Peculiarities of the magnetic-history-dependent phase in CePtSn

Peculiarities of the magnetic-history-dependent phase in CePtSn

ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 322 (2010) 1120–1122 Contents lists available at ScienceDirect Journal of Magnetism and...

235KB Sizes 0 Downloads 1 Views

ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 322 (2010) 1120–1122

Contents lists available at ScienceDirect

Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm

Peculiarities of the magnetic-history-dependent phase in CePtSn Jan Prokleˇska a,, Blanka Detlefs b, Vladim´ır Sechovsky´ a, Martin M´ısˇ ek a a b

Charles University, Department of Condensed Matter Physics, Ke Karlovu 5, 121 16, Prague, Czech Republic European Commission, Institute for Transuranium Elements, Postfach 2340, D-76125 Karlsruhe, Germany

a r t i c l e in f o

a b s t r a c t

Available online 9 September 2009

Due to its peculiar properties and presence of many types of different basic mechanisms determining the magnetic ground state, CePtSn compound is one of the well-studied alloys from the CeTX group with TiNiSi-type structure. In this article we present the results of elastic and transport properties, and we concentrate our efforts on the description of the evolution for field applied along the b-axis, where a magnetic-history-dependent (MHD) phenomenon occurs. Based on this results and the fact that the low-temperature zero-field-cooled state may be described as the presence of two kinds of spin slips in magnetic structure, we conclude that the MHD behavior may be interpreted as the magnetic field annealing of one set of spin stacking faults in the magnetic structure in favor of the other one. & 2009 Elsevier B.V. All rights reserved.

Keywords: CePtSn Thermal expansion Magnetostriction Anisotropy

1. Introduction CePtSn belongs to the group of CeTX compounds crystallizing in the orthorhombic TiNiSi-type structure. Similar to some CeTX companions, a strong anisotropy of magnetic and other electronic properties is observed in magnetic fields [1]. This compound orders antiferromagnetically (TN = 7.5 K), with subsequent order-to-order transition at Tt =5 K (characterized by the appearance of second propagation vector). The easy magnetization axis is the a-axis, with field-induced transition at 12.5 T. The hard axis in this compound is the c-axis; application of the magnetic field along this direction does not influence the ordering temperatures and no additional features are observable on the field dependencies. The most interesting and unusual behavior of this antiferromagnet (AF) is observed for the magnetic field applied along the b-axis. At low temperatures (To3 K), two field-induced (AF-to-AF) transitions are observed at fields of 3.5 and 11 T, respectively. Both these transitions are accompanied with pronounced magnetoresistivity steps ( 30% and 10%, respectively); however, only minor features are visible on the magnetization curve. The 3.5 T transition is irreversible; it is observed only when the sample was cooled in zero magnetic field prior to the measurement from temperatures above 3.5 K, but no transition is found when sweeping the field down through 3.5 T. In contrary, the 11 T transition is fully reversible. In this paper we investigated the magnetic-history-dependent phenomena in detail and we present the results of comparative

 Corresponding author. Tel.: + 420 221 911 653; fax: + 420 224 911 061.

E-mail address: [email protected] (J. Prokleˇska). 0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2009.09.017

measurements of transport and elastic properties with special care to the anisotropy of these properties.

2. Experimental All measurements were done on properly oriented and well-shaped piece (cuboid cut along main crystallographic directions, with dimensions 1.50  3.29  1.47 mm3) of a single crystal sample grown by the Czochralski method. Electrical resistivity was measured using the standard four-point method in the PPMS (Quantum Design). The thermal expansion and magnetostriction were measured using the miniature capacitance cell [2] connected to the Andeen Hagerling 2500A capacitance bridge. In this case, the PPMS was used for the temperature and magnetic field control. In order to have well-defined conditions, the thermal expansion temperature dependencies were measured in the sweep mode with the typical speed of 50 mK/min. For the measurements of the magnetostriction curves, the field was held constant during the data acquisition. All longitudinal components of the elastic tensor were measured as well as transversal components for the field applied along the b-axis.

3. Results All zero-field thermal expansion data as well as resulting volume change are shown in Fig. 1. The temperature evolution of the volume change is monotonous with clear indication for both transitions; however, there is a strong anisotropy in the contribution from individual directions, where the a- and c- axis contributions are complementary above the Tt transition and

ARTICLE IN PRESS J. Prokleˇska et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 1120–1122

1121

a-axis b-axis c-axis

volume a-axis b-axis

dx/x

c-axis

4

2

6

8

10

12

14

T (K)

dx/x

10-5

10-5

Fig. 1. Thermal expansion measured along main crystallographic axis at zero magnetic field, depicted together with volume change. The curves are shifted for clarity (indicated by arrows).

6 volume a-axis b-axis c-axis

dx/x (10-5)

4 2

-6

0

-4

-2

0

2

4

6

B (T) -2

Fig. 3. Full magnetostriction curves measured for field applied along the b-axis. The arrows indicate the evolution of the magnetic history of the sample (T= 1.8 K).

-4 0

2

4

6

8

10

12

14

B (T)

10

Fig. 2. Magnetostriction measured along principal axis for field applied along the b-axis (T= 1.8 K).

