Antiferromagnetism, spin-glass state, H-T phase diagram, and inverse magnetocaloric effect in Co2RuO4

Interplay of lattice-spin-orbital coupling and Jahn-Teller effect in noncollinear spinel TixMn1-x(FeyCo1-y)2O4: a neutron diffraction study

Local magnetostructural changes and dynamical spin fluctuations in doubly diluted spinel TixMn1‒x(FeyCo1‒y)2O4has been reported by means of neutron diffraction and magnetization studies. Two distinct sets of compositions (i)x(Ti) = 0.20 andy(Fe) = 0.18; (ii)x(Ti) = 0.40 andy(Fe) = 0.435 have been considered for this study. The first compound of equivalent stoichiometry Ti0.20Mn0.80Fe0.36Co1.64O4exhibits enhanced tetragonal distortion across the ferrimagnetic transition temperatureTC= 258 K in comparison to the end compound MnCo2O4(TC∼ 180 K) with a characteristic ratioct/√2atof 0.99795(8) demonstrating robust lattice-spin-orbital coupling. However, in the second case Ti0.40Mn0.60Fe0.87Co1.13O4with higherB-site compositions, the presence of Jahn-Teller ions with distinct behavior appears to counterbalance the strong tetragonal distortion thereby ceasing the lattice-spin-orbital coupling. Both the investigated systems show the coexistence of noncollinear antiferromagnetic and ferrimagnetic components in cubic and tetragonal settings. On the other hand, the dynamical ac-susceptibility,χac(T) reveals a cluster spin-glass state with Gabay-Toulouse (GT) like mixed phases behaviour belowTC. Such dispersive behaviour appears to be sensitive to the level of octahedral substitution. Further, the field dependence ofχac(T) follows the weak anisotropic GT-line behaviour with crossover exponent Φ lies in the range 1.38-1.52 on theH-Tplane which is in contrast to theB-site Ti substituted MnCo2O4spinel that appears to follow irreversible non-mean-field AT-line behaviour (Φ ∼ 3 +δ). Finally, the Arrott plots analysis indicates the presence of a pseudo first-order like transition (T< 20 K) which is in consonance with and zero crossover of the magnetic entropy change within the frozen spin-glass regime.

Magnetic exchange interactions and non-Debye relaxation in spin-3/2 frustrated Kagomé magnet Co3V2O8

We report the experimental determination of the magnetic exchange parameter (J/kB= 2.88 ± 0.02 K) for the Spin-3/2 ferromagnetic (FM) Kagomé lattice system: Co3V2O8using the temperature dependence of dc-magnetic susceptibilityχ(T) data by employing the fundamental Heisenberg linear chain model. Our results are quite consistent with the theoretically reported nearest neighbor dominant FM exchange coupling strengthJex-NN∼2.45 K. Five different magnetic phase transitions (6.2-11.2 K) and spin-flip transitions (9.6-7.7 kOe) have been probed using the∂(χT)/∂Tvs.T, heat capacity (CP-T) and differential isothermal magnetization curves. Among such sequence of transitions, the prominent ones being incommensurate antiferromagnetic (AFM) state at 11.2 K, commensurate AFM state at 8.8 K, and commensurate FM state across 6.2 K. All the successive magnetic phase transitions have been mapped onto a single H-T plane through which one can easily distinguish the above-mentioned different phases. The magnetic contribution of theCP-TnearTN(11.2 K) has been analyzed using the power-law expressionCM=A|T-TN|-αresulting in the critical exponentα= 0.18 ± 0.01 (0.15 ± 0.003) forTTN), respectively for the Co3V2O8. It is interesting to note that non-Debye type dipole relaxation is quite prominent in Co3V2O8and was evident from the Kohlrausch-Williams-Watts analysis of complex modulus and impedance spectra (0⩽β⩽1). Mott's variable-range hopping of charge carriers process is evident through the resistivity analysis (ρac-T-1/4) in the temperature range 275 ∘C-350 ∘C. Moreover, the frequency-dependent analysis ofσac(ω) follows Jonscher's power law yielding two distinct activation energies (Ea∼0.37 and 2.29 eV) between the temperature range 39 ∘C-99 ∘C and 240 ∘C-321 ∘C.

