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Article Reference Article Weak ferromagnetism in the antiferromagnetic magnetoelectric crystal LiCoPO4 KHARCHENKO, N. F., et al. Abstract A study of the magnetization of the antiferromagnetic magnetoelectric crystal LiCoPO4 as a function of temperature and the strength of a magnetic field oriented along the antiferromagnetic vector reveals features due to the presence of a weak ferromagnetic moment. The value of the magnetic moment along the b axis at 15 K is approximately 0.12 G. The existence of a ferromagnetic moment can account for the anomalous behavior of the magnetoelectric effect observed previously in this crystal. Reference KHARCHENKO, N. F., et al. Weak ferromagnetism in the antiferromagnetic magnetoelectric crystal LiCoPO4. Low Temperature Physics, 2001, vol. 27, no. 9-10, p. 895-898 DOI : 10.1063/1.1414584 Available at: http://archive-ouverte.unige.ch/unige:31080 Disclaimer: layout of this document may differ from the published version. 1 / 1 LOW TEM,PERATURE PHYSICS VOLUME 27, NUMBER 9-10 SEPTEMBER-OCTOBER 2001 Weak ferromagnetism in the antiferromagnetic magnetoelectric crystal LiCoP04 N. F. Kharchenko and Vu. N. Kharchenko* B. Verkin Institute for Low Temperature Physics and Engineering, National Academy of Sciences of Ukraine, pr. Lenina 47, 61103 Kharkov, Ukraine R. Szymczak and M. Baran Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, PL-02-668 Warsaw, Poland H. Schmid University of Geneva, Department of Inorganic, Analytical and Applied Chemistry, CJ-12[] Geneva 4, Switzerland (Submitted August 3, 2001) Fiz. Nizk. Temp. 27, 1208-1213 (September-October 2001) A study of the magnetization of the antiferromagnetic magnetoelectric crystal LiCoP04 as a function of temperature and the strength of a magnetic field oriented along the antiferromagnetic vector reveals features due to the presence of a weak ferromagnetic moment. The value of the magnetic moment along the b axis at 15 K is approximately 0.12 G. The existence of a ferromagnetic moment can account for the anomalous behavior of the magnetoelectric effect observed previously in this crystal. © 2001 American Institute of Physics. [DOl: 10.1063/1.1414584] Cobalt lithium orthophosphate is a well-known magne­ have been studied, for which it is necessary to apply mag­ toelectric crystal of the family of orthorhombic antiferromag­ netic and electric fields simultaneously in order to prepare a nets with the olivine structure and having the general for­ single-domain state. The behavior of the magnetoelectric ef­ 2 2 2 mula LiMP04 (where M=Fe2+, Mn +, Co +, Ni +). It has fect observed by the authors of Refs. 1-3 could be due to the been attracting attention because it has large values of the existence of a weak ferromagnetic moment in the crystal. 1 2 constants axv and ayx of the linear magnetoelectric effect • However, previous studies of the magnetic properties of both 7 8 9 and unusual and as yet unexplained behavior of the magne­ polycrystalline • and single-crystal LiCoP04 have not de­ toelectric effect in a magnetic field?-4 The magnetoelectric tected weak ferromagnetism. effect in this crystal is the largest among the compounds of The creation of a homogeneous magnetic state in a com­ 3d elements. The crystal structure of LiCoP04 , like that of pensated antiferromagnet in the presence of only a magnetic the other lithium phosphates of transition elements of the field may also be caused by quadratic (in the field) magneti­ olivine family, has a symmetry described by the space group zation effect.10 In this case the magnetic field induces in the Pnma (Di~) (Refs. 5,6). In this structure the unit cell (a crystal a magnetic moment that is even with respect to the ---. 10.20 A, b=5.92 A, c=4.70 A) contains four formula field strength. In antiferromagnetic (AFM) states with oppo­ - units, and the magnetic ions are crystallographically equiva- sitely directed sublattice moments, oppositely directed mag­ lent and occupy four c positions. According to the results of netic moments will be induced. Therefore, when a magnetic neutron-diffraction studies7 carried out on polycrystalline field is applied in a certain direction, the energy of the col­ samples of LiCoP04 , upon antiferromagnetic ordering ( T N linear antiferromagnetic domains will be different. When the = 21.9 K) 2 the number of formula units in the unit cell re­ difference of the energies of the antiferromagnetic domains, 3 mains unchanged ( z = 4), and the magnetic moments of the which varies in proportion to H , reaches a threshold value Co2 + ions are collinear and directed along the b axis, com­ determined by the coercivity of the