Temperature and thickness dependence of the magnetization reversal in DyFe2/YFe2 exchange-coupled superlattices

[ DyFe₂/YFe₂] superlattices have been grown by Molecular Beam Epitaxy. The magnetic properties of this hard/soft composite system, the components of which are exchange-coupled at the interfaces, have been investigated in the 10 K–300 K temperature range, with a specific attention paid to the influence of the soft and hard materials thicknesses. In order to unravel the very rich magnetization reversal processes, conventional susceptibility and magnetization measurements have been combined with element-selective X-ray Magnetic Circular Dichroism analysis. The superlattice with thin individual thicknesses ([ 1nm DyFe₂/4 nm YFe₂] ₇₀) reverses as a unique giant ferrimagnetic block in which the exchange-favoured antiparallel arrangement between net magnetizations is maintained under a magnetic field. In the superlattices with large individual thicknesses ([ 10 nm DyFe₂/13 nm YFe₂] ₁₈ and [ 10 nm DyFe₂/20 nm YFe₂] ₁₃), the expected exchange spring behaviour is observed: the soft YFe₂ layers reverse for positive bias fields, followed by the irreversible switch of the hard DyFe₂ layers. In the case of intermediate thickness for the individual DyFe₂ layers ([ 3 nm DyFe₂/12 nm YFe₂] ₂₂, [ 5 nm DyFe₂/20 nm YFe₂] ₁₃, [ 7 nm DyFe₂/28 nm YFe₂] ₁₀), the magnetization reversal process strongly depends on temperature. In particular, an unusual magnetization reversal process occurs in the high temperature range where it becomes easier to reverse the hard DyFe₂ layers for positive fields, while maintaining the dominant YFe₂ magnetization along the field direction.     

Magnetic properties of YFe12−xMox compounds and magnetocaloric effect of YFe9.5Mo2.5

The characterization and magnetic properties of YFe_12−xMo_x (x=2.0, 2.5 and 3.0) with the ThMn₁₂-type structure, and the magnetocaloric effect of YFe_9.5Mo_2.5 were investigated. A directional growth was observed in YFe₁₀Mo₂ alloy. A broad peak in the zero-field-cooling (ZFC) magnetization curve of the YFe_12−xMo_x compounds is ascribed to the existence of ferromagnetic clusters with different site moments and scattered orientations of the moments. The broad range of the peak is reduced with increasing Mo content. A weak peak is observed near 190 K in the ZFC curve of YFe₉Mo₃, which is associated with the 8i sites being mostly occupied by Mo atoms. YFe_9.5Mo_2.5 has a magnetic entropy change of −1.09 J/kg K for a field change of 5 T at 277 K.     

YFe2B2电子结构的第一性原理计算

基于密度泛函理论,从头计算了具有ThCr₂Si₂型四方晶系的稀土金属化合物Yfe₂B₂体相的物理特性.能量计算结果表明Yfe₂B₂体相处于顺磁金属态;而能带结构、态密度、布居数以及差分电荷　分布的计算结果表明Y原子的5s,5p电子具有很强的局域性;Fe原子的3d电子和B的1s,2s和2p电子强烈耦合,使得最近邻Fe原子与B原子形成了Fe—B共价键;最近邻的两个Fe原子之间由于 关键词： 稀土金属化合物 第一性原理计算 能带结构 态密度     

Magnetic anisotropy in multilayers

Magnetic anisotropy between in-plane and out of plane magnetic alignments is studied in a variety of multilayer systems using Mössbauer spectrosopy to observe the (Fe) magnetic orientation. The surface anisotropy in Fe/Au (1 1 1) multilayers is measured as K _s = 0.9 × 10^?3 Jm^?2. In Fe/Ni multilayers the dependence of magnetic orientation on external field applied normal to the layers enables volume and interface anisotropies K _v = (?5 ± 1) × 10⁴ Jm^?3 and K _s = (?0.6 ± 0.4)× 10^?3 Jm^?2 to be evaluated. In similar applied field experiments coherent rotation of the magnetic Fe and NiFe layers in Fe/Cu/NiFe/Cu multilayers was observed for intervening Cu layer thickness x = 5 Å but independent rotation for x = 50 Å. Out of plane magnetic components are observed for DyFe₂, YFe₂ thin films and DyFe₂/YFe₂ multilayers. In fields of up to 0.25 T applied inplane only the moments of the YFe₂ film showed significant rotation.     

