The giant volume magnetostriction and colossal magnetoresistance in Eu<Subscript>0.55</Subscript>Sr<Subscript>0.45</Subscript>MnO<Subscript>3</Subscript> manganite

A doped manganite with the composition Eu_0.55Sr_0.45MnO₃ exhibits giant negative magnetostriction and colossal negative magnetoresistance at temperatures in the vicinity of the magnetic phase transformation (T～41 K). In the temperature interval 4.2 K≤T ≤40 K, the isotherms of magnetization, volume magnetostriction, and resistivity exhibit jumps at the critical field strength H_c1, which decreases with increasing temperature. At 70 K ≤T ≤120 K, the jumps on the isotherms are retained, but the shapes of these curves change and the H_c1 value increases with the temperature. At H<H_c1, the magnetoresistance is positive and exhibits a maximum at 41 K; at H>H_c1, the magnetoresistance becomes negative, passes through a minimum near 41 K and then reaches a colossal value. The observed behavior is explained by the existence of three phases in Eu_0.55Sr_0.45MnO₃, including a ferromagnetic (in which the charge carriers concentrate due to a gain in the s-d exchange energy) and two antiferromagnetic phases (of the A and CE types). The volumes of these phases at low temperatures are evaluated. It is shown that the colossal magnetoresistance and the giant volume magnetostriction are related to the ferromagnetic phase formed as a result of the magnetic-field-induced transition of the CE-type antiferromagnetic phase to the ferromagnetic state.     

Magnetic and transport properties of colossal magnetoresistance compound EuB<Subscript>6</Subscript>

The galvanomagnetic and magnetic properties of EuB₆ single crystal have been measured over wide temperature (1.8–300 K) and magnetic-field (up to 70 kOe) ranges, and the parameters of charge carriers and the characteristics of the magnetic subsystem are estimated in the paramagnetic and ferromagnetic (T < T _C ≈ 13.9 K) phases of this compound with strong electron correlations. In the temperature range T < T* ≈ 80 K, a magnetoresistance hysteresis Δρ(H)/ρ(0) is detected; it reaches a maximum amplitude of about 5% at T ≈ 12 K. The anomalies of charge transport observed in the temperature range T _C < T < T* are shown to be related to the magnetic scattering of charge carriers (m _eff = (15–30)m ₀, where m ₀ is the free-electron mass) that results from a short-range magnetic order appearing upon the formation of ferromagnetic nanoregions (ferrons).     

Peculiarities of magnetic,galvanomagnetic, elastic,and magnetoelastic properties of Eu<Subscript>0.55</Subscript>Sr<Subscript>0.45</Subscript>MnO<Subscript>3</Subscript>

Magnetic, elastic, magnetoelastic, transport, and magnetotransport properties of the Eu_0.55Sr_0.45MnO₃ ceramics have been studied. A break was detected in the temperature dependence of electrical resistivity ρ(T) near the temperature of the magnetic phase transformation (41 K), with the material remaining an insulator down to the lowest measurement temperature reached (ρ=10⁶ Ω cm at 4.2 K). In the interval 4.2≤T≤50 K, the isotherms of the magnetization, volume magnetostriction, and ρ were observed to undergo jumps at the critical field H_C1, which decreases with increasing T. For 50≤T≤120 K, the jumps in the above curves persist, but the pattern of the curves changes and H_C1 grows with increasing T. The magnetoresistance Δρ/ρ = (ρ _H ?ρ_H=0)/ρ _H is positive for H<H_C1 and passes through a maximum at 41 K, where Δρ/ρ = 6%. For H>H_C1, the magnetoresistance is negative, passes through a minimum near 41 K, and reaches a colossal value of 3×10⁵ % at H=45 kOe. The volume magnetostriction is negative and attains a giant value of 4.5×10^?4atH=45 kOe. The observed properties are assigned to the existence of three phases in Eu_0.55Sr_0.45MnO₃, namely, a ferromagnetic (FM) phase, in which carriers are concentrated because of the gain in s-d exchange energy, and two antiferromagnetic (AFM) phases of the A and CE types. Their fractional volumes at low temperatures were estimated to be as follows: ～3% of the sample volume is occupied by the FM phase; ～67%, by the CE-type AFM phase; and ～30%, by the A-type AFM phase.     

