The discovery of the colossal magnetocaloric effect in a seriesof amorphous ribbons based on Finemet N.. Two criteria producing the colossal magnetocaloric effect CMCE in magnetic materi
Trang 1The discovery of the colossal magnetocaloric effect in a series
of amorphous ribbons based on Finemet
N Chaua, N.D Thea,b, N.Q Hoaa,c, C.X Huua, N.D Thoa, S.-C Yuc,∗
aCenter for Materials Science, College of Science, Vietnam National University, 334 Nguyen Trai, Hanoi, Vietnam
bDepartment of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom
cDepartment of Physics, Chungbuk National University, Cheongju 361-763, Republic of Korea
Received 23 August 2005; received in revised form 10 February 2006; accepted 24 February 2006
Abstract
A large number of amorphous ribbons based on Finemet have been prepared by rapid quenching on a single copper wheel with linear
speed of v = 25–30 m/s The ribbons are 20–25m thick and 6–8 mm wide All as-cast samples are amorphous Two criteria producing the colossal magnetocaloric effect (CMCE) in magnetic materials working as magnetic refrigerants are high saturation magnetization and sharp ferromagnetic–paramagnetic phase transition The Fe-based amorphous ribbons fit these cretia Thermomagnetic curves as well as isothermal magnetization curves around the Curie temperature of all the studied samples have been determined The results show that the magnetic entropy change,|Sm|, belongs to a class of materials with CMCE and the |Sm|maxvalues have been obtained at a moderately low magnetic field change
of 1.35 T, moreover|Sm|maxoccurred at quite high temperature
© 2006 Elsevier B.V All rights reserved
Keywords: Amorphous magnetic materials; Magnetocaloric effect; Nanocrystalline materials
1 Introduction
When an external magnetic field is applied to a material,
magnetic moments in material attempt to align with the
mag-netic field, thereby reducing the magmag-netic entropy of the spin
system If this process is performed adiabatically, the reduction
in spin entropy is offset by an increase in lattice entropy, and
temperature of the sample will rise When an applied magnetic
field is removed, the temperature of specimen will drop This
magnetocaloric effect (MCE), or adiabatic temperature change,
which is detected as the heating or cooling of magnetic materials,
is due to the varying magnetic field Magnetic refrigeration
pro-vides an alternative method for cooling Recently, there has been
interest in extending the magnetic refrigeration technique to near
and higher than room temperature region because of the desire
to eliminate chlorofluoro-carbons present in high-temperature
gas-cycle systems and to save energy[1]
As we well known, the adiabatic magnetic entropy change,
Sm, and temperature change,Tad, are correlated with
mag-netization, magnetic field change, heat capacity and absolute
∗Corresponding author Tel.: +82 43 2612269; fax: +82 43 2756415.
E-mail address:scyu@chungbuk.ac.kr (S.-C Yu).
temperature by Maxwell’s fundamental relations[2]:
Sm(T, H) =
Hmax 0
∂M(T, H)
∂T
Tad(T, H) = −
Hmax 0
T C(T, H) H
∂M(T, H)
∂T
H
dH (2)
where Hmaxis the final applied magnetic field
From Eq (1), we see that two criteria forming large MCE in magnetic materials are a high saturation mag-netization and a sharp change in magmag-netization at the ferromagnetic–paramagnetic (FM–PM) phase transition The prototype material for room temperature range is lan-thanide metal Gd which orders ferromagnetically at 294 K[3]
A series of Gd5(Ge1−xSix)4 alloys was reported[4,5] to dis-play a Sm at least two times larger than that of Gd near room-temperature The compound La(Fe,Co)11.83Al1.17has also showed a considerable MCE near room temperature[6] Recently, a new class of magnetic refrigerant materials MnFe(P, As) and related compounds for room-temperature applications have been discovered[7,8], also, interstitial mod-ifications of compounds La(Fe, Si)13 with hydrogen, carbon and nitrogen[9,10]have attracted much attention These new 0921-5093/$ – see front matter © 2006 Elsevier B.V All rights reserved.
doi: 10.1016/j.msea.2006.02.354
Trang 2Fig 1 X-ray diffraction patterns of as-cast samples Fe73.5−xCrxSi13.5B9
Nb3Au1
materials have important advantages over existing magnetic
coolants: they exhibit a large MCE and the operating
tempera-ture can be ranged from below 200 to about 400 K by adjusting
the chemical composition or the content of interstitial atoms
Another class of materials also displaying a large MCE is
based on perovskite [11,12], where we examined the positive
entropy change in manganite with charge-ordering[13]
In this report we present our discovery of colossal
magne-tocaloric effect (CMCE) in a series of amorphous ribbons based
on Finemet
2 Experiments
Amorphous ribbons with nominal compositions (number
indicate at.%)
No 1: Fe73.5Si13.5B9Nb3Au1;
No 2: Fe73.5 −xCrxSi13.5B9Nb3Cu1(x = 1–9);
No 3: Fe73.5 −xCrxSi13.5B9Nb3Au1(x = 1–5);
No 4: Fe73.5 −xMnxSi13.5B9Nb3Cu1(x = 1, 3 and 5).
have been prepared by rapid quenching melting alloys (using
elements of 99.9% purity) on a copper wheel with wheel speeds
v = 25–30 m/s The ribbons are 20–25m thick and 6–8 mm
wide
Fig 2 Thermomagnetic curves of as-cast ribbon Fe73.5Si13.5B9Nb3Au1 : (1)
heating cycle and (2) cooling cycle.
