Effect of the Non-Magnetic Ion Doping on the Magnetic Behavior of MgCr₂O₄

Abstract

Geometrically frustrated magnets exhibit exotic excitations due to competing interactions between spins. The spinel compound MgCr₂O₄, a three-dimensional Heisenberg antiferromagnet, hosts both spin-wave and spin-resonance modes, but the origin of its resonant excitations remains debated. Suppressing magnetic order via non-magnetic doping can help isolate these modes in neutron scattering studies. We synthesized Ga³⁺ and Cd²⁺-doped MgCr₂O₄ via solid-state reaction and analyzed their structure and magnetism. Ga³⁺ doping (0–20%) causes anomalous lattice shrinkage due to site disorder from Ga³⁺ occupying both Mg²⁺ and Cr³⁺ sites. Magnetically, Ga³⁺ doping drives the system from the antiferromagnetic order to a spin-glass state, fully suppressing magnetic ordering at 20% doping. In contrast, Cd²⁺ replaces only Mg²⁺, expanding the lattice and meantime inducing strong spin-glass behavior. At 10% Cd²⁺, long-range antiferromagnetic order is entirely suppressed. Thus, 10% Cd-doped MgCr₂O₄ offers an ideal platform to study the resonant magnetic excitations without any spin-wave interference.

1. Introduction

Magnetic frustration emanating from competing exchange interactions at the microscopic scale engenders macroscopic degeneracy of spin configurations and exotic magnetic excitations [1,2]. Predominantly, such phenomena manifest in materials known as geometrically frustrated magnets, wherein magnetic ions occupy the vertices of specific lattices. Examples include YbMgGaO₄ [3], which features a triangular lattice of Yb³⁺ ions and exhibits a quantum spin liquid state with itinerant excitations and quantum spin fluctuations. ZnCu₃(OH)₆Cl₂ [4,5], which hosts a spin-1/2 kagome lattice of Cu²⁺ ions and whose ground state may lie near a quantum critical point or resemble a critical spin liquid, exhibits highly diffusive scattering intensity in neutron scattering experiments, as reported by Tian-Heng Han et al. [6], distinct from that of non-frustrated quantum magnets and indicative of strong quantum fluctuations—providing direct evidence of its frustrated antiferromagnetic nature. Neutron diffuse scattering measurements on the kagome lattice compound Y_0.5Ca_0.5BaCo₄O₇ reveal the presence of only a short-range chiral order, without the emergence of long-range antiferromagnetic ordering, making it a compelling realization of a frustrated antiferromagnet [7]. In addition, neutron scattering experiments on this system reveal the onset of spin-freezing at low temperatures, accompanied by a broad dynamic response with significant temperature dependence [8]. Additionally, Na₄Ir₃O₈ [9] is present, a three-dimensional frustrated magnet with a hyperkagome lattice of Ir⁴⁺, whose ground state has been proposed as a rare 3D realization of a spin liquid. The pyrochlore lattice, another archetype of 3D geometry-frustrated lattice, has garnered enormous attention for decades as it could present a plethora of unique quantum behaviors, including spin ice, magnetic monopoles, quantum spin liquids, and spin resonances [10,11,12,13]. The lattice structure of magnetic ions is the motif of a 3D network of corner-sharing tetrahedra and is identified in two main categories of natural materials: pyrochlore oxides A₂B₂O₇ [12,14,15,16] with dual pyrochlore lattices on both A and B atomic sites and spinel chalcogenides AB₂X₄ (X = O, S, Se) [17,18,19,20,21,22,23], where only the B-site cations form a pyrochlore lattice.

