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.

Alloys of rare earths (R) and Fe (with a high Fe:R ratio) have been a subject of scientific interest for decades due to their unique physical properties.1–4 Among the compounds with important practical applications are the binary R2Fe17 and pseudobinary compositions R2Fe14B crystallizing in the hexagonal (with heavy rare earths, HR) and tetragonal crystal structures, respectively.5,6 These are mainly used as materials for permanent magnets operating in various environments, including the aggressive ones.7,8 Recently, other high-Fe-content rare-earth-based compounds have also found applications as a working body in the magnetic refrigerator.9,10

It is known3 that the R–Fe compounds with light rare earths (LR) are ferromagnets (R and Fe magnetic moments align collinear), while the compounds with heavy rare earths (HR) are ferrimagnets (R and Fe moments align antiparallel). Sufficiently high magnetic fields induce spin–reorientation transitions in the latter samples, which, when measured near absolute zero and analyzed, allow us to obtain reliable information on the inter-sublattice exchange interactions.11–20 

Magnetic characteristics of the R–Fe compounds can be influenced by several means. Combining interstitial and substitutional atoms is an efficient way to control the magnetization, temperature of magnetic phase transition, magnetocrystalline anisotropy (MCA), etc.21–22 We have been able to find new compositions of the R2Fe14B- and R2Fe17-type with light interstitial elements having novel unique magnetic properties. Hydrogen introduced into the crystal lattice of these intermetallics significantly boosts the volume (and hence, increases distances between the magnetoactive atoms) and changes in the electronic structure of the materials.23–26 

R–Fe readily absorbs hydrogen (often at room temperature and pressure 105–106 Pa). The amount of hydrogen absorbed depends on the composition. The maximum amount of hydrogen in R2Fe17 and R2Fe14B can reach 5.5 hydrogen atoms per formula unit (at. H/f.u.).27–30 Although the hydrogenation effects on the magnetic properties of R–Fe compounds have been studied quite thoroughly, several important issues should still be clarified.

First, observation of the full magnetization processes in ferrimagnetic R–Fe–H hydrides requires the use of high magnetic fields, often above 50 T. Second, the stability of hydrides with time should be addressed. Hydrides need to be properly stored and their properties periodically checked. Recently, we were not able to reproduce the structural and magnetic properties of Tm2Fe17H5.5 hydride with the highest hydrogen concentration after a year of storage because hydrogen partially escaped the sample.31 The effect of hydrogen on the magneto-thermal properties of the Fe-rich intermetallic compounds (R2Fe17, R2Fe14B) is another important and poorly studied issue.

The aim of this paper is to investigate the structure and high magnetic-field behavior of the magnetization of the multicomponent R–Fe–H compounds with a high Fe content. The use of magnetic fields up to 58 T at low temperatures allows us to observe the rotations of the magnetic moments of the individual sublattices and to investigate the spin–reorientation phase transitions and, thus, determine the parameters of the inter-sublattice exchange interactions. Special attention is paid to the phenomenon of magnetic compensation in the Dy2(Fe,Al)17 compounds and their hydrides and to their magnetocaloric effect.

To obtain the parent compositions, the arc melting method was used for (Nd0.5R0.5)2Fe14B (R = Dy, Ho, Er, Tm) while the induction melting method was employed for Er2(Fe,Co)14B and Dy2(Fe,Al)17; for details of the synthesis of initial alloys, see Refs. 32–34. Some samples were obtained as single crystal. Backscattered Laue patterns were used to confirm the single-crystalline state of the samples and to orient them for magnetic measurements. The phase composition and lattice parameters were determined by powder x-ray diffraction (XRD). The elemental composition was assessed by energy-dispersive x-ray spectroscopy with a simultaneous study of the microstructure. The compositions were found to depart from the reported ones by no more than 3%.

Synthesis of the hydrides of (Nd0.5R0.5)2Fe14B (R = Dy, Ho, Er, Tm) with a maximum hydrogen concentration of 5.5 hydrogen atoms per formula unit was carried out using special equipment, the schematic diagram of which is shown in Fig. 1.

