Diamagnetically diluted iron borate-based single crystals: EMR and SQUID studies

Iron borate FeBO
3
is an excellent example of the materials called “transparent magnets” [1], associating room temperature magnetic ordering (weak ferromagnetism) with transmission windows in visible spectral range [2]. The persisting research interest in iron borate is stimulated by its outstanding magnetic, magneto-acoustical, optical, magneto-optical, resonance, etc. characteristics [3–7]. Recently, new iron borate-based materials - Fe
x
Ga
1-x
BO
3
crystals - have been synthesized by the solution in the melt technique [8] and studied by Electron Paramagnetic Resonance (EPR) [9], Nuclear Magnetic Resonance [10, 11] as well as by optical and magnetooptical techniques [12]. These crystals: (i) per se possess extraordinary physical characteristics suitable for practical applications, and these characteristics can be monitored in the synthesis process; (ii) allow comprehensive studies of diamagnetic dilution - isomorphous substitution of iron by gallium - effect on the properties of magnetic materials, viz., gradual transition from magnetically ordered to paramagnetic state; (iii) allow understanding the nature of various mechanisms responsible for magnetic properties of iron borate, e.g., magnetocrystalline anisotropy, the former having different concentration and temperature dependences. Different types of Electron Magnetic Resonance (EMR) have been observed in Fe
x
Ga
1-x
BO
3
crystals depending on iron contents and the temperature. Figure 1 (a) shows the spectra transformation with x. At x = 1 (pure iron borate), only a low-field resonance is observed, earlier identified as Antiferromagnetic Resonance (AFMR) [7]. At a lower iron content, x = 0.75, besides the low-field line a new broad resonance emerges at higher magnetic fields, with the effective g-factor g≈2. Since iron substitution for gallium occurs more or less randomly, such crystals are expected to contain regions with different local iron concentrations, implying different magnetic ordering. The low-field line observed in the mixed crystals, by analogy with iron borate [7], can be identified as AFMR line arising from magnetically ordered regions, whereas the high-field line can be ascribed to Cluster Magnetic Resonance (CMR), i.e., EMR arising from only partially magnetically ordered regions. Both the low- and highfield EMR lines are present in all crystals with 0.35 ≤ x < 1; thus, one can conclude that in such crystals both long-range and short-range (cluster-type) magnetic ordering coexist. Figure 1 (b) shows the temperature dependence of the CMR line intensity for x = 0.65. With decreasing temperature this line considerably broadens and its intensity does not follow the T
⁻¹
Curie law, confirming its attribution to magnetic clusters. At still lower iron contents, x = 0.2, the AFMR line disappears and the high-field line increases in intensity. The EMR spectra for x = 0.2 crystal consist of a single line at g≈2, quite similar to the high-field line observed for higher x, consequently, in this case the antiferromagnetic regions are absent in the whole temperature range. The temperature dependence of the intensity of this line shown in Figure 1 (c), confirms that the Curie law for this resonance is not respected. For x = 0.04, the latter line also disappears and the EPR spectrum of diluted Fe
³⁺
ions, broadened by dipole-dipole interactions, comes into view. At a still lower iron content, x = 0.003, this spectrum is spectacularly narrowed. A detailed account of the EPR studies of these crystals has been recently carried out using laboratory-developed codes [9]. In order to confirm or infirm the existence of magnetic clusters in the mixed iron-gallium borates, we have carried out SQUID measurements. The temperature dependence of the magnetic susceptibility in crystals with intermediate x-values, e.g., see Figure 2, reveals the presence of a strong out-of-phase component, thereby confirming the existence of magnetic clusters at intermediate x values. Acknowledgments This work was partially supported by the V.I. Vernadsky Crimean Federal University Development Program for 2015 – 2024.