Frustrated antiferromagnets offer a captivating platform to study the intricate relationship of magnetic interactions, geometric constraints, and emergent phenomena. By controlling spin orientations, these materials can be tailored for applications in spintronics and quantum information processing. The research focuses on the interplay of magnetic and exchange anisotropy effects in artificial heterostructures based on a canonical frustrated antiferromagnet, UO2. The potential to manipulate the spin directions in this material and switch between distinct antiferromagnetic states is investigated using substrate-induced strain. The phenomenon is probed using exchange bias (EB) effects in stoichiometric UO2/Fe3O4 bilayers. By employing many-body first-principles calculations magnetic configurations in the UO2 layers are identified. Even a minor tetragonal distortion triggers a transition between antiferromagnetic states of different symmetries, driven by a robust alteration of single-ion anisotropy due to the distortion. Consequently, this change influences the arrangement of magnetic moments at the UO2/Fe3O4 interface, affecting the magnitude of exchange bias. The findings showcase how epitaxial strain can effectively manipulate the antiferromagnetic states in frustrated antiferromagnets by controlling single-site anisotropy.

Formation of thin uranium hydrides films, UH2 and \b{eta}-UH3, synthesized by a reactive dc sputtering of uranium metal, was explored using variable deposition conditions. Obtained stable oxygen-free hydride films were studied by a variety of methods, both in situ (photoelectron spectroscopy - XPS), and ex-situ (x-ray diffraction - XRD, transmission electron microscopy - TEM), electrical resistivity, and magnetometry). Both types of hydrides are ferromagnetic, the Curie temperatures of UH2 and \b{eta}-UH3 are approx. 120 and 170 K, respectively. Ferromagnetism in the thin films is robust and does not depend on structure details while electrical resistivity data reflect disorder in both types of hydrides.

The magnetocaloric effect (MCE) and anomalies of magnetostriction behavior were studied at the order-order and order-disorder magnetic phase transitions in hydrided Gd single crystal grown by a modified Czochralski method. The composition GdH0.15 was obtained using a Sievert-type apparatus. While parent Gd shows an isotropic MCE at the order-disorder phase transition, the effect is anisotropic in GdH0.15 due to the appearance of local anisotropy. We investigate in detail the temperature variation of the longitudinal, transverse, volume, and anisotropic magnetostriction. Hydrogenation is found to influence both the magnitude and the sign of the magnetostriction constants $\lambda_{\rm ij}^{\alpha}$.

Crystal structure, magnetic and electrical properties of thin U-Mo films and their hydrides prepared by reactive sputter deposition from metallic targets were studied. The films are metallic but with high values of electrical resistivity. Both bcc U(0.7)9Mo(0.21) thin film and the hydride (UH3)(0.74)Mo-0.26 (beta-UH3 structure type) have a very strong texture. Superconductivity is identified below T-c = 0.55 K for U0.79Mo0.21. The critical field of 1.0 T is exceeding that of alpha-U (0.3 T). The hydride film (UH3)(0.74)Mo-0.26 is a ferromagnet with the Curie temperature T-C = 165 K, higher concentration of Mo somewhat reduces the TC value. Deposition on a cooled substrate reduces the grain size of the hydride, approaching the limit of amorphous state. The U-Mo films and hydrides exhibit the net negative temperature coefficient of resistivity, TCR = d(rho)/d(T), attributed to the randomness on atomic scale, yielding very strong scattering of electrons and weak localization.

In this work we report on the structural, magnetic and magnetocaloric properties of polycrystalline HoNi2 and ErNi2 Laves-phase compounds. Powder X-ray diffraction studies at room temperature show that the compounds crystallize in the cubic C15 structure (MgCu2 type). The magnetic measurements reveal that the second-order magnetic phase transition from ferromagnetic to paramagnetic state appears at TC equal 13.5 K for HoNi2 and 6.5 K for ErNi2. Using direct measurements over a wide field range (up to 11 T), along with indirect methods, we have found low and high field regularities of the magnetocaloric effect characterized by the adiabatic temperature change, DTad, and the isothermal magnetic entropy change, DSmag. The magnetocaloric properties near the phase transition are discussed in the framework of the Landau theory for the second-order phase transitions. (C) 2017 Elsevier B.V. All rights reserved.