FIG. 1.

Principal scheme of the hydrogenation equipment. 1—Reactor with a sample, 2 and 8—induction furnace, 3 and 9—temperature control system, 4—vacuum gauge, 5—computer, 6, 10, and 11—valve, 7—reactor with LaNi5H6, and 12—vacuum pump.

FIG. 1.

Principal scheme of the hydrogenation equipment. 1—Reactor with a sample, 2 and 8—induction furnace, 3 and 9—temperature control system, 4—vacuum gauge, 5—computer, 6, 10, and 11—valve, 7—reactor with LaNi5H6, and 12—vacuum pump.

Close modal

The hydrogenation process was performed in several stages, and the main parameters of the synthesis are listed in Table I.

TABLE I.

Hydrogenation process stages.

Sample and system preparationActivation of the sampleHydrogenationCooling
Sample grinding
System evacuation to high vacuum (p = 0.1 mPa) at room temperature 
dT = 2 K/min to temperature 623 K
stable pressure p = 2 mPa
dT = −1 oC/min
to temperature 573 K.
24 h at a temperature of 573 K.
Final pressure p = 0.35 mPa 
72 h
Hydrogen pressure over 8 MPa
Temperature 573 K 
dT = −0.5 K/min
to room temperature 
Sample and system preparationActivation of the sampleHydrogenationCooling
Sample grinding
System evacuation to high vacuum (p = 0.1 mPa) at room temperature 
dT = 2 K/min to temperature 623 K
stable pressure p = 2 mPa
dT = −1 oC/min
to temperature 573 K.
24 h at a temperature of 573 K.
Final pressure p = 0.35 mPa 
72 h
Hydrogen pressure over 8 MPa
Temperature 573 K 
dT = −0.5 K/min
to room temperature 

When polycrystalline samples were hydrogenated, the homogenized alloy was ground in an agate mortar in acetone to avoid powder oxidation during grinding. The fine powder was placed in an aluminum oxide crucible. The crucible with the powder was placed in the stainless steel reactor 1 (Fig. 1). Then the system was pumped out to high vacuum using the vacuum pump 12. The pumping process was carried out at room temperature and took in some cases up to 72 h. During the pumping, the system was leak-checked.

The next step—sample activation—was necessary for degassing the sample. The sample was activated in a dynamic vacuum, with indirect heating of reactor 1 with the sample in induction furnace 2. Temperature controller 3 controlled the temperature. The following activation procedure was experimentally selected for this series of samples: the reactor was heated up at a rate of 2 degrees per minute to a temperature of 623 K. Then, the temperature of 623 K was held until the pressure in the system stabilized. After stabilizing the pressure, the system was cooled at a rate of 1 degree per minute to a temperature of 573 K. The system was kept at a temperature of 573 K for about 24 h. The final pressure in the system did not exceed 0.4 mPa. After that, the system was isolated from the pumping system, and high-purity hydrogen (impurity content 10−3–10−4wt. %) was introduced into the system. Hydrogen was obtained by decomposing hydride LaNi5 by indirect heating of reactor 7 by induction furnace 8.

The sample was held for at least 72 h at a temperature of 573 K and a pressure of not less than 8 MPa.35 Then, the system was cooled to room temperature at a cooling rate of 0.5 K/min. After that, the sample was removed from the system and sent for XRD analysis. Data collection and analysis were performed in real-time using computer 5 and software written on National Instruments LabView.

Based on the collected data, using the van der Waals equation, the hydrogen content of the hydrogenated sample was calculated. To confirm the estimated hydrogen concentration, a part of the sample was used for desorption. Desorption is a reverse hydrogenation procedure. Based on the amount of hydrogen released during desorption, the quantitative composition of hydrogen in the hydride was adjusted. The error in determining the hydrogen concentration was 0.05 at. H/f.u. Hydrides of the composition (Nd0.5R0.5)2Fe14BH5.5 were obtained.

