6
PHYSICAL REVIE%' B VOLUME 43, NUMBER 16 1 JUNE 1991 Anisotropic magnetoresistance of the semimetallic antiferromagnet EuAs3 W. Bauhofer' Max Pla-nck Ins-titut fiir Festkorperforschung, D 7000 -Stuttgart 80, Germany K. A. McEwen Department of Physics, Birkbeck College, Uniuersity of London, Malet Street, London WC1F. 7HX, United Kingdom (Received 20 February 1990; revised manuscript received 11 July 1990) The Hall effect and the magnetoresistance of single crystals of EuAs3 have been measured in the temperature region of magnetic ordering. We observe an extreme anisotropic behavior. When the magnetic IIIeld is applied perpendicular to the buckled As layers of the EuAs3 crystal structure, a strong positive magnetoresistance is found, similar to the case of a-As. The situation changes com- pletely with the magnetic field para11el to [010), the direction of the ordered Eu + moments in the low-temperature AF1 phase of EuAs3. In this case, the magnetoresistance is much weaker and reflect; . ne riel. d-lnducea magnetic phase transitions, similar to the case of the rare-earth metals Dy and Ho. We discuss these features on the basis of superzone and spin-Auctuation scattering. I. INTRODUCTION The semimetal europium triarsenide, EuAs3, shows a remarkable variety of electric' and magnetic proper- ties. We present here a further example of the unusual behavior of this compound: its anisotropic magneto- resistance (MR). EuAs3 is particularly interesting since it combines ideally the pronounced galvanomagnetic prop- erties of a group-V semimetal (originating from the high mobility of the charge carriers) with the magnetism of a rare-earth metal. The monoclinic crystal structure (space group C2/m) of EuAs3 consists of puckered As layers [the (001) planes] which can be derived from the crystal structure of black phosphorous by removing one third of the phosphorous atoms. ' The Eu + ions may be con- sidered to occupy the sites of the missing P atoms. The magnetic (B, T) phase diagram of EuAs3 is surprisingly complex ' despite the S7/2 magnetic ground state of Eu +, which has been verified by Mossbauer spectroscopy. " In zero field, EuAs3 exhibits two successive magnetic phase transitions at T+=11. 2 and TI =10.3 K. The antiferromagnetic collinear AF1 phase, observed for T(TI, contains ferromagnetically ordered (201) planes; adjacent planes have moment direc- tions parallel and antiparallel to the twofold [010] axis. ' The magnetic structure of the IC phase found for Tl & T(T& represents an incommensurate variant of AF1. ' ' Figure 1 illustrates the sequence of magnetic phase transitions which can be induced by applying a magnetic field in the AF1 phase parallel to the magnetic moments. The AF1-to-SF1 transition at 0.9 T for 5 K represents a spin-Aop into an incommensurate spiral ar- rangement of the moments within the (010) plane. The commensurate collinear SF2 phase appears between 2.2 and 3.5 T, and is followed by another incommensurate phase (SF3) for 3. 5 ( B (4.4 T. The resistivity of EuAs3 is of the order of 10 A cm at room temperature, and decreases by a factor of 1600 at C3 LLj LL D I— LLI 2 EuAs& B11 I3 0 0 6 8 10 TEMPERATURE (K) I 12 FIG. 1. Magnetic (8, T) phase diagram of EuAs, as obtained from magnetization and neutron scattering investigations (Refs. 2, 4, and 6). 0.35 K. The C2/m space group permits five independent Hall tensor components. Three components have mutu- ally different indices and describe the usual or transverse Hall effect. The other two components have two equal indices and denote a longitudinal geometry. In contrast to the case of isostructural SrAs3, ' longitudinal Hall coefficients could not be detected for EuAs3. ' Following Ref. 15 we use for the labeling of the Hall tensor com- ponents p, "& an orthogonal laboratory system x&, xz, x3, with x2 parallel to the crystallographic a axis (~~[100]) and x3 parallel to the b axis (~~[010]). The indices i, j, k refer to the directions of Hall electric field, current, and magnetic field. To give an example of the meaning of the indices pe&3 describes a situation where the Hall voltage is measured along [100], the current fiows perpendicular to the (001) plane, and the magnetic field is applied along [010]. Figure 2 shows the temperature dependencies of the transverse Hall tensor components of EuAs3 mea- 13 450 Qc1991 The American Physical Society

