Electron spin resonance analysis of magnetic structures in La2/3Ca1/3MnO3

Measurements of electron spin resonance (ESR) of La_2/3Ca_1/3MnO₃ (LCMO) in the ferromagnetic and paramagnetic phases were carried out. Phase transition and temperature dependence of the peak-to-peak ESR linewidth were determined. The transition temperature between ferromagnetic and paramagnetic phases was observed at 265 K. A prominent increase of the peak-to-peak linewidth with decreasing temperature below T_c was observed. Using the dynamic scale theory and block spin transformation in critical phenomenon, the quantitative calculation of peak-to-peak linewidth at near T_c was made, which was in good agreement with the experimental data. It was believed that the long interactions between the ferromagnetic microregions for LCMO played a key role in determining the ESR linewidth.     

Role of inter-site exchange and hybrid interactions in itinerant ferromagnetism

In this work, we study the magnetic properties of itinerant electron systems using the Hubbard-like tight binding Hamiltonian along with inter-site exchange and hybrid interactions. We have used the mean-field approximation to deal with the exchange and hybrid interactions. It is found that hybrid interaction is more effective than exchange interaction for the on-set of ferromagnetic state. We have studied the effect of hybrid interaction on various physical quantities at different temperatures. The effective mass (m*/m) of up spin electrons increases slowly as the temperature decreases but below the critical temperature (T_c), it decreases rapidly. For down spin electrons effective mass increases slowly as the temperature decreases and below T_c, it increases more rapidly. Spectral weight (n/m*) for up spin electrons decreases slowly upto T_c and below T_c, it increases rapidly. For down spin electrons spectral weight decreases slowly upto T_c and below T_c, it decreases rapidly. Our results for both the effective mass and spectral weight are in good agreement with recently observed experimental behaviour in itinerant ferromagnet Ga_1−xMn_xAs [Phys. Rev. Lett. 89 (2002) 097203]. We have also studied variation of the spectral weight and optical absorption with temperature in presence of magnetic field. We found that these two quantities for up spin electrons increase as applied magnetic field increases at all temperatures (∼4T_c). For down spin electrons these two quantities decrease as applied magnetic field increases.     

Critical nuclear relaxation in cobalt metal

Nuclear magnetic resonance of cobalt metal was investigated in the paramagnetic and ferromagnetic states and in the critical region below T_c. The Knight shift and spin lattice relaxation times were measured in the paramagnetic phase in the solid and liquid states from 1578 K to 1825 K. The resonant frequency, spin-lattice and spin-spin relaxation times were measured in the ferromagnetic phase from room temperature to 1385 K. The main part of (T₁T)^-1 results from fluctuating orbital moments in both phases except near T_c where this process forms the background for critical spin relaxation. The critical exponents for T^-1₁ and for the magnetization in the ferromagnetic state were found to be n' = 0.96 ± 0.07 and β = 0.308 ± 0.012, respectively.     

Peculiarities of the magnetic structure of the cryogenic permanent magnet Tb<Subscript>3</Subscript>Co

The magnetic state of the Tb₃Co compound with orthorhombic structure has been investigated by measuring the magnetization in static and pulsed magnetic fields and using neutron diffraction analysis. It is shown that the antiferromagnetic modulated structure, arising in this material at T < T _N = 82 K, passes upon cooling below T _t ≈ 72 K to an incommensurate magnetic structure with a ferromagnetic component along the c axis.     

Valence and magnetic states of impurity iron ions in the La<Subscript>0.75</Subscript>Sr<Subscript>0.25</Subscript>Co<Subscript>0.98</Subscript>Fe<Subscript>0.02</Subscript>O<Subscript>3</Subscript> perovskite

The local magnetic and valence states of impurity iron ions in the rhombohedral La_0.75Sr_0.25Co_0.98 ⁵⁷Fe_0.02O₃ perovskite were studied using Mössbauer spectroscopy in the temperature range 87–293 K. The Mössbauer spectra are described by a single doublet at 215–293 K. The spectra contained a paramagnetic and a ferromagnetic component at 180–212 K and only a broad ferromagnetic sextet at T < 180 K. The results of the studies showed that, over the temperature range 87–295 K, the iron ions are in a single (tetrahedral) state with a valence of +3. In the temperature range 180–212 K, two magnetic states of Fe³⁺ ions were observed, one of which is in magnetically ordered microregions and the other, in paramagnetic microregions; these states are due to atomic heterogeneity. In the magnetically ordered microregions in the temperature range 87–212 K, the magnetic state of the iron ions is described well by a single state with an average spin S = 1.4 ± 0.2 and a magnetic moment μ(Fe) = 2.6 ± 0.4μ _B .     