Влияние легирования алюминием и железом на структуру, магнитные и магнитокалорические свойства многокомпонентных сплавов Tb-Dy-Ho-Co

A study is made of the effects of various factors such as time (7 years), temperature, high magnetic field up to 580 kOe and heat treatment (HT) on the morphological structure and magnetic hysteresis properties of a high-coercive nanocrystalline (Nd0.55Ho0.45)2.7(Fe0.8Co0.2)14B1.2 alloy with a low temperature coefficient of remanence. We find a rather weak time effect on (Nd0.55Ho0.45)2.7(Fe0.8Co0.2)14B1.2. After 7 years, the loss in the maximum magnetic energy product (BH)max is no more than 5%. Annealing of the sample at 250 °C for 30 min decreases the amount of amorphous phase from 7.2 to 1.7%, while the grains’ size of the 2-14-1 phase increases from 83 to 109 nm. For the HT alloy, a magnetization jump is observed at H ~500 kOe. It can be attributed to the first-order magnetization process or a spin-flip magnetic transition. Rectangularity of the hysteresis loop degrades after annealing. In case of the short-time heat treatment, losses in (BH)max are ~10%.

Ferrimagnet HoFe6Al6 (tetragonal ThMn12-type crystal structure) has a compensation point for the Ho and Fe magnetic sublattices at a temperature close to absolute zero. The experimental study was carried out in fields up to 60 T. H-T phase diagrams and a full magnetization process along the principal crystallographic directions of a single-crystalline sample are obtained theoretically by using a model of a fd ferrimagnet with two anisotropic sublattices, coupled by weak exchange interaction. The two firstorder phase transitions, found experimentally along the [110] and [100] axes, were explained theoretically. The transition along the [110] direction occurs between noncollinear and collinear phases, it starts at the compensation point and ends at a tricritical point around 60 T. The transition along the [100] direction goes also from the point of compensation to the point of the liquid-vapor type at 40 T. This transition was shown to occur between two distinct noncollinear phases. The crucial role of the strong rare-earth anisotropy for the positions of the obtained critical points was revealed. (C) 2017 Elsevier B.V. All rights reserved.

A comprehensive analysis of the structure, phase composition, surface topology features, and magnetic and magnetocaloric properties of Tb0.3Dy0.35Ho0.35Co1.75 T (0.25) (T = Al, Fe) multicomponent alloys has been performed. The specifics of variations in the structure and functional properties induced by the partial substitution of cobalt atoms in the 3d sublattice of RCo2 with aluminum or iron atoms have been determined.

We report the attainment of the ferromagnetic state in an interstitially modified heavy rare-earth-iron intermetallic compound in an external magnetic field. The starting composition is RE2Fe17, which is the RE-Fe binary richest in iron. We concentrate on the Tm-Fe compound, which is the most sensitive to magnetic field. The maximum possible amount of hydrogen (5 at.H/f.u.) is inserted into a Tm2Fe17 single crystal. We demonstrate that in a magnetic field of 57 T Tm2Fe17H5 reaches the ferromagnetic state with an enviably high polarization of 2.25 T.

The effect of Tb and Al substitution within the rare earth and cobalt sublattices on structural and magnetothermal properties of Dy0.5Ho0.5Co2 has been studied. Multicomponent Laves phase alloys Tb-x(Dy0.5Ho0.5)(1) Co-x(2) Al-y(y) (x = 0, 0.3, 0.4, 0.5; y = 0, 0.25) synthesized using high-purity metals have been studied using X-ray diffraction analysis, heat capacity and magnetocaloric measurements. Dy0.5Ho0.5Co2 has a first order phase transition at the Curie temperature T-C approximate to 110 K. Both Tb and Al substitution leads to increase of the T-C. The increasing Tb content leads to the decreases slightly the MCE and all the transitions near the Curie temperature are of the first order. As for the Al-containing compounds, MCE measurements show that the phase transition type changes from the first to the second-order. The advantage of Tb-x(Dy0.5Ho0.5)(1) xCo1.75Al0.25 as compared with Al-free alloys is 'table-like' behavior of MCE. (C) 2017 Elsevier B.V. All rights reserved.

Gadolinium is a nearly ideal soft-magnetic material. However, one cannot take advantage of its properties at temperatures higher than the room temperature where Gd loses the ferromagnetic ordering. By using high-purity bulk samples with grains ~200 nm in size, we present proof-of-concept measurements of an increased Curie point (TC) and spontaneous magnetization in Gd due to hydrogenation. From first-principles we explain increase of TC in pure Gd due to the addition of hydrogen. We show that the interplay of the characteristic features in the electronic structure of the conduction band at the Fermi level in the high-temperature paramagnetic phase of Gd and "negative" pressure exerted by hydrogen are responsible for the observed effect.

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.

The influence of simultaneous substitution within the rare earth (R) and Co sublattices on the structural, magnetic, and magnetocaloric properties of the Laves phase RCo2-type compounds is studied. Main attention is devoted to the studies of the magnetostructural phase transitions and the transition types with respect to the alloy composition. Multicomponent alloys Tbx(Dy0.5Ho0.5)1−xCo2 and Tbx(Dy0.5Ho0.5)1−xCo1.75Al0.25 were prepared with the use of high purity metals. Majority of the Tbx(Dy0.5Ho0.5)1−xCo2 alloys exhibit magnetic transitions of the first-order type and a large magnetocaloric effect. The substitution of Al for Co in Tbx(Dy0.5Ho0.5)1−xCo2 increases the Curie temperature (TC) but changes the transition type from first-to the second-order. The discussion of the physical mechanisms behind the observed phenomena is given on the basis of the first principles electronic-structure calculations taking into account both the atomic disorder and the magnetic disorder effects at finite temperatures. The advantage of Al-containing materials is that sufficiently high magnetocaloric effect values are preserved at T > TC.

