Dr. V. Ganesan
Phys. Status Solidi RRL, 7, 184 (2013)
Skyrmions in MnSi: Basic research on materials of technological applications
The fundamental and applied research in condensed matter physics focused on two
important aspects. Primarily, it aims at investigating the nature of materials and
secondly bringing up potential technological applications. The research of first and
the latter are interdependent and should progress parallel. Importantly it should
take into consideration of the present-day needs. One such mostly discussed topic is
spintronics, a future demanding technology. Although, rare-earth based
intermetallic and oxide compounds like manganites show good magneto-caloric
properties and colossal magnetoresistance, they fail to be fruitful on the actual
application front for various known reasons. Therefore recent trend is to use the
transition metal based compounds (either metals or semiconductors) which are less
expensive.
Recent evolution is the observation of skyrmions in MnSi, a new type of spin
ordering (typical schematic is shown in the figure 1), different from conventional
ferromagnetic (↑↑↑↑) and antiferromagnetic (↑↓↑↓) spin arrangements. It is said to
revolute the spintronic technology in years to come for the reason that its energy
scales are at par with that of nuclear spin. Hence speeding up the flip-flop process by
the supply of minimal Zeeman energy.
Our current interest goes with the exploring the physical properties of MnSi based
on thermodynamic and electrical properties under the ambience of low temperature
T, and magnetic fields H. Broadly classifying, in the absence of magnetic fields MnSi
is a helimagnet below 30 K with a helical wavelength of about 180 Å. Depending on
the strength of H, the magnetic structure gets modified into various chiral phases.
One such phase just below magnetic ordering temperature, Tc, within a small H-T
window is the skyrmion phase. Detailed thermo-magnetic phase diagram of MnSi
based on exclusive investigation of specific heat is reported [1, 2].
Figure-1: left-Schematic of skyrmion [3], right-types of skyrmions
The magnetic phase diagram of MnSi is constructed based on the variation of
specific heat with temperature (figure 2 left) and magnetic field (figure 2 right). As a
broad classification, helical, conical, ferromagnetic, A-phase (skyrmion phase),
intermediate and paramagnetic phases are identified (figure 3 left) [1].
Dr. V. Ganesan
Phys. Status Solidi RRL, 7, 184 (2013)
Figure 2: left-temperature dependent specific heat of MnSi; right-field dependent Cp
Figure 3: left-broad phase diagram; right-exclusive division of A-phase region
A phase is successfully identified by the local minima formation in Cp(H) that relates
to the relatively low entropy state. This indicates the presence of a kind of magnetic
ordering, suggesting the formation of A-phase as a precursor state to the stable SkX.
A first to second order transition in field from helical to fluctuation disordered phase
is reported. The confinement temperature TL is 27.83 K and the Vollhardt invariant T
& H are found to be 31.5 K & 4 kOe respectively.
However, close inspection of the data reveals the different pockets of regions within
the A-phase (figure 3 right) [2]. The phase diagram identifies several important
theoretically proposed precursor phenomena like (a) repulsive skyrmions (I) in the
regular modulations region, (b) attractive (II) and confined (III) skyrmionic phases in
the precursor states and (c) nucleation phase with highly chiral fluctuations between
the precursor and fluctuation disordered regions.
References
[01] S. S. Samatham and V. Ganesan Phys. Status Solidi RRL, 7, 184 (2013)
[02] S. Shanmukharao Samatham and V. Ganesan Phys. Status Solidi B (2015)
[03] Christian Pfleiderer and Achim Rosch, Nature 465, 880 (2010)