\PERGAMON International Journal of Hydrogen Energy 13 "0888# 106Ð108

9259!2088:88:,08[99 Þ 0888 International Association for Hydrogen Energy[ Published by Elsevier Science Ltd[ All rights reservedPII] S 9 2 5 9 ! 2 0 8 8 " 8 7 # 9 9 9 3 2 ! 2

Magnetic anistropy of YFe00Ti single crystal and its hydrideS[A[ Nikitina\ I[S[ Tereshinaa\ V[N[ Verbetskyb\�\ A[A[ Salamovab

a Faculty of Physics\ Moscow State University\ 008788 Moscow\ Russiab Faculty of Chemistry\ Moscow State University\ 008788 Moscow\ Russia

Abstract

The magnetocrystalline anisotropies of YFe00Ti and its hydride are studied by torque and pendulum magnetometerin the temperature range 3[1Ð699 K in magnetic _elds up to 02 kOe both on single crystals and magnetically alignedpowder samples[ The temperature dependence of the anisotropy constants K0"T# and K1"T# is determined[ The Curietemperature\ saturation magnetization and easy c!axis anisotropy are increased by hydrogenation[ Þ 0888 InternationalAssociation for Hydrogen Energy[ Published by Elsevier Science Ltd[ All rights reserved[

0[ Introduction

Rare!earth intermetallic compounds RFe00Ti havebeen attracting interest as candidates for permanent mag!nets because of fairly high values of Curie temperature\saturation magnetization and magnetocrystalline ani!sotropy ð0\ 1Ł[ Recently\ interstitial atoms such as H\ Nand C have been introduced into the crystal to modifythe magnetic properties ð2\ 3Ł[ To understand this e}ect\ itis important to examine the properties of the Fe sublatticemagnetism[ The purpose of the present paper is a detailedstudy of magnetocrystalline anisotropy of YFe00Ti singlecrystal and its hydride[

1[ Experimental

The YFe00Ti alloy was prepared by induction meltingunder argon atmosphere of constituent elements of purityat least 88[84 wt)[ The ingot was remelted for homo!genization[ X!ray di}raction was used to control the sin!gle phase in samples[ The ingots were preheated andcooled slowly in order to increase the grain size[ Thesingle crystals were extracted from the solidi_ed ingotsand were oriented by the conventional back Laue re~ec!tion method[ For investigation of the YFe00Ti interactionwith hydrogen\ a procedure was employed as described

� Corresponding author[ E!mail] magÝrem[phys[msu[su

in Ref ð4Ł[ Magnetic measurements were made on singlecrystal and the magnetically aligned powder samplesusing a torque and pendulum magnetometer in the tem!perature range 67Ð699 K in magnetic _elds up to 02 kOe[The magnetization curves of YFe00Ti single crystal werealso measured in impulse _elds up to 099 kOe using theinduction method in the temperature range 3[1Ð299 K[

2[ Result and discussion

Figure 0 shows the temperature dependence of satu!ration magnetization ss for the YFe00Ti single crystalspecimens and its hydride[ It is noted that for ss forYFe00Ti is much smaller than YFe00Ti H "see Table 0#[Table 0 also lists the lattice parameters "a and c# andunit!cell volume V at T � 299 K for YFe00Ti and itshydride[ The unit!cell volume expansion is 9[8) uponhydrogenation[ The increase of the saturation mag!netization of the hydride results from the volume expan!sion[ Thermomagnetic analysis was used to measure theCurie temperature Tc[ The Tc � 427 K for YFe00Ti andTc � 599 K for YFe00Ti H "Fig[ 0\ inset#[ The Y2¦ is non!magnetic and the Curie temperature of this compound isdetermined by the FeÐFe exchange interactions[ It canbe seen from the experimental results\ that the FeÐFeexchange interactions increase upon hydrogenation[ Theexchange coupling constants were obtained earlier in Refð3Ł[

Figure 1 shows the experimental torque curve L"8#\where 8 is the angle between H and the c!axes\ for aligned

S[A[ Nikitin et al[ : International Journal of Hydrogen Energy 13 "0888# 106Ð108107

