Temperature and field dependence of magnetic transitionsin the rare earth alloy Dy0.965Y0.035

Archana Lakhani n

UGC-DAE, Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452001, India

a r t i c l e i n f o

Keywords:Rare earth metals and alloysFirst order phase transitionPhase coexistenceMagnetization

a b s t r a c t

A systematic magnetization study on the field and temperature dependence of paramagnetic to spiral(second order) and spiral to ferromagnetic (first order) phase transitions has been carried out on thealloy Dy0.965Y0.035. The first order magnetic phase transition (FOMT) is confirmed by the hysteresis acrossTC where ferro and spiral phases coexist and the ferromagnetic phase grows with increase in magneticfield. An organized thermo magnetization study on this compound reveals interesting results includinggradual reduction of Néel temperature, enhancement of Curie temperature and a union of first andsecond order phase transitions seen at low fields into a direct transition from PM to FM phase avoidingthe FOMT at higher fields.

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1. Introduction

The magnetic materials having coupled structural and mag-netic transition are interesting materials for both practical appli-cations as well as fundamental studies. Dysprosium (Dy) is onesuch example among rare earth elements whose structural andmagnetic transitions occur simultaneously. Magnetically, Dy is aparamagnet at room temperature and shows a transition fromparamagnetic (PM) to antiferromagnetic spiral (S) state at Néeltemperature (TN) �180 K and to ferromagnetic (FM) state at Curietemperature (TC) �90 K when cooled in zero field [1]. Thetransition from PM to S phase is of second order in nature whileS to FM state is a first order magnetic phase transition (FOMT)[2,3]. This FOMT is associated with structural change from hex-agonal to orthorhombic structure below TC. The spontaneouschange in structure is due to the magnetoelastic coupling whichresults in the c- axis expansion of the Dy lattice [4]. Dysprosiumbeing susceptible to magnetic field and having interesting mag-netic properties like large magnetocrystalline anisotropy, complexstructure and having high localized moment; various magneto-transport and structural studies have been performed on thesingle crystals as well as on polycrystalline samples [5–9].

There are few studies on Dy alloys diluted with nonmagneticelements like Y and Lu [10–13]. One of these is an interesting studyon the time evolution of FOMT from spiral to ferromagnetic stateon the single crystals of Dy–4% Y alloy by Tajima et al. using

synchrotron X-ray diffraction and magnetization measurements[12,13]. However there are no organized magnetization andmagnetotransport studies on the lightly diluted Dy–Y alloys, whichreveal the systematic evolution of the various transitions especially theFOMT, influenced by temperature and magnetic field both.

Substituting the non magnetic Yttrium in Dy lattice changesthe strength of magnetoelastic coupling and hence the associatedmagnetic transitions [14]. TC varies drastically in comparison to TN.Up to 5% Yttrium substitution, the TC decreases significantly onincreasing the Y concentration whereas above 5% Y substitution,ferromagnetic transition is not observed [10]. In order to observethe effect of temperature and magnetic field on the magnetictransitions and explore the change in nature of the first ordertransition we have performed various thermomagnetization mea-surements on the polycrystalline alloy formed by 3.5% Yttriumdoped in Dy.

2. Experimental

The rare earth elements Dy and Y having purity better than99.9% were arc melted in stoichiometric amounts under highpurity argon atmosphere for preparing polycrystalline Dy and thecompound Dy0.965Y0.035. The samples were melted several times toensure homogeneity. No impurity phase was detected by X-raydiffraction study. Magnetization measurements as a function oftemperature and magnetic field are performed using a 14 T VibratingSample Magnetometer (14 T-VSM) in the Physical Property Measure-ment System (PPMS) from M/s. QD, USA. Magnetic measurements

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n Tel.: þ91 731 246 3913.E-mail addresses: archnalakhani@gmail.com, archnalakhani@csr.res.in

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are carried out in the temperature range of 2–300 K and in themagnetic field range of 0–10 T.

3. Results and discussions

Fig. 1 shows the temperature dependent magnetization in threedifferent measurement protocols viz; zero field cooled warming(ZFC), field cooled cooling (FCC) and field cooled warming (FCW)at 0.05 T for Dy and Dy0.965Y0.035 alloy (hereafter Dy–3.5% Y). Thesmall peak at �179 K is the Néel temperature (TN) at 0.05 T forDysprosium, where PM to S phase transition takes place; whichreduces to 175 K for Dy–3.5% Y alloy. Enlarged view of TN at 0.05 Tfor Dy and Dy–3.5% Y is shown in the inset of Fig. 1. A sharp rise inmagnetization and a clear hysteresis in cooling and heating cyclesof M(T) at 0.05 T indicate the first order nature of the spiral toferromagnetic transition at TC �90 K for Dy and at �71 K forDy–3.5% Y. It is clearly visible from these two M(T) curves that theFOMT is much broader for Dy–3.5% Y. The width of hysteresismeasured at the center of extreme values of magnetization at TCfor Dy sample is �7 K whereas for Dy–3.5% Y, it is �17 K. Ondilution with non magnetic impurity Y, the FOMT gets broader dueto the disorder influenced by Yttrium atoms in the Dy lattice. Inrecent times there have been various interesting studies ondisorder broadened first order phase transitions influenced bychemical inhomogeneities [15–16].

