of 7
Rev. Sci. Instrum.
91
, 113902 (2020);
https://doi.org/10.1063/5.0026311
91
, 113902
© 2020 Author(s).
Optical Raman measurements of low
frequency magnons under high pressure
Cite as: Rev. Sci. Instrum.
91
, 113902 (2020);
https://doi.org/10.1063/5.0026311
Submitted: 23 August 2020 . Accepted: 18 October 2020 . Published Online: 04 November 2020
Xiang Li
, S. E. Cooper
, A. Krishnadas
,
D. M. Silevitch
, T. F. Rosenbaum
, and
Yejun Feng
Review of
Scientific Instruments
ARTICLE
scitation.org/journal/rsi
Optical Raman measurements of low frequency
magnons under high pressure
Cite as: Rev. Sci. Instrum.
91
, 113902 (2020); doi: 10.1063/5.0026311
Submitted: 23 August 2020
Accepted: 18 October 2020
Published Online: 4 November 2020
Xiang Li,
1,2
S. E. Cooper,
2
A. Krishnadas,
2
D. M. Silevitch,
1
T. F. Rosenbaum,
1
and Yejun Feng
1,2,a)
AFFILIATIONS
1
Division of Physics, Mathematics, and Astronomy, California Institute of Technology, Pasadena, California 91125, USA
2
Okinawa Institute of Science and Technology Graduate University, Onna, Okinawa 904-0495, Japan
a)
Author to whom correspondence should be addressed:
yejun@oist.jp
ABSTRACT
The application of giga-Pascal scale pressures has been widely used as a tool to systematically tune the properties of materials in order to access
such general questions as the driving mechanisms underlying phase transitions. While there is a large and growing set of experimental tools
successfully applied to high-pressure environments, the compatibility between diamond anvil cells and optical probes offers further potential
for examining lattice, magnetic, and electronic states, along with their excitations. Here, we describe the construction of a highly efficient
optical Raman spectrometer that enables measurements of magnetic excitations in single crystals down to energies of 9 cm
1
(1.1 meV or
13 K) at cryogenic temperatures and under pressures of tens of GPa.
©
2020 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license
(http://creativecommons.org/licenses/by/4.0/).
https://doi.org/10.1063/5.0026311
.,
s
I. INTRODUCTION
Magnetism and magnetic materials are central topics in a wide
range of research fields such as geology, chemistry, materials sci-
ence, and physics, along with many practical applications, from
imaging to information storage. For many of these fields, an ather-
mal tuning mechanism for the magnetic state is a powerful method
employed to reveal the underlying energy scales, interactions, order-
ing, and transitions as the magnetism evolves. Hydrostatic pressure
is a preferred technique for such studies as it can drive a change
in energy density comparable to that of the magnetic couplings.
Many of the standard techniques for probing magnetic ground states
and their excitations have been adapted for compatibility with high-
pressure environments, including neutron magnetic diffraction,
1
μ
SR spectroscopy,
2
x-ray magnetic diffraction,
3,4
synchrotron Möss-
bauer spectroscopy,
5
and x-ray magnetic dichroism.
6
These, how-
ever, often require the use of international user facilities. Optical
Raman scattering provides a means to explore magnetic excitations
under pressure within an individual research laboratory. Moreover,
it can offer unique insights into symmetry and local structure.
Optical Raman scattering is capable of probing lattice,
7
mag-
netic,
8
and electronic
9
degrees of freedom, with correspondingly
increasing experimental difficulty. Despite the wide popularity of
optical Raman techniques even before the invention of the laser
7
and their application to high pressure sample environments start-
ing soon after the introduction of the diamond anvil cell,
10
exist-
ing high-pressure Raman studies have focused primarily on rela-
tively high energies, above 100 cm
1
, examining the evolution of
lattice phonons and pressure-driven structural transitions. To date,
there are only a few examples in the literature
10–17
of Raman scat-
tering at low frequencies (
<
100 cm
1
) and high pressures, thereby
permitting measurement of single magnons or charge-density-wave
fluctuations.
Here, we describe the construction of an instrument capable of
obtaining Raman spectra down to 9 cm
1
under multi-GPa pres-
sures and cryogenic temperatures. By carefully designing the opti-
cal, cryogenic, and high-pressure components, our Raman probe
offers spatial sensitivity, polarization analysis, and a high signal-
to-background ratio. It is a highly efficient tool for exploring the
evolution of magnetic excitations under pressure.