Anisotropic magnetoresistance of the semimetallic antiferromagnet

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PHYSICAL REVIE%' B VOLUME 43, NUMBER 16 1 JUNE 1991

Anisotropic magnetoresistance of the semimetallic antiferromagnet EuAs3

W. Bauhofer'Max Pla-nck Ins-titut fiir Festkorperforschung, D 7000-Stuttgart 80, Germany

K. A. McEwenDepartment ofPhysics, Birkbeck College, Uniuersity of London, Malet Street, London WC1F. 7HX, United Kingdom

(Received 20 February 1990; revised manuscript received 11 July 1990)

The Hall effect and the magnetoresistance of single crystals of EuAs3 have been measured in thetemperature region of magnetic ordering. We observe an extreme anisotropic behavior. When themagnetic IIIeld is applied perpendicular to the buckled As layers of the EuAs3 crystal structure, astrong positive magnetoresistance is found, similar to the case of a-As. The situation changes com-pletely with the magnetic field para11el to [010), the direction of the ordered Eu + moments in thelow-temperature AF1 phase of EuAs3. In this case, the magnetoresistance is much weaker andreflect; .ne riel.d-lnducea magnetic phase transitions, similar to the case of the rare-earth metals Dyand Ho. We discuss these features on the basis of superzone and spin-Auctuation scattering.

I. INTRODUCTION

The semimetal europium triarsenide, EuAs3, shows aremarkable variety of electric' and magnetic proper-ties. We present here a further example of the unusualbehavior of this compound: its anisotropic magneto-resistance (MR). EuAs3 is particularly interesting since itcombines ideally the pronounced galvanomagnetic prop-erties of a group-V semimetal (originating from the highmobility of the charge carriers) with the magnetism of arare-earth metal. The monoclinic crystal structure (spacegroup C2/m) of EuAs3 consists of puckered As layers[the (001) planes] which can be derived from the crystalstructure of black phosphorous by removing one third ofthe phosphorous atoms. ' The Eu + ions may be con-sidered to occupy the sites of the missing P atoms.

The magnetic (B,T) phase diagram of EuAs3 issurprisingly complex ' despite the S7/2 magneticground state of Eu +, which has been verified byMossbauer spectroscopy. " In zero field, EuAs3 exhibitstwo successive magnetic phase transitions at T+=11.2and TI =10.3 K. The antiferromagnetic collinear AF1phase, observed for T(TI, contains ferromagneticallyordered (201) planes; adjacent planes have moment direc-tions parallel and antiparallel to the twofold [010] axis. '

The magnetic structure of the IC phase found forTl & T(T& represents an incommensurate variant ofAF1. ' ' Figure 1 illustrates the sequence of magneticphase transitions which can be induced by applying amagnetic field in the AF1 phase parallel to the magneticmoments. The AF1-to-SF1 transition at 0.9 T for 5 Krepresents a spin-Aop into an incommensurate spiral ar-rangement of the moments within the (010) plane. Thecommensurate collinear SF2 phase appears between 2.2and 3.5 T, and is followed by another incommensuratephase (SF3) for 3.5 (B (4.4 T.

The resistivity of EuAs3 is of the order of 10 A cm atroom temperature, and decreases by a factor of 1600 at

C3

LLj

LL

DI—LLI

2

EuAs&B11 I3

00 6 8 10

TEMPERATURE (K)

I

12

FIG. 1. Magnetic (8, T) phase diagram of EuAs, as obtainedfrom magnetization and neutron scattering investigations (Refs.2, 4, and 6).

