Pressure effect on the magnetic transition temperature of Sm1−xNdxMn2Ge2

journal of lnetism ‘1’; ELSEVIER

Journal

of Magnetism

and Magnetic

Materials

magnetic erials

132 (1994) 67-70

Pressure effect on the magnetic transition temperature of Sm,_,Nd,Mn,Ge, R DuraJ a,*, M kraj

a, A Szytula b

aZnstztute of Physzcs, Technzcal Unzuerszty of Cracow, ul Podchorqrych ’ Znstztute of Physzcs, Jagzellonzan Unzuerszty, ul Reymonta

I, 30-084 Cracow, Poland 4, 30-059 Cracow, Poland

(Received 20 October 1993)

Abstract Magnetic properties of Sm,_,Nd,Mn,Ge, are reported The pressure effect on the magnetic phase transition of Sm,,Nd, ,Mn,Ge, has been measured m order to study the atomic distance dependence of the Mn-Mn mteractlon The value of dT,/dP, dT,/dP, dT,/dP, dT,/dP are found to be -0 8 K/GPa, -4 4 K/GPa, - 220 K/GPa, 176 K/GPa, respectively The (P, T) diagram has also been determined The results indicate that the magnetic phase transItIon 1s strongly correlated with the mteratomlc distances

1. Introduction Intermetalllc compounds RMn,X, where R 1s a rare earth and X 1s Sl or Ge crystallize m the body-centered tetragonal ThCr,Sl, type structure (space group 14/mmm) which can be described as a stackmg of atomic layers m the direction of the c-axe with sequence R-X-Mn-X-R In RMn,Ge, compounds at temperatures above 100 K, the magnetic moments of Mn atoms order ferromagnetlcally for light R (La-Nd), except for Sm, and antlferromagnetlcally for heavy R (EuLu), respectively There 1s a critical value of %-Mn = 0 285 nm for couplmg between the mterlayer Mn moments An antlferromagnetlc order appears for Rbn_Mn< 0 285 nm and a ferromagnetic one for Rhn_Mn> 0 285 nm [l]

* Correspondmg

author

0304-8853/94/$07 00 0 1994 Elsevler SSDZ 0304-8853(93)E0669-4

Science

At room temperature the Rhn_Mn distance m SmMn,Ge, 1s close to the crltlcal value, separatmg antlferro- and ferromagnetic phases Magnetic characterlstlcs of SmMn,Ge, show that the sample 1s ferromagnetic m the temperature range of 153 K < T < 341 K, and becomes antlferromagnetic for 106 5 K < T < 153 K and a reentrant ferromagnetlsm appears below 106 5 K [2] The easy axis of magnetic moments 1s parallel to the [llOl direction at low temperatures, while at high temperatures it 1s parallel to the [OOl] direction [3,41 NdMn,Ge, is a ferromagnet with Curie temperature T,= 336 K The spontaneus magnetlzatlon of NdMn,Ge, 1s perpendicular to the tetragonal axis below 250 K and parallel to that axis above 250 K [5] The experimental results indicate that both the external pressure and pressure stimulated by chemical substltutlon give a similar effect for Sm,_,R,Mn,Ge, (R = Y, Gd) [6,71 Therefore,

B V All rights reserved

R Aura] et al /Journal

68

of Magnetwn and Magnettc Materials 132 (1994) 67-70

it IS Interesting to examme the hydrostatic pressure effect and substitution of Sm atoms by Nd atoms on magnetic transition temperatures of Sm,_,Nd,Mn,Ge, The present paper reports the results of mvestlgatlon of temperature, pressure and composltlon dependence of the Sm,_,R,Mn,Ge, system obtained by magnetometric and X-ray diffraction methods

2. Experiment

and results

The samples were prepared by a combmatlon of arc melting and solid state diffusion techniques The purity was 3N for Sm and Nd, 4N for Mn and 5N for Ge The samples were melted and annealed for 100 h at 800°C and then cooled down to the room temperature

V[~lO-~nm] -i

1850 1830 181 0

c/c I f 2 653 _m,

I

Ll79

-+__L

2 68

---+--___

2 67

---f

c[nm] 1 096

2.66 1

_- /- _-/ _Cr

i

i

x

a

[nml

c [nml

0 01 02 04 06 10

0 0 0 0 0 0

4062 4065 4076 4079 4089 4103

10895 10899 1 0900 10932 1 0929 10939

(1) (1) (1) (1) (3) (5)

