Journal of ELECTRONIC MATERIALS, Vol. 43, No. 1, 2014
`DOI: 10.1007/s11664-013-2725-6
`Ó 2013 TMS
`
`Effect of Mo Addition on Structure and Magnetocaloric Effect
`in c-FeNi Nanocrystals
`
`HUSEYIN UCAR,1,3 MARK CRAVEN,2 D.E. LAUGHLIN,1
`and M.E. MCHENRY1
`
`1.—Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA, USA.
`2.—Department of Biology, Carlow University, Pittsburgh, PA, USA. 3.—e-mail: hucar@andrew.
`cmu.edu
`
`Nanocrystalline powders of (Fe70Ni30)100xMox (x = 1 to 4) were produced by
`high-energy (SPEX) mechanical alloying. Increasing the Mo content was
`found to stabilize the face-centered cubic phase in mechanically alloyed
`nanopowders. To obtain a single c-phase, a powdered sample was solution
`annealed in the c-phase field and water quenched. The Curie temperature, TC,
`of the alloys was lowered with Mo addition, without decreasing the refriger-
`ation capacity (RC), due to the additional temperature broadening of the
`magnetic entropy change. Based on previous study on the role of disorder, the
`additional temperature broadening was attributed to increased positional
`disorder and changes in the distribution of ferromagnetic exchange bonds
`introduced by Mo addition into the c-FeNi system. (Fe70Ni30)97Mo3 and
`(Fe70Ni30)96Mo4 alloys have RCFWHM values of 440 J/kg and 432 J/kg at
`5 T, comparable to other prominent magnetic refrigerants operating near
`room temperature. The economic viability of these alloys, along with their
`competitive magnetocaloric properties and potential for scalable production,
`make them good candidate magnetic refrigerants without critical rare-earth
`materials.
`
`Key words: Nanostructured FeNi, magnetocaloric effect, magnetic
`refrigeration, mechanical alloying, ball milling
`
`INTRODUCTION
`
`The magnetocaloric effect (MCE) is defined as the
`temperature change of a magnetic material upon
`the application of a magnetic field. As the magnetic
`field increases entropy of the spin subsystem de-
`creases, it is balanced by an increase in the entropy
`of the lattice under adiabatic conditions. This in-
`crease in the lattice subsystem promotes heating of
`the material.1 The temperature rise due to losses is
`a limiting factor in magnetic components for high-
`frequency power electronics,2 and MCE can con-
`tribute to passive cooling to control heating in high-
`power conversion applications. The MCE has been
`investigated for various materials that operate at
`
`(Received April 24, 2013; accepted August 10, 2013;
`published online August 29, 2013)
`
`high temperatures. However, the trend in MCE re-
`search is shifting towards synthesizing magnetoca-
`loric materials for room-temperature applications
`with high efficiency and reduced cost, as well as
`trying to reduce or eliminate reliance on strategic
`rare-earth materials. To achieve this goal, transi-
`tion metals have been investigated to replace rare-
`earth metals for cost reduction. Researchers have
`been able to reduce cost through use of transition
`metals while obtaining magnetocaloric responses
`comparable to those of rare-earth metals. Recently,
`Fe-Ni alloys were suggested as economical alterna-
`tives3 whose RC could be tuned by alloying and the
`breadth of
`the magnetic
`transition controlled
`by
`impurity and disorder-derived distributed
`exchange interactions.4,5
`Our earlier study showed that binary FeNi with
`Fe70Ni30 stoichiometry has the highest magnetic
`
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`138
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`Ucar, Craven, Laughlin, and McHenry
`
`moment in the c-phase, with TC slightly above room
`temperature.6 The aim of this study is to show the
`effect of the c-stabilizer, Mo, on the structure and
`magnetic properties of c-FeNi nanocrystals. The
`possibility of tuning the Curie temperature (TC) of
`c-FeNi near room temperature by small additions of
`Mo is also discussed. Addition of Mo to the c-FeNi
`system also promotes high initial permeability and
`increased electrical resistivity.7
`
`EXPERIMENTAL PROCEDURES
`
`(Fe70Ni30)100xMox (x = 1 to 4) were
`Alloys of
`produced via ball milling. For simplicity, Fe70Ni30 is
`designated as FeNi and (Fe70Ni30)100xMox alloys
`from x = 1 to 4 are designated as Mo1, Mo2, Mo3, and
`Mo4, respectively. Elemental Fe (125-mesh particle
`size), Ni (100-mesh particle size), and Mo (100-mesh
`particle size) were mixed and sealed in a steel vial in
`Ar atmosphere. Ball milling was performed with a
`SPEX 8000 mixer/mill using hardened-steel vials
`and balls with ball-to-powder weight ratio of 10:1 for
`30 h. Previous studies indicated that 12 h is suffi-
`cient time to reach a steady-state microstructure.8
`After 30 h of milling, powders were characterized
`using a Philips X’Pert multipurpose diffractometer
`(MPD) working in continuous scanning mode with
`Cu Ka radiation (k = 0.154056 nm). Magnetic prop-
`erties were studied using a Lakeshore 7407 vibrat-
`ing-sample magnetometer (VSM) using a maximum
`applied field of 0.55 T. For low-temperature mag-
`netic properties, a physical properties measurement
`system (PPMS) with a VSM head was used instead
`with a maximum applied field of 5 T. The magnetic
`entropy change due to the application of a magnetic
`
`where DSM is the magnetic entropy change, M is the
`magnetization, and T is the temperature. The par-
`tial derivative is replaced by finite differences, and
`the integration is performed numerically from zero
`to the maximum value of the applied magnetic field.
