ENHANCEMENT OF FERROMAGNETIC CLUSTER INDUCED BY MAGNETIC FIELD IN THE PHASE-SEPARATED La0.5Ca0.5MnO3

Magnetic and transport properties of La_0.5Ca_0.5MnO₃ have been investigated by measuring the magnetization and resistance in zero-field-cooled (ZFC) and field-cooled (FC) modes. Conspicuously irreversible behaviors of magnetization/resistance in the two different modes were observed below the charge ordering transition temperature (T_CO). The ZFC and FC magnetizations at 5K, as functions of the magnetic field, coincide for μ₀H≤1T. Afterwards, the ZFC magnetization tends to an approximate constant, but the FC one increases linearly with increasing field. There exists an excellent correspondence between magnetization and resistance below T_CO. All the results suggest that the ferromagnetic clusters embed in the charge-ordered matrix. The phenomenon of ferromagnetic clusters growing up easily in the FC procedure has been interpreted according to the model of thermally activated two-level system.     

Anisotropic magnetoelastic response in the magnetic Weyl semimetal Co_3Sn_2S_2

Co_3Sn_2S_2 is a recently identified magnetic Weyl semimetal in Shandite compounds. Upon cooling, Co_3Sn_2S_2 undergoes a ferromagnetic transition with c-axis polarized moments(~0.3 μB/Co) around T_C= 175 K, followed by another magnetic anomaly around T_A≈ 140 K. A large intrinsic anomalous Hall effect is observed in the magnetic state below T_C with a maximum of anomalous Hall angle near T_A. Here, we report an elastic neutron scattering on the crystalline lattice of Co_3Sn_2S_2 in a magnetic field up to 10 T. A strongly anisotropic magnetoelastic response is observed, while only a slight enhancement of the Bragg peaks is observed when B//c. The in-plane magnetic field(B//ab) dramatically suppresses the Bragg peak intensity probably by tilting the moments and lattice toward the external field direction. The in-plane magnetoelastic response commences from T_C, and as it is further strengthened below T_A, it becomes nonmonotonic against the field between T_A and T_C because of the competition from another in-plane magnetic order. These results suggest that a magnetic field can be employed to tune the Co_3Sn_2S_2 lattice and its related topological states.     

Spin frustration and magnetic ordering in triangular lattice antiferromagnet Ca_3CoNb_2O_9

We synthesized a quasi-two-dimensional distorted triangular lattice antiferromagnet Ca3 Co Nb2O9, in which the effective spin of Co2+is 1/2 at low temperatures, whose magnetic properties were studied by dc susceptibility and magnetization techniques. The x-ray diffraction confirms the quality of our powder samples. The large Weiss constant θCW ~-55 K and the low Neel temperature TN~ 1.45 K give a frustration factor f =| θCW/TN|≈ 38, suggesting that Ca3 Co Nb2O9resides in strong frustration regime. Slightly below TN, deviation between the susceptibility data under zero-field cooling(ZFC)and field cooling(FC) is observed. A new magnetic state with 1/3 of the saturate magnetization Ms is suggested in the magnetization curve at 0.46 K. Our study indicates that Ca3 Co Nb2O9is an interesting material to investigate magnetism in triangular lattice antiferromagnets with weak anisotropy.     

Magnetic properties and magnetoresistance of HfFe2Ge2-type Dy1-xGdxMn6Ge6 (x=0.1－0.6) compounds

The magnetic properties and magnetoresistance effects of Dy_{1-x}Gd_xMn_6Ge_6 (x=0.1－0.6) compounds have been studied by magnetic properties and resistivity measurements in applied magnetic fields up to 5T. The compounds with x=0.1, 0.2, 0.4 and 0.5 order antiferromagnetically at 425, 428, 430 and 432K, respectively, and there are second magnetic phase transitions below 100K. The compound with x=0.6 exhibits a transition from ferrimagnetic to antiferromagnetic, then to ferrimagnetic state again with decreasing temperature. Furthermore, it displays a field-induced metamagnetic transition, and its threshold field decreases with increasing temperature. The magnetoresistance curve of the compound with x=0.6 in applied magnetic fields up to 5T is presented and the magnetoresistance effects are related to the metamagnetic transitions.     