0.35 K. The C2/m space group permits five independentHall tensor components. Three components have mutu-ally different indices and describe the usual or transverseHall effect. The other two components have two equalindices and denote a longitudinal geometry. In contrastto the case of isostructural SrAs3, ' longitudinal Hallcoefficients could not be detected for EuAs3. ' FollowingRef. 15 we use for the labeling of the Hall tensor com-ponents p,"& an orthogonal laboratory system x&,xz, x3,with x2 parallel to the crystallographic a axis (~~[100])and x3 parallel to the b axis (~~[010]). The indices i,j,krefer to the directions of Hall electric field, current, and

magnetic field. To give an example of the meaning of theindices pe&3 describes a situation where the Hall voltage ismeasured along [100], the current fiows perpendicular tothe (001) plane, and the magnetic field is applied along[010]. Figure 2 shows the temperature dependencies ofthe transverse Hall tensor components of EuAs3 mea-

13 450 Qc1991 The American Physical Society

43 ANISOTROPIC MAGNETORESISTANCE OF THE. . . 13 451

sured at a magnetic field of 0.8 T. No field dependencehas been found up to this field strength. Above 100 K,two Hall coefficients, pi32 and p32„are positive, whereas

p2&3 stays negative over the entire temperature range.Below 100 K, both p & 32 and p3z, undergo two signchanges and become positive again for T~0 K. This in-dicates that EuAs3 is predominantly p conducting whenthe charge carriers are deAected in planes containing theb axis ([010]) direction. Only for B~~[010], when thecharge carriers move within the (010) plane, do holes playa negligible role for the electrical transport. The ob-served anisotropy in EuAs3 is in agreement with resultsderived from Shubnikov —de Haas oscillations in SrAs3. '

The unusual temperature dependence of the Hallcoefficients of EuAs3 could be due to multiband conduc-tion. An interesting alternative explanation is based onthe anomalous Hall effect caused by the asymmetricscattering at magnetic moments. Further experimentaldetails are needed to discriminate between these twomodels. The low crystal symmetry complicates the eval-uation of mobilities and carrier densities from Hall effectand conductivity data, but we estimate the hole concen-tration is =5 X 10 cm

The structural anisotropy of EuAs3 is ref1ected by itsMR. With the magnetic field B perpendicular to the Aslayers the resistivity of EuAs3 increases strongly, in closeanalogy to the behavior of n-As. ' However, with 8parallel to the As layers this contribution to the MR ismuch weaker and we observe a variety of field-inducedmagnetic phase transitions reminiscent of those found inthe heavy rare-earth metals. ' '

In this paper we present measurements of the MR ofEuAs3 for various field and current configurations. Wethen discuss the observed effects on the basis of the con-tributions from the different possible spin-dependentscattering mechanisms. A correlation is given betweenthe behavior of the MR at the magnetic phase transitionsand the corresponding changes of the propagation wavevectors describing the magnetic structures.

orientation by x-rays, using the back reAection Laue tech-nique, samples of typical dimension 1X 1X6 mm werecut out with a diamond saw. Four probes of 100-pm goldwire were spot-welded directly onto the polished surfaces.Most of the experiments were carried out with a 5-Tmagnet of a SHE magnetometer. For some experimentsa 7-T coil was employed.

III. RESULTS

A. Zero-field resistivity

The temperature dependence of the zero-field resistivi-ty of EuAs3 is shown in Fig. 3. For 10(T ( 15 K we ob-serve an increase of the resistivity caused by criticalscattering (CS) of the charge carriers on spin fiuctuations.This culminates in the extra magnetic superzone scatter-ing which appears in the IC phase below T~. We expectthis contribution to be most important for p, since thewave vector of the IC phase is close to the [001] direc-tion. The increasing order of the IC phase between T&and T~ gradually reduces the magnetic contribution tothe resistivity. With the IC-AF1 transition at TL a sud-den decrease of the resistivity occurs. This indicates that,while significant regions of the Fermi surface are des-troyed by the additional magnetic superzone boundariesdue to the IC phase, the effect is much less important inthe AF1 phase. The [010] direction is not affected by themagnetic superzones and no resistivity jump is expectedfor p& in contrast to the experimental results, which indi-cate a small effect here.

