Abstract
CuAl2O4 is a ternary oxide spinel with Cu2+ ions (\(s=1/2\)) primarily populating the A-site diamond sublattice. The compound is reported to display evidence of spin glass behavior but possess a non-frozen magnetic ground state below the transition temperature. On the other hand, the spinel CuGa2O4 displays spin glass behavior at ~ 2.5 K with Cu2+ ions more readily tending to the B-site pyrochlore sublattice. Therefore, we investigate the magnetic and structural properties of the solid solution CuAl2(1-x)Ga2xO4 examining the evolution of the magnetic behavior as Al3+ is replaced with a much larger Ga3+ ion. Our results show that the Cu2+ ions tend to migrate from tetrahedral to octahedral sites as the Ga3+ ion concentration increases, resulting in a concomitant change in the glassy magnetic properties of the solution. Results indicate glassy behavior for much of the solution with a general trend towards decreasing magnetic frustration as the Cu2+ ion shifts to the B-site. However, the \(x=0.1\) and 0.2 members of the system do not show glassy behavior down to our measurement limit (1.9 K) suggesting a delayed spin glass transition. We suggest that these two members are additional candidates for investigation to access highly frustrated exotic quantum states.
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Introduction
Frustrated magnetic systems continue to be an area of intense interest due to the emergence of novel magnetic ground states 1. Among the many molecular geometries leading to magnetic frustration are the spinels (AB2X4, X = O, S, Sr). These materials are composed of a close packed X anion lattice containing alternating tetrahedral and octahedral sites that accommodate A2+ and B3+ cations, respectively. Population of either site with magnetic cations can lead to frustration. For instance, the octahedral sites create a corner-sharing tetrahedral pyrochlore network which can lead to geometric frustration if populated with magnetic moments with antiferromagnetic nearest neighbor (NN) exchange 2,3,4,5. The tetrahedral sites, which connect to form a diamond lattice, may also lead to frustration, as observed in FeSc2S4, MnSc2S46, FeAl2O47 and CoAl2O48. In this case, frustration arises due to competing interactions between NN and next-nearest neighbors (NNN) on the lattice resulting in an interesting phase diagram7. In particular, if the exchange ratio \({J}_{\mathrm{NNN}}/{J}_{\mathrm{NN}}\) is above 1/8 and \(s\le 1\) there is no thermal phase transition, but rather these compounds settle in a highly degenerate ground state associated with a spiral spin liquid9,10. For \(s\ge 3/2\) a crossover to classical behavior occurs accompanied by a thermal phase transition to an ordered ground state. Therefore, spinels with spin at the quantum limit (\(s=1/2\)) populating the A-site are of particular interest in accessing highly degenerate quantum ground states.
While magnetic frustration may prevent a system from arriving at a well-ordered ground state, the addition of quenched disorder may lead to a glassy magnetic state such as a spin glass, where the energetics of the system are dominated by a rough free energy landscape, and hence slow relaxation dynamics. Despite intensive investigation, a consensus is still lacking as to the nature of the ground state and dynamics of the spin glass 11. One source of disorder in the spinel lattice leading to the spin glass state comes from cation mixing between sites. The fractional amount of B3+ atoms sitting on the A site is indicated by an inversion parameter \(\eta\) such that the compound formula may be written as \([{\mathrm{A}}_{(1-\upeta)} {\mathrm{B}_{\upeta}}]_{\mathrm{tet}}[{\mathrm{A}}_{\upeta} {\mathrm{B}}_{(2-\upeta)}]_{\mathrm{oct}}{\mathrm{X}}_{4}\). Spinels with \(0<\eta <2/3\) are classified as “normal” spinels; otherwise, they are classified as “inverse” spinels.
It has recently been observed that the \(x=0\) (CuAl2O4) member possesses indications of an exotic quantum ground state. Magnetic measurements reveal a high frustration factor \(f=\frac{\left|{\Theta }_{cw}\right|}{{T}_{sg}}=67\) with a spin glass like cusp in the magnetic susceptibility at \({T}_{\mathrm{sg}}\sim 2\) K, a high Curie Weiss temperature \({\Theta }_{\mathrm{cw}}=-140\) K indicating a large antiferromagnetic exchange, and the onset of a metastability between the FC and ZFC spectra12. However, muon spin resonance (µSR) reveals a non-frozen state with significant dynamic spin fluctuation below the spin glass-like freezing transition. Site dislocation plays a role in introducing glassy behavior, but nevertheless spin fluctuations associated with the liquid state persists13. Prior investigation of this end member show that there is some inter-site mixing, with \(\eta = 0.35 - 0.4\), indicating that the Cu2+ ions primarily populate the diamond sublattice14.
At the other end of the solid solution, CuGa2O4 is reported as an inverse spinel with the copper atoms primarily populating the pyrochlore sublattice \((\eta\) = 0.85) 15. Spin glass behavior is reported, with an onset at \(3.8\) K indicated by µSR and a cusp in the susceptibility at \(2.5\) K. It is argued that, in addition to the pyrochlore geometry, Jahn–Teller distortion plays a role in the frustration by introducing a random anisotropic exchange between Cu2+ spins16. Both end members of this system are also reported to have a non-zero zero-point entropy, characteristic of frustrated systems, and on the order of the predictions from the Sherrington-Kirkpatrick XY spin glass model17.