Zero—field—cooled and field—cooled magnetizations and magnetic susceptibility of itinerant ferromagnet SrRuO3

Zero-field-cooled(ZFC) magnetization,field-cooled(FC) magnetization,ac magnetic susceptibility and major hysteresis loops of itinerant ferromagnet SrRuO3 have been measured at magnetic ordering temperatures ranging from 5 to 160K.An empirical model is proposed to calculate the measured ZFC magnetization.The result indicates that the calculated ZFC magnetization compares well with the measured one.Based on the generalized Preisach model.both the ZFC and FC curves are reproduced by numerical simulations.The critical temperature and critical exponents are determined by measuring the ac magnetic susceptibility in different bias magnetic fields at temperatures in the vicinity of the point of phase transition.     

Magnetic dynamics of two-dimensional itinerant ferromagnet Fe_3GeTe_2

Among the layered two-dimensional ferromagnetic materials(2 D FMs),due to a relatively high T_C,the van der Waals(vdW) Fe_3 GeTe_2(FGT) crystal is of great importance for investigating its distinct magnetic properties.Here,we have carried out static and dynamic magnetization measurements of the FGT crystal with a Curie temperature TC ≈ 204 K.The M-H hysteresis loops with in-plane and out-of-plane orientations show that FGT has a strong perpendicular magnetic anisotropy with the easy axis along its c-axis.Moreover,we have calculated the uniaxial magnetic anisotropy constant(K_1)from the SQUID measurements.The dynamic magnetic properties of FGT have been probed by utilizing the high sensitivity electron-spin-resonance(ESR) spectrometer at cryogenic temperatures.Based on an approximation of single magnetic domain mode,the K_1 and the effective damping constant(α_(eff)) have also been determined from the out-of-plane angular dependence of ferromagnetic resonance(FMR) spectra obtained at the temperature range of 185 K to T_C.We have found large magnetic damping with the effective damping constant α_(eff) ～ 0.58 along with a broad linewidth(ΔH_(pp) 1000 Oe at 9.48 GHz,H ‖ c-axis).Our results provide useful dynamics information for the development of FGT-based spintronic devices.     

Nodal superconducting gap in LiFeP revealed by NMR:Contrast with LiFeAs

Identifying the uniqueness of FeP-based superconductors may shed new lights on the mechanism of superconductivity in iron-pnictides.Here,we report nuclear magnetic resonance(NMR) studies on LiFeP and LiFeAs which have the same crystal structure but different pnictogen atoms.The NMR spectrum is sensitive to inhomogeneous magnetic fields in the vortex state and can provide the information on the superconducting pairing symmetry through the temperature dependence of London penetration depth λ_L.We find that λ_L saturates below T～0.2 T_C in LiFeAs,where T_C is the superconducting transition temperature,indicating nodeless superconducting gaps.Furthermore,by using a two-gaps model,we simulate the temperature dependence of λ_L and obtain the superconducting gaps of LiFeAs,as Δ_1=1.2 k_B T_C and Δ_2=2.8 k_BT_C,in agreement with previous result from spin-lattice relaxation.For LiFeP,in contrast,λ_L does not show any saturation down to T～0.03 T_C,indicating nodes in the superconducting gap function.Finally,we demonstrate that strong spin fluctuations with diffusive characteristics exist in LiFeP,as in some cuprate high temperature superconductors.     

