Uniaxial Ferromagnet Research Articles

Domain engineering of the metastable domains in the 4f-uniaxial-ferromagnet CeRu2Ga2B

In search of novel, improved materials for magnetic data storage and spintronic devices, compounds that allow a tailoring of magnetic domain shapes and sizes are essential. Good candidates are materials with intrinsic anisotropies or competing interactions, as they are prone to host various domain phases that can be easily and precisely selected by external tuning parameters such as temperature and magnetic field. Here, we utilize vector magnetic fields to visualize directly the magnetic anisotropy in the uniaxial ferromagnet CeRu2Ga2B. We demonstrate a feasible control both globally and locally of domain shapes and sizes by the external field as well as a smooth transition from single stripe to bubble domains, which opens the door to future applications based on magnetic domain tailoring.

Analysis of Magnetization Curves and Magnetocrystalline Anisotropy of Uniaxial Ferromagnets

Theoretical analysis of the processes of magnetization of uniaxial ferromagnetic materials is performed within the Neel theory of magnetic phases. Relations are obtained for the constants of magnetic crystal anisotropy K 1, K 2, the form factor N, and the saturation magnetization I s , at which the magnetization curves exhibit jumps (FOMP). Formulas for computing the saturation fields H s and the jump fields H FOMP are derived for crystals with different types of magnetocrystalline anisotropy MCA. It is shown that the Sucksmith–Thompson method is applicable for computing the first too MCAconstants of uniaxial ferromagnets with any type of MCA. Constants K 1 and K 2 are computed with allowance for the form factor of the specimen. Model magnetization curves are plotted for uniaxial ferromagnets with different types of MCA along and perpendicularly to crystallographic axis c. The analytical results match the model curves well.

К вопросу об эффективной массе структурных неоднородностей доменной границы в одноосных ферромагнетиках

К вопросу об эффективной массе структурных неоднородностей доменной границы в одноосных ферромагнетиках

Magnetic hysteresis properties of nanocrystalline (Nd,Ho)-(Fe,Co)-B alloy after melt spinning, severe plastic deformation and subsequent heat treatment

A study is made of the effect of multistage treatment on the magnetic hysteresis properties at room temperature and below of high coercive (Nd0.55Ho0.45)2.7(Fe0.8Co0.2)14B1.2 alloy including melt spinning (MS), severe plastic deformation (SPD) and heat treatment (HT). It is shown that SPD and HT of MS samples improves the rectangularity of the second quadrant part of the hysteresis loop resulting in a more than 25% increase of the maximal magnetic energy product as compared to ordinary MS processing. The observed changes in the magnetic properties of the samples are discussed and modelled within the framework of Stoner-Wohlfarth theory of magnetization reversal processes in uniaxial ferromagnets.

Superconductor-ferromagnet bilayer under external drive: The role of vortex-antivortex matter

Using advanced Ginzburg-Landau simulations, we study the superconducting state of a thin superconducting film under a ferromagnetic layer, separated by an insulating oxide, in applied external magnetic field and electric current. The taken uniaxial ferromagnet is organized into a series of parallel domains with alternating polarization of out-of-plane magnetization, sufficiently strong to induce vortex-antivortex pairs in the underlying superconductor in absence of other magnetic field. We show the organization of such vortex-antivortex matter into rich configurations, some of which are not matching the periodicity of the ferromagnetic film. The variety of possible configurations is enhanced by applied homogeneous magnetic field, where additional vortices in the superconductor may lower the energy of the system by either annihilating the present antivortices under negative ferromagnetic domains or by lowering their own energy after positioning under positive ferromagnetic domains. As a consequence, both the vortex-antivortex reordering in increasing external field and the evolution of the energy of the system are highly nontrivial. Finally, we reveal the very interesting effects of applied dc electric current on the vortex-antivortex configurations, since resulting Lorentzian force has opposite direction for vortices and antivortices, while direction of the applied current with respect to ferromagnetic domains is of crucial importance for the interaction of the applied and the Meissner current, as well as the consequent vortex-antivortex dynamics—both of which are reflected in the anisotropic critical current of the system.

