Magnetic ordering in CeMnCuSi2

Temperature and field-dependent magnetization measurements on polycrystalline CeMnCuSi₂ reveal that the Mn moments in this compound exhibit ordering with a ferromagnetic (FM) component ordered instead of the previously reported purely antiferromagnetic (AFM) ordering. The FM ordering temperature, T_c, is about 120 K and almost unchanged with external fields up to 50 kOe. Furthermore, an AFM component (such as in a canted spin structure) is observed to be present in this phase, and its orientation is modified rapidly by the external magnetic field. The Ce L₃-edge X-ray absorption result shows that the Ce ions in this compound are nearly trivalent, very similar to that in the heavy fermion system CeCu₂Si₂. Large thermomagnetic irreversibility is observed between the zero-field-cooled (ZFC) and field-cooled (FC) M(T) curves below T_c indicating strong magnetocrystalline anisotropy in the ordered phase. At 5 K, a metamagnetic-type transition is observed to occur at a critical field of about 8 kOe, and this critical field decreases with increasing temperature. The FM ordering of the Mn moments in CeMnCuSi₂ is consistent with the value of the intralayer Mn–Mn distance R^a_Mn–Mn=2.890 Å, which is greater than the critical value 2.865 Å for FM ordering. Finally, a magnetic phase diagram is constructed for CeMnCuSi₂.     

Anticrossing spectroscopy in multi-nanolayer structures

Presently we explored nanosandwich structures with graphite (Gt) and graphene (Gn) nanolayers. We found that in Pt–SiO₂–Gt, Pt–BN–Gt and Pt–SiO₂–Ni–Gn structures the spectra may be decomposed into several components, each corresponding to a different value of the total spin angular momentum S. Only one component was required to describe the Pt–SiO₂–Ni–Gn spectra at 5.3 K, with additional components appearing at higher temperatures. On the other hand, a single component described the Pt–BN–Ni–Gn spectra at all temperatures. Temperature dependence of the spectra of the Pt–SiO₂–Ni–Gn system was studied in the 5.3–75.3 K range. Presently we obtained experimental results for novel sandwich systems, with the Gn layer only two monoatomic layers thick. Thus, we compared experimental spectra of a three-nanolayer sandwich system containing a Gt nanolayer with those of a four-nanolayer system containing a diatomic Gn layer. The experimental results were discussed using a theoretical model of the respective physical mechanisms. We propose an exchange anticrossing mechanism, whereby the spin-state polarization of the given Zeeman?s substate in the Pt nanolayer is transported to Gt or Ni–Gn nanolayer by the exchange interaction between the two layers. As long as exchange interaction coupling spin states in different nanolayers is involved, we term the respective spectra the “spin anticrossing exchange-resonance spectra”. This clarifies the physical origins of some of the model parameters, i.e. the growing external magnetic field shifts the Zeeman?s substates in the different layers differently, producing the anticrossing spectrum. In the frameworks of the developed model, we propose spin–orbit (SO) interaction as the main factor inducing the spin–lattice relaxation, which is one of the important factors determining the line shape. We performed ab initio calculations of the SO interaction in carbon and metal nanolayers, finding that the SO interactions monotonously increase with the atomic number.     

Mössbauer characterization of tin dopant ions in the antiferromagnetic ilmenite MnTiO3

Mössbauer parameters of ¹¹⁹Sn diamagnetic dopant cations in an antiferromagnetic compound having the ilmenite structure are for the first time reported. The spectra reveal a well resolved hyperfine splitting pattern of combined magnetic and quadrupole interactions (at 5 K, δ=0.19 mm/s, H₁=52.5 kOe, eV_ZZQ_3/2=−0.80 mm/s, θ≈0°). This spectral component whose contribution (A₁=82%) represents more than four fifths of the total amount of the dopant (Sn/(Mn+Ti)=1/200) is assigned to Sn(IV) ions located in the bulk of MnTiO₃, on the Mn(II) site, and with a Mn(II) vacancy in their nearest surrounding. Two spectral components with minor contributions are also observed: one of them (H₂≈25 kOe, A₂=8%) can be assigned to Sn(IV) ions, in the MnTiO₃ lattice as well, on a site where they exhibit a weaker spin polarization (this site could be the Ti(IV) one) and the other (H₃=0 kOe, A₃=10%) to SnO₂ or/and Ti_1−xSn_xO₂ clusters. The Néel temperature of MnTiO₃ probed by the ¹¹⁹Sn dopant (T_N=69±2 K) agrees well with the values previously provided by ESR and antiferromagnetic resonance measurements. Variation of H₁ with temperature follows close the Brillouin function for S=5/2. No perturbation appears in the Mössbauer spectra around T=90 K where a broad peak, characteristic of 2D magnetic interactions, is observed on the static magnetic susceptibility curve.     

