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## Creator

[Yasuyuki Shimura](https://orcid.org/0000-0002-2598-1277), [Ryoma Yokoo](https://orcid.org/0009-0009-0421-515X), [Kanta Watanabe](https://orcid.org/0009-0000-1195-5409), Hiroto Furuie, [Naohito Tsujii](https://orcid.org/0000-0002-6181-5911), [Kazunori Umeo](https://orcid.org/0000-0001-9442-4143), [Takahiro Onimaru](https://orcid.org/0000-0001-9990-3098)

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This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Yasuyuki Shimura, Ryoma Yokoo, Kanta Watanabe, Hiroto Furuie, Naohito Tsujii, Kazunori Umeo, Takahiro Onimaru; YbCo2: Large magnetic entropy change per volume in Yb-based metallic magnetic refrigerants for sub-Kelvin temperature. Appl. Phys. Lett. 8 September 2025; 127 (10): 102403 and may be found at https://doi.org/10.1063/5.0287094.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[YbCo2: Large magnetic entropy change per volume in Yb-based metallic magnetic refrigerants for sub-Kelvin temperature](https://mdr.nims.go.jp/datasets/9ca24899-ce1c-45bb-9b14-e1f254559cb7)

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YbCo2: Large Magnetic Entropy Change per Volumein Yb-based Metallic Magnetic Refrigerants for Sub-Kelvin TemperatureYasuyuki Shimura,1, a) Ryoma Yokoo,1 Kanta Watanabe,1 Hiroto Furuie,1 Naohito Tsujii,2 Kazunori Umeo,3 andTakahiro Onimaru11)Department of Quantum Matter, Graduate School of Advanced Science and Engineering, Hiroshima University,Higashi-Hiroshima 739-8530, Japan2)Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-2-1 Sengen,Tsukuba, Ibaraki 305-0047, Japan3)Department of Low Temperature Experiment, Integrated Experimental Support/Research Division, N-BARD,Hiroshima University, Higashi-Hiroshima 739-8526, Japan(Dated: 4 August 2025)A Yb-based intermetallic compound YbCo2 exhibiting a magnetic-field-induced order is known to show a giant specific heat divided by temperature, C/T ∼ 6.5 J/K2mol, around 0.3 K. We investigate the potential of this substance as the magnetic refrigerants by measuring the thermodynamic properties down to 0.1 K. The entropy change per volume by applying magnetic field of 3 T is found to be −∆SM ∼ 0.15 J/K cm3 around 1 K. This value is particularly large in the Yb-based metallic magnetic refrigerants to provide the sub-Kelvin temperature. In addition, we demonstrate that a 2.2 gram sample is cooled down to 0.26 K by the adiabatic demagnetization refrigeration from 1.8 K and 12 T. This cooling performance is attributed to the ground state, at zero field, locating around the boundary between the field-induced ordered phase and the paramagnetic one. Our study clarifies the utility of the sub-Kelvin magnetic refrigerants which combine the large −∆SM and the high thermal conductivity of metal.Adiabatic demagnetization refrigeration (ADR) is a tech-nique to cool down by demagnetizing the magnetic refriger-ants on the quasi-adiabatic environments. Since this techniqueneeds only a magnet and magnetic materials as the refriger-ants, we can reach the cryogenic temperature below 1 K with-out complex refrigerators using precious 3He. Though para-magnetic salts including 3d-transition metals have been usedas the cryogenic magnetic refrigerants for decades, it is notconvenient to use them because of the chemical instability1,2.In recent years, various stable magnetic refrigerants includ-ing rare-earth ions have been studied. The low magnetic-transition temperature TM in rare-earth ions retains the mag-netic entropy used for magnetic refrigeration around absolutezero.The performance as the magnetic refrigerants is evaluatedby two parameters of the adiabatic temperature change, ∆Tad,and the magnetic entropy change, −∆SM. The former is de-fined as ∆Tad = Ti −Tf, where Ti (Tf) indicates the initial (fi-nal) temperature before (after) demagnetization in the ADRexperiments. The value of −∆SM is evaluated as the differ-ence between the entropy under the zero field and the mag-netic field. We often need to mount the magnetic refrigerantson the narrow space in a refrigerator. In addition, the spatialarea where the magnetic field is applied is limited. Therefore,the magnitude of −∆SM per volume is of importance for thereal application.Main route to increase the amount of −∆SM is focusingon the Gd-based compounds since a Gd3+ has an entropyof ln8 = 3ln2 arising from the spin S = 7/2. This value is3 times as large as that in the ground