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The performance of sintered permanent magnets with nucleation-type coercivity mechanism is largely governed by the magnetic state of thin surface layers of constituent grains, and a deeper insight into magnetization–demagnetization processes occurring in the shell part of the grains is very important for further improvement of hard magnetic materials. In this work, we used Nd2Fe14B and SmCo5 single crystals as model objects. By applying magneto-optical Kerr microscopy and conventional magnetometry, we compare the magnetization–demagnetization processes occurring in the thin surface layer and in the volume of both single crystals. We show that upon magnetization along the c-axis, the volume of the single crystals saturates in the field, rigorously determined by demagnetization factor of the bulk sample, whereas in the surface layer a magnetic domain structure exists up to 1.88 T in Nd2Fe14B and 1.19 T in SmCo5 regardless of their bulk demagnetization factors. This means that the surface layer with orientation perpendicular to c-axis magnetizes as a thin magnetic film and has an effective demagnetization factor N eff ∼ 1. We also show that this effect can be reproduced in the framework of conventional finite element method modeling but the analytical solution of this problem still needs to be found. We believe that our findings can be useful for understanding of the formation of a high coercive state in nucleation-type permanent magnets, where the phenomenological concept of the large effective demagnetization factor N eff plays an important role.

The possibilities of application of known models of domain structure to multiphase alloys are analyzed with the aim to assess the anisotropy constants and to search for magnetically uniaxial highly anisotropic phases. It is shown in what cases and how the standard models of domain structure may be applied to such alloys. Recommendations on the necessary amendments of the model to be applied to multiphase materials are given.

The simulation results on the magnetization curves I(H) and the d2I(H)/dH2 dependences are presented. For this purpose models of rotation of the magnetization vector and the Néel model of magnetic phases were used. Recommendations for implementing the SPD-method for the anisotropy field determining for polycrystalline multidomain samples of ferromagnetic compounds with easy-axis MCA are proposed.

This paper presents the study of the magnetic properties and magnetocaloric effect (MCE) of the rare-earth ferrimagnetic Dy 2 Fe 10 Al 7 compound. The field and temperature dependencies of the magnetization in the temperature range 4.2–300 K were studied in magnetic fields up to 100 kOe. The MCE was studied by direct method in magnetic fields up to 70 kOe. It was established that the sign of the MCE changes from positive to negative near the compensation temperature. It was shown from the obtained experimental results and the theoretical estimations that the temperature and field dependencies of the MCE in rare-earth ferrimagnet differ from those in ferromagnetic materials. The effective field ( H eff 2 ) determined from the values of the MCE near to the compensation temperature are in a good agreement with those reported earlier.

Magnetic domain structure is studied in single crystals of R2Fe17 (R = Tb, Dy, Ho, Er) intermetallic compounds in a wide range of temperatures (10 – 300 K) and magnetic fields (μ0 H = 0 – 0.1 T). It is shown that single crystal specimens of hexagonal magnetics with planar-type anisotropy acquire domain structures containing not only 180-degree domain boundaries but also 60- and 120-degree ones.

Magnetic refrigerants are used as heat exchangers to provide rapid heat transfer between magnetocaloric materials and heat-transfer liquid. An important question is how to turn bulk magnetocaloric materials into porous structures with superior heat transfer properties and cooling performance. We discuss two methods for assembling Y2Fe17 rapidly quenched ribbons into heat exchangers of desired geometry stacked 100 mm thick plates with 100 mm gaps: the first method consists in gluing rapidly quenched ribbons using thermoconductive epoxy; the second is sintering stacked ribbons at a temperature of 30 K below the melting point of the Y2Fe17 phase. These approaches are promising with regards to making near-net shaped magnetic refrigerants. We report on adiabatic temperature change Delta T-ad, magnetic entropy change Delta S-m and thermal conductivity lambda of rapidly quenched Y2Fe17 ribbons, obtained at different quenching rates. A direct correlation between the lattice parameters of the Y2Fe17, Delta T-ad and Delta S-m in rapidly quenched samples is observed. (C) 2016 Elsevier B.V. All rights reserved.