Visualization of the magnetic flux structure in phosphorus-doped EuFe<Subscript>2</Subscript>As<Subscript>2</Subscript> single crystals

Magnetic flux structure on the surface of EuFe₂(As_1-x P _x )₂ single crystals with nearly optimal phosphorus doping levels x = 0.20 and x = 0.21 is studied by low-temperature magnetic force microscopy and decoration with ferromagnetic nanoparticles. The studies are performed in a broad temperature range. It is shown that the single crystal with x = 0.21 in the temperature range between the critical temperatures T _SC= 22 K and T _C = (18 ± 0.3) K of the superconducting and ferromagnetic phase transitions, respectively, has the vortex structure of a frozen magnetic flux, typical for type-II superconductors. The magnetic domain structure is observed in the superconducting state below T _C. The nature of this structure is discussed.     

Experimental evidence of a two-phase magnetic state in thin epitaxial films of Re<Subscript>0.6</Subscript>Ba<Subscript>0.4</Subscript>MnO<Subscript>3</Subscript> (Re = La,Pr, Nd,Gd)

Thin epitaxial films of Re_0.6Ba_0.4MnO₃ (Re = La, Pr, Nd, Gd) on (001)-oriented single crystal SrTiO₃ and ZrO₂(Y₂O₃) substrates have been prepared and studied. All films possess a cubic perovskite structure, except for the film with Re = La, which exhibited a rhombohedral distortion of the perovskite lattice. The results show evidence for the presence of two magnetic phases, ferromagnetic (FM) and antiferromagnetic (AFM), in the films studied: (i) the magnetization isotherm M(H) appears as a superposition of a linear component (characteristic of antiferromagnets) and a small spontaneous magnetization component; (ii) the magnetic moment per formula unit is significantly reduced as compared to the value expected for the complete FM or ferrimagnetic ordering; (iii) there is a difference between magnetizations of the samples cooled with and without an applied magnetic field, which is preserved in the entire range of magnetic fields studied (50 kOe); (iv) the temperature dependence of the magnetization M(T) in strong magnetic fields is close to linear (for the composition with Re = Gd, M(T) is described by a Langevin function for superparamagnets with a cluster moment of 2μ_B); and (v) the magnetization hysteresis loops of the field-cooled samples are shifted along the field axis. The exchange integral (characterizing the Mn-O-Mn coupling via the FM-AFM phase boundary) estimated from the latter shift is | J|=10^?6 eV. This value is two orders of magnitude lower than the negative exchange integral between the FM layers in ReMnO₃, which makes the presence of a transition layer at the FM-AFM phase boundary unlikely. The temperature dependences of electrical resistance and magnetoresistance exhibit maxima at the Curie temperature (T_C), where the magnetoresistance reaches a colossal value. This behavior indicates that the two-phase magnetic state is caused by a strong s-d exchange.     

Magnetic and magneto-lattice dynamics in GdMn<Subscript>2</Subscript>O<Subscript>5</Subscript>

The dynamics of magnetoelectric RMn₂O₅ crystals (R=Eu and Gd) was studied in the frequency and temperature ranges 20–300 GHz and 5–50 K, respectively. The crystals possessed magnetic and ferroelectric long-range order and had close transition temperatures, T_{N, C}?36 and 30 K for R=Eu and Gd, respectively. Mixed magneto-lattice excitations were observed in GdMn₂O₅; the excitations were most intense near the transition temperature T?30 K at frequencies close to the antiferromagnetic resonance frequencies of the Mn subsystem. Along with the antiferromagnetic resonance of the Mn subsystem, the ferromagnetic resonance of the Gd subsystem was observed in GdMn₂O₅ in an external magnetic field. No such dynamics was characteristic of EuMn₂O₅.     

Magnetic Resonance Study of the Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> Doped with Manganese

Crystals of 3D topological insulators, bismuth telluride Bi₂Te₃, doped with manganese were studied using electron spin resonance (ESR) spectroscopy together with the SQUID magnetometry, transport measurements, and X-ray characterization. The obtained ESR data, such as the temperature and the angular dependence of the resonance field, reveal the specific critical behavior and confirm the ferromagnetic ordering of Mn spins even at modest doping. In addition to the studies of the critical behavior of diluted ferromagnet Bi_2?x Mn _x Te_3, we also discuss the effects of the limited solubility of Mn ions giving rise to microscopic inclusions of the spurious magnetic phases which were revealed using ESR technique.     

