Enhancement of magnetocaloric effect in B-rich FeZrBCu amorphous alloys
L.F. Kiss
a,⇑, T. Kemény
a, V. Franco
b, A. Conde
baInstitute for Solid State Physics and Optics, Wigner Research Centre for Physics, Hungarian Academy of Sciences, H-1525 Budapest, P.O. Box 49, Hungary
bDpto. Física de la Materia Condensada, ICMSE-CSIC, Universidad de Sevilla, P.O. Box 1065, 41080 Sevilla, Spain
a r t i c l e i n f o
Article history:
Received 13 August 2014
Received in revised form 13 October 2014 Accepted 25 October 2014
Available online 31 October 2014
Keywords:
FeZrBCu amorphous alloys Early transition metals Magnetic moment Magnetocaloric effect
a b s t r a c t
Magnetic properties and magnetocaloric effect were studied in the Fe92xZr7BxCu1amorphous alloy series (x= 0–23 at.%). Enhancement of the magnetocaloric effect was observed in the B-rich side (x> 15 at.%) which correlates well with the anomalous increase of the saturation magnetic moment per Fe atom in this concentration range. Research into Fe2(B1yETMy) amorphous alloys (ETM = Zr, Nb, Ti and V;
y= 0–0.55 atomic fraction for Zr and to 0.25 atomic fraction for the rest) reveals an unexpected increase of the iron magnetic moment when early transition metals are substituted for B up toy= 0.1–0.25 atomic fraction. This behavior is attributed to the highly attractive B–ETM interaction. Similar mechanism is thought to explain the anomalous increase of the iron magnetic moment and hence the magnetocaloric effect in the boron-rich Fe1xZr7BxCu1amorphous alloys. Universal scaling of the magnetic entropy change curves is also used to detect differences between Fe92xZr7BxCu1alloys.
Ó2014 Elsevier B.V. All rights reserved.
1. Introduction
The magnetic properties of FeZrB(Cu) amorphous alloys have been intensively studied in the past two decades since (1) these materials are promising candidates for magnetocaloric applica- tions and (2) nanocrystalline alloys prepared by annealing from FeZrBCu amorphous precursors (Nanoperm type alloys) are good and cheap soft magnetic materials with a high application poten- tial [1–18,20–29]. The Curie temperature (TC) of the FeZrB(Cu) amorphous alloys can be easily tuned by varying the Zr and/or B content over a large temperature region around room temperature.
The Zr and B concentrations used for this purpose were around 0–
10 and 0–20 at.%, respectively [16–18]. The magnetic entropy change for a single composition is not too big (around 1.3–
1.6 J kg1K1 for a magnetic field change of 2 T with respect to the value of 5.4 J kg1K1for Gd[30]). To overcome this deficiency, recently, the use of two-ribbon composites with quite different Curie points has been proposed to obtain a table-like behavior of the entropy change vs. temperature curve [25–27], maximizing the effective refrigerant capacity. Therefore, it would be desirable to prepare FeZrB(Cu) amorphous alloys having relatively high Curie point and high magnetic entropy change at the same time in order
to extend the upper range of the operational region of the magnetic cooling.
The magnetic and magnetocaloric properties of Fe90xZr10Bx
(x= 5–20 at.%) and Fe93xZr7Bx (x= 0–13 at.%) amorphous alloys were studied intensively[16–18]. The magnetic entropy change increases with the increase of the B content for both alloy series, reaching a maximum at aroundx= 5–10 and 9 at.% B for the alloys containing 10 and 7 at.% Zr, respectively. This dependence is found to be correlated with the average magnetic moment per Fe atom. In the present paper we studied the magnetic and magnetocaloric behavior of a very similar amorphous alloy series, Fe92xZr7BxCu1, up to somewhat higher B concentrations (x= 0–23 at.%). We dis- covered that after an initial maximum in the magnetic entropy change (DSM) as a function of the B concentration (which was observed earlier[16,17]), DSMincreases further above 15 at.% B.
Similar correlation is found with the saturation magnetic moment per Fe atom. A possible explanation for the unexpected increase of the iron magnetic moment with increasing B content in the high B-concentration range will be proposed, which is in accord with the structural modification suggested in a recent ab initio molecu- lar dynamics study of the closely related Fe–B–Y system[19].
