🔬 materials science

Specific features of the magnetic-field dependences of electrical resistivity in Bi--Mn solid solutions with low Mn content

This study demonstrates that while the magnetoresistance of a textured polycrystalline Bi $_{88.08}$ Mn $_{11.92}$ solid solution differs significantly from a lower-concentration Bi-Mn-Fe sample at low temperatures due to varying amounts of magnetic $\alpha$ -BiMn, their behaviors converge near room temperature, with the higher manganese content sample exhibiting substantially lower maximum magnetoresistance values attributed to internal magnetism influencing electrical transport.

Original authors: A. V. Terekhov, V. M. Yarovyi, Yu. A. Kolesnichenko, K. Rogacki, E. Lähderanta, E. V. Khristenko, A. L. Solovjov

Published 2026-02-24

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: A. V. Terekhov, V. M. Yarovyi, Yu. A. Kolesnichenko, K. Rogacki, E. Lähderanta, E. V. Khristenko, A. L. Solovjov

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you have a very special, almost magical metal called Bismuth. For over a century, scientists have loved studying it because it behaves strangely when you put it in a magnetic field. It's like a shy dancer who suddenly starts spinning wildly when music (magnetism) plays.

Recently, a team of scientists decided to mix this magical Bismuth with a little bit of Manganese (a magnetic metal) to see what happens. They created two different "recipes":

Recipe A: Mostly Bismuth with a tiny sprinkle of Manganese.
Recipe B: Bismuth with a much heavier dose of Manganese.

Their goal was to see how these mixtures conduct electricity when you turn up the magnetic "volume." Here is what they found, explained simply:

The Setup: A Highway with Roadblocks

Think of the Bismuth as a super-highway where electricity (cars) flows very smoothly.

In Recipe A, the highway is mostly clear, with just a few small construction zones (Manganese bits) scattered around.
In Recipe B, the highway is crowded with many more construction zones.

The scientists wanted to know: If we blast the highway with a giant magnetic field, do the cars speed up, slow down, or get stuck?

The Big Discovery: The "Goldilocks" Temperature

The most surprising thing they found was that the behavior of these mixtures depends entirely on the temperature, acting like a thermostat for the magnetic field's effect.

1. The Cold Zone (Below 100°C / -173°C):
When it's very cold, the two recipes behave very differently.

Recipe A (Less Manganese): The magnetic field acts like a super-strength booster. The resistance to electricity goes up massively (by nearly 4,000%!). It's like the magnetic field turns the smooth highway into a muddy swamp, making it incredibly hard for the cars to move.
Recipe B (More Manganese): Surprisingly, adding more Manganese actually made the "muddy swamp" effect weaker. The resistance still went up, but not as much as in Recipe A. It's as if the extra construction zones somehow helped the traffic flow a bit better than expected, or perhaps they changed the rules of the road entirely.

2. The Warm Zone (Above 100°C):
When the temperature gets warmer, the two recipes start to act the same again. The difference between them disappears, and they both behave like normal, predictable materials.

The "Spin" Mystery

Why did adding more Manganese make the effect weaker in the cold?
The scientists believe it has to do with spinning.

The Manganese atoms are like tiny magnets that can spin.
In the cold, these spins like to line up in a specific direction (perpendicular to the highway).
When you add too many Manganese atoms (Recipe B), they start bumping into each other and changing how they line up. This "rearranging" of the tiny magnets changes how the electricity flows through the Bismuth highway, canceling out some of the giant resistance effect seen in the lighter mixture.

The Two Ways to Point the Magnet

The scientists also tested pointing the magnetic field in two directions:

Perpendicular (⊥): The magnetic field hits the highway from the side. This caused the biggest resistance changes.
Parallel (//): The magnetic field runs along the highway. This caused smaller changes.

Interestingly, adding more Manganese hurt the "Parallel" performance much more than the "Perpendicular" one. It's like adding more construction zones to a straight road (Parallel) causes more traffic jams than adding them to a side road (Perpendicular).