In this work we examine the magnetic and structural properties of the entire solid solution of this chemical system. We find magnetic behavior consistent with either an insulating spin glass, or a cluster glass where domains of net spin act as the magnetic entity of interest18. For \(x=0\), we find a high frustration value similar to what is reported above12. We also show that the behavior of the ac susceptibility for the \(x=0\) member fits a typical dynamical phase transition model for spin glasses. For \(x>0.2\), as Al is replaced with Ga, we observe a decrease in the frustration in the system which correlates with Cu2+ ions shifting to the octahedral sites. However, for the \(x = 0.1\) and \(0.2\) compositions, we detect no peak in the ac susceptibility. We speculate that glassy behavior for these members occurs below the minimum accessible temperature of our apparatus; this would suggest a high degree of magnetic frustration, and therefore of interest for further investigation.
Results
Structural characterization
Room temperature synchrotron X-ray diffraction results show a well formed CuAl2(1−x)Ga2xO4 phase for the entire solid solution. As an example, the results for the \(x=0.4\) member are shown in Fig. 1a. Small amounts of residual CuO are visible in the \(x=0\) and \(x=1\) members. All the members are found to be in the Fd \(\overline{3 }m\) space group and obey Vegard’s law as shown in Fig. 2a indicating that the Ga3+ is taken up into the lattice replacing the Al3+. We also obtained neutron diffraction data for the even members of the solution. We present results for the \(x=0.8\) member on the right in Fig. 1b along with its refinement fit. Refinement for CuAl2O4 agrees well with published results in terms of lattice parameter and chemical occupancy14,15 with \(\eta =0.38\). On the other hand CuGa2O4 differs from prior reported results15 with \(\eta =0.47\).
For the intermediate members of the solution, determination of the occupancy with X-ray diffraction required a two-step process due to challenges in discriminating between nearly isoelectronic ions such as Cu2+ and Ga3+. This difficulty was exacerbated by having to determine simultaneously the location of three cations. To deconvolute the metal occupancies, the location of the Al3+ cation was first determined for all compositions by performing a refinement on our high-resolution X-ray data with Ga3+ acting as a surrogate for Cu2+. Using these values as a starting point, we repeated the refinement process with the neutron diffraction data. The difference in coherent neutron scattering cross section was great enough to allow a determination of the occupancies of the Cu2+ (\({\sigma }_{c}=7.485\)) and Ga3+ (\({\sigma }_{c}=6.675\)) cations. In the final step, all cation occupancies were allowed to simultaneously refine. We obtained similar Al3+ occupancies for both the synchrotron and neutron data as shown in Fig. 2b,c. We further note that we also obtained similar Al3+ occupancies by using Cu2+ as the surrogate for Ga3+ (not shown).
The occupancies for the tetrahedral and octahedral sites are displayed in Fig. 2b,c. For the \(x = 0\) member, Al3+ has a strong tendency to occupy the octahedral sites. The Cu2+ ions prefer, but do not completely populate the tetrahedral sites, as previously reported14. We note that structural results reported in the literature show no structural distortion or magnetic long range order down to 0.4 K for the \(x=0\) member12. As \(x\) increases (i.e., Al3+ is replaced by Ga3+) the Cu2+ ions are partially shift to the octahedral sites.
High temperature susceptibility
We describe all high temperature paramagnetic behavior for all members of the solid solution with the Curie–Weiss Law, \({\left(\chi -{\chi }_{0}\right)}^{-1}=\frac{3{k}_{B}(T-{\Theta }_{\mathrm{cw}})}{{A}_{v}{{p}_{\mathrm{eff}}}^{2}{{\mu }_{b}}^{2}}\). In this equation \({\chi }_{0}\) is the temperature independent portion of the magnetic susceptibility,\({A}_{v}\) is Avogadro’s number, \({k}_{B}\) is the Boltzmann constant, \({\Theta }_{\mathrm{cw}}\) is the Curie–Weiss temperature, and \({p}_{\mathrm{eff}}\) is the effective magnetic moment in units of Bohr magnetons,\({\mu }_{b}\). A characteristic result for the \(x = 1\) member is plotted in Fig. 3a. The inverse susceptibility begins to deviate from linear paramagnetic behavior around 130 K similar to prior reported results16. \({\Theta }_{\mathrm{cw}}\) for the entire solid solution is reported in Fig. 3b and \({p}_{\mathrm{eff}}\) in Fig. 3c. The solid solution is found to have high temperature effective magnetic moments ranging between 1.8 and 2.4 \({\mu }_{b}\) with a maximum at \(x = 0.2\). Typical observed values for materials with Cu2+ range from \(1.7\) to \(2.2{ \mu }_{b}\); similar to the theoretical value \({p}_{\mathrm{eff}}=g\sqrt{s(s+1)}=1.7 {\mu }_{b}\) 19. All Curie Weiss temperatures are found to be negative, indicating a short-range antiferromagnetic exchange interaction between spins. Concomitant with the Cu2+ cations repositioning to the octahedral sites (cf. Fig. 2), we find that \({\Theta }_{\mathrm{cw}}\) decreases in magnitude for \(x>0.2\).