Magnetization of a Ho2Fe14Si3 single crystal was measured in a steady magnetic field of up to 9 T and in pulsed fields of up to 60 T applied along the principal axes. Ho2Fe14Si3 is a ferrimagnet below T-c = 480 K, has a spontaneous magnetic moment of about 8 mu B/f.u. (at T = 4.2 K) and exhibits a large easy-plane magnetic anisotropy. There is also a certain anisotropy within the basal plane, the b axis [120] being the easy-magnetization direction. In fields applied along the a and b axes field-induced first-order phase transitions are observed at 29 T and at 22 T, respectively. Along the easy axis b we observe also an Sshaped anomaly at about 47 T, which does not correspond to a phase transition. A simple model predicts that the two observed first-order transitions are the only ones taking place in Ho2Fe14Si3; the magnetization should subsequently grow continuously and arrive at saturation at similar to 100 T. This is in stark contrast to the behavior of the parent compound Ho2Fe17, where as many as three sequential first-order transitions are expected for H parallel to b. The reason for the disparity is that the basal-plane anisotropy constant K-Ho is at least one order of magnitude smaller in Ho2Fe14Si3 than it is in Ho2Fe17. (C) 2016 Elsevier B.V. All rights reserved.

The compound Tm 2 Fe 17 , the only one among R 2 Fe 17 (where R is a rare earth metal), exhibits uniaxial magnetic anisotropy at cryogenic temperatures. Its Curie temperature is close to room temperature, T C = 295 K. Magnetic phase diagrams for the Tm 2 Fe 17 –H system have been constructed on the basis of measuring the temperature and field dependences of magnetization performed for different crystallographic directions of single-crystalline samples of Tm 2 Fe 17 and its hydrides Tm 2 Fe 17 H x ( x = 1, 2, 3, 4). It has been found that the hydrogen atoms, located in the interstices of the crystal lattice of this compound, have a significant effect on both its Curie temperature and the type of magnetic anisotropy.

Crystal structure and magnetic properties were studied on a single crystal of HoFe6Al6H and compared with those of the parent HoFe6Al6 compound with a tetragonal crystal structure of the ThMn12 type. Hydrogenation leads to a 1% volume expansion. HoFe6Al6 is a ferrimagnet with exact compensation of the Ho and Fe sublattices magnetizations at low temperatures. Both the hydride and the parent compound display a high magnetic anisotropy of the easy-plane type, a noticeable anisotropy exists also within the easy plane with the [110] axis as the easy magnetization direction. The hydrogenation increases slightly (from 10 to 10.45 μB) the magnetic moment of the Fe sublattice as a result of volume expansion. It leads to a decompensation of the Fe and Ho sublattices and HoFe6Al6H has a spontaneous moment 0.45 μB/f.u. The enhancement of the Fe–Fe intra-sublattice exchange interaction results in a higher Curie temperature (TC) value, 350 K in the hydride as compared to 315 K of HoFe6Al6. The Ho–Fe inter-sublattice interaction is also enhanced in the hydride. The molecular field Hmol created on Ho ions by Fe sublattice is 38 T in HoFe6Al6 and 48 T in HoFe6Al6H. The inter-sublattice exchange constant nHoFe is 3.8 T/μB and 4.6 T/μB, respectively. High-field measurements confirm the enhancement of the Ho–Fe exchange interaction in the hydride found from the temperature dependence of magnetization.

The magnetic anisotropy (MA) of trihydrides of single-crystalline rare-earth ( R ) intermetallics R 2 Fe 17 H 3 ± x ( R = Tb and Dy) with small deviations in the hydrogen concentration ( x = 0 – 0.4) was investigated via the magnetic and magnetoelastic measurements in a wide temperature region. The easy-axis MA of Tb 2 Fe 17 H 3 ± x is preserved in the entire range of magnetically ordered state while an “easy-cone – easy-plane” transition takes place in Dy 2 Fe 17 H 3 ± x above ambient temperatures. The trihydrides demonstrate a significant decrease of magnetic anisotropy as the temperature increases. The temperature variation of thermal expansion of Dy 2 Fe 17 H 3 displays anomalies in the vicinity of a spin-reorientation transition. The longitudinal magnetostriction λ | | ( T ) of the same compound changes its sign at 340 K. The compounds studied can find specific applications in practice.