Dy2(Fe,Al)17 materials were hydrogenated in a similar fashion to those described above. Although compounds of the R2Fe17 and R2Fe14B types can absorb up to 5–5.5 at. H/f.u., we only managed to obtain stable hydrides of Dy2(Fe,Al)17 with the hydrogen content not exceeding 3.2 at. H/f.u. The concentration of hydrogen was calculated based on the desorption procedure first performed within the first 24 h after the hydrogenation process and then after 1 year.

The process of preparation and hydrogenation of the single crystal is different from that of polycrystals as they are not ground into fine powder but were hydrogenated as they are. Er2(Fe,Co)14B single crystals were hydrogenated as follows. The pieces of crystal were placed in the crucible, which is set in reactor 1 in Fig. 1. Then the system was pumped out to high vacuum using a vacuum pump 12. The pumping process was carried out at room temperature and can take up to 72 h. During the pumping process, the system was checked for leaking.

The activation procedure follows the same scenario as described above, with the only difference that the rate of temperature change is always half a degree per minute so that the temperature change occurs more slowly and should not lead to the destruction of the single crystal. After the sample was activated, the system is cooled to room temperature at the same rate of half a degree per minute. The hydration process occurs at room temperature and low hydrogen pressure (p = 3 kPa). The hydrogen supply occurs in small portions to avoid an uncontrolled reaction and destruction of the single crystal. For homogenization, the sample was kept in a hydrogen environment for at least 72 h. The amount of absorbed hydrogen was calculated based on the pressure drop of hydrogen in the reaction chamber using the van der Waals equation. We have been able to successfully obtain Er2(Fe,Co)14BH3 hydrides without destroying the single-crystalline state.

High-field magnetization measurements were performed using high, nondestructive magnetic-field pulses up to 58 T at 4.2 K at the Dresden High Magnetic Field Laboratory.36 The magnetization was measured using a compensated pair of coils.20 During the nondestructive pulses, the field change up to 58 T within the rise time of 7 ms leads to a field sweep rate of about 0.1 ms/T. The experimental results obtained using the nondestructive pulses were normalized to steady-field magnetization data up to 14 T obtained using a commercial PPMS 14 (Quantum Design, USA). The measurements were carried out on free powder samples, except for some samples for which the single crystals were available.

The extraction method was used for direct measurements of adiabatic temperature change ΔT in magnetic fields up to 14 T in 4.2–300 K temperature range.37 This method was implemented using a bitter coil magnet and an experimental setup in the Institute of Low Temperatures and Structure Research (Wroclaw, Poland). The magnetocaloric effect was also studied by an indirect method, when the magnetic entropy change ΔSm was calculated based on data of the temperature dependencies of the magnetization, studied in fields up to 5 T in a temperature range of 60–115 K, with a temperature step of 5 K. The isothermal entropy change is defined as the ratio of the area between two magnetic isotherms at close temperatures T1 and T2, to the difference between these temperatures. To estimate ΔSm upon changing the external field from zero to H, the following expression can be derived allowing for the Maxwell relation:

ΔSm=1T1T2[0HM(H,T2)dH0HM(H,T1)dH].
(1)

The magnetocaloric effect was studied on polycrystalline samples.

X-ray diffraction patterns at room temperature showed that all parent compounds (Nd0.5R0.5)2Fe14B (R = Dy, Ho, Er, Tm), Er2(Fe,Co)14B, Dy2(Fe,Al)17) are single phase. The lattice parameters a and c, relation c/a, and unit cell volume V are given in Table II.

TABLE II.

Structural parameters for (Nd,R)2Fe14BHy, and R2(Fe,Co)14BHy compounds (tetragonal crystal structure, space group P42/mnm) and for Dy2(Fe,Al)17Hy compounds (rhombohedral crystal structure of the Th2Zn17 type).

