Ferromagnetic resonance in a system of magnetic films with different Curie temperatures

The peculiarities of absorption of rf electromagnetic radiation (ferromagnetic resonance) in multilayer NiFe/Ni_0.65Cu_0.35(d)/CoFe structures in a wide temperature range are analyzed. It is shown that the type of interaction of the NiFe and CoFe ferromagnetic films via a “weak” ferromagnetic Ni_0.65Cu_0.35 interlayer changes from antiferromagnetic to ferromagnetic upon cooling and a decrease in interlayer thickness d. The detected temperature dependence of the interlayer interaction indicates the possibility of observation of a strong magnetocaloric effect in the structures under investigation.     

Magnetic and structural transition of Ni50+xMn25−x/2Ga25−x/2 (x=2–5) alloys

Magnetic and structural transitions of non-stoichiometric Ni_50+xMn_25−x/2Ga_25−x/2 (x=2–5) alloys are systematically investigated. Differential scanning calorimetry and modified thermogravimetry (TG) are used to measure magnetic and structural transitions simultaneously. The structural transition temperatures increase monotonically with increasing Ni substitution for Mn and Ga. Different magnetic transition sequences on heating were observed from ferromagnetic martensite to ferromagnetic autensite, then to paramagnetic autensite, from ferromagnetic martensite to paramagnetic austensite or from ferromagnetic martensite to paramagnetic martensite, respectively. Three kinds of NiMnGa alloys were obtained according to the sequence of the structural and magnetic transition, whose structural transition temperatures are lower, equal to or higher than the magnetic transition temperatures.     

Theory of polarized electron beam production by field emission from ferromagnets

A theory is given for field emission from ferromagnetic solids in which the two spin subbands of the conduction band are split by exchange forces. Using a simple plane wave model for the conduction band states it is shown that the familiar WKB approximation for the tunneling probability has to be improved by momentum dependent factors to account for the spin polarization of the emission current observed in a recent experiment which is 8% in the case of Gd. The calculations are extended to describe emission from Bloch states. Applications of the theory to various ferromagnetic systems are discussed, the use of ferromagnetic Eu_1?xGd_xS as a source for highly polarized beams is suggested.     

Magnetic structure and phase diagram of Sr<Subscript>5</Subscript>Rh<Subscript>4</Subscript>O<Subscript>12</Subscript> frustrated Ising magnet

The magnetic structure of Sr₅Rh₄O₁₂ is based on Ising chains of rhodium ions with a variable valence, Rh³⁺-Rh⁴⁺. The ordering in the chains is assumed to be ferromagnetic. It has been shown that the magnetic structure and phase diagram of Sr₅Rh₄O₁₂ are well described in a model taking into account weak antiferromagnetic interactions between the nearest and next-nearest neighbors on the triangular lattice of ferromagnetic Ising chains. The ground state at low temperatures is the two-sublattice stripe phase; this phase in the magnetic field is transformed to the ferrimagnetic phase and, then, to the ferromagnetic phase. Small plateaus can be observed in the region of the transition from the ferrimagnetic phase to the ferromagnetic one.     

Features of magnetic properties of La x MnO3 + δ (0.815 ≤ x ≤ 1.0)

Magnetic and resonance studies of the system of polycrystalline samples of self-doped manganites La _x MnO_{3 + δ} (x = 0.815, 0.90, 0.94, 0.97, and 1.0) have been performed in a temperature range of 77–300 K. According to ⁵⁵Mn NMR data, all samples contain a ferromagnetic phase at 77 K. As the defect density increases (x changes from 1.0 to 0.815), samples become more magnetically ordered. In this case, the ferromagnetic state of the system gradually changes from a mixed state in which both ferromagnetic insulating (basic) and ferromagnetic metal (for x = 0.97 and 1.0) phases coexist to only the ferromagnetic metal state (for x = 0.815 and 0.90). It has been shown that both ferromagnetic metal and ferromagnetic insulating phases are inhomogeneous, and either phase consists of two phases with different dynamics of nuclear spins and different Curie temperatures. The diagram of the magnetic phase state of the La _x MnO_{3 + δ} system (x = 0.815, 0.90, 0.94, 0.97, 1.0) has been constructed for a temperature range of 120–240 K and Mn⁴⁺ contents of 12–30%.     

