Intersublattice Exchange Interaction Research Articles

Spin reorientation in GdMn2(Ge1-xSix)2 compounds

Structure and magnetic properties of layered GdMn2(Ge1-xSix)2 (0 ≤ x ≤ 1) compounds were studied. All the compounds crystallize in the tetragonal ThCr2Si2-type structure. It was shown by magnetization measurements at low temperature on quasi-single crystals that, with increasing Si concentration, the easy magnetization direction reorients from the c-axis to the basal plane. The spin reorientation occurs via an angular phase. A model of three magnetic sublattices coupled by negative intersublattice exchange interactions was used to describe the field dependences of the magnetization. For GdMn2Ge2 and GdMn2(Ge0.9Si0.1)2 in the fields applied along the c-axis, seven different magnetic structures were predicted, including two angular structures considered for the first time. The model explains formation of angular magnetic structures in zero field in GdMn2(Ge1-xSix)2 system by taking into account magnetic anisotropy of Mn sublattices with a positive anisotropy constant K1 and negative K2.

Role of intersublattice exchange interaction in ultrafast longitudinal and transverse magnetization dynamics in permalloy

We report on element-specific measurements of ultrafast demagnetization and magnetization precession damping in permalloy thin films. Magnetization dynamics induced by an optical pump at 1.5 eV is probed simultaneously at the ${M}_{2,3}$ edges of Ni and Fe with high-order harmonics for moderate demagnetization rates (less than 50%). The role of the intersublattice exchange interaction in both longitudinal and transverse dynamics is analyzed with a Landau-Lifshitz-Bloch description of ferromagnetically coupled Fe and Ni sublattices. It is shown that the intersublattice exchange interaction governs the dissipation during demagnetization as well as precession damping of the magnetization vector.

Influence of substitutions and hydrogenation on the structural and magnetic properties of (R’R’’)2Fe17 (R’, R’’ = Sm, Er, Ho): Compositions with promising fundamental characteristics

The structural and magnetic properties of the (R,R’)2Fe17-type (R and R’ are heavy (Ho, Er) and light (Sm) rare earth metals, respectively) compounds and the (R,R’)2Fe17Hy hydrides are reported. The hydrides with a high hydrogen concentration (y ≥ 4) are obtained by direct hydrogen absorption by the intermetallics. The rhombohedral Th2Zn17-type of structure is inherent in the Sm containing parent compounds and hydrides. The unit cell increase and increased Curie temperature are characteristic of the hydrides. Magnetic properties of the samples are investigated in pulsed magnetic fields up to 60 T. Strong magnetic fields induce phase transitions in the compounds with a high content of heavy rare earth elements. The parameter of the intersublattice exchange interaction is estimated.

Perspective on synthesis, structure, and magnetic properties of R–Fe–H hydrides

The structural and magnetic properties of the multicomponent R–Fe–H compounds with a high content of Fe and H are reported. The process of synthesis of the hydrides (R,R′)2Fe14BH5.5 [where R and R′ are light (Nd) and heavy (Dy, Ho, Er, Tm) rare earth metals, respectively] with a maximum hydrogen content is described in detail. The paper also provides insights into the synthesis of single-crystalline hydrides using the example of the R2(Fe,Co)14BH3 series. The hydrides (R,Nd)2Fe14BH5.5, R2(Fe,Co)14BH3, R2(Fe,Al)17H3 have a significantly increased volume as compared to the parent materials. High-field magnetization results of both parent and hydrogenated compounds at low temperatures are presented. Spin–reorientation phase transitions induced by an external magnetic field are observed. The parameter of the intersublattice exchange interaction and the influence of hydrogen on it are estimated within the framework of the mean field theory. The magnetocaloric effect of the compounds with a magnetic compensation point is studied with a special emphasis placed on the change of the sign of the effect.

Study of the effect of nitrogen and hydrogen on the structure and magnetic properties of (Sm, Er)2Fe17 alloys

Materials based on the Sm2Fe17 compound with nitrogen have great potential for the manufacture of highly efficient permanent magnets. The initial, nitrided, and hydrogenated alloys based on the Sm2Fe17 intermetallic compound with partial substitution of erbium atoms for samarium atoms Sm1.2Er0.8Fe17(H, N)x have been studied by X-ray phase analysis and scanning electron microscopy. Nanopowders of Sm1.2Er0.8Fe17N2 were obtained by mechanical grinding. The grinding time was varied from 0 to 60 minutes. The magnetic hysteresis properties of all powder samples were studied in static magnetic fields up to 7 T, as well as in pulsed magnetic fields up to 60 T. The strength of the intersublattice exchange interaction was estimated. The obtained values λ are valid only for the explored concentration of Sm/Er ions.