Probing the strongly correlated magnetic state of Co2C nanoparticles at low temperatures usingμSR

Co2C nanoparticles (NPs) are amongst transition metal carbides whose magnetic properties have not been well explored. An earlier study (Royet al2021J. Phys.: Condens. Matter33375804) showed that a pellet made from Co2C NPs exhibits exchange bias (EB) effect below a temperature,TEB= 50 K and a spin glass (SG) feature emerges belowTSG= 5 K. In the current study we use magnetic, electrical transport, specific heat, and muon spin rotation (μSR) measurements to explore further the magnetic properties of a pellet made with 40 nm diameter pure Co2C NPs. We uncover the onset of Kondo localization at Kondo temperatureTK(= 40.1 K), which is close to the onset temperature (TEB) of the EB effect. A crossover from the Kondo-screened scenario to the Ruderman-Kittel-Kasuya-Yosida interaction-dominated regime is also observed forT Collapse

Key Words

• Kondo effect
• exchange bias
• magnetic nanoparticles
• muon spin relaxation
• spin glass

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Affiliation(s)

Nirmal Roy Indian Institute of Technology Kanpur, Kanpur, Uttar Pradesh 208016, India
P C Mahato Indian Institute of Technology Kanpur, Kanpur, Uttar Pradesh 208016, India
Suprotim Saha Indian Institute of Technology Kanpur, Kanpur, Uttar Pradesh 208016, India
M Telling ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot Oxon OX11 0QX, United Kingdom
J S Lord ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot Oxon OX11 0QX, United Kingdom
D T Adroja ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot Oxon OX11 0QX, United Kingdom Highly Correlated Matter Research Group, Physics Department, University of Johannesburg, PO Box 524, Auckland Park 2006, South Africa
S S Banerjee Indian Institute of Technology Kanpur, Kanpur, Uttar Pradesh 208016, India

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Reentrant canonical spin-glass dynamics and tunable field-induced transitions in (GeMn)Co2O4Kagomé lattice

We report on the reentrant canonical semi spin-glass characteristics and controllable field-induced transitions in distorted Kagomé symmetry of (GeMn)Co2O4. ThisB-site spinel exhibits complicated, yet interesting magnetic behaviour in which the longitudinal ferrimagnetic (FiM) order sets in below the Néel temperatureTFN∼ 77 K due to uneven moments of divalent Co (↑ 5.33μB) and tetravalent Mn (↓ 3.87μB) which coexists with transverse spin-glass state below 72.85 K. Such complicated magnetic behaviour is suggested to result from the competing anisotropic superexchange interactions (JAB/kB∼ 4.3 K,JAA/kB∼ -6.2 K andJBB/kB∼ -3.3 K) between the cations, which is extracted following the Néel's expression for the two-sublattice model of FiM. Dynamical susceptibility (χac(f, T)) and relaxation of thermoremanent magnetization,MTRM(t) data have been analysed by means of the empirical scaling-laws such as Vogel-Fulcher law and Power law of critical slowing down. Both of which reveal the reentrant spin-glass like character which evolves through a number of intermediate metastable states. The magnitude of Mydosh parameter (Ω ∼ 0.002), critical exponentzυ= (6.7 ± 0.07), spin relaxation timeτ0= (2.33 ± 0.1) × 10-18s, activation energyEa/kB= (69.8 ± 0.95) K and interparticle interaction strength (T0= 71.6 K) provide the experimental evidences for canonical spin-glass state below the spin freezing temperatureTF= 72.85 K. The field dependence ofTFobtained fromχac(T) follows the irreversibility in terms of de Almeida-Thouless mean-field instability in which the magnitude of crossover scaling exponent Φ turns out to be ∼2.9 for the (Ge0.8Mn0.2)Co2O4. Isothermal magnetization plots reveal two field-induced transitions across 9.52 kOe (HSF1) and 45.6 kOe (HSF2) associated with the FiM domains and spin-flip transition, respectively. Analysis of the inverse paramagnetic susceptibilityχp-1χp=χ-χ0after subtracting the temperature independent diamagnetic termχ0(=-3 × 10-3emu mol-1Oe-1) results in the effective magnetic momentμeff= 7.654μB/f.u. This agrees well with the theoretically obtainedμeff= 7.58μB/f.u. resulting the cation distributionMn0.24+↓A[Co22+↑]BO4in support of the Hund's ground state spin configurationS=3/2andS= 1/2of Mn4+and Co2+, respectively. TheH-Tphase diagram has been established by analysing all the parameters (TF(H),TFN(H),HSF1(T) andHSF2(T)) extracted from various magnetization measurements. This diagram enables clear differentiation among the different phases of the (GeMn)Co2O4and also illustrates the demarcation between short-range and long-range ordered regions.