Compoundsa (nm)c (nm)c/aV (nm3)ΔV/V0 (%)
(Nd0.5Dy0.5)2Fe140.8770 1.2120 1.38 0.932 … 
(Nd0.5Dy0.5)2Fe14BH5.5 0.8884 1.2278 1.38 0.969 4.0 
(Nd0.5Ho0.5)2Fe140.8776 1.2098 1.38 0.932 … 
(Nd0.5Ho0.5)2Fe14BH5.5 0.8906 1.2227 1.37 0.970 4.1 
(Nd0.5Er0.5)2Fe140.8771 1.2061 1.38 0.928 … 
(Nd0.5Er0.5)2Fe14BH5.5 0.8881 1.2215 1.38 0.963 3.8 
(Nd0.5Tm0.5)2Fe140.8851 1.2055 1.38 0.926 … 
(Nd0.5Tm0.5)2Fe14BH5.5 0.8885 1.2209 1.37 0.964 4.1 
Er2Fe13Co10.8728 1.1939 1.37 0.909 … 
Er2Fe13Co1BH3 0.878 1.2024 1.37 0.927 1.9 
Er2Fe12Co20.8718 1.1928 1.37 0.907 … 
Er2Fe12Co2BH3 0.8766 1.2001 1.37 0.922 1.7 
Dy2Fe10Al7 0.8672 1.2569 1.45 2.456 … 
Dy2Fe10Al7H3.2 0.8753 1.2761 1.46 2.538 3.4 
Dy2Fe10.5Al6.5 0.8457 1.2517 1.48 2.326 … 
Dy2Fe10.5Al6.5H3.2 0.8564 1.2712 1.48 2.422 4.1 
Compoundsa (nm)c (nm)c/aV (nm3)ΔV/V0 (%)
(Nd0.5Dy0.5)2Fe140.8770 1.2120 1.38 0.932 … 
(Nd0.5Dy0.5)2Fe14BH5.5 0.8884 1.2278 1.38 0.969 4.0 
(Nd0.5Ho0.5)2Fe140.8776 1.2098 1.38 0.932 … 
(Nd0.5Ho0.5)2Fe14BH5.5 0.8906 1.2227 1.37 0.970 4.1 
(Nd0.5Er0.5)2Fe140.8771 1.2061 1.38 0.928 … 
(Nd0.5Er0.5)2Fe14BH5.5 0.8881 1.2215 1.38 0.963 3.8 
(Nd0.5Tm0.5)2Fe140.8851 1.2055 1.38 0.926 … 
(Nd0.5Tm0.5)2Fe14BH5.5 0.8885 1.2209 1.37 0.964 4.1 
Er2Fe13Co10.8728 1.1939 1.37 0.909 … 
Er2Fe13Co1BH3 0.878 1.2024 1.37 0.927 1.9 
Er2Fe12Co20.8718 1.1928 1.37 0.907 … 
Er2Fe12Co2BH3 0.8766 1.2001 1.37 0.922 1.7 
Dy2Fe10Al7 0.8672 1.2569 1.45 2.456 … 
Dy2Fe10Al7H3.2 0.8753 1.2761 1.46 2.538 3.4 
Dy2Fe10.5Al6.5 0.8457 1.2517 1.48 2.326 … 
Dy2Fe10.5Al6.5H3.2 0.8564 1.2712 1.48 2.422 4.1 

We find that hydrogenation does not alter the crystal lattice type in the studied compounds and all hydrides are also single phase. The structural parameters of the hydrides are also given in Table II. The relative volume change ΔV/V upon hydrogen absorption in the R–Fe–H system can exceed 4%.