`
`RESULTS AND DISCUSSION
`
`X-Ray Diffraction
`
`of mechanically alloyed
`Structural analysis
`(Fe70Ni30)100xMox powders was performed by x-ray
`diffraction (XRD). Figure 1 shows the XRD pat-
`terns, exhibiting both body-centered cubic (bcc) and
`face-centered cubic (fcc) phases for all Mo concen-
`trations. However, the fcc-to-bcc volume fraction
`ratio was found to increase with increasing Mo
`content.
`The fractions of fcc and bcc phases were deter-
`mined by comparing the intensities of the bcc(211)
`and fcc(220) peaks. Unlike the bcc(110) and fcc(111)
`peaks, the selected peaks do not overlap with each
`other, which would otherwise lead to more difficult
`interpretation of results. To correctly estimate the
`fractions of each phase, XRD peaks need to be cor-
`rected for Lorentz polarization, multiplicity, and
`structure factors.9 The equation for calculating the
`phase fraction of each phase is8
`
`Fig. 1. x-Ray diffraction patterns of as-milled (Fe70Ni30)100xMox (x = 0 to 4) alloys, labeled FeNi, Mo1, Mo2, Mo3, and Mo4, respectively. Inset:
`fcc(220) and bcc(221) peaks of as-milled (Fe70Ni30)100xMox (x = 0 to 4) alloys.
`
`field was calculated using a numerical approxima-
`tion to the equation
`
`
`
`
`
`@M
`@T
`
`dH;
`
`H
`
`(1)
`
`ZH
`
`max
`
`0
`
`DSM ¼
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`Effect of Mo Addition on Structure and Magnetocaloric Effect in c-FeNi Nanocrystals
`
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`

`
`as described by the virtual bound state model.11,12
`Virtual bound state and magnetic valence models13
`have been used ubiquitously in interpreting the
`moment reduction in crystalline and amorphous
`alloys14 due to early transition-metal addition. Even
`though the TC of the alloys systematically decreases
`with Mo addition, the trend does not seem to be
`followed for the FeNi and Mo1 alloys; i.e., the TC of
`Mo1 is slightly higher than that of FeNi. While the
`Mo alloys were synthesized consecutively in this
`study, the FeNi alloy was synthesized in a different
`batch, results of which were presented in our earlier
`study.6 Standardizing the parameters such as the
`temperature, the atmosphere, and most importantly
`the degree of contamination during milling is not a
`trivial task, which might account for the slight shift
`of magnetic properties for the alloys produced by
`mechanical alloying.
`Since Mo4 has the TC closest to room temperature,
`the magnetocaloric response of this alloy was also
`measured from 150 K to 480 K at field of 5 T using
`the PPMS (Fig. 3b). The magnetocaloric response of
`materials is compared using a parameter called the
`refrigeration capacity (RC). One well-known defini-
`tion of the RC is the product of the peak entropy
`change times the full-width at half-maximum
`(FWHM) of the peak DT, i.e., RCFWHM = DSpk
`DT.
`M
`According to this definition,
`the experimental
`RCFWHM of Mo4 is calculated to be 432 J/kg. For
`materials that did not cover sufficient area around
`the peak, e.g., Mo2 and Mo3, RCFWHM was estimated
`by extrapolating the experimental data to the tem-
`peratures required for this calculation (Fig. 3).
`Moreover, for the alloys lacking experimental data
`at high fields, e.g., Mo1, Mo2, and Mo3, the RCFWHM
`can be expressed as a power law,15
`RCFWHM ¼ AHn;
`where A is a prefactor. By extrapolating the exper-
`imental data to higher fields, one can compare the
`response of the alloy of interest to some benchmark
`refrigerants. The extrapolated and experimental
`values (in bold) of the alloys in this study are pre-
`sented in Table I with their peak temperatures.
`It can be concluded from Table I that Mo can be
`used as a means to tune TC without compromising
`RCFWHM. Even though the magnetic moment is
`slightly suppressed with addition of Mo into FeNi,
`the RCFWHM of the Mo alloys remains virtually the
`same. This is attributed to the fact that the mag-
`netic entropy curve broadens with Mo addition,
`which balances out the reduction resulting from
`the magnetic moment suppression. For maximum
`thermodynamic efficiency, a thermodynamic cycle is
`typically operated for a range of temperatures, so a
`large peak entropy is not necessarily desirable. This
`technique offers advantages over previously pub-
`lished routes designed to produce nanocrystal/
`amorphous nanocomposites18–20 with near-room-
`temperature TC values in terms of the size of the RC
`
`(5)
`
`fbcc
`ffcc
`
`¼ CF
`
`Ibcc
`Ifcc
`
`;
`
`(2)
`
`:
`
`(3)
`
`where fbcc and ffcc are the fractions of the phases,
`and Ibcc and Ifcc are the intensities measured from
`the experimental data. CF, the correction factor, can
`be calculated by dividing the theoretical intensity
`Itheoretical(bcc) by Itheoretical(fcc), thus
`CF ¼ ItheoreticalðbccÞ
`ItheoreticalðfccÞ
`
`
`!