state doublet realizedin most other rare-earth ions affected by the crystalline elec-tric field. Indeed, some magnetic refrigerants of Gd-baseda)Electronic mail: simu@hiroshima-u.ac.jpoxides/fluoride where TM is suppressed by the geometrical frustration effect exhibit large −∆SM exceeding 0.1-0.2 J/K cm3 3–9. Instead of such a large −∆SM, if we demand Tf be-low ∼ 0.1 K, the Yb-based oxides should be selected9–12.Another significant point to cool down to below 1 K is the thermal conductivity of the magnetic refrigerants because the thermal conductivity in the insulators mentioned above sharply decays as a power of temperature on cooling. In this point, Yb-based intermetallic compounds are advantageous as the cryogenic magnetic refrigerants to cool down to ∼ 0.2 K13–16. The strongly localized nature of 4 f electrons in the heavy rare-earth ion Yb3+ (4 f 13) results in the low TM related to Tf. Another advantage peculiar to the strongly correlated metals including Yb or Ce is that Tf can be further lowered with the chemical substitution by tuning the ground state to the quantum critical points where TM is absolute zero17–19.Thus, Yb-based intermetallic compounds are advantageous as the cryogenic magnetic refrigerants because of the low Tf and the high thermal conductivity. In this paper, we report the performance of YbCo2 as magnetic refrigerants. YbCo2 is one of the C15 Laves-phase compounds with the cubic MgCu2-type structure20. The specific heat divided by temperature, C/T , divergently increases on cooling and exhibits a maxi-mum at ∼ 0.3 K, where the value of C/T is ∼ 6.5 J/K2mol 21,22. A magnetically ordered state appears to set in at 0.3 K as inferred from the zero-field µ SR experiments. This giant C/T and the density of Yb, accounting for 1/3 in the con-stituent atoms, are expected to provide the sizable entropy per volume at zero field . Notably, YbCo2 exhibits a field-induced order above 1 T and 2 K21. In this field-induced phase, both moments in Yb and Co order magnetically22. Since the field-induced order takes away the entropy under magnetic field around absolute zero, enhancement of −∆SM is expected.Polycrystalline samples of YbCo2 were prepared from Yb ingots and Co powder. An open tungsten crucible containing2FIG. 1. (a) Electronic specific heat divided by temperature Cel/T un-der the magnetic field up to 8 T in YbCo2. The closed and open ar-rows show the drops in Cel/T on heating by the field-induced phasetransition and the peak, respectively. (b) Electronic entropy Se de-rived from Cel/T .them was encapsulated in a quartz ampoule under Ar atmo-sphere. This crucible in this quartz ampoule was heated upby a induction furnace. Eventually, YbCo2 were obtained byreacting the Co powder with the melted Yb for 20-30 minutes.The cubic MgCu2-type structure was confirmed by the X-raypowder diffraction. The ratio of Yb : Co in the sample were 1: 2.03(1) determined by the electron-probe microanalysis.Specific heat was measured down to 0.4 K by a 3He optionof PPMS (Quantum Design Co.) by the thermal relaxationmethod. Measurements at lower temperatures down to 0.1 Kwere conducted by using a laboratory-made calorimeter in-stalled in the commercial Cambridge Magnetic RefrigeratormFridge in the magnetic field up to 4 T. Magnetization wasmeasured down to 1.8 K and up to 5 T by MPMS (QuantumDesign Co.). We measured magnetization down to 0.4 K bya capacitive Faraday magnetometer installed in a 3He single-shot refrigerator (Heliox, Oxford Instruments)23,24. The ADRexperiments were performed by a laboratory-made ADR cellattachable to PPMS. The setup and the procedure of our ADRexperiments are described in Ref. [19].Figure 1(a) shows the electronic specific heat divided bytemperature, Cel/T , as a function of temperature in the mag-netic field up to 8 T. Cel is obtained as Cel = C−Cnuc −Cph,where C, Cnuc, and Cph are the raw data of the specific heat,nuclear, and phonon contributions, respectively. In Fig. S1 ofFIG. 