Phase domain structures in cylindrical magnets under conditions of a first-order magnetic phase transition

The magnetic and resonance properties of cylindrical magnets at first-order phase transition from paramagnetic to ferromagnetic state were theoretically studied. It has been shown that in the external magnetic field directed perpendicularly to the rotation axis, formation of a specific domain structure of paramagnetic and ferromagnetic layers can be energetically favorable. The parameters of cylindrical phase domains as well as their dependences on temperature, magnetic field and material characteristics have been calculated. Peculiarities of the magnetic resonance spectra appearing as a result of the phase domain formation have been considered. Dependence of the resonance field of the system of ferromagnetic domains on magnetization and temperature has been obtained.     

Angular dependence of the photothermally modulated magnetic resonance in Gd thin films

In this paper we use electron spin resonance and photothermally modulated magnetic resonance techniques to investigate gadolinium thin films as a function of the orientation of the film surface with respect to the external magnetic field and of the temperature, around the magnetic phase transition temperature. We observe that, in the ferromagnetic phase, the resonance line is shifted up to higher external magnetic fields when the angle between the film surface and the field increases, revealing the magnetic anisotropy of the sample. At the same time, when the temperature is augmented to values higher than the phase transition temperature, the external field of the resonance collapses back to the expected value in the paramagnetic phase for all orientations. We also demonstrated that, even for the perpendicular orientation (magnetic field perpendicular to the sample surface), the photothermally modulated magnetic resonance signal is maximized near the magnetic phase transition temperature. Furthermore, in the ferromagnetic phase the photothermally modulated magnetic resonance intensity is very sensitive to the orientation, showing a significant enhancement in the perpendicular direction.     

Magnetic properties of the charge ordered Nd0.75Na0.25MnO3

Nd_0.75Na_0.25MnO₃ polycrystalline ceramic is prepared via sol-gel process and its magnetic properties and electron spin resonance (ESR) spectra have been investigated experimentally. As the compound is cooled from room temperature, a charge-ordered state first develops below 170 K. A high magnetic field melts the charge ordered state and stabilizes a ferromagnetic (FM) state below 170 K. A field induced transition, analogous to a spin flip transition, is observed between 40 and 170 K. The critical temperature for spin flip increases with increasing temperature. Below 130 K, the compound tends to be intrinsically inhomogeneous, i.e. FM clusters and paramagnetic domains coexist in this system at least, which is confirmed by ESR measurements. When the external magnetic field is zero, long range FM interaction is not developed in this system; however, a tendency of re-entrant FM transition is observed in this compound.     

Precursor state of skyrmions in MnSi: a heat capacity study

The magnetic phase diagram of MnSi based on the detailed magneto‐heat capacity study is presented. It shows the A‐phase, a precursor to the formation of stable skyrmion lattice, along with the intermediate, helical, conical, ferromagnetic and paramagnetic phases. The field isotherms of specific heat in and around the transition temperature reveal the different magnetic modulations. The local minima represent the relatively low entropy state due to the formation of A‐phase as a precursor to the stable skyrmion lattice. The field‐induced second‐order phase transition is observed by melting the intermediate phase. The region of existence of first‐order phase transition is found to be, effectively, from helical to the intermediate phase. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

一级相变磁制冷材料的基础问题探究

由于一级相变磁制冷材料发生磁相变时有晶胞体积的突变,相变过程中有相变潜热存在,其磁化过程中有许多磁学问题有待于进一步探究.本文以LaFe13-xSix合金为研究对象,在现有对磁一级相变基础问题的分析基础上,对一级相变材料中系统熵变、等温熵变、绝热温变、热滞、磁滞、铁磁与顺磁态两相共存的温度区间和磁场区间、制冷能力的计算等磁学基础问题进行了较为细致的探究.分析表明,在忽略完全铁磁态和顺磁态对磁热效应的贡献时,Maxwell方程和Clausius-Clapeyron方程计算熵变的值具有等效性.等温磁化过程中升温和降温曲线包围的面积SABCE(磁滞的大小),实际上是升温过程和降温过程中磁场做的净功,等于相变潜热之差.磁滞和热滞的大小与磁化过程数据测量的时间有关,测量时间越长则滞后越小,当相变是平衡相变则滞后为零.另外,对温度和磁场诱导磁相变过程进行了分析,提出了一级相变磁制冷材料制冷能力的不同计算模型.本文对一级相变磁制冷材料的磁学基础问题研究有一定的参考价值.     