Antiferromagnetic short-range order and cluster spin-glass state in diluted spinel ZnTiCoO4

The nature of magnetism in the doubly-diluted spinel ZnTiCoO₄= (Zn²⁺)_A[Ti⁴⁺Co²⁺]_BO₄is reported here employing the temperature and magnetic field (H) dependence of dc susceptibility (χ), ac susceptibilities (χ' andχ″), and heat capacity (C_p) measurements. Whereas antiferromagnetic (AFM) Néel temperatureT_N= 13.9 K is determined from the peak in the ∂(χT)/∂TvsTplot, the fit of the relaxation timeτ(determined from the peak in theχ″ vsTdata at different frequencies) to the Power law:τ=τ₀[(T-T_SG)/T_SG]^-zνyields the spin glass freezing temperatureT_SG= 12.9 K,zν∼ 11.75, andτ₀∼ 10^-12s. Since the magnitudes ofτ₀andzνdepend on the magnitude ofT_SG, a procedure is developed to find the optimum value ofT_SG= 12.9 K. A similar procedure is used to determine the optimumT₀= 10.9 K in the Vogel-Fulcher law:τ=τ₀ exp[E_a/k_B(T-T₀)] yieldingE_a/k_B= 95 K, andτ₀= 1.6 ×  10^-13s. It is argued that the comparatively large magnitude of the Mydosh parameter Ω = 0.026 andk_BT₀/E_a= 0.115 (≪1) suggests cluster spin-glass state in ZnTiCoO₄below T_SG. In theC_pvsTdata from 1.9 K to 50 K, only a broad peak near 20 K is observed. This and absence ofλ-type anomaly nearT_NorT_SGcombined with the reduced value of change in magnetic entropy from 50 K to 1.9 K suggests only short-range AFM ordering in the system, consistent with spin-glass state. The field dependence ofT_SGshows slight departure (ϕ∼ 4.0) from the non-mean-field Almeida-Thouless lineT_SG(H) =T_SG(0) (1 -AH^2/ϕ). Strong temperature dependence of magnetic viscositySand coercivityH_Cwithout exchange bias, both tending to zero on approach toT_SGfrom below, further support the spin-glass state which results from magnetic dilution driven by diamagnetic Zn²⁺and Ti⁴⁺ions leading to magnetic frustration. Magnetic phase diagram in theH-Tplane is established using the high-field magnetization dataM(H,T) forT<T_Nwhich reveals rapid decrease ofT_SGwith increase inHwhereas decrease inT_Nwith increase inHis weaker, typical of AFM systems. ForT>T_N, the data ofχvsTare fit to the modified Curie-Weiss law,χ=χ₀+C/(T+θ), withχ₀= 3.2 × 10^-4emu mol^-1Oe^-1yieldingθ= 4 K andC= 2.70 emu K mol^-1Oe^-1. This magnitude ofCyields effective magnetic moment = 4.65μ_Bfor Co²⁺, characteristic of Co²⁺ions with some contribution from spin-orbit coupling. Molecular field theory with effective spinS= 3/2 of Co²⁺is used to determine the nearest-neighbor exchange constantJ₁/k_B= 2.39 K AFM and next-nearest-neighbor exchange constantJ₂/k_B= -0.66 K (ferromagnetic).

Anisotropic thermal expansion and electronic transitions in the Co3BO5 ludwigite

The investigations of the crystal structure, magnetic and electronic properties of Co₃BO₅ at high temperatures were carried out using powder X-ray diffraction, magnetic susceptibility, electrical resistivity, and thermopower measurements. The orthorhombic symmetry (Sp.gr. Pbam) was observed at 300 K and no evidence of structural phase transitions was found up to 1000 K. The compound shows a strong anisotropy of the thermal expansion. A large negative thermal expansion along the a-axis is observed over a wide temperature range (T = 300-600 K) with α_a = -35 M K^-1 at T = 500 K with simultaneous expansion along the b- and c-axes with α_b = 70 M K^-1 and α_c = 110 M K^-1, respectively. The mechanisms of thermal expansion are explored by structural analysis. The activation energy of the conductivity decreases significantly above 700 K. Electronic transport was found to be a dominant conduction mechanism in the entire temperature range. The correlations between the thermal expansion, electrical resistivity, and effective magnetic moment were revealed and attributed to the evolution of the spin state of Co³⁺ ions towards the spin crossover and gradual charge-ordering transition.

Tricritical-point phase diagram in PrCu9Sn4

Tricritical phenomenon appearing in multiple phases is a fundamental and attractive issue in condensed-matter physics. In this work, a field-modulated tricritical phenomenon is realized in single-crystal PrCu₉Sn₄. The magnetization under variable directions of field indicates strong magnetic anisotropy in PrCu₉Sn₄, which reveals ferromagnetic coupling forH//c. A paramagnetic-to-ferromagnetic magnetic transition occurs withH//catT_C= 11.7 K, which is evidenced to be of a first-ordered type. The systematical study of the critical behavior gives thatβ= 0.195(8),γ= 0.911(1), andδ= 0.0592(1) forH//cconsistent with a tricritical mean-field model, which suggests a field-modulated tricritical phenomenon. A detailedH-Tphase diagram around the tricritical point (TCP) is constructed for single-crystal PrCu₉Sn₄forH//c, where ferromagnetic state, forced ferromagnetic phase and paramagnetic state meet at the TCP (H_tr= 799 kOe,T_tr= 11.3 K). The single-crystal PrCu₉Sn₄supplies a platform to deep investigate the field-modulated magnetic couplings and tricritical phenomenon.