Drastic reduction of the R -Fe exchange in interstitially modified (Nd,Ho)2Fe14B compounds probed by megagauss magnetic fields

In this paper, the full magnetization process demonstrated by the series of ferrimagnetic intermetallic compounds ${(\mathrm{Nd},\mathrm{Ho})}_{2}{\mathrm{Fe}}_{14}\mathrm{B}$ and ${\mathrm{Ho}}_{2}{\mathrm{Fe}}_{14}\mathrm{B}$ and their hydrides with the maximum possible hydrogen content (for the given crystal structure type) is studied theoretically and experimentally using megagauss magnetic fields. We observe field-induced phase transitions from the initial ferrimagnetic to the forced-ferromagnetic state in magnetic fields up to 130 T and describe the magnetization process analytically. We find a drastic decrease of the critical transition fields in the hydrogenated compounds. This is due to extremely strong, nearly twofold reduction of the $R$-Fe intersublattice exchange interaction because of the combined substitution and hydrogenation effects. A comparative analysis of the magnetization behavior for the system ${\mathrm{Ho}}_{2}{\mathrm{Fe}}_{17}\text{\ensuremath{-}}\mathrm{H}$ is also performed.

Ab-initio prediction of high TC half-metallic ferrimagnetism in Li-based Heusler compounds Mn2LiZ (Z = Si, Ge and Sn)

Using density functional theory, we have investigated the structural, electronic, magnetic and thermoelectric properties of the new full-Heusler compounds Mn2LiZ (Z = Si, Ge and Sn). We showed that the three studied compounds favour the inverse Heusler structure with a ferrimagnetic ground-state. In the basis of the formation energy calculation the compounds are thermodynamically stable. The magneto-electronic investigation shows for all compounds a half-metallic ferrimagnetism with a fairly large gap and a low net spin magnetic moment of only 1 μB, obeying to the Slater-Pauling rule. Strong inter-sublattice exchange interactions are found and a mean field approximation reveals a high Curie temperature TC which exceeds 1000 K. Thermoelectric and electronic transports were determined within Boltzmann’s transport theory and their analysis results are discussed in detail.

Thresholding of the Elliott-Yafet spin-flip scattering in multi-sublattice magnets by the respective exchange energies

How different microscopic mechanisms of ultrafast spin dynamics coexist and interplay is not only relevant for the development of spintronics but also for the thorough description of physical systems out-of-equilibrium. In pure crystalline ferromagnets, one of the main microscopic mechanism of spin relaxation is the electron-phonon (el-ph) driven spin-flip, or Elliott-Yafet, scattering. Unexpectedly, recent experiments with ferro- and ferrimagnetic alloys have shown different dynamics for the different sublattices. These distinct sublattice dynamics are contradictory to the Elliott-Yafet scenario. In order to rationalize this discrepancy, it has been proposed that the intra- and intersublattice exchange interaction energies must be considered in the microscopic demagnetization mechanism, too. Here, using a temperature-dependent x-ray emission spectroscopy (XES) method, we address experimentally the element specific el-ph angular momentum transfer rates, responsible for the spin-flips in the respective (sub)lattices of Fe_{20}Ni_{80}, Fe_{50}Ni_{50} and pure nickel single crystals. We establish how the deduced rate evolution with the temperature is linked to the exchange coupling constants reported for different alloy stoichiometries and how sublattice exchange energies threshold the related el-ph spin-flip channels. Thus, these results evidence that the Elliott-Yafet spin-flip scattering, thresholded by sublattice exchange energies, is the relevant microscopic process to describe sublattice dynamics in alloys and elemental magnetic systems.

Behavior of magnetization in high magnetic fields of (Sm,Ho)2Fe17N2.4 powders obtained by mechanical grinding

Fundamental magnetic properties of the R2Fe17 compounds are highly sensitive to the atomic substitutions and interstitial absorption of light elements (nitrogen or hydrogen). In our work, both were combined, and the influence of the substitutions in the R-sublattice and nitrogen absorption on the magnetization behavior of Sm2Fe17 compound in magnetic fields up to 58 T was studied. (Sm,Ho)2Fe17N2.4 nitride was prepared. Magnetisation measurements were carried out on powder samples. Nanopowders of (Sm,Ho)2Fe17N2.4 were obtained by mechanical grinding. The grinding time was varied from 0 to 60 minutes. The strength of the inter-sublattice coupling in all samples is estimated by analyzing high-field magnetization data. In our work is shown that nitrogenation weakens the inter-sublattice exchange interaction in the compounds up to 45 - 50 %. Grinding time and, as a consequence, particle size affects the inter-sublattice exchange interaction. To achieve a ferromagnetic state, fields are needed well in excess of 58 T.