Magnetic anisotropy and reduced neodymium magnetic moments inNd3Ru4Al12

This paper addresses the electronic properties of $\mathrm{N}{\mathrm{d}}_{3}\mathrm{R}{\mathrm{u}}_{4}\mathrm{A}{\mathrm{l}}_{12}$ (hexagonal crystal structure) with focus on its magnetic anisotropy that allows a comparison between single-ion and two-ion mechanisms when comparing to ${\mathrm{U}}_{3}\mathrm{R}{\mathrm{u}}_{4}\mathrm{A}{\mathrm{l}}_{12}$. We performed magnetization measurements on a single crystal in static and pulsed magnetic fields as well as neutron-diffraction experiments. $\mathrm{N}{\mathrm{d}}_{3}\mathrm{R}{\mathrm{u}}_{4}\mathrm{A}{\mathrm{l}}_{12}$ is a strongly anisotropic uniaxial ferromagnet with a Curie temperature of 39 K. The magnetic moments are aligned collinearly along the [001] axis. The magnetic structure of $\mathrm{N}{\mathrm{d}}_{3}\mathrm{R}{\mathrm{u}}_{4}\mathrm{A}{\mathrm{l}}_{12}$ has orthorhombic symmetry for which the crystallographic Nd site is split into two magnetically inequivalent positions, Nd1 and Nd2. The Nd1 and Nd2 atoms exhibit reduced magnetic moments, 0.95 and $2.66\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$, as compared to the free $\mathrm{N}{\mathrm{d}}^{3+}$-ion value ($3.28\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$). We discuss this finding in terms of crystal-field effects and competing exchange and anisotropy interactions. Since the single-ion mechanism in $\mathrm{N}{\mathrm{d}}_{3}\mathrm{R}{\mathrm{u}}_{4}\mathrm{A}{\mathrm{l}}_{12}$ leads to uniaxial anisotropy and the two-ion mechanism of the actinide analog ${\mathrm{U}}_{3}\mathrm{R}{\mathrm{u}}_{4}\mathrm{A}{\mathrm{l}}_{12}$ is known to lead to planar anisotropy, this paper demonstrates the decisive influence of these different mechanisms on the magnetic anisotropy.

Взаємодія пружних та спінових хвиль в феромагнетику одноосної симетрії

The dispersion laws of coupled magnetoelastic waves have been calculated for all ground states in a uniaxial ferromagnet. The magnetoelastic interaction is shown to take place not for all sound modes in those ground states. The obtained dispersion laws testify that the magnetoelastic interaction coefficient depends on both the magnetization direction and the wave vector direction. It is demonstrated that the magnetoelastic interaction between sound and spin waves in the uniaxial ferromagnet is characterized by the constants B44 and B66, whereas the other magnetoelastic constants govern only the formation of a magnetoelastic gap in the spectrum of coupled waves.

Stable states of magnetic inhomogeneities localized in the region of defects

The topology and conditions of nucleation of magnetic inhomogeneities at defects of a uniaxial ferromagnet have been investigated theoretically. It has been shown that, for specific material parameters, two types of inhomogeneities corresponding to a 0-degree domain wall, which differ in the size, amplitude, and energy, can arise at defects. It has been found that the large-amplitude inhomogeneity is a metastable formation. However, with inclusion of demagnetizing fields of the sample, it can become stable, i.e., energetically more favorable in comparison with the small-amplitude inhomogeneity. The possible contribution from this inhomogeneity to the static and dynamic processes of magnetization reversal of magnetic materials has been considered.

Dipole-exchange spin waves in nanotubes composed of uniaxial ferromagnets with “easy-plane” and “easy-axis” anisotropies

Dipole-exchange spin waves in nanotubes composed of uniaxial ferromagnets are studied. An equation for the magnetic potential of linear spin waves in uniaxial “easy plane” ferromagnets is obtained in the magnetostatic approximation taking into account magnetic dipole-dipole interactions, exchange interactions, and anisotropy effects. A solution is found for this equation and a dispersion relation for these types of spin waves is obtained. The dependence of the spin wave frequency on the total wave number is found for the case of a thin nanotube. An expression is derived for the transverse wave number spectrum.

Generation of magnetic inhomogeneities at solitary ferromagnet defects

In the framework of micromagnetic theory, we develop a general approach to the problem of determining conditions for generating a magnetic inhomogeneity at uniaxial ferromagnet defects. We establish that for the same material parameters, two types of magnetic inhomogeneities corresponding to the zero-degree domain wall and having different amplitudes can form at solitary defects. We find diagrams of their existence and show that the obtained regularities of inhomogeneity generation have a general character independent of the form of the chosen defect model.