antiferromagnetic do­ pletely compensating each other. The magnetic structure main wall, the crystal will go to a single-domain state or a of the crystal is described in a collinear four-sublattice magnetization reversal of its antiferromagnetic state will model with the Shubnikov symmetry group Sh~i 5 (Pnma') occur.U A quadratic magnetization effect is allowed by sym­ (Ref. 7). metry only in AFM crystals which are not symmetric with respect to the operation of anti-inversion (or complete inver­ In studying the magnetoelectric effect in LiCoP04 , it 2 3 was found • that for preparation of a homogeneous (single­ sion): f' = r. 1'. However, the group mm m I that has been domain) antiferromagnetic state of the crystal, as must be established for LiCoP04 , although it does not contain the done in order to measure the magnetoelectric effect, it is operation of spatial inversion, does have a center of anti­ sufficient to decrease the temperature of the sample from inversion. Consequently, the quadratic magnetization and T> T N to T < T N in a magnetic field H oriented along the B weak ferromagnetic moment (WFM) should be forbidden in axis, or to apply a sufficiently strong magnetic field along the LiCoP04 . 12 b axis at temperatures T< T N. This is atypical for all of the Magnetooptic studies of LiCoP04 have revealed new compensated antiferromagnetic magnetoelectric crystals that features of the behavior of this antiferromagnetic crystal in a 1063-777X/2001/27(9-1 0)/4/$20.00 895 © 2001 American Institute of Physics 896 Low Temp. Phys. 27 (99-10), September-October 2001 Kharch_qnko-et al. 10 a 8 FC(H = 1 T) 6 H= 1 T 4 2 0 0 5 10 15 20 25 T, K 0.075 0.050 (!) 0.4 0.025 0.6 ~~----~r=----;o 1:::: ~ 0.2 C!l b 0.5 0 :::c....._ 0 5 10 15 20 25 :a; 0.4 T, K 0.55 1.04 c T=21.3 K (!)0.54 ZFC H = 0.05T ~ tTN 0.53 21 22 23 24 25 26 27 28 29 30 :::a;1.01L---~---L~~=-~U-~~--~--~~~ T, K -2.0 -1.5 -1.0 -0.5 0 0.5 1.0 1.5 2.0 H,nt • T FIG. 1. Temperature dependence of the magnetization of the LiCoP04 crys­ tal in an external magnetic field Hllb with a field strength of 1 T (a) and 0.05 FIG. 2. Magnetization of the LiCoP04 crystal as a function of the internal T (b); the MCn curves near the Nee! temperature at H=0.05 T (c). FC magnetic field Hint in an external magnetic field Hllb (a); the M(H)IH (H= I T) - the value of the magnetic field in which the sample was curves at crystal temperatures of 15 K (b) and 21.3 K (c). cooled. a) In the temperature interval 5-10.5 K the projection of magnetic fields. It was found that the birefringence of lin­ the magnetic moment of the sample on the magnetic field early polarized light induced by a magnetic field Hllb is com­ direction is negative. With increasing temperature its abso­ parable to the spontaneous magnetic linear birefringence at a lute value initially increases and then, at T> 8 K, decreases value of the external field much smaller than the value of the to zero, changes sign, and increases monotonically with tem­ effective exchange field. This property suggests the presence perature out to the neighborhood of T 1 (see Fig. lb and le). of transverse projections of the magnetic moments in the b) At a temperature T 1 = 20.9 K there is a jump in the crystal and, hence, a noncollinear magnetic structure in it. magnetization (occurring in a single interval between expe,..,......._ Given this situation, it is advisable to make highly sen­ mental points, which in this part of the curve is 0.1 K). sitive measurements of the magnetization of the LiCoP04 c) Near the Neel temperature at 21.6 K (see Fig. le) a crystal. In this paper we report the results of measurements weak but clearly registered peak is observed. The increase of of the magnetization M as a function of temperature and the magnetization with further increase in temperature begins magnetic field strength. All of the measurements were made only at T>TN. with a SQUID magnetometer (Quantum Design MPMS-5). All of these anomalies can be explained by assuming The sample studied had a mass of 7.46 mg and was in the that the sample in the initial state had a ferromagnetic mo­ form a parallelepiped with dimensions of 0.96X 1.22X 1.76 ment directed oppositely to the direction of the applied mag­ mm. netic field. Thus the sample was initially cooled in "zero" The experimental results are presented in Figs. 1-3. Fig­ field ( ZFC) and then a field H = 0.05 T, in which the mea­ ure 1 shows the curves of the temperature dependence of the surements were made, was applied. The residual field of the magnetization obtained in magnetic fields H of 1.0 and 0.05 superconducting solenoid can be as high as 0.002 T.
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