Synthesis, structure and magnetic properties of Sm1.2Ho0.8Fe17Нx (x = 0; 4.4)

Intermetallic compounds based on rare-earth metals and iron are by far the most promising materials for permanent magnets. In this work, the multicomponent compound Sm1.2Ho0.8Fe17 was prepared by induction melting. The hydride Sm1.2Ho0.8Fe17H4.4 with a high hydrogen content was obtained by direct hydrogenation of the intermetallic compound. The rhombohedral Th2Zn17-type of structure (space group R3¯m) is inherent to parent compound and hydride as well. The effect of hydrogenation on the magnetic properties of Sm1.2Ho0.8Fe17 was investigated. Curie temperature of the hydride Sm1.2Ho0.8-Fe17H4.4 is higher than that of parent compound by ΔTC = 138 K. The hydrogen embedded in Sm1.2Но0.8Fe17 crystal lattice increases the saturation magnetization (σS) at T = 300 K, but does not significantly affect σS at Т = 4.2 K. The ferrimagnetic structure is retained in magnetic fields up to 58 T in the parent compound, while, in the hydride, there is a spin-reorientation phase transition observed at 55 T. It is found that the parameter of the intersublattice exchange interaction decreases significantly in the hydride Sm1.2Ho0.8Fe17H4.4 (and in the nitride Sm1.2Ho0.8Fe17N2.4) which is associated with boosting of the unit-cell volume and distances between magnetic ions.

Strengthening the Intersublattice Exchange Interaction in R(Fe,Ti)$${}_{12}$$N (R = Ho and Er) Nitrides

The field dependences of the magnetization of nitrogen-containing compounds R(Fe,Ti) $${}_{12}$$ N (R = Ho and Er) are analyzed. It was found that the initial HoFe $${}_{11}$$ Ti sample at $$T=4.2$$ K exhibits the eaxy-axis anisotropy; however, as a result of nitriding, the anisotropy type changes to the easy cone of magnetization. As in the original samples of HoFe $${}_{11}$$ Ti and ErFe $${}_{11}$$ Ti, in HoFe $${}_{11}$$ TiN and ErFe $${}_{11}$$ TiN nitrides the cascade of spin-reorientation phase transitions is observed which is induced by external magnetic field. In the framework of the molecular field theory, the effect of nitriding on the interlattice exchange interactions is estimated. It is established that the molecular field that acts from the iron sublattice on rare-earth ions (holmium and erbium) is enhanced. The effect of nitriding and hydrogenation on the intersublattice exchange is compared.

Anisotropic exchange in GdGa

In this work we discuss the non-collinear ground-state reported for GdGa on the basis of a model Hamiltonian considering anisotropic exchange interactions. We show that a competition between intra and inter-sublattice exchange interactions can lead to a canted structure, which competes with a collinear (ferro or antiferromagnetic) order. The mean-field thermodynamic analysis of the model for S=7/2 shows a good agreement between calculated and reported experimental data of the canting angle and isothermal entropy change for GdGa.

Tailoring the ferrimagnetic-to-ferromagnetic transition field by interstitial and substitutional atoms in the R–Fe compounds

Fundamental characteristics of rare-earth (R) – iron intermetallics R2Fe14B are highly sensitive to the atomic substitutions and interstitial absorption of light elements. We studied a combined influence of the substitutions in the rare-earth sublattice and hydrogen absorption on the magnetization behavior in magnetic fields up to 60 T Er2Fe14B and Tm2Fe14B ferrimagnets chosen for the study showed that the substitution of Nd for Er or Tm increases the saturation magnetization as a result of ferromagnetic ordering of Nd and Fe moments. Under sufficiently high magnetic fields the magnetic moments rotate and the field-induced ferromagnetic state may be observed. The field at which a transition occurs is related to the strength of the inter-sublattice exchange interaction. The role of hydrogen is primarily to weaken the inter-sublattice ferrimagnetic coupling so that the reorientation becomes achievable at the available magnetic field strength (in hydrides Tm2Fe14BH5.5 and (Tm0.5Nd0.5)2Fe14BH5.5). We analyze the volume dependence of the R–Fe magnetic interaction in R2Fe14B and compare it with other R–Fe compounds.

Microscopic Nature of the First-Order Field-Induced Phase Transition in the Strongly Anisotropic Ferrimagnet HoFe_{5}Al_{7}.