2. Experimental details and data analysis
Fe92xZr7BxCu1(x= 0–23 at.%) and Fe2(B1yETMy) (ETM = Zr, Nb, Ti and V;y= 0–
0.55 atomic fraction for Zr and to 0.25 atomic fraction for the rest) amorphous alloys were prepared by melt-spinning in vacuum. The former alloys were obtained
http://dx.doi.org/10.1016/j.jallcom.2014.10.127 0925-8388/Ó2014 Elsevier B.V. All rights reserved.
⇑Corresponding author.
E-mail addresses:kissl@szfki.hu,kiss.laszlo.ferenc@wigner.mta.hu(L.F. Kiss).
Contents lists available atScienceDirect
Journal of Alloys and Compounds
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j a l c o m
in the form of ribbons with a cross section of 12lm1 mm, the latter alloys consisted of flakes with similar thickness. The amorphous nature of the alloys was checked by X-ray diffraction and Mössbauer spectroscopy. The magnetic mea- surements were performed using a Quantum Design MPMS 5S superconducting quantum interference device (SQUID) magnetometer in the temperature range of 5–400 K up to a magnetic field of 5 T; and using a Lakeshore 7407 vibrating sample magnetometer (VSM) above 300 K up to 1.5 T. The saturation magnetization (saturation moment per Fe atom) was derived from the magnetization (M) vs.
magnetic field (H) curve measured at 5 K by linear extrapolation toH= 0 from 3 to 5 T fields (for the Fe92xZr7BxCu1alloys) and from 2 to 5 T (for the Fe2(B1yETMy) alloys), except forx= 2, 4, 6 at.% B where the extrapolation interval was between 4 and 5 T because of the non-saturating feature of the magnetization curves. The temperature dependence of the magnetization for Fe90Zr7B2Cu1was measured firstly in a magnetic field of 10 Oe during warming after cooling the sample from 300 K in a field close to zero ( 1 Oe), called ZFC (zero-field cooling) mode;
secondly the sample was cooled from 300 K in 10 Oe and the magnetization was measured again in 10 Oe during warming, called FC (field cooling) mode.
The magnetocaloric properties were calculated indirectly from the isothermal magnetization curves. The magnetic entropy change due to the application of a magnetic fieldHwas evaluated using a numerical approximation to the equation DSM¼
ZH 0
@M
@T
H
dH
where the partial derivative is replaced by finite differences and the integration is performed numerically. A universal curve is constructed by normalizing the magnetic entropy change curves with respect their peakDSpkMand rescaling the tem- perature axis[31]. Here two different reference temperatures were used for scaling:
h1¼ ðTTCÞ=ðTr1TCÞ TTC
h2¼ ðTTCÞ=ðTr2TCÞ T>TC
whereTr1andTr2are the temperatures of the two reference points of each curve and TCis the Curie temperature.
The refrigerant capacity,RC, has been calculated as the product of the absolute value of theDSMpeak times the full width at half maximum.
3. Results and discussion
Fig. 1 shows the temperature dependence of the magnetic entropy change for the whole Fe92xZr7BxCu1 amorphous alloy series for a field change of 0–1.5 T. Similar curves were obtained [16–18]in the 0–20 at.% B-content range where the effect saturates or displays a shallow maximum after an initial increase as the boron concentration increases. The slight deviations between our findings and those of[16–18]are caused by small differences in the compositions of the alloys studied. Upon further increasing the boron content, the magnetic entropy change begins to increase again up tox= 23 at.%.
The corresponding refrigeration capacity (RC) is shown inFig. 2 for the same alloy series. The increase of RC is quite evident above x= 15 at.% B.
The Curie temperature of the Fe92xZr7BxCu1amorphous alloy series increases linearly with the B content abovex= 6 at.% and
can be well correlated with the peak temperature of the magnetic entropy change (Tpk) as shown in Fig. 3. The two curves almost coincide reflecting the linear relation between TC and Tpk. In Fig. 4 the peak value of the magnetic entropy change (DSpkM) is plotted as a function of both Tpk and the boron concentration.
Fig. 4 shows explicitly the unexpected enhancement of the magnetic entropy change abovex= 15 at.% B content.