Spin-glass and magnetic blocking transitions in amorphous YFe2

Low field dc magnetic susceptibility measurements on amorphous YFe₂ show a distinct cusp-like peak at T_S.G. = 58 K. This result, together with earlier Mőssbauer and neutron scattering measurements, indicates that a true thermodynamic spin-glass transition occurs at T_S.G.. In addition, susceptibility and coercive field data are presented which strongly suggest a magnetic freezing or blocking temperature near T = 20 K. This is the first time these two magnetic phenomena have been observed in the same magnetic system.     

Atomistic simulation for the phase stability, site preference and thermal expansion of YFe12−xTx (T=Ti, V, Cr, Mn, Zr, Nb, Mo, W)

The effect of the ternary element on the structural properties of YFe_12−xT_x (T=Ti, V, Cr, Mn, Zr, Nb, Mo, W) has been studied by using interatomic pair potentials based on ab initio method and lattice inversion equations. Calculated results show that adding ternary element T makes the crystal cohesive energy of YFe_12−xT_x decrease markedly, which indicates that T helps stabilize the 1:12 phases. The ternary elements T prefer to occupy the 8i sites in these compounds. The calculated results are found in good agreement with the values deduced from experiments. Furthermore, we have calculated the thermal expansion of YFe₁₁V compounds with the ThMn₁₂ structure. The method utilized in the present investigation offers a rather easy and direct way to study the structural properties of YFe_12−xT_x.     

Magnetic anisotropy of YFe11Ti and its hydride

A study of the magnetic anisotropy of single-crystal YFe₁₁Ti and of its hydride is reported. The measurements were performed on a vibrating-sample and a torsion magnetometer on field-oriented single-crystal and powder samples. The temperature dependence of the magnetic anisotropy constants K ₁ and K ₂ was determined by a mathematical treatment of experimental torque curves and by applying the Sucksmith-Thompson technique to magnetization data. It is shown that the temperature dependence of the magnetic anisotropy constants of YFe₁₁Ti and of its hydride does not follow the prediction of the single-ion exchange model. It is found that hydrogenenation results in a growth of the magnetic anisotropy constant, which can be explained by an electron-density redistribution near the lattice positions occupied by iron atoms. Fiz. Tverd. Tela (St. Petersburg) 40, 285–289 (February 1998)     

Magnetism and electronic correlations in the iron aluminides R Fe2Al10 (R = Y,Yb)

YFe₂Al₁₀ and YbFe₂Al₁₀ are new additions to the currently investigated rare‐earth (R) series of compounds RT₂Al₁₀ in which T is Fe, Ru, or Os. Unusual physical properties are drawing considerable attention to this group of compounds. Intriguingly, very small moments order in CeRu₂Al₁₀ at as high as 27 K, whereas CeFe₂Al₁₀ is a Kondo insulator with very strong hybridization of the Ce magnetic moments and no magnetic ordering at all. Here we present the first results of two further iron‐based compounds YFe₂Al₁₀ and YbFe₂Al₁₀ as a timely contribution to the knowledge of the physics at work in this system. In YFe₂Al₁₀ we find signatures of correlated electrons and the low‐temperature results of specific heat, magnetic susceptibility and electrical resistivity are indicative of ferromagnetic quantum criticality. In YbFe₂Al₁₀ localized f‐electron spins of Yb become involved in correlations with a moderately enhanced electronic specific heat that classifies it as a non‐ordered heavy‐fermion compound. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Magnetic properties of Y(Fe0.95Mn0.05)2 compound

Mössbauer spectroscopy, magnetization and X-ray diffrraction measurements of the ternary compound Y(Fe_0.95Mn_0.05)₂ prepared by melting were performed in order to investigate the effect of the substitution of Mn on the magnetic properties of YFe₂. The experimental results show change of the lattice parameter. The magnetization and the Curie temperature decrease with substitution of Fe by Mn.     

Hydrogen absorption and its effect on the magnetic properties of rare-earth iron intermetallics

X-ray diffraction studies of the hydrogen absorption in several YFe and CeFe intermetallic compounds showed that no structural changes occur upon hydrogen absorption in Y₆Fe₂₃, YFe₃, YFe₂. The lattice constants of the hydrides were found to be appreciably larger than those of the pure intermetallic compounds. The magnetic properties of the hydrides were determined and compared with the original compounds. In all cases the magnetic moment per Fe atom proved to be much larger in the hydride phases. Hydrogen absorption can lead to a decrease as well as to an increase of the magnetic ordering temperature (T_c). These changes in T_c could adequately be explained in terms of the observed increases in lattice constant and the data available for the pressure derivative of T_c of these compounds.     