We report on x-ray magnetic circular dichroism experiments in pulsed fields up to 30T to follow the rotations of individual magnetic moments through the field-induced phase transition in the ferrimagnet HoFe_{5}Al_{7}. Near the ground state, we observe simultaneous stepwise rotations of the Ho and Fe moments and explain them using a two-sublattice model for an anisotropic ferrimagnet with weak intersublattice exchange interactions. Near the compensation point, we find two phase transitions. The additional magnetization jump reflects the fact that the Ho moment is no longer rigid as the applied field acts against the intersublattice exchange field.

Probing the exchange coupling in the complex modified Ho-Fe-B compounds by high-field magnetization measurements

By examining the Ho2Fe14B case, we explored the influence of substitution and absorption atoms on the high-field behavior of magnetization of rare-earth (R)-Fe intermetallics. The value of the first critical field shows that the inter-sublattice exchange interactions remain practically unchanged when the substitution takes place in the R sublattice (replacement of up to 50 % of Ho by Nd). On the contrary, hydrogen absorption by Ho2Fe14B and Ho1Nd1Fe14B of the maximum possible hydrogen concentration 5.5 at./f.u. decreases the strength of the R-Fe exchange by 30%. Remarkably, the influence of hydrogenation is stronger in the compound modified by substitution.

Tuning magnetization, blocking temperature, cation distribution of nanosized Co0.2Zn0.8Fe2O4 by mechanical activation

The study on structural, microstructural, magnetic, and hyperfine properties of nanosized Co0.2Zn0.8Fe2O4 having particle size ∼18 nm (CZM) synthesized by high energy ball milling of Co0.2Zn0.8Fe2O4 nanoparticles of size ∼20 nm (CZ) produced by flow rate controlled coprecipitation method has revealed that the inclusion of strain induced anisotropy produced by mechanical treatment and escalation of oxygen mediated intersublattice exchange interaction of spinel ferrites by tuning cation distribution properly, can improve the magnetic quality of nanosized ferrites significantly. This upshot will be of immense help in promoting the technological application of nanostructured ferrites. The Rietveld refinement of powder x-ray diffraction pattern and the analysis of transmission electron micrographs, energy dispersive x-ray spectrum, and FTIR spectrum of the sample have confirmed that CZM is single phase cubic nanometric spinel ferrite of Fd3¯m symmetry and it possesses large microstrain within its crystal lattice. The dc magnetic and Mössbauer spectroscopic studies together with indicate that the particles in the sample are composed of ferrimagnetically aligned core and spin-glass like shell and the system behaves superparamagnetically at 300 K. The saturation magnetization (44 and 87 emu g−1 at 300 and 10 K) and hyperfine field of the sample are substantially higher than its counterparts reported earlier. In spite of its lower size compared to CZ, the blocking temperature (∼220 K) of CZM is higher than that of CZ (70 K) and also that of its counterparts synthesized by chemical methods. The strengthening of the intersublattice A-O-B superexchange interaction because of migration of Fe3+ ions from octahedral [B] to tetrahedral (A) sites in lieu of the relocation of Zn2+ among (A) and [B] sites helps in enhancement of magnetization and hyperfine field of CZM. The giant coercivity (HC ∼ 5600 Oe at 10 K) of CZM is accounted by the presence of spin glass like surface layer in the sample. Moreover, the system exhibits striking memory effect which can be suitably utilized in storing binary bits (0, 1) through magnetic field change in the cooling cycle of magnetization versus temperature profile, and the stored binary coded number can be faithfully retrieved in the heating cycle.

First-principles calculations of exchange interactions, spin waves, and temperature dependence of magnetization in inverse-Heusler-based spin gapless semiconductors