In order to explain the enhancement of the magnetocaloric effect on the B-rich side of the Fe92xZr7BxCu1amorphous alloys, the B-concentration dependence of the saturation magnetic moment per Fe atom (
l
Fe) was also determined as shown inFig. 5.Similar curves were obtained in [16–18] below x= 20 at.% B content. Room temperature data on saturation magnetization are also available for FexZr7B93xand FexZr7B91xCu2amorphous alloys (796x689) with the same trend [3]. The minor differences between our results and those of [16–18] are caused partly by the slightly different compositions of the investigated alloys.
Besides that, we determined the saturation magnetic moment at 5 K in contrast with the other studies where data measured at 200–300 K were used. The iron magnetic moment initially increases rapidly up tox= 4–6 at.% B where it shows a maximum and begins to decrease up to x= 15 at.% B. Increasing further the boron content, the iron magnetic moment begins to increase again.
Since the magnetic entropy change is proportional to the magnetic moment[32], it is established that the enhancement of the mag- netic entropy change on the B-rich side is related to the increase of the iron magnetic moment. Two composition ranges with an approximate proportionality betweenDSpkM and
l
Feare shown in Fig. 6.0 100 200 300 400 500 600 700
-1.8 -1.5 -1.2 -0.9 -0.6 -0.3 0.0
ΔH = 15 kOe
ΔSM(J kg-1 K-1 )
T (K)
B2 B4 B6 B8 B10 B15 B21 B23
Fe92-xZr7BxCu1
Fig. 1.Magnetic entropy change as a function of temperature for Fe92xZr7BxCu1
amorphous alloys for a magnetic field change of 1.5 T. The boron contents are shown by labels. The lines are guide to the eye.
5 10 15 20 25
40 50 60 70 80 90
RC (J kg-1 )
x (at% B) Fe92-xZr7BxCu1
Fig. 2.Refrigeration capacity as a function of boron content for Fe92xZr7BxCu1
amorphous alloys for a magnetic field change of 1.5 T. The line is guide to the eye.
0 10 20
200 300 400 500
Tpk (K)
x (at% B)
200 300 400 500
Tc (K) Fe92-xZr7BxCu1
Fig. 3.Left axis: peak value of the magnetic entropy change as a function of boron concentration for Fe92xZr7BxCu1amorphous alloys for a magnetic field change of 1.5 T (stars). The line is a linear fit to the points above 8 at.% B.Right axis: Curie temperature of the alloys (crosses) determined from Arrott plots.
The initial increase of the iron magnetic moment when small amount of iron is replaced by boron is associated with the gradu- ally eliminated spin-glass behavior of the alloy series. The hypo- thetic composition of this alloy series for x= 0, Fe92Zr7Cu1, is close to Fe93Zr7 which shows a clear spin-glass transition [33].
Forx= 2 at.% B a typical reentrant-spin-glass behavior is observed as shown inFig. 7by the temperature dependence of the low-field magnetization (note the bifurcation of the ZFC and FC curves).
Another feature related to the spin-glass-like nature of these amorphous alloys with low B content is the significant high-field
susceptibility. On the other hand, the alloys with B contents above x= 8 at.% behave as soft ferromagnets with small high-field suscep- tibility. The smaller iron magnetic moments obtained for low B content is the consequence of the fact that these alloys cannot be saturated in our maximum field of 5 T. Addition of B atoms suppresses the number of low-moment Fe atoms having only Fe nearest neighbors (compressed regions with small atomic volume per Fe atom), moreover it leads to the expansion of the Fe–Fe interatomic distance. Both phenomena result in the increase of the iron magnetic moment.
The saturation magnetic moment per Fe atom decreases between 6 and 13 at.% B content (Fig. 5). This dependence is the normal behavior in Fe-rich Fe–B based alloys, e.g. it is observed also in binary Fe–B metallic glasses in the 13–25 at.% B-concentration range[34,35]. Earlier, an increasing iron magnetic moment with increasing B content[36] had also been published for the latter alloys; however, this was shown[37]to be caused by incomplete magnetic saturation. On the contrary, the dependence of the Curie temperature on the B content is anomalous in both the Fe92xZr7BxCu1 (Fig. 3) and the binary Fe–B amorphous alloys:
TC increases in the same B-content range. In these itinerant ferromagnets, the Curie point is determined by the details of the electronic structure and theTC increase with B content reflects the delicate balance between the atomic-volume and magnetic- moment effects.
