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The giant magnetocaloric effect between 190 and 300K in the

*Corresponding author.Tel.:+1-515-294-7931;fax:+1-

515-294-9579.

E-mail address:cagey@https://www.wendangku.net/doc/1f13263098.html,

(K.A.Gschneidner Jr.).

occur at B20K lower than room temperature,and therefore,new materials with a large magnetoca-loric effect between B280and300K are desired for future,near room temperature magnetic refrigeration applications.

The large magnitude of the MCE,i.e.the giant magnetocaloric effect,in the Gd5Si2Ge2and related alloys is associated with a reversible?rst order phase transformation,during which the ferromagnetic to paramagnetic transition is coupled with the crystallographic phase change from the orthorhombic Gd5Si4-type to the mono-clinic Gd5Si2Ge2-type crystal structure[3].Pe-charsky and Gschneidner[5]proposed a phase diagram for the pseudo-binary Gd5Si4–Gd5Ge4 system in the as-arc-melted state.A total of three solid solutions and a narrow two-phase region at 0.8o x o0.96were found in the Gd5Si x Ge4àx alloys.The Si-rich solid solution with the ortho-rhombic Gd5Si4-type crystal structure at 2.0o x p4.0,an intermediate intermetallic phase with the monoclinic Gd5Si2Ge2-type crystal struc-ture at0.96p x p2.0,and the Ge-rich solid solution with the orthorhombic Sm5Ge4-type crystal structure at0p x p0.8were reported.

All alloys from the Si-rich orthorhombic solid solution region(2.0o x p4.0)undergo a second order ferromagnetic to paramagnetic phase trans-formation on heating and exhibit a moderate MCE above B300K.The alloys with the room temperature monoclinic crystal structure (0.96p x p2.0)undergo a?rst order ferromagnetic to paramagnetic phase transition,which occurs simultaneously with the crystallographic phase change from the Gd5Si4-to the Gd5Si2Ge2-type crystal structure during heating.The Ge-rich orthorhombic alloys(0o x p0:8)have two phase transformations on heating:at low temperatures (B20to B120K)they rearrange magnetically and structurally via a?rst order phase transition where the ferromagnetic to antiferromagnetic phase change is coupled with the crystallographic transi-tion from the Gd5Si4-to the Sm5Ge4-type crystal structure,and at B125–B135K the materials undergo a second order antiferromagnetic to paramagnetic transformation.The alloys with the room temperature monoclinic(Gd5Si2Ge2-type) and orthorhombic(Sm5Ge4-type)crystal struc-tures exhibit the giant magnetocaloric effect around their respective?rst order phase transition temperatures;the latter are lower than B277K and vary as a function of the Si and Ge concentrations.In the single-phase regions,the unit cell volumes increase while the temperatures of the combined magnetic–crystallographic trans-formations decrease nearly linearly with the increasing Ge content[5],thus pointing to close relationships between the chemical composition, crystal structure,and magnetic properties of these complex metallic alloys.

Recently,a quite detailed phase diagram of the Gd5Si x Ge4àx system for alloys in the as-arc melted state has been reported by Pecharsky et al.[6].The three extended solid solutions reported earlier in Ref.[5]were con?rmed and the phase boundaries were also re?ned(only10ternary alloys were studied in1997leaving large compositional gaps uncharacterized,while more than30alloys were used in the2002study).The homogeneity range of the Gd5Si4-based solid solution covers the region 2.3p x p4.0;the monoclinic solid solution with the Gd5Si2Ge2-type crystal structure exists at 1.6o x p2.01,and the Gd5Ge4-based solid solu-tion forms at0p x p1.2.Rao[7]and Liu et al.[8] reported their versions of the Gd5Si4–Gd5Ge4 phase diagram for alloys in the as-prepared state. According to these authors,the Gd5Si1.6Ge2.4 alloy is included in the monoclinic Gd5Si2Ge2-type homogeneity range.However,according to Pe-charsky et al.[6],the as-arc-melted Gd5Si1.6Ge2.4 lies in a two-phase region between the Ge-rich orthorhombic and the monoclinic solid solutions. This disagreement may be related to the purity of Gd,which was used in alloy preparation:our unpublished data[9]indicate that interstitial impurities such as C and F may stabilize the monoclinic crystal structure at room temperature in the Gd5Si x Ge4àx alloys near x?1:5:Rao[7] and Liu et al.[8]used commercial Gd to prepare their alloys,and all commercial grades are inferior to Ames Laboratory Gd;the former have between 2and5at%interstitial impurities,primarily H,C and O,while the latter Gd contains less than0.05 at%each H,C,and O.