Mössbauer studies on some new magnetic oxides

Three new magnetic oxides have been synthesised, viz., Li₂Fe₃VO₈, YFe₃TiO₈ and CaFe₃VO₈. XRD studies show that the first compound has a cubic symmetry of spinel type while the latter two show orthorhombic symmetry. All the three compounds are magnetic at room temperature, however, the first compound shows a large magnetic moment as compared to the other two. Their Mössbauer spectra at room temperature show distinct hyperfine split spectral lines. The present preliminary investigation indicates that the long range order is ferrimagnetic in Li₂Fe₃VO₈ and CaFe₃VO₈ while in YFe₃TiO₈ it is more likely to be canted antiferromagnetic.     

Fermi liquid breakdown and evidence for superconductivity in YFe2Ge2

In the d‐electron system YFe₂Ge₂, an unusually high and temperature dependent Sommerfeld ratio of the specific heat capacity C /T ～ 100 mJ/(mol K²) and an anomalous power law temperature dependence of the electrical resistivity signal Fermi liquid breakdown, probably connected to a close‐by quantum critical point. Full resistive transitions and DC diamagnetic screening fractions of up to 80% suggest that pure samples of YFe₂Ge₂ superconduct below 1.8 K. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

A first-principle study of the magnetic,electronic and elastic properties of the hypothetical YFe5 compound

We present a DFT-based study of the magnetic properties, electronic structure and bulk modulus of YFe₅ at ambient and higher hydrostatic pressures. The LSDA and GGA approximations, as implemented in the electronic structure code FPLO-09, are used throughout the scalar relativistic calculation in this work. Charge and spin density maps using the WIEN2k are also reported for the equilibrium lattice constants. Our study shows that the magnetic phase of this hypothetical compound is more stable than the nonmagnetic phase, and that the application of pressure on magnetic YFe₅ has a prominent effect on its magnetic and electronic properties, e.g. the reduction of the magnetic moment and finally the disappearance of ferromagnetism.     

过渡族金属替代元素M对YFe11M系统的磁性质的影响

基于密度泛函理论(DFT),采用线性缀加平面波展式结合改进的局域轨道方法(APW+lo),对具有ThMn₁₂结构的永磁材料YFe₁₁M(M=Sc,V等)的结构和磁性进行了计算和分析.探讨了过渡族金属替代元素M在YFe₁₁M中系统的最佳可能占位.讨论了不同替代元素M对YFe₁₁M系统的磁性质的影响. 关键词： 11M')" href="#">YFe₁₁M 密度泛函 晶体结构 磁矩     

Conductivity and specific heat anomalies at the low temperature transition in the stoichiometric YFe2O4

Anomalies were found in electrical conductivity and specific heat at the two-step transition in the stoichiometric YFe₂O₄. It is concluded that the transition is a new type of Verwey transition: i.e., ordering of ionic charge, Fe²⁺ and Fe³⁺, accompanied by lattice distortion and antiferromagnetic spin ordering.     

Ab initio calculations of magnetic properties of the interstitially doped YFe<Subscript>11</Subscript>Mo compound

The recent increase in the number of studies of RFe_11–xM_x compounds is related to their promising application as permanent magnets. However, the insufficiently high value of the Curie temperature T_C of these compounds is a barrier to their widespread use. The increase in the Curie temperature of these compounds is achieved by doping with the light nonmetallic atoms such as hydrogen, nitrogen, and carbon. In this paper, it is shown numerically that this doping leads to drastic changes of the electronic band dispersions in a wide energy region around the Fermi level. This in turn changes values of the magnetic moments of ions and Heisenberg exchange interaction parameters. The values of ab initio calculated magnetic moments and direct exchange interaction parameters make it possible to calculate the Curie temperatures for both parent and nitrogen-doped compounds within the mean-field approach to the Heisenberg model in the sample of YFe₁₁Mo, a typical representative of the R(Fe,M)₁₂L class. Theoretical values of T_C obtained for YFe₁₁Mo and YFe₁₁MoN (514 and 723 K respectively) are consistent with experimental ones (472 and 664 K) with an accuracy of 10%. Also, the calculated increase in T_C upon nitrogenization (about 200 K) is in good agreement with the experimental data.     