A similar nonlinear decrease in \({\Theta }_{\mathrm{cw}}\) has been observed in the system CoAl2(1−x)Ga2xO4. This decrease is associated with an increasing bond length that is geometrically necessary with the presence of the larger Ga atoms, and in turn decreases the exchange strength between the Co ions. Further, a linear decrease in \({\Theta }_{\mathrm{cw}}\) with bond distance is observed when comparing the end members of this Co solid solution and the well-ordered A-stie compounds Co3O4 and CoRh2O4. This suggests that the non-linearity in \({\Theta }_{cw}\) versus \(x\) for both CoAl2(1−x)Ga2xO4 and CuAl2(1−x)Ga2xO4 is due to intersite mixing20. To a lesser extent we also see a decrease in \({p}_{\mathrm{eff}}\) after \(x = 0.2\). Increasing orbital quench on the octahedral sites may play a role.
DC susceptibility
We have obtained FC-ZFC dc magnetic susceptibility curves in magnetic fields ranging from 100 to 500 Oe for all solid solution members. We observe bifurcation for members \(x=0\) and \(x\ge 0.3\). Typical results are plotted in Fig. 4a. Above 500 Oe the bifurcation is suppressed in the detectable range of measurement. We note that the FC curve does not flatten below the bifurcation temperature as observed in many spin glasses. This may suggest a portion of the spins are free from the glassy dynamics.
The de Almeida-Thouless (AT) phase boundary separating the spin glass state from the paramagnetic state is proposed in the Sherrington-Kirkpatrick (SK) mean field Ising spin glass model21, and is described as a power law of the form
where \({T}_{\mathrm{irr}}\) is the irreversibility temperature, \(H\) is the externally applied dc field, C is a constant, and \(\Phi =3\). In Heisenberg systems a second boundary, the Gabay-Tolouse (GT) line, is associated with the freezing of the longitudinal components of the magnetic moment and has a predicted exponent of \(\Phi =1\)1 22 . Non-mean field predictions allow \(\Phi\) to vary, and experimental results capturing deviation from mean field predictions have been observed in systems such as GdAl23, CuMn0.2524 , and Cr0.5Fe0.5Ga11.
In our measurements, the FC-ZFC separation temperature \(({T}_{\mathrm{irr}})\) is indistinguishable from the ZFC peak temperature \(\left({T}_{p}\right)\). Therefore, we track \({T}_{p}\) as a function \(H\) and find that best fit values for \(\Phi\) reported in Table 1 range from 2.9 to 4.6. We do not observe any clear trend for \(\Phi\) with material composition. Though \(\Phi\) tends to be slightly larger than the mean field value, best fit curves with a fixed exponent of \(3\) also yield statistically reasonable results. Typical results are displayed in Fig. 4b. Additional measurements below 100 Oe are required for accurate estimates for \({T}_{\mathrm{irr}}\left(0\right)\). Evidence for a GT line is not observed at these magnetic field values.
Slow relaxation and memory
In Fig. 5 we show the time dependent magnetization for solution members \(x=0.4, 0.6, 0.8\), and \(1.0\). Each sample was field cooled below \({T}_{p}\) down to 1.9 K in the presence of a 200 Oe field. After a wait time \({t}_{w}=7200\) s the field was set to zero at a rate of 5 Oe/s. As shown in the graph we observe slow relaxation curves, and in all cases the data is reasonably fit to a stretched exponential of the form
where M0 is the FC magnetization and \({\tau }^{^{\prime}}\) is the characteristic relaxation time. An additional offset \({M}_{1}\) was included for an improved quality of fit. However, \({M}_{1}\ll\) \({M}_{0}\) and was neglected in the fits shown in Fig. 5a. Such a model has been used to describe slow relaxation results for a number of glassy systems including spin glasses 25,26. The exponent \(1-n\) ranges from 0.39 to 0.68 with no clear trend in composition. The percolation model for spin glasses puts a bound on the exponent such that \(\frac{1}{3}<1-n<1\) 25. The value \({\tau }^{^{\prime}}\) ranges from 200 s for the \(x = 1\) member to 2200 s for \(x = 0.8\). Additional investigation is required to show if varying \({t}_{w}\) leads to aging.
Memory effects are also well known in magnetic glassy systems. We have performed two sets of memory experiments on the \(x=0.8\) member of the solid solution. In the first set, we pause the cooling of the system at 2.2 K, and on a separate run at 2.4 K, as the temperature is cooled to 1.9 K in zero field. We then apply a dc field of 50 Oe and measure the magnetization as the temperature is increased. Results in Fig. 6a show a deviation from the continuous (unpaused) ZFC temperature sweep. To better demonstrate the memory effect, we also plot the difference between the unpaused ZFC curve and each paused curve such that
\(\Delta M\) shows minima at the two temperatures at which the system was paused in Fig. 6b. For the second set of experiments, we paused the system at 2.3 K for 250 s and 2500 s. Again \(\Delta M\) shows minima at 2.3 K in Fig. 6c with the longer pause time showing the larger effect. Results clearly indicate memory effects and hence the presence of a glassy magnetic state below \({T}_{p}\) for \(x=0.8\).