Magnetic phase transition in (Mn0.95Ni0.05)3B4

It was found that the metallic compound (Mn_0.95Ni_0.05)₃B₄ was ferromagnetic below 195 K and antiferromagnetic between 195 and 354 K. The transition temperature from ferromagnetic to antiferromagnetic increases with increasing external magnetic field. On the other hand, the transition temperature from antiferromagnetic to paramagnetic decreases with increasing magnetic field. It is expected that the present results might be explained by the theoretical results on the coexistence of ferro- and antiferromagnetism in the itinerant electron system reported by Moriya and Usami.     

Untersuchung der Neutronendepolarisation von Dy in der Umgebung von magnetischen Umwandlungspunkten

It is possible to determine the dimensions of the ferromagnetic correlation range by depolarisation measurements of polarized thermal neutrons near magnetic transition points, where small magnetic domains (< 10^?4 cm) are present. The dimensions of the magnetic domains of Dy were determined in the temperature range from 4,2 °K to room temperature and in an external magnetic field from 0 to 2,4 kOe. The size of the domains increases with decreasing temperature and increasing external field. For low temperatures a sort of internal coercive force for the wall mobility was observed, which strongly hinders the formation of greater ferromagnetic domains. At the Curie-point the ferromagnetic correlation range shows a continuous transition and goes only slowly to zero when the temperature increases.     

Stochastic resonance and nonequilibrium dynamic phase transition of Ising spin system driven by a joint external field

The dynamic response and stochastic resonance of a kinetic Ising spin system （ISS） subject to the joint action of an external field of weak sinusoidal modulation and stochastic white-nolse are studied by solving the mean-field equation of motion based on Glauber dynamics. The periodically driven stochastic ISS shows that the characteristic stochastic resonance as well as nonequilibrium dynamic phase transition （NDPT） occurs when the frequency ω and amplitude h0 of driving field, the temperature t of the system and noise intensity D are all specifically in accordance with each other in quantity. There exist in the system two typical dynamic phases, referred to as dynamic disordered paramagnetic and ordered ferromagnetic phases respectively, corresponding to a zero- and a unit-dynamic order parameter. The NDPT boundary surface of the system which separates the dynamic paramagnetic phase from the dynamic ferromagnetic phase in the 3D parameter space of ho-t-D is also investigated. An interesting dynamical ferromagnetic phase with an intermediate order parameter of 0.66 is revealed for the first time in the ISS subject to the perturbation of a joint determinant and stochastic field. The intermediate order dynamical ferromagnetic phase is dynamically metastable in nature and owns a peculiar characteristic in its stability as well as the response to external driving field as compared with a fully order dynamic ferromagnetic phase.     

Reorientation phase transitions in planar arrays of dipolarly interacting ferromagnetic particles

Reorientation phase transitions (RPT) taking place in regular arrays of rectangular submicron-size ferromagnetic particles due to the competition between the external magnetic field of arbitrary direction and internal dipolar fields are analysed in this article. Dipolar interaction between particles is taken into account via real-space calculations of magnetometric demagnetizing factors. Long stripe arrays are also under consideration. I find that the direction of the external magnetic field determines the kind of the phase transition, while the dipolar interaction between particles can significantly change the values of RPT critical field. Calculations were presented for a set of submicron particles/stripe arrays, which were under experimental investigations recently.     

On the theory of phase transitions in magnetic fluids

Particles of magnetic fluids (ferrofluids), as is known from experiments, can condense to bulk dense phases at low temperatures (that are close to room temperature) in response to an external magnetic field. It is also known that a uniform external magnetic field increases the threshold temperature of the observed condensation, thus stimulating the condensation process. Within the framework of early theories, this phenomenon is interpreted as a classical gas-liquid phase transition in a system of individual particles involved in a dipole-dipole interaction. However, subsequent investigations have revealed that, before the onset of a bulk phase transition, particles can combine to form a chain cluster or, possibly, a topologically more complex heterogeneous cluster. In an infinitely strong magnetic field, the formation of chains apparently suppresses the onset of a gas-liquid phase transition and the condensation of magnetic particles most likely proceeds according to the scenario of a gas-solid phase transition with a wide gap between spinodal branches. This paper reports on the results of investigations into the specific features of the condensation of particles in the absence of an external magnetic field. An analysis demonstrates that, despite the formation of chains, the condensation of particles in this case can proceed according to the scenario of a gas-liquid phase transition with a critical point in the continuous binodal. Consequently, a uniform magnetic field not only can stimulate the condensation phase transition in a system of magnetic particles but also can be responsible for a qualitative change in the scenario of the phase transition. This inference raises the problem regarding a threshold magnetic field in which there occurs a change in the scenario of the phase transition.     