The magnesium trinary compound MgCr₂O₄, as a member of chromium-based spinel oxides ACr₂O₄ (A = Zn, Mg, Cd, Hg) [19,24,25,26,27,28,29], is considered as a classical Heisenberg pyrochlore antiferromagnet. The sole magnetic ion Cr³⁺ (3d³, $S=3/2$) in MgCr₂O₄ [25], with half-filled $t_{2g}$ orbitals and quenched orbital momentum, resides on a pyrochlore lattice and is antiferromagnetically coupled with its six nearest Cr³⁺ neighbors. The Curie–Weiss temperature ${\theta }_{\mathrm{CW}}$ of MgCr₂O₄ varies from $-346$ to $-430$ K [25,26,30], indicating strong antiferromagnetic interactions among Cr³⁺, and an apparent hump around 50 K in the magnetic susceptibility provides evidence of short-range spin ordering. Upon cooling well below $|{\theta }_{\mathrm{CW}}|$, MgCr₂O₄ undergoes intricate phase transitions with antiferromagnetic transition temperatures $T_{\mathrm{N}}$ ranging from 12 to 16 K [25,31,32] and a concomitant decrease in lattice symmetry from cubic (space group: $Fd\overline{3}m$) to tetragonal phase (space group: $I{4}_1/amd$). The large value of $f=|{\theta }_{\mathrm{CW}}|/T_{\mathrm{N}}>20$ clearly indicates that the whole spin system in MgCr₂O₄ is highly frustrated. Neutron powder diffraction (NPD) experiments have shown multiple magnetic propagation vectors within the antiferromagnetic phase of MgCr₂O₄ below $T_{\mathrm{N}}$ [24,26,31]. Notably, these wave vectors exhibit various combinations across different NPD experiments, yet the magnetic moment of Cr³⁺ remains unsaturated and consistently smaller than the fully ordered moment of Cr³⁺ ($3.87{\mu }_{\mathrm{B}}$) in most instances. Below the Néel temperature, magnetic Bragg reflections are indexed by multiple propagation vectors, including k_L,1 = (1/2, 1/2, 0) and k_L,2 = (1, 0, 1/2) [26,31], associated with a partially resolved “L phase” [26,31,33]. In addition, some samples exhibit a distinct “H phase” above $T_N$ but below $T_H\approx 16$ K, characterized by a propagation vector k_H = (0, 0, 1) [26,32]. Some reports of $\mathit{k}=(1/2,1/2,1/2)$ [24] further emphasize the complexity of the system’s magnetic ordering and suggest the coexistence of multiple magnetic phases. Additionally, sharp and non-dispersive spin resonances at energy transfer $E=4.5$, 9.0, 13.5 … meV observed in inelastic neutron scattering (INS) spectra were interpreted as transitions between two quantum levels of spin clusters, leading to the classification of MgCr₂O₄ as a potential molecular spin liquid [24,31,34]. However, this hypothesis has recently been contested from a classical perspective, and the debate surrounding its magnetic ground state remains unresolved [33,34].

Chemical doping is usually considered a promising method to understand complicated magnetic orders and underlying spin–lattice couplings in MgCr₂O₄, particularly by modulating its low-temperature antiferromagnetic behaviors [29,35,36,37]. Differences in ionic radius ratios and volatilization rates among different elements often hinder the formation of a pure phase, making detailed synthesis and doping studies essential. Moreover, understanding how doping affects the magnetic ion sites is crucial for identifying appropriate doping levels that can effectively suppress magnetic ordering. According to the report in (S. E. Dutton, PRB, 2011) [25], the magnetic properties of MgCr₂O₄ are profoundly sensitive to metal atoms’ nonstoichiometry both at Mg²⁺ and Cr³⁺ sites, and even minor alterations in the microscopic chemical environment significantly impact its Néel temperature T_N and spin correlations above T_N. A similar effect is also observed in its homologous compound, ZnCr₂O₄ [35,36,37,38,39], where Cd²⁺ substitution for Mg²⁺ and Ga³⁺ substitution for Cr³⁺ effectively suppress its T_N. To our knowledge, there has been no systematic investigation of chemical doping in magnesium spinel oxide MgCr₂O₄.

