L. H. Yin

Magnetic and dielectric properties of Aurivillius phase Bi6Fe2Ti3O18 and the doped compounds

The magnetic and dielectric properties of Bi6Fe2Ti3O18, Bi6FeCoTi3O18, and Bi5LaFeCoTi3O18 are investigated. The room-temperature ferromagnetism is observed in the samples Bi6FeCoTi3O18 and Bi5LaFeCoTi3O18 compared with the paramagnetic behavior in Bi6Fe2Ti3O18 at room temperature. The ferromagnetism in Bi6FeCoTi3O18 and Bi5LaFeCoTi3O18 can be ascribed to spin canting of the Fe-based and Co-based sublattices via the antisymmetric Dzyaloshinskii-Moriya interaction. The frequency-dependent behavior of the dielectric loss peak in Bi6FeCoTi3O18 and Bi5LaFeCoTi3O18 manifests itself a thermally activated relaxation process. The rather large activation energy (2.62 ± 0.11 eV for Bi6FeCoTi3O18 and 1.97 ± 0.07 eV for Bi5LaFeCoTi3O18) implies that the relaxation process is not due to the thermal motion of oxygen vacancies inside ceramics.

Multiferroic properties of Aurivillius phase Bi6Fe2−xCoxTi3O18 thin films prepared by a chemical solution deposition route

The magnetic and ferroelectric properties of Co-doped Bi6Fe2Ti3O18 thin films Bi6Fe2−xCoxTi3O18 (BFCTO) (0 ≤x≤ 1) are investigated. The coexistence of room-temperature ferromagnetism and ferroelectricity is observed in BFCTO thin films. The x = 0.6 sample exhibits a higher remnant magnetization Mr of 8.41 emu/cm3 and a remnant polarization Pr of 17.6 μC/cm2 compared with other BFCTO thin films. The ferromagnetism can be ascribed to the spin canting of Fe- and Co-based sublattices via the Dzyaloshinskii-Moriya interaction [I. Dzyaloshinsky, J. Phys. Chem. Solids 4, 241 (1958); T. Moriya, Phys. Rev. 120, 91 (1960)]. The change in the remnant polarization is discussed in terms of the variation of grain size and oxygen vacancies caused by Co-doping.

MAGNETIC-PROPERTIES OF SM2(FE1-XMX)17NY NITRIDES (M = CO, NI, AL, TI, V)

Abstract The thermal stability, structural and magnetic properties of Sm 2 (Fe 1− x M x ) 17 N y nitrides (M = Co, Ni, Al, Ti and V) have been investigated. The Curie temperature increases with Co concentration but decreases with Ni or Al concentration. The saturation magnetization M s and the anisotropy field B a at room temperature of Sm 2 (Fe 1 - x Co x ) 17 N y both reach their maxima of 147 J/T kg and 15.5 T respectively. All nitrides start to decompose at a similar temperature of ≈ 650°C.

Multiferroicity and magnetoelectric coupling enhanced large magnetocaloric effect in DyFe0.5Cr0.5O3

DyFe0.5Cr0.5O3 has been synthesized using a sol-gel method. It exhibits ferroelectricity at the antiferromagnetic ordering temperature TN1∼261 K. Large magnetocaloric effect (MCE) (11.3 J/kg K at 4.5 T) enhanced by magnetoelectric coupling due to magnetic field and temperature induced magnetic transition was observed. Temperature-dependent Raman study shows an anomalous behavior near TN1 in the phonon modes related to the vibration of Dy atoms and stretching of CrO6/FeO6 octahedra, suggesting the ferroelectricity in DyFe0.5Cr0.5O3 is associated with the spin-phonon coupling with respect to both Dy and Cr/Fe ions. These results suggest routes to obtain high-temperature multiferroicity and large MCE for practical applications.

Structural and magnetic properties of NdFe12−xMoxN1−δ compounds

A series of nitrides NdFe12−xMoxNi1−δ with x=1–2.5 and δ≤0.2 have been investigated. All nitrides are of the ThMn12 tetragonal structure with the unit cell volumes about 3% larger than their parent alloys. Curie temperatures are enhanced by nitrogenation about 160 K and increase linearly with decreasing content x of Mo. NdFe12−xMox exhibits a weak uniaxial anisotropy with a value of BA less than 0.6 T when x≥1.75. After nitrogenation, all nitrides have a strong uniaxial anisotropy. Values of the anisotropy field BA increase monotonically from 6.35 T for x=2.5 up to 9.5 T for x=1 as decreasing content x of Mo. Saturation magnetizations also increase monotonically with decreasing content x.

