Presence Of Magnetic Inhomogeneities Research Articles

Evolution of short-range magnetic correlations in ferromagnetic Ni-V alloys

We experimentally study how the magnetic correlations develop in a binary alloy close to the ferromagnetic quantum critical point with small-angle neutron scattering (SANS). Upon alloying the itinerant ferromagnet nickel with vanadium, the ferromagnetic order is continuously suppressed. The critical temperature Tc vanishes when vanadium concentrations reach the critical value of xc=0.116 indicating a quantum critical point separating the ferromagnetic and paramagnetic phases. Earlier magnetization and $\mu$SR data have indicated the presence of magnetic inhomogeneities in Ni(1-x)V(x) and, in particular, recognize the magnetic clusters close to xc, on the paramagnetic and on the ferromagnetic sides with nontrivial dynamical properties [R. Wang et al., Phys. Rev. Lett. 118, 267202 (2017)]. We present the results of SANS study with full polarization analysis of polycrystalline Ni(1-x)V(x) samples with x=0.10 and x=0.11 with low critical temperatures Tc below 50 K. For both Ni-V samples close to xc we find isotropic magnetic short-range correlations in the nanometer-scale persisting at low temperatures. They are suppressed gradually in higher magnetic fields. In addition, signatures of long-range ordered magnetic domains are present below Tc. The fraction of these magnetic clusters embedded in the ferromagnetic ordered phase grows towards xc and agrees well with the cluster fraction estimate from the magnetization and $\mu$SR data. Our SANS studies provide new insights into the nature of the inhomogeneities in a ferromagnetic alloy close to a quantum critical point.

Magnetocaloric effect in La0.70Ag0.25MnO[formula omitted] magnetic nanoparticles

Experimental study of magnetocaloric effect (MCE) was performed by direct and indirect methods on La0.70Ag0.25MnO3+δ nanoparticles with rhombohedral LaAlO3 – type crystal structure (R3¯c space group – No. 167, hP30). The content of oxygen was modified by annealing at 800 °C/48h in different atmosphere (air, O2 and Ar). The Curie temperature TC increases from 250 K to 319.5 K and to 322.9 K with the content of oxygen. The adiabatic temperature change ΔT(4T) = 1.52 K, the change of magnetic entropy −ΔSmax(4T) = 3.96 Jkg−1K−1 and the relative cooling power RCT(4T) = 172 J/kg are maximal for sample treated in Ar and decrease with oxygen content. The frequency dependence of ΔT (f) and the shape of maximum in ΔT (T) and −ΔSmax (T) indicate relaxation processes and presence of magnetic inhomogeneities which are present mostly in sample treated in Ar. The samples demonstrate the stability of the MCE up to 1000 cycles of switching on and off the magnetic field without any signs of degradation.

SMALL-ANGLE SCATTERING OF NEUTRONS ON Sr<sub>2</sub>FeMoO<sub>6–δ</sub> SAMPLES WITH DIFFERENT-DEGREE SUPERSTRUCTURAL ORDERING OF Fe/Mo CATIONS

Single-phase Sr2FeMoO6-δ samples with different-degreesuperstructiral ordering Fe/Mo cations superstructural ordering (P, 76, 86 and 93 %) were obtained by the solid-phase technique. Based on the results of measuring the magnetic characteristics in the samples, we found that an increase in magnetization (26.41, 32.36 and 42.66 A·m2·kg–1), magnetic moment (1.33, 3.07 and 3.58 μB/f.u.) and Curie temperatures (422, 428 and 437 K) withparameter P (76, 86 and 93 %) can be explained by the presence of antistructural defects, as well as antiferromagnetic inclusions. This determines the redistribution of electron density, which is accompanied by the change in electronic configuration of a part of Fe/Mo cations. Based on the temperature dependences of the magnetic moment of the samples measured in ZFC and FC modes, and on small-angle polarized neutron scattering (SANS), we found that the samples are in a magnetically inhomogeneous state. An important result to mention is that we detected the difference between the slope of the SANS curves of samples with different oxygen content, which demonstrates a different microstructure of inhomogeneities. The main inhomogeneities are magnetic inclusions with the dimensions depending on the superstructural ordering of Fe/Mo cations. According to the Porod law, it was shown that the Sr2FeMoO6-δ samples with wave vector values 0.1 > q > 0.002 Å–1 contain polydisperse grains with a smooth surface. For q > 0.1 Å–1 a deviation from the Porod law is observed, confirming the presence of magnetic inhomogeneities with a diameter < 6 nm in the grains.