In this work, we report the effects of Ga³⁺ and Cd²⁺ doping on the crystal structure and magnetic properties of MgCr₂O₄. Ga³⁺ doping effectively modulates the magnetic properties of the MgCr₂O₄ system by weakening its antiferromagnetic (AFM) order, inducing a transition to a spin-glass state, and eventually driving the system into a paramagnetic state at higher doping levels. Similarly, the introduction of Cd²⁺ also suppresses AFM interactions. However, due to the larger ionic radius of Cd²⁺ and the resulting stronger lattice distortion, this effect is more sensitive, enabling spin-freezing to occur at relatively low doping concentrations and temperatures.

2. Materials and Methods

The spinel chromium oxides Mg(Cr_1−xGa_x)₂O₄ (x = 0, 0.05, 0.10, 0.15, and 0.20) and Mg_1−xCd_xCr₂O₄ (x = 0, 0.05, 0.10, 0.15, and 0.20) were synthesized via a conventional high-temperature solid-state reaction method [37,38]. Based on the X-ray powder diffraction (XRD) refinement results, we found that a 1% reduction in the Cr₂O₃ (99.99%, Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China) stoichiometric amount was necessary to obtain a pure phase. Mg(Cr_1−xGa_x)₂O₄ was prepared by mixing stoichiometric amounts of Cr₂O₃, Ga₂O₃ (99.99%, Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China), and MgO (99.99%, Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China) in air, with a 1% reduction in Cr₂O₃. The mixture was pre-sintered at 800 °C for 12 h in air. After cooling, it was reground and subsequently reacted at 1200 °C for 24 h to obtain Mg(Cr_1−xGa_x)₂O₄(x = 0, 0.05, 0.10, 0.15, and 0.20). Similarly, MgO, Cr₂O₃, and CdO (99.99%, Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China) were mixed in stoichiometric ratios and subjected to the same synthesis procedure to obtain Mg_1−xCd_xCr₂O₄(x = 0, 0.05, 0.10, 0.15, and 0.20).

The crystallographic structures of the samples were characterized at room temperature using XRD (Rigaku Smartlab 3kW, Rigaku Corporation, Tokyo, Japan) with Cu Kα radiation (λ = 1.54 Å). The DC magnetic susceptibilities of all samples were measured using the Physical Property Measurement System (PPMS) from Quantum Design (San Diego, CA, USA), under the condition of zero-field cooling (ZFC) and field cooling (FC) processes. The time-of-flight neutron powder diffraction (NPD) measurements were conducted on Mg(Cr_1−xGa_x)₂O₄ ($\mathit{x}=0.20$) samples using the General Purpose Powder Diffractometer (GPPD) at China Spallation Neutron Source (CSNS), and the diffraction data were refined by using the Rietveld method with FullProf (Version: January-2021).

3. Results and Discussions

3.1. MgCr₂O₄ Doped with Ga

To elucidate the structural modifications induced by Ga doping in the spinel compound MgCr₂O₄, a combination of X-ray and neutron diffraction analysis was employed. The phases and the cation site occupancy were carefully determined. Figure 1b shows the X-ray diffraction patterns of Mg(Cr_1−xGa_x)₂O₄ ($x=0$, 0.05, 0.10, 0.15, and 0.20) measured at room temperature over the 2$\theta$ range of 10°–80°. All diffraction peaks can be indexed to the cubic spinel structure with space group $Fd\overline{3}m$ (No. 227), and no impurity phases were observed in any of the synthesized samples. A representative Rietveld refinement profile for the undoped MgCr₂O₄ sample is shown in Figure 1c. Initial XRD refinements with the model of Ga substitution only on Cr (B-site) yielded poor agreement, implying possible Ga on the A-site. However, due to the poor sensitivity of the light elements, such as Mg, XRD is not able to determine the ratio of Mg/Ga on the A-site. Thus, the neutron powder diffraction and refinement were employed because of the strong contrast of the neutron coherent scattering length of Mg (5.375 fm), Ga (7.288 fm), and Cr (3.635 fm) element [40]. The typical tof-NPD refinement for the sample Mg (Cr_1−xGa_x)₂O₄, x = 0.20, is presented in Figure 1d, indicating that the actual composition is (Mg_0.754Ga_0.246)(Cr_0.923Ga_0.077)₂O₄. This demonstrates that Ga³⁺ ions substitute on both the A-site (Mg²⁺) and B-site (Cr³⁺), resulting in a complex compound with the nominal formula (Mg_1−xGa_x)(Cr_1−yGa_y)₂O₄ while preserving the spinel structure. The site disorder between Mg²⁺ and Ga³⁺ ions has been also observed in other compounds, such as the the nonmagnetic layers in YbMgGaO₄ [3].