Ferromagnetic resonance observation of a phase transition in magnetic field-aligned Fe2O3 nanoparticles

Fe₂O₃ hematite (alpha) nanoparticles suspended in the liquid phase of the liquid crystal 4,4-azoxyanlsole (PAA) are cooled below the freezing temperature (397 K) in a 4000 G dc magnetic field. The in field solidification locks the direction of maximum magnetization of the particles parallel to the direction of the applied dc magnetic field removing the effects of dynamical fluctuations of the nanoparticles on the magnetic properties allowing a study of the intrinsic magnetic properties of the nanoparticles as well as the anisotropic behavior of the ferromagnetic resonance (FMR) signal. Freezing in PAA allows temperature-dependent measurements to be made at much higher temperature than previous measurements. The field position, line width and intensity of the FMR signal as a function of temperature as well as the magnetization show anomalies in the vicinity of 200 K indicative of a magnetic transition, likely the previously observed Morin transition shifted to lower temperature due to the small particle size. Weak ferromagnetism is observed below T_c in contrast to the bulk material where it is antiferromagnetic below T_c. The Raman spectrum above and below 200 K shows no evidence of a change in lattice symmetry associated with the magnetic transition.     

Effect of pressure and magnetic field on bilayer La1.25Sr1.75Mn2O7 single crystal

We report the temperature dependence of susceptibility for various pressures, magnetic fields and constant magnetic field of 5 T with various pressures on La_2−2xSr_1+2xMn₂O₇ single crystal to understand the effectiveness of pressure and magnetic field in altering the magnetic properties. We find that the Curie temperature, T_c, increases under pressure (dT_c/dP=10.9 K/GPa) and it indicates the enhancement of ferromagnetic phase under pressure up to 2 GPa. The magnetic field dependence of T_c is about 26 K for 3 T. The combined effect of pressure and constant magnetic field (5 T) shows dT_c/dP=11.3 K/GPa and the peak structure is suppressed and broadened. The application of magnetic field of 5 T realizes 3D spin ordered state below T_c at atmospheric pressure. Both peak structure in χ_c and 3D spin ordered state are suppressed, and changes to 2D-like spin ordered state by increase of pressure. These results reveal that the pressure and the magnetic field are more competitive in altering the magnetic properties of bilayer manganite La_1.25Sr_1.75Mn₂O₇ single crystal.     

Single-site functional-integral approach to itinerant-electron ferromagnetism II. Thermodynamical property

Thermodynamical properties of ferromagnetic metals are discussed by using the single-site functional-integral method developed recently by the present author. It is shown that the entropy and the specific heat consist of two terms; one is due to the thermal excitations of electrons near the Fermi level, and the other arises from the disordered local magnetic moment. Numerical calculations show a huge specific-heat-peak due to magnetic contributions at the Curie temperature, T_c, and a large electronic specific heat at low temperatures and at T>T_c, which are qualitatively in agreement with experimental data of ferromagnetic metals like iron.     

Magnetic structure of the random system near the ferro-antiferromagnetic transition

The magnetic structure of the disordered alloy Fe₆₅Ni₂₈Mn₇ was investigated in the temperature 4.2–300 K by the methods: small angle scattering of neutrons, Mössbauer effect, magnetization, magnetic contribution to the thermal coefficient of the thermal expansion, and resistivity. All measurements show that long-range ferromagnetic order appears below T_c ? 160 K. At the same time for T ? 100 K, a dramatic change of magnetic state takes place which is interpreted as the freezing of “spin glass”. An increase of the magnetic contribution to the resistivity with decreasing temperature was also found. This increase was attributed to the existence of poor-bonded magnetic moments of the Kondo-type. A model of the magnetic ground state is proposed which includes the details of magnetic behavior such as long-range ferromagnetic order, spin glass, finite ferro-and antiferromagnetic clusters, and Kondo-type states. A magnetic phase diagram of the system Fe₆₅(Ni_1?xMn_x)₃₅ is also proposed.     

