Neutron diffraction evidence for local spin canting, weak Jahn–Teller distortion, and magnetic compensation in Ti_1−x Mn _x Co₂O₄ spinel

Abstract

A systematic study using neutron diffraction and magnetic susceptibility are reported on Mn substituted ferrimagnetic inverse spinel Ti 1−x MnxCo 2 O 4 in the temperature interval 1.6 K ≤ T ≤ 300 K. Our neutron diffraction study reveals cooperative distortions of the T O 6 octahedra in Ti 1−x MnxCo 2 O 4 system for all the Jahn-Teller active ions T = Mn 3+ , Ti 3+ and Co 3+ , having the electronic configurations 3d 1 , 3d 4 and 3d 6 , respectively which are confirmed by the Xray photoelectron spectroscopy. Two specific compositions (x = 0.2 and 0.4) have been chosen in this study because these two systems show unique features such as; (i) Noncollinear Yafet-Kittel type magnetic ordering, and (ii) Weak tetragonal distortion with c/a < 1, in which the apical bond length dc(T B -O) is longer than the equatorial bond length d ab (T B -O) due to the splitting of the eg level of Mn 3+ ions into d x 2 −y 2 and d z 2 . For the composition x = 0.4, the distortion in the T B O 6 octahedra is stronger as compared to x = 0.2 because of the higher content of trivalent Mn. Ferrimagnetic ordering in Ti 0.6 Mn 0.4 Co 2 O 4 and Ti 0.8 Mn 0.2 Co 2 O 4 sets in at 110.3 and 78.2 K, respectively due to the presence of unequal magnetic moments of cations, where Ti 3+ , Mn 3+ , and Co 3+ occupying the octahedral, whereas, Co 2+ sits in the tetrahedral site. For both compounds an additional weak antiferromagnetic component could be observed lying perpendicular to the ferrimagnetic component. The analysis of static and dynamic magnetic susceptibilities combined with the heat-capacity data reveals a magnetic compensation phenomenon (MCP) at T COMP = 25.4 K in Ti 0.8 Mn 0.2 Co 2 O 4 and a reentrant spin-glass behaviour in Ti 0.6 Mn 0.4 Co 2 O 4 with a freezing temperature ∼110.1 K. The MCP in this compound is characterized by sign reversal of magnetization and bipolar exchange bias effect below T COMP with its magnitude depending on the direction of external magnetic field and the cooling protocol. arXiv:2003.09308v1 [cond-mat.str-el]

Magnetic properties of Sn- and Mn-incorporated Co2TiO4 from single-step calcination

“…Sn 4+ (0.69 Å)-ion has a larger ionic radius than Ti 4+ , and the lattice expands (1.1%) in CSTO compared to that in CTO. 15 Since the ionic radius of Mn 4+ (0.53 Å, VI coordination) ions is smaller than Ti 4+ (0.60 Å, VI coordination), the lattice contracts (1.6%) in CMTO, which is almost double as compared to the reported values by Pramanik et al 31 This is due to the presence of Mn 4+ in CMTO differing from the report by Pramanik et al 31 where the presence of Mn 3+ resulted in the Jahn-Teller distorted spinel.…”

“…From the site occupancies after the final cycle of refinement, the samples have the compositions of (Co 0.99 )[Co ] is the cation distribution. 31 This difference is primarily attributed to the wet chemical route adopted in our study compared to the conventional solid-state method. 31 The refined lattice parameter values and other details of structural refinements are listed in Tables 1 and 2.…”

“…31 This difference is primarily attributed to the wet chemical route adopted in our study compared to the conventional solid-state method. 31 The refined lattice parameter values and other details of structural refinements are listed in Tables 1 and 2. In our case, the lattice parameter of CTO is 8.3730 (22) Å, lower than the value listed in the PDF entry , and it may be due to the cobalt excess scenario in Co 3−x TiO 4 .…”

“…These characteristics confirm divalent cobalt's presence in the sample. 31 The existence of peaks with binding energies of 463.7 and 458.0 eV suggests trivalent titanium (Fig. 5f ).…”

Lattice dynamics and magnetic exchange interactions in GeCo2O4, a spinel with S = 1/2 pyrochlore lattice

“…The closeness between the magnetic and structural transition temperatures reveals the existence of competing spin-orbit coupling and Jahn-Teller effects in GCO [26,31,32]. Currently, there exists a fair amount of debate on the fact that T S is below the T N , which is uncommon when compared to other spinels that exhibit magnetostructural quantum phase transitions [33][34][35][36][37][38][39][40][41][42][43][44][45].…”