Effects of the Mn/Co ratio on the magnetic transition and magnetocaloric properties of Mn1+xCo1-xGe alloys

We have investigated the magnetic transition and magnetocaloric effects of Mn 1+x Co 1 x Ge alloys by tuning the ratio of Mn/Co.With increasing Mn content,a series of first-order magnetostructural transitions from ferromagnetic to paramagnetic states with large changes of magnetization are observed at room temperature.Further increasing the content of Mn (x=0.11) gives rise to a single second-order magnetic transition.Interestingly,large low-field magnetic entropy changes with almost zero magnetic hysteresis are observed in these alloys.The effects of Mn/Co ratio on magnetic transition and magnetocaloric effects are discussed in this paper.     

Electron paramagnetic resonance spectroscopy of Cr 3+ in hexagonal Cs 2 NaGaF 6 crystals

Powder samples of hydrothermally grown Cr 3+ -doped Cs 2 NaGaF 6 crystals have been investigated with electron paramagnetic resonance spectroscopy at X - (9.5 v GHz) and Q -band (34 v GHz). Analysis of the spectra clearly demonstrates that there are two distinct Cr 3+ centres in the Cs 2 NaGaF 6 crystal, having nearly identical g factors, but differing largely from the viewpoint of their zero field splitting. By using the 53 Cr hyperfine spectra observed with electron nuclear double resonance spectroscopy, it is deduced that these centres have opposite signs for the zero field splitting. The spectroscopic properties of the Cr 3+ centres in the isostructural Cs 2 NaGaF 6 and Cs 2 NaAlF 6 crystals are compared and discussed.     

First-principles linear muffin-tin orbital investigations in the electronic and magnetic structures of double perovskites Ba2TMoO6 (T=V,Cr, Mn,Fe and Co)

The electronic and magnetic structures of ordered double perovskites Ba₂TMoO₆ (T=V, Cr, Mn, Fe and Co) are systematically investigated by means of the first-principle linear muffin-tin orbitals with the atomic-sphere approximation (LMTO-ASA) method. The calculations are performed by using the both local spin density approximation (LSDA) and the LSDA+U Coulomb interaction schemes. The results show a half-metallic ferrimagnetic ground states for T=Cr, Fe and Co in LSDA+U treatment, whereas half-metallic ferromagnetic character is observed for T=V. For T=Mn, insulating ground state is obtained, stabilized in the antiferromagnetic state. The LSDA+U calculations yield better agreement with the theoretical and the experimental results than do the LSDA.     

MnxGe1−x thin layers studied by TEM, X-ray absorption spectroscopy and SQUID magnetometry

Mn_0.06Ge_0.94 samples have been grown by molecular-beam epitaxy on Ge(0 0 1)2 × 1. High-resolution transmission electron microscopy shows the coexistence of an ordered diluted Mn_0.06Ge_0.94 film and of nanoscopic crystallites, which were identified as Mn₅Ge₃ by electron diffraction. The magnetic properties of the Mn_0.06Ge_0.94 samples show a superposition of a paramagnetic behavior, due to the interaction of Mn atoms diluted in the Ge host, and a ferromagnetic behavior attributed to the Mn₅Ge₃ crystallites dispersed into the films. The Mn L_2,3 X-ray absorption spectra of the Mn_0.06Ge_0.94 films exhibit a lineshape typical of metallic Mn, with considerably reduced multiplet structure.     

The effect of 3d-transition metal solutes on the ferrimagnetism of Fe7Se8

The effect of 3d-transition metal solutes on the magnetization of Fe₇Se₈ has been investigated by studying two series of alloys with constant solute concentrations of 3 and 10 at%, respectively. Small additions of Ti and V were sufficient to reduce the magnetization by one order of magnitude, whereas Ni slightly increased the magnetization. The changes due to the presence of Cr, Mn and Co were less noticeable. In certain cases the thermal treatment turned out to be a decisive factor in determining the type of magnetism. Alloys with Cr quenched from 1070 K were clearly antiferromagnetic. Quenched samples containing ten percent Ti were paramagnetic following a Curie-Weiss type law, but slow cooling from 1070 K reestablished the ferrimagnetic state. Order-disorder phenomena were assumed to be responsible for the changes due to the thermal treatment.     