Determination of the tricritical point,H-Tphase diagram and exchange interactions in the antiferromagnet MnTa2O6

Using the analysis of the temperature and magnetic field dependence of the magnetization (M) measured in the temperature range of 1.5 K to 400 K in magnetic fields up to 250 kOe, the magnetic field-temperature (H-T) phase diagram, tricritical point and exchange constants of the antiferromagnetic MnTa₂O₆are determined in this work. X-ray diffraction/Rietveld refinement and x-ray photoelectron spectroscopy of the polycrystalline MnTa₂O₆sample verified its phase purity. Temperature dependence of the magnetic susceptibilityχ(=M/H) yields the Néel temperatureT_N= 5.97 K determined from the peak in the computed ∂(χT)/∂TvsTplot, in agreement with theT_N= 6.00 K determined from the peak in theC_PvsTdata. The experimental data ofC_PvsTnearT_Nis fitted toC_P=A|T-T_N|^-αyielding the critical exponentα= 0.10(0.13) forT>T_N(T<T_N). TheχvsTdata forT> 25 K fits well with the modified Curie-Weiss law:χ=χ₀+C/(T-θ) withχ₀= -2.12 × 10^-4emu mol^-1 Oe^-1yieldingθ= -24 K, andC= 4.44 emu K mol^-1 Oe^-1, the later giving magnetic momentμ= 5.96 μ_Bper Mn²⁺ion. This yields the effective spinS= 5/2 andg= 2.015 for Mn²⁺, in agreement withg= 2.0155 measured using electron spin resonance spectroscopy. Using the magnitudes ofθandT_Nand molecular field theory, the antiferromagnetic exchange constantsJ₀/k_B= -1.5 ± 0.2 K andJ_⊥/k_B= -0.85 ± 0.05 K for Mn²⁺ions along the chainc-axis and perpendicular to thec-axis respectively are determined. TheχvsTdata when compared to the prediction of a Heisenberg linear chain model provides semiquantitative agreement with the observed variation. TheH-Tphase diagram is mapped using theM-Hisotherms andM-Tdata at differentHyielding the tricritical pointT_TP(H,T) = (17.0 kOe, 5.69 K) separating the paramagnetic, antiferromagnetic, and spin-flop phases. At 1.5 K, the experimental magnitudes of the exchange fieldH_E= 206.4 kOe and spin-flop fieldH_SF= 23.5 kOe yield the anisotropy fieldH_A= 1.34 kOe. These results for MnTa₂O₆are compared with those reported recently in the isostructural MnNb₂O₆.

Tailoring the electronic structure and magnetic properties of pyrochlore Co2Ti1-xGexO4: a GGA +Uab initiostudy

We report the electronic structure and magnetic properties of Co₂Ti_1-xGe_xO₄(0 ⩽x⩽ 1) spinel by means of the first-principle methods of density functional theory involving generalized gradient approximation along with the on-site Coulomb interaction (U_eff) in the exchange-correlation energy functional. Special emphasis has been given to explore the site occupancy of Ge atoms in the spinel lattice by introducing the cationic disorder parameter (y) which is done in such a way that one can tailor the pyrochlore geometry and determine the electronic/magnetic structure quantitatively. For all the compositions (x), the system exhibits weak tetragonal distortion (c/a≠ 1) due to the non-degeneratedz2anddx2-y2states (e_gorbitals) of the B-site Co. We observe large exchange splitting (Δ_EX∼ 9 eV) between the up and down spin bands oft_2gande_gstates, respectively, of tetrahedral and octahedral Co²⁺(⁴A_2(g)(F)) and moderate crystal-field splitting (Δ_CF∼ 4 eV) and the Jahn-Teller distortion (Δ_JT∼ 0.9 eV). These features indicate the strong intra-atomic interaction which is also responsible for the alteration of energy band-gap (1.7 eV ⩽E_g⩽ 3.3 eV). The exchange interaction (J_BB∼ -4.8 meV, for (x,y) = (0.25, 0)) between the Co²⁺dominates the overall antiferromagnetic behaviour of the system for all 'x' as compared toJ_AA(∼-2.2 meV, for (x,y) = (0.25, 0)) andJ_AB(∼-1.8 meV, for (x,y) = (0.25, 0)). For all the compositions without any disorderness in the system, the net ferrimagnetic moment (Δμ) remains constant, however, increases progressively with increasingxdue to the imbalance of Co spins between the A- and B-sites.