First-principles Study on the Magnetic,Half-metal and Thermoelectric Transport Properties of Inorganic-Organic Hybrid Compounds [C_4N_2H_(12)][Fe_4~Ⅱ(HPO_3)_2(C_2O_4)_3]

The electronic structure,magnetic and half-metal properties of inorganic-organic hybrid compound [C_4N_2H_(12)][Fe_4~Ⅱ(HPO_3)_2(C_2O_4)_3] are investigated by using the full-potential linearized augmented plane wave (FPLAPW) method within density-functional theory (DFT) calculations.The density of states (DOS),the total energy of the cell and the spontaneous magnetic moment of [C_4N_2H_(12)][Fe_4~Ⅱ(HPO_3)_2(C_2O_4)_3] are calculated.The calculation results reveal that the low-temperature phase of [C_4N_2H_(12)][Fe_4~Ⅱ(HPO_3)_2(C_2O_4)_3] exhibits a stable ferromagnetic (FM) ground state,and we find that this organic compound is a half-metal in FM state.In addition,we have calculated antiferromagnetically coupled interactions,revealing the existence of antiferromagnetic (AFM),which is in agreement with the experiment.We have also found that [C_4N_2H_(12)][Fe_4~Ⅱ(HPO_3)_2(C_2O_4)_3] is a semiconductor in the AFM state with a band gap of about 0.40 eV.Subsequently,the transport properties for potential thermoelectric applications have been studied in detail based on the Boltzmann transport theory.     

Magnetic properties of Nd0.5Pb0.5—xSrxMnO3 materials

The structure and magnetic properties of Nd_{0.5}Pb_{0.5-x}Sr_xMnO_3 (0≤x≤0.4) manganites were systematically investigated. Significant changes in Curie temperature and metal-insulator (MI) transition temperature of the samples were observed. All samples exhibited a transition from paramagnetic semiconducting to ferromagnetic metallic state. Curie temperature T_C and the MI transition temperature T_p increased with increasing Sr content. We attributed these behaviours to the enhancing of both the double exchange mechanism and the Jahn－Teller electron－phonon coupling.     

Large magnetic entropy change and magnetic properties in La （Fel－xMnx）ll.TSil.3Hy compounds

Magnetic properties and magnetic entropy change in La(Fe_{1-x}Mn_x)_{11.7}Si_{1.3}H_y compounds have been investigated. A significant increase of the Curie temperature T_C and a small increase of the saturation magnetizations μ_S have been observed after the introduction of interstitial H, which caused a slight volume expansion. The first-order field-induced itinerant-electron metamagnetic (IEM) transition remains and brings about a large magnetic entropy change around room temperatures for the compounds. The maximal magnetic entropy change is about 23.4, 17.7 and 15.9J/kg·K under a magnetic field change from 0 to 5T for x=0.01, 0.02 and 0.03, respectively. Therefore, the compounds appear to be potential candidates for magnetic refrigerants around room temperatures.     

Sm3（F3,Co,Mn）29 compounds：promising materials for permanent magnets

The outstanding hard-magnetic properties are reported of Sm_3Fe_{28.1-x}Co_xMo_{0.9} compounds with x=12, 14, 16. In this alloy system, only a small amount of Mo is needed to stabilize the 3:29 structure so that the magnetic properties are not seriously affected by the presence of this nonmagnetic element. Substitution of Co for Fe leads to a significant increase of the magnetic anisotropy, and for x≥14 the easy magnetization direction changes from easy plane to the easy axis. In this alloy system, the compound Sm_3Fe_{12.1}Co_{16}Mo_{0.9} is a very promising candidate for permanent magnet applications. Its room temperature saturation magnetization (μ_0M_s=1.5 T) and anisotropy field (B_{an}=6.5 T) are comparable to the values for Nd_2Fe_{14}B (μ_0M_s=1.6 T and B_{an}=7 T). However, the Curie temperature of Sm_3Fe_{12.1}Co_{16}Mo_{0.9} is 1020 K, which is appreciably higher than that for Nd_2Fe_{14}B (T_C=588 K).     

Strain-tuned magnetic properties in (Ga,Fe)Sb:First-principles study

In view of the importance of enhancing ferromagnetic(FM) coupling in dilute magnetic semiconductors(DMSs),the effects of strain on the electronic structures and magnetic properties of(Ga,Fe)Sb were examined by a first-principles study.The results of the investigation indicate that Fe_(Ga) substitution takes place in the low-spin state(LSS) with a total magnetic moment of 1μB in the strain range of-3% to 0.5%,which transitions to the high-spin state(HSS) with a total magnetic moment of 5μB as the strain changes from 0.6% to 3%.We attribute the changes in the amount and distribution of the total moment to the influence of the crystal field under different strains.The FM coupling is strongest under a strain of about0.5%,but gradually becomes weaker with increasing compressive and tensile strains.The magnetic coupling mechanism is discussed in detail.Our results highlight the important contribution of strain to magnetic moment and FM interaction intensity,and present an interesting avenue for the future design of high Curie temperature(T_C) materials in the(Ga,Fe)Sb system.     