The obtained crystallographic parameters mainly from the XRD Rietveld refinement of the Gd-doped sample MgCr₂O₄ are presented in Figure 2. As shown in Figure 2a,b, with the increases in the named Ga³⁺ doping amount, both Cr³⁺ and Mg²⁺ occupancies of the A-site and B-site decrease. While the Cr occupancy nearly remains unchanged, the Mg occupancy increases drastically as the total doping amount of the Ga is constant. The above result suggests Ga ions prefer to stay at the A-site in the low doping level. Although Ga³⁺ doping does not change the overall symmetry of the spinel structure, the lattice constant a slightly decreases with Ga doping increasing due to the smaller ionic radius of Ga³⁺. A monotonic decrease in the Cr–Cr and Mg–Mg bond lengths is also observed, consistent with the decrease in lattice constant [Figure 2d,e]. Furthermore, Figure 2f shows that the decrease in the weighted average ionic radius at the A-site outweighs the slight increase at the B-site, suggesting that lattice shrinkage is primarily driven by substitution at the A-site. Such lattice contraction arises from the distinct size mismatch between dopant and host cations: Ga³⁺ in octahedral coordination (B-site) has a slightly larger ionic radius (62 pm) than Cr³⁺ (61.5 pm), while Ga³⁺ in tetrahedral coordination (A-site) is substantially smaller (47 pm) than Mg²⁺ (57 pm). In previously reported studies on Ga-doped ZnCr₂O₄ [25,37], no evidence of Ga incorporation at the A-site was observed, which may be attributed to the large ionic radius mismatch between Zn²⁺ (60 pm) and Ga³⁺ (47 pm) [41], resulting in an unfavorable substitution environment.

Figure 3 presents the temperature-dependent DC magnetic susceptibility, $\chi \left(T\right)$, of Mg(Cr_1−xGa_x)₂O₄ ($x=0$, 0.05, 0.10, 0.15, and 0.20) measured under zero-field-cooled (ZFC) and field-cooled (FC) conditions with an applied magnetic field of 0.2 T in the temperature range 2.0–300 K. The Néel temperature ($T_{\mathrm{N}}$) is determined by the drop temperature in $\chi \left(T\right)$. For the undoped sample ($x=0$), $\chi \left(T\right)$ drops sharply at 12.6 K, indicating the antiferromagnetic ordering as reported [24,42]. With increasing Ga doping, $T_{\mathrm{N}}$ gradually shifts toward lower temperatures and falls below 2.0 K for $x=0.20$ (see Figure 4a), which is out of the measurable temperature range on a conventional PPMS. Further magnetic susceptibility measurement of this sample with He3 or Dilution refrigerator equipment is needed to clarify the possible antiferromagnetic order at extreme low temperature. Unlike the $T_{\mathrm{N}}$ of MgCr₂O₄ increase with the lattice constants’ decrease under pressure [43], our results of Ga-doping reveal contrary behavior. One possibility is that the incorporation of Ga³⁺ ions partially replaces the magnetic Cr³⁺ ions, resulting in magnetic site dilution. This weakens the magnetic interactions and ultimately leads to a reduction in the $T_{\mathrm{N}}$ value.

