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Real-space observation of magnetic domain structure at metamagnetic

tansition in a triple-layer ruthenate Sr4Ru3O10

Article  in  Journal of Physics Conference Series · March 2009

DOI: 10.1088/1742-6596/150/4/042134

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2009 J. Phys.: Conf. Ser. 150 042134

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Real-space observation of magnetic domain structure

at metamagnetic tansition in a triple-layer ruthenate

Sr4Ru3O10

Y Nakajima1, Y Matsumoto1, D Fobes2, M Zhou2, Z Q Mao2, and TTamegai11 Department of Applied Physics, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo113-8656, Japan2 Department of Physics, Tulane University, New Orleans, Louisiana 70118, USA

E-mail: yasuyuki@ap.t.u-tokyo.ac.jp

Abstract. We report the real-space observation of magnetic domains in the ac−plane atthe metamagnetic transition in a triple-layer ruthenate Sr4Ru3O10 by magneto-optical imagingand scanning Hall probe measurements. We find anisotropic magnetic domain structureswith weak c−axis correlation at low fields. The magnetic inhomogeneities are removed abovethe metamagnetic transition field. These changes of magnetic structure support the phase-separation scenario for the ultra-sharp steps in magnetoresistance.

1. IntroductionRuthenates of the Ruddlesden-Popper series Srn+1RunO3n+1 have attracted great interestbecause of the variety of ground states, such as spin-triplet superconductivity in Sr2RuO4(n=1) [1] and metamagnetic quantum criticality in Sr3Ru2O7 (n=2) [2]. A triple-layerruthenate Sr4Ru3O10 (n=3) [3] is one of the members of this series and a ferromagnet withCurie temperature TC = 105 K. Below TM ∼ 50 K, a metamagnetic transition occurs whenthe magnetic field is applied along the ab−plane. Recent resistivity measurements at lowtemperature reveal that the resistivity displays step-like changes with decreasing in-plane fieldthrough the transition, suggesting the presence of phase separation with ‘magnetic domain’formation through the metamagnetic transition [4]. In order to clarify the magnetic domainstructure near the metamagnetic transition field, we have performed the real space observationof magnetic domain structure in the ac-plane at metamagnetic transition in Sr4Ru3O10 bymagneto-optical imaging and scanning Hall probe microscopy (SHPM).

2. ExperimentsHigh quality single crystals of Sr4Ru3O10 were grown by the floating-zone technique [5].Magneto-optical images are obtained by using the local-field-dependent Faraday effect in the in-plane magnetized garnet indicator film. SHPM measurements are performed by ion-doped GaAsmicro-Hall probe with 10×10 µm2 active area with a mechanical scanner. Bulk magnetizationis measured by a SQUID magnetometer (MPMS-XL5, Quantum Design).

25th International Conference on Low Temperature Physics (LT25) IOP PublishingJournal of Physics: Conference Series 150 (2009) 042134 doi:10.1088/1742-6596/150/4/042134

c© 2009 IOP Publishing Ltd 1

3. Results and discussionFigure 1 depicts the field dependence of the bulk magnetization obtained by SQUIDmagnetometer for Sr4Ru3O10 with magnetic field parallel to the ab-plane at T = 2 K. Weobserve a metamagnetic transition accompanied by significant hysteresis at H .20 kOe, whichis similar to previous reports [4].

1.5

1.0

0.5

0.0

4pM

(m

B/R

u)

50403020100

H (kOe)

H // abT = 2 K

Figure 1. Field dependence of bulk magnetization for Sr4Ru3O10 applying magnetic fieldparallel to the ab-plane.

Figures 2 (a) (b) show the zero-field magneto-optical images for the ac-plane in Sr4Ru3O10,which is perpendicular to the ferromagnetic easy-axis. The image at 110 K is subtracted fromboth images as a background. We find the inhomogeneity of the local magnetization and it isalmost constant with decreasing temperatures. In addition, local magnetization on the ab-planeof the sample is ∼ 30 G at 80 K, much smaller than the saturation magnetization of ∼ 600 G at 80K measured by bulk magnetization. We observe a relatively large local magnetization inside thesample and do not observe branching of the magnetic domains near the surface. These resultsindicate that the local magnetic distribution is produced by stray fields from ferromagneticdomains with opposite polarities with a weak c−axis correlation, in contrast to ferromagnetswith the stripe-shape magnetic domain structure such as La1.36Sr1.64Mn2O7 [6].

Figure 3 (a) shows the SHPM image of magnetic domain structures in the ac-plane ofSr4Ru3O10 at T = 3 K, when sweeping up magnetic field perpendicular to the ac-plane. Scannedarea is about 100×100 µm2. At zero field, the inhomogeneity of magnetization is observed asobtained by magneto-optical image. The inhomogeneity is almost unchanged when applyingmagnetic field up to 12.5 kOe. With further increasing field up to 20 kOe, local magnetizationsuddenly becomes homogeneous. We attribute this change to the metamagnetic transition. Thehomogeneous field profile is unchanged above this field. Figure 2 (b) shows the SHPM imagefor down-sweep after applying 30 kOe. The characteristic field, where the drastic change ofthe magnitude of local magnetization corresponding to metamagnetic transition is observed, isshifted to lower field. These behaviors are consistent with the bulk magnetization measurementsas shown in Fig. 1. When returning to zero field though the metamagnetic taransition, the localmagnetizaiton distribution becomes almost the same as that before applying field.

