Magnetic ordering in bulk MnSi crystals with chemically induced negative pressure

Abstract

MnSi crystals with chemically induced negative pressure (doped by less than 1% Ge) have been synthesized by the Czochralski method. X-ray powder diffraction has revealed that the samples are crystallized in the B20 structure, inherent to pure MnSi, without any impurity phases. The lattice constant a is slightly larger than that of undoped MnSi. The samples have a spiral spin structure with the wave vector |k| = 0.385 nm −1 at low temperatures. The ordering temperature is enhanced up to T C = 39 K. The critical field H C2 shows an increase of about 25% for the doped samples. Close to the critical temperature the A phase occurs. The temperature range of the A phase in the (H-T) phase diagram for the doped compound ranges from T A = 27.5 K, characteristic for pure MnSi, to T C = 39 K in the zero-field cooled (ZFC) regime of magnetization. The magnetic features of the (H-T) phase diagram of the compounds MnSi are reminiscent of those observed for the MnSi thin films on the Si substrate.

Effect of negative chemical pressure on the prototypical itinerant magnet MnSi

“…Despite the indirect role of Si on the magnetism of this compound, we find that these substitutions significantly increase the transition temperature and the saturation moment in addition to stabilizing the skyrmion lattice phase over a wider temperature and field range while preserving all the qualitative features of magnetism found in the complex MnSi magnetic phase diagram [1]. Our results indicate that the negative chemical pressure is the primary factor for such a change in magnetic behavior consistent with previous results [24]. We support our findings by performing positive pressure experiments, which reverse the effect produced by negative pressure.…”

“…There have been relatively few studies focusing on chemical substitution on the Si site [24,25]. One of these reported a significant increase in T C, by 10 K, with less than 1% Ge substitution for Si [24]. The extreme sensitivity of T C to chemical substitution was thought to be due to an increased localization of the magnetic moments stemming from an increased negative chemical pressure.…”

Effect of Ge substitution on magnetic properties in the itinerant chiral magnet MnSi

“…From the phase diagram, one can see that the effect of the Ge-substitution is not only the change of the helical ordering temperature T c but also enlargement of the skyrmion phase region. Typically, for MnSi, the temperature width of the skyrmion-phase region is about 2 ∼ 3 K as reported in earlier works [9,14]. In contrast, the width of the skyrmion-phase region increases in Ge-substituted MnSi, and is about 7 K for x = 0.144 (5).…”

“…[13], slight increase of T c (less than 3 K) was reported for MnSi with 10% Si substituted by Ge. In contrast, the other experiment shows that the helical ordering temperature can reach 39 K by replacing only 1% Si with Ge [14]. Quite recently, there appears a report on magnetic properties of MnSi 1−x Ge x in a wide x range 0 ≤ x ≤ 1, using high-pressure synthesis [15].…”

Фазовые Переходы В Геликоидальных Ферромагнетиках С Концентрационными Флуктуациями Локальной Намагниченности

“…Отметим также, что в не стехиометрических образцах MnSi c заметным дефицитом марганца имеет место увеличение почти на порядок температурного интервала скирмионной фазы по сравнению со стехиометрическим составом [7]. Поэтому концентрационным флуктуациям модуля локальной намагниченности, возникающие при заметных отклонениях от стехиометрии, могут оказаться решающими для формирования вихревых структур в области фазового перехода первого рода, что требует отдельного рассмотрения.…”