Effect of low-valent atom substitution on electronic structure and magnetic properties of Fe1.5M0.5CoSi (M=V, Cr, Mn, Fe) Heusler alloys

Quaternary Heusler alloys Fe_1.5M_0.5CoSi with M=V, Cr, Mn and Fe have been investigated theoretically and experimentally. All of these samples crystallize in the ordered Heusler-type structure. The calculated electronic structure shows a pseudogap around E_F in the minority spin states of Fe₂CoSi. With the substitution of low-valent atoms for Fe, the majority antibonding peak is shifted to higher energy and a minority gap around the Fermi level is opened. High spin polarization ratio is obtained in Fe_1.5M_0.5CoSi (M=V, Cr, Mn) alloys. The calculated total spin moments decrease with decreasing number of valence electrons and follow the Slater-Pauling curve, which agree with the experimental results well. The Curie temperature decreases as M atom varies from Fe to V, but is always higher than 650 K, which is suitable for technical applications.     

Mössbauer spectroscopic study of the thermal spin crossover in [Fe(II)(isoxazole)6](ClO4)2

The ⁵⁷Fe Mössbauer spectroscopy of mononuclear [Fe(II)(isoxazole)₆](ClO₄)₂ has been studied to reveal the thermal spin crossover of Fe(II) between low-spin (S=0) and high-spin (S=2) states. Temperature-dependent spin transition curves have been constructed with the least-square fitted data obtained from the Mössbauer spectra measured at various temperatures between 84 and 270 K during a cooling and heating cycle. This compound exhibits an unusual temperature-dependent spin transition behaviour with T_C(↓)=223 and T_C(↑)=213 K occurring in the reverse order in comparison to those observed in SQUID observation and many other spin transition compounds. The compound has three high-spin Fe(II) sites at the highest temperature of study of which two undergo spin transitions. The compound seems to undergo a structural phase transition around the spin transition temperature, which plays a significant role in the spin crossover behaviour as well as the magnetic properties of the compound at temperatures below T_C. The present study reveals an increase in high-spin fraction upon heating in the temperature range below T_C, and an explanation is provided.     

The ferromagnetic and half-metallic properties of the layered organic–inorganic hybrid Fe[CH3(CH2)2PO3(H2O)]

The density-functional theory (DFT) within the full potential linearized augmented plane wave (FPLAPW) method was applied to study the layered organic–inorganic hybrid Fe[CH₃(CH₂)₂PO₃(H₂O)]. The relative stability of the ground state, the electronic band structure, the magnetic and the conducting properties were investigated. The calculations reveal that the compound has a stable ferromagnetic ground state and the spin magnetic moment per molecule is about 4.0 μ_B, which is mainly from Fe(II) ion. By analysis of the band structure, we find that the compound has half-metallic properties.     

Effects of B′-site transition metal on the properties of double perovskites Sr2FeMO6 (M=Mo, W): B′ 4d-5d system

The crystal structure, magnetic and magnetotransport properties of the variation of B′-site transition metal in Sr₂FeMO₆ (M=Mo, W) with double perovskites structure have been investigated systematically. Measurements of magnetization vs. temperature at H=5 T show that Sr₂FeMoO₆ is a ferromagnet and Sr₂FeWO₆ is an antiferromagnet with T_N∼35 K. Additionally, the large magnetoresistance ratio (MR) of ∼22% (H=3 T) at room temperature (RT) was observed in the Sr₂FeWO₆ compound. However, the Sr₂FeMoO₆ compound did not show any significant MR even at high fields and RT (MR∼1%; H=3 T and 300 K). The implications of these findings are supported by band structure calculations to explain the interaction between the 4d(Mo) and 5d(W) orbitals of transition metal ions and oxygen ions.     