Complex exchange mechanism driven ferromagnetism in half-metallic Heusler Co2TiGe : Evidence from critical behavior

We have investigated the critical phenomenon associated with the magnetic phase transition in the half-metallic full-Heusler Co$_2$TiGe. The compound undergoes a continuous ferromagnetic to paramagnetic phase transition at the Curie temperature $T_{C}$=371.5 K. The analysis of magnetization isotherms in the vicinity of $T_{c}$, following modified Arrott plot method, Kouvel-Fisher technique, and critical isotherm plot, yields the asymptotic critical exponents $\beta$=0.495, $\gamma$=1.324, and $\delta$=3.67. The self-consistency and reliability of the obtained exponents are further verified by the Widom scaling relation and scaling equation of states. The mean-field-like value of the critical exponent $\beta$ suggests long-range nature of the exchange interactions, whereas the values of the critical exponents $\gamma$ and $\delta$, imply sizeable critical spin fluctuations. The half-metallic itinerant character of Co$_{2}$TiGe in the presence of magnetic inhomogeneity may result in such a strong deviation from the three-dimensional Heisenberg values ($\beta$=0.369, $\gamma$=1.38 and $\delta$=4.8) of the critical exponents towards the mean field values ($\beta$=0.5, $\gamma$=1 and $\delta$=3). The results suggest complex nature of exchange couplings that stabilize the long-range ferromagnetic ordering in the system and are consistent with the earlier theoretical studies on the exchange mechanism in Co$_2$TiGe.

Singlet-triplet conversion and the long-range proximity effect in superconductor-ferromagnet structures with generic spin dependent fields.

The long-range proximity effect in superconductor-ferromagnet (S/F) hybrid nanostructures is observed if singlet Cooper pairs from the superconductor are converted into triplet pairs which can diffuse into the ferromagnet over large distances. It is commonly believed that this happens only in the presence of magnetic inhomogeneities. We show that there are other sources of the long-range triplet component (LRTC) of the condensate and establish general conditions for their occurrence. As a prototypical example, we consider first a system where the exchange field and spin-orbit coupling can be treated as time and space components of an effective SU(2) potential. We derive a SU(2) covariant diffusive equation for the condensate and demonstrate that an effective SU(2) electric field is responsible for the long-range proximity effect. Finally, we extend our analysis to a generic ferromagnet and establish a universal condition for the LRTC. Our results open a new avenue in the search for such correlations in S/F structures and make a hitherto unknown connection between the LRTC and Yang-Mills electrostatics.

Measurement of spin memory lengths in PdNi and PdFe ferromagnetic alloys

Weakly ferromagnetic alloys are being used by several groups in the study of superconducting/ferromagnetic hybrid systems. Because spin-flip and spin-orbit scattering in such alloys disrupt the penetration of pair correlations into the ferromagnetic material, it is desirable to have a direct measurement of the spin memory length in such alloys. We have measured the spin memory length at 4.2 K in sputtered Pd0.88Ni0.12 and Pd0.987Fe0.013 alloys using methods based on current-perpendicular-to-plane giant magnetoresistance. The alloys are incorporated into hybrid spin valves of various types, and the spin memory length is determined by fits of the Valet-Fert spin-transport equations to data of magnetoresistance vs. alloy thickness. For the case of PdNi alloy, the resulting values of the spin memory length are lsf(PdNi) = 2.8 +/- 0.5 nm and 5.4 +/- 0.6 nm, depending on whether or not the PdNi is exchange biased by an adjacent Permalloy layer. For PdFe, the spin memory length is somewhat longer, lsf(PdFe) = 9.6 +/- 2 nm, consistent with earlier measurements indicating lower spin-orbit scattering in that material. Unfortunately, even the longer spin memory length in PdFe may not be long enough to facilitate observation of spin-triplet superconducting correlations predicted to occur in superconducting/ferromagnetic hybrid systems in the presence of magnetic inhomogeneity.

