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.

Rare-earth (R)-iron intermetallics have a prominent role among magnetic materials ranging between daily life and space industry.1–6 The interest in their fundamental properties does not wane in the context of emerging new as well as already implemented research methods, which still get further developed (for instance, the high magnetic fields methodology7,8). The magnetism of the R-Fe compounds is substitution-sensitive9–11 and varies strongly upon absorption of interstitial elements.12 Both effects are studied separately in great detail in literature. However, the combined effect of substitution and absorption impurities is rather scarcely covered. Complex modifications may alter not only fundamental but also functional properties of the intermetallic compounds. (Note that light interstitial atoms (e.g. hydrogen, nitrogen, oxygen, carbon) normally surround the R-Fe functional materials (in air, water)).

Compounds of the R2Fe14B type have been studied intensively since the discovery of the best permanent magnet Nd2Fe14B in 1984.13,14 Recently high magnetic field instruments7,8 – a very informative tool in the studies of ferrimagnets15–17 – became more accessible for research. The literature survey shows that for the compounds of the R2Fe14B family materials with R = Dy and Tm were studied using ultra-high magnetic fields up to 120 T.18 This field was sufficient to observe a field-induced ferromagnetism in Tm2Fe14B while for Dy2Fe14B fields exceeding 120 T were required. Other R2Fe14B ferrimagnets were studied in much lower fields, 20 - 30 T,9 and up to 55 T.19 

In the present work we describe results of investigations of Ho2Fe14B. Nd was chosen for partial substitution for Ho in order to boost interest to the work from the point of view of production of thermally stable magnets based on (Nd,Ho)2Fe14B and (Nd,Ho)2(Fe,Co)14B. (These materials have a very low temperature coefficient of remanence in the temperature range between -60 and +120 °C, which is demanded for a number of instruments using permanent magnets20–22). Further, hydrogen was chosen as an interstitial element since the NdFeB-type compounds absorb hydrogen very readily – the property of immense importance for the hydrogen-assisted processing of materials (HD and HDDR processes23) for industrial use. Furthermore, magnets prepared from the compounds can change their functional properties with time by operating in an aggressive H-containing environment.24 

Earlier studies25 of the influence of hydrogen on magnetocrystalline anisotropy (MCA) of R2Fe14B were conducted for the systems Ho2Fe14B-H, Nd2Fe14B-H, Y2Fe14B-H and others. Hydrogen was shown to suppress the contributions from both the Fe- and R-sublattices to the total MCA. The calculations of the crystal fields, however, were performed under an assumption of nearly unchanged inter-sublattice exchange interactions in the hydrogen-charged materials. We studied the process of magnetization of the compounds in fields up to 60-80 T. Based on the shape of the magnetization M(H) curves, information on the inter-sublattice exchange interactions and the influence of substitution and interstitial atoms were extracted.

Detailed information on the preparation and characterization of the parent samples Ho2Fe14B and Ho1Nd1Fe14B and their hydrides can be found elsewhere.26–28 Both samples were hydrided at the Department of Condensed Matter, Charles University in Prague. The reaction chamber with sample was first evacuated to high vacuum and subsequently filled by hydrogen, obtained by thermal desorption of LaNi5Hx. To obtain a maximum hydrogen concentration in the samples, the H2 pressure of 100 bar in the reaction chamber was used. The hydrogenation was indicated by a pressure decrease in the closed volume. The amount of absorbed hydrogen was determined with precision better than 10% by thermally induced desorption in a closed evacuated reactor. As a result of hydrogen absorption, samples decrepitated to fine powder. All subsequent studies of both parent materials (Ho2Fe14B and Ho1Nd1Fe14B) and the hydrides (Ho2Fe14BHx with x = 0; 2.3; 3.8; 5.5 and Ho1Nd1Fe14BHx with x = 3 and 5.5) were performed on free powder samples.

The crystal structure of the materials was characterized by standard (DRON-3M) x-ray diffraction (XRD) at room temperature. The composition of the alloys was verified by the Energy-dispersive X-ray Spectroscopy (EDXS).

