Magnetic structure and magnetic properties of the spinel solid solutions ZnCr_2xAl_2-2xS₄(0.85⩽x⩽1). I. Neutron diffraction study

Abstract

The spinel solid solutions ZnCr2xAl2-2xS4 have been studied by means of neutron diffraction in the concentration range 0.85<or=x<or=1. For the pure compound (x=1), several powder samples with different heat treatments and a single crystal were studied. At TN(x=1)=15.5, TN(x=0.9)=14.5 and TN(x=0.85)=14K a helical structure with k1=(0, 0, 0.79) and mk1 perpendicular to (001) is built up in a second-order phase transition ( beta =0.3). At T0=12K, a first-order transition takes place and the helical structure begins to transform into two collinear structures corresponding to k2=(1/2, 1/2, 0), mk2//(110) and k3=(1, 1/2, 0); mk3//(001). At low temperature, the three phases coexist in different percentages depending on the number of sulphur vacancies. This mixed phase results from a competition between the ferromagnetic first- and third-nearest-neighbour couplings (j1 approximately=2K and J3 approximately=1K, respectively) and the antiferromagnetic second-nearest-neighbour interaction (J2 approximately=-1K).

Magnetostructural Transitions in a Frustrated Magnet at High Fields

“…Its step-like shape is only slightly affected by temperature indicating a welldefined phase transformation. Since the magnetization shows only a gradual change in this temperature and field range, and taking into account neutron diffraction and x-ray data [18][19][20] we associate the anomaly 2 with the second structural transition from the tetragonal phase III to the cubic phase II. It is also important to note that the magnitude of the total change of ∆v/v 0 by magnetic field is close to that expected for the full recovery of the cubic state estimated from a fit to the experimental data in the true paramagnetic state (above 60 K) shown by the dashed line in Fig.…”

“…In contrast, at T N 2 = 7.3 K where ZnCr 2 S 4 transforms into the orthorhombic structure, only a smooth change of the sound velocity is found, and a broad peak in the attenuation appears. According to [18] the collinear structure evolves already at 12 K which could explain the observation of two peaks in the attenuation below T N 1 and of a weak anomaly in the sound velocity at T N 2 . The anomaly in the attenuation at 11 K shifts to lower temperatures with increasing fields.…”

“…Neutron-diffraction measurements [18][19][20] established two subsequent magnetic transitions in ZnCr 2 S 4 : the first one to an incommensurate helical AFM order at T N 1 ≈ 14 K, and the second one, to coexisting commensurate spin order at T N 2 ≈ 7 K. The helical state is characterized by a spin spiral with a propagation vector k 1 ≈ (0, 0, 0.787) along the crystallographic c direction and with the spins rotating in the a-b plane. This is also the ground-state structure of the bond-frustrated AFM ZnCr 2 Se 4 with increased FM exchange as compared to ZnCr 2 S 4 [12].…”

Structural and magnetic properties of single-crystalline spinel systems ZnCr2−xAlxSe4 (x=0.15 and 0.23)

“…Various types of magnetic order together with concomitant semiconducting behaviour have been reported for chalcogenide spinel materials [2][3][4][5][6][7]. These properties can be modified in a broad range of compound composition, as the spinel crystal structure allows for different cationic substitutions [8][9][10].…”

“…The compound exhibits semiconducting properties with p-type conductivity [15]. Recently, much interest has been paid to various diluted systems, based on ZnCr 2 Se 4 , where effects of site disorder, lattice frustration and random distribution of spin interactions can be observed [4,5,16].…”