1. (Nd0.5R0.5)2Fe14B (R = Dy, Ho, Er, Tm) and their hydrides

Figure 2 shows the magnetization curves for the ferrimagnetic (Nd0.5R0.5)2Fe14B (R = Dy, Ho, Er, Tm) compounds and their hydrides with 5.5 hydrogen atoms per formula unit (the maximum possible H amount for this type of crystal structure) in magnetic fields up to 58 T at 4.2 K. It can be seen that for all parent compounds, (Nd0.5R0.5)2Fe14B, the magnetic field strength, is insufficient for observing the rotation of the individual sublattices moments. Hydrogenation completely changes this picture. All the compositions, except for the compound with Dy, exhibit magnetic phase transitions induced by an external magnetic field. It can be seen that when Er or Tm replaces Ho, the value of the first critical field HCR decreases. In (Nd0.5Tm0.5)2Fe14BH5.5 hydride, a transition from the ferri- to the ferro-magnetic state takes place in the available field range. Since thulium is the heavy rare earth with a Lande factor closest to unity, it takes the smallest applied magnetic field to reach the ferromagnetic state. Moreover, in the case of Tm3+ ion, the ground state belongs to a block of three states, which are allowed to mix. Due to this, the magnetization curve of (Nd0.5Tm0.5)2Fe14BH5.5 hydride has two jumps, each by about 7 μB (Fig. 2).28,38,39

FIG. 2.

Field dependencies of magnetization of (Nd0.5R0.5)2Fe14B (a) and (Nd0.5R0.5)2Fe14BH5.5 (b) in pulsed magnetic fields up to 58 T at 4.2 K.

FIG. 2.

Field dependencies of magnetization of (Nd0.5R0.5)2Fe14B (a) and (Nd0.5R0.5)2Fe14BH5.5 (b) in pulsed magnetic fields up to 58 T at 4.2 K.

Close modal

For the ferrimagnetic R–Fe compounds, the first critical field is proportional to the parameter of the inter-sublattice exchange interaction λ,

HCR=λ(MRMT),
(2)

where MR and MT — are magnetization of the rare-earth and Fe sublattices. So, using the procedure described in detail in Refs. 40 and 41, the parameters of the inter-sublattice exchange interaction (λ) were calculated, and the following feature was established: the decrease in the parameter λ is 30, 38, and 50% for hydrides with the maximum hydrogen content (Nd0.5Ho0.5)2Fe14BH5.5, (Nd0.5Er0.5)2Fe14BH5.5 and (Nd0.5Tm0.5)2Fe14BH5.5, respectively. For the hydride (Nd0.5Ho0.5)2Fe14BH5.5, we performed structural and high-field magnetic studies both immediately after the hydrogenation, and later, after one year. It was found that hydrides are stable over time; their magnetic properties are fully reproduced.

2. Er2(Fe,Co)14B and their hydrides

Field-induced transition in high magnetic fields leads to the destruction of the collinear ferrimagnetic structure and a canting between the magnetic moments of the R and 3d-sublattices. We observed this phenomenon both in the Er2(Fe, Co)14B systems and in their hydrides when measuring the magnetization in pulsed magnetic fields up to 48 T. Magnetization curves for Er2Fe12Co2B and Er2Fe12Co2BH3 single crystals for the field along and perpendicular to the tetragonal c axis at 4.2 K are shown in Fig. 3. The c axis is the hard magnetization axis. The magnetization for Hc quickly reaches the ferrimagnetic saturation, and then, slowly increases up to a critical field μ0HCR1 = 42 T, where a sharp jump is observed. We observed a magnetization jump in the fields up to 48 T in single crystals of Er2(Fe, Co)14B and their hydrides Er2(Fe, Co)14BH3.32 

FIG. 3.

Magnetization curves of Er2Fe12Co2B single crystal (a) and its hydride Er2Fe12Co2BH3 (b) at 4.2 K in pulsed magnetic up to 48 T applied along and perpendicular to axis c.

FIG. 3.

Magnetization curves of Er2Fe12Co2B single crystal (a) and its hydride Er2Fe12Co2BH3 (b) at 4.2 K in pulsed magnetic up to 48 T applied along and perpendicular to axis c.