Multifractal analysis of the electronic states in the Fibonacci superlattice under weak electric fields

We report the effect of substitution of Cr by V and Mn in the spin- \frac12\frac{1}{2} 1 2 dimer compound Sr₃Cr₂O₈. In the pentavalent Cr site, V ion has zero effective moment, while Mn remains in the spin-1 state. For both V and Mn substitutions, the gap in the spin excitation spectra remains almost unchanged in the dilute doping regime. The low temperature Curie tail, expected to be originated from free or quasi-free magnetic ions, is found to be larger in case of V doped samples. However, inter-dimer interaction is found to be stronger in the Mn doped samples. The underlying magnetic behaviour of these doped samples is presumably related to the effect of broken dimers due to random substitutions.     

Ferromagnetism in MnxGe1−x films prepared by magnetron sputtering

Mn_xGe_1−x thin films were prepared by magnetron sputtering with a substrate temperature of 673 K and subsequently annealed at 873 K. The X-ray diffraction (XRD) measurements showed that all samples had a single Ge cubic structure. No films showed clear magnetic domain structure under a magnetic force microscope (MFM). Atom force microscope (AFM) measurements showed that the films had an uniform particle size distribution, and a columnar growth pattern. X-ray photoelectron spectroscopy (XPS) measurements indicated that the valences of both Mn and Ge atoms increase with the Mn concentration. The resistance decreased with increasing temperature, suggesting that the films were typical semiconductors. Magnetic measurements carried out using a Physical Property Measurement System (PPMS) showed that all samples exhibited ferromagnetism at room temperature. There was a small concentration of Mn₁₁Ge₈ in the films, but the ferromagnetism was mainly induced by Mn substitution for Ge site.     

非晶态合金FeTmB的磁矩和居里温度

本文系统地报道单辊液淬方法制备FeTmB(Tm=Ti,V,Cr,Mn,Zr,Nb,Mo,Ta,W)非晶态合金的磁性,讨论了3d,4d,5d元素的加入对非晶态FeB合金的磁矩和居里温度的影响。实验结果表明在非晶态FeTmB合金系中Fe原子磁矩都在2.0μ_B左右。Tm原子在非晶态Fe基合金中比在相应的晶态合金中显示更强的局域特性。Tm原子的磁矩与元素的外层电子数有关,IVB(Ti),VB(V),VIB(Cr),VIIB(Mn)族原子的磁矩分别约为4,5,4,3μ_B,Tm的磁矩与铁原子磁矩反平行耦合。合金磁矩随Tm含量的变化率dμ/dx与混合模型的计算值相符合。用虚拟束缚态讨论,得到IVB(Ti),VB(V)族元素的虚拟束缚态在费密面以上,VIB(Cr),VIIB(Mn)族元素的虚拟束缚态与费密面交迭。 关键词：     

非晶态Fe86M4Zr10合金的磁性和电性

本文研究了非晶态Fe₈₆M₄Zr₁₀(M=V,Cr,Mn,Fe,CO,Ni,Cu,B,Si)合金的基本磁性和低温电性,讨论了不同元素M的掺杂对FeZr合金居里温度和磁矩的影响,并用相干交换散射模型解释了样品在居里温度附近出现的电阻率极小。 关键词：     

Electronic structure and half-metallicity in Heusler alloys Fe2YB (Y=Ti, V, Mn, Cr)

The electronic structure and magnetic properties of B-based Heusler alloys Fe₂YB (Y=Ti, V, Cr and Mn) have been studied theoretically. These alloys are all ferrimagnets except for Fe₂VB. The latter has 24 valence electrons and is a paramagnetic semimetal. Fe₂CrB is predicted to be half-metals at equilibrium lattice constant. The spin polarization of Fe₂MnB is also quite high. The calculated total moments are 1.00 μ_B for Fe₂CrB and 2.04 μ_B for Fe₂MnB. In Fe₂CrB and Fe₂MnB, the total moments are mainly determined by the partial moment of Cr or Mn. The Fe moment is relatively small and antiparallel to that of Cr or Mn. Under uniform lattice distortion, the half-metallicity of Fe₂CrB is more stable than Fe₂MnB, which is related to the detailed DOS structure of them near E_F.     

