The Ghana international, 22, made 51 appearances for Bundesliga side Augsburg before moving to Stamford Bridge on a five-year deal last August.
Raman made his Chelsea debut in the Champions League and went on to make 23 appearances for the Blues.
His return to Germany reunites him with former Augsburg manager Markus Weinzierl, who took over as Schalke manager in June.
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Yanic Wildschut went close early on for the Latics when he cut inside and drove a shot against the upright.
Billy Clarke failed to make contact with Jamie Proctor's dangerous cross for the visitors after the interval.
And Vuckic ensured Wigan would remain second in the table with a composed finish before Wildschut drove wide as he attempted to add a second.
Defeat cost play-off hopefuls Bradford their place in the top six, with Yorkshire rivals Barnsley winning at Fleetwood.
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Wigan manager Gary Caldwell told BBC Radio Manchester: "I think we were always the team on the front foot. They made it difficult for us.
"I thought we kept going and showed great character, great belief in what we were doing, kept passing the ball and got our reward in the end."

Máiría Cahill won a by-election to fill the seat left vacant after Jimmy Harte of the Labour party resigned.
Labour candidate Ms Cahill won 122 of the 188 valid votes cast by serving senators and members of parliament.
Last year, she told the BBC she had been raped by an IRA man when she was 16, with the organisation forcing her into silence to protect itself.
The man she accused of raping her was prosecuted but later acquitted of all charges against him after two separate court cases in 2014.
Earlier this year, Northern Ireland's Public Prosecution Service apologised to Ms Cahill after a report into how it handled her allegations found delays in her case had been "unacceptable".
Ms Cahill is a member of one of the Irish republican movement's most prominent families.
But on Thursday, ahead of the by-election, she told Irish state broadcaster RTÉ she no longer considers herself a republican.
She said she was "deeply sorry" for her involvement in the dissident group Republican Network for Unity, adding that it coincided with a vulnerable point in her life.
"I didn't think about it at the time and I've had a lot of reflection since" she said.
After her election, Ms Cahill said she would invite sex abuse organisations to the senate to discuss support for victims.
"I want to use my time [in the senate] to continue to work for victims of abuse," she said.
"That is the issue that has brought me to where I am today and it is the issue that I will continue to campaign on during my time as a senator.
"I also want to use my time to explore ways of engaging young jobseekers in this country and to help change their lives for the better."
Ms Cahill had been the favourite to win the by-election.
The other candidates who stood were Fianna Fáil's Keith Swanick, who received 38 votes, Sinead Burke of Sinn Féin, who won 18 votes, and independent candidate Gerry Beades, who received 10.
Schalke have signed Chelsea defender Baba Rahman on a season-long loan.
[NEXT_CONCEPT]
Wigan extended their unbeaten run to 16 games as Haris Vuckic's goal gave them victory over Bradford City.
[NEXT_CONCEPT]
A Belfast woman who has accused the IRA of a sex abuse cover-up has won a seat in the Irish senate.