Further on we focus on the anomalous increase of the iron mag- netic moment above 15 at.% B (Fig. 5). In order to understand this unexpected behavior, we prepared some amorphous alloy series of the composition Fe2(B1yETMy) where the Fe content does not vary while B is replaced by early transition metals (ETM = Zr, Nb, Ti and V).Fig. 8shows the saturation magnetic moment per Fe atom for this alloy series as a function of the ETM content.
Replacement of B by ETM causes evidently the iron magnetic moment to increase up to 0.1–0.25 atomic fraction ETM content depending on the type of ETM atom. InFig. 8the data for ETM = Zr were taken from our earlier paper[38]where the proportionality between the iron magnetic moment and the average Fe hyperfine field was also shown. In view of the literature results available, this initial increase is quite unexpected. Room-temperature saturation magnetization shows a monotonous decrease for Fe80ZrxB20xand Fe75ZrxB25x[39,40]amorphous alloys with increasing Zr content and similar trends have been observed for Fe80ETMxB20x
(ETM = Cr, V, Nb, Mo, Ta, W)[41]metallic glasses. Systematic com- position dependences of the low-temperature saturation moments for FeZrB(Cu) amorphous alloys with constant Fe content are not available. Another study [42] indicates an approximate 10–20%
decrease for the low-temperature saturation moment of the alloys
250 300 350 400 450 500 550
1.0 1.2 1.4 1.6
0 5 10 15 20 25
ΔS
pk M
(J kg-1 K-1 )
Tpk (K)
Fe92-xZr7BxCu1 x (at% B)
Fig. 4.Peak value vs. peak temperature of the magnetic entropy change (solid square) and vs. boron concentration (open circle) for Fe92xZr7BxCu1amorphous alloys for a magnetic field change of 1.5 T.
0 5 10 15 20 25
1.6 1.8 2.0
μFe (μB)
x (at% B) Fe92-xZr7BxCu1
Fig. 5.Saturation magnetic moment per Fe atom at 5 K as a function of boron content for Fe92xZr7BxCu1amorphous alloys. The line is guide to the eye.
1.5 1.6 1.7 1.8 1.9 2.0
1.0 1.2 1.4 1.6
ΔS
pk M
(J kg-1 K-1 )
μFe (μB) Fe92-xZr7BxCu1
Fig. 6.Peak temperature of the magnetic entropy change as a function of saturation magnetic moment per Fe atom for Fe92xZr7BxCu1amorphous alloys. The line is guide to the eye.
0 50 100 150 200 250 300
0 10 20 30 40
H = 10 Oe FC
ZFC
M (emu/g)
T (K) Fe90Zr7B2Cu1
Fig. 7.Magnetization as a function of temperature measured in 10 Oe after zero- field cooling (ZFC) and field cooling (FC) for Fe90Zr7B2Cu1.
in a very narrow (4–6 at.%) ETM-concentration range, which is in contrast with the present results.
This way the unexpected increase of the iron magnetic moment when replacing B by ETM both in the high B content Fe92xZr7BxCu1 (Fig. 5) and the Fe2(B1yETMy) amorphous alloy series (Fig. 8) is observed beyond any doubts for a wide range of ETM additives.
We attribute this to the highly negative B–ETM pair energy. This strong interaction is expected to shorten the B–ETM separation increasing the Fe atomic volume and hence its magnetic moment.
Note that this trend is effective only when the boron content is large enough for a high fraction of B–ETM pairs to be formed.
Therefore, the same mechanism is proposed to explain the increase of the iron magnetic moment for the B-rich Fe92xZr7BxCu1amor- phous alloys (abovex= 15 at.% B) studied in the present paper.