II. OPTICAL CONFIGURATION
Several groups have described high-pressure Raman systems
using a traditional triple-grating spectrometer to collect the spec-
tra.
13–17
The first two stages are combined in a subtraction mode
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, 113902 (2020); doi: 10.1063/5.0026311
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, 113902-1
© Author(s) 2020
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to remove excessive spectral weight outside the measurement range,
and a physical block between the two stages eliminates the elastic
laser line. We instead elect to use Bragg-diffraction-based volume
grating bandpass and notch filters (BragGrate OD3 or OD4 types,
OptiGrate Corp. USA) to focus on the removal of the elastic line.
This allows the measurement of the inelastic spectrum much closer
to the elastic frequency (within 10 cm
1
) and also provides a much
higher throughput efficiency compared to the significant photon
flux loss through 8–9 reflective mirrors in the first two stages of
a triple-grating spectrometer. The tradeoff associated with the use
of these volume Bragg gratings is that it essentially entails choos-
ing one specific laser wavelength for the whole system. Changing
the wavelength is economically demanding and time consuming as
it requires both the purchase of a whole new set of volume Bragg
grating bandpass and notch filters specifically for the desired laser
wavelength and rearrangement/realignment of large portions of the
optical path. Fixing the laser wavelength would prevent studies using
resonant Raman scattering. It also rules out the possibility of vary-
ing the incident laser wavelength, taking advantage of the changing
balance between Raman and fluorescence efficiencies at different
wavelengths to improve the signal-to-noise ratio. Nevertheless, for
the desired magnetic measurements, the advantage in throughput
outweighs the above considerations.
We choose a single-frequency, continuous wave diode-pumped
659.6 nm laser (100-mW, Cobolt AB, Sweden). The long wavelength
reduces the Raman efficiency. However, it also significantly reduces
the fluorescence, which is advantageous in high pressure measure-
ments as the sample surface is covered by a pressure medium rather
than being in vacuum.
The general layout of our optical setup is presented in Fig. 1(a).
Along the incident beam path, the laser first passes through a beam
expander to generate a relatively constant Gaussian beam profile
over a long distance. The intensity, polarization state, and spectral
profile of the incident laser beam are shaped by neutral density fil-
ters, a quarter wave plate plus a linear polarizer, and volume Bragg
grating bandpass filters, respectively.
While the incident and Raman scattered light paths can be
chosen independently, here we limit our probe to a backscatter-
ing Raman geometry,
̄
z
(
P
1
P
2
)
z
, where
̄
z
and
z
are the diametrically
opposed propagation directions of the incident and back-scattered
light and
P
1
and
P
2
are the polarization states of the incident and
scattered photons, respectively. This collinear geometry is dictated
by the typical geometry of an opposite-anvil high-pressure diamond
anvil cell, and to a lesser extent by the optical window of the cryo-
stat [Fig. 2(a)]. The incident laser path converges with the scattered
light path at the first of the five volume Bragg grating notch filters
[Fig. 1(a)].
The set of five notch filters in series along the scattered
light path rejects elastically scattered laser light that originates
from the focused spot on the sample surface. In addition, random
scattered light from multiple surfaces and fluorescence along the
beam path are rejected by a home-built confocal microscopy struc-
ture to further enhance the signal-to-background ratio. After the
Raman signal passes through a polarization analyzer to define the
detection scheme, the light is fiber-coupled to the entrance of a
single-grating spectrometer. We use a single-grating spectrometer of
700-mm or 750-mm length (such as SpectraPro HRS-750 or the
third stage of TriVista-557, Princeton Instruments), with three
grating choices of 600 lines/mm, 1200 lines/mm, and 1800 lines/mm,
to provide a flexible combination of spectral range and resolu-
tion. A liquid-nitrogen-cooled CCD camera with 1340
×
400 pixel
2
(PyLoN-400BR-eXcelon, Princeton Instruments) serves as a photon
detector with low electronic noise and a consistently high quan-
tum efficiency (
>
90%) in the spectral range between 600 nm and
900 nm.