A theoretical consideration of the processes of magnetization of orthorhombic crystals has been performed based on the Neel theory of magnetic phases. It has been established that orthorhombic magnets are characterized by linear magnetization curves. The main mechanisms of achieving the state of magnetic saturation of orthorhombic magnets along various directions of magnetization have been considered. A comparison of the theoretical and experimental magnetization curves for single crystals of the orthorhombic compound FeB has been performed. A good coincidence of the theoretical and experimental curves made it possible to determine the saturation field for FeB single crystals in different crystallographic directions and the magnitudes of the constants of magnetocrystalline anisotropy of the FeB compound.

Processes of magnetization of orthorhombic single crystals of FeB are studied and the magnetocrystalline anisotropy is analyzed theoretically. The domain structure of the FeB single crystals is determined and its theoretical model is suggested.

This paper presents an experimental study of the magnetoelectric (ME) properties of layered heterostructures consisting of PZT-46, PZT-19, and Ni-Zn ferrites (1000NN and 2000NN). Their magnetoelectric response is examined as a function of the configuration, shape, and thickness of the layers in the composites. The results demonstrate that increasing the thickness of the magnetostrictive layer from 0.35 to 3.5 mm has an adverse effect on the magnitude of the ME effect. A comparative analysis of the data for rectangular and disk-shaped samples shows that the ME response of the disks is a factor of 2.5 stronger.

Single crystals of an orthorhombic compound FeB have been synthesized. Based on the data of magnetic measurements and X-ray diffraction analysis, it has been shown that two mutually perpendicular preferred magnetization axes exist in the orthorhombic single crystals of FeB: the easy axis (EA) corresponds to the crystallographic axis [010], and the hard axis (HA), to the [100] axis. The function of the energy of magnetocrystalline anisotropy (MCA) of the orthorhombic magnets has been analyzed for extrema. Orientations of the EAs and HAs have been determined. It has been established that in the orthorhombic magnets there is always realized a single type of MCA: two preferred mutually perpendicular axes always exist in the crystals, one of which is an EA, and the other is an HA. Expressions for the anisotropy fields of orthorhombic magnets have been obtained. The constants of MCA of the orthorhombic compound FeB have been calculated to be K1* ≈ 4 × 105 J/m3 and K2* 7×104 J/m3. The domain structure on the planes (100), (010), and (001) of the FeB single crystals has been studied. It has been shown that the main volume of the sample is occupied by stripe domains ∼40 μm wide with the walls oriented along (100) crystallographic planes. It has been found that the structure of closure domains on the plane (010) of the FeB single crystals has some specific features, namely, there exists a preferred direction in the structure for the orientation of the boundaries of the main domains, and the closure domains have the shape of regular rhombs. A model of the domain structure of FeB single crystals is suggested.

A study on the magnetocaloric effect (MCE) has been carried out on the HoCo 5 single crystal using a direct method in the temperature range 78–300 K and in a magnetic field of 1.3 T at various orientations of the single crystal with respect to the direction of external magnetic field. Our research has shown that magnetic anisotropy strongly affects magnitude and sign of the MCE in the regions of spin-reorientation phase transitions. In addition, the magnetic domain structure of the HoCo 5 single crystal in (0 0 1) plane in the temperature range from room temperature down to 4.2 K was investigated.

The magnetic domain structure on (1 0 0), (1 1 0), (0 0 1) and arbitrary oriented planes of the R(FeCo) 11 Ti single crystals (R = Dy, Tb) is investigated. The role of the stress-induced magnetic anisotropy in the DS formation and the spin-reorientation transitions is analyzed. It is shown that the magnetoelastic contribution to the anisotropy in these compounds is large and can cause changes of the spin-reorientation temperatures and special domain patterns formation.