Furthermore, the ZFC and FC $\chi \left(T\right)$ curves of Ga-doped samples bifurcate below $T_{\mathrm{N}}$, with the downturn in the ZFC branch and the upturn in the FC branch, implying the emergence of spin-glass behaviors [44,45]. The inset in Figure 3 displays the inverse magnetic susceptibility curve and its Curie–Weiss fit for MgCr₂O₄. The data in the high-temperature range (200 K ≤ T ≤ 300 K) were fitted using the Curie–Weiss law, $\chi$ = C/(T $-{\theta }_{\mathrm{CW}}$), where $\chi$ denotes the magnetic susceptibility, $\mathit{C}$ represents the Curie constant, and ${\theta }_{\mathrm{CW}}$ stands for the Curie–Weiss temperature, as shown in Figure 4. The fitted Weiss temperature |${\theta }_{\mathrm{CW}}$| for MgCr₂O₄ is $-516$ K, which deviates significantly from previously reported values [25,30,46]. This discrepancy may be attributed to the modified synthesis process in which a 1% reduction in the Cr₂O₃ precursor mass leading to a pure phase MgCr₂O₄ product. Enhanced phase purity likely strengthens intrinsic antiferromagnetic interactions, thereby increasing the magnitude of ${\theta }_{\mathrm{CW}}$.

With increasing Ga³⁺ doping, ${\theta }_{\mathrm{CW}}$ decreases monotonically [Figure 4b], indicating suppression of antiferromagnetic interactions. The effective magnetic moment per Cr³⁺ ion (${\mu }_{\mathrm{eff}}$) remains essentially unchanged upon doping [Figure 4c]. The magnetic frustration parameter, defined as $f=|{\theta }_{\mathrm{CW}}|/T_{\mathrm{N}}$, further increases with Ga doping [Figure 4d], indicating enhanced magnetic frustration. This effect may arise from the disruption of lattice periodicity due to partial substitution of magnetic B-site ions, leading to the formation of spatially heterogeneous [Cr₃Ga₁] tetrahedral clusters. The competition between Cr–Cr superexchange interactions in undoped regions and Cr–Ga–Cr exchange pathways at dopant boundaries introduces additional magnetic frustration within the system.

The XRD refinement confirms that Ga³⁺ ions occupy both A and B-sites, where A-sites are typically non-magnetic, and Cr³⁺ ions at magnetic B-sites are diluted. This substitution leads to structural distortions in the Mg(Cr_1−xGa_x)₂O₄ lattice. Figure 3 illustrates the evolution of magnetic phases with increasing Ga doping. Intermediate doping levels ($x=0.05$–0.10) display spin-glass-like behavior, evidenced by the bifurcation between ZFC and FC curves. At higher doping levels ($x\ge 0.15$), the system gradually transitions into a paramagnetic state, evidenced by the disappearance of the ZFC–FC splitting.

In ZnCr₂O₄, Ga³⁺ ions predominantly substitute for Cr³⁺ ions at the B-site, with minimal incorporation into the A-site (Zn²⁺), resulting in a relatively well-defined doping process [37]. This “magnetic site dilution” disrupts the Cr³⁺-based magnetic tetrahedral network, progressively weakening the antiferromagnetic order and inducing a spin-glass state. In contrast, in MgCr₂O₄, due to the closer ionic radii of Mg²⁺ and Ga³⁺, Ga³⁺ ions not only occupy the B-site Cr³⁺ positions but also partially substitute for A-site Mg²⁺, leading to A/B-site mixing and cation disorder. This more complex site occupancy results in an anomalous lattice contraction and introduces stronger perturbations to the magnetic structure, thereby suppressing the antiferromagnetic order at lower doping levels and giving rise to more pronounced spin-glass behavior.