Magnetic properties of Pb<Subscript>2</Subscript>Fe<Subscript>2</Subscript>Ge<Subscript>2</Subscript>O<Subscript>9</Subscript> single crystals

Single crystals of Pb₂Fe₂Ge₂O₉ have been grown. They were subjected to X-ray diffraction, magnetic, neutron diffraction, Mössbauer and spin resonance studies. It has been established that Pb₂Fe₂Ge₂O₉ is a weak ferromagnet with a Néel temperature T _N = 46 K, and the exchange and spin-flop transition fields have been estimated. It has been demonstrated that the weak ferromagnetic moment is actually the result of the single-ion anisotropy axes for the magnetic moments of different magnetic sublattices being not collinear.     

Experimental study of antiferromagnetic resonance in noncollinear antiferromagnetic Mn<Subscript>3</Subscript>Al<Subscript>2</Subscript>Ge<Subscript>3</Subscript>O<Subscript>12</Subscript>

The structural and magnetic properties of the mesoporous systems based on silicon dioxide with a regular hexagonal arrangement of pores several microns in length and several nanometers in diameter, which are filled with iron compound nanofilaments in various chemical states, are studied in detail. The studies are performed using the following mutually complementary methods: transmission electron microscopy, SQUID magnetometry, electron spin resonance, Mössbauer spectroscopy, polarized neutron small-angle diffraction, and synchrotron radiation diffraction. It is shown that the iron nanoparticles in pores are mainly in the γ phase of Fe₂O₃ with a small addition of the α phase and atomic iron clusters. The effective magnetic field acting on a nanofilament from other nanofilaments is 11 mT and has a dipole nature, the ferromagnetic–paramagnetic transition temperature is in the range 76–94 K depending on the annealing temperature of the samples, and the temperature that corresponds to the change in the magnetic state of the iron oxide nanofilaments is T ≈ 50–60 K at H = 0 and T ≈ 80 K at H = 300 mT. It is also shown that the magnetization reversal of an array of nanofilaments is caused by the magnetostatic interaction between nanofilaments at the fields that are lower than the saturation field.     

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 .     

Crossover from polaron to band conduction upon doping CaMnO<Subscript>3</Subscript> with La ions

In studying the specular reflection IR spectra of manganite polycrystals with electron doping Ca_1?x La _x MnO₃ (0 ≤ x ≤ 0.050) at room temperature, a crossover from polaron to band conduction is observed at x = 0.030. It has been shown that the observed changes in the electronic subsystem is associated with the crossover in the behavior of the magnetization and magnetoresistance in the magnetically ordered and paramagnetic phases that occurs at the same concentration and is described in [C. Chiorescu et al., Phys. Rev. B 73, 014406 (2006)].     

Effect of a magnetic field on the thermal and kinetic properties of the Sm<Subscript>0.55</Subscript>Sr<Subscript>0.45</Subscript>MnO<Subscript>3.02</Subscript> manganite

The temperature and magnetic-field dependences of the heat capacity, thermal conductivity, thermopower, and electrical resistivity of the Sm_0.55Sr_0.45MnO_3.02 ceramic material are studied in the temperature range 77–300 K and in magnetic fields up to 26 kOe. It is revealed that the quantities under investigation exhibit anomalous behavior due to a magnetic phase transition at the Curie temperature T_C. An increase in the magnetic field strength H leads to an increase in the Curie temperature T_C and a jump in the heat capacity ΔC_p at T_C. The temperature dependences of the measured quantities are characterized by hystereses that are considerably suppressed in a magnetic field of 26 kOe and depend neither on the thermocycling range nor on the rate of change in the temperature. The thermal conductivity K at temperatures above T_C shows unusual behavior for crystalline solids (dK/dT>0) and, upon the transition to a ferromagnetic state, drastically increases as a result of a decrease in the phonon scattering by Jahn-Teller distortions. It is demonstrated that the hystereses of the studied properties of the Sm_0.55Sr_0.45MnO_3.02 manganite are caused by a jumpwise change in the critical temperature due to variations in the lattice parameters upon the magnetic phase transition.     