Under the terms of the Creative Commons Attribution 3.0 Unported License., An investigation of the Y2Fe17 compound belonging to the class of intermetallic alloys of rareearth and 3d-transition metals is presented. The magnetization, magnetostriction, and thermal expansion of the Y2Fe17 single crystal were studied. The forced magnetostriction and magnetostriction constants were investigated in the temperature range of the magnetic ordering close to the room temperature. The giant field induced volume magnetostriction was discovered in the room temperature region in the magnetic field up to 1.2 T. The contributions of both anisotropic singleion and isotropic pair exchange interactions to the volume magnetostriction and magnetostriction constants were determined. The experimental results were interpreted within the framework of the Standard Theory of Magnetostriction and the Landau thermodynamic theory. It was found out that the giant values of the volume magnetostriction were caused by the strong dependence of the 3delectron Coulomb charge repulsion on the deformations and width of the 3d-electron energy band., The work was supported by RFBR Grant Nos. 13-02-00916 and 12-02-31516.

Magnetization and anisotropy field of Nd2Fe14−xCoxBH4.1 (x=0–8) hydrides have been measured on powder aligned specimens. It was found that the unit cell volume increases after hydrogenation. At room temperature all compounds show “easy axis” anisotropy and “easy cone” anisotropy at temperatures below the spin-reorientation temperature (TSR). TSR of the initial compounds and hydrides decreases by the replacement of Fe by Co. Hydrogen insertion in the crystal lattice of Nd2Fe14−xCoxBH4.1 leads to a decrease of TSR by 15 K at Co concentrations x ⩽ 6 . The anisotropy field at room temperature decreases under hydrogenation. We can conclude that the replacement of Fe by Co causes both the unit cell volume and TSR decrease. The TSR decrease is determined by a change of electron structure and magnetocrystalline anisotropy rather than by a decrease of the unit cell volume with the Co concentration increasing.

In ternary R 3 Fe 33 - x Ti 3 ( x = 0 , 1 , … , 7 , R = Gd , Y ) and R 3 Fe 33 - x Ti 3 ( R = Tb , Ho , Dy , Er , x = 0 , 3 , 6 , 9 ) alloys, the anomalous wide homogeneous region of ThMn12 structure has been obtained. The single-crystal samples for each composition R3Fe33−xTi3 were prepared. By means of thermomagnetic analysis, it is shown that the TC of these single crystals monotonically decreases with the decrease in Fe concentration. With the decrease of iron concentration, the magnitude of magnetization saturation steadily decreases too. On the other hand, the temperatures of spin reorientation phases transition TSR for R = Tb , Ho , Dy , Er increase. From this, it follows that by the small changes in the Fe concentration it is possible to make a significant change of parameters of exchange interaction between R and 3d sublattices.

Magnetic characteristics of the Nd 2 Fe 14 BH x single crystals and polycrystalline samples were investigated in the temperature range 4.2-750 K. It is established that hydrogen absorption does not change the crystal structure of these compounds but is accompanied by an expansion of a unit cell volume. The obtained data show that the Curie temperature and magnetic moment are increasing in the all investigated hydrides whereas the temperatures of a spin-reorientation transition remain unchanged.

The tetragonal ThMn 12 -type, single crystalline DyFe 10 CoTi sample has been investigated by torque and magnetization measurements and observation of domain structure at various temperatures between 10–300 K and in magnetic field from B =0 to 0.15 T. These examinations showed that the magnetic structure of DyFe 10 CoTi changes from “easy axis” ( c -axis) type to conical at 225 K and to “easy plane” ( ab plane) type at 100 K.