For the alloys from the intermediate phase region with the monoclinic crystal structure,the

A.O.Pecharsky et al./Journal of Magnetism and Magnetic Materials267(2003)60–6861

heat treatment at1570K for1h has been shown to enhance the value of the MCE considerably[6]. After the heat treatment,the MCE peak tempera-ture is lower than that for the as-arc-melted alloys by about5K.Pecharsky et al.[6]also pointed out the possibility that the extent of the phase regions in the Gd5Si x Ge4àx system can be modi?ed by using an appropriate heat treatment.Recently,we reported that heat treatment at different tempera-tures for different periods has a strong effect on the value of the MCE in the Gd5Si2Ge2alloy,and that the heat treatment at1570K appears to be the most suitable processing for the monoclinic alloy phases in the Gd5Si x Ge4àx system[10].

This paper reports the most recent results obtained during continuing studies of the Gd5Si x Ge4àx pseudo-binary system,including: the magnetic and thermodynamic properties,the existence of the giant magnetocaloric effect at room temperature,and the in?uence of heat treatment on both the phase relationships and properties of the monoclinic Gd5Si2Ge2-type solid solution region.

2.Alloy preparation and characterization

A total of15alloys in the Gd5Si x Ge4àx system with x varying from1.4to2.2were prepared from high purity components as described in Ref.[6];all of the samples were heat treated at1570K for different periods ranging from1to24h.The heat treatment was carried out in B10à6Torr vacuum using an induction furnace.After the heat treat-ment,the alloys were cooled by shutting the power to the furnace.The volume of the alloys for heat treatment did not exceed1cm3to provide fast cooling.

The alloys were examined by X-ray powder diffraction,magnetic,and calorimetric(heat capa-city)measurements.The X-ray powder diffraction data were collected at room temperature on an automated Scintag diffractometer using Cu K a radiation between20 and80 2y with data collection step2y?0:02 .The crystal structures of single-phase alloys were re?ned in the P1121/a space group symmetry in an isotropic approxima-tion using the Rietveld technique;in all cases,the

re?nement converged to R Bragg of the order of9%.

The magnetization isotherms,MeHTT;were

measured in a Lake Shore AC/DC magnetometer

(model7225)in the vicinity of the magnetic phase

transition temperatures in a DC magnetic?eld

varying from0to50kOe.Magnetization data

were collected with a B5K steps in temperature

and2kOe steps in the magnetic?eld beginning

from the lowest selected temperature.The mea-

surement sequence at each temperature was

carried out during a?eld increase beginning from

zero magnetic?eld,after sample temperature has

been stabilized and held constant for5–7min.

After completion of the?eld dependent measure-

ments at a speci?c temperature,the sample was

slowly warmed(B1.5K/min)to the next tempera-

ture in zero magnetic?eld.The heat capacity,

C PeTTH;of selected alloys was measured using a

semiadiabatic heat pulse calorimeter[11]during

heating from B4to350K in constant DC

magnetic?elds ranging from0to100kOe.Each

measurement sequence was begun after cooling the

sample to the lowest temperature(B3.8to B4.2K)in zero magnetic?eld followed by the application of a speci?c magnetic?eld.The

isothermal magnetic entropy change,àD S M;was

calculated from the isothermal magnetization and,

independently,from heat capacity data as de-

scribed by Pecharsky and Gschneidner[12,13].The

phase transition and the MCE maximum tempera-

tures were established from calorimetric and

magnetic measurements with the estimated accu-

racy of72and75K,respectively.

3.Results and discussion

According to the earlier report[6],the X-ray

diffraction patterns of six Gd5Si x Ge4àx alloys with

x ranging from 1.72to 2.01,which were heat

treated at1570K for1h,belong to the monoclinic

solid solution region and are single-phase materi-

als.When heat-treated for longer periods,i.e.,2–

4h,the result was a slight decrease in the values of

àD S M and a slight increase of the?rst order phase

transition temperatures.The X-ray powder dif-

fraction patterns of the alloys heat treated for

A.O.Pecharsky et al./Journal of Magnetism and Magnetic Materials267(2003)60–68 62

longer periods showed evidence of small amounts of the Gd(Si1ày Ge y)and Gd5(Si1àz Ge z)3phases, which have a different stoichiometry with respect to the ratio between the Gd and(Si+Ge)atoms, i.e.1:1and5:3,respectively.Both the magnetic and calorimetric results con?rm the appearance of impurity phases and lead to a conclusion that there are slow solid state reactions,which result in the decomposition of the intermetallic compounds with the5:4stoichiometry to the1:1and5:3 stoichiometries at temperatures near and above B1570K for1.72p x p2.01.