AC susceptibility
Fixed \({H}_{ac}\)
Further insight into the dynamic properties of glassy magnetic systems including spin glasses can be obtained through ac susceptibility measurements. We performed measurements on all members of the CuAl2(1−x)Ga2xO4 solid solution at frequencies ranging from 25 Hz to 2.5 kHz with an excitation field \({H}_{\mathrm{ac}}=3\) Oe. The real part of the susceptibility \({\chi }^{{\prime}}\) shows a peak for \(x=0\) (Fig. 7a) and \(x\ge 0.3\). The corresponding imaginary part of the susceptibility \({\chi }^{{{\prime \prime}}}\) (Fig. 7b) displays a maximum at temperatures lower than the peak in \({\chi }^{^{\prime}}\) followed by an inflection point at higher temperatures. Both show a frequency dependent trend as expected for glassy systems with the peak/inflection point shifting to higher temperatures as the frequency increases. The relative shift in temperature per frequency decade may be quantified as
where \({T}_{f}\) is the temperature of the peak in \({\chi }^{{\prime}}\), \(\omega /2\pi\) is the excitation frequency, and ∆ indicates the change between frequencies. \({T}_{f}\) is estimated using a spline curve to locally smooth the data. As indicated in Table 2, \(K\) ranges from 0.03 to 0.04. Such values are an order of magnitude larger than canonical metallic spin glasses but smaller than insulator spin glasses (0.06–0.08)27,28. For a superparamagnet, \(K > 0.2\)27.
The frequency dependent shift in temperature can be described in terms of a classical phase transition, where the correlation time diverges as a power law such that \(\tau \propto {\xi }^{z}\). \(\xi\) is the correlation length and z is the dynamical scaling exponent. The correlation length is in turn related to the reduced temperature \(\xi \propto {\left(\frac{{T}_{f}-{T}_{sg}}{{T}_{sg}}\right)}^{-\nu }\) where \(\nu\) is the correlation length critical exponent. The correlation time near the critical point is then \(\tau ={\tau }_{0}{\left(\frac{{T}_{f}-{T}_{sg}}{{T}_{sg}}\right)}^{-z\nu }\) where \({\tau }_{0}\) is the characteristic magnetic moment spin-flip time. Associating the excitation frequency with the inverse of the correlation time we rewrite the relationship as
The susceptibility should then diverge as the temperature approaches the spin glass temperature \({T}_{s\mathrm{g}}\) at zero frequency. From these temperatures we obtain parameters from a 3-variable fit to the equation \(\mathrm{ln}\omega =\mathrm{ln}{\omega }_{0}+z\nu \mathrm{ln}\left(\frac{{T}_{f}}{{T}_{sg}}-1\right)\). Results for frequencies above 2.5 kHz are not included in the fit due to evidence of Joule heating from eddy currents induced by the ac field. We also find that results below 25 Hz are too noisy to obtain a precise estimate of the peak location. Fit parameters for the even members of the solution are reported in Table 2. Values obtained for \({\tau }_{0}\) derived from these fittings are accurate to only the nearest order of magnitude. Nevertheless the values fall within the range for canonical spin glasses (10–12–10–14 s)25,29,30 while values as low as 10–7 have been reported for insulator spin glasses such as Eu0.4Sr0.6S31 and cluster glasses such as Nd5Ge332. Values for cluster glasses range from 10–6 to 10−11 s28. We were also able to estimate the dynamical exponent \(z\nu\) from the data. Typical values for \(z\nu\) range from 4 to 12 for glassy magnetic systems 27. However, results as high as 13 have been reported in a number of cases 33,34,35,36 while 14 has been associated with the random field Ising magnet37. Results obtained in this study range from 9 to 17; however, we hesitate to draw any strong conclusions due to the large uncertainty associated with these results, necessitating a more careful approach in subsection “Variable Hac”.
\({\chi }^{{{\prime \prime}}}\) displays a similar shift in temperature with increasing frequency. The maximum of the slope \({\left(\frac{d{\chi }^{{{\prime \prime}}}}{dT}\right)}_{\mathrm{max}}\) has been associated with \({T}_{f}\)27,38, but in our data we clearly see the maximum occurring at a slightly higher temperature than the peak in \({\chi }^{^{\prime}}\). Due to a much smaller signal no frequency dependent trends could be extracted from this data. Plots of \({\chi }^{{{\prime \prime}}}\) and \(\frac{d{\chi }^{{{\prime \prime}}}}{dT}\) versus T for the \(x=0\) member of the solution are displayed in Fig. 7b.
We also examined the shift in \({T}_{f}\) in the presence of an external dc field. Results for \(x=0.5\) are shown in Fig. 8a. As the field increases the peak both broadens and shifts to lower temperatures, as has been observed for many magnetic glassy systems. Results for the entire solid solution were obtained at \(\frac{\omega }{2\pi }=\) 1 kHz and \({H}_{\mathrm{ac}}=3\) Oe. As shown in Fig. 8b, above 500 Oe we observe an AT like power law trend similar to Eq. (1). Best fits for \(\Phi\) are reported in Table 1 and vary from 3.2 to 5; all values are higher than the mean field value of 3. Non-mean field analysis on a number of spin glasses show \(\Phi\) varying from 3.2 (random anisotropic \(\alpha -\) DyNi39) to 5 (CuMn40), suggesting that values obtained for this solid solution are reasonable in a spin glass framework. As a benchmark, a value of 3.8 is obtained for the cluster glass Cr0.5Fe0.5Ga11. For \(H < 500\) Oe we observe a crossover from AT like behavior to \(T\propto {H}^{2}\). However, we do not believe this is associated with the GT line. Similar behavior is observed in ac susceptibility measurements of insulating spin glass Eu0.4Sr0.6S41, and is due to shortened relaxation times (higher frequencies) in low fields. The behavior diminishes for lower frequencies for this solid solution (not shown) and in published results (e.g. 42).
