Magnetic correlations in Cr–Fe–Mn alloy

Small angle neutron scattering (SANS) measurement has been performed on the reentrant ferromagnet Cr₆₀Fe₂₀Mn₂₀ alloy. An increase in the SANS intensity is observed below 100 K coinciding with an increase in the magnetization. It exhibits a maximum at 40 K and shifts to lower temperatures with increase in Q. The magnetic part of the SANS intensity fits to a Lorentzian at all temperatures. The temperature dependence of the inverse correlation length exhibits a minimum at 40 K. In the antiferromagnetic spin-glass phase at 15 K ferromagnetic correlations of ~40 Å is observed.     

Spin correlations and a mesoscopic structure in Ni-Mn-Ga

The mesoscopic structures of the Heusler alloys Ni_49.1Mn_29.4Ga_21.5 and Ni₂MnGa are studied by small-angle polarized neutron scattering in the temperature range 15 < T < 400 K. The characteristic temperatures of phase transformations (ferromagnetic, martensitic, and premartensitic transformations) and the characteristic sizes of mesoscopic inhomogeneities in them have been determined. Differences in the spin dynamics of these phases and magnetic-nuclear interference upon neutron scattering have been revealed. The evolutions of the mesoscopic structures in the nonstoichiometric and stoichiometric alloys are found to be substantially different.     

Magnetic properties, magnetoresistance, and magnetic-field-induced phase transitions in Tb0.95Bi0.05MnO3 and Eu0.8Ce0.2Mn2O5 multiferroics

This paper reports on a study of the magnetic properties, magnetoresistance, and phase transitions in the semiconducting manganite multiferroics Tb_0.95Bi_0.05MnO₃ and Eu_0.8Ce_0.2Mn₂O₅ whose dielectric properties have been a subject of an earlier study. An analysis of these properties has led us to the conclusion that the above crystals at temperatures T ≥ 180 K undergo phase separation with the formation of a dynamic periodic alternation of quasi-2D layers of manganese ions in different valence states, i.e., charge-induced ferroelectricity. This state exhibits a giant permittivity and ferromagnetism in the layers containing Mn³⁺ and Mn⁴⁺ ions. At low temperatures (T < 100 K), the phase volume is occupied primarily by the dielectric phase. Studies of the magnetic properties and the effect of the magnetic field on the dielectric properties of crystals substantiate the scenario of the formation of a state with giant permittivity put forward by us. At low temperatures, Tb_0.95Bi_0.05MnO₃ exhibits features at the points of phase transitions in pure TbMnO₃. A ferromagnetic moment is observed to exist at all the temperatures covered. At the boundaries of the quasi-2D layers, magnetic-field-induced jumps of the electrical resistivity caused by metamagnetic transitions in the layers with Mn³⁺ and Mn⁴⁺ ions are observed. At temperatures T ≥ 180 K, the electrical resistivity undergoes a periodic variation in a magnetic field which is a manifestation of carrier self-organization. A high magnetic field is capable of shifting the phase transition from 180 K to higher temperatures and inducing additional phase transitions.     

Doping-induced temperature evolution of a helicoidal spin structure in the MnGe compound

The helicoidal magnetic structure of a MnGe compound doped with 25% Fe is studied by means of small-angle neutron scattering in a wide temperature range of 10–300 K. Analysis of the scattering-function profile demonstrates that magnetic structures inherent to both pure MnGe and its doped compounds are unstable. The doping of manganese monogermanide is revealed to lead to higher destabilization of the magnetic system. In passing from MnGe to Mn_0.75Fe_0.25Ge, the magnetic-ordering temperature T _N decreases from 130 to 95 K, respectively. It is demonstrated that, at temperatures close to 0 K, the intensity of the contribution to scattering from stable spin helices decreases and the intensity of scattering by spin helix fluctuations increases with increasing impurity-metal concentration. An increased intensity of anomalous scattering caused by spin excitations existing in the system is observed. Helicoidal fluctuations and spin excitations corresponding to low temperatures indicate the quantum nature of the instability in the doped compound. However, MnGe doping with Fe atoms has no influence on the compound’s magnetic properties at temperatures of higher than T _N. The temperature range of short-range ferromagnetic correlations is independent of concentrations and is restricted by temperatures T ranging from 175 to 300 K.     

