Noncollinear ferrimagnetic structures and spin dynamics in Al-doped lithium ferrites

The new non-collinear magnetic order, in an Al-doped lithium ferrite (Al-LFO), <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:mrow> <a:mo>(</a:mo> <a:mrow> <a:msub> <a:mi mathvariant="normal">Li</a:mi> <a:mrow> <a:mn>0.5</a:mn> </a:mrow> </a:msub> <a:msub> <a:mi mathvariant="normal">Al</a:mi> <a:mrow> <a:mn>0.5</a:mn> </a:mrow> </a:msub> </a:mrow> <a:mo>)</a:mo> </a:mrow> <a:msub> <a:mrow> <a:mo>(</a:mo> <a:mrow> <a:msub> <a:mi mathvariant="normal">Fe</a:mi> <a:mrow> <a:mn>0.75</a:mn> </a:mrow> </a:msub> <a:msub> <a:mi mathvariant="normal">Li</a:mi> <a:mrow> <a:mn>0.25</a:mn> </a:mrow> </a:msub> </a:mrow> <a:mo>)</a:mo> </a:mrow> <a:mn>2</a:mn> </a:msub> <a:msub> <a:mi mathvariant="normal">Fe</a:mi> <a:mn>3</a:mn> </a:msub> <a:msub> <a:mi mathvariant="normal">O</a:mi> <a:mn>8</a:mn> </a:msub> </a:mrow> </a:math> (denoted as Li50Al50), could be determined in the magnetic space group <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"> <h:mrow> <h:mi mathvariant="bold-italic">P</h:mi> <h:msub> <h:mn>4</h:mn> <h:mn>3</h:mn> </h:msub> <h:msub> <h:mrow> <h:msup> <h:mn>2</h:mn> <h:mo>′</h:mo> </h:msup> </h:mrow> <h:mn>1</h:mn> </h:msub> <h:msup> <h:mn>2</h:mn> <h:mo>′</h:mo> </h:msup> </h:mrow> </h:math> using high-resolution neutron powder diffraction (HRNPD) at 3 and 50 K. The largest spin-canting angle of the non-collinear octahedral B-sublattice determined from the structural analysis using 3K-HRNPD approaches the Yafet-Kittel angle 21° at 0 K, extrapolated from saturation magnetization. The spin-canting textures shown at 3 and 50 K are thermally averaged at elevated temperatures to reveal collinear spin alignments along [001] at 300 K under zero external fields. The non-collinearity of magnetic spins in Al-LFOs arises from multiple spin-reorientation (SR) processes without magnetic phase transitions below <j:math xmlns:j="http://www.w3.org/1998/Math/MathML"> <j:msub> <j:mi>T</j:mi> <j:mi>c</j:mi> </j:msub> </j:math> (e.g., 788 K in Li50Al50). The SRs are activated above 100 K, showing Arrhenius behavior, but blocked at 5 K without altering the spontaneous magnetic polarization, as indicated by ac magnetic susceptibility data. The dilute effect of <k:math xmlns:k="http://www.w3.org/1998/Math/MathML"> <k:mrow> <k:mi mathvariant="normal">L</k:mi> <k:msup> <k:mrow> <k:mi mathvariant="normal">i</k:mi> </k:mrow> <k:mo>+</k:mo> </k:msup> </k:mrow> </k:math> replacing <n:math xmlns:n="http://www.w3.org/1998/Math/MathML"> <n:mrow> <n:mi mathvariant="normal">F</n:mi> <n:msup> <n:mrow> <n:mi mathvariant="normal">e</n:mi> </n:mrow> <n:mrow> <n:mn>3</n:mn> <n:mo>+</n:mo> </n:mrow> </n:msup> </n:mrow> </n:math> ( <q:math xmlns:q="http://www.w3.org/1998/Math/MathML"> <q:mrow> <q:mn>20</q:mn> <q:mo>%</q:mo> </q:mrow> </q:math> in Li60Al40 or <r:math xmlns:r="http://www.w3.org/1998/Math/MathML"> <r:mrow> <r:mn>25</r:mn> <r:mo>%</r:mo> </r:mrow> </r:math> in Li50Al50) because of the respective <s:math xmlns:s="http://www.w3.org/1998/Math/MathML"> <s:mrow> <s:mi mathvariant="normal">A</s:mi> <s:msup> <s:mrow> <s:mi mathvariant="normal">l</s:mi> </s:mrow> <s:mrow> <s:mn>3</s:mn> <s:mo>+</s:mo> </s:mrow> </s:msup> </s:mrow> </s:math> -substitution for <v:math xmlns:v="http://www.w3.org/1998/Math/MathML"> <v:mrow> <v:mn>40</v:mn> <v:mo>%</v:mo> </v:mrow> </v:math> and <w:math xmlns:w="http://www.w3.org/1998/Math/MathML"> <w:mrow> <w:mn>50</w:mn> <w:mo>%</w:mo> </w:mrow> </w:math> <x:math xmlns:x="http://www.w3.org/1998/Math/MathML"> <x:mrow> <x:mi mathvariant="normal">L</x:mi> <x:msup> <x:mrow> <x:mi mathvariant="normal">i</x:mi> </x:mrow> <x:mo>+</x:mo> </x:msup> </x:mrow> </x:math> in the geometrically frustrated B-sublattice (the hyperkagome lattice) enhances not only the lattice-distortion-induced electric polarization, but also the intrinsic spin dynamics. All SR-related features in both Al-LFOs are effectively quenched above 0.5 kOe, where all magnetic spins of <ab:math xmlns:ab="http://www.w3.org/1998/Math/MathML"> <ab:mrow> <ab:mi mathvariant="normal">F</ab:mi> <ab:msup> <ab:mrow> <ab:mi mathvariant="normal">e</ab:mi> </ab:mrow> <ab:mrow> <ab:mn>3</ab:mn> <ab:mo>+</ab:mo> </ab:mrow> </ab:msup> </ab:mrow> </ab:math> are collinearly aligned parallel to the external magnetic fields, independent of frequency and temperature. These findings could be applied to further develop LFO-type multiferroics, e.g., for photovoltaic applications.