5 research outputs found
Visualization of superparamagnetic dynamics in magnetic topological insulators.
Quantized Hall conductance is a generic feature of two-dimensional electronic systems with broken time reversal symmetry. In the quantum anomalous Hall state recently discovered in magnetic topological insulators, time reversal symmetry is believed to be broken by long-range ferromagnetic order, with quantized resistance observed even at zero external magnetic field. We use scanning nanoSQUID (nano-superconducting quantum interference device) magnetic imaging to provide a direct visualization of the dynamics of the quantum phase transition between the two anomalous Hall plateaus in a Cr-doped (Bi,Sb)2Te3 thin film. Contrary to naive expectations based on macroscopic magnetometry, our measurements reveal a superparamagnetic state formed by weakly interacting magnetic domains with a characteristic size of a few tens of nanometers. The magnetic phase transition occurs through random reversals of these local moments, which drive the electronic Hall plateau transition. Surprisingly, we find that the electronic system can, in turn, drive the dynamics of the magnetic system, revealing a subtle interplay between the two coupled quantum phase transitions
Confined Chemical Fluid Deposition of Ferromagnetic Metalattices
A magnetic, metallic
inverse opal fabricated by infiltration into
a silica nanosphere template assembled from spheres with diameters
less than 100 nm is an archetypal example of a āmetalatticeā.
In traditional quantum confined structures such as dots, wires, and
thin films, the physical dynamics in the free dimensions is typically
largely decoupled from the behavior in the confining directions. In
a metalattice, the confined and extended degrees of freedom cannot
be separated. Modeling predicts that magnetic metalattices should
exhibit multiple topologically distinct magnetic phases separated
by sharp transitions in their hysteresis curves as their spatial dimensions
become comparable to and smaller than the magnetic exchange length,
potentially enabling an interesting class of āspin-engineeredā
magnetic materials. The challenge to synthesizing magnetic inverse
opal metalattices from templates assembled from sub-100 nm spheres
is in infiltrating the nanoscale, tortuous voids between the nanospheres
void-free with a suitable magnetic material. Chemical fluid deposition
from supercritical carbon dioxide could be a viable approach to void-free
infiltration of magnetic metals in view of the ability of supercritical
fluids to penetrate small void spaces. However, we find that conventional
chemical fluid deposition of the magnetic late transition metal nickel
into sub-100 nm silica sphere templates in conventional macroscale
reactors produces a film on top of the template that appears to largely
block infiltration. Other deposition approaches also face difficulties
in void-free infiltration into such small nanoscale templates or require
conducting substrates that may interfere with properties measurements.
Here we report that introduction of āspatial confinementā
into the chemical fluid reactor allows for fabrication of nearly void-free
nickel metalattices by infiltration into templates with sphere sizes
from 14 to 100 nm. Magnetic measurements suggest that these nickel
metalattices behave as interconnected systems rather than as isolated
superparamagnetic systems coupled solely by dipolar interactions