of 30 Hz [30] and 1000 Hz [21,24]. Based on these studies, phase diagrams have been deduced for a wide range of chemical substitution and field directions but no attention has been devoted to the frequency dependence [24,30].…”