A survey on the effect of vanadium content on the magnetoelastic properties of YFe12−xVx alloys

Experimental results on the thermal expansion and magnetostriction of YFe_12−xV_x (1.5≤x≤3.5) alloys are reported. The results show that the anisotropic magnetostriction (Δλ) at a finite field (1.5 T) increases with increasing vanadium content in the range of x<2. But for x>2, a decrease in the magnetic anisotropy with increasing vanadium content causes a decrease in the saturation values of Δλ. In addition, the thermal expansion coefficient becomes a minimum for x≈2. Experimental curves exhibit that the forced volume magnetostriction (ΔV/V) is positive and increases linearly with the applied field at high fields. But in the low field region (≤0.5 T), a minimum appears in the isothermal curves of ΔV/V around the saturation field. The results are explained by considering the influence of vanadium content on the magnetization anisotropy of YFe_12−xV_x compounds.     

Changes in vibrational modes near the magnetic phase transition in magnetostrictive materials

Mössbauer studies of ⁵⁷Fe in RFe₂ (R = Y, Tb, Dy, Ho) at temperatures 300 to 800 K and in R(Co_0.98Fe_0.02)₂ (R = Tb, Dy) at temperatures 85 to 300 K have been performed. The spectra yield the temperature dependence of the easy axis of magnetization, the magnetic hyperfine fields, electric field gradients and isomer shift. From the total Mössbauer absorption area the recoil free fraction was determined. The relative intensities of the various Mössbauer absorption lines yield the anisotropy in the recoil free fraction. In all compounds, except in YFe₂, all measured quantities show large variations near the magnetic phase transition temperature (T_c). The variation in the recoil free fraction and in its anisotropy and the variation in the electric field gradient are all consistent, quantitatively, with anisotropic softening in the vibration modes of the Fe nucleus near T_c In RFe₂ the modes in which the iron vibrates along the local C_3h symmetry axes are softened less than the perpendicular modes. In RCo₂ it is the other way around. It seems, that these critical softening phenomena are due to the strong magnon-phonon coupling present in these materials and absent in YFe₂. This conclusion is confirmed by a theoretical analysis given in an adjacent paper. In it we use the Green function method to calculate the average of the square of vibrational amplitudes in a Debye solid with a strong magnon-phonon coupling at temperatures close to the magnetic phase transition. We show that these amplitudes may increase or decrease near T_c depending on the details of the coupled system.     

Correlation of soft magnetic properties with free volume and medium range ordering in metallic glasses probed by fluctuation microscopy and positron annihilation technique

Amorphous ribbons of different thicknesses of Co_64.5Fe_3.5Si₁₆B₁₄Ni₂ alloy were synthesized using the melt spinning technique by varying wheel speed. The effect of cooling rate on the ribbon thickness and their soft magnetic properties have been studied. The amorphous structure has been characterized in terms of structural free volume and medium range order (MRO) by positron annihilation spectroscopy and fluctuation electron microscopy techniques. Positron lifetime spectra of amorphous samples showed two lifetime components. The first component was found to be correlated with MRO whereas, the second lifetime component was found to be associated with nanovoid type of defects, and the second component was strongly dependent on processing conditions. It could be established that the coercivity of the amorphous samples produced by the rapid solidification technique mainly depends on the defects formed during processing rather than change induced in MRO.     

Magnetic properties of RFe6Al6 alloys with R=Y,Dy

Intermetallics crystallizing in ThMn₁₂ type structure were investigated. Magnetostatic measurements showed that the magnetic ordering temperature and the magnetic moment of YFe₆Al₆ samples depend strongly on thermal and mechanical treatment. These measurements for a powdered sample of YFe₆Al₆ showed that the alloy was a ferromagnet with a Curie temperatureT _C =265 K and a magnetic moment μ=5.1 μ_B/f.u. at 77.4 K. From X-ray, magnetostatic and Mössbauer effect measurements it appears that the Fe atoms prefer the 8j and 8f crystallographic positions. Magnetostatic measurements for a powdered sample of DyFe₆Al₆ showed that this alloy was a ferrimagnet with the ordering temperatureT ₀=311 K and magnetic moment μ=1.1 μ_B/f.u. at 77.4 K.