The Higgs effect and the magnetoelastic gap in ferromagnets

It is shown that the manifestation of a magnetoelastic gap in a ferromagnet is caused by the same effect that endows the Higgs boson with mass. The “easy plane” state is studied in a uniaxial ferromagnet, taking into account magnetoelastic interaction. The strain tensor components of the ferromagnet in the ground state, magnetoelastic gap, and the renormalization of the magnetic anisotropy constant, are determined.

A general formalism for the determination of the effective mass of the nanoscale structural inhomogeneities of the domain wall in uniaxial ferromagnets.

On the basis of the method of gyrotropic Thiele forces, we build a formalism that allows the determination of the effective mass of the nanoscales structural elements of the domain wall (DW): vertical Bloch line and Bloch point in uniaxial ferromagnets. As shown, the effective mass of these magnetic inhomogeneities depends on the value of the gyrotropic domain wall bend that is created by their movement.Electronic supplementary materialThe online version of this article (doi:10.1186/s11671-015-0861-z) contains supplementary material, which is available to authorized users.

Concepts on statistical physics - fluctuations in equilibrium

This essay corresponds to the content of three lectures about statistical physics delivered to the audience of the 2014 section of the R. A. Salmeron School of Physics, at the UnB. Our starting point was very simple statistical models (lattice gas, spin-1/2 ferromagnet), used as illustrations of the competencies and methods in statistical physics. Thus we introduce the Gibbs ensembles, defining a connection with thermodynamics and discussing the role played by fluctuations and large numbers. We present phenomenological aspects of phase transitions and critical phenomena in simple fluids and in uniaxial ferromagnets, emphasizing the universal character of the critical exponents. We describe the phenomenological van der Waals and Curie-Weiss theories and the Landau expansion, which are present-day relevant methods, despite the fact that such theories give rise to critical exponents in disagreement with experiments. We present then the paradigmatic Ising model, which points us to a way to overcome the phenomenological results. A brief presentation of the scale phenomenological methods and the contemporaneous renormalization group are considered at the end of these lectures.

Anomalous magnetoresistance in nanocrystalline gadolinium at low temperatures

The results of a detailed investigation of electrical resistivity, ρ(T) and transverse magnetoresistance (MR) in nanocrystalline Gd samples with an average grain size d = 12 nm and 18 nm reveal the following. Besides a major contribution to the residual resistivity, ρr(0), arising from the scattering of conduction electrons from grain surfaces/interfaces/boundaries (which increases drastically as the average grain size decreases, as expected), coherent electron–magnon scattering makes a small contribution to ρr(0), which gets progressively suppressed as the applied magnetic field (H) increases in strength. At low temperatures (T ≲ 40 K) and fields (H = 0 and H = 5 kOe), ρH(T) varies as T3/2 with a change in slope at T+ ≃ 16.5 K. As the field increases beyond 5 kOe, the T3/2 variation of ρH(T) at low temperatures (T ≲ 40 K) changes over to the T2 variation and a slight change in the slope dρH/dT2 at T+(H) disappears at H ⩾ 20 kOe. The electron–electron scattering (Fermi liquid) contribution to the T2 term, if present, is completely swamped by the coherent electron–magnon scattering contribution. As a function of temperature, (negative) MR goes through a dip at a temperature Tmin ≃ T+, which increases with H as H2/3. MR at Tmin also increases in magnitude with H and attains a value as large as ∼15% (17%) for d = 12 nm (18 nm) at H = 90 kOe. This value is roughly five times greater than that reported earlier for crystalline Gd at Tmin ≃ 100 K. Unusually large MR results from an anomalous softening of magnon modes at T ≃ Tmin ≈ 20 K. In the light of our previous magnetization and specific heat results, we show that all the above observations, including the H2/3 dependence of Tmin (with Tmin(H) identified as the Bose–Einstein condensation (BEC) transition temperature, TBEC(H)), are the manifestations of the BEC of magnons at temperatures T ⩽ TBEC. Contrasted with crystalline Gd, which behaves as a three-dimensional (3D) pure uniaxial dipolar ferromagnet in the asymptotic critical region, ρH=0(T) of nanocrystalline Gd, in the critical region near the ferromagnetic-paramagnetic phase transition, is better described by the model proposed for a 3D random uniaxial dipolar ferromagnet.