The "Why Should We Care?" Part

Why does this matter?

Valleytronics: The scientists mention a new field of electronics called "Valleytronics." Imagine electrons not just as cars, but as cars that can choose different "valleys" (lanes) on a mountain road. Bismuth is a perfect place to study this. Understanding how to control these lanes with magnets could lead to super-fast, low-energy computers in the future.
Quantum Computers: Some scientists think these "valleys" could be used to build the memory chips for quantum computers.

The Bottom Line

This paper is a story about balance.

Too little Manganese, and the magnetic field creates a massive traffic jam (high resistance).
Too much Manganese, and the tiny magnets start rearranging themselves, changing the traffic rules and making the jam less severe.
The "sweet spot" for seeing the most dramatic effects is at a specific cold temperature (around 100 Kelvin).

The scientists concluded that while Bismuth is the main "highway," the Manganese "construction zones" are the ones actually controlling the traffic flow, especially when it's cold and the tiny magnets are spinning. This helps us understand how to design better materials for the electronics of tomorrow.

1. Problem Statement and Motivation

Bismuth (Bi) is a well-known semimetal with unique physical properties, including large positive magnetoresistance (MR) and topological features relevant to valleytronics. While Bi-Mn solid solutions are known for their ferromagnetic properties (Curie temperature ~640 K), their electronic transport behavior remains underexplored compared to their magnetic characteristics.

Previous work by the authors on a lower-Mn concentration sample ( $Bi_{95.69}Mn_{3.69}Fe_{0.62}$ ) revealed giant positive magnetoresistance and anomalies explained by a two-band conductivity model influenced by internal magnetism. However, the specific magnetic-field dependence of electrical resistance for a higher-Mn concentration sample ( $Bi_{88.08}Mn_{11.92}$ ) had not been studied. The primary objective of this research was to fill this gap, specifically investigating how increasing the manganese concentration (and consequently the volume fraction of the magnetic $\alpha$ -BiMn phase) affects the magnetotransport properties, particularly at low temperatures where magnetic reorientation occurs.

2. Methodology

Sample Synthesis: High-purity Bi and Mn ( $\ge$ 99.999%) were used to synthesize textured polycrystalline $Bi_{88.08}Mn_{11.92}$ via the Bridgman method in evacuated graphitized quartz ampoules at 630 K.
Sample Structure: The material consists of a bismuth matrix containing inclusions of the magnetic $\alpha$ -BiMn phase. The $Bi_{88.08}Mn_{11.92}$ sample contains a significantly higher volume fraction of these magnetic inclusions compared to the previously studied $Bi_{95.69}Mn_{3.69}Fe_{0.62}$ .
Experimental Setup:
- Measurement: Electrical resistance was measured using a standard four-probe configuration on a Quantum Design PPMS system.
- Conditions: Measurements were conducted in magnetic fields up to 90 kOe at temperatures ranging from 5 K to 300 K.
- Configurations: Two orientations were tested: Magnetic field perpendicular to current ( $H \perp I$ ) and parallel to current ( $H \parallel I$ ).
- Comparison: Data for $Bi_{88.08}Mn_{11.92}$ were directly compared with literature data for $Bi_{95.69}Mn_{3.69}Fe_{0.62}$ and pure bismuth.

3. Key Results

A. Magnetoresistance Behavior ( $H \perp I$ )

Low Temperature (5–80 K): The MR curves for different temperatures overlap up to ~22 kOe. Above this field, the curves diverge, and MR magnitude increases with temperature.
High Temperature (100–300 K): The behavior reverses; MR decreases as temperature increases. The maximum MR was observed at 100 K, reaching 3170% at 90 kOe.
Field Dependence: At 5 K, the dependence is linear in weak fields and follows a weak power law in strong fields. The exponent for the power law is higher in the high-Mn sample ( $Bi_{88.08}Mn_{11.92}$ ) than in the low-Mn sample.
Comparison: At temperatures above 150 K, the MR behavior of both Bi-Mn compositions becomes nearly identical.