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.     

Correlation of electrical properties with magnetic properties for ferromagnetic (Ga1−xMnx)As epilayer

The correlation of electrical properties with magnetic properties for the (Ga_1−xMn_x)As epilayer was investigated. For electrical transport measurements, it was clearly observed that the electrical mobility is increased with decreasing temperatures for both the (Ga_0.974Mn_0.026)As/LT-GaAs epilayer and the LT-GaAs:Be epilayer. However, a different behavior was observed at the cryogenic temperature region. The electrical mobility of (Ga_0.974Mn_0.026)As/LT-GaAs epilayer increases with decreasing temperature, while the mobility of LT-GaAs:Be epilayer decreases with decreasing temperature. In Arrhenius plots of carrier mobility for the (Ga_0.974Mn_0.026)As/LT-GaAs epilayer, the critical point is observed at 69 K, and this value is almost the same as the T_C. This result indicates that the carrier transport in ferromagnetic (Ga_1−xMn_x)As epilayers might be related to a spin-ordering effect because the spins will be arranged with the same direction below the T_C, and this will lead to reducing the probability of spin-disorder scattering. Therefore, the observation of a gradual drop below the T_C in the temperature-dependent resistivity curve is expected to be a result of the spin-ordering effect in the ferromagnetic (Ga_0.974Mn_0.026)As/LT-GaAs epilayer.     

Microwave magnetoresistance and electron spin resonance in Ge:Mn thin films and nanowires

Resonant and nonresonant absorption of microwave radiation is found to occur in germanium films implanted with manganese at concentrations of 2, 4, and 8 at %. Electron spin resonance is observed in two temperature ranges: (i) in the vicinity of the phase transition of Mn₅Ge₃ clusters to the ferromagnetic state at T = 295 K; and (ii) in the range of temperatures below 60 K, at which collective ordering of Mn spins in the crystal lattice and spin-wave resonance take place. The dependence of the nonresonant signal of the microwave magnetoresistance on the magnetic field exhibits a nonmonotonic behavior identical for the X and K microwave bands. An analysis of the field dependence of the microwave magnetoresistance makes it possible to separate two components of the derivative of the magnetoresistance: the quasi-linear Lorentzian component observed in strong fields and the negative exponential anisotropic component determined by spin-dependent scattering of charge carriers from magnetic impurities. The length of the phase relaxation of charge carriers is estimated to be 350 nm at T = 2 K and exceeds the thickness of the film (120 nm) and the sizes of clusters and precipitates (3–5 nm). In quasi-one-dimensional nanowires of the composition Ge:Mn at the same impurity concentrations, microwave magnetoresistance is absent. These facts suggest that conduction in thin films has a quasi-two-dimensional character and that the measured microwave magnetoresistance is associated with charge carriers in the crystal lattice rather than with impurity clusters.     