The dominant role of the high B content in the glass formation and in the related structural and magnetic properties is confirmed in a recent ab initio molecular dynamics (AIMD) simulation[19]of the closely related Fe–B–Y amorphous alloy system. They have found that the Fe72.5Y3.5B24alloy has stronger compositional short range order (CSRO) referring to Y–B and B–Y pairs than Fe72Y6B22. It is in fact the structural modification which results in the increase of the Fe magnetic moment found in our study. The undisputable feature of this AIMD study is the strong affinity between B and Y which is the real driving force of the changes detected by us. While quantitative figures are not quoted in this report, the published fig- ures hint at an increase of the Fe atomic volume, a decreasing total coordination number and a practically constant Fe–Fe coordination with increasing B content in this high-B content range. All of these features are well known to increase the atomic magnetic moment.
It is also quite reasonable to assume that this effect is relevant for all Fe–ETM–B amorphous alloys. In the low-B content alloy the Fe moment is diminished by the d–d hybridization caused by the early transition metals. When the B content is increased, the strong B–ETM attraction increases the fraction of the B–ETM pairs and reduces their effect on the Fe magnetic moment which accordingly increases.
The temperature dependence of the magnetic entropy change for the Fe2(B1yETMy) (ETM = Nb, Ti and V) amorphous alloys for a field change of 0–1.5 T is shown inFig. 9. For Ti and V the change of DSM follows the same decreasing trend with increasing ETM content as does the saturation magnetic moment (Fig. 8).
For the Fe92xZr7BxCu1amorphous alloys, the curves of the mag- netic entropy change as a function of temperature collapse into universal curve abovex= 10 at.% B (Fig. 10) using the transforma- tion described above. This collapse of the different curves close to the transition temperature is a clear indication that the alloys of this series possess similar values of the critical exponents. When
the reduced temperature reaches values lower than 1, with specific values depending on the B content, the deviation from the universal curve shows the presence of additional magnetic phenomena. The deviation from the universal curve below x= 10 at.% B might reflect the spin-glass behavior of the alloys. This deviation is larger as the B content decreases, in agreement with this interpretation.
4. Conclusions
Anomalous increase of the magnetic entropy change was observed for the Fe92xZr7BxCu1 amorphous alloy series with increasing B content abovex> 15 at.% B, which was explained by the anomalous increase of the iron magnetic moment in the same (B-rich) concentration range. The underlying physical phenome- non is suggested to be the highly negative B–Zr pair energy, sup- ported by magnetization measurements on Fe2(B1yETMy) metallic glasses. This effect might be used to produce amorphous ribbons with compositions that have highTCandDSpkM at the same time. These ribbons can form part of a composite material showing a table-like behavior of the magnetic entropy vs. temperature curve, increasing the refrigeration capacity.
Acknowledgments
This work was supported by the Hungarian Scientific Research Fund through Grant OTKA K 101456, the Hispano-Hungarian Bilateral Cooperation Program (Acción Integrada HH2004-0015;
TÉT E-21/04), the Spanish MINECO and EU FEDER (Project MAT
0.0 0.1 0.2 0.3 0.4 0.5 0.6
1.4 1.6 1.8 2.0 2.2
V Zr Nb Ti
Fe2(B1-yETMy)
μFe (μB)
yETM
Fig. 8.Saturation magnetic moment per Fe atom as a function of early transition metal (ETM) content for Fe2(B1yETMy) amorphous alloys. Data on Zr from Ref.[32], the rest from this work. The lines are guide to the eye.
300 400 500 600 700
-2.0 -1.5 -1.0 -0.5 0.0
Fe2(B1-yETMy) ΔH = 15 kOe
ΔSM (J/kgK)
T (K)
yNi = 0.25 yTi = 0.25 yTi = 0.125 yV = 0.25 yV = 0.125
Fig. 9.Magnetic entropy change as a function of temperature for Fe2(B1yETMy) (ETM = Nb, Ti and V) amorphous alloys for a magnetic field change of 1.5 T. The ETM contents are shown by labels. The lines are guide to the eye.
0.0 0.2 0.4 0.6 0.8 1.0
Fe92-xZr7BxCu1 B2 B4 B6 B8 B10 B15 B21 B23
ΔSM/ΔSpk M
θ
-6 -4 -2 0 2 4 6
Fig. 10.Normalized magnetic entropy as a function of reduced temperature using two reference temperatures for Fe92xZr7BxCu1amorphous alloys.
2013-45165-P) and the PAI of the Regional Government of And- alucía (Project P10-FQM-6462). We thank L. Bujdosó for the fabri- cation of the amorphous alloys.
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