Large spontaneous exchange bias in a weak ferromagnet Pb6Ni9(TeO6)5

Zero-field-cooling exchange bias up to room temperature in the strained kagome antiferromagnet Mn3.1Sn0.9

Zero-field-cooling exchange bias (ZFC EB) has always been a research hotspot for researchers, because it can realize the movement of the magnetization hysteresis loop along the field axis without field cooling, which greatly expands the universality and convenience of the application of the exchange bias effect. Achieving ZFC EB at room temperature is an ongoing challenge. To this end, a design strategy from the sublattice level is proposed, and a wide temperature range ZFC EB up to room temperature with a vertical magnetization shift is observed in the strained kagome antiferromagnet Mn3.1Sn0.9. Magnetic analysis and first-principles calculations reveal that the ZFC EB arises from the strong exchange interaction between the non-coplanar antiferromagnetic Mn kagome sublattice occupying normal Mn sites and the collinear ferromagnetic Mn sublattice occupying Sn sites. This discovery is of great significance for the application of ZFC EB in antiferromagnetic spintronic devices.

Exchange bias effect in polycrystalline Bi0.5Sr0.5Fe0.5Cr0.5O3 bulk

Bulk Bi_0.5Sr_0.5Fe_0.5Cr_0.5O₃ (BSFCO) is a new compound comprising the R3c structure. The structural, magnetic property and exchange bias (EB) details are investigated. The material was in the super-paramagnetic (SP) state at room temperature. Exchange bias usually occurs at the boundary between different magnetic states after field cooling (H_FC) acts on the sample. Here the result shows that changing H_FC from 1 to 6 T reduces the H_EB value by 16% at 2 K at the same time. Meanwhile, H_EB diminishes as the ferromagnetic layer thickness increases. The variation of (the thickness of ferromagnetic layer) t_FM with the change of H_FC leads to the tuning of H_EB by H_FC in BSFCO bulk. These effects are obviously different from the phenomenon seen in other oxide types.

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).

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

Static and dynamic magnetic properties of normal spinel Co2RuO4= (Co2+)A[Co3+Ru3+]BO4are reported based on our investigations of the temperature (T), magnetic field (H) and frequency (f) dependence of the ac-magnetic susceptibilities and dc-magnetization (M) covering the temperature rangeT= 2 K-400 K and H up to 90 kOe. These investigations show that Co2RuO4exhibits an antiferromagnetic (AFM) transition atTN∼ 15.2 K, along with a spin-glass state at slightly lower temperature (TSG) near 14.2 K. It is argued thatTNis mainly governed by the ordering of the spins of Co2+ions occupying theA-site, whereas the exchange interaction between the Co2+ions on theA-site and randomly distributed Ru3+on theB-site triggers the spin-glass phase, Co3+ions on theB-site being in the low-spin non-magnetic state. Analysis of measurements ofM(H,T) forTTN, analysis of the paramagnetic susceptibility (χ) vs.Tdata are fit to the modified Curie-Weiss law,χ=χ0+C/(T+θ), withχ0= 0.0015 emu mol-1Oe-1yieldingθ= 53 K andC= 2.16 emu-K mol-1Oe-1, the later yielding an effective magnetic momentμeff= 4.16μBcomparable to the expected value ofμeff= 4.24μBper Co2RuO4. UsingTN,θand high temperature series forχ, dominant exchange constantJ1/kB∼ 6 K between the Co2+on theA-sites is estimated. Analysis of the ac magnetic susceptibilities nearTSGyields the dynamical critical exponentzν= 5.2 and microscopic spin relaxation timeτ0∼ 1.16 × 10-10sec characteristic of cluster spin-glasses and the observed time-dependence ofM(t) is supportive of the spin-glass state. LargeM-Hloop asymmetry at low temperatures with giant exchange bias effect (HEB∼ 1.8 kOe) and coercivity (HC∼ 7 kOe) for a field cooled sample further support the mixed magnetic phase nature of this interesting spinel. The negative magnetocaloric effect observed belowTNis interpreted to be due to the AFM and SG ordering. It is argued that the observed change from positive MCE (magnetocaloric effect) forT>TNto inverse MCE forT Collapse

Key Words

• antiferromagnetism
• exchange bias
• spin-glass state

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Affiliation(s)

Sayandeep Ghosh Department of Physics, Indian Institute of Technology, Guwahati-781039, Assam, India
Deep Chandra Joshi Department of Materials Science and Engineering, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden
Prativa Pramanik Department of Physics, Indian Institute of Technology, Guwahati-781039, Assam, India
Suchit K Jena Department of Physics, Indian Institute of Technology, Guwahati-781039, Assam, India
Suresh Pittala Department of Physics, Indian Institute of Science, Bangalore-560012, Karnataka, India
Tapati Sarkar Department of Materials Science and Engineering, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden
Mohindar S Seehra Department of Physics & Astronomy, West Virginia University, Morgantown, WV 26506, United States of America
Subhash Thota Department of Physics, Indian Institute of Technology, Guwahati-781039, Assam, India

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