The remaining Gd5Si x Ge4àx alloys,with 1.4p x o1.72and2.01o x p2.2,were annealed at 1570K for1–24h.The X-ray powder diffraction data were collected after the heat treatment at 1570K for1–8h,depending on the composition. As shown in Table1,single-phase materials exist over the range1.5p x p2.1.Two of the15alloys, Gd5Si1.4Ge2.6and Gd5Si2.2Ge1.8,were predomi-nantly orthorhombic but contained detectable amounts of the neighboring monoclinic Gd5Si2Ge2-type phase when heat treated at 1570K for short periods of time(from1–2and 1–4h,respectively).Longer heat treatments (3–24h)resulted in single-phase Gd5Si1.4Ge2.6with the Sm5Ge4-type structure and single-phase Gd5Si2.2Ge1.8with the Gd5Si4-type structure.It appears,therefore,that the monoclinic solid solution in the Gd5Si x Ge4àx system extends from x?1:5to2.1at1570K.The lattice parameters of the monoclinic Gd5Si2Ge2-type single-phase alloys are shown in Fig.1,and they are in a good agreement with previously published data[5–8]. Fig.2illustrates the heat capacity of seven single-phase alloys measured in zero magnetic ?eld.All materials exhibit a?rst order phase transformation as evidenced by the characteristic behavior of the heat capacity near their respective phase transition temperatures.The phase transi-tion temperature systematically decreases with the increasing Ge content,which agrees with the results reported for the as-arc-melted monoclinic Gd5Si x Ge4àx[5].The peak values of the heat capacity for the alloys near both ends of the homogeneity range are noticeably lower,and the corresponding heat capacity anomalies are broad-er,when compared with the alloys in the middle of the same phase region.

A possible explanation of the observed broad-ening of the heat capacity anomalies is microscopic chemical inhomogeneities retained in the

Table1

The as-arc melted alloy stoichiometry and the phase compositions after heat treatment at1570K

Alloy stoichiometry Time of heat treatment at1570K,

hours

Phase composition a(structure type)

Gd5Si1.40Ge2.601–2Sm5Ge4+Gd5Si2Ge2

Gd5Si1.40Ge2.603–24Sm5Ge4

Gd5Si1.50Ge2.505Gd5Si2Ge2

Gd5Si1.52Ge2.483Gd5Si2Ge2

Gd5Si1.60Ge2.401Gd5Si2Ge2

Gd5Si1.72Ge2.281Gd5Si2Ge2

Gd5Si1.80Ge2.201Gd5Si2Ge2

Gd5Si1.95Ge2.051Gd5Si2Ge2

Gd5Si1.98Ge2.021Gd5Si2Ge2

Gd5Si2.00Ge2.001Gd5Si2Ge2

Gd5Si2.01Ge1.991Gd5Si2Ge2

Gd5Si2.02Ge1.982Gd5Si2Ge2

Gd5Si2.06Ge1.944Gd5Si2Ge2

Gd5Si2.09Ge1.917Gd5Si2Ge2

Gd5Si2.10Ge1.908Gd5Si2Ge2

Gd5Si2.20Ge1.801–4Gd5Si4+Gd5Si2Ge2

Gd5Si2.20Ge1.805–24Gd5Si4

a The majority phase,if two are listed,is shown?rst.

A.O.Pecharsky et al./Journal of Magnetism and Magnetic Materials267(2003)60–6863

heat-treated alloys upon rapid cooling.As follows from the X-ray powder diffraction and magnetic property data,the as-arc-melted samples with the stoichiometries close to the ends of the homo-geneity range contain both the monoclinic (the majority)and the orthorhombic (the minority)Sm 5Ge 4-or Gd 5Si 4-type phases for Ge-rich and Ge-poor compositions,respectively.We expect that chemical compositions of the majority phases are slightly different from the minority phases upon the solidi?cation,as was observed in the slowly solidi?ed large single crystalline grains of Gd 5Si 2Ge 2[14].During the annealing,the entire sample transforms into the monoclinic Gd 5Si 2Ge 2-type phase.From the similarity of both the Si and Ge,the differences in the respective chemical potentials are small and,therefore,driving force,which controls their redistribution in the solid state during short time heat treatments (see Table 1)is negligible.Hence,the microscopic chemical inhomogeneities are likely preserved in the annealed alloys when they are close to the ends of the single-phase region.Since the Curie temperatures in the monoclinic Gd 5Si 2Ge 2-type materials are strongly dependent on the Si to Ge ratio (e.g.,see Refs.[3,4]),this results in different fractions of the sample undergoing combined magnetic-crystallographic transformation at slightly different temperatures,which is manifested as the considerable broadening of the respective heat capacity anomalies.On the contrary,the alloys in the middle of the solid solution are quite homogeneous in the as-arc melted state because they only contain the single monoclinic phase and this results in much narrower and higher heat capacity peaks after the heat treatment.Related to chemical inhomogeneity is the following argu-ment:considering that the structural transition is

x (at.%Si)