Variable Hac
To decrease the uncertainty in the estimates obtained in the dynamical scaling analysis we repeated the measurements while varying the magnitude of the ac driving field. We chose the magnitude for each frequency such that it maximizes the ac signal while also avoiding Joule heating. This careful approach also extends the frequency range over which models may be fit to the data. Results are reported for comparison in Table 3.
Such ac measurements are typically performed with \({H}_{\mathrm{ac}}\) held constant across all frequencies. The excitation magnitude is chosen to be small so as to avoid introducing nonlinearity to the measurement 18,43. However, we demonstrate that the excitation field can be varied to maximize signal to noise while leaving \({T}_{f}\) unaffected. We demonstrate this in Fig. 9a where we measured \({T}_{f}\) as a function of excitation amplitude from 10 Hz to 10 kHz for \(x=0\). We also report the values of \({H}_{\mathrm{ac}}\) selected for the measurements presented in this section. Below 1 kHz we observe no significant shift in freezing temperature for the range of dc fields chosen, while at 10 kHz we observe a deviation to lower temperatures at higher fields. Further, we observe no significant change in the shape or magnitude of the peak in \({\chi }^{^{\prime}}\) for different \({H}_{\mathrm{ac}}\) as demonstrated in Fig. 9b for 10 Hz.
Susceptibility results for a varying \({\mathrm{H}}_{\mathrm{ac}}\) for the \(x=0\) end member are shown in Fig. 10a,b. In this more comprehensive analysis, we find that the dynamical exponents and the spin-flip times fall within the uncertainty of the first set of measurements. These results reinforce our conclusions that the magnetic properties of the solid solution ought to be interpreted as either an insulating spin glass or cluster glass. We also performed a second series of fits using the inflection point of the imaginary part of the susceptibility \({\left(\frac{d{\chi }^{{{\prime \prime}}}}{dT}\right)}_{\mathrm{max}}\) (Table 4). Spin flip times are longer than those derived from \({\chi }^{{\prime}}\) and range from \({10}^{-8}\) to \({10}^{-11}\) s, while \(z\upsilon\) values tend to be smaller ranging from 5 to 13. A similar discrepancy between the parameters derived from \({\chi }^{^{\prime}}\) and \({\chi }^{{{\prime \prime}}}\) is observed, for instance, in the insulating spin glass La0.9Sr0.1CoO344. Nevertheless, the dynamic scaling analysis confirms the general observation that the majority of the solid solution most closely resembles that of either a cluster glass or insulating spin glass.
With an estimate for \({T}_{sg}\) reported in Table 3, we can now estimate how the magnetic frustration evolves with composition in the CuAl2(1-x)Ga2xO4 system. We find a negative linear correlation between the frustration and the occupancy of the octahedral site as shown in Fig. 11. The trend departs from linearity for the \(x=1\) member of the solution. Using the occupancy obtained for \(x=0.2\), we extrapolate out that \(f\sim 110\), and hence \({T}_{sg}\sim 1.5\) K. This temperature is consistent with the fact that we are unable to detect a freezing transition at the low temperature limit of 1.9 K for our apparatus.
Summary and discussion
We have investigated the magnetic and structural properties of the solid solution CuAl2(1−x)Ga2xO4. Our structural analyses show that the spin 1/2 Cu2+ ions favor the tetrahedral sites for CuAl2O4 (\(x=0, \eta =0.38\)) and generally redistribute toward the octahedral sites for \(x>0.2\). This is similar to the general trend observed for CoAl2(1−x)Ga2xO420 with the larger Ga atom increasing the bond length, leading to a decrease in \({\Theta }_{\mathrm{CW}}\) indicating a decrease in magnetic exchange interactions. However, unlike the Co solid solution, we do not observe a purely paramagnetic phase at \(x=0\) due to intersite mixing which results in glassy dynamics spanning the observable portion of the solution.
When detectable via our apparatus, all members of the solution display a bifurcation in the FC-ZFC magnetization data below 3 K with a decrease in transition temperature as a function of applied field. In all cases the FC susceptibility curve continues to increase below the bifurcation temperature suggesting a portion of the spins not participating in the glassy dynamics. This is consistent with the interpretation suggested by Cho et al.13 for CuAl2O4 that a dynamic and glassy phase coexist with the glassy phase corresponding to the degree of site disorder. Further, the applied magnetic field dependent bifurcation temperature deviates from the mean field AT line. We observe similar AT-like behavior in ac susceptibility measurements in the presence of an external dc field. Prior investigation for the \(x=1\) member includes modeling that assumes a Heisenberg magnetic system16, therefore separating out whether there is an additional GT transition is a topic for further research. Other evidence of spin glass behavior includes slow relaxation dynamics for even members of the solid solution (\(x\ge 0.4)\), and memory effects for \(x=0.8\).