Low-frequency electrical resistance of iron,cobalt, and nickel in the vicinity of their Curie temperatures

Alternating-current electrical resistance measurements between 17 Hz and 100 kHz were made on high purity Fe, Co, and Ni wires in the vicinity of their Curie temperatures (T _c). The electrical resistance was independent of frequency for temperatures (T) aboveT _c. As the temperature was lowered, however, there was an abrupt jump in the electrical resistance atT _c followed by a gradual decrease toward its dc value. The magnitude of the electrical resistance jump atT _c increased as the square root of the frequency. The enhancement of the electrical resistance forT≦T _c is produced by an abrupt decrease of the skin depth atT _c which, in turn, is due to the sudden increase in the initial magnetic permeability atT _c. Measurements of the ac electrical resistance in the vicinity of the Curie temperature of certain ferromagnetic metals can be utilized to 1) accurately determine the Curie temperature using frequencies as low as 17 Hz, and 2) quantitatively determine the initial magnetic permeability as a function of temperature and heat treatment.     

Magnetic properties of MgCo2, MgNi2 and Mg2Ni

The magnetic properties of MgCo₂, MgNi₂ and Mg₂Ni were investigated in the range 4–400 K. The compound MgCo₂ is ferromagnetic below T_c = 321 but gives rise to a first order ferromagnetic-antiferromagnetic transition near 45 K. The compounds MgNi₂ and Mg₂Ni are both Pauli paramagnetic; they have an almost temperature-independent magnetic susceptibility. For all compounds the lattice constants were determined.     

Magnetic resonance spectrum of a two-phase state in single crystals of La0.7Pb0.3MnO3 lanthanum manganite

Two absorption lines are observed over a wide temperature range below T _c in the magnetic resonance spectrum of an La_0.7Pb_0.3MnO₃ single crystal. These lines correspond to two magnetic phases in the sample. The frequency-field dependence of spectra obtained in the range of microwave radiation frequencies 10–77 GHz allows these phases to be interpreted as ferromagnetic and paramagnetic phases. The phase volume ratio depends on the temperature and the magnitude of the external magnetic field. Features in the temperature behavior of parameters of the magnetic absorption line are observed in the region of the highest magnetic resistance of the sample. The results are interpreted within the mechanism of electronic phase separation.     

Spin-reorientation in GdGa

We have determined the magnetic structure of the intermetallic compound GdGa by ¹⁵⁵Gd Mössbauer spectroscopy and neutron powder diffraction. This compound crystallizes in the orthorhombic (Cmcm) CrB-type structure. It orders ferromagnetically at T _c ?=?190(2) K and then undergoes a spin reorientation at T _SR ?=?68(2) K. Between T _c and T _SR , the magnetic structure is characterized by ferromagnetic order of the Gd moments along the b-axis. On cooling below T _SR , the Gd 4c magnetic moments split into two groups (2:2). At 3.6 K, the Gd moment is 6.7(4) μ _B , and the Gd magnetic moments are in the bc-plane, canted by 84(3)° and 46(4)° with respect to the crystallographic b-axis. This splitting into two magnetically inequivalent sites is confirmed by our 5 K ¹⁵⁵Gd Mössbauer results.     

Magnetic properties of SrRu0.9Fe0.1O3 thin films grown on different surfaces of SrTiO3 substrates

We stabilized SrRu_0.9Fe_0.1O₃ single-crystalline films on SrTiO₃ (001) and SrTiO₃ (110) substrates using epitaxial strain during thin-film growth. X-ray diffraction (XRD) θ–2θ scans showed strong peaks demonstrating single-crystal quality. Fe doping in SrRuO₃ had negative effects on the ferromagnetic properties, such as decreasing the T_c and saturated magnetic moment, as well as weakening the ferromagnetism. The negative effects were reduced when a suitable surface of the cubic substrate was selected for thin-film SrRuO₃ growth. We found that the ferromagnetic properties, such as the T_c and saturated magnetic moment, differed depending on the substrate surface. The observed differences are discussed in terms of Ru–Ru nearest-neighbor distance.     