Employing first-principles electronic-structure calculations in conjunction with the frozen-magnon method, we calculate exchange interactions, spin-wave dispersion, and spin-wave stiffness constants in inverse-Heusler-based spin gapless semiconductor (SGS) compounds ${\text{Mn}}_{2}\text{CoAl}, {\text{Ti}}_{2}\text{MnAl}, {\text{Cr}}_{2}\text{ZnSi}, {\text{Ti}}_{2}\text{CoSi}$, and ${\text{Ti}}_{2}\text{VAs}$. We find that their magnetic behavior is similar to the half-metallic ferromagnetic full-Heusler alloys, i.e., the intersublattice exchange interactions play an essential role in the formation of the magnetic ground state and in determining the Curie temperature ${T}_{\mathrm{c}}$. All compounds, except ${\text{Ti}}_{2}\text{CoSi}$, possess a ferrimagnetic ground state. Due to the finite energy gap in one spin channel, the exchange interactions decay sharply with the distance, and hence magnetism of these SGSs can be described considering only nearest- and next-nearest-neighbor exchange interactions. The calculated spin-wave dispersion curves are typical for ferrimagnets and ferromagnets. The spin-wave stiffness constants turn out to be larger than those of the elementary $3d$ ferromagnets. Calculated exchange parameters are used as input to determine the temperature dependence of the magnetization and ${T}_{\mathrm{c}}$ of the SGSs. We find that the ${T}_{\mathrm{c}}$ of all compounds is much above the room temperature. The calculated magnetization curve for ${\text{Mn}}_{2}\text{CoAl}$ as well as the Curie temperature are in very good agreement with available experimental data. This study is expected to pave the way for a deeper understanding of the magnetic properties of the inverse-Heusler-based SGSs and enhance the interest in these materials for application in spintronic and magnetoelectronic devices.

High magnetic field study of the Dy2Fe17Hx compounds with x = 0–3.8

The Dy2Fe17Hx compounds crystallizing in a hexagonal Th2Ni17-like structure are studied on aligned powder by magnetization measurements carried out in steady (up to 5T) and pulsed (60T) magnetic fields at 4.2–300K. Dy2Fe17 is a ferrimagnet with TC=375K and a spontaneous moment of 16.5μB/f.u. Magnetization curves recorded for two different crystal orientations for Dy2Fe17Hx polycrystalline oriented samples show that the insertion of hydrogen in the Dy2Fe17 crystal lattice significantly modifies the magnetization, Curie temperature and intersublattice exchange interactions of the Dy2Fe17 phase. The high-field behavior of Dy2Fe17 is compared with that of its hydrides derivatives. Despite different types of magnetic anisotropy, the high-field behavior of the hydrides and the parent compound is qualitatively similar. Depending upon the composition, they can exhibit easy plane or else conical type anisotropy as for x=0 and 3, respectively. The low temperature spontaneous magnetization exhibits a moderate composition dependence, it first decreases continuously upon increasing the hydrogen composition up to x=3 then slightly increases for x=3.8 reflecting the Fe sublattice evolution. A mean Fe moment of about 2.1μB is derived for Dy2Fe17H3.8 composition, this magnetization value is close to that of the original Dy2Fe17 compound. It is found that the intersublattice coupling between the Fe and Dy sublattices is reduced as illustrated by the decrease of the nDy–T coefficient from about 3.3 down to 2.82T f.u./μB for Dy2Fe17 and Dy2Fe17H3.8, respectively.

Magnetic anisotropy and magnetic phase transitions in RFe5Al7

RFe5Al7 (R – Gd, Tb, Dy, Ho, Er and Tm) single crystals have been studied by measurements of magnetization, sound propagation (in static and pulsed magnetic fields up to 60T) and specific heat. Fundamental magnetic properties have been determined and compared for all these materials. RFe5Al7 are highly anisotropic ferrimagnets. Spontaneous and field-induced magnetic phase transitions of anisotropic and exchange nature have been observed in RFe5Al7. Strong magnetoelastic interactions are manifested by pronounced acoustic anomalies at the phase transformations. The detected magnetization jumps provide important information on the R–Fe inter-sublattice exchange interactions.

Phase transitions of anisotropic and exchange origins inTmFe5Al7

Magnetization and sound propagation reveal a number of unusual spontaneous and field-induced transformations in ferrimagnetic ${\mathrm{TmFe}}_{5}{\mathrm{Al}}_{7}$ (${T}_{\mathrm{C}}$ = 193 K). The rare-earth sublattice was found to provide a uniaxial magnetic anisotropy, whereas the iron sublattice favors an easy-plane anisotropy. A competition between them results in a first-order spin-reorientation transition at 64 K as the magnetic moments rotate from the $c$ axis to the basal plane of a tetragonal structure. The transition is preceded by a first-order magnetization process of type II along the hard axis. Remarkably, the intersublattice Tm-Fe exchange interaction is weakened at the spin-reorientation transition. Concomitantly, the spontaneous magnetic moment disappears, and the ferrimagnetic state changes to antiferromagnetic. With increasing temperature, the strength of the Tm-Fe exchange is recovered, and the ferrimagnetism is restored at 82 K through another first-order phase transformation. Below 40 K, a first-order field-induced transition occurs for a magnetic field applied along the easy [001] axis. It reflects a rotation of the magnetic moments towards the forced ferromagnetic state observed above 30 T. Along the hard [100] axis the ferromagnetic saturation is not reached even at 60 T.