B. Magnetic field perpendicular to (001)

The transverse MR of EuAs3 at 4.2 K for Bi(001) isdepicted in Figs. 4(a) and 4(b). The MR is strongly posi-tive and varies quadratically with the field up to 4 T: theincrease in p„ is more than twice that in p&. For largerfields, the increase is stronger than quadratic, indicatingthe onset of long-period quantum oscillations. ' The MR

II. EXPERIMENTAL

Single crystals of EuAs3 were prepared by a Bridgmangrowth method originally proposed by Wittman. ' After

40— ~L & ~N Eu

o

O.O5

II I I I

lI 1 I

30—CQ

I

C)

20—

10—LLI

5loo 2OO 3OO

0l2 20

TEMPERATURE (K)24 28

FIG. 2. Temperature dependence of the transverse Hall ten-sor components of EuAs3.

FIG. 3. Temperature dependence of the electrical resistivityof EuAs3 in the region of magnetic ordering. CS stands for crit-ical scattering, SZC for superzone scattering.

13 452 W. BAUHOFER AND K. A. McEWEN 43

for Bl(001) is determined by the mobility of the chargecarriers in the (001) plane, which is parallel to the As lay-ers of the EuAs3 structure. The strong MR for thisorientation, where the carriers move within the As layers,clearly shows the close relationship to a-As.

C. Magnetic field parallel to [100]

Figure 5(a) shows measurements, at temperatures from4.8 to 20 K, of bp/p(0) with BII[100]. The transverseMR is again positive but one order of magnitude smallerthan the preceding case for Bl(001) (compare with Fig.4). As a consequence, the sudden increase of the scatter-ing at the transition to the paramagnetic state (see Fig. 3)becomes observable: at 3.8 T for 8.2 K, and at 1.6 T for10.2 K. It may be noted that bp/p(0) has a minimumaround Tz and hence the MR at 20 K is larger than at 12K. This suggests that at 12 K the reduction of the resis-tivity due to the suppression of spin fiuctuation (fiuctua-tion suppression, FS) is of the same order as the increasecaused by the cyclotron motion of the carriers.

The longitudinal MR at 4.2 K for BII [100] is shown inFig. 4(b). The field dependence is clearly nonquadratic.A maximum at 4.8 T is followed by saturation for 8 & 5.5T.

D. Magnetic field parallel to [010]

A magnetic field parallel to [010] produces a drasticchange of the MR of EuAS3 which is now dominated bythe field-induced magnetic phase transitions. Both thetransverse and the longitudinal MR become negative forhigh magnetic fields, indicating that for this orientationthe normal positive MR contribution due to the cyclo-tron motion of the carriers is subsidiary to other contri-butions.

The trans'«~e MR, &pb/p(0) f'or BII[010],III[100], ofEuAs3 is depicted for different temperatures in Fig. 6(a).It should be noted that these measurements and thoseshown in Fig. 4(a) were carried out on the same sample.At 20 K, well above T~, the FS and cyclotron orbit

01 2 5 ~(7) 68K

20

10

10 20 g2 (T&) 30

20.0K

10.6K

12.0K

1 2 3 4 E)(T)

(a)

1~01 2 3

Q

= I

6 e (7)(a)

( Q

(b)

10 20I

30 ~2 (7~) 40

Q

b

03 - 8 IIIIIQ

CL ~ 02—CL

O. l

2 3 4 5

Q

(b)

6 7

e (T)

FICx. 4. Transverse magnetoresistance of EuAs3 at 4.2 K. (a)B.t(oo1 )„ III [oo1 ]; (b) Bl(001), II I

[o1o].