Magnetic structure of the cubic U2Te3 with a non-stoichiometric Th3P4 type of structure

The neutron diffraction patterns of the cubic U₂Te₃ have been obtained at 15 and 150 K. There are no extrapeaks of magnetic origin below the Curie point T_c = 70 ± 5 K. Thus the ordering corresponds to a collinear ferromagnetic structure. The value of the magnetic moment is 1.78 μ_B which is close to those values determined for a few cubic uranium pnictides and chalcogenides.     

Re-entry in ferromagnetic superconductors

The re-entry phenomenon in magnetic superconductors is studied using the generalized Ginzburg-Landau free energy introduced by Blount and Varma. The re-entry temperature T_c2 is simply that temperature at which the magnetization acts as a source of induction strong enough to destroy superconductivity. Above T_c2 ferromagnetism and superconductivity coexist. The structure is an Abrikosov vortex lattice, with ferromagnetic magnetization spreading widely around the vortex cores. Within our approximations, the phase transition at T_c2 is of second order.     

The large anisotropy of the magnetic and transport properties in the Ba5Co5ClO13 single crystal

In this Letter, the single crystals of Ba₅Co₅ClO₁₃ were grown by the flux method successfully. Their structure, magnetic and transport properties were studied. A large anisotropy of the magnetic and transport properties has been detected in this compound. Below the TN∼108 K, the magnetic susceptibility exhibits an antiferromagnetic peak in χc and an upturn transition in χab. We suggest that this behavior is consistent with the competition of the ferromagnetic (FM) intra-blocks coupling and antiferromagnetic (AFM) inter-blocks coupling in this compound. The temperature dependence of the resistivity displays a hump in ρab and a kink in ρc around TN, suggesting the strong coupling between the transport and magnetic properties. Above and below the transition, the transport properties in ab plane follow the three-dimensional (3D) variable range hopping (VRH) mechanism.     

The low field magnetisation of Y4Co3

The onset of magnetic order at 6 K and a superconducting transition at 2 K was previously observed in Y₄Co₃. In this paper we investigate the magnetisation in very low fields, in the vicinity of the magnetic and superconducting transition (T_c). Below T_c the M-H curves are characterised by broad hysteresis loops. This type of the behaviour indicates that the sample consists of different regions. Some of these regions are superconducting below T_c and others are magnetically ordered below 6 K.     

A neutron small angle scattering study of the onset of ferromagnetism in TiBe2-xCux alloys

The onset of ferromagnetic order in TiBe_2-xCu_x alloys, with x = 0.5, 0.4, 0.3, 0.2 and 0.15, has been examined using the technique of neutron small-angle scattering. The Curie temperature, T_c, for these alloys has been determined from the temperature dependence of the magnetic critical scattering. A linear extrapolation of T_c versus copper concentration yields a critical concentration for ferromagnetic order of x_c = 0.05 ± 0.02. For the alloys with x = 0.4, 0.5 the lineshape of the magnetic critical scattering, at and above T_c, is well explained by the Ornstein-Zernicke form of the spin correlation function. For the lower concentration alloys the exact form of the spin correlation function is still unclear.     

Magnetostriction and thermal expansion anomalies near the Curie point of the compound La0.7Sr0.3MnO3 with the perovskite structure

It is found that the bulk part ω of the magnetostriction near the Curie temperature T _c in a La_0.7Sr_0.3MnO₃ single crystal with the perovskite structure is negative and that the temperature dependence of |ω| has a maximum near T _c . The quantity |ω| at the maximum increases rapidly with increasing magnetic field. The thermal expansion coefficient near T c increases with temperature much faster than linearly. The paramagnetic Curie temperature determined from the Curie-Weiss law, which the paramagnetic susceptibility of this crystal satifies, was found to be lower than T _c . These anomalies and also the near-T _c metal-insulator transition which is characteristic for this material are explained by the existence of a magnetically two-phase state consisting of a conducting ferromagnetic matrix containing antiferromagnetic insulating microregions occupying not more than 5% of the sample volume. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 6, 449–453 (25 March 1997)