B. Magnetoresistance Behavior ( $H \parallel I$ )

Low Temperature (5–80 K): MR increases monotonically up to ~26 kOe, then reaches a maximum before decreasing as the field increases further. The peak MR value increases linearly with temperature.
High Temperature (100–300 K): MR increases monotonically with the field, showing a tendency toward saturation. The maximum MR at 100 K was 380% at 90 kOe.
Field Dependence: Similar to the perpendicular case, the low-Mn sample exhibits a linear dependence in strong fields at 5 K, while the high-Mn sample shows power-law behavior with a tendency toward saturation.

C. Comparative Analysis

Magnitude: Increasing the Mn concentration (and thus the $\alpha$ $α$ -BiMn phase fraction) significantly reduces the maximum magnetoresistance.
- At 90 kOe and 100 K:
  - $Bi_{95.69}Mn_{3.69}Fe_{0.62}$ : 3877% ( $H \perp I$ ) / 742% ( $H \parallel I$ ).
  - $Bi_{88.08}Mn_{11.92}$ : 3170% ( $H \perp I$ ) / 380% ( $H \parallel I$ ).
Saturation: Unlike pure bismuth (which shows linear MR at high fields) or high-Mn ferromagnetic matrices (which show linear MR without saturation), the low-Mn Bi-Mn solid solutions exhibit complex power-law behaviors and saturation tendencies at low temperatures.

4. Discussion and Physical Interpretation

The authors propose that the transport current flows predominantly through the bismuth matrix, while the magnetic $\alpha$ -BiMn inclusions influence charge transport via internal magnetism.

Spin-Reorientation Transition: A critical factor is the magnetic structure change in Bi-Mn solid solutions below 100 K, where Mn magnetic moments reorient from perpendicular to parallel relative to the crystallographic c-axis. This transition explains the qualitative differences in MR behavior between the two samples below 100 K.
Fermi Surface Overlap: Bismuth is a nearly compensated semimetal with small Fermi energy. The internal magnetic field from $\alpha$ -BiMn inclusions likely alters the overlap of electron and hole pockets in k-space.
Quantum Limit vs. Magnetic Order: While a maximum in MR at low fields was previously attributed to the quantum limit (Landau levels) in pure bismuth, the observation of MR maxima up to 80 K in these samples suggests that changes in the magnetic order of the Mn inclusions are the dominant mechanism, rather than purely quantum effects.
Anisotropy: The significantly larger drop in MR for the $H \parallel I$ configuration compared to $H \perp I$ as Mn concentration increases is attributed to the higher magnetization of the $\alpha$ -BiMn phase in the parallel configuration due to the spin-reorientation transition.

5. Significance and Conclusions

Novelty: This is the first comprehensive study of the magnetic-field dependence of resistivity for $Bi_{88.08}Mn_{11.92}$ in both $H \perp I$ and $H \parallel I$ configurations.
Mechanism Insight: The study confirms that in low-Mn Bi-Mn solid solutions, the bismuth matrix governs conductivity, but the magnetic inclusions significantly modulate transport properties, especially at low temperatures where magnetic reorientation occurs.
Material Design: The findings highlight the sensitivity of magnetotransport to the concentration of magnetic phases. The reduction in giant MR with higher Mn content suggests a trade-off between magnetic phase volume and transport efficiency.
Future Work: The authors plan to conduct detailed magnetic property investigations of $Bi_{88.08}Mn_{11.92}$ to verify the hypothesis regarding the link between Mn magnetic order and the observed MR anomalies.

In summary, the paper elucidates the complex interplay between the semimetallic transport of bismuth and the internal magnetism of Mn-rich inclusions, demonstrating that magnetic reorientation transitions below 100 K are the primary drivers of anomalous magnetoresistance behavior in these solid solutions.