The magnetic anisotropy and magnetostriction of a Lu2Fe17 single crystal are investigated. The temperature dependence of the magnetic anisotropy constant K 1 is measured in the range 4.2–300 K. The results obtained are compared with the data calculated using the Callen theoretical formula. It is found that the temperature dependence of K 1 for the Lu2Fe17 single crystal deviates from the temperature curve predicted by the localized single-ion model. The inference is drawn that a certain contribution to the magnetic anisotropy of the Lu2Fe17 compound is made by the magnetic anisotropy of band electrons. The longitudinal, transverse, and volume magnetostrictions of the Lu2Fe17 single crystal are studied, and the magnetostriction constants are calculated. It is demonstrated that the exchange integral of Lu2Fe17, as for the Y2Fe17 compound, substantially depends on the atomic volume. This dependence is responsible for the considerable difference between the Curie temperatures of the Lu2Fe17 and Y2Fe17 compounds. It is revealed that the magnetostriction of the Lu2Fe17 single crystal in the temperature range of the magnetic phase transition is determined by the two-ion exchange and single-ion contributions.

On the basis of magnetic domain structure investigations in single crystals of R2Fe14B, RFe11Ti, RCo5 intermetallic compounds (where R is a rare earth metal) in the temperature region 4.2–400 K, the features of domain structure transformation during spin reorientation transitions of the first and second order are analyzed.

The effect of the substitution of Co for Fe on the magnetic anisotropy in YTiFe11−xCox (0≤x≤5) single crystals has been studied by using torque and vibrating-sample magnetometry techniques. The K1 and K2 values were calculated. The Co ion gives a resultant anisotropy which is opposite to that of Fe. A competition is expected to take place in the composition range 5

The aim of this work is to study the influence of substitution of Co for Fe on the magnetocrystalline anisotropy and spin reorientation transition in TbFe 11− x Co x Ti ( x =0;0.5;1;2;3;4;5) compounds. The anisotropy study has been performed by means of torque and magnetization measurements on single crystal samples. It was shown that increase of the Co content in Tb(FeCo) 11 Ti compounds leads to an increase of the Curie temperatures T c . The temperature of the magnetic phase transition during which the easy magnetic direction (EMD) changes from the c-axis via a cone to the basal plane has been found to decrease with increasing Co concentration. On the basis of the experimental results a T − x magnetic phase diagram was constructed. The appearance of phase transitions can be explained by the temperature-dependent competition between the planar Tb sublattice anisotropy and the transition metal (FeCo) 11 sublattice anisotropy.

The performance characteristics of linear Hall element sensors for non-contact determination of the position of a moveable permanent magnet along a pre-established path are analysed. Simple theoretical expressions suitable for computer modelling of previously described sensors are presented. Practical measurements have verified the theoretical predictions arrived at by simulation means. Having established the validity of the basic formulae predictions of performance for a variety of magnet configurations can now be attempted using superposition principles. As an application, a method of improving the linearity of a simple sensor with a single cylinder magnet by means of shaping the cylinder to a special profile is described. An alternative example is a new high-gradient configuration of a radially symmetric sensor construction on a base of two oppositely magnetized rings.

We have investigated the anisotropy of the magnetocaloric effect in a NdCo₅ single crystal in a wide range of temperatures, including the spin-reorientation temperature region. In the field μ(0)H =1.3 T in the spin-reorientation region 250-310 K, we discovered a giant rotating magnetocaloric effect of ~ 1.6 K, caused by rotation of the magnetization vector. The calculations of the anisotropy magnetocaloric effect for the field μ(0)H =1.3 T have been carried out.

The magnetic domain structure of Fe14Nd2B single crystals has been studied from room temperature down to 4.2 K by means of the magneto-optical Kerr effect. It was found that the coercivity of different domain walls essentially depends on their type and orientation. The general expressions for the domain wall energy density γ have been performed for Fe14Nd2B compounds in the low-temperature region. The role of the new domain wall types in the low-temperature magnetization reversal process in FeNdB permanent magnets is discussed.