III. HIGH-PRESSURE CRYOGENIC SAMPLE
ENVIRONMENT
Here, we discuss the specific considerations to meet the con-
straints associated with the high-pressure and low temperature sam-
ple environment. The Raman spectrometer was constructed around
a commercial microscope objective instead of assembling our own
compound lens. We use a 20
×
microscope lens with an extra-long
25.5 mm working distance [Fig. 2(a)] and a numerical aperture (NA)
of 0.40 (M Plan Apo NIR B, Mitutoyo Co., Japan). This lens was
designed by the manufacturer to provide a wide range of apochro-
matic correction from 420 nm to 1064 nm and has achieved a tight
focusing spot of
5
μ
m lateral size and
5
μ
m depth of focus with
our optical arrangement. The narrow depth of focus together with
the confocal design eases the need for ultrapure diamonds with very
low fluorescence; all our data presented here were collected using
cells assembled with type I diamond anvils.
To position our micrometer-sized focused laser beam spot on
the sample under high pressure, we mount the Raman optical setup
and the cryostat on heavy-duty x–y and z translational stages (Mod-
els 5102.40 and 5103.C30, Huber Diffraktionstechnik GmbH & Co.
KG) [Fig. 1(b)]. All three degrees of movement have a repeatability
of about 2
μ
m as specified by the manufacturer. This fine transla-
tional precision allows surveying many different parts of the sam-
ple surface to assess potential damage areas, intrinsically heteroge-
neous phase coexistence (for instance, near a first-order quantum
phase transition), or other circumstances where sample heterogene-
ity is germane. To assist in positioning the focused laser beam on
particular locations on the sample, a white-light illuminated CCD
camera-based imaging system can be inserted into the beam path
while maintaining the presence of the laser beam.
Both the cryostat and the high-pressure cells are constructed
to be compatible with the 25.5-mm optical working distance. We
use a cryogen-free Gifford–McMahon (GM) cryostat (0.5 W cool-
ing power at 4.2 K, Sumitomo RDK-205E) with an optical window
and a thermal radiation shield anchored at the first (60 K) cooling
stage. Our pressure cell is directly mounted on the cold finger to
optimize the cooling power and typically can be cooled below 4.5 K
despite the large surface area of the pressure cell and the optical-
access opening on the 60-K radiation shield [Fig. 2(a)]. At
T
= 4.5 K,
the pressure cell has a peak-to-peak vibration amplitude of
13
μ
m.
However, it is in the plane transverse to the incident laser beam, so
the vibration does not affect the laser focusing condition on the sam-
ple surface. Replacing the GM cryocooler with a pulse-tube based
cryocooler would offer both larger cooling power in the range of
1 W–2 W at 4.2 K and a reduction in the vibration amplitude to
10
μ
m. The primary effect of the vibration is to increase the effec-
tive lateral spot size, which serves to distribute the heat load from
the laser over a larger area and thereby reduces the amount of local
heating. The cryostat vibration should not affect the detection of
Rev. Sci. Instrum.
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, 113902 (2020); doi: 10.1063/5.0026311
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, 113902-2
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FIG. 1
. (a) Optical schematic for the
high-pressure Raman instrument. (1)
Continuous-wave
laser;
(2)
beam
expander; (3) neutral density attenu-
ators; (4a–b) quarter wave plate and
polarization
analyzers;
(5)
volume
Bragg bandpass filter; (6) volume Bragg
notch filters; (7) high-NA long-working
distance
microscope
objective;
(8)
diamond anvil cell; (9) cryostat with
thermal radiation shield; (10) confocal
microscopy structure; and (11) white
light imaging system. (b) Aerial and (c)
side view CAD rendering of the Raman
instrument showing the optical bench
and cryostat mounted on x–y and z
translational stages, respectively. The
numerical labels in panel (b) denote the
same parts as in panel (a).
coexisting phases and magnetic domains, as long as the vibration
amplitude is smaller than the typical high-pressure sample size of
70
μ
m–200
μ
m.
4,18,19
While individual domain sizes are not always
larger than the focused laser spot, the surface/volume ratio of differ-
ent domains could vary significantly over the spatial area and as a
function of pressure.
20
We construct our high-pressure diamond anvil cell to have
an asymmetric structure to minimize the access distance from one
side. While such asymmetric cell designs are well-known in the
literature,
10,21
here we have the additional design constraint of
incorporating a helium membrane mechanism to allow tuning pres-
sure
in situ
at low temperature.
4,18
We take a modified Merrill–
Bassett type three-pin cell design
4
and add an access hole that allows
the membrane tubing to pass unconstrained through the cell body
after the cell is sealed (Fig. 2), similar to the approach used in a
pressure cell designed for the Physical Property Measurement Sys-
tem (Quantum Design, Inc. USA) family of cryostats.