FICx. 5. Magnetoresistance of EuAs3 with BII [1QQ] forditferent temPeratures: (a) transverse MR, III[Q1Q]; (b) longitudinal MR, III [100].

43 ANISOTROPIC MAGNETORESISTANCE OF THE. . . 13 453

B lib, I lla (a)

0.4

01 2 3

4.2 K

5.2 K

6.OK

O.OK

4 ':, B(T)'

~ 14.0K

mechanisms are of comparable magnitude: The MR hasinitially positive values, but changes sign around 4 T.However, at 14 K the negative MR for all fields rejectsthe rapid suppression (FS) of the strong spin fluctuationsclose to the ordering temperature.

In the ordered phase, the results are quite diA'erent. At6 K, we observe a two-stage increase of the resistivity upto a maximal value of h)o/p(0) around 2 T followed by adecrease leading to negative MR values for 8 & 5 T.

The most unusual features of the MR of EuAS3 arefound at lower temperatures and may be correlated with

the field-induced phase transitions. As the field is in-creased, the transverse MR with BE~[010] increases rapid-ly [see Fig. 6(a)] and exhibits an anomaly at theAF1~SF1 transition. At the SF1~SF2 transition thereis a dramatic drop in the resistivity by about 25% of)o(0),which is subsequently restored at the SF2~SF3 transi-tion. The resistivity then drops rather rapidly, with achange of slope at the SF3~SF4 transition seen in the5.2 K measurements.

The longitudinal MR at 4.2 K is presented in Fig. 6(b)together with the magnetic phase diagram of EuAs3. Thefollowing features may be noted. At the AF1~SF1 tran-sition we observe a weak anomaly with a change in slope;at the SF1—+SF2 transition there is a pronounced in-crease in the resistivity, which then increases only ratherslowly in the SF2 phase (similar to the transverse MR inthis phase). The onset of the SF2~SF3 transition ischaracterized by a sudden increase of the resistivity byabout 20%%uo of )o(0). The magnitude of the increase in pbin this configuration is thus about two-thirds of the in-crease in p, in the transverse case. The MR then de-creases monotonically with increasing field, exhibitingweak anomalies at both the SF3—+SF4 and theSF4—+ paramagnetic state transitions. After this, theresistivity (which is now less than the zero-field value) de-creases more slowly and would appear to saturate at avalue around b p/p(0) = —0.5.

IV. DISCUSSION

08- B IIIIIb

)b

We first consider the MR contributions due to the cy-clotron motion of the charge carriers. With the approxi-mate relation

&p/p(0) = (p,& )',we can relate the observed extreme anisotropy of the MRwith corresponding hole mobilities pz and hole e6'ectivemasses m~ using

0.4pz= ew/mz, (2)

0.2CL ~o ~ 0

—044.2

A

6Q-LLJLX

8.0Q

QZLLJ

10.0-

P

where p~ is an averaged mobility in the plane perpendicu-lar to the applied magnetic field, ~ is the carrier scatter-ing time. Table I summarizes the results at 5 T for threemutually perpendicular orientations a, b, c'. With therelations (1) and (2), and neglecting the anisotropic of r,we obtain

m& «m, &m +,C

thereby tracing back the extremely anisotropic MR of

TABLE I. Magnetoresistance and related eA'ective masses fordiff'erent orientations of the magnetic field.

FIG. 6. Magnetoresistance of EuAs3 with B~~ [010]: (a) trans-verse MR, 1~~[100], at different temperatures; (b) longitudinalMR, I~i [010],at 4.2 K; the lower part shows the magnetic phasediagram of EuAs3, and the dashed lines illustrate the connectionbetween magnetic phase transitions and corresponding featuresof the MR.