Ion dependence of magnetic anisotropy in MFe2O4 (MFe,Co, Mn) nanoparticles synthesized by high-temperature reaction

The influence of different M²⁺ cations on the effective magnetic anisotropy of systems composed of MFe₂O₄ (M=Fe, Co and Mn) nanoparticles was investigated. Samples were prepared by the high-temperature (538 K) solution phase reaction of Fe (acac)₃, Co (acac)₂ and Mn (acac)₂ with 1,2 octanodiol in the presence of oleic acid and oleylamine. The final particles are coated by an organic layer of oleic acid that prevents agglomeration. Transmission electron microscopy (TEM) images show that particles present near spherical form and a narrow grain size distribution, with mean diameters in the range of 4.5–7.6 nm. Powder samples were analyzed by ac susceptibility and Mössbauer measurements, and K_eff for all samples was evaluated using both techniques, showing a strong dependence on the nature of the divalent cation.     

Magnetic behavior of MnAs precipitates in Ga1−xMnxAs diluted magnetic semiconductor

Ferromagnetic Ga_1−xMn_xAs layers (where x≈4.7–5.5%) were grown on (1 0 0) GaAs substrates by molecular beam epitaxy. These p-type (Ga,Mn)As films were revealed to have a ferromagnetic structure and ferromagnetism is observed up to a Curie temperature of 318 K, which is ascribed to the presence of MnAs secondary magnetic phases within the film. It is highly likely that the phase segregation occurs due to the high Mn cell temperature around 890–920 °C, as it is well established that GaMnAs is unstable at such a high temperature. The MnAs precipitate in the samples with x≈4.7–5.5% has a Curie temperature T_c≈318 K, which was characterized from field-cooled and zero-field-cooled magnetization curves.     

A comparison between shake-up and UV charge-transfer transitions

The He(II) spectra of the unsubstituted metallocenes {M(η-C₅H₅)₂}, M  V, Cr, Mn, Fe, Co, Ni and Ru, and of {Mn(η-C₅H₄Me)₂} are reported; both He(I) and He(II) spectra of some decamethylmetallocenes {M(η-C₅Me₅)₂}, where M  Mg, V, Cr, Mn, Fe, Co and Ni, are also given. Intensity changes between the He(I) and He(II) spectra are shown to provide a reliable guide to band assignment. A ligand field treatment of the decamethylmanganocene cation, including limited configuration interaction, gives values for Δ₂, B and C; these values are also in good agreement with the photoelectron spectra of {M(η-C₅Me₅)₂} where M  V, Cr and Fe. Overlap between the ligand and metal “d” band structures prevents complete assignment in the cases of Co and Ni.     

Magnetic,electrical transport and electron spin resonance studies of Fe-doped manganite LaMn0.7Fe0.3O3+δ

We have investigated the magnetic, electrical transport and electron spin resonance (ESR) properties of polycrystalline Fe-doped manganite LaMn_0.7Fe_0.3O_3+δ prepared by sol–gel method. A typical cluster-glass feature is presented by DC magnetization and AC susceptibility measurements and a sharp but shallow memory effect was observed. Symmetrical Lorentzian lines of the Mn/Fe spectra were detected above 120 K, where the sample is a paramagnetic (PM) insulator. When the temperature decreases from 120 K, magnetic clusters contributed from ferromagnetic (FM) interaction between Mn³⁺ and Mn³⁺/Fe³⁺ ions develop and coexist with PM phase. At lower temperature, these FM clusters compete with antiferromagnetic (AFM) ones between Fe³⁺ ions, which are associated with a distinct field-cooled (FC) effect in characteristic of cluster-glass state.     

High sensitivity of magnetism of the “114” ferrimagnet CaBaCo4O7 to stoichiometry deviation

The effect of oxygen/cobalt off-stoichiometry upon magnetism in CaBaCo₄O₇ has been investigated. It is shown that the oxides CaBaCo₄O_7+δ and CaBaCo_4−xO_7−δ (0≤x≤0.20) synthesized below 1100 °C in air exhibit phase separation, where ferrimagnetic regions with T_C~56 K to 64 K coexist with regions of magnetic clusters. The latter are detected from ac-susceptibility measurements, which show various frequency dependent peaks at ~14–20 K, 37 K, and 45 K, depending on the stoichiometry. The origin of this phenomenon is attributed to the great sensitivity of the material to oxidation as the synthesis of temperature is lowered, leading to the introduction of additional Co³⁺ cations, with respect to the ideal formula CaBaCo₂²⁺Co₂³⁺O_7. This excess Co³⁺ tends to destroy the ferromagnetic zig-zag chains of the ferrimagnetic structure and creates various cobalt spin clusters, leading to the inherent phase separation in the samples.     