Magnetocaloric effect and magnetic properties in YMnO3 perovskite

The magnetic properties and magnetocaloric effect in YMnO₃ have been investigated using Monte Carlo simulations. The thermal magnetization, specific heat and magnetic entropy have been obtained for different values of exchange interactions and for a several external magnetic field values. The variation of adiabatic temperature change with the temperatures has been obtained for several values of external magnetic field. It has been found that the sample exhibited a paramagnetic to ferromagnetic phase transition at 30 K. The transition temperature of YMnO₃ has been deduced for different values of size (1/L) and different values of exchange interactions. The relative cooling power with several values of external magnetic field has been established.     

Room-temperature ferromagnetism in alkylamine-intercalated vanadium oxide

Electron paramagnetic resonance measurements revealed a ferromagnetic phase transition taking place near room temperature in quasi-two-dimensional dodecylamine-intercalated vanadium pentoxide with a mixed structure of coexisting lamellar and rectangular phases. It is shown that the ferromagnetism can be ascribed to the rectangular phase, whereas the paramagnetic lamellar phase undergoes no magnetic transition.     

Domain structure of the antiferromagnetic insulating state in Nd0.5Sr0.5MnO3

Optical reflectivity studies of the ferromagnetic metal (FMM) to antiferromagnetic insulator (AFI) phase transition were performed on Nd_0.5Sr_0.5MnO₃ single crystals in a wide temperature and magnetic field range. The formation of a domain structure in the AFI state was observed. On the basis of the experimental results and symmetry analysis we conclude that these domains are crystal twins, confirming the symmetry lowering of the crystal at this phase transition. The twin domain structure of the AFI state in the Nd_0.5Sr_0.5MnO₃ is visible in reflected unpolarized light due to a different tilting of the surface of the domains. We find no evidence for intermediate FMM+AFI stripe phase.     

Metamagnetic phase transitions induced by static and pulsed fields in (Sm0.5Gd0.5)0.55Sr0.45MnO3 ceramics

It has been found that the magnetic susceptibility of (Sm_0.5Gd_0.5)_0.55Sr_0.45MnO₃ ceramic samples in zero external magnetic field exhibits a sharp peak near the temperature of 48.5 K with a small temperature hysteresis that does not depend on the frequency of measurements and is characteristic of the phase transition to an antiferromagnetic state with a long-range charge orbital ordering, which is accompanied by an increase in the magnetic susceptibility with a decrease in the temperature. The magnetization isotherms in static and pulsed magnetic fields at temperatures below 60 K demonstrate the occurrence of an irreversible metamagnetic transition to a homogeneous ferromagnetic state with a critical transition field independent of the measurement temperature, which, apparently, is associated with the destruction of the insulating state with a long-range charge ordering. In the temperature range 60 K ?? T ?? 150 K, the ceramic samples undergo a magnetic-field-induced reversible phase transition to the ferromagnetic state, which is similar to the metamagnetic transition in the low-temperature phase and is caused by the destruction of local charge/orbital correlations. With an increase in the temperature, the critical transition fields increase almost linearly and the field hysteresis disappears. Near the critical fields of magnetic phase transitions, small ultra-narrow magnetization steps have been revealed in pulsed fields with a high rate of change in the magnetic field of ??400 kOe/??s.     

Dynamic phase transitions and the effects of frequency of oscillating magnetic field on the dynamic phase diagrams in the bilayer honeycomb lattice with AB stacking geometry

We study some dynamic properties of the bilayer honeycomb lattice with AB stacking geometry in the presence of a time-dependent oscillating external magnetic field. We employ the Glauber transition rates to construct the mean-field dynamical equations. First, we obtain dynamic phases in the system and observe the paramagnetic (p), ferromagnetic (f), compensated (c) antiferromagnetic (af), surface ferromagnetic (sf) and mixed (m) phases. Besides, coexistence phase regions also exist in the system. Second, we investigate the thermal behavior of the dynamic order parameters. From these study, the natures (first- or second-order) of the transitions are characterized and the dynamic phase transition (DPT) points are presented. The DPTs are obtained and the dynamic phase diagrams (DPD) are constructed plane of the temperature versus the amplitude of the magnetic field. We investigate the effect of the frequency of the oscillating external magnetic field on the DPD.     