Determination par diffraction des neutrons de la structure antiferromagnetique du grenat Mn3Al2Ge3O12

The magnetic structure of the antiferromagnet Mn₃Al₂Ge₃O₁₂ with Mn²⁺ on dodecahedral site is determined by neutron diffraction. At the difference of the magnetic structures of isomorphous rare earth garnets, the observed structure is rhombohedral. The importance of super-superexchange interactions of the type MnOOMn or MnOGeO is pointed out.     

Magnetic ordering in digital alloys of group-IV semiconductors with 3<Emphasis Type="Italic">d</Emphasis>-transition metals

The ab initio investigation of the magnetic ordering in digital alloys consisting of monolayers of 3d-transition metals Ti, V, Cr, Mn, Fe, Co, and Ni introduced into the Si, Ge, and Si_0.5Ge_0.5 semiconductor hosts is reported. The calculations of the parameters of the exchange interactions and total-energy calculations indicate that the ferromagnetic order appears only in the manganese monolayers, whereas the antiferromagnetic order is more probable in V, Cr, and Fe monolayers, and Ti, Co, and Ni monolayers are nonmagnetic. The stability of the ferromagnetic phase in digital alloys containing manganese monolayers has been analyzed using the calculations of magnon spectra.     

Microscopic magnetic properties of nonstoichiometric V2O3+x—NMR and inelastic spin-flip neutron scattering measurements

The local magnetic properties of the V sites in the nonstoichiometric V₂O_3+x (0 ? x <0.08) have been examined by nuclear magnetic resonance and inelastic spin-flip neutron scattering techniques. The samples with x = 0.01 and 0.02 show a paramagnetic metal (PM)-antiferromagnetic insulator (AFI) transition. In the AFI phase, two distinct ⁵¹V NMR signals with hyperfine fields H_n = 184.9±0.5 kOe and 71±1 kOe were observed at 1.8 K, which were assigned as due to V³⁺ and V³⁺ sites, respectively. On the other hand, the samples with x = 0.04 and 0.06 were metallic down to 1.4K, and showed a paramagnetic (PM)-antiferromagnetic (AFM) transition at about 10 K. In these samples, a ⁵¹V NMR signal with H_n = 58±2 k0e and one with 〈H_n〉 = 9kOe were observed at 1.8 K, which were assigned as due to V³⁺-like sites and the matrix V sites, respectively. These results are entirely consistent with those obtained from the neutron experiment. We propose that in the metallic phase (0.04 ? x < 0.08) the minority V⁴⁺-like sites are magnetically localized in the delocalized V matrix and may be responsible for the antiferromagnetic long range order below 10 K.     

Bulk Sn1−xMnxO2 magnetic semiconductors without room-temperature ferromagnetism

Polycrystalline Sn_1−xMn_xO₂ (0≤x≤0.05) diluted magnetic semiconductors were prepared by solid-state reaction method and their structural and magnetic properties had been investigated systematically. The three Mn-doped samples (x=0.01, 0.03, 0.05) undergo paramagnetic to ferromagnetic phase transitions upon cooling, but their Curie temperatures are far lower than room temperature. The magnetization cannot be attributed to any identified impurity phase. It is also found that the magnetization increases with increasing Mn doping, while the ratio of the Mn ions contributing to ferromagnetic ordering to the total Mn ions decreases.     

From antiferromagnetic to ferromagnetic coupling for V adatoms on Co(001) substrates

We discuss the polarization of V atoms on Co(001) substrates within density functional calculations. For sub-monolayer coverage the coupling between V and Co is clearly of antiferromagnetic type whereas it changes to ferromagnetic coupling in the case of a full V monolayer on Co(001). The results obtained in the case of a sub-monolayer coverage are in agreement with recent X-ray magnetic circular dichroism by Huttel et al. [Phys. Rev. B 68, 174405 (2003)]. The transition from antiferromagnetic coupling (in the case of sub-monolayer coverage) to ferromagnetic coupling (for a full monolayer coverage) is discussed in terms of local coordination numbers and V-Co hybridization. Comparison with Cr and Mn coverages on Co(001) complicates the problem: i) submonolayer Cr coverage stabilizes the antiferromagnetic coupling between Cr and Co atoms (like for V on Co(001) whereas a Cr monolayer on Co presents in-plane antiferromagnetic coupling; ii) submonolayer Mn coverage stabilizes now the ferromagnetic coupling between Mn and Co whereas a Mn monolayer on Co(001) presents an in-plane antiferromagnetic coupling. Competition between Co induced magnetism and surface induced magnetism at V sites is discussed.     