Fig. 2 shows magnetization as a function of temperature forDy–3.5% Y alloy at various fields during cooling and heating cyclesranging from 0.05 T to 2 T indicated on the respective curves. InDy–3.5% Y, the second order PM to S phase transition at TN shifts tolower temperatures with increase in magnetic field and vanishesabove 1.3 T as shown in Fig. 2. The drop in TN values with increasein field is shown in the inset-I of Fig. 2 by triangular symbols. Onthe other hand the first order spiral to ferromagnetic (FM)transition occurs at a lower temperature having TC �70.5 K at0.05 T. Here TC and TN are defined as the average temperaturevalues obtained by differentiating the FCC and FCW curves. Incontrast to TN, TC values increase on increasing the magnetic fieldgradually from 0.05 T to 2 T as shown by the circles in the inset-I ofFig. 2. The first order S to FM transition tends to merge with PM toS phase transition at 1.5 T and above. Inset-II of Fig. 2 shows theFCC and FCW curves at 1.5 T and 2 T clearly showing the absence of

two distinct transitions viz: PM–S and S–FM transitions at fieldshigher than 1.3 T.

The ferromagnetic (FM) and spiral (S) phases coexist across TCover a wide temperature range of �140–55 K at 0.05 T. Thecoexisting S and FM phases are also observed by X-ray diffractionpatterns in Dy–4% Y [12,13]. In conjunction with increase in TC onincreasing the field, the width of the hysteresis decreases and theFM fraction grows gradually with field. The system turns com-pletely ferromagnetic at low temperature as shown by isothermalmagnetization curves at 2 K and 10 K in Fig. 3. The technicalsaturation at 2 K is reached above 4 T.

MH isotherm at T¼180 K shows a paramagnetic behavior,which is above TN (�175 K) for Dy–3.5% Y. At To175 K the systemis in S phase at lower fields and remains in S phase until TC isreached; however the S phase transforms to FM phase on increasingthe field. At temperatures in between TN and TC i.e. in the range of175 K–70 K, the Dy–3.5% Y alloy shows a metamagnetic transition

Fig. 1. Magnetization as a function of temperature M(T) measured in ZFC, FCC andFCW protocols at 0.05 T for Dy and Dy0.965Y0.035. Inset shows enlarged view of PMto S phase transition at TN in FCC and FCW protocols.

Fig. 2. M versus T plots for Dy0.965Y0.035 sample during cooling (blue triangles) andheating (red circles) cycles in applied fields of m0H¼0.05, 0.1, 0.2, 0.5, 0.7, 1, 1.3 and2 T. Inset-I shows the variation of TC and TN with increasing magnetic field. Inset-IIshows the M(T) plots for 1.5 T and 2 T.

Fig. 3. The isothermal magnetization curves at various temperatures 2 to 180 Kafter cooling the sample in zero field to the respective temperature. Inset shows thefive quadrants of M–H isotherm at T¼2 K.

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from S to FM phase at the critical field (HC) varying with temperatureas shown in Fig. 3. A hysteresis is also observed in the forward andreverse cycles of field which reveals the first order nature of this fieldinduced transition; though, for the sake of clarity M–H isotherms foronly forward cycles are shown here. The FM phase increases onincreasing the field and HC increases with increase in temperature.This can be explained in terms of Landau's free energy diagram [2].At TC, the S and FM phases both have the same free energy separatedby a certain energy barrier known as ‘nucleation energy’. Thepresence of magnetic field reduces the free energy of ferromagneticphase and therefore the S–FM transformation takes place at a highertemperature and hence HC increases with increase in temperature.As a result of H–T induced FOMT, the co-existing S and FM phasesare observed in the H–T space.

One of the interesting features in the M–H isotherms is theobservation of a virgin magnetization curve being outside theenvelope curve as seen at 2 K in the inset of Fig. 3 for Dy–3.5% Y. Inearlier studies on disorder broadened FOMT materials like man-ganites, some intermetallic and shape memory alloys referred toas magnetic glasses, the virgin curve being outside the envelopecurve has been recognized as a signature of arrested kineticsacross the FOMT [17]. The virgin curve outside the hysteresis curveis prominently observed in the single crystal study of the Dy–4% Ystudied by Oguchi et al. [13]. The confirmation of the presence ofkinetic arrest in Dy–Y alloys requires a comprehensive study onDy–Y alloys.

Thus, this systematic magnetization study signifies three note-worthy phenomena viz. reduction of TN, enhancement of FM phasewith increase in TC and suppression of thermal hysteresis onincreasing the magnetic field. These results indicate a directtransition from PM to FM phase, avoiding the first order S to FMphase transition. This is evident from dM/dT curves shown inFig. 4, which shows two clear transitions at TC and TN form0H¼0.05 T while these two transitions are merged into a singletransition at m0H¼2 T.

4. Conclusion

A systematic magnetization study on Dy0.965Y0.035 alloy showsmainly three simultaneously occurring phenomena in differentranges of temperatures namely reduction of TN, enhancement of TCaccompanied by suppression of thermal hysteresis and growth ofFM phase with increasing field. This leads to the fusion of twotransitions seen at low fields into a direct transition from PM to FMphase avoiding the first order phase transition at fields above 1.5 T.

Acknowledgment

I thank A. Banerjee for the support. DST, India is acknowledgedfor funding the 14 T VSM-PPMS system used in this study. M.Gupta is acknowledged for X-ray diffraction measurements.

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Fig. 4. The differential magnetization of FCC and FCW curves as a function oftemperature for fields at 0.05 T and 2 T.

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