Magnetic ordering in Ni–Cd ferrite

A series of Ni_1−xCd_xFe₂O₄ (0.0≤x≤0.8) were prepared by conventional double sintering ceramic method and sintered at 1200 °C for 6 h. X-ray diffraction results confirmed the single-phase spinel structures of all the samples. The Curie temperature decreases linearly with increasing Cd content, which is explained due to the weakening of the A–B exchange interaction. The sample with x=0.7 shows re-entrant type of spin glass phase transitions. The magnetic moment and saturation magnetization at 20 K are found to increase with Cd content up to x=0.5 and then tends to decrease for x>0.5. The increase in magnetic moment with cadmium is attributed to Neel's two sublattice (A- and B-sublattice) collinear models according to which the magnetic moment is the vector sum of the lattice magnetic moment. The decrease in magnetization for x>0.5 obeys the Yafet–Kittel (Y–K) model. The increase in Y–K angles for x>0.3 indicates the increased tendency for triangular spin arrangements on B-sites. This suggests the existence of a canted spin structure in the ferrite system with higher content of Cd.     

Crystallographic and magnetic structures of Y0.8Sr2.2Mn2GaO8−δ: a new vacancy-ordered perovskite structure

The anion-deficient perovskite Y_0.8Sr_2.2Mn₂GaO_8−δ (where δ∼0.1) has been synthesised and the crystal and magnetic structures determined by Rietveld analysis of neutron powder diffraction (NPD) data. The material has body-centred tetragonal symmetry (I4/mmm, a=7.6373(3) Å and c=15.6636(10) Å) and consists of alternating layers of octahedral and tetrahedral polyhedra, the layers being perpendicular to [001]. The octahedral layers are preferentially occupied by manganese and the tetrahedral layers are a mixture of manganese and gallium. The precise cation distribution depends critically on preparative conditions. An unusual structural feature of these materials is the arrangement of oxygen vacancies in the tetrahedral layers: in the basic structure, isolated squares of corner-linked tetrahedra are formed instead of the chains that are observed in brownmillerite phases. Additional oxide ions in this layer probably allow the Mn ions to achieve distorted square pyramidal coordination. Low temperature NPD and magnetisation data indicate antiferromagnetic ordering below 100 K.     

Magnetic susceptibility of (CH3NH3)2FeCl3Br: An example of a canted spin system

Magnetic susceptibility of (CH₃NH₃)₂FeCl₃Br is measured in the temperature range 80–200°K. A sharp peak characteristic of a spin canted system is found at T_N(H=0)=98° K. Magnetic field dependence of the susceptibility and the effect of the halide ion size on the canted spin is discussed.     

Exchange coupling in CoO–Co bilayer

In this work, exchange bias and coercivity enhancement in ferromagnet (FM)–antiferromagnet (AFM) bilayer have been investigated. CoO film (50 nm) was deposited by sputtering with a relatively high oxygen partial pressure. The deposited films were subsequently annealed at varied temperature up to 973 K in the air atmosphere. The CoO film shows a disordered structure in the as-deposited state and an increase of crystallinity after annealing characterized by XRD and Raman spectra. A 40-nm Co film was deposited on the as-deposited CoO and annealed films. The Co–CoO bilayer shows a large exchange bias up to 1600 Oe and relatively high coercivity up to 3200 Oe (H_C−) at 5 K, which is much larger than that of crystalline Co–CoO bilayer films without any treatment. The spin glass behavior combined with increasing crystallinity, surface roughness of CoO after annealing may be attributed to the large exchange bias and high coercivity.     

High-temperature ferromagnetism in laser-deposited layers of silicon and germanium doped with manganese or iron impurities

The paper reports on the results of a study of the synthesis conditions effects on magnetic and transport properties of nanosized layers of high-T_c diluted magnetic semiconductors (DMS), such as Ge:Mn, Si:Mn and Si:Fe, fabricated by laser-plasma deposition over a wide range of the growth temperature, T_g=(20-550) °C on single-crystal GaAs or Al₂O₃ substrates. Ferromagnetism of the layers was detected by measurement data of the magneto-optical Kerr effect, anomalous Hall effect, negative magnetoresistance and ferromagnetic resonance (FMR) at 5-500 K. The optimum growth temperature, T_g, for Si:Mn/GaAs layers with T_c≈400 K is shown to be about 400 °C. The Si:Mn/Al₂O₃ layers with 35% of Mn have the metal-type of conductivity with manifestation of magnetization up to room temperature. Different types of uniformly doped structures and digital alloys have been investigated. In contrast to GaSb:Mn films, Si-based ferromagnetic layers have strongly different magnetic and electric properties in case of uniformly doped structures and digital alloys. Positive results of the Fermi level variation effect on the improvement of Si- and Ge-based DMS layers have been gained on the use of additional doping with shallow acceptor Al impurity which contributes to the increase of the hole concentration and the RKKY exchange interaction of 3d-ions. The Ge:(Mn, Al)/GaAs or Ge (Mn, Al)/Si layers grown at 20 °C feature surprising extraordinary angular dependence of FMR.     