2. (a) Time dependence of temperature in YbCo2 by ADR fromthe initial temperature of Ti = 1.8 K and the various initial fieldsof Bi = 1, 2, 3, 5, 8, 10, and 12 T. The inset shows the picture ofthe sample used in this ADR experiment. This sample was mounton the laboratory-made ADR cell attachable to PPMS19. (b) Finaltemperature Tf from Ti = 1.8 K as a function of Bi. Tf are determinedby the electronic entropy Sel in Fig. 1(b) (Open Circles) and theADR experiment (Closed Circles). The inset displays the adiabatictemperature change ∆Tad = Ti −Tf which is derived from Sel in Fig.1(b).the Supplementary Material, we plot C/T and Cph/T whereCph extract from Ref. [21] is the specific heat of the non-magnetic reference compound YNi2. In this paper, we focuson the low-temperature range below 6 K where Cph is ignor-able small, compared with Cel. In the magnetic fields up to4 T, we observed a convex downward curvature in C/T be-low ∼ 0.3 K as shown in Fig. S1. This curvature followingCnuc/T ∼ 1/T 3 is subtracted from C/T .As shown by the solid arrows in Fig. 1(a), a drop in Cel/Ton heating is observed above 1 T and 2 K. As observed anddiscussed in Ref. [21, 22]. this drop is due to the field-inducedphase transition whose phase boundary is plotted in the insetof Fig. 3(b). In addition, as indicated with the open arrows inFig. 1(a), a sharp peak suggesting another phase transition isobserved at 0.3 K at zero field. This peak is proposed to bethe magnetic order because the internal field is observed be-low 0.4 K in the muon spin resonance experiments22. Figure1(b) displays the temperature dependence of the electronic en-tropy Sel derived by integrating Cel(T )/T with respect to the3FIG. 3. (a) Magnetic field dependence of magnetization M downto 0.4 K up to 8 T in YbCo2. (b) Field derivative of magnetizationdM/dB. The peaks by the metamagnetic transition are indicated bythe arrows. The inset shows the magnetic phase diagram constructedfrom the peaks in dM/dB (squares), the peaks (open circles) and thedrops (closed circles) observed in Cel/T in Fig. 1(a). The dashedgreen line shows the phase boundary reported in Ref. [21].temperature. At zero field, Sel(T ) reaches R ln2 J/Kmol at ∼1 K resultant from the Kramers doublet of a Yb3+ ion. Asdescribed later, −∆SM and ∆Tad are obtained from Sel at zerofield and the magnetic field.Figure 2(a) displays the time dependence of the sampletemperature in the ADR experiments from Ti = 1.8 K withvarious initial fields up to Bi = 12 T. Demagnetization fromt = 0 was performed with a speed of 0.6 T/min. The YbCo2sample with 2.2 gram mass was mount on a Cu-stage whichis thermally insulated from the heat bath at 1.8 K as shownin the inset of Fig. 2(a). The temperature of the sample mea-sured by an attached RuO2 thermometer exhibits a minimumafter demagnetization. This minimum temperature is Tf fromTi = 1.8 K by ADR. In Fig. 2(b), we plot the Bi dependenceof Tf obtained from our ADR experiments and those evaluatedfrom Sel in Fig. 1(b). Here, Tf is defined as the temperatureat the intersection between Sel(T,B = 0) and a line parallelto the horizontal (temperature) axis from Sel(T = Ti,B ̸= 0).FIG. 4. The magnetic entropy change per volume −∆SM as a func-tion of temperature, determined by the magnetization M in Fig. 3and the electronic entropy Sel in Fig. 1. For comparison, we also plot−∆SM in the reported Yb-based metallic magnetic refrigerants forsub-Kelvin temperature under the magnetic field of 3 T13–16,25. InYbPt2Sn, the average of entropy at 2 T and 4 T is adopted as entropyat 3 T13.These two Tf evaluated from Sel and the ADR experimentswell overlap with each other. Tf decreases with increasing Biand reaches 0.26 K at 12 T. The adiabatic temperature change∆Tad = Ti −Tf evaluated from Sel is displayed in the inset ofFig. 2(b) as a function of Ti.Figure 3(a) and (b) display the magnetic-field dependenceof the magnetization M and the field derivative dM/dB downto 0.4 K, respectively. The value of M even above 5 T is ∼2 µB/Yb which is about half of 4.00 µB/Yb expected fromthe Yb3+ free ion without the crystalline-electric-field effect.dM/dB in Fig. 3(b) exhibits a peak due to the metamagnetictransition between 0.9 K and 3 K as shown by the arrows.In the inset of Fig. 3(b), we show the B− T phase diagramconstructed with the peak in dM/dB, the drop due to the field-induced phase transition above 2 K, and the peak below 0.5K in Cel/T of Fig. 1(a). This phase boundary of the field-induced phase transition is almost consistent with the reportedone22. Notably, absence of the peak in dM/dB below 0.5 Ksuggests this phase boundary reaches the finite temperature inthe zero magnetic field.From two different measurements of specific heat and mag-netization, −∆SM individually can be evaluated as shownin Fig. 4. Through the Maxwell relation of (∂S/∂B)T =(∂M/∂T )B, −∆SM as a function of B at a temperature T =(T1 +T2)/2 is derived from a pair of the isothermal magneti-4zation data at two temperatures of T1 and T2 as,−∆SM(B,T ) =−∫ B0dM(B′,T )dTdB′∼=−∫ B0M(B′,T1)−M(B′,T2)T1 −T2dB′.