1.5

1.6

1.7

1.8

1.9

2.0

2.1

L a t t i c e p a r a m e t e r (?)

Fig.1.The lattice parameters of the monoclinic Gd 5Si 2Ge 2-type alloys in the Gd 5Si x Ge 4àx system determined at room temperature after the heat treatment at 1570K.The angle g varies from 93.246(5) for x ?1:5to g ?93:176e5To for x ?2:1:

T (K)

050100150200250300

C p (J /g -a t K )

100

200

300

400

500

Fig.2.The heat capacity vs.temperature of selected mono-clinic alloys in the Gd 5Si x Ge 4àx system measured in a zero magnetic ?eld.

A.O.Pecharsky et al./Journal of Magnetism and Magnetic Materials 267(2003)60–68

coupled with the magnetic disordering on heating,another possible explanation of this behavior is the higher strain levels that develop in the alloys at the two ends of the homogeneity range when com-pared to those in the middle [15].

Fig.3shows the effect of the magnetic ?eld on the heat capacity of Gd 5Si 2.09Ge 1.91,which became monoclinic after it was heat treated at 1570K for 7h.The behavior is typical for ?rst order transformations exhibited by other alloys from the same monoclinic solid solution (e.g.,see Ref.[4]).With increasing magnetic ?eld,the maximum of the heat capacity is shifted to a higher temperature at a rate B 0.6K/kOe,which is similar to other related alloys, e.g.Gd 5Si 2Ge 2[1]and Gd 5Si 1.8Ge 2.2[16].The transformation remains a ?rst order phase transition in the 75kOe magnetic ?eld.The heat capacity anomaly is not seen in a 100kOe magnetic ?eld because the phase transi-tion in this magnetic ?eld occurs above 350K,

which exceeds the high temperature limit of our calorimeter.

The isothermal magnetization as a function of a magnetic ?eld in the vicinities of the respective transition temperatures was measured for all single-phase materials with the room temperature monoclinic crystal structure.It is shown in Fig.4for the two terminal compositions:Gd 5Si 1.5Ge 2.5(T C ?B 195K)and Gd 5Si 2.1Ge 1.9(T C ?B 297K).The metamagnetic-like behavior of the magnetiza-tion in the transition region is typical for all other monoclinic alloys in the Gd 5Si x Ge 4àx system.The isothermal magnetic entropy change (àD S M )was calculated from the magnetization isotherms for all of the single-phase alloys,and it is shown in Fig.5for a 0–50kOe magnetic ?eld change with a few omissions for clarity.The maximum value of àD S M systematically increases in a non-linear fashion with the decreasing Si concentration (see Fig.6)and all alloys exhibit the giant magnetocaloric effect.Most importantly,the two alloys,Gd 5Si 2.09Ge 1.91and Gd 5Si 2.1Ge 1.9,exhibit the giant magneto-caloric effect at and slightly above room temperature:the àD S M reaches 18and 16J/kg K at 292and 301K,respectively,for the 0–50kOe magnetic ?eld change.For comparison,high purity Gd exhibits àD S M ?11J/kg K at 293K for the same magnetic ?eld change [17].A second trend is clearly observed in àD S M as a function of temperature (and composition):although the maximum values of àD S M are lower for the Si-rich alloys,the width of the MCE peak increases and becomes nearly 30%broader for the Gd 5Si 2.1Ge 1.9when compared with the Gd 5Si 1.5Ge 2.5stoichiometry.It is,therefore,apparent that the overall cooling capacity (which can be estimated as the area under the àD S M peaks [18])of the monoclinic Gd 5Si x Ge 4àx alloys remains nearly constant regardless of their composition.