Fig 3 Magnetization as a function of applied field of the sample Fe73.5Si13.5B9Nb3Au1 at different temperatures.
The structure of the ribbons was examined by X-ray diffrac-tometery (D5005 Bruker) with Cu K␣ radiation (λ = 1.54056 ´˚A) Isothermal magnetization curves and thermomagnetic curves were measured by vibrating sample magnetometer (VSM-DMS 880, Digital Measurement Systems) with maximal
Fig 4 Thermomagnetic curves of as-cast ribbons Fe73.5−xCrxSi13.5B9Nb3Cu1 (a) and magnetic entropy change,|Sm|, vs temperature (b).
Trang 3applied 13.5 kOe The heat capacity measurements of the
studied samples were performed by DSC TA 2960
Instru-ments and the results showed that these values are in
order of that of pure Fe (the maximum values of
capac-ity reached at respective Curie temperatures, C were around
400 J/kg K)
3 Results and discussion
Fig 1shows the XRD patterns of as-cast ribbons No 3 These
patterns exhibit only one broad peak around 2θ = 45◦, showing
that the samples are amorphous The same behavior is observed
for all ribbons studied
The thermomagnetic curves of all samples have been
measured at a low applied magnetic field of 50 Oe Fig 2
presents the M(T) curves for as-cast ribbon No 1 as an example.
It is clear that at the Curie temperature, TC, of amorphous
state, magnetization suddenly decreases, after that the sample
is in a (super)paramagnetic state to above 550◦C, then starts
to increase due to crystallization To study the magnetocaloric
effect of the samples, a series of isothermal magnetization
curves around their respective TC have been measured in a
Fig 5 Temperature dependence of magnetic entropy change,|Sm |, of ribbons
Fe73.5−xCr Si13.5B9Nb3 Au1 : (a) and Fe73.5−xMn Si13.5B9 Nb3 Cu1 (b).
magnetic field up to 13.5 kOe.Fig 3shows the results for sample
No 1
When magnetization is measured at a small discrete field and temperature interval,Smcould be determined from Eq.(1)by formula:
Sm= M i − M i+1
where M i and M i+1are the experimental values of magnetization
at Tiand T i+1 , respectively, under an applied magnetic field of Hi The magnetic entropy change,|Sm|, of sample No 1 has been calculated and has a maximum value of 7.8 J/kg K This value indicated that the mentioned sample has colossal magnetocaloric effect (CMCE) We note that this CMCE was reached at a quite low magnetic field variation of 13.5 kOe
Figs 4–5display the thermomagnetic curves of as-cast rib-bons as well as|Sm| versus temperature of the other samples These figures show that the doping of Cr and Mn in Finemet-type alloys significantly decreased the Curie temperature of amorphous state of respective compositions In the systems with
Cr doping this could be explained by ferromagnetic dilution
as well as by the existence of FeCr at the grain boundary [14] In the case of Mn substituted for Fe in Finemet, the authors of [15] reported that the migration of Mn atoms to the grain boundary region would promote a reduction of the magnetic coupling in the system We also see that all studied samples exhibit a large MCE and the temperature at which
|Sm| reached a maximum (close to the respective TC of amorphous phase) could be controlled by adjusting the doping content
According to our knowledge, CMCE was first discovered
by us for amorphous phase of Finemet compound with very high|Sm|max= 13.9 J/kg K [16] The studied samples in the present work belong to ultrasoft nanocomposite materials after appropriate annealing similar to that obtained in the ribbons
Fe73.5−xCoxSi13.5B9Nb3Cu1[17]and in Finemet with Cu sub-stituted by Ag[18]
4 Conclusions
The amorphous magnetic alloys based on Finemet have essential advantages: high saturation magnetization Ms, sharp
change of Ms at FM–PM phase transition of the amorphous state, high working temperature (∼TC) and low heat capacity (400 J/kg K) Therefore they are very well adapted for magne-tocaloric materials and:
(i) colossal magnetic entropy change,|Sm|, is discovered in
a large number of amorphous ribbons;
(ii) |Sm|maxoccurred at quite high temperature, which could
be controlled by substitution effect;
(iii) the|Sm|maxvalue has been obtained at a moderately low magnetic field change of 1.35 T and as a consequence, the studied samples could be considered as promising mag-netic refrigerant materials working in the high temperature region
Trang 4The authors express sincere thanks to Vietnam National
Fun-damental Research Program for financial support and this work
was supported by Korea Science and Engineering Foundation
through the Research Center for Advanced Magnetic Materials
at Chungnam National University
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