According to Ch. Kant et al. [42], both nearest-neighbor ($J_1$) and next-nearest-neighbor ($J_2$) magnetic exchange interactions are present among B-site Cr³⁺ ions, with $J_2$ generally weaker than $J_1$. Their theoretical model indicates that the $J_2$ is antiferromagnet in MgCr₂O₄ and ZnCr₂O₄ while transitioning to ferromagnetic in highly structurally distorted analogs like CdCr₂O₄. These findings are corroborated by previous reports on Zn(Cr_1−xGa_x)₂O₄ [37]. In Mg(Cr_1−xGa_x)₂O₄, the structural distortion may be strong enough to trigger the change of the magnetic interactions. A ferromagnetic $J_2$ might be induced with the sample of $x=0.15$, leading to the observed ferromagnetic behavior in this system.

3.2. MgCr₂O₄ Doped with Cd

X-ray powder diffraction experiments were conducted at room temperature on polycrystalline Mg(Cr_1−xCd_x)₂O₄($x=0$, 0.05, 0.10, 0.15, and 0.20). As shown in Figure 5, the diffraction patterns confirm phase purity for all samples, with no detectable secondary phases. Rietveld refinement of the X-ray data reveals crystallization in the cubic spinel structure with space group Fd$\overline{3}$m (No. 227). As shown in Figure 5a, the normalized X-ray diffraction patterns suggest a systematic decrease in the intensity of the (111) reflection with increasing Cd content, indicating the successful Cd²⁺ substitution into the MgCr₂O₄ lattice.

Figure 5b presents a representative X-ray Rietveld refinement profile of Mg_1−xCd_xCr₂O₄ for $x=0.05$. The full refinement results for all Cd-doped samples are summarized in Figure 6. Figure 6a shows good agreement between the actual Cd concentrations and the nominal doping levels, confirming the accuracy of the composition control. Figure 6d displays the average ionic radius at the A-site, derived from the actual Cd doping levels. The radius increases linearly with doping, indicating effective substitution of Mg²⁺ by the larger Cd²⁺ ions. There is also an implication that no Cd ions take the site of Cr. Figure 6b,c present the refined lattice parameter a, along with the interatomic distances $d_1$ (Cr–Cr) and $d_2$ (Mg–Mg), respectively. All structural parameters increase monotonically with Cd content, indicating that Cd doping leads to lattice expansion and structural distortion in the MgCr₂O₄ system.

Figure 7 displays the ZFC and FC susceptibility curves of Mg_1−xCd_xCr₂O₄ ($x=0$, 0.05, 0.10, 0.15, and 0.20), measured under an external magnetic field of 0.2 T. As temperature decreases, larger bifurcation between ZFC and FC susceptibility curves were observed. A distinct bifurcation emerges at the spin-glass transition temperature ($T_{\mathrm{f}}$), which becomes more pronounced with higher Cd content. As shown in Figure 8a, $T_{\mathrm{N}}$ decreases from 12.6 K to 6 K with increasing Cd doping. Figure 8b shows that the Weiss temperature (${\theta }_{\mathrm{CW}}$), obtained from Curie–Weiss fitting, decreases with increasing x in the range $0\le x\le 0.125$ but slightly increases for $0.125\le x\le 0.20$. This suggests that the suppression of antiferromagnetic interactions saturates near $x=0.125$. Figure 8c shows that the frustration factor $f=|{\theta }_{\mathrm{CW}}|/T_{\mathrm{N}}$ reaches a minimum at $x=0.125$, indicating maximal suppression of magnetic frustration. The effective magnetic moment (${\mu }_{\mathrm{eff}}$) remains nearly constant across all doping levels. Previous investigations on Cd-doped ZnCr₂O₄ have demonstrated that the introduction of Cd²⁺ ions not only reduces the $T_{\mathrm{N}}$ but also effectively suppresses long-range antiferromagnetic ordering, leading to a spin-glass-like state [38]. Similarly, in the present study on MgCr₂O₄, Cd²⁺ doping exhibits a comparable trend, with $T_{\mathrm{N}}$ decreasing and AFM order gradually replaced by spin-glass behavior. This consistency across both Zn and Mg based spinels underscores the crucial role of A-site non-magnetic ion substitution in disrupting magnetic interactions and tuning the magnetic ground state in geometrically frustrated spinel systems.

