This research, presented at the 2014 APS March Meeting in Denver, Colorado, characterizes magnetic phase transitions in the manganese-doped dichalcogenide TaS2.

1. Magnetic Phase
Transitions in Intercalated
Dichalcogenide
Nanostructures
CORBYN MELLINGER
UNIVERSITY OF NORTHERN IOWA

2. Intercalated Dichalcogenide
Nanostructures
 Layered material made up of TaS2 with
magnetic Mn ions between layers
 Concentration of Mn determines
magnetic properties
 Our sample: Mn0.235TaS2
(Layered structure of MnxTaS2))
(SEM image of MnxTaS2 nanotubes)

3. Magnetism Background
 Atoms possess orbital
and spin angular
momentum
 These spins contribute
to “magnetic
moments”
 Magnetic phase of
material depends on
short-range and long-
range order of
individual moments
A: Paramagnetic
B: Ferromagnetic

4. Objectives
 Characterize magnetic phase transitions in Mn0.235TaS2
nanostructures
 Previous research characterize transitions for 15%, 20%, and 23%
intercalation
 Build phase diagram for general MnxTaS2 as a function of
x

5. AC Susceptibility vs. Temperature
 Data down to T ~ 75 K behaves
in accordance with Curie-Weiss
Law
 χ =
C
T−θ
 Indicative of paramagnetic
to ferromagnetic transition
 Peak at T ~ 40 K indicates
second magnetic phase
transition
 Ferromagnetic to cluster-
glass

6. Upper Transition
 Behavior of transition governed by critical exponents β
and γ
 χ0 T ∝
T−Tc
Tc
−γ
(T > Tc)
 Ms(T) ∝
T−Tc
Tc
β
(T < Tc)
 Given appropriate β and γ, plot of M
1
β vs. χ0
−1 1
γ will
produce straight lines of given isotherms
 Arrott-Noakes Analysis

7. Arrott-Noakes Analysis
 Exponents selected using iterative
code in MATLAB
 Many sets of values tried; set
returning straightest lines reported
back by program
 Tc given as isotherm which
intersects origin

8. Kouvel-Fisher Analysis
 Related to equations of
state
 χ0
−1 ∙
dχ0
−1
dT
−1
=
T−Tc
γ
 Ms ∙
dMs
dT
−1
=
T−Tc
β
 Does not require
exponents to be known
before analysis

9. Lower Transition
Spin Glass
 Individual spins “locked” in
place
 Small time dependence for
alignment of moments with
external field
Cluster Glass
 Groups of spins “locked”
 Similarly small time
dependence on external field

10. Vogel-Fulcher Plot
 AC susceptibility
measurements
performed at
frequency 𝑓

1
f
= τ = τ0e
E0
kB T−Tg

11. Phase Diagram
 Adjusted phase
diagram based on
new x = 0.235 data
 More points desired
near multicritical
point (x~0.225) and
for higher x

12. Conclusions & Analysis
Upper Transition
 PM→FM transition has critical
exponent values β =0.86 and
γ =1.22 and takes place at
Tc = 74.4 K
 γ is in line with 3D Heisenberg
models, but β is larger than
model predicts
 Reentrant magnetic state
likely contains mixture of FM
and CG states, leading to
suppressed FM behavior
Lower Transition
 FM→CG transition occurs at
T = 39.6 K
 τ0 and
E0
kB
values are within
reasonable bounds to
accept the conclusion that
the lowest magnetic state is
that of cluster glass

13. Acknowledgements
 National Science Foundation Award No. DMR-1206530
 Department of Physics, University of Northern Iowa
 Dr. Paul Shand