(2) (2) (3) (3) (3) (3)

compounds

c/a

V [nm3

2681 2 681 2 674 2 679 2 673 2 666

1797 1800 181 1 181 9 182 7 184 2

X 10m3]

X-ray diffraction studies were carried out usmg the DRON-3 diffractometer with CoK, radlatlon At room temperature, the X-ray diffraction pattern of all samples consist of peaks characteristic for the ThCr,Sl, type structure only The lattice parameters were derived usmg the leastsquares method with the Nelson-Riley extrapolation function The concentration dependence of the lattice constant, unit cell volume and c/a ratio are shown m Fig 1 and Table 1 The lattice constant variation with composltlon obeys Vegard’s law, suggesting no change m the bonding character when Sm atoms are replaced by Nd atoms Magnetometric measurements were carried out m the temperature range SO-500 K by means of the modified Faraday method with a maximum external field of 1 T The values of T, and magnetic moments p (at T = 80 K and B = 1 T) for 0 1
: 1 090

1

.' / -t

_+ ’ ’

Fig 1 Varlatlon with concentration and c, the cell volume and the c/a for Sm,_,Nd,Mn,Ge,

1 094 1092

a[nm]fy~-[exxx 0410

0

Table 1 Crystal data for Sm,_,Nd,Mn,Ge,

1 088

’

of the lattice constants a ratio at room temperature

Table 2 Magnetic properties T=80Kand B=l

of Sm,_,Nd,Mn,Ge, T)

compounds

[CLBI

x

T, [Kl

P

01 02 04 06 10

344*1 336+1 326+1 325+1 336+1

33+01 43*01 42+01 42kO 44*01

1

(at

R DuraJ et al /Journal

of Magnetrsm and Magnetic Materrak 132 (1994) 67-70

69

Smo gNdo 1Mn2Ge2 l234567-

Fig 2 Magnetic

susceptlbdlty

,yac vs temperature

external hydrostatic pressures up to 0 6 GPa The UNIPRESS compressor of IF-012A type was used and gaseous hehum was the pressure transmitting medmm The voltage induced m the pick-up coils was measured by a selective nanovoltmeter The whole experiment was performed under constant pressure The samples were heated and cooled with a speed of 30 K/h The temperature was

’ F, \T, 52..,.,,,.,I,,,,,‘00

01

02

AF 05

06

Fig 3 (P, T) phase diagram data for Sm,,Ndo lMn,Gez, F2 - ferromagnetic phases, AF - antlferromagnetlc phase

F,,

for Sm o,Nd,

0 0 0 0 0 0 0

GPa 2 GPa 25 GPa 3 GPa 35 GPa 47 GPa 55 GPa

,Mn,Ge,

at various

pressures

determined by the use of a copper-constantan thermocouple placed m direct contact with the sample Fig 3 shows the (P, T) magnetic phase diagram for sample Sm,_,R,Mn,Ge,

3. DiscussIon At high temperatures ferro- or antlferromagnetlc ordermg m the Mn sublattlce 1s strong and depends crltlcally on the Mn-Mn mtralayer dlstance The calculation of an electronic structure of (Y, La)Mn,Ge, mdlcates an itinerant character of magnetism for these compounds [8,9] At room temperature the R&n_Mn distances m Sm 1_,R,Mn,Ge2 are greater than the critical value, separatmg antlferromagnetlc and ferromagnetic phases, so all these samples are ferromagnets below T, These experiment results show that temperature CT), pressure (PI, and content (x1 induce the changes of lattice constants, and that magnetic propertles of Sm,_,R,Mn,Ge, are very sensitive to bond lengths (or mteratomlc dlstances or overlap between atoms or hybndlzatlon, 1 The substltutmg rare earth atoms have dlf-

