Chapter 8
Conclusions
Noise is an entity that most scientists attempt to sideline in their measurements.
There may however be some benefits to examining the characteristics of noise
intrinsic to a black box that one is studying. A famous example of the usefulness of
such a study is the discovery of the Cosmic Microwave Background that stemmed
from an unprecedented measurement [1]. In this thesis, the intrinsic noise coming
from a single crystal of Dy 2 Ti 2 O 7 was studied and a microscopic understanding
of the inner workings of this material were developed. In conclusion we observe
that virtually all the elements of S (ω, T ) predicted for a magnetic monopole
plasma, including the existence of intense magnetization noise and its characteristic
frequency and temperature dependence, are detected. Moreover, comparisons of
simulated and measured correlation functions C (t) of the magnetic-flux noise (t)
imply that the motion of magnetic charges is strongly correlated. At the end of the
chapter, directions for some future experiments are presented.
We introduced a novel SQUID-based spin noise spectroscopy technique to
studies of lanthanide-pyrochlores, namely Dy 2 Ti 2 O 7 which is a Dipolar Spin Ice
material. Theoretical predictions for the magnetic-flux signature of a plasma of
magnetic charges ±m ∗ spin ice are tested for Dy 2 Ti 2 O 7 . Monte-Carlo simulations
predict the existence of a strong magnetization noise intrinsic to spin ice materials.
Our work reports the observation of the predicted magnetic-flux noise for the
first time [2]. The frequency and temperature dependence of the magnetic-flux
noise spectrum S (ω, T ) predicted for ±m ∗ magnetic charges undergoing thermal
generation and recombination (Figs. 4.2, 4.3, 4.4) is confirmed directly and in detail
(Fig. 5.1). The expected transition from a plateau of constant magnetic-flux noise
[3, 4] for ωτ (T ) 1, due to random-fluctuations, to a power-law falloff [4]
for ωτ (T ) 1, is observed throughout (Fig. 8.1). The prediction of S (0, T )
proportional to τ (T ) holds true for both MC simulations of DSIM for DTO and
our experiment (Fig. 8.1bottom)
These S (ω, T ) characteristics are distinctive to spin ices. This is because the
magnetization-noise spectral density signature of a ferromagnet [5], a classic spin
© Springer Nature Switzerland AG 2021
R. Dusad, Magnetic Monopole Noise, Springer Theses,
https://doi.org/10.1007/978-3-030-58193-0_8
61
Conclusions
Noise is an entity that most scientists attempt to sideline in their measurements.
There may however be some benefits to examining the characteristics of noise
intrinsic to a black box that one is studying. A famous example of the usefulness of
such a study is the discovery of the Cosmic Microwave Background that stemmed
from an unprecedented measurement [1]. In this thesis, the intrinsic noise coming
from a single crystal of Dy 2 Ti 2 O 7 was studied and a microscopic understanding
of the inner workings of this material were developed. In conclusion we observe
that virtually all the elements of S (ω, T ) predicted for a magnetic monopole
plasma, including the existence of intense magnetization noise and its characteristic
frequency and temperature dependence, are detected. Moreover, comparisons of
simulated and measured correlation functions C (t) of the magnetic-flux noise (t)
imply that the motion of magnetic charges is strongly correlated. At the end of the
chapter, directions for some future experiments are presented.
We introduced a novel SQUID-based spin noise spectroscopy technique to
studies of lanthanide-pyrochlores, namely Dy 2 Ti 2 O 7 which is a Dipolar Spin Ice
material. Theoretical predictions for the magnetic-flux signature of a plasma of
magnetic charges ±m ∗ spin ice are tested for Dy 2 Ti 2 O 7 . Monte-Carlo simulations
predict the existence of a strong magnetization noise intrinsic to spin ice materials.
Our work reports the observation of the predicted magnetic-flux noise for the
first time [2]. The frequency and temperature dependence of the magnetic-flux
noise spectrum S (ω, T ) predicted for ±m ∗ magnetic charges undergoing thermal
generation and recombination (Figs. 4.2, 4.3, 4.4) is confirmed directly and in detail
(Fig. 5.1). The expected transition from a plateau of constant magnetic-flux noise
[3, 4] for ωτ (T ) 1, due to random-fluctuations, to a power-law falloff [4]
for ωτ (T ) 1, is observed throughout (Fig. 8.1). The prediction of S (0, T )
proportional to τ (T ) holds true for both MC simulations of DSIM for DTO and
our experiment (Fig. 8.1bottom)
These S (ω, T ) characteristics are distinctive to spin ices. This is because the
magnetization-noise spectral density signature of a ferromagnet [5], a classic spin
© Springer Nature Switzerland AG 2021
R. Dusad, Magnetic Monopole Noise, Springer Theses,
https://doi.org/10.1007/978-3-030-58193-0_8
61