Spintronics, Spin Caloritronics, and Skyrmion

C. L. Chien
Department of Physics and Astronomy
The Johns Hopkins University
Baltimore, MD 21218

Spintronic phenomena in magnetic nanostructures, including interlayer coupling, giant magnetoresistance (GMR), spin transfer torque (STT), tunneling magnetoresistance (TMR) in magnetic tunnel junctions (MTJs) and others, have provided some of the most exciting development in condensed matter physics for the past 25 years.  These discoveries have also expeditiously led to new technologies in field sensing and non-volatile memories. These accomplishments notwithstanding, innovation and developments continue unabated.  In this talk, some recent results in spintronics, spin caloritronics, and Skyrmion are described. 
              Spintronic devices have evolved from field-driven devices early on, to current-driven STT devices, and most recently to voltage-controlled devices (VCD).  We describe some recent development in VCD in which the high and the low resistance states of an MTJ can be repeatedly accessed by voltage pulses less than 1.5 V in magnitude and with switching current densities of only 10⁴ A/cm², two orders of magnitude lower than those of STT switching.¹ 
              Spin caloritronics concern phenomena where a heat current can generate a pure spin current and vice versa.  Of those, the spin Seebeck effect (SSE) has been most extensively explored.  However, in some cases the SSE may be inadvertently entangled with other thermal effects, most notably the anomalous Nernst effect.²  Since Pt in contact has most often been employed to detect the pure spin current via the inverse spin Hall effect, the SSE signal may also be contaminated by the magnetic proximity effects in Pt.  The realization of intrinsic SSE remains an outstanding and unresolved issue.
              Parallel alignment of magnetic moments is the hallmark of most ferromagnets. In some ferromagnetic systems the presence of the Dzyaloshinskii-Moriya interaction leads to more exotic spin arrangement such as the Skyrmion state with a double-twist structure, which carries a topological charge and a Berry phase.  Skyrmion has recently been realized in noncentrosymmetric cubic B20 chiral magnet single crystals (e.g., MnSi, FeGe), but only within a small region in the phase space at a finite field range and at temperatures just below T_C.  However, the Skyrmion phase can be greatly expanded and manipulated in epitaxial thin films. ³