Physical properties and magnetic structure of a layered antiferromagnet PrPd_(0.82)Bi_2

We report the physical properties, crystalline and magnetic structures of singe crystals of a new layered antiferromagnetic(AFM) material PrPd_(0.82)Bi_2. The measurements of magnetic properties and heat capacity indicate an AFM phase transition at T_N～7K. A large Sommerfeld coefficient of 329.23 m J·mol~(-1)·K~(-2) is estimated based on the heat capacity data, implying a possible heavy-fermion behavior. The magnetic structure of this compound is investigated by a combined study of neutron powder and single-crystal diffraction. It is found that an A-type AFM structure with magnetic propagation wavevector k =(0 0 0) is formed below TN. The Pr~(3+) magnetic moment is aligned along the crystallographic c-axis with an ordered moment of 1.694(3) μBat 4K, which is smaller than the effective moment of the free Pr~(3+) ion of 3.58 μB.PrPd_(0.82)Bi_2 can be grown as large as 1 mm×1 cm in area with a layered shape, and is very easy to be cleaved, providing a unique opportunity to study the interplay between magnetism, possible heavy fermions, and superconductivity.     

Nonequilibrium dynamical phase transition of 3D kinetic Ising／Heisenberg spin system

We have studied the nonequilibrium dynamic phase transitions of both three-dimensional (3D) kinetic Ising and Heisenberg spin systems in the presence of a perturbative magnetic field by Monte Carlo simulation. The feature of the phase transition is characterized by studying the distribution of the dynamical order parameter. In the case of anisotropic Ising spin system (ISS), the dynamic transition is discontinuous and continuous under low and high temperatures respectively, which indicates the existence of a tri-critical point (TCP) on the phase boundary separating low-temperature order phase and high-temperature disorder phase. The TCP shifts towards the higher temperature region with the decrease of frequency, i.e. T_{TCP}=1.33×exp(-ω/30.7). In the case of the isotropic Heisenberg spin system (HSS), however, the situation on dynamic phase transition of HSS is quite different from that of ISS in that no stable dynamical phase transition was observed in kinetic HSS after a threshold time. The evolution of magnetization in the HSS driven by a symmetrical external field after a certain duration always tends asymptotically to a disorder state no matter what an initial state the system starts with. The threshold time τ depends upon the amplitude H_{0}, reduced temperature T/T_C and the frequency ω as τ=C·ω^α·H_0^{-β}·(T/T_C)^{-γ}.     

Magnetic Behaviour and Heating Effect of Fe3O4 Ferrofluids Composed of Monodisperse Nanoparticles

Fe3O4 ferrofluids containing monodisperse Fe3O4 nanoparticles with different diameters of 8, 12, 16 and 18nm are prepared by using high-temperature solution phase reaction. The particles have single crystal structures with narrow size distributions. At room temperature, the 8-nm ferrofluid shows superparamagnetic behaviour, whereas the others display hysteresis properties and the coercivity increases with the increasing particle size. The spin glass-like behaviour and cusps near 190K are observed on all ferrofluids according to the temperature variation of field-cooled （FC） and zero-field-cooled （ZFC） magnetization measurements. The cusps are found to be associated with the freezing point of the solvent. As a comparison, the ferrofluids are dried and the FC and ZFC magnetization curves of powdery samples are also investigated. It is found that the blocking temperatures for the powdery samples are higher than those for their corresponding ferrofluids. Moreover, the size dependent heating effect of the ferrofluids is also investigated in ac magnetic field with a frequency of 55 kHz and amplitude of 200 Oe.     

Magnetocaloric effect in the layered organic–inorganic hybrid(CH_3NH_3)_2CuCl_4

We present a study of magnetocaloric effect of the quasi-two-dimensional(2D) ferromagnet(CH_3NH_3)_2CuCl_4 in ab plane(easy-plane). From the measurements of magnetic field dependence of magnetization at various temperatures,we have discovered a large magnetic entropy change associated with the ferromagnetic–paramagnetic transition. The heat capacity measurements reveal an abnormal adiabatic change below the Curie temperature T_c~8.9 K, which is caused by the nature of quasi-2D layered crystal structure. These results suggest that perovskite organic–inorganic hybrids with a layered structure are suitable candidates as working substances in magnetic refrigeration technology.     

Lateral magnetic stiffness under different parameters in a high-temperature superconductor levitation system

Magnetic stiffness determines the stability of a high-temperature superconductor(HTS) magnetic levitation system.The quantitative properties of the physical and geometrical parameters that affect the stiffness of HTS levitation systems should be identified for improving the stiffness by some effective methods. The magnetic stiffness is directly related to the first-order derivative of the magnetic force with respect to the corresponding displacement, which indicates that the effects of the parameters on the stiffness should be different from the relationships between the forces and the same parameters.In this paper, we study the influences of some physical and geometrical parameters, including the strength of the external magnetic field(B0) produced by a rectangular permanent magnet(PM), critical current density(Jc), the PM-to-HTS area ratio(α), and thickness ratio(β), on the lateral stiffness by using a numerical approach under zero-field cooling(ZFC)and field cooling(FC) conditions. In the first and second passes of the PM, the lateral stiffness at most of lateral positions essentially increases with B0 increasing and decreases with β increasing in ZFC and FC. The largest lateral stiffness at every lateral position is almost produced by the minimum value of Jc, which is obviously different from the lateral force–Jc relation. The α-dependent lateral stiffness changes with some parameters, which include the cooling conditions of the bulk HTS, lateral displacement, and movement history of the PM. These findings can provide some suggestions for improving the lateral stiffness of the HTS levitation system.     

Electronic and magnetic properties of CrI_3 nanoribbons and nanotubes

CrI_3 in two-dimensional(2D) forms has been attracting much attention lately due to its novel magnetic properties at atomic large scale.The size and edge tuning of electronic and magnetic properties for 2D materials has been a promising way to broaden or even enhance their utility, as the case with nanoribbons/nanotubes in graphene, black phosphorus, and transition metal dichalcogenides.Here we studied the CrI_3 nanoribbon(NR) and nanotube(NT) systematically to seek the possible size and edge control of the electronic and magnetic properties.We find that ferromagnetic ordering is stable in all the NR and NT structures of interest.An enhancement of the Curie temperature T_C can be expected when the structure goes to NR or NT from its 2D counterpart.The energy difference between the FM and AFM states can be even improved by up to 3–4 times in a zigzag nanoribbon(ZZNR), largely because of the electronic instability arising from a large density of states of iodine-5p orbitals at E_F.In NT structures, shrinking the tube size harvests an enhancement of spin moment by up to 4%, due to the reduced crystal-field gap and the re-balance between the spin majority and minority populations.     

Field-tuned magnetic structure and phase diagram of the honeycomb magnet YbCl_3

We report thermodynamic and neutron diffraction measurements on the magnetic ordering properties of the honeycomb lattice magnet YbCl_3. We find YbCl_3 exhibits a Ne′el type long-range magnetic order at the wavevector(0, 0, 0) below TN= 600 mK.This magnetic order is associated with a small sharp peak in heat capacity and most magnetic entropy release occurs above the magnetic ordering temperature. The magnetic moment lies in-plane, parallel to the monoclinic a-axis, whose magnitude mYb= 0.86(3) μBis considerably smaller than the expected fully ordered moment of 2.24 μBfor the doublet crystal-field ground state. The magnetic ordering moment gradually increases with increasing magnetic field perpendicular to the ab-plane, reaching a maximum value of 1.6(2) μBat 4 T, before it is completely suppressed above ～ 9 T. These results indicate the presence of strong quantum fluctuations in YbCl_3.