−∆SM(T ) evaluated from the magnetization in Fig. 3(a) isplotted by the open circles in Fig. 4. Sel(T ) obtained from thespecific heat data in Fig. 1 also provides −∆SM(B) = Sel(B =0)− Sel(B) as a function of temperature. At 3 T, −∆SM(T )derived from the specific heat and magnetization data exhibitsa peak around 1 K, where the value of −∆SM is ∼ 0.15 J/Kcm3. This peak goes to the higher temperature side with in-creasing magnetic fields. For comparison, we plot −∆SM(T )at 3 T in the reported Yb-based metallic magnetic refrigerantsto provide sub-Kelvin temperature13–16,25. The coexistence ofthe largest −∆SM(T ) of YbCo2 among them and Tf below 0.3K as shown in Fig. 2 indicate the utilities as the cryogenicmagnetic refrigerants. As the magnetic refrigerants exhibitingTf below ∼ 1 K and evidently exceeding −∆SM ∼ 0.15 J/Kcm3 around 3 T, some rare-earth oxides/fluorides of the insu-lators have been reported3,5,6,8. Notably, −∆SM ∼ 0.2 J/K cm3has been reported in a Er(4 f 11)-based intermetallic compoundErPd2Sb26.Materials containing high-density magnetic atoms likeYbCo2 are expected to possess a large −∆SM per volume.However, in general, since the strength of the magnetic in-teraction or magnitude of TM is correlated with the densityof magnetic ions, such a large −∆SM competes with the lowTM ≈ Tf below 1 K. The field-induced order in YbCo2 is suf-ficiently suppressed by decreasing the magnetic field to zero,as shown in the inset of Fig. 3(b). In other words, YbCo2 atzero field is located around the boundary between the mag-netically ordered phase and the paramagnetic disordered oneas the magnetic quantum critical point. This suppression ofTM results in the low Tf even in the high-Yb-density envi-ronment. Indeed, the minimization of Tf is observed in thevicinity of the boundary between magnetically ordered phaseand paramagnetic disordered one by substituting Cu with Niin an antiferromagnet Ce2Cu2In19. In addition, since emer-gence of field-induced order in YbCo2 suppresses the entropyunder the magnetic field Sel(B ̸= 0), this field-induced ordercontributes to increase of −∆SM = Sel(B = 0)− Sel(B ̸= 0).Thus, the large −∆SM(T ) can coexist with the low Tf below ∼1 K because the ground state at zero field is located around theboundary between the field-induced ordered phase and para-magnetic one.In conclusion, we report the performance of a Yb-basedintermetallic compound YbCo2 as the magnetic refrigerantsfor cooling down to sub-Kelvin temperature by measuring thespecific heat and magnetization, in addition to the ADR exper-iments. The magnetic entropy change −∆SM(T ) per volumeexhibits a maximum in B = 3 T at ∼ 1 K, where the value of−∆SM is ∼ 0.15 J/K cm3. This value is particularly large inthe reported Yb-based intermetallic magnetic refrigerants forsub-Kelvin temperature. The large −∆SM(T ) and the low Tfbelow 0.3 K are attributed to the ground state, at zero field, lo-cating around the boundary between the field-induced orderedphase and paramagnetic one.In Supplementary Material, we show the raw data of thespecific heat before subtracting the nuclear and lattice contri-bution.This work was financially supported by the JST FORESTProgram (Grant Number JPMJFR2233) and JSPS KAKENHI(Grant Numbers JP22K03529, 23H04866, 23H04870, and24K21692). YS thanks the Japanese research grants from"The Kyoto Technoscience Center" (No. 8 in 2024), and"The Mazda Foundation" (No. 21KK-191). The measure-ments at the cryogenic temperature and composition analysiswere performed at N-BARD in Hiroshima University. We ac-knowledge for the useful comments by Yoshifumi Tokiwa andToshiro Takabatake.AUTHOR DECLARATIONSCONFLICT OF INTERESTThe authors have no conflicts to disclose.AUTHOR CONTRIBUTIONSYasuyuki Shimura: Conceptualization (lead); Investiga-tion (supporting); Data curation (supporting); Writing - orig-inal draft (lead); Writing - review & editing (equal); Supervi-sion (lead). Ryoma Yokoo: Investigation (lead); Data cura-tion (lead); Writing - review & editing (equal). Kanta Watan-abe: Investigation (supporting); Data curation (supporting);Writing - review & editing (equal). Hiroto Furuie: In-vestigation (supporting). Naohito Tsujii: Conceptualization(supporting); Writing - review & editing (equal). KazunoriUmeo: Investigation (supporting); Writing - review & edit-ing (equal). 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