Direction of themagnetic field

Bii(001)B]][100]Bii [010]

Magnetoresistance~p/p(0)

=10 mambmbm

C

mamC

13 454 W. BAUHOFER AND K. A. McEWEN 43

EuAs3 to an extremely anisotropic Fermi surface. Thisassumption is supported by the experimental result thatShubnikov —de Haas oscillations of the MR in EuAs3(Ref. 21) as in isostructural SrAs3 (Ref. 16) are only ob-served when the magnetic field direction does not coin-cide with the b axis. For a quantitative discussion the an-isotropy of the carrier scattering times has to be con-sidered.

Some more information on the Fermi surface of EuAs3can be inferred from the high-field limits of the MR.The observed behavior characterized by saturation of thelongitudinal MR in combination with no saturation ofthe transverse MR suggests the case of a compensatedmetal with only closed orbits within the (001) and (100)planes. Again, further experimental work is needed toverify this suggestion.

The initial increase of the MR can be well understoodin terms of the enhancement of the spin Iluctuations (FE)which occurs in collinear antiferromagnets when a field isapplied parallel to the direction of the ordered momentsp. A positive, quadratic MR results from a competitionbetween (a) the suppression of spin Auctuations for thesublattice with p parallel to B and (b) the enhancement ofspin fluctuations for the sublattice with p antiparallel toB.

We discuss next the anomalies at the field-inducedmagnetic phase transitions in the context of magnetic su-perzone scattering. Magnetic superzone boundaries arisefrom the additional potential, periodic in the magneticpropagation wave vector Q of the ordered structure, ex-perienced in a magnetically ordered state by the conduc-tion electrons via the s fexchange inte-raction. New en-

ergy gaps are thus introduced in the electronic bandstructure.

These gaps will, in general, lead to a modification ofthe shape of the Fermi surface and hence the transportproperties and the effective f fexchange -interaction.The effect of magnetic superzone scattering is normallyto increase the component of the electrical resistivity inthe direction of the propagation vector of the magneticstructure. For example, in the heavy rare-earth metals,the onset of antiferromagnetism with a Q parallel to the cdirection produces superzone contributions to the resis-tivity below the Neel temperature which are typically ofthe order of 10 pA cm.

Abrupt changes of the MR at field-induced phase tran-sitions are caused by different magnetic superzones of themagnetic structures involved. Such eff'ects were firstrecognized in magnetoresistance measurements on singlecrystals of holmium. ' At the field-induced transitionsbetween the helical, fan, and recently recognized helifanphases, the resistivity jumps are typically a few percent.For EuAs3, however, we observe jumps in the MR of upto 30% [see Fig. 6(a)]. The interplay of the differentproperties of the constituents of EuAs3 could explain thisbehavior: the high mobility of the group-V element Asenhances the galvanomagnetic eff'ects of the rare-earthmetal europium.

The magnitude of the superzone energy gaps intro-duced in the band structure is proportional to (S,z ),where S,d is the amplitude of the modulated magnetic

1.0—

0.8—EU (AS0.4 0.6 3

B II I II [010]

0.6—C3

CL 0.4CL

0.2—

-0,20.0 1.0 2.0

B (T)3.0 4.0

FIG. 7. Longitudinal magnetoresistance of Eu(Asp 6Pp 4)3 at4.2 K, BIII II [o)o].

structure. In the AF1 and SF1 phases of EuAs3, S,d isalmost the full 7p~ of the Eu + ion. The substantial sizeof these gaps may be expected to create significantmodifications of the relatively small Fermi surface ofEuAs3 when Q changes at the phase transitions.

