Development of multi-frequency ESR system for high-pressure measurements up to 2.5 GPa
Graphical abstract
Introduction
Pressure is an indispensable parameter for condensed matter physics because the application of pressure is the only way to tune the parameters of Hamiltonians continuously and cleanly. In particular, the pressure effect on quantum spin systems has attracted much attention over the past decade due to its potential for realizing novel pressure-induced quantum phenomena. For the spin gap system, several attempts have been made to collapse the spin gap by applying pressure to explore the novel phase. It has been suggested or found experimentally that the spin gap was reduced in a spin dimer system KCuCl3 [1], [2], a two-dimensional spin dimer system (C4H12N2) Cu2Cl6 [3], [4], two-leg spin ladder systems IPA-CuCl3 [5], Cu2(C5H12N2)2Br4 [6], and so on. In the well-known two-dimensional orthogonal spin dimer system SrCu2(BO3)2, the suppression of the gap energy with pressure and the existence of a novel magnetic phase were suggested [7]. In our previous high-pressure ESR of up to 1 GPa, direct suppression of the gap energy was also observed [8]. One of the most remarkable pressure effects on the spin gap system is the appearance of the longitudinal mode for a spin- dimer system TlCuCl3 above its quantum critical point [9]. This novel mode is considered to be similar to the Higgs boson and thus intensive study of this mode has been performed recently [10]. Other interesting pressure-induced quantum critical transitions in a Kagome lattice system herbertsmithite [11] and a pyrochlore system Tb2Ti2O7 [12] have also been reported.
In order to observe such pressure effects of these systems, magnetic susceptibility measurements and inelastic neutron scattering measurements are useful, and are usually performed using a single-layer type pressure cell [2], [4], [5], [6], [9], [10]. It is known that the upper limit of a pressure cell with a single-layer cylinder is 1.5 GPa [13], [14]. Therefore, the measured pressure ranges were limited to 1.5 GPa or below in these experiments and there have been only a few studies examining the quantum critical points within this pressure range [6], [9], [10]. Since the hybrid-type pressure cell used in NMR [7], the anvil-type one used in neutron diffraction [11], [12] or the specifically designed cell for μSR [4] have a pressure range above 1.5 GPa, they exhibit the pressure-induced transitions at higher pressures. However, neutron diffraction and μSR are not particularly powerful means of investigating a disordered state, although they can clarify the existence of a pressure-induced ordered state.
The high-pressure, high-field and multi-frequency ESR is one of the most powerful means of studying quantum spin systems in both the disordered and ordered states, particularly in the ground state and low-lying excited states. We have developed ESR systems by combining electromagnetic wave transmission-type piston-cylinder pressure cells with a pulsed high magnetic field ESR system up to 16 T or 55 T [1], [8], [15]. However, the small bores of our pulse magnets allow us to use only the single-layer type pressure cell and the pressure range is limited to 1 GPa at most. Although an anvil-type pressure cell might be used to realize the higher pressure, the very small sample space in such cells would lead to major problems with the sensitivity. The combination of an anvil-type pressure cell and resonators actually achieves practical sensitivity, but the frequency is fixed at the X-band [16], [17]. In addition to our group, another team has developed a high-field and multifrequency ESR system [18]. However, the pressure range of that system is also limited up to 1.6 GPa due to the single-layer cylinder.
A hybrid-type pressure cell with a cylinder consisting of an inner NiCrAl cylinder and outer CuBe sleeve has been developed to overcome this upper limit [14], [19]. Compared to the cell with a single-layer cylinder, this cell can double the maximum pressure of the single-layer pressure cell, to a maximum pressure of 4 GPa. In this paper, we describe a newly developed hybrid-type pressure cell for transmission-type ESR measurement and an ESR system using a cryogen-free superconducting magnet with a wide bore. Although the maximum field is limited to 10 T, we have performed ESR measurement up to 2.5 GPa successfully. We present sample applications of this high pressure ESR system to two magnetic materials. These examples show the potential of this system for exploring the novel phenomena of the quantum spin systems under pressure.
Section snippets
Overview of the experimental setup
Fig. 1 shows the newly developed hybrid-type piston-cylinder pressure cell for transmission-type ESR measurement. The major difference between this new pressure cell and the similar pressure cells used in the resistivity measurement, the AC susceptibility measurement, and other physical property measurements [14], [19] is that all inner components of the backups and pistons are made of ZrO2-based ceramic (FCY20A; Fuji Die Co., Ltd.) instead of metal alloys such as CuBe or WC, and this enables
Application to magnetic materials
High-pressure ESR measurements were performed on two magnetic materials as sample applications of this newly developed ESR system. One material was an paramagnet NiSnCl6 · 6H2O, which is known to exhibit single ion anisotropy [23], [24]. This paramagnet was originally studied as one of the candidates for exhibiting field-induced magnetic ordering above a critical field where the branch crosses the branch [24]. Recently, the longitudinal mode in the pressure-induced ordered phase,
Conclusion
We have developed a hybrid-type pressure cell with inner components made of ZrO2-based ceramic for ESR measurement. We confirmed that the pressure of 2 GPa can be generated repeatedly. The maximum pressure reached 2.44 GPa in this study. The relationship between the pressure at low temperature and the load at room temperature was determined and was found to follow the quadratic expression of the load. The reproducibility for the pressure generation was very high and the expression could be used
Acknowledgment
This research was partially supported by a Grant-in-Aid for Scientific Research (C) (No. 25400341) from the Japan Society for the Promotion of Science.
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