Magnetic structure of a CaMnO2.75 crystal with ordered oxygen vacancies

The magnetic state of a CaMnO_{3 ? δ} crystal with ordered oxygen vacancies (for δ = 0.25, when the numbers of Mn⁴⁺ and Mn³⁺ ions in the manganite are equal to each other) is studied using neutron diffraction. Magnetic scattering in the CaMnO_2.75 crystal in the ground state is determined by the wave vector (1/2, 1/2, 1/2)2π/a _c (G-type antiferromagnetic order). In the crystal, long-range magnetic order disappears at the temperature T _N = 116 K, whereas short-range magnetic order is retained up to 240 K. It is shown that the instability of the G-type structure in the temperature range 60 K < T < T _N is associated, in many respects, with the formation of the C′ antiferromagnetic phase in the bulk of the crystal. The structure of the C′ antiferromagnetic phase involves chains with Mn³⁺-Mn⁴⁺ ferromagnetic interaction. A comparison of the results of the neutron diffraction investigations with the experimental data on the magnetic characteristics and electrical resistivity demonstrates that the specific features revealed in the spin system of the CaMnO_2.75 crystal are governed directly by the competition of the Mn³⁺-Mn⁴⁺ ferromagnetic double exchange with the antiferromagnetic superexchange between manganese ions.     

Structural and magnetic inhomogeneities, phase transitions, 55Mn NMR, and magnetoresistive properties of La0.6Sr0.2Mn1.2 − x Nb x O3 ceramics

Ceramic samples of lanthanum strontium manganite perovskites La_0.6Sr_0.2Mn_{1.2 ? x} Nb _x O₃ (x = 0–0.3) annealed at temperatures of 1260 and 1500°C have been investigated using the X-ray diffraction, electron microscopic, resistive, magnetoresistive, and magnetic (χ_ac, ⁵⁵Mn NMR) methods. It has been found that there is a correlation between the increasing unit cell parameter a of the rhombohedral R $\bar 3$ c structure and the average ionic radius with increasing niobium concentration x and annealing temperature for the case where the lattice contains anion vacancies, cation vacancies, and nanostructured clusters. The observed increase in the electrical resistivity and decrease in the temperatures of metal-semiconductor phase transition T _ms and ferromagnetic-paramagnetic phase transition T _C with an increase in the niobium concentration x and the annealing temperature have been explained by the decrease in the content of the ferromagnetic phase, as well as by changes in the ratio Mn³⁺/Mn⁴⁺, the oxygen nonstoichiometry, and the concentration of defects weakening the high-frequency electronic exchange of the ions Mn³⁺ ? Mn⁴⁺. The presence of nanostructured clusters in the lattice has been confirmed by an anomalous hysteresis associated with the unidirectional exchange anisotropy of the interaction between the ferromagnetic matrix and antiferromagnetic clusters with Mn²⁺ and Nb³⁺ in distorted A-positions. An analysis of the asymmetrically broadened ⁵⁵Mn NMR spectra and their computer decomposition have revealed a high-frequency electronic exchange and an inhomogeneity of the magnetic and valence states of manganese due to the nonuniform distribution of all ions and defects. Two types of magnetoresistive effects have been found: one effect, which is observed near the phase transition temperatures T _C and T _ms, is caused by scattering at intracrystalline nanostructured heterogeneities of the imperfect perovskite structure, and the other effect, which is observed in the low-temperature range, is induced by tunneling through intercrystalline mesostructured boundaries. The phase diagram has demonstrated that there is a strong correlation between the composition, structure, resistive and magnetic properties of rare-earth manganites.     

Mesoscopic structure of Ni<Subscript>2 + <Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ga alloys in the concentration range corresponding to a coupled magnetic-structural phase stransition

The Ni_{2 + x} Mn_{1 − x} Ga magnetic shape memory alloys at concentrations x = 0.18–0.36 have been studied using small-angle polarized neutron scattering in magnetic fields 0 < H ≤ 5.7 kOe. In this concentration range, the alloy undergoes a coupled magnetic-structural phase transition. At x < 0.18, the martensitic phase transition occurs, as usual, in the ferromagnetic state, i.e., T _m < T _C (T _m and T _C are the temperatures of the martensitic and magnetic phase transitions, respectively). However, at x > 0.27, the relationship between the characteristic temperatures is changed, i.e., T _m > T _C. The small-angle polarized neutron scattering data indicate that, in the concentration range under investigation, there occurs a complex transformation of the magnetic and atomic structures. All phase transitions exhibit a temperature hysteresis of scattering and polarization, which is characteristic of first-order phase transitions. A comparative analysis of the mesoscopic structures of the Ni_{2 + x} Mn_{1 − x} Ga and Ni_{2 + x + y} Mn_{1 − x} Ga_{1 − y} alloys studied using the small-angle polarized neutron scattering technique has been carried out.     

