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Register a new userSpin-state driven ferromagnetic and spin glass states in layered LaSrCoO$_4$
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Famous for its spin-state puzzle, LaSrCoO$_4$ (Co$^{3+}$) is an intermediate between antiferromagnetic (AFM) La$_2$CoO$_4$ (Co$^{2+}$) and ferromagnetic (FM) Sr$_2$CoO$_4$ (Co$^{4+}$). The appearance of the Co$^{3+}$ valence state (not present in the end compounds) is intriguing because of the spin-state transitions associated with it. In this work, we report two magnetic transitions in LaSrCoO$_4$: (i) a transition at T $=$ T$_c$ $simeq$ 225 K, from the paramagnetic state to a state with an inhomogeneous long-range ferromagnetic (FM) order wherein finite FM clusters coexist with infinite FM matrix in the percolation sense, and (ii) the transition to the cluster spin glass (CSG) state at T $=$ T$_g$ $simeq$ 8 K. Finite FM clusters (which at low temperatures give rise to the cluster spin glass state) and infinite FM matrix are formed due to the spin-spin interactions brought about by the inhomogeneously distributed Co$^{3+}$ high spin (HS) and Co$^{3+}$ low spin (LS) ions. A firm support to the presence of an unconventional (inhomogeneous) ferromagnetic order comes from the anomalous values of the critical exponents $beta$, $gamma$ and $delta$ for the spontaneous magnetization, `zero-field magnetic susceptibility and the critical M - H isotherm, while the coexistence of HS Co$^{3+}$ and LS Co$^{3+}$ ions is confirmed by the results of the extended X-ray absorption fine structure spectroscopy.
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Cobaltates have rich spin-states and diverse properties. Using spin-state pictures and firstprinciples calculations, here we study the electronic structure and magnetism of the mixed-valent double perovskite YBaCo2O6. We find that YBaCo2O6 is in the formal intermediate-spin (IS) Co3+/low-spin (LS) Co4+ ground state. The hopping of eg electron from IS-Co3+ to LS-Co4+ via double exchange gives rise to a ferromagnetic half-metallicity, which well accounts for the recent experiments. The reduction of both magnetization and Curie temperature by oxygen vacancies is discussed, aided with Monte Carlo simulations. We also explore several other possible spin-states and their interesting electronic/magnetic properties. Moreover, we predict that a volume expansion more than 3% would tune YBaCo2O6 into the high-spin (HS) Co3+/LS Co4+ ferromagnetic state and simultaneously drive a metal-insulator transition. Therefore, spin-states are a useful parameter for tuning the material properties of cobaltates.
As a sister compound and isostructural of MnBi2Te4, the high quality MnSb2Te4 single crystals are grown via solid-state reaction where prolonged annealing and narrow temperature window play critical roles on account of its thermal metastability. X-ray diffraction analysis on MnSb2Te4 single crystals reveals pronounced cation intermixing, 28.9(7)% Sb antisite defects on the Mn (3a) site and 19.3(6)% Mn antisite defects on the Sb (6c) site, compared with MnBi2Te4. Unlike antiferromagnetic (AFM) nature MnBi2Te4, MnSb2Te4 contains magnetic and antiferromagnetic competition and exhibits a spin glass (SG) state below 24 K. Its magnetic hysteresis, anisotropy, and relaxation process are investigated in detail with DC and AC magnetization measurements. Moreover, anomalous Hall effect as a p-type conductor is demonstrated through transport measurements. This work grants MnSb2Te4 a possible access to the future exploration of exotic quantum physics by removing the odd/even layer number restraint in intrinsic AFM MnBi2Te4-family materials as a result of the crossover between its magnetism and potential topology in the Sb-Te layer.
Magnetism of the spin-$frac12$ $alpha$-KVOPO$_4$ is studied by thermodynamic measurements, $^{31}$P nuclear magnetic resonance (NMR), neutron diffraction, and density-functional band-structure calculations. Ferromagnetic Curie-Weiss temperature of $theta_{rm CW}simeq 15.9$ K and the saturation field of $mu_0H_ssimeq 11.3$ T suggest the predominant ferromagnetic coupling augmented by a weaker antiferromagnetic exchange that leads to a short-range order below 5 K and the long-range antiferromagnetic order below $T_{rm N}simeq 2.7$ K in zero field. Magnetic structure with the propagation vector $mathbf k=(0,frac12,0)$ and the ordered magnetic moment of 0.58 $mu_B$ at 1.5 K exposes a non-trivial spin lattice where strong ferromagnetic dimers are coupled antiferromagnetically. The reduction in the ordered magnetic moment with respect to the classical value (1 $mu_{rm B}$) indicates sizable quantum fluctuations in this setting, despite the predominance of ferromagnetic exchange. We interpret this tendency toward ferromagnetism as arising from the effective orbital order in the folded chains of the VO$_6$ octahedra.
The ac-susceptibility of the electron doped single-layered manganite La$_{1.1}$Sr$_{0.9}$MnO$_4$ is analyzed in detail. A quasi two-dimensional (2$D$) antiferromagnetic (AFM) order with Ising anisotropy is stabilized below $T_N$ $sim$ 80K. We show that below $T_N$, a rare 2$D$ spin-glass (SG) correlation develops with the same Ising anisotropy as the AFM state. Using simple scaling arguments of the droplet model, we derive a scaling form for the ac-susceptibility data of a 2$D$ SG, which our experimental data follows fairly well. Due to simplifications in this 2$D$ case, the proposed scaling form only contains two unknown variables $psi u$ and $tau_0$. Hence, the logarithmic growth law of the SG correlation predicted by the droplet model is convincingly evidenced by the scaling of our experimental data. The origin and nature of this 2$D$ SG state is also discussed.
We have grown single crystals of Na$_2$BaNi(PO$_4$)$_2$, a new spin-1 equilateral triangular lattice antiferromagnet (ETLAF), and performed magnetic susceptibility, specific heat and thermal conductivity measurements at ultralow temperatures. The main results are (i) at zero magnetic field, Na$_2$BaNi(PO$_4$)$_2$ exhibits a magnetic ordering at 430 mK with a weak ferromagnetic moment along the $c$ axis. This suggests a canted 120$^circ$ spin structure, which is in a plane including the crystallographic $c$ axis due to the existence of an easy-axis anisotropy and ferromagnetically stacked along the $c$ axis; (ii) with increasing field along the $c$ axis, a 1/3 magnetization plateau is observed which means the canted 120$^circ$ spin structure is transformed to a up up down (UUD) spin structure. With even higher fields, the UUD phase further evolves to possible V and V phases; (iii) with increasing field along the $a$ axis, the canted 120$^circ$ spin structure is possibly transformed to a umbrella phase and a V phase. Therefore, Na$_2$BaNi(PO$_4$)$_2$ is a rare example of spin-1 ETLAF with single crystalline form to exhibit easy-axis spin anisotropy and series of quantum spin state transitions.
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