Magnetoresistance of La<Subscript>0.67</Subscript>Sr<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> epitaxial films grown on a substrate with low lattice mismatch

The structure, electrical resistivity, and magnetoresistance of La_0.67Sr_0.33MnO₃ heteroepitaxial films (120-nm thick) practically unstrained by lattice mismatch with the substrate were studied. A strong maximum of negative magnetoresistance of ≈27% (for μ₀H = 4 T) was observed at T ≈360 K. While the magnetoresistance decreased monotonically in magnitude with decreasing temperature, it was still in excess of 2% at 150 K. For T < 250 K, the temperature dependence of the electrical resistivity ρ of La_0.67Sr_0.33MnO₃ films is fitted well by the relation ρ = ρ₀ + ρ ₁(H)T^2.3, where ρ₀ = 1.1×10^?4 Ω cm, ρ₁(H = 0) = 1.8×10^?9 Ω cm/K^2.3, and ρ₁(μ₀H = 4 T)/ρ₁(H = 0) ≈0.96. The temperature dependence of a parameter γ characterizing the extent to which the electrical resistivity of the ferromagnetic phase of La_0.67Sr_0.33MnO₃ films is suppressed by a magnetic field (μ ₀H = 5 T) was determined.     

Magneto-optical spectroscopy and optical detection of EPR spectra of Yb<Superscript>3+</Superscript> paramagnetic centers with cubic symmetry in single crystals <Emphasis Type="Italic">Me</Emphasis>F<Subscript>2</Subscript> (<Emphasis Type="Italic">Me</Emphasis> = Cd,Ca, Pb)

Cubic paramagnetic centers formed by Yb³⁺ impurity ions in fluorite-type crystals MeF₂ (Me = Cd, Ca, Pb) have been investigated using electron paramagnetic resonance, magnetic circular dichroism, magnetic circular polarization of luminescence, Zeeman splitting of optical absorption and luminescence lines, and optical detection of electron paramagnetic resonance. The g factors of the ²Γ₇ state in the excited multiplet ² F _5/2 of Yb³⁺ ions in Me F₂ crystals, the hyperfine interaction constant ¹⁷¹ A (¹⁷¹Yb) for the excited multiplet ² F _5/2 in the CaF₂ crystal, and the energies and symmetry properties of all energy levels of Yb³⁺ ions in MeF₂ crystals are determined. The crystal-field parameters for the crystals under investigation are calculated.     

Electrical resistivity and magnetotransport properties of La<Subscript>0.67</Subscript>Ba<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> epitaxial films biaxially compressed by the substrate

The structure, electrical resistivity, and magnetoresistance of La_0.67Ba_0.33MnO₃(20 nm) films grown coherently on an La_0.3Sr_0.7Al_0.65Ta_0.35O₃(001) substrate with a lattice misfit of about 1% were studied. The rigid connection of the manganite layer with the bulk substrate brought about the unit cell distortion of the substrate (a _⊥/a _‖ = 1.02) and a decrease in the unit cell volume as compared to that of the corresponding bulk crystals (a _‖ and a _⊥ are the unit cell parameters measured in the substrate plane and along the surface normal, respectively). The temperature T _M ≈ 295 K, at which the electrical resistivity ρ of the (20 nm)La_0.67Ba_0.33MnO₃ films reached a maximum, was 40–45 K lower than that for the corresponding bulk crystals. The negative magnetoresistance (MR ≈ ?0.25 for μ₀ H = 1 T) attained a peak value at T _MR ≈ 270 K. The response of ρ to a magnetic field depended substantially on the angle between the current flow in the film and the direction of the magnetic field.     

Electronic Structure and Magnetic Phase Transition in Helicoidal Fe<Subscript>1 - <Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Si Ferromagnets

LSDA + U + SO calculations of the electronic structure of helicoidal Fe_{1 - x}Co _x Si ferromagnets within the virtual crystal approximation have been supplemented with the consideration of the Dzyaloshinski-Moriya interaction and ferromagnetic fluctuations of the spin density of collective d electrons with the Hubbard interactions at Fe and Co atoms randomly distributed over sites. The magnetic-state equation in the developed model describes helicoidal ferromagnetism and its disappearance accompanied by the occurrence of a maximum of uniform magnetic susceptibility at temperature T _C and chiral fluctuations of the local magnetization at T > T _C . The reasons why the magnetic contribution to the specific heat at the magnetic phase transition changes monotonically and the volume coefficient of thermal expansion (VCTE) at low temperatures is negative and has a wide minimum near T _C have been investigated. It is shown that the VCTE changes sign when passing to the paramagnetic state (at temperature T _S ).     