The magnetotransport properties of La0.7Sr0.3MnO3∕BaTiO3 superlattices grown by pulsed laser deposition technique

We have investigated the magnetotransport properties of La0.7Sr0.3MnO3∕BaTiO3 superlattices, grown on SrTiO3 substrate by pulsed laser deposition technique, both with current-in-plane and current-perpendicular-to-the-plane directions. Several features indicate the presence of magnetic inhomogeneities at the interfaces, which are independent of BaTiO3 layer thickness variation. First, the magnetic property in the superlattices decreases. Second, a hysteresis in magnetoresistance due to the relaxation of the resistive state is observed. Third, a threshold under an applied magnetic field in the magnetoresistance is seen. Such behaviors are in agreement with the phase separation scenario, which could be the possible reason for these magnetic inhomogeneities at the interfaces. On the contrary, the magnetoresistance with the current-perpendicular-to-the-plane direction is mostly attributed to the tunneling effect along with the ordering of the spin at the interface. This study confirms the importance of the interfaces in superlattices that can be used to control physical properties in oxide materials.

Magnetic and magnetoresistivity properties of nanocrystalline Nd 0.7Sr 0.3MnO 3

Nanocrystalline Nd 0.7Sr 0.3MnO 3 sample with an average particle size of 40 nm has been synthesized by citrate-complex method. The temperature-dependent magnetization shows the absence of long-range ferromagnetic (FM) order. The electron magnetic resonance study indicates the presence of magnetic inhomogeneity below 290 K. Electrical conductivity follows variable range hopping mechanism in paramagnetic (PM) regime. The low-temperature insulator-like resistivity seems to obey non-tunneling conductivity mechanism. The magnetoresistivity (MR) increases with decreasing temperature down to magnetic transition temperature ( T C) and saturates below T C. The MR does not follow spin polarized tunneling mechanism in FM regime. The above results could be best described by spin-dependent hopping between the localized spin clusters together with the phase separation phenomenon. It is assumed that the clusters form due to the distribution of canted spins all over the volume of the nanoparticles and these canted spins prompt the itinerant electrons to localize. The analysis of the MR reveals that FM phase contributes to the total MR at low applied magnetic fields, whereas the PM phase contributes at relatively high fields. The above model quantitatively explains the observed MR.

Theory of two magnon scattering microwave relaxation and ferromagnetic resonance linewidth in magnetic thin films

A detailed analysis of the two magnon scattering contribution to the microwave relaxation and ferromagnetic resonance linewidth in isotropic and anisotropic films and disks has been made. The analysis is based on the Sparks, Loudon, and Kittel (SLK) theory for the scattering of uniform mode magnons into degenerate spin wave states for isotropic spherical samples in the presence of magnetic inhomogeneities in the form of spherical voids or pores. The SLK theory has been extended to include: (i) thin film and thick film samples magnetized in an oblique out-of-plane direction; (ii) uniaxially anisotropic materials with either easy-axis or easy-plane anisotropy and an anisotropy axis perpendicular to the disk plane; (iii) a modified density of degenerate states to account for the nonzero relaxation rate of the scattered spin waves; and (iv) two limiting cases of the scattering interaction: (a) the original SLK case where the inhomogeneities are modeled as spherical voids and the coupling to the degenerate spin waves varies with the spin wave propagation direction and (b) an isotropic scattering model where the coupling is independent of the propagation direction. The formulation is valid for thick films for which the discrete nature of the spin wave modes may be neglected. The two magnon linewidth as a function of field orientation is calculated for three classes of material parameters corresponding to yttrium iron garnet and barium M-type and zinc Y-type hexagonal ferrites. The linewidth versus static field angle profiles show characteristic profiles which depend on the crystalline anisotropy, the sample dimensions, the nature of the scattering interaction, the inhomogeneity size, and the inhomogeneity volume fraction. These parameters, as well as the shape and evolution of the spin wave band as a function of the field angle under ferromagnetic resonance conditions, play critical roles in determining the linewidth versus angle profiles.