Close modal

Er2(Fe, Co)14B exhibits easy-plane magnetocrystalline anisotropy (MCA) with an easy axis [100] at low temperatures. The M(H) curves show a jump-like transition for the field along the easy axis (Hc). When Fe is partly replaced by Co, the critical field decreases (μ0HCR = 41 T and μ0HCR = 40 T for Er2Fe13Co1B and Er2Fe12Co2B, respectively). Our experimental M(H) curves for the field along the hard axis (Hc) show no jumps up to 48 T, and are characterized by a rapid increase in fields μ0H < 5 Т and a slower monotonic increase in stronger fields. To determine the features of the spin reorientation in strong fields, we consider the following expression for the energy E of a two-sublattice ferrimagnet taking into account the magnetocrystalline anisotropy:

E=λMRMTcosαHMsinφ+K1sin2φ+K2Rsin4φ,
(3)

where MR and MT are magnetizations of the rare-earth and 3d-sublattice (Fe–Co); λ is the inter-sublattice exchange interaction parameter; α is the angle between μR and MT; φ is the angle between the net magnetization vector M and the tetragonal axis c (along which the magnetic field H is applied); K1 is the effective MCA constant; K2R is the second MCA constant of the R sublattice. According to Ref. 12, the effective anisotropy constant depends on the bevel angle of the sublattices

K1(α)=K1R2+K1T2+2K1RK1Tcos(2α),
(4)

where K1R and K1T are the first MCA constants of the R and 3d-sublattices, respectively. Our experimental data show that taking into account the second constant K2 is of fundamental importance. In the field dependence of the magnetization along the hard axis (Hc), three ranges can be distinguished: H<HCR1=λ|MTMR|, HCR1<H<HCR2, and H>HCR2. The collinear structure is violated at HCR1, and an angle α arises between the magnetic moments of the R and 3d-sublattices (α ≤ π) at H > HCR1.

We found a peculiar feature when studying the magnetization processes of the anisotropic ferrimagnets with easy-plane anisotropy. The magnetization for strong field applied along the hard direction exceeds the magnetization for field applied along the easy direction. This can be explained by the fact that in a field applied along the hard magnetization axis, the collinear ferrimagnetic structure is broken in a field less than HCR1 (the critical field at Hc). In the noncollinear phase, the net magnetization grows due to a continuous rotation of the magnetic moments of both sublattices.

For the easy magnetization direction, the critical field increases by μ0ΔH = 2 T for the hydrides Er2(Fe,Co)14BH3 in comparison with the initial compositions. The magnetization jump determined by the angle α between the directions of magnetization of the sublattices is practically independent of the concentration of cobalt and hydrogen.

The Er2Fe14B–H system has been investigated in detail.42–45 We found the following correlation: the critical field and inter-sublattice exchange parameter λ46,47 decreases with increasing Co concentration, while hydrogenation (up to 3 at. H/f.u.) of substituted compositions Er2(Fe, Co)14B leads to an increase in the inter-sublattice exchange parameter. As a result, λ retains its value.

3. Dy2(Fe,Al)17 and their hydrides

Figure 4 presents the magnetization curves of ferrimagnetic Dy2(Fe,Al)17 compounds and their hydrides at 4.2 K in magnetic fields up to 58 T. All compounds have nonzero spontaneous magnetization. Hydrogenation decreases the spontaneous magnetization from 3.8 to 2.8 μB/f.u., in case of the Dy2Fe10Al7H3.2 hydride. The magnetization increases strongly for all compounds when an external magnetic field is applied. Our experimental data for parent compounds are similar to Refs. 48–52. The increase of magnetization is due to the higher Dy magnetic moments, which are ordered antiparallel to the magnetic moments of the Fe sublattice: the antiparallel coupling between the Dy and Fe magnetic moments is broken by the magnetic fields of sufficient strength.

FIG. 4.

Field dependencies of magnetization of Dy2(Fe,Al)17 alloys [(a) and (c)] and their hydrides Dy2(Fe,Al)17H3.2 [(b) and (d)] in pulsed magnetic up to 58 T at 4.2 K.

FIG. 4.

Field dependencies of magnetization of Dy2(Fe,Al)17 alloys [(a) and (c)] and their hydrides Dy2(Fe,Al)17H3.2 [(b) and (d)] in pulsed magnetic up to 58 T at 4.2 K.

