PPMS-based set-up for Raman and luminescence spectroscopy at high magnetic field, high pressure and low temperature
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We present an experimental set-up permitting Raman and luminescence spectroscopy studies in a commercial Physical Properties Measurement System (PPMS) from Quantum Design. Using this experimental set-up, gaseous, liquid and solid materials, in bulk or thin film form, may be investigated. The set-up is particularly suitable for the study of the spin-lattice coupling in strongly correlated oxide materials utilizing several different stimuli, e.g. magnetic and electric fields, high pressure and low temperatures. Details for the Raman extension, sample holder assembly and optical design, as well as data acquisition and measurement routine are described. Finally, we present exemplary results collected using the set-up, measured on reference materials, as well as on a correlated transition metal oxide.
KeywordsRaman spectroscopy Quantum Design PPMS High pressure Ruby Hydrogen conversion Strongly correlated materials Langasite
Large efforts in the field of material science are spent on the development of novel functional materials. In this context, strongly correlated electron systems are a class of materials that promise a multitude of applications. Several transition metal oxides are being considered for the designing of new devices, owing e.g. to their magnetic, electric and dielectric [1-3] or optical and photovoltaic properties [4,5]. In such materials, the functionalities or physical properties are determined by the localization and delocalization of charge carriers . As a result, magnetic, (di)electric, thermal, and optical properties are correlated with each other . Inelastic light scattering from electronic charge density and phonons in correlated materials is a promising approach to evaluate such correlation properties . This approach is deemed to be particularly fruitful if it can be done under the simultaneous influence of several stimuli that probe these properties such as, e.g. magnetic and/or electric fields, pressure and/or temperature.
Raman spectroscopy for instance is a versatile technique for studying structural properties, lattice distortions, and e.g. the coupling of spin and lattice degrees of freedom. This is due to different types of excitations that can be studied under the influence of external stimuli leading to observation of shifts in vibrational frequencies, changes in peak intensities and polarization states. This was successfully employed to study structure-property relationships in strongly correlated transition metal oxides [8,9]. A particular strength of electronic Raman scattering is the possibility to detect information on the electron dynamics from different regions of the Brillouin zone.
A Raman set-up for in-situ measurements of such properties at low temperature, magnetic field, electrical field and high pressure was constructed as an extension to the Quantum Design, Inc. Physical Properties Measurement System (PPMS)  - a standard cryogenic device in many laboratories world wide. An optical probe and an automatized data acquisition system were designed and linked with the already automatized temperature and magnetic field control provided by the PPMS. Several material properties can thus be investigated in the same PPMS set-up, using the standard set of options such as resistivity, magnetization or heat capacity, as well as using extensions permitting e.g. magneto(di)electric measurements  or the here proposed Raman spectroscopy set-up. Dedicated custom-made devices for Raman spectroscopy with multiple excitations not using a Quantum Design PPMS were reported in literature previously [12,13].
We present results collected on different reference systems, such as ruby and H 2, as well as a transition metal oxide, Ba 3 NbFe 3 Si 2 O 14, a langasite material with magnetically induced electronic polarization.
Experimental apparatus and methods
PPMS host system and Raman extension
The Raman set-up is based on a commercial Quantum Design, Inc. PPMS  which incorporates (i) a cryostat providing a controllable variable temperature in the 2 - 400 K interval and (ii) a superconducting magnet up to 9 T (superconducting magnets up to 16 Tesla are available). The native hardware control of temperature and magnetic field provided by the PPMS was used and a new extension, permitting the collection of Raman spectroscopy data, was designed and fabricated. The Raman extension comprises the following parts: (a) The custom-made Raman top part (including several imaging lenses, beamsplitter, notch filter, white light illumination and CCD camera) and a modified PPMS Raman insert (using multifunction probe temperature sensor) serving as a sample/DAC holder. (b) A miniature nonmagnetic diamond anvil cell (DAC). (c) A 488 nm argon-ion CW laser system. (d) A Shamrock spectrometer with thermoelectrically cooled CCD detector.
Raman spectra are recorded with a SR-303I-A Shamrock 303i spectrometer (Andor Technology) coupled by a single mode optical fiber to the PPMS Raman extension. The spectrometer is equipped with 1200l/mm grating and thermoelectrically cooled multichannel CCD detector (iDus, Andor Technology, 1024 ×256 pixels, -50°C). A linearly polarized argon ion laser (488 nm line) is used for the excitation at powers up to 15 mW. The spectral axis is calibrated by the fluorescence lines of a neon lamp. Inside the PPMS, Raman spectra are collected in the back scattering geometry, at a resolution of about 4 cm −1. Elastically backscattered fundamental light (488 nm) is filtered out using a narrow band notch filter (Semrock Inc.). The accuracy of the spectral measurements, resulting from the wavelength calibration procedure and experimental conditions, is estimated to be about 2 cm −1. Typical acquisition times vary between 20 - 120 sec. Experimentally accessible ranges of temperature, pressure, magnetic and electric fields available for spectroscopic acquisitions correspond to 3 - 350 K (higher temperatures with additional heating possible), 0 - 60 GPa, 0 - ±9 Tesla and 0 - ±10 MV/m, respectively. So far, spectral acquisitions involving strong electric fields have been carried out outside the PPMS only. Typically, the experimental arrangement involves application of hundreds of volts across a sample which is few tens of micrometers thick.
