Jahn–Teller distortions in (Co_1–xCu_x)Cr₂O₄ (x = 0.5, 0.25) nanoparticles: Structural, magnetic and electronic properties

Multiferroic CoCr₂O₄ forms a normal cubic spinel structure of the form AB₂O₄ (A = tetrahedral site occupied by Co²⁺ ions, B = octahedral site occupied by Cr³⁺ ions).¹ The compound undergoes a magnetic phase transition to ferrimagnetic ordering from paramagnetic phase at Curie temperature, T_C ≈93 K.¹ The most fascinating features of this compound relates to the magnetic transitions that occur in the temperature range below the T_C. These transitions observed at the temperatures associated with the spiral ordering and lock-in transitions, that occur at T_S ≈ 25 K and T_L ≈ 15 K.¹ Experimental findings evidenced that the origin of the multiferroicity stems from the conical spin modulation below T_S.¹ In order to examine the low temperature magnetic structure, temperature dependent neutron diffraction measurements of the polycrystalline samples were carried out.² Lawes et al.² confirmed the stability of the cubic phase up to 11 K and indicated the signature of complex magnetic ordering in this compound.² The observed low temperature magnetic transitions were also substantiated through heat capacity (C_p) and dielectric measurements.² 

On the other hand CuCr₂O₄, having a normal spinel structure with Cu²⁺ at A site, demonstrates orbital ordering.³ The structure undergoes a cubic to compressed tetragonal symmetry on cooling at 850 K (= T_JT) because of the cooperative Jahn-Teller (J-T) ordering, and a tetragonal to orthorhombic structural change below 130 Kat the onset of its transition from paramagnetic to ferrimagnetic regime, at the Curie temperature (T_C).^3,4 The Cu²⁺ and Cr³⁺ ions have 3d⁹ and 3d³spin configurations, respectively. The cooperative J-T effect destabilizes the crystal structure leading to a splitting of the triply degenerate t_2g orbital into a non-degenerate orbital at elevated energy and doubly degenerate orbitals at lower energy.³ The orbital ordering in CuCr₂O₄ is expected because of the existence of unpaired 3d electrons related to the Cu²⁺ ion occupying the xy orbital.³ The orthorhombic crystal distortion of the J-T effect induced tetragonal CuCr₂O₄ phase persists concomitantly with ferrimagnetic phase transition at 130 K, as evidenced by temperature dependent high-resolution synchrotron XRD results.⁴ C_p measurements of CuCr₂O₄ show a transition at 130 K and an additional anomaly at 155 K, indicating a second magnetic phase.⁴ Tomiyasu et al.⁵ explored this new magnetic structure by conducting neutron diffraction studies within 155 K and 125 K and described it as almost collinear magnetic arrangement in pyrochlore lattice formed by Cr. However, under 125 K, a ferrimagnetic component of non-collinear nature evolves due to the Cu-Cr interaction.⁵ 

Looking at the uniqueness of both CoCr₂O₄ and CuCr₂O₄ motivated the current work to investigate the crystal structure, magnetism and electronic behaviour with Co site substituted by J-T active Cu ions. Besides the effect of cationic site substitution, the possible role of particle size on crystal structure is also investigated.

Chemical sol-gel method was used to synthesize powders of (Co_1−xCu_x)Cr₂O₄ nanoparticles, with x = 0.25, 0.5 and 0.75, using 0.5 M stock solutions of commercially available respective metal nitrates. After gelation the gel residue was dried over a hotplate. Separate portions of the dried powder were calcined in a box furnace at 600 °C and 800 °C for 1 hour, respectively. X-ray diffraction (XRD) with Cu–K_α radiation was employed for structural characterization and phase identification and microstructure was analyzed using transmission electron microscopy (TEM). Magnetic field and temperature dependent magnetization studies were carried with a 14 T Cryogenic Measurement System having VSM insert. Room temperature electronic properties were probed using X-ray photoelectron spectroscopy (XPS) experiments employing a SPECS PHOIBOS 150 electron energy analyser, and monochromatised photon source produced by Al–K_α radiation with energy 1486.71 eV.