Close modal

Since we consider the magnetization at low temperatures, the magnetic moment of the Dy sublattice MR is close to the free-ion value, 10 μB/f.u. For the ferrimagnetic (antiparallel) arrangement of Dy and Fe moments, the 3d-sublattice moment MT equals 13.8 μB/f.u. (for the Y2Fe10Al7 compound).53–54 The magnetic field is a tool to convert the ferrimagnet with Mferri=MRMT to the field-induced ferromagnet with the saturation magnetization Mferro=MR+MT. In Fig. 4(a), the magnetization curve of the Dy2Fe10Al7 compound should saturate at 33.6 μB/f.u. in a field of about 110 T.

The resulting curve M(H) will contain a change in the slope (an increase of magnetization at a certain field, which is called critical HCR), by analyzing which the coupling strength between the sublattices can be estimated. Equation (2) was used for the system Dy2Fe17−xAlxHy (x = 6.5, 7; y = 0; 3.2). The smooth increase of the magnetization of Dy2Fe10Al7 is observed when the magnetic field grows above the critical field μ0HCR = 21.7 T. The critical field was determined by a peak of the second field derivative of the magnetization, |d2M/dH2|. Below HCR (in collinear ferrimagnetic phase) the net magnetization is MR—MT = 6.2 μB/f.u., which leads to the parameter of the inter-sublattice exchange interaction λ = 3.5 T/μB. Due to hydrogenation, this parameter decreases to 3.2 T/μB in Dy2Fe10Al7H3.2. Thus, hydrogenation decreases the inter-sublattice exchange interaction by about 7%.

For the Dy2Fe10.5Al6.5 compound, the spontaneous magnetization decreases to 1.8 μB/f.u. (in comparison to Dy2Fe10Al7). This decrease is due to an increase of MT to 15.6 μB/f.u. The Dy2Fe10.5Al6.5 compound is close to the compensated composition, where the magnetic moments of the rare earth and 3d-sublattices should be equal to each other. A decrease of λ by about 5% is obtained by analyzing the critical field of Dy2Fe10.5Al6.5 and its hydride. λ equals 3.2 T/μB for the parent compound and 3.0 T/μB for the Dy2Fe10.5Al6.5H3 hydride.

Thus, we were able to study three multicomponent systems: (Nd0.5R0.5)2Fe14B (R = Dy, Ho, Er, Tm), Er2(Fe,Co)14B, and Dy2(Fe,Al)17 with different effects of hydrogenation on the inter-sublattice exchange interactions.

Temperature dependencies of the magnetization in the field of 1 T for Dy2Fe10Al7 and its hydride are presented in Fig. 5. A sharp decrease in the magnetization is observed at a temperature of magnetic ordering, TC. The minimum of the temperature dependencies of the magnetization is near the magnetic compensation point, Tcomp. In hydride Tcomp = 85 K, it is lower by 40 K than in the parent compound (Tcomp = 135 K). The magnetization of Dy and Fe sublattices becomes equal at Tcomp.

FIG. 5.

Temperature dependencies of magnetization of Dy2Fe10Al7 (a) alloy and Dy2Fe10Al7H3.2 (b) hydride in the field of 1 T. Insertion: temperature dependence of MCE in field of 5 T.

FIG. 5.

Temperature dependencies of magnetization of Dy2Fe10Al7 (a) alloy and Dy2Fe10Al7H3.2 (b) hydride in the field of 1 T. Insertion: temperature dependence of MCE in field of 5 T.

Close modal

The magnetocaloric effect in the parent compound was investigated by the direct method. The temperature dependence of the adiabatic temperature change is shown in the insert in Fig. 5. It shows a sharp peak at the Curie temperature and a change of sign at Tcomp.

The magnetization isotherms of Dy2Fe10Al7H3.2 compound are shown in Fig. 6. No saturation is observed up to 5 T. A linear growth field dependence is observed at Tcomp.55 The temperature dependencies of the isothermal entropy change (ΔSM) near Tcomp are shown in Fig. 7. The sign of ΔSm changes at the compensation point.

FIG. 6.

Magnetization curves in static magnetic field up to 5 T of Dy2Fe10Al7H3.2 hydride at various temperatures.