Temperature variation of phonon frequency distribution in the fast ion conductor lead fluoride

A weighted phonon frequency distribution has been measured in PbF₂ at temperatures 10, 302, 660 and 910 K, using a neutron scattering technique. At 10 K good agreement is found between the measured distribution and the phonon density-of-states calculated from the low temperature dispersion relation of PbF₂. At the higher temperatures, near the ionic conductivity transition temperature, T_c ～ 700 K, the optic modes are observed to broaden into a high energy tail consistent with strong anharmonicity or extensive disorder. A low energy peak arising from transverse acoustic modes remains well defined even at temperatures above T_c.     

Long-wavelength magnetic excitations in the Zn0.55Mn0.45Fe2O4 ferrite

The small-angle neutron scattering energy spectra of the Zn_0.55Mn_0.45Fe₂O₄ ferrite are analyzed at different temperatures (both below and above T _C ? 390 K) and scattering angles. The thermal expansion coefficient α(T) is measured in the temperature range 80–600 K. It is revealed that inelastic neutron scattering is governed not only by spin waves of the Holstein-Primakoff type but also by the substantial contribution of additional long-wavelength magnetic excitations. The physical nature of these low-energy magnetic excitations is discussed.     

Electron hopping and optic phonons in Eu3S4

Raman scattering on single crystals of Eu₃S₄ does not show the allowed q=o phonon modes in the cubic phase and exhibits no new modes in the distorted low temperature phase (T<186 K). Above the Curie temperature T_c=3.8 K the scattering is dominated by a spin-disorder induced one-phonon density of states allowing for the observation of the zone boundary phonon breathing mode of the S^2?ions. This mode does not show any anomaly near the charge order -disorder phase transition T_t=186 K. Temperature tunable spin fluctuations associated with the temperature activated Eu²⁺→Eu³⁺ electron hopping are detected in the scattering intensity, superimposed on the usual thermal spin disorder.     

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.     

Magnetic coupling between the different phases in nanocrystalline Fe-Si-B studied by small angle neutron scattering

The magnetic coupling in the two-phase nanostructure of Fe_73.5Si_15.5B₇Nb₃Cu₁ has been investigated by SANS technique. The scattering function for partially aligned magnetic moments of ferromagnetic single domain particles following the Langevin statistics of superparamagnetism has been derived which describes the field and temperature dependency of the SANS profiles. Above the Curie temperature of the amorphous phase T _c ^am the nanocrystals show superparamagnetic scattering behaviour whereas below T _c ^am they interact by magnetostatic forces only and not by exchange coupling via the amorphous matrix.     

Magnetic excitations in neodymium,a sinusoidally modulated antiferromagnet

We report the first unambiguous observations of the magnetic excitations in neodymium metal, which exhibits complex antiferromagnetic structures below T_N = 20 K. The wavevector and temperature dependence of the excitations have been determined from neutron inelastic scattering measurements made at the Institut Laue-Langevin. Two district, low-energy, magnetic modes are observed for the [100] basal plane direction (TM) whilst there is only a single mode in the c-direction (TA) These modes are only observed clearly at low temperatures (T?8 K): they broaden and soften as the temperature is raised through T_N. Measurements were also made of the longitudinal and transverse acoustic phonon spectra in neodymium. The predictions of current theories of the excitations in sinusoidally modulated magnetic structures are discussed and compared with the results of our observations.     