Antiferromagnetic resonance studies in molecular magnetic materials

EPR spectroscopy has become an increasingly powerful tool to examine the spin states and dynamics of single-molecule magnets, but has not been exploited to probe bulk magnetically ordered phases of molecular magnets. In this article, we review the EPR spectra of antiferromagnets and canted antiferromagnets below T_N with particular reference to our own studies on the canted antiferromagnet, p-NCC₆F₄CNSSN (T_N=36 K). The antiferromagnetic resonance experiment allows the saturation of the sublattice magnetisation to be probed. In addition, the exchange and anisotropy fields (H_e and H_a), the spin-flop field (H_sf) and for canted antiferromagnets, the Dzyaloshinskii-Moriya field (H_DM), which gives rise to the spontaneous moment, can be determined.     

Heat capacity of paramagnetic nickelocene: Comparison with diamagnetic ferrocene

Nickelocene [bis(η⁵-cyclopentadienyl)nickel: Ni(C₅H₅)₂, electron spin S=1, the ground state configuration ³A_2g] is paramagnetic and belongs to a typical molecule-based magnet. Heat capacities of nickelocene have been measured at temperatures in the 3−320 K range by adiabatic calorimetry. By comparing with those of diamagnetic ferrocene crystal, a small heat capacity peak centered at around 15 K and a sluggish hump centered at around 135 K were successfully separated. The low-temperature peak at 15 K caused by the spin is well reproduced by the Schottky anomaly due to the uniaxial zero-field splitting of the spin S=1 with the uniaxial zero-field splitting parameter D/k=45 K (k: the Boltzmann constant). The magnetic entropy 9.7 J K⁻¹ mol⁻¹ is substantially the same as the contribution from the spin-manifold R ln 3=9.13 J K⁻¹ mol⁻¹ (R: the gas constant). The sluggish hump centered at around 135 K arises from rotational disordering of the cyclopentadienyl rings of nickelocene molecule. The enthalpy and entropy gains due to this anomaly are 890 J mol⁻¹ and 6.9 J K⁻¹ mol⁻¹, respectively. As the hump spreads over a wide temperature region, separation of the hump from the observed heat capacity curve involves a little bit ambiguity. Therefore, these values should be regarded as being reasonable but tentative. The present entropy gain is comparable with 5.5 J K⁻¹ mol⁻¹ for the sharp phase transition at 163.9 K of ferrocene crystal. This fact implies that although the disordering of the rings likewise takes place in both nickelocene and ferrocene, it proceeds gradually in nickelocene and by way of a cooperative phase transition in ferrocene. A reason for this originates in loose molecular packing in nickelocene crystal. Molar heat capacity and the standard molar entropy of nickelocene are larger than those of ferrocene beyond the mass effect over the whole temperature region investigated. This fact provides with definite evidences for the loose molecular packing in nickelocene crystal.     

Temperature and pressure effects on the spin state of ferric ions in the [Fe(sal2-trien)][Ni(dmit)2] spin crossover complex

Thermal and pressure effects have been investigated on the [Fe(sal₂-trien)][Ni(dmit)₂] spin crossover complex by means of Mössbauer spectroscopic, calorimetric, X-ray diffraction and magnetic susceptibility measurements. The complex displays a complete thermal spin transition between the and spin states of Fe^III near 245 K with a hysteresis loop of ca. 30 K. This transition is characterised by a change of the enthalpy, ΔH_HL=7 kJ/mol, entropy, ΔS_HL=29 J/Kmol, and the unit cell volume, ΔV_HL=15.4 Å³. Under hydrostatic pressures up to 5.7 kbar the thermal transition shifts to higher temperatures by ca. 16 K/kbar. Interestingly, at a low applied pressure of 500 bar the hysteresis loop becomes wider (ca. 61 K) and the transition is blocked at ∼50% upon cooling, indicating a possible (irreversible) structural phase transition under pressure.