FIG. 6.

Magnetization curves in static magnetic field up to 5 T of Dy2Fe10Al7H3.2 hydride at various temperatures.

Close modal
FIG. 7.

Temperature dependencies of the magnetic part entropy change in magnetic field up to 5 T.

FIG. 7.

Temperature dependencies of the magnetic part entropy change in magnetic field up to 5 T.

Close modal

Below Tcomp, the magnetic moment of the Dy sublattice, is larger than the Fe sublattice and is oriented along the field. On the contrary, the magnetic field disorders the magnetic moments of the Fe sublattice at T < Tcomp, resulting in an increase of the magnetic entropy of the Fe sublattice. In this case, the field causes a decrease in magnetic entropy. Above Tcomp, the Dy magnetic moment becomes smaller than that of the Fe sublattice and orients antiparallel to the external magnetic field. At the same time, the magnetic moment of the Fe sublattice is collinear with an applied field. Therefore, now the field causes the growth of the magnetic part of the entropy. In this case, the magnetic field disorders the rare-earth magnetic sublattice, and the additional ordering in the Fe sublattice occurs. At the compensation point, the total magnetic moment is zero, and hence, the magnetic entropy does not change when an external field is applied.

MCE is not large near Tcomp; nevertheless, materials having a compensation point could be employed as temperature sensors in magnetic refrigerators, which absorb or release heat into the system departed from the correct temperature, thus returning the system to the desired state.

A study of the hydrogen influence on the strength of the R–Fe exchange coupling was carried out using compounds (R,R′)2Fe14B with a partial replacement of Nd by heavy rare earths and compounds with a substitution of Fe by Co or Al [R2(Fe,Co)14B and R2(Fe,Al)17] in high magnetic fields. A strong dependence of the critical transition field HCR on the hydrogen content was found. Hydrogenation serves as a tool allowing us to decrease the R–Fe inter-sublattice exchange and, thus, to lower the critical field to the forced ferromagnetic state. From a simple mean-field model, the inter-sublattice exchange interaction was estimated for all of the investigated compounds. It was shown that hydrogenation could dramatically change the inter-sublattice exchange interaction (by ∼30%) in the compounds with the R substitution. At the same time, in the compounds with partly replaced Fe, the hydrogenation has a weak effect (<5%).

In the case of compounds with partly substituted Fe for Co, the critical field (and the inter-sublattice exchange) decrease with an increase in the cobalt concentration, while hydrogenation (up to 3 at. H/f.u.) of substituted compositions Er2(Fe,Co)14B leads to an increase in the inter-sublattice exchange. As a result, the inter-sublattice exchange remains almost unchanged.

On the other hand, the substitution of Fe by a non-magnetic Al leads to the appearance of magnetic compensation in the compounds. These compounds (and their hydrides) feature a change in the MCE sign with temperature. Materials demonstrating magnetic compensation point can be employed as temperature sensors in magnetic refrigerators, which absorb or release heat into the system departed from the correct temperature, thus returning the system to the desired state. Moreover, hydrogenation allows us to tune the compensation temperature.

To conclude, hydrogenation is an efficient tool to tune the strength of the R–Fe exchange coupling of pseudobinary rare-earth-iron compounds. One should also take into account that the operation of Nd2Fe14B-based magnets with added heavy rare earths in an aggressive hydrogen-containing surrounding may change their properties, as the absorbed hydrogen content increases.

The structural studies are supported by the project “Nanomaterials Centre for Advanced Applications,” Project No. CZ.02.1.01/0.0/0.0/15_003/0000485, financed by ERDF. Magnetic studies in steady fields were performed under the support of Project No. 19–00925S of the Czech Science Foundation and by MGML (https://mgml.eu) within the Program of Czech Research Infrastructures (Project No. LM2018096). For the high-field studies, we acknowledge the support of HLD at HZDR, a member of the European Magnetic Field Laboratory (EMFL).

The data that support the findings of this study are available within the article.

The authors have no conflicts to disclose.

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