Galvanomagnetic properties of atomically disordered Sr<Subscript>2</Subscript>RuO<Subscript>4</Subscript> single crystals

The effect of neutron-bombardment-induced atomic disorder on the galvanomagnetic properties of Sr₂RuO₄ single crystals has been experimentally studied in a broad range of temperatures (1.7–380 K) and magnetic fields (up to 13.6 T). The disorder leads to the appearance of negative temperature coefficients for both the in-plane electric resistivity (ρ_a) and that along the c axis (ρ_c), as well as the negative magnetoresistance Δρ, which is strongly anisotropic to the magnetic field orientation (H ∥ a and H ∥ c), with the easy magnetization direction along the c axis and a weak dependence on the probing current direction in the low-temperature region. The experimental ρ_a(T) and ρ_c(T) curves obtained for the initial and radiation-disordered samples can be described within the framework of a theoretical model with two conductivity channels. The first channel corresponds to the charge carriers with increased effective masses (～10m _e , where m _e is the electron mass) and predominantly electron-electron scattering, which leads to the quadratic temperature dependences of ρ_a and ρ_c. The second channel corresponds to the charge carriers with lower effective masses exhibiting magnetic scattering at low temperatures, which leads to the temperature dependence of the ρ_{a, c}(T) ∝ 1/T type.     

Positive magnetoresistance of La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript>/C Composites at room temperature

Two-phase composites xLa_0.7Sr_0.3MnO₃/(100–x)C (x = 5–85 mass %) have been synthesized. The magnetoresistive properties of these materials in magnetic fields from 0 to 15 kOe have been investigated. It has been shown that, at room temperature, the positive isotropic magnetoresistance for samples with x = 50–60 mass % reaches 15%.     

Crystallographic,magnetic, and electrical properties of thin <Emphasis Type="Italic">Re</Emphasis><Subscript>0.6</Subscript>Ba<Subscript>0.4</Subscript>MnO<Subscript>3</Subscript> epitaxial films (<Emphasis Type="Italic">Re</Emphasis> = La,Pr, Nd,Gd)

Thin Re_0.6Ba_0.4MnO₃ epitaxial films (Re = La, Pr, Nd, Gd) grown on (001)SrTiO₃ and (001)ZrO₂(Y₂O₃) single crystal substrates have been prepared and studied. All the films were found to have a cubic perovskite structure, with the exception of the film with Re = La, which revealed rhombohedral distortion of the perovskite cell. The temperature dependences of the electrical resistivity and magnetoresistance pass through a maximum near the Curie point T_C, where the magnetoresistance reaches a colossal value. The magnetization isotherms M(H) are superpositions of a magnetization that is linear in field (like that of an antiferromagnet) and a weak spontaneous magnetization. The magnetic moment per formula unit is substantially smaller than that expected under complete ferro-or ferrimagnetic ordering. The magnetizations of samples cooled in a magnetic field (FC samples) and with no field applied (ZFC samples) differ by an amount that persists up to the highest measurement fields (50 kOe). The M(T) dependence obtained in strong magnetic fields is close to linear. Hysteresis loops of the FC samples are shifted along the field axis. The above magnetic and electric properties of thin films are explained in terms of two coexisting magnetic phases, which are due to strong s-d exchange coupling.     

On the <Emphasis Type="Italic">g</Emphasis>-factor value of canted antiferromagnet CoCO<Subscript>3</Subscript>

Experimental data on the magnetization of canted antiferromagnet CoCO₃ (T_N = 18.1 K) in the paramagnetic region are described by the isotropic g factor g_‖ = g_⊥ = 6.5 that differs from the anisotropic values g_‖ = 3.05 and g_⊥ = 4.95 obtained in electron paramagnetic resonance (EPR) measurements at T = 4.2 K on Co²⁺ ions in magnetically diluted crystals. The g-factor values calculated in the Abragam-Pryce and Weiss molecular field approximations using the magnetization data in the magnetic ordered region correspond to data obtained in EPR measurements. It is shown that the absence of the anisotropy of the g factor at high temperatures cannot be explained in the approximations used. Causes of the observed discrepancies are discussed.