Fig[ 0[ Temperature dependence of saturation magnetization s

for 0*YFe00TiH^ 1*YFe00Ti single crystal[ Inset] ther!momagnetic curves of YFe00Ti and its hydride[

Table 0Lattice parameters a and c\ unit!cell volume V\ second!order anisotropy constant K0 and saturation

magnetization ss at T � 299 K for YFe00Ti and its hydride

a ð_Ł c ð_Ł V ð_2Ł c:a K0×096 ss

ðerg cm−2Ł ðemu:gŁ

YFe00Ti 7[498 3[672 235[5 9[45100 9[74 019YFe00TiH 7[436 3[675 238[5 9[44885 0[14 027

Fig[ 1[ Observed torque curves L obtained in an applied _eld of 02 kOe for 0*YFe00TiH^ 1*YFe00Ti aligned powder samples atT � 299 K[ Inset] magnetization curves of YFe00Ti ð2\ 3Ł and its hydride ð0\ 1Ł along and perpendicular to the alignment direction atT � 299 K[

powder samples YFe00Ti and YFe00TiH at T � 299 K[The shape of the torque curve is a typical uniaxial type[The easy axis is c axis[ We have also obtained the torquecurves for the "009# disk of YFe00Ti single crystal atdi}erent temperatures[ Anisotropy constants K0 and K1

were determined by using a procedure as described inRef[ ð5Ł[ The magnetization curves of the aligned sampleswere measured in magnetic _elds applied parallel andperpendicular to the alignment direction for YFe00Ti andYFe00TiH at T � 299 K "Fig[ 1\ inset#[ Anisotropy con!stants K0 and K1 were determined by using Sucksmith|smethod ð6Ł[

The temperature dependence of anisotropy constantsK0 and K1 for the YFe00Ti single crystal is presentedin Fig[ 2 "open circles] SucksmithÐThompson method\closed circles] method of analysis of the corrected torquecurves#[ The magnetic anisotropy constant K1

"K1 � 9[985×096 erg cm−2 at T � 3[1 K# is negligible

S[A[ Nikitin et al[ : International Journal of Hydrogen Energy 13 "0888# 106Ð108 108

Fig[ 2[ Experimental temperature dependence of the magneticanisotropy constants] 0\ K0^ 1\ K1 for YFe00Ti single crystal "opensymbols*SucksmithÐThompson method\ and _lled symbols*method of analysis of the corrected torque curves#[ Inset] 0*K0"T#:K"9# approximations by 1*calculated curve for a local!ized model ð7Ł[

compared to K0 "K0 � 0[829[0#×096 erg cm−2 at T � 3[1K#[ The decrease of K0 with increasing temperature isfaster than the predicted theory of localized magneticmoment system\ namely K0"T#:K0"9# � ðMs"T#:Ms"9#Ł2

"Fig[ 3 inset# ð7Ł[

Fig[ 3[ Dependence of anisotropy constants K0 vs the T:Tc for]0*YFe00TiH^ 1 YFe00Ti[

Figure 3 shows the dependence of anisotropy constantsK0 vs the T:Tc for YFe00Ti and its hydride[ It is notedthat hydrogenation enhances the easy c!axis anisotropyof the YFe00Ti compound[ From pure crystal! _eld con!sideration the second!order anisotropy constant K0 isexpected to be proportional to "0−a"c:a#1# ð8Ł[ Com!paring the experimental data with calculated data showsthat the change of c:a ratio and saturation magnetizationby hydrogenation cannot adequately explain the changeof value of second!order anisotropy constant K0[

Interstitial hydrogen e}ect on the crystal _eld par!ameters is complex[ Hydrogen atoms are located at theYTi1Fe tetrahedral sites ð3Ł[ From experimental data"Table 0#\ it follows that introduction of H increases thelattice parameters and the unit!cell volume[ This leads tothe narrowing 2d band ð3Ł[ The change of local environ!ment of Fe ions resulting from hydrogenation makesvalency bonds between Fe ions weaker and rearrangeselectron density of valency electrons ð09Ł[ These e}ectsare probably responsible for the signi_cant increase ofmagnetic anisotropy in YFe00TiH which is reported inthis paper[

Acknowledgements

This work was supported by the Russian Foundationfor Fundamental Science "85!91!07160#

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