We have also shown that the ac susceptibility reveals typical glassy behavior, with a peak in \({\chi }^{^{\prime}}(T)\) and an inflection in \({\chi }^{{{\prime \prime}}}(T)\) shifting to lower temperatures with decreasing frequency. The frequency dependent shift fits to a dynamical phase transition typical for spin glasses. Parameters derived from this model fit within ranges for both insulating spin glasses and cluster glasses, although with the higher spin flip times reported in Table 3 we lean towards an insulating spin glass. Therefore, we believe the best interpretation of our solid solution from this analysis is that of a spin glass-like system shifting from a dilute diamond lattice to a dilute pyrochlore as Al is replaced with Ga13,45, with weaker frustration as Cu2+ shifts to the pyrochlore lattice and bond length increases.
The ac susceptibility results are interesting to compare to a similar A-site spinel FeAl2O4 (\(s=2\)) in light of a proposed magnetic phase diagram as a function of \(\eta\) 46. This diagram is based on CoAl2O4 and shows an overlapping region separating a spin liquid regime at low values of \(\eta\) and spin glass region at higher values. In the case of FeAl2O4 the ac susceptibility peak does not shift with changing frequency. Hence, assuming a similar phase diagram, the compound is interpreted as residing near the spin liquid/spin glass boundary47. By contrast, our results add weight to the idea that the \(x=0\) and the \(x>2\) portions of the solid solution are either closer to or reside within the spin glass portion of the diagram. \(\mu SR\) measurements of the entire solution would reveal the degree to which any additional non-glassy dynamic spin fluctuation state persists and evolves for \(x>0\).
Finally, we have obtained frustration values for the even members of the solution. Frustration values greater than 10 are considered large8, and are therefore of interest in accessing highly degenerate quantum ground states. A value of \(f=67\) and a \({T}_{\mathrm{sg}} \sim 2\) K have been reported by Nirmala et al. 12 for \(x=0\), slightly lower than the values of 69–78 ( \({T}_{\mathrm{sg}}\sim 1.82\) K) we achieved for a sample with the same chemistry. We note a linear trend in magnetic frustration associated with the relocation of the Cu ion as Al is replaced with Ga. Extrapolating from this trend, we predict a frustration value of ~ 110 for \(x=0.2\) and a \({T}_{\mathrm{sg}}\)~1.5 K. Such a high frustration value makes this instance of the solid solution a candidate for additional investigation as a potential quantum spin glass or spin liquid.
Experimental details
Synthesis
The solid solution CuAl2(1−x)Ga2xO4 (\(x = 0\) to \(1\) in steps of 0.1) was created by mixing stoichiometric amounts of CuO, Al2O3, and Ga2O3, and allowing them to react at \(950^\circ\) C for 100 h in atmosphere in alumina crucibles. The reactants were supplied by Alfa Aesar and possess a purity \(\ge 99.99 \%\). These smaller batches (< 1 g) were used for high-resolution structural analysis via synchrotron diffraction and magnetic analysis. A second set of samples (> 4 g per sample), with compositions corresponding to increments of \(x=0.2\), were synthesized at \(1000^\circ\) C for 20 h. These samples were used for our neutron diffraction studies. In all cases, phase purity was initially assessed via X-ray powder diffraction (Bruker D8 Discover, Cu Kα = 1.5408 Å). Trace amounts of CuO and GaO impurities were indexed and included in Rietveld refinement via the Fullprof suite48.
Structural characterization
High-resolution synchrotron X-ray powder diffraction data were collected at room temperature (\(295\) K) at the Advanced Photon Source at Argonne National Laboratory using beamline 11-BM (\(\lambda =0.412826 \mbox {\normalfont \AA}\)). Neutron powder diffraction measurements were performed at Oak Ridge National Laboratory using two complementary techniques: time of flight data were collected on the BL-11A POWGEN instrument at the Spallation Neutron Source and constant-wavelength neutron powder diffraction data were collected at the High-Flux Isotope Reactor facility (HFIR) using HB-2A49. Rietveld refinements were performed on all collected data using the Fullprof refinement software suite 48.
Magnetic characterization
Both ac and dc magnetic measurements were performed with an Evercool Physical Property Measurement System (PPMS, Quantum Design) using the vibrating sample magnetometer (VSM) and ac measurement system (ACMS II) options, respectively. Parameters particular to various magnetic measurements performed in this study are discussed in the appropriate Results subsections.
References
Balents, L. Spin liquids in frustrated magnets. Nature 464, 199–208 (2010).
Anderson, P. W. Ordering and antiferromagnetism in ferrites. Phys. Rev. 102, 1008 (1956).
Martinho, H. et al. Magnetic properties of the frustrated antiferromagnetic spinel ZnCr2O4 and the spin-glass Zn1 − x CdxCr2 O4 ( x = 0.05 , 0.10 ). Phys. Rev. B 64, 024408 (2001).