Polarized SSXANES study of spin ordering in ferromagnetic and paramagnetic phases of La1.2Sr1.65Ca0.15Mn2O7

Spin-selected polarized X-ray absorption near-edge structures (SSXANES) at the Mn K-edge from a bilayer La_1.2Sr_1.65Ca_0.15Mn₂O₇ single crystal have been studied with high resolution, both in the ferromagnetic (15 K) as well as paramagnetic phase (300 K). The orientation-dependent SSXANES spectra show unmistakable temperature dependence as the system makes the ferromagnetic to paramagnetic phase transition. The pre-edge structures are too intense to be ascribed to weak quadrupole transitions and are interpreted in terms of hybridization of Mn 3d orbitals with O2p and Mn 4p orbitals over and above similar onsite hybridization. The results also indicate possible existence of a small local (time-frozen) ferromagnetic ordering in the macroscopically disordered state. Need for further experimental and theoretical work on the SSXANES spectra from the bilayer system is emphasized.     

Electronic structure of the ordered ferromagnetic alloys Ni3Mn,Ni3Fe,and Ni3Co

Calculations of the band structure of the ferromagnetic alloys Ni₃Mn, Ni₃Fe, and Ni₃Co are presented. The results are compared with experimental data. Change in the electronic structure as one goes from Ni₃Mn to Ni₃Co is analyzed. The occurrence of ferromagnetism in the alloys studied and the possibility of their existence in an ordered ferromagnetic state are examined.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 82–88, June, 1988.     

The role of Co impurities and oxygen vacancies in the ferromagnetism of Co-doped SnO2: GGA and GGA+U studies

The electronic structure and ferromagnetic stability of Co-doped SnO₂ are studied using the first-principle density functional method within the generalized gradient approximation (GGA) and GGA+U schemes. The addition of effective U_Co transforms the ground state of Co-doped SnO₂ to insulating from half-metallic and the coupling between the nearest neighbor Co spins to weak antimagnetic from strong ferromagnetic. GGA+U_Co calculations show that the pure substitutional Co defects in SnO₂ cannot induce the ferromagnetism. Oxygen vacancies tend to locate near Co atoms. Their presence increases the magnetic moment of Co and induces the ferromagnetic coupling between two Co spins with large Co-Co distance. The calculated density of state and spin density distribution calculated by GGA+U_Co show that the long-range ferromagnetic coupling between two Co spins is mediated by spin-split impurity band induced by oxygen vacancies. More charge transfer from impurity to Co-3d states and larger spin split of Co-3d and impurity states induced by the addition of U_Co enhance the ferromagnetic stability of the system with oxygen vacancies. By applying a Coulomb U_O on O 2 s orbital, the band gap is corrected for all calculations and the conclusions derived from GGA+U_Co calculations are not changed by the correction of band gap.     

Structures and magnetic behaviours of TiO2-Mn-TiO2 multilayers

The TiO2-Mn-TiO2 multilayers are successfully grown on glass and silicon substrates by alternately using radio frequency reactive magnetron sputtering and direct current magnetron sputtering. The structures and the magnetic behaviours of these films are characterised with x-ray diffraction, transmission electron microscope (TEM), vibrating sample magnetometer, and superconducting quantum interference device (SQUID). It is shown that the multi-film consists of a mixture of anatase and rutile TiO2 with an embedded Mn nano-film. It is found that there are two turning points from ferromagnetic phase to antiferromagnetic phase. One is at 42 K attributed to interface coupling between ferromagnetic Mn3O4 and antiferromagnetic Mn2O3, and the other is at 97 K owing to the interface coupling between ferromagnetic Mn and antiferromagnetic MnO. The samples are shown to have ferromagnetic behaviours at room temperature from hysteresis in the M-H loops, and their ferromagnetism is found to vary with the thickness of Mn nano-film. Moreover, the Mn nano-film has a critical thickness of about 18.5 nm, which makes the coercivity of the multi-film reach a maximum of about 3.965×10 2 T.     

Ferromagnetic insulator effects in ferromagnetic semiconductor/ferromagnetic insulator/p-wave superconductor junctions

The effect of the ferromagnetic insulator on tunneling conductance in ferromagnetic semiconductor/ferromagnetic insulator/p-wave superconductor (FS/FI/P) junctions is studied based on a scattering theory. Three kinds of pairings for the P side are chosen: p_x, p_{y ,}p_x+ip_y waves. It is shown that the spin filtering effect originating from the exchange field in the FI strongly modifies the normalization conductance. Many novel features including the zero-bias conductance dip and splitting are exhibited for fixed spin polarization in the FS. The tunneling spectrum for the heavy holes is much different from that for the light holes due to the different mismatches in the effective mass and Fermi velocity between FS and P.     