`
`The theoretical intensity can be expressed as
`1 þ cos2ð2hÞ
`sin2ð2hÞ cosðhÞ
`
`Itheoretical ¼ Fj
`
`jp
`
`;
`
`(4)
`
`where F is the structure factor, p is the multiplicity,
`and the term in parenthesis is the Lorentz polari-
`zation factor. For the bcc(211) and fcc(220) planes,
`the CF was calculated to be 2.62. Following this
`procedure, the phase fractions were calculated and
`are plotted versus Mo concentration in Fig. 2.
`
`Magnetocaloric Properties
`
`response was calculated
`The magnetocaloric
`according to Eq. 1 using isothermal magnetization
`curves. The magnetic entropy changes, DSM, for the
`FeNi, Mo1, Mo2, Mo3, and Mo4 alloys are illustrated
`in Fig. 3 for a maximum applied field of 0.55 T, or
`5 T for the Mo4 alloy.
`From Fig. 3a, it is obvious that small additions of
`Mo into Fe70Ni30 decrease the TC of the alloy and
`the magnetic moment, which agrees well with pre-
`vious studies.10 There are two reasons for the
`reduction in magnetic moment with Mo addition.
`With addition of Mo, TC reaches near room tem-
`perature, which brings about reduction in the
`magnetic moment. Secondly, Mo decreases the spin-
`up electron density of the d-band state, n
`, in the
`FeNi system, which reduces the magnetic moment,
`
`"d
`
`Fig. 2. Fractions of bcc and fcc phases in as-milled (Fe70-
`Ni30)100xMox (x = 0 to 4) alloys, determined by x-ray diffractometry.
`
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`

`140
`
`(a)
`
`Ucar, Craven, Laughlin, and McHenry
`
`(b)
`
`Fig. 3. Temperature dependence of the magnetic entropy change, DSM, of (a) solution-annealed (Fe70Ni30)100xMox (x = 0 to 4) at 0.55 T (data
`for FeNi adapted from Ref. 6) and (b) solution-annealed (Fe70Ni30)96Mo4 at 5 T.
`
`Table I. Peak temperature (Tpk), peak magnetic entropy change ( DSpk
`M
`magnetocaloric refrigerants operating near room temperature
`
`

`
`

`
`), and RCFWHM values of promising
`
`Nominal
`Composition
`
`Gd5Ge1.9Si2Fe0.1
`Fe88Zr7B4Cu1
`(Fe70Ni30)99Mo1
`(Fe70Ni30)98Mo2
`(Fe70Ni30)97Mo3
`(Fe70Ni30)96Mo4
`
`Tpk (K)
`
`pk| (5 T)
`|DSM
`(J kg21)
`
`RCFWHM
`(5 T)
`
`pk| (0.5 T)
`|DSM
`(Exp)
`
`RCFWHM (0.5 T)
`(Exp)
`
`300
`300
`373
`353
`320
`300
`
`7.1
`3.31
`1.74
`1.70
`1.69
`1.67
`
`630
`654
`460
`445
`440
`432
`
`–
`–
`0.21
`0.19
`0.19
`0.18
`
`–
`–
`56
`53
`52
`50
`
`Ref.
`
`16
`17
`This study
`This study
`This study
`This study
`
`Experimental values in bold.
`
`and cost of the alloy. It also has advantages over
`chemical synthesis techniques21 in terms of the po-
`tential scalability of the process. Besides magnet-
`ocaloric applications, it may also offer possibilities
`for self-regulated radiofrequency (RF) heating for
`hyperthermia applications.22,23
`
`ACKNOWLEDGEMENTS
`
`H.U., M.E.M., and D.E.L. acknowledge support of
`the NSF through Grant No. DMR #0804020. M.C.
`acknowledges support of the REU Program Grant
`through DMR #1005076.
`
`CONCLUSIONS
`
`Ball milling of Fe, Ni, and Mo particles for 30 h led
`to alloy formation as indicated by structural data.
`Increasing the Mo content was found to promote fcc
`c-phase formation over the bcc a-phase. Mo was used
`to tune the TC of the FeNi alloy without changing the
`RCFWHM value significantly. The Mo3 and Mo4 alloys
`had peak temperatures of 320 K and 300 K, respec-
`tively, making these alloys appropriate for applica-
`tions operating near room temperature such as
`magnetic refrigeration and self-regulated hyper-
`thermia for cancer treatments. Even though the
`RCFWHM values of the Mo alloys are slightly lower
`than those of other important refrigerants (Table I),
`their attractive economic viability would make them
`preferable alternatives for large-scale production.
`
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