Temperature-induced magnetic phase transitions in crystals with competing single-ion and interionic magnetic anisotropies

Temperature-induced phase transitions in a uniaxial ferromagnetic system of spins S = 1 with competing one-particle and two-particle anisotropies are studied. It is shown that, in the case where easy-plane single-ion anisotropy dominates over easy-axis two-particle anisotropy, the transition from the paramagnetic state to a ferromagnetic state with magnetization perpendicular to the anisotropy axis is a second-order displacive magnetic phase transition. In the opposite case, where two-particle anisotropy dominates over single-particle anisotropy, the transition to a ferromagnetic state with magnetization perpendicular to the anisotropy axis is also continuous but of the order-disorder type. In a system with competing second-order one-and two-particle anisotropies, the orientational first-order phase transition can occur to a state with the magnetization directed along or perpendicular to the anisotropy axis.     

Interaction of separated ferromagnetic domains in a hole-doped manganite achieved by a magnetic field

We report the change in the magnetic microstructure with the application of a magnetic field to a hole-doped manganite La0.81Sr0.19MnO3 in the mixed-phase state, in which ferromagnetic and paramagnetic phases coexist. In situ observations by electron holography have revealed that the applied magnetic field generates a "channel" of the magnetic flux in the paramagnetic phase region, thereby connecting the separated ferromagnetic domains. The magnetic flux density of this channel is estimated at 0.33 T, which is comparable with that of the ferromagnetic domains. The connection of the separated ferromagnetic domains appears to promote the conduction in the mixed-phase state as predicted for many manganites exhibiting the magnetoresistance effect.     

Magnetic phase diagrams of axial third-nearest-neighbour Ising model

The magnetic phase diagrams of axial third-nearest-neighbour Ising (A3NNI) model under an external field have been studied by means of the molecular field approximation. The phase boundaries among paramagnetic, (anti)ferromagnetic and various modulated phases are determined by analysing the frequency-dependent susceptibility, or by solving the coupled equations for magnetisations for spins up to 17, iteratively. The resultant phase diagram is found to be very consistent with the exact results for the ground state spin ordering and to be far improved than existing finite temperature phase diagrams for the same model.     

The order of magnetic phase transition in La(Fe1−xCox)11.4Si1.6 compounds

Magnetic transitions in La(Fe_1−xCo_x)_11.4Si_1.6 compounds with x=0–0.08, have been studied by DC magnetic measurements and Mössbauer spectroscopy. The temperature dependence of the Landau coefficients has been derived by fitting the magnetization, M(μ₀H), using the Landau expansion of the magnetic free energy. For x0.02 there is a strongly first-order magnetic phase transition between ferromagnetic and paramagnetic (F–P) states in zero external field and a metamagnetic transition from paramagnetic to ferromagnetic (P–F) above T_c. Increasing the cobalt content drives the F–P transition towards second order and eliminates the metamagnetic transition.     

Magnetic measurements of Fe-Ni-Nb-B and Fe-Co-Mo-Cu-B in the vicinity of the Curie temperature

The study of the transition between ferromagnetic and paramagnetic states has been investigated on selected metallic glass systems based on Fe-Ni-Nb-B and Fe-Co-Mo-Cu-B with T_C close to room temperature. Samples in the form of ribbons were prepared by planar flow casting and magnetostriction in parallel and perpendicular directions and saturation magnetostrictions have been determined on these samples in as-cast states together with hysteresis loops. In addition, a magneto-optic device for dynamic domain observation has been used for observation of domain structure. Magnetostriction measurements using direct method of measurement show the decrease of saturation magnetostriction towards zero upon approaching T_C. In paramagnetic state the field dependencies of magnetostriction in parallel and perpendicular configurations exhibit a linear dependence on the external magnetic field. In the transition region of temperatures the dependencies are a combination of ferromagnetic and paramagnetic field dependencies. The coercivity H_C in the materials investigated exhibits values below 20 A/m. The observed magnetic domains are typical for this class of amorphous alloys. The polarization in paramagnetic state increases gradually with increase in magnetizing field, reflecting the increasing amount of polarized regions.