High-pressure dc magnetic measurements on a bisdiselenazolyl radical ferromagnet using a vibrating-coil SQUID magnetometer

The high-pressure magnetic properties of the iodo-substituted bisdiselenazolyl radical ferromagnet IBPSSEt have been studied by vibrating-coil SQUID magnetometry. The magnetic state at a pressure ($P$) of approximately 2 GPa has the highest Curie temperature ($T_{\mathrm{C}}$) of 27.5 K, and displays an ideal three-dimensional (3D) ferromagnetic interaction network. The value of $T_{\mathrm{C}}$ observed by ac magnetic susceptibility measurements is consistent with that obtained from dc measurements below approximately 4 GPa. Field-cooled dc measurements at more elevated pressures reveal a slow evolution of magnetic ordering, so that at $P>6$ GPa the structure may be described in terms of a 1D ferromagnetic chain with predominantly antiferromagnetic lateral (interchain) interactions, in accord with the results of density functional theory calculations.

Figure 1

Pressure dependence of $T_{\mathrm{C}}$ for representative radical ferromagnets; see Table 1 for the numbering scheme. The initial response of $T_{\mathrm{C}}$ to pressure for thiazyl and selenazyl radicals 5–9 is positive, contrary to that of nitroxyl radicals except for 3. The inset shows the detailed pressure dependence of $T_{\mathrm{C}}$ for the three bisdiselenazolyl radicals $X$BPSSEt ($X=\text{Cl}$, Br, I) [20, 21, 22, 31].

Figure 2

(a) Molecular framework of bisdiselenazolyl radicals $X$BPSSEt ($X=\text{Cl}$, Br, I). (b) Unit cell of IBPSSEt, space group $P\overline{4}{2}_{1}m$. (c), (d) Intermolecular magnetic exchange interactions around $\overline{4}$ points ($J_1,J_2$) and along $\pi$ stacks ($J_{\pi }$). (e) Variation in DFT-BS calculated magnetic exchange energies as a function or pressure, $J_{\pi }$ in green, $J_1$ in red, and $J_2$ in blue [22]; ferromagnetic regions are highlighted in blue.

Figure 3

Temperature dependence of the in-phase component $m^{\prime }$ of ac magnetic susceptibility measurement on IBPSSEt, using a miniature DAC and SQUID, performed over low-pressure [(a) $P\le 2.4$ GPa] and high-pressure [(b) $P\le 8.0$ GPa] ranges [22]. The out-of-phase component mainly reflects large ac magnetic responses due to the eddy current of DAC, and it is not useful to detect any magnetic signal of the target material.

Figure 4

(a) Overview of SQUID-VCM: The use of diamond anvils with different culet size (i.e., 500 and 550 $\mu \mathrm{m}$) allows the CuBe-NiCrAl composite gasket to be deformed away from the NbTi detection coil. The detection coil is ideally vibrated at the place where the slope of magnetic flux against the distance from the sample (shown as the sensitivity curve) is maximum. (b) Photograph of the DAC and actuating unit in the cryostat. (c) An example of a performance test for IBPSSEt at $P=0.2\mathrm{GPa}$: It contains one heating process after ZFC, three field-cooling (FC) ones, and two heating ones after FC. By investigating the influence of field cooling, the development of ferromagnetic correlation can be observed.

Figure 5

SQUID-VCM measurement for IBPSSEt using (a) Apiezon-J oil as the PTM for both the FC process of 30 Oe and heating process maintaining its $H_{\mathrm{dc}}$ and (b) Daphne oil 7373 as the PTM for the heating process after FC of 30 Oe. The insets of (a) and (b) show $M\left(T\right)$ for the heating process at (a) $P=2.1\mathrm{GPa}$ and (b) $P=6.3$ and 9.7 GPa, respectively.

Figure 6

The analyses of $M\left(T\right)$ for both a $T$ decrease and $T$ increase in the FC process at $P=2.1\mathrm{GPa}$ using the Brillouin function (a) and scaling curve with critical component ($\beta =0.369$) for the spontaneous magnetization in the three-dimensional Heisenberg system (b). In (a), $M\left(T\right)$ observed in the warming process can be reproduced with the Brillouin function with a spin quantum number $S=1/2$ and $T_{\mathrm{C}}=27.0–27.5\mathrm{K}$. In (b), there are three scaling curves proportional to ${(T_{\mathrm{C}}-T)}^{\beta }$ with $\beta =0.369$ and $T_{\mathrm{C}}=26.0–28.0\mathrm{K}$. $A$ is an arbitrary constant.

Figure 7

Pressure dependence of Curie temperature $T_{\mathrm{C}}$ for IBPSSEt; solid blue circles stand for the characteristic temperature estimated in the dc measurements, which corresponds to $T_{\mathrm{C}}$ for $P<4.1$ GPa. For $P>6$ GPa, there are two characteristic temperatures. For reference, red symbols present the results of the ac measurements by piston cylinder cell ($\square$) and DAC ($\circ ,•,▴$) [22].