Unlike Ga doping, XRD refinement confirms that Cd²⁺ substitutes exclusively at the A-site and does not affect the magnetic Cr³⁺ ions at the B-site. The $\chi \left(T\right)$ data reveal that even at $x=0.05$, Cd doping induces a magnetic transition from antiferromagnetic to spin-glass behavior, which persists even at high doping concentrations, similar to the previously reported behavior of Zn_1−xCd_xCr₂O₄ [38]. This early transition is likely driven by the large ionic radius mismatch (21 pm) between Cd²⁺ and Mg²⁺, compared to the smaller mismatches between Ga³⁺ and Cr³⁺ (0.5 pm) or Mg²⁺ (10 pm). The pronounced ionic size mismatch induces lattice distortions, which mediate the antiferromagnetic-to-spin-glass crossover at dilute doping concentrations. In studies on ZnCr₂O₄ [38], Ga³⁺ doping has been found to dilute the Cr³⁺ sublattice, introducing site disorder that suppresses the long-range antiferromagnetic ordering. In contrast, Cd²⁺ doping induces bond disorder by randomly modifying Cr–Cr exchange interactions, thereby promoting a transition into a spin-glass state. Given the high degree of similarity between MgCr₂O₄ and ZnCr₂O₄, it is reasonable to extend this interpretation to the MgCr₂O₄ system. Therefore, Cd²⁺ doping in MgCr₂O₄ effectively suppresses magnetic ordering while preserving a corner sharing tetrahedral lattice composed entirely of magnetic Cr³⁺ ions, making it an ideal candidate for investigating the origin of resonance magnetic excitations in MgCr₂O₄.

4. Conclusions

In summary, we successfully synthesized a series of Ga and Cd-doped MgCr₂O₄ spinel oxides via solid-state reaction. Doping with Ga³⁺ or Cd²⁺ ions preserved the cubic spinel structure of the parent MgCr₂O₄ system. Structural refinement of the XRD data reveals an anomalous decrease in the lattice constant of MgCr₂O₄ with increasing Ga³⁺ doping concentration. This behavior is attributed to cation site disorder and structural distortions induced by doping. Furthermore, NPD refinement performed on the $x=0.20$ sample confirms that Ga³⁺ ions not only substitute for Cr³⁺ at the B-site but also partially occupy the A-site positions originally held by Mg²⁺, resulting in a complex composition with the chemical formula (Mg_1−xGa_x)(Cr_1−yGa_y)₂O₄. This provides direct experimental evidence for the dual-site occupation of Ga³⁺ in the MgCr₂O₄ lattice. Magnetic susceptibility measurements show that Ga³⁺ doping progressively transforms the system from an antiferromagnetic state into a spin-glass-like state in the $x=0$–$0.10$ range and completely suppresses the antiferromagnetic transition at $x=0.20$, likely due to magnetic site dilution by Ga³⁺. In contrast, Cd²⁺ doping leads to lattice expansion, and Rietveld refinements confirm that Cd²⁺ exclusively substitutes for Mg²⁺ at the A-sites. Magnetic measurements demonstrate that even low levels of Cd²⁺ doping induce spin-glass-like behavior, with complete suppression of the antiferromagnetic transition at 10% doping level. This is attributed to lattice distortions caused by the large ionic radius of Cd²⁺, which disrupt magnetic exchange paths and destabilize long-range order. Accordingly, considering both the preserved Cr³⁺ magnetic tetrahedral lattice and the enhanced suppression of magnetic order, 10% Cd-doped MgCr³⁺ serves as an ideal material platform for future inelastic neutron scattering studies aimed at isolating and understanding the origin of resonant magnetic excitations. This work provides both sample materials and preliminary research foundations for understanding the origin of the resonant magnetic excitation modes in MgCr³⁺.