19
By putting
all tubing, sealing screws with Belleville washers, and even the ther-
mometer on one side of the pressure cell, we can comfortably con-
struct the vacuum window and radiation shield in the space between
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, 113902 (2020); doi: 10.1063/5.0026311
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, 113902-3
© Author(s) 2020
Review of
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FIG. 2
. (a) Side view CAD rendering detailing the environment around the sam-
ple at high pressure and cryogenic temperature. From left to right: microscope
lens (gray), 1.5 mm-thick fused quartz window on the cryostat (gray blue), brass
radiation shield with optical opening (gold), pressure cell cap (red-brown), mem-
brane (fluorescent green), and the high-pressure diamond anvil cell (yellow).
The pressure chamber between the anvils is within the working distance of
the microscope lens. The membrane line is passed through the cell body toward
the back, alongside the sealing screws and Belleville washer stacks. A ruler
included in the CAD rendering marks 1 cm. (b) Side view photograph of the pres-
sure cell, helium membrane, and cell cap assembly following the rendering in panel
(a). (c) A fully assembled cell with the membrane line lead-soldered to an adaptor
for further connection to the regular tubing system.
the microscope lens and the high-pressure cell (Fig. 2) with a couple
of millimeters of clearance on both sides of the vacuum window.
The numerical aperture of 0.40 of the microscope lens matches
up well with the 1-mm diameter optical opening in the tungsten car-
bide seats supporting the Drukker type diamond anvils in our pres-
sure cell. Our pressure cells have been constructed with
±
25
or
±
35
conical angles; both are compatible with the specified numerical
aperture.
Since the light path is parallel to the axial direction of the dia-
mond anvils, the restriction to a single probing geometry,
̄
z
(
P
1
P
2
)
z
,
might be of concern for non-cubic systems as potentially not all
Raman active modes could be explored under high pressure. This
issue is most acute for layered materials as typical pressure chambers
are disk shaped and the layered samples are often mounted inside
the pressure chamber with the naturally cleaving
a
b
plane paral-
lel to the diamond culet surface. To overcome this limitation, we
employ a culet-perforated diamond anvil design
19,22
to allow mount-
ing plate-like samples on their side inside the perforated diamond
cavity. Assuming that the
c
-axis is the stacking axis of
a
b
layers,
the crystal can be placed with its large
a
b
plane surface parallel to
the laser beam direction, as shown in Fig. 3. This innovation makes
possible the exploration of the pressure evolution of Raman active
modes in the
a
c
or
b
c
planes.
Our general approach to maintaining a highly hydrostatic sam-
ple environment has been discussed extensively in the past.
4,18
Here,
we follow the parameters listed in Ref. 4 for the initial sample pres-
sure chamber construction. We have considered several possible
choices for pressure medium. Our past experience has established
that argon is not a sufficiently hydrostatic pressure medium. High
pressure loading of supercritical neon
3
or liquid helium in the super-
fluid state is not easily available, and they do not necessarily maintain
a large sample chamber relative to the size of the sample, which
is important to sustain hydrostaticity.
18
Here, we have used both
a methanol:ethanol 4:1 (ME 4:1) mixture and liquid nitrogen as
pressure media. Under pressure and at low temperature, nitrogen
has several different phases and many strong vibration modes in
FIG. 3
. Culet-perforated diamond anvils for studying Raman modes scattered from
the side surface of layered materials. (a) Schematic of the culet-perforated dia-
mond anvil in the high-pressure assembly. The two diamond anvils (blue) sandwich
the gasket (brown) and a sample (gray) inside an optically transparent pressure
medium, with one of the anvils having a perforation in its culet. (b) Enlarged three-
dimensional cut-out view near the sample chamber. (c) Micrograph of a sample in
a pressure chamber at 1.8 GPa. Illumination is from below to illustrate the sample
chamber enclosed by the metallic gasket and the perforated hole in the middle.
A piece of single crystal Sr
2
IrO
4
is placed inside the perforated hole, fixed by a
tiny amount of Apiezon grease at the bottom. A few ruby balls are placed on the
edge of the perforation for
in situ
pressure measurements. The scale bar repre-
sents 200
μ
m. (d) Enlarged view of the sample inside the perforation showing the
shining surface condition of a
b
c
plane cross section. The parallel edges of the
sample are the naturally cleaving
a
b
planes of Sr
2
IrO
4
on both sides. The scale
bar represents 50
μ
m.
Rev. Sci. Instrum.
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, 113902 (2020); doi: 10.1063/5.0026311
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, 113902-4
© Author(s) 2020