Alignment and measurement routine
Measurements are taken in the back scattering geometry. Due to space limitation inside the cryostat and low temperature conditions right angle scattering geometry is difficult to achieve in the described set-up. For sample illumination and focusing, white light is coupled via an optical fiber into the set-up and applied in transmission. Prior the measurements the optical system is pre-aligned and tested outside the cryostat using a room-temperature probe stand.
A standard (high-pressure) experiment is done in the following way. (i) The sample is mounted onto the Raman probe using ambient-pressure sample holder or DAC. For high-pressure experiments specific sample-, ruby- and pressure transmitting medium loading procedures have to be applied as described in Ref. . (ii) The sample is aligned with the optical set-up and inserted into the PPMS cryostat. (iii) Single measurements are performed manually or an automatized measurement sequence is programmed using the PPMS third-party option together with the spectrometer software scripting option.
Exemplary results and discussion
Ruby fluorescence lines
Spin-phonon coupling in a strongly correlated oxide
Langasite materials are well known for their piezoelectric and non-linear optical properties [21,22]. A recent interest in the magnetic and electrical properties of langasite materials is due to the discovery that they could crystalize in a Kagomé-like lattice structure consisting of rare-earth and transition metal cations [23,24]. While a detailed report of the study of the spin-phonon coupling in those materials is beyond the scope of this article, we present here Raman spectroscopy data collected in our set-up as a function of temperature and magnetic fields. Simple qualitative analysis are also included, in order to illustrate the typical studies which may be performed on such materials.
Ba 3 NbFe 3 Si 2 O 14 crystallizes in the non-centrosymmetric trigonal P321 space group with 23 unit cell atoms. Magnetic Fe 3+ ions are arranged in triangle units on a triangular lattice . The structure is non-polar and hence no spontaneous polarization can be expected for point group 32 . It has been reported that Ba 3 NbFe 3 Si 2 O 14 undergoes an antiferromagnetic phase transition at approximately 27 K . A much larger Curie-Weiss temperature of θ≈−175 K , reflects the large magnetic frustration in the system . The magnetic transition is accompanied by a symmetry lowering from P321 to C2 . Further details on the magnetic structure and Raman mode assignment can be found in literature [25,29,30].
In a quasi-harmonic approximation, the frequency shift can be attributed to the volume change only and anharmonic effects due to phonon-phonon and spin-phonon interaction are ignored. Nevertheless, additional changes in the phonon frequencies when approaching T N are evident, as shown in Figure 5(d). For instance, the 506 cm −1 phonon mode reveals softening behavior with an onset above T N and a subsequent hardening below 25 K. Similar behavior is observed for the other modes, suggesting spin-phonon coupling. Figure 5(d) also illustrates precision and stability of the Raman set-up, which allow detection of a magnetic transition involving a change of only a fraction of wavenumber.
The magnetic field dependence of the unpolarized Raman spectra of Ba 3 NbFe 3 Si 2 O 14 at 5 K is shown in Figure 5(b)+(c). In this measurement the magnetic field was set to the highest value (9 T) and the first Raman spectrum was collected. Subsequently, the magnetic field was swept from 9 T to -9 T. At a first glance, a comparably ‘strong’ change in the integrated intensity of the 310, 416 and 506 cm −1 modes and in the position of the 615 cm −1 mode are found. Furthermore, several new modes appear at 250, 322, 760 and 840 cm −1 when the field is changed from 9 T to -9 T. The influence of the magnetic field on phonon modes 506, 573, 675 and 982 cm −1 is weak and only the 416 and 615 cm −1 modes exhibit major changes in the frequency .
Recently, the magnetic (and structural) properties of related langasites were found to be greatly affected by application of pressure , suggesting the relevance of T,P,H-dependent Raman studies on Ba 3 NbFe 3 Si 2 O 14 and other strongly correlated materials.
A Raman and luminescence spectroscopy system for the study of strongly correlated electron materials is presented. The system is designed as an extension of the Quantum Design, Inc. PPMS and allows for measurements in a large temperature range (3 - 350 K), high magnetic fields (0 - 9 T), high pressure (0 - 60 GPa) and electric field strengths up to 10 MV/m. Gaseous, liquid and solid materials, in amorphous, poly- and single-crystalline bulk form or as a thin film can be studied. Exemplary results for ruby and solid hydrogen at low temperature and high hydrostatic pressure and high magnetic field are shown. Finally results of the study of the spin-phonon coupling in the complex transition metal oxide Ba 3 NbFe 3 Si 2 O 14 are presented and discussed.
We are very grateful to Dr. Y. Tokunaga, and Profs Y. Taguchi and Y. Tokura for advices and support in the floating zone growth of Ba 3 NbFe 3 Si 2 O 14 single-crystal. Financial support from the Carl Tryggers Stiftelse för Vetenskaplig Forskning (project CTS 09:217), the Swedish Foundation for International Cooperation in Research and Higher Education (STINT), the Swedish Research Council (VR) and the Göran Gustafsson Foundation is acknowledged. MH thanks the Swedish Research Council (VR, contract 2012-6562) for financial support and Profs M. Fiebig and O. Tjernberg for hosting him during his postdoc. AG thanks the Russian Foundation for Basic Researches (grant 14-02-00483-a), and the program of RAS “Physics of elementary particles, fundamental nuclear physics and nuclear technologies” for financial support.
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