Fig. 1(a) depicts the X-ray diffraction (XRD) plots of (Co_1−xCu_x)Cr₂O₄ (x = 0.5 and 0.25) powders calcined at 600 °C that show reflections related to CoCr₂O₄ (JCPDS: 221084) with Fd3m symmetry. The broadening of XRD peaks as observed in the present case is ascribed to the nano size effect.⁶ The Scherrer equation,⁶ was used to calculate the crystallite size for (Co_0.5Cu_0.5)Cr₂O₄ and (Co_0.75Cu_0.25)Cr₂O₄ nanoparticles that turns out to be ∼ 11 nm and ∼ 18 nm, respectively. Careful analyses of the obtained microscopic pictures of (Co_0.5Cu_0.5)Cr₂O₄ and (Co_0.75Cu_0.25)Cr₂O₄ powder samples confirm the non-uniform size distribution (Figs. 1(b) and (c)). Formation of ring like shapes composed of spots as observed in selected area electron diffraction (SAED) (insets of Figs. 1(b) and (c)) confirms crystalline nature of the nanoparticles. Estimation of the average particle sizes, obtained from the TEM results in the present work, yields 32(6) nm for (Co_0.5Cu_0.5)Cr₂O₄ and 20(8) nm for (Co_0.75Cu_0.25)Cr₂O₄ nanoparticles. Comparison of the particle size with the crystallite size indicates the polycrystalline nature of (Co_0.5Cu_0.5)Cr₂O₄ particles.

Here it is important to note that the CuCr₂O₄ prefers to crystallize in the tetrahedral structure below 850 K due to the cooperative J-T effect.^3,4 Increasing the Cu content (x = 0.75) resulted in the onset of the tetragonal phase; as indicated from the splitting of (220) peak for the sample calcined at 600 °C (Fig. 2(a)).⁴ To probe the role of calcination temperature on structure, the three powder samples were further calcined at 800 °C. The selected XRD pattern around (220) plane is shown in Fig. 2(b). It is noted from this that the (Co_0.75Cu_0.25)Cr₂O₄ sample does not show any splitting of the peak related (220) reflection which is indicative of J-T distortion despite calcination.⁴ It is observed in Fig. 2 that calcination of the (Co_0.5Cu_0.5)Cr₂O₄ sample at 800 °C results in the splitting of the (220) peak. Consequently, it is evident that both Co concentration and calcination temperature determines the crystal structure of this kind of chromite samples.

As the XRD results provide evidence of the unusual cubic phase retention of the x = 0.5 and 0.25 samples calcined at 600 °C further magnetic and electronic properties will focus only on these samples. Temperature $T$ dependent magnetization $M$ experiments were done for both zero (M_ZFC) and field (M_FC) cooled conditions. For both measurements the applied probing field was 0.1 T. Figs. 3(a) and (b) show the temperature dependent magnetization data obtained for the samples (Co_0.5Cu_0.5)Cr₂O₄ and (Co_0.75Cu_0.25)Cr₂O₄ calcined at 600 °C, where T_C was calculated following the reported methods^7–9 and found to be 103 K and 99 K, respectively. M_ZFC for both samples becomes negative at a particular temperature identified as the compensation point, T_comp, below T_C.^10,11 (Co_0.5Cu_0.5)Cr₂O₄ compound shows T_comp ≈ 84 K and (Co_0.75Cu_0.25)Cr₂O₄ sample indicates T_comp ≈ 79 K (Figs. 3(a) and (b)). The feature related to T_S becomes prominent for (Co_0.75Cu_0.25)Cr₂O₄ (Fig. 3(b)). The difference between M_FC and M_ZFC is significant for x = 0.5, indicating more frustration in the system when compared to x = 0.25.⁸ Hence, it appears that the frustration possibly drives the cubic to tetragonal phase transition for samples calcined at 800 °C.^4,5

Additionally, M(µ₀H) measurements were done at distinct temperatures and are depicted in Figs. 3(c) and (d). Significant enhancement of coercivity occurs with increasing x from 0.25 to 0.5, however, the magnetization does not saturate in line with previous findings for undoped and Ni substituted CoCr₂O₄ nanoparticles.^11,12 The ferrimagnetic hysteresis observed at 10 K for (Co_0.5Cu_0.5)Cr₂O₄ shows an abrupt kink marked by arrows in Fig. 3(c) around zero field that can be compared to the observation of a wasp-waist like feature as detected in Ni substituted CoCr₂O₄ nanoparticles.¹¹ With increasing temperature the coercivity decreases and at 103 K (close to the T_C) the M(μ₀H) plot indicates a superparamagnetic phase with non-linear behaviour of magnetization and zero-coercivity.^11,12

