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Influence of the magnetic field direction on the magnetoelectric properties of an incommensurate conical improper multiferroic

The complete magnetic and ferroelectric H-T phase diagrams for magnetic fields along the special magnetic directions were determined in a Dzyaloshinskii–Moriya multiferroic crystal with conical antiferromagnetic structure

Understanding the interplay between magnetism and ferroelectricity in improper multiferroics is of interest in fundamental and applied research. In previous studies we showed the appearance of ferroelectricity in Dzyaloshinskii-Moriya Mn1-xCoxWO4 crystals (x≥0.15) due to the stabilization of a rare double-k spin configuration: a transverse conical AFM order, composed of AFM collinear (AF4) and incommensurate (ICOM) cycloidal orders. The objective of the present study was the construction of the complete magnetoelectric phase diagrams for the principal magnetic directions.

The magnetic and pyroelectric responses to magnetic fields were characterized along the main axes of the conical spin arrangement in quality crystals of Mn0.80Co0.20WO grown by floating zone: the easy α and hard ω axes, and the b axis. The rotation plane (ωb) of the cycloidal spins is perpendicular to the easy magnetic axis of AF4 (α). ω is the magnetically hard direction within the ac plane (α ω). The magnetic order evolution was studied by single-crystal neutron diffraction (D23, ILL) up to 12 T, and by magnetometry (ac, dc) up to 60 T (at ICMAB and the EMFL Lab. at Dresden). The dielectric polarization was measured up to 20/60 T in static/pulsed fields. Several magnetoelectric transitions were thoroughly investigated:

     A: Suppression of the conical antiferromagnetic structure under field Hα. The complete magnetoelectric phase diagrams were determined for H//ω and H//b, with the field applied along the two elliptical axes of the cycloid in the conical structure. These components can be separately suppressed under field producing a fan-like magnetic configuration. In this phase transition (AF2+AF4 →AF3+AF4) the ferroelectric Pb state transforms to a paraelectric phase. At higher fields the paraelectric AF3 (COM) and AF4 (ICOM) components are successively suppressed.   

     B: Conical to cycloidal structure transformation and polarization in H//α axis. Marked differences were found in the H//α topology respect to the magnetoelectric transitions in the previous configurations (H//ω and H//b). An increase of the electric polarization accompanies the first metamagnetic transition (Pb(H)> Pb(0)) with H//α. The COM AF4 spin ordering is transferred to the AF2* magnetic cycloid, which exhibits enhanced elliptical amplitudes (AF2+AF4 →AF2*). Increasing further the field, and before the forced FM (paramagnetic) state, another intermediate magnetic phase (Y phase, with no polarization) was detected.

For all the phases the symmetry dictated relationships between magnetic order and the polarization tensor have been analyzed. The obtained results might be common for other magnetic materials possessing conical antiferromagnetic structures.

Authors:
Irene Urcelay-Olabarria,1 Eric Ressouche,2 Vsevolod Yu Ivanov,3 Vassil Skumryev,4 Zhaosheng Wang,5 Yurii Skourski,5 Anatoly M. Balbashov,6 Yu. F. Popov,7 Gennadii P. Vorob'ev,7 Navid Qureshi,8 José Luís García-Muñoz,9 and Alexander A. Mukhin3

Affiliations:
1 Zientzia eta Teknologia Fakultatea, Universidad del País Vasco, UPV/EHU, Spain
2Université Grenoble Alpes, CEA, INAC, MEM, France
3 Prokhorov General Physics Institute, RAS, Russia
4 Institució Catalana de Recerca i Estudis Avancats (ICREA) and Universitat Autònoma de Barcelona, Spain
5Hochfeld-Magnetlabor Dresden (HLD-EMFL) Helmholtz-Zentrum Dresden-Rossendorf, Germany
6Moscow Power Engineering Institute, Russia
7Faculty of Physics, M.V. Lomonosov Moscow State University, Russia
8Institut Laue Langevin, France
9Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Spain

Publication:
Influence of the magnetic field on the stability of the multiferroic conical spin arrangement of Mn0.80Co0.20WO4
Physical Review B 98 (13), 134430 [10pp] (2018)
DOI: 10.1103/PhysRevB.98.134430

Figure:
Conical antiferromagnetic structure in the ground state of ferroelectric Mn0.80Co0.20WO4. H-T magnetoelectric phase diagram of a Mn0.80Co0.20WO4 crystal for magnetic field along the α easy axis. Sketch of the magnetic and crystallographic axes drawn together with the conical surface that envelopes the spins.

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