Single crystals of TbFe11-xCoxTiHy (x = 5; y = 0, 1) with the tetragonal ThMn12-type structure were obtained. Magnetisation measurements along main crystallographic directions of TbFe6Co5TiHy (y = 0, 1) single crystals have been performed in the temperature range 5-340 K. It was established that TbFe6Co5Ti compounds show "easy axis" anisotropy in whole temperature range below Curie temperature (TC). It was concluded that hydrogenation leads to an increase of magnetic anisotropy, and TbFe6Co5TiH hydride demonstrates "easy plane" anisotropy with the easy axis oriented along the [100] crystallographic direction. It was found that the replacement of the Fe by Co and hydrogen insertion into the crystal lattice of Tb(Fe,Co)11Ti compounds have an opposite influence on the magnetocrystalline anisotropy.

Crystallographic structure, domain structure and magnetic properties of ternary Gd 3 Fe x Ti 3 ( x =34, 33, …, 24), TbFe 11 Ti and TbFe 10 Ti alloys and single crystals are investigated. It is shown that all these compounds have a ThMn 12 tetragonal crystal structure. The wide homogeneous region for the ThMn 12 structure detected for this group of compounds is used to explain the differences in the values of spin-reorientation temperatures of RFe 11 Ti compounds found by different authors.

Ferromagnetic compound NdCo2 has complex magnetic transformation, which are accompanied by changes of the crystal lattice. The purpose of this paper was the investigation of influence of these transformations on the field and temperature dependence of magnetostriction. It was found that near the Curie temperature Tc = 98K, where the transition from cubic to tetragonal phase is observed, there is a maximum in the temperature dependence of the transverse magnetostriction λ⊥(T) in the field B = 4T. In this field λ⊥ reaches high values of λ⊥~200·10−6 at temperatures higher than Tc. This shows that in this field, the tetragonal distortion and band magnetism of 3d-subsystem remain even at T>Tc. At T

The analysis of magnetocaloric effect in microcrystalline and nanocrystalline TbFe11Ti and TbFe8Ti intermetallic compounds was carried out. It was found that the dependence between the reduction of the grain size below a single domain size and value of magnetocaloric effect has a complicated nature. Probably, it is connected with the influence of intergrain exchange interaction in nanocrystalline alloys.

The magnetic structure of single-crystal TbFe11−xCoxTi compounds has been studied over a broad temperature range and in strong magnetic fields (up to 14 T). Measurements of magnetization and magnetostriction and a study of the domain structure revealed that spin-reorientation transitions (SRTs) in TbFe11−xCoxTi single crystals depend substantially on the cobalt concentration. It was established that the SRT temperatures and threshold magnetic fields are governed by the interplay between the magnetic anisotropies of the 3d and terbium sublattices. It is shown that, in these compounds, the low-temperature phase with planar anisotropy is separated in temperature from the high-temperature phase with uniaxial anisotropy by an intermediate metastable phase containing domains of the uniaxial or planar phase.

The angular dependence of the nucleation field H n in high-coercivity grains of sintered NdFeB and SmCo 5 magnets has been studied by direct observations of the domain structure in an external magnetic field. The effect of the stray field of surrounding grains on the angular dependence of H n has been estimated quantitatively. It is found that changes in H n of individual grains as large as 0.45 T may be observed, depending on the micromagnetic state of neighbouring grains.

Thermal remagnetization was studied on samples with different M H c distribution. The process is computer simulated on the basis of a model for which the internal magnetic field distribution is calculated with given experimental M H c distribution and M H c and M s temperature dependence. Peculiarities of the open-circuit thermal remagnetization of non-ellipsoidal magnets are examined.

The magnetization reversal behaviour of individual grains was studied by direct domain structure observations. On this basis the angular dependence of the saturation field Hs, critical field of reverse domain removal, Hk, and nucleation field Hn was found to obey the law 1/cos(ϕ − ϕm), where ϕm is the angle between the grain easy axis and the axis of magnet texture.