The behavior discussed in the previous para-graph may be understood from the following considerations.The magnetocaloric effect is di-rectly proportional to the derivative of the magnetization with respect to temperature at a constant magnetic ?eld,eq M =q T TH :Assuming that the magnetically ordered structures of the

T (K)

C P (J /g -a t K )

100

120

Fig.3.The heat capacity vs.temperature of the Gd 5Si 2.09Ge 1.91heat treated at 1570K for 7h measured in 0,10,20,50,and 75kOe magnetic ?elds.

A.O.Pecharsky et al./Journal of Magnetism and Magnetic Materials 267(2003)60–68

Gd 5Si x Ge 4àx alloys with 1.5p x p 2.1remain identical,the overall change of the bulk magneti-zation upon the transition from the magnetically ordered to the magnetically disordered states

should remain nearly constant (on per mole basis).However,since the transformation temperature increases from B 200to B 300K,the increased thermal ?uctuations of the crystal lattice broaden the FM -PM phase transition and reduce the corresponding |eq M =q T TH |thus systematically reducing the maximum àD S M :On the other hand,because the overall change of the magnetization is expected to be the same,so is the cooling capacity of these alloys.A small dependence of the àD S M (expressed in units J/kg K)on the composition (when the lighter Si is substituted for the heavier Ge)can be neglected because it corresponds to less than 3%reduction of the molecular weight when x changes from 1.5to 2.1.

Fig.7shows the transition temperatures determined from the locations of the heat capacity maximum (zero magnetic ?eld data),and the temperatures at which the maximum D S M (calculated from the magnetization isotherms for a 0–50kOe magnetic ?eld change)occur.The two sets of data are in excellent agreement.The temperature at which the maximum magnetocaloric effect is observed in the

H (kOe)0

M (e m u /g )

03060

120150

180H (kOe)

3040

030

120

150

Gd 5Si 1.50Ge 2.50

Gd 5Si 2.10Ge 1.90

Fig.4.The magnetization vs.magnetic ?eld of the Gd 5Si 1.5Ge 2.5and Gd 5Si 2.1Ge 1.9alloys heat treated at 1570K for 5and 8h,respectively,in the vicinities of their phase transition temperatures.

T (K)

180200220240260280300320

-?S M (J /k g K )

Fig.5.The MCE for a 0to 50kOe magnetic ?eld change calculated from magnetization data for selected monoclinic Gd 5Si 2Ge 2-type alloys in the Gd 5Si x Ge 4àx system.

A.O.Pecharsky et al./Journal of Magnetism and Magnetic Materials 267(2003)60–68

monoclinic Gd 5Si x Ge 4àx alloys increases with increasing Si content and reaches B 300K for Gd 5Si 2.1Ge 1.9.It is interesting to note that there is a tendency to a deviation from linearity at either end of the homogeneity range.This trend,once again may be the result of intrinsic compositional inhomogeneities expected to persist in the alloys near x ?1:5and 2.1.

4.Conclusions

As a result of this study we have shown that the intermediate intermetallic phase with the monoclinic Gd 5Si 2Ge 2-type crystal structure at room temperature can be retained at ambient conditions after short (1–8h)heat treatment at 1570K within the range of compositions from Gd 5Si 1.5Ge 2.5to Gd 5Si 2.1Ge 1.9.All alloys exhibit the giant magnetocaloric,which varies from 46J/kg K at 195K in Gd 5Si 1.5Ge 2.5to

16J/kg K at 301K in Gd 5Si 2.1Ge 1.9for the 0–50kOe magnetic ?eld change,and is associated with the coupled ?rst order magnetic–crystallo-graphic phase transformation occurring in the title alloys.The temperature,where the large magnetocaloric effect is observed can be easily adjusted between B 190and 300K by varying the chemical composition (i.e.the Si to Ge ratio)of the materials between Gd 5Si 1.5Ge 2.5and Gd 5Si 2.1Ge 1.9.Acknowledgements

This manuscript has been authored by Iowa State University of Science and Technology under Contract No.W-7405-ENG-82with the US Department of Energy.This research was sup-ported by the Of?ce of Basic Energy Sciences,Material Sciences Division of the US DOE.

x (at.%Si)

1.5

1.6 1.7 1.8 1.9

2.0 2.1

T T R f r o m -?S M (K )

Fig.7.Transition temperatures determined from the locations of the heat capacity maximum observed in zero magnetic ?eld data and the temperatures at which the maximum j àD S M j occur.

x (at.%Si)

-?S M (J /k g K )

Fig. 6.The maximum value of àD S M as a function of Si content (x ).The dashed line drawn through the data points is a guide for the eye.

A.O.Pecharsky et al./Journal of Magnetism and Magnetic Materials 267(2003)60–68

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