The samples magnetization was measured in steady and pulsed magnetic fields. The magnetization study in steady magnetic fields up to 14 T was conducted at T = 4.2 K using a PPMS-14 magnetometer (Quantum Design, USA) in Prague. High-field magnetization study in pulsed magnetic fields up to 60 T (and in the case of Ho1Nd1Fe14B up to 80 T) at 4.2 K was performed at the High-Field Laboratory in Dresden. All pulsed-field data were calibrated against magnetization measured in steady fields. Magnetization data presented below were corrected for demagnetization.

Single-phase parent materials Ho2Fe14B and Ho1Nd1Fe14B with well-defined directional structure26 and single crystalline structure,27 respectively, were obtained. Ho2Fe14B remains single-phase upon hydrogenation. However, after hydrogenation of Ho1Nd1Fe14B to the maximum hydrogen concentration 5.5 at./f.u., traces of the foreign phases (α-Fe and 2:17 phase) with total amount not exceeding 7 % were detected. X-ray diffraction analysis showed that hydrogen absorption by the Ho2Fe14B compound does not change symmetry and crystal structure type but leads to an increase of the unit cell volume (V) which agrees well with literature data for the Ho2Fe14B-H system.29 For Ho2Fe14BH5.5 and Ho1Nd1Fe14BH5.5, the relative volume change ΔV/V = 3.4 and 3.5 %, respectively, was recorded, while no changes of the crystal structure type was detected.

Figure 1(a and b) demonstrates the magnetization curves of Ho2Fe14B, Ho2Fe14BH5.5, Ho1Nd1Fe14B and Ho1Nd1Fe14BH5.5 powder samples measured at T = 4.2 K in pulsed fields. It is seen that magnetization in Ho2Fe14B (Fig. 1a) saturates to MS = 11.3 μB/f.u. in rather weak magnetic fields in the ferrimagnetic state (MS(ferri)). Indeed, considering that the saturation magnetization for the Fe-sublattice in Ho2Fe14B is the same as in the other compounds with non-magnetic rare earths, e.g. Y2Fe14B, MS = 31.4 μB/f.u.,9 the magnetic moment on the Ho sublattice (oriented antiparallel to the Fe-moment) is close to that of a free ion Ho3+ (≈10 μB/f.u.). However, in fields of approx. 35 T a smooth increase of magnetization (first critical field of the transition μ0Hcr1) related to the change of direction of magnetic moments of Ho and Fe commences. The process is, however, not complete in fields up to 60 T, and higher magnetic fields are required to observe a field-induced ferromagnetic state. However, the obtained Hcr1 value allows us to estimate the parameter λ of the inter-sublattice exchange interaction using the following expression:19,30,31

Hcr1=λMFe2MHo
(1)

where MFe and MHo are the magnetizations of the Fe and Ho sublattices, respectively. For Hcr1 = 35 T (found by analyzing the field derivatives of the magnetization; we estimate the uncertainty in the field determination is ± 0.3 T), λ equals to 3.07 T/μB that agrees very well with data of Ref. 32.

FIG. 1.

Magnetization curves of Ho2Fe14B, Ho2Fe14BH5.5, Ho1Nd1Fe14B and Ho1Nd1Fe14BH5.5 powder samples measured at T = 4.2 K in pulsed fields.

FIG. 1.

Magnetization curves of Ho2Fe14B, Ho2Fe14BH5.5, Ho1Nd1Fe14B and Ho1Nd1Fe14BH5.5 powder samples measured at T = 4.2 K in pulsed fields.

Close modal

The substitution of one half of Ho atoms for Nd increases considerably the saturation magnetization to 24.5 μB/f.u. An almost twofold increase of the critical field value is observed, that was anticipated for the studied composition Ho1Nd1Fe14B. To evaluate the parameter λ of the inter-sublattice exchange interaction in this case we used a modified expression:

Hcr1=λMFeMHoξ
(2)

where ξ=11+λNdχNd. Here λNd and χNd are the exchange parameter and susceptibility of the Nd sublattice, respectively.31 According to our estimates, the product λNdχNd does not exceed 0.1. The earlier studies of the pseudoternary compounds (Er,R)2Fe14B (R = Nd, Gd, Dy)33 showed that λ does not change considerably when the composition varies. Indeed, in Ho1Nd1Fe14B we only record a 3% increase of λ (cf. 3.17 T/μB and 3.07 T/μB obtained in Ho2Fe14B). In practice, heavy rare earths such as Ho and Dy are added into the Nd2Fe14B-based magnets during technological cycle of the permanent magnets production. Our study shows that the inter-sublattice interaction in the pseudoternary compound Nd1Ho1Fe14B does not change considerably as compared to the parent Ho2Fe14B while the saturation magnetization increases is more than twofold.