First principles study the ferromagnetic properties and electronic structure of boron doped ZnSe

Using the full-potential linearized augmented plane wave method with generalized gradient approximation, the magnetic properties and the electronic structure of the boron-doped ZnSe (zinc blende phase) are investigated. Spin polarization calculations show the magnetic moment of the 64-atoms supercell containing one B_Se (B_Zn) is 3.00 (0.015) μ_B. The density of states indicates the magnetic moments of the B_Se doped configuration mainly come from the doped boron atoms and a few from its neighboring zinc atoms. The ferromagnetic and antiferromagnetic calculations for several doped configurations suggest B_Se could induce stable ferromagnetic ground state in ZnSe hosts and ferromagnetic couplings exist between the doped boron atoms. Electronic structures show that B_Se is p-type ferromagnetic semiconductor and hole-mediated double exchange is responsible for the ferromagnetism, while the B_Zn doped configuration is n-type semiconductor. Relative shallow acceptor and donor levels indicate boron-doped ZnSe is ionized easily at working temperatures.     

Magnetic properties of La0.70Sr0.30MnO2.85 anion-deficient manganite under hydrostatic pressure

The magnetic properties of La_0.70Sr_0.30MnO_2.85 anion-deficient manganite are studied experimentally under hydrostatic pressure. The results show that, in the whole pressure range under investigation (0–1 GPa), the sample is a spin glass with a smeared phase transition to the paramagnetic state. It is found that the spin glass state arises from the frustration of the exchange coupling of the ferromagnetic clusters embedded in the antiferromagnetic matrix. The fraction of the sample volume occupied by the ferromagnetic phase is found to be V _fer ～ 13%. Under hydrostatic pressure, the freezing temperature T _f of the magnetic moments of the ferromagnetic clusters increases at a rate of 4.30 K/GPa and the magnetic ordering temperature T _MO increases at a rate of 12.90 K/GPa. In addition, the ferromagnetic part of the sample increases by ΔV _fer ～ 5%. The enhancement of the ferromagnetic properties of La_0.70Sr_0.30MnO_2.85 anion-deficient manganite under hydrostatic pressure is explained by the redistribution of oxygen vacancies and a decrease in the unit-cell parameters.     

Magnetoresistances and magnetic entropy changes associated with negative lattice expansions in NaZn13-type compounds LaFeCoSi

Magnetoresistances and magnetic entropy changes in NaZn13-type compounds La（Fel-xCox）11.9Si1.1 （x=0.04, 0.06, and 0.08） with Curie temperatures of 243 K, 274 K, and 301 K, respectively, are studied. The ferromagnetic ordering is accompanied by a negative lattice expansion. Large magnetic entropy changes in a wide temperature range from ～230 K to ～320 K are achieved. Raising Co content increases the Curie temperature but weakens the magnetovolume effect, thereby causing a decrease in magnetic entropy change. These materials exhibit a metallic character below Tc, whereas the electrical resistance decreases abruptly and then recovers the metal-like behaviour above Tc. Application of a magnetic field retains the transitions via increasing the ferromagnetic ordering temperature. An isothermal increase in magnetic field leads to an increase in electrical resistance at temperatures near but above Tc, which is a consequence of the field-induced metamagnetic transition from a paramagnetic state to a ferromagnetic state.     

Magnetic patterns and flux pinning in Pd0.99Fe0.01-Nb hybrid structures

The magnetic properties of hybrid thin-film Pd_0.99Fe_0.01-Nb structures are studied by a magneto-optical technique. It is shown that, below 14 K, the samples exhibit the ferromagnetic ordering corresponding to the formation of weakly coupled ferromagnetic nanoclusters. In the clusters, the effective spin polarization of Fe ions is about 4μ_B, corresponding to that in the bulk Pd₃Fe alloy. The proximity of the ferromagnetic layer does not suppress the superconductivity in niobium. It does not affect the superconducting transition temperature but leads to an enhanced pinning and results in an increase in the critical current by about 30%. This behavior agrees well with the existence of the nanocluster structure in the ferromagnetic film.     

Ordered magnetic frustration: IX. Magnetic structure of Ba2Ni3F10 at 2.5 K

The magnetic structure of Ba₂Ni₃F₁₀ at 2.5 K has been solved and refined from powder neutron diffraction data. Magnetic moments are oriented along [-1 0 1] perpendicularly to the rutile chains axis of the structure as in rutile NiF₂. Competition between strong antiferromagnetic and weak ferromagnetic interactions within even cycles of corner-sharing and edge-sharing octahedra leads to a frustrated antiferromagnetic coupling in bioctahedra units while non-frustrated rutile-like chains exhibit a normal ferromagnetic coupling.     