Magnetic properties of transition metal substituted La0.85Ag0.15Mn1−yMyO3 compounds (M=Co, Cr and Al)

In this paper we report a systematic study of Mn-site substitution by M=Co, Cr and Al in La_0.85Ag_0.15MnO₃ series to understand the magnetic interactions between Mn and other transition metals. The long-range ferromagnetic (FM) ordering of the parent compound was significantly affected by Mn-site substitution. The measured magnetic properties of Co-doped samples have been explained on the basis of FM interactions in Mn³⁺-O-Mn⁴⁺, Co²⁺-O-Mn⁴⁺, Co³⁺-O-Mn⁴⁺ networks and simultaneous antiferromagnetic (AFM) interactions in Mn⁴⁺-O-Mn⁴⁺, Co²⁺-O-Mn³⁺ networks. The magnetic properties of Cr-doped compounds could be understood on the basis of double exchange FM interactions in Mn³⁺-O²⁻-Mn⁴⁺ networks and competing AFM in Cr³⁺-O-Mn⁴⁺, Mn⁴⁺-O-Mn⁴⁺, Cr³⁺-O-Mn³⁺ networks. However, it is found that the doping of Al ions play a role of magnetic dilution, without contributing any other competing magnetic interaction. The field variations of magnetization of all the above three series could be analysed by fitting to Brillouin function model and the effective spin contribution for FM has been determined. The measured saturation magnetization has been explained quantitatively.     

Magnetic characterization of vanadium oxide/polyaniline nanotubes

We present a magnetic study of vanadium oxide nanotubes (VO_x-NTs) with polyaniline (PAni). Transmission electron microscopy (TEM) shows the tubular shape and the multi-wall structure of the nanotubes. The static magnetic susceptibility measured at different magnetic fields shows a Curie behavior, while the magnetization versus magnetic field presents a non-linear dependence at low temperatures. Both experiments can be explained by the presence of paramagnetic ions with S = 1/2. Using Electron Paramagnetic Resonance technique, we identified as V⁴⁺(3d¹) the paramagnetic ions present in the nanostructures. All the experimental results can be explained by a fraction between 14% and 18% of V⁴⁺ with respect to the total V atoms in the system.     

Effect of annealing temperature on structure, magnetic properties and optical characteristics in Zn0.97Cr0.03O nanoparticles

The Cr-doped zinc oxide (Zn_0.97Cr_0.03O) nanoparticles were successfully synthesized by sol-gel method. The relationship between the annealing temperature (400 °C, 450 °C, 500 °C and 600 °C) and the structure, magnetic properties and the optical characteristics of the produced samples was studied. The results indicate that Cr (Cr³⁺) ions at least partially substitute Zn (Zn²⁺) ions successfully. Energy dispersive spectroscopy (EDS) measurement showed the existence of Cr ion in the Cr-doped ZnO. The samples sintered in air under the temperature of 450 °C had single wurtzite ZnO structure with prominent ferromagnetism at room temperature, while in samples sintered in air at 500 °C, a second phase-ZnCr₂O₄ was observed and the samples were not saturated in the field of 10000 Oe. This indicated that they were mixtures of ferromagnetic materials and paramagnetic materials. Compared with the results of the photoluminescence (PL) spectra, it was reasonably concluded that the ferromagnetism observed in the studied samples was originated from the doping of Cr in the lattice of ZnO crystallites.     

Co和Cr掺杂对层状钙钛矿锰氧化物La1.2Sr1.8Mn2O7磁性的影响

研究了层状钙钛矿锰氧化物La_1.2Sr_1.8Mn₂O₇中Mn被Co和Cr替代对磁性能的影响.Co替代Mn后,长程铁磁序被破坏,铁磁性减弱,出现团簇玻璃态和自旋玻璃态,表明Co离子和Mn离子之间不存在双交换作用.而Mn被Cr替代后长程铁磁序仍然保持,证实Cr³⁺和Mn³⁺之间存在铁磁性交换作用. 关键词：