The asymmetric feature seen in the hysteresis loops at 10 K can arise due to several reasons such as the bi- or multimodal distribution of magnetic granules with dissimilar coercivities,¹³ the concurrence of hard and soft kind of magnetic entities,¹⁴ or due to the differences in the super-exchange interaction that can result as a consequence of spinel structure.¹⁵ In a similar spinel compound CoFe₂O₄ nanoparticles this behaviour was attributed to the presence of particles belonging to two separate groups with dissimilar anisotropies,¹⁶ or the presence of canted spins that can favour the spin re-orientation transitions that can occur at low temperature.¹⁷ Both the samples demonstrate decrease in coercivity with advancement in temperature and the hysteresis loops approach a superparamagnetic type curve when measured at temperature less than T_C.^11,12 However, in the current scenario the occurrence of an anomaly in the hysteresis at 10 K is attributed to the Cu substitution that resembles the characteristic feature of CuCr₂O₄ (Fig. 3(d)).⁵ 

Substituting Cu at the Co site leads to an enhancement in the probability that the cationic oxidation state can change and that can alter the magnetic exchange interactions.¹¹ Fig. 4 show the 2p XPS core levels of Co and Cu. The spin orbit splitting (SOS) ≥ 15.2 eV for Co 2p (Fig. 4(a)) and the presence of a strong satellite confirm the dominance of the Co²⁺ oxidation state.^18,19 However, the presence of weak satellites at a binding energy (BE) of approximately 10 eV higher than the main spin orbit peaks, suggests the presence of fractional Co³⁺ ions. In the case of the Cu 2p core level, the presence of a broad O KVV Auger line with a centroid at ∼ 975 eV BE significantly affects the shape of the background in the Cu 2p_1/2 BE region (not shown here). Therefore the focus is on the Cu 2p_3/2 BE region (Fig 4(b)). The main peak is composed of a more intense feature at ∼ 933.6 eV with a clear shoulder on the lower BE side corresponding to 2+ and 1+/0+ oxidation states, respectively. The strong characteristic satellite feature centred at ∼ 940.5 eV confirms the Cu²⁺ majority oxidation state in both compounds.¹⁸ The majority of Cr ions are found to be in the 3+ oxidation state in both compounds (not shown in this work).

Smart and Greenwald,²⁰ showed that different exchange coupling mechanisms in the distorted orthorhombic structure play a key role in the magnetostructural coupling. As observed, antiferromagnetism can only be achieved by two conditions.²⁰ Firstly, the crystal structure should have two magnetic sublattices, A having nearest neighbours only on sublattice B, and vice versa.²⁰ Secondly, the exchange integral, J, between neighbouring magnetic atoms should be negative leading to an antiparallel alignment.²⁰ For a particular temperature perfect antiparallel ordering is highly sensitive towards thermal agitation.²⁰ The value of J is a function of interatomic separation, r. In addition, in the process of building the crystal, the exchange interaction will arrange the atoms to increase $J$⁠, consequently the magnetic atoms either pull closer together or push apart on the basis of the sign of dJ/dr.¹⁹ Since the number of nearest neighbours increases with decreasing temperature, more deformation is expected at low temperature.²⁰ Hence, the population of Co atoms at A sites manipulates the exchange interaction so that it retains the cubic phase up to x = 0.75.

Sol-gel method was used to synthesize nanosized (Co_1−xCu_x)Cr₂O₄ with x = 0.5, 0.25 and 0.75. XRD results confirm crystallinity and cubic phase retention up to x = 0.5 for the samples calcined at 600 °C. Further increasing x or the calcination temperature to 800 °C leads to a tetragonal distortion in crystal structure (for x ≥ 0.5) due to J-T effect. Ferrimagnetic T_C of (Co_0.5Cu_0.5)Cr₂O₄ samples was observed to be higher in comparison to the (Co_0.75Cu_0.25)Cr₂O₄ powders. Ferrimagnetism persist below T < T_C for both the samples having a non-saturated magnetization. M(μ₀H) experiments performed at 10 K for both the samples demonstrated a kink-like feature which is attributed to CuCr₂O₄. In addition, a superparamagnetic feature was observed at the onset of T_C. XPS data revealed that Co and Cu core levels contain majority 2+ oxidation states of the cations. The cubic phase, as well as the T_C, is found to be strongly controlled by the A site cations.

Financial grant from the SANRF (Grant No. 93551, 120856, 126978, 93205, 119314, 90698, 126911), and the funding from URC and FRC of University of Johannesburg (UJ) is acknowledged. The utilization of the NEP Cryogenic Physical Properties Measurement System at UJ, procured from the funding of SANRF (Grant No: 88080) and UJ, South Africa, is acknowledged. Authors thank the Spectrum Analytical Facility within FoS at UJ for characterizations.

The data that support the findings of this study are available from the corresponding author upon reasonable request.