national high magnetic field laboratory
PULSED FIELD FACILITY
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OPTICAL FBG DILATOMETRY
FIGURE 2 & 3
The magnetocaloric effect (MCE), central to the principle of magnetic refrigeration, can be used to determine the (T,H) phase diagram of materials, often much more quickly and efficiently than the specific heat. MCE data can also be used to extract the entropy vs temperature and magnetic field, and thereby predict the magnetization, and the rest of the thermodynamic parameters of a material. It provides useful information about the nature of phase transitions, e.g. whether they are 1st or 2nd order, and it can be used to estimate the entropy involved in any phase transition or crossover. A typical MCE measurement measures the temperature of the latticevia a thermometer that is in intimate thermal contact with the sample, as the magnetic field is varied. The sample and thermometer should be somewhat isolated from the external environment, and depending on the degree of this isolation, the MCE can be measured in different limits, e.g. adiabatic, quasi-adiabatic or equilibrium. Under adiabatic conditions encountered in pulsed magnetic fields the temperature evolution of the sample is given by the Langevin equation ∂T/∂H|S = -T/CH ∂M/∂T|H and hence, depending on the properties of the material under study the MCE (sample temperature changes observed as the magnetic field changes) can be positive or negative as shown in animation 2.
optical FBG dilatometry
Novel techniques to modulate the refraction index used to ‘write’ Bragg gratings directly on the core of optical fibers have recently opened the way to high sensitivity optical-based strain measurements. The method, known as Fiber Bragg grating (FBG) dilatometry has a resolution L/L < 10-6, and can be used at cryogenic temperatures, in high magnetic fields, to measure samples as small as one millimeter in length. Most importantly, FBG dilatometry delivers data that is unaffected by electromagnetic noise ‘at the speed of light’!! In other words, the speed at which reflection spectra from the FBG can be read and recorded is only limited by the speed at which In-GaAs line array detectors can presently be read. Current state-of-the-art commercially available cameras operate acquiring one full 1024-pixel reflection spectrum with 14 bits resolution every 22 microseconds. These characteristics set optical FBG dilatometry apart as the only nanometer-resolution strain measurement method available and suitable for the very high magnetic fields produced at the NHMFL with resistive, resistive-superconductive hybrid and pulsed magnetic fields.
The schematic diagram in Fig. 1 on the page describes the operation principles of optical FBG strain measurements. A ‘white’ light source, in our case a solid state light emiting diode (SLED), is used to send broad spectrum infrared light into an optical fiber furbished with a 1mm-long Bragg grating that reflects light at λB = 1550 nm at room temperature. By means of an optical circulator (not shown) the reflected light is diverted into an spectrometer where the beam is spread, and then used to iluminate an In-GaAs line array camera acquiring the data at 46kHz. The sample is attached to the FBG section of the 9μm silica core/125 μm cladding single mode fiber with a cyano-acrilate bond as shown in Fig. 2 , and cooled to cryogenic temperatures using standar 3He/4He cryostats. Magnetic fields generated with continuous or pulsed resistive magnets are applied parallel or perpendicular to the strain measurement, to a recent world record of 100.75 T. The upper left pannel in Fig. 1 show data recently obtained in the spin transition system LaCoO₃, where the single crystal sample lattice and magnetization response to a change of spin state induced by an applied magnetic field was observed. The results were interpreted as a transition of some of the Co³⁺ ions in the perovskite material from a spin S = 0 state to a spin S = 1 state. It is believed that a similar lattice effect takes places in related ferric perovskites in Earth’s lower mantle under pressure. A full understanding of spin state transitions is yet ellusive, but these studies in high magnetic fields allowed for useful comparisons with theoretical predictions.
The optical FBG dilatometry technique is now available to all qualified NHMFL users.
• Magnetostriction and magnetic texture to 100.75 Tesla in frustrated
SrCu2(BO3)2. M. Jaime et al., Proc. Natl. Acad. Sci. 109, 12404 (2012).
• Cascade of Magnetic Field Induced Spin Transitions in LaCoO3. M.
Altarawneh et al., Phys. Rev. Lett. 109, 037201 (2012).
• Anisotropic cascade of field-induced phase transitions in the
frustrated spin-ladder system BiCu2PO6. Y. Kohama et al., Phys., Rev.
Lett. (2012) in the press.
Figure 2: Schematics of an optical FBG magnetostriction experiment in high magnetic fields. Upper left panel: bundle of optical fibers. Upper righ panel: recent magnetization and magnetostriction data measured to 97.4 T in LaCoO3, a spin-transition perovskite material that is related to ferric perovskites found in Earth’s lower mantle.
Figure 3: CeInRh₅ single crystal sample mounted with a 9μmcore/125μm cladding optical fiber, and a Cernox ⁽ᴿ⁾ bare chip thermometer for simultaneous magnetostriction and magnetocaloric effect measurements in continuous or pulsed magnetic fields to 100 Tesla.