The rapid increase of the transverse MR in the a direc-tion (p, ) at the AF1~SF1 direction, coupled with therelatively small change in the longitudinal MR (i.e., p&),demonstrates that the a* component of the magneticpropagation wave vector (measured in units of the re-ciprocal lattice vectors a*, b*, c*) of the SF1 phase pro-duces profound modifications of the Fermi surface. Fur-ther evidence for the importance of the a * component isprovided by a rapid drop in p, at the SF1~SF2 transi-tion and the subsequent restoration of this fraction of theresistivity at the SF2~SF3 transition. We recall that thea* component of Q changes from 0. 1~0~0.05~0. 11in the SF1~SF2~SF3~SF4 sequence, whereas the c*component follows the sequences 0.25 ~0.25~0.225~0. 213. Major perturbations of the Fermi sur-face are thus to be expected from the large number of ex-tra Brillouin zone boundaries introduced along the a*direction in the SF1, SF3, and SF4 phases.

Magnetization and neutron measurements ' indicatethat the component of the magnetic moments along thefield direction has reached about 40% of the saturationvalue at 2.4 T (just above the onset of the SF2 phase) sothat 90% remains perpendicular to the field in the modu-lated structure. By the time the SF3 phase is reached at 4T, the amplitude of both the ferromagnetic and themodulated moments are approximately equal. Therefore,the rapid decrease in (S ) for the modulated componentof the moment and the concomitant reduction in the su-perzone energy gaps explains the fall in the transverseMR with increasing field above 4 T, and the absence of aparticularly significant anomaly at the SF3~SF4 transi-tions.

It is instructive to compare the MR results for EuAs3with those of the isostructural Eu(Aso 6Po&)3. In zerofield, this compound exhibits the same AF1 magnetic

43 ANISOTROPIC MAGNETORESISTANCE OF THE. . . 13 455

structure as EuAs3, and a similar sequence of field-induced transitions to the so-called SFM1, SFM2, andSFM3 phases has been found. ' Here, we use the nota-tion defined in Ref. 8. Figure 7 shows our measurementsof the longitudinal MR in the [010] direction ofEu(Aso 6Po4)3. In comparison with Fig. 6(b), the mixedcompound displays very little MR at 4.2 K in the AF1phase, but it reveals much sharper resistivity jumps at theAF1 ~SFM1 and SFM1~SFM2 transitions. NeutrondiA'raction studies show that the propagation wave vec-tors of these three SFM phases are (0.106,1,0.315),(0,1,0.5) with p~~c, and (0.088, 1,0.323) respectively. Werecall that the corresponding wave vectors of the SF1,SF2, and SF3 phases in EuAs3 are (0.1,1,0.25), (0, 1,0.25),and (0.05, 1,0.225). The qualitative differences in the MRanomalies of EuAs3 and Eu(Aso 6PO 4)3 may be due to thesensitivity of the transport properties to small changes inthe electronic band structure and Fermi surface and tothe magnetic ordering wave vectors. An alternative ex-planation is that the jumps of the MR in the [010] direc-tion are mainly due to changes of the superzones in the[001] direction. Further analysis must await more de-tailed information of their electronic properties.

V. CONCLUSION

We have shown that the magnetoresistance (MR) is afurther example of the exotic properties of EuAs3. Thebehavior of the MR changes from arseniclike, stronglypositive for one orientation, to rare-earth-like behavior,merely by a 90 rotation of the sample with respect to thefield. The MR due to the cyclotron motion of the car-riers is extremely anisotropic. The field-induced phasetransitions produce jumps in the MR up to 30%, tentimes larger than in rare-earth metals. Jumps in the MReven occur when the current is perpendicular to thedirection of the changes of the propagation vector. Thisunusual behavior is not fully understood. A quantitativedescription of the superzone scattering necessitates amore detailed knowledge of the structure and Fermi sur-face of EuAs3.

ACKNO% LEDGMENTS

We wish to thank Dr. M. Hartweg for growing thecrystals, M. Krempel for sample preparation, and S.Sachs for technical assistance. We acknowledge the sup-port of Professor H. G. von Schnering.

Present address: Technische Universitat Hamburg-HarburgPostfach 90 10 52, 2100 Hamburg 90, Germany.

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