Annealing and magnetic field effects on the resistivity of Fe-rich FeZr glasses

Electrical resistivity at zero and applied magnetic fields has been measured in as-quenched and annealed Fe 91Zr 9 and Fe 90Zr 10 amorphous alloys between 4 and 320 K. Fe 91Zr 9 remained amorphous after annealing at 400 K (1 h) while in Fe 90Zr 10, traces of crystallites were observed. Similar annealings at 600 K were sufficient to crystallize both alloys, with α-Fe as the main product. A.c. susceptibility shows small changes in the Curie and spin-glass temperatures upon annealing. The resistivity displays a minimum at high temperatures (≅ 250 K), slightly influenced by a magnetic field and thermal treatment. The resistivity is a competition of contributions in which the structure plays the most important role. The presence of magnetic inhomogeneities could also favor an enhanced electronic localization.

Magnetostatic coupling in multilayered films

The possibility of magnetostatic coupling in multilayered magnetic films due to the presence of magnetic inhomogeneities is discussed. The reason of magnetic inhomogeneities formation is the regions with different magnetic phases or noncoherent magnetization rotation during remagnetization process of the films. This kind of magnetostatic coupling depends on the correlation between sizes of regions with different magnetic phases. It can result in positive or negative coupling between layers.

X-ray and neutron topographical studies of magnetic materials

Neutron and synchrotron topography have proved to be complementary tools for the investigation of magnetic domains of all kinds (ferro-, ferri- and antiferromagnetic) and of magnetic phase coexistence in single crystals. These imaging techniques, which map the special variations of the reflecting power, allow the simultaneous observation of the crystalline (dislocations, twins, precipitates,…) and ‘magnetic’ defects. The origin of the magnetic related contrast in the case of X-rays resides mainly in the variation of magnetostrictive distortion between neighbouring domains. For neutrons contrast mainly arises from changes in the domains or phase structure factors. X-ray topographic experiments lead to a better understanding of the strains created by the presence of magnetic inhomogeneities. Neutron topography is the only technique that is capable of visualizing ‘exotic’ magnetic domains like 180° antiferromagnetic domains (MnF 2, α-Fe 2O 3,…) or chirality domains (right hand/left hand helix) in helimagnets. These investigations show the relationship between domain size and sample purity, domain shape and magnetic history of the crystal, and highlight several striking ‘memory effects’. Synchrotron topography, in the ‘real time’ or the ‘stroboscopic’ versions, allows the observation of the shapes and movements of the domains and interfaces as a function of the temperature or the applied field, as well as their interaction with crystalline defects. Several examples are presented to illustrate the capabilities of these imaging techniques.

Magnetostatic field computations in terms of two‐component vector potentials

AbstractIn this paper the magnetostatic problem is stated in terms of two‐component electric and magnetic vector potentials. An associated numerical method, based on the adoption of edge elements, is proposed. This procedure overcomes the cancellation problems and the complexity of the interface conditions encountered by similar approaches in the presence of magnetic inhomogeneities and discontinuities of currents and magnetic fields.

Exact results in the three-dimensional Landau-Ginzburg model of magnetic inhomogeneities in uniaxial ferromagnets: I. Continuous transitions

The Landau-Ginzburg model is applied to a three-dimensional uniaxial ferromagnet in the presence of magnetic inhomogeneities. The equation of motion for the order parameter is derived and its steady-state exact, analytical solutions are found using the method of symmetry reduction for partial differential equations. All translationally invariant solutions are studied in detail and their energies are calculated exactly. Some results on spherically and cylindrically symmetric solutions are presented, as well as on solutions invariant under subgroups involving dilations.

Magnetization and FMR studies in Ni-Ag multilayers

We have prepared Ni-Ag multilayers and studied their magnetization (M) (68 - 293 K) and FMR properties (290 K). The Ag layer thickness (t Ag) was kept constant at 5 nm and that of Ni (t Ni) varied from 0.7 to 10 nm. At 4 K, M decreases slightly from the bulk value but remains much higher than that reported so far in other systems. We attribute this to the presence of Ag in the pores of thin layers of Ni and their mixing which leads to an increase in Ni moment. FMR results are shown to indicate to relatively high M for t < 3 nm where no anisotropy expected. The large increase in the line width in these cases also confirms the presence of magnetic inhomogeneities.