Zhang, Z. et al. Local order and frustration in the geometrically frustrated spinels Cd1−xZnxV2O4. Phys. Rev. B 74, 014108 (2006).
Dey, K., Majumdar, S. & Giri, S. Ferroelectricity in spiral short-range-ordered magnetic state of spinel MnCr2O4: Significance of topological frustration and magnetoelastic coupling. Phys. Rev. B 90, 184424 (2014).
Fritsch, V. et al. Spin and orbital frustration in MnSc2S 4 and FeSc2S4. Phys. Rev. Lett. 92, 116401 (2004).
Nair, H. S., K., R. K. & Strydom, A. M. Spin freezing in the spin-liquid compound FeAl2O4. Phys. Rev. B 91, 054423 (2015).
Zaharko, O. et al. Spin liquid in a single crystal of the frustrated diamond lattice antiferromagnet CoAl2O4. Phys. Rev. B 84, 094403 (2011).
Bergman, D., Alicea, J., Gull, E., Trebst, S. & Balents, L. Order-by-disorder and spiral spin-liquid in frustrated diamond-lattice antiferromagnets. Nat. Phys. 3, 487–491 (2007).
Pradhan, S. K., Dalal, B., Sarkar, A. & De, S. K. Spectroscopic and magnetic investigations of a spin-frustrated Mn-doped CoAl2O4 spinel. Phys. Chem. Chem. Phys. 21, 842–850 (2019).
Bag, P., Baral, P. R. & Nath, R. Cluster spin-glass behavior and memory effect in Cr0.5Fe0.5Ga. Phys. Rev. B 98 (2018).
Nirmala, R. et al. Spin glass behavior in frustrated quantum spin system CuAl2O4 with a possible orbital liquid state. J. Phys. Condens. Matter 29, 13LT01 (2017).
Cho, H. et al. Dynamic spin fluctuations in the frustrated A-site spinel CuAl2O4. Phys. Rev. B 102, 014439 (2020).
O’Neill, H. St. C., James, M., Dollase, W. A. & Redfern, S. A. T. Temperature dependence of the cation distribution in CuAl2O4 spinel. Eur. J. Miner. 17, 581–586 (2005).
Gonzalez, J. M. R. & Arean, C. O. X-ray diffraction determination of the cation distribution and oxygen positional parameter in polycrystalline spinels. J. Chem. Soc. Dalton Trans. 2155 (1985).
Petrakovskii, G. A. et al. Spin-glass state in CuGa2O4. Phys. Rev. B 63, 184425 (2001).
Fenner, L. A., Wills, A. S., Bramwell, S. T., Dahlberg, M. & Schiffer, P. Zero-point entropy of the spinel spin glasses CuGa2O4 and CuAl2O4. J. Phys. Conf. Ser. 145, 012029 (2009).
Mukherjee, S., Ranganathan, R., Anilkumar, P. S. & Joy, P. A. Static and dynamic response of cluster glass in La0.5Sr0.5CoO3. Phys. Rev. B 54, 9267–9274 (1996).
Ashcroft, N. & Mermin, N. Solid State Physics. (Saunders College, 1976).
Melot, B. C. et al. Magnetic frustration on the diamond lattice of the A-site magnetic spinels CoAl2−xGaxO4 : The role of lattice expansion and site disorder. Phys. Rev. B 80 (2009).
de Almeida, J. R. & Thouless, D. J. Stability of the Sherrington-Kirkpatrci solution of a spin glass model. J. Phys. Math. Gen. 11, 983 (1977).
Gabay, M. & Toulouse, G. Coexistence of spin-glass and ferromagnetic orderings. Phys. Rev. Lett. 47, 201–204 (1981).
Malozemoff, A. P., Barnes, S. E. & Barbara, B. Non-mean-field interpretation of the irreversibility line in spin-glasses. Phys. Rev. Lett. 51, 1704 (1983).
Berton, A., Chaussy, J., Odin, J., Rammal, R. & Tournier, R. Magnetocaloric investigation of (H, T) phase diagram of CuMn spin glass. J. Phys. Lett. 43, 153–158 (1982).
Malinowski, A. et al. Spin-glass behavior in Ni-doped La1.85Sr0.15CuO4. Phys. Rev. B 84, 024409 (2011).
Lago, J., Blundell, S. J., Eguia, A., Jansen, M. & Rojo, T. Three-dimensional Heisenberg spin-glass behavior in SrFe0.90Co0.10O3.0. Phys. Rev. B 86, 064412 (2012).
Mydosh, J. A. Spin Glasses: An Experimental Introduction. (Taylor & Francis, 1993).
Patra, M., Majumdar, S. & Giri, S. Grain size effect on the magnetic cluster-glass properties of La 0.88 Sr 0.12 CoO 3. J. Phys. Condens. Matter 22, 116001 (2010).
Souletie, J. & Tholence, J. L. Critical slowing down in spin glasses and other glasses: Fulcher versus power law. Phys. Rev. B 32, 516–519 (1985).
Sandlund, L. et al. Dynamics of Cu-Mn spin-glass films. Phys. Rev. B 40, 869–872 (1989).
Bontemps, N., Rajchenbach, J., Chamberlin, R. V. & Orbach, R. Dynamic scaling in the Eu0.4Sr0.6S spin-glass. Phys. Rev. B 30, 6514–6520 (1984).