MR (%)

almost cancels each other. With application of the field along the a and c directions, no unexpected behavior was found and the obtained data fit well in the previously published magnetic phase diagrams. Interesting behavior is observed for field applied along the b-axis. First of all, the overall magnetostriction curves up to the 14 T magnetic field applied along the b-axis are shown in Fig. 2. Similar to the thermal expansion evolution the a and c axis contributions are complementary, leading together with the b-axis magnetostriction to the monotonous increase of the volume change interrupted by minor positive steps (  5  10 6) at the transition fields. The changes along the main crystallographic directions are of one order of magnitude larger and for a given direction they have opposite signs at the 3.5 and 11 T transition. Secondly, when entering the magnetic-history-dependent (MHD) phase present in the magnetic phase diagram for field applied along the b-axis, the sample partially restores its zero-field-cooled (ZFC) properties (see Fig. 3). While lowering the magnetic field, the drop in the magnetostriction is partially (approximately 12) restored and the subsequent field cycling is almost reversible, with further small but notable features. Also, after first field cycling, the field-induced transition occurs at a significantly lower magnetic field.

magnetoresistivity

0 -10 -20 -30 -40 -4

-2

0

2

4

B (T) Fig. 4. Full magnetoresistivity curve for field applied along the b-axis (T= 1.8 K).

In order to exclude possible (however unlikely) instrumental effects, we performed the detailed longitudinal magnetoresistivity measurements in a similar way as the magnetostriction measurements (see Fig. 4). As expected, the low field transition is accompanied by  30% drop in magnetoresistivity. Further field cycling reveals that the resistivity reflects features similar to the magnetostriction when cycling field across the MHD area in the phase diagram, i.e. partial restoration of the ZFC state and shift of the lower transition field. The restoration of the original ZFC

ARTICLE IN PRESS 1122

J. Prokleˇska et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 1120–1122

resistivity is smaller than in the case of elastic properties; however, all features including the minor evolution with subsequent magnetic field cycling is present.

4. Discussion In order to properly discuss the observed phenomena, a few remarks from the previous microscopic experiments should be noted. The spin slip scheme for the magnetic ordering on the microscopic level was proposed [3] based on the neutron diffraction and mSR experiments. The field-dependent neutron diffraction in the MHD phase revealed [4] that the irreversible transition is connected with the redistribution of magnetic domains characterized by different q, more specifically, the volume fraction of q1 =(0, 0.470, 0)-related domains is increased on the account of the q2 = (0, 0.428, 0)-related volume fraction. This redistribution leads to dramatic drop in magnetoresistivity due to the different resistivity in these domains. Similar to magnetoresistivity, magnetostriction is even more sensitive to the population of these two domain types. The situation when the magnetostriction (also the magnetoresistivity, but on a different scale) partially restores its ZFC values may reflect the fact that after the first field cycle the domains are not fully re-populated and both types are still present and their ratio is modified with further field cycling (leading to the presently observed minor evolution in the subsequent field cycling). In the framework of the spin slip model, this can be understood as the gradual annealing of one type of spin slips (related to the q2 propagation vector) by the application of the magnetic field along the b-axis, whereas spin slip related to the q1 propagation vector is favored. Also, the results from the thermal expansion measurements explain rather unexpected results from the hydrostatic pressure experiment [5], where no significant shift of ordering temperatures was observed. The elastic properties of CePtSn are strongly anisotropic with small and monotonous evolution of the volume change and, consequently, small sensitivity to the applied

hydrostatic pressure is observed (detailed study of the behavior under applied pressure is in preparation, including the results from the uniaxial pressure experiment where strong sensitivity of ordering temperatures is expected). This study reveals several unknown aspects of the ordering schemes in the CePtSn compound. Primarily, the partial restoration of the ZFC state is observed on both elastic and transport properties, accompanied with shift of the transition field after first field cycle. This phenomenon as well as the overall magnetichistory-dependent behavior is explained from the spin slip model of the magnetic structure as a consequent preferential annealing of spin slips in the q2-related domains by application of external magnetic field. Secondly, the elastic properties measured along the principal axis in zero field are presented, reflecting the strong anisotropy in this compound and its influence on the mechanisms of magnetic ordering. Consequently, it shows an importance and necessity for the uniaxial pressure experiments, which can disentangle the anisotropic sensitivities to the applied pressure, indicated by strong anisotropy in elastic properties. Also, the complete magnetic structure determination is highly desirable in order to properly understand the magnetic phase diagram evolution.

Acknowledgements This work is a part of the research plan MSM 0021620834 that is financed by the Ministry of Education of the Czech Republic. Part of this work was also supported by GAUK 209/2006.

References [1] [2] [3] [4] [5]

T. Khmelevska, et al., J. Appl. Phys. 89 (2001) 7189. M. Rotter, et al., Rev. Sci. Instr. 69 (1998) 2742. D.R. Noakes, et al., Physica B 289–290 (2000) 248. B. Janousova, et al., Physica B 328 (2003) 145. B. Janousova, et al., Acta Phys. Pol. B 34 (2003) 1039.