We here show the details of the inhomogeneity of the local magnetization. Figure 4 (a) showsthe average local magnetization in the scanned area obtained from the data shown in Fig. 3,which is similar to bulk magnetization. The finite magnetization at zero field is due to excess

25th International Conference on Low Temperature Physics (LT25) IOP PublishingJournal of Physics: Conference Series 150 (2009) 042134 doi:10.1088/1742-6596/150/4/042134

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number of magnetic domains with positive magnetization in the present scanned area. Figure4 (b) shows the standard deviation of local magnetization in the scanned area obtained by thedata shown in Fig. 3. The peak at 17.5 kOe for up-sweep and 15 kOe for down-sweep possiblysuggests the coexistence of lowly polarized (LP) phase at lower fields and forced ferromagnetic(FFM) phase at higher fields through the metamagnetic transition, which is proposed to explainthe ultra-sharp resistivity steps [4]. The maximum of standard deviation and its change at themetamagnetic transition for down-sweep are larger than those for up-sweep, indicating that themagnetic domain structures for down-sweep are more inhomogeneous than up-sweep through( a ) ( b )

1 0 0 K � 1 1 0 K 5 . 4 5 K � 1 1 0 Ka x i s 2 0 0 � m

Figure 2. The zero-field magneto-optical image in the ac-plane for Sr4Ru3O10 at (a) 100 Kand (b) 5.45 K. We subtract the image at 110 K as a background from both images.� �( a ) ( b )

0 O e 1 0 k O e 1 2 . 5 k O e1 5 k O e 1 7 . 5 k O e 2 0 k O e

2 2 . 5 k O e 2 5 k O e 3 0 k O e5 0 � mT = 3 K1 0 0 � m

c a x i s0 O e 1 0 k O e 1 2 . 5 k O e

1 5 k O e 1 7 . 5 k O e 2 0 k O e2 2 . 5 k O e 2 5 k O e 3 0 k O e5 0 + m

c a x i s

T = 3 K1 0 0 � m0 G 1 k G 0 G 1 k GFigure 3. The SHPM image of magnetic domain structures in the ac-plane for Sr4Ru3O10 atT = 3 K, when sweeping (a) up and (b) down magnetic field perpendicular to the ac-plane,respectively.

25th International Conference on Low Temperature Physics (LT25) IOP PublishingJournal of Physics: Conference Series 150 (2009) 042134 doi:10.1088/1742-6596/150/4/042134

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the metamagnetic transition. The fact that ultra-sharp resistivity steps have been observed onlyfor down-sweep can be attributed to the lager inhomogeneity in the down sweep.

140

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40

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0S

tand

ard

devi

atio

n (G

)302520151050

H (kOe)

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up-sweep down-sweep

800

700

600

500

400

300

200

B-H

(G

)

302520151050

H (kOe)

(a)

T = 3 K

Figure 4. (a) Average local magnetization in the scanned area obtained from the data shownin Fig. 3. Arrows indicate the sweep direction of the field. (b) Standard deviation of localmagnetization in the scanned area obtained from the data shown in Fig. 3. Filled and opencircles represent up-sweep and down-sweep data, respectively.

4. SummaryWe have performed the real-space observation of magnetic domain structures in the ac-plane forSr4Ru3O10 by magneto-optical imaging and SHPM techniques. We observe an inhomogeneity oflocal magnetization originating from anisotropic magnetic domain structures with weak c-axiscorrelation. We find that the inhomogeneity is strongly suppressed above the metamagnetictransition field. These changes of magnetic structures support the phase-separation scenario forthe ultra-sharp resistivity steps. The more inhomogeneous local magnetization for down-sweepat metamagnetic transition than that for up-sweep is observed, which may explain the fact thatthe resistivity ultra-sharp steps are observed for only down-sweep.

AcknowledgmentsThis work is partly supported by a Grant-in-Aid for Scientific Research from the Ministry ofEducation, Culture, Sports, Science, and Technology. Work at Tulane is supported by the NSFunder grant DMR-0645305 and by the DOE under grant DE-FG02-07ER46358.

References

[1] Ishida K, Mukuda H, Kitaoka Y, Asayama K, Mao Z Q, Mori Y and Maeno Y 1998 Nature 396 658[2] Perry R S, Galvin L M , Grigera S A , Capogna L, Schofield A J , Mackenzie A P, Chiao M, Julian S R,

Ikeda S I, Nakatsuji S, Maeno Y and Pfleiderer C 2001 Phys. Rev. Lett. 86 2661[3] Crawford M K, Harlow R L, Marshall W, Li Z, Cao G, Lindstrom R L, Huang Q and Lynn J W 2002 Phys.

Rev. B 65 214412[4] Mao Z Q, Zhou M, Hooper J, Golub V and O ’Connor C J 2006 Phys. Rev. Lett. 96 077205[5] Zhou M, Hooper J, Fobes D, Mao Z Q , Golub V, O ’Connor C J 2005 Mater. REs. Bull. 40 942[6] Welp U, Berger A, Vlasko-Vlasov V K, Li Q, Gray K and Mitchell J F 2006 Phys. Rev. B 62 8616

25th International Conference on Low Temperature Physics (LT25) IOP PublishingJournal of Physics: Conference Series 150 (2009) 042134 doi:10.1088/1742-6596/150/4/042134

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