Maji, B., Suresh, K. G. & Nigam, A. K. Low temperature cluster glass behavior in Nd5Ge3. J. Phys. Condens. Matter 23, 506002 (2011).
Kumar, V. et al. Evidence for spin glass state of NdCo1−xNixO3 ( x = 0.3−0.5). J. Appl. Phys. 116, 073903 (2014).
Vincent, E., Hammann, J. & Alba, M. Dynamical critical behavior of the CdCr2x0.85In2x0.15S4 spin-glass. Solid State Commun. 58, 57 (1986).
Eftimova, K. & Laiho, R. Low-temperature dynamic magnetic properties of PdxCo50 Al50-x alloys. J. Magn. Magn. Mater. 166, 179 (1997).
Mantilla, J., Ter Haar, E., Coaquira, J. A. H. & Bindilatti, V. Dynamic susceptibility measurements at the spin-glass transition in the Zn1−xMnxIn2Se4 semiconductor. J. Phys. Condens. Matter 19, 386225 (2007).
King, A. R., Mydosh, J. A. & Jaccarino, V. Ac susceptibility study of the d = 3 random-field critical dynamics. Phys. Rev. Lett. 56, 2525–2528 (1986).
Gunnarsson, K. et al. Dynamics of an Ising spin-glass in the vicinity of the spin-glass temperature. Phys. Rev. Lett. 61, 754–757 (1988).
Dieny, B. & Barbara, B. Critical properties of a random-anisotropy system a−DyNi : A new universality class in disordered systems?. Phys. Rev. Lett. 57, 1169–1172 (1986).
Barbara, B., Malozemoff, A. P. & Imry, Y. Scaling of nonlinear susceptibility in MnCu and GdAl spin-glasses. Phys. Rev. Lett. 47, 1852–1855 (1981).
Paulsen, C. C., Williamson, S. J. & Maletta, H. Evidence for a phase transition in the spin glass Eu0.4Sr0.6S from dynamic susceptibility measurements. Phys. Rev. Lett. 59, 128–131 (1987).
Fischer, K. H. Dynamics of vector spin glasses with random anisotropy. Z. Phys. B Condens. Matter 60, 151–159 (1985).
Maignan, A., Martin, C., Damay, F., Raveau, B. & Hejtmanek, J. Transition from a paramagnetic metallic to a cluster glass metallic state in electron-doped perovskite manganites. Phys. Rev. B 58, 2758–2763 (1998).
Khan, N., Midya, A., Mandal, P. & Prabhakaran, D. Critical exponents and irreversibility lines of La0.9Sr0.1CoO3 single crystal. J. Appl. Phys. 113, 183909 (2013).
Zakrzewski, A. V. et al. Evolution of magnetic and orbital properties in the magnetically diluted A-site spinel Cu1 − xZnxRh2O4. Phys. Rev. B 97, 214411 (2018).
Hanashima, K., Kodama, Y., Akahoshi, D., Kanadani, C. & Saito, T. Spin glass order by antisite disorder in the highly frustrated spinel oxide CoAl2O4. J. Phys. Soc. Jpn. 82, 024702 (2013).
Nair, H. S., Fu, Z., Voigt, J., Su, Y. & Brückel, Th. Approaching the true ground state of frustrated A-site spinels: A combined magnetization and polarized neutron scattering study. Phys. Rev. B 89 (2014).
Rodríguez-Carvajal, J. Recent advances in magnetic structure determination by neutron powder diffraction. Phys. B Condens. Matter 192, 55–69 (1993).
Calder, S. et al. A suite-level review of the neutron powder diffraction instruments at Oak Ridge National Laboratory. Rev. Sci. Instrum. 89, 092701 (2018).
Acknowledgements
This work was supported by the Air Force Office of Scientific Research (AFOSR) LRIR Grant #’s 14RQ08COR, 18RQCOR100, and 19RXCOR052, the AFRL Aerospace Systems Directorate (AFRL/RQ), and the National Research Council (NRC). Part of this research was conducted at the Oak Ridge National Laboratory (ORNL) High Flux Reactor and Spallation Neutron Source, which was sponsored by the Scientific User Facilities Division Office of Basic Energy Sciences, U.S. Department of Energy (DOE). ORNL is managed by UT-Battelle, LLC under contract No. DE-AC05-00OR22725 for the U.S. DOE. This research also used resources of the Advanced Photon Source; a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. This research was performed while J. A. B. and M. A. S. held NRC Research Associateship awards at AFRL/RQ.
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T.B. and T.H. initiated the concept. T.B. synthesized the solid solution and performed the magnetic measurements. T.B. and M.S. performed the data analysis on the magnetic measurements. T.B., M.S., K. T., and J. B. performed the diffraction measurements. M.S. and T.B. analyzed the diffraction data. T.B., M.S. and T.H. wrote the main manuscript.
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Bullard, T.J., Susner, M.A., Taddei, K.M. et al. Magnetic and structural properties of the solid solution CuAl2(1−x)Ga2xO4. Sci Rep 11, 11355 (2021). https://doi.org/10.1038/s41598-021-89197-1
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DOI: https://doi.org/10.1038/s41598-021-89197-1
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