The problem of the influence of hydrogen absorption on the strength of the R-Fe exchange coupling for multicomponent (RR’)2Fe14B compounds has remained open until now. It is well-known34 that hydrogenation increases the saturation magnetization in Y2Fe14B. The mean magnetic moment on Fe atoms increases almost linearly with an increase of the amount of hydrogen in the samples from 2.2 to 2.4 μB in the hydrogen concentration range 0 ≤ x ≤ 4.5(5.5).9,35 The Curie temperatures increase with the hydrogen content increase in Y2Fe14B-H is related to the strengthening of the Fe-Fe exchange interactions. When Y is substituted for other (magnetic) rare earths, the Curie temperature of the parent compound is obviously different however the tendency (and rate) for the TC increase with the increasing hydrogen uptake is the same for various compounds (see Ref. 9 and references therein], thus pointing to the fact that the size effects (changes in the bond lengths) can be ignored in the case of hydrogenation. Moreover Ref. 25 claims that the parameter λ of the inter-sublattice exchange interaction changes weakly after hydrogenation of R2Fe14B with R of only one kind.

We have studied the influence of hydrogenation on the first critical field of transition in the Ho2Fe14B-H system.28 It is found that (see Fig. 2) upon the increase of hydrogen concentration from x = 0 to x = 5.5, the critical field of the transition decreases from 35 T to 30 T and follows the linear trend

Hcr1=Hcr100.91x
(3)

Calculation using Eq. (1) provides the decrease of the λ parameter in Ho2Fe14BHx from 3.07 T/μB at x = 0 to 2.2 T/μB at x = 5.5. This implies the 28.3 % decrease of the strength of R-Fe exchange. For Ho1Nd1Fe14BH5.5 we observe a drastic decrease of the critical field of the transition from 70 to 50 T upon absorption of 5.5 at.H/f.u. Such a decrease of the Hcr1 is several times larger than in the hydride Ho2Fe14BH5.5. At the same time, calculations using Eq. (2) show that λ decreases from 3.17 T/μB at x = 0 to 2.06 T/μB for x = 5.5 (that is close to the value 2.2 T/μB for Ho2Fe14BH5.5) and this gives us the 35 % decrease of the strength of R-Fe exchange in Ho1Nd1Fe14BH5.5 (cf. 28.3 % in Ho2Fe14BH5.5).

FIG. 2.

Concentration dependence of first critical field for Ho2Fe14BHx and Ho1Nd1Fe14BHx systems.

FIG. 2.

Concentration dependence of first critical field for Ho2Fe14BHx and Ho1Nd1Fe14BHx systems.

Close modal

To conclude, hydrogenation is an efficient tool to influence the strength of the R-Fe exchange coupling in case of ternary R2Fe14B and pseudoternary (RR’)2Fe14B compounds. Similar studies conducted for a wide class of substituted compounds (RR’)2Fe14B would be desirable. In our work, the study was carried out using the compounds with Ho and Nd in the high magnetic fields, which demonstrated strong dependence of the magnitude of the first critical field of the transition μ0Hcr1 on the hydrogen content. From a simple mean-field model the parameter of the inter-sublattice exchange interaction was estimated for the two systems: Ho2Fe14BHx and Ho1Nd1Fe14BHx. It was shown that while hydrogenation may dramatically change the inter-sublattice exchange interaction (by approx. 30%), the R-ion substitution produces only a minor effect (∼ 3 %). Moreover, in the substituted compound Ho1Nd1Fe14B, hydrogenation effects have stronger impact (∼ 7%) than that in Ho2Fe14B. One should also take into account that operation of the Nd2Fe14B-based magnets with the added heavy rare earths in an aggressive hydrogen-containing surrounding may change their properties as the absorbed hydrogen content increases.

This work is performed with financial support of the grant of Russian Science Foundation (project N 18-13-00135). Part of the measurements has been performed in the Materials Growth and Measurement Laboratory (http://mgml.eu/). We also acknowledge the support of HLD at HZDR, member of the European Magnetic Field Laboratory (EMFL).

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