Neutron diffraction studies of the magnetic properties of Pr0.5Sr0.5Co1 ? x Mn x O3 solid solutions

The crystal structure and magnetic properties of a system of Pr_0.5Sr_0.5Co_{1 ? x} Mn _x O₃ solid solutions were studied by neutron diffraction and magnetization measurements. It is shown that, at a low manganese concentration, the structure can be described by the I/2a monoclinic space group; with increasing substitution level x the structure becomes orthorhombic. For x > 0.9 the crystal structure is tetragonal at high temperatures and the symmetry is lowered to orthorhombic with lowering the temperature. The substitution of cobalt for manganese leads to the destruction of long-range ferromagnetic order near x ?? 0.25. A transition from the high-temperature ferromagnetic phase to the A-type low-temperature antiferromagnetic phase is observed at x ?? 0.93 in the temperature range 110?C160 K.     

Spin correlations in Ni-Mn-Ga

Results of measuring small-angle neutron scattering and neutron depolarization in a Ni_49.1Mn_29.4Ga_21.5 single crystal in the temperature range 15<T<400 K and in the range of magnetic fields 0<H<4.5 kOe are presented. The characteristic temperatures of the alloy were found to be as follows: T _C=373.7 K and the martensite transition temperature T _m=301–310 K. The magnetic critical scattering at T _C and the scattering at T<T _C were adequately described by the relationship I _m=A(q ²+κ²)^?2 (q is the transferred wave vector and R _c=1/κ is the correlation radius), and the temperature dependences of the A and R _c scattering parameters were determined. Left-right asymmetry was observed at 150<T< T _m in the scattering of neutrons polarized along or opposite to the applied field. This asymmetry was due to the inelastic magnetic interaction of neutrons in the sample. The magnetization of the alloy at T _m, critical scattering at T?T _C, anomalies in scattering, and the softening of magnetic excitations at 150 <T<T _m are discussed.     

Magnetism in a UNi2/3Rh1/3Al single crystal

We report a magnetization, magnetostriction, electrical resistivity, specific heat and neutron scattering study of a UNi_2/3Rh_1/3Al single crystal, a solid solution of an antiferromagnet UNiAl and a ferromagnet URhAl. The huge uniaxial magnetic anisotropy confining the principal magnetic response to the c axis in the parent compounds persists also for the solid solution. The magnetization curve at 1.6 K has a pronounced S shape with an inflection at 12 T. The temperature dependence of magnetic susceptibility exhibits a maximum around 10 K and is magnetic history dependent at lower temperatures where the resistivity increases linearly with decreasing temperature. The low-temperature ρ(T) anomaly is removed in a magnetic field applied along c, which yields a large negative magnetoresistance amounting to m46 zin 14T (at 2 K). The C/T values exhibit a minimum around 12 K and below 8 K they become nearly constant (about 250 mJ mol^?1 K^?2), which is strongly affected by magnetic fields. Neutron scattering data confirm a non-magnetic ground state of UNi_2/3Rh_1/3Al. The bulk properties at low temperatures are tentatively attributed to the freezing of U magnetic moments with antiferromagnetic correlations. The additional intensities detected on top of nuclear reflections in neutron diffraction in a magnetic field applied along c are found to be proportional to the field-induced magnetization, which reflects field-induced ferromagnetic coupling of U magnetic moments. This scenario is corroborated also by finding low-temperature magnetostriction data that also scale with the square of magnetization.     

Magnetic study of CaMn0.96Mo0.04O3, canting vs. phase separation

CaMn_0.96Mo_0.04O₃ is an example of Mn⁴⁺ rich perovskite manganites, which exhibits a net ferromagnetic component at low temperature, observed by dc magnetization and ac susceptibility. To characterize the magnetic state of this compound, neutron powder diffraction was carried out in the 2-400 K temperature range, showing that it is necessary to use three components (ferromagnetic and G- and A-type antiferromagnetic) to describe it. This particular state is in agreement with the unusual magnetic behaviour observed by macroscopic measurements and is compared to the one observed for manganites with similar Mn valence but obtained by A-site substitution.