70

R Aura] et al /Journal

of Magneturn and Magnetic Materials 132 (1994) 67-70

ferent atomic radn R(Sm3+) = 0 1804 nm, R(Nd3+) = 0 1821 nm The substitution causes an increase m lattice parameters for Sm, _xRXMn, Ge, with the constant number of valence electrons and unchanged magnetic Mn sublattice m the former phases (negative chemical pressure effect) Sm, ,Nd, ,Mn,Ge, represents a very mterestmg case The observed temperature dependence of the susceptlblhty at P = 0 GPa (Fig 2) shows, that there are two different ferromagnetic regions below T, = 147 K and 147 K < T < 344 K Above 344 K the sample 1s paramagnetlc The appearance of the cusp on X-T curves at T, = 147 K corresponds to the spin-flop transItion which was observed for NdMn,Ge, [5] The mfluence of the external magnetic field on the magnetic phase transition at T, = 147 K 1s observed External pressure causes a decrease m lattice constant and a change of magnetic properties of Sm,,Nd, ,Mn,Ge, (Fig 3) For P r 0 25 GPa the antlferromagnetlc order appears and two metamagnetlc phase transitions F, Tl AF and AF Tz F, are observed The temperature of the ferro-antlferromagnetlc phase transition mcreases for the first phase transition (dT,/dP = 176 K/GPa) and decreases for the second one with pressure (dT,/dP = - 220 K/GPa) The Curie temperature decreases slowly with pressure The signs of dT,/dP (dT,/dP= -0 8 K/GPa) of the ferromagnetic compounds (PrMn,Ge, [lo], NdMn,Ge, [lo], SmMn,Ge, [2,41) are also negative (- 19 K/GPa, - 2 0 K/GPa, - 6 0 K/GPa, - 11 K/GPa, respectlvely) A similar dependence of the critical temperatures T,,T, and the Curie temperature T,ISobserved for Sm, _,Gd,Mn,Ge, [6] and Sm, _,Y,Mn,Ge, [7] For P = 0 28 GPa the observed temperature dependence of the ac susceptlblhty, and values T,,T2,and T, are similar as for the SmMn,Ge, samples This suggests that m this case, external pressure may counterbalance the negative chemlcal pressure effect A small sharp peak seen at 150 5 K on the slope of magnetization vs temperature curve for SmMn,Ge, [21 can be explained If we consider the results obtained for Sm,,Nd, ,

Mn,Ge, for different pressure Thus, the AF Tz F, first order magnetic phase transition m Mn sublattice, associated with dlscontmous change m structural parameters IS probably connected with the change of easy axis of magnetlzatlon at TK The spin-flop temperature transitIon TK decreases with pressure almost lmeary with dT,/dP = -4 4 K/GPa The pressure effect of the spm-flop transition temperature T, for NdMn,Ge, was measured by Kawashlma et al [lo] and the value of dT,/dP was found to be -33 K/GPa At low temperatures the magnetic moments of the Sm and Nd sublattice are ordered, and for light rare earths the ferromagnetic couplmg between the rare earth and Mn sublattice appears The easy axis IS m the base plane The change of the easy axis of magnetization for (Nd, Sm)Mn,Ge, can be explained as the result of competltlon between R and Mn sublattice amsotroples [51, so the present results of the pressure effect on magnetic phase transitions m Sm,, Nd, ,Mn,Ge, are consistent with this fact

References

[11A

Szytuta and J LecleJewlcz, III Handbook on the Physics and Chemistry of Rare Earths, Vol 12, Eds K A Gschneldner, Jr and L Eyrmg (North-Holland, Amsterdam, 1989) p 133 Dl M DuraJ, R DuraJ, A Szytuka and Z Tomkowlcz, J Magn Magn Mater 73 (1988) 240 T Shlgeoka and N Iwata, Solid [31 H FUJII, T Okamoto, State Commun 53 (1985) 715 [41 E M Gyorgy, B Batlogg, J P Remelka, R B Van Dover, R M Flemmg, HE Balr, G P Espmosa, AS Cooper and R G Mames, J Appl Phys 61 (1987) 4237 N Iwata and H FUJII, J Magn Magn 151 T Shlgeoka, Mater 76 & 77 (1988) 189 [61M DuraJ, R Dura) and A Szytula, J Magn Magn Mater 79 (1989) 61 Magn [71 M Dura], R DuraJ and A Szytuia, J Magn Mater 82 (1989) 319 181 S Ishlda, S Asano, J Ishlda, J Phys Sot Jpn 55 No 3 (1986) 936 Acta Phys [91 E Kulatov, V Veselago and L Vmokurova, Pol A77 (1990) 709 H Yoshlda and T Kaneko, 1101 T Kawashlma, T Kanomata, J Magn Magn Mater 90 & 91 (1990) 721