Pressure-induced change of the interlayer exchange interaction from ferromagnetic to antiferromagnetic in CS2CuF4 observed by neutron scattering experiments up to 2 GPa

Abstract

Neutron scattering experiments on the two-dimensional Heisenberg ferromagnets Cs₂ CuF₄ and K₂CuF₄ have been performed around 2 ～ 3 GPa over 1·4–15 K. At ambient pressure both the intralayer and the interlayer exchange interactions in these two compounds are ferromagnetic. At about 2 GPa, the interlayer exchange coupling in Cs₂ CuF₄ is found to change from ferromagnetic to antiferromagnetic, while the ferromagnetic intralayer exchange interaction is maintained. Contrary to Cs₂CuF₄, the ferromagnetism in K₂Cuf₄ is not destroyed by pressure up to 9 GPa, that was confirmed in the early study of the magnetic susceptibility measurements.     

Pressure-induced weak moment in the two-dimensional antiferromagnet (C2H5NH3)2CuCl4

Abstract

The two-dimensional Heisenberg antiferromagnet (C₂H₅NH₃)₂CuCl₄ has the ferromagnetic intralayer exchange interaction, while the extremely weak interlayer exchange interaction is antiferromagnetic. Neutron scattering experiments under high pressures have been performed on this compound. We confirm that the spin structure changes around 1～2 GPa from the collinear alignment along the a-axis to a spin-canting one. The weak moment due to the canting is parallel to the c-axis. The results indicate that the ferromagnetic intralayer and the antiferromagnetic interlayer exchange interactions are maintained up to 1～2 GPa. Why the weak ferromagnetic moment along the c-axis occurs is due to a lowering of crystal symmetry by pressure.     

Ferromagnetic superstructure of an La0.85Sr0.15MnO3 manganite single crystal

The elastic thermal-neutron scattering patterns of a La_0.85Sr_0.15MnO₃ manganite orthorhombic single crystal are investigated in the temperature range 4.2–300 K. It is found that, in addition to the known ferromagnetic ordering (T _C=240 K), this compound exhibits a ferromagnetic superstructure with the (010)2π/b wave vector (in the Pnma setting of the space group D _2h ¹⁶ ). The ferromagnetic superstructure is observed in the studied crystal at temperatures ranging from 4.2 to 200 K. It is shown that the formation of the ferromagnetic superstructure in this compound is directly associated with a 1/8-type charge ordering of Mn³⁺ and Mn⁴⁺ ions.     

Magnetic properties of electron-doped La0.23Ca0.77MnO3 nanoparticles

Magnetic properties of electron-doped La_0.23Ca_0.77MnO₃ manganite nanoparticles, with average size of 12 and 60?nm, prepared by the glycine?Cnitrate method, have been investigated in the temperature range 5?C300?K and magnetic fields up to 90?kOe. It is suggested that weak ferromagnetic moment results from ferromagnetic shells of the basically antiferromagnetic nanoparticles and from domains of frustrated disordered phase in the core. Assumption of two distinct sources of ferromagnetism is supported by the appearance of two independent ferromagnetic contributions in the fit of the T ^3/2 Bloch law to spontaneous magnetization. The ferromagnetic components, which are more pronounced in smaller particles, occupy only a small fraction of the nanoparticle volume and the antiferromagnetic ground state remains stable. It is found that the magnetic hysteresis loops following field cooled processes, display size-dependent horizontal and vertical shifts, namely, exhibiting exchange bias effect. Time-dependent magnetization dynamics demonstrating two relaxation rates were observed at constant magnetic fields upon cooling to T?<?100?K.     

Magnetism and magnetocaloric properties of Mn0.95Cr0.05As

In this study the magnetic properties of Mn_0.95Cr_0.05As, prepared by mechanical milling, have been investigated. The results suggest that the presence of strains is very important for the magnetic state of the compound. In the presently studied compound, a combined magnetic and structural transition occurs from paramagnetic MnP phase to ferromagnetic NiAs phase at about 234 K. With further decreasing temperature, at 159 K, a transition from ferromagnetic NiAs phase to helimagnetic (H_a-type) MnP phase is observed, which is accompanied by an inverse magnetocaloric effect. The ferromagnetic phase is recovered when the temperature is increased. At both first-order transitions, at 159 and 234 K, large magnetic-entropy changes are observed.