The Bloch point in uniaxial ferromagnets as a quantum mechanical object

Quantum effects such as tunneling through pinning barrier of the Bloch Point and over-barrier reflection from the defect potential of one have been investigated in ferromagnets with uniaxial strong magnetic anisotropy. It is found that these phenomena can be appeared only in subhelium temperature range.

Reflection of Surface Spin Waves from the Interface of Uniaxial and Biaxial Ferromagnets in a Planar Magnetic Field

The article investigates the process of reflection of surface spin waves passing through the interface of uniaxial and biaxial ferromagnets in a planar external magnetic field directed along the hard axis of ferromagnet. The problem is solved using the spin density formalism and the Landau-Lifshitz equations for the case of the absence of dissipation in the system. Geometrical optics formalism is used to describe the processes of refraction of surface spin waves propagating in the ferromagnetic medium with non uniform distribution of magnetic parameters. Quantum mechanical approach is used for calculation of the amplitudes of reflected and transmitted waves. It is shown that spin wave birefringence phenomenon appears at the interface of two uniform ferromagnetic components. Frequency and field dependencies of reflection coefficients for different branches of spin waves are obtained in the study. It is shown that it is possible to change the “optical” parameters of the system by only changing a magnitude of the external homogeneous magnetic field. It is also shown that reflection amplitude depends heavily on the angle of incidence.

Reflection of Surface Spin Waves from the Interface of Uniaxial and Biaxial Ferromagnets in a Planar Magnetic Field

The article investigates the process of reflection of surface spin waves passing through the interface of uniaxial and biaxial ferromagnets in a planar external magnetic field directed along the hard axis of ferromagnet. The problem is solved using the spin density formalism and the Landau-Lifshitz equations for the case of the absence of dissipation in the system. Geometrical optics formalism is used to describe the processes of refraction of surface spin waves propagating in the ferromagnetic medium with non uniform distribution of magnetic parameters. Quantum mechanical approach is used for calculation of the amplitudes of reflected and transmitted waves. It is shown that spin wave birefringence phenomenon appears at the interface of two uniform ferromagnetic components. Frequency and field dependencies of reflection coefficients for different branches of spin waves are obtained in the study. It is shown that it is possible to change the “optical” parameters of the system by only changing a magnitude of the external homogeneous magnetic field. It is also shown that reflection amplitude depends heavily on the angle of incidence.

Critical behaviour of nanocrystalline gadolinium: evidence for random uniaxial dipolar universality class

We report on how nanocrystal size affects the critical behaviour of the rare-earth metal Gd near the ferromagnetic-to-paramagnetic phase transition. The asymptotic critical behaviour of the coarse-grained polycrystalline sample (with an average crystallite size of L≅100 μm) is that of a (pure) uniaxial dipolar ferromagnet, as is the case with single crystal Gd, albeit the width of the asymptotic critical region (ACR) is reduced. As the grain size approaches ∼30 nm, the ACR is so narrow that it could not be accessed in the present experiments. Inaccessibly narrow ACR for L ∼ 30 nm and continuous increase in the width of the ACR as L decreases from 16 to 9.5 nm basically reflect a crossover to the random uniaxial dipolar fixed point caused by the quenched random exchange disorder prevalent at the internal interfaces (grain boundaries).

Phase diagrams and solvation forces of a uniaxial ferromagnet in a slit—the double-parabola approach

We obtain the phase diagrams of a uniaxial ferromagnet enclosed in a slit and analyse the properties of the solvation forces acting between the slit walls. Our analysis is based on the mean-field theory applied to the double-parabola Hamiltonian for a uniaxial ferromagnet in a slit, which allows us to derive several results in analytic form. In particular, the Kelvin law and the law describing the influence of the slit width on the position of the delocalization transition as compared to the wetting transition are derived and the corresponding critical exponent is evaluated. The influence of the thermodynamic state of the system on the properties of the solvation forces, in particular their discontinuous behaviour at coexistence lines, is discussed.

Remagnetization of permanent magnets in closed and open magnetic circuits

The homogeneity of the strength of the magnetic field on the surface of a magnet placed in open and closed magnetic circuits is investigated using a program for modeling the remagnetization processes in highly anisotropic uniaxial ferromagnets. It is shown that the inhomogeneity of the magnetic field on the surface of a magnet is a consequence of magnetostatic interaction between the magnet’s microvolumes. A comparison of the calculation results and experimental data shows their close resemblance.