Magnetic and dielectric properties of Aurivillius phase Bi6Fe2Ti3−2xNbxCoxO18 (0 ≤ x ≤ 0.4)

We investigate the structural, magnetic, and dielectric properties of Bi6Fe2Ti3−2xNbxCoxO18 (0 ≤ x ≤ 0.4). The room-temperature ferromagnetism is observed in the Nb and Co co-doped samples compared with the paramagnetic behavior in Bi6Fe2Ti3O18. The ferromagnetism in Bi6Fe2Ti3−2xNbxCoxO18 can be understood in terms of spin canting of the antiferromagnetic coupling of the Fe-based and Co-based sublattices via Dzyaloshinsky-Moriya interaction. Moreover, doping Co at Ti sites can significantly enhance the ferromagnetic Curie temperature compared with the substitution of Co for Fe in the Aurivillius compounds. The dielectric loss of Bi6Fe2Ti3−2xNbxCoxO18 (0.1 ≤ x ≤ 0.4) exhibits a relaxation process. The rather large activation energy in the 0.1 ≤ x ≤ 0.3 samples implies that the relaxation process is not due to the thermal motion of oxygen vacancies inside ceramics.

Giant magnetocaloric effect and temperature induced magnetization jump in GdCrO3 single crystal

We report on a systematic study of the single-crystal GdCrO3, which shows various novel magnetic features, such as temperature-induced magnetization reversal (TMR), temperature-induced magnetization jump (TMJ), spin reorientation, and giant magnetocaloric effect (MCE). In the field-cooled cooling process with modest magnetic field along the c axis, GdCrO3 first shows a TMR at Tcomp∼120−130 K and then an abrupt TMJ with a sign change of magnetization at Tjump∼52−120 K, and finally a spin reorientation at TSR∼4−7 K. Interestingly, the remarkable TMJ behavior, which was not reported ever before, persists at higher fields up to 10 kOe even when TMR disappears. In addition, giant MCE with the maximum value of magnetic entropy change reaching ∼31.6 J/kg K for a field change of 44 kOe was also observed in GdCrO3 single crystal, suggesting it could be a potential material for low-T magnetic refrigeration. A possible mechanism for these peculiar magnetic behaviors is discussed based on the various competing magnet...

Large remnant polarization and magnetic field induced destruction of cycloidal spin structure in Bi1−xLaxFeO3 (0 ≤ x ≤ 0.2)

We prepared a series of Bi1−xLaxFeO3 (0 ≤ x ≤ 0.2) ceramics with a sol-gel method and find that both the magnetization and dielectric constant show an abrupt anomaly near a critical field Hc, which is attributed to the destruction of the cycloidal antiferromagnetic spin structure. The critical field Hc decreases substantially from ∼20 T for the x = 0 sample [Y. F. Popov et al., JETP Lett. 57, 69 (1993)] to ∼2.8 T for the x = 0.17 sample and finally to 0 T for the x = 0.2 sample at room temperature (RT). It is also found that Hc increases with decreasing temperature. The variation of Hc with La substitution and temperature can be ascribed to the change in the magnetic anisotropy and isotropic superexchange interaction, respectively. We have also discussed the magnetodielectric effects in these samples in terms of the Ginzburg-Landau theory and the spin-phonon model. Moreover, increasing the doping level of La to 0.15 greatly improves the RT leakage-current and ferroelectric (FE) properties. A RT square-sha...

Role of rare earth ions in the magnetic, magnetocaloric and magnetoelectric properties of RCrO3 (R = Dy, Nd, Tb, Er) crystals

We report a systematic study on the magnetic, magnetocaloric (MC), specific heat and magnetoelectric (ME) properties of the single crystals of rare-earth orthochromites, RCrO3 (R = Dy, Nd, Er, Tb). Interesting stair-like metamagnetic transitions were observed for the first time in TbCrO3 crystals. Intrinsic large anisotropy in the magnetic and ME properties were also revealed in all of the RCrO3 crystals. The reorientation/ordering of the rare earth R3+ spins was observed to be accompanied with large MC, rotating MC and ME effects. These behaviors are found be closely related to the R–R and R–Cr interactions in these chromites. In particular, the ME effect can be ascribed to the spin–phonon coupling. All these results suggest the essential and unique role of 4f electrons of rare earth ions in these chromites.

MAGNETIC PHASE-DIAGRAM OF ND(FE,MO)12 ALLOYS

The magnetic phase diagram of the alloy series NdFe12−xMox (x=1.0∼2.5) is obtained. Below the Curie temperature, the ferromagnetic phase is divided into three regions: Uniaxial, canted 1, and canted 2. At a temperature of Tsr1 (below 200 K) a spin reorientation transition (SRT) was observed for all the samples, and at Tsr2 (above 200 K) another SRT was found for samples with Mo concentration 1.5<x<1.75. At room temperature the easy direction of magnetization (EDM) of NdFe12−xMox changes from the c axis to a canted structure with increasing Mo concentration, with a critical composition